Presence and Importance of Connexin43 During Myogenesis

We analyzed the expression of connexin(Cx)43 in proliferating and differentiating C₂C₁₂cells and in myoblasts obtained from newborn mice. Cx43 was present in both cell types and under both conditions. The functional role of gap junctional communication (GJC) during terminal differentiation was evaluated in C₂C₁₂myoblasts in the presence or absence of the gap junction blocker 18β-glycyrrhetinic acid (β-GA). Differentiation was temporally analyzed through myogenin expression, activity of creatine kinase (CK), and yield of multinucleated cells. In cells treated with β-GA, the CK activity and myotube formation were reversibly blocked. While in control cultures positive myogenin expression was seen in cell clusters, in β-GA treated cultures the myogenin immunoreactivity was detected in few, preferentially sparse cells. The role of Cx43 during terminal differentiation was evaluated in cultures of myoblasts obtained from Cx43^Cre-ER(T)/fltransgenic mice. Inducible deletion of Cx43 was obtained upon activation of Cre-ER(T) via 4-OH-tamoxifen applications. Cx43 deletion led to a drastic decrease in myogenin expression at 24 h of differentiation as compared to myoblasts from control mice. Our results indicate that Cx43-containing gap junctions are required for normal skeletal muscle terminal differentiation. These channels might provide a pathway for the intercellular transfer of signals involved in myogenesis.     

Spatiotemporal expression of connexin 39 and -43 during myoblast differentiation in cultured cells and in the mouse embryo

Connexin39 (Cx39) and connexin43 (Cx43) are known to be expressed during development of skeletal muscles. Here we have compared the expression pattern of both connexins during differentiation of established C(2)C(12) mouse myoblasts and in the mouse embryo. Cx43 is highly abundant in undifferentiated myoblasts, but no Cx39 protein was detected in these cells. Upon differentiation into myotubes, Cx39 expression increased. The consecutive expression of these connexins was also observed in the mouse embryo. Cx39 and Cx43 were found in different plaques in accordance with the notion that Cx43 is exclusively expressed in myoblasts and Cx39 in myotubes. Thus, differentiating C(2)C(12) cells in culture can serve to study the involvement of gap junctions in myogenesis, since expression of corresponding Cx39 and Cx43 proteins appears to be very similar as in the mouse embryo.     

Connexin39 deficient mice display accelerated myogenesis and regeneration of skeletal muscle

During muscle development and regeneration of skeletal muscle in mice connexin43 (Cx43) and connexin39 (Cx39) are specifically expressed: Cx43 in satellite cells and myoblasts, whereas Cx39 is exclusively expressed in myogenin-positive cells. We generated Cx39 deficient mice by replacing the coding region of the Gjd4 gene by DNA coding for the enhanced green fluorescent protein eGFP. Adult Cx39 deficient mice exhibit no obvious phenotypic alterations of skeletal muscle compared to wild type mice in the resting state. However, myogenesis in Cx39 deficient embryos is accelerated as indicated by increased myogenin expression on ED13.5 and ED16.5 and increased expression of Cx43 in developing skeletal muscle. In addition, the regeneration process of skeletal muscle in Cx39 deficient mice is accelerated as shown by a 2 day earlier onset of MyoD and myogenin expression, relative to wild type littermates. Interestingly, Cx43 expression was also upregulated in Cx39 deficient mice during regeneration of skeletal muscle. We hypothesize that Cx43 may compensate for the loss of Cx39 during myogenesis and regeneration.     

In differentiating prefusion myoblasts connexin43 gap junction coupling is upregulated before myoblast alignment then reduced in post-mitotic cells

Previously we have shown that during in vivo muscle regeneration differentiating rat primary myoblasts transiently upregulate connexin43 (Cx43) gap junctions and leave cell cycle synchronously. Here, we studied the temporal regulation of Cx expression in relation to functional dye coupling in allogenic primary myoblast cultures using western blotting, immuno-confocal microscopy and dye transfer assays. As in vivo, Cx43 was the only Cx isotype out of Cx26, 32, 37, 40, 43 and 45 found in cultured rat myoblasts by immunostaining. Cultured myoblasts showed similar temporal regulation of Cx43 expression and phenotypic maturation to those regenerating in vivo. Cx43 protein was progressively upregulated in prefusion myoblasts, first by the cytoplasmic assembly in sparse myoblast meshworks and then in cell membrane particles in aligned cells. Dye injection using either Lucifer Yellow alone, Cascade Blue with a non-junction permeant FITC-dextran revealed an extensive gap junction coupling between the sparse interacting myoblasts and a reduced communication between the aligned, but still prefused cells. The aligned myoblasts, uniformly upregulate p21^waf1/cip1 and p27^kip1 cell cycle control proteins. Taken together, in prefusion myoblasts less membrane-bound Cx43 was found to mediate substantially more efficient dye coupling in the growing cell fraction than those in the aligned post-mitotic myoblasts. These and our in vivo results in early muscle differentiation are consistent with the role of Cx43 gap junctions in synchronizing cell cycle control of myoblasts to make them competent for a coordinated syncytial fusion.     

Spatiotemporal Expression of Connexin 39 and -43 During Myoblast Differentiation in Cultured Cells and in the Mouse Embryo

Connexin39 (Cx39) and connexin43 (Cx43) are known to be expressed during development of skeletal muscles. Here we have compared the expression pattern of both connexins during differentiation of established C₂C₁₂ mouse myoblasts and in the mouse embryo. Cx43 is highly abundant in undifferentiated myoblasts, but no Cx39 protein was detected in these cells. Upon differentiation into myotubes, Cx39 expression increased. The consecutive expression of these connexins was also observed in the mouse embryo. Cx39 and Cx43 were found in different plaques in accordance with the notion that Cx43 is exclusively expressed in myoblasts and Cx39 in myotubes. Thus, differentiating C₂C₁₂ cells in culture can serve to study the involvement of gap junctions in myogenesis, since expression of corresponding Cx39 and Cx43 proteins appears to be very similar as in the mouse embryo.     

Myogenin expression, cell cycle withdrawal, and phenotypic differentiation are temporally separable events that precede cell fusion upon myogenesis

During terminal differentiation of skeletal myoblasts, cells fuse to form postmitotic multinucleated myotubes that cannot reinitiate DNA synthesis. Here we investigated the temporal relationships among these events during in vitro differentiation of C2C12 myoblasts. Cells expressing myogenin, a marker for the entry of myoblasts into the differentiation pathway, were detected first during myogenesis, followed by the appearance of mononucleated cells expressing both myogenin and the cell cycle inhibitor p21. Although expression of both proteins was sustained in mitogen-restimulated myocytes, 5- bromodeoxyuridine incorporation experiments in serum-starved cultures revealed that myogenin-positive cells remained capable of replicating DNA. In contrast, subsequent expression of p21 in differentiating myoblasts correlated with the establishment of the postmitotic state. Later during myogenesis, postmitotic (p21-positive) mononucleated myoblasts activated the expression of the muscle structural protein myosin heavy chain, and then fused to form multinucleated myotubes. Thus, despite the asynchrony in the commitment to differentiation, skeletal myogenesis is a highly ordered process of temporally separable events that begins with myogenin expression, followed by p21 induction and cell cycle arrest, then phenotypic differentiation, and finally, cell fusion.     

Sphingosine 1-phosphate induces myoblast differentiation through Cx43 protein expression: a role for a gap junction-dependent and -independent function

Although sphingosine 1-phosphate (S1P) has been considered a potent regulator of skeletal muscle biology, acting as a physiological anti-mitogenic and prodifferentiating agent, its downstream effectors are poorly known. In the present study, we provide experimental evidence for a novel mechanism by which S1P regulates skeletal muscle differentiation through the regulation of gap junctional protein connexin (Cx) 43. Indeed, the treatment with S1P greatly enhanced Cx43 expression and gap junctional intercellular communication during the early phases of myoblast differentiation, whereas the down-regulation of Cx43 by transfection with short interfering RNA blocked myogenesis elicited by S1P. Moreover, calcium and p38 MAPK-dependent pathways were required for S1P-induced increase in Cx43 expression. Interestingly, enforced expression of mutated Cx43(Delta130-136) reduced gap junction communication and totally inhibited S1P-induced expression of the myogenic markers, myogenin, myosin heavy chain, caveolin-3, and myotube formation. Notably, in S1P-stimulated myoblasts, endogenous or wild-type Cx43 protein, but not the mutated form, coimmunoprecipitated and colocalized with F-actin and cortactin in a p38 MAPK-dependent manner. These data, together with the known role of actin remodeling in cell differentiation, strongly support the important contribution of gap junctional communication, Cx43 expression and Cx43/cytoskeleton interaction in skeletal myogenesis elicited by S1P.     

Lithium chloride regulates connexin43 in skeletal myoblasts in vitro: possible involvement in Wnt/beta-catenin signaling

Gap junction channels composed of connexin43 (Cx43) are essential for normal myogenic differentiation and skeletal muscle regeneration. Here, the aim was to study whether lithium chloride (LiCl) could regulate Cx43 expression and gap junction channel function by mimicking the Wnt/beta-catenin pathway in primary myoblasts. Cx43 mRNA expression in myoblasts was up-regulated in response to 5 mM LiCl. The enhanced Cx43 protein expression resulting from treatment with 5 and 10 mM LiCl for 24 h increased gap-junctional coupling in myoblasts. However, no obvious changes were observed with 20 mM LiCl. Furthermore, chronic treatment with 10 mM LiCl decreased Cx43 protein expression compared with untreated cells. The authors showed that LiCl mimicked the active canonical Wnt/beta-catenin signaling by glycogen synthase kinase-3beta (GSK-3beta) inactivation and accumulation of the effector protein beta-catenin into the nucleus. These results suggest that LiCl regulates Cx43 expression in skeletal myoblasts in vitro partly by a Wnt/beta-catenin-dependent pathway.     

Deletion of a single allele of Cx43 is associated with a reduction in the gap junctional intercellular communication and increased cell proliferation of mouse lung pneumocytes type II

OBJECTIVES: Connexins (Cx) are proteins that form the gap junctional channels at neighbouring plasma membranes between adjacent cells. Cxs are involved in cell communication, which is reportedly correlated with cell proliferation and differentiation. Alterations in connexin expression and/or gap junctional intercellular communication (GJIC) capacity have long been postulated to be important in a number of pathological conditions including cancer. This study was performed to determine the consequences of the deletion of a single allele of Gja1 (Cx43 gene) in Alveolar Type II cells (APTIIs), and its impact on GJIC and cell proliferation. MATERIAL AND METHODS: In order to do so, APTIIs from wild type (Cx43(+/+)) and heterozygous (Cx43(+/-)) mice were harvested and cultured for 4 days. The GJIC capacity was evaluated by scrape-loading method, with the transfer of lucifer yellow dye. The expression of Cx43 was evaluated by immunofluorescence method and Western blotting. Cell proliferation was evaluated by 3-(4,5-dimethylthazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Results: It was observed that GJIC capacity was significantly reduced and cell proliferation index was significantly higher in Cx43(+/-) cells compared to Cx43(+/+) cells. CONCLUSIONS: These results show that knocking out one allele of Cx43 leads to a lower cell to cell communication capacity, and consequently induces a higher cell proliferation. Because chemically induced lung adenomas in mice are known to originate from APTIIs, these alterations may play a critical role in their susceptibility to lung carcinogenesis.     

Effects of a reduction in the number of gap junction channels or in their conductance on ischemia-reperfusion arrhythmias in isolated mouse hearts

A transient reduction of cell coupling during reperfusion limits myocardial necrosis, but little is known about its arrhythmogenic effects during ischemia-reperfusion. Thus, we analyzed the effect of an extreme reduction in the number of gap junction channels or in their unitary conductance on ventricular arrhythmias during myocardial ischemia-reperfusion. Available gap junction uncouplers have electrophysiological effects independent from their uncoupling actions. Thus, isolated hearts from Cx43(Cre-ER(T)/fl) mice treated with 4-hydroxytamoxifen (4-OHT), from Cx43KI32 mice [in which connexin (Cx)43 was replaced with Cx32], and from control animals were submitted to regional ischemia and reperfusion, and spontaneous and induced ventricular arrhythmias were monitored. In additional hearts, changes in activation time and electrical impedance during global ischemia-reperfusion were assessed. In contrast to treatment with 4-OHT, replacement of Cx43 with Cx32 did not modify baseline activation time or electrical impedance. However, the number of extrasistole and ventricular tachyarrhythmias was higher in isolated hearts from Cx43KI32 and 4-OHT-treated Cx43(Cre-ER(T)/fl) animals versus wild-type animals during normoxia, ischemia (12.29 ± 3.26 and 52.17 ± 22.51 vs. 3.00 ± 1.46 spontaneous tachyarrhythmias, P < 0.05), and reperfusion. The impairment in conduction during ischemia was steeper in isolated hearts from Cx43KI32 animals, whereas changes in myocardial impedance were attenuated during ischemia in both transgenic models, suggesting altered cell-to-cell coupling at baseline. In conclusion, both reduction of Cx43 with 4-OHT and replacement of Cx43 by less-conductive Cx32 were arrhythmogenic under normoxia and ischemia-reperfusion, despite no major effects on baseline electrical properties. These results suggest that modifications in gap junction communication silent under normal conditions may be arrhythmogenic during ischemia-reperfusion.     

Morphofunctional integration between skeletal myoblasts and adult cardiomyocytes in coculture is favored by direct cell-cell contacts and relaxin treatment

The success of cellular cardiomyoplasty, a novel therapy for the repair of postischemic myocardium, depends on the anatomical integration of the engrafted cells with the resident cardiomyocytes. Our aim was to investigate the interaction between undifferentiated mouse skeletal myoblasts (C₂C₁₂ cells) and adult rat ventricular cardiomyocytes in an in vitro coculture model. Connexin43 (Cx43) expression, Lucifer yellow microinjection, Ca²⁺ transient propagation, and electrophysiological analysis demonstrated that myoblasts and cardiomyocytes were coupled by functional gap junctions. We also showed that cardiomyocytes upregulated gap junctional communication and expression of Cx43 in myoblasts. This effect required direct cell-to-cell contact between the two cell types and was potentiated by treatment with relaxin, a cardiotropic hormone with potential effects on cardiac development. Analysis of the gating properties of gap junctions by dual cell patch clamping showed that the copresence of cardiomyocytes in the cultures significantly increased the transjunctional current and conductance between myoblasts. Relaxin enhanced this effect in both the myoblast-myoblast and myoblast-cardiomyocyte cell pairs, likely acting not only on gap junction formation but also on the electrical properties of the preexisting channels. Our findings suggest that myoblasts and cardiomyocytes interact actively through gap junctions and that relaxin potentiates the intercellular coupling. A potential role for gap junctional communication in favoring the intercellular exchange of regulatory molecules, including Ca²⁺, in the modulation of myoblast differentiation is discussed. gap junctions; connexin43     

Effects of 18-glycyrrhetinic acid on serine 368 phosphorylation of connexin43 in rat neonatal cardiomyocytes

18Beta-glycyrrhetinic acid (18beta-GA) regulates serine/threonine dephosphorylation of connexin43 (Cx43). Phospho-specific antibodies were used here to determine the effect of 18beta-GA on serine 368-phosphorylated Cx43 (pSer368Cx43) in cultured rat neonatal cardiomyocytes by immunofluorescence microscopy and immunoblot analyses. 18beta-GA caused a time-dependent increase in pSer368Cx43 levels and induced gap junction disassembly, shown by a change in pSer368Cx43 immunostaining from large aggregates to dispersed punctates at cell-cell contact areas. 18beta-GA also induced a time-dependent increase in the levels of serine 729-phosphorylated PKCepsilon, the active form of PKCepsilon. The 18beta-GA-induced increase in pSer368Cx43 levels and changes in pSer368Cx43 staining pattern were abolished by the PKC inhibitor, chelerythrine. Furthermore, 18beta-GA increased the co-immunoprecipitation of Cx43 with PKCepsilon. However, the 18beta-GA-induced increase in pSer368Cx43 levels and increased association of Cx43 with PKCepsilon were inhibited by co-treatment with the protein phosphatase type 1 and type 2A inhibitor, calyculin A. We conclude that 18beta-GA induces Ser368 phosphorylation of Cx43 via PKCepsilon.     

Transient upregulation of connexin43 gap junctions and synchronized cell cycle control precede myoblast fusion in regenerating skeletal muscle in vivo

The spatio-temporal expression of gap junction connexins (Cx) was investigated and correlated with the progression of cell cycle control in regenerating soleus muscle of Wistar rats. Notexin caused a selective myonecrosis followed by the complete recapitulation of muscle differentiation in vivo, including the activation, commitment, proliferation, differentiation and fusion of myogenic cells. In regenerating skeletal muscle, only Cx43 protein, out of Cx-s 26, –32, –37, –40, –43 and –45, was detected in desmin positive cells. Early expression of Cx43 in the proliferating single myogenic progenitors was followed by a progressive upregulation in interacting myoblasts until syncytial fusion, and then by a rapid decline in multinucleate myotubes. The significant upregulation of Cx43 gap junctions in aligned myoblasts preceding fusion was accompanied by the widespread nuclear expression of cyclin-dependent kinase inhibitors p21^waf1/Cip1 and p27^kip1 and the complete loss of Ki67 protein. The synchronized exit of myoblasts from the cell cycle following extensive gap junction formation suggests a role for Cx43 channels in the regulation of cell cycle control. The potential of Cx43 channels to stimulate p21^waf1/Cip1 and p27^kip1 is known. In the muscle, proving the involvement of Cx43 in either a direct or a bystander cell cycle regulation requires functional investigations.     

Possible involvement of gap junctions in the barrier function of tight junctions of brain and lung endothelial cells

Gap-junction plaques are often observed with tight-junction strands of vascular endothelial cells but the molecular interaction and functional relationships between these two junctions remain obscure. We herein show that gap-junction proteins connexin40 (Cx40) and Cx43 are colocalized and coprecipitated with tight-junction molecules occludin, claudin-5, and ZO-1 in porcine blood-brain barrier (BBB) endothelial cells. Gap junction blockers 18beta-glycyrrhetinic acid (18beta-GA) and oleamide (OA) did not influence expression of Cx40, Cx43, occludin, claudin-5, junctional adhesion molecule (JAM)-A, JAM-B, JAM-C, or ZO-1, or their subcellular localization in the porcine BBB endothelial cells. In contrast, these gap-junction blocking agents inhibited the barrier function of tight junctions in cells, determined by measurement of transendothelial electrical resistance and paracellular flux of mannitol and inulin. 18beta-GA also significantly reduced the barrier property in rat lung endothelial (RLE) cells expressing doxycycline-induced claudin-1, but did not change the interaction between Cx43 and either claudin-1 or ZO-1, nor their expression levels or subcellular distribution. These findings suggest that Cx40- and/or Cx43-based gap junctions might be required to maintain the endothelial barrier function without altering the expression and localization of the tight-junction components analyzed.     

Cardiac mitochondrial connexin 43 regulates apoptosis

Connexin 43 (Cx43) is thought to be present largely in the plasma membrane and its function solely to provide low resistance electrical connection between myocytes. A recent report suggested the presence of Cx43 in the mitochondria as well. We confirmed the presence of Cx43 in the mitochondria isolated from adult rat ventricles with the Cx43 immunoreactivity fractionating to the outer mitochondrial membrane. Mitochondrial Cx43 is mostly phosphorylated only detected by a phospho-specific antibody. Using a Ca2+ -sensitive electrode and Western blot, we showed that the gap junction inhibitors 18-beta-glycyrrhetinic acid (beta-GA), oleamide, and heptanol all induced concomitant release of Ca2+ and cytochrome C in isolated mitochondria whereas the inactive analog 18-beta-glycyrrhizic acid failed to do so. In low density neonatal myocyte culture with no appreciable cell-cell contacts, beta-GA induced apoptosis as assessed by TUNEL staining. Our results suggest a novel role of Cx43 as a regulator of mitochondrial physiology and myocyte apoptosis.     

Gene expression alterations in connexin null mice extend beyond the gap junction

Connexin43 (Cx43) is the principal gap junction protein between astrocytes in the neonatal brain and also interconnects neural precursor cells during CNS development. In an attempt to understand global effects of expression of the Cx43 gap junction gene on development and function of the nervous system, we have compared gene expression patterns in cultured astrocytes and brains from wildtype mice with those in which Cx43 is deleted as well as in spinal cords of experimental autoimmune encepahlomyelitis (EAE) mice. One surprising result obtained from high densitity mouse cDNA studies was the large number of genes that were statistically altered in mice with decreased expression of Cx43. These altered genes encode proteins with a wide range of functions within cells, and thus deletion of normal gap junction expression appears to result in globally altered glial functions in addition to disruption of intercellular communication. Here we discuss those results in the context of the strategies and data analysis paradigms that we are using in such studies.     

Hepatic granulomas induced by Schistosoma mansoni in mice deficient for connexin 43 present lower cell proliferation and higher collagen content

Granuloma formation involves a coordinated interaction between monocytes and macrophages, epithelioid cells, lymphocytes, eosinophils, neutrophils and fibroblasts. It has been established that extracellular communication via cytokines is important for the assembly of granulomas. However, the importance of gap junctions and intercellular communication to granuloma formation and development had never been assessed. Connexins are proteins that form gap junctions, and connexin 43 (Cx43) is present in macrophages, lymphoid cells, myelogenous cells, fibroblasts and others. We analyzed the effect of heterologous deletion of Gja1 (Cx43 gene) on the formation and development of hepatic granulomas induced by Schistosoma mansoni eggs. Heterozygous (Cx43(+/-)) and wild-type (Cx43(+/+)) mice were infected subcutaneously with S. mansoni cercarie and evaluated after 6, 8 and 12 weeks. Granuloma cells express Cx43, as revealed by real-time PCR in isolated granulomas, and by immunohistochemistry. Cx43 expression was reduced in Cx43(+/-) mice, as expected. No differences in the average area of granulomas or number of cells per granuloma were observed between mice of different genotypes. However, granuloma cells from Cx43(+/-) mice displayed a reduced index of the proliferating cell nuclear antigen (PCNA) labeling at 8 and 12 weeks post-infection. Moreover, Cx43(+/-) granulomas unexpectedly presented a higher degree of fibrosis, quantified by morphometric analysis in Sirius Red-stained slides. Our results indicate that the deletion of one allele of the Cx43 gene, and possibly the reduced gap junction intercellular communication capacity (GJIC), may impair the interactions between granuloma cells, reducing their proliferation and increasing their collagen content, thereby modifying the characteristics of S. mansoni granuloma in mice.