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Simard, M. & Nedergaard, M. The neurobiology of glia in the context of water and ion homeostasis. Neuroscience 129, 877-896
Abstract
Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output.
... The cessation of cerebral blood flow (CBF) during LVO stroke results in a severe reduction of oxygen and glucose supply within the downstream vascular territory [5] and is accompanied by functionally relevant alterations of intracellular and extracellular ion composition in the affected brain region [6,7]. These events are part of a complex set of mechanisms in response to the ischemic stimulus which has been coined the ischemic cascade. ...
... These events are part of a complex set of mechanisms in response to the ischemic stimulus which has been coined the ischemic cascade. Key parenchymal and intravascular processes that parallel and/or define tissue damage include: bioenergetic failure, excitotoxicity, intracellular calcium (Ca 2+ ) and sodium (Na + ) overload, extracellular accumulation of potassium (K + ), oxidative stress, and neuroinflammation [5][6][7][8][9][10]. These processes are closely interrelated as, e.g., severely reduced CBF leads to peri-infarct depolarization which is characterized by a regional switch from blood hypoxygenation to hyperoxygenation during propagation [11,12]. ...
... Normal neuroelectric activity and water content of the brain require the careful orchestration and proper distribution of intracellular and extracellular ions. The observed relative hypokalemia and increase in Na + /K + ratios support the notion of significant ion movements within ischemic brain regions which are characterized by net K + losses and/or Na + gains [6,7]. The literature suggests that astrocytes may form a functional syncytium for extracellular and intravascular potassium clearance as a means to control neuronal excitability in viable tissue [7]. ...
Purpose
Disturbances of blood gas and ion homeostasis including regional hypoxia and massive sodium (Na⁺)/potassium (K⁺) shifts are a hallmark of experimental cerebral ischemia but have not been sufficiently investigated for their relevance in stroke patients.
Methods
We report a prospective observational study on 366 stroke patients who underwent endovascular thrombectomy (EVT) for large-vessel occlusion (LVO) of the anterior circulation (18 December 2018–31 August 2020). Intraprocedural blood gas samples (1 ml) from within cerebral collateral arteries (ischemic) and matched systemic control samples were obtained according to a prespecified protocol in 51 patients.
Results
We observed a significant reduction in cerebral oxygen partial pressure (−4.29%, paO2ischemic = 185.3 mm Hg vs. paO2systemic = 193.6 mm Hg; p = 0.035) and K⁺ concentrations (−5.49%, K⁺ischemic = 3.44 mmol/L vs. K⁺systemic = 3.64 mmol/L; p = 0.0083). The cerebral Na⁺:K⁺ ratio was significantly increased and negatively correlated with baseline tissue integrity (r = −0.32, p = 0.031). Correspondingly, cerebral Na⁺ concentrations were most strongly correlated with infarct progression after recanalization (r = 0.42, p = 0.0033). We found more alkaline cerebral pH values (+0.14%, pHischemic = 7.38 vs. pHsystemic = 7.37; p = 0.0019), with a time-dependent shift towards more acidotic conditions (r = −0.36, p = 0.055).
Conclusion
These findings suggest that stroke-induced changes in oxygen supply, ion composition and acid-base balance occur and dynamically progress within penumbral areas during human cerebral ischemia and are related to acute tissue damage.
... Subsequently, in the second half of the 19th century, the neuroanatomist Santiago Ramón y Cajal, was able to visualize astrocytes for the first time by using a gold and mercury chloride-sublimate staining (Ramón y Cajal, 1913) labeling a protein later identified by Eng et al. (1971) as glial fibrillary acidic protein (GFAP). These pioneering discoveries paved the path to countless studies that served to highlight the plethora of functions operated by astrocytes in the CNS, such as synapse maturation and elimination (Chung et al., 2015), ion and neurotransmitters homeostasis (Simard and Nedergaard, 2004), regulation of functional hyperemia (Macvicar and Newman, 2015), and modulation of synaptic plasticity (Bains and Oliet, 2007;Ota et al., 2013). ...
... As structural components of the neurovascular unit, astrocytes are essential for the formation and maintenance of the blood brain barrier (BBB) (Virchow, 1858;Bélanger et al., 2011;Cabezas et al., 2014;Macvicar and Newman, 2015), for the transport of cerebrospinal fluid (CSF) in the glymphatic system (Iliff and Nedergaard, 2013;Jessen et al., 2015), and for metabolic support (Bélanger et al., 2011). Astrocytes also notably assist synapse formation and maintenance (Chung et al., 2015), participate in the tripartite synapse (Araque et al., 1999;Perea et al., 2009;Farhy-Tselnicker and Allen, 2018), modulate synaptic plasticity (Bains and Oliet, 2007;Ota et al., 2013), and regulate neurotransmitters uptake and recycling (Sonnewald et al., 1997;Simard and Nedergaard, 2004). For a systematic and general description of all the functions operated by astrocytes we refer to other reviews (Khakh and Sofroniew, 2015;Verkhratsky and Nedergaard, 2018). ...
The vision of astroglia as a bare scaffold to neuronal circuitry has been largely overturned. Astrocytes exert a neurotrophic function, but also take active part in supporting synaptic transmission and in calibrating blood circulation. Many aspects of their functioning have been unveiled from studies conducted in murine models, however evidence is showing many differences between mouse and human astrocytes starting from their development and encompassing morphological, transcriptomic and physiological variations when they achieve complete maturation. The evolutionary race toward superior cognitive abilities unique to humans has drastically impacted neocortex structure and, together with neuronal circuitry, astrocytes have also been affected with the acquisition of species-specific properties. In this review, we summarize diversities between murine and human astroglia, with a specific focus on neocortex, in a panoramic view that starts with their developmental origin to include all structural and molecular differences that mark the uniqueness of human astrocytes.
... Reactive astrogliosis describes a spectrum of heterogeneous changes in gene expression [46][47][48], cell morphology [47][48][49][50][51][52], and overall function including fluid and ion homeostasis [53,54], oxidative stress response [55][56][57] and synapse formation [58,59]. These changes can also range from reversible to permanent [60,61]. ...
... Conversely, "regulation of protein phosphorylation" pathways are enriched across datasets, suggesting a fundamental role for cellular regulatory processes in astrocytes and psychiatric disorders. Overall, pathway analysis identified biological processes that are commonly dysregulated in psychiatric disorders [53][54][55][56][57][58][59], and supports a role for astrocytes in a subset of these pathological processes. Color intensity is proportional to -log10 (p-value). ...
Astrocytes have many important functions in the brain, but their roles in psychiatric disorders and their responses to psychotropic medications are still being elucidated. Here, we used gene enrichment analysis to assess the relationships between different astrocyte subtypes, psychiatric diseases, and psychotropic medications (antipsychotics, antidepressants and mood stabilizers). We also carried out qPCR analyses and “look-up” studies to assess the chronic effects of these drugs on astrocyte marker gene expression. Our bioinformatic analysis identified gene enrichment of different astrocyte subtypes in psychiatric disorders. The highest level of enrichment was found in schizophrenia, supporting a role for astrocytes in this disorder. We also found differential enrichment of astrocyte subtypes associated with specific biological processes, highlighting the complex responses of astrocytes under pathological conditions. Enrichment of protein phosphorylation in astrocytes and disease was confirmed by biochemical analysis. Analysis of LINCS chemical perturbagen gene signatures also found that kinase inhibitors were highly discordant with astrocyte-SCZ associated gene signatures. However, we found that common gene enrichment of different psychotropic medications and astrocyte subtypes was limited. These results were confirmed by “look-up” studies and qPCR analysis, which also reported little effect of psychotropic medications on common astrocyte marker gene expression, suggesting that astrocytes are not a primary target of these medications. Conversely, antipsychotic medication does affect astrocyte gene marker expression in postmortem schizophrenia brain tissue, supporting specific astrocyte responses in different pathological conditions. Overall, this study provides a unique view of astrocyte subtypes and the effect of medications on astrocytes in disease, which will contribute to our understanding of their role in psychiatric disorders and offers insights into targeting astrocytes therapeutically.
... Even though all cells are perfect osmometers, glial cells and particularly astrocytes and MC play a pivotal role in the mobilization of osmolytes and water in the direction required to counteract the osmotic change. Astrocytes and MC are strategically located between the vasculature and the synaptic structure, sharing functional and neurochemical properties with neuronal cells and regulating the homeostasis of extracellular fluids through several membrane proteins responsible for the active and passive transport of ions, organic osmolytes, and osmotically obligated water (Bormann and Kettenmann, 1988;O'Neill, 1999;Simard and Nedergaard, 2004;Pasantes-Morales and Vázquez-Juárez, 2012;Reed and Blazer-Yost, 2022). In this work, we are considering the systems responsible for cell volume control points in astrocytes and MC as key players in water homeostasis in the brain and retina (Nicchia et al., 2004;Simard and Nedergaard, 2004;Bringmann et al., 2005;Kuhrt et al., 2008). ...
... Astrocytes and MC are strategically located between the vasculature and the synaptic structure, sharing functional and neurochemical properties with neuronal cells and regulating the homeostasis of extracellular fluids through several membrane proteins responsible for the active and passive transport of ions, organic osmolytes, and osmotically obligated water (Bormann and Kettenmann, 1988;O'Neill, 1999;Simard and Nedergaard, 2004;Pasantes-Morales and Vázquez-Juárez, 2012;Reed and Blazer-Yost, 2022). In this work, we are considering the systems responsible for cell volume control points in astrocytes and MC as key players in water homeostasis in the brain and retina (Nicchia et al., 2004;Simard and Nedergaard, 2004;Bringmann et al., 2005;Kuhrt et al., 2008). Cell volume regulation. ...
Brain edema is a pathological condition with potentially fatal consequences, related to cerebral injuries such as ischemia, chronic renal failure, uremia, and diabetes, among others. Under these pathological states, the cell volume control processes are fully compromised, because brain cells are unable to regulate the movement of water, mainly regulated by osmotic gradients. The processes involved in cell volume regulation are homeostatic mechanisms that depend on the mobilization of osmolytes (ions, organic molecules, and polyols) in the necessary direction to counteract changes in osmolyte concentration in response to water movement. The expression and coordinated function of proteins related to the cell volume regulation process, such as water channels, ion channels, and other cotransport systems in the glial cells, and considering the glial cell proportion compared to neuronal cells, leads to consider the astroglial network the main regulatory unit for water homeostasis in the central nervous system (CNS). In the last decade, several studies highlighted the pivotal role of glia in the cell volume regulation process and water homeostasis in the brain, including the retina; any malfunction of this astroglial network generates a lack of the ability to regulate the osmotic changes and water movements and consequently exacerbates the pathological condition.
... It is widely acknowledged that syncytial systems formed of multiple cells of the same type exist in the CNS. Links between astrocytes, for example, can be formed via gap junctions (Simard and Nedergaard 2004;Wallraff et al. 2006;Dallérac et al. 2018). When many astrocytes are connected in this way, the resulting syncytial system effectively acts as a single cell with an expansive receptive field, permitting homeostasis of global conditions and thus global activity (Simard and Nedergaard 2004;Wallraff et al. 2006;Dallérac et al. 2018). ...
... Links between astrocytes, for example, can be formed via gap junctions (Simard and Nedergaard 2004;Wallraff et al. 2006;Dallérac et al. 2018). When many astrocytes are connected in this way, the resulting syncytial system effectively acts as a single cell with an expansive receptive field, permitting homeostasis of global conditions and thus global activity (Simard and Nedergaard 2004;Wallraff et al. 2006;Dallérac et al. 2018). Until this point, however, most studies have conceptualised the carotid body as either a complete organ or a collection of individual cells that communicate using the linear logic of the canonical model of carotid body function. ...
The classic peripheral chemoreflex response is a critical homeostatic mechanism. In healthy individuals, appropriate chemoreflex responses are triggered by acute activation of the carotid body - the principal chemosensory organ in mammals. However, the aberrant chronic activation of the carotid body can drive the elevated sympathetic activity underlying cardio-respiratory diseases such as hypertension, diabetes and heart failure. Carotid body resection induces intolerable side effects and so understanding how to modulate carotid body output without removing it, and whilst maintaining the physiological chemoreflex response, represents the next logical next step in the development of effective clinical interventions. By definition, excessive carotid body output must result from altered intra-carotid body inter-cellular communication. Alongside the canonical synaptic transmission from glomus cells to petrosal afferents, many other modes of information exchange in the carotid body have been identified, for example bidirectional signalling between type I and type II cells via ATP-induced ATP release, as well as electrical communication via gap junctions. Thus, herein we review the carotid body as an integrated circuit, discussing a variety of different inter-cellular signalling mechanisms and highlighting those that are potentially relevant to its pathological hyperactivity in disease with the aim of identifying novel therapeutic targets.
... In a healthy central nervous system (CNS), astrocytes participate in neural/synaptic development, blood flow regulation, ion, and neurotransmitter balance, blood-brain barrier (BBB) formation, and synaptic function (Barres, 2008;Pereira & Furlan, 2010;Santello et al., 2019;Schummers et al., 2008;Simard & Nedergaard, 2004). Under normal conditions, astrocytes exist in a 'naïve' state with diverse morphology but can also be activated to undergo complex changes to their structural-functional properties in a process known as reactive astrogliosis (Pekny & Pekna, 2014;Sofroniew, 2015;Verkhratsky, Ho, et al., 2019). ...
A better understanding of nicotine neurobiology is needed to reduce or prevent chronic addiction, ameliorate the detrimental effects of nicotine withdrawal, and increase successful cessation of use. Nicotine binds and activates two astrocyte-expressed nicotinic acetylcholine receptors (nAChRs), α4β2 and α7. We recently found that Protein kinase B-β (Pkb-β or Akt2) expression is restricted to astrocytes in mice and humans. To determine if AKT2 plays a role in astrocytic nicotinic responses, we generated astrocyte-specific Akt2 conditional knockout (cKO) and full Akt2 KO mice for in vivo and in vitro experiments. For in vivo studies, we examined mice exposed to chronic nicotine for two weeks in drinking water (200 μg/mL) and following acute nicotine challenge (0.09, 0.2 mg/kg) after 24 hrs. Our in vitro studies used cultured mouse astrocytes to measure nicotine-dependent astrocytic responses. We validated our approaches using lipopolysaccharide (LPS) exposure inducing astrogliosis. Sholl analysis was used to measure glial fibrillary acidic protein responses in astrocytes. Our data show that wild-type (WT) mice exhibit increased astrocyte morphological complexity during acute nicotine exposure, with decreasing complexity during chronic nicotine use, whereas Akt2 cKO mice showed increased astrocyte morphology complexity. In culture, we found that 100μM nicotine was sufficient for morphological changes and blocking α7 or α4β2 nAChRs prevented observed morphologic changes. Finally, we performed conditioned place preference (CPP) in Akt2 cKO mice and found that astrocytic AKT2 deficiency reduced nicotine preference compared to controls. These findings show the importance of nAChRs and Akt2 signaling in the astrocytic response to nicotine.
... 2,3 They are enriched in channels and transport mechanisms that take up water and solutes and thereby drive perfusion of the brain parenchyma. 4,5 Cerebrospinal fluid (CSF) flows from subarachnoid sinuses through perivascular spaces between vascular cells and astrocytes, forming the so-called glymphatic circulation. 6,7 The topology, volume, and content of perivascular space through which the glymphatic circulation flows have been subjects of considerable experimental research and hydraulic modeling studies. ...
Astrocyte endfeet enwrap brain vasculature, forming a boundary for perivascular glymphatic flow of fluid and solutes along and across the astrocyte endfeet into the brain parenchyma. We evaluated astrocyte sensitivity to shear stress generated by such flow, finding a set point for downstream calcium signaling that is below about 0.1 dyn/cm². This set point is modulated by albumin levels encountered in cerebrospinal fluid (CSF) under normal conditions and following a blood-brain barrier breach or immune response. The astrocyte mechanosome responsible for the detection of shear stress includes sphingosine-1-phosphate (S1P)-mediated sensitization of the mechanosensor Piezo1. Fluid flow through perivascular channels delimited by vessel wall and astrocyte endfeet thus generates sufficient shear stress to activate astrocytes, thereby potentially controlling vasomotion and parenchymal perfusion. Moreover, S1P receptor signaling establishes a set point for Piezo1 activation that is finely tuned to coincide with CSF albumin levels and to the low shear forces resulting from glymphatic flow.
... Glia are known to be important for water regulation. [29][30][31] In insect eyes, this cell type is best understood in Drosophila melanogaster, where genetic evidence suggests that Semper cells are resident eye glia, sharing distinct similarities with vertebrate eye-specific Mü ller glia. 22 Mü ller glia have an osmoregulatory role in maintaining eye function by utilizing water (aqp4) and solute transport (kir4.1) ...
... Emerging evidence indicates a potential contribution of hypothalamic non-neuronal cells glia in regulating body weight and peripheral nutrient metabolism 4,5 . Astrocytes, a prominent class of glial cells in the mammalian brain, play a vital role in supporting neuronal metabolism 6 , modulating synaptic activity 7 , and maintaining electrolyte and water balance 8 . In addition, astrocytes uphold the integrity of the blood-brain barrier 9 and regulate the transport of glucose into the brain 10 . ...
Nicotinamide adenine dinucleotide (NAD)⁺ serves as a crucial coenzyme in numerous essential biological reactions, and its cellular availability relies on the activity of the nicotinamide phosphoribosyltransferase (NAMPT)-catalyzed salvage pathway. Here we show that treatment with saturated fatty acids activates the NAD⁺ salvage pathway in hypothalamic astrocytes. Furthermore, inhibition of this pathway mitigates hypothalamic inflammation and attenuates the development of obesity in male mice fed a high-fat diet (HFD). Mechanistically, CD38 functions downstream of the NAD⁺ salvage pathway in hypothalamic astrocytes burdened with excess fat. The activation of the astrocytic NAMPT–NAD⁺–CD38 axis in response to fat overload induces proinflammatory responses in the hypothalamus. It also leads to aberrantly activated basal Ca²⁺ signals and compromised Ca²⁺ responses to metabolic hormones such as insulin, leptin, and glucagon-like peptide 1, ultimately resulting in dysfunctional hypothalamic astrocytes. Our findings highlight the significant contribution of the hypothalamic astrocytic NAD⁺ salvage pathway, along with its downstream CD38, to HFD-induced obesity.
... Astrocytes play an important role in brain energy metabolism [1][2][3][4], but have also crucial functions in brain development [5], (ion) homeostasis [6][7][8][9], the regulation and modulation of neuronal signals [10,11], memory formation [12] and the protection against toxins and oxidative stress [13][14][15]. Although astrocytes are considered as a rather glycolytic cell type [16], also the oxidative metabolism plays an important role for astrocytic energy regeneration [17][18][19]. ...
Astrocyte-derived pyruvate is considered to have neuroprotective functions. In order to investigate the processes that are involved in astrocytic pyruvate release, we used primary rat astrocyte cultures as model system. Depending on the incubation conditions and medium composition, astrocyte cultures established extracellular steady state pyruvate concentrations in the range between 150 µM and 300 µM. During incubations for up to 2 weeks in DMEM culture medium, the extracellular pyruvate concentration remained almost constant for days, while the extracellular lactate concentration increased continuously during the incubation into the millimolar concentration range as long as glucose was present. In an amino acid-free incubation buffer, glucose-fed astrocytes released pyruvate with an initial rate of around 60 nmol/(h × mg) and after around 5 h an almost constant extracellular pyruvate concentration was established that was maintained for several hours. Extracellular pyruvate accumulation was also observed, if glucose had been replaced by mannose, fructose, lactate or alanine. Glucose-fed astrocyte cultures established similar extracellular steady state concentrations of pyruvate by releasing pyruvate into pyruvate-free media or by consuming excess of extracellular pyruvate. Inhibition of the monocarboxylate transporter MCT1 by AR-C155858 lowered extracellular pyruvate accumulation, while inhibition of mitochondrial pyruvate uptake by UK5099 increased the extracellular pyruvate concentration. Finally, the presence of the uncoupler BAM15 or of the respiratory chain inhibitor antimycin A almost completely abolished extracellular pyruvate accumulation. The data presented demonstrate that cultured astrocytes establish a transient extracellular steady state concentration of pyruvate which is strongly affected by modulation of the mitochondrial pyruvate metabolism.
... In the CNS, glial cells significantly influence cell structure similar to cell homeostasis. Astrocytes, also known collectively as astroglia, participate in all essential CNS functions including homeostasis and neurogenesis (Aschner, 1998;Becher et al., 2000;Simard and Nedergaard, 2004;Otaegi et al., 2011;Oberheim et al., 2012;Howarth, 2014;Tay et al., 2015). Previous studies suggest that microRNAs can directly modify the glial phenotype and cause astrocyte dysfunction and promote astrocyte proliferation in models of chronic spinal cord injury (Hoye et al., 2018;Wang et al., 2018). ...
Neural stem cells (NSCs) are defined by their ability to self-renew and generate various cell types within the nervous system. Understanding the underlying mechanism by which NSCs proliferate and differentiate is crucial for the efficient modulation of in vivo neurogenesis. MicroRNAs are small non-coding RNAs controlling gene expression concerned in post-transcriptional control by blocking messenger RNA (mRNA) translation or degrading mRNA. MicroRNAs play a role as modulators by matching target mRNAs. Recent studies have discussed the biological mechanism of microRNA regulation in neurogenesis. To investigate the role of microRNAs in NSCs and NSC-derived glial cells, we screened out NSC-specific microRNAs by using miRNome-wide screening. Then, we induced downregulation by the sponge against the specific microRNA to evaluate the functional role of the microRNA in proliferation, differentiation, and apoptosis in NSCs and NSC-derived astrocytes. We found that microRNA-325-3p is highly expressed in NSCs and astrocytes. Furthermore, we showed that microRNA-325-3p is a regulator of apoptosis by targeting brain-specific angiogenesis inhibitor (BAI1), which is a receptor for apoptotic cells and expressed in the brain and cultured astrocytes. Downregulation of microRNA-325-3p using an inducible sponge system induced cell death by regulating BAI1 in NSCs and NSC-derived astrocytes. Overall, our findings can provide an insight into the potential roles of NSC-specific microRNAs in brain neurogenesis and suggest the possible usage of the microRNAs as biomarkers of neurodegenerative disease.
... During development, astrocytes play a role in guiding the migration of neuronal axons and neuroblast (Powell and Geller, 1999), and the formation of developing synapses (Ullian et al., 2001;Christopherson et al., 2005); moreover, they can drive microglial synapse engulfment, or actively engulf synapses and sculpt neuronal circuits (Chung et al., 2013;Vainchtein et al., 2018). With their terminal processes (end-feet), astrocytes contribute to the formation and maintenance of brain-blood integrity (Abbott, 2002); thank to the presence of several plasma membrane transporters, during neuronal activity they can buffer extracellular K + concentration and water content (Simard and Nedergaard, 2004), regulate the extracellular pH and remove excessive glutamate from the synapses (Rose et al., 2018). Astrocytes sense neuronal activity via metabotropic neurotransmitter receptors, and are able to provide energy substrate to neurons through the so call "astrocyteneuron lactate shuttle" (Magistretti and Pellerin, 1999); in addition, astrocytic networks can support the high energy demand of neuronal activity, also at site distant from blood vessels (Rouach et al., 2008), thus ensuring glia-neurons metabolic coupling necessary for memory formation (Suzuki et al., 2011;Gao et al., 2016). ...
Astrocytes are highly plastic cells whose activity is essential to maintain the cerebral homeostasis, regulating synaptogenesis and synaptic transmission, vascular and metabolic functions, ions, neuro- and gliotransmitters concentrations. In pathological conditions, astrocytes may undergo transient or long-lasting molecular and functional changes that contribute to disease resolution or exacerbation. In recent years, many studies demonstrated that non-neoplastic astrocytes are key cells of the tumor microenvironment that contribute to the pathogenesis of glioblastoma, the most common primary malignant brain tumor and of secondary metastatic brain tumors. This Mini Review covers the recent development of research on non-neoplastic astrocytes as tumor-modulators. Their double-edged capability to promote cancer progression or to represent potential tools to counteract brain tumors will be discussed.
... The cascade-heterogated effect allowed for the preferential transport of multiple ions by the sorting of ionic transfer energy barriers. Meanwhile, such transmitted ionic signal strengths of HBGs have already achieved levels compatible with those of specific physiological bioactivities (28,29). Unlike ion-selective membranes and hydrogels, our HBGs can store a stable ion source on demand and efficiently process intrinsic ionic signals to the ambient aqueous environment. ...
Currently, electronics and iontronics in abiotic-biotic systems can only use electrons and single-species ions as unitary signal carriers. Thus, a mechanism of gating transmission for multiple biosignals in such devices is needed to match and modulate complex aqueous-phase biological systems. Here we report the use of cascade-heterogated biphasic gel iontronics to achieve diverse electronic-to-multi-ionic signal transmission. The cascade-heterogated property determined the transfer free energy barriers experienced by ions and ionic hydration-dehydration states under an electric potential field, fundamentally enhancing the distinction of cross-interface transmission between different ions by several orders of magnitude. Such heterogated or chemical-heterogated iontronics with programmable features can be coupled with multi-ion cross-interface mobilities for hierarchical and selective cross-stage signal transmission. We expect that such iontronics would be ideal candidates for a variety of biotechnology applications.
... These processes involve a variety of channels and transporters, enabling astrocytes to monitor essential elements, such as fluids, ions, and neurotransmitters. They regulate these components to maintain a healthy synaptic microenvironment 42,43 . In addition to their role in regulating synaptic homeostasis, astrocytes act as "tuners" on a larger scale during CNS development. ...
Glial cell activation precedes neuronal cell death during brain aging and the progression of neurodegenerative diseases. Under neuroinflammatory stress conditions, lipocalin-2 (LCN2), also known as neutrophil gelatinase-associated lipocalin or 24p3, is produced and secreted by activated microglia and reactive astrocytes. Lcn2 expression levels are known to be increased in various cells, including reactive astrocytes, through the activation of the NF-κB signaling pathway. In the central nervous system, as LCN2 exerts neurotoxicity when secreted from reactive astrocytes, many researchers have attempted to identify various strategies to inhibit LCN2 production, secretion, and function to minimize neuroinflammation and neuronal cell death. These strategies include regulation at the transcriptional, posttranscriptional, and posttranslational levels, as well as blocking its functions using neutralizing antibodies or antagonists of its receptor. The suppression of NF-κB signaling is a strategy to inhibit LCN2 production, but it may also affect other cellular activities, raising questions about its effectiveness and feasibility. Recently, LCN2 was found to be a target of the autophagy‒lysosome pathway. Therefore, autophagy activation may be a promising therapeutic strategy to reduce the levels of secreted LCN2 and overcome neurodegenerative diseases. In this review, we focused on research progress on astrocyte-derived LCN2 in the central nervous system.
... For instance, brain water homeostasis plays a remarkable role in neural activity balance. In this context, ionic equilibrium and water distribution are milestones in brain tissue activity [65]. ...
Aquaporins (AQPs) are a family of membrane proteins involved in the transport of water and ions across cell membranes. AQPs have been shown to be implicated in various physiological and pathological processes in the brain, including water homeostasis, cell migration, and inflammation, among others. Epileptogenesis is a complex and multifactorial process that involves alterations in the structure and function of neuronal networks. Recent evidence suggests that AQPs may also play a role in the pathogenesis of epilepsy. In animal models of epilepsy, AQPs have been shown to be upregulated in regions of the brain that are involved in seizure generation, suggesting that they may contribute to the hyperexcitability of neuronal networks. Moreover, genetic studies have identified mutations in AQP genes associated with an increased risk of developing epilepsy. Our review aims to investigate the role of AQPs in epilepsy and seizure onset from a pathophysiological point of view, pointing out the potential molecular mechanism and their clinical implications.
... Astrocytic Ca 2+ signal is key step in gliotransmitter release, potassium buffering, and activity transferring through the astrocytic network. 7,14,15 However, whether the astrocytic Ca 2+ signal influences the sensory information processing by cortical networks remains unknown. ...
Cortical neuron-astrocyte communication in response to peripheral sensory stimulation occurs in a topographic-, frequency-, and intensity-dependent manner. However, the contribution of this functional interaction to the processing of sensory inputs and consequent behavior remains unclear. We investigate the role of astrocytes in sensory information processing at circuit and behavioral levels by monitoring and manipulating astrocytic activity in vivo. We show that astrocytes control the dynamic range of the cortical network activity, optimizing its responsiveness to incoming sensory inputs. The astrocytic modulation of sensory processing contributes to setting the detection threshold for tactile and thermal behavior responses. The mechanism of such astrocytic control is mediated through modulation of inhibitory transmission to adjust the gain and sensitivity of responding networks. These results uncover a role for astrocytes in maintaining the cortical network activity in an optimal range to control behavior associated with specific sensory modalities.
... Glial cells are functionally divided into separate groups such as astrocytes, microglia, oligodendrocytes, etc. Astrocytes affect neuronal function in various ways, for example, by regulating the concentration of ions and neurotransmitters [1,2], releasing gliotransmitters that can act directly on neuronal receptors [3,4], and modulating neuronal excitability, synaptic transmission and plasticity [5,6]. Moreover, astrocytes establish direct connections with neurons by forming tripartite synapses [7,8]. ...
Alzheimer’s disease (AD) is one of the most widespread neurodegenerative diseases. Most of the current AD therapeutic developments are directed towards improving neuronal cell function or facilitating Aβ amyloid clearance from the brain. However, some recent evidence suggests that astrocytes may play a significant role in the pathogenesis of AD. In this paper, we evaluated the effects of the optogenetic activation of Gq-coupled exogenous receptors expressed in astrocytes as a possible way of restoring brain function in the AD mouse model. We evaluated the effects of the optogenetic activation of astrocytes on long-term potentiation, spinal morphology and behavioral readouts in 5xFAD mouse model of AD. We determined that in vivo chronic activation of astrocytes resulted in the preservation of spine density, increased mushroom spine survival, and improved performance in cognitive behavioral tests. Furthermore, chronic optogenetic stimulation of astrocytes resulted in the elevation of EAAT-2 glutamate uptake transporter expression, which could be a possible explanation for the observed in vivo neuroprotective effects. The obtained results suggest that the persistent activation of astrocytes may be considered a potential therapeutic approach for the treatment of AD and possibly other neurodegenerative disorders.
... As ROS accumulate, peroxidation of membrane lipids and proteins impairs smooth muscle ion channels, thus reducing myogenic reactivity [46,47]. Concurrently, astrocytic endfeet edema may lead to microvascular compression, further impairing vascular reactivity [48]. ...
Poor outcomes in Subarachnoid Hemorrhage (SAH) are in part due to a unique form of secondary neurological injury known as Delayed Cerebral Ischemia (DCI). DCI is characterized by new neurological insults that continue to occur beyond 72 h after the onset of the hemorrhage. Historically, it was thought to be a consequence of hypoperfusion in the setting of vasospasm. However, DCI was found to occur even in the absence of radiographic evidence of vasospasm. More recent evidence indicates that catastrophic ionic disruptions known as Cortical Spreading Depolarizations (CSD) may be the culprits of DCI. CSDs occur in otherwise healthy brain tissue even without demonstrable vasospasm. Furthermore, CSDs often trigger a complex interplay of neuroinflammation, microthrombi formation, and vasoconstriction. CSDs may therefore represent measurable and modifiable prognostic factors in the prevention and treatment of DCI. Although Ketamine and Nimodipine have shown promise in the treatment and prevention of CSDs in SAH, further research is needed to determine the therapeutic potential of these as well as other agents.
... Mounting evidence suggests that astrocytes directly modulate synaptic activity, including synaptic transmission, formation, and elimination as well as neuronal repair (Newman, 2003;Achour and Pascual, 2010;Chung et al., 2015;Burda et al., 2016). Moreover, astrocytes can affect neuronal activity indirectly via regulation of the transfer of metabolites through the blood-brain barrier, provision of metabolic support, and maintenance of ionic homeostasis in the extracellular environment (Simard and Nedergaard, 2004;Deitmer et al., 2019;Rose et al., 2020b), see also Table 1. In the below section, we have attempted to summarise the molecular signalling pathways underlying the astrocytic effect on neuronal signalling and function. ...
Neuronal signalling is a key element in neuronal communication and is essential for the proper functioning of the CNS. Astrocytes, the most prominent glia in the brain play a key role in modulating neuronal signalling at the molecular, synaptic, cellular, and network levels. Over the past few decades, our knowledge about astrocytes and their functioning has evolved from considering them as merely a brain glue that provides structural support to neurons, to key communication elements. Astrocytes can regulate the activity of neurons by controlling the concentrations of ions and neurotransmitters in the extracellular milieu, as well as releasing chemicals and gliotransmitters that modulate neuronal activity. The aim of this review is to summarise the main processes through which astrocytes are modulating brain function. We will systematically distinguish between direct and indirect pathways in which astrocytes affect neuronal signalling at all levels. Lastly, we will summarize pathological conditions that arise once these signalling pathways are impaired focusing on neurodegeneration.
... Astrocytes are star-shaped glial cells with radial processes that play essential functions as blood-brain barrier formation and maintenance (Abbott et al., 2006;Daneman and Prat, 2015;Janzer and Raff, 1987), ionic environment regulation (Anderson and Swanson, 2000;Sattler and Rothstein, 2006;Seifert et al., 2006;Simard and Nedergaard, 2004;Strohschein et al., 2011), control of neurogenesis and glycogen storage (Brown et al., 2005;Brown and Ransom, 2007;Matsui et al., 2017), neurometabolic uncoupling (Magistretti, 2006), iron-induce antioxidant protection (Hoepken et al., 2004;Oide et al., 2006;Regan et al., 2002), among others. When the brain tissue is damaged, astrocytes exhibit hypertrophy (Hol and Pekny, 2015;Kimelberg, 2004), and alter gene expression of glial fibrillary acidic protein (GFAP) resulting in a reactive state (Sofroniew, 2009). ...
Astrocytes perform multiple essential functions in the brain showing morphological changes. Hypertrophic astrocytes are commonly observed in cognitively healthy aged animals, implying a functional defense mechanism without losing neuronal support. In neurodegenerative diseases, astrocytes show morphological alterations, such as decreased process length and reduced number of branch points, known as astroglial atrophy, with detrimental effects on neuronal cells. The common marmoset (Callithrix jacchus) is a non-human primate that, with age, develops several features that resemble neurodegeneration. In this study, we characterize the morphological alterations in astrocytes of adolescent (mean 1.75 y), adult (mean 5.33 y), old (mean 11.25 y), and aged (mean 16.83 y) male marmosets. We observed a significantly reduced arborization in astrocytes of aged marmosets compared to younger animals in the hippocampus and entorhinal cortex. These astrocytes also show oxidative damage to RNA and increased nuclear plaques in the cortex and tau hyperphosphorylation (AT100). Astrocytes lacking S100A10 protein show a more severe atrophy and DNA fragmentation. Our results demonstrate the presence of atrophic astrocytes in the brains of aged marmosets.
... Astrocytes exist ubiquitously throughout the central nervous system (CNS), performing numerous key functions, including maintaining extracellular ionic balance, neurotransmitter clearance, modulating the synaptic connectivity and plasticity, providing metabolic support to neurons, and protecting nearby cells by secreting neurotrophic factors [7,13,45,51,56]. Given the important roles and supportive capacity in the CNS, it is not surprising that malfunction of astrocytes is directly responsible for various pathological conditions. ...
Molecular and functional diversity among region-specific astrocytes is of great interest in basic neuroscience and the study of neurological diseases. In this study, we present the generation and characterization of astrocytes from human embryonic stem cells with the characteristics of the ventral midbrain (VM). Fine modulation of WNT and SHH signaling during neural differentiation induced neural precursor cells (NPCs) with high expression of EN1 and NKX6.1, but less expression of FOXA2. Overexpression of nuclear factor IB in NPCs induced astrocytes, thereby maintaining the expression of region-specific genes acquired in the NPC stage. When cocultured with dopaminergic (DA) precursors or DA neurons, astrocytes with VM characteristics (VM-iASTs) promoted the differentiation and survival of DA neurons better than those that were not regionally specified. Transcriptomic analysis showed that VM-iASTs were more closely related to human primary midbrain astrocytes than to cortical astrocytes, and revealed the upregulation of WNT1 and WNT5A, which supports their VM identity and explains their superior activity in DA neurons. Taken together, we hope that VM-iASTs can serve to improve ongoing DA precursor transplantation for Parkinson’s disease, and that their transcriptomic data provide a valuable resource for investigating regional diversity in human astrocyte populations.
Graphical Abstract
... Astrocytes sustain numerous functions in the central nervous system, from controlling the water homeostasis and extracellular ion concentrations [1,2], regulating the blood flow [3], secreting humoral and trophic factors [4] providing metabolic support for neurones [5] and maintaining glutamate, GABA and adenosine homeostasis [6,7]. Furthermore, astroglia contributes to synaptogenesis, synaptic development, regulation and remodelling [8,9]. ...
Astrocytes contribute to the progression of neurodegenerative diseases, including Alzheimer's disease (AD). Here, we report the neuroanatomical and morphometric analysis of astrocytes in the entorhinal cortex (EC) of the aged wild type (WT) and triple transgenic (3xTg-AD) mouse model of AD. Using 3D confocal microscopy, we determined the surface area and volume of positive astrocytic profiles in male mice (WT and 3xTg-AD) from 1 to 18 months of age. We showed that S100β-positive astrocytes were equally distributed throughout the entire EC in both animal types and showed no changes in Nv (number of cells/mm3) nor in their distribution at the different ages studied. These positive astrocytes, demonstrated an age-dependent gradual increase in their surface area and in their volume starting at 3 months of age, in both WT and 3xTg-AD mice. This last group demonstrated a large increase in both surface area and volume at 18 months of age when the burden of pathological hallmarks of AD is present (69.74% to 76.73% in the surface area and the volume, for WT and 3xTg-AD mice respectively). We observed that these changes were due to the enlargement of the cell processes and to less extend the somata. In fact, the volume of the cell body was increased by 35.82% in 18-month-old 3xTg-AD compared to WT. On the other hand, the increase on the astrocytic processes were detected as soon as 9 months of age where we found an increase of surface area and volume (36.56% and 43.73%, respectively) sustained till 18 month of age (93.6% and 113.78%, respectively) when compared age-matched non-Tg mice. Moreover, we demonstrated that these hypertrophic S100β-positive astrocytes were mainly associated with Aβ plaques. Our results show a severe atrophy in GFAP cytoskeleton in all cognitive areas; whilst within the EC astrocytes independent to this atrophy show no changes in GS and S100β; which can play a key role in the memory impairment.
... Most notably, researchers must ensure that the appropriate cell types are included within their culture model to effectively recapitulate the in vivo disease state. This is especially true for CNS models as crosstalk between neurons and glia play significant roles in both pathological conditions [1][2][3][4][5] and maintaining homeostasis [6,7]. The microelectrode array (MEA) technology is a popular method to study in vitro neural networks, as it provides a non-invasive method to simultaneously record electrophysiological activity from multiple sites [8,9]. ...
Neuroinflammation plays a central role in many neurological disorders, ranging from traumatic brain injuries to neurodegeneration. Electrophysiological activity is an essential measure of neuronal function, which is influenced by neuroinflammation. In order to study neuroinflammation and its electrophysiological fingerprints, there is a need for in vitro models that accurately capture the in vivo phenomena. In this study, we employed a new tri-culture of primary rat neurons, astrocytes, and microglia in combination with extracellular electrophysiological recording techniques using multiple electrode arrays (MEAs) to determine the effect of microglia on neural function and the response to neuroinflammatory stimuli. Specifically, we established the tri-culture and its corresponding neuron-astrocyte co-culture (lacking microglia) counterpart on custom MEAs and monitored their electrophysiological activity for 21 days to assess culture maturation and network formation. As a complementary assessment, we quantified synaptic puncta and averaged spike waveforms to determine the difference in excitatory to inhibitory neuron ratio (E/I ratio) of the neurons. The results demonstrate that the microglia in the tri-culture do not disrupt neural network formation and stability and may be a better representation of the in vivo rat cortex due to its more similar E/I ratio as compared to more traditional isolated neuron and neuron-astrocyte co-cultures. In addition, only the tri-culture displayed a significant decrease in both the number of active channels and spike frequency following pro-inflammatory lipopolysaccharide exposure, highlighting the critical role of microglia in capturing electrophysiological manifestations of a representative neuroinflammatory insult. We expect the demonstrated technology to assist in studying various brain disease mechanisms.
... However, there are inherent limitations within these models that limit their physiological relevance, most notably researchers much ensure that the appropriate cell types are included within their culture model to effectively recapitulate the in vivo disease state. This is especially true for CNS models as crosstalk between neurons and glia play significant roles in both pathological conditions [1][2][3][4][5] and maintaining homeostasis [6,7]. Microelectrode array (MEA) technology is a popular method to study in vitro neural networks, as it provides a non-invasive method to simultaneously record electrophysiological activity from multiple sites [8,9]. ...
BACKGROUND
We have previously described a tri-culture of neurons, astrocytes, and microglia that accurately mimics the in vivo neuroinflammatory response (both neurotoxic and neuroprotective) to a wide range of neuroinflammatory stimuli. Electrophysiological activity is an essential measure of neuronal function, which is influenced by neuroinflammation. Microelectrode array (MEA) technology is a versatile tool to non-invasively study in vitro neural networks by simultaneously recording electrophysiological activity from multiple sites. In this study, we used extracellular recordings to determine the effect of microglia on neural network formation and stability in primary cortical cultures and monitor the changes in neural activity in response to neuroinflammatory stimuli.
METHODS
Primary neonatal rat cortical tri-cultures of neurons, astrocytes, and microglia or co-cultures of neurons and astrocytes were cultured on custom MEAs and the neural activity was monitored for 21 days in vitro to assess culture maturation and network formation. Quantification of synaptic puncta and averaged spike waveforms were used to determine the difference in excitatory to inhibitory neuron ratio (E/I ratio) of the neurons in tri- and co-cultures. The electrophysiological response to lipopolysaccharide (LPS) treatment of both culture types were compared.
RESULTS
The tri- and co-culture showed minimal difference in electrophysiological markers of neural network formation and stability with the exception of a significant increase in spike frequency in the tri-culture at later timepoints (DIV 17 and 21). Additionally, there was no significant difference in the density of either post-synaptic or excitatory pre-synaptic puncta between the culture types. However, characterization of the average spike waveforms revealed that the tri-culture had an E/I ratio much closer to that found in the rat cortex. Finally, only the tri-culture displayed a significant decrease in both the number of active channels and spike frequency following LPS exposure.
CONCLUSIONS
This study demonstrates that the microglia in the tri-culture do not disrupt neural network formation and stability as quantified using extracellular recordings and may be a better representation of the in vivo cortex due to the closer E/I ratio than more traditional isolated neuron and neuron-astrocyte co-cultures. Additionally, the tri-culture is better able to mimic the neuroinflammatory response to LPS, which was quantified via changes in neural electrophysiological activity.
... In the CNS, astrocytes maintain ion and water homeostasis by clearing excess neuroactive substances (such as glutamate or K + ions) from sites with high neuronal activity to the extracellular space in a neuronal activity-dependent manner [46,47]. TRPV4 is particularly abundant in astrocytic membranes at the interface between brain tissues and extracerebral liquid spaces [26,34]. ...
Transient receptor potential vanilloid 4 (TRPV4) is a nonselective cation channel that can be activated by diverse stimuli, such as heat, mechanical force, hypo-osmolarity, and arachidonic acid metabolites. TRPV4 is widely expressed in the central nervous system (CNS) and participates in many significant physiological processes. However, accumulative evidence has suggested that deficiency, abnormal expression or distribution, and overactivation of TRPV4 are involved in pathological processes of multiple neurological diseases. Here, we review the latest studies concerning the known features of this channel, including its expression, structure, and its physiological and pathological roles in the CNS, proposing an emerging therapeutic strategy for CNS diseases.
... Astrocytes play a critical role in shuttling energy resources to neurons, responding to glutamatergic activity by producing lactate and releasing it into the extracellular space, which neurons then utilize for metabolic demands [65][66][67]. Astrocytes also regulate neurotransmitter release and re-uptake, to help maintain neuronal homeostasis [13,[68][69][70][71]. In humans, regarding glial cells, the fovea contains only MG subtypes, indicating that these cells alone are sufficient to supply the metabolic needs of the RGCs [13,71]. ...
Glaucoma is a progressive age-related disease of the visual system and the leading cause of irreversible blindness worldwide. Currently, intraocular pressure (IOP) is the only modifiable risk factor for the disease, but even as IOP is lowered, the pathology of the disease often progresses. Hence, effective clinical targets for the treatment of glaucoma remain elusive. Glaucoma shares comorbidities with a multitude of vascular diseases, and evidence in humans and animal models demonstrates an association between vascular dysfunction of the retina and glaucoma pathology. Integral to the survival of retinal ganglion cells (RGCs) is functional neurovascular coupling (NVC), providing RGCs with metabolic support in response to neuronal activity. NVC is mediated by cells of the neurovascular unit (NVU), which include vascular cells, glial cells, and neurons. Nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signaling is a prime mediator of NVC between endothelial cells and neurons, but emerging evidence suggests that cGMP signaling is also important in the physiology of other cells of the NVU. NO-cGMP signaling has been implicated in glaucomatous neurodegeneration in humans and mice. In this review, we explore the role of cGMP signaling in the different cell types of the NVU and investigate the potential links between cGMP signaling, breakdown of neurovascular function, and glaucoma pathology.
... Increased swelling of astrocytic end-feet, which then compress the capillary lumen, was observed in a rodent model of SAH [5]. Multiple lines of evidence suggest that upregulation of the AQP4 expression, which is predominantly found in capillaries surrounding astrocyte end-feet, is associated with cytotoxic edema formation [6]. Currently, available treatments for cerebral edema are limited to hypothermia, osmotherapy and surgical decompression, which are usually administered based on symptoms and often lead to adverse side effects [7]. ...
Aneurysmal subarachnoid hemorrhage (SAH) can cause severe neurological deficits and high mortality. Early brain edema following SAH contributes to the initiation of microcirculation impairment and may further lead to delayed ischemic neurologic deficit (DIND). This study aimed to investigate whether dental pulp stem cell conditioned medium (DPSC-CM) ameliorates SAH-induced microcirculation impairment and the underlying mechanisms. SAH was induced via intrathecal injection of fresh autologous blood in Wistar male adult rat. DPSC-CM or DPSC-CM + insulin growth factor-1 (IGF-1) antibody was randomly administered by intrathecal route 5 min after SAH induction. To evaluate the underlying mechanisms of DPSC-CM in the treatment of SAH, primary rat astrocyte and microglia co-cultures were challenged with hemolysate or SAH-patient CSF in the presence or absence of DPSC-CM. The results showed that in vivo, DPSC-CM treatment decreased the brain water content, improved microcirculation impairment and enhanced functional recovery at 24 h post-SAH. DPSC-CM treatment also alleviated the expressions of water channel protein aquaporin-4 (AQP4) and pro-inflammatory cytokines, and enhanced the expressions of anti-inflammatory factors in the cortical region. However, all the beneficial effects of DPSC-CM were abrogated after treatment with IGF-1 neutralizing antibody. The in vitro results further showed that DPSC-CM treatment reduced hemolysate/SAH-patient CSF-induced astrocyte swelling and promoted M2 microglia polarization, partially through IGF-1/AKT signaling. The data suggested that DPSC-CM significantly reduced brain edema and rescued microcirculation impairment with concomitant anti-inflammatory benefits after SAH, and may potentially be developed into a novel therapeutic strategy for SAH.
... Astrocytes maintain a functional environment in the CNS by controlling fluid and ion homeostasis and modulate synaptic transmission by influencing synaptic plasticity and by regulating extracellular electrolytes and neurotransmitters (Danbolt, 2001;Simard & Nedergaard, 2004; D. G. Souza et al., 2019). They are also capable of releasing neuroactive molecules themselves, so called gliotransmitters (Araque et al., 2014;Verkhratsky et al., 2016). ...
... Glial cells, in particular astrocytes, which are one of the most common glial cell types in the brain [8], are essential components of the osmotic regulatory system and play crucial roles in water and ion homeostasis. Astrocytes undergo rapid volume changes during different neuronal activities, and this process can induce a redistribution of water and ions with the local swelling of astrocytes at the sites of neuronal activities together with the shrinkage at distant sites [9][10][11][12][13][14]. Astrocytes adjacent to neurons during osmotic challenges can control swelling of their end-feet to regulate vasopressin secretion in the supraoptic nucleus and the paraventricular nucleus, and to produce other osmolality-modulating hormones [15,16]. In addition, astrocyte swelling occurs not only at the cellular level but also in their subcellular organelles such as the endoplasmic reticulum, the mitochondria, as well as the nucleus. ...
Astrocytes have distinctive morphological and functional characteristics, and are found throughout the central nervous system. Astrocytes are now known to be far more than just housekeeping cells in the brain. Their functions include contributing to the formation of the blood–brain barrier, physically and metabolically supporting and communicating with neurons, regulating the formation and functions of synapses, and maintaining water homeostasis and the microenvironment in the brain. Aquaporins (AQPs) are transmembrane proteins responsible for fast water movement across cell membranes. Various subtypes of AQPs (AQP1, AQP3, AQP4, AQP5, AQP8 and AQP9) have been reported to be expressed in astrocytes, and the expressions and subcellular localizations of AQPs in astrocytes are highly correlated with both their physiological and pathophysiological functions. This review describes and summarizes the recent advances in our understanding of astrocytes and AQPs in regard to controlling water homeostasis in the brain. Findings regarding the features of different AQP subtypes, such as their expression, subcellular localization, physiological functions, and the pathophysiological roles of astrocytes are presented, with brain edema and glioma serving as two representative AQP-associated pathological conditions. The aim is to provide a better insight into the elaborate “water distribution” system in cells, exemplified by astrocytes, under normal and pathological conditions.
... The NVU plays an essential role in maintaining cerebral blood flow and BBB integrity (Zlokovic, 2008). Astrocytes support brain health by interacting with the NVU and other cell types in the brain parenchyma (Colombo & Farina, 2016;Szu & Binder, 2016) and by providing essential growth factors and metabolites (Eidsvaag et al., 2017;Hoddevik et al., 2017;Seifert et al., 2006;Simard & Nedergaard, 2004;Zeppenfeld et al., 2017). Expression of the astrocyte marker glial fibrillary acidic protein (GFAP) increases with age in humans and mice (Kimbroughet al., 2015;Kovacs et al., 2018;Kress et al., 2014;Stichel & Luebbert, 2007;Wruck & Adjaye, 2020;Zhuang et al., 2019), plays an important role in astrogliosis (Faulkner et al., 2004;Lundkvist et al., 2004;McLean & Lane, 1995;Nawashiro et al., 1998;Pekny & Pekna, 2004;Sofroniew & Vinters, 2010), and its increased expression is correlated with Alzheimer's disease (AD) (Wruck et al., 2016). ...
Introduction:
Increasing age is the number one risk factor for developing cognitive decline and neurodegenerative disease. Aged humans and mice exhibit numerous molecular changes that contribute to a decline in cognitive function and increased risk of developing age-associated diseases. Here, we characterize multiple age-associated changes in male C57BL/6J mice to understand the translational utility of mouse aging.
Methods:
Male C57BL/6J mice from various ages between 2 and 24 months of age were used to assess behavioral, as well as, histological and molecular changes across three modalities: neuronal, microgliosis/neuroinflammation, and the neurovascular unit (NVU). Additionally, a cohort of 4- and 22-month-old mice was used to assess blood-brain barrier (BBB) breakdown. Mice in this cohort were treated with a high, acute dose of lipopolysaccharide (LPS, 10 mg/kg) or saline control 6 h prior to sacrifice followed by tail vein injection of 0.4 kDa sodium fluorescein (100 mg/kg) 2 h later.
Results:
Aged mice showed a decline in cognitive and motor abilities alongside decreased neurogenesis, proliferation, and synapse density. Further, neuroinflammation and circulating proinflammatory cytokines were increased in aged mice. Additionally, we found changes at the BBB, including increased T cell infiltration in multiple brain regions and an exacerbation in BBB leakiness following chemical insult with age. There were also a number of readouts that were unchanged with age and have limited utility as markers of aging in male C57BL/6J mice.
Conclusions:
Here we propose that these changes may be used as molecular and histological readouts that correspond to aging-related behavioral decline. These comprehensive findings, in the context of the published literature, are an important resource toward deepening our understanding of normal aging and provide an important tool for studying aging in mice.
... Astrocytes are the most abundant glial cells in the brain(Booth et al 2017). They regulate brain homeostasis and microenvironment through the uptake of potassium ions and glutamate through the inward rectifier potassium channels and the excitatory amino acid transporter, (EAAT)-1/2(Olsen et al 2006, Rothstein et al 1996, Simard & Nedergaard 2004. They also form a major component of the BBB and help to maintain this structure that limits the entry of blood-borne elements in the CNS(Dong & Benveniste 2001a). ...
Parkinson’s disease (PD), which is the most common motor neurodegenerative disorder has attracted a tremendous amount of research advancement amid the challenges of the lack of an appropriate model that summate all the features of the human disease. Nevertheless, an aspect of the disease that is yet to be fully elucidated is the role of the immune system particularly the adaptive arm in the pathogenesis of PD. The focus of this study therefore was to characterize the contribution of lymphocytes in PD using the AAV1/2-A53T-α-synuclein mouse model of the disease that encodes for human mutated A53T-α-synuclein. This model was suitable for this research because it reflects more faithfully the molecular pathology underlying the human disease by exhibition of insoluble α-synuclein containing Lewy-like protein aggregates as compared to the more classical toxin models used in PD research. The outcome of this study showed that stereotaxic delivery of pathogenic α-synuclein via a viral vector into the substantia nigra engender the invasion of activated CD4+ and CD8+ T lymphocytes in the brain. The invasion of activated T cells in the brain especially in the substantia nigra then results in enhanced microglial activation and the disintegration of dopaminergic neurons. In addition, it was also discovered that CD4+ T cells augmented dopaminergic cell death to a greater extent than CD8+ T cells although; axonal degeneration occurred relatively independent from T cells contribution. The ex vivo and in vitro, experiments also indicated that the T cells were not only activated but they were specific to the mutated human α-synuclein antigen. As a result, they demonstrated selectivity in inducing more cell death to primary hippocampal neurons transduced with AAV1/2-A53T-α-synuclein vector than neurons with empty viral vector infection. The mechanism of T cell induced neuronal cell loss could not be attributed to the presence of cytokines neither was it mediated through MHC I and II. On the whole, this research has established that the presence of pathogenic α-synuclein in the substantia nigra has the potential to trigger immune responses that involve the transmigration of adaptive immune cells into the brain. The infiltration of the T cells consequently has a detrimental effect on the survival of dopaminergic neurons and the progression of the disease
... Liberan glutamato, D-serina, GABA y ATP a las neuronas. Los astrocitos expresan transportadores de glutamato, canales de K + y canales de agua, lo que les permite contribuir al mantenimiento de la homeostasis del glutamato, la homeostasis del potasio y la homeostasis del intercambio de agua con los puentes de H, respectivamente [ 25 ]. ...
El cerebro humano ha evolucionado a partir de un órgano olfativo mamífero, que permite en el corto tiempo una capacidad para la coordinación continua y el control muscular. En los humanos, el nivel de comunicación con feromonas permanece durante un tiempo prolongado hasta que podría ser reemplazado por una nueva memoria de recuperación audiovisual, a nivel consciente. Este contiene memoria de la etapa evolutiva previa como vestigios a un nivel subconsciente, que estructura los complejos psicosomáticos. En la etapa de crianza, la creación de una memoria transitoria no podría ser el ADN nuclear de las neuronas de los circuitos neuronales.
... Astrocytes are the most abundant cell type in the central nervous system (CNS) and are involved in key functions such as maintaining ionic homeostasis (Simard and Nedergaard, 2004), regulating the blood-brain barrier (BBB) (Abbott, 2002), and providing metabolic support (Brown and Ransom, 2007). Because astrocytes display such diverse functions, any changes in astrocyte molecules significantly impact epileptogenesis (Binder and Steinhäuser, 2021). ...
Epilepsy is a chronic brain disorder characterized by unprovoked seizures. Mechanisms underlying seizure activity have been intensely investigated. Alterations in astrocytic channels and transporters have shown to be a critical player in seizure generation and epileptogenesis. One key protein involved in such processes is the astrocyte water channel aquaporin-4 (AQP4). Studies have revealed that perivascular AQP4 redistributes away from astrocyte endfeet and toward the neuropil in both clinical and preclinical studies. This subcellular mislocalization significantly impacts neuronal hyperexcitability and understanding how AQP4 becomes dysregulated in epilepsy is beginning to emerge. In this review, we evaluate the role of AQP4 dysregulation and mislocalization in epilepsy.
... Astrocytes are known components of the neurovascular unit and are part of the blood-brain barrier, mediating nutrient exchange from bloodstream to nerve cells (Simard and Nedergaard, 2004). In agreement with this function, CETN2-positive protoplasmic astrocytes were detected around vasculature with their end feet tightly juxtaposed onto the vessel surface (Figures 3F-F"). ...
As microtubule-organizing centers (MTOCs), centrosomes play a pivotal role in cell division, neurodevelopment and neuronal maturation. Among centrosomal proteins, centrin-2 (CETN2) also contributes to DNA repair mechanisms which are fundamental to prevent genomic instability during neural stem cell pool expansion. Nevertheless, the expression profile of CETN2 in human neural stem cells and their progeny is currently unknown. To address this question, we interrogated a platform of human neuroepithelial stem (NES) cells derived from post mortem developing brain or established from pluripotent cells and demonstrated that while CETN2 retains its centrosomal location in proliferating NES cells, its expression pattern changes upon differentiation. In particular, we found that CETN2 is selectively expressed in mature astrocytes with a broad cytoplasmic distribution. We then extended our findings on human autoptic nervous tissue samples. We investigated CETN2 distribution in diverse anatomical areas along the rostro-caudal neuraxis and pointed out a peculiar topography of CETN2-labeled astrocytes in humans which was not appreciable in murine tissues, where CETN2 was mostly confined to ependymal cells. As a prototypical condition with glial overproliferation, we also explored CETN2 expression in glioblastoma multiforme (GBM), reporting a focal concentration of CETN2 in neoplastic astrocytes. This study expands CETN2 localization beyond centrosomes and reveals a unique expression pattern that makes it eligible as a novel astrocytic molecular marker, thus opening new roads to glial biology and human neural conditions.
... Astrocytes have functional and structural domains that are estimated to be in contact with 200-600 dendrites and about 10 5 synapses [122,123] and their extensive contacts with synaptic sites ensure strict control of local ions [124], pH homeostasis [125], the delivery of metabolic substrates to neurons [126][127][128][129], control of the microvasculature [130], and modulation of synaptic activity and plasticity by releasing neuroactive substances [42,[131][132][133][134][135][136][137][138]. Perisynaptic astrocytes also express many transporters of amino acid neurotransmitters, including glutamate and GABA, and remove neurotransmitters to maintain transmitter homeostasis [139], thus assuring the rapid and efficient control of the speed and extent of neurotransmitter clearance, a mechanism involved in synaptic plasticity [140,141]. ...
The 22q11 deletion syndrome (DS) is the most common microdeletion syndrome in humans and gives a high probability of developing psychiatric disorders. Synaptic and neuronal malfunctions appear to be at the core of the symptoms presented by patients. In fact, it has long been suggested that the behavioural and cognitive impairments observed in 22q11DS are probably due to alterations in the mechanisms regulating synaptic function and plasticity. Often, synaptic changes are related to structural and functional changes observed in patients with cognitive dysfunctions, therefore suggesting that synaptic plasticity has a crucial role in the pathophysiology of the syndrome. Most interestingly, among the genes deleted in 22q11DS, six encode for mitochondrial proteins that, in mouse models, are highly expressed just after birth, when active synaptogenesis occurs, therefore indicating that mitochondrial processes are strictly related to synapse formation and maintenance of a correct synaptic signalling. Because correct synaptic functioning, not only requires correct neuronal function and metabolism, but also needs the active contribution of astrocytes, we summarize in this review recent studies showing the involvement of synaptic plasticity in the pathophysiology of 22q11DS and we discuss the relevance of mitochondria in these processes and the possible involvement of astrocytes.
... However, recent studies have established several roles of glia in regulating brain functions. These functions include the formation of the blood-brain barrier by supporting endothelial cells , regulation of blood flow during enhanced neuronal activity, bidirectional communications with neurons in regulating synaptogenesis, myelination (Nave 2010), neurotransmission, provision of substrates for neuronal energy metabolism (Pellerin et al. 1998), and maintenance of extracellular ion homeostasis (Simard and Nedergaard 2004). The current understanding of functional and spatial relationship of astrocytes to neurons has led to the tripartite concept of the synaptic cleft. ...
Major depressive disorder is a debilitating psychiatric condition that affects millions of people worldwide. Until recently, the neuronal component(s) of the brain was considered to be major players in the pathophysiology of depression. Recent advancements in Neuroscience research has suggested the involvement of glila cells in the biology of depression. Glial cell morphology, density, and glial markers were found to be altered in the brain of both, depressed subjects and preclinical models of depression. Moreover, perturbed neuron-glial communication in depressed subjects reveals a critical role of neurotransmitter cycling in the progression of the disease. Many ongoing studies are focused on uncovering the therapeutic potential of glial cells’ manipulation in the treatment of depression. In this chapter, we have focused not only on the importance of glial cells in the efficient functioning of the brain but also on how neurons and glial cells communicate with each other and how their involvement could facilitate the progression of MDD. In addition, we have also reviewed the advancements in biophysical techniques like nuclear magnetic resonance (NMR) and positron emission tomography (PET) that are presently used to study different aspects of neuroenergetics. We conclude by proposing that further in-depth glia-centric studies pertaining to depression and associated psychiatric conditions would be radical for developing more efficient therapeutic strategies.
The creatine‐phosphocreatine cycle serves as a crucial temporary energy buffering system in the brain, regulated by brain creatine kinase (CKB), in maintaining Adenosine triphosphate (ATP) levels. Alzheimer's disease (AD) has been linked to increased CKB oxidation and loss of its regulatory function, although specific pathological processes and affected cell types remain unclear. In our study, cerebral cortex samples from individuals with AD, dementia with Lewy bodies (DLB), and age‐matched controls were analyzed using antibody‐based methods to quantify CKB levels and assess alterations associated with disease processes. Two independently validated antibodies exclusively labeled astrocytes in the human cerebral cortex. Combining immunofluorescence (IF) and mass spectrometry (MS), we explored CKB availability in AD and DLB cases. IF and Western blot analysis demonstrated a loss of CKB immunoreactivity correlated with increased plaque load, severity of tau pathology, and Lewy body pathology. However, transcriptomics data and targeted MS demonstrated unaltered total CKB levels, suggesting posttranslational modifications (PTMs) affecting antibody binding. This aligns with altered efficiency at proteolytic cleavage sites indicated in the targeted MS experiment. These findings highlight that the proper function of astrocytes, understudied in the brain compared with neurons, is highly affected by PTMs. Reduction in ATP levels within astrocytes can disrupt ATP‐dependent processes, such as the glutamate‐glutamine cycle. As CKB and the creatine‐phosphocreatine cycle are important in securing constant ATP availability, PTMs in CKB, and astrocyte dysfunction may disturb homeostasis, driving excitotoxicity in the AD brain. CKB and its activity could be promising biomarkers for monitoring early‐stage energy deficits in AD.
Lipids are an extremely heterogeneous group of compounds, resulting in a wide variety of biological functions they perform. The traditional view of lipids as important structural components of the cell and compounds playing a trophic role is currently being supplemented by information on the possible participation of lipids in signaling, not only intracellular, but also intercellular. The review article discusses current data on the role of lipids and their metabolites formed in glial cells (astrocytes, oligodendrocytes, microglia) in the communication of these cells with neurons. In addition to the metabolic transformations of lipids in each type of glial cells, special attention is paid to the lipid signal molecules (phosphatidic acid, arachidonic acid and its metabolites, cholesterol, etc.) and the possibility of their participation in the implementation of the synaptic plasticity, as well as in other possible mechanisms associated with the realization of the neuroplasticity. The generalization of these new data can significantly expand knowledge about the regulatory functions of lipids in neuroglial relationships.
Multiple sclerosis (MS) is a neurodegenerative disease, which is also referred to as an autoimmune disorder with chronic inflammatory demyelination affecting the core system that is the central nervous system (CNS). Demyelination is a pathological manifestation of MS. It is the destruction of myelin sheath, which is wrapped around the axons, and it results in the loss of synaptic connections and conduction along the axon is also compromised. Various attempts are made to understand MS and demyelination using various experimental models out of them. The most popular model is experimental autoimmune encephalomyelitis (EAE), in which autoimmunity against CNS components is induced in experimental animals by immunization with self-antigens derived from basic myelin protein. Astrocytes serve as a dual-edged sword both in demyelination and remyelination. Various drug targets have also been discussed that can be further explored for the treatment of MS. An extensive literature research was done from various online scholarly and research articles available on PubMed, Google Scholar, and Elsevier. Keywords used for these articles were astrocyte, demyelination, astrogliosis, and reactive astrocytes. This includes articles being the most relevant information to the area compiled to compose a current review.
Hyponatremia is the most common electrolyte abnormality encountered in critically ill patients and is linked to heightened morbidity, mortality, and healthcare resource utilization. However, its causal role in these poor outcomes and the impact of treatment remain unclear. Plasma sodium is the main determinant of plasma tonicity; consequently, hyponatremia commonly indicates hypotonicity but can also occur in conjunction with isotonicity and hypertonicity. Plasma sodium is a function of total body exchangeable sodium and potassium and total body water. Hypotonic hyponatremia arises when total body water is proportionally greater than the sum of total body exchangeable cations, that is, electrolyte-free water excess; the latter is the result of increased intake or decreased (kidney) excretion. Hypotonic hyponatremia leads to water movement into brain cells resulting in cerebral edema. Brain cells adapt by eliminating solutes, a process that is largely completed by 48 h. Clinical manifestations of hyponatremia depend on its biochemical severity and duration. Symptoms of hyponatremia are more pronounced with acute hyponatremia where brain adaptation is incomplete while they are less prominent in chronic hyponatremia. The authors recommend a physiological approach to determine if hyponatremia is hypotonic, if it is mediated by arginine vasopressin, and if arginine vasopressin secretion is physiologically appropriate. The treatment of hyponatremia depends on the presence and severity of symptoms. Brain herniation is a concern when severe symptoms are present, and current guidelines recommend immediate treatment with hypertonic saline. In the absence of significant symptoms, the concern is neurologic sequelae resulting from rapid correction of hyponatremia which is usually the result of a large water diuresis. Some studies have found desmopressin useful to effectively curtail the water diuresis responsible for rapid correction.
The aim of this review is to explore the relationship between melatonin, free radicals, and non-excitatory amino acids, and their role in stroke and aging. Melatonin has garnered significant attention in recent years due to its diverse physiological functions and potential therapeutic benefits by reducing oxidative stress, inflammation, and apoptosis. Melatonin has been found to mitigate ischemic brain damage caused by stroke. By scavenging free radicals and reducing oxidative damage, melatonin may help slow down the aging process and protect against age-related cognitive decline. Additionally, non-excitatory amino acids have been shown to possess neuroprotective properties, including antioxidant and anti-inflammatory in stroke and aging-related conditions. They can attenuate oxidative stress, modulate calcium homeostasis, and inhibit apoptosis, thereby safeguarding neurons against damage induced by stroke and aging processes. The intracellular accumulation of certain non-excitatory amino acids could promote harmful effects during hypoxia-ischemia episodes and thus, the blockade of the amino acid transporters involved in the process could be an alternative therapeutic strategy to reduce ischemic damage. On the other hand, the accumulation of free radicals, specifically mitochondrial reactive oxygen and nitrogen species, accelerates cellular senescence and contributes to age-related decline. Recent research suggests a complex interplay between melatonin, free radicals, and non-excitatory amino acids in stroke and aging. The neuroprotective actions of melatonin and non-excitatory amino acids converge on multiple pathways, including the regulation of calcium homeostasis, modulation of apoptosis, and reduction of inflammation. These mechanisms collectively contribute to the preservation of neuronal integrity and functions, making them promising targets for therapeutic interventions in stroke and age-related disorders.
Subarachnoid hemorrhage (SAH) is a type of hemorrhagic stroke resulting from the rupture of an arterial vessel within the brain. Unlike other stroke types, SAH affects both young adults (mid-40s) and the geriatric population. Patients with SAH often experience significant neurological deficits, leading to a substantial societal burden in terms of lost potential years of life. This review provides a comprehensive overview of SAH, examining its development across different stages (early, intermediate, and late) and highlighting the pathophysiological and pathohistological processes specific to each phase. The clinical management of SAH is also explored, focusing on tailored treatments and interventions to address the unique pathological changes that occur during each stage. Additionally, the paper reviews current treatment modalities and pharmacological interventions based on the evolving guidelines provided by the American Heart Association (AHA). Recent advances in our understanding of SAH will facilitate clinicians’ improved management of SAH to reduce the incidence of delayed cerebral ischemia in patients.
Ceruloplasmin (Cp) is a ferroxidase enzyme that is essential for cell iron efflux. The absence of this protein in humans and rodents produces progressive neurodegeneration with brain iron accumulation. Astrocytes express high levels of Cp and iron efflux from these cells has been shown to be central for oligodendrocyte maturation and myelination. To explore the role of astrocytic Cp in brain development and aging we generated a specific conditional KO mouse for Cp in astrocytes (Cp cKO). Deletion of Cp in astrocytes during the first postnatal week induced hypomyelination and a significant delay in oligodendrocyte maturation. This abnormal myelin synthesis was exacerbated throughout the first two postnatal months and accompanied by a reduction in oligodendrocyte iron content, as well as an increase in brain oxidative stress. In contrast to young animals, deletion of astrocytic Cp at 8 months of age engendered iron accumulation in several brain areas and neurodegeneration in cortical regions. Aged Cp cKO mice also showed myelin loss and oxidative stress in oligodendrocytes and neurons, and at 18 months of age, developed abnormal behavioral profiles, including deficits in locomotion and short-term memory. In summary, our results demonstrate that iron efflux-mediated by astrocytic Cp-is essential for both early oligodendrocyte maturation and myelin integrity in the mature brain. Additionally, our data suggest that astrocytic Cp activity is central to prevent iron accumulation and iron-induced oxidative stress in the aging CNS.
While pain is sensed and conducted by neurons, including primary sensory neurons (nociceptors) and spinal cord pain transmission neurons, mounting evidence suggests that non-neuronal cells such as immune cells and glial cells in the peripheral nervous system (PNS) and central nervous system (CNS) play active roles in the pathogenesis and resolution of pain. We review how immune cells and glial cells interact with peripheral and central nociceptive neurons by secreting neuroactive signaling molecules (neuromodulators), leading to altered pain sensitivity. It is generally believed that chronic pain is maintained by central sensitization, that is, increased synaptic and neuronal responsiveness (synaptic or neural plasticity) in central pain pathways, after painful injuries and insults. Recent studies also suggest that central sensitization is driven by neuroinflammation. We also discuss how immune cells and glial cells regulate central sensitization and neuroinflammation in the context of chronic pain.KeywordsAstrocytesB cellsDRGFibroblastsMacrophagesMast cellsMicrogliaNeutrophilsOligodendrocytesSatellite glial cellsSchwann’s cellsSpinal cordT cells
Neuroelectronic devices are essential tools in neuroscience research, diagnosis, and/or treatment of neurological diseases, as well as in neuro-prosthetics and brain–computer interfaces. Despite a long history of application, neuroelectronic devices are still facing challenges of unsatisfactory chronic stability and a lack of understanding of cellular mechanisms for recording and stimulation. To improve the information transfer between the neural tissue and electronic devices, a comprehensive understanding of the biological activities around the neural electrode is critical. In vivo fluorescent microscopy technologies are rapidly developing and have revolutionized our understanding of cellular dynamics in response to neural interfacing materials. Here, we will provide an overview of the in vivo fluorescence microscopy systems and imaging configurations for studying the neural electronic interface, as well as recent findings in biological mechanisms learned using these advanced optical imaging modalities. Finally, we will discuss the current challenges and future directions.
Graphical abstract
Neurodegenerative diseases are broadly characterized neuropathologically by the degeneration of vulnerable neuronal cell types in a specific brain region. The degeneration of specific cell types has informed on the various phenotypes/clinical presentations in someone suffering from these diseases. Prominent neurodegeneration of specific neurons is seen in polyglutamine expansion diseases including Huntington's disease (HD) and spinocerebellar ataxias (SCA). The clinical manifestations observed in these diseases could be as varied as the abnormalities in motor function observed in those who have Huntington's disease (HD) as demonstrated by a chorea with substantial degeneration of striatal medium spiny neurons (MSNs) or those with various forms of spinocerebellar ataxia (SCA) with an ataxic motor presentation primarily due to degeneration of cerebellar Purkinje cells. Due to the very significant nature of the degeneration of MSNs in HD and Purkinje cells in SCAs, much of the research has centered around understanding the cell autonomous mechanisms dysregulated in these neuronal cell types. However, an increasing number of studies have revealed that dysfunction in non-neuronal glial cell types contributes to the pathogenesis of these diseases. Here we explore these non-neuronal glial cell types with a focus on how each may contribute to the pathogenesis of HD and SCA and the tools used to evaluate glial cells in the context of these diseases. Understanding the regulation of supportive and harmful phenotypes of glia in disease could lead to development of novel glia-focused neurotherapeutics.
Loss of function of the astrocyte membrane protein MLC1 is the primary genetic cause of the rare white matter disease Megalencephalic Leukoencephalopathy with subcortical Cysts (MLC), which is characterized by disrupted brain ion and water homeostasis. MLC1 is prominently present around fluid barriers in the brain, such as in astrocyte endfeet contacting blood vessels and in processes contacting the meninges. Whether the protein plays a role in other astrocyte domains is unknown. Here, we show that MLC1 is present in distal astrocyte processes, also known as perisynaptic astrocyte processes (PAPs) or astrocyte leaflets, which closely interact with excitatory synapses in the CA1 region of the hippocampus. We find that the PAP tip extending toward excitatory synapses is shortened in Mlc1-null mice. This affects glutamatergic synaptic transmission, resulting in a reduced rate of spontaneous release events and slower glutamate re-uptake under challenging conditions. Moreover, while PAPs in wildtype mice retract from the synapse upon fear conditioning, we reveal that this structural plasticity is disturbed in Mlc1-null mice, where PAPs are already shorter. Finally, Mlc1-null mice show reduced contextual fear memory. In conclusion, our study uncovers an unexpected role for the astrocyte protein MLC1 in regulating the structure of PAPs. Loss of MLC1 alters excitatory synaptic transmission, prevents normal PAP remodeling induced by fear conditioning and disrupts contextual fear memory expression. Thus, MLC1 is a new player in the regulation of astrocyte-synapse interactions.
Lipids comprise an extremely heterogeneous group of compounds that perform a wide variety of biological functions. Traditional view of lipids as important structural components of the cell and compounds playing a trophic role is currently being supplemented by information on the possible participation of lipids in signaling, not only intracellular, but also intercellular. The review article discusses current data on the role of lipids and their metabolites formed in glial cells (astrocytes, oligodendrocytes, microglia) in communication of these cells with neurons. In addition to metabolic transformations of lipids in each type of glial cells, special attention is paid to the lipid signal molecules (phosphatidic acid, arachidonic acid and its metabolites, cholesterol, etc.) and the possibility of their participation in realization of synaptic plasticity, as well as in other possible mechanisms associated with neuroplasticity. All these new data can significantly expand our knowledge about the regulatory functions of lipids in neuroglial relationships.
Intracellular cell signaling is a well understood process. However, extracellular signals such as hormones, adipokines, cytokines and neurotransmitters are just as important but have been largely ignored in other works. They are causative agents for diseases including hypertension, diabetes, heart disease, and arthritis so offer new, and often more approachable, targets for drug design. Aimed at medical professionals and pharmaceutical specialists, this book integrates extracellular and intracellular signalling processes and offers a fresh perspective on new drug targets. Written by colleagues at the same institution, but with contributions from leading international authorities, it is the result of close cooperation between the authors of different chapters. Readers are introduced to a new approach to disease causation by adipokines and toxic lipids. Heart disease, migraines, stroke, Alzheimer's disease, diabetes, cancer, and arthritis are approached from the perspective of prevention and treatment by alteration of extracellular signalling. Evidence is presented that the avoidance of toxic lifestyles can reduce the incidence of such illnesses and new therapeutic targets involving adipokines, ceramide and endocannabinoids are discussed.
Network dysfunction has been implicated in numerous diseases and psychiatric disorders, and the hippocampus serves as a common origin for these abnormalities. In this study, we tested the hypothesis that chronic induction of local changes in neurons and astrocytes is sufficient to induce impairments in cognition and behavior. We chronically activated the hM3D(Gq) pathway in CaMKII+ neurons or GFAP+ astrocytes within the ventral hippocampus across 3, 6 and 9 months. We observed that CaMKII-hM3Dq activation impaired fear acquisition, decreased anxiety and social interaction, and modified spatial odor memory and novel environment exploration, while GFAP-hM3Dq activation impaired fear acquisition and enhanced recall. CaMKII-hM3Dq activation modified the number of microglia, while GFAP-hM3Dq activation impacted microglial morphological characteristics, but neither affected astrocytes. Manipulation of both cell types increased the presence of phosphorylated tau at the earliest time point. Overall, our study provides evidence for how each of these cell types are uniquely engaged in disorders that have characteristic network dysfunction while adding a more direct role for glia in modulating behavior.
Main Points
Behaviorally, neuronal Gq activation modified fear, anxiety, social and exploration behaviors, while astrocytic Gq activation induced changes in fear acquisition and recall.
Cellularly, CaMKII-Gq activation modified microglial number, while GFAP-Gq activation affected microglial morphology. Neither cell manipulation affected astrocytic number or morphology.
pTau is increased in vHPC at the 3 month time point for both neuronal and astrocytic Gq activation.
Astrocyte glutamate release can modulate synaptic activity and participate in brain intercellular signaling. P2X7 receptors form large ion channels when activated by ATP or other ligands. Here we show that P2X7 receptors provide a route for excitatory amino acid release from astrocytes. Studies were performed using murine cortical astrocyte cultures. ATP produced an inward current in patch-clamped astrocytes with properties characteristic of P2X7 receptor activation: the current was amplified in low divalent cation medium, blocked by pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and more potently activated by 3'-O-(4-benzoyl)benzoyl ATP (BzATP) than by ATP itself. Measurement of current reversal potentials showed the relative BzATP-induced permeabilities to different substrates to be Na+, 1 > Cl-, 0.34 > N-methyl-D-glucamine, 0.27 > L-glutamate, 0.15 approximately D-aspartate, 0.16. Astrocytes exposed to BzATP also became permeable to Lucifer yellow, indicating a large channel opening. Release of L-glutamate and D-aspartate through P2X7 channels was confirmed using radiolabeled tracers. As with the inward current, release of glutamate and D-aspartate was induced by BzATP more potently than ATP, amplified in Ca2+/Mg2+-free medium, and blocked by PPADS or oxidized ATP. Efflux through P2X7 channels is a previously unrecognized route of ligand-stimulated, nonvesicular astrocyte glutamate release.
Membrane water transport is critically involved in brain volume homeostasis and in the pathogenesis of brain edema. The cDNA encoding aquaporin-4 (AQP4) water channel protein was recently isolated from rat brain. We used immunocytochemistry and high-resolution immunogold electron microscopy to identify the cells and membrane domains that mediate water flux through AQP4. The AQP4 protein is abundant in glial cells bordering the subarachnoidal space, ventricles, and blood vessels. AQP4 is also abundant in osmosensory areas, including the supraoptic nucleus and subfornical organ. Immunogold analysis demonstrated that AQP4 is restricted to glial membranes and to subpopulations of ependymal cells. AQP4 is particularly strongly expressed in glial membranes that are in direct contact with capillaries and pia. The highly polarized AQP4 expression indicates that these cells are equipped with specific membrane domains that are specialized for water transport, thereby mediating the flow of water between glial cells and the cavities filled with CSF and the intravascular space.
Glial–neuronal communication was studied by monitoring the effect of intercellular glial Ca ²⁺ waves on the electrical activity of neighboring neurons in the eyecup preparation of the rat. Calcium waves in astrocytes and Müller cells were initiated with a mechanical stimulus applied to the retinal surface. Changes in the light-evoked spike activity of neurons within the ganglion cell layer occurred when, and only when, these Ca ²⁺ waves reached the neurons. Inhibition of activity was observed in 25 of 53 neurons (mean decrease in spike frequency, 28 ± 2%). Excitation occurred in another five neurons (mean increase, 27 ± 5%). Larger amplitude Ca ²⁺ waves were associated with greater modulation of neuronal activity. Thapsigargin, which reduced the amplitude of the glial Ca ²⁺ increases, also reduced the magnitude of neuronal modulation. Bicuculline and strychnine, inhibitory neurotransmitter antagonists, as well as 6-Nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX) and d (−)-2-amino-7-phosphonoheptanoic acid (D-AP7), glutamate antagonists, reduced the inhibition of neuronal activity associated with glial Ca ²⁺ waves, suggesting that inhibition is mediated by inhibitory interneurons stimulated by glutamate release from glial cells. The results suggest that glial cells are capable of modulating the electrical activity of neurons within the retina and thus, may directly participate in information processing in the CNS.
Postembedding immunogold labeling was used to examine the subcellular distribution of the inwardly rectifying K⁺ channel Kir4.1 in rat retinal Müller cells and to compare this with the distribution of the water channel aquaporin-4 (AQP4). The quantitative analysis suggested that both molecules are enriched in those plasma membrane domains that face the vitreous body and blood vessels. In addition, Kir4.1, but not AQP4, was concentrated in the basal ∼300–400 nm of the Müller cell microvilli. These data indicate that AQP4 may mediate the water flux known to be associated with K⁺ siphoning in the retina. By its highly differentiated distribution of AQP4, the Müller cell may be able to direct the water flux to select extracellular compartments while protecting others (the subretinal space) from inappropriate volume changes. The identification of specialized membrane domains with high Kir4.1 expression provides a morphological correlate for the heterogeneous K⁺ conductance along the Müller cell surface. GLIA 26:47–54, 1999. © 1999 Wiley-Liss, Inc.
There are multiple isoforms of the Na,K-ATPase in the nervous system, three isoforms of the a subunit, and at least two of the b subunit. The a subunit is the catalytic subunit. The b subunit has several roles. It is required for enzyme assembly, it has been implicated in neuron-glia adhesion, and the experimental exchange of b subunit isoforms modifies enzyme kinetics, im- plying that it affects functional properties. Here we describe the specificities of antibodies against the Na,K-ATPase b subunit isoforms b1 and b2. These antibodies, along with antibodies against the a subunit isoforms, were used to stain sections of the rat cerebellum and cultures of cerebellar granule cells to ascertain expression and subcellular distribution in identifiable cells. Comparison of a and b isoform distribution with double- label staining demonstrated that there was no preferential as- sociation of particular a subunits with particular b subunits, nor was there an association with excitatory or inhibitory neuro- transmission modes. Isoform composition differences were seen when Purkinje, basket, and granule cells were compared. Whether b1 and b2 are specific for neurons and glia, respec- tively, has been controversial, but expression of both b subunit types was seen here in granule cells. In rat cerebellar astro- cytes, in sections and in culture, a2 expression was prominent, yet the expression of either b subunit was low in comparison. The complexity of Na,K-ATPase isoform distribution under- scores the subtlety of its regulation and physiological role in excitable cells.
Vasopressin and oxytocin pathways were specifically localized in glutaraldehyde-paraformaldehyde fixed rat brains, with the use of the unlabelled antibody enzyme method and purification of the first antiserum. Vasopressin and oxytocin containing pathways were traced from the paraventricular nucleus towards the dorsal and ventral hippocampus, the nuclei of the amygdala, substantia nigra and substantia grisea, nucleus tractus solitarius, nucleus ambiguus and to the substantia gelatinosa of the spinal cord. In addition, a vasopressin containing pathway between the suprachiasmatic nucleus and the lateral habenular nucleus was demonstrated. The possible nature (axons or dendrites) and role of these extrahypothalamic fibres is discussed in relation to water balance, milk ejection and avoidance behaviour.
Maintenance of osmotic pressure is a primary regulatory process essential for normal cell function. The osmolarity of extracellular fluids is regulated by modifying the intake and excretion of salts and water. A major component of this regulatory process is the neuroendocrine hypothalamo-neurohypophysial system, which consists of neurons located in the paraventricular and supraoptic nuclei. These neurons synthesize the neurohormones vasopressin and oxytocin and release them in the blood circulation. We here review the mechanisms responsible for the osmoregulation of the activity of these neurons. Notably, the osmosensitivity of the supraoptic nucleus is described including the recent data that suggests an important participation of taurine in the transmission of the osmotic information. Taurine is an amino acid mainly known for its involvement in cell volume regulation, as it is one of the major inorganic osmolytes used by cells to compensate for changes in extracellular osmolarity. In the supraoptic nucleus, taurine is highly concentrated in astrocytes, and released in an osmodependent manner through volume-sensitive anion channels. Via its agonist action on neuronal glycine receptors, taurine is likely to contribute to the inhibition of neuronal activity induced by hypotonic stimuli. This inhibitory influence would complement the intrinsic osmosensitivity of supraoptic neurons, mediated by excitatory mechanoreceptors activated under hypertonic conditions. These observations extend the role of taurine from the regulation of cell volume to that of the whole body fluid balance. They also point to a new role of supraoptic glial cells as active components in a neuroendocrine regulatory loop.
Gap junctions are highly conductive channels that allow the direct transfer of intracellular messengers such as Ca2+ and inositol triphosphate (IP3) between interconnected cells. In brain, astrocytes are coupled extensively by gap junctions. We found here that gap junctions among astrocytes in acutely prepared brain slices as well as in culture remained open during ischemic conditions. Uncoupling first occurred after the terminal loss of plasma membrane integrity. Gap junctions therefore may link ischemic astrocytes in an evolving infarct with the surroundings. The free exchange of intracellular messengers between dying and potentially viable astrocytes might contribute to secondary expansion of ischemic lesions.
Neuronal and glial sodium-dependent transporters are crucial for the control of extracellular glutamate levels in the CNS. The regulation of these transporters is relatively unexplored, but the activity of other transporters is regulated by protein kinase C (PKC)- and phosphatidylinositol 3-kinase (PI3K)-mediated trafficking to and from the cell surface. In the present study the C6 glioma cell line was used as a model system that endogenously expresses the excitatory amino acid carrier 1 (EAAC1) subtype of neuronal glutamate transporter. As previously observed, phorbol 12-myristate 13-acetate (PMA) caused an 80% increase in transporter activity within minutes that cannot be attributed to the synthesis of new transporters. This increase in activity correlated with an increase in cell surface expression of EAAC1 as measured by using a membrane-impermeant biotinylation reagent. Both effects of PMA were blocked by the PKC inhibitor bisindolylmaleimide II (Bis II). The putative PI3K inhibitor, wortmannin, decreased L-[3H]-glutamate uptake activity by >50% within minutes. Wortmannin decreased the Vmax of L-[3H]-glutamate and D-[3H]-aspartate transport, but it did not affect Na+-dependent [3H]-glycine transport. Wortmannin also decreased cell surface expression of EAAC1. Although wortmannin did not block the effects of PMA on activity, it prevented the PMA-induced increase in cell surface expression. This trafficking of EAAC1 also was examined with immunofluorescent confocal microscopy, which supported the biotinylation studies and also revealed a clustering of EAAC1 at cell surface after treatment with PMA. These studies suggest that the trafficking of the neuronal glutamate transporter EAAC1 is regulated by two independent signaling pathways and also may suggest a novel endogenous protective mechanism to limit glutamate-induced excitotoxicity.
Vasopressin and oxytocin pathways were specifically localized in glutaraldehyde-paraformaldehyde fixed rat brains, with the use of the unlabelled antibody enzyme method and purification of the first antiserum. Vasopressin and oxytocin containing pathways were traced from the paraventricular nucleus towards the dorsal and ventral hippocampus, the nuclei of the amygdala, substantia nigra and substantia grisea, nucleus tractus solitarius, nucleus ambiguus and to the substantia gelatinosa of the spinal cord. In addition, a vasopressin containing pathway between the suprachiasmatic nucleus and the lateral habenular nucleus was demonstrated. The possible nature (axons or dendrites) and role of these extrahypothalamic fibres is discussed in relation to water balance, milk ejection and avoidance behaviour.
The electrophysiologist's view of brain astrocytes has changed markedly in recent years. In the past astrocytes were viewed as passive, K+ selective cells, but it is now evident that they are capable of expressing voltage- and ligand-activated channels previously thought to be restricted to neurons. The functional importance of most of these ion channels is not understood at present. However, from studies of astrocytes cultured from different species and brain regions, we learned that like their neuronal counterparts astrocytes are a heterogeneous group of brain cells showing similar heterogeneity in their ion-channel expression. Not only are subpopulations of astrocytes within areas of the brain equipped with specific sets of ion channels but, furthermore, regional heterogeneity is apparent. In addition, astrocyte ion channel expression is dynamic and changes during development. Some ion channels are only expressed postnatally, yet others appear to be expressed only during certain stages of development. Interestingly, the expression of some astrocyte channels, including Na+, Ca2+, and some K+ channels, appears to be controlled by neurons via mechanisms that are presently unknown. Some studies suggest roles for astrocyte channels in basic cell processes such as cell proliferation. Thus, although the role of some astrocyte channels remains unclear, our understanding of astrocyte physiology is starting to take shape and points towards roles of ion channels not involved in electrogenesis.
Chloride channels have several functions, including the regulation of cell volume, stabilizing membrane potential, signal transduction and transepithelial transport. The plasma membrane Cl- channels already cloned belong to different structural classes: ligand-gated channels, voltage-gated channels, and possibly transporters of the ATP-binding-cassette type (if the cystic fibrosis transmembrane regulator is a Cl- channel). The importance of chloride channels is illustrated by the phenotypes that can result from their malfunction: cystic fibrosis, in which transepithelial transport is impaired, and myotonia, in which ClC-1, the principal skeletal muscle Cl- channel, is defective. Here we report the properties of ClC-2, a new member of the voltage-gated Cl- channel family. Its sequence is approximately 50% identical to either the Torpedo electroplax Cl- channel, ClC-0 (ref. 8), or the rat muscle Cl- channel, ClC-1 (ref. 9). Isolated initially from rat heart and brain, it is also expressed in pancreas, lung and liver, for example, and in pure cell lines of fibroblastic, neuronal, and epithelial origin, including tissues and cells affected by cystic fibrosis. Expression in Xenopus oocytes induces Cl- currents that activate slowly upon hyperpolarization and display a linear instantaneous current-voltage relationship. The conductivity sequence is Cl- greater than or equal to Br- greater than I-. The presence of ClC-2 in such different cell types contrasts with the highly specialized expression of ClC-1 (ref. 9) and also with the cloned cation channels, and suggests that its function is important for most cells.
The Na,K-ATPase plays an active role in glial physiology, contributing to K+ uptake as well as to the Na+ gradients used by other membrane carriers. There are multiple isoforms of Na,K-ATPase alpha and beta subunits, and different combinations result in different affinities for Na+ and K+. Isoform choice should thus influence K+ and Na+ homeostasis in astrocytes. Prior studies of astrocyte Na,K-ATPase subunit composition have produced apparently conflicting results, suggesting plasticity of gene expression. Purified hat astrocytes from the cerebral cortex and cerebellum of both mouse and rat were systematically investigated here. Using antibodies specific for the alpha 1, alpha 2, alpha 3, beta 1, beta 2, and beta 3 subunits, isoform level was assessed with Western blots, and cellular distribution was visualized with immunofluorescence. Although alpha 1 was always expressed, differences were observed in the expression of alpha 2 and beta 2, subunits that can be expressed in astrocytes in vivo and in coculture with neurons. In addition, abundant or subunit was expressed in rat astrocytes and in mouse cerebellar astrocytes without an equivalent level of any of the known beta isoforms, suggesting that an additional beta subunit important for glia is yet to be discovered. Conditions that have been shown to increase Na,K-ATPase activity in astrocyte cultures, such as dibutyryl cAMP, high extracellular K+, and glutamate, did not specifically induce missing subunits, suggesting that cellular interactions are required to alter the ion transporter phenotype. (C) 1998 Wiley-Liss, Inc.
The Na,K-ATPase is a dominant factor in retinal energy metabolism, and unique combinations of isoforms of its alpha and beta subunits are expressed in different cell types and determine its functional properties. We used isoform-specific antibodies and fluorescence confocal microscopy to determine the expression of Na,K-ATPase alpha and beta subunits in the mouse and rat retina. In the adult retina, alpha 1 was found in Muller and horizontal cells, alpha 2 in some Muller glia, and alpha 3 in photoreceptors and all retinal neurons. beta 1 was largely restricted to horizontal, amacrine, and ganglion cells; beta 2 was largely restricted to photoreceptors, bipolar cells, and Muller glia; and beta 3 was largely restricted to photoreceptors. Photoreceptor inner segments have the highest concentration of Na,K-ATPase in adult retinas. Isoform distribution exhibited marked changes during postnatal development. alpha 3 and beta 2 were in undifferentiated photoreceptor somas at birth but only later were targeted to inner segments and synaptic terminals. beta 3, in contrast, was expressed late in photoreceptor differentiation and was immediately targeted to inner segments. A high level of beta 1 expression in horizontal cells preceded migration, whereas increases in beta 2 expression in bipolar cells occurred very late, coinciding with synaptogenesis in the inner plexiform layer. Most of the spatial specification of Na,K-ATPase isoform expression was completed before eye opening and the onset of electroretinographic responses on postnatal day 13 (P13), but quantitative increase continued until P22 in parallel with synaptogenesis.
This study evaluates the hypothesis that arginine vasopressin (AVP) and atriopeptin, peptide hormones synthesized and released within the brain, are regulators of brain cell volume using cultured astroglial cells derived from newborn rats. Cell water content, regarded as volume, was measured in defined, serum-free medium as the 3-O-methylglucose (3-MG) space. Initial experiments established conditions such that glucose, which competes with 3-MG for the glucose carrier, would not interfere with the measurement of the 3-MG space. AVP increased the 3-MG space of glial cells by an average of 25% between 30 and 120 min of exposure, whereas atriopeptin decreased it by 32%. The 3-MG space remained close to normal after coadministration of both peptides. The AVP-dependent increase in 3-MG space was blocked both by the V1 antagonist d(CH2)5Tyr(Me)AVP (Manning compound) and by the cotransport inhibitor, bumetanide. Results are consistent with a role for AVP and atriopeptin in the homeostasis of atroglial cell volume.
Several lines of evidence indicate that the extent of ischemic injury is not defined immediately following arterial occlusion; rather that infarction expands over time. Episodes of spreading depression have been linked to this secondary increase in infarct volume. Tissue bordering the infarct fails to repolarize following spreading depression and is incorporated into the infarction. The result is that ischemic infarcts expand stepwise following each episode of spreading depression. Another line of evidence has demonstrated that gap junction blockers effectively inhibit spreading depression.
These observations suggest that the efflux of potentially harmful cytosolic messengers from ischemic cells into surrounding nonischemic cells might cause amplification of injury in focal stroke. It is therefore conceivable that minimizing gap junction permeability might reduce final infarct volume. To test this hypothesis, the authors pretreated rats with the gap junction blocker, octanol, before occluding the middle cerebral artery and compared the sizes of the ischemic lesions to those in rats that received vehicle dimethyl sulfoxide prior to arterial occlusion. Histopathological analysis was performed 24 hours later. The 12 octanol-treated animals showed a significantly decreased mean infarction volume (80 ± 16 mm ³ ) compared with the nine control rats (148 ± 9 mm ³ ). In a separate set of experiments, the frequency of experimentally induced waves of spreading depression was evaluated following octanol treatment. Octanol pretreatment resulted in complete inhibition in two of nine animals, transient inhibition in five of nine, and no inhibition in two of nine.
The results indicate that gap junction inhibitors, when not limited by toxicity, have significant therapeutic potential in the treatment of acute stroke.
The activity of high-affinity glutamate transporters is essential for the normal function of the mammalian central nervous system. Using a combined pharmacological, confocal immunocytochemical, enzyme-based microsensor and fluorescence imaging approach, we examined glutamate uptake and transporter protein localization in single astrocytes of neuron-containing and neuron-free microislands prior to pre-synaptic transmitter secretion and during functional neuronal activity. Here, we report that the presence or absence of neurons strikingly affects the uptake capacity of the astroglial glutamate transporters GLT1 and GLAST1. Induction of transporter function is activated by neurons and this effect is mimicked by pre-incubation of astrocytes with micromolar concentrations of glutamate. Moreover, increased glutamate transporter activation is reproduced by endogenous release of glutamate via activation of neuronal nicotinic receptors. The increase in transport activity is dependent on neuronal release of glutamate, is associated with the local redistribution (clustering) of GLT1 and GLAST1 but is independent of transporter synthesis and of glutamate receptor activation. Together, these results suggest an activity-dependent neuronal feedback system for rapid astroglial glutamate transporter regulation where neuron-derived glutamate is the physiological signal that triggers transporter function.
New imaging techniques can see into the functioning brain, revealing the regions that are active during certain tasks and sensations. Magistretti et al ., in their Perspective, take this notion one step further and, by careful analysis of the biochemistry that gives rise to imaging signals, they deduce some of the cellular and molecular events that accompany neuronal activity in the working brain, ultimately laying the groundwork for determining the biochemistry that underlies human cognition.
The water channel AQP4 is concentrated in perivascular and subpial membrane domains of brain astrocytes. These membranes form
the interface between the neuropil and extracerebral liquid spaces. AQP4 is anchored at these membranes by its carboxyl terminus
to α-syntrophin, an adapter protein associated with dystrophin. To test functions of the perivascular AQP4 pool, we studied
mice homozygous for targeted disruption of the gene encoding α-syntrophin (α-Syn−/−). These animals show a marked loss of AQP4 from perivascular and subpial membranes but no decrease in other membrane domains,
as judged by quantitative immunogold electron microscopy. In the basal state, perivascular and subpial astroglial end-feet
were swollen in brains of α-Syn−/− mice compared to WT mice, suggesting reduced clearance of water generated by brain metabolism. When stressed by transient
cerebral ischemia, brain edema was attenuated in α-Syn−/− mice, indicative of reduced water influx. Surprisingly, AQP4 was strongly reduced but α-syntrophin was retained in perivascular
astroglial end-feet in WT mice examined 23 h after transient cerebral ischemia. Thus α-syntrophin-dependent anchoring of AQP4
is sensitive to ischemia, and loss of AQP4 from this site may retard the dissipation of postischemic brain edema. These studies
identify a specific, syntrophin-dependent AQP4 pool that is expressed at distinct membrane domains and which mediates bidirectional
transport of water across the brain–blood interface. The anchoring of AQP4 to α-syntrophin may be a target for treatment of
brain edema, but therapeutic manipulations of AQP4 must consider the bidirectional water flux through this molecule.
Glutamate uptake and metabolism was studied in cerebral cortical astrocytes. The expression of the astrocytic glutamate transporter GLAST was found to be stimulated by extracellular glutamate through activation of kainate receptors on the astrocytes. Energy metabolism and ammonia homeostasis are two important aspects of glutamate handling in astrocytes. It is well known that glutamate transport into astrocytes and glutamine formation are energy consuming processes. Furthermore, ammonia is required for glutamine production. On the other hand, glutamate metabolism through the tricarboxylic acid cycle is an energy and ammonia producing pathway. In the present study it was shown that at an extracellular glutamate concentration of 0.5 mM, high energy phosphates were reduced, and more than 50% of the glutamate carbon skeleton entered the tricarboxylic acid cycle to yield products like lactate, aspartate, and additionally glutamate and glutamine derived from tricarboxylic acid cycle intermediates. Entry into the cycle was not affected by the transaminase inhibitor aminooxyacetic acid, indicating that deamination is the major route for 2-oxoglutarate formation from glutamate. Synthesis of glutamate from 2-oxoglutarate, however, proceeded via transamination. In an earlier study it was shown that at glutamate concentrations at and below 0.2 mM, glutamine appears to be the major product and entry of glutamate into the tricarboxylic acid cycle is decreased 70% by aminooxyacetic acid. In an attempt to unify the above mentioned results, it is suggested that availability of ammonia and energy demands are major factors determining the metabolic fate of glutamate in astrocytes. GLIA 21:56–63, 1997. © 1997 Wiley-Liss, Inc.
Steroid hormones alter several aspects of micro-vascular function within the CNS. Both microvessel formation and blood-brain barrier expression appear to be influenced by interactions between astrocytes and endothelial cells. To determine if steroids alter astrocyte-endothelial interactions, we studied their effects on astroglial-induced micro-vessel morphogenesis in vitro. Q astroglial cells induce bovine retinal microvascular endothelial cells to differentiate into capillary-like structures. Dexamethasone, hydrocortisone, and progesterone at 10 nM inhibited C6-induced microvessel morphogenesis by 75, 35, and 30%, respectively. Inhibition by dexamethasone was both time and concentration dependent, reaching 80-100% at 1 μM. Tetrahydrocortisone and 17α-hydroxyprogesterone had only marginal inhibitory effects. Cortexolone, a glucocorticoid receptor antagonist, blocked inhibition by dexamethasone. Progesterone receptors were expressed in C6 but not bovine retinal microvascular endothelial cells, identifying the astroglial cell as the likely effector of progesterone-mediated inhibition. Astroglial cells were further implicated as the effectors of steroid-mediated inhibition because none of the steroids inhibited astroglial-independent capillary-like structure formation in response to a reconstituted extracellular matrix, Matrigel. These findings are evidence that steroids modulate neural microvascular endothelial cell functions indirectly through perivascular astrocytes via a receptor-mediated mechanism.
This special issue on steroids and glia represents the intersection of two emerging themes in the neurosciences: (a) Glia actively modulate and participate in brain function throughout life, and (b) glia are sensitive to steroid hormones. This overview begins by reviewing some of the basic principles of steroid hormone action on the brain and introducing the various glia that inhabit the peripheral and central nervous system. A prominent theme among the articles that follow is that glia may be direct targets for steroid hormones since they possess steroid receptors and the promoter region of glial-specific genes such as glutamine synthetase contain hormone-responsive elements. The articles in this special issue discuss evidence that glia may mediate steroid action on the nervous system in the context of (a) steroid metabolism, which may control the hormonal microenvironment of neurons both in the normal and injured brain; (b) brain development including sexual differentiation; (c) synaptic plasticity which may underlie the cyclic release of luteinizing hormone releasing hormone in the female rodent brain; (d) neural repair and aging; and (e) brain immune function. Another theme among these articles is that glia influence neurons via specific secreted and cell-surface molecules, and that steroids affect this mode of communication by altering the level of glial production of these signaling molecules and/or the sensitivity of neurons to such signals. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 434–445, 1999
Glucocorticoid hormones affect gene expression directly at the level of transcription via intracellular receptors that translocate to the nucleus in the presence of steroid. In the brain, two types of high-affinity receptors bind glucocorticoids, the type I, mineralocorticoid receptor and the type II, glucocorticoid receptor (GR). Both receptor types are expressed by many types of neurons. Although binding studies have suggested that glial cells may also express receptors, the expression of these receptors in specific classes of glia has not been studied previously. This immunocytochemical study was undertaken to determine which of the different classes of glial cells express type II GR. Primary cultures of mixed glial cells from rat cerebrum and cerebellum, purified oligodendrocytes and astrocytes. as well as two glial tumor cell lines were screened for the expression of glucocorticoid receptors using a mouse monoclonal antibody directed against rat liver GR (BuGR-2). Glial cell types were identified by morphology and immunoreactivity (IR) with antibodies directed against glial fibrillary acidic protein (GFAP), cyclic nucleotide phosphodiesterase (CNP), or myelin basic protein (MBP). Double immunofluorescence microscopy revealed that all GFAP-IR cells (type 1 and type 2 astrocytes), all CNP- or MBP-IR cells (oligodendrocytes), as well as immature and intermediate cell types expressed GR, although at different levels. C6 glioma and JScl1 Schwannoma cells were observed to express moderate to high levels of GR. Furthermore, cells grown in the absence of glucocorticoids had diffuse GR staining over the cytoplasm, whereas cells grown in the presence of the synthetic glucocorticoid dexamethasone had strong nuclear staining. These results demonstrate that, in vitro, all classes of glial cells express glucocorticoid receptors that can translocate to the nucleus in the presence of hormone. These observations suggest that glial cells are major targets for glucocorticoid-directed control of gene transcription in the nervous system.
We have used postnatal rat cerebellar astrocyte-enriched cultures to study the excitatory amino acid receptors present on these cells. In the cultures used, type-2 astrocytes (recognized by the monoclonal antibodies A2B5 and LB1) selectively took up γ-[3H]aminobutyric acid ([3H]GABA) and released it when incubated in the presence of micromolar concentrations of kainic and quisqualic acids. The releasing effect of kainic acid was concentration dependent in the range of 5–100 μM. Quisqualate was more effective than kainate in the lower concentration range but less effective at concentrations at which its releasing activity was maximal (∼50 μM). N-Methyl-d-aspartic acid and dihydrokainate (100 μM) did not stimulate [3H]GABA release from cultured astrocytes. l-Glutamic acid (20–100 μM) stimulated [3H]GABA release as effectively as kainate. The stimulatory effects of kainate and quisqualate on [3H]GABA release were completely Na+ dependent; that of kainate was also partially Ca2+ dependent. Kynurenic acid (50–200 μM) selectively antagonized the releasing effects of kainic acid and also that of l-glutamate; quisqualate was unaffected. Quisqualic acid inhibited the releasing effects of kainic acid when both agonists were used at equimolar concentrations (50 μM). d-[3H]aspartate was taken up by both type-1 and type-2 astrocytes, but only type-2 astrocytes released it in the presence of kainic acid. Excitatory amino acid receptors with a pharmacology similar to that of the receptors present in type-2 astrocytes were also expressed by the immature, bipotential progenitors of type-2 astrocytes and oligodendrocytes.
It has been demonstrated that agents which inhibit chloride influx and, therefore, lower intracellular chloride levels in the astrocyte, a major cell type in the cerebral gray matter, inhibit astrocytic swelling in vitro and in vivo. Herein, we report additional examples of a series of [(N-alkyl-1,3-dihydro-1-oxoisoindolin-5-yl)oxy] alkanoic acids and their effects upon ion transport in primary rat astrocyte cultures. The 4-chloro-substituted 1-oxoisoindolines demonstrated superior astrocytic chloride influx inhibitory activity as compared to the 6-chloro and non-chlorinated analogs. The four-carbon acid side chain derivatives were more active than the three- and two-carbon analogs. The pharmacological profile of these compounds was examined with respect to inhibition of the Cl−-Cl−/Cl−-HCO anion exchanger and Na+-K+-2Cl− cotransport mechanisms in glia, and the compounds were found to exhibit a similar profile to that of furosemide by inhibiting both transporters.
There are at least three subtypes of cloned metabotropic P2 receptors linked to intracellular Ca2+ rises in rat brain cells, namely, P2Y1, P2Y2 and P2Y4. In this study we explore the subtypes of the metabotropic P2 receptors seen in freshly isolated astrocytes (FIAs) from P8-P25 rats. We found by single cell RT-PCR that in process-bearing FIAs from hippocampi of P8-P12 rats, 31% of the glial fibrillary acidic protein (GFAP) mRNA (+) cells expressed P2Y1 mRNA while only 5% of the cells tested expressed P2Y2 mRNA. The expression of P2Y1 receptor mRNA was not changed in FIAs from the hippocampi of P18-P25 rats, but 38% of the GFAP mRNA (+) cells in the P18-P25 age group then showed P2Y2 mRNA. We also studied whether the mRNA was expressing functional receptor protein by measuring Ca2+ responses to specific agonists for P2Y1 and P2Y2. We found that similar proportions of GFAP mRNA (+) FIAs responded to ATP or UTP as showed mRNAs for P2Y 1 and P2Y2, respectively. Total tissue RNA from P9 and P24 rat hippocampus showed a 2.8-fold increase in P2Y2 mRNA levels from P9 to P24 with a decrease in P2Y1 mRNA. Thus, this study shows a marked up-regulation of mRNA for P2Y2 from 9 to 24 days in rat hippocampus, and some of this increase is likely due to the protoplasmic astrocytes which is being translated into functional receptor protein in these cells.
In the nucleus accumbens (NAc) of rats, the involvement of P2X and P2Y receptors in the generation of astrogliosis in vivo, was investigated by local application of their respective ligands. The agonists used had selectivities for P2X1,3 (α,β-methylene adenosine 5′-triphosphate; α,β-meATP), P2Y1,12 (adenosine 5′-O-(2-thiodiphosphate; ADP-β-S) and P2Y2,4,6 receptors (uridine 5′-O-(3-thiotriphosphate; UTP-γ-S). Pyridoxalphosphate-6-azophenyl-2,4-disulphonic acid (PPADS) was used as a non-selective antagonist. The astroglial reaction was studied by means of immunocytochemical double-labelling with antibodies to glial fibrillary acidic protein (GFAP) and 5-bromo-2′-deoxyuridine (BrdU).
The agonist-induced changes in comparison to the artificial cerebrospinal fluid (aCSF)-treated control side reveal a strong mitogenic potency of ADP-β-S and α,β-meATP, whereas UTP-γ-S was ineffective. The P2 receptor antagonist PPADS decreased the injury-induced proliferation when given alone and in addition inhibited all agonist effects.
The observed morphogenic changes included hypertrophy of astrocytes, elongation of astrocytic processes and up-regulation of GFAP. A significant increase of both GFAP-immunoreactivity (IR) and GFA-protein content (by using Western blotting) was found after microinfusion of α,β-meATP or ADP-β-S. In contrast, UTP-γ-S failed to increase the GFAP-IR. The morphogenic effects were also inhibited by pre-treatment with PPADS.
A double immunofluorescence approach with confocal laser scanning microscopy showed the localisation of P2X3 and P2Y1 receptors on the GFAP-labelled astrocytes.
In conclusion, the data suggest that P2Y (P2Y1 or P2Y12) receptor subtypes are involved in the generation of astrogliosis in the NAc of rats, with a possible minor contribution of P2X receptor subtypes.
British Journal of Pharmacology (2001) 134, 1180–1189; doi:10.1038/sj.bjp.0704353
Glutamate is the major excitatory neurotransmitter of the mammalian retina and glutamate uptake is essential for normal transmission at glutamatergic synapses. The reverse transcriptase-polymerase chain reaction (RT-PCR) has revealed the presence of three different high-affinity glutamate transporters in the rat retina, viz. GLAST-1, GLT-1 and EAAC-1. No message has been found in the retina for EAAT-4, a transporter recently cloned from human brain. By using membrane vesicle preparations of total rat retina, we show that glutamate uptake in the retina is a high-affinity electrogenic sodium-dependent transport process driven by the transmembrane sodium ion gradient. Autoradiography of intact and dissociated rat retinae indicates that glutamate uptake by Mller glial cells dominates total retinal glutamate transport and that this uptake is strongly influenced by the activity of glutamine synthetase. RT-PCR, immunoblotting and immunohistochemistry have revealed that Mller cells express only GLAST-1. The Km for glutamate of GLAST-1 is 2.1ǂ.4 M. This study suggests a major role for the Mller cell glutamate transporter GLAST-1 in retinal transmitter clearance. By regulating the extracellular glutamate concentration, the action of GLAST-1 in Mller cells may extend beyond the protection of neurons from excitotoxicity; we suggest a mechanism by which Mller cell glutamate transport might play an active role in shaping the time course of excitatory transmission in the retina.
Seizures, neuronal damage and extracellular Ca2+ concentration were studied in rats unilaterally injected in the dorsal hippocampus with quinolinic acid, a brain metabolite with excitotoxic properties. In freely moving animals, in the first 2 h after the injection of a convulsant and neurotoxic dose (156 nmol), quinolinic acid induced a tetrodotoxin-insensitive decrease in the extracellular Ca2+ concentration (nadir 40%) in the injected area, as assessed by brain dialysis coupled to a fluorimetric method for Ca2+ detection. Blockade of quinolinic acid-induced decrements in Ca2+ by 15.6 nmol d-(−)2-amino-7-phosphonoheptanoic acid indicated that this effect was receptor-mediated. Dose-response relationships showed a close association between seizure activity (measured by EEG) and extracellular Ca2+ changes in the injected area. Changes in Ca2+ were apparent at the site of injection prior to the onset of focal seizures and they were not found in the homotypic structure where siezures were conducted. Drugs effective in blocking seizures (carbamazepine and flunarizine) prevented the fall in extracellular Ca2+, while drugs without anticonvulsant activity (ethosuximide and nifedipine) did not. Destruction of nerve cells by quinolinic acid was not prevented by treatment with carbamazepine and flunarizine. The results suggest that the fall in extracellular Ca2+ observed in the first 2 h after quinolinic acid, probably reflecting the ion influx into neurons, is involved in triggering focal seizures but is not related to the occurrence of nerve cell death.
The most prominent sites of vasopressin (VP) production in the rat brain are the paraventricular nucleus, the supraoptic nucleus, the suprachiasmatic nucleus, the bed nucleus of the stria terminalis (BST), and the medial amygdaloid nucleus (MA). Recently a number of new sites have been suggested, including the hippocampus, the diagonal band of Broca, and the choroid plexus. This chapter shows how differential regulation of these VP systems can be exploited to identify the contributions of individual VP systems to the various central functions in which VP has been implicated. It will focus on the development, anatomy, and function of the sexually dimorphic VP projections of the BST and MAThis system contains more cells and has denser projections in males than in females. This system is also extremely responsive to gonadal steroids as it only produces VP in the presence of gonadal steroids. It has been implicated in sexually dimorphic functions such as aggressive behavior as well as in non-sexually dimorphic functions such as social recognition memory. Using comparative studies done in prairie voles as an example, this chapter makes the case that the VP projections of the BST and MA may simultaneously generate sex differences in some brain functions and behaviors and prevent them in others.
Thirst, the longing or compelling desire to drink, arises physiologically by two main mechanisms-extracellular and cellular dehydration. The hormone angiotensin II has been implicated in the former but not in the latter brain mechanism. To test this apparent difference, experiments in 5 mammalian species examined the effect of intracerebroventricular infusion of losartan, an angiotensin II type I receptor antagonist, on the thirst induced by intracerebroventricular infusion of an artificial cerebrospinal fluid made hypertonic by the inclusion of 500 mM NaCl. The losartan infusion reduced the water intake due to increased brain sodium concentration in all 5 species, cattle, sheep, rabbits, rats and mice. Thus, the thirst evoked by cellular dehydration, as well as the thirst evoked by extracellular dehydration, may be mediated by angiotensin II.
Ion and water homeostasis in the CNS is subjected to a neuroendocrine control exerted by neuropeptides formed within the brain. In order to gain information on this neuroendocrine control of Cl- homeostasis, 36Cl- uptake was measured in cultured Type-I astrocytes exposed to the neuropeptides [Arg8]Vasopressin (AVP), and atriopeptin (AP) and to various Cl- transport modifiers. AVP increased while AP decreased 36Cl- uptake of cultured astrocytes in a dose-dependent manner. Both effects became statistically significant at greater than 10(-9) M concentration of the peptides. For the appearance of the effects at least 30-min exposure was necessary. AVP and AP extinguished each other's effect by almost stochiometric manner. When administered together with AVP, the VIA vasopressin receptor antagonist "Manning compound" inhibited, while V2 vasopressin receptor agonist did not influence the 36Cl- uptake-increasing effect of AVP. However, bumetanide, a specific inhibitor of Na+-K+-2Cl- cotransport, inhibited the effect of vasopressin and also inhibited the 36Cl- uptake of AVP non-treated, control cells. Our findings suggest that brain Cl- homeostasis is controlled by neuroendocrine system in the CNS.
Recovery from neuronal activation requires rapid clearance of potassium ions (K+) and restoration of osmotic equilibrium. The predominant water channel protein in brain, aquaporin-4 (AQP4), is concentrated in the astrocyte end-feet membranes adjacent to blood vessels in neocortex and cerebellum by association with alpha-syntrophin protein. Although AQP4 has been implicated in the pathogenesis of brain edema, its functions in normal brain physiology are uncertain. In this study, we used immunogold electron microscopy to compare hippocampus of WT and alpha-syntrophin-null mice (alpha-Syn-/-). We found that <10% of AQP4 immunogold labeling is retained in the perivascular astrocyte end-feet membranes of the alpha-Syn-/- mice, whereas labeling of the inwardly rectifying K+ channel, Kir4.1, is largely unchanged. Activity-dependent changes in K+ clearance were studied in hippocampal slices to test whether AQP4 and K+ channels work in concert to achieve isosmotic clearance of K+ after neuronal activation. Microelectrode recordings of extracellular K+ ([K+]o) from the target zones of Schaffer collaterals and perforant path were obtained after 5-, 10-, and 20-Hz orthodromic stimulations. K+ clearance was prolonged up to 2-fold in alpha-Syn-/- mice compared with WT mice. Furthermore, the intensity of hyperthermia-induced epileptic seizures was increased in approximately half of the alpha-Syn-/-mice. These studies lead us to propose that water flux through perivascular AQP4 is needed to sustain efficient removal of K+ after neuronal activation.
Progress curves of the enzymatic reactions show that ATPases of bulk isolated glial cells, perikarya and synaptosomes exhibit hysteretic change. Initial velocities of enzyme activities were therefore obtained according to the equation valid for the hysteretic model. The (Na+, K+)-ATPase activities of the same brain fractions were measured before or after NaI treatment. Only glial and synaptosomal enzyme could be adequately extracted by using this procedure. Attempts to purify the (Na+, K+)-ATPase from brain perikarya by NaI extraction were unsuccessful. In order to determine the effect of the K+ ions on enzymic physiological efficiency (phys. eff.; i.e., the ratio Vmax/Kmapp) the variation of (Na+, K+)-ATPase activities from each brain fraction was measured as a function of Mg.ATP2- concentration in the presence of 5 and 20 mM K+ ions. High K+ ion concentrations (20 mM) increased the physiological efficiency of glial enzyme and decreased the same kinetic parameter in neuronal (perikaryal as well as synaptosomal) enzyme preparations. Results are discussed in relation to a possible distribution of distinct enzyme in different brain cell populations as well as a possible role of glial cells in an active regulation of K+ ion extracellular fluid in the CNS.
Two sets of new observations are reported: (i) astrocytes in primary cultures show an increased potassium-induced swelling in the presence of 1-100 x 10(-12) M vasopressin, whereas no similar phenomenon is found in primary cultures of neurons, and (ii) the furosemide-sensitive cotransport system for uptake of K+, Na+, and Cl-, which is known to exist in astrocytes, is absent in neurons. On the basis of these findings and observations by other investigators on transport of ions and water in the brain in vivo, a novel mechanism is suggested, according to which all boundaries of brain parenchymal tissue (perivascular astrocytic end-feet, glia limitans, and ependyma) in the absence of vasopressin are capable of performing a net uptake of K+, Na+, and Cl- without uptake of water, and that the resulting hyperosmolarity in the presence of vasopressin leads to water uptake (cell swelling), which causes a reduction in the amount of water in the interstitial fluid and thus an increase in extracellular concentrations of ions.
The excitatory and metabolic events in nervous tissue lead to localized increases in extracellular potassium (K+) and intracellular hydrogen (H+) and calcium (Ca2+) ion concentrations. Even more pronounced increases are seen under pathological conditions and may interfere with the maintenance of cellular function and structure. Most presentations on the second day focused on these processes and the mechanisms for the clearance of K+, H+, and Ca2+ from intra- or extra-cellular compartments. The essential role of glial cells was a returning theme. Extracellular K+ is transported into cells by the Na-K pump and by two other processes, Na-K-Cl2 cotransport and spatial buffering, which both depend on the operation of the Na-K pump. The clearance of H+ from the cytoplasm and into the extracellular space is mediated by Na+ gradient dependent processes, Na+/H+ antiport, Cl-/HCO3- exchange, and Na(+)-HCO3- cotransport. Also the clearance of cytoplasmic Ca2+ is to a large extent mediated by a Na+ gradient dependent process, the Na+/Ca2+ antiport. There is a wide divergence between the rates of Na-K pump mediated K+ influx measured in various cultures of glial and neuronal cells. There is a considerable need for systematic comparison between the functional capacity and the concentration of Na-K pumps in cell cultures and intact nervous tissue. It has not yet been ascertained whether K+ transport as measured in cultured astrocytes is representative for K+ transport in the in situ functioning astrocyte. In astrocytes, glutamate was shown to elicit a rapid intercellular propagation of a rise in cytoplasmic Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)
Evidence is accumulating that interactions between different cell types are of paramount importance for CNS function, for example, release of the excitatory transmitter glutamate from neurons and its preferential uptake into astrocytes. Some information is also available about energy metabolism in different cell types, or more often in models of different cell types (e.g., synaptosomes, cultured neurons, cultured astrocytes). In this review an attempt is made not only to correlate information obtained with different cell models but also to integrate this information with in vivo data, with histochemical observations, and with results obtained using brain slices. The emerging patterns indicate that neurons, synaptosomes, and astrocytes are all capable of complete glycolysis and oxidation of glucose. Elevated extracellular concentrations of potassium, known to occur in vivo, enhance energy metabolism by mechanisms that differ between neurons and astrocytes and to a large extent serve to reaccumulate extracellular potassium ions into adjacent cells. Monoaminergic agonists also stimulate energy metabolism, but mainly or exclusively in astrocytes. Profound differences are found between the effects of excess potassium and of aminergic transmitters, suggesting that high potassium concentrations enhance neuronal-astrocytic interactions, whereas the monoamines may tend to dissociate metabolic events in neurons and in astrocytes.
Brain glycogen is localized almost exclusively to glia, where it undergoes continuous utilization and resynthesis. We have shown that glycogen utilization increases during tactile stimulation of the rat face and vibrissae. Conversely, decreased neuronal activity during hibernation and anesthesia is accompanied by a marked increase in brain glycogen content. These observations support a link between neuronal activity and glial glycogen metabolism. The energetics of glycogen metabolism suggest that glial glycogen is mobilized to meet increased metabolic demands of glia rather than to serve as a substrate for neuronal activity. An advantage to the use of glycogen may be the potentially faster generation of ATP from glycogen than from glucose. Alternatively, glycogen could be utilized if glucose supply is transiently insufficient during the onset of increased metabolic activity. Brain glycogen may have a dynamic role as a buffer between the abrupt increases in focal metabolic demands that occur during normal brain activity and the compensatory changes in focal cerebral blood flow or oxidative metabolism.
In the present work we studied the effect of serotonin (5-HT) on the kinetics of Na+/K(+)-ATPase in subcellular preparations of the cerebral cortex from male Wistar rats using various concentrations of ATP and K+ with and without added 5-HT. Also we studied the effect of 5-HT on the enzyme in glial or neuronal preparations. The results indicated that there was a significant increase (P < 0.05) of the Vmax in the presence of 5-HT in the whole tissue preparation (homogenate) but not in the subcellular fractions, suggesting that the interaction could be preferentially with the glial pump. Further results supported that this was the case since activation by 5-HT was mainly in the glial preparations. Kinetic data and the binding of [3H]ouabain supported that the enzyme is activated by 5-HT through the exposure of more enzymatic active sites.
Changes in intra- and extracellular [Ca2+] and [H+], together with alterations in tissue Po2 and local blood flow, were measured in areas CA1 and CA3 of the hippocampus during recovery (up to 8 h) after an 8-min period of low-flow ischemia. Restoration of blood supply was followed by an immediate rise in flow and tissue Po2 above normal, with large fluctuations in both persisting for up to 4 h. In area CA1, [Ca2+]i decreased rapidly from an ischemic mean value of 30 μM to a control mean level of 73.1 nM in 20-30 min, whereas normalization of (Ca2+]e took ∼1 h. Recovery of [Ca2+]i was accelerated by preischemic administration of a calcium antagonist, nifedipine, and a free radical scavenger, N-tert-butyl-α-phenylnitrone (PBN), but not by MK-801, a blocker of N-methyl-D-aspartate receptors. There was a secondary rise in [Ca2+]i in many cells beginning ∼2 h after reperlusion. This was attenuated somewhat by PBN but not clearly influenced by either nifedipine or MK-801. Changes of [Ca2+]i in area CA3 were much smaller and slightly slower than in area CA1 and were not affected by the drugs mentioned above. In both areas CA1 and CA3, pHe and pHi fell during ischemia to an average value of 6.2, from which there was a rapid initial recovery in the first 5-10 min when blood flow was restored. Thereafter tissue pH rose slowly and did not reach control levels for ∼1 h, and in some microareas not at all. It is concluded that (a) effective mechanisms for restoring normal [Ca2+]i remain intact after 8 min of low-flow ischemia; (b) in neurons of area CA1, some insidious change in the homeostasis of calcium triggers a secondary rise in its free cytosolic concentration, which may be causally related to activation of irreversible cell damage; and (c) the changes in [Ca2+]i and [Ca2+]e during and following 8 min of ischemia can be adequately accounted for by movements of a fixed pool of Ca between intra- and extracellular compartments, and possible mechanisms are discussed.
Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.
The mechanisms of glutamate-induced glial swelling have been studied using an in vitro model that permits detection of cell volume changes with high accuracy. The model allows for a close control of the extracellular environment to study in isolation the effect of defined extracellular alterations occurring in brain under pathophysiologic conditions. Glutamate was applied in concentrations between 50 microM and 10 mM to either C6 glioma cells or astrocytes from primary culture. Glutamate uptake was assessed by HPLC measurements of amino acids in the extracellular medium. Glutamate at all concentrations tested caused glial swelling, which, however, was moderate, with maximal average volume increases between 5.0 +/- 1.92 and 18.38 +/- 1.6% of control at 50 microM and 5 mM glutamate, respectively. Swelling was concentration dependent and correlated with glutamate uptake. After removal of all extracellular glutamate by glial uptake, cell volume spontaneously normalized. Pretreatment of the cells for 90 min with ouabain (1 mM) to abolish the extracellular/intracellular Na+ gradient, prevented glutamate-induced swelling. It is concluded that while glial cells readily accumulate glutamate from the extracellular environment to protect neurons from excitotoxic effects, swelling results from the increase of intracellular osmotic activity due to the uptake of Na+ and glutamate.
The pHi regulation from intracellular acidosis in the central nervous system appears to be mediated by mechanisms driven by the large inwardly directed Na+ gradient. The involvement of these mechanisms in pHi regulation of neurones and glial cells has been investigated in the leech central nervous system using ion-selective microelectrodes. For recovery from acidification, there appear to be three separate mechanisms: Na+/H+ exchange, Na(+)-dependent Cl-/HCO3- exchange, and Na+-HCO3- cotransport. All three mechanisms have a profound effect on the maintenance of pHi homeostasis in glial cells; whereas in leech neurones, as in other neuronal cells studied previously, the predominant mechanisms are Na+/H+ and Na(+)-dependent Cl-/HCO3- exchange. In addition to acid extrusion mechanisms we also found evidence for Na(+)-independent Cl-/HCO3- exchange. At alkaline pHi this exchanger may mediate some of the pHi recovery from intracellular alkalinization.
Neurons and glia exhibit complex homeostatic interactions via shared extracellular space which can involve metabolites, inorganic ions, and neurotransmitters. Focal application of glutamate to both human and rat central nervous system astrocytes in primary culture produced a rapid, transient increase in both cytoplasmic and nuclear Ca2+. These Ca2+ waves can propagate at up to 15-20 micron/s for long distances (millimetres) through the astrocyte syncitium. Oscillatory Ca2+ signals were frequently observed under control conditions and were enhanced by glutamate application. These Ca2+ signals were paralleled by rapid extensions of filopodia from the astrocyte cell margin and apical surface near the point of glutamate application. Focal application of glutamate to rat hippocampal neurons also elicited rapid, transient increases in intracellular Ca2+. Levels of Ca2+ signals were consistently two- to three-fold greater in pyramidal neurons cultured from CA1 than in those from CA3. Filopodial extension was extensive in CA1 neurons, but rare in CA3 neurons, and in either case observable only during the first few days of primary culture. Diversity of glial and neuronal responses to binding the glutamate receptors may reflect their roles in homeostatic interactions.