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.     

Epithelial cell polarity as reflected in enterocytes

Absorptive cells are the main cells present in the intestinal epithelium. The plasma membrane of these tall columnar cells reflects their high degree of polarization, by dividing into apical and basolateral domains with different compositions. The most characteristic structure of these cells consists of closely packed apical microvilli with the same height, looking like a brush, which is why they were named the brush border. The concentrated pattern of some apical markers observed in a restricted brush border domain shows that mature enterocytes are hyperpolarized epithelial cells: the filamentous brush border glycocalyx is anchored at the top of the microvilli and the annexin XIII is concentrated in the lower three fourths. Many studies have been carried out on the biosynthesis and intracellular pathway of domain markers. The results show clearly that the basolateral markers take a direct pathway from the trans-Golgi network to the basolateral membrane. However, the two apical pathways, one direct and one indirect pathway via the basolateral membrane, are used, depending on the apical protein involved. Efficient protein sorting and addressing are essential to the establishment and maintenance of cell polarity, on which the integrity of the epithelial barrier depends.     

Porcine Choroid plexus epithelial cells in culture: regulation of barrier properties and transport processes

The epithelial cells of the choroid plexus are the structural basis of the blood-cerebrospinal fluid (CSF)-barrier. Here we summarise our recent efforts to culture those cells mainly on permeable supports in vitro. Isolated from porcine brains, we report a simple protocol for the primary culture using cytosine arabinoside as an additive that is cytotoxic for other cells except the plexus epithelial cells. Enhanced barrier properties are obtained by withdrawal of serum from the culture medium after confluency is reached. Cells improve their polarity, permeability for hydrophilic substrates is lowered, electrical resistance is increased tenfold, and a pH-gradient is built up across the cell monolayer. Polarised secretion of proteins and most importantly fluid secretion into the apical filter compartment was attained and proven to be dependent on the Na(+),K(+)-ATPase activity. Active transport processes (penicillin G, riboflavin, myo-inositol, ascorbic acid) were studied and clearly showed the involvement of the organic anion transporter. The permeability of the barrier was found to be regulated by cyclic adenosine monophosphate (cAMP). Moreover, we report that cell proliferation and differentiation is controlled by components of the extracellular matrix. The present culture model could now be used as an in vitro system to quantify drug transport across the blood-CSF-barrier.     

Junctional complexes and cell polarity in the urinary tubule

In this review, we demonstrate how differentiated membrane domains can be detected in epithelial cells using conventional light and electron microscopy, freeze-fracture electron microscopy and the immunoand cytochemical detection of membrane components. Using specific examples from the kidney, we show how the polarized insertion of these components into either apical or basolateral plasma membrane regions on either side of the tight junction barrier is related to specific functions of principal and intercalated cells in the collecting duct. In addition, distinct basal and lateral membrane domains have been revealed in some cells that are maintained in the absence of a tight junctional barrier in the plane of the membrane. This suggests that other factors, possibly related to cytoskeletal elements, may be involved in the functional segregation of these membrane areas. We propose that epithelial cell plasma membranes should be subdivided into apical, lateral and basal regions, and that the term “basolateral” may be an oversimplification.     

Searching for specific binding sites of the secretory glycoproteins of the subcommissural organ

The molecular organization of Reissner's fiber (RF), the structure of its proteins, and the permanent turnover of these proteins are all facts supporting the possibility that RF may perform multiple functions. There is evidence that CSF-soluble RF-glycoproteins may occur under physiological conditions. The present investigation was designed to investigate the probable existence within the CNS of specific binding sites for RF-glycoproteins. Three experimental protocols were used: (1) immunocytochemistry of the CNS of bovine fetuses using anti-idiotypic antibodies, raised against monoclonal antibodies developed against bovine RF-glycoproteins; (2) in vivo binding of the RF glycoproteins, perfusing into the rat CSF 125I-labeled RF-glycoproteins, or grafting SCO into a lateral ventricle of the rat; (3) in vitro binding of unlabeled RF-glycoproteins to rat and bovine choroid plexuses maintained in culture. One of the anti-idiotypic antibody generated by a Mab raised against RF-glycoproteins binds to choroidal cells. Furthermore, binding of RF-glycoproteins to the rat choroid plexus was obtained when: (1) the choroid plexus was cultured in the presence of unlabeled RF-glycoproteins; (2) the concentration of soluble RF-glycoproteins in the CSF was increased by isografting SCOs into a lateral ventricle; (3) radiolabeled glycoproteins were perfused into the ventricular CSF. This evidence suggests that the apical plasma membrane of the ependymal cells of the choroid plexus has specific binding sites for RF-glycoproteins, of unknown functional significance. The radiolabeled RF-glycoproteins perfused into the rat CSF also bound to the paraventricular thalamic nucleus, the floor of the Sylvian aqueduct and of the rostral half of the fourth ventricle, and the meninges of the brain and spinal cord. The labeling of the paraventricular thalamic nucleus points to a functional relationship between this nucleus and the SCO. The possibility that the SCO may be a component of the circadian timing system is discussed.     

Improved visualization and quantitative analysis of fluorescent membrane sterol in polarized hepatic cells

Dehydroergosterol is a natural yeast sterol which has recently been employed for direct observation of intracellular sterol transport by UV microscopy. Here, methods are described for improved visualization and quantification of dehydroergosterol in the membranes of polarized HepG2 cells. Using a new online assay, it is shown that dehydroergosterol derived from a cyclodextrin complex inserted into the plasma membrane with a half time of t_1/2 ∼ 34 s. Based on a detailed bleaching analysis of dehydroergosterol, slightly different bleaching rates for dehydroergosterol in the basolateral and canalicular membrane were found, indicating different fluorophore environments. Bleaching correction in concert with 3D imaging allows for detection of dehydroergosterol enrichment in microvilli of the canalicular membrane forming the biliary canaliculus. Evidence is provided that some dehydroergosterol accumulating in a subapical compartment or apical recycling compartment can rapidly (t_1/2 ∼ 2 min) exchange in vesicles towards the biliary canaliculus while the majority of dehydroergosterol does not redistribute from this compartment. The rapidly exchanging pool resembles only a small portion of the total subapical compartment or apical recycling compartment-associated dehydroergosterol (about 15–30%). Kinetic modelling supports the theory that the subapical compartment or apical recycling compartment to biliary canaliculus transport pathway for sterol is unidirectional. This pathway might be important for rapid biliary transport of free sterol produced by hydrolysis of cholesteryl esters derived from high density lipoprotein.     

Involvement of the choroid plexus in central nervous system inflammation

During inflammatory conditions in the central nervous system (CNS), immune cells immigrate into the CNS and can be detected in the CNS parenchyma and in the cerebrospinal fluid (CSF). The most comprehensively investigated model for CNS inflammation is experimental autoimmune encephalomyelitis (EAE), which is considered the prototype model for the human disease multiple sclerosis (MS). In EAE autoagressive CD4(+), T cells gain access to the CNS and initiate the molecular and cellular events leading to edema, inflammation, and demyelination in the CNS. The endothelial blood-brain barrier (BBB) has been considered the obvious place of entry for the circulating immune cells into the CNS. A role of the choroid plexus in the pathogenesis of EAE or MS, i.e., as an alternative entry site for circulating lymphocytes directly into the CSF, has not been seriously considered before. However, during EAE, we observed massive ultrastructural changes within the choroid plexus, which are different from changes observed during hypoxia. Using immunohistochemistry and in situ hybridization, we observed expression of VCAM-1 and ICAM-1 in the choroid plexus and demonstrated their upregulation and also de novo expression of MAdCAM-1 during EAE. Ultrastructural studies revealed polar localization of ICAM-1, VCAM-1, and MAdCAM-1 on the apical surface of choroid plexus epithelial cells and their complete absence on the fenestrated endothelial cells within the choroid plexus parenchyme. Furthermore, ICAM-1, VCAM-1, and MAdCAM-1 expressed in choroid plexus epithelium mediated binding of lymphocytes via their known ligands. In vitro, choroid plexus epithelial cells can be induced to express ICAM-1, VCAM-1, MAdCAM-1, and, additionally, MHC class I and II molecules on their surface. Taken together, our observations imply a previously unappreciated function of the choroid plexus in the immunosurveillance of the CNS.     

Cell death in the choroid plexus following transient forebrain global ischemia in the rat

Following a complete disruption of blood flow to the brain, cerebral ischemia, a specific neuronal population, namely the CA1 pyramidal neurons in the hippocampus, will die a delayed type of cell death. This is often referred to as "delayed neuronal death" (DND). It is not known why it takes around 48 hours for these cells to die. It is very often speculated that events, intrinsic to the CA1 neurons, regulate their demise, whereas it is less often considered that extrinsic mechanisms also could play an important role for the development of DND. We discovered that in addition to the CA1 pyramidal neurons, cells in the choroid plexus were TUNEL (terminaldeoxynucleotidyl-mediated biotin-dUTP nick-end labeling)-positive following transient forebrain global ischemia. The time course and the number of TUNEL-positive cells were determined. A dramatic increase in the number of TUNEL-positive cells in the choroid plexus was seen at 18, 24, and at 36 hours of recovery, but not at 48 hours of recovery following 15 minutes of transient forebrain global ischemia. No TUNEL-positive cells were seen at 24 hours of recovery in the CA1 region. The cell death in the choroid plexus thus preceded the occurrence of cell death in the CA1 region. Massive cell death in the choroid plexus will inevitably lead to a leaky blood-CSF barrier, which in turn will allow substances to enter the ventricular system and from there reach the brain parenchyma. We, therefore, conclude that choroid plexus cell death may adversely affect the outcome of CA1 pyramidal neurons following transient forebrain global ischemia, through, e.g., a disruption of the blood-cerebro spinal fluid barrier. Alternatively, the choroid plexus may produce factors, which can affect the outcome of neurons.     

Tumors of the choroid plexus

Choroid plexus tumors are rare intraventricular papillary neoplasms derived from choroid plexus epithelium, which account for only between 0.4-0.6% of all intracranial and 2-3% of pediatric neoplasms. Plexus papillomas outnumber choroid plexus carcinomas by a ratio of 5:1 and around 80% of choroid plexus carcinomas arise in children. Plexus tumors are most common in the lateral and fourth ventricles; while 80% of lateral ventricle tumors present in children, fourth ventricle tumors are evenly distributed in all age groups. Clinically, choroid plexus tumors tend to cause hydrocephalus and increased intracranial pressure. Histologically, choroid plexus papillomas correspond to WHO grade I, choroid plexus carcinomas to WHO grade III. Immunohistochemically, cytokeratins and vimentin are expressed by virtually all choroid plexus papillomas and most choroid plexus carcinomas while transthyretin and S-100 protein are present in 80-90% of cases, less frequently, though, in choroid plexus carcinomas. Glial fibrillary acidic protein can be found focally in about 25-55% of choroid plexus papillomas and 20% of choroid plexus carcinomas. The mean Ki67/MIB1 labeling index for choroid plexus papillomas is 1.9%, for choroid plexus carcinomas 13. 8%. Choroid plexus papillomas typically show hyperdiploidy with gains particularly on chromosomes 7, 9, 12, 15, 17, and 18 while one choroid plexus carcinoma showed rearrangements of chromosomes 7p11-12, 9q11-12, 15q22, and 19q13.4. Choroid plexus papillomas can usually be cured by surgery alone with a 5-year survival rate of up to 100% with occasional recurrences while choroid plexus carcinomas grow more rapidly and have a less favorable outcome with a 5-year survival rate of 26-40%.     

Brain drug delivery, drug metabolism, and multidrug resistance at the choroid plexus

The choroid plexuses (CPs) have the capability to modulate drug delivery to the cerebrospinal fluid (CSF) and to participate in the overall cerebral biodisposition of drugs. The specific morphological properties of the choroidal epithelium and the existence of a CSF pathway for drug distribution to different targets in the central nervous system suggest that the CP-CSF route is more significant than previously thought for brain drug delivery. In contrast to its role in CSF penetration of drugs, CP is also involved in brain protection in that it has the capacity to clear the CSF from numerous potentially harmful CSF-borne exogenous and endogenous organic compounds into the blood. Furthermore, CP harbors a large panel of drug-metabolizing enzymes as well as transport proteins of the multidrug resistance phenotype, which modulate the cerebral bioavailability of drugs and toxins. The use of an in vitro model of the choroidal epithelium suitable for drug transport studies has allowed the demonstration of the choroidal epithelium acting as an effective metabolic blood-CSF barrier toward some xenobiotics, and that a vectorial, blood-facing efflux of conjugated metabolites occurs at the choroidal epithelium. This efflux involves a specific transporter with characteristics similar to those of the multidrug resistance associated protein (MRP) family members. Indeed, at least one member, MRP1, is largely expressed at the CP epithelium, and localizes at the basolateral membrane. These metabolic and transport features of the choroidal epithelium point out the CP as a major detoxification site within the brain.     

Immunohistochemical study on the distribution of sodium-dependent vitamin C transporters in the respiratory system of adult rat

As vitamin C (L-ascorbic acid, VC) is known to be essential for many enzymatic reactions, the study on the transport mechanism of VC through cytoplasmic membrane is crucial to understanding physiological role of VC in cells and the respiratory system. In this regard, the study on the newly identified sodium-dependent VC transporters (SVCTs), SVCT1 and SVCT2, is required in organs that contain high concentration of VC. We have shown the distribution of SVCT proteins in the respiratory system, which has been reported to be one of the organs with a high concentration of VC, using immunohistochemical techniques. In the present study, intense SVCT immunoreactivities (IRs) were mainly localized in the respiratory system epithelial cells. In the trachea, both SVCT1 and 2 were localized in the psuedostratified ciliated columnar epithelium. In the terminal bronchiole, SVCT1 and 2 IRs were mainly observed in the apical portion of the simple columnar epithelium. In addition, SVCT IRs was localized within the cell membrane of some alveolar cells, even though we could not identify the exact cell types. These results provide the first evidence that intense SVCT1 and 2 IRs were found in the apical portion of the respiratory epithelial cells, suggesting that SVCT proteins in the apical portion could transport the reduced form of VC included in the airway surface liquid into the respiratory epithelial cells.     

Imaging the dynamics of intracellular protein translocation by photoconversion of phamret-cybr/ROM

Cybr/Reduced On-random Motile (ROM) is a scaffold protein, containing a postsynaptic density protein-95/discs-large/ZO-1 (PDZ) domain, a LEU region and a PDZ domain binding region at the C-terminus. In the immune system, Cybr/ROM was found to localize in vesicles and at the plasma membrane, through interactions with cytohesin-1. In this investigation, we reported Cybr/ROM as occurring in vesicles, the cytoplasm and at membrane ruffles of H1299 lung cancer cells. Its localization at the ruffles was dependent on intact actin structures as indicated by latrunculin A treatment, which abrogated ruffle formation and staining of Cybr/ROM at the cells' periphery. Transfection of truncation mutants consisting of either the PDZ or LEU domain showed that the LEU domain of ROM was localized to membrane ruffles, vesicles and the cytoplasm, whereas, the PDZ domain localized to the membrane ruffles and cytoplasm only. There was therefore, domain/molecular segregation of Cybr/ROM in different cellular compartments. Cybr/ROM was subcloned into a plasmid carrying the photoactivation-mediated resonance energy transfer (Phamret) protein. The photoconversion experiments demonstrated the diffusion of ROM from the cytoplasm to the membrane ruffling sites and conversely from membrane ruffles to the cytoplasm. Large variances in the transport velocity of Cybr/ROM in the cytoplasm suggested that its movements were facilitated by other mechanisms in addition to diffusion.     

Quantitative microscopy reveals 3D organization and kinetics of endocytosis in rat hepatocytes

In order to demonstrate the power of quantitative microscopy, the endocytic apparatus of rat hepatocytes was reexamined using in situ liver and short term cultured hepatocyte couplets that were allowed to internalize endocytic markers for various time intervals. Correlative confocal light and electron microscopy demonstrate a tubulovesicular reticulum representing the endocytic apparatus. Volume and membrane area account for 2% of cell volume and 30% plasma membrane surface. Colocalization analysis demonstrated that pathway-specific ligands and fluid-phase markers enter EEA1-positive vesicles, the early endosomal compartment, immediately after internalization. These vesicles are translocated rapidly from basolateral to perinuclear and apical locations. Ligands are sorted within 5 min to their respective pathways. Sequential colocalization of an asialoglycoprotein-pulse with rab7 and lamp3 demonstrates that early endosomes change into or fuse with late endosomes and lysosomes. Alternatively, markers are sequestered into the common endosome consisting of rab11-positive, long tubules that originate from early endosomes and show an affinity for the transcytotic marker pIgA and its receptor. This compartment mediates transcytosis by delivering the receptor-ligand complex to the subapical compartment, a set of apical, rab11-positive vesicles, which are connected to the tubular reticulum. We conclude that vesicular traffic between preexisting compartments, maturation or fusion of endocytic organelles, and transport in tubules act in concert and together mediate transport between compartments of a tubulovesicular endocytic apparatus. In addition, we show that quantitative microscopy using high resolution data sets can detect and characterize kinetics of various parameters thus adding a dynamic component to 3D information.     

Structure and expression of the transthyretin gene in the choroid plexus: a model for the study of the mechanism of evolution

Thyroid hormones are key regulators of brain differentiation and function. They permeate strongly into lipid membranes. However, a substantial portion of thyroid hormone is retained in the intravascular/extracellular compartments by binding to plasma proteins. In the brain, transthyretin is the most important of these proteins. This transthyretin is synthesized in the epithelial cells of the choroid plexus and exclusively secreted towards the brain. A net movement of thyroid hormones from the blood to the brain ensues. During evolution, transthyretin synthesis in the choroid plexus and the beginnings of a neocortex first appeared at the stage of the stem reptiles. The affinity of transthyretin for thyroxine increased and that for triiodothyronine decreased during evolution. This could augment the importance of deiodination for regulation of metabolism and gene expression by thyroid hormones in the brain. Successive shifts of the splice site at the 5' end of exon 2 of transthyretin precursor mRNA in the 3' direction led to a shortening of the N-terminal sections and to an increase in hydrophilicity of the N-terminal regions of transthyretin. This shift can be explained by a sequence of single base mutations. It could be an example for a molecular mechanism of positive Darwinian evolution. The selection pressure, which led to the expression of the transthyretin gene in the choroid plexus during evolution, might have been the maintenance of thyroid hormone homeostasis in the extracellular compartment of the brain in the presence of the greatly increasing volume of the lipid phase.     

Transplanted choroidal plexus epithelial cells can integrate with organotypic spinal cord slices into a new system

This study aimed to evaluate the integration of transplanted choroidal plexus epithelial cells with organotypic spinal cord slices. Organotypic spinal cord slices, normally cultured for 6 days, were divided into control group (Ctrl) and transplanted group (T). The choroidal plexus epithelial cells were dissociated and primary cultured (C group). The choroidal plexus epithelial cells cultured for 6–7 days were labeled by 1,1’-dioctadecyl-3,3,3’,3’-tetramethyl-indocarbocyanineperchlorate (CM-Dil), and were identified by transthyretin (TTR) in immunocytochemistry. They were adjusted to the density of 0.5–1 × 10⁷/ml, then 2 μl cells suspension were transplanted to the spinal cord slices in the T group. The same amount of basal medium was dripped on the spinal cord slices in the Ctrl group. After 14 days of transplantation, the differentiations into neurons and astrocytes, and the synapses were identified by immunofluorescence histochemistry. At the same time, the ratios of cell differentiations and synapses in new system, and the changes of MAPK signaling pathway were tested by western blotting. The choroid plexus epithelial cells were well labeled by CM-Dil and were immune-stained by TTR in immunocytochemistry. The choroid plexus epithelial cells bodies were small when transplanted on the spinal cord slices, but big when transplanted on the polyester membrane inserts. The transplanted cells could differentiate into astrocytes, and possibly differentiate into neurons, and there were a large number of synaptophysin positive vesicles between transplanted cells and organotypic spinal cord slices in immunofluorescence histochemistry. The levels of GFAP, TUB-III and synaptophysin in the T group were higher than which in the Ctrl and C groups in western blotting (P < 0.05). And the ratios of p-JNK/JNK and p-P38/P38 in the T group were significantly lower than which in the Ctrl and C groups (P < 0.05). But the ratio of p-ERK/ERK in the three groups was of no significant difference. The transplanted choroidal plexus epithelial cells can integrate with organotypic spinal cord slices into a new system.     

In situ freeze-fracture of monolayer cell cultures grown on a permeable support.

The growth of cultured epithelial cells on permeable supports allows increased cell differentiation and the assessment of a variety of transcellular and paracellular transport processes. The need to assess the corresponding ultrastructural characteristics of these cells under identical conditions prompted this laboratory to develop a reliable method for producing freeze-fracture replicas of these cultures. Sections of filter inserts with the cell-side facing up are placed between layers of polyvinyl alcohol with a strip of mylar positioned on the layer of polyvinyl alcohol. Following freezing, the monolayer is fractured by lifting the mylar strip from the assembly. The result is a consistent fracture of the apical membrane sufficient for analysis of tight junction sealing strands, microvilli distribution, and intramembranous particle (IMP) distribution between apical and lateral membrane domains. This method utilizes standard equipment and readily available materials and, most importantly, allows the freeze-fracture and replication of an undisturbed cell monolayer.     

Intracellular membrane trafficking in bone resorbing osteoclasts

There is ample evidence now that the two major events in bone resorption, namely dissolution of hydroxyapatite and degradation of the organic matrix, are performed by osteoclasts. The resorption cycle involves several specific cellular activities, where intracellular vesicular trafficking plays a crucial role. Although details of these processes started to open up only recently, it is clear that vesicular trafficking is needed in several specific steps of osteoclast functioning. Several plasma membrane domains are formed during the polarization of the resorbing cells. Multinucleated osteoclasts create a tight sealing to the extracellular matrix as a first indicator of their resorption activity. Initial steps of the sealing zone formation are alpha(v)beta(3)-integrin mediated, but the final molecular interaction(s) between the plasma membrane and mineralized bone matrix is still unknown. A large number of acidic intracellular vesicles then fuse with the bone-facing plasma membrane to form a ruffled border membrane, which is the actual resorbing organelle. The formation of a ruffled border is regulated by a small GTP-binding protein, rab7, which indicates the late endosomal character of the ruffled border membrane. Details of specific membrane transport processes in the osteoclasts, e.g., the formation of the sealing zone and transcytosis of bone degradation products from the resorption lacuna to the functional secretory domain remain to be clarified. It is tempting to speculate that specific features of vesicular trafficking may offer several potential new targets for drug therapy of bone diseases.     

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.     

Membrane traffic in myelinating oligodendrocytes

In the central nervous system (CNS), the myelin sheath is synthesised by oligodendrocytes as a specialised subdomain of an extended plasma membrane, reminiscent of the segregated membrane domains of polarised cells. Myelination takes place within a relatively short period of time and oligodendrocytes must have adapted membrane sorting and transport mechanisms to achieve such a high rate of myelin synthesis and to maintain the unique organisation of the myelin membrane. In adult life, maintenance of the functional myelin sheath requires a carefully orchestrated balance of myelin synthesis and turnover. Imbalance in these processes may cause dys- or demyelination and disease. This review summarises what is currently known about myelin protein trafficking and mistrafficking in oligodendrocytes. We also present data demonstrating distinct transport pathways for myelin structural proteins and the expression of SNARE proteins in differentiating oligodendrocytes. Myelinating glial cells may well serve as a model system for studying general aspects of membrane trafficking and organisation of membrane domains.     

Development of the choroid plexus

Mammalian choroid plexuses develop at four sites in the roof of the neural tube shortly after its closure, in the order IVth, lateral, and IIIrd ventricles. Bone morphogenetic proteins and tropomyosin are involved in early specification of these sites and in early plexus growth. Four stages of lateral ventricular plexus development have been defined, based on human and sheep fetuses; these depend mainly on the appearance of epithelial cells and presence or absence of glycogen. Other plexuses and other species are probably similar, although marsupials may lack glycogen. Choroid plexuses form one of the blood-brain barrier interfaces that control the brain's internal environment. The mechanisms involved combine a structural diffusion restraint (tight junctions between the plexus epithelial cells) and specific exchange mechanisms. In this review, it is argued that barrier mechanisms in the developing brain are different in important respects from those in the adult brain, but these differences do not necessarily reflect immaturity of the system. Absence of a barrier mechanism or presence of one not found in the adult may be a specialisation that is appropriate for that stage of brain development. Emphasis is placed on determining which mechanisms are present in the immature brain and relating them to brain development. One mechanism unique to the developing brain transfers specific proteins from blood to cerebrospinal fluid (CSF), via tubulocisternal endoplasmic reticulum in plexus epithelial cells. This results in a high concentration of proteins in early CSF. These proteins do not penetrate into brain extracellular space because of "strap" junctions between adjacent neuroependymal cells, which disappear later in development, when the protein concentration in CSF is much lower. Functions of the proteins in early CSF are discussed in terms of generation of a "colloid" osmotic pressure that expands the ventricular system as the brain grows; the proteins may also act as specific carriers and growth factors in their own right. The pathway for low molecular weight compounds, which is much more permeable in the developing choroid plexuses, appears also to be a transcellular one, rather than paracellular via tight junctions. There is thus good evidence to support a novel view of the state of development and functional significance of barrier mechanisms in the immature brain. It grows in an environment that is different from that of the rest of the fetus/neonate and that is also different in some respects from that of the adult. But these differences reflect developmental specialisation rather than immaturity.