Immunorecognition, ultrastructure and phosphorylation status of astrocytic gap junctions and connexin43 in rat brain after cerebral focal ischaemia

Regulation of connexin43-protein binding in astrocytes in response to chemical ischemia/hypoxia

Connexin-protein interactions are believed to be critical for the regulation of gap junctional intercellular communication and for the function of gap junctions formed by these complexes. We have primarily used immunoprecipitation strategies to investigate whether connexin43 binds to selected signaling and cytoskeletal proteins and whether connexin43-protein binding is altered in cultured astrocytes exposed to chemical ischemia/hypoxia, a treatment that resembles ischemia in vivo. Chemical ischemia/hypoxia induced marked dephosphorylation of connexin43, which was accompanied by increased association of connexin43 with c-Src, ERK1/2, and mitogen-activated protein kinase phosphatase-1 and by decreased association between connexin43 and beta-actin. Moreover, we found that endogenous c-Src in normal astrocytes exists primarily in the Triton X-100-soluble membrane fraction, distinct from the Triton-insoluble fraction, which contains gap junctions. After chemical ischemia/hypoxia, c-Src appeared in the Triton-insoluble fraction and was co-immunoprecipitated with connexin43, suggesting that chemical ischemia/hypoxia induced translocation of c-Src to the Triton-insoluble fraction and association with connexin43. Furthermore, the "dephosphorylated" form of connexin43 was immunoprecipitated by a phosphotyrosine antibody, suggesting tyrosine phosphorylation of connexin43 by c-Src. In addition, the association between connexin43 and c-Src was blocked by inhibition of connexin43 dephosphorylation, suggesting that the interaction between connexin43 and c-Src can be regulated by alterations in the phosphorylation state of connexin43. These results identify new binding partners for connexin43 and demonstrate that interactions between connexin43 and protein kinases and phosphatases are dynamically altered as a consequence of connexin43 phosphorylation.

Connexin47, connexin29 and connexin32 co-expression in oligodendrocytes and Cx47 association with zonula occludens-1 (ZO-1) in mouse brain

Gap junctions between glial cells in mammalian CNS are known to contain several connexins (Cx), including Cx26, Cx30 and Cx43 at astrocyte-to-astrocyte junctions, and Cx29 and Cx32 on the oligodendrocyte side of astrocyte-to-oligodendrocyte junctions. Recent reports indicating that oligodendrocytes also express Cx47 prompted the present studies of Cx47 localization and relationships to other glial connexins in mouse CNS. In view of the increasing number of connexins reported to interact directly with the scaffolding protein zonula occludens-1 (ZO-1), we investigated ZO-1 expression and Cx47/ZO-1 interaction capabilities in brain, spinal cord and Cx47-transfected HeLa cells. From counts of over 9000 oligodendrocytes labeled by immunofluorescence in various brain regions, virtually all of these cells were found to express Cx29, Cx32 and Cx47. Oligodendrocyte somata displayed robust Cx47-immunopositive puncta that were co-localized with punctate labeling for Cx32 and Cx43. By freeze-fracture replica immunogold labeling, Cx47 was abundant on the oligodendrocyte-side of oligodendrocyte/astrocyte gap junctions. By immunofluorescence, labeling for Cx47 along myelinated fibers was sparse in most brain regions, whereas Cx29 and Cx32 were previously found to be concentrated along these fibers. By immunogold labeling, Cx47 was found in numerous small gap junctions linking myelin to astrocytes, but not within deeper layers of myelin. Brain subcellular fractionation revealed a lack of Cx47 enrichment in myelin fractions, which nevertheless contained an enrichment of Cx32 and Cx29. Oligodendrocytes were immunopositive for ZO-1, and displayed almost total Cx47/ZO-1 co-localization. ZO-1 was found to co-immunoprecipitate with Cx47, and pull-down assays indicated binding of Cx47 to the second PDZ domain of ZO-1. Our results indicate widespread expression of Cx47 by oligodendrocytes, but with a distribution pattern in relative levels inverse to the abundance of Cx29 in myelin and paucity of Cx29 in oligodendrocyte somata. Further, our findings suggest a scaffolding and/or regulatory role of ZO-1 at the oligodendrocyte side of astrocyte-to-oligodendrocyte gap junctions.

Increased apoptosis and inflammation after focal brain ischemia in mice lacking connexin43 in astrocytes

Astrocytes secrete cytokines and neurotrophic factors to neurons, consistent with a neurosupportive role for astrocytes. However, in ischemic or metabolic insults, the function of astrocytic gap junctions composed mainly from connexin43 (Cx43) remains controversial. We have previously shown that heterozygous Cx43 null mice subjected to middle cerebral artery occlusion exhibited significantly enhanced stroke volume and apoptosis compared to wild-type mice. In this study, we used mice in which the human GFAP promoter-driven cre transgene deletes the floxed Cx43 gene in astrocytes, excluding the effects from reduced Cx43 expression in many other cell types as well as astrocytes. We induced focal brain ischemia in mice lacking Cx43 in astrocytes [Cre(+)] and control littermates [Cre(-)]. Cre(+) mice showed a significantly increased stroke volume and enhanced apoptosis, detected by terminal dUTP nick-end labeling and caspase-3 immunostaining, compared to Cre(-) mice. Inflammatory response assessed by the microglial marker CD11b was amplified in the penumbra of Cre(+) mice compared to that of Cre(-) mice. Our results suggest that astrocytic gap junctions could be important for the regulation of neuronal apoptosis and the inflammatory response after stroke. These findings support the view that astrocytes play a critical role in neuroprotection during ischemic insults.

Regulation of Connexin43 Protein Complexes by Intracellular Acidification

Ischemia-induced acidification of astrocytes or cardiac myocytes reduces intercellular communication by closing gap junction channels and subsequently internalizing gap junction proteins. To determine whether such coupling changes might be attributable to altered interactions between connexin43 (Cx43) and other proteins, we applied the nigericin/high K + method to vary intracellular pH (pHi) in cultured cortical astrocytes. Intracellular acidification was accompanied by internalization of Cx43 with retention of Cx43 scaffolding protein Zonula Occludens-1 (ZO-1) at cell surfaces, suggesting that ZO-1 and Cx43 dissociate at low pHi. Coimmunoprecipitation studies revealed decreased binding of ZO-1 and increased binding of c-Src to Cx43 at low pHi. Resonant mirror spectroscopy was used to quantify binding of the SH3 domain of c-Src and the PDZ domains of ZO-1 to the carboxyl terminal domain of Cx43 (Cx43CT). Data indicate that the c-Src/Cx43CT interaction is highly pH dependent whereas the ZO-1/Cx43CT interaction is not. Moreover, binding of c-Src to Cx43CT prevented and reversed ZO-1/Cx43CT binding. We hypothesize that increased affinity of c-Src for Cx43 at low pHi aids in separation of Cx43 from ZO-1 and that this may facilitate internalization of Cx43. These data suggest that protracted acidification may remodel protein-protein interactions involving Cx43 and thus provide an important protective mechanism to limit lesion spread after ischemic injury.

Coupling of astrocyte connexins Cx26, Cx30, Cx43 to oligodendrocyte Cx29, Cx32, Cx47: Implications from normal and connexin32 knockout mice

Oligodendrocytes in vivo form heterologous gap junctions with astrocytes. These oligodendrocyte/astrocyte (A/O) gap junctions contain multiple connexins (Cx), including Cx26, Cx30, and Cx43 on the astrocyte side, and Cx32, Cx29, and Cx47 on the oligodendrocyte side. We investigated connexin associations at A/O gap junctions on oligodendrocytes in normal and Cx32 knockout (KO) mice. Immunoblotting and immunolabeling by several different antibodies indicated the presence of Cx32 in liver and brain of normal mice, but the absence of Cx32 in liver and brain of Cx32 KO mice, confirming the specificity and efficacy of the antibodies, as well as allowing the demonstration of Cx32 expression by oligodendrocytes. Oligodendrocytes throughout brain were decorated with numerous Cx30-positive puncta, which also were immunolabeled for both Cx32 and Cx43. In Cx32 KO mice, astrocytic Cx30 association with oligodendrocyte somata was nearly absent, Cx26 was partially reduced, and Cx43 was present in abundance. In normal and Cx32 KO mice, oligodendrocyte Cx29 was sparsely distributed, whereas Cx47-positive puncta were densely localized on oligodendrocyte somata. These results demonstrate that astrocyte Cx30 and oligodendrocyte Cx47 are widely present at A/O gap junctions. Immunolabeling patterns for these six connexins in Cx32 KO brain have implications for deciphering the organization of heterotypic connexin coupling partners at A/O junctions. The persistence and abundance of Cx43 and Cx47 at these junctions after Cx32 deletion, together with the paucity of Cx29 normally present at these junctions, suggests Cx43/Cx47 coupling at A/O junctions. Reductions in Cx30 and Cx26 after Cx32 deletion suggest that these astrocytic connexins likely form junctions with Cx32 and that their incorporation into A/O gap junctions is dependent on the presence of oligodendrocytic Cx32.

Connexin29 and connexin32 at oligodendrocyte and astrocyte gap junctions and in myelin of the mouse central nervous system

The cellular localization, relation to other glial connexins (Cx30, Cx32, and Cx43), and developmental expression of Cx29 were investigated in the mouse central nervous system (CNS) with an anti-Cx29 antibody. Cx29 was enriched in subcellular fractions of myelin, and immunofluorescence for Cx29 was localized to oligodendrocytes and myelinated fibers throughout the brain and spinal cord. Oligodendrocyte somata displayed minute Cx29-immunopositive puncta around their periphery and intracellularly. In developing brain, Cx29 levels increased during the first few postnatal weeks and were highest in the adult brain. Immunofluorescence labeling for Cx29 in oligodendrocyte somata was intense at young ages and was dramatically shifted in localization primarily to myelinated fibers in mature CNS. Labeling for Cx32 also was localized to oligodendrocyte somata and myelin and absent in Cx32 knockout mice. Cx29 and Cx32 were minimally colocalized on oligodendrocytes somata and partly colocalized along myelinated fibers. At gap junctions on oligodendrocyte somata, Cx43/Cx32 and Cx30/Cx32 were strongly associated, but there was minimal association of Cx29 and Cx43. Cx32 was very sparsely associated with astrocytic connexins along myelinated fibers. With Cx26, Cx30, and Cx43 expressed in astrocytes and Cx29, Cx32, and Cx47 expressed in oligodendrocytes, the number of connexins localized to gap junctions of glial cells is increased to six. The results suggested that Cx29 in mature CNS contributes minimally to gap junctional intercellular communication in oligodendrocyte cell bodies but rather is targeted to myelin, where it, with Cx32, may contribute to connexin-mediated communication between adjacent layers of uncompacted myelin.

Gap junctional hemichannels in the heart

Upon contacting each other, cells form gap junctions, in which each cell contributes half of the channel linking their cytoplasms, enabling them to share their metabolome up to a molecular weight of 1000. Each hemichannel (or connexon) is randomly inserted into the plasma membrane and then migrates to the site of cell-to-cell contact before pairing with the neighbouring cell's hemichannel to form a communicating conduit. This review summarizes the evidence for hemichannels in heart ventricular myocytes. Morphological findings are summarized describing how hemichannels are inserted into the plasma membrane. Once in the plasma membrane, hemichannels can be functionally detected electrophysiologically or by dye uptake assays. Each technique reveals specific aspects of hemichannel function. Using dye uptake studies, it is possible to investigate the biological regulation of hemichannels in vivo. Evidence is summarized which indicates that hemichannels are normally kept closed in the presence of normal extracellular Ca because they are phosphorylated at residues in the C-terminus regulated by the MAPK signalling pathway. When hemichannels are dephosphorylated, the channels open and allow dye uptake into the cells, as well as potentially deleterious ion exchange. Biological stresses, such as hyperosmolarity and metabolic inhibition, open hemichannels by this mechanism through activating phosphatases. The resulting ion fluxes may have important roles in heart physiology and pathophysiology.

Unusual topographical pattern of proximal astrogliosis around a cortical devascularizing lesion

Class II vessels were disrupted on the cortical surface of adult rats within a circular 5-mm-diameter area. This consistently resulted in the formation of a conical lesion by day 1, with a cystic cavity forming by day 21. Four markers were used to identify the glial response surrounding the lesion. The antibody used against S100beta marked the largest astrocytic pool in the gray matter of the cerebral cortex; only approximately 5% of astrocytes were glial fibrillary acidic protein (GFAP)(+) in control animals. GFAP served as a marker for distal reactive gliosis and vimentin (VIM) for proximal gliosis. Isolectin B4 was used as an additional marker to distinguish VIM(+) microglia from astrocytes inside the lesion area. Three immunohistochemically distinct areas of reactive astrocytes surrounding the lesion were found within 24 hr of injury and lasted through day 6. The first area, in contrast to focal traumatic injuries, consisted of a 196-microm-thick boundary layer of S100beta(+) cells immediately surrounding the lesion that never expressed GFAP or VIM by day 6. This boundary layer turns into a GFAP(+) glial limitans encasing the cystic cavity by day 21. A second unusual extended area around the base of the lesion reaching partly into the corpus callosum consisted of S100beta(+)/GFAP(+)/VIM(+) cells. This region appears to be compatible with the local or proximal gliotic response usually found completely surrounding other focal-type injuries. The proximal response at the base of the lesion developed over the first 3 days in the following sequence: S100beta(+)/GFAP(-)/VIM(-) to S100beta(+)/GFAP(+)/VIM(-) to S100beta(+)/GFAP(+)/VIM(+). Ninety percent of the astrocytes in this area express VIM. This is very high compared with findings in stab-wound preparations, where only 10% of astrocytes (surrounding entire lesion) are found to be VIM(+). A third region, consistent with a remote or distal reactive gliotic response, demonstrated staining for S100beta and had increased GFAP contents throughout the neocortical hemisphere. Cells in this region were never found to be VIM(+). Among S100beta(+) cells close to the boundary region, more than 80% expressed detectable GFAP by 2 days after lesioning. S100beta(+) cells 1 mm more laterally (distal to lesion) did not express GFAP to the same level until day 6. Thus, we find three immunohistochemically distinct populations of reactive astrocytes surrounding the focal ischemic lesion. In contrast to the case for stab-wound traumatic injury, the response closest to and surrounding the lesion did not up-regulate GFAP or VIM by day 6. The proximal response was, instead, more remote and only at the base of the lesion, extending partly into the corpus callosum.

The SHB adapter protein is required for efficient multilineage differentiation of mouse embryonic stem cells

The SH2 domain-containing adapter protein SHB transmits signals from receptor tyrosine kinases regulating diverse processes such as apoptosis and differentiation. To elucidate a role for SHB in cell differentiation, wild-type and R522K (inactive SH2 domain-mutant) SHB were transfected and expressed in mouse embryonic stem (ES) cells. Microarray analysis using Affymetrix U74A chips on undifferentiated ES cells and expression of selected differentiation markers after generation of embryoid bodies were subsequently assessed. Wild-type SHB altered the expression of 16 genes in undifferentiated ES cells, many of which have been found to relate to neural cell function. R522K-SHB altered the expression of 128 genes in undifferentiated ES cells, the majority of which were decreased, including several transcription factors related to development. When grown as embryoid bodies, after 4 days R522K-SHB ES cells were already found to display a different morphological appearance, with an impaired cavity formation that occurred in the absence of altered OCT4 expression. This impairment was reversed by exogenous addition of Matrigel. In addition, R522K-SHB embryoid bodies displayed reduced mRNA contents of the liver protein albumin, the pancreatic proteins amylase, glucagon and insulin after 20 days of differentiation. Matrigel did not restore the impaired expression of albumin in the R522K-SHB cells. Expression of the mesodermal marker cardiac actin and the neural marker neurofilament heavy chain alpha was not affected by wild-type or R522K-SHB overexpression. It is concluded that SHB is required for efficient differentiation of ES cells into embryoid bodies with normal cavities and cells belonging to endodermal lineages.

Gap junctions and neuronal injury: protectants or executioners?

The authors review concepts and recent experimental observations that relate gap junctional communication to the pathophysiology of neuronal injury, specifically ischemic or traumatic damage. The role played by this type of direct intercellular communication during the progression of the injuries can be conceived to be either detrimental or beneficial, depending on the arguments employed. The data indicate that, far from being a simple matter of judgment, the contribution of gap junctions to cell injury is a complicated phenomenon that depends on the specific insult and network in which it operates.

Connexin immunoreactivity in glial cells of the rat retina

The rat retina contains two types of macroglial cells, Müller cells, radial glial cells that are the principal macroglial cells of vertebrate retinas, and astrocytes associated with the surface vasculature. In addition to the often-described gap-junctional coupling between astrocytes, coupling also occurs between astrocytes and Müller cells. Immunohistochemistry and confocal microscopy were used to identify connexins in the retinas of pigmented rats. Several antibodies directed against connexin43 stained astrocytes, identified using antibodies directed against glial fibrillary acidic protein (GFAP). In addition, two connexin43 antibodies stained Müller cells, identified with antibodies directed against S100 or glutamine synthetase. Connexin30-immunoreactive puncta were confined to the vitreal surface of the retina and colocalized with GFAP-immunoreactive astrocyte processes. Connexin45 immunoreactivity was associated with both astrocytes and Müller cells. We conclude that retinal glial cells express multiple connexins, and the patterns of immunostaining that we observe in this study are consistent with the expression of connexins30, -43, and possibly -45 by astrocytes and the expression of connexins43 and -45 by Müller cells. As gap-junction channels may be formed by both homotypic and heterotypic hemichannels, and the hemichannels may themselves be homomeric or heteromeric, there exists a multitude of possible gap-junction channels that could underlie the homotypic coupling between retinal astrocytes and the heterotypic coupling between astrocytes and Müller cells.

Anticonvulsant actions of gap junctional blockers in an in vitro seizure model

Gap junctions (gjs) are increasingly recognized as playing a significant role in seizures. We demonstrate that different types of gap junctional blocking agents reduce the duration of evoked seizure-like primary afterdischarges (PADs) in the rat in vitro CA1 hippocampal pyramidal region, following repetitive tetanization of the Schaffer collaterals. Intracellular acidosis, which is known to block gap junctional communication, decreased the PADs, whereas alkalinization increased the PADs. Cellular excitability was not significantly depressed as determined by input/output relations recorded before and during perfusion of the gj blockers blockers carbenoxolone and sodium propionate. There was a small decrease following 1-octanol perfusion and a large decrease following NH(4)Cl application. Carbenoxolone diminished PAD duration, but increased neuronal excitability in whole-cell recordings. After robust PADs were established, the expression of several gj proteins including connexins (Cxs) 26, 32, 36, and 43, as measured by Western blotting, was unchanged, although the level of nonphosphorylated Cx43 was decreased. Our data support the concept that blocking gap junctional communication is an anticonvulsant mechanism.

Detection of a novel pattern of connexin 43 immunoreactivity responsive to dehydration in rat hypothalamic magnocellular nuclei

Immunocytochemical expression of Connexin 43 (Cx 43) in the rat Supraoptic Nucleus was analyzed following dehydration, using sequence-specific anti-Cx 43 antibodies (designated 13-8300, 71-0700, and sc-9059) that exhibit differential recognition of Cx 43. Punctate and longitudinally arranged immunostaining patterns of Cx 43 labeling, as evidenced by antibody sc-9059, was detected overlaying the nucleus of magnocellular neuroendocrine cells. This novel form of longitudinally arranged Cx 43 immunoreactivity was modified by dehydration and halothane exposure, but not lactation.

The ovarian gap junction protein connexin43: regulation by gonadotropins

The major role of the ovarian follicle is the timely production of a mature fertilizable oocyte. This mission is accomplished by a gonadotropin-regulated, gap junction-mediated alteration between established and interrupted cell-cell communication. Recent studies have revealed that gonadotropin action on ovarian gap junctions is elicited at the transcriptional, translational and post-translational levels. Here, we review the existing information generated on the molecular mechanisms employed by the gonadotropins to elicit their effect on the ovarian gap junction protein Cx43.

Ischemia-induced brain damage depends on specific gap-junctional coupling

Ischemic brain injury results in neuronal loss and associated neurologic deficits. Although there is some evidence that intercellular communication via gap junctions can spread oxidative cell injury, the possible role of gap-junctional communication in ischemia-induced cell death is the object of debate. Because gap junctions directly connect the cytoplasms of coupled cells, they offer a way to propagate stress signals from cell to cell. The authors investigated the contribution of gap-junctional communication to cell death using an in vitro ischemia model, which was reproduced by submersion of organotypic hippocampal slices into glucose-free deoxygenated medium. The gap-junctional blocker carbenoxolone significantly decreased the spread of cell death, as measured by propidium iodide staining, over a 48-hour period after the ischemic episode. Carbenoxolone ameliorated the hypoxia-induced impairment of the intrinsic neuronal electrophysiologic characteristics, as measured by whole-cell patch clamp recordings. To determine whether specific connexins were involved in the spread of postischemic cell death, the authors partially reduced the synthesis of specific connexins using antisense oligodeoxynucleotides. Simultaneous knockdown of two connexins localized mostly in neurons, connexins 32 and 26, resulted in significant neuroprotection 48 hours after the hypoxic-hypoglycemic episode. Similarly, partial reduction of the predominant glial connexin 43 significantly decreased cell death. These results indicate that gap-junctional communication contributes to the propagation of hypoxic injury and that specific gap junctions could be a novel target to reduce brain damage.

Specific gap junctions enhance the neuronal vulnerability to brain traumatic injury

Traumatic brain injury results in neuronal loss and associated neurological deficits. Although most research on the factors leading to trauma-induced damage focuses on synaptic or ionic mechanisms, the possible role of direct intercellular communication via gap junctions has remained unexplored. Gap junctions connect directly the cytoplasms of coupled cells; hence, they offer a way to propagate stress signals from cell to cell. We investigated the contribution of gap junctional communication (GJC) to cell death using an in vitro trauma model. The impact injury, induced by a weight dropped on the distal CA1 area of organotypic hippocampal slices, results in glutamate-dependent cell loss. The gap junctional blockers carbenoxolone and octanol decreased significantly post-traumatic cell death, measured by propidium iodide staining over a 72 hr period after the impact. Dye coupling in the pyramidal layers was enhanced immediately after the injury and decreased over the following 24 hr. To determine whether specific connexins were involved in the spread of trauma-induced cell death, we used organotypic slices from connexin43 (Cx43) knock-out mice, as well as acute knock-outs by incubation with antisense oligodeoxynucleotides. Simultaneous knockdown of two neuronal connexins resulted in significant neuroprotection. Slices from the null-mutant Cx43 mice, as well as the acute Cx43 knockdown, also showed decreased cell death after the impact. The gap junctional blockers alleviated the trauma-induced impairment of synaptic function as measured by electrophysiological field potential recordings. These results indicate that GJC enhances the cellular vulnerability to traumatic injury. Hence, specific gap junctions could be a novel target to reduce injury and secondary damage to the brain and maximize recovery from trauma.

Connexin26 in adult rodent central nervous system: demonstration at astrocytic gap junctions and colocalization with connexin30 and connexin43

The connexin family of proteins (Cx) that form intercellular gap junctions in vertebrates is well represented in the mammalian central nervous system. Among these, Cx30 and Cx43 are present in gap junctions of astrocytes. Cx32 is expressed by oligodendrocytes and is present in heterologous gap junctions between oligodendrocytes and astrocytes as well as at autologous gap junctions between successive myelin layers. Cx36 mRNA has been identified in neurons, and Cx36 protein has been localized at ultrastructurally defined interneuronal gap junctions. Cx26 is also expressed in the CNS, primarily in the leptomeningeal linings, but is also reported in astrocytes and in neurons of developing brain and spinal cord. To establish further the regional, cellular, and subcellular localization of Cx26 in neural tissue, we investigated this connexin in adult mouse brain and in rat brain and spinal cord using biochemical and immunocytochemical methods. Northern blotting, western blotting, and immunofluorescence studies indicated widespread and heterogeneous Cx26 expression in numerous subcortical areas of both species. By confocal microscopy, Cx26 was colocalized with both Cx30 and Cx43 in leptomeninges as well as along blood vessels in cortical and subcortical structures. It was also localized at the surface of oligodendrocyte cell bodies, where it was coassociated with Cx32. Freeze-fracture replica immunogold labeling (FRIL) demonstrated Cx26 in most gap junctions between cells of the pia mater by postnatal day 4. By postnatal day 18 and thereafter, Cx26 was present at gap junctions between astrocytes and in the astrocyte side of most gap junctions between astrocytes and oligodendrocytes. In perinatal spinal cord and in five regions of adult brain and spinal cord examined by FRIL, no evidence was obtained for the presence of Cx26 in neuronal gap junctions. In addition to its established localization in leptomeningeal gap junctions, these results identify Cx26 as a third connexin (together with Cx30 and Cx43) within astrocytic gap junctions and suggest a further level of complexity to the heterotypic connexin channel combinations formed at these junctions.

Connexin43 null mutation increases infarct size after stroke

Glial-neuronal interactions have been implicated in both normal information processing and neuroprotection. One pathway of cellular interactions involves gap junctional intercellular communication (GJIC). In astrocytes, gap junctions are composed primarily of the channel protein connexin43 (Cx43) and provide a substrate for formation of a functional syncytium implicated in the spatial buffering capacity of astrocytes. To study the function of gap junctions in the brain, we used heterozygous Cx43 null mice, which exhibit reduced Cx43 expression. Western blot analysis showed a reduction in the level of Cx43 protein and GJIC in astrocytes cultured from heterozygote mice. The level of Cx43 is reduced in the adult heterozygote cerebrum to 40% of that present in the wild-type. To assess the effect of reduced Cx43 and GJIC on neuroprotection, we examined brain infarct volume in wild-type and heterozygote mice after focal ischemia. In our model of focal stroke, the middle cerebral artery was occluded at two points, above and below the rhinal fissure. Four days after surgery, mice were killed, the brains were sectioned and analyzed. Cx43 heterozygous null mice exhibited a significantly larger infarct volume compared with wild-type (14.4 +/- 1.4 mm(3) vs. 7.7 +/- 0.82 mm(3), P < 0.002). These results suggest that augmentation of GJIC in astrocytes may contribute to neuroprotection after ischemic injury.

Global ischemia-induced increases in the gap junctional proteins connexin 32 (Cx32) and Cx36 in hippocampus and enhanced vulnerability of Cx32 knock-out mice

Gap junctions are conductive channels that connect the interiors of coupled cells. In the hippocampus, GABA-containing hippocampal interneurons are interconnected by gap junctions, which mediate electrical coupling and synchronous firing and thereby promote inhibitory transmission. The present study was undertaken to examine the hypothesis that the gap junctional proteins connexin 32 (Cx32; expressed by oligodendrocytes, interneurons, or both), Cx36 (expressed by interneurons), and Cx43 (expressed by astrocytes) play a role in defining cell-specific patterns of neuronal death in hippocampus after global ischemia in mice. Global ischemia did not significantly alter Cx32 and Cx36 mRNA expression and slightly increased Cx43 mRNA expression in the vulnerable CA1, as assessed by Northern blot analysis and in situ hybridization. Global ischemia induced a selective increase in Cx32 and Cx36 but not Cx43 protein abundance in CA1 before onset of neuronal death, as assessed by Western blot analysis. The increase in Cx32 and Cx36 expression was intense and specific to parvalbumin-positive inhibitory interneurons of CA1, as assessed by double immunofluorescence. Protein abundance was unchanged in CA3 and dentate gyrus. The finding of increase in connexin protein without increase in mRNA suggests regulation of Cx32 and Cx36 expression at the translational or post-translational level. Cx32(Y/-) null mice exhibited enhanced vulnerability to brief ischemic insults, consistent with a role for Cx32 gap junctions in neuronal survival. These findings suggest that Cx32 and Cx36 gap junctions may contribute to the survival and resistance of GABAergic interneurons, thereby defining cell-specific patterns of global ischemia-induced neuronal death.

Enrichment of neuronal and glial connexins in the postsynaptic density subcellular fraction of rat brain

The similar dense, protein-rich and detergent-resistant characteristics of postsynaptic densities (PSDs) and gap junctions led us to investigate the distribution of gap junctions and their constituent connexins in CNS subcellular fractions containing PSDs. Western blot analysis showed these fractions to be enriched in both neuronal and glial connexins, namely, connexin26, connexin30, connexin36 and connexin43. Connexins were retained in these fractions after treatment with n-lauroyl sarcosine to remove loosely associated proteins. Confocal double immunofluorescence confirmed the presence of connexins in PSD fractions and showed a near total co-localization of glial connexin30 and connexin43, demonstrating preservation of inter-connexin relationships that have been observed in vivo. In contrast, none of the connexins were co-localized with the PSD structural protein PSD-95, indicating their lack of direct association with PSDs. These results show that PSD preparations contain significant levels of connexin proteins, which appear to remain assembled as gap junctions. Thus, protocols used to isolate PSDs may serve as a basis for development of methods to isolate CNS gap junctions, which would aid biochemical identification of regulatory and structural proteins associated with these structures.

Pharmacological modulation of the bystander effect in the herpes simplex virus thymidine kinase/ganciclovir gene therapy system: effects of dibutyryl adenosine 3',5'-cyclic monophosphate, alpha-glycyrrhetinic acid, and cytosine arabinoside

The herpes simplex virus type 1 thymidine kinase (HSV1-tk) suicide gene/ganciclovir system was first applied to the treatment of glioblastoma tumors, but was hampered by the low gene transfection yield. Fortunately, the gap junction-dependent diffusion of phosphorylated ganciclovir metabolites from transfected cells to their neighbors proved to enhance the overall benefit of this strategy. However, as tumor cells are often gap junction-deficient, we sought to restore this property pharmacologically and hence to improve the efficacy of the treatment. We demonstrated that this approach was feasible in glioblastoma cells using dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) (100 microM) as a pharmacological inducer of gap junctions. alpha-Glycyrrhetinic acid (25 microM), on the other hand, strongly inhibited both gap junction-mediated intercellular communication and the bystander effect, thus confirming the role of gap junctions in HSV-tk-mediated bystander killing. Using cytosine arabinoside as a growth inhibitor, we underlined the role of tumor cell proliferation in the sensitivity of HSV-tk-transfected cells to ganciclovir and demonstrated its correlation with the importance of the bystander effect.

Activation of fibres in rat sciatic nerve alters phosphorylation state of connexin-43 at astrocytic gap junctions in spinal cord: evidence for junction regulation by neuronal-glial interactions

Intercellular communication via gap junction channels composed of connexin-43 is known to be regulated by phosphorylation of this protein. We investigated whether connexin-43 at astrocytic gap junctions is similarly regulated in response to neural activation. The effect of peripheral nerve stimulation on connexin-43 phosphorylation state in the spinal cord of rats was examined with a monoclonal antibody (designated 13-8300) shown previously to recognize selectively a dephosphorylated form of connexin-43. Immunolabelling with 13-8300 was absent in the lumbar spinal cord in control animals, but was induced in the dorsal horn ipsilateral to sciatic nerve electrical stimulation for 15min or 1h at a frequency of 1 or 100Hz. Immunorecognition of connexin-43 by a polyclonal anti-connexin-43 antibody, shown previously to undergo epitope masking under various conditions, was reduced in the dorsal horn on the stimulated side. These responses were abolished by local anaesthetic or tetrodotoxin application proximal to the site of nerve stimulation. Selective electrical stimulation of A-fibres or activation of cutaneous C-fibres by capsaicin evoked labelling with 13-8300 in deep and superficial laminae of the dorsal horn, respectively. Nerve stimulation increased the number of 13-8300-positive astrocytic gap junctions, as well as the levels of dephosphorylated connexin-43 in the dorsal horn on the stimulated side. Sciatic nerve transection produced results similar to those seen after C-fibre activation with capsaicin.Thus, peripheral nerve stimulation evokes astrocytic connexin-43 dephosphorylation in the spinal cord dorsal horn, suggesting that gap junctional coupling between astrocytes in vivo is subject to regulation by neuronal-glial interactions following neural activation.

Connexin43 phosphorylation state and intercellular communication in cultured astrocytes following hypoxia and protein phosphatase inhibition

The effects of hypoxia and phosphatase inhibitors on connexin43 (Cx43) phosphorylation state, gap junctional intercellular communication (GJIC) and immunolabelling with anti-Cx43 antibodies were investigated in cultured astrocytes. Astrocytes contained predominantly phosphorylated forms of Cx43 and these underwent dephosphorylation 30 min after hypoxia. This was preceded by a 77% reduction in GJIC 15 min after hypoxia, indicating that reduced GJIC occurs prior to Cx43 dephosphorylation. Hypoxia caused a reduction in punctate immunostaining (epitope masking) at cell-cell contacts with one anti-Cx43 antibody, and increased labelling with another antibody (13-8300) that detects only a dephosphorylated form of Cx43. Inhibition of protein phosphatase (PP)-1 and PP-2A with okadaic acid or calyculin A had little effect on hypoxia-induced Cx43 dephosphorylation. Inhibition of PP-2B (calcineurin) with cyclosporin A or FK506 reduced Cx43 dephosphorylation and junctional uncoupling seen after hypoxia. These results demonstrate that responses of astrocytic Cx43 to hypoxia in vitro are similar to those seen after ischaemia in vivo, and that inhibition of protein phosphatase protects astrocytes from hypoxia-induced Cx43 dephosphorylation and junctional uncoupling. In addition, calcineurin may play a direct role in the regulation of astrocytic GJIC and Cx43 phosphorylation state.

Regulation of astrocyte gap junctions by hypoxia-reoxygenation

Confluent cultures of rat cortical astrocytes were subjected to 12-h hypoxia (<1% O(2)) followed by reoxygenation. Just after hypoxia, the cellular distribution, phosphorylation state and levels of connexin43 (Cx43), as well as the extent of dye coupling were as in control conditions. Nonetheless, 15-30 min after reoxygenation, dye coupling was transiently reduced by approximately 70%. The reduction in dye coupling occurred without changes in the state of phosphorylation or levels of Cx43. Nevertheless, it was correlated with a decrease in Cx43 reactivity found at membrane appositions and the appearance of intracellular Cx43-positive vesicle-like structures of variable size, suggesting internalization of gap junction channels. Reoxygenation-induced cellular uncoupling and redistribution of Cx43 were prevented by melatonin (500 microM), a potent-free radical scavenger, or indomethacin (50 microM), an inhibitor of the cyclooxygenase-dependent arachidonic acid metabolism. In astrocytes cultured under normoxia, the state of phosphorylation of Cx43 was not affected by antimycin A, a blocker of the mitochondrial oxidative metabolism, but phosphorylation was drastically reduced by iodoacetate, a blocker of anaerobic glycolysis. Thus, these results strongly suggest that reoxygenation-induced uncoupling is mediated by arachidonic acid byproducts that induce, at least, disorganization of Cx43 gap junction channels.

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.

Use of antibodies in the analysis of connexin 43 turnover and phosphorylation

A series of antipeptide antibodies designed to recognize specific sequences of the gap junction protein connexin 43 (Cx43) were developed and characterized immunochemically and immunohistologically. These antibodies bound to gap junctions and, on Western blots, to 43-kDa (often resolved as a doublet) and 41-kDa proteins in samples from heart, leptomeningeal cells, and brain. Relatively little of the 41-kDa protein was detectable in heart homogenates. Cultured rat leptomeningeal cells expressed high levels of the gap junction protein Cx43 and were used to analyze its turnover and phosphorylation. Pulse-chase experiments in leptomeningeal cells with [(35)S]methionine indicated that the 41-kDa form of connexin 43 was the first immunoprecipitable translation product. Radiolabel subsequently appeared in the lower band of the doublet at 43 kDa, followed by a shift into the higher band and turnover of the protein with a t(1/2) of 2.7 h. Pulse-chase labeling with [(32)P]P(i) indicated that phosphorylation of connexin 43 was limited to the 43-kDa protein, with a t(1/2) of 1.7 h. Treatment with alkaline phosphatase shifted the apparent molecular mass of the 43-kDa protein doublet such that it comigrated with the 41-kDa form. Hence, the 43-kDa protein observed on Western blots of both leptomeningeal cells and heart arises by phosphorylation of the 41 kDa precursor. Phosphorylation of serine residues accounts for most, if not all, of Cx43 phosphorylation in this system.

Stimulated phosphorylation of intracellular connexin43

A monoclonal antibody, Zymed 13-8300, was previously reported to only detect nonphosphorylated connexin43 (Nagy et al., Exp. Cell Res. 236, 127-136, 1997). We show that 13-8300 can detect several phosphorylated species of connexin43 in Western blots after stimulation of two fibroblast cell systems with fresh growth medium, 12-O-tetradecanoyl phorbol-13-acetate, pervanadate, or permolybdate. In one of the cell systems, at least three forms of phosphorylated connexin43 could migrate at the same position during electrophoresis. The comigration of differentially phosphorylated species may complicate the molecular and functional analysis of phosphorylation sites in Cx43. Immunofluorescence experiments indicated that the newly generated phosphorylated Cx43 forms mainly had a perinuclear location. Also, in cells treated with brefeldin A for 8 h, in which the majority of connexin43 was intracellular, phosphorylation was induced by the agents. Phosphorylation of intracellular connexin43 can therefore be induced by several stimuli.

Connexin30 in rodent, cat and human brain: selective expression in gray matter astrocytes, co-localization with connexin43 at gap junctions and late developmental appearance

We previously presented evidence [Nagy et al. (1997) Neuroscience 78, 533-548] that, in addition to their ubiquitous expression of connexin43, astrocytes produce a second connexin suggested to be connexin30, a recently discovered member of the family of gap junction proteins. A connexin30 specific antibody was subsequently developed and utilized here to confirm and extend our earlier observations. On western blots, this antibody detected a 30,000 mol. wt protein in rat, mouse, cat and human brain, and exhibited no cross-reaction with connexin43, connexin26 or any other known connexins expressed in brain. Immunohistochemically, connexin30 was localized in astrocytes, at gap junctions between these cells and on the astrocyte side of gap junctions between astrocytes and oligodendrocytes. Double labelling revealed co-localization of connexin30 and connexin43 at astrocytic gap junctions. Punctate immunolabelling patterns for both connexins were qualitatively similar, but differences were evident. In contrast to regional connexin43 expression, diencephalic and hindbrain areas exhibited considerably greater expression than forebrain areas, subcortical perivascular astrocytic endfeet were more heavily labelled for connexin30, white matter tracts such as corpus callosum, internal capsule and anterior commissure were devoid of connexin30, and appreciable levels of connexin30 during development were not seen until about postnatal day 15. These results indicate that connexin30 is expressed by gray, but not white matter astrocytes, its distribution is highly heterogeneous in gray matter, it is co-localized with connexin43 at astrocytic gap junctions where it forms homotypic or heterotypic junctions, and its emergence is delayed until relatively late during brain maturation. Taken together, these results suggest that astrocytic connexin30 expression at both regional and cellular levels is subject to regulation in adult brain as well as during brain development.