Alteration of sodium transport by the choroid plexus with amiloride

Mechanisms of cerebrospinal fluid secretion by the choroid plexus epithelium: Application to various intracranial pathologies

The choroid plexus (CP) is a small yet highly active epithelial tissue located in the ventricles of the brain. It secretes most of the CSF that envelops the brain and spinal cord. The epithelial cells of the CP have a high fluid secretion rate and differ from many other secretory epithelia in the organization of several key ion transporters. One striking difference is the luminal location of, for example, the vital Na+-K+-ATPase. In recent years, there has been a renewed focus on the role of ion transporters in CP secretion. Several studies have indicated that increased membrane transport activity is implicated in disorders such as hydrocephalus, idiopathic intracranial hypertension, and posthemorrhagic sequelae. The importance of the CP membrane transporters in regulating the composition of the CSF has also been a focus in research in recent years, particularly as a regulator of breathing and hemodynamic parameters such as blood pressure. This review focuses on the role of the fundamental ion transporters involved in CSF secretion and its ion composition. It gives a brief overview of the established factors and controversies concerning ion transporters, and finally discusses future perspectives related to the role of these transporters in the CP epithelium.

Cerebrospinal fluid pH regulation

The cerebrospinal fluid (CSF) fills the brain ventricles and the subarachnoid space surrounding the brain and spinal cord. The fluid compartment of the brain ventricles communicates with the interstitial fluid of the brain across the ependyma. In comparison to blood, the CSF contains very little protein to buffer acid-base challenges. Nevertheless, the CSF responds efficiently to changes in systemic pH by mechanisms that are dependent on the CO2/HCO3- buffer system. This is evident from early studies showing that the CSF secretion is sensitive to inhibitors of acid/base transporters and carbonic anhydrase. The CSF is primarily generated by the choroid plexus, which is a well-vascularized structure arising from the pial lining of the brain ventricles. The epithelial cells of the choroid plexus host a range of acid/base transporters, many of which participate in CSF secretion and most likely contribute to the transport of acid/base equivalents into the ventricles. This review describes the current understanding of the molecular mechanisms in choroid plexus acid/base regulation and the possible role in CSF pH regulation.

Outcomes of the 2019 hydrocephalus association workshop, "Driving common pathways: extending insights from posthemorrhagic hydrocephalus"

The Hydrocephalus Association (HA) workshop, Driving Common Pathways: Extending Insights from Posthemorrhagic Hydrocephalus, was held on November 4 and 5, 2019 at Washington University in St. Louis. The workshop brought together a diverse group of basic, translational, and clinical scientists conducting research on multiple hydrocephalus etiologies with select outside researchers. The main goals of the workshop were to explore areas of potential overlap between hydrocephalus etiologies and identify drug targets that could positively impact various forms of hydrocephalus. This report details the major themes of the workshop and the research presented on three cell types that are targets for new hydrocephalus interventions: choroid plexus epithelial cells, ventricular ependymal cells, and immune cells (macrophages and microglia).

The choroid plexus and its role in the pathogenesis of neurological infections

The choroid plexus is situated at an anatomically and functionally important interface within the ventricles of the brain, forming the blood-cerebrospinal fluid barrier that separates the periphery from the central nervous system. In contrast to the blood-brain barrier, the choroid plexus and its epithelial barrier have received considerably less attention. As the main producer of cerebrospinal fluid, the secretory functions of the epithelial cells aid in the maintenance of CNS homeostasis and are capable of relaying inflammatory signals to the brain. The choroid plexus acts as an immunological niche where several types of peripheral immune cells can be found within the stroma including dendritic cells, macrophages, and T cells. Including the epithelia cells, these cells perform immunosurveillance, detecting pathogens and changes in the cytokine milieu. As such, their activation leads to the release of homing molecules to induce chemotaxis of circulating immune cells, driving an immune response at the choroid plexus. Research into the barrier properties have shown how inflammation can alter the structural junctions and promote increased bidirectional transmigration of cells and pathogens. The goal of this review is to highlight our foundational knowledge of the choroid plexus and discuss how recent research has shifted our understanding towards viewing the choroid plexus as a highly dynamic and important contributor to the pathogenesis of neurological infections. With the emergence of several high-profile diseases, including ZIKA and SARS-CoV-2, this review provides a pertinent update on the cellular response of the choroid plexus to these diseases. Historically, pharmacological interventions of CNS disorders have proven difficult to develop, however, a greater focus on the role of the choroid plexus in driving these disorders would provide for novel targets and routes for therapeutics.

In Vitro Models of the Blood–Cerebrospinal Fluid Barrier and Their Applications in the Development and Research of (Neuro)Pharmaceuticals

The pharmaceutical research sector has been facing the challenge of neurotherapeutics development and its inherited high-risk and high-failure-rate nature for decades. This hurdle is partly attributable to the presence of brain barriers, considered both as obstacles and opportunities for the entry of drug substances. The blood–cerebrospinal fluid (CSF) barrier (BCSFB), an under-studied brain barrier site compared to the blood–brain barrier (BBB), can be considered a potential therapeutic target to improve the delivery of CNS therapeutics and provide brain protection measures. Therefore, leveraging robust and authentic in vitro models of the BCSFB can diminish the time and effort spent on unproductive or redundant development activities by a preliminary assessment of the desired physiochemical behavior of an agent toward this barrier. To this end, the current review summarizes the efforts and progresses made to this research area with a notable focus on the attribution of these models and applied techniques to the pharmaceutical sector and the development of neuropharmacological therapeutics and diagnostics. A survey of available in vitro models, with their advantages and limitations and cell lines in hand will be provided, followed by highlighting the potential applications of such models in the (neuro)therapeutics discovery and development pipelines.

Preclinical update on regulation of intracranial pressure in relation to idiopathic intracranial hypertension

BACKGROUND

Elevated intracranial pressure (ICP) is observed in association with a range of brain disorders. One of these challenging disorders is idiopathic intracranial hypertension (IIH), characterized by raised ICP of unknown cause with significant morbidity and limited therapeutic options. In this review, special focus is put on the preclinical research performed in order to understand the pathophysiology behind ICP regulation and IIH. This includes cerebrospinal fluid dynamics, molecular mechanisms underlying disturbances in brain fluids leading to elevated ICP, role of obesity in IIH, development of an IIH model and ICP measurements in rodents. The review also discusses existing and new drug targets for IIH that have been evaluated in vivo.

CONCLUSIONS

ICP monitoring in rodents is challenging and different methods have been applied. Some of these methods are invasive, depend on use of anesthesia and only allow short-term monitoring. Long-term ICP recordings are needed to study IIH but existing methods are hampered by several limitations. As obesity is one of the most common risk factors for IIH, a rodent obese model has been developed that mimics some key aspects of IIH. The most commonly used drugs for IIH have been evaluated in vivo for their efficacy at lowering ICP in the existing animal models. These studies suggest these drugs, including acetazolamide, might have limited or no reducing effect on ICP. Two drug targets that can impact ICP in healthy rodents are topiramate and a glucagon-like peptide-1 receptor (GLP-1R) agonist. However, it remains to evaluate their effect in an IIH model with more precise and valid ICP monitoring system. Therefore, continued evaluation in the preclinical research with refined tools is of great importance to further understand the pathophysiology behind disorders with raised ICP and to explore new drug targets.

European headache federation guideline on idiopathic intracranial hypertension

Background

Idiopathic Intracranial Hypertension (IIH) is characterized by an elevation of intracranial pressure (ICP no identifiable cause. The aetiology remains largely unknown, however observations made in a number of recent clinical studies are increasing the understanding of the disease and now provide the basis for evidence-based treatment strategies.

Methods

The Embase, CDSR, CENTRAL, DARE and MEDLINE databases were searched up to 1st June 2018. We analyzed randomized controlled trials and systematic reviews that investigate IIH.

Results

Diagnostic uncertainty, headache morbidity and visual loss are among the highest concerns of clinicians and patients in this disease area. Research in this field is infrequent due to the rarity of the disease and the lack of understanding of the underlying pathology.

Conclusions

This European Headache Federation consensus paper provides evidence-based recommendations and practical advice on the investigation and management of IIH.

Electronic supplementary material

The online version of this article (10.1186/s10194-018-0919-2) contains supplementary material, which is available to authorized users.

Topiramate is more effective than acetazolamide at lowering intracranial pressure

Background

The management of idiopathic intracranial hypertension focuses on reducing intracranial pressure to preserve vision and reduce headaches. There is sparse evidence to support the use of some of the drugs commonly used to manage idiopathic intracranial hypertension, therefore we propose to evaluate the efficacy of these drugs at lowering intracranial pressure in healthy rats.

Methods

We measured intracranial pressure in female rats before and after subcutaneous administration of acetazolamide, topiramate, furosemide, amiloride and octreotide at clinical doses (equivalent to a single human dose) and high doses (equivalent to a human daily dose). In addition, we measured intracranial pressure after oral administration of acetazolamide and topiramate.

Results

At clinical and high doses, subcutaneous administration of topiramate lowered intracranial pressure by 32% (p = 0.0009) and 21% (p = 0.015) respectively. There was no significant reduction in intracranial pressure noted with acetazolamide, furosemide, amiloride or octreotide at any dose. Oral administration of topiramate significantly lowered intracranial pressure by 22% (p = 0.018), compared to 5% reduction with acetazolamide (p = >0.999).

Conclusion

Our in vivo studies demonstrated that both subcutaneous and oral administration of topiramate significantly lowers intracranial pressure. Other drugs tested, including acetazolamide, did not significantly reduce intracranial pressure. Future clinical trials evaluating the efficacy and side effects of topiramate in idiopathic intracranial hypertension patients would be of interest.

Transport across the choroid plexus epithelium

The choroid plexus epithelium is a secretory epithelium par excellence. However, this is perhaps not the most prominent reason for the massive interest in this modest-sized tissue residing inside the brain ventricles. Most likely, the dominant reason for extensive studies of the choroid plexus is the identification of this epithelium as the source of the majority of intraventricular cerebrospinal fluid. This finding has direct relevance for studies of diseases and conditions with deranged central fluid volume or ionic balance. While the concept is supported by the vast majority of the literature, the implication of the choroid plexus in secretion of the cerebrospinal fluid was recently challenged once again. Three newer and promising areas of current choroid plexus-related investigations are as follows: 1) the choroid plexus epithelium as the source of mediators necessary for central nervous system development, 2) the choroid plexus as a route for microorganisms and immune cells into the central nervous system, and 3) the choroid plexus as a potential route for drug delivery into the central nervous system, bypassing the blood-brain barrier. Thus, the purpose of this review is to highlight current active areas of research in the choroid plexus physiology and a few matters of continuous controversy.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

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.

Cerebrospinal Fluid Secretion by the Choroid Plexus

The choroid plexus epithelium is a cuboidal cell monolayer, which produces the majority of the cerebrospinal fluid. The concerted action of a variety of integral membrane proteins mediates the transepithelial movement of solutes and water across the epithelium. Secretion by the choroid plexus is characterized by an extremely high rate and by the unusual cellular polarization of well-known epithelial transport proteins. This review focuses on the specific ion and water transport by the choroid plexus cells, and then attempts to integrate the action of specific transport proteins to formulate a model of cerebrospinal fluid secretion. Significant emphasis is placed on the concept of isotonic fluid transport across epithelia, as there is still surprisingly little consensus on the basic biophysics of this phenomenon. The role of the choroid plexus in the regulation of fluid and electrolyte balance in the central nervous system is discussed, and choroid plexus dysfunctions are described in a very diverse set of clinical conditions such as aging, Alzheimer's disease, brain edema, neoplasms, and hydrocephalus. Although the choroid plexus may only have an indirect influence on the pathogenesis of these conditions, the ability to modify epithelial function may be an important component of future therapies.

Epithelial pathways in choroid plexus electrolyte transport

A stable intraventricular milieu is crucial for maintaining normal neuronal function. The choroid plexus epithelium produces the cerebrospinal fluid and in doing so influences the chemical composition of the interstitial fluid of the brain. Here, we review the molecular pathways involved in transport of the electrolytes Na+, K+, Cl-, and HCO3(-)across the choroid plexus epithelium.

Sodium Transport in the Choroid Plexus and Salt-Sensitive Hypertension

To elucidate the role of epithelial sodium channels (ENaCs) and Na + -K + -ATPase in Na + transport by the choroid plexus, we studied ENaC expression and Na + transport in the choroid plexus. Lateral ventricle choroid plexuses were obtained from young male Wistar, Dahl salt–resistant (SS.BN13), and Dahl salt–sensitive (SS/MCW) rats on a regular (0.3%) or high- (8.0%) salt diet. The effects of ENaC blocker benzamil and Na + -K + -ATPase blocker ouabain on sodium transport were evaluated by measuring the amounts of retained 22 Na + and by evaluating intracellular [Na + ] with Sodium Green fluorescence. In Wistar rats, ENaC distribution was as follows: microvilli, 10% to 30%; cytoplasm, 60% to 80%; and basolateral membrane, 5% to 10%. Benzamil (10 −8 m ) decreased 22 Na + retention by 20% and ouabain (10 −3 m ) increased retention by 40%, whereas ouabain and benzamil combined caused no change. Similar changes were noted in intracellular [Na + ]. In Dahl rats on a regular salt diet, intracellular [Na + ] was similar, but the amount of retained 22 Na + was less in sensitive versus resistant rats. High salt did not affect ENaC mRNA or protein, nor the benzamil induced decreases in retained 22 Na + or intracellular [Na + ] in either strain. However, high salt increased intracellular [Na + ] and attenuated the increase in uptake of 22 Na + by ouabain in resistant but not sensitive rats, suggesting a decrease in Na + -K + -ATPase activity only in resistant rats. These findings suggest that both ENaC and Na + -K + -ATPase regulate Na + transport in the choroid plexus. Aberrant regulation of Na + transport and of Na + -K + -ATPase activity, but not of ENaCs, might contribute to the increase in cerebrospinal fluid [Na + ] in Dahl salt-sensitive rats on a high-salt diet.

Nhe1 is a luminal Na+/H+ exchanger in mouse choroid plexus and is targeted to the basolateral membrane in Ncbe/Nbcn2-null mice

The choroid plexus epithelium (CPE) secretes the major fraction of the cerebrospinal fluid (CSF). The Na(+)-HCO(3)(-) transporter Ncbe/Nbcn2 in the basolateral membrane of CPE cells is important for Na(+)-dependent pH(i) increases and probably for CSF secretion. In the current study, the anion transport inhibitor DIDS had no effect on the residual pH(i) recovery in acidified CPE from Ncbe/Nbcn2 knockout mouse by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF)-fluorescence microscopy in the presence of CO(2)/HCO(3)(-) (Ncbe/Nbcn2-ko+DIDS 109% of control, P = 0.76, n = 5). Thus Ncbe/Nbcn2 mediates the DIDS-sensitive Na(+)-dependent pH(i) recovery in the CPE. The Na(+)/H(+) exchanger-1 Nhe1 is proposed to mediate similar functions as Ncbe/Nbcn2 in CPE. Here, we immunolocalize the Nhe1 protein to the luminal membrane domain in mouse and human CPE. The Na(+)-dependent pH(i) recovery of Nhe1 wild-type (Nhe1-wt) mice in the absence of CO(2)/HCO(3)(-) was abolished in the Nhe1 knockout CPE (Nhe1-ko 0.37% of Nhe1-wt, P = 0.0007, n = 5). In Ncbe/Nbcn2-ko mice, Nhe1 was targeted to the basolateral membrane. Nevertheless, the luminal Na(+)-dependent pH(i) recovery was increased in Ncbe/Nbcn2-ko compared with wild-type littermates (Nhe1-ko 146% of Nhe1-wt, P = 0.007, n = 5). Whereas the luminal Nhe activity was inhibited by the Nhe blocker EIPA (10 microM) in the Ncbe/Nbcn2-wt, it was insensitive to the inhibitor in Ncbe/Nbcn2-ko (Ncbe/Nbcn2-ko+EIPA 100% of control, P = 0.98, n = 5). This indicates that a luminal EIPA-insensitive Nhe was induced in Ncbe/Nbcn2-ko CPE and that EIPA-sensitive Nhe activity was basolateral. The Nhe1 translocation in Ncbe/Nbcn2-ko CPE may reflect a compensatory response, which provides the cells with better means of regulating pH(i) or transporting Na(+) after Ncbe/Nbcn2 disruption.

Multiplicity of cerebrospinal fluid functions: New challenges in health and disease

This review integrates eight aspects of cerebrospinal fluid (CSF) circulatory dynamics: formation rate, pressure, flow, volume, turnover rate, composition, recycling and reabsorption. Novel ways to modulate CSF formation emanate from recent analyses of choroid plexus transcription factors (E2F5), ion transporters (NaHCO3 cotransport), transport enzymes (isoforms of carbonic anhydrase), aquaporin 1 regulation, and plasticity of receptors for fluid-regulating neuropeptides. A greater appreciation of CSF pressure (CSFP) is being generated by fresh insights on peptidergic regulatory servomechanisms, the role of dysfunctional ependyma and circumventricular organs in causing congenital hydrocephalus, and the clinical use of algorithms to delineate CSFP waveforms for diagnostic and prognostic utility. Increasing attention focuses on CSF flow: how it impacts cerebral metabolism and hemodynamics, neural stem cell progression in the subventricular zone, and catabolite/peptide clearance from the CNS. The pathophysiological significance of changes in CSF volume is assessed from the respective viewpoints of hemodynamics (choroid plexus blood flow and pulsatility), hydrodynamics (choroidal hypo- and hypersecretion) and neuroendocrine factors (i.e., coordinated regulation by atrial natriuretic peptide, arginine vasopressin and basic fibroblast growth factor). In aging, normal pressure hydrocephalus and Alzheimer's disease, the expanding CSF space reduces the CSF turnover rate, thus compromising the CSF sink action to clear harmful metabolites (e.g., amyloid) from the CNS. Dwindling CSF dynamics greatly harms the interstitial environment of neurons. Accordingly the altered CSF composition in neurodegenerative diseases and senescence, because of adverse effects on neural processes and cognition, needs more effective clinical management. CSF recycling between subarachnoid space, brain and ventricles promotes interstitial fluid (ISF) convection with both trophic and excretory benefits. Finally, CSF reabsorption via multiple pathways (olfactory and spinal arachnoidal bulk flow) is likely complemented by fluid clearance across capillary walls (aquaporin 4) and arachnoid villi when CSFP and fluid retention are markedly elevated. A model is presented that links CSF and ISF homeostasis to coordinated fluxes of water and solutes at both the blood-CSF and blood-brain transport interfaces.

OUTLINE

1 Overview2 CSF formation2.1 Transcription factors2.2 Ion transporters2.3 Enzymes that modulate transport2.4 Aquaporins or water channels2.5 Receptors for neuropeptides3 CSF pressure3.1 Servomechanism regulatory hypothesis3.2 Ontogeny of CSF pressure generation3.3 Congenital hydrocephalus and periventricular regions3.4 Brain response to elevated CSF pressure3.5 Advances in measuring CSF waveforms4 CSF flow4.1 CSF flow and brain metabolism4.2 Flow effects on fetal germinal matrix4.3 Decreasing CSF flow in aging CNS4.4 Refinement of non-invasive flow measurements5 CSF volume5.1 Hemodynamic factors5.2 Hydrodynamic factors5.3 Neuroendocrine factors6 CSF turnover rate6.1 Adverse effect of ventriculomegaly6.2 Attenuated CSF sink action7 CSF composition7.1 Kidney-like action of CP-CSF system7.2 Altered CSF biochemistry in aging and disease7.3 Importance of clearance transport7.4 Therapeutic manipulation of composition8 CSF recycling in relation to ISF dynamics8.1 CSF exchange with brain interstitium8.2 Components of ISF movement in brain8.3 Compromised ISF/CSF dynamics and amyloid retention9 CSF reabsorption9.1 Arachnoidal outflow resistance9.2 Arachnoid villi vs. olfactory drainage routes9.3 Fluid reabsorption along spinal nerves9.4 Reabsorption across capillary aquaporin channels10 Developing translationally effective models for restoring CSF balance11 Conclusion.

Ion channel diversity, channel expression and function in the choroid plexuses

Knowledge of the diversity of ion channel form and function has increased enormously over the last 25 years. The initial impetus in channel discovery came with the introduction of the patch clamp method in 1981. Functional data from patch clamp experiments have subsequently been augmented by molecular studies which have determined channel structures. Thus the introduction of patch clamp methods to study ion channel expression in the choroid plexus represents an important step forward in our knowledge understanding of the process of CSF secretion.

Two K⁺ conductances have been identified in the choroid plexus: Kv1 channel subunits mediate outward currents at depolarising potentials; Kir 7.1 carries an inward-rectifying conductance at hyperpolarising potentials. Both K⁺ channels are localised at the apical membrane where they may contribute to maintenance of the membrane potential while allowing the recycling of K⁺ pumped in by Na⁺-K⁺ ATPase. Two anion conductances have been identified in choroid plexus. Both have significant HCO₃^- permeability, and may play a role in CSF secretion. One conductance exhibits inward-rectification and is regulated by cyclic AMP. The other is carried by an outward-rectifying channel, which is activated by increases in cell volume. The molecular identity of the anion channels is not known, nor is it clear whether they are expressed in the apical or basolateral membrane. Recent molecular evidence indicates that choroid plexus also expresses the non-selective cation channels such as transient receptor potential channels (TRPV4 and TRPM3) and purinoceptor type 2 (P2X) receptor operated channels. In conclusion, good progress has been made in identifying the channels expressed in the choroid plexus, but determining the precise roles of these channels in CSF secretion remains a challenge for the future.

Altered pH(i) regulation and Na(+)/HCO3(-) transporter activity in choroid plexus of cilia-defective Tg737(orpk) mutant mouse

Tg737(orpk) mice have defects in cilia assembly and develop hydrocephalus in the perinatal period of life. Hydrocephalus is progressive and is thought to be initiated by abnormal ion and water transport across the choroid plexus epithelium. The pathology is further aggravated by the slow and disorganized beating of motile cilia on ependymal cells that contribute to decreased cerebrospinal fluid movement through the ventricles. Previously, we demonstrated that the hydrocephalus phenotype is associated with a marked increase in intracellular cAMP levels in choroid plexus epithelium, which is known to have regulatory effects on ion and fluid movement in many secretory epithelia. To evaluate whether the hydrocephalus in Tg737(orpk) mutants is associated with defects in ion transport, we compared the steady-state pH(i) and Na(+)-dependent transport activities of isolated choroid plexus epithelium tissue from Tg737(orpk) mutant and wild-type mice. The data indicate that Tg737(orpk) mutant choroid plexus epithelium have lower pH(i) and higher Na(+)-dependent HCO(3)(-) transport activity compared with wild-type choroid plexus epithelium. In addition, wild-type choroid plexus epithelium could be converted to a mutant phenotype with regard to the activity of Na(+)-dependent HCO(3)(-) transport by addition of dibutyryl-cAMP and mutant choroid plexus epithelium toward the wild-type phenotype by inhibiting PKA activity with H-89. Together, these data suggest that cilia have an important role in regulating normal physiology of choroid plexus epithelium and that ciliary dysfunction in Tg737(orpk) mutants disrupts a signaling pathway leading to elevated intracellular cAMP levels and aberrant regulation of pH(i) and ion transport activity.

Water and solute secretion by the choroid plexus

The cerebrospinal fluid (CSF) provides mechanical and chemical protection of the brain and spinal cord. This review focusses on the contribution of the choroid plexus epithelium to the water and salt homeostasis of the CSF, i.e. the secretory processes involved in CSF formation. The choroid plexus epithelium is situated in the ventricular system and is believed to be the major site of CSF production. Numerous studies have identified transport processes involved in this secretion, and recently, the underlying molecular background for some of the mechanisms have emerged. The nascent CSF consists mainly of NaCl and NaHCO(3), and the production rate is strictly coupled to the rate of Na(+) secretion. In contrast to other secreting epithelia, Na(+) is actively pumped across the luminal surface by the Na(+),K(+)-ATPase with possible contributions by other Na(+) transporters, e.g. the luminal Na(+),K(+),2Cl(-) cotransporter. The Cl(-) and HCO(3) (-) ions are likely transported by a luminal cAMP activated inward rectified anion conductance, although the responsible proteins have not been identified. Whereas Cl(-) most likely enters the cells through anion exchange, the functional as well as the molecular basis for the basolateral Na(+) entry are not yet well-defined. Water molecules follow across the epithelium mainly through the water channel, AQP1, driven by the created ionic gradient. In this article, the implications of the recent findings for the current model of CSF secretion are discussed. Finally, the clinical implications and the prospects of future advances in understanding CSF production are briefly outlined.

Distribution of sodium transporters and aquaporin-1 in the human choroid plexus

The choroid plexus epithelium secretes electrolytes and fluid in the brain ventricular lumen at high rates. Several channels and ion carriers have been identified as likely mediators of this transport in rodent choroid plexus. This study aimed to map several of these proteins to the human choroid plexus. Immunoperoxidase-histochemistry was employed to determine the cellular and subcellular localization of the proteins. The water channel, aquaporin (AQP) 1, was predominantly situated in the apical plasma membrane domain, although distinct basolateral and endothelial immunoreactivity was also observed. The Na(+)-K(+)-ATPase alpha(1)-subunit was exclusively localized apically in the human choroid plexus epithelial cells. Immunoreactivity for the Na(+)-K(+)-2Cl(-) cotransporter, NKCC1, was likewise confined to the apical plasma membrane domain of the epithelium. The Cl(-)/HCO(3)(-) exchanger, AE2, was localized basolaterally, as was the Na(+)-dependent Cl(-)/HCO(3)(-) exchanger, NCBE, and the electroneutral Na(+)-HCO(3)(-) cotransporter, NBCn1. No immunoreactivity was found toward the Na(+)-dependent acid/base transporters NHE1 or NBCe2. Hence, the human choroid plexus epithelium displays an almost identical distribution pattern of water channels and Na(+) transporters as the rat and mouse choroid plexus. This general cross species pattern suggests central roles for these transporters in choroid plexus functions such as cerebrospinal fluid production.

Distribution of epithelial sodium channels and mineralocorticoid receptors in cardiovascular regulatory centers in rat brain

Epithelial sodium channels (ENaC) are important for regulating sodium transport across epithelia. Functional studies indicate that neural mechanisms acting through mineralocorticoid receptors (MR) and sodium channels (presumably ENaC) are crucial to the development of sympathoexcitation and hypertension in experimental models of salt-sensitive hypertension. However, expression and localization of the ENaC in cardiovascular regulatory centers of the brain have not yet been studied. RT-PCR and immunohistochemistry were performed to study ENaC and MR expression at the mRNA and protein levels, respectively. Both mRNA and protein for alpha-, beta-, and gamma-ENaC subunits and MR were found to be expressed in the rat brain. All three ENaC subunits and MR were present in the supraoptic nucleus, magnocellular paraventricular nucleus, hippocampus, choroid plexus, ependyma, and brain blood vessels, suggesting the presence of multimeric channels and possible regulation by mineralocorticoids. In most cortical areas, thalamus, amygdala, and suprachiasmatic nucleus, notable expression of gamma-ENaC was undetectable, whereas alpha- and beta-ENaC were abundantly expressed pointing to the possibility of a heterogeneous population of channels. The findings suggest that stoichiometrically different populations of ENaC may be present in both epithelial and neural components in the brain, which may contribute to regulation of cerebrospinal fluid and interstitial Na+ concentration as well as neuronal excitation.

Molecular mechanisms of cerebrospinal fluid production

The epithelial cells of the choroid plexuses secrete cerebrospinal fluid (CSF), by a process which involves the transport of Na(+), Cl(-) and HCO(3)(-) from the blood to the ventricles of the brain. The unidirectional transport of ions is achieved due to the polarity of the epithelium, i.e. the ion transport proteins in the blood-facing (basolateral) membrane are different to those in the ventricular (apical) membrane. The movement of ions creates an osmotic gradient which drives the secretion of H(2)O. A variety of methods (e.g. isotope flux studies, electrophysiological, RT-PCR, in situ hybridization and immunocytochemistry) have been used to determine the expression of ion transporters and channels in the choroid plexus epithelium. Most of these transporters have now been localized to specific membranes. For example, Na(+)-K(+)ATPase, K(+) channels and Na(+)-2Cl(-)-K(+) cotransporters are expressed in the apical membrane. By contrast the basolateral membrane contains Cl(-)- HCO(3) exchangers, a variety of Na(+) coupled HCO(3)(-) transporters and K(+)-Cl(-) cotransporters. Aquaporin 1 mediates water transport at the apical membrane, but the route across the basolateral membrane is unknown. A model of CSF secretion by the mammalian choroid plexus is proposed which accommodates these proteins. The model also explains the mechanisms by which K(+) is transported from the CSF to the blood.

A SCL4A10 gene product maps selectively to the basolateral plasma membrane of choroid plexus epithelial cells

The choroid plexus epithelium of the brain ventricular system produces the majority of the cerebrospinal fluid and thereby defines the ionic composition of the interstitial fluid in the brain. The transepithelial movement of Na+ and water in the choroid plexus depend on a yet-unidentified basolateral stilbene-sensitive Na+-HCO3- uptake protein. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed the expression in the choroid plexus of SLC4A10 mRNA, which encodes a stilbene-sensitive Na+-HCO3- transporter. Anti-COOH-terminal antibodies were developed to determine the specific expression and localization of this Na+-HCO3- transport protein. Immunoblotting demonstrated antibody binding to a 180-kDa protein band from mouse and rat brain preparations enriched with choroid plexus. The immunoreactive band migrated as a 140-kDa protein after N-deglycosylation, consistent with the predicted molecular size of the SLC4A10 gene product. Bright-field immunohistochemistry and immunoelectron microscopy demonstrated strong labeling confined to the basolateral plasma membrane domain of the choroid plexus epithelium. Furthermore, the stilbene-insensitive Na+-HCO3- cotransporter, NBCn1, was also localized to the basolateral plasma membrane domain of the choroid plexus epithelium. Hence, we propose that the SLC4A10 gene product and NBCn1 both function as basolateral HCO3- entry pathways and that the SLC4A10 gene product may be responsible for the stilbene-sensitive Na+-HCO3- uptake that is essential for cerebrospinal fluid production.

Mechanisms of CSF secretion by the choroid plexus

The epithelial cells of the choroid plexus secrete cerebrospinal fluid (CSF), by a process that involves the movement of Na(+), Cl(-) and HCO(3)(-) from the blood to the ventricles of the brain. This creates the osmotic gradient, which drives the secretion of H(2)O. The unidirectional movement of the ions is achieved due to the polarity of the epithelium, i.e., the ion transport proteins in the blood-facing (basolateral) are different to those in the ventricular (apical) membranes. Saito and Wright (1983) proposed a model for secretion by the amphibian choroid plexus, in which secretion was dependent on activity of HCO(3)(-) channels in the apical membrane. The patch clamp method has now been used to study the ion channels expressed in rat choroid plexus. Two potassium channels have been observed that have a role in maintaining the membrane potential of the epithelial cell, and in regulating the transport of K(+) across the epithelium. An inward-rectifying anion channel has also been identified, which is closely related to ClC-2 channels, and has a significant HCO(3)(-) permeability. This channel is expressed in the apical membrane of the epithelium where it may play an important role in CSF secretion. A model of CSF secretion by the mammalian choroid plexus is proposed that accommodates these channels and other data on the expression of transport proteins in the choroid plexus.

Involvement of non-neuronal brain cells in AVP-mediated regulation of water space at the cellular, organ, and whole-body level

Vasopressin (AVP) influences non-neuronal brain cells in cell-type specific manners: (1) it regulates water balance at the cellular level of brain parenchyma by adjusting astrocytic water permeability; (2) it contributes to the control of extracellular K(+) concentration ([K(+)](e)) in brain by stimulation of K(+) transfer from blood to brain, due to activation of an inwardly directed Na(+),K(+),Cl(-) cotransporter at the luminal membrane of capillary endothelial cells and opening of K(+) channels at their abluminal membrane; (3) it decreases formation of cerebrospinal fluid (CSF) by decreasing Cl(-) secretion into CSF by epithelial cells of the choroid plexus, probably by inhibition of Cl(-)/HCO(-)(3) exchange at their basolateral membrane; (4) it contributes to regulation of intracellular volume within the brain by regulation of water permeability in ependymal cells and subpial astrocytes; and (5) it exerts effects on specialized astrocytes in circumventricular organs, their adjacent glia limitans, and the neural pituitary, which regulate AVP release to the systemic circulation by altering the spatial relationship between neurons and their adjacent glial cells. A unified mechanism is proposed, which integrates most of the effects of AVP and may be of considerable importance for neuronal excitability and, thus, for behavior.

The choroid plexuses and the barriers between the blood and the cerebrospinal fluid

1. The fluid homeostasis of the brain depends both on the endothelial blood-brain barrier and on the epithelial blood-cerebrospinal fluid (CSF) barrier located at the choroid plexuses and the outer arachnoid membrane. 2. The brain has two fluid environments: the brain interstitial fluid, which surrounds the neurons and glia, and the CSF, which fills the ventricles and external surfaces of the central nervous system. 3. CSF acts as a fluid cushion for the brain and as a drainage route for the waste products of cerebral metabolism. 4. Recent findings suggest that CSF may also act as a "third circulation" conveying substances secreted into the CSF rapidly to many brain regions.

Functional demonstration of Na+-K+-2Cl- cotransporter activity in isolated, polarized choroid plexus cells

The function of the apical Na+-K+-2Cl- cotransporter in mammalian choroid plexus (CP) is uncertain and controversial. To investigate cotransporter function, we developed a novel dissociated rat CP cell preparation in which single, isolated cells maintain normal polarized morphology. Immunofluorescence demonstrated that in isolated cells the Na+-K+-ATPase, Na+-K+-2Cl- cotransporter, and aquaporin 1 water channel remained localized to the brush border, whereas the Cl-/HCO-3 (anion) exchanger type 2 was confined to the basolateral membrane. We utilized video-enhanced microscopy and cell volume measurement techniques to investigate cotransporter function. Application of 100 microM bumetanide caused CP cells to shrink rapidly. Elevation of extracellular K+ from 3 to 6 or 25 mM caused CP cells to swell 18 and 33%, respectively. Swelling was blocked completely by Na+ removal or by addition of 100 microM bumetanide. Exposure of CP cells to 5 mM BaCl2 induced rapid swelling that was inhibited by 100 microM bumetanide. We conclude that the CP cotransporter is constitutively active and propose that it functions in series with Ba2+-sensitive K+ channels to reabsorb K+ from cerebrospinal fluid to blood.

Identification and expression of the Na+/H+ exchanger in mammalian cerebrovascular and choroidal tissues: characterization by amiloride-sensitive [3H]MIA binding and RT-PCR analysis

We report the initial characterization of [3H]5-(N-methyl-N-isobutyl)amiloride (MIA) binding to the Na+/H+ exchanger (NHE) and expression of its gene in mammalian cerebrovascular, choroidal and neocortical tissues. [3H]MIA bound reversibly to particulate fractions of rat, pig and human cerebral microvessels, choroid plexus and cerebral cortex. Scatchard analyses revealed binding to a single amiloride-sensitive site with dissociation constants (Kd) ranging from 20 to 90 nM for the various tissue preparations. The maximal binding capacities (Bmax) were between 2 to 17 pmol/mg protein and were several-fold greater in cerebral microvessels compared to the cerebral cortex. Amiloride, MIA, 5-(N, N-hexamethylene)amiloride (HMA), 5-(N, N-dimethyl)amiloride (DMA) and 5-(N-methyl-N-isopropyl)amiloride (IPA) variably displaced [3H]MIA binding to the microvessels in the following rank order: MIA>HMA>/=IPA>DMA>amiloride. Benzamil, a potent ligand of the Na+/Ca+ transporter was the least sensitive. These binding results were most compatible with the existence of the amiloride-sensitive NHE type 1 in the brain vascular and choroidal tissues. To substantiate this, we utilized reverse transcription polymerase chain reaction (RT-PCR) techniques to search for NHE-1 mRNA. Using primers corresponding to conserved sequences of the human growth factor-activatable NHE gene, RT-PCR revealed strong expression of NHE-1 mRNA in cerebral microvessels, choroid plexus, pial vessels and vascular smooth muscle cells relative to neocortical tissues from several species including rat, pig, cow, monkey and human subjects. Further confirmation of NHE-1 isoform mRNA expression in the cerebrovascular tissues was obtained by HpaII restriction digestion analysis and by subcloning and sequencing of the PCR amplified products. Our study suggests that mammalian cerebrovascular and choroidal tissues contain high amounts of the ubiquitous amiloride-sensitive [3H]MIA binding proteins consistent with the expression of NHE type 1 mRNA.

Further characterization of the sodium-dependent nucleoside transporter (N3) in choroid plexus from rabbit

The Na+/nucleoside cotransporter in rabbit choroid plexus differs from Na+/nucleoside cotransporters in other tissues in terms of substrate selectivity and stoichiometry. The overall goal of this study was to further characterize the kinetics of this system (N3). Choroid plexus tissue slices obtained from rabbit brain were depleted of ATP and treated with valinomycin and K+. Na+/thymidine uptake at 30 s in the presence of an inside negative potential difference was significantly greater than in the absence of a potential difference. Na+/thymidine uptake was not significantly affected by replacing chloride with either thiocyanate or sulfate. The Km of Na+/guanosine uptake was 149, 85.2 and 30.5 microM in the presence of a 25, 50 and 100 mM Na+ gradient, respectively, whereas the Vmax was unaffected, suggesting that Na+ binds first to the cotransporter, then, the nucleoside. Therapeutically relevant base-modified nucleoside analogs, 5-fluorouridine, 2-chloroadenosine and 5-iododeoxyuridine, significantly inhibited Na+/thymidine uptake with IC50 values (mean +/- S.E.) of 12.0 +/- 2.3, 21.3 +/- 2.2 and 24.4 +/- 2.1 microM, respectively, whereas nucleoside analogs structurally modified on the ribose ring, 3'-azidothymidine, dideoxyinosine and dideoxycytidine (100 microM) did not. These studies suggest that Na+/nucleoside cotransport in the choroid plexus is electrogenic and is not dependent on chloride. This cotransporter, which is present in choroid plexus but not in renal brush-border membrane vesicles from rabbit, may play a role in the disposition of clinically relevant base-modified nucleoside analogs into and out of the brain.

Formycin B elimination from the cerebrospinal fluid of the rat

The goal of this study was to determine whether specific transport systems are involved in nucleoside elimination from the cerebrospinal fluid (CSF). First, in vitro studies were carried out in isolated choroid plexus tissue slices from rat to ascertain the mechanisms of transport of formycin B, a model nucleoside analogue. 3H-Formycin B accumulated against a concentration gradient in the presence of an Na+ gradient in the isolated ATP-depleted choroid plexus tissue slices. This accumulation was reduced by high concentrations of unlabeled formycin B. Nitrobenzylthioinosine (NBMPR), an equilibrative nucleoside transport inhibitor, inhibited the uptake of formycin B in the absence of an Na+ gradient. These data suggest that both equilibrative and secondary active Na(+)-nucleoside transport systems are present in rat choroid plexus. In vivo, formycin B, together with inulin as a bulk flow marker, was injected into the lateral ventricle of the anesthetized rat with the aid of a stereotaxic device, and CSF was sampled from the cisterna magna at various times after injection. Twelve rats were randomized and divided into a low- and a high-dose group. The CSF clearance (CLCSF) of formycin B was significantly higher than the CLCSF of inulin in both animal groups (P < 0.01), indicating that formycin B is cleared from CSF by a pathway(s) in addition to bulk flow. Formycin B CLCSF was significantly lower in the high-dose group than in the low-dose group (P < 0.05), suggesting a saturable CSF elimination. The CLCSF of formycin B was also significantly reduced in animals treated with NBMPR (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-dependent antiporters in choroid plexus epithelial cultures from rabbit

The mechanism of recovery from an acid load in primary cultures of rabbit choroid plexus epithelium (CPE) was examined, with emphasis on Na(+)-dependent antiports. Cells were incubated in saline solutions buffered to pH 7.38 with either HEPES or HCO3- plus 95% O2/5% CO2. Intracellular pH (pHi) was determined from the steady-state distribution of [14C]benzoate. Recovery after acidification with NH4Cl was rapid (t1/2 = 5 min) and was dependent on external Na+ (EC50 = 12 mM). Hexamethyleneamiloride and ethylisopropylamiloride, potent inhibitors of the Na+/H+ antiport, blocked 80% of recovery when [Na+] was 5 mM with IC50 values of 100 nM. However, neither drug blocked recovery in normal [Na+]. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of Cl-/HCO3- antiports, blocked recovery of pHi in a dose-related fashion in the presence of bicarbonate, but not in the presence of HEPES. No inhibition occurred with benzamil, an amiloride congener with high affinity for the Na+ channel, nor with dimethylbenzamil, an inhibitor of Na+/Ca2+ exchange. The carbonic anhydrase inhibitor acetazolamide also did not alter recovery from acidification. In CPE that had been pH-clamped with nigericin and KCl, the initial rate of 22Na+ uptake was very rapid (227 pmol/micrograms of DNA/min at pH 6.2), was dependent on external [Na+] with an EC50 value of 8 mM, and was inversely related to the pH of the medium. The maximal inhibition of 22Na+ uptake by hexamethyleneamiloride was 60% with an IC50 value of 76 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Cisterna magna microdialysis of 22Na to evaluate ion transport and cerebrospinal fluid dynamics

Microdialysis is used in vivo for measuring compounds in brain interstitial fluid. The authors describe another application of this technique to the central nervous system, namely microprobe dialysis in the cisterna magna to study the dynamics of ion transport and cerebrospinal fluid (CSF) formation in the rat. The choroid plexus is the major source of CSF, which is produced by active transport of Na from blood into the cerebral ventricles. Formation of CSF is directly proportional to the blood-to-CSF transport of Na. By injecting 22Na into the systemic circulation and quantifying its movement into CSF by microdialysis, one can reliably estimate alterations in the rate of CSF formation. The sensitivity of this system was determined by administering acetazolamide, a standard inhibitor of CSF production. Because acetazolamide is known to decrease CSF formation by 40% to 50%, the cisternal microdialysis system in animals treated with this drug should detect a corresponding decrease in the amount of 22Na dialyzed. This hypothesis is supported by the 22Na uptake curves for control versus treated animals: that is, by the acetazolamide-induced average diminution of about 45% in both the rate and extent of tracer accession to dialysate. Bumetanide, a loop diuretic, reduced by 30% the 22Na entry into dialysate. Microprobe dialysis of fluid in the cisterna magna is thus a minimally invasive and economical method for evaluating effects of drugs and hormones on the choroid plexus-CSF system.

Potassium cotransport with sodium and chloride in the choroid plexus

The effects of loop diuretics and ion substitution on the 2-min uptake of K (86Rb as marker) were analyzed to obtain evidence for K cotransport with Na and Cl in the choroid plexus epithelium. The isolated plexuses, which were excised from lateral ventricles of adult rats, were bathed in artificial cerebrospinal fluid (aCSF). At concentrations of 10(-6) to 10(-4) M, the specific cotransport inhibitors, bumetanide and piretanide, suppressed uptake of K in a dose-dependent manner. Ouabain-insensitive K uptake was stimulated by preincubating the choroid plexus in aCSF very low in [Na] and [K], then incubating it in much higher concentrations of these cations; bumetanide (10(-4)M) blocked this stimulated uptake by 52%. Moreover, tissue preincubation in Na- or Cl-free medium, followed by incubation with normal concentrations of both ions, stimulated the ouabain-insensitive uptake of K from 15 (baseline) to 35 nmol/mg dry weight. This stimulation of K transport depended on the simultaneous presence of both Na and Cl in aCSF, and replacing either ion alone did not stimulate the ouabain-insensitive K uptake. Collectively, these findings, together with those from a previous pharmacological study of 22Na and 36Cl transport, constitute strong evidence for the cotransport of Na, K, and Cl in rat choroid plexus.

Stilbenes inhibit exchange of chloride between blood, choroid plexus and cerebrospinal fluid

Disulfonic stilbenes (DIDS and SITS) were used to probe the nature of Cl transport in choroid plexus (CP) of adult rats. DIDS was more effective than SITS in reducing in vitro choroid cell [Cl]. Intravenously- or intraventricularly-administered DIDS substantially decreased 36Cl transport from blood to CP to CSF. Rate of CSF renewal was diminished nearly 30% by DIDS. These inhibitory findings implicate an anion antiporter for mediating Cl flux (and thus water) between blood and CSF.