Connexin-43 interactions with ZO-1 and alpha- and beta-tubulin

Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation

Gap junction proteins, connexins, are dynamic polytopic membrane proteins that exhibit unprecedented short half-lives of only a few hours. Consequently, it is well accepted that in addition to channel gating, gap junctional intercellular communication is regulated by connexin biosynthesis, transport and assembly as well as the formation and removal of gap junctions from the cell surface. At least nine members of the 20-member connexin family are known to be phosphorylated en route or during their assembly into gap junctions. For some connexins, notably Cx43, evidence exists that phosphorylation may trigger its internalization and degradation. In recent years it has become apparent that the mechanisms underlying the regulation of connexin turnover are quite complex with the identification of many connexin binding molecules, a multiplicity of protein kinases that phosphorylate connexins and the involvement of both lysosomal and proteasomal pathways in degrading connexins. This paper will review the evidence that connexin phosphorylation regulates, stimulates or triggers gap junction disassembly, internalization and degradation.

Connexin43 Associated with an N-cadherin-containing Multiprotein Complex Is Required for Gap Junction Formation in NIH3T3 Cells

Previous studies have indicated an intimate linkage between gap junction and adherens junction formation. It was suggested this could reflect the close membrane-membrane apposition required for junction formation. In NIH3T3 cells, we observed the colocalization of connexin43 (Cx43alpha1) gap junction protein with N-cadherin, p120, and other N-cadherin-associated proteins at regions of cell-cell contact. We also found that Cx43alpha1, N-cadherin, and N-cadherin-associated proteins were coimmunoprecipitated by antibodies to either Cx43alpha1, N-cadherin, or various N-cadherin-associated proteins. These findings suggest that Cx43alpha1 and N-cadherin are coassembled in a multiprotein complex containing various N-cadherin-associated proteins. Studies using siRNA knockdown indicated that cell surface expression of Cx43alpha1 required N-cadherin, and conversely, N-cadherin cell surface expression required Cx43alpha1. Pulse-chase labeling and cell surface biotinylation experiments indicated that in the absence of N-cadherin, Cx43alpha1 cell surface trafficking is blocked. Surprisingly, siRNA knockdown of p120, an N-cadherin-associated protein known to modulate cell surface turnover of N-cadherin, reduced N-cadherin cell surface expression without altering Cx43alpha1 expression. These observations suggest that in contrast to the coregulated cell surface trafficking of Cx43alpha1 and N-cadherin, N-cadherin turnover at the cell surface may be regulated independently of Cx43alpha1. Functional studies showed gap junctional communication is reduced and cell motility inhibited with N-cadherin or Cx43alpha1 knockdown, consistent with the observed loss of both gap junction and cadherin contacts with either knockdown. Overall, these studies indicate that the intracellular coassembly of connexin and cadherin is required for gap junction and adherens junction formation, a process that likely underlies the intimate association between gap junction and adherens junction formation.

News and views in Histochemistry and Cell Biology

Advances in histochemical methodology and ingenious applications of novel and improved methods continue to confirm the standing of morphological means and approaches in research efforts, and contribute significantly to increasing our knowledge about structures and functions in all areas of the life sciences from cell biology to pathology. Reports published during recent months documenting this progress are summarized in the present review.

Structural Changes in the Carboxyl Terminus of the Gap Junction Protein Connexin43 Indicates Signaling between Binding Domains for c-Src and Zonula Occludens-1

Regulation of cell-cell communication by the gap junction protein connexin43 can be modulated by a variety of connexin-associating proteins. In particular, c-Src can disrupt the connexin43 (Cx43)-zonula occludens-1 (ZO-1) interaction, leading to down-regulation of gap junction intercellular communication. The binding sites for ZO-1 and c-Src correspond to widely separated Cx43 domains (approximately 100 residues apart); however, little is known about the structural modifications that may allow information to be transferred over this distance. Here, we have characterized the structure of the connexin43 carboxyl-terminal domain (Cx43CT) to assess its ability to interact with domains from ZO-1 and c-Src. NMR data indicate that the Cx43CT exists primarily as an elongated random coil, with two regions of alpha-helical structure. NMR titration experiments determined that the ZO-1 PDZ-2 domain affected the last 19 Cx43CT residues, a region larger than that reported to be required for Cx43CT-ZO-1 binding. The c-Src SH3 domain affected Cx43CT residues Lys-264-Lys-287, Ser-306-Glu-316, His-331-Phe-337, Leu-356-Val-359, and Ala-367-Ser-372. Only region Lys-264-Lys-287 contains the residues previously reported to act as an SH3 binding domain. The specificity of these interactions was verified by peptide competition experiments. Finally, we demonstrated that the SH3 domain could partially displace the Cx43CT-PDZ-2 complex. These studies represent the first structural characterization of a connexin domain when integrated in a multimolecular complex. Furthermore, we demonstrate that the structural characteristics of a disordered Cx43CT are advantageous for signaling between different binding partners that may be important in describing the mechanism of channel closure or internalization in response to pathophysiological stimuli.

Shear stress-induced upregulation of connexin 43 expression in endothelial cells on upstream surfaces of rat cardiac valves

Endothelial expression of the gap junction proteins, connexin (Cx) 37, Cx40, and Cx43, varies within the vascular network. While previous studies suggest that shear stress may upregulate Cx43, it is not well understood if shear stress affects the expression of all endothelial connexins and to what extent. Endothelial cells on the upstream and downstream surfaces of cardiac valves are subjected to considerably different intensities of shear stress. We therefore reasoned that we could determine the extent hemodynamic forces affect the expression of Cx37, Cx40, and Cx43 by comparing their immunohistochemical distribution on the upstream and downstream surfaces of rat cardiac valves. We found 70- to 200-fold greater expression of Cx43 in the endothelial cells on the upstream than on the downstream surfaces. However, Cx37 was expressed almost equally in the endothelial cells on upstream and downstream surfaces, and Cx40, a major connexin in most vascular endothelial cells, was not detected on either surface. In addition to the heterogeneity in Cx43 expression, endothelial cells on the upstream surface were 35% to 65% smaller than those on the corresponding downstream surface. These results suggest that shear stress may affect endothelial cell size and Cx43 expression but not Cx37 expression.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

Connexin 26-mediated gap junctional intercellular communication suppresses paracellular permeability of human intestinal epithelial cell monolayers

In some cell types, gap junctional intercellular communication (GJIC) is associated with tight junctions. The present study was performed to determine the roles of GJIC in regulation of the barrier function of tight junctions. Caco-2 human colonic cells were used as a monolayer model, and barrier function was monitored by measuring mannitol permeability and transepithelial electrical resistance (TER). The monolayers were chemically disrupted by treatment with oleic acid and taurocholic acid. Western blotting analyses were performed to evaluate the protein levels of connexins, which are components of gap junctional intercellular channels. Cx26 expression was detected in preconfluent Caco-2 cells, and its level increased gradually after the monolayer reached confluency. These results prompted us to examine whether overexpression of Cx26 affects barrier function. Monolayers of Caco-2 cells stably expressing Cx26 showed significantly lower mannitol permeability and higher TER than mock transfectants when the monolayers were chemically disrupted. The levels of claudin-4, an important component of tight junctions, were significantly increased in the stable Cx26 transfectant. These results suggest that Cx26-mediated GJIC may play a crucial role in enhancing the barrier function of Caco-2 cell monolayers.

p38/SAPK2 controls gap junction closure in astrocytes

Astrocyte gap junction communication (GJC) is thought to contribute to death signal propagation following central nervous system injury, noteworthy in some ischemia/anoxia models. The inhibition of p38/stress-activated protein kinase 2 (p38/SAPK2) by a pyrimidyl imidazole derivative has been reported to reduce the extent of the lesion area after cerebral ischemia. Therefore, interleukin-1beta (IL-1beta), which contributes to stroke-induced brain injury and activates p38/SAPK2, and hyperosmolarity induced by sorbitol, a potent stimulus of p38/SAPK2 in non-neuronal cells, were used to investigate a possible involvement of p38/SAPK2 in GJC modulation in mouse cultured astrocytes. Both stimuli inhibited dye coupling within minutes. The IL-1beta effect was transient, while that of sorbitol lasted up to 90 min. Both stimuli induced a rapid p38/SAPK2 activation, the kinetic of which matched that of induction of dye coupling inhibition. Immunocytochemical studies showed that IL-1beta and sorbitol induced a p38/SAPK2 translocation from the nucleus to the cytoplasm. The pharmacological agent SB203580 specifically blocked p38/SAPK2 activation, cytoplasmic translocation and reversed the IL-1beta and sorbitol-induced inhibition of GJC. Further characterization of the p38/SAPK2 mode of action on GJC, performed with sorbitol, revealed an increased phosphorylation of protein kinase C (PKC) substrates abolished by both PKC inhibitors and SB203580. Expression and serine phosphorylation of connexin 43, the main component of astrocyte gap junctions, were unchanged, suggesting the existence of additional intracellular signaling mechanisms modulating the channel gating. Altogether, these results demonstrate that p38/SAPK2 is a central mediator of IL-1beta and sorbitol inhibitory actions on GJC and establish PKC among the distal effectors of p38/SAPK2.

Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks

Gap junctions consist of intercellular channels dedicated to providing a direct pathway for ionic and biochemical communication between contacting cells. After an initial burst of publications describing electrical coupling in the brain, gap junctions progressively became less fashionable among neurobiologists, as the consensus was that this form of synaptic transmission would play a minimal role in shaping neuronal activity in higher vertebrates. Several new findings over the last decade (e.g. the implication of connexins in genetic diseases of the nervous system, in processing sensory information and in synchronizing the activity of neuronal networks) have brought gap junctions back into the spotlight. The appearance of gap junctional coupling in the nervous system is developmentally regulated, restricted to distinct cell types and persists after the establishment of chemical synapses, thus suggesting that this form of cell-cell signaling may be functionally interrelated with, rather than alternative to chemical transmission. This review focuses on gap junctions between neurons and summarizes the available data, derived from molecular, biological, electrophysiological, and genetic approaches, that are contributing to a new appreciation of their role in brain function.

Diversity in protein–protein interactions of connexins: emerging roles

Gap junctions, specialised membrane structures that mediate cell-to-cell communication in almost all tissues, are composed of channel-forming integral membrane proteins termed connexins. The activity of these intercellular channels is closely regulated, particularly by intramolecular modifications as phosphorylations of proteins by protein kinases, which appear to regulate the gap junction at several levels, including assembly of channels in the plasma membrane, connexin turnover as well as directly affecting the opening and closure ("gating") of channels. The regulation of membrane channels by protein phosphorylation/dephosphorylation processes commonly requires the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, catalytic and regulatory subunits), that play essential roles in channel localisation and activity, linking signalling enzymes, substrates and effectors into a structure frequently anchored to the cytoskeleton. The present review summarises the up-to-date progress regarding the proteins capable of interacting or at least of co-localising with connexins and their functional importance.

Phosphorylation of serine 262 in the gap junction protein connexin-43 regulates DNA synthesis in cell-cell contact forming cardiomyocytes

Mitogenic stimulation of cardiomyocytes is associated with decreased gap junction coupling and protein kinase C (PKC)-mediated phosphorylation of the gap junction protein connexin43 (Cx43). Identification of and interference with the amino acid(s) that becomes phosphorylated in response to stimulation are important steps towards defining the relationship between Cx43 phosphorylation and cell cycle. Using immunoblotting and phosphospecific antibodies we were able to show that serine-262 (S262) on Cx43 becomes phosphorylated in response to growth factor or PKC stimulation of cardiomyocytes. To examine the effect of Cx43, S262 phosphorylation and cell-cell contact (and/or coupling) on DNA synthesis, we overexpressed wild-type (wt) or mutant Cx43, carrying a S262-to-alanine (S262A, simulating the unphosphorylated state) or a S262-to-aspartate (S262D, simulating constitutive phosphorylation) substitutions in cultures of cell-cell contact forming or isolated cardiomyocytes. Overexpression of wt-Cx43 caused a significant decrease in DNA synthesis irrespective of the presence of cell-cell contact. In cell-cell contact forming cultures, the S262D mutation reversed while the S262A mutation increased the inhibitory effect of Cx43. In the absence of cell-cell contact, the S262-Cx43 mutations had no significant effect on Cx43 inhibition of DNA synthesis. Dye-coupling, evaluated by scrape-loading, indicated increased gap junction permeability in S262A (compared to wt or S262D) overexpressing myocytes. We conclude that Cx43 inhibits cardiomyocyte DNA synthesis irrespectively of cell-cell contact or coupling. Cell-cell contact, and possibly gap junction-mediated communication is required, however, in order to reverse Cx43 inhibition of DNA synthesis by S262 phosphorylation.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin (Cx) protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of Cx40 and Cx43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Canonical transient receptor potential channel 4 (TRPC4) co-localizes with the scaffolding protein ZO-1 in human fetal astrocytes in culture

Members of the mammalian transient receptor potential (TRP) family form cation-permeable channels at the plasma membrane implicated in capacitative calcium influx after activation by either second-messenger-mediated pathways or store depletion, or both. This study shows that with the use of RT-PCR, Western blotting, and immunohistochemistry, resting astrocytes express TRPC4 at the cell membrane, particularly at sites of cell-to-cell contact. By confocal imaging and immunoelectron microscopy, we detected co-localization of TRPC4 with the scaffolding protein zonula occludens 1 (ZO-1), and demonstrated that immunoprecipitation with antibodies to ZO-1 brought down TRPC4, and vice-versa. It has been proposed that the targeting of TRPC4 to the cell membrane is dependent on the interaction of the C-terminal TRL motif with PDZ domains. Using transfection of astrocytes with myc-tagged TRPC4 or TRL-motif truncated TRPC4 (deltaTRL), we found that deltaTRL localized predominantly to a juxtanuclear compartment, whereas the wild-type protein showed cell surface distribution. Deletion of the TRL motif also reduced plasma membrane expression as assessed by cell surface biotinylation experiments. Using GST fusion proteins, we found that TRPC4 interacted with the PDZ1 domain of ZO-1 and that this was also dependent on the TRL motif. Thus, our data demonstrate that the PDZ-interacting domain of TRPC4 controls its cell surface localization. These data implicate TRPC4 in the regulation of calcium homeostasis in astrocytes, particularly as part of a signaling complex that forms at junctional sites between astrocytes.

Gap junction-microtubule associations in rat alveolar epithelial cells

Connexin 43 (Cx43) is a predominant gap junction (GJ) protein expressed by alveolar epithelial cells (AEC) in primary cell culture. Cx43 trafficking, assembly, and turnover are regulated by multiple mechanisms, including those mediated by integrins, by extracellular matrix, and by the cytoskeleton. Immunocytochemical double labeling demonstrates association of microtubules with internalization of Cx43-positive GJ plaques. Antibodies against the α5-integrin subunit block cell-matrix interactions without effect on tubulin expression, whereas inhibition of MAP kinase kinase by PD-98059 reduces tubulin expression, based on both Western blot and immunostaining. To examine direct association of microtubules (MT) with GJ plaques, we treated day 3 AEC for 0.5-24 h with colchicine, an inhibitor of tubulin polymerization. After 60 min, MTs were disassembled, whereas Western blot analysis showed no change in tubulin expression. In parallel, colchicine initiated redistribution of immunopositive Cx43 from the membrane to the cytosol. These observations support the premise that direct association of the cytoskeleton with gap junctions plays a significant role in regulation of Cx43 expression and distribution through integrin-mediated signal transduction pathways.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of connexins (Cx) 40 and 43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed approximately 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on the intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Endothelin-1 decreases gap junctional intercellular communication by inducing phosphorylation of connexin 43 in human ovarian carcinoma cells

Endothelin-1 (ET-1) is overexpressed in ovarian carcinoma and acts as an autocrine factor selectively through the ETA receptor (ETAR) to promote tumor cell proliferation, survival, neovascularization, and invasiveness. Loss of gap junctional intercellular communication (GJIC) is critical for tumor progression by allowing the cells to escape growth control. Exposure of HEY and OVCA 433 ovarian carcinoma cell lines to ET-1 led to a 50-75% inhibition in intercellular communication and to a decrease in the connexin 43 (Cx43)-based gap junction plaques. To investigate the phosphorylation state of Cx43, ovarian carcinoma cell lysates were immunoprecipitated and transient tyrosine phosphorylation of Cx43 was detected in ET-1-treated cells. BQ 123, a selective ETAR antagonist, blocked the ET-1-induced Cx43 phosphorylation and cellular uncoupling. Gap junction closure was prevented by tyrphostin 25 and by the selective c-Src inhibitor, PP2. Furthermore, the increased Cx43 tyrosine phosphorylation was correlated with ET-1-induced increase of c-Src activity, and PP2 suppressed the ET-1-induced Cx43 tyrosine phosphorylation, indicating that inhibition of Cx43-based GJIC is mainly mediated by the Src tyrosine kinase pathway. In vivo, the inhibition of human ovarian tumor growth in nude mice induced by the potent ETAR antagonist, ABT-627, was associated with a reduction of Cx43 phosphorylation. These findings indicate that the signaling mechanisms involved in GJIC disruption on ovarian carcinoma cells depend on ETAR activation, which leads to the Cx43 tyrosine phosphorylation mediated by c-Src, suggesting that ETAR blockade may contribute to the control of ovarian carcinoma growth and progression also by preventing the loss of GJIC.

Connexin 36 in bovine retina: lack of phosphorylation but evidence for association with phosphorylated proteins

In vertebrate retina interneuronal communication through gap junctions is involved in light adaptation and in the transfer of visual information from the rod pathway to the cone pathway. Reports over the last two decades have indicated that these gap junctions are regulated by cyclic nucleotide-dependent protein kinases suggesting that the gap junction proteins, connexins, are phosphorylated. Though all the connexins involved in light adaptation and information transfer from rod to cone pathway are not yet known, connexin 36 has been shown to be definitively involved in the latter process. We have therefore attempted to investigate the cyclic nucleotide-dependent phosphorylation of this connexin in bovine retina. We found several soluble and membrane proteins in bovine retina whose phosphorylation was regulated by cyclic nucleotides. However, no protein of about 36 kDa with cyclic nucleotide-regulated phosphorylation was found in gap junction-enriched membrane preparations. A 36-kDa phosphorylated protein was found in gap junction-enriched membranes phosphorylated in the presence of calcium. However, this protein was not immunoprecipitated by anti-connexin 36 antibodies indicating that it was not connexin 36 in spite of its similarity in molecular weight. Immunoprecipitation did reveal phosphorylated proteins coimmunoprecipitated with connexin 36. Two of these proteins were identified as beta and alpha tubulin subunits. Though cyclic GMP and calcium did not greatly influence the association of these proteins with connexin 36, the results suggest the possibility of connexin 36 associating with other proteins. Together, these observations indicate that interneuronal communication at gap junctions made by connexin 36 may not be regulated by direct phosphorylation of connexin 36, but possibly by phosphorylation of associated proteins.

Lens connexins alpha3Cx46 and alpha8Cx50 interact with zonula occludens protein-1 (ZO-1)

Connexin alpha1Cx43 has previously been shown to bind to the PDZ domain-containing protein ZO-1. The similarity of the carboxyl termini of this connexin and the lens fiber connexins alpha3Cx46 and alpha8Cx50 suggested that these connexins may also interact with ZO-1. ZO-1 was shown to be highly expressed in mouse lenses. Colocalization of ZO-1 with alpha3Cx46 and alpha8Cx50 connexins in fiber cells was demonstrated by immunofluorescence and by fracture-labeling electron microscopy but showed regional variations throughout the lens. ZO-1 was found to coimmunoprecipitate with alpha3Cx46 and alpha8Cx50, and pull-down experiments showed that the second PDZ domain of ZO-1 was involved in this interaction. Transiently expressed alpha3Cx46 and alpha8Cx50 connexins lacking the COOH-terminal residues did not bind to the second PDZ domain but still formed structures resembling gap junctions by immunofluorescence. These results indicate that ZO-1 interacts with lens fiber connexins alpha3Cx46 and alpha8Cx50 in a manner similar to that previously described for alpha1Cx43. The spatial variation in the interaction of ZO-1 with lens gap junctions is intriguing and is suggestive of multiple dynamic roles for this association.

Regulation of epidermal growth factor-induced connexin 43 gap junction communication by big mitogen-activated protein kinase1/ERK5 but not ERK1/2 kinase activation

The gap junction protein, Cx43, plays a pivotal role in coupling cells electrically and metabolically, and the putative phosphorylation sites that modulate its function are reflected as changes in gap junction communication. Growth factor stimulation has been correlated with a decrease in gap junction communication and a parallel activation of ERK1/2; the inhibition of epidermal growth factor (EGF)-induced Cx43 gap junction uncoupling was observed by using the MEK1/2 inhibitor, PD98059. Because 1) BMK1/ERK5, another MAPK family member also activated by growth factors, possesses a phosphorylation motif similar to ERK1/2, and 2) it has been reported that PD98059 can inhibit not only MEK1/2-ERK1/2 but also MEK5-BMK1 activation, we investigated whether BMK1 can regulate EGF-induced Cx43 gap junction uncoupling and phosphorylation, comparing this to the role of ERK1/2 on Cx43 function and phosphorylation induced by EGF. Selective activation or inactivation of ERK1/2 by using a constitutively active form or a dominant negative form of MEK1 did not regulate Cx43 gap junction coupling. In contrast, we found that BMK1, selectively activated by constitutively active MEK5alpha, induced gap junction uncoupling, and the inhibition of BMK1 activation by transfection of dominant negative BMK1 prevented EGF-induced gap junction uncoupling. Activated BMK1 selectively phosphorylates Cx43 on Ser-255 in vitro and in vivo, but not on S279/S282, which are reported as the consensus phosphorylation sites for MAPK. Furthermore, by co-immunoprecipitation, we found that BMK1 directly associates with Cx43 in vivo. These data indicate that BMK1 is more important than ERK1/2 in EGF-mediated Cx43 gap junction uncoupling by association and Cx43 Ser- 255 phosphorylation.

Multiple connexins contribute to intercellular communication in the Xenopus embryo

To explore the role of gap junctional intercellular communication (GJIC) during Xenopus embryogenesis, we utilized the host-transfer and antisense techniques to specifically deplete Cx38, the only known maternally expressed connexin. Cx38-depleted embryos developed normally but displayed robust GJIC between blastomeres at 32-128 cell stages, suggesting the existence of other maternal connexins. Analysis of embryonic cDNA revealed maternal expression of two novel connexins, Cx31 and Cx43.4, and a third, Cx43, that had been previously identified as a product of zygotic transcription. Thus, the early Xenopus embryo contains at least four maternal connexins. Unlike Cx38, expression of Cx31, Cx43 and Cx43.4 continue zygotically. Of these, Cx43.4 is the most abundant, accumulating significantly in neural structures including the brain, the eyes and the spinal cord.