Autocellular coupling by gap junctions in cultured astrocytes: a new view on cellular autoregulation during process formation

Glial Patchwork: Oligodendrocyte Progenitor Cells and Astrocytes Blanket the Central Nervous System

Tiling is a developmental process where cell populations become evenly distributed throughout a tissue. In this review, we discuss the developmental cellular tiling behaviors of the two major glial populations in the central nervous system (CNS)—oligodendrocyte progenitor cells (OPCs) and astrocytes. First, we discuss OPC tiling in the spinal cord, which is comprised of the three cellular behaviors of migration, proliferation, and contact-mediated repulsion (CMR). These cellular behaviors occur simultaneously during OPC development and converge to produce the emergent behavior of tiling which results in OPCs being evenly dispersed and occupying non-overlapping domains throughout the CNS. We next discuss astrocyte tiling in the cortex and hippocampus, where astrocytes migrate, proliferate, then ultimately determine their exclusive domains by gradual removal of overlap rather than sustained CMR. This results in domains that slightly overlap, allowing for both exclusive control of “synaptic islands” and astrocyte-astrocyte communication. We finally discuss the similarities and differences in the tiling behaviors of these glial populations and what remains unknown regarding glial tiling and how perturbations to this process may impact injury and disease.

Cx43 carboxyl terminal domain determines AQP4 and Cx30 endfoot organization and blood brain barrier permeability

The neurovascular unit (NVU) consists of cells intrinsic to the vessel wall, the endothelial cells and pericytes, and astrocyte endfeet that surround the vessel but are separated from it by basement membrane. Endothelial cells are primarily responsible for creating and maintaining blood-brain-barrier (BBB) tightness, but astrocytes contribute to the barrier through paracrine signaling to the endothelial cells and by forming the glia limitans. Gap junctions (GJs) between astrocyte endfeet are composed of connexin 43 (Cx43) and Cx30, which form plaques between cells. GJ plaques formed of Cx43 do not diffuse laterally in the plasma membrane and thus potentially provide stable organizational features to the endfoot domain, whereas GJ plaques formed of other connexins and of Cx43 lacking a large portion of its cytoplasmic carboxyl terminus are quite mobile. In order to examine the organizational features that immobile GJs impose on the endfoot, we have used super-resolution confocal microscopy to map number and sizes of GJ plaques and aquaporin (AQP)-4 channel clusters in the perivascular endfeet of mice in which astrocyte GJs (Cx30, Cx43) were deleted or the carboxyl terminus of Cx43 was truncated. To determine if BBB integrity was compromised in these transgenic mice, we conducted perfusion studies under elevated hydrostatic pressure using horseradish peroxide as a molecular probe enabling detection of micro-hemorrhages in brain sections. These studies revealed that microhemorrhages were more numerous in mice lacking Cx43 or its carboxyl terminus. In perivascular domains of cerebral vessels, we found that density of Cx43 GJs was higher in the truncation mutant, while GJ size was smaller. Density of perivascular particles formed by AQP4 and its extended isoform AQP4ex was inversely related to the presence of full length Cx43, whereas the ratio of sizes of the particles of the AQP4ex isoform to total AQP4 was directly related to the presence of full length Cx43. Confocal analysis showed that Cx43 and Cx30 were substantially colocalized in astrocyte domains near vasculature of truncation mutant mice. These results showing altered distribution of some astrocyte nexus components (AQP4 and Cx30) in Cx43 null mice and in a truncation mutant, together with leakier cerebral vasculature, support the hypothesis that localization and mobility of gap junction proteins and their binding partners influences organization of astrocyte endfeet which in turn impacts BBB integrity of the NVU.

Comprehensive Transcriptome and Pathway Analyses Revealed Central Role for Fascin in Promoting Triple-Negative Breast Cancer Progression

Recent years have witnessed major progress in development of novel therapeutic agents such as chemotherapy, targeted therapy and immune checkpoint inhibitors for breast cancer. However, cancer-related death remains high especially in triple-negative breast cancer (TNBC) due limited therapeutic options. Development of targeted therapies for TNBC requires better understanding of biology and signaling networks that promote disease progression. Fascin, an actin bundling protein, was identified as a key regulator of many signaling pathways that contribute to breast cancer progression. Herein, fascin ShRNA was used to generate stable fascin knockdown (FSCN1^KD) in the MDA-MB-231 TNBC cell line and then were subjected to comprehensive mRNA and miRNA transcriptome analysis. We identified 129 upregulated and 114 downregulated mRNA transcripts, while 14 miRNAs were differentially expressed in FSCN1^KD. Ingenuity pathway analysis (IPA) was used to predict the impact of differentially expressed transcripts on signaling pathways and functional categories and to construct miRNA-mRNA regulatory networks in the context of FSCN1 knockdown. Compared to FSCN1^KD, fascin-positive (FSCN1^CON) breast cancer cells showed enrichment in genes promoting cellular proliferation, migration, survival, DNA replication and repair. Expression of FSCN1^high (identified in BRCA dataset from TCGA) in conjunction with elevated expression of the top 10 upregulated or decreased expression of the top 10 downregulated genes (identified in our FSCN1^CON vs. FSCN1^KD) correlates with worst survival outcome. Taken together, these data confirmed fascin's role in promoting TNBC progression, and identified a novel opportunity for therapeutic interventions via targeting those FSCN1-related transcripts.

The many ways astroglial connexins regulate neurotransmission and behavior

Astrocytes have emerged as major players in the brain, contributing to many functions such as energy supply, neurotransmission, and behavior. They accomplish these functions in part via their capacity to form widespread intercellular networks and to release neuroactive factors, which can modulate neurotransmission at different levels, from individual synapses to neuronal networks. The extensive network communication of astrocytes is primarily mediated by gap junction channels composed of two connexins, Cx30 and Cx43, which present distinct temporal and spatial expression patterns. Yet, astroglial connexins are also involved in direct exchange with the extracellular space via hemichannels, as well as in adhesion and signaling processes via unconventional nonchannel functions. Accumulating evidence indicate that astrocytes modulate neurotransmission and behavior through these diverse connexin functions. We here review the many ways astroglial connexins regulate neuronal activity from the molecular level to behavior.

Modeling of Astrocyte Networks: Toward Realistic Topology and Dynamics

Neuronal firing and neuron-to-neuron synaptic wiring are currently widely described as orchestrated by astrocytes—elaborately ramified glial cells tiling the cortical and hippocampal space into non-overlapping domains, each covering hundreds of individual dendrites and hundreds thousands synapses. A key component to astrocytic signaling is the dynamics of cytosolic Ca²⁺ which displays multiscale spatiotemporal patterns from short confined elemental Ca²⁺ events (puffs) to Ca²⁺ waves expanding through many cells. Here, we synthesize the current understanding of astrocyte morphology, coupling local synaptic activity to astrocytic Ca²⁺ in perisynaptic astrocytic processes and morphology-defined mechanisms of Ca²⁺ regulation in a distributed model. To this end, we build simplified realistic data-driven spatial network templates and compile model equations as defined by local cell morphology. The input to the model is spatially uncorrelated stochastic synaptic activity. The proposed modeling approach is validated by statistics of simulated Ca²⁺ transients at a single cell level. In multicellular templates we observe regular sequences of cell entrainment in Ca²⁺ waves, as a result of interplay between stochastic input and morphology variability between individual astrocytes. Our approach adds spatial dimension to the existing astrocyte models by employment of realistic morphology while retaining enough flexibility and scalability to be embedded in multiscale heterocellular models of neural tissue. We conclude that the proposed approach provides a useful description of neuron-driven Ca²⁺-activity in the astrocyte syncytium.

Connexins as therapeutic targets in neurological and neuropsychiatric disorders

Astrocytes represent the reticular part of the central nervous system; gap junctions formed by connexins Cx43, Cx30- and Cx26 provide for homocellular astrocyte-astrocyte coupling, whereas connexins Cx30, Cx32, Cx43, and Cx47 connect astrocytes and oligodendrocytes. Astroglial networks are anatomically and functionally segregated being homologous to neuronal ensembles. Connexons, gap junctions and hemichannels (unpaired connexons) are affected in various neuropathologies from neuropsychiatric to neurodegenerative diseases. Manipulation of astrocytic connexins modulates the size and outreach of astroglial syncytia thus affecting astroglial homeostatic support. Modulation of astrocytic connexin significantly modifies pharmacological profile of many CNS drugs, which represents an innovative therapeutic approach for CNS disorders; this approach is now actively tested in pre-clinical and clinical studies. Wide combination of connexin modulators with CNS drugs open new promising perspectives for fundamental studies and therapeutic strategies.

LncRNAs regulate the cytoskeleton and related Rho/ROCK signaling in cancer metastasis

Some of the key steps in cancer metastasis are the migration and invasion of tumor cells; these processes require rearrangement of the cytoskeleton. Actin filaments, microtubules, and intermediate filaments involved in the formation of cytoskeletal structures, such as stress fibers and pseudopodia, promote the invasion and metastasis of tumor cells. Therefore, it is important to explore the mechanisms underlying cytoskeletal regulation. The ras homolog family (Rho) and Rho-associated coiled-coil containing protein serine/threonine kinase (ROCK) signaling pathway is involved in the regulation of the cytoskeleton. Moreover, long noncoding RNAs (lncRNAs) have essential roles in tumor migration and guide gene regulation during cancer progression. LncRNAs can regulate the cytoskeleton directly or may influence the cytoskeleton via Rho/ROCK signaling during tumor migration. In this review, we focus on the regulatory association between lncRNAs and the cytoskeleton and discuss the pathways and mechanisms involved in the regulation of cancer metastasis.

Monitoring gap junctional communication in astrocytes from acute adult mouse brain slices using the gap-FRAP technique

Intercellular communication through gap junction channels plays a key role in cellular homeostasis and in synchronizing physiological functions, a feature that is modified in number of pathological situations. In the brain, astrocytes are the cell population that expresses the highest amount of gap junction proteins, named connexins. Several techniques have been used to assess the level of gap junctional communication in astrocytes, but so far they remain very difficult to apply in adult brain tissue. Here, using specific loading of astrocytes with sulforhodamine 101, we adapted the gap-FRAP (Fluorescence Recovery After Photobleaching) to acute hippocampal slices from 9 month-old adult mice. We show that gap junctional communication monitored in astrocytes with this technique was inhibited either by pharmacological treatment with a gap junctional blocker or in mice lacking the two main astroglial connexins, while a partial inhibition was measured when only one connexin was knocked-out. We validate this approach using a mathematical model of sulforhodamine 101 diffusion in an elementary astroglial network and a quantitative analysis of the exponential fits to the fluorescence recovery curves. Consequently, we consider that the adaptation of the gap-FRAP technique to acute brain slices from adult mice provides an easy going and valuable approach that allows overpassing this age-dependent obstacle and will facilitate the investigation of gap junctional communication in adult healthy or pathological brain.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Morphological study of a connexin 43-GFP reporter mouse highlights glial heterogeneity, amacrine cells, and olfactory ensheathing cells

Connexin 43 (Cx43) is the main astrocytic connexin and forms the basis of the glial syncytium. The morphology of connexin-expressing cells can be best studied in transgenic mouse lines expressing cytoplasmic fluorescent reporters, since immunolabeling the plaques can obscure the shapes of the individual cells. The Cx43kiECFP mouse generated by Degen et al. (FASEBJ 26:4576, 2012) expresses cytosolic ECFP and has previously been used to establish that Cx43 may not be expressed by all astrocytes within a population, and this can vary in a region-dependent way. To establish this mouse line as a tool for future astrocyte and connexin research, we sought to consolidate reporter authenticity, studying cell types and within-region population heterogeneity. Applying anti-GFP, all cell types related to astroglia were positive-namely, protoplasmic astrocytes in the hippocampus, cortex, thalamus, spinal cord, olfactory bulb, cerebellum with Bergmann glia and astrocytes also in the molecular layer, and retinal Müller cells and astrocytes. Labeled cell types further comprise white matter astrocytes, olfactory ensheathing cells, radial glia-like stem cells, retinal pigment epithelium cells, ependymal cells, and meningeal cells. We furthermore describe a retinal Cx43-expressing amacrine cell morphologically reminiscent of ON-OFF wide-field amacrine cells, representing the first example of a mammalian CNS neuron-expressing Cx43 protein. In double staining with cell type-specific markers (GFAP, S100ß, glutamine synthetase), Cx43 reporter expression in the hippocampus and cortex was restricted to GFAP⁺ astrocytes. Altogether, this mouse line is a highly reliable tool for studies of Cx43-expressing CNS cells and astroglial cell morphology. © 2017 Wiley Periodicals, Inc.

Astrocytic GAP43 Induced by the TLR4/NF-κB/STAT3 Axis Attenuates Astrogliosis-Mediated Microglial Activation and Neurotoxicity

Growth-associated protein 43 (GAP43), a protein kinase C (PKC)-activated phosphoprotein, is often implicated in axonal plasticity and regeneration. In this study, we found that GAP43 can be induced by the endotoxin lipopolysaccharide (LPS) in rat brain astrocytes both in vivo and in vitro. The LPS-induced astrocytic GAP43 expression was mediated by Toll-like receptor 4 and nuclear factor-κB (NF-κB)- and interleukin-6/signal transducer and activator of transcription 3 (STAT3)-dependent transcriptional activation. The overexpression of the PKC phosphorylation-mimicking GAP43(S41D) (constitutive active GAP43) in astrocytes mimicked LPS-induced process arborization and elongation, while application of a NF-κB inhibitory peptide TAT-NBD or GAP43(S41A) (dominant-negative GAP43) or knockdown of GAP43 all inhibited astrogliosis responses. Moreover, GAP43 knockdown aggravated astrogliosis-induced microglial activation and expression of proinflammatory cytokines. We also show that astrogliosis-conditioned medium from GAP43 knock-down astrocytes inhibited GAP43 phosphorylation and axonal growth, and increased neuronal damage in cultured rat cortical neurons. These proneurotoxic effects of astrocytic GAP43 knockdown were accompanied by attenuated glutamate uptake and expression of the glutamate transporter excitatory amino acid transporter 2 (EAAT2) in LPS-treated astrocytes. The regulation of EAAT2 expression involves actin polymerization-dependent activation of the transcriptional coactivator megakaryoblastic leukemia 1 (MKL1), which targets the serum response elements in the promoter of rat Slc1a2 gene encoding EAAT2. In sum, the present study suggests that astrocytic GAP43 mediates glial plasticity during astrogliosis, and provides beneficial effects for neuronal plasticity and survival and attenuation of microglial activation.

SIGNIFICANCE STATEMENT

Astrogliosis is a complex state in which injury-stimulated astrocytes exert both protective and harmful effects on neuronal survival and plasticity. In this study, we demonstrated for the first time that growth-associated protein 43 (GAP43), a well known growth cone protein that promotes axonal regeneration, can be induced in rat brain astrocytes by the proinflammatory endotoxin lipopolysaccharide via both nuclear factor-κB and signal transducer and activator of transcription 3-mediated transcriptional activation. Importantly, LPS-induced GAP43 mediates plastic changes of astrocytes while attenuating astrogliosis-induced microglial activation and neurotoxicity. Hence, astrocytic GAP43 upregulation may serve to indicate beneficial astrogliosis after CNS injury.

Disentangling calcium-driven astrocyte physiology

Astrocytes seem to rely on relatively sluggish and spatially blurred Ca(2+) waves to communicate with fast and point-precise neural circuits. This apparent discrepancy could, however, reflect our current inability to understand the microscopic mechanisms involved. Difficulties in detecting and interpreting astrocyte Ca(2+) signals may have led to some prominent controversies in the field. Here, we argue that a deeper understanding of astrocyte physiology requires a qualitative leap in our experimental and analytical strategies.

Proximity of excitatory synapses and astroglial gap junctions in layer IV of the mouse barrel cortex

Neurons and astrocytes, the two major cell populations in the adult brain, are characterized by their own mode of intercellular communication--the synapses and the gap junctions (GJ), respectively. In addition, there is increasing evidence for dynamic and metabolic neuroglial interactions resulting in the modulation of synaptic transmission at the so-called "tripartite synapse". Based on this, we have investigated at the ultrastructural level how excitatory synapses (ES) and astroglial GJ are spatially distributed in layer IV of the barrel cortex of the adult mouse. We used specific antibodies for connexin (Cx) 30 and 43 to identify astroglial GJ, these two proteins are known to be present in the majority of astroglial GJ in the cerebral cortex. In electron-microscopic images, we measured the distance between two ES, between two GJ and between a GJ and its nearest ES. We found a ratio of two GJ per three ES in the hollow and septal areas. Taking into account the size of an astrocyte domain, the high density of GJ suggests the occurrence of reflexive type, i.e. GJ between processes of the same astrocyte. Interestingly, the distance between an ES and an astroglial GJ was found to be significantly lower than that between either two synapses or between two GJ. These observations indicate that the two modes of cell-to-cell communication are not randomly distributed in layer IV of the barrel cortex. Consequently, this feature may provide the morphological support for the recently reported functional interactions between neuronal circuits and astroglial networks.

Connexins-mediated glia networking impacts myelination and remyelination in the central nervous system

In the central nervous system (CNS), the glial gap junctions are established among astrocytes (ASTs), oligodendrocytes (OLs), and/or between ASTs and OLs due to the expression of membrane proteins called connexins (Cxs). Together, the glial cells form a network of communicating cells that is important for the homeostasis of brain function for its involvement in the intercellular calcium wave propagation, exchange of metabolic substrates, cell proliferation, migration, and differentiation. Alternatively, Cxs are also involved in hemichannel function and thus participate in gliotransmission. In recent years, pathologic changes of oligodendroglia or demyelination found in transgenic mice with different subsets of Cxs or pharmacological insults suggest that glial Cxs may participate in the regulation of the myelination or remyelination processes. However, little is known about the underlying mechanisms. In this review, we will mainly focus on the functions of Cx-mediated gap junction channels, as well as hemichannels, in brain glial cells and discuss the way by which they impact myelination and remyelination. These aspects will be considered at the light of recent genetic and non-genetic studies related to demyelination and remyelination.

Studying subcellular detail in fixed astrocytes: dissociation of morphologically intact glial cells (DIMIGs)

Studying the distribution of astrocytic antigens is particularly hard when they are localized in their fine, peripheral astrocyte processes (PAPs), since these processes often have a diameter comparable to vesicles and small organelles. The most appropriate technique is immunoelectron microscopy, which is, however, a time-consuming procedure. Even in high resolution light microscopy, antigen localization is difficult to detect due to the small dimensions of these processes, and overlay from antigen in surrounding non-glial cells. Yet, PAPs frequently display antigens related to motility and glia-synaptic interaction. Here, we describe the dissociation of morphologically intact glial cells (DIMIGs), permitting unambiguous antigen localization using epifluorescence microscopy. Astrocytes are dissociated from juvenile (p13-15) mouse cortex by applying papain treatment and cytospin centrifugation to attach the cells to a slide. The cells and their complete processes including the PAPs is thus projected in 2D. The entire procedure takes 2.5-3 h. We show by morphometry that the diameter of DIMIGs, including the PAPs is similar to that of astrocytes in situ. In contrast to cell culture, results derived from this procedure allow for direct conclusions relating to (1) the presence of an antigen in cortical astrocytes, (2) subcellular antigen distribution, in particular when localized in the PAPs. The detailed resolution is shown in an exemplary study of the organization of the astrocytic cytoskeleton components actin, ezrin, tubulin, and GFAP. The distribution of connexin 43 in relation to a single astrocyte's process tree is also investigated.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

Connexin and pannexin mediated cell-cell communication

In this review, we briefly summarize what is known about the properties of the three families of gap junction proteins, connexins, innexins and pannexins, emphasizing their importance as intercellular channels that provide ionic and metabolic coupling and as non-junctional channels that can function as a paracrine signaling pathway. We discuss that two distinct groups of proteins form gap junctions in deuterostomes (connexins) and protostomes (innexins), and that channels formed of the deuterostome homologues of innexins (pannexins) differ from connexin channels in terms of important structural features and activation properties. These differences indicate that the two families of gap junction proteins serve distinct, complementary functions in deuterostomes. In several tissues, including the CNS, both connexins and pannexins are involved in intercellular communication, but have different roles. Connexins mainly contribute by forming the intercellular gap junction channels, which provide for junctional coupling and define the communication compartments in the CNS. We also provide new data supporting the concept that pannexins form the non-junctional channels that play paracrine roles by releasing ATP and, thus, modulating the range of the intercellular Ca(2+)-wave transmission between astrocytes in culture.

Glial cells in (patho)physiology

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites, astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca(2+) signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca(2+) -dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer's disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review not only sheds new light on the brain operation in health and disease, but also points to many unknowns.

Role of gap junctions in epilepsy

Epilepsy is a common neurological disorder characterized by periodic and unpredictable seizures. Gap junctions have recently been proposed to be involved in the generation, synchronization and maintenance of seizure events. The present review mainly summarizes recent reports concerning the contribution of gap junctions to the pathophysiology of epilepsy, together with the regulation of connexin after clinical and experimental seizure activity. The anticonvulsant effects of gap junction blockers both in vitro and in vivo suggest that the gap junction is a candidate target for the development of antiepileptic drugs. It is also of interest that the roles of neuronal and astrocytic gap junctions in epilepsy have been investigated independently, based on evidence from pharmacological manipulations and connexin-knockout mice. Further studies using more specific manipulations of gap junctions in different cell types and in human epileptic tissue are needed to fully uncover the role of gap junctions in epilepsy.

Morphology and properties of astrocytes

Astrocytes were identified about 150 years ago, and, for the longest time, were considered to be supporting cells in the brain providing trophic, metabolic, and structural support for neural networks. Research in the last 2 decades has uncovered many novel molecules in astrocytes and the finding that astrocytes communicate with neurons via Ca2+ signaling, which leads to release of chemical transmitters, termed gliotransmitters, has led to renewed interest in their biology. This chapter will briefly review the unique morphology and molecular properties of astrocytes. The reader will be introduced to the role of astrocytes in blood-brain barrier (BBB) maintenance, in Ca2+ signaling, in synaptic transmission, in CNS synaptogenesis, and as neural progenitor cells. Mention is also made of the diseases in which astrocyte dysfunction has a role.

PKC inhibition increases gap junction intercellular communication and cell adhesion in human neuroblastoma

Gap junction intercellular communication and cell-cell adhesion are essential for maintaining a normal cellular phenotype, including the control of growth and proliferation. Loss of either cell-cell adhesion or communication is common in cancers, while restoration of function is associated with tumor suppression. Protein kinase C (PKC) isozymes regulate a broad spectrum of cellular functions including growth and proliferation, and their overexpression has been correlated with carcinogenesis. Consequently, PKC inhibitors are currently undergoing clinical trials as an anti-cancer agents although the precise cellular alterations induced by PKC inhibitors remain to be elucidated. In the current study, the effects of PKC inhibitors on cell interactions were investigated using human neuroblastoma (IMR32, SKNMC, and SHSY-5Y) cell lines. An analysis of intercellular communication revealed an increase in gap junctional coupling with PKC inhibition. The observed increase in coupling was not associated with a change in Connexin 43 distribution or an alteration of phosphorylation status of the protein. There was also an increase in cell-cell adhesion with PKC inhibitor treatment as indicated by a cell aggregation assay. Therefore, the growth suppressive abilities of PKC inhibition on tumors may be due to the cancer suppressive effects of increased gap junction intercellular communication and cell-cell adhesion.

Hyperglycaemia and diabetes impair gap junctional communication among astrocytes

Sensory and cognitive impairments have been documented in diabetic humans and animals, but the pathophysiology of diabetes in the central nervous system is poorly understood. Because a high glucose level disrupts gap junctional communication in various cell types and astrocytes are extensively coupled by gap junctions to form large syncytia, the influence of experimental diabetes on gap junction channel-mediated dye transfer was assessed in astrocytes in tissue culture and in brain slices from diabetic rats. Astrocytes grown in 15–25 mmol/l glucose had a slow-onset, poorly reversible decrement in gap junctional communication compared with those grown in 5.5 mmol/l glucose. Astrocytes in brain slices from adult STZ (streptozotocin)-treated rats at 20–24 weeks after the onset of diabetes also exhibited reduced dye transfer. In cultured astrocytes grown in high glucose, increased oxidative stress preceded the decrement in dye transfer by several days, and gap junctional impairment was prevented, but not rescued, after its manifestation by compounds that can block or reduce oxidative stress. In sharp contrast with these findings, chaperone molecules known to facilitate protein folding could prevent and rescue gap junctional impairment, even in the presence of elevated glucose level and oxidative stress. Immunostaining of Cx (connexin) 43 and 30, but not Cx26, was altered by growth in high glucose. Disruption of astrocytic trafficking of metabolites and signalling molecules may alter interactions among astrocytes, neurons and endothelial cells and contribute to changes in brain function in diabetes. Involvement of the microvasculature may contribute to diabetic complications in the brain, the cardiovascular system and other organs.

Cx43 has distinct mobility within plasma-membrane domains, indicative of progressive formation of gap-junction plaques

Connexin 43 (Cx43) is a dynamic molecule, having a short half-life of only a few hours. In this study, we use fluorescent-protein-tagged Cx43 variants to examine Cx43 delivery to the cell surface, its residency status in various cell-surface membrane domains and its mobility characteristics. Rapid time-lapse imaging led to the identification of Cx43 being delivered to cell-surface domains that lacked a contacting cell, and also to its localization within membrane protrusions. Fluorescence recovery after photobleaching (FRAP) was used to investigate the mobility state of cell-surface-localized Cx43. Cx43 mobility within clustered cell-surface profiles of Cx43 could be categorized into those with generally a high degree of lateral mobility and those with generally a low degree of lateral mobility. Cx43 mobility was independent of cluster size, yet the C-terminal domain of Cx43 regulated the proportion of gap-junction-like clusters that acquired a low Cx43 mobility state. Collectively, these studies show that Cx43 establishes residency at all cell-surface membrane domains, and progressively acquires assembly states that probably reflect differences in either channel packing and/or its interactions with Cx43-binding proteins.

A role for Connexin43 during neurodevelopment

Connexin43 (Cx43) is the predominant gap junction protein expressed in premitotic radial glial cells and mature astrocytes. It is thought to play a role in many aspects of brain development and physiology, including intercellular communication, the release of neuroactive substances, and neural and glial proliferation and migration. To investigate the role of Cx43 in brain physiology, we generated a conditional knockout (cKO) mouse expressing Cre recombinase driven by the human GFAP promoter and a floxed Cx43 gene. The removal of Cx43 from GFAP-expressing cells affects the behavior of the mice and the development of several brain structures; however, the severity of the phenotype varies depending on the mouse background. One mouse subline, hereafter termed Shuffler, exhibits cellular disorganization of the cortex, hippocampus, and cerebellum, accompanied by ataxia and motor deficits. The Shuffler cerebellum is most affected and displays altered distribution and lamination of glia and neurons suggestive of cell migration defects. In all Shuffler mice by postnatal day two (P2), the hippocampus, cortex, and cerebellum are smaller. Disorganization of the ventricular and subventricular zone of the cortex is also evident. Given that these are sites of early progenitor cell proliferation, we suspect production and migration of neural progenitors may be altered. In conclusion, neurodevelopment of Shuffler/Cx43 cKO mice is abnormal, and the observed cellular phenotype may explain behavioral disturbances seen in these animals as well as in humans carrying Cx43 mutations.

Astrocytic complexity distinguishes the human brain

One of the most distinguishing features of the adult human brain is the complexity and diversity of its cortical astrocytes. Human protoplasmic astrocytes manifest a threefold larger diameter and have tenfold more primary processes than those of rodents. In all mammals, protoplasmic astrocytes are organized into spatially non-overlapping domains that encompass both neurons and vasculature. Yet unique to humans and primates are additional populations of layer 1 interlaminar astrocytes that extend long (millimeter) fibers, and layer 5-6 polarized astrocytes that also project distinctive long processes. We propose that human cortical evolution has been accompanied by increasing complexity in the form and function of astrocytes, which reflects an expansion of their functional roles in synaptic modulation and cortical circuitry.

Sensitivity of the brain transcriptome to connexin ablation

Extensive studies on mice with total or partial disruption of either connexin43 (Cx43) or connexin32 (Cx32) have detected only subtle changes in central nervous system structure, growth, development, or function. We have used high density cDNA arrays to analyze the regulation, control, and coordination of the abundances of 7446 distinct transcripts in four brains, each of Cx43 null (K43), Cx43 heterozygous (H43), and Cx32 null (K32) mice as compared to the brains of wildtype (W) mice. The use of multiple samples allowed the determination of the statistical significance of gene regulation. Significantly regulated genes encoded proteins of all functional categories, extending beyond those that might be expected to depend on junctional communication. Moreover, we found a high degree of similarity between genes regulated in the K43 and H43 brains and a remarkable overlap between gene regulation in brains of K43 and K32. The regulated genes in both K43 and H43 brains showed an outstanding inverse coordination with the levels of expression of Cx43 in W brain, indicating that the regulated genes are largely predictable from their co-variance with Cx43 in the wildtype samples. These findings lead to the hypothesis that connexin expression may represent a central node in the regulation of gene expression patterns in brain.

New insights into the expression and function of neural connexins with transgenic mouse mutants

Gap junctions represent direct intercellular conduits between contacting cells. The subunit proteins of these conduits are called connexins. To date, 20 and 21 connexin genes have been described in the mouse and human genome, respectively, many of them represent sequence-orthologous pairs. Targeted deletion of connexin genes in the mouse genome opened new insights into the biological function of these channel forming proteins, which, in some cases, could be correlated to phenotypic abnormalities in humans, suffering from inherited diseases caused by mutations in the corresponding orthologous connexin gene. Replacing the connexin coding DNA by an appropriate reporter gene has clarified in several cases its cell type specific expression in mouse brain. Various studies demonstrated that connexin36 is mainly expressed in interneurons of retina and brain. Targeted deletion of connexin36 evoked a loss of electrical signal transduction and interferes with synchrony which probably leads to defects in visual transmission and memory. Deletion of connexin43 in astrocytes of mouse brain resulted in increased spreading depression consistent with the notion of altered "spatial buffering" of K(+) ions and glutamate secreted by active neurons. General connexin30-deficiency led to hearing impairment and apoptosis of hair cells, similar to that observed in mice with cochlea specific deletion of connexin26. Reporter gene expression in connexin30-deficient mice indicated that astrocytes in certain brain regions and leptomeningeal as well as ependymal cells are labelled. Reporter gene expression in connexin45- and connexin47-deficient mice was used to reassign connexin45 expression to certain CNS neurons and connexin47 expression to oligodendrocytes.

Alterations in metabolism and gap junction expression may determine the role of astrocytes as ?good samaritans? or executioners

Our knowledge of astroglia and their physiological and pathophysiological role(s) in the central nervous system (CNS) has grown during the past decade, revealing a complex picture. It is becoming increasingly clear that glia play a significant role in the homeostasis and function of the CNS and that neurons should no longer be considered the only cell type that responds, both rapidly and slowly, to electrochemical activity. We discuss recent advances in the field with an emphasis on the impact of hypoxia and ischemia on astrocytic metabolism and the functional relationship between glucose metabolism and gap junctions in astrocytes. We also address the controversy over whether astrocytic gap junctions mediate protection or killing of neurons during or after hypoxic or ischemic insults.

Failure to express GAP-43 leads to disruption of a multipotent precursor and inhibits astrocyte differentiation

The nervous system-specific protein GAP-43 is significantly upregulated in neurons and glia that are differentiating. In P19 EC cells that do not express GAP-43, neurogenesis is inhibited; many immature neurons apoptose and the survivors do not mature morphologically. Here we show that the initial defect is in an early precursor with characteristics of a neural stem cell, which failed to respond normally to retinoic acid (RA). As a consequence, its progeny had altered cell fates: In addition to the neuronal defects previously reported, RC1-labeled radial glia failed to exit the cell cycle, accumulated, and failed to acquire GFAP immunoreactivity. However, leukemia inhibitory factor (LIF) could stimulate GFAP expression suggesting that astrocytes not derived from radial glia are less affected by absence of GAP-43. Differentiation of radial glia-derived astrocytes was also inhibited in glial cultures from GAP-43 (-/-) cerebellum, and in GAP-43 (-/-) telencephalon in vivo, differentiation of astrocytes derived from both radial and nonradial glia lineages were both affected: In the glial wedge, GFAP-labeled radial glia-derived astrocytes were reduced consistent with the interpretation that they may be unable to deflect GAP-43 (-/-) commissural axons toward the midline. At the midline, both radial and nonradial glia-derived astrocytes were also decreased although it fused normally. The results demonstrate that GAP-43 expressed in multipotent precursors is required for appropriate cell fate commitment, and that its absence affects astrocyte as well as neuronal differentiation.

Nutrition during brain activation: does cell-to-cell lactate shuttling contribute significantly to sweet and sour food for thought?

Functional activation of astrocytic metabolism is believed, according to one hypothesis, to be closely linked to excitatory neurotransmission and to provide lactate as fuel for oxidative metabolism in neighboring neurons. However, review of emerging evidence suggests that the energetic demands of activated astrocytes are higher and more complex than recognized and much of the lactate presumably produced by astrocytes is not locally oxidized during activation. In vivo activation studies in normal subjects reveal that the rise in consumption of blood-borne glucose usually exceeds that of oxygen, especially in retina compared to brain. When the contribution of glycogen, the brain's major energy reserve located in astrocytes, is taken into account the magnitude of the carbohydrate-oxygen utilization mismatch increases further because the magnitude of glycogenolysis greatly exceeds the incremental increase in utilization of blood-borne glucose. Failure of local oxygen consumption to equal that of glucose plus glycogen in vivo is strong evidence against stoichiometric transfer of lactate from astrocytes to neighboring neurons for oxidation. Thus, astrocytes, not nearby neurons, use the glycogen for energy during physiological activation in normal brain. These findings plus apparent compartmentation of metabolism of glycogen and blood-borne glucose during activation lead to our working hypothesis that activated astrocytes have high energy demands in their fine perisynaptic processes (filopodia) that might be met by glycogenolysis and glycolysis coupled to rapid lactate clearance. Tissue culture studies do not consistently support the lactate shuttle hypothesis because key elements of the model, glutamate-induced increases in glucose utilization and lactate release, are not observed in many astrocyte preparations, suggesting differences in their oxidative capacities that have not been included in the model. In vivo nutritional interactions between working neurons and astrocytes are not as simple as implied by "sweet (glucose-glycogen) and sour (lactate) food for thought."

Neighborly interactions of metabolically-activated astrocytes in vivo

Metabolic responses of brain cells to a stimulus are governed, in part, by their enzymatic specialization and interrelationships with neighboring cells, and local shifts in functional metabolism during brain activation are likely to be influenced by the neurotransmitter system, subcellular compartmentation, and anatomical structure. Selected examples of functional activation illustrate the complexity of metabolic interactions in working brain and of interpretation of changes in brain lactate levels. The major focus of this article is the disproportionately higher metabolism of glucose compared to oxygen in normoxic brain, a phenomenon that occurs during activation in humans and animals. The glucose utilized in excess of oxygen is not fully explained by accumulation of glucose, lactate, or glycogen in brain or by lactate efflux from brain to blood. Thus, any lactate derived from the excess glucose could not have been stoichiometrically exported to and metabolized by neighboring neurons because oxygen consumption would have otherwise increased and matched that of glucose. Metabolic labeling of tricarboxylic acid cycle-derived amino acids increased during brief sensory stimulation, reflecting a rise in oxidative metabolism. Brain glycogen is mainly in astrocytes, and its level falls throughout the stimulus and early post-activation interval. Glycogenolysis cannot be accounted for by lactate accumulation or oxidation; there must be rapid product clearance. Glycogen restoration is slow and diversion of glucose from oxidative pathways for its re-synthesis could reduce the global O(2)/glucose uptake ratio; astrocytes could downshift this ratio for up to an hour after 5 min stimulus. Morphological studies of astrocytes reveal a paucity of cytoplasm and organelles in the fine processes that surround synapses and form gap junction connections with neighboring astrocytes. Specialized regions of astrocytes, e.g. their endfeet and thin peripheral lamellae, are likely to have compartmentalized metabolic activities. Anatomical constraints imposed upon the fine processes might require preferential utilization of glycolysis to satisfy their energy demands, but rapid lactate clearance would then be essential, since its accumulation would inhibit glycolysis. Gap junctional connections between neighboring astrocytes provide a mechanism for rapid metabolite spreading via the astrocytic syncytium and elimination of by-products. Local structure-function relationships need to be incorporated into experimental models of neuron-astrocyte and astrocyte-astrocyte interactions in working brain.

Functional hemichannels in astrocytes: a novel mechanism of glutamate release

Little is known about the expression and possible functions of unopposed gap junction hemichannels in the brain. Emerging evidence suggests that gap junction hemichannels can act as stand-alone functional channels in astrocytes. With immunocytochemistry, dye uptake, and HPLC measurements, we show that astrocytes in vitro express functional hemichannels that can mediate robust efflux of glutamate and aspartate. Functional hemichannels were confirmed by passage of extracellular lucifer yellow (LY) into astrocytes in nominal divalent cation-free solution (DCFS) and the ability to block this passage with gap junction blocking agents. Glutamate/aspartate release (or LY loading) in DCFS was blocked by multivalent cations (Ca2+, Ba2+, Sr2+, Mg2+, and La3+) and by gap junction blocking agents (carbenoxolone, octanol, heptanol, flufenamic acid, and 18alpha-glycyrrhetinic acid) with affinities close to those reported for blockade of gap junction intercellular communication. Glutamate efflux via hemichannels was also accompanied by greatly reduced glutamate uptake. Glutamate release in DCFS, however, was not significantly mediated by reversal of the glutamate transporter: release did not saturate and was not blocked by glutamate transporter blockers. Control experiments in DCFS precluded glutamate release by volume-sensitive anion channels, P2X7 purinergic receptor pores, or general purinergic receptor activation. Blocking intracellular Ca2+ mobilization by BAPTA-AM or thapsigargin did not inhibit glutamate release in DCFS. Divalent cation removal also induced glutamate release from intact CNS white matter (acutely isolated optic nerve) that was blocked by carbenoxolone, suggesting the existence of functional hemichannels in situ. Our results indicated that astrocyte hemichannels could influence CNS levels of extracellular glutamate with implications for normal and pathological brain function.

Rapid morphological changes in astrocytes are accompanied by redistribution but not by quantitative changes of cytoskeletal proteins

Astrocytes have the potential to acquire very different morphologies, depending on their regional location in the CNS and on their functional interactions with other cell types. Morphological changes between a flat or a fibroblast-like and a stellate or process-bearing appearance, and vice versa, can occur rapidly, but very little is known as to whether morphological transformations are based on quantitative changes of cytoskeletal proteins in microfilaments, intermediate filaments, and/or microtubules. Using a cell culture of selective type 1 astrocytes, we compared the distribution and protein amounts of a number of cytoskeletal proteins both during primary process growth induced by specific media conditions and after secondary transformations induced by dBcAMP. Our data presented in this report support the idea that astrocytes can undergo dramatic changes in their morphology requiring subcellular redistribution of most cytoskeletal proteins but no quantitative modifications of the amount of the respective proteins. After pharmacological treatment with lysophosphatic acid and genistein we show that astrocytes can acquire intermediate morphologies reminiscent of both fibroblast and stellate-like cells. These experiments demonstrate that the recently described RhoA-mediated signaling cascade between the cell surface and cytoskeletal proteins is only one of several signaling pathways acting on the astrocytic cytoskeleton.

Expression and function of astrocytic gap junctions in aging

Astrocytic gap junctions have been implicated in a variety of signaling pathways essential to normal brain function. However, no information exists on the prevalence of gap junction channels and their function in the aging brain. Here we have compared the expression of the two most abundant astrocytic gap junction proteins in young and senescent brains and quantified the extent of functional gap junction coupling. The expression level of Cx43 peaked in 7-month-old mice. The relative numbers of Cx43 immunoreactive plaques were 596+/-61, 734+/-62, and 755+/-114 in 3-, 7-, and 21-month-old mice, whereas plaques size averaged 0.9+/-0.1 microm(2) (3 months), 1.3+/-0.1 microm(2) (7 months), and 0.7+/-0.1 microm(2) (21 months). The expression level of Cx30 was also highest in 7-month-old animals (315+/-49 plaques, size 0.8+/-0.07 microm(2) vs. 585+/-51 plaques, size 0.9+/-0.1 microm(2) in 3- and 7-month-old mice, respectively), but only 262+/-63 plaques (size 0.4+/-0.04 microm(2)) in 21-month-old mice. Western blot analysis revealed that the content of both Cx43 and Cx30 remained relatively constant at 3, 7, and 21 months. The fluorescence recovery of photobleach technique (FRAP) was used to evaluate coupling in freshly prepared hippocampal slices. Gap junction coupling did not change significantly as a function of aging, but a tendency towards reduced coupling was observed as the animals aged. Average fluorescence recovery after 2 min was 63+/-6% in younger animals, 59+/-5% in adult animals, and 54+/-4% in old brain. These observations indicate that although astrocytic gap junction proteins are maintained at high levels through the entire lifespan of mice, aging is associated with changes in the number and size of both Cx30 and Cx43 gap junction plaques.

Cell-specific expression of connexins and evidence of restricted gap junctional coupling between glial cells and between neurons

The transmembrane connexin proteins of gap junctions link extracellularly to form channels for cell-to-cell exchange of ions and small molecules. Two primary hypotheses of gap junction coupling in the CNS are the following: (1) generalized coupling occurs between neurons and glia, with some connexins expressed in both neurons and glia, and (2) intercellular junctional coupling is restricted to specific coupling partners, with different connexins expressed in each cell type. There is consensus that gap junctions link neurons to neurons and astrocytes to oligodendrocytes, ependymocytes, and other astrocytes. However, unresolved are the existence and degree to which gap junctions occur between oligodendrocytes, between oligodendrocytes and neurons, and between astrocytes and neurons. Using light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling of adult rat CNS, we investigated whether four of the best-characterized CNS connexins are each present in one or more cell types, whether oligodendrocytes also share gap junctions with other oligodendrocytes or with neurons, and whether astrocytes share gap junctions with neurons. Connexin32 (Cx32) was found only in gap junctions of oligodendrocyte plasma membranes, Cx30 and Cx43 were found only in astrocyte membranes, and Cx36 was only in neurons. Oligodendrocytes shared intercellular gap junctions only with astrocytes, with each oligodendrocyte isolated from other oligodendrocytes except via astrocyte intermediaries. Finally, neurons shared gap junctions only with other neurons and not with glial cells. Thus, the different cell types of the CNS express different connexins, which define separate pathways for neuronal versus glial gap junctional communication.

Identification of cells expressing Cx43, Cx30, Cx26, Cx32 and Cx36 in gap junctions of rat brain and spinal cord

We have identified cells expressing Cx26, Cx30, Cx32, Cx36 and Cx43 in gap junctions of rat central nervous system (CNS) using confocal light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling (FRIL). Confocal microscopy was used to assess general distributions of connexins, whereas the 100-fold higher resolution of FRIL allowed co-localization of several different connexins within individual ultrastructurally-defined gap junction plaques in ultrastructurally and immunologically identified cell types. In >4000 labeled gap junctions found in >370 FRIL replicas of gray matter in adult rats, Cx26, Cx30 and Cx43 were found only in astrocyte gap junctions; Cx32 was only in oligodendrocytes, and Cx36 was only in neurons. Moreover, Cx26, Cx30 and Cx43 were co-localized in most astrocyte gap junctions. Oligodendrocytes shared intercellular gap junctions only with astrocytes, and these heterologous junctions had Cx32 on the oligodendrocyte side and Cx26, Cx30 and Cx43 on the astrocyte side. In 4 and 18 day postnatal rat spinal cord, neuronal gap junctions contained Cx36, whereas Cx26 was present in leptomenigeal gap junctions. Thus, in adult rat CNS, neurons and glia express different connexins, with "permissive" connexin pairing combinations apparently defining separate pathways for neuronal vs. glial gap junctional communication.

Gap junctions in the nervous system

Synapses are classically defined as close connections between two nerve cells or between a neuronal cell and a muscle or gland cell across which a chemical signal (i.e., a neurotransmitter) and/or an electrical signal (i.e., current-carrying ions) can pass. The definition of synapse was developed by Charles Sherrington and by Ramon y Cajal at the beginning of this century and refined by John Eccles and Bernard Katz 50 years later; in this collection of papers, the definition of synapses is discussed further in the chapter by Mike Bennett. who provided the first functional demonstration of electrical transmission via gap junction channels between vertebrate neurons. As is evidenced by the range of topics covered in this issue, research dealing with gap junctions in the nervous system has expanded enormously in the past decade, major findings being that specific cell types in the brain expresses specific types of connexins and that expression patterns coincide with tissue compartmentalization and function and that these compartments change during development.

Connexin channels in Schwann cells and the development of the X-linked form of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease comprises a group of genetically heterogenous disorders of the peripheral nervous system. The X-linked form of Charcot-Marie-Tooth (CMTX) is associated with mutations in the gene encoding the gap junction protein connexin32 (Cx32), which is expressed in Schwann cells. Immunocytochemical evidence suggests that Cx32 is localized to the incisures of Schmidt-Lanterman and the paranodes of myelinating Schwann cells, where it appears to form reflexive gap junctions. It is currently thought that this cytoplasmic continuity provides a much shorter diffusion pathway for the transport of ions, metabolites and second messenger molecules through intracellular channels between the adaxonal and peri-nuclear regions of Schwann cells, across the myelin sheath. This review summarizes our current understanding of the role of connexins in Schwann cells and focuses on the lessons for channel function and disease pathophysiology derived from the functional analysis of Cx32 mutations. One of the most intriguing aspects emerging from this work is that several mutations retain functional competence, although the mutated channels exhibit altered gating properties. This suggests that partial and/or selective disruption of the radial communication pathway formed by Cx32 is sufficient to cause a functional deficit and lead to the development of CMTX. The next challenge will be to define, at the molecular level, the sequence of events involved in the disease process. The presence of a group of functional mutations should help understand the cellular basis of CMTX, by allowing the identification of the specific molecules that need to be exchanged through Cx32 channels, but are excluded from the mutated ones.

Connexins and gap junctions of astrocytes and oligodendrocytes in the CNS

This review article summarizes early and recent literature on the structure, distribution and composition of gap junctions between astrocytes and oligodendrocytes, and the differential expression of glial connexins in adult and developing mammalian CNS. In addition to an overview of the topic, discussion is focused on the organization of homologous gap junctional interactions between astrocytes and between oligodendrocytes as well as on heterologous junctional coupling between astrocytes and oligodendrocytes. The homotypic and heterotypic nature of these gap junctions is related to the connexins known to be produced by glial cells in the intact brain and spinal cord. Emphasis is placed on the ultrastructural level of analysis required to attribute gap junction and connexin deployment to particular cell types and subcellular locations. Our aim is to provide a firm basis for consideration of anticipated rapid advances in understanding of structural relationships of gap junctions and connexins within the glial gap junctional syncytium. Conclusions to date suggest that the glial syncytium is more complex than previously appreciated and that glial pathways of junctional communication may not only be determined by the presence of gap junctions, but also by the connexin composition and conductance regulation of junctional channels.