Choroid plexus: biology and pathology

The choroid plexus is an epithelial–endothelial vascular convolute within the ventricular system of the vertebrate brain. It consists of epithelial cells, fenestrated blood vessels, and the stroma, dependent on various physiological or pathological conditions, which may contain fibroblasts, mast cells, macrophages, granulocytes or other infiltrates, and a rich extracellular matrix. The choroid plexus is mainly involved in the production of cerebrospinal fluid (CSF) by using the free access to the blood compartment of the leaky vessels. In order to separate blood and CSF compartments, choroid plexus epithelial cells and tanycytes of circumventricular organs constitute the blood–CSF–brain barrier. As non-neuronal cells in the brain and derived from neuroectoderm, choroid plexus epithelia are defined as a subtype of macroglia. The choroid plexus is involved in a variety of neurological disorders, including neurodegenerative, inflammatory, infectious, traumatic, neoplastic, and systemic diseases. Aβ and Biondi ring tangles accumulate in the Alzheimer’s disease choroid plexus. In multiple sclerosis, the choroid plexus could represent a site for lymphocyte entry in the CSF and brain, and for presentation of antigens. Recent studies have provided new diagnostic markers and potential molecular targets for choroid plexus papilloma and carcinoma, which represent the most common brain tumors in the first year of life. We here revive some of the classical studies and review recent insight into the biology and pathology of the choroid plexus.     

THE DEVELOPMENT OF THE HUMAN BLOOD-BRAIN AND BLOOD-CSF BARRIERS

The commonly held belief that the fetal blood-brain and blood-CSF barriers are immature is reviewed. Results obtained from carefully conducted experiments with horseradish peroxidase and optimal freeze-fracturing suggest that the chick, rat and monkey brain barrier systems to proteins are tight from the earliest stages of development. Previous studies are reviewed in the light of new information on retrograde axonal transport, circumventricular organs, the proper use of horseradish peroxidase, freeze-fracturing, immunocytochemistry and plasma protein gene expression in the developing human brain. Original data on the development of human brain barrier systems are included. Tight junctions between cerebral endothelial and choroid plexus epithelial cells form the morphological basis for these systems. CSF in the fetus contains a remarkably high concentration of protein in contrast to adult CSF which is characterized by a very low protein concentration. This has previously been interpreted as due to immaturity of barriers in the fetal brain. Tight junctions between cerebral endothelial cells and between choroid plexus epithelial cells have been investigated in human embryos and fetuses by freeze fracture and thin section electron microscopy. As soon as the choroid plexus and the brain capillaries differentiated they exhibited well formed tight junctions. These junctions were very complex at early stages of development. A new barrier consisting of 'strap junctions' was found in the developing germinal matrix. The very high concentration of protein in early human fetal CSF cannot be accounted for by a lack of tight junctions in the developing brain barrier systems. Some transfer of proteins from blood to CSF, possibly via an intracellular route, has been demonstrated in immature experimental animals, but it seems that an important contribution to CSF proteins in the fetus may be synthesis by the developing brain and choroid plexuses with subsequent release into the CSF.     

The choroid plexus-cerebrospinal lfuid interface in Alzheimer’s disease：more than just a barrier

The choroid plexus is a complex structure which hangs inside the ventricles of the brain and consists mainly of choroid plexus epithelial (CPE) cells surrounding fenestrated capillaries. These CPE cells not only form an anatomical barrier, called the blood-cerebrospinal lfuid barrier (BCSFB), but also present an active interface between blood and cerebrospinal lfuid (CSF). CPE cells perform indispensable functions for the development, maintenance and functioning of the brain. Indeed, the primary role of the choroid plexus in the brain is to maintain homeostasis by secreting CSF which contains different molecules, such as nutrients, neurotrophins, and growth factors, as well as by clearing toxic and undesirable molecules from CSF. The choroid plexus also acts as a selective entry gate for leukocytes into the brain. Recent ifndings have revealed distinct changes in CPE cells that are associated with aging and Alzheimer’s disease. In this review, we review some recent ifndings that highlight the importance of the CPE-CSF system in Alzheimer’s dis-ease and we summarize the recent advances in the regeneration of brain tissue through use of CPE cells as a new therapeutic strategy.     

The choroid plexus-cerebrospinal fluid interface in Alzheimer's disease:more than just a barrier

The choroid plexus is a complex structure which hangs inside the ventricles of the brain and consists mainly of choroid plexus epithelial(CPE) cells surrounding fenestrated capillaries.These CPE cells not only form an anatomical barrier,called the blood-cerebrospinal fluid barrier(BCSFB),but also present an active interface between blood and cerebrospinal fluid(CSF).CPE cells perform indispensable functions for the development,maintenance and functioning of the brain.Indeed,the primary role of the choroid plexus in the brain is to maintain homeostasis by secreting CSF which contains different molecules,such as nutrients,neurotrophins,and growth factors,as well as by clearing toxic and undesirable molecules from CSF.The choroid plexus also acts as a selective entry gate for leukocytes into the brain.Recent findings have revealed distinct changes in CPE cells that are associated with aging and Alzheimer's disease.In this review,we review some recent findings that highlight the importance of the CPE-CSF system in Alzheimer's disease and we summarize the recent advances in the regeneration of brain tissue through use of CPE cells as a new therapeutic strategy.     

Human choroid plexus growth factors: What are the implications for CSF dynamics in Alzheimer's disease?

The choroid plexus plays a key role in supporting neuronal function by secreting cerebrospinal fluid (CSF) and may be involved in the regulation of various soluble factors. Because the choroid plexus is involved in growth factor secretion as well as CSF dynamics, it is important to understand how growth factors in CSF interact with the brain parenchyma as well as with cells in direct contact with the flowing CSF, i.e., choroid plexus and arachnoid villi. While the existence of growth factors in the choroid plexus has been documented in several animal models, the presence and distribution of growth factors in the human choroid plexus has not been extensively examined. This study describes the general distribution and possible functions of a number of key proteins in the human choroid plexus and arachnoid villi, including basic fibroblast growth factor, FGF receptor, and vascular endothelial growth factor. FGF and VEGF could both be readily demonstrated in choroid plexus epithelial cells. The presence of FGF and VEGF within the choroid plexus was also confirmed by ELISA analysis. Since Alzheimer's disease (AD) is known to be associated with a number of growth factor abnormalities, we examined the choroid plexus and arachnoid villi from AD patients. Immunohistochemical studies revealed the presence of FGF and VEGF within the AD choroid plexus and an increased density of FGFr in both the choroid plexus and the arachnoid villi of AD patients. No qualitative changes in the distribution of FGF and VEGF were observed in the AD choroid plexus. The appearance of FGFr in AD arachnoid was associated with robust amyloid and vimentin immunoreactivity. These findings confirm the presence of FGF and VEGF within the normal and AD choroid plexus and suggest that the alteration of growth factors and their receptors may contribute to the pathogenesis of the hydrocephalus ex vacuo that is characteristically seen in AD.     

Role of choroid plexus epithelium in the removal of valproic acid from the central nervous system

Previous experiments suggest the primary route of valproic acid (VPA) removal from the rabbit central nervous system (CNS) is by probenecid-sensitive transporters at the blood-brain barrier but not at the choroid plexus. The purpose of this study was to determine if other transport mechanisms at the choroid plexus played a significant role in the removal of VPA from the CNS. In six rabbits, silicone oil was perfused into both cerebral ventricles and out through the cisterna magna to physically block exchange of VPA between cerebrospinal fluid (CSF) and blood and between brain and CSF. In six control rabbits, perfusion was performed with mock CSF. Both groups received a loading dose followed by continuous intravenous infusion of VPA for 2.10 min. Ventriculocisternal perfusion with silicone oil had no significant effect on the steady-state brain concentrations or brain-to-plasma concentration ratios of VPA, further confirming that efflux of VPA at the choroid plexus is negligible.     

Transthyretin: a choroid plexus-specific transport protein in human brain. The 1986 S. Weir Mitchell award

Plasma transthyretin (TTR, formerly called prealbumin) is a 55-kd protein that participates in the plasma transport of both thyroxine and retinol (vitamin A). TTR concentrations are disproportionately high in human ventricular CSF, suggesting that TTR is either selectively transported across or synthesized de novo within the blood-CSF barrier. To address this question, we adopted a molecular genetic approach; after isolating a cDNA clone encoding human TTR, we previously demonstrated specific TTR messenger RNA (mRNA) synthesis in rat choroid plexus. We have now extended these investigations to the human brain. Northern analysis of postmortem brain homogenates revealed abundant TTR mRNA in choroid plexus, but not in cerebellum or cerebral cortex. Choroid plexus mRNA was readily translated into TTR preprotein in an in vitro translation system. An immunocytochemical survey of human postmortem brain sections revealed the presence of TTR protein specifically and uniquely in the cytoplasm of choroid plexus epithelial cells; these results were corroborated at the mRNA level by an extensive survey of whole rat-brain sections by in situ hybridization. Therefore, within the mammalian CNS, TTR is the first known protein synthesized solely by the choroid plexus, suggesting a special role for TTR in the brain or CSF. Whether this function differs from its established plasma transport functions is presently unknown.     

Uptake and concentrations of calcium in rat choroid plexus during chronic hypo- and hypercalcemia

The choroid plexus has been implicated in the regulation of cerebrospinal fluid (CSF) [Ca], but little information is available concerning Ca transport by this epithelium. We determined the transfer coefficients for 45Ca uptake into choroid plexus from blood, as well as tissue [Ca], in weanling Fischer-344 rats fed low, normal, or high Ca diets for 8 weeks. Plasma [Ca] decreased by 45% with low Ca diet and increased by 25% with high Ca diet. Choroid plexus 45Ca uptake varied inversely with plasma [Ca]. This relation was due largely to changes in extracellular Ca binding rather than to entry from blood, as the transfer coefficient was independent of plasma [Ca]. The extracellular Ca distribution in choroid plexus, the intercept of a plot of tissue 45Ca distribution against time, was reciprocally related to plasma [Ca]. Changes in total cell [Ca] during hypercalcemia were equivalent to those in plasma, and in hypocalcemia were 70% of those in plasma. These findings indicate that regulation of CSF [Ca] does not involve saturable transport of Ca into the choroid epithelium from blood, and that the apical membrane of the choroid epithelium is involved in homeostasis of CSF [Ca].     

Leptin transport at the blood--cerebrospinal fluid barrier using the perfused sheep choroid plexus model

Leptin is secreted by adipose tissue and thought to regulate appetite at the central level. Several studies have explored the central nervous system (CNS) entry of this peptide across the blood-brain and blood-cerebrospinal fluid (CSF) barriers in parallel, but this is the first to explore the transport kinetics of leptin across the choroid plexus (blood-CSF barrier) in isolation from the blood-brain barrier (BBB). This is important as the presence of both barriers can lead to ambiguous results from transport studies. The model used was the isolated Ringer perfused sheep choroid plexus. The steady-state extraction of [(125)I]leptin (7.5 pmol l(-1)) at the blood face of the choroid plexus was 21.1+/-5.7%, which was greater than extraction of the extracellular marker, giving a net cellular uptake for [(125)I]leptin (14.0+/-3.7%). In addition, trichloroacetic acid precipitable [(125)I] was detected in newly formed CSF, indicating intact protein transfer across the blood-CSF barrier. Human plasma concentrations of leptin are reported to be 0.5 nM. Experiments using 0.5 nM leptin in the Ringer produced a concentration of leptin in the CSF of 12 pM (similar to that measured in humans). [(125)I]Leptin uptake at the blood-plexus interface using the single-circulation paired tracer dilution technique (uptake in <60 s) indicated the presence of a saturable transport system, which followed Michaelis-Menten-type kinetics (K(m)=16.3+/-1.8 nM, V(max)=41.2+/-1.4 pmol min(-1) g(-1)), and a non-saturable component (K(d)=0.065+/-0.002 ml min(-1) g(-1)). In addition, secretion of new CSF by the choroid plexuses was significantly decreased with leptin present. This study indicates that leptin transport at the blood-CSF barrier is via saturable and non-saturable mechanisms and that the choroid plexus is involved in the regulation of leptin availability to the brain.     

Autonomic nerves in the mammalian choroid plexus and their influence on the formation of cerebrospinal fluid

The choroid plexuses of all ventricles receive a well-developed adrenergic and cholinergic innervation reaching both the secretory epithelium and the vascular smooth muscle cells. Also peptidergic nerves, containing vasoactive intestinal polypeptide, are present but primarily associated only with the vascular bed. A sympathetic inhibitory effect on the plexus epithelium has been indicated in determinations of carbonic anhydrase activity and by studies of various aspects of active transport in isolated plexus tissue. Pharmacological analysis in vitro has shown the choroidal arteries to possess both vasoconstrictory alpha-adrenergic and vasodilatory beta-adrenergic receptors. Electrical stimulation of the sympathetic nerves, which originate in the superior cervical ganglia, induces as much as 30% reduction in the net rate of cerebrospinal fluid (CSF) production, while sympathectomy results in a pronounced increase, about 30% above control, in the CSF formation. There is strong reason to believe that the choroid plexus is under the influence of a considerable sympathetic inhibitory tone under steady-state conditions. From pharmacological and biochemical experiments it is suggested that the sympathomimetic reduction in the rate of CSF formation is the result of a combined beta-receptor-mediated inhibition of the secretion from the plexus epithelium and a reduced blood flow in the choroid plexus tissue resulting from stimulation of the vascular alpha-receptors. The choroid plexus probably also represents an important inactivation site and gate mechanism for sympathomimetic amines, as evidenced by considerable local activity of catechol-O-methyl transferase and monoamine oxidase, primarily type B. The CSF production rate is also reduced by cholinomimetic agents, suggesting the presence of muscarinic-type cholinergic receptors in the choroid plexus.     

Leptin transport at the blood–cerebrospinal fluid barrier using the perfused sheep choroid plexus model

Leptin is secreted by adipose tissue and thought to regulate appetite at the central level. Several studies have explored the central nervous system (CNS) entry of this peptide across the blood–brain and blood–cerebrospinal fluid (CSF) barriers in parallel, but this is the first to explore the transport kinetics of leptin across the choroid plexus (blood–CSF barrier) in isolation from the blood–brain barrier (BBB). This is important as the presence of both barriers can lead to ambiguous results from transport studies. The model used was the isolated Ringer perfused sheep choroid plexus. The steady-state extraction of [¹²⁵I]leptin (7.5 pmol l⁻¹) at the blood face of the choroid plexus was 21.1±5.7%, which was greater than extraction of the extracellular marker, giving a net cellular uptake for [¹²⁵I]leptin (14.0±3.7%). In addition, trichloroacetic acid precipitable [¹²⁵I] was detected in newly formed CSF, indicating intact protein transfer across the blood–CSF barrier. Human plasma concentrations of leptin are reported to be 0.5 nM. Experiments using 0.5 nM leptin in the Ringer produced a concentration of leptin in the CSF of 12 pM (similar to that measured in humans). [¹²⁵I]Leptin uptake at the blood–plexus interface using the single-circulation paired tracer dilution technique (uptake in <60 s) indicated the presence of a saturable transport system, which followed Michaelis–Menten-type kinetics (K_m=16.3±1.8 nM, V_max=41.2±1.4 pmol min⁻¹ g⁻¹), and a non-saturable component (K_d=0.065±0.002 ml min⁻¹ g⁻¹). In addition, secretion of new CSF by the choroid plexuses was significantly decreased with leptin present. This study indicates that leptin transport at the blood–CSF barrier is via saturable and non-saturable mechanisms and that the choroid plexus is involved in the regulation of leptin availability to the brain.     

Disturbed function of the blood–cerebrospinal fluid barrier aggravates neuro-inflammation

Multiple sclerosis (MS) is a chronic neuro-inflammatory disorder, which is marked by the invasion of the central nervous system by monocyte-derived macrophages and autoreactive T cells across the brain vasculature. Data from experimental animal models recently implied that the passage of leukocytes across the brain vasculature is preceded by their traversal across the blood–cerebrospinal fluid barrier (BCSFB) of the choroid plexus. The correlation between the presence of leukocytes in the CSF of patients suffering from MS and the number of inflammatory lesions as detected by magnetic resonance imaging suggests that inflammation at the choroid plexus contributes to the disease, although in a yet unknown fashion. We here provide first insights into the involvement of the choroid plexus in the onset and severity of the disease and in particular address the role of the tight junction protein claudin-3 (CLDN3) in this process. Detailed analysis of human post-mortem brain tissue revealed a selective loss of CLDN3 at the choroid plexus in MS patients compared to control tissues. Importantly, mice that lack CLDN3 have an impaired BCSFB and experience a more rapid onset and exacerbated clinical signs of experimental autoimmune encephalomyelitis, which coincides with enhanced levels of infiltrated leukocytes in their CSF. Together, this study highlights a profound role for the choroid plexus in the pathogenesis of multiple sclerosis, and implies that CLDN3 may be regarded as a crucial and novel determinant of BCSFB integrity.     

The blood-brain barrier. II. Physiological data

In the first part of this review, the morphological characteristics of the interfaces between blood and some compartments of the central nervous system have been described. The blood-brain barrier (BBB) is situated between the blood and the brain extracellular space (ECS) at the endothelial cells of brain capillaries joined by tight junctions. The blood-cerebrospinal fluid (CSF) barrier is mainly situated at the epithelium of the choroid plexuses. In this second part, the general mechanisms and methods of study of the transport of low molecular weight substances across the BBB are reviewed. In addition water and solute transports across the interfaces are described. The third part will deal with the transport of metabolic substrates and of drugs, with the barrier to neurotransmitters and with the physiological alterations of the permeability of the blood-brain barrier. Blood-brain transport is transcellular. According to the nature of the transported molecule it can be either by diffusion through the membranes (lipophilic molecules) or carrier-mediated (hydrophilic molecules such as metabolic substrates). In vivo methods of study have first been experimental. The recent use of short-lived positron-emitting radioisotopes should extent their use to clinical studies. In vitro measurement of transport on isolated capillaries or choroid plexuses give additional information. Water exchange between plasma and the brain is rapid. However, the permeability of the brain capillaries for water is lower than that of other capillaries and of most membranes, but more similar to that of lipid bilayers and of most tight epithelia. The permeability for water of the choroid plexuses which secrete CSF is much higher than that of brain capillaries. CSF and ECS electrolyte composition is under strict homeostatic regulation, leaving it independent of fluctuations in plasma composition. This constant composition is the consequence of the low permeability of brain capillaries to electrolytes as well as that of the presence of a sodium potassium dependent ATPase on the abluminal membrane of capillaries and the apical membrane of choroid plexuses. This enzyme contributes to the extrusion of potassium from the CSF and ECS and allows a large gradient of potassium to be created between ECS and plasma. A low level of potassium in CSF and ECS is in fact needed for normal nerve conduction.     

Mechanisms for the elimination of 5-hydroxyindoleacetic acid from brain and cerebrospinal fluid of the rat during postnatal development

Mechanisms employed for the elimination of 5-hydroxyindoleacetic acid (5-HIAA) from brain and cerebrospinal fluid (CSF) were studied in rats during early postnatal development. Probenecid was used to inhibit the removal of 5-HIAA by active transport mechanisms. In the 1- and 4-day-old animals, the major route for efflux of 5-HIAA from brain to blood was indirectly via CSF pathways, and was mediated, presumably at the choroid plexus, by an active transport mechanism possessing an efflux rate sufficient to remove all the 5-HIAA formed in the immature brain. The slow rate of bulk flow of CSF could account for the removal of only a small percentage of the 5-HIAA formed in brain, and active transport of the catabolite from brain capillaries could not be demonstrated. In contrast, the elimination of 5-HIAA via CSF pathways was of minor importance in the 30-day-old rat. More than 75% of the major metabolite of 5-hydroxytryptamine was removed directly from brain to blood by an active transport mechanism, presumably located at the glia-capillary interphase.     

Transport of 5-aminolevulinic acid between blood and brain

Little is known about the movement of 5-aminolevulinic acid (delta-aminolevulinic acid; ALA) between blood and brain. This is despite the fact that increases in brain ALA may be involved in generating the neuropsychiatric symptoms in porphyrias and that systemic administration of ALA is currently being used to delineate the borders of malignant gliomas. The current study examines the mechanisms involved in the movement of [(14)C]ALA across the blood-brain and blood-CSF barriers in the rat. In the adult rat, the influx rate constant (K(i)) for [(14)C]ALA movement into brain was low ( approximately 0.2 microl/g per min), was unaffected by increasing plasma concentrations of non-radioactive ALA or probenecid (an organic anion transport inhibitor) and, therefore, appears to be a diffusional process. The K(i) for [(14)C]ALA was 3-fold less than that for [(14)C]mannitol, a molecule of similar size. This difference appears to result from a lower lipid solubility rather than saturable [(14)C]ALA transport from brain to blood. The K(i) for [(14)C]ALA for uptake into the neonatal brain was 7-fold higher than in the adult. However, again, this was unaffected by increasing plasma ALA concentrations suggesting a diffusional process. In contrast, at the blood-CSF barrier, there was evidence of carrier-mediated [(14)C]ALA transport from blood to choroid plexus and blood to CSF. Both processes were inhibited by administration of non-radioactive ALA and probenecid. However, experiments in choroid plexus epithelial cell primary cultures indicated that transport in these cells was polarized with [(14)C]ALA uptake from the apical (CSF) side being about 7-fold greater than uptake from the basolateral (blood) side. In total, these results suggest that the brain is normally fairly well protected from changes in plasma ALA concentration by the very low blood-brain barrier permeability of this compound and by a saturable efflux mechanism present at the choroid plexus.     

Radioiodide uptake in brain, CSF, thyroid, and salivary glands of audiogenic seizure mice

DBA/2J (DBA) mice are susceptible to audiogenic seizures (ASs) in an age-dependent manner. Anion transport as measured by radioiodide uptake was determined in thyroid gland, salivary gland, skeletal muscle, cerebral cortex, cerebellum, brainstem, and CSF from these mice at various ages. Anion transport was also determined in C57BL/6J(C57) mice, an AS-resistant strain. In thyroid, DBA mice had an enhanced ability to concentrate iodide at 21 days of age when they have maximal AS susceptibility, as compared with the same-aged C57 mice. This difference in thyroid function was less marked at 40 days of age, when DBA mice are less AS susceptible, and was absent at 110 days of age, when DBA mice are AS resistant. In brain, differences in iodide uptake were also noted between these two strains of mice at 21 days of age. DBA mice had an increased concentration of iodide in CSF, an indication that they have a defect in the transport of iodide out of the CSF across the choroid plexus. In addition, DBA mice had a lower ratio of cerebral cortex to CSF iodide, which suggests that DBA mice have a defect in the transport of this anion into cerebral cortical cells from brain interstitial fluid. These differences in iodide transport in brain decreased with age as the AS susceptibility of DBA mice decreased. These results suggest a relation between anion transport in thyroid gland, cerebral cortex, and choroid plexus and AS susceptibility in DBA mice at 21 days of age.     

The uptake of delta9-tetrahydrocannabinol in choroid plexus and brain cortex in vitro and in vivo.

[3H]delta9 Tetrahydrocannabinol (delta9-THC) was actively transported by the choroid plexus and cerebral cortical slices of the rabbit when incubated as a BSA-microsuspension in artificial rabbit CSF. The transport system for delta9-THC in choroid plexus had a V max of 174 nmoles/mg tissue/h, approximately 9-fold greater than that observed for cortical slices. In vivo experiments demonstrated a preferential distribution of delta9-THC in choroid plexus at 1 h after intravenous injection. These results indicate that delta9-THC is actively accumulated by choroidal epithelium and may also be transported across the epithelial stroma into the capillary circulation. This suggests that the choroid plexus participates in the regulation of delta9-THC concentration in CSF and indirectly in brain by means of the "sink" function of the CSF.     

Evidence for ascorbic acid transport system in rat brain capillaries

Although ascorbic acid (AA) crosses the choroid plexus and may enter the brain at an appreciable rate, it is not clearly established that there exist transport system(s) carrying this vitamin from blood into the brain cells across the brain capillaries. Thus the rate of its uptake by choroid plexus and cerebral capillaries were evaluated in vitro in this study. Choroid plexus and brain capillaries were isolated from two-month-old male Sprague-Dawley rats. Time course of AA incorporation in micro vessels and choroid plexus was studied up to 30 min. After stopping the incorporation with the excess of cold isotonic saline, micro vessels were filtered and sonicated. The intracellular incorporated AA radioactivity was measured by liquid scintillation counting. AA uptake by micro vessel was tested for Na+-dependence and saturability. The time course studies showed linear increase in total uptake and accumulation of AA by choroid plexus and endothelial cells up to 30 min. Treatment with oubain or replacement with sodium chloride showed that uptake is an Na+- independent process. Transport of AA to cerebrospinal fluid and brain was also shown to be readily saturated by increasing the level of cold AA. These results document that the brain capillary endothelial cells are able to transport and accumulate AA, and may have a critical role in the homeostasis and regulation of cerebral ascorbic acid concentration.     

Choline and PAH transport across blood-CSF barriers: The effect of lithium

The components of the blood-CSF barrier responsible for the transport of p-aminohippuric acid (PAH) and choline from CSF to blood were identified using in vitro preparations of frog choroid plexus and arachnoid membranes. Choline was transported out of CSF across the arachnoid, while PAH was transported out across the choroid plexus. Probenecid and ouabain blocked both processes. The effect of Li on these transport processes was tested by the addition of 5 mM LiCl to the incubation media. Li increased, by a factor of two, choline transport across the arachnoid, but there was no effect of Li on PAH transport across the plexus. Lithium was passively transported across the choroid plexus, and we suggest that the major transport pathway is through the tight junctions. The steady-state distribution of Li between the choroidal epithelium and the incubation medium was only half that expected for passive distribution. This suggests the existence of sodium/lithium countertransport in these epithelial cell membranes.     

Molecular mechanism of distorted iron regulation in the blood-CSF barrier and regional blood-brain barrier following in vivo subchronic manganese exposure

Previous studies in this laboratory indicated that manganese (Mn) exposure in vitro increases the expression of transferrin receptor (TfR) by enhancing the binding of iron regulatory proteins (IRPs) to iron responsive element-containing RNA. The current study further tested the hypothesis that in vivo exposure to Mn increased TfR expression at both blood-brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier (BCB), which contributes to altered iron (Fe) homeostasis in the CSF. Groups of rats (10-11 each) received oral gavages at doses of 5 mg Mn/kg or 15 mg Mn/kg as MnCl(2) once daily for 30 days. Blood, CSF, and choroid plexus were collected and brain capillary fractions were separated from the regional parenchyma. Metal analyses showed that oral Mn exposure decreased concentrations of Fe in serum (-66%) but increased Fe in the CSF (+167%). Gel shift assay showed that Mn caused a dose-dependent increase of binding of IRP1 to iron responsive element-containing RNA in BCB in the choroid plexus (+70%), in regional BBB of capillaries of striatum (+39%), hippocampus (+56%), frontal cortex (+49%), and in brain parenchyma of striatum (+67%), hippocampus (+39%) and cerebellum (+28%). Real-time RT-PCR demonstrated that Mn exposure significantly increased the expression of TfR mRNA in choroid plexus and striatum with concomitant reduction in the expression of ferritin (Ft) mRNA. Collectively, these data indicate that in vivo Mn exposure results in Fe redistribution in body fluids through regulating the expression of TfR and ferritin at BCB and selected regional BBB. The disrupted Fe transport by brain barriers may underlie the distorted Fe homeostasis in the CSF.