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Localization of proteins for ion transporters, channels and associated enzymes and identification of their corresponding genes in adult and immature rat choroid plexus. Data for the localisation of the proteins are from Damkier et al. (2010) and see also Tables 3 and 4. CSF secretion results from coordinated transport of ions and water from basolateral membrane to cytoplasm, then sequentially across apical membrane into ventricles (for review see Davson & Segal, 1996). On the plasma-facing membrane is parallel Cl-/HCO 3-exchange (AE2, Slc4a2) and Na + /HCO 3-co-transport (NBC1, Slc4a4) with net function bringing Cl-into cells in exchange for HCO 3-(Murphy & Johanson, 1989). Also basolaterally located is Na-dependent Cl-/HCO 3-exchange (NCBn2 Slc4a10) that modulates pH and perhaps CSF formation (Damkier et al., 2007). Apical Na + influx by NHE5 (Slc9a5) and ATB1 (Atb1b1, Na + /K +ATPase, asterisk) maintains a low cell Na + that sets up a favorable basolateral gradient to drive Na + uptake (Pollay et al., 1985). Na + is extruded into CSF mainly via the Na + /K +-ATPase pump (ATB1, Atb1b1) and, under some conditions, the Na + /K +/-Cl-co-transporter NKCC1, Slc12a2, see Johanson et al., 2008 for review). Overall cell volume is maintained by the K + /Cl-co-transporters NCCT (Slc12a3) and KCC1 (Slc12a4). Aquaporin (AQP1/3/4) channels on CSFfacing membrane mediate water flux into ventricles (Oshio et al., 2005). Polarized distribution of carbonic anhydrase (CAR) and Na + /K +-ATPase, and aquaporins, enable net ion and water translocation to CSF (see Johanson et al., 2008 and Brian et al., 2010 for review). The gene Slc4a7 (NBCn1) was not detected by RNA-Seq, although it has been reported in both rat and mouse choroid plexus (Praetorius et al. 2004); this may have been for technical reasons or because of lack of antibody specificity see section "Limitations of study". The genes for Clir (chloride inwardly rectifying) channels has not been previously identified but are probably Clica and Clicb. The gene for VRAC (volume regulated anion channels) is not known (Alexander et al., 2011); see also Table 4. The carbonic anhydrases CAR2 and CAR8 have an intracellular distribution; CAR8 has been shown to lack the characteristic enzyme activity of these proteins (Picaud et al., 2009). It is not known whether it is functional in the embryo. The CLIC chloride channels are also intracellular (Edwards & Kahl, 2010) and as such we have placed them cytosolically, however it is more likely that they sit on the internal membrane of the cell and aid in movement of Cl-to other channels. The inset boxes show the fold differences for genes expressed at a higher level in the embryonic (red) or in the adult (green) choroid plexus.
Citations
... In addition to passive transport, multiple molecules can be shuttled across the BBB through a variety of ion channels and selective transporters. The main transport mechanisms can be divided into endothelial cell and pericytal transport with machineries across both cell types including active efflux [98], carrier-mediated (CMT [99]) ion-transport [100] and receptor-mediated transport (RMT) [101], with exception of active efflux, which is a specific property of BMVECs. In addition to these, there is also a BMVEC/pericyte independent mechanism, vascular-mediated transport [102]. ...
The blood–brain barrier (BBB) is a highly specialized and dynamic compartment which regulates the uptake of molecules and solutes from the blood. The relevance of the maintenance of a healthy BBB underpinning disease prevention as well as the main pathomechanisms affecting BBB function will be detailed in this review. Barrier disruption is a common aspect in both neurodegenerative diseases, such as amyotrophic lateral sclerosis, and neurodevelopmental diseases, including autism spectrum disorders. Throughout this review, conditions altering the BBB during the earliest and latest stages of life will be discussed, revealing common factors involved. Due to the barrier’s role in protecting the brain from exogenous components and xenobiotics, drug delivery across the BBB is challenging. Potential therapies based on the BBB properties as molecular Trojan horses, among others, will be reviewed, as well as innovative treatments such as stem cell therapies. Additionally, due to the microbiome influence on the normal function of the brain, microflora modulation strategies will be discussed. Finally, future research directions are highlighted to address the current gaps in the literature, emphasizing the idea that common therapies for both neurodevelopmental and neurodegenerative pathologies exist.
... There have been a few studies of lithium transfer across the mammalian choroid plexus involving experimental evidence for the exchange of lithium between blood and CSF, but disagreement exists about the mechanisms involved [17][18][19][20]. Recent studies investigating localisation and function of ion channels and transporters in choroid plexus epithelial cells, in both adult and during development [20][21][22][23] may eventually provide a better understanding. ...
... There are numerous ion channels and exchangers in choroid plexus epithelial cells [21]. Many of these are expressed in immature rat choroid plexus [22,23]. However, their permeability to lithium appears not to have been investigated. ...
Background
Little is known about the extent of drug entry into developing brain, when administered to pregnant and lactating women. Lithium is commonly prescribed for bipolar disorder. Here we studied transfer of lithium given to dams, into blood, brain and cerebrospinal fluid (CSF) in embryonic and postnatal animals as well as adults.
Methods
Lithium chloride in a clinically relevant dose (3.2 mg/kg body weight) was injected intraperitoneally into pregnant (E15–18) and lactating dams (birth-P16/17) or directly into postnatal pups (P0–P16/17). Acute treatment involved a single injection; long-term treatment involved twice daily injections for the duration of the experiment. Following terminal anaesthesia blood plasma, CSF and brains were collected. Lithium levels and brain distribution were measured using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry and total lithium levels were confirmed by Inductively Coupled Plasma-Mass Spectrometry.
Results
Lithium was detected in blood, CSF and brain of all fetal and postnatal pups following lithium treatment of dams. Its concentration in pups’ blood was consistently below that in maternal blood (30–35%) indicating significant protection by the placenta and breast tissue. However, much of the lithium that reached the fetus entered its brain. Levels of lithium in plasma fluctuated in different treatment groups but its concentration in CSF was stable at all ages, in agreement with known stable levels of endogenous ions in CSF. There was no significant increase of lithium transfer into CSF following application of Na⁺/K⁺ ATPase inhibitor (digoxin) in vivo, indicating that lithium transfer across choroid plexus epithelium is not likely to be via the Na⁺/K⁺ ATPase mechanism, at least early in development. Comparison with passive permeability markers suggested that in acute experiments lithium permeability was less than expected for diffusion but similar in long-term experiments at P2.
Conclusions
Information obtained on the distribution of lithium in developing brain provides a basis for studying possible deleterious effects on brain development and behaviour in offspring of mothers undergoing lithium therapy.
... There are numerous ion channels and exchangers in choroid plexus epithelial cells (21). Many of these are expressed in immature rat choroid plexus (22,23). ...
Background
Little is known about the extent of drug entry into developing brain, when administered to pregnant and lactating women. Lithium is commonly prescribed for bipolar disorder. Here we studied transfer of lithium given to dams, into blood, brain and cerebrospinal fluid (CSF) in embryonic and postnatal animals as well as adults.
Methods
Lithium chloride in a clinically relevant dose (3.2mg/kg body weight) was injected intraperitoneally into pregnant (E15-18) and lactating dams (birth-P16/17) or directly into postnatal pups (P0-P16/17). Acute treatment involved a single injection; long-term treatment involved twice daily injections for the duration of the experiment. Following terminal anaesthesia blood plasma, CSF and brains were collected. Lithium levels and brain distribution were measured using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry and total lithium levels were confirmed by Inductively Coupled Plasma-Mass Spectrometry.
Results
Lithium was detected in blood, CSF and brain of all fetal and postnatal pups following lithium treatment of dams. Its concentration in pups’ blood was consistently below that in maternal blood (30-35%) indicating significant protection by the placenta and breast tissue. However, much of the lithium that reached the fetus entered its brain. Levels of lithium in plasma fluctuated in different treatment groups but its concentration in CSF was stable at all ages, in agreement with known stable levels of endogenous ions in CSF. There was no significant increase of lithium transfer into CSF following application of Na⁺/K⁺ ATPase inhibitor (Digoxin) in vivo, indicating that lithium transfer across choroid plexus epithelium is not likely to be via the Na⁺/K⁺ ATPase mechanism, at least early in development. Comparison with passive permeability markers suggested that in acute experiments lithium permeability was less than expected for diffusion but similar in long-term experiments at P2.
Conclusions
Information obtained on the distribution of lithium in developing brain provides a basis for studying possible deleterious effects on brain development and behaviour in offspring of mothers undergoing lithium therapy.
... There are many more channels that show age-related differential expression in choroid plexus, the functions of which are unclear. Redrawn from Liddelow et al. (2016) with additional data from Liddelow et al. (2013). ...
Properties of the local internal environment of the adult brain are tightly controlled providing a stable milieu essential for its normal function. The mechanisms involved in this complex control are structural, molecular and physiological (influx and efflux transporters) frequently referred to as the “blood‐brain barrier”. These mechanisms include regulation of ion levels in brain interstitial fluid essential for normal neuronal function, supply of nutrients, removal of metabolic products and prevention of entry or elimination of toxic agents. A key feature is cerebrospinal fluid secretion and turnover. This is much less during development, allowing greater accumulation of permeating molecules. The overall effect of these mechanisms is to tightly control the exchange of molecules into and out of the brain. This review presents experimental evidence currently available on the status of these mechanisms in developing brain. It has been frequently stated for over nearly a century that the blood‐brain barrier is not present or at least is functionally deficient in the embryo, fetus and newborn. We suggest the alternative hypothesis that the barrier mechanisms in developing brain are likely to be appropriately matched to each stage of its development. The contributions of different barrier mechanisms, such as changes in constituents of cerebrospinal fluid in relation to specific features of brain development, for example neurogenesis, are only beginning to be studied. The evidence on this previously neglected aspect of brain barrier function is outlined. We also suggest future directions this field could follow with special emphasis on potential applications in a clinical setting. This article is protected by copyright. All rights reserved
... The whole profile of transporters expressed in the choroid plexus has been the subject of major transcriptome studies comparing adult and embryonic tissue [25,167]. ...
The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na⁺-pumps. K⁺ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na⁺ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K⁺ concentration within tight limits. This is most likely because Na⁺-pumps vary the rate at which K⁺ is transported out of ISF in response to small changes in K⁺ concentration. There is also evidence for functional regulation of K⁺ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3⁻ and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3⁻ transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3⁻ concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.
Hydrocephalus is caused by an overproduction of cerebrospinal fluid (CSF), an obstruction of fluid movement, or improper reabsorption. CSF accumulation in the brain’s ventricles causes ventriculomegaly, increased intracranial pressure, inflammation, and neural cell injury. Hydrocephalus can arise from brain trauma, hemorrhage, infection, tumors, or genetic mutations. Currently, there is no cure for hydrocephalus. Treatments like shunting and endoscopic third ventriculostomies are used, but, unfortunately, these therapeutic approaches require brain surgery and have high failure rates. The choroid plexus epithelium (CPe) is thought to be the major producer of CSF in the brain. It is a polarized epithelium that regulates ion and water movement from a fenestrated capillary exudate to the ventricles. Despite decades of research, control of electrolyte movement in the CPe is still not fully understood. This review discusses important transporters on the CPe, how some of these are regulated, and which of them could be potential targets for hydrocephalus treatment. To advance the development of hydrocephalus treatments, physiologically relevant preclinical models are crucial. This review covers some of the current animal and cell culture methods used to study hydrocephalus and highlights the need to develop standardized preclinical models that are used by multiple investigators in order to replicate critical findings and resolve controversies regarding potential drug targets.
Background
Little is known about the extent of drugs administered to pregnant and lactating women entering the developing brain. Lithium is one of the most commonly prescribed drugs for bipolar disorder. Here we studied transfer of lithium given to dams, into blood, brain and cerebrospinal fluid (CSF) in embryonic, postnatal and adult rats.
Methods
Lithium chloride in a clinically relevant dose (3.2mg/Kg body weight) was injected intraperitoneally into pregnant (E15-18) and lactating dams (birth-P16) or directly into postnatal pups (P0-P16). Acute treatment involved a single injection; chronic treatment involved twice daily injections for the duration of the experiment. Following terminal anaesthesia blood plasma, CSF and brains were collected. Lithium levels and brain distribution were measured using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).
Results
Lithium was detected in blood, CSF and brain of all fetal and postnatal pups following lithium treatment of dams. Its concentration in pups’ blood was consistently below that in maternal blood (30-35%) indicating significant protection by the placenta and breast tissue. Levels of lithium in plasma fluctuated in different treatment groups but its concentration in CSF was stable at all ages, in agreement with known stable levels of endogenous ions in CSF. Only a small non-significant increase of lithium transfer into CSF occurred following application of Na⁺/K⁺ ATPase inhibitor (Digoxin) in vivo, indicating that Na⁺/K⁺ ATPase is at most only a minor mechanism for lithium transfer across choroid plexus cells into CSF early in development. Presumably lithium transfer across choroid plexus epithelial cell is occurring predominantly via other ion channels.
Conclusions
Information obtained on the distribution of lithium in developing brain provides a basis for studying possible deleterious effects on brain development and behaviour in offspring of mothers undergoing lithium therapy.
This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g⁻¹ min⁻¹). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood–brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood–brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood–brain barrier, have higher total CL values. Substances crossing the blood–brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood–brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na⁺ and Cl⁻ ions across the blood–brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood–brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood–brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.
Electronic supplementary material
The online version of this article (10.1186/s12987-018-0113-6) contains supplementary material, which is available to authorized users.
There are five exchange interfaces between the peripheral circulation (blood), the cerebrospinal fluid (CSF) and the brain: (i) meninges, (ii) blood vessels, (iii) choroid plexuses, (iv) circumventricular organs and (v) ependyma (neuroependyma in embryos). All five interfaces have distinctive morphological and physiological properties; the first three are characterised by intercellular tight junctions that provide important structural basis for limiting molecular exchange across their interfaces. Cells that form these interfaces are also sites of extensive exchange mechanisms (transporters) that control entry and exit of a wide variety of molecules into the brain. Secretion of CSF by the choroid plexuses which flows through the ventricular system, and the exchange of substances between the CSF and brain is an important mechanism for the control of the characteristic composition of the brain interstitial fluid. Understanding of the complexity of barrier mechanisms is essential for evaluation of the effects of inflammatory conditions affecting the brain, whether in the adult or during development.
