Choroid plexus NKCC1 mediates cerebrospinal fluid clearance during mouse early postnatal development.

Cerebrospinal fluid (CSF) provides vital support for the brain. Abnormal CSF accumulation, such as hydrocephalus, can negatively affect perinatal neurodevelopment. The mechanisms regulating CSF clearance during the postnatal critical period are unclear. Here, we show that CSF K+, accompanied by water, is cleared through the choroid plexus (ChP) during mouse early postnatal development. We report that, at this developmental stage, the ChP showed increased ATP production and increased expression of ATP-dependent K+ transporters, particularly the Na+, K+, Cl-, and water cotransporter NKCC1. Overexpression of NKCC1 in the ChP resulted in increased CSF K+ clearance, increased cerebral compliance, and reduced circulating CSF in the brain without changes in intracranial pressure in mice. Moreover, ChP-specific NKCC1 overexpression in an obstructive hydrocephalus mouse model resulted in reduced ventriculomegaly. Collectively, our results implicate NKCC1 in regulating CSF K+ clearance through the ChP in the critical period during postnatal neurodevelopment in mice.

Glucose, Fructose, and Urate Transporters in the Choroid Plexus Epithelium.

The choroid plexus plays a central role in the regulation of the microenvironment of the central nervous system by secreting the majority of the cerebrospinal fluid and controlling its composition, despite that it only represents approximately 1% of the total brain weight. In addition to a variety of transporter and channel proteins for solutes and water, the choroid plexus epithelial cells are equipped with glucose, fructose, and urate transporters that are used as energy sources or antioxidative neuroprotective substrates. This review focuses on the recent advances in the understanding of the transporters of the SLC2A and SLC5A families (GLUT1, SGLT2, GLUT5, GLUT8, and GLUT9), as well as on the urate-transporting URAT1 and BCRP/ABCG2, which are expressed in choroid plexus epithelial cells. The glucose, fructose, and urate transporters repertoire in the choroid plexus epithelium share similar features with the renal proximal tubular epithelium, although some of these transporters exhibit inversely polarized submembrane localization. Since choroid plexus epithelial cells have high energy demands for proper functioning, a decline in the expression and function of these transporters can contribute to the process of age-associated brain impairment and pathophysiology of neurodegenerative diseases.

Angiomodulin (IGFBP7) is a cerebral specific angiocrine factor, but is probably not a blood-brain barrier inducer.

BACKGROUND: Several secreted factors have been identified as drivers of cerebral vasculature development and inducers of blood-brain barrier (BBB) differentiation. Vascular endothelial growth factor A (VEGF-A) is central for driving cerebral angiogenesis and Wnt family factors (Wnt7a, Wnt7b and norrin) are central for induction and maintenance of barrier properties. Expressed by developing neural tissue (neuron and glia progenitors), they influence the formation of central nervous system (CNS) vascular networks. Another type of factors are tissue-specific paracrine factors produced by endothelial cells (ECs), also known as 'angiocrine' factors, that provide instructive signals to regulate homeostatic and regenerative processes. Very little is known about CNS angiocrine factors and their role in BBB development. Angiomodulin (AGM) was reported to be expressed by developing vasculature and by pathological tumor vasculature. Here we investigated AGM in the developing CNS and its function as a potential BBB inducer. METHODS: We analyzed microarray data to identify potential angiocrine factors specifically expressed at early stages of barrier formation. We then tested AGM expression with immunofluorescence and real-time PCR in various organs during development, post-natal and in adults. Permeability induction with recombinant proteins (Miles assay) was used to test potential interaction of AGM with VEGF-A. RESULTS: Several angiocrine factors are differentially expressed by CNS ECs and AGM is a prominent CNS-specific angiocrine candidate. Contrary to previous reports, we found that AGM protein expression is specific to developing CNS endothelium and not to highly angiogenic developing vasculature in general. In skin vasculature we found that AGM antagonizes VEGF-A-induced vascular hyperpermeability. Finally, CNS AGM expression is not specific to BBB vasculature and AGM is highly expressed in non-BBB choroid-plexus vasculature. CONCLUSIONS: We propose AGM as a developmental CNS vascular-specific marker. AGM is not a pan-endothelial marker, nor a general marker for developing angiogenic vasculature. Thus, AGM induction in the developing CNS might be distinct from its induction in pathology. While AGM is able to antagonize VEGF-A-induced vascular hyperpermeability in the skin, its high expression levels in non-BBB CNS vasculature does not support its potential role as a BBB inducer. Further investigation including loss-of-function approaches might elucidate AGM function in the developing CNS.

Accurate, strong, and stable reporting of choroid plexus epithelial cells in transgenic mice using a human transthyretin BAC.

BACKGROUND: Choroid plexus epithelial cells express high levels of transthyretin, produce cerebrospinal fluid and many of its proteins, and make up the blood-cerebrospinal fluid barrier. Choroid plexus epithelial cells are vital to brain health and may be involved in neurological diseases. Transgenic mice containing fluorescent and luminescent reporters of these cells would facilitate their study in health and disease, but prior transgenic reporters lost expression over the early postnatal period. METHODS: Human bacterial artificial chromosomes in which the transthyretin coding sequence was replaced with DNA for tdTomato or luciferase 2 were used in pronuclear injections to produce transgenic mice. These mice were characterized by visualizing red fluorescence, immunostaining, real-time reverse transcription polymerase chain reaction, and luciferase enzyme assay. RESULTS: Reporters were faithfully expressed in cells that express transthyretin constitutively, including choroid plexus epithelial cells, retinal pigment epithelium, pancreatic islets, and liver. Expression of tdTomato in choroid plexus began at the appropriate embryonic age, being detectable by E11.5. Relative levels of tdTomato transcript in the liver and choroid plexus paralleled relative levels of transcripts for transthyretin. Expression remained robust over the first postnatal year, although choroid plexus transcripts of tdTomato declined slightly with age whereas transthyretin remained constant. TdTomato expression patterns were consistent across three founder lines, displayed no sex differences, and were stable across several generations. Two of the tdTomato lines were bred to homozygosity, and homozygous mice are healthy and fertile. The usefulness of tdTomato reporters in visualizing and analyzing live Transwell cultures was demonstrated. Luciferase activity was very high in homogenates of choroid plexus and continued to be expressed through adulthood. Luciferase also was detectable in eye and pancreas. CONCLUSIONS: Transgenic mice bearing fluorescent and luminescent reporters of transthyretin should prove useful for tracking transplanted choroid plexus epithelial cells, for purifying the cells, and for reporting their derivation from stem cells. They also should prove useful for studying transthyretin synthesis by other cell types, as transthyretin has been implicated in many functions and conditions, including clearance of ß-amyloid peptides associated with Alzheimer's disease, heat shock in neurons, processing of neuropeptides, nerve regeneration, astrocyte metabolism, and transthyretin amyloidosis.

The ESCRT-III Protein CHMP1A Mediates Secretion of Sonic Hedgehog on a Distinctive Subtype of Extracellular Vesicles.

Endosomal sorting complex required for transport (ESCRT) complex proteins regulate biogenesis and release of extracellular vesicles (EVs), which enable cell-to-cell communication in the nervous system essential for development and adult function. We recently showed human loss-of-function (LOF) mutations in ESCRT-III member CHMP1A cause autosomal recessive microcephaly with pontocerebellar hypoplasia, but its mechanism was unclear. Here, we show Chmp1a is required for progenitor proliferation in mouse cortex and cerebellum and progenitor maintenance in human cerebral organoids. In Chmp1a null mice, this defect is associated with impaired sonic hedgehog (Shh) secretion and intraluminal vesicle (ILV) formation in multivesicular bodies (MVBs). Furthermore, we show CHMP1A is important for release of an EV subtype that contains AXL, RAB18, and TMED10 (ART) and SHH. Our findings show CHMP1A loss impairs secretion of SHH on ART-EVs, providing molecular mechanistic insights into the role of ESCRT proteins and EVs in the brain.

Glutathione Conjugation at the Blood-CSF Barrier Efficiently Prevents Exposure of the Developing Brain Fluid Environment to Blood-Borne Reactive Electrophilic Substances.

Exposure of the developing brain to toxins, drugs, or deleterious endogenous compounds during the perinatal period can trigger alterations in cell division, migration, differentiation, and synaptogenesis, leading to lifelong neurological impairment. The brain is protected by cellular barriers acting through multiple mechanisms, some of which are still poorly explored. We used a combination of enzymatic assays, live tissue fluorescence microscopy, and an in vitro cellular model of the blood-CSF barrier to investigate an enzymatic detoxification pathway in the developing male and female rat brain. We show that during the early postnatal period the choroid plexus epithelium forming the blood-CSF barrier and the ependymal cell layer bordering the ventricles harbor a high detoxifying capacity that involves glutathione S-transferases. Using a functional knock-down rat model for choroidal glutathione conjugation, we demonstrate that already in neonates, this metabolic pathway efficiently prevents the penetration of blood-borne reactive compounds into CSF. The versatility of the protective mechanism results from the multiplicity of the glutathione S-transferase isoenzymes, which are differently expressed between the choroidal epithelium and the ependyma. The various isoenzymes display differential substrate specificities, which greatly widen the spectrum of molecules that can be inactivated by this pathway. In conclusion, the blood-CSF barrier and the ependyma are identified as key cellular structures in the CNS to protect the brain fluid environment from different chemical classes of potentially toxic compounds during the postnatal period. This metabolic neuroprotective function of brain interfaces ought to compensate for the liver postnatal immaturity.SIGNIFICANCE STATEMENT Brain homeostasis requires a stable and controlled internal environment. Defective brain protection during the perinatal period can lead to lifelong neurological impairment. We demonstrate that the choroid plexus forming the blood-CSF barrier is a key player in the protection of the developing brain. Glutathione-dependent enzymatic metabolism in the choroidal epithelium inactivates a broad spectrum of noxious compounds, efficiently preventing their penetration into the CSF. A second line of detoxification is located in the ependyma separating the CSF from brain tissue. Our study reveals a novel facet of the mechanisms by which the brain is protected at a period of high vulnerability, at a time when the astrocytic network is still immature and liver xenobiotic metabolism is limited.

Highly segregated localization of the functionally related vps10p receptors sortilin and SorCS2 during neurodevelopment.

Nervous system development is a precisely orchestrated series of events requiring a multitude of intrinsic and extrinsic cues. Sortilin and SorCS2 are members of the Vps10p receptor family with complementary influence on some of these cues including the neurotrophins (NTs). However, the developmental time points where sortilin and SorCS2 exert their activities in conjunction or independently still remain unclear. In this study we present the characterization of the spatiotemporal expression pattern of sortilin and SorCS2 in the developing murine nervous system. Sortilin is highly expressed in the fetal nervous system with expression localized to distinct cell populations. Expression was high in neurons of the cortical plate and developing allocortex, as well as subpallial structures. Furthermore, the neuroepithelium lining the ventricles and the choroid plexus showed high expression of sortilin, together with the developing retina, spinal ganglia, and sympathetic ganglia. In contrast, SorCS2 was confined in a marked degree to the thalamus and, at E13.5, the floor plate from midbrain rostrally to spinal cord caudally. SorCS2 was also found in the ventricular zones of the ventral hippocampus and nucleus accumbens areas, in the meninges and in Schwann cells. Hence, sortilin and SorCS2 are extensively present in several distinct anatomical areas in the developing nervous system and are rarely co-expressed. Possible functions of sortilin and SorCS2 pertain to NT signaling, axon guidance and beyond. The present data will form the basis for hypotheses and study designs for unravelling the functions of sortilin and SorCS2 during the establishment of neuronal structures and connections.

The Type I Interferon Response Determines Differences in Choroid Plexus Susceptibility between Newborns and Adults in Herpes Simplex Virus Encephalitis.

UNLABELLED: Newborns are significantly more susceptible to severe viral encephalitis than adults, with differences in the host response to infection implicated as a major factor. However, the specific host signaling pathways responsible for differences in susceptibility and neurologic morbidity have remained unknown. In a murine model of HSV encephalitis, we demonstrated that the choroid plexus (CP) is susceptible to herpes simplex virus 1 (HSV-1) early in infection of the newborn but not the adult brain. We confirmed susceptibility of the CP to HSV infection in a human case of newborn HSV encephalitis. We investigated components of the type I interferon (IFN) response in the murine brain that might account for differences in cell susceptibility and found that newborns have a dampened interferon response and significantly lower basal levels of the alpha/beta interferon (IFN-α/ß) receptor (IFNAR) than do adults. To test the contribution of IFNAR to restricting infection from the CP, we infected IFNAR knockout (KO) adult mice, which showed restored CP susceptibility to HSV-1 infection in the adult. Furthermore, reduced IFNAR levels did not account for differences we found in the basal levels of several other innate signaling proteins in the wild-type newborn and the adult, including protein kinase R (PKR), that suggested specific regulation of innate immunity in the developing brain. Viral targeting of the CP, a region of the brain that plays a critical role in neurodevelopment, provides a link between newborn susceptibility to HSV and long-term neurologic morbidity among survivors of newborn HSV encephalitis. IMPORTANCE: Compared to adults, newborns are significantly more susceptible to severe disease following HSV infection. Over half of newborn HSV infections result in disseminated disease or encephalitis, with long-term neurologic morbidity in 2/3 of encephalitis survivors. We investigated differences in host cell susceptibility between newborns and adults that contribute to severe central nervous system disease in the newborn. We found that, unlike the adult brain, the newborn choroid plexus (CP) was susceptible early in HSV-1 infection. We demonstrated that IFN-α/ß receptor levels are lower in the newborn brain than in the adult brain and that deletion of this receptor restores susceptibility of the CP in the adult brain. The CP serves as a barrier between the blood and the cerebrospinal fluid and plays a role in proper neurodevelopment. Susceptibility of the newborn choroid plexus to HSV-1 has important implications in viral spread to the brain and, also, in the neurologic morbidity following HSV encephalitis.

Development and functions of the choroid plexus-cerebrospinal fluid system.

The choroid plexus (ChP) is the principal source of cerebrospinal fluid (CSF), which has accepted roles as a fluid cushion and a sink for nervous system waste in vertebrates. Various animal models have provided insights into how the ChP-CSF system develops and matures. In addition, recent studies have uncovered new, active roles for this dynamic system in the regulation of neural stem cells, critical periods and the overall health of the nervous system. Together, these findings have brought about a paradigm shift in our understanding of brain development and health, and have stimulated new initiatives for the treatment of neurological disease.

Expression of tight junction proteins and transporters for xenobiotic metabolism at the blood-CSF barrier during development in the nonhuman primate (P. hamadryas).

The choroid plexus (CP) is rich in barrier mechanisms including transporters and enzymes which can influence drug disposition between blood and brain. We have limited knowledge of their state in fetus. We have studied barrier mechanisms along with metabolism and transporters influencing xenobiotics, using RNAseq and protein analysis, in the CP during the second-half of gestation in a nonhuman primate (Papio hamadryas). There were no differences in the expression of the tight-junctions at the CP suggesting a well-formed fetal blood-CSF barrier during this period of gestation. Further, the fetal CP express many enzymes for phase I-III metabolisms as well as transporters suggesting that it can greatly influence drug disposition and has a significant machinery to deactivate reactive molecules with only minor gestational changes. In summary, the study suggests that from, at least, midgestation, the CP in the nonhuman primate is restrictive and express most known genes associated with barrier function and transport.

Selenoprotein P and apolipoprotein E receptor-2 interact at the blood-brain barrier and also within the brain to maintain an essential selenium pool that protects against neurodegeneration.

Selenoprotein P (Sepp1) and its receptor, apolipoprotein E receptor 2 (apoER2), account for brain retaining selenium better than other tissues. The primary sources of Sepp1 in plasma and brain are hepatocytes and astrocytes, respectively. ApoER2 is expressed in varying amounts by tissues; within the brain it is expressed primarily by neurons. Knockout of Sepp1 or apoER2 lowers brain selenium from ∼120 to ∼50 ng/g and leads to severe neurodegeneration and death in mild selenium deficiency. Interactions of Sepp1 and apoER2 that protect against this injury have not been characterized. We studied Sepp1, apoER2, and brain selenium in knockout mice. Immunocytochemistry showed that apoER2 mediates Sepp1 uptake at the blood-brain barrier. When Sepp1(-/-) or apoER2(-/-) mice developed severe neurodegeneration caused by mild selenium deficiency, brain selenium was ∼35 ng/g. In extreme selenium deficiency, however, brain selenium of ∼12 ng/g was tolerated when both Sepp1 and apoER2 were intact in the brain. These findings indicate that tandem Sepp1-apoER2 interactions supply selenium for maintenance of brain neurons. One interaction is at the blood-brain barrier, and the other is within the brain. We postulate that Sepp1 inside the blood-brain barrier is taken up by neurons via apoER2, concentrating brain selenium in them.

Ontogeny of inter-alpha inhibitor proteins in ovine brain and somatic tissues.

Inter-alpha inhibitor proteins (IAIPs) found in relatively high concentrations in human plasma are important in inflammation. IAIPs attenuate brain damage in young and adult subjects, decrease during sepsis and necrotizing enterocolitis in premature infants, and attenuate sepsis-related inflammation in newborn rats. Although a few studies have reported adult organ-specific IAIP expression, information is not available on age-dependent IAIP expression. Given evidence suggesting IAIPs attenuate brain damage in young and adult subjects, and inflammation in newborns, we examined IAIP expression in plasma, cerebral cortex (CC), choroid plexus (CP), cerebral spinal fluid (CSF), and somatic organs in fetal, newborn, and adult sheep to determine the endogenous expression patterns of these proteins during development. IAIPs (enzyme-linked immunosorbent assay) were higher in newborn and adult than fetal plasma (P < 0.05). Western immunoblot detected 125 kDa PaI (Pre-alpha Inhibitor) and 250 kDa IaI (Inter-alpha Inhibitor) in plasma, CNS, and somatic organs. PaI expression in CC and CP was higher in fetuses than newborns and adults, but IaI expression was higher in adults than fetuses and newborns. Both PaI and IaI were higher in fetal than newborn CSF. IAIPs exhibited organ-specific ontogenic patterns in placenta, liver, heart, and kidney. These results provide evidence for the first time that plasma, brain, placenta, liver, heart, and kidney express IAIPs throughout ovine development and that expression patterns are unique to each organ. Although exact functions of IAIPs in CNS and somatic tissues are not known, their presence in relatively high amounts during development suggests their potential importance in brain and organ development.

Cortical hypoplasia and ventriculomegaly of p73-deficient mice: Developmental and adult analysis.

Trp73, a member of the p53 gene family, plays a crucial role in neural development. We describe two main phenotypic variants of p73 deficiency in the brain, a severe one characterized by massive apoptosis in the cortex leading to early postnatal death and a milder, non-/low-apoptosis one in which 50% of pups may reach adulthood using an intensive-care breeding protocol. Both variants display the core triad of p73 deficiency: cortical hypoplasia, hippocampal malformations, and ventriculomegaly. We studied the development of the neocortex in p73 KO mice from early embryonic life into advanced age (25 months). Already at E14.5, the incipient cortical plate of the p73 KO brains showed a reduced width. Examination of adult neocortex revealed a generalized, nonprogressive reduction by 10-20%. Area-specific architectonic landmarks and lamination were preserved in all cortical areas. The surviving adult animals had moderate ventricular distension, whereas pups of the early lethal phenotypic variant showed severe ventriculomegaly. Ependymal cells of wild-type ventricles strongly express p73 and are particularly vulnerable to p73 deficiency. Ependymal denudation by apoptosis and reduction of ependymal cilia were already evident in young mice, with complete absence of cilia in older animals. Loss of p73 function in the ependyma may thus be one determining factor for chronic hydrocephalus, which leads to atrophy of subcortical structures (striatum, septum, amygdala). p73 Is thus involved in a variety of CNS activities ranging from embryonic regulation of brain size to the control of cerebrospinal fluid homeostasis in the adult brain via maintenance of the ependyma.

The transcription factor Otx2 regulates choroid plexus development and function.

The choroid plexuses (ChPs) are the main regulators of cerebrospinal fluid (CSF) composition and thereby also control the composition of a principal source of signaling molecules that is in direct contact with neural stem cells in the developing brain. The regulators of ChP development mediating the acquisition of a fate that differs from the neighboring neuroepithelial cells are poorly understood. Here, we demonstrate in mice a crucial role for the transcription factor Otx2 in the development and maintenance of ChP cells. Deletion of Otx2 by the Otx2-CreERT2 driver line at E9 resulted in a lack of all ChPs, whereas deletion by the Gdf7-Cre driver line affected predominately the hindbrain ChP, which was reduced in size, primarily owing to an increase in apoptosis upon Otx2 deletion. Strikingly, Otx2 was still required for the maintenance of hindbrain ChP cells at later stages when Otx2 deletion was induced at E15, demonstrating a central role of Otx2 in ChP development and maintenance. Moreover, the predominant defects in the hindbrain ChP mediated by Gdf7-Cre deletion of Otx2 revealed its key role in regulating early CSF composition, which was altered in protein content, including the levels of Wnt4 and the Wnt modulator Tgm2. Accordingly, proliferation and Wnt signaling levels were increased in the distant cerebral cortex, suggesting a role of the hindbrain ChP in regulating CSF composition, including key signaling molecules. Thus, Otx2 acts as a master regulator of ChP development, thereby influencing one of the principal sources of signaling in the developing brain, the CSF.

SPARC/osteonectin, an endogenous mechanism for targeting albumin to the blood-cerebrospinal fluid interface during brain development.

Specialized populations of choroid plexus epithelial cells have previously been shown to be responsible for the transfer of individual plasma proteins from blood to the cerebrospinal fluid (CSF), contributing to their characteristically high concentrations in CSF of the developing brain. The mechanism of this protein transfer remains elusive. Using a marsupial, Monodelphis domestica, we demonstrate that the albumin-binding protein SPARC (osteonectin/BM-40/culture-shock protein) is present in a subset of choroid plexus epithelial cells from its first appearance, throughout development, and into adulthood. The synthesis of SPARC by the lateral ventricular plexus was confirmed with real-time PCR. The expression level of SPARC was higher in plexuses of younger than older animals. Western blot analysis of the gene product confirmed the quantitative PCR results. The co-localization of SPARC and albumin shown by immunocytochemistry and its cellular location indicate that this glycoprotein may act as a recognition site for albumin. In addition, the numbers of SPARC-immunopositive cells and its expression were responsive to experimental changes of albumin concentration in the blood. It is suggested that SPARC may be one of the molecules that govern the uptake and delivery of proteins from blood to the CSF. The results also confirm that protein transfer across the blood-CSF barrier is developmentally and physiologically regulated.

Change of neural stem cells in the choroid plexuses of developing rat.

Neural stem cells reside in various brain regions. However, neural stem cells in the choroid plexuses are poorly understood. This study was conducted to corroborate the hypotheses that there are neural stem cells in the choroid plexuses, and the change of neural stem cells is age dependent. We examined neural stem cells from rats at postnatal 1, 3, 7 days, 2, 4, 6, and 8 weeks to investigate the distribution and change of the cells in the choroid plexuses. We found nestin-positive cells in the choroid plexuses and these nestin-expressing cells were located principally at the boundary between the choroid plexus epithelium and the underlying connective tissue stroma. Some choroid plexuses of the postnatal 1-, 3-, 7-day, and 2-week rats were stained with line-like markers of nestin. We also observed nestin-positive cells in 4-week rats, but no such cells were detected in the 6- and 8-week rats. These findings indicate that neural stem cells exist in the rat choroid plexuses, and the change of neural stem cells is age-dependent.

Lack of cadherins Celsr2 and Celsr3 impairs ependymal ciliogenesis, leading to fatal hydrocephalus.

Ependymal cells form the epithelial lining of cerebral ventricles. Their apical surface is covered by cilia that beat in a coordinated fashion to facilitate circulation of the cerebrospinal fluid (CSF). The genetic factors that govern the development and function of ependymal cilia remain poorly understood. We found that the planar cell polarity cadherins Celsr2 and Celsr3 control these processes. In Celsr2-deficient mice, the development and planar organization of ependymal cilia are compromised, leading to defective CSF dynamics and hydrocephalus. In Celsr2 and Celsr3 double mutant ependyma, ciliogenesis is markedly impaired, resulting in lethal hydrocephalus. The membrane distribution of Vangl2 and Fzd3, two key planar cell polarity proteins, was disturbed in Celsr2 mutants, and even more so in Celsr2 and Celsr3 double mutants. Our findings suggest that planar cell polarity signaling is involved in ependymal cilia development and in the pathophysiology of hydrocephalus, with possible implications in other ciliopathies.

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. Abeta 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.

Cellular transfer of macromolecules across the developing choroid plexus of Monodelphis domestica.

Choroid plexus epithelial cells secrete cerebrospinal fluid (CSF) and transfer molecules from blood into CSF. Tight junctions between choroidal epithelial cells are functionally effective from early in development: the route of transfer is suggested to be transcellular. Routes of transfer for endogenous and exogenous plasma proteins and dextrans were studied in Monodelphis domestica (opossum). Pups at postnatal (P) days 1-65 and young adults were injected with biotinylated dextrans (3-70 kDa) and/or foetal protein fetuin. CSF, plasma and brain samples were collected from terminally anaesthetized animals. Choroid plexus cells containing plasma proteins were detected immunocytochemically. Numbers of plasma protein-positive epithelial cells increased to adult levels by P28, but their percentage of plexus cells declined. Numbers of cells positive for biotinylated probes increased with age, while their percentage remained constant. Colocalization studies showed specificity for individual proteins in some epithelial cells. Biotinylated probes and endogenous proteins colocalized in about 10% of cells in younger animals, increasing towards 100% by adulthood. Injections of markers into the ventricles demonstrated that protein is transferred only from blood into CSF, whereas dextrans pass in both directions. These results indicate that protein and lipid-insoluble markers are transferred by separate mechanisms present in choroid plexuses from the earliest stage of brain development, and transfer of proteins from plasma across choroid plexus epithelial cells contributes to the high protein concentration in CSF in the immature brain.