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Astrocytes accumulate A beta 42 and give rise to astrocytic amyloid plaques in Alzheimer disease brains
Abstract
beta-Amyloid(1-42) (A beta 42), a major component of amyloid plaques, accumulates within pyramidal neurons in the brains of individuals with Alzheimer's disease (AD) and Down syndrome. In brain areas exhibiting AD pathology, A beta 42-immunopositive material is observed in astrocytes. In the present study, single- and double-label immunohistochemistry were used to reveal the origin and fate of this material in astrocytes. Our findings suggest that astrocytes throughout the entorhinal cortex of AD patients gradually accumulate A beta 42-positive material and that the amount of this material correlates positively with the extent of local AD pathology. A beta 42-positive material within astrocytes appears to be of neuronal origin, most likely accumulated via phagocytosis of local degenerated dendrites and synapses, especially in the cortical molecular layer. The co-localization of neuron-specific proteins, alpha 7 nicotinic acetylcholine receptor and choline acetyltransferase, in A beta 42-burdened, activated astrocytes supports this possibility. Our results also suggest that some astrocytes containing A beta 42-positive deposits undergo lysis, resulting in the formation of astrocyte-derived amyloid plaques in the cortical molecular layer in brain regions showing moderate to advanced AD pathology. These astrocytic plaques can be distinguished from those arising from neuronal lysis by virtue of their smaller size, their nearly exclusive localization in the subpial portion of the molecular layer of the cerebrocortex, and by their intense glial fibrillary acidic protein immunoreactivity. Overall, A beta 42 accumulation and the selective lysis of A beta 42-burdened neurons and astrocytes appear to make a major contribution to the observed amyloid plaques in AD brains.
... Within the brain of AD patients, increased microglial activation is observed during the prodromal and potentially preclinical stages of AD and is present in both mildly and severely cognitively impaired individuals [9,10]. Additionally, astrocytes in the post-mortem brain of AD patients exhibit significant cellular atrophy, upregulate the cytoskeletal protein glial fibrillary acidic protein (GFAP) and internalise Aβ [11][12][13][14]. While post-mortem studies find microglia and astrocytes surrounding Aβ aggregates in high numbers, this research fails to answer when and how microglia and astrocytes specifically respond to Aβ, and whether this is beneficial or detrimental for AD progression [15][16][17]. ...
... A large body of evidence has implicated neuroinflammatory changes as a key neuropathological feature within AD progression, largely associated with changes in the states of astrocytes and microglia [13,68,69]. Unexpectedly, we found preliminary evidence that in iPSC-derived astrocytes the level of IL-6 secretion upon LPS stimulation was determined by APOE genotype and was independent of PSEN2 status. ...
Background
Widescale evidence points to the involvement of glia and immune pathways in the progression of Alzheimer’s disease (AD). AD-associated iPSC-derived glial cells show a diverse range of AD-related phenotypic states encompassing cytokine/chemokine release, phagocytosis and morphological profiles, but to date studies are limited to cells derived from PSEN1, APOE and APP mutations or sporadic patients. The aim of the current study was to successfully differentiate iPSC-derived microglia and astrocytes from patients harbouring an AD-causative PSEN2 (N141I) mutation and characterise the inflammatory and morphological profile of these cells.
Methods
iPSCs from three healthy control individuals and three familial AD patients harbouring a heterozygous PSEN2 (N141I) mutation were used to derive astrocytes and microglia-like cells and cell identity and morphology were characterised through immunofluorescent microscopy. Cellular characterisation involved the stimulation of these cells by LPS and Aβ42 and analysis of cytokine/chemokine release was conducted through ELISAs and multi-cytokine arrays. The phagocytic capacity of these cells was then indexed by the uptake of fluorescently-labelled fibrillar Aβ42.
Results
AD-derived astrocytes and microglia-like cells exhibited an atrophied and less complex morphological appearance than healthy controls. AD-derived astrocytes showed increased basal expression of GFAP, S100β and increased secretion and phagocytosis of Aβ42 while AD-derived microglia-like cells showed decreased IL-8 secretion compared to healthy controls. Upon immunological challenge AD-derived astrocytes and microglia-like cells showed exaggerated secretion of the pro-inflammatory IL-6, CXCL1, ICAM-1 and IL-8 from astrocytes and IL-18 and MIF from microglia.
Conclusion
Our study showed, for the first time, the differentiation and characterisation of iPSC-derived astrocytes and microglia-like cells harbouring a PSEN2 (N141I) mutation. PSEN2 (N141I)-mutant astrocytes and microglia-like cells presented with a ‘primed’ phenotype characterised by reduced morphological complexity, exaggerated pro-inflammatory cytokine secretion and altered Aβ42 production and phagocytosis.
... The role of glial cells in the pathogenesis of several neurodegenerative diseases has been extensively studied (Bennett and Viaene, 2021;Brück et al., 2016;Gleichman and Carmichael, 2020;Mrak and Griffin, 2005). Astrocytes in particular have been demonstrated to associate closely with Aβ plaques, and reactive astrocytes are increased both in human AD brains and AD mouse models (Funato et al., 1998;Nagele et al., 2003). We hypothesized that alleviating Aβ burden via reduction of α-syn could also provide a means to reduce neuroinflammation in the brains of AD mice. ...
... Astrocytes are capable of accumulating substantial amounts of neuronderived, Aβ as a consequence of their debris-clearing role during neurodegeneration. Astrocytes eventually undergo lysis and contribute to the dispersal of Aβ, leading to the deposition of GFAP-rich, astrocytic Aβ plaques, first appearing in the molecular layer of the cerebral cortex, as we observed in our study (Bido et al., 2021;Nagele et al., 2003;Nagele et al., 2004). Microglia are then recruited to plaques. ...
Neurodegenerative disorders of aging are characterized by the progressive accumulation of proteins such as α-synuclein (α-syn) and amyloid beta (Aβ). Misfolded and aggregated α-syn has been implicated in neurological disorders such as Parkinson's disease, and Dementia with Lewy Bodies, but less so in Alzheimer's Disease (AD), despite the fact that accumulation of α-syn has been confirmed in over 50% of postmortem brains neuropathologically diagnosed with AD. To date, no therapeutic strategy has effectively or consistently downregulated α-syn in AD. Here we tested the hypothesis that by using a systemically-delivered peptide (ApoB11) bound to a modified antisense oligonucleotide against α-syn (ASO-α-syn), we can downregulate α-syn expression in an AD mouse model and improve behavioral and neuropathologic phenotypes. Our results demonstrate that monthly systemic treatment with of ApoB11:ASO α-syn beginning at 6 months of age reduces expression of α-synuclein in the brains of 9-month-old AD mice. Downregulation of α-syn also led to reduction in Aβ plaque burden, prevented neuronal loss and astrogliosis. Furthermore, we found that AD mice treated with ApoB11:ASO α-syn had greatly improved hippocampal and spatial memory function in comparison to their control counterparts. Collectively, our data supports the reduction of α-syn through use of systemically-delivered ApoB11:ASO α-syn as a promising future disease-modifying therapeutic for AD.
... 3,17,29-31 GFAP is a marker of astrocyte activation, the brain's response to neuronal injury, including amyloid-β and tau pathology. [32][33][34] Although the exact role of astrocyte activation in early Alzheimer's disease pathology needs further study, it has been suggested that reactive astrocytes may act as initiators and/or modulators of early 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Alzheimer's disease pathology and progression. [32][33][34][35] Our findings that amyloid-β positive individuals show higher GFAP levels compared to amyloid-β negative individuals and subtle increases in the early predementia stage support this hypothesis and suggest increases in plasma GFAP levels to be a direct effect of brain amyloid-β pathology. ...
... [32][33][34] Although the exact role of astrocyte activation in early Alzheimer's disease pathology needs further study, it has been suggested that reactive astrocytes may act as initiators and/or modulators of early 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Alzheimer's disease pathology and progression. [32][33][34][35] Our findings that amyloid-β positive individuals show higher GFAP levels compared to amyloid-β negative individuals and subtle increases in the early predementia stage support this hypothesis and suggest increases in plasma GFAP levels to be a direct effect of brain amyloid-β pathology. Resemblance of monozygotic twins for plasma GFAP levels was high, indicating that the process of astrocytosis may be, to a large extent, genetically determined. ...
Blood-based biomarkers could prove useful to predict Alzheimer’s Disease core pathologies in advance of clinical symptoms. Implementation of such biomarkers requires a solid understanding of their long-term dynamics, and the contribution of confounding to their association with Alzheimer's disease pathology. Here we assess the value of plasma amyloid-β1-42/1-40, phosphorylated-tau181 and glial fibrillary acidic protein to detect early Alzheimer's disease pathology, accounting for confounding by genetic and early environmental factors.
Participants were 200 monozygotic twins, aged ≥60 years with normal cognition from the EMIF-AD PreclinAD study. All twins had amyloid-β status and plasma samples available at study enrollment. For 80 twins, additional plasma samples were available that had been collected approximately 10 years prior to amyloid-β status assessment. Single Molecule Array assays were applied to measure amyloid-β1-42/1-40, phosphorylated-tau181 and glial fibrillary acidic protein. Predictive value of, and longitudinal change in these biomarkers were assessed using Receiver Operating Characteristic curve analysis and linear mixed models.
Amyloid pathology could be predicted using blood-based biomarkers obtained at time of amyloid status assessment (amyloid-β1-42/1-40: Area Under Curve = 0·65, p = 0·01; phosphorylated-tau181: Area Under Curve = 0·84, p < 0·001; glial fibrillary acidic protein: Area Under Curve = 0·74, p < 0·001), as well as, using those obtained 10 years prior to amyloid status assessment (amyloid-β1-42/1-40: Area Under Curve = 0·69, p = 0·03; phosphorylated-tau181: Area Under Curve = 0·92, p < 0·001; glial fibrillary acidic protein: Area Under Curve = 0·84, p < 0·001). Longitudinally, amyloid-β1-42/1-40 levels decreased (β(SE)=-0·12(0·01), p < 0·001) and phosphorylated-tau181 levels increased (β(SE) = 0·02(0·01), p = 0·004). Amyloid-β+ individuals showed a steeper increase in phosphorylated-tau181 compared to amyloid-β- individuals (β(SE) = 0·06(0·02), p = 0·004). Also amyloid-β+ individuals tended to show a steeper increase in glial fibrillary acidic protein (β(SE) = 0·04(0·02), p = 0·07). Within monozygotic twin-pairs, those with higher plasma phosphorylated-tau181 and lower amyloid-β1-42/1-40 levels were more likely to be amyloid-β+ (β(SE) = 0·95(0·26), p < 0·001; β(SE)=-0·28(0·14), p < 0·05) indicating minimal contribution of confounding by genetic and early environmental factors.
Our data support the use of amyloid-β1-42/1-40, phosphorylated-tau181 and glial fibrillary acidic protein as screening tools for Alzheimer's disease pathology in the normal aging population, which is of importance for enrollment of high-risk subjects in secondary, or even primary, prevention trials. Furthermore, these markers show potential as low-invasive monitoring tool of disease progression and possibly treatment effects in clinical trials.
... Reactive astrocytes cluster around amyloid plaques, and the brain area occupied by these reactive astrocytes shows a significant increase in AD patients [30]. In addition, there are several lines of evidence that astrocytes can also eliminate amyloid plaques and neurons containing Aβ and enhance phagocytosis by microglia [31][32][33]. To investigate the number of amyloid plaque-associated astrocytes, we assessed the number of astrocytes within 20 μm from amyloid plaques. ...
... In the brains of AD patients and mouse models, there is a significant increase in GFAP immunoreactivities in plaque-associated astrocytes. Similar to the activation of microglia, reactive astrocytes phagocytose amyloid aggregates and dystrophic neurites and are involved in the inflammatory response to Aβ [33,50]. Additionally, knockout of GFAP in an AD mouse model showed a twofold increase in amyloid plaque burden and twice the amounts of dystrophic neurites [51]. ...
Background
Aducanumab (Adu), which is a human IgG1 monoclonal antibody that targets oligomer and fibril forms of beta-amyloid, has been reported to reduce amyloid pathology and improve impaired cognition after administration of a high dose (10 mg/kg) of the drug in Alzheimer’s disease (AD) clinical trials. The purpose of this study was to investigate the effects of a lower dose of Adu (3 mg/kg) with enhanced delivery via focused ultrasound (FUS) in an AD mouse model.
Methods
The FUS with microbubbles opened the blood–brain barrier (BBB) of the hippocampus for the delivery of Adu. The combined therapy of FUS and Adu was performed three times in total and each treatment was performed biweekly. Y-maze test, Brdu labeling, and immunohistochemical experimental methods were employed in this study. In addition, RNA sequencing and ingenuity pathway analysis were employed to investigate gene expression profiles in the hippocampi of experimental animals.
Results
The FUS-mediated BBB opening markedly increased the delivery of Adu into the brain by approximately 8.1 times in the brains. The combined treatment induced significantly less cognitive decline and decreased the level of amyloid plaques in the hippocampi of the 5×FAD mice compared with Adu or FUS alone. Combined treatment with FUS and Adu activated phagocytic microglia and increased the number of astrocytes associated with amyloid plaques in the hippocampi of 5×FAD mice. Furthermore, RNA sequencing identified that 4 enriched canonical pathways including phagosome formation, neuroinflammation signaling, CREB signaling and reelin signaling were altered in the hippocami of 5×FAD mice receiving the combined treatment.
Conclusion
In conclusion, the enhanced delivery of a low dose of Adu (3 mg/kg) via FUS decreases amyloid deposits and attenuates cognitive function deficits. FUS-mediated BBB opening increases adult hippocampal neurogenesis as well as drug delivery. We present an AD treatment strategy through the synergistic effect of the combined therapy of FUS and Adu.
... Pro-inflammatory cytokine levels are enhanced in the brain of AD patients by reactive astrocytes that respond to interleukin 6, interleukin 1β, and TNFα. 119,120 During the progression of AD, astrocytes release feed-forward signals that stimulate other CNS cells to interact with neurons and microglia, contributing to the complex chain of events that ultimately results in neurodegeneration. Reactive astrocytes may play both protective and harmful roles in AD; atrophic astrocytes may lose their homeostatic capabilities. ...
... Glial cell dysfunction observed in the postmortem human AD brain has been reported in various clinical studies 62,63 . In the present study, we also found that MT reduced microgliosis in APP NL-G-F/NL-G-F mice. ...
Education, occupation, and an active lifestyle, comprising enhanced social, physical, and mental components are associated with improved cognitive functions in aged people and may delay the progression of various neurodegenerative diseases including Alzheimer’s disease. To investigate this protective effect, 3-month-old APPNL-G-F/NL-G-F mice were exposed to repeated single- or multi-domain cognitive training. Cognitive training was given at the age of 3, 6, & 9 months. Single-domain cognitive training was limited to a spatial navigation task. Multi-domain cognitive training consisted of a spatial navigation task, object recognition, and fear conditioning. At the age of 12 months, behavioral tests were completed for all groups. Then, mice were sacrificed, and their brains were assessed for pathology. APPNL-G-F/NL-G-F mice given multi-domain cognitive training compared to APPNL-G-F/NL-G-F control group showed an improvement in cognitive functions, reductions in amyloid load and microgliosis, and a preservation of cholinergic function. Additionally, multi-domain cognitive training improved anxiety in APPNL-G-F/NL-G-F mice as evidenced by measuring thigmotaxis behavior in the Morris water maze. There were mild reductions in microgliosis in the brain of APPNL-G-F/NL-G-F mice with single-domain cognitive training. These findings provide causal evidence for the potential of certain forms of cognitive training to mitigate the cognitive deficits in Alzheimer disease.
... Accordingly, increasing evidence supports a role of astrocytes in AD onset and progression, as heralded by several morphological and molecular changes of astrocytes in AD mouse models (e.g. APP/PS1) and in human patients [6,7] as well as by the link between Aβ plaques formation and changes in astrocytes morphology and activity [8,9]. ...
Increasing evidence implicates astrocytic dysfunction in Alzheimer’s disease (AD), a neurodegenerative disorder characterised by progressive cognitive loss. The accumulation of amyloid-β (Aβ) plaques is a histopathological hallmark of AD and associated with increased astrocyte reactivity. In APP/PS1 mice modelling established AD (9 months), we now show an altered astrocytic morphology and enhanced activity of astrocytic hemichannels, mainly composed by connexin 43 (Cx43). Hemichannel activity in hippocampal astrocytes is also increased in two models of early AD: (1) mice with intracerebroventricular (icv) administration of Aβ1-42, and (2) hippocampal slices superfused with Aβ1-42 peptides. In hippocampal gliosomes of APP/PS1 mice, Cx43 levels were increased, whereas mice administered icv with Aβ1-42 only displayed increased Cx43 phosphorylation levels. This suggests that hemichannel activity might be differentially modulated throughout AD progression. Additionally, we tested if adenosine A2A receptor (A2AR) blockade reversed alterations of astrocytic hemichannel activity and found that the pharmacological blockade or genetic silencing (global and astrocytic) of A2AR prevented Aβ-induced hemichannel dysregulation in hippocampal slices, although A2AR genetic silencing increased the activity of astroglial hemichannels in control conditions. In primary cultures of astrocytes, A2AR-related protective effect was shown to occur through a protein kinase C (PKC) pathway. Our results indicate that the dysfunction of hemichannel activity in hippocampal astrocytes is an early event in AD, which is modulated by A2AR.
Supplementary Information
The online version contains supplementary material available at 10.1007/s00018-023-04983-6.
... On the other hand, astrocyte and microglial activation are also related with the pathogenesis of AD. In fact, astrogliosis is classically considered as the astrocytic reaction to AD pathology in human brain and animal models (Rodríguez et al. 2016;Nagele et al. 2003). ...
Pathophysiology of sporadic Alzheimer’s disease (SAD) and familial Alzheimer’s disease (FAD) remains poorly known, including the exact role of neuroglia and specifically astroglia, in part because studies of astrocytes in human Alzheimer’s disease (AD) brain samples are scarce. As far as we know, this is the first study of a 3-D immunohistochemical and microstructural analysis of glial fibrillary acidic protein (GFAP)- and glutamine synthetase (GS)-positive astrocytes performed in the entorhinal cortex (EC) of human SAD and FAD samples. In this study, we report prominent atrophic changes in GFAP and GS astrocytes in the EC of both SAD and FAD characterised by a decrease in area and volume when compared with non-demented control samples (ND). Furthermore, we did not find neither astrocytic loss nor astrocyte proliferation or hypertrophy (gliosis). In contrast with the astrogliosis classically accepted hypothesis, our results show a highly marked astrocyte atrophy that could have a major relevance in AD pathological processes being fundamental and key for AD mnesic and cognitive alterations equivalent in both SAD and FAD.
... Emerging lines of evidence have confirmed that massive reactive astrogliosis is an archetypical morphological feature in the brain of AD mouse models [133] and AD patients [134]. In AD, astrocytes experience remodeling in morphology, transcriptional profile, and function. ...
Microglia and astrocytes are regarded as active participants in the central nervous system under various neuropathological conditions, including Alzheimer’s disease (AD). Both microglia and astrocyte activation have been reported to occur with a spatially and temporarily distinct pattern. Acting as a double-edged sword, glia-mediated neuroinflammation may be both detrimental and beneficial to the brain. In a variety of neuropathologies, microglia are activated before astrocytes, which facilitates astrocyte activation. Yet reactive astrocytes can also prevent the activation of adjacent microglia in addition to helping them become activated. Studies describe changes in the genetic profile as well as cellular and molecular responses of these two types of glial cells that contribute to dysfunctional immune crosstalk in AD. In this paper, we construct current knowledge of microglia-astrocyte communication, highlighting the multifaceted functions of microglia and astrocytes and their role in AD. A thorough comprehension of microglia-astrocyte communication could hasten the creation of novel AD treatment approaches.
... Thus, considering the important role of astrocytes in PD, studies of AEVs in a prospective cohort in future experiments may improve the performance of the biomarker in early diagnosis of PD. Finally, astrocytes are activated at the site of Aβ deposition in AD [61,62], and Aβ can be internalized by and detected in astrocytes in AD brains [63,64]. This points to the potential of plasma AEVs as a biomarker to differentiate other neurodegenerative diseases. ...
Background
The accumulation of α-synuclein (α-syn), an essential step in PD development and progression, is observed not only in neurons but also in glia, including astrocytes. The mechanisms regulating astrocytic α-syn level and aggregation remain unclear. More recently, it has been demonstrated that a part of α-syn spreading occurs through extracellular vesicles (EVs), although it is unknown whether this process is involved in astrocytes of PD. It is known, however, that EVs derived from the central nervous system exist in the blood and are extensively explored as biomarkers for PD and other neurodegenerative disorders.
Methods
Primary astrocytes were transfected with A53T α-syn plasmid or exposed to α-syn aggregates. The level of astrocyte-derived EVs (AEVs) was assessed by nanoparticle tracking analysis and immunofluorescence. The lysosomal function was evaluated by Cathepsin assays, immunofluorescence for levels of Lamp1 and Lamp2, and LysoTracker Red staining. The Apogee assays were optimized to measure the GLT-1 ⁺ AEVs in clinical cohorts of 106 PD, 47 multiple system atrophy (MSA), and 103 healthy control (HC) to test the potential of plasma AEVs as a biomarker to differentiate PD from other forms of parkinsonism.
Results
The number of AEVs significantly increased in primary astrocytes with α-syn deposition. The mechanism of increased AEVs was partially attributed to lysosomal dysfunction. The number of α-syn-carrying AEVs was significantly higher in patients with PD than in HC and MSA. The integrative model combining AEVs with total and aggregated α-syn exhibited efficient diagnostic power in differentiating PD from HC with an AUC of 0.915, and from MSA with an AUC of 0.877.
Conclusions
Pathological α-syn deposition could increase the astrocytic secretion of EVs, possibly through α-syn-induced lysosomal dysfunction. The α-syn-containing AEVs in the peripheral blood may be an effective biomarker for clinical diagnosis or differential diagnosis of PD.
... Neuroinflammation in the AD brain involves glial cells accumulation [49,50]. Microglial activation and astrocyte reactivity have been observed in the brains of AD humans and animal models [51][52][53]. In line with a previous study [54], we revealed that Aβ 1-42 could induce astrocytes reactivity and microglial activation in the CA1 subregion of the hippocampus. ...
Background
Amyloid-β1-42 (Aβ1-42) plays an essential role in the development of the early stage of Alzheimer’s disease (AD). Asiatic acid (AA), an active compound in Centella asiatica L, exhibit neuroprotective properties in previous studies. Due to its low bioavailability, the nose-to-brain delivery technique was used to enhance AA penetration in the brain. In this study, AA was also loaded in solid lipid nanoparticles (SLNs) as a strategy to increase its absorption in the nasal cavity.
Methods
Memory impairment was induced via direct intracerebroventricular injection of Aβ1-42 oligomer into mouse brain. The neuroprotective effect and potential underlying mechanisms were investigated using several memory behavioral examinations and molecular techniques.
Results
The intranasal administration of AA in SLNs attenuated learning and memory impairment induced by Aβ1-42 in Morris water maze and novel object recognition tests. AA significantly inhibited tau hyperphosphorylation of pTau-S396 and pTau-T231 and prevented astrocyte reactivity and microglial activation in the hippocampus of Aβ1-42-treated mice. It is also decreased the high levels of IL-1β, TNF-α, and malondialdehyde (MDA) in mouse brain.
Conclusions
These results suggested that nose-to-brain delivery of AA in SLNs could be a promising strategy to treat the early stage of AD.
... It is also observed that microglia and astrocytes are increased in areas near the Aβ plaques in AD [67]. The reactive astrocytes around the Aβ plaques induce inflammatory reactions and alter Ca 2+ signaling [68,69]. A loss of astrocytic function and reactivity contribute to AD [70]. ...
Oxidative stress with a depletion of glutathione is a key factor in the initiation and progression of Alzheimer's disease (AD). N-Acetylcysteine (NAC), a glutathione precursor, provides neuroprotective effects in AD animal models. Its amide form, N-Acetylcysteine amide (NACA), has an extended bioavailability compared to NAC. This study evaluates the neuroprotective effects of NACA against Aβ1-42 peptide-induced AD-like pathology in rats. Male Wistar rats (2.5 months old) were divided into five groups: Normal Control (NC), Sham (SH), Aβ, Aβ + NACA and NACA + Aβ + NACA (n = 8 in all groups). AD-like pathology was induced by the intracerebroven-tricular infusion of Aβ1-42 peptide into the lateral ventricle. NACA (75 mg/kg) was administered either as a restorative (i.e., injection of NACA for 7 consecutive days after inducing AD-like pathology (Aβ + N group)), or as prophylactic (for 7 days before and 7 days after inducing the pathology (N + Aβ + N group)). Learning and memory, neurogenesis, expression of AD pathology markers, antioxidant parameters, neuroprotection, astrogliosis and microgliosis were studied in the hippocam-pus and the prefrontal cortex. All data were analyzed with a one-way ANOVA test followed by Bonferroni's multiple comparison test. NACA treatment reversed the cognitive deficits and reduced oxidative stress in the hippocampus and prefrontal cortex. Western blot analysis for Tau, Synapto-physin and Aβ, as well as a histopathological evaluation through immunostaining for neurogenesis, the expression of neurofibrillary tangles, β-amyloid peptide, synaptophysin, neuronal morphology and gliosis, showed a neuroprotective effect of NACA. In conclusion, this study demonstrates the neuroprotective effects of NACA against β-amyloid induced AD-like pathology.
... The role of glial cells in the pathogenesis of several neurodegenerative diseases has been extensively studied [42][43][44][45]. Astrocytes in particular have been demonstrated to associate closely with A plaques, and reactive astrocytes are increased both in human AD patient brains and AD mouse models [46,47]. We hypothesized that by alleviating A burden via reduction of ␣-syn, we could also provide a means to reduce neuroinflammation in the brains of AD mice. ...
Background:
Alzheimer's disease (AD) cases are often characterized by the pathological accumulation of α-synuclein (α-syn) in addition to amyloid-β (Aβ) and tau hallmarks. The role of α-syn has been extensively studied in synucleinopathy disorders, but less so in AD. Recent studies have shown that α-syn may also play a role in AD and its downregulation may be protective against the toxic effects of Aβ accumulation.
Objective:
We hypothesized that selectively knocking down α-syn visa RNA interference improves the neuropathological and biochemical findings in AD mice.
Methods:
Here we used amyloid precursor protein transgenic (APP-Tg) mice to model AD and explore pathologic and behavioral phenotypes with knockdown of α-syn using RNA interference. We selectively reduced α-syn levels by stereotaxic bilateral injection of either LV-shRNA α-syn or LV-shRNA-luc (control) into the hippocampus of AD mice.
Results:
We found that downregulation of α-syn results in significant reduction in the number of Ab plaques and tau. In addition, mice treated with LV-shRNA α-syn had amelioration of abnormal microglial activation (Iba1) and astrocytosis (GFAP) phenotypes in AD mice.
Conclusion:
This suggests a link between Aβ and α-syn in pathology pointing to a possible therapeutic angle for AD targeting α-syn.
... It is postulated that stress conditions associated with the disease microenvironment may trigger reinforcing interactions between astrocytes and other glial cells resulting in a compromise of astrocyte and neuronal function as well as chronic neuroinflammation (Wheeler et al. 2019;Joshi et al. 2019;Sofroniew 2020). This could result in astrocyte detrimental effects like altered homeostasis of neurotransmitters glutamate and GABA, K + and Ca + ion homeostasis, release of pro-inflammatory mediators, buildup of intracellular Abeta, and a reduced proliferative capacity of reactive astrocytes during aging (Nagele et al. 2003;Grimaldi et al. 2019;Sofroniew 2020). Deleterious effects of astrocytic populations have been observed in other diseases and animal models (Varcianna et al. 2019;Spanos and Liddelow 2020). ...
Neurodegenerative diseases such as Alzheimer’s disease have been classically studied from a purely neuronocentric point of view. More recent evidences support the notion that other cell populations are involved in disease progression. In this sense, the possible pathogenic role of glial cells like astrocytes is increasingly being recognized. Once faced with tissue damage signals and other stimuli present in disease environments, astrocytes suffer many morphological and functional changes, a process referred as reactive astrogliosis. Studies from murine models and humans suggest that these complex and heterogeneous responses could manifest as disease-specific astrocyte phenotypes. Clear understanding of disease-associated astrocytes is a necessary step to fully disclose neurodegenerative processes, aiding in the design of new therapeutic and diagnostic strategies. In this work, we present the transcriptomics characterization of neurotoxic astrocytic cultures isolated from adult symptomatic animals of the triple transgenic mouse model of Alzheimer’s disease (3xTg-AD). According to the observed profile, 3xTg-AD neurotoxic astrocytes show various reactivity features including alteration of the extracellular matrix and release of pro-inflammatory and proliferative factors that could result in harmful effects to neurons. Moreover, these alterations could be a consequence of stress responses at the endoplasmic reticulum and mitochondria as well as of concomitant metabolic adaptations. Present results support the hypothesis that adaptive changes of astrocytic function induced by a stressed microenvironment could later promote harmful astrocyte phenotypes and further accelerate or induce neurodegenerative processes.
... Aβ oligomers are phagocytosed and degraded by microglia, while astrocytes are crucial for their destruction and clearance (Phillips et al., 2014). It has been found that astrocytes accumulate Aβ-42 peptide in the cortex, which inevitably worsens Aβ-pathologies, and supports the idea that astrocytes are involved in chronic neuroinflammation (Nagele et al., 2003). Furthermore, the presence of active astrocytes in the brains of AD patients supports the critical function of these brain cells in neurodegeneration (Phillips et al., 2014). ...
... 113 Finally, there is some evidence for astrocytic bamyloid presence in the entorhinal cortex of AD patients. 114 ...
... 109,110 Reactive astrogliosis is observed in astrocytes surrounding amyloid plaques in the AD brain, and these cells are positive for IL-6, a key SASP component. 111,112 Astrogliosis is comparable to cellular senescence in that proinflammatory factors are secreted and cause neuroinflammation. 113,114 Several senescent phenotypes were noted in the astrocytes of patients with AD. ...
Neurodegenerative diseases, such as Alzheimer's and Parkinson's, are characterized by several pathological features, including selective neuronal loss, aggregation of specific proteins, and chronic inflammation. Aging is the most critical risk factor of these disorders. However, the mechanism by which aging contributes to the pathogenesis of neurodegenerative diseases is not clearly understood. Cellular senescence is a cell state or fate in response to stimuli. It is typically associated with a series of changes in cellular phenotypes such as abnormal cellular metabolism and proteostasis, reactive oxygen species (ROS) production, and increased secretion of certain molecules via senescence-associated secretory phenotype (SASP). In this review, we discuss how cellular senescence contributes to brain aging and neurodegenerative diseases, and the relationship between protein aggregation and cellular senescence. Finally, we discuss the potential of senescence modifiers and senolytics in the treatment of neurodegenerative diseases.
... Recent studies have indicated that SMA is no longer an exclusive motor neuron disorder due to the wide expression of the SMN protein in the CNS, suggesting SMA is a multisystem disorder [71]. The crucial contribution of astrocytes to diseases of the CNS has been demonstrated for disorders, such as Alzheimer's disease or epilepsy [6,31,53,56]. Astrocytes are also suggested to be critically involved in the pathogenesis of SMA. Most of these studies demonstrated early enhanced expression of GFAP as a sign of increased reactivity of spinal astrocytes before the loss of spinal MN appeared in severe mouse models or have recently demonstrated higher levels in CSF of SMA patients [22,33,45,59]. ...
Spinal muscular atrophy (SMA) is a progressive neuromuscular disorder caused by a loss of the survival of motor neuron 1 (SMN1) gene, resulting in a loss of spinal motor neurons (MNs), leading to muscle weakness and wasting. The pathogenesis of MN loss in SMA and the selective vulnerability in different cellular populations are not fully understood. To investigate the role of spinal astrocytes in the pathogenesis of late-onset SMA, we used a mouse model in addition to in vitro approaches. Immunostaining, Western blot analysis, small interfering ribonucleic acid (siRNA) transfections, functional assays, enzyme-linked immunosorbent assay (ELISA), behavioral tests, and electrophysiological measurements were performed. Early activation of spinal astrocytes and a reduction of the excitatory amino acid transporter 1 (EAAT1) on postnatal day (P) 20 preceded the loss of spinal MNs in SMA mice occurring on P42. EAAT1 reduction resulted in elevated glutamate levels in the spinal cord of SMA mice at P20 and P42. SMA-like astrocytes generated by siRNA and an ex vivo model of glutamate excitotoxicity involving organotypic spinal cord slice cultures revealed the critical role of glutamate homeostasis in the degeneration of MNs. The pre-emptive administration of arundic acid (AA), as an inhibitor of astrocyte activation, to SMA mice prior to the loss of motor neurons (P28) resulted in elevated EAAT1 protein levels compared to vehicle-treated SMA mice and prevented the increase of glutamate in the spinal cord and the loss of spinal MNs. Furthermore, AA preserved motor functions during behavioral experiments, the electrophysiological properties, and muscle alteration of SMA mice. In a translational approach, we transfected healthy human fibroblasts with SMN1 siRNA, resulting in reduced EAAT1 expression and reduced uptake but increased glutamate release. These findings were verified by detecting elevated glutamate levels and reduced levels of EAAT1 in cerebrospinal fluid of untreated SMA type 2 and 3 patients. In addition, glutamate was elevated in serum samples, while EAAT1 was not detectable. Our data give evidence for the crucial role of spinal astrocytes in the pathogenesis of late-onset SMA, a potential driving force for MN loss by glutamate excitotoxicity caused by EAAT1 reduction as an early pathophysiological event. Furthermore, our study introduces EAAT1 as a potential therapeutic target for additional SMN-independent therapy strategies to complement SMN-enhancing drugs.
Supplementary Information
The online version contains supplementary material available at 10.1007/s00401-023-02554-4.
... The contribution of astrocytes to the AD pathology, already suggested by Alois Alzheimer [17], is well documented and reported [12,[18][19][20]. Astrocytes and microglial cells are key players in the limitation of Aβ plaques; as has been revealed by 3-D analysis showing that astrocytes are in the dense core of the amyloid plaque, as well as surrounding them, with their primary processes, robustly enlarged, penetrating towards the core [21,22] in both major affected areas (Hipocampus and Entorhinal Cortex -EC-), despite the fact that the full CNS is totally compromised. EC is the first zone affected by AD before it extends to other brain regions [23]. ...
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.
... Astrocytes with high Aβ load are frequently found in the human AD brain [13][14][15][16][17]. We have previously reported that cultured astrocytes engulf large amounts of aggregated Aβ, but then store, rather than degrade the ingested material [18][19][20]. ...
Background
Astrocytes play a central role in maintaining brain energy metabolism, but are also tightly connected to the pathogenesis of Alzheimer’s disease (AD). Our previous studies demonstrate that inflammatory astrocytes accumulate large amounts of aggregated amyloid-beta (Aβ). However, in which way these Aβ deposits influence their energy production remain unclear.
Methods
The aim of the present study was to investigate how Aβ pathology in astrocytes affects their mitochondria functionality and overall energy metabolism. For this purpose, human induced pluripotent cell (hiPSC)-derived astrocytes were exposed to sonicated Aβ42 fibrils for 7 days and analyzed over time using different experimental approaches.
Results
Our results show that to maintain stable energy production, the astrocytes initially increased their mitochondrial fusion, but eventually the Aβ-mediated stress led to abnormal mitochondrial swelling and excessive fission. Moreover, we detected increased levels of phosphorylated DRP-1 in the Aβ-exposed astrocytes, which co-localized with lipid droplets. Analysis of ATP levels, when blocking certain stages of the energy pathways, indicated a metabolic shift to peroxisomal-based fatty acid β-oxidation and glycolysis.
Conclusions
Taken together, our data conclude that Aβ pathology profoundly affects human astrocytes and changes their entire energy metabolism, which could result in disturbed brain homeostasis and aggravated disease progression.
... Furthermore, amyloid associated proteins, like ApoE, significantly altered the astrocytic uptake of oligomeric Aß, suggesting a mechanism that could ultimately effect clearance and deposition at different disease stages (Nielsen et al., 2010). Reactive astrocytes found in postmortem AD tissue of the entorhinal cortex have a significant Aß load (Nagele et al., 2003), however astrocytes with large phagocytic burden become inefficient at degradation of cell debris (Loov et al., 2012). One study found that Aß accumulation in astrocytes resulted in enlarged endosomes, where engulfed Aß 42 was stored instead of being actively degraded (Söllvander et al., 2016). ...
Many neurodegenerative diseases fall under the class of diseases known as proteinopathies, whereby the structure and localization of specific proteins become abnormal. These aberrant proteins often aggregate within cells which disrupts vital homeostatic and physiological cellular functions, ultimately contributing to cell death. Although neurodegenerative disease research is typically neurocentric, there is evidence supporting the role of non-neuronal cells in the pathogenesis of these diseases. Specifically, the role of astrocytes in neurodegenerative diseases has been an ever-growing area of research. Astrocytes are one of the most abundant cell types in the central nervous system (CNS) and provide an array of essential homeostatic functions that are disrupted in neurodegenerative diseases. Astrocytes can exhibit a reactive phenotype that is characterized by molecular changes, as well as changes in morphology and function. In neurodegenerative diseases, there is potential for reactive astrocytes to assume a loss-of-function phenotype in homeostatic operations such as synapse maintenance, neuronal metabolic support, and facilitating cell-cell communication between glia and neurons. They are also able to concurrently exhibit gain-of-function phenotypes that can be destructive to neural networks and the astrocytes themselves. Additionally, astrocytes have been shown to internalize disease related proteins and reflect similar or exacerbated pathology that has been observed in neurons. Here, we review several major neurodegenerative disease-specific proteinopathies and what is known about their presence in astrocytes and the potential consequences regarding cell and non-cell autonomous neurodegeneration.
... Astrocyte-derived ApoE4 may drive ApoE-dependent BBB alterations [138] and thus induce BBB leakage in AD patients [139]. Reactive astrogliosis occurs in the vicinity of amyloid plaques [140], and reactive astrocytes can undergo atrophy with subsequent reactive astrocytic hypertrophy and form part of the plaque in the hippocampal formation in AD patients [141]. Furthermore, studies have suggested that astrocytic processes undergo alterations before Ab plaque formation [142]. ...
Background
Synaptic dysfunction is a major contributor to Alzheimeŕs disease (AD) pathogenesis in addition to the formation of neuritic β-amyloid plaques and neurofibrillary tangles of hyperphosphorylated Tau protein. However, how these features contribute to synaptic dysfunction and axonal loss remains unclear. While years of considerable effort have been devoted to gaining an improved understanding of this devastating disease, the unavailability of patient-derived tissues, considerable genetic heterogeneity, and lack of animal models that faithfully recapitulate human AD have hampered the development of effective treatment options. Ongoing progress in human induced pluripotent stem cell (hiPSC) technology has permitted the derivation of patient- and disease-specific stem cells with unlimited self-renewal capacity. These cells can differentiate into AD-affected cell types, which support studies of disease mechanisms, drug discovery, and the development of cell replacement therapies in traditional and advanced cell culture models.
Aim of Review
To summarize current hiPSC-based AD models, highlighting the associated achievements and challenges with a primary focus on neuron and synapse loss.
Key Scientific Concepts of Review
We aim to identify how hiPSC models can contribute to understanding AD-associated synaptic dysfunction and axonal loss. hiPSC-derived neural cells, astrocytes, and microglia, as well as more sophisticated cellular organoids, may represent reliable models to investigate AD and identify early markers of AD-associated neural degeneration.
... Indeed, several post-mortem immunohistochemical and immunoelectron microscopy studies found that astrocytes contained non-fibrillar deposits of Aβ, likely derived from extracellular deposits of diffuse amyloid (Akiyama, Schwab et al. 1996. However, this observation has also been interpreted as suggesting a direct contribution of reactive astrocytes to plaque formation (Nagele, D'Andrea et al. 2003), rather than clearance. ...
Alzheimer's disease (AD) is characterized by the extracellular aggregation of the Amyloid-ß peptide (Aß) in amyloid plaques and by the intraneuronal accumulation of aggregated hyperphosphorylated tau (pTau) in neurofibrillary tangles (NFTs). To date, our understanding of the molecular pathways and cellular circuits involved in the toxic effects of Aß and pTau is still incomplete. To better understand how pTau exerts its toxicity in AD, we used two complementary localized proteomics approaches: (i) a quantitative proteomics method performed on microdissected NFTs from AD patients, and (ii) an Affinity-Purification Mass Spectrometry method to identify NFT-associated proteins that specifically bind to pTau. In this study, we identified 542 proteins in NFTs, and confirmed that 75 proteins present in NFTs interacted with pTau. In addition, we also identified 13 novel interactors. Among these, SCRN1 was selected for further characterization. Immunohistochemistry showed that SCRN1 was a neuronal protein that abundantly accumulated in NFTs and plaque-associated dystrophic neurites throughout the progression of AD. Quantification of SCRN1 immunohistochemistry confirmed that SCRN1 preferentially accumulated in NFTs in comparison to surrounding non-tangle containing neurons, at both early and late stages of AD. Furthermore, this association between SCRN1 and pTau was unique to AD, and was not observed in other tauopathies. Thus, SCRN1 may be a novel therapeutic target and serve as a useful biomarker to distinguish AD from other tauopathies. In conclusion, we hope that proteomic studies such as ours will lead to a better understanding of the disease mechanisms occurring in AD and other tauopathies.
... Both microglia and astrocytes exhibit activated phenotypes and are often co-localized with plaques [2]. While microglia are known to exhibit both positive and negative outcomes in AD, much less is known about the role of astrocytes, even though their presence near plaques is well-documented [3]. ...
The most significant genetic risk factor for developing late-onset Alzheimer's disease (AD) is the ε4 allele of apolipo-protein E (APOE4). APOE genotype and biological sex are key modulators of microglial and astroglial function, which exert multiple effects on AD pathogenesis. Here, we show astroglial interactions with amyloid plaques in the EFAD transgenic mouse model of AD. Using confocal microscopy, we observed significantly lower levels of astrocytic plaque coverage and plaque compaction (beneficial effects of glial barrier formation) with APOE4 genotype and female sex. Conversely, neurite damage and astrocyte activation in the plaque environment were significantly higher in APOE4 carriers and female mice. Astrocyte coverage of plaques was highest in APOE3 males and poorest in APOE4 females. Collectively, our findings provide new insights into the roles of astroglia and highlight the importance of addressing independent and interactive effects of APOE genotype and biological sex in understanding processes contributing to AD pathogenesis.
Nucleocytoplasmic transport of macromolecules is essential in eukaryotic cells. In this process, the karyopherins play a central role when they transport cargoes across the nuclear pore complex. Importin 4 belongs to the karyopherin β family. Many studies have focused on finding substrates for importin 4, but no direct mechanism studies of its precise transport function have been reported. Therefore, this paper mainly aimed to study the mechanism of nucleoporins in mediating nuclear import and export of importin 4. To address this question, we constructed shRNAs targeting Nup358, Nup153, Nup98, and Nup50. We found that depletion of Nup98 resulted in a shift in the subcellular localization of importin 4 from the cytoplasm to the nucleus. Mutational analysis demonstrated that Nup98 physically and functionally interacts with importin 4 through its N‐terminal phenylalanine‐glycine (FG) repeat region. Mutation of nine of these FG motifs to SG motifs significantly attenuated the binding of Nup98 to importin 4, and we further confirmed the essential role of the six FG motifs in amino acids 121–360 of Nup98 in binding with importin 4. In vitro transport assay also confirmed that VDR, the substrate of importin 4, could not be transported into the nucleus after Nup98 knockdown. Overall, our results showed that Nup98 is required for efficient importin 4‐mediated transport. This is the first study to reveal the mechanism of importin 4 in transporting substrates into the nucleus.
Accumulation of amyloid-β (Aβ) and tau tangles are hallmarks of Alzheimer’s disease. Aβ is extracellular while tau tangles are typically intracellular, and it is unknown how these two proteinopathies are connected. Here, we use data of 1206 elders and test that RNA expression levels of GPER1, a transmembrane protein, modify the association of Aβ with tau tangles. GPER1 RNA expression is related to more tau tangles (p = 0.001). Moreover, GPER1 expression modifies the association of immunohistochemistry-derived Aβ load with tau tangles (p = 0.044). Similarly, GPER1 expression modifies the association between Aβ proteoforms and tau tangles: total Aβ protein (p = 0.030) and Aβ38 peptide (p = 0.002). Using single nuclei RNA-seq indicates that GPER1 RNA expression in astrocytes modifies the relation of Aβ load with tau tangles (p = 0.002), but not GPER1 in excitatory neurons or endothelial cells. We conclude that GPER1 may be a link between Aβ and tau tangles driven mainly by astrocytic GPER1 expression.
Neuroinflammation is the major cause of neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease. Currently available drugs present relatively low efficacy and are not capable of modifying the course of the disease or delaying its progression. Identifying well-tolerated and brain-penetrant agents of plant origin could fulfil the pressing need for novel treatment techniques for neuroinflammation. Attention has been drawn to a large family of flavonoids in citrus fruits, which may function as strong nutraceuticals in slowing down the development and progression of neuroinflammation. This review is aimed at elucidating and summarizing the effects of the flavonoid tangeretin (TAN) in the management of neuroinflammation-mediated neurodegenerative disorders. A literature survey was performed using various resources, including ScienceDirect, PubMed, Google Scholar, Springer, and Web of Science. The data revealed that TAN exhibited immense neuroprotective effects in addition to its anti-oxidant, anti-diabetic, and peroxisome proliferator-activated receptor-γ agonistic effects. The effects of TAN are mainly mediated through the inhibition of oxidative and inflammatory pathways via regulating multiple signaling pathways, including c-Jun N-terminal kinase, phosphoinositide 3-kinase, mitogen-activated protein kinase, nuclear factor erythroid-2-related factor 2, extracellular-signal-regulated kinase, and CRE-dependent transcription. In conclusion, the citrus flavonoid TAN has the potential to prevent neuronal death mediated by neuroinflammatory pathways and can be developed as an auxiliary therapeutic agent in the management of neurodegenerative disorders.
The search for new therapies to reduce symptoms and find a cure for Parkinson's disease has focused attention on two key points: the accumulation of alpha-synuclein aggregates and astrocytes. The former is a hallmark of the disease, while the latter corresponds to a type of glial cell with an important role in both the prevention and development of this neurodegenerative disorder. Traditionally, research has focused on therapies targeting dopaminergic neurons. Currently, as more is known about the genetic and molecular factors and the neuroglial interaction in the disease, great emphasis has been placed on the neuroprotective role of astrocytes in the early stages of the disease and on the astrocytic capture of alpha-synuclein under both physiological and pathological conditions. This review aims to analyze the contribution of alpha-synuclein and astrocytes to the development and progression of Parkinson's disease, as well as to evaluate recent therapeutic proposals specifically focused on synucleopathies and astroglial cells as potential therapies for the disease.
Alzheimer's disease (AD), a progressive neurodegenerative disorder, is the most common cause of dementia in elderly people and substantially affects patient quality of life. Oxidative stress is considered a key factor in the development of AD. Nrf2 plays a vital role in maintaining redox homeostasis and regulating neuroinflammatory responses in AD. Previous studies show that potassium 2-(1-hydroxypentyl)-benzoate (PHPB) exerts neuroprotective effects against cognitive impairment in a variety of dementia animal models such as APP/PS1 transgenic mice. In this study we investigated whether PHPB ameriorated the progression of AD by reducing oxidative stress (OS) damage. Both 5- and 13-month-old APP/PS1 mice were administered PHPB (100 mg·kg-1·d-1, i.g.) for 10 weeks. After the cognition assessment, the mice were euthanized, and the left hemisphere of the brain was harvested for analyses. We showed that 5-month-old APP/PS1 mice already exhibited impaired performance in the step-down test, and knockdown of Nrf2 gene only slightly increased the impairment, while knockdown of Nrf2 gene in 13-month-old APP/PS1 mice resulted in greatly worse performance. PHPB administration significantly ameliorated the cognition impairments and enhanced antioxidative capacity in APP/PS1 mice. In addition, PHPB administration significantly increased the p-AKT/AKT and p-GSK3β/GSK3β ratios and the expression levels of Nrf2, HO-1 and NQO-1 in APP/PS1 mice, but these changes were abolished by knockdown of Nrf2 gene. In SK-N-SH APPwt cells and primary mouse neurons, PHPB (10 μM) significantly increased the p-AKT/AKT and p-GSK3β/GSK3β ratios and the level of Nrf2, which were blocked by knockdown of Nrf2 gene. In summary, this study demonstrates that PHPB exerts a protective effect via the Akt/GSK3β/Nrf2 pathway and it might be a promising neuroprotective agent for the treatment of AD.
Background:
Increased blood-brain barrier (BBB) permeability and amyloid-β (Aβ) peptides (especially Aβ1-42) (Aβ42) have been linked to Alzheimer's disease (AD) pathogenesis, but the nature of their involvement in AD-related neuropathological changes leading to cognitive changes remains poorly understood.
Objective:
To test the hypothesis that chronic extravasation of bloodborne Aβ42 peptide and brain-reactive autoantibodies and their entry into the brain parenchyma via a permeable BBB contribute to AD-related pathological changes and cognitive changes in a mouse model.
Methods:
The BBB was rendered chronically permeable through repeated injections of Pertussis toxin (PT), and soluble monomeric, fluorescein isothiocyanate (FITC)-labeled or unlabeled Aβ42 was injected into the tail-vein of 10-month-old male CD1 mice at designated intervals spanning ∼3 months. Acquisition of learned behaviors and long-term retention were assessed via a battery of cognitive and behavioral tests and linked to neuropathological changes.
Results:
Mice injected with both PT and Aβ42 demonstrated a preferential deficit in the capacity for long-term retention and an increased susceptibility to interference in selective attention compared to mice exposed to PT or saline only. Immunohistochemical analyses revealed increased BBB permeability and entry of bloodborne Aβ42 and immunoglobulin G (IgG) into the brain parenchyma, selective neuronal binding of IgG and neuronal accumulation of Aβ42 in animals injected with both PT and Aβ42 compared to controls.
Conclusion:
Results highlight the potential synergistic role of BBB compromise and the influx of bloodborne Aβ42 into the brain in both the initiation and progression of neuropathologic and cognitive changes associated with AD.
Alzheimer’s disease (AD) is the main neurodegenerative disorder characterized by several pathophysiological features, including the misfolding of the tau protein and the amyloid beta (Aβ) peptide, neuroinflammation, oxidative stress, synaptic dysfunction, metabolic alterations, and cognitive impairment. These mechanisms collectively contribute to neurodegeneration, necessitating the exploration of therapeutic approaches with multiple targets. Physical exercise has emerged as a promising non-pharmacological intervention for AD, with demonstrated effects on promoting neurogenesis, activating neurotrophic factors, reducing Aβ aggregates, minimizing the formation of neurofibrillary tangles (NFTs), dampening inflammatory processes, mitigating oxidative stress, and improving the functionality of the neurovascular unit (NVU). Overall, the neuroprotective effects of exercise are not singular, but are multi-targets. Numerous studies have investigated physical exercise’s potential in both AD patients and animal models, employing various exercise protocols to elucidate the underlying neurobiological mechanisms and effects. The objective of this review is to analyze the neurological therapeutic effects of these exercise protocols in animal models and compare them with studies conducted in AD patients. By translating findings from different approaches, this review aims to identify opportune, specific, and personalized therapeutic windows, thus advancing research on the use of physical exercise with AD patients.
This review examines the role of impaired amyloid-β clearance in the accumulation of amyloid-β in the brain and the periphery, which is closely associated with Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The molecular mechanism underlying amyloid-β accumulation is largely unknown, but recent evidence suggests that impaired amyloid-β clearance plays a critical role in its accumulation. The review provides an overview of recent research and proposes strategies for efficient amyloid-β clearance in both the brain and periphery. The clearance of amyloid-β can occur through enzymatic or non-enzymatic pathways in the brain, including neuronal and glial cells, blood-brain barrier, interstitial fluid bulk flow, perivascular drainage, and cerebrospinal fluid absorption-mediated pathways. In the periphery, various mechanisms, including peripheral organs, immunomodulation/immune cells, enzymes, amyloid-β-binding proteins, and amyloid-β-binding cells, are involved in amyloid-β clearance. Although recent findings have shed light on amyloid-β clearance in both regions, opportunities remain in areas where limited data is available. Therefore, future strategies that enhance amyloid-β clearance in the brain and/or periphery, either through central or peripheral clearance approaches or in combination, are highly encouraged. These strategies will provide new insight into the disease pathogenesis at the molecular level and explore new targets for inhibiting amyloid-β deposition, which is central to the pathogenesis of sporadic AD (amyloid-β in parenchyma) and CAA (amyloid-β in blood vessels).
INTRODUCTION
We studied how biomarkers of reactive astrogliosis mediate the pathogenic cascade in the earliest Alzheimer's disease (AD) stages.
METHODS
We performed path analysis on data from 384 cognitively unimpaired individuals from the ALzheimer and FAmilies (ALFA)+ study using structural equation modeling to quantify the relationships between biomarkers of reactive astrogliosis and the AD pathological cascade.
RESULTS
Cerebrospinal fluid (CSF) amyloid beta (Aβ)42/40 was associated with Aβ aggregation on positron emission tomography (PET) and with CSF p‐tau181, which was in turn directly associated with CSF neurofilament light (NfL). Plasma glial fibrillary acidic protein (GFAP) mediated the relationship between CSF Aβ42/40 and Aβ‐PET, and CSF YKL‐40 partly explained the association between Aβ‐PET, p‐tau181, and NfL.
DISCUSSION
Our results suggest that reactive astrogliosis, as indicated by different fluid biomarkers, influences the pathogenic cascade during the preclinical stage of AD. While plasma GFAP mediates the early association between soluble and insoluble Aβ, CSF YKL‐40 mediates the latter association between Aβ and downstream Aβ‐induced tau pathology and tau‐induced neuronal injury.
Highlights
Lower CSF Aβ42/40 was directly linked to higher plasma GFAP concentrations.
Plasma GFAP partially explained the relationship between soluble Aβ and insoluble Aβ.
CSF YKL‐40 mediated Aβ‐induced tau phosphorylation and tau‐induced neuronal injury.
Curcumin (Curc) has been shown to have the potential to ameliorate or prevent the development of Alzheimer's disease (AD). However, most of them are in vitro and in vivo short-term studies. This study was conducted to investigate whether long-term, low-dose dietary Curc intake in mouse of AD might suppress short-term memory retention, amyloid-beta (Aβ) deposition and tau phosphorylation, delaying the onset of AD and prolonging the lifespan of the animals. Short-term memory was examined by the Y-maze method after 6 months old. Immunohistochemical analysis was performed at 10 months old to determine changes in Aβ deposition, tau phosphorylation, and glial cell number in brain tissue. Furthermore, we investigated the survival rate for 12 months old and evaluated the AD prevention effect. The alternation rates of short-time memory in the wild type and AD mice were 56.2% and 25.9%, respectively. These rates in the experimental groups (0.02% and 0.5% Curc) were in the range of 44.4–45.7%. The area of Aβ42 deposition in AD mice was approximately 25,000 µm2, while the experimental groups had a significantly reduced area of 5000–10,000 µm2. Survival rate was 34% in the AD control group, 100% in the 0.02% Curc, and 83% in the 0.5% Curc group, significantly longer in the Curc groups than the AD control group. This study demonstrates that long-term intake of low concentrations of Curc may act on the tau- phosphorylation, suppress brain inflammation, delay the onset of AD, and prolong the lifespan of the mouse.
As a classic animal model of Alzheimer's disease (AD), the 3 × Tg-AD mouse not only recapitulates most of anatomical hallmarks observed in AD pathology but also displays cognitive alterations in memory and learning tasks. The 3 × Tg-AD can better show the two characteristics of AD, amyloid β (Aβ) and neurofibrillary tangles (NFT). Therefore, 3 × Tg-AD strain is widely used in AD pathogenesis research and new drug development of AD. In this paper, the construction methods, pathological changes, and treatment characteristics of 3 × Tg-AD mouse models commonly used in AD research are summarized and commented, hoping to provide reference for researchers to choose and establish experimental patterns.
Objective:
Long non-coding RNAs (IncRNAs) are regulatory RNA transcripts that have recently been associated with the onset of many neurodegenerative illnesses, including Alzheimer's disease (AD). Several IncRNAs have been found to be associated with AD pathophysiology, each with a distinct mechanism. In this review, we focused on the role of IncRNAs in the pathogenesis of AD and their potential as novel biomarkers and therapeutic targets.
Methods:
Searching for relevant articles was done using the PubMed and Cochrane library databases. Studies had to be published in full text in English in order to be considered.
Results:
Some IncRNAs were found to be upregulated, while others were downregulated. Dysregulation of IncRNAs expression may contribute to AD pathogenesis. Their effects manifest as the synthesis of beta-amyloid (Aβ) plaques increases, thereby altering neuronal plasticity, inducing inflammation, and promoting apoptosis.
Conclusion:
Despite the need for more investigations, IncRNAs could potentially increase the sensitivity of early detection of AD. Until now, there has been no effective treatment for AD. Hence, InRNAs are promising molecules and may serve as potential therapeutic targets. Although several dysregulated AD-associated lncRNAs have been discovered, the functional characterization of most lncRNAs is still lacking.
Objectives:
β-Amyloid protein (Aβ) plays pivotal roles in pathogenesis of Alzheimer's disease (AD) and triggers various pathophysiological events. Lycopene is a promising neuroprotector with multiple bioactivities, while its bioavailability is limited. Lycopene-loaded microemulsion (LME) possessing superior bioavailability and brain-targeting efficiency was developed in our previous study. In this investigation, we aimed to comprehensively evaluate its neuroprotective effects and underlying mechanisms using intracerebroventricular (ICV) Aβ1-42 injection mice.
Methods:
Mice were assigned to the Sham, Aβ, Aβ + LME and Aβ + lycopene dissolved in olive oil (LOO) groups. ICV Aβ1-42 administration was performed, followed by oral gavage of brain-targeted LME or conventional LOO formulation for 3 weeks. Brain samples were harvested for immunohistochemistry, biochemical assays and western blotting analyses.
Results:
Our findings verified Aβ-induced neurotoxicity on neuroinflammation, oxidative stress, apoptosis, Aβ metabolisms and synaptic plasticity. LME supplementation dramatically attenuated astrocytosis and microgliosis, decreased malondialdehyde production and rescued antioxidant capacities, normalized apoptotic parameters and alleviated neuronal loss, inhibited amyloidogenic processing and activated non-amyloidogenic pathway, together with upregulating synaptic protein expressions and restoring synaptic plasticity. Nevertheless, most of these phenomena were not observed for mice treated with LOO, implying that LME showed significantly higher therapeutic efficacy against Aβ injury.
Discussion:
In summary, brain-targeted LME could exert neuroprotective function via suppressing a series of cascades triggered by Aβ aggregates, thus ameliorating Aβ neurotoxicity and associated abnormalities. Given this, LME may serve as an attractive candidate for AD prevention and treatment, and superiority of brain-targeting delivery is highlighted.
Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that leads to dementia and patient death. AD is characterized by intracellular neurofibrillary tangles, extracellular amyloid beta (Aβ) plaque deposition, and neurodegeneration. Diverse alterations have been associated with AD progression, including genetic mutations, neuroinflammation, blood–brain barrier (BBB) impairment, mitochondrial dysfunction, oxidative stress, and metal ion imbalance.Additionally, recent studies have shown an association between altered heme metabolism and AD. Unfortunately, decades of research and drug development have not produced any effective treatments for AD. Therefore, understanding the cellular and molecular mechanisms underlying AD pathology and identifying potential therapeutic targets are crucial for AD drug development. This review discusses the most common alterations associated with AD and promising therapeutic targets for AD drug discovery. Furthermore, it highlights the role of heme in AD development and summarizes mathematical models of AD, including a stochastic mathematical model of AD and mathematical models of the effect of Aβ on AD. We also summarize the potential treatment strategies that these models can offer in clinical trials.
Several studies have provided interesting evidence about the role of the bidirectional communication between the gut and brain in the onset and development of several pathologic conditions, including inflammatory bowel diseases (IBDs), neurodegenerative diseases, and related comorbidities. Indeed, patients with IBD can experience neurologic disorders, including depression and cognitive impairment, besides typical intestinal symptoms. In parallel, patients with neurodegenerative disease, such as Parkinson disease and Alzheimer disease, are often characterized by the occurrence of functional gastrointestinal disorders. In this context, enteric glial cells and brain astrocytes are emerging as pivotal players in the initiation/maintenance of neuroinflammatory responses, which appear to contribute to the alterations of intestinal and neurologic functions observed in patients with IBD and neurodegenerative disorders. The present review was conceived to provide a comprehensive and critical overview of the available knowledge on the morphologic, molecular, and functional changes occurring in the enteric glia and brain astroglia in IBDs and neurologic disorders. In addition, our intent is to identify whether such alterations could represent a common denominator involved in the onset of comorbidities associated with the aforementioned disorders. This might help to identify putative targets useful to develop novel pharmacologic approaches for the therapeutic management of such disturbances.
The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.
The progress in Alzheimer’s disease (AD) treatment suggests a combined therapeutic approach targeting the two lesional processes of AD, which include amyloid plaques made of toxic Aβ species and neurofibrillary tangles formed of aggregates of abnormally modified Tau proteins. A pharmacophoric design, novel drug synthesis, and structure-activity relationship enabled the selection of a polyamino biaryl PEL24-199 compound. The pharmacologic activity consists of a non-competitive β-secretase (BACE1) modulatory activity in cells. Curative treatment of the Thy-Tau22 model of Tau pathology restores short-term spatial memory, decreases neurofibrillary degeneration, and alleviates astrogliosis and neuroinflammatory reactions. Modulatory effects of PEL24-199 towards APP catalytic byproducts are described in vitro, but whether PEL24-199 can alleviate the Aβ plaque load and associated inflammatory counterparts in vivo remains to be elucidated. We investigated short- and long-term spatial memory, Aβ plaque load, and inflammatory processes in APPSwe/PSEN1ΔE9 PEL24-199 treated transgenic model of amyloid pathology to achieve this objective. PEL24-199 curative treatment induced the recovery of spatial memory and decreased the amyloid plaque load in association with decreased astrogliosis and neuroinflammation. The present results underline the synthesis and selection of a promising polyaminobiaryl-based drug that modulates both Tau and, in this case, APP pathology in vivo via a neuroinflammatory-dependent process.
Growing evidence indicates that astrocytes are tightly connected to Alzheimer's disease (AD) pathogenesis. However, the way in which astrocytes participate in AD initiation and progression remains to be clarified. Our previous data show that astrocytes engulf large amounts of aggregated amyloid-beta (Aβ) but are unable to successfully degrade the material. In this study, we aimed to evaluate how intracellular Aβ-accumulation affects the astrocytes over time. For this purpose, human induced pluripotent cell (hiPSC)-derived astrocytes were exposed to sonicated Aβ-fibrils and then cultured further for one week or ten weeks in Aβ-free medium. Cells from both time points were analyzed for lysosomal proteins and astrocyte reactivity markers and the media were screened for inflammatory cytokines. In addition, the overall health of cytoplasmic organelles was investigated by immunocytochemistry and electron microscopy. Our data demonstrate that long-term astrocytes retained frequent Aβ-inclusions that were enclosed within LAMP1-positive organelles and sustained markers associated with reactivity. Furthermore, Aβ-accumulation resulted in endoplasmic reticulum and mitochondrial swelling, increased secretion of the cytokine CCL2/MCP-1 and formation of pathological lipid structures. Taken together, our results provide valuable information of how intracellular Aβ-deposits affect astrocytes, and thereby contribute to the understanding of the role of astrocytes in AD progression.
Alzheimer’s disease (AD) is considered by many to be a synaptic failure. Synaptic function is in fact deeply affected in the very early disease phases and recognized as the main cause of AD-related cognitive impairment. While the reciprocal involvement of amyloid beta (Aβ) and tau peptides in these processes is under intense investigation, the crucial role of extracellular vesicles (EVs) released by different brain cells as vehicles for these molecules and as mediators of early synaptic alterations is gaining more and more ground in the field. In this review, we will summarize the current literature on the contribution of EVs derived from distinct brain cells to neuronal alterations and build a working model for EV-mediated propagation of synaptic dysfunction in early AD. A deeper understanding of EV–neuron interaction will provide useful targets for the development of novel therapeutic approaches aimed at hampering AD progression.
Introduction:
Sleep disruption is associated with astrocyte activation and impaired cognition in model organisms. However, the relationship among sleep, astrocyte activation, and cognition in humans is uncertain.
Methods:
We used RNA-seq to quantify the prefrontal cortex expression of a panel of human activated astrocyte marker genes in 1076 older adults in the Religious Orders Study and Rush Memory and Aging Project, 411 of whom had multi-day actigraphy prior to death. We related this to rest fragmentation, a proxy for sleep fragmentation, and to longitudinal cognitive function.
Results:
Fragmentation of rest periods was associated with higher expression of activated astrocyte marker genes, which was associated with a lower level and faster decline of cognitive function.
Discussion:
Astrocyte activation and fragmented rest are associated with each other and with accelerated cognitive decline. If experimental studies confirm a causal relationship, targeting sleep fragmentation and astrocyte activation may benefit cognition in older adults.
Highlights:
Greater fragmentation of rest periods, a proxy for sleep fragmentation, is associated with higher composite expression of a panel of genes characteristic of activated astrocytes. Increased expression of genes characteristic of activated astrocytes was associated with a lower level and more rapid decline of cognitive function, beyond that accounted for by the burden of amyloid and neurofibrillary tangle pathology. Longitudinal and experimental studies are needed to delineate the causal relationships among sleep, astrocyte activation, and cognition.
Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that requires further pathological elucidation to establish effective treatment strategies. We previously showed that amyloid β (Aβ) toxic conformer with a turn at positions 22–23 is essential for forming highly toxic oligomers. In the present study, we evaluated phenotypic changes with aging in AD model AppNL-P-F/NL-P-F (NL-P-F) mice with Swedish mutation (NL), Iberian mutation (F), and mutation (P) overproducing E22P-Aβ, a mimic of toxic conformer utilizing the knock-in technique. Furthermore, the role of the toxic conformer in AD pathology was investigated. NL-P-F mice produced soluble toxic conformers from an early age. They showed impaired synaptic plasticity, glial cell activation, and cognitive decline, followed by the accumulation of Aβ plaques and tau hyperphosphorylation. In addition, the protein expression of hypoxia-inducible factor (HIF)-1α was increased, and gene expression of HIF-3α was decreased in NL-P-F mice. HIF dysregulation due to the production of soluble toxic conformers may be involved in AD pathology in NL-P-F mice. This study could reveal the role of a highly toxic Aβ on AD pathogenesis, thereby contributing to the development of a novel therapeutic strategy targeting the toxic conformer.
ALZHEIMER'S disease (AD) is the most common cause of progressive intellectual failure in aged humans. AD brains contain numerous amyloid plaques surrounded by dystrophic neurites, and show profound synaptic loss, neurofibrillary tangle formation and gliosis. The amyloid plaques are composed of amyloid beta-peptide (A beta), a 40-42-amino-acid fragment of the beta-amyloid precursor protein (APP)(1). A primary pathogenic role for APP/A beta is suggested by missense mutations in APP that are tightly linked to autosomal dominant forms of AD(2,3). A major obstacle to elucidating and treating AD has been the lack of an animal model. Animals transgenic for APP have previously failed to show extensive AD-type neuropathology(4-10), but we now report the production of transgenic mice that express high levels of human mutant APP (with valine at residue 717 substituted by phenylalanine) and which progressively develop many of the pathological hallmarks of AD, including numerous extracellular thioflavin S-positive AP deposits, neuritic plaques, synaptic loss, astrocytosis and microgliosis. These mice support a primary role for APP/A beta in the genesis of AD and could provide a preclinical model for testing therapeutic drugs.
The role of glial inflammatory processes in Alzheimer's disease has been highlighted by recent epidemiological work establishing head trauma as an important risk factor, and the use of anti-inflammatory agents as an important ameliorating factor, in this disease. This review advances the hypothesis that chronic activation of glial inflammatory processes, arising from genetic or environmental insults to neurons and accompanied by chronic elaboration of neuroactive glia-derived cytokines and other proteins, sets in motion a cytokine cycle of cellular and molecular events with neurodegenerative consequences. In this cycle, interleukin-1 is a key initiating and coordinating agent. Interleukin-1 promotes neuronal synthesis and processing of the β-amyloid precursor protein, thus favoring continuing deposition of β-amyloid, and activates astrocytes and promotes astrocytic synthesis and release of a number of inflammatory and neuroactive molecules. One of these, S100β, is a neurite growth-promoting cytokine that stresses neurons through its trophic actions and fosters neuronal cell dysfunction and death by raising intraneuronal free calcium concentrations. Neuronal injury arising from these cytokine-induced neuronal insults can activate microglia with further overexpression of interleukin-1, thus producing feedback amplification and self-propagation of this cytokine cycle. Additional feedback amplification is provided through other elements of the cycle. Chronic propagation of this cytokine cycle represents a possible mechanism for progression of neurodegenerative changes culminating in Alzheimer's disease.
One of the most remarkable characteristics of astrocytes is their vigorous response to diverse neurologic insults, a feature that is well conserved across a variety of different species. The astroglial response occurs rapidly and can be detected within one hour of a focal mechanical trauma (Mucke et al., 1991). Prominent reactive astrogliosis is seen; in AIDS dementia; a variety of other viral infections; prion associated spongiform encephalopathies; inflammatory demyelinating diseases; acute traumatic brain injury; neurodegenerative diseases such as Alzheimer's disease. The prominence of astroglial reactions in various diseases, the rapidity of the astroglial response and the evolutionary conservation of reactive astrogliosis indicate that reactive astrocytes fulfill important functions of the central nervous system (CNS). Yet, the exact role reactive astrocytes play in the injured CNS has so far remained elusive. This chapter summaries the various experimental models and diseases that exhibit astrogliosis and increase in glial fibrillary acidic protein (GFAP). Recent in vitro studies to inhibit GFAP synthesis are also presented.
Alzheimer's disease is characterized by the extracellular deposition of β-amyloid peptide (Aβ) in cerebral plaques and evidence
is accumulating that amyloid is neurotoxic. Aβ is derived from the β-amyloid precursor protein (APP). Proteolytic processing
of APP by the enzyme, β-secretase, produces the N terminus of Aβ, and releases a secreted ectodomain of APP (β-s-APP). To
develop animal models for measuring β-secretase activity in specific brain cells in vivo, we have targeted the expression of the full-length human APP to either neurons or astrocytes in transgenic mice using the
neuron- specific enolase (NSE) promoter or a modified glial fibrillary acidic protein (GFAP) gene, respectively. The APP cDNAs
expressed were mutated (KM to NL at 670/671) to encode amino acid substitutions that enhance amyloidogenic processing in vitro. Western analyses revealed abundant production of β-s-APP in the brains of NSE-APP mice and enzyme-linked immunosorbent assay
analyses showed production of Aβ in fetal primary mixed brain cultures and brain homogenates from these transgenic animals.
Because the NSE promoter drives expression primarily in neurons, this provides in vivo evidence that the β-secretase cleavage necessary for generation of β-s-APP and Aβ is efficiently performed in neurons. In
contrast, only little β-s-APP was detected in brain homogenates of GFAP-APP mice, indicating that astrocytes show very little
β-secretase activity in vivo. This provides strong in vivo evidence that the major source of Aβ in brain is from neurons and not from astrocytes.
Astrocytes and other glia in the central nervous system are now thought to produce molecules that negatively modulate axon growth, thereby influencing axon pathfinding in both development and regeneration. The relevant evidence for glial cell boundaries and the inhibitory molecules present in these extracellular matrix structures is discussed in this minireview.
Amyloid beta-protein (Abeta) is the major component of senile plaques that emerge in the cortex during aging and appear most abundantly in Alzheimer's disease. In the course of our immunocytochemical study on a large number of autopsy cases, we noticed, in many aged nondemented cases, the presence of unique diffuse plaques in the cortex distinct from ordinary diffuse plaques by immunocytochemistry. The former were amorphous, very faintly Abeta-immunoreactive plaques resembling diffuse plaques, but they stained for Abeta40 and were associated with small cells containing Abeta-positive granules. A panel of amino- and carboxyl-terminal-specific Abeta antibodies showed that such Abeta40-positive diffuse plaques and cell-associated granules were composed exclusively of amino-terminally deleted Abeta terminating at Abeta40, -42, and -43. Double immunostaining also showed that those Abeta-immunoreactive granules are located in astrocytes and not in microglia or neurons. Immunoelectron microscopy revealed that nonfibrillar Abeta immunoreactivity was located within lipofuscin-like granules in somewhat swollen astrocytes. These findings raise the possibility that astrocytes take up Abeta and attempt to degrade it in lysosomes in the aged brain.
Transgenic mice overexpressing different forms of amyloid precursor protein (APP), i.e. wild type or clinical mutants, displayed an essentially comparable early phenotype in terms of behavior, differential glutamatergic
responses, deficits in maintenance of long term potentiation, and premature death. The cognitive impairment, demonstrated
in F1 hybrids of the different APP transgenic lines, was significantly different from nontransgenic littermates as early as
3 months of age. Biochemical analysis of secreted and membrane-bound APP, C-terminal “stubs,” and Aβ(40) and Aβ(42) peptides
in brain indicated that no single intermediate can be responsible for the complex of phenotypic dysfunctions. As expected,
the Aβ(42) levels were most prominent in APP/London transgenic mice and correlated directly with the formation of amyloid
plaques in older mice of this line. Plaques were associated with immunoreactivity for hyperphosphorylated tau, eventually
signaling some form of tau pathology. In conclusion, the different APP transgenic mouse lines studied display cognitive deficits
and phenotypic traits early in life that dissociated in time from the formation of amyloid plaques and will be good models
for both early and late neuropathological and clinical aspects of Alzheimer’s disease.
Context.—Down syndrome patients who live to middle age invariably develop the neuropathologic features of Alzheimer disease, providing a unique situation in which to study the early and sequential development of these changes.
Objective.—To study the development of amyloid deposits, senile plaques, astrocytic and microglial reactions, and neurofibrillary tangles in the brains of young individuals (<30 years of age) with Down syndrome.
Methods.—Histologic and immunocytochemical study of a series of autopsy brains (n = 14, from subjects aged 11 months to 56 years, with 9 subjects <30 years) examined at the Office of the Chief Medical Examiner of the State of Maryland and The Johns Hopkins Hospital.
Results.—The principal observations included the presence of intraneuronal Aβ immunostaining in the hippocampus and cerebral cortex of very young Down syndrome patients (preceding the extracellular deposition of Aβ) and the formation of senile plaques and neurofibrillary tangles.
Conclusions.—We propose the following sequence of events in the development of neuropathologic changes of Alzheimer disease in Down syndrome: (1) intracellular accumulation of Aβ in neurons and astrocytes, (2) deposition of extracellular Aβ and formation of diffuse plaques, and (3) development of neuritic plaques and neurofibrillary tangles with activation of microglial cells.
Diffuse amyloid β-protein (Aβ) deposits with numerous glial cells containing C-terminal Aβ fragments occur in the cerebral cortex of patients with Alzheimer's disease. By using a panel of antibodies specific for various epitopes in the Aβ peptide, we have investigated the immunohistochemical nature of the diffuse Aβ deposits. The extracellular material contains Aβ with a C-terminus at residue valine⁴⁰ (Aβ40) as well as residues alanine⁴²/threonine⁴³ (Aβ42). The N-termini include aspartate¹, pyroglutamate³, and pyroglutamate¹¹, with pyroglutamate³ being dominant. Microglia and astrocytes in and around these deposits contain intensely staining granules. Most of these granules are negative for antibodies to the N-terminally located sequences of Aβ. These include 6E10 (Aβ1–17), 6F/3D (Aβ8–17), and the N-terminal antibodies specific to aspartate¹, pyroglutamate³, and pyroglutamate¹¹. The C-termini of intraglial Aβ are comparable with those of the extracellular deposits. The microglia and astrocytes have quiescent morphology compared with those associated with senile plaques and other lesions such as ischemia. Complement activation in these deposits is not prominent and often below the sensitivity of immunohistochemical detection. Although factors which may cause this type of deposit remain unclear, lack of strong tissue responses suggests that these deposits are a very early stage of Aβ deposition. They were found only inconsistently and were absent in a number of cases examined in this study. Further analysis of these deposits might provide important clues regarding the accumulation and clearance of Aβ in Alzheimer's disease brain. GLIA 25:324–331, 1999. © 1999 Wiley-Liss, Inc.
Alzheimer's disease pathology is characterized by the presence of neuritic plaques enriched in ;-amyloid peptide1-42 (A ;1-42) and the loss of cholinergic functions in the brain . A ;1-42 binds to a neuronal pentameric cation channel , the ;7 nicotinic acetylcholine receptor ( ;7nAChR) , with high affinity and modulates two processes critical in memory formation , calcium homeostasis and neurotransmitter release . We compared the protein levels of intact ;7nAChR in the hippocampi of Alzheimer's disease patients (n=10) and age-matched non-demented control subjects (n=10) using gel electrophoresis and Western blot analysis . We showed that the ;7nAChR protein was significantly reduced by 54% (p ;0 .0001) in Alzheimer's disease hippocampi . The possibility of developing a biochemical marker of Alzheimer's disease by monitoring the changes in ;7nAChR protein levels in a relevant accessible tissue is discussed .
This report summarizes the consensus recommendations of a panel of neuropathologists from the United States and Europe to improve the postmortem diagnostic criteria for Alzheimer's disease. The recommendations followed from a two-day workshop sponsored by the National Institute on Aging (NIA) and the Ronald and Nancy Reagan Institute of the Alzheimer's Association to reassess the original NIA criteria for the postmortem diagnosis of Alzheimer's disease published in 1985 (2). The consensus recommendations for improving the neuropathological criteria for the postmortem diagnosis of Alzheimer's disease are reported here, and the ''position papers'' by members of the Working Group that accompany this report elaborate on the research findings and concepts upon which these recommendations were based. Further, commentaries by other experts in the field also are included here to provide additional perspectives on these recommendations. Finally, it is anticipated that future meetings of the Working Group will reassess these recommendations and the implementation of postmortem diagnostic criteria for Alzheimer's disease. (C) 1997 Elsevier Science Inc.
Context A central tenet of Alzheimer disease (AD) is the loss
of cortical cholinergic function and cholinergic markers in postmortem
brain specimens. Whether these profound deficits in cholinergic markers
found in end-stage patients are also found in patients with much
earlier disease is not known.Objective To determine whether cholinergic deficits in AD precede,
follow, or occur in synchrony with the earliest signs of cognitive
deterioration.Design, Setting, and Patients Postmortem study of nursing
home residents with Clinical Dementia Rating (CDR) Scale scores of 0.0
to 2.0 and 4.0 to 5.0 who underwent autopsy between 1986 and 1997,
comparing the activity of the cholinergic marker enzymes in the
cortices of 66 elderly subjects with no (CDR score=0.0;
n=18), questionable (CDR score=0.5;
n=11), mild (CDR score=1.0;
n=22), or moderate (CDR score=2.0;
n=15) dementia vs subjects with severe dementia (CDR
score=4.0-5.0; n=15).Main Outcome Measures Activity of the cholinergic marker enzymes
choline acetyltransferase and acetylcholinesterase in 9 neocortical
brain regions.Results The activity of choline acetyltransferase and
acetylcholinesterase in 9 neocortical brain regions did not differ
significantly in subjects with CDR scores of 0.0 to 2.0, but was
significantly lower in subjects with severe dementia (CDR
score=4.0-5.0). Choline acetyltransferase levels were
significantly correlated with severity of neuropathological lesions of
AD, as measured by density of neuritic plaques and neurofibrillary
tangles.Conclusions Although neocortical cholinergic deficits are
characteristic of severely demented AD patients, in this study,
cholinergic deficits were not apparent in individuals with mild AD and
were not present until relatively late in the course of the disease.
These results suggest that patients with more severe disease should be
a target for cholinergic treatment.
Granular structures that are recognized by antibodies specific for the C-terminal but not the N-terminal sequences of the β-amyloid protein (Aβ) fragments are present in a subset of microglia and astrocytes in Alzheimer brain tissue. The immunohistochemical profile indicates that the Aβ in these granules is truncated between the residues 17 and 31 and terminates at the residue 42 or 43. Such granule-containing glia occur only in brain areas with the heavy Aβ deposits. Whether the intraglial Aβ fragments accumulate as a result of phagocytosis of extracellular Aβ or are formed intracellularly by glial cells from amyloid precursor protein (APP) remains unknown.
Because the mechanisms of Aβ degradation in normal and Alzheimer's disease brain are poorly understood, we have examined whether various cortical cells are capable of processing this peptide. Rat microglia and astrocytes, as well as the human THP-1 monocyte cell line, degraded Aβ1−42 added to culture medium. In contrast, neither rat cortical neurons or meningeal fibroblasts effectively catabolized this peptide. When Aβ fibrils were immobilized as plaque-like deposits on culture dishes, both microglia and THP-1 cells removed the peptide. Astrocytes were incapable of processing the Aβ deposits, but these cells released glycosaminoglycase-sensitive molecules that inhibited the subsequent removal of Aβ by microglia. This implied that astrocyte-derived proteoglycans associated with the amyloid peptide and slowed its degradation. The addition of purified proteoglycan to Aβ that was in medium or focally deposited also resulted in significant inhibition of peptide removal by microglia. These data suggest that Aβ can be catabolized by microglia and proteoglycans which co-localize with senile plaques may slow the degradation of Aβ within these pathologic bodies.
Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main termination zone in the outer molecular layers of the dentate gyrus. Concomitantly, astrocytes become hypertrophic, and microglial cells alter their phenotype, suggesting participation in anterograde degeneration. This study analyzes the involvement of these lesion-induced activated glial cells in the process of phagocytosis of degenerated axonal debris. We established a phagocytosis-dependent labeling technique that allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells. Stereotaxic application of small crystals of the biotin- and rhodamine-conjugated dextran amine Mini Ruby (MR) into the entorhinal cortex led to strong and stable axonal staining of perforant path axons. Following entorhinal lesion, labeled terminals and fibers condensed and formed small granules. Incorporation of these rhodamine-fluorescent granules resulted in a phagocytosis-dependent cell labeling. During the first 3 days, we were able to identify these cells as microglia by using double-fluorescence and confocal microscopy. The first unequivocally double-labeled astrocytes were found 6 days post lesion (dpl). Whereas in all stages a subpopulation of microglial cells remained devoid of MR-labeled granules, all astrocytes in the middle molecular layer were double-labeled after long survival times (20 dpl). On the ultrastructural level, labeled granules appeared to be perforant path axons containing the tracer. Both terminals and myelinated fibers could be seen inside the cytoplasm of microglial cells and astrocytes. Thus, anterograde degeneration is a sufficient stimulus to induce axon incorporation by both astrocytes and a subpopulation of microglial cells. GLIA 20:145–154, 1997. © 1997 Wiley-Liss Inc.
The deposition of the amyloid β-protein (Aβ) is a pathological hallmark of Alzheimer’s disease (AD). Aβ is a peptide consisting
of 39–43 amino acids and is derived by β- and γ-secretase cleavage from the Aβ protein precursor (AβPP). An N-terminal-truncated
form of Aβ can occur following α- and γ-secretase cleavage of AβPP. Fleecy amyloid is a recently identified distinct type
of Aβ deposits occurring in the internal layers (pri-α, pri-β and pri-γ) of the human entorhinal cortex. Fleecy amyloid consists
exclusively of N-terminal-truncated Aβ and is a transient form of Aβ deposits, which disappears in late-stage β-amyloidosis.
In this study, the entorhinal cortex of 15 cases with AD-related pathology was used to examine astrocytes in the vicinity
of N-terminal-truncated Aβ in fleecy amyloid of the layers pri-α, pri-β, and pri-γ in comparison to astrocytes in the vicinity
of full-length Aβ in layers pre-β and pre-γ. Immunohistochemistry was performed with antibodies directed against AβPP, Aβ40, Aβ42, Aβ17–24, Aβ1–17 and Aβ8–17 as well as by double-labeling with antibodies directed against Aβ17–24, Aβ42, and glial fibrillary acid protein (GFAP). A large number of GFAP-positive astrocytes containing N-terminal-truncated Aβ
fragments appeared in the vicinity of N-terminal-truncated Aβ, whereas Aβ-containing astrocytes were rarely seen in the vicinity
of full-length Aβ. These results suggest that N-terminal-truncated Aβ peptide may be cleared preferentially from the extracellular
space by astrocytic uptake and processing. Such an astroglial uptake of N-terminal-truncated Aβ may account for the transient
nature of fleecy amyloid and point to the use of N-terminal truncation of Aβ in potential therapeutic strategies aimed at
preventing the brain from amassing full-length Aβ deposits.
To clarify whether senile plaques disappear, we examined amyloid β protein (Aβ) deposits in non-demented subjects, and found
novel diffuse plaques associated with astroglial Aβ. Formalin-fixed paraffin-embedded sections from cortical areas were immunolabeled
with a panel of Aβ antibodies, and astroglial and microglial markers. Cerebral Aβ deposition was primarily found as diffuse
plaques (DP) in these subjects. A subset of DP was associated with clusters of intensely Aβ-positive small granules. The clusters,
which were located just adjacent to astroglial nucleus, had the characteristics of lipofuscin granules and, therefore, were
quite different from “small stellate deposits”. Substantial amounts of Aβ-positive granules were found inside astrocytes by
dual labeling of Aβ and glial fibrillary acid protein, and the majority of astroglial Aβ immunoreactivity was located on lipofuscin
granules. Aβ-positive granules lacked immunoreactivity with antisera for the N-terminal region of Aβ. These peculiar DP showed
a much weaker staining than ordinary DP. The DP associated with astroglial Aβ were found in about one third of the subjects,
although the density varied widely among individuals. From these findings, we propose that DP, which are associated with the
N-terminal truncated Aβ in astrocytes, represent the disappearing stage of senile plaques.
Reactive glial cell properties could contribute to pathomechanisms underlying Alzheimer's disease by favoring oxidative neuronal damage and β-amyloid toxicity. A critical step is apparently reached when pathological glia activation is no longer restricted to micoglia and includes astrocytes. By giving up their differentiated state, astrocytes may lose their physiological negative feed-back control on microglial NO production and even contribute to neurotoxic peroxynitrate formation. Another consequence is the impairment of the astrocyte-maintained extracellular ion homeostasis favoring excitotoxic damage. By the production of apolipoprotein-E, triggered by the microglial cytokine interleukine-1β, reactive astrocytes could promote the transformation of β-amyloid into the toxic form. A pharmacologically reinforced cAMP signaling in rat glial cell cultures depressed oxygen radical formation in microglia and their release of TNF-α and interleukine-1β, feed-forward signals which mediate oxidative damage and secondary astrocyte activation. Cyclic AMP also favored differentiation and expression of a mature ion channel pattern in astrocytes improving their glutamate buffering. A deficient cholinergic signaling that increases the risk of pathological APP processing was compensated by an adenosine-mediated reinforcement of the second messenger calcium. A combination therapy with acetylcholine-esterase inhibitors together with adenosine raising pharmaca, therefore, may be used to treat cholinergic deficiency in Alzheimer's disease.
Deficits of cortical nicotinic acetylcholine receptors (nAChRs) have been observed in Alzheimer's disease (AD) by receptor binding assays. Little is known about the receptor subunit specificity influenced by AD, and it might be of importance for therapeutic strategies. In the present study, the protein levels of nAChR alpha3, alpha4, alpha7, and beta2 subunits were investigated using western blot analysis on postmortem brains of patients with AD and age-matched controls. The results showed that in human postmortem brain samples, bands with molecular masses of 52, 42, and 50 kDa were detected by anti-alpha4, anti-alpha7, and anti-beta2 antibodies, respectively. When anti-alpha3 antibody was used, one major band of 49 kDa and two minor bands of 70 and 38 kDa were detected. In AD patients, as compared with age-matched controls, the alpha4 subunit was reduced significantly by approximately 35 and 47% in the hippocampus and temporal cortex, respectively. A significant reduction of 25% in the alpha3 subunit was also observed in the hippocampus and a 29% reduction in the temporal cortex. For the alpha7 subunit, the protein level was reduced significantly by 36% in the hippocampus of AD patients, but no significant change was detected in the temporal cortex. In neither the hippocampus nor the temporal cortex was a significant difference observed in the beta2 subunit between AD patients and controls. These results reveal brain region-specific changes in the protein levels of the nAChR alpha3, alpha4, and alpha7 subunits in AD.
A genetic locus associated with familial Alzheimer disease (FAD) and a candidate gene, APP, encoding the amyloid protein precursor have both been assigned previously to chromosome 21, and, in a few FAD families, mutations of APP have been detected. However, obligate crossovers between APP and FAD have also been reported in several FAD pedigrees, including FAD4, a large kindred showing highly suggestive evidence for linkage of the disorder to chromosome 21. In case the apparent APP crossover in FAD4 actually represented an intragenic recombination event or segregation of different mutations in different family branches, we have performed a more detailed assessment of APP as a candidate gene in this family. The entire coding region of the APP gene was sequenced for FAD4 and for FAD1, a second large kindred. No mutations were found, indicating that, in at least one chromosome 21–linked FAD pedigree, the gene defect is not accounted for by a mutation in the known coding region of the APP gene. A total of 25 well-characterized early- and late-onset FAD pedigrees were typed for genetic linkage to APP, to assess the percentage of FAD families predicted to carry mutations in the APP gene. None of the FAD families yielded positive lod scores at a recombination fraction of 0.0. To estimate the overall prevalence of FAD-associated mutations in the βA4 domain of APP, we sequenced exons 16 and 17 in 30 (20 early- and 10 late-onset) FAD kindreds and in 11 sporadic AD cases, and we screened 56 FAD kindreds and 81 cases of sporadic AD for the presence of the originally reported FAD-associated mutation, APP717 Val→Ile (by BclI digestion). No APP gene mutations were found in any of the FAD families or sporadic-AD samples examined in this study, suggesting that the mutations in exons 16 and 17 are a rare cause of FAD. Overall, these data suggest that APP gene mutations account for a very small portion of FAD.
The chapter discusses the role of the astrocyte in the central nervous system (CNS) injury and disease. Astrocytes comprise as much as 25% of the cells and 35% of the total mass of the CNS. Astrocytes form barriers around blood vessels and connections between nerve cells. Numerous functions have been assigned to the astrocyte depending on its stage of maturation, location in the CNS, and response to CNS insult. Some signals that regulate gene expression in development and response to astrocyte injury are: growth factors, prion protein from Scrapies, neural and immunological adhesion molecules, such as NCAM, LFA-1, gangliosides, low density lipoproteins, cytokines from T-cells, macrophages and other glia, neurotransmitters and neuropeptides, such as catecholamines, monoamines, glutamate, ATP, substance P, and antigen-antibody complexes. Astrocytic responses to these signals include: (1) proliferation, movement and differentiation; (2) changes in shape, cell volume, cytoskeletal organization, endocytic activity, lysosomal fragility, and enzyme content; (3) buffering capacity for K+ , glutamate and GABA; (4) expression of nerve growth factor, tumor necrosis factor, interferon α and β, interleukin 1 and 6, colony stimulating factor-1, fibroblast growth factor, neurotropic factors, neurite promoting agents, MHC class I and I1 histocompatibility antigens, amyloid protein, GD3 ganglioside, ICAM- 1, Na+ channel protein, GFAP, crystallin, vimentin and heat shock proteins.
We present here both linear regressions and multivariate analyses correlating three global neuropsychological tests with a number of structural and neurochemical measurements performed on a prospective series of 15 patients with Alzheimer's disease and 9 neuropathologically normal subjects. The statistical data show only weak correlations between psychometric indices and plaques and tangles, but the density of neocortical synapses measured by a new immunocytochemical/densitometric technique reveals very powerful correlations with all three psychological assays. Multivariate analysis by stepwise regression produced a model including midfrontal and inferior parietal synapse density, plus inferior parietal plaque counts with a correlation coefficient of 0.96 for Mattis's Dementia Rating Scale. Plaque density contributed only 26% of that strength.
Brains of rats that had received needle stab wounds were examined sequentially for 72 h by immunohistochemistry using antibodies to beta-amyloid precursor protein (APP). Immunoreactivity appeared 0.5 h after the injury in axons adjacent to the needle tract, becoming stronger over 15 h in neighboring swollen axons. APP-immunoreactive glial cells appeared 6 h after the injury around the needle tract and persisted throughout the experiment. Morphologically, these cells were thought to be mainly astrocytes.
Ultrastructural studies of biopsied cortical tissue from the right frontal lobe of 8 patients with mild to moderate Alzheimer's disease (AD) revealed that the number of synapses in lamina III of Brodmann's area 9 was significantly decreased when compared with the number in age-matched control brains (n = 9; postmortem time, less than 13 hours). Further decline in synaptic number was seen in age-matched autopsied AD specimens. In the AD brains there was significant enlargement of the mean apposition length, which correlated with degree of synapse loss; as synapse density declined, synapse size increased. The enlargement of synapses, coupled with the decrease in synaptic number, allowed the total synaptic contact area per unit volume to remain stable in the patients who underwent biopsy. In autopsied subjects who had AD, there was no further enlargement of mean synaptic contact area. There was a significant correlation between synapse counts and scores on the Mini-Mental State examination in the patients who underwent biopsy. Lower mental status scores were associated with greater loss of synapses. Choline acetyltransferase activity was significantly decreased in the biopsied group and declined further in the autopsied specimens of AD. There was no relationship between choline acetyltransferase activity and scores on the Mini-Mental State examination or synapse number. There is evidence of neural plasticity in the AD neuropil; synaptic contact size increased in patients who had biopsy and possibly compensated for the numerical loss of synapses. But by end stage of the disease, the ability of the cortex to compensate was exceeded and both synapse number and synaptic contact area declined.(ABSTRACT TRUNCATED AT 250 WORDS)
In patients with Alzheimer's disease, amyloid fibrils that are aggregates of A4 protein subunits are deposited in the brain. A similar process occurs at an earlier age in persons with Down's syndrome. To investigate the deposition of amyloid in these diseases, we used a radioimmunoassay to measure levels of the amyloid precursor (PreA4) in the serum of 17 patients with Down's syndrome, 15 patients with Alzheimer's disease, and 33 normal elderly controls. The mean (+/- SD) concentration of serum PreA4 was increased 1.5-fold in patients with Down's syndrome (2.49 +/- 1.13 nmol per liter) as compared with that in controls (1.68 +/- 0.49 nmol per liter; P less than 0.007); the levels in patients with Alzheimer's disease were similar to those in controls (1.83 +/- 0.78; P less than 0.98). We also found that the concentration of PreA4 in the brain tissue of two adults with Down's syndrome (100 and 190 pmol per gram) was higher than that in the brain tissue of either 26 patients with Alzheimer's disease (64.4 +/- 17.3 pmol per gram) or 17 elderly controls with neurologic disease (68.5 +/- 26.3 pmol per gram). Immunocytochemical studies of brain tissue from 26 patients with Down's syndrome showed that the deposition of A4 protein amyloid began in these patients approximately 50 years earlier than it began in 127 normal aging subjects studied previously, although the rate of deposition was the same. We conclude that, since the gene for PreA4 is on the long arm of chromosome 21, which is present in triplicate in Down's syndrome, overexpression of this gene may lead to increased levels of PreA4 and amyloid deposition in Down's syndrome. However, since increased levels of PreA4 are not present in Alzheimer's disease, additional factors must account for the amyloid deposition in that disorder.
The relationship of microglia and astrocytes to deposits of beta-amyloid protein (BAP) was studied in Alzheimer's disease by immunohistochemistry. BAP was detected in forms varying from diffuse amorphous deposits to compact spherical masses. These latter corresponded in frequency and distribution to senile plaques revealed by Bielschowsky and thioflavine S staining. Approximately half the diffuse deposits had no human leukocyte antigen (HLA)-DR-positive reactive microglia associated with them while nearly all compact deposits had single or multiple HLA-DR-positive reactive microglia embedded in their core. Electron microscopy showed these reactive microglia to be in intimate contact with amyloid fibrils. These data suggest that amyloid deposition may precede the activation of microglia.
Alzheimer's disease is characterized by a widespread functional disturbance of the human brain. Fibrillar amyloid proteins are deposited inside neurons as neurofibrillary tangles and extracellularly as amyloid plaque cores and in blood vessels. The major protein subunit (A4) of the amyloid fibril of tangles, plaques and blood vessel deposits is an insoluble, highly aggregating small polypeptide of relative molecular mass 4,500. The same polypeptide is also deposited in the brains of aged individuals with trisomy 21 (Down's syndrome). We have argued previously that the A4 protein is of neuronal origin and is the cleavage product of a larger precursor protein. To identify this precursor, we have now isolated and sequenced an apparently full-length complementary DNA clone coding for the A4 polypeptide. The predicted precursor consists of 695 residues and contains features characteristic of glycosylated cell-surface receptors. This sequence, together with the localization of its gene on chromosome 21, suggests that the cerebral amyloid deposited in Alzheimer's disease and aged Down's syndrome is caused by aberrant catabolism of a cell-surface receptor.
One hundred brains of patients with Down's syndrome (DS) who died in institutions for chronic care were examined for clinicopathological correlation of Alzheimer's disease. Fifty-one were below and 49 were above age 30 years at death. Tissues from the right, prefrontal, and hippocampal cortices were processed for microscopy using H&E and Bodian-periodic acid-Schiff impregnation. Morphometric evaluations of plaques and tangles were carried out. Plaques or plaques and tangles were found in the brains of 56 patients with DS, 7 below age 30 and 49 above that age. A history of dementia was evident in the medical records of 15 of these patients; of these only 2 were below the age of 30. The brains of the patients with DS who also had clinical dementia had more than twenty plaques or plaques and tangles per 1.5 X 10(6) micron 2 of cortex. The numbers of plaques and tangles found in the brains of the patients with DS above the age of 30 greatly increased with age but varied from brain to brain. These observations suggest a correlation among dementia, the density of plaques and tangles, and age. All 100 brains studied showed early arrest of brain growth and brain atrophy, a condition that may have been due to prenatal arrest of neurogenesis mainly in the granular cell layers, prenatal and postnatal arrest of synaptogenesis, and early aging. Plaques and tangles developed twenty to thirty years earlier and dementia was clinically detected at least three times more frequently (20 to 30%) in DS than it is known to occur in the non-DS population.
A quantitative morphometric analysis was used to estimate neurone and synapse densities in cerebral cortical biopsy tissues from patients with dementia under 65 years of age and pathologically verified as suffering from Alzheimer's disease. Estimates of the numerical density of neurones and synapses were made in layers II-III and V of both frontal and temporal cortex. A greater loss of synapses than that of neurones was found in Alzheimer's disease, amounting to a minimum (uncorrected for atrophy) of 25% in layers II-III and 36% in layer V of the temporal cortex, and 27% in layer V of the frontal cortex. Values of synapse to neurone ratio also demonstrated this greater loss of synapses, there being on average 38% fewer synapses associated with each surviving neurone in layers II-III of the temporal cortex, 30% fewer in layer V, and a deficit of 14% in layer V of the frontal cortex. It is concluded that a major loss of synapses occurred in this group of patients with Alzheimer's disease, probably at an early stage of the disease, and that the loss is likely to form a fundamental part of the pathological process that underlies the cortical damage of this condition.
Four clones were isolated from an adult human brain complementary DNA library with an oligonucleotide probe corresponding
to the first 20 amino acids of the beta peptide of brain amyloid from Alzheimer's disease. The open reading frame of the sequenced
clone coded for 97 amino acids, including the known amino acid sequence of this polypeptide. The 3.5-kilobase messenger RNA
was detected in mammalian brains and human thymus. The gene is highly conserved in evolution and has been mapped to human
chromosome 21.
To test whether the hypothesis that the Alzheimer's A beta peptide is neurotoxic, we introduced a transgene into mice to direct expression of this peptide to neurons. We show that the transgene is expressed in brain regions which are severely affected in Alzheimer's disease resulting in extensive neuronal degeneration. Morphological and biochemical evidence indicates that the eventual death of these cells occurs by apoptosis. Coincident with the cell degeneration and cell death is the presence of a striking reactive gliosis. Over 50% of the transgenic mice die by 12 months of age, half the normal life span of control mice. These data show that A beta is neurotoxic in vivo and suggest that apoptosis may be responsible for the accompanying neuronal loss, the principal underlying cellular feature of Alzheimer's disease.
This ultrastructural and light microscopic immunocytochemical study describes the time course of anatomical changes that occur in striatal astrocytes in response to neuronal deafferentation in young adult rats and the coordinate distribution of two astrocytic proteins involved in reactive synaptogenesis, glial fibrillary acidic protein and clusterin. We found that following a unilateral lesion of the cerebral cortex, striatal astrocytes undergo a rapid ultrastructural transformation from a protoplasmic to a reactive type of astroglia and are the primary cells involved in the removal of degenerating axon terminals, but not axons of passage, from the neuropil. In addition, at 10 and 27 days postlesion, processes of reactive astrocytes are also seen to occupy vacant postsynaptic spines after degenerating presynaptic terminals are removed, suggesting that they may also participate in the reinnervation of the deafferented neurons.
Multiple nicotinic receptors seem to exist in brain as revealed by neurophysiological, neurochemical, molecular and immunological studies. The mechanisms for their involvement in higher functions including learning and memory are still relatively unknown. The nicotinic receptor subtypes in human brain undergo changes during aging. Deficits of brain nicotinic receptors have been traced in neurodegenerative disorders as Alzheimer's disease and Parkinson's disease. Brain imaging studies in patients and neurochemical studies in autopsy brain tissue from Alzheimer patients reveal significant losses of the nicotinic receptors. New therapeutic compounds tried in Alzheimer's disease, aiming to increase cholinergic activity in the brain, act via the nicotinic receptors in brain. Augmentation of nicotinic receptor function in brain might be of importance for alleviating some of the cognitive impairments in Alzheimer's disease.
Nicotinic cholinergic agonists represent a relatively newly developing area for therapeutic intervention in Alzheimer disease (AD) and related neurodegenerative dementias. Loss of cholinergic receptors has been reported not only in AD but also in Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. Clinical studies suggest that compounds that act to stimulate nicotinic receptors may improve learning and memory in a variety of models of cognitive impairment in animals. Early clinical studies have suggested positive effects on cognition of nicotine in human beings with and without AD. Finally, nicotinic compounds might show the progression of AD, as suggested by preclinical models of cell death as well as epidemiological evidence of a protective effect of smoking in AD and Parkinson's disease.
To determine whether the presenilin 1 (PS1), presenilin 2 (PS2) and amyloid beta-protein precursor (APP) mutations linked to familial Alzheimer's disease (FAD) increase the extracellular concentration of amyloid beta-protein (A beta) ending at A beta 42(43) in vivo, we performed a blinded comparison of plasma A beta levels in carriers of these mutations and controls. A beta 1-42(43) was elevated in plasma from subjects with FAD-linked PS1 (P < 0.0001), PS2N1411 (P = 0.009), APPK670N,M671L (P < 0.0001), and APPV7171 (one subject) mutations. A beta ending at A beta 42(43) was also significantly elevated in fibroblast media from subjects with PS1 (P < 0.0001) or PS2 (P = 0.03) mutations. These findings indicate that the FAD-linked mutations may all cause Alzhelmer's disease by increasing the extracellular concentration of A beta 42(43), thereby fostering cerebral deposition of this highly amyloidogenic peptide.
It is established that astrocytes are the intimate partner of neurons throughout their lifespan. However, astrocytes play different roles at different stages of the lifespan. During neurogenesis and early development, glial cells provide a scaffold for the correct migration of neurons and growth cones. They provide guidance cues and may also be involved in neuronal proliferation. In the adult, astrocytes maintain neuronal homeostasis and synaptic plasticity. This review discusses some of the cell-cell communication signals which are involved in the maintenance of synaptic plasticity. They are divided into: (a) glia-to-glia signaling, which involves non-synaptic communication by coupling of astrocytes. In this cell communication, cytoplasmic exchange of ions and small molecules among cells is accomplished through cell coupling of cells via cell-to-cell contacts, termed gap junctions; (b) neuron-to-glia signaling, which involves synaptic interactions. This cell-cell intercommunication has received considerable attention. Of special interest is the role of astrocytes in glutamic acid compartmentation and in preventing glutamic acid neurotoxicity. A glutamate-induced calcium signaling appears to be involved in this neuron-astrocyte interaction. The intriguing possibility is that neuronally induced astrocyte calcium signals may feed back to influence neuronal excitability or regulate synaptic transmission; (c) signals from astrocytes to neurons: a new concept of neuron-glia intercommunication. As mentioned, calcium appears to be the molecule in this glia-neuron signaling, although glial cell receptor-mediated signals are involved also. Receptor interactions on astrocytes through a cascade of events lead to modification in the extracellular concentration of glutamate. The role of astrocytes in synaptic plasticity is not as well understood during ageing and their role in neuronal cell death during ageing and neurodegeneration can only be speculated. However, astrocytes in the aged brain exhibit various receptors, including glutamate receptors. Thus, astrocytes can be expected to modify the expression of endogenous neurotoxins and thus contribute to synaptic plasticity in ageing. Synaptic plasticity continues to be a homeostatic relationship between neurons and glial cells. The possibility of signaling from astrocytes to neurons has opened new horizons for glial cell function and new challenges of research for gliobiologists.
Cerebral deposition of the amyloid beta protein (A beta) is an early and invariant feature of Alzheimer disease (AD). Whereas the 40-amino acid form of A beta (A beta 40) accounts for approximately 90% of all A beta normally released from cells, it appears to contribute only to later phases of the pathology. In contrast, the longer more amyloidogenic 42-residue form (A beta 42), accounting for only approximately 10% of secreted A beta, is deposited in the earliest phase of AD and remains the major constituent of most amyloid plaques throughout the disease. Moreover, its levels have been shown to be increased in all known forms of early-onset familial AD. Thus, inhibition of A beta 42 production is a prime therapeutic goal. The same protease, gamma-secretase, is assumed to generate the C termini of both A beta 40 and A beta 42. Herein, we analyze the effect of the compound MDL 28170, previously suggested to inhibit gamma-secretase, on beta-amyloid precursor protein processing. By immunoprecipitating conditioned medium of different cell lines with various A beta 40- and A beta 42-specific antibodies, we demonstrate a much stronger inhibition of the gamma-secretase cleavage at residue 40 than of that at residue 42. These data suggest that different proteases generate the A beta 40 and A beta 42 C termini. Further, they raise the possibility of identifying compounds that do not interfere with general beta-amyloid precursor protein metabolism, including A beta 40 production, but specifically block the generation of the pathogenic A beta 42 peptide.
Alzheimer's disease (AD) is the most common form of dementia in the aged population. Early-onset familial AD (FAD) involves mutations in a gene on chromosome 21 encoding the amyloid protein precursor or on chromosomes 14 or 1 encoding genes known as presenilins. All mutations examined have been found to increase the production of amyloidogenic forms of the amyloid protein (A beta), a 4 kDa peptide derived from APP. Despite the remarkable progress in elucidating the biochemical mechanisms responsible for AD, little is known about the normal function of APP. A model of how APP and A beta are involved in pathogenesis is presented. This model may explain why certain neuronal populations are selectively vulnerable in AD. It is suggested that those neurons which more readily undergo neuritic sprouting and synaptic remodelling are more vulnerable to A beta neurotoxicity.
The loss of neocortical synapses that occurs in Alzheimer's disease (AD) has been shown to correlate with cognitive decline. In addition, marked losses in the cholinergic system in AD, specifically choline acetyltransferase (ChAT) activity and high affinity presynaptic neuronal nicotinic cholinergic receptors (nAChRs), have also been described. We hypothesized that in AD, the loss of [3H]-ligand binding to nAChRs, which are largely presynaptic, would correlate with changes in two other presynaptic markers: synaptophysin (Syn), a measure of synaptic density, and ChAT activity. The midfrontal (MF) cortex of 36 autopsy confirmed (NIA and CERAD criteria) AD patients (mean death age +/- SD 80.1 +/- 8.4 years) who met NINDS-ADRDA criteria for a clinical diagnosis of probable or possible AD, and 11 nondemented controls (mean death age +/- SD 77.9 +/- 8.0) were examined. Synapse counts were quantified by a dotimmunobinding assay for Syn. ChAT activity was assessed by standard biochemical assays. Nicotinic cholinergic receptor binding was assayed using the high affinity nicotinic agonist [3H]-(+/-)-epibatidine ([3H]-EPI). The mean +/- SD Syn in AD (83.4 +/- 31.9 arbitrary units (AU)/mg protein) was significantly lower than controls (126.1 +/- 19.9, p = 0.0003; t-test). The mean ChAT activity in AD (139.0 +/- 75.6 nmol ACh/hr/100 mg protein) was significantly lower than controls (219.6 +/- 70.8, p = 0.004). The mean [3H]-EPI total binding in AD (6.2 +/- 2.8 fmol/mg protein) was significantly lower than controls (14.8 +/- 3.2; p < 0.0001). Syn correlated with [3H]-EPI binding in AD (r = 0.48, p = 0.006; Pearson) but ChAT did not (r = -0.20, p = 0.34). We conclude that loss of high affinity nAChR binding correlates with loss of synapses in AD. The lack of correlation between [3H]-EPI binding and ChAT activity suggests that the targeted receptor populations may not be located exclusively on cholinergic neurons.
It is a truism of modern biomedical science that the development of therapies expected to slow or arrest the progression of a disease requires as detailed an understanding of its molecular and cellular pathogenesis as possible. In turn, the cloning of novel gene products implicated in a disease often leads to new insights about fundamental features of protein structure and function. A particularly compelling example of this beneficial interplay between basic and applied cell biology arises from the exciting recent progress in deciphering Alzheimer's disease (AD). This review discusses the current understanding of the cell biology of two proteins crucial for the pathogenesis of AD, the beta-amyloid precursor protein and presenilin.
We examined the inner molecular layer (IML) of the hippocampal dentate gyrus for possible changes in synaptic density. Material was obtained from 9 individuals with Alzheimer disease (AD) and compared to samples obtained from 10 age-matched, postmortem-matched neurologically normal controls, employing standard ultrastructural techniques. Statistical analyses demonstrated a significant decline in synaptic numbers between controls and AD subjects. This decline was accompanied by a significant increase in apposition length and resulted in a significant correlation with the synaptic density. As the number of synapses declined, the apposition length increased. Assessment was also made of the granule cells density and the analyses showed a significant decline in the synapse to granule cell ratio in the AD group. This decline in the density of synaptic contacts in the IML reflects a more widespread decline in plasticity in AD and may be related to the memory problems associated with the disease.
Upregulation of the glial fibrillary acidic protein (GFAP) in astrocytes is a hallmark of the phenomenon known as reactive gliosis and, yet, the function of GFAP in this process is largely unknown. Our previous studies have shown that mature astrocytes react vigorously to substrate bound beta-amyloid protein (BAP) in a variety of ways (i.e., increased GFAP, enhanced motility, unusual aggregation patterns, inhibitory ECM production). In order to uncover which, if any, of these phenomena are causally related to the function of GFAP, primary cortical astrocytes from transgenic mice lacking GFAP were cultured on BAP substrates at low or high density and at various lengths of time following in vitro maturation. Differences between mutant and control cells became progressively more obvious when cells were matured in vitro for two weeks or longer and especially in cultures that were at high density. Mature control astrocytes show a dramatic response to BAP by aggregating into a meshwork of rope-like structures that completely bridge over the peptide surface. In marked contrast, mature GFAP-null astrocytes initiate the response much more slowly and had a much reduced ability to aggregate tightly. Furthermore, we prepared hippocampal slice cultures from GFAP-/- and GFAP+/+ mice and compared their astrocytic responses to injected BAP. GFAP-/- astrocytes of hippocampal slice cultures failed to form a barrier-like structure around the edge of the BAP deposit as did GFAP+/+ astrocytes. Our data suggest that GFAP may be essential for mature astrocytes to constrain certain types of highly inflammatory lesions in the brain.
Diffuse amyloid beta-protein (Abeta) deposits with numerous glial cells containing C-terminal Abeta fragments occur in the cerebral cortex of patients with Alzheimer's disease. By using a panel of antibodies specific for various epitopes in the Abeta peptide, we have investigated the immunohistochemical nature of the diffuse Abeta deposits. The extracellular material contains Abeta with a C-terminus at residue valine40 (Abeta40) as well as residues alanine42/threonine43 (Abeta42). The N-termini include aspartate1, pyroglutamate3, and pyroglutamate11, with pyroglutamate3 being dominant. Microglia and astrocytes in and around these deposits contain intensely staining granules. Most of these granules are negative for antibodies to the N-terminally located sequences of Abeta. These include 6E10 (Abeta1-17), 6F/3D (Abeta8-17), and the N-terminal antibodies specific to aspartate1, pyroglutamate3, and pyroglutamate11. The C-termini of intraglial Abeta are comparable with those of the extracellular deposits. The microglia and astrocytes have quiescent morphology compared with those associated with senile plaques and other lesions such as ischemia. Complement activation in these deposits is not prominent and often below the sensitivity of immunohistochemical detection. Although factors which may cause this type of deposit remain unclear, lack of strong tissue responses suggests that these deposits are a very early stage of Abeta deposition. They were found only inconsistently and were absent in a number of cases examined in this study. Further analysis of these deposits might provide important clues regarding the accumulation and clearance of Abeta in Alzheimer's disease brain.
The deposition of amyloid beta (A beta) protein plays a central role in the neuropathology of Alzheimer's disease (AD) and it constitutes the core of classical senile plaques. However, little is known about its intracellular distribution. An immunogold electron microscope study was therefore carried out on biopsies of brain tissue from patients with AD using a monoclonal antibody raised against residues 8 to 17 of the A beta protein. Specific A beta immunogold labeling was observed over extracellular amyloid fibrils associated with senile plaques. In addition, widespread intracellular A beta immunolabeling was observed adjacent to granular structures (30-40 nm in diameter) within membrane-bound processes. Pretreatment of some sections with amylase or omission of lead citrate staining from others strongly suggests that the electron-dense granular structures associated with A beta immunoreactivity are glycogen. Some of the A beta-immunolabeled processes contained gliofilaments and immunolabeling of alternate sections for glial fibrillary acidic protein confirmed that the A beta-immunolabeled processes were astrocytic. A beta immunolabeling was not observed over neuronal or microglial processes. Whether the presence of A beta protein in astrocytes is the result of synthetic or degradation processes requires further investigation.