Connexins: Synthesis, Post-Translational Modifications, and Trafficking in Health and Disease

Connexins are tetraspan transmembrane proteins that form gap junctions and facilitate direct intercellular communication, a critical feature for the development, function, and homeostasis of tissues and organs. In addition, a growing number of gap junction-independent functions are being ascribed to these proteins. The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional level, and undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function. Here, we summarize these key regulatory events, with emphasis on how these affect connexin multifunctionality in health and disease.

Cross-talk between angiotensin II and IGF-1-induced connexin 43 expression in human saphenous vein smooth muscle cells

Vascular restenosis following coronary artery bypass graft can cause major clinical complications due to intimal hyperplasia in venous conduits. However, the precise underlying mechanisms of intimal hyperplasia are still unclear. We have recently reported that increased expression of connexin43 (Cx43) is involved in the proliferation of vascular smooth muscle cells (SMCs) in human saphenous vein (SV). In this study, we investigated the signalling transduction pathway involved in Cx43 expression and SV SMC proliferation. Angiotensin-II (AT-II, 100 ng/ml) increased AT-II receptor 1 (AT-1R) protein expression and insulin-like growth factor-1 (IGF-1) (100 ng/ml) up-regulated IGF-1 receptor (IGF-1R) protein expression in SV SMCs. Interestingly, AT-1R expression was also increased by IGF-1 treatment, and IGF-1R expression was increased by AT-II treatment, which was blocked by siRNA-IGF-1R and siRNA-AT-1R, respectively. Furthermore, the effect of AT-II and IGF-1 signal cross-talk i nducing up-regulation of their reciprocal receptors was blocked by siRNA against extracellular signal-regulated kinases 1/2 (Erk 1/2) in SMCs of SV. Moreover, AT-II and IGF-1-induced Cx43 expression via phosphorylation of Erk 1/2 and activation of transcription factor activator protein 1 (AP-1) through their reciprocal receptors in SV SMCs. These data demonstrate a cross-talk between IGF-1R and AT-1R in AT-II and IGF-1-induced Cx43 expression in SV SMCs involving Erk 1/2 and downstream activation of the AP-1 transcription factor.

Regulation of connexin expression by transcription factors and epigenetic mechanisms

Gap junctions are specialized cell-cell junctions that directly link the cytoplasm of neighboring cells. They mediate the direct transfer of metabolites and ions from one cell to another. Discoveries of human genetic disorders due to mutations in gap junction protein (connexin [Cx]) genes and experimental data on connexin knockout mice provide direct evidence that gap junctional intercellular communication is essential for tissue functions and organ development, and that its dysfunction causes diseases. Connexin-related signaling also involves extracellular signaling (hemichannels) and non-channel intracellular signaling. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. In recent years, it has become clear that epigenetic processes are also essentially involved in connexin gene expression. In this review, we summarize recent knowledge on regulation of connexin expression by transcription factors and epigenetic mechanisms including histone modifications, DNA methylation, and microRNA. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Regulation of the connexin 43 promoter in the brook trout testis: role of the thyroid hormones and cAMP

Gap junctions are critical for spermatogenesis. They are composed of integral proteins, the connexins. In mammals, a loss of Cx43 expression results in the inhibition of spermatogenesis. We have shown that Cx43 is expressed in the Sertoli cells of rainbow trout and that cAMP and triiodothyronine (T(3)) regulate testicular Cx43 expression in brook trout testis. The objective of this study was to determine if cAMP and T(3) act at the level of the cx43 promoter to regulate its expression. A 607 bp 5' flanking sequence of the cx43 promoter was obtained by Genome Walking. A TATA box was predicted to be located between positions -36 and -30 relative to the transcriptional initiation site. 5'-Rapid amplification of cDNA ends indicated a single transcriptional start site. Single C/EBP (-164 to -156) and tr-beta (-112 to -107) response elements were identified and electrophoretic mobility shift assays indicated the presence of competitive protein binding sites at each region. Immortalized rainbow trout gonadal cell line (RTG-2) which express cx43 and tr-beta transcripts were transfected with a vector containing the Cx43 promoter inserted into a luciferase expression vector. Transactivation of the reporter genes was stimulated by either cAMP or T(3). Sequential deletion and point mutations in either the C/EBP or tr-beta response element indicated that T(3) but not cAMP directly induced luciferase transactivation of the luciferase gene by acting on different sites of the Cx43 promoter. Together, these data indicate that T(3) stimulates cx43 expression via direct regulation of gene transcription.

Immunomodulation by atorvastatin upregulates expression of gap junction proteins in coxsackievirus B3 (CVB3)-induced myocarditis

OBJECTIVE

To investigate the effect of atorvastatin on myocardial expression of gap junction proteins, connexins (Cxs), during coxsackievirus B3 (CVB3)-induced myocarditis.

METHODS

Viral myocarditis was induced in mice by inoculation with CVB3. Atorvastatin (5 or 10 mg kg(-1) day(-1)) or saline was administered by daily oral gavage from the day of induction of viral myocarditis to the day of sacrifice. Fourteen days after injection of CVB3, animals were sacrificed. Alterations in myocardial Cxs expression were examined by RT-PCR, immunoblot, and immunohistochemistry. Plasma levels of TNF-alpha and IFN-gamma were measured by ELISA.

RESULTS

Fourteen days after inoculation with CVB3, myocardial expression of Cx43 and Cx45 was significantly downregulated. Treatment with atorvastatin not only reduced the overproduction of TNF-alpha and IFN-gamma, but also enhanced the expression of Cx43 and Cx45, therefore attenuating myocardial injury and improving the survival rate of viral myocarditis.

CONCLUSION

This study shows for the first time that myocardial expression of Cxs is downregulated during CVB3-induced myocarditis and that immunomodulation by atorvastatin could restore the impaired gap junction channels and improve the outcome of viral myocarditis.

Involvement of connexin 43 in angiotensin II-induced migration and proliferation of saphenous vein smooth muscle cells via the MAPK-AP-1 signaling pathway

Proliferation and migration of vascular smooth muscle cells (VSMCs) lead to intimal thickening and influence the long-term patency of venous graft post coronary arterial bypass graft. There is increasing evidence that connexins are involved in the development of intimal hyperplasia and restenosis. We assessed connexin 43 (Cx43) expression and its role in angiotensin II-induced proliferation and migration of smooth muscle cells and the signal pathways involved in human saphenous vein bypass conduits. Angiotensin II significantly increased gap junctional intercellular communication and induced the expression of Cx43 in human saphenous vein SMCs in a dose- and time-dependent manner through angiotensin II type 1 receptor. The effect of angiotensin II was blocked by siRNA of ERK 1/2, p38 MAPK and JNK, respectively. Overexpression of Cx43 markedly increased the proliferation of saphenous vein SMCs. However, siRNA for Cx43 inhibited angiotensin II-induced proliferation, cyclin E expression and migration of human saphenous vein SMCs. In dual-luciferase reporter assay, angiotensin II markedly activated AP-1 transcription factor, which was significantly attenuated by a dominant-negative AP-1 (A-Fos) with subsequent inhibition of angiotensin II-induced transcriptional expression of Cx43. These data demonstrate the role of Cx43 in the proliferation and migration of human saphenous vein SMCs and angiotensin II-induced Cx43 expression via mitogen-activated protein kinases (MAPK)-AP-1 signaling pathway.

Upregulation of connexin 43 by retinoids but not by non-provitamin A carotenoids requires RARs

Expression of connexin 43 (Cx43) is correlated with reduced indexes of neoplasia and is upregulated by cancer-preventive retinoids and carotenoids in nontransformed human and murine fibroblasts and keratinocytes. The molecular mechanism of upregulation, however, is poorly understood. Three retinoic acid receptor (RAR) antagonists (Ro 41-5253, BMS453, and BMS493) were capable of suppressing retinoid-induced Cx43 protein expression in 10T1/2 cells. However, Ro 41-5253 did not suppress protein expression by the non-provitamin A carotenoids astaxanthin or lycopene. In contrast, Cx43 induction by astaxanthin but not by a RAR-specific retinoid was inhibited by GW9662, an antagonist of peroxisome proliferator activated receptor-gamma activation. Simultaneous treatment with the maximally effective concentration of a retinoid and with beta-carotene or the non-provitamin A carotenoid astaxanthin resulted in supraadditive upregulation of Cx43 expression, again indicating separate mechanisms of gene regulation by these two cancer preventive agents.

Regulation of connexin expression

Gap junctions contain cell-cell communicating channels that consist of multimeric proteins called connexins and mediate the exchange of low-molecular-weight metabolites and ions between contacting cells. Gap junctional communication has long been hypothesized to play a crucial role in the maintenance of homeostasis, morphogenesis, cell differentiation, and growth control in multicellular organisms. The recent discovery that human genetic disorders are associated with mutations in connexin genes and experimental data on connexin knockout mice have provided direct evidence that gap junctional communication is essential for tissue functions and organ development. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Cell coupling via gap junctions is dependent on the specific pattern of connexin gene expression. This pattern of gene expression is altered during development and in several pathological conditions resulting in changes of cell coupling. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. However, transcriptional control is one of the most important points. In this review, we summarize recent knowledge on transcriptional regulation of connexin genes by describing the structure of connexin genes and transcriptional factors that regulate connexin expression.

Gap junctional communication in tissue inflammation and repair

Local injury induces a complex orchestrated response to stimulate healing of injured tissues, cellular regeneration and phagocytosis. Practically, inflammation is defined as a defense process whereby fluid and white blood cells accumulate at a site of injury. The balance of cytokines, chemokines, and growth factors is likely to play a key role in regulating important cell functions such as migration, proliferation, and matrix synthesis during the process of inflammation. Hence, the initiation, maintenance, and resolution of innate responses depend upon cellular communication. A process similar to tissue repair and subsequent scarring is found in a variety of fibrotic diseases. This may occur in a single organ such as liver, kidneys, pancreas, lung, skin, and heart, but fibrosis may also have a more generalized distribution such as in atherosclerosis. The purpose of this review is to summarize recent advances on the contribution of gap junction-mediated intercellular communication in the modulation of the inflammatory response and tissue repair.

Transcriptional regulation of connexin 43 expression by retinoids and carotenoids: similarities and differences

Gap junctions, connexons, are formed by assembly of trans-membrane connexin proteins and have multiple functions including the coordination of cell responses. Most human tumors are deficient in gap junctional communication (GJC) and restoration of GJC by forced expression of connexins reduces indices of neoplasia. Expression of connexin 43 (Cx43), the most widely-expressed connexin family member, is upregulated by cancer-preventive retinoids and carotenoids in normal and preneoplastic cells; an action considered of mechanistic significance. However, the molecular mechanism for upregulated expression is poorly understood. The retinoic acid receptor antagonist Ro 41-5253 was capable of suppressing retinoid-induction Cx43 luciferase reporter construct in F9 cells, but did not suppress reporter activity induced by the non-pro-vitamin A carotenoids astaxanthin or lycopene, indicating that retinoids have separate mechanisms of gene activation than non-pro-vitamin A carotenoids. Neither class of compound required protein synthesis for induction of Cx43 mRNA, nor was the 5.0 h half-life of Cx43 mRNA altered, indicating direct transcriptional activation. The responsive region was found within -158 bp and +209 bp of the transcription start site; this contains a Sp1/Sp3 GC-box to which Sp1 and Sp3 were bound, as revealed by electrophoretic mobility shift assays (EMSA), but no retinoic acid response element (RARE). Site directed mutagenesis of this GC-box resulted in increased basal levels of transcription and loss of responsiveness to a synthetic retinoid. In this construct astaxanthin and lycopene produced marginally, but not significantly higher, reporter activity than the control.

Analysis of the rat connexin 43 proximal promoter in neonatal cardiomyocytes

Altered transcriptional control is likely to contribute to the down-regulation of connexin 43 (Cx43) expression observed in many forms of heart disease. However, little is known about the factors regulating Cx43 transcription in the heart under (patho)physiological conditions. Therefore, a systematic study of rat Cx43 (rCx43) proximal promoter regulation in rat primary neonatal ventricular cardiomyocytes (NCM) and, for comparison, different cell types was initiated. Luciferase assays revealed that, in NCM, the proximal promoter is preserved in a conserved region extending from 148 nucleotides upstream towards 281 nucleotides downstream relative to the transcription initiation site (TIS). Further deletional analysis suggested the involvement of four putative Sp- and two AP1-binding sites. The binding of both Sp1 and Sp3 to the Sp-binding elements and AP1 to the AP1-binding elements was demonstrated by electrophoretic mobility shift assays (EMSA). Promoter-luciferase assays using the natural rCx43 proximal promoter and mutated derivatives in NCM, HL-1 and A7r5 cells revealed that all sites contribute to basal promoter activity. Trans-activation of the Cx43 proximal promoter with Sp1 and Sp3 in Drosophila Schneider line 2 (SL2) cells demonstrated that Sp1 and, to a lesser extent, Sp3 determine rCx43 promoter activation. Thus Sp1, Sp3 and AP1 determine basal Cx43 expression. In addition, we studied the effect of the cardiac transcription factor Nkx2.5 on Cx43 regulation. NCM were infected with adenovirus encoding either beta-galactosidase (control) or Nkx2.5. Cx43 protein and mRNA were significantly decreased after Nkx2.5 infection as shown by Western and Northern blot analyses. Promoter-reporter assays demonstrated that the rCx43 promoter was down-regulated approximately twofold upon Nkx2.5 overexpression. Therefore, in NCM, Nkx2.5 appears to play a role in the regulation of Cx43 expression.

Critical role of the transcriptional repressor neuron-restrictive silencer factor in the specific control of connexin36 in insulin-producing cell lines

Connexin36 (Cx36) is specifically expressed in neurons and in pancreatic beta-cells. Cx36 functions as a critical regulator of insulin secretion and content in beta-cells. In order to identify the molecular mechanisms that control the beta-cell expression of Cx36, we initiated the characterization of the human 5' regulatory region of the CX36 gene. A 2043-bp fragment of the human CX36 promoter was identified from a human BAC library and fused to a luciferase reporter gene. This promoter region was sufficient to confer specific expression to the reporter gene in insulin-secreting cell lines. Within this 5' regulatory region, a putative neuron-restrictive silencer element conserved between rodent and human species was recognized and binds the neuron-restrictive silencing factor (NRSF/REST). This factor is not expressed in insulin-secreting cells and neurons; it functions as a potent repressor through the recruitment of histone deacetylase to the promoter of neuronal genes. The NRSF-mediated repression of Cx36 in HeLa cells was abolished by trichostatin A, confirming the functional importance of histone deacetylase activity. Ectopic expression, by viral gene transfer, of NRSF/REST in different insulin-secreting beta-cell lines induced a marked reduction in Cx36 mRNA and protein content. Moreover, mutations in the Cx36 neuron-restrictive silencer element relieved the low transcriptional activity of the human CX36 promoter observed in HeLa cells and in INS-1 cells expressing NRSF/REST. The data showed that cx36 gene expression in insulin-producing beta-cell lines is strictly controlled by the transcriptional repressor NRSF/REST indicating that Cx36 participates to the neuronal phenotype of the pancreatic beta-cells.

Unexpected induction of the human connexin 43 promoter by the ras signaling pathway is mediated by a novel putative promoter sequence

Connexin 43 (Cx43) is essential for survival and is tightly regulated at the transcriptional and post-transcriptional levels. A number of previous studies have demonstrated altered expression in malignant tissues, and in the presence of carcinogenic factors. We examined the effect of protooncogenes of Cx43 expression, and found no effect on Cx43 promoter activity in cells transformed with Src or erbB2. On the other hand, we identified and characterized a novel sequence that mediates Cx43 promoter regulation in cell lines engineered to overexpress H-Ras. Compared with wild-type NIH3T3 cells, both Cx43 mRNA and protein levels are increased in NIH3T3-Ras cells. The H-Ras+ cells also have enhanced Cx43 promoter activation, which is inhibited by the MEK1 inhibitor 2'-amino-3'-methoxyflavone (PD98059), suggesting that Ras-mediated Cx43 overexpression is via the mitogen activated protein kinase kinase/extracellular signal-regulated pathway. Deletion analysis of the Cx43 promoter revealed a 200-bp region downstream of the Cx43 transcription start site as the minimal sequence essential for the Ras-mediated Cx43 up-regulation. Using this 200-base pair fragment in electrophoretic mobility shift assays, we identified one main protein complex that binds efficiently and is more abundant in nuclear extracts from NIH3T3-Ras and MCF7-Ras cells compared with their matched controls. This complex selectively recognizes a consensus sequence, AGTTCAATCA, located at positions +149 to +158 of the Cx43 promoter. Supershift assays identified the 90-kDa heat shock protein (HSP90) and c-Myc as constituents of this DNA-binding complex. Treatment of cells with the HSP90 inhibitor geldanamycin resulted in repression of the Cx43 promoter activity, and inhibits binding of the complex to the Cx43 promoter. Coimmunoprecipitation studies confirmed the interaction between endogenous HSP90 and c-Myc. This study provides evidence that the transcriptional up-regulation of Cx43 by Ras-Raf-MAPK is mediated via the interaction of a novel Cx43 promoter element with a protein complex that contains both HSP90 and c-Myc.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

Sp1 and Sp3 activate the rat connexin40 proximal promoter

The rat gap junction protein connexin40 (rCx40) has a characteristic developmental and regional expression pattern, for which the exact regulatory mechanisms are not known. To identify the molecular factors controlling Cx40 expression, its proximal promoter was characterized. The proximal rCx40 promoter is the most conserved noncoding region within the Cx40-gene known thus far and contains five potential binding sites for Sp-family transcription factors. The binding of both Sp1 and Sp3 to each of these DNA elements was demonstrated by EMSA. Luciferase assays of the natural rCx40 proximal promoter or mutated derivatives in Cx40-expressing (NCM, primary rat neonatal cardiomyocytes and A7r5, rat smooth muscle embryonic thoracic aorta cells) and -nonexpressing cells (N2A, mouse neuroblastoma cells) revealed that all sites are contributing to basal promoter activity. Trans-activation assays in Drosophila Schneider line 2 cells demonstrated that Sp1 and Sp3 activate the rCx40 proximal promoter in a dose-dependent and additive manner.