Relationship between orthogonal arrays of particles and tight junctions as demonstrated in cells of the ventricular wall of the rat brain

Aquaporin-1 and Aquaporin-4 Expression in Ependyma, Choroid Plexus and Surrounding Transition Zones in the Human Brain

The choroid plexus (CP) is a structure in the brain ventricles that produces the main part of the cerebrospinal fluid (CSF). It is covered with specialized cells which show epithelial characteristics and are the site of the blood-CSF barrier. These cells form a contiguous cell sheet with ventricle-lining ependymal cells which are known to express aquaporin-4 (AQP4). In contrast, CP epithelial cells express aquaporin-1 (AQP1) apically. We investigated the expression patterns of aquaporins in the CP-ependyma transition from human body donors using immunofluorescence and electron microscopy. Ependymal cells and subependymal astrocytes at the base of the CP showed a particularly high AQP4 immunoreactivity. Astrocytic processes formed a dense meshwork or glial plate around the blood vessels entering the CP. Interestingly, some of these astrocytic processes were in direct contact with the CP stroma, which contains fenestrated blood vessels, separated only by a basal lamina. Electron microscopy confirmed the continuity of the subastrocytic basal lamina with the CP epithelium. We also probed for components of the AQP4 anchoring dystrophin-dystroglycan complex. Immunolabeling for dystrophin and AQP4 showed an overlapping staining pattern in the glial plate but not in previously reported AQP4-positive CP epithelial cells. In contrast, dystroglycan expression was associated with laminin staining in the glial plate and the CP epithelium. This suggests different mechanisms for AQP4 anchoring in the cell membrane. The high AQP4 density in the connecting glial plate might facilitate the transport of water in and out of the CP stroma and could possibly serve as a drainage and clearing pathway for metabolites.

Adherens, tight, and gap junctions in ependymal cells: A systematic review of their contribution to CSF-brain barrier

Introduction

The movement of fluids and solutes across the ependymal barrier, and their changes in physiologic and disease states are poorly understood. This gap in knowledge contributes strongly to treatment failures and complications in various neurological disorders.

Methods

We systematically searched and reviewed original research articles treating ependymal intercellular junctions on PubMed. Reviews, opinion papers, and abstracts were excluded. Research conducted on tissue samples, cell lines, CSF, and animal models was considered.

Results

A total of 45 novel articles treating tight, adherens and gap junctions of the ependyma were included in our review, spanning from 1960 to 2022. The findings of this review point toward a central and not yet fully characterized role of the ependymal lining ultrastructure in fluid flow interactions in the brain. In particular, tight junctions circumferentially line the apical equator of ependymal cells, changing between embryonal and adult life in several rodent models, shaping fluid and solute transit in this location. Further, adherens and gap junctions appear to have a pivotal role in several forms of congenital hydrocephalus.

Conclusions

These findings may provide an opportunity for medical management of CSF disorders, potentially allowing for tuning of CSF secretion and absorption. Beyond hydrocephalus, stroke, trauma, this information has relevance for metabolite clearance and drug delivery, with potential to affect many patients with a variety of neurological disorders. This critical look at intercellular junctions in ependyma and the surrounding interstitial spaces is meant to inspire future research on a central and rather unknown component of the CSF-brain interface.

Ependyma: a new target for autoantibodies in neuromyelitis optica?

Neuromyelitis optica (NMO) is an autoimmune demyelinating disease of the central nervous system characterized by the presence of autoantibodies (called NMO-IgG) targeting aquaporin-4. Aquaporin-4 is expressed at the perivascular foot processes of astrocytes, in the glia limitans, but also at the ependyma. Most studies have focused on studying the pathogenicity of NMO-IgG on astrocytes, and NMO is now considered an astrocytopathy. However, periependymal lesions are observed in NMO suggesting that ependymal cells could also be targeted by NMO-IgG. Ependymal cells regulate CSF-parenchyma molecular exchanges and CSF flow, and are a niche for sub-ventricular neural stem cells. Our aim was to examine the effect of antibodies from NMO patients on ependymal cells. We exposed two models, i.e. primary cultures of rat ependymal cells and explant cultures of rat lateral ventricular wall whole mounts, to purified IgG of NMO patients (NMO-IgG) for 24 hours. We then evaluated the treatment effect using immunolabelling, functional assays, ependymal flow analysis and bulk RNA sequencing. For each experiment, the effects were compared with those of purified IgG from a healthy donors and non-treated cells. We found that: (i) NMO-IgG induced aquaporin-4 agglomeration at the surface of ependymal cells and induced cell enlargement in comparison to controls. In parallel, it induced an increase in gap junction connexin-43 plaque size; (ii) NMO-IgG altered the orientation of ciliary basal bodies and functionally impaired cilia motility; (iii) NMO-IgG activated the proliferation of sub-ventricular neural stem cells; (iv) treatment with NMO-IgG up-regulated the expression of pro-inflammatory cytokines and chemokines in the transcriptomic analysis. Our study showed that NMO-IgG can trigger an early and specific reactive phenotype in ependymal cells, with functional alterations of intercellular communication and cilia, activation of sub-ventricular stem cell proliferation and the secretion of pro-inflammatory cytokines. These findings suggest a key role for ependymal cells in the early phase of NMO lesion formation.

Aquaporin-4 in Astroglial Cells in the CNS and Supporting Cells of Sensory Organs-A Comparative Perspective

The main water channel of the brain, aquaporin-4 (AQP4), is one of the classical water-specific aquaporins. It is expressed in many epithelial tissues in the basolateral membrane domain. It is present in the membranes of supporting cells in most sensory organs in a specifically adapted pattern: in the supporting cells of the olfactory mucosa, AQP4 occurs along the basolateral aspects, in mammalian retinal Müller cells it is highly polarized. In the cochlear epithelium of the inner ear, it is expressed basolaterally in some cells but strictly basally in others. Within the central nervous system, aquaporin-4 (AQP4) is expressed by cells of the astroglial family, more specifically, by astrocytes and ependymal cells. In the mammalian brain, AQP4 is located in high density in the membranes of astrocytic endfeet facing the pial surface and surrounding blood vessels. At these locations, AQP4 plays a role in the maintenance of ionic homeostasis and volume regulation. This highly polarized expression has not been observed in the brain of fish where astroglial cells have long processes and occur mostly as radial glial cells. In the brain of the zebrafish, AQP4 immunoreactivity is found along the radial extent of astroglial cells. This suggests that the polarized expression of AQP4 was not present at all stages of evolution. Thus, a polarized expression of AQP4 as part of a control mechanism for a stable ionic environment and water balanced occurred at several locations in supporting and glial cells during evolution. This initially basolateral membrane localization of AQP4 is shifted to highly polarized expression in astrocytic endfeet in the mammalian brain and serves as a part of the neurovascular unit to efficiently maintain homeostasis.

Distribution of the Immunoreactivity for Glycoprotein M6B in the Neurogenic Niche and Reactive Glia in the Injury Penumbra Following Traumatic Brain Injury in Mice

The location and morphology of astrocytes are known to contribute to their diversity, and this diversity is often associated with their selective functions. However, molecular markers for astrocyte subtypes are largely unknown. In this study, we found that the immunoreactivity for glycoprotein GPM6B (M6B-IR) is preferentially expressed in the astrocytes associated with ventricles or neurogenic regions of the adult mouse brain. In particular, M6B-IR in the neurogenic niche was confined to glial fibrillary acidic protein- or nestin-expressing neural stem cells. Furthermore, in the injury penumbra, reactive astrocytes expressing nestin also exhibited strong M6B-IR. These results reveal that GPM6B is a potential molecular marker for a subset of astrocytes, as well as for the injury-dependent activation of astrocytes.

A novel look at astrocytes: aquaporins, ionic homeostasis, and the role of the microenvironment for regeneration in the CNS

Aquaporin-4 (AQP4) water channels are located at the basolateral membrane domain of many epithelial cells involved in ion transport and secretion. These epithelial cells separate fluid compartments by forming apical tight junctions. In the brain, AQP4 is located on astrocytes in a polarized distribution: At the border to blood vessels or the pial surface, its density is very high. During ontogeny and phylogeny, astroglial cells go through a stage of expressing tight junctions, separating fluid compartments differently than in adult mammals. In adult mammals, this barrier is formed by arachnoid, choroid plexus, and endothelial cells. The ontogenetic and phylogenetic barrier transition from glial to endothelial cells correlates with the regenerative capacity of neuronal structures: Glial cells forming tight junctions, and expressing no or unpolarized AQP4 are found in the fish optic nerve and the olfactory nerve in mammals both known for their regenerative ability. It is hypothesized that highly polarized AQP4 expression and the lack of tight junctions on astrocytes increase ionic homeostasis, thus improving neuronal performance possibly at the expense of restraining neurogenesis and regeneration.

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

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.

Regional analysis of the ependyma of the third ventricle of rat by light and electron microscopy

Ependymal lining of cerebral ventricles lies at the interface between the ventricular cavities and the brain parenchyma. Ependymal cells are involved in various functions within the brain and play a major role in the production of the chemical principals of the cerebrospinal fluid. Histological studies on the regional variation of the third ventricular ependyma and the subependyma of adult rats were carried out by light and electron microscopic methods. For light microscopic analysis, methacrylate sections were used. In addition to the routine haematoxylin and eosin (H and E) staining for histological studies, the sections were stained with toluidine blue, cresyl violet and periodic acid Schiff's reagent (PAS). A regional analysis of the ependyma of the third ventricle showed that in most regions the ependyma was monolayered. The sidewalls and floor of the ventral portion of the third ventricle showed a multilayered ependyma. For descriptive purposes at the light microscopic level, the ependymal cells were classified, based on the cell shape (flat, cuboidal or columnar), presence or absence of cilia and the number of cytoplasmic granules present in the cells. Studies of transmission electron microscope have shown that these granules represent the cell organelles of the ependyma. The subependyma also showed a regional morphological variation, and, in most instances, contained glial and neuronal elements. In regions of specific brain nuclei, neurons were the major cell type of the subependyma. PAS staining did not show any positive granules in the ependymal cytosol. Characteristic supraependymal elements were present at the ependymal surface of the third ventricle.

Epithelial and endothelial barriers in the olfactory region of the nasal cavity of the rat

The olfactory ensheathing (glial) cells (OECs) have been identified to be useful candidate cells to support regeneration after being transplanted into injured fiber tracts of the central nervous system. We investigated by means of immunocytochemistry and freeze-fracturing the morphology and molecular composition of OEC tight junctions in the rat olfactory system. In addition, we tested the hypothesis whether tight junctions and orthogonal arrays of particles (OAPs) which contain the water channel protein aquaporin-4 (AQP4), are mutually exclusive as suggested in previous studies. In OECs, we found neither OAPs nor AQP4, but tight junctions immunoreactive for ZO-1, occludin, and claudin-5, but immunonegative for ZO-2 and claudin-3. To shed more light on the function of OEC tight junctions, we tested the permeability and tight junction composition of blood vessels and fila olfactoria. We found them both, permeable for infused lanthanum nitrate, and to be immunopositive for ZO-1 and claudin-5. The tight junctions of the OECs are discussed to be responsible for micro-compartmentalization within the olfactory fiber tract providing a benefit for axonal growth.

Connexin 50 gene on human chromosome 1q21 is associated with schizophrenia in matched case control and family-based studies

BACKGROUND

The gap junction subunit connexin permits direct intercellular exchange of ions and molecules including glutamate, and plays an important role in the central nervous system. The connexin 40 (Cx40) and connexin 50 (Cx50) genes are located on chromosome 1q21.1, a region strongly linked with schizophrenia. These lines of evidence suggest that Cx40 and Cx50 may play a role in schizophrenia.

METHODS

Using an allele-specific PCR assay, four polymorphisms each were genotyped for Cx40 and Cx50 in 190 Caucasian patients with schizophrenia and 190 controls matched for sex, age and ethnicity. Following up, Cx50 rs989192 and rs4950495 were investigated in 99 Canadian and 163 Portuguese trios and nuclear families with schizophrenia probands. Hardy-Weinberg equilibrium and linkage disequilibrium (LD) block identification was carried out with HaploView, and association analysis for alleles and haplotypes with a permutation test of 10 000 simulations was carried out using the UNPHASED software program.

RESULTS

Distributions of genotype frequencies of all markers were in Hardy-Weinberg equilibrium in Caucasian patients, controls and families. One rs989192-rs4950495 LD block was found in patients but not in controls. We found a significant association between the Cx50 rs989192-rs4950495 haplotype and schizophreniay (chi(2) = 29.55, p<0.01). The A-C haplotype had a higher frequency in patients (chi(2) = 7.153, p<0.01). Family studies also showed that the A-C haplotype was transmitted more often to patients with schizophrenia (chi(2) = 8.43, p<0.01). No association of Cx40 with schizophrenia was found for allele, genotype or haplotype analyses.

CONCLUSIONS

Our matched case-control and family study indicate that Cx50, but not Cx40, may play a role in the genetic susceptibility to schizophrenia.

Growing axons in fish optic nerve are accompanied by astrocytes interconnected by tight junctions

Mammalian astrocytes are in general interconnected by gap but not by tight junctions and play an ambiguous and controversially discussed role in central nervous system regeneration. At different neuroanatomical sites, fish astrocytes are interconnected by tight junctions and desmosomes and are involved in the successful regeneration of lesioned fiber tracts. In fish, newly generated retinal ganglion cells continuously grow new axons to the optic tectum but the interrelationship between glial tight junctions and axonal growth is undefined so far. We therefore investigated the occurrence of tight junctional structures and molecules within the ribbon-shaped optic nerve of a teleost fish (Astatotilapia burtoni) and found a predominant expression of zonula occludens protein-1 and claudin-1 in astrocytes where axons of new ganglion cells are assembled retinotopically within the optic nerve. This may support a previously formulated hypothesis according to that different properties of astrocytic membranes could be responsible for different glio-neuronal interactions which in turn may determine the micro-environmental conditions of growing axons.

Heterogeneous occurrence of aquaporin-4 in the ependyma and in the circumventricular organs in rat and chicken

Aquaporins are selective water channel proteins critical in volume homeostasis. In the CNS AQP4 predominates, localized mainly in the glia limitans, the perivascular endfeet and ependyma. The present immunofluorescent study reveals the distribution of aquaporin-4 in the circumventricular organs in rat and chicken brains. The ventricular ependyma (especially in the third one), the subfornical organ, the area postrema, the rat pineal body (in part), and the vascular organ of lamina terminalis were marked by intense immunopositivity. Several areas, however, proved to be immunonegative: the central canal, the subcommissural organ, the ependymal zone of the median eminence in rat but its whole thickness in chicken, the subtrochlear organ, and the paraventricular organ. The immunostaining of the lateral septal and subseptal organs were similar to their environment. Results on developing rats suggested that the aquaporin-4 immunonegativity is a secondary phenomenon. Surveying other structural and functional features, no clear explanation of the heterogeneous occurrence of aquaporin-4 was found. The absence of aquaporin-4 seems to correlate with some features of the "ependymal organs" (thickened, pseudostratified ependyma, presence of blood-brain barrier) and with the avoidance of GFAP. On the other hand, the organs rich in aquaporin-4 have features of the "hypendymal organs" (glial and vascular plexus but no blood-brain barrier). There are organs, however, which do not fit into either group completely, i.e. the lateral septal and subseptal organs. Presence of tight junctions coincides with the absence of aquaporin-4 in the ependyma of spinal cord, the subcommissural organ and the ependyma of median eminence.

Claudin-1, claudin-2 and claudin-11 are present in tight junctions of choroid plexus epithelium of the mouse

The choroid plexus epithelium forms the blood-cerebrospinal fluid (CSF) barrier and is responsible for the secretion of the CSF from the blood. The morphological correlate of the blood-CSF barrier are the tight junctions of choroid plexus epithelium. By freeze-fracture electron microscopy it has been demonstrated that choroid plexus epithelial tight junctions form parallel strands resembling those of Sertoli cells building the blood-testis barrier and those of the myelin sheaths of oligodendrocytes. As the oligodendrocyte specific protein/claudin-11 has been shown to be the central mediator of parallel-array tight junctions in Sertoli cells and myelin sheaths in mice, we asked whether claudin-11 is present in the tight junctions of choroid plexus epithelial cells of the mouse. Here, we present the first direct evidence that claudin-11 besides claudin-1 and -2, occludin and the zonula occludens protein ZO-1 is present in choroid plexus epithelial tight junctions. During inflammation in the central nervous system such as experimental autoimmune encephalomyelitis, the molecular composition of choroid plexus epithelial tight junctions does not change considerably. Their unique molecular composition, with claudin-11 accompanied by claudin-1 and claudin-2 points to a unique regulatory mechanism of the blood-CSF-barrier function.

Organization of choroid plexus epithelial and endothelial cell tight junctions and regulation of claudin-1, -2 and -5 expression by protein kinase C

Claudins are components of the tight junctional complex in epithelial and endothelial cells. We characterized the composition of tight junctions in the choroid plexus of the lateral ventricle in the rat brain and tested whether protein kinase C induced changes in their composition. Claudin-1, -2 and -5 were present in the epithelial cells at and near the tight junctions, respectively. In the endothelial cells, claudin-5 was stronger expressed than claudin-1 and -2. Twenty-four hours after the phorbolester injection into the ventricle, claudin-1 immunoreactivity of the epithelial cells was increased and spread to the cytoplasm. The claudin-2 and -5 immunoreactivities were reduced. These findings are consistent with an influence of protein kinase C on the composition of the tight junctions in the choroid plexus.

Tight junctions of the blood-brain barrier

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The blood-brain barrier endothelial cells comprise an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. The latter is realized by the tight junctions between the endothelial cells of the brain microvasculature, which are subject of this review. Morphologically, blood-brain barrier-tight junctions are more similar to epithelial tight junctions than to endothelial tight junctions in peripheral blood vessels. 2. Although blood-brain barrier-tight junctions share many characteristics with epithelial tight junctions, there are also essential differences. However, in contrast to tight junctions in epithelial systems, structural and functional characteristics of tight junctions in endothelial cells are highly sensitive to ambient factors. 3. Many ubiquitous molecular constituents of tight junctions have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin, and 7H6. Signaling pathways involved in tight junction regulation comprise, among others, G-proteins, serine, threonine, and tyrosine kinases, extra- and intracellular calcium levels, cAMP levels, proteases, and TNF alpha. Common to most of these pathways is the modulation of cytoskeletal elements which may define blood-brain barrier characteristics. Additionally, cross-talk between components of the tight junction- and the cadherin-catenin system suggests a close functional interdependence of the two cell-cell contact systems. 4. Recent studies were able to elucidate crucial aspects of the molecular basis of tight junction regulation. An integration of new results into previous morphological work is the central intention of this review.

Phorbol ester induced changes in tight and adherens junctions in the choroid plexus epithelium and in the ependyma

The molecular composition and functional properties of cell-cell junctions of choroid plexus epithelial cells and the ependyma of the lateral ventricular wall were investigated in the rat brain. Expression studies of cadherin and alpha- and beta-catenins, as well as expression of occludin and ZO-1, indicated that cell adherens and tight junctions were present in both choroid plexus epithelial cells and in ependymal cells. We then tested the hypothesis that phorbolester in vivo can induce changes in the expression level of adherens and tight junction molecules at the blood-cerebrospinal fluid (CSF) barrier as well as in the ependyma. In addition, the functional properties of the ependymal junctions were tested by injection of dextran 3000 into the striatum after phorbolester application. Twenty-four hours after phorbolester-injection into the lateral ventricle of the rat brain, the expression patterns of tight and adherens junction molecules were markedly changed in the epithelial cells of the choroid plexus. The adherens junction proteins cadherin and beta-catenin were reduced in both the ependymal cells of the lateral ventricle and choroid plexus epithelial cells. In addition, the occludin-immunoreactivity of the choroid plexus epithelial cells was strongly reduced. However, the ZO-1 immunoreactivity was not affected by the phorbol ester-treatment and the alpha-catenin immunoreactivity was not changed. Furthermore, phorbol ester injection induced a reduction of the volume of intrastriatal injected biotinylated dextran (m.w. 3000), which is consistent with a modulatory influence of protein kinase C activation on the clearance capacity of the brain.

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

On the organization of astrocytic gap junctions in rat brain as suggested by LM and EM immunohistochemistry of connexin43 expression

Gap junctions and the intercellular communication syncytium they form between glial cells are thought to play a critical role in glial maintenance of appropriate metabolic environments in neural tissues. We have previously suggested (Yamamoto et al., Brain Res. 508:313-319, '90) that the vast majority of astrocytes in rat brain express connexin43, one of several recently identified gap junction proteins. Here, we confirm ultrastructurally that astrocytes in a number of brain regions of rat are immunolabelled with an antibody against connexin43 and that neurons and oligodendrocytes are devoid of labelling. The distribution of connexin43 immunoreactivity throughout the brain is presented at the light microscope (LM) level. By LM, immunoreactive structures consisted primarily of round or elongated puncta ranging from 0.3 microns to 4 microns in length and of annular profiles ranging from 1 to 10 microns in diameter. Immunolabelled fibrous processes were only occasionally seen and no labelling was observed in astrocytic cell bodies. Long, linear arrays of puncta were rare in gray matter but were common in white matter where they were arranged parallel to myelinated fibers. Puncta organized in a honeycomb pattern were seen near the cerebral cortical surface and frequently around blood vessels. Regional immunoreaction density, which was a reflection of either the concentration or staining intensity of immunoreactive elements, was remarkably heterogeneous; dramatic differences in labelling intensity frequently delineated anatomical boundaries between adjacent nuclei. Abrupt as well as graded fluctuations of immunoreaction intensity were also observed within nuclear structures. By electron microscopy (EM), gap junctions of fibrous and protoplasmic astrocytes were intensely stained and labelled organelles were often observed intracellularly in areas near gap junctions. These junctions and the spread of immunoreaction product to perijunctional organelles in their vicinity were considered to correspond to puncta seen by LM. Labelling within astrocytic cell bodies was seen in only a few instances. In some brain areas, astrocytic processes commonly gave rise to immunoreactive lamellae that partially ensheathed neuronal cell bodies, axon terminals, dendrites, and synaptic glomeruli. Such lamellae were considered to correspond to immunoreactive annular profiles seen by LM. Perivascular endfoot processes of astrocytes displayed intense staining of their gap junctions and portions of their apposing membranes.(ABSTRACT TRUNCATED AT 400 WORDS)

Rabbit retinal Müller cells in cell culture show gap and tight junctions which they do not express in situ

Retinae of early postnatal rabbits were enzymatically dissociated and explanted in a culture system. The prospective myelinated region was discarded in order to avoid the presence of astrocytic or mesenchymal cells. After about 14 days in vitro (DIV), outgrowing glial (Müller) cells formed what light optically appeared to be confluent monolayers but by electron microscopy was shown to consist of flat epithelioid cells which overlapped considerably by extension of cytoplasmic tongues. Applying the freeze-fracture technique, apposed membranes of these cells were demonstrated to express infrequently but consistently both gap and tight junctions. This kind of junctions has never been observed on the membrane of rabbit Müller cells in situ. In comparison with Müller cell membranes in situ, the density of intramembrane particles was considerably reduced. Orthogonal arrays of particles which are characteristic elements of Müller cells in situ were not detected. Our results suggest that in homogeneous cell culture, Müller cells form some kind of epithelium-like specialized intercellular junctions. This situation resembles that of closely related glial cell types which form homogeneous layers in situ as e.g. retinal pigment epithelium cells expressing tight junctions, and marginal astrocytes being coupled by extensive gap junctions.

Orthogonal arrays of particles in astroglial cells: quantitative analysis of their density, size, and correlation with intramembranous particles

Astroglial cells were investigated by means of freeze-fracture in normal rat and mouse brain, cell culture and human gliomas. Membranes of these cells were quantitatively analyzed for their intramembranous particles (IMPs) and orthogonal arrays of particles (OAPs). Measurement of the size of OAPs and IMPs has permitted the search for a correlation between the 7-nm IMPs, which are distributed randomly in the membrane, and the subunits of OAPs (OAP-Su, also 7 nm in diameter). Using cultured astroglial cells treated with basic fibroblast growth factor (bFGF), arginine vasopressin, or sorbitol, good evidence for a relationship between the density of 7-nm IMPs and the size of OAPs can be demonstrated. These findings led to a hypothetical model of OAP modulation. A preliminary report has been published elsewhere (Neuhaus and Wolburg, 1989).

Orthogonal arrays of intramembrane particles in the supporting cells of the guinea-pig vestibular sensory epithelium

Membrane specializations of the contact region between afferent nerve endings and supporting cells of the sensory epithelia of guinea-pig vestibular endorgans were examined by thin-section and freeze-fracture electron microscopy. The calyx-type nerve endings (C-endings) are separated from supporting cells (SC) by a 25-30 nm space. At irregular intervals along the upper lateral surface of supporting cells, the intercellular space narrows markedly to form special close contacts between the C-ending and SC plasma membranes. Freeze-fracture replicas reveal membrane specializations--orthogonal arrays of particulate units--in the region where the close intercellular contacts were found in sections. Orthogonal arrays consisting of from 5 to 20 units were observed on the cytoplasmic (P) fracture face of the lateral SC plasma membrane. These particulate units from a 12 x 12-nm square, and each unit is composed of four 6-nm subunits. Possible roles of the orthogonal arrays are discussed.