Regulation of gap junctions by phosphorylation of connexins

Connexin 43 in LA-25 cells with active v-src is phosphorylated on Y247, Y265, S262, S279/282, and S368 via multiple signaling pathways

Modulation of gap junction structures and gap junctional communication is important in maintaining tissue homeostasis and can be controlled via phosphorylation of connexin 43 (Cx43) through several different signaling pathways. Transformation of cells by v-src has been shown to down-regulate gap junction communication coincident with an increase in tyrosine phosphorylation on Cx43. Activation of mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) also lead to down-regulation via phosphorylation on specific serine residues. Using phosphospecific anti-Cx43 antibodies generated by the authors' laboratory to specific tyrosines (src substrates) and serine residues (MAPK and PKC substrates) to probe LA-25 cells (which express temperature-sensitive v-src), the authors show that distinct tyrosine and serines residues are phosphorylated in response to v-src activity. They show that tyrosine phosphorylation appears to occur predominantly in gap junction plaques when src is active. In addition, src activation led to increased phosphorylation of apparent MAPK and PKC sites in Cx43. These results indicate all three signaling pathways could contribute to gap junction down-regulation during src transformation in LA-25 cells.

No impact of protein phosphatases on connexin 43 phosphorylation in ischemic preconditioning

Cardiac connexin 43 (Cx43) is involved in infarct propagation, and the uncoupling of Cx43-formed channels reduces infarct size. Cx43-formed channels open upon Cx43 dephosphorylation, and ischemic preconditioning (IP) prevents the ischemia-induced Cx43 dephosphorylation. In addition to the sarcolemma, Cx43 is also present in the cardiomyocyte mitochondria. We now examined the interaction of Cx43 with protein phosphatases PP1alpha, PP2Aalpha, and PP2Balpha and the role of such interaction for Cx43 phosphorylation in preconditioned myocardium. Infarct size (in %area at risk) in left ventricular anterior myocardium of Göttinger minipigs subjected to 90 min of low-flow ischemia and 120 min of reperfusion was 23.1 +/- 2.7 [n = 7, nonpreconditioned (NIP) group] and was reduced by IP to 10.0 +/- 3.2 (n = 6, P < 0.05). Mitochondrial and gap junctional Cx43 dephosphorylation increased after 85 min of ischemia in NIP myocardium, whereas Cx43 phosphorylation was preserved with IP. PP2Aalpha and PP1alpha, but not PP2Balpha, were detected by Western blot analysis in the left ventricular myocardium. Cx43 coprecipitated with PP2Aalpha but not with PP1alpha. Although the total PP2Aalpha immunoreactivity (confocal laser scan) was increased to 154 +/- 24% and 194 +/- 13% of baseline (P < 0.05) after 85 min of ischemia in NIP and IP myocardium, respectively, the PP2A activities were similar between the groups. The amount of PP2Aalpha coimmunoprecipitated with Cx43 remained unchanged. Only PP2Aalpha coprecipitates with Cx43 in pig myocardium. This interaction is not affected by IP, suggesting that PP2Aalpha is not involved in the prevention of the ischemia-induced Cx43 dephosphorylation by IP.

Role of connexin-43 hemichannels in the pathogenesis of Yersinia enterocolitica

Connexin (Cx) channels are sites of cytoplasmic communication between contacting cells. Evidence indicates that the opening of hemichannels occurs under both physiological and pathological conditions. In this paper, the involvement of Cx-43 hemichannels is demonstrated in the pathogenesis of Yersinia. Parental HeLa cells and transfected HeLa cells stably expressing Cx-43 (HCx43) were infected with Yersiniaenterocolitica, and bacterial uptake was measured by the colony-forming unit method. Bacterial uptake was higher in HCx43 cells than in parental cells and was inhibited by the Cx channel blocker, 18-alpha-glycyrrhetinic acid (AGA). The inhibitory effect of AGA was more pronounced on the Y. enterocolitica uptake by HCx43 cells than by parental cells. The ability of HCx43 cells to incorporate the permeable fluorescent tracer Lucifer Yellow (LY) was assessed. Dye incorporation was inhibited by AGA, whereas Y. enterocolitica infection of HCx43 cells increased LY incorporation. Western blotting analysis demonstrated that Y. enterocolitica infection of HCx43 cells induced tyrosine phosphorylation of Cx-43, thus supporting a critical role for Cx-43 in the strategies exploited by bacterial pathogens to invade non-phagocytic cells.

Hepatic gap junctions in the hepatocarcinogen-resistant DRH rat

Although the gap junction or connexin (Cx) is considered to be a tumor-suppressor, it is also required for tumor promotion. Therefore, we examined hepatic gap junctions in hepatocarcinogen-resistant (DRH) rats. Specifically, we investigated gap junction structure and Cx32 expression during normal conditions and in response to a hepatocarcinogen, 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). On a basal diet without 3'-MeDAB, hepatic gap junctions and Cx32 protein expression were greater in DRH rats than in control Donryu rats, as evidenced by morphometry, immunohistochemistry and immunoblotting. On a diet containing 3'-MeDAB, gap junctions and expressed Cx32 were increased significantly in Donryu rats, but not in DRH rats. In this condition, Donryu rats lost weight but DRH rats increased relative liver weight. After 3'-MeDAB treatment, cathepsin D expression in hepatocytes was significantly increased only in Donryu rats, indicating that DRH rats were less susceptible to 3'-MeDAB. The abundance of mitogen-activated protein kinase, some constituent of which might be associated with the degree of Cx protein phosphorylation, was reduced to a greater extent in Donryu than in DRH rats after 3'-MeDAB treatment. The resistance of DRH rats to carcinogenesis may be due partially to their stabilized gap junctions, which could coordinate metabolic coupling to evade 3'-MeDAB toxicity.

Phosphorylation in the C-terminus of the rat connexin46 (rCx46) and regulation of the conducting activity of the formed connexons

To analyse the role of PKC-dependent phosphorylation in the C-terminus of rCx46 in regulation of rCx46 connexons, truncated mutants rCx46(45.3) and rCx46(44.2) which end before and after PKC-dependent phosphorylation sites respectively were generated. Both rCx46(45.3) and rCx46(44.2) formed connexons in Xenopus oocytes similar to Cx46(wt)-connexons. They were activated by depolarisation above -40 mV and at voltages above 50 mV, inactivation was spontaneously observed or induced by PKC activator TPA, suggesting that inactivation does not require PKC-dependent phosphorylation in the C-terminus. Three casein-kinase-II-(CKII)-dependent phosphorylation sites were also identified. rCx46(37.7) and rCx46(28.2) respectively without two or all of these sites were generated. rCx46(37.7)-connexons were similar to rCx46(wt)-connexons. rCx46(28.2)-connexons comparable to rCx46(wt)-connexons were observed after injection of 50 times more rCx46(28.2)-mRNA (25 ng per oocyte). CKII-blocker inhibited depolarisation-evoked currents in oocytes injected with 0.5 ng per oocyte rCx46(37.7)-mRNA or rCx46(wt)-mRNA. Injection of 25 ng per oocyte rCx46(37.7)-mRNA or rCx46(wt)-mRNA overcame the effect of CKII-inhibitor. We propose that CKII-dependent phosphorylation in the C-terminus accelerates formation of rCx46-connexons.

Incidence of protein on actin bridges between endothelium and smooth muscle in arterioles demonstrates heterogeneous connexin expression and phosphorylation

Although much physiology in resistance vessels has been attributed to the cytoplasmic connection between endothelial cells (ECs) and vascular smooth muscle cells (VSMCs), little is known of the protein expression between the two cell types. In an attempt to identify the proteins between ECs and VSMCs, mouse cremaster arterioles were stained with phalloidin-Alexa 594 and viewed on a confocal microscope that resolved "actin bridges" within the internal elastic lamina between ECs and VSMCs. To determine the incidence of protein, the pixel intensity from the antibodies on actin bridges were compared with the pixel intensity from antibodies within ECs or VSMCs. N-cadherin, desmin, connexin (Cx)40, and Cx43 and phosphorylated Cx43 at serine-368 were identified on actin bridges, but NG2, CD31, and Cx45 were not evident. Cx37 expression was more variable than the other connexins examined. Using this method on rat mesentery, we confirm the previously published predominance of Cx37 and Cx40 at the myoendothelial junction that was determined using electron microscopy. We conclude that this new method represents an important screening mechanism in which to rapidly test for protein expression between ECs and VSMCs and possibly a first-step in quantifying protein expression at the myoendothelial junction.

Generation of mouse oocytes defective in cAMP synthesis and degradation: endogenous cyclic AMP is essential for meiotic arrest

Although it is established that cAMP accumulation plays a pivotal role in preventing meiotic resumption in mammalian oocytes, the mechanisms controlling cAMP levels in the female gamete have remained elusive. Both production of cAMP via GPCRs/Gs/adenylyl cyclases endogenous to the oocyte as well as diffusion from the somatic compartment through gap junctions have been implicated in maintaining cAMP at levels that preclude maturation. Here we have used a genetic approach to investigate the different biochemical pathways contributing to cAMP accumulation and maturation in mouse oocytes. Because cAMP hydrolysis is greatly decreased and cAMP accumulates above a threshold, oocytes deficient in PDE3A do not resume meiosis in vitro or in vivo, resulting in complete female infertility. In vitro, inactivation of Gs or downregulation of the GPCR GPR3 causes meiotic resumption in the Pde3a null oocytes. Crossing of Pde3a(-/-) mice with Gpr3(-/-) mice causes partial recovery of female fertility. Unlike the complete meiotic block of the Pde3a null mice, oocyte maturation is restored in the double knockout, although it occurs prematurely as described for the Gpr3(-/-) mouse. The increase in cAMP that follows PDE3A ablation is not detected in double mutant oocytes, confirming that GPR3 functions upstream of PDE3A in the regulation of oocyte cAMP. Metabolic coupling between oocytes and granulosa cells was not affected in follicles from the single or double mutant mice, suggesting that diffusion of cAMP is not prevented. Finally, simultaneous ablation of GPR12, an additional receptor expressed in the oocyte, does not modify the Gpr3(-/-) phenotype. Taken together, these findings demonstrate that Gpr3 is epistatic to Pde3a and that fertility as well as meiotic arrest in the PDE3A-deficient oocyte is dependent on the activity of GPR3. These findings also suggest that cAMP diffusion through gap junctions or the activity of additional receptors is not sufficient by itself to maintain the meiotic arrest in the mouse oocyte.

Purification and reconstitution of the connexin43 carboxyl terminus attached to the 4th transmembrane domain in detergent micelles

In recent years, reports have identified that many eukaryotic proteins contain disordered regions spanning greater than 30 consecutive residues in length. In particular, a number of these intrinsically disordered regions occur in the cytoplasmic segments of plasma membrane proteins. These intrinsically disordered regions play important roles in cell signaling events, as they are sites for protein-protein interactions and phosphorylation. Unfortunately, in many crystallographic studies of membrane proteins, these domains are removed because they hinder the crystallization process. Therefore, a purification procedure was developed to enable the biophysical and structural characterization of these intrinsically disordered regions while still associated with the lipid environment. The carboxyl terminal domain from the gap junction protein connexin43 attached to the 4th transmembrane domain (TM4-Cx43CT) was used as a model system (residues G178-I382). The purification was optimized for structural analysis by nuclear magnetic resonance (NMR) because this method is well suited for small membrane proteins and proteins that lack a well-structured three-dimensional fold. The TM4-Cx43CT was purified to homogeneity with a yield of approximately 6 mg/L from C41(DE3) bacterial cells, reconstituted in the anionic detergent 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-RAC-(1-glycerol)], and analyzed by circular dichroism and NMR to demonstrate that the TM4-Cx43CT was properly folded into a functional conformation by its ability to form alpha-helical structure and associate with a known binding partner, the c-Src SH3 domain, respectively.

Altered expression of connexin43 contributes to the arrhythmogenic substrate during the development of heart failure in cardiomyopathic hamster

Heart failure is known to predispose to life-threatening ventricular tachyarrhythmias even before compromising the systemic circulation, but the underlying mechanism is not well understood. The aim of this study was to clarify the connexin43 (Cx43) gap junction remodeling and its potential role in the pathogenesis of arrhythmias during the development of heart failure. We investigated stage-dependent changes in Cx43 expression in UM-X7.1 cardiomyopathic hamster hearts and associated alterations in the electrophysiological properties using a high-resolution optical mapping system. UM-X7.1 hamsters developed left ventricular (LV) hypertrophy by ages 6∼10 wk and showed a moderate reduction in LV contractility at age 20 wk. Appreciable interstitial fibrosis was recognized at these stages. LV mRNA and protein levels of Cx43 in UM-X7.1 were unaffected at age 10 wk but significantly reduced at 20 wk. The expression level of Ser255-phosphorylated Cx43 in UM-X7.1 at age 20 wk was significantly greater than that in control golden hamsters at the same age. In UM-X7.1 at age 10 wk, almost normal LV conduction was preserved, whereas the dispersion of action potential duration was significantly increased. UM-X7.1 at age 20 wk showed significant reduction of cardiac space constant, significant decrease in conduction velocity, marked distortion of activation fronts, and pronounced increase in action potential duration dispersion. Programmed stimulation resulted in sustained ventricular tachycardia or fibrillation in UM-X7.1. LV activation during polymorphic ventricular tachycardia was characterized by multiple phase singularities or wavebreaks. During the development of heart failure in the cardiomyopathic hamster, alterations of Cx43 expression and phosphorylation in concert with interstitial fibrosis may create serious arrhythmogenic substrate through an inhibition of cell-to-cell coupling.

Dopamine and cyclic-AMP regulated phosphoprotein-32-dependent modulation of prefrontal cortical input and intercellular coupling in mouse accumbens spiny and aspiny neurons

The roles of dopamine and cyclic-AMP regulated phosphoprotein-32 (DARPP-32) in mediating dopamine (DA)-dependent modulation of corticoaccumbens transmission and intercellular coupling were examined in mouse accumbens (NAC) neurons by both intracellular sharp electrode and whole cell recordings. In wild-type (WT) mice bath application of the D2-like agonist quinpirole resulted in 73% coupling incidence in NAC spiny neurons, compared with baseline (9%), whereas quinpirole failed to affect the basal coupling (24%) in slices from DARPP-32 knockout (KO) mice. Thus, D2 stimulation attenuated DARPP-32-mediated suppression of coupling in WT spiny neurons, but this modulation was absent in KO mice. Further, whole cell recordings revealed that quinpirole reversibly decreased the amplitude of cortical-evoked excitatory postsynaptic potentials (EPSPs) in spiny neurons of WT mice, but this reduction was markedly attenuated in KO mice. Bath application of the D1/D5 agonist SKF 38393 did not alter evoked EPSP amplitude in WT or KO spiny neurons. Therefore, DA D2 receptor regulation of both cortical synaptic (chemical) and local non-synaptic (dye coupling) communications in NAC spiny neurons is critically dependent on intracellular DARPP-32 cascades. Conversely, in fast-spiking interneurons, blockade of D1/D5 receptors produced a substantial decrease in EPSP amplitude in WT, but not in KO mice. Lastly, in putative cholinergic interneurons, cortical-evoked disynaptic inhibitory potentials (IPSPs) were attenuated by D2-like receptor stimulation in WT but not KO slices. These data indicate that DARPP-32 plays a central role in 1) modulating intercellular coupling, 2) cortical excitatory drive of spiny and aspiny GABAergic neurons, and 3) local feedforward inhibitory drive of cholinergic-like interneurons within accumbens circuits.

Gap junctions in the ovary: expression, localization and function

Gap junctions that allow the direct communication between cytoplasmic compartments of neighboring cells are present in a variety of tissues and organs and play pivotal roles in a wide range of physiological processes. In the ovary, gap junctions consist mainly of connexin (Cx) 43 and Cx37, and their indispensable role in regulating folliculogenesis and oogenesis is well established. The ovarian Cx43 is regulated by gonadotropins at the transcriptional, translational and post-translational levels whereas the regulation of the ovarian Cx37 is yet unknown. In addition to their involvement in normal ovarian functions, gap junction proteins, particularly Cx43, seem to act as cancer suppressors. A summary of our present knowledge regarding gap junctional communication (GJC) and the ovarian gap junction proteins in normally developing ovaries and under pathological conditions is presented in this review.

Function of connexins in the renal circulation

Connexins form intercellular channels that span two plasma membranes and directly couple the cytoplasm of adjacent cells. This morphological contact enables the exchange of ions, second messengers, and metabolites, which act to regulate several biological functions. This review focuses on the significance of connexins in the renal circulation. Cells of the renal vasculature are coupled and express connexins in a vessel and cell-specific pattern. This finding indicates that renal connexins likely play an important role in renal autoregulatory mechanisms (Bayliss effect, tubuloglomerular feedback) and in the control of vasomotor responses. The described coupling of endothelial and vascular smooth muscle cells in the afferent arterioles may also contribute to the communication of neighboring nephrons, called 'nephron coupling.' Furthermore, deletion of the Cx40 and Cx43 genes results in an altered functional behavior of the renin-producing cells, suggesting involvement of these connexin isoforms in the regulation of renin secretion and synthesis. In addition, this review discusses the role of renal connexin expression in the pathogenesis of hypertension or diabetes.

Changes of gap junctional cell-cell communication in overactive detrusor in rats

To evaluate the changes in intercellular communication through gap junctions in detrusor overactivity (DO), we studied 23 adult female Wistar rats with DO after partial outflow obstruction (DO group) and 13 sham-operated rats (control group). The two groups were compared by means of urodynamics, light and electron microscopy, expression of Cx40, Cx43, and Cx45 mRNA genes with RT-PCR, Cx43 protein with Western blot analysis, and functional intercellular communication with scrape loading dye transfer (SLDT) and fluorescence recovery after photobleaching (FRAP). The number of gap junctions and the expression of connexin mRNA and Cx43 protein were increased in DO rats, and intercellular communication through gap junctions increased after 6 wk of partial outflow obstruction as assessed with SLDT and FRAP techniques. The findings provide a theoretical rationale for using Cx43 antagonists and gap junction inhibitors in the treatment of patients with overactive detrusor secondary to partial bladder outflow obstruction.

Intracellular calcium regulation of connexin43

The mechanism by which intracellular Ca(2+) concentration ([Ca(2+)](i)) regulates the permeability of gap junctions composed of connexin43 (Cx43) was investigated in HeLa cells stably transfected with this connexin. Extracellular addition of Ca(2+) in the presence of the Ca(2+) ionophore ionomycin produced a sustained elevation in [Ca(2+)](i) that resulted in an inhibition of the cell-to-cell transfer of the fluorescent dye Alexa fluor 594 (IC(50) of 360 nM Ca(2+)). The Ca(2+) dependency of this inhibition of Cx43 gap junctional permeability is very similar to that described in sheep lens epithelial cell cultures that express the three sheep lens connexins (Cx43, Cx44, and Cx49). The intracellular Ca(2+)-mediated decrease in cell-to-cell dye transfer was prevented by an inhibitor of calmodulin action but not by inhibitors of Ca(2+)/calmodulin-dependent protein kinase II or protein kinase C. In experiments that used HeLa cells transfected with a Cx43 COOH-terminus truncation mutant (Cx43(Delta257)), cell-to-cell coupling was similarly decreased by an elevation of [Ca(2+)](i) (IC(50) of 310 nM Ca(2+)) and similarly prevented by the addition of an inhibitor of calmodulin. These data indicate that physiological concentrations of [Ca(2+)](i) regulate the permeability of Cx43 in a calmodulin-dependent manner that does not require the major portion of the COOH terminus of Cx43.

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

Connexin 43 expression of foreign body giant cells after implantation of nanoparticulate hydroxyapatite

In bone a role of connexin 43 has been implicated with the fusion of mononuclear precursors of the monocyte/macrophage lineage into multinucleated cells. In order to investigate the putative role of connexin 43 in formation of bone osteoclast-like foreign body giant cells which are formed in response to implantation of biomaterials, nanoparticulate hydroxyapatite had been implanted into defects of minipig femura. After 20 days the defect areas were harvested and connexin 43 expression and synthesis were investigated by using immunohistochemistry, Western Blot, and in situ hybridization within macrophages and osteoclast-like foreign body giant cells. Morphological analysis of gap junctions is performed ultrastructurally. As shown on protein and mRNA level numerous connexin 43 positive macrophages and foreign body giant cells (FBGC) were localized within the granulation tissue and along the surfaces of the implanted hydroxyapatite (HA). Besides, the formation of FBGC by fusion of macrophages could be shown ultrastructurally. Connexin 43 labeling observed on the protein and mRNA level could be attributed to gap junctions identified ultrastructurally between macrophages, between FBGC, and between FBGC and macrophages. Annular gap junctions in the cytoplasm of FBGC pointed to degradation of the channels, and the ubiquination that had occurred in the course of degradation was confirmed by Western blot analysis. All in all, the presently observed pattern of connexin 43 labeling refers to an functional role of gap junctional communication in the formation of osteoclast-like foreign body giant cells formed in response to implantation of the nanoparticulate HA.

Role of gap junctions in regulation of progesterone secretion by ovine luteal cells in vitro

To evaluate the role of gap junctions in the regulation of progesterone secretion, two experiments were conducted. In Experiment 1, luteal cells obtained on days 5, 10, and 15 were cultured overnight at densities of 50 x 10(3), 100 x 10(3), 300 x 10(3), and 600 x 10(3) cells/dish in medium containing: (1) no treatment (control), (2) LH, or (3) dbcAMP. In Experiment 2, luteal cells from days 5 and 10 of the estrous cycle were transfected with siRNA, which targeted the connexin (Cx) 43 gene. In Experiment 1, progesterone secretion, Cx43 mRNA expression, and the rates of gap junctional intercellular communication (GJIC), were affected by the day of the estrous cycle, cell density, and treatments (LH or dbcAMP). The changes in progesterone secretion were positively correlated with the changes in Cx43 mRNA expression and the rates of GJIC. Cx43 was detected on the luteal cell borders in every culture, and luteal cells expressed 3beta-hydroxysteroid dehydrogenase. In Experiment 2, two Cx43 gene-targeted sequences decreased Cx43 mRNA expression and progesterone production by luteal cells. The changes in Cx43 mRNA expression were positively correlated with changes in progesterone concentration in media. Thus, our data demonstrate a relationship between gap junctions and progesterone secretion that was supported by (1) the positive correlations between progesterone secretion and Cx43 mRNA expression and GJIC of luteal cells and (2) the inhibition of Cx43 mRNA expression by siRNA that resulted in decreased production of progesterone by luteal cells. This suggests that gap junctions may be involved in the regulation of steroidogenesis in the ovine corpus luteum.

Enhancing effect of connexin 32 gene on vinorelbine-induced cytotoxicity in A549 lung adenocarcinoma cells

PURPOSE

Connexin (Cx) genes exert negative growth effects on tumor cells with certain cell specificity, and tumor-suppressive effects of the Cx genes contribute to enhancement of chemotherapeutical agents-induced cytotoxicity in some cancer cells. Since we and others have been reported that Cx32 acts as a tumor suppressor gene in lung adenocarcinomas, this study was undertaken to estimate if the combination of Cx32-dependent tumor-suppressive effect and vinorelbine (VBN), a chemotherapeutic agent which has been utilized for clinical lung adenocarcinoma treatment, could be effective in enhancing the sensitivity of the lung cancer to VBN treatment.

METHODS

We established the A549 cells (a human lung adenocarcinoma cell line) which had stable expression of Cx32 and estimated effect of Cx32 on VBN-induced cytotoxicity in the established cells.

RESULTS

Cx32 expression in A549 cells significantly potentiated VBN-induced cytotoxicity on the cells due to enhancement of apoptosis induction. The enhancing cytotoxicity in A549 cells by Cx32 mainly depended on a decrease in expression of multi-drug resistance-1 (MDR-1) gene responsible for reduction of VBN accumulation into the cells. We also observed that silencing of Cx32 by siRNA treatment elevated the expression level of MDR-1 mRNA in A549 cells and that inhibition of MDR-1 gene product-dependent function enhanced VBN-induced cytotoxicity in the cells.

CONCLUSION

These results suggest that Cx32 contributes to the enhancement of VBN-induced cytotoxicity in A549 cells via the reduction of MDR-1 expression.

Oscillating fluid flow activation of gap junction hemichannels induces ATP release from MLO-Y4 osteocytes

Mechanical loads are required for optimal bone mass. One mechanism whereby mechanical loads are transduced into localized cellular signals is strain-induced fluid flow through lacunae and canaliculi of bone. Gap junctions (GJs) between osteocytes and osteoblasts provides a mechanism whereby flow-induced signals are detected by osteocytes and transduced to osteoblasts. We have demonstrated the importance of GJ and gap junctional intercellular communication (GJIC) in intracellular calcium and prostaglandin E(2) (PGE(2)) increases in response to flow. Unapposed connexons, or hemichannels, are themselves functional and may constitute a novel mechanotransduction mechanism. Using MC3T3-E1 osteoblasts and MLO-Y4 osteocytes, we examined the time course and mechanism of hemichannel activation in response to fluid flow, the composition of the hemichannels, and the role of hemichannels in flow-induced ATP release. We demonstrate that fluid flow activates hemichannels in MLO-Y4, but not MC3T3-E1, through a mechanism involving protein kinase C, which induces ATP and PGE(2) release.

Inhibitory effect of bisphenol A on gap junctional intercellular communication in an epithelial cell line of rat mammary tissue

An endocrine disruptor, bisphenol-A (BPA), has been reported to have several short-term actions in various cells and tissues. However, the mechanisms of these actions have not been fully elucidated. In order to assess the effect of BPA on the intercellular communication mediated by gap junctions, we conducted the present study in the rat epithelium-derived BICR-M1Rk cell-line, in which connexin 43 (Cx43) is a major gap junction channel-forming protein. The cytotoxicity of BPA toward the cultured cells was evaluated by using both MTT reduction and LDH leakage assay systems. The results showed no appreciable loss in cell viability in the presence of increasing concentrations of BPA (from 0.1 to 3.2 microM) for 1 h incubation. However, most of cell viability was lost when cells were incubated for 24 hr with the same concentrations of BPA. The BPA acted as an antagonist on gap junction-mediated intercellular communication (GJIC), and the phenomenon was dose-dependent and irreversible. According to the data obtained from scrape-loading dye-transfer experiments, three quarters of normal GJIC was reduced by concentration of 0.4 microM BPA for 1 h incubation. To identify the relevance of this retardation upon BPA treatment, the GJIC to Cx43 synthesis, the mRNA and protein levels of Cx43 were assessed with RT-PCR and Western-blotting, respectively. The total protein level of Cx43 was almost constant in a wide range of BPA concentrations, as well as in Cx43 mRNA level. These results suggest that BPA inhibits GJIC through a modulation of the gating of gap junction channels, not through a genomic modulation of Cx43.

Interferon-gamma inhibits intestinal restitution by preventing gap junction communication between enterocytes

BACKGROUND & AIMS

Necrotizing enterocolitis (NEC) is characterized by interferon-gamma (IFN-gamma) release and inadequate intestinal restitution. Because enterocytes migrate together, mucosal healing may require interenterocyte communication via connexin 43-mediated gap junctions. We hypothesize that enterocyte migration requires interenterocyte communication, that IFN impairs migration by impairing connexin 43, and that impaired healing during NEC is associated with reduced gap junctions.

METHODS

NEC was induced in Swiss-Webster or IFN(-/-) mice, and restitution was determined in the presence of the gap junction inhibitor oleamide, or via time-lapse microscopy of IEC-6 cells. Connexin 43 expression, trafficking, and localization were detected in cultured or primary enterocytes or mouse or human intestine by confocal microscopy and (35)S-labeling, and gap junction communication was assessed using live microscopy with oleamide or connexin 43 siRNA.

RESULTS

Enterocytes expressed connexin 43 in vitro and in vivo, and exchanged fluorescent dye via gap junctions. Gap junction inhibition significantly reduced enterocyte migration in vitro and in vivo. NEC was associated with IFN release and loss of enterocyte connexin 43 expression. IFN inhibited enterocyte migration by reducing gap junction communication through the dephosphorylation and internalization of connexin 43. Gap junction inhibition significantly increased NEC severity, whereas reversal of the inhibitory effects of IFN on gap junction communication restored enterocyte migration after IFN exposure. Strikingly, IFN(-/-) mice were protected from the development of NEC, and showed restored connexin 43 expression and intestinal restitution.

CONCLUSIONS

IFN inhibits enterocyte migration by preventing interenterocyte gap junction communication. Connexin 43 loss may provide insights into the development of NEC, in which restitution is impaired.

Alpha-synuclein overexpression reduces gap junctional intercellular communication in dopaminergic neuroblastoma cells

Alpha-synuclein has been implicated in the pathology of certain neurodegenerative diseases, including Parkinson disease (PD) and dementia with Lewy bodies (LBs). Overexpression of human alpha-synuclein in neuronal cells reduces cell viability, but the precise cellular and molecular mechanisms remain poorly understood. Gap junctional intercellular communication (GJIC) is thought to be essential for maintaining cellular homeostasis and growth control. In the present study, the effect of alpha-synuclein overexpression on GJIC in human dopaminergic neuroblastoma SH-SY5Y cells was investigated. Cells overexpressing wild-type alpha-synuclein were more vulnerable to hydrogen peroxide and 6-hydroxydopamine. GJIC was decreased in cells overexpressing alpha-synuclein. In addition, alpha-synuclein binds directly to connexin-32 (Cx32). As such, the post-translational modification of Cx32 was enhanced in cells overexpressing alpha-synuclein. These findings suggest that alpha-synuclein can modulate GJIC in a dopaminergic neuronal cell line through specific binding to Cx32.

Influence of anisotropic conduction properties in the propagation of the cardiac action potential

Anisotropy, the property of being directionally dependent, is ubiquitous in nature. Propagation of the electrical impulse in cardiac tissue is anisotropic, a property that is determined by molecular, cellular, and histological determinants. The properties and spatial arrangement of connexin molecules, the cell size and geometry, and the fiber orientation and arrangement are examples of structural determinants of anisotropy. Anisotropy is not a static property but is subject to dynamic functional regulation, mediated by modulation of gap junctional conductance. Tissue repolarization is also anisotropic. The relevance of anisotropy extends beyond normal propagation and has important implications in pathological states, as a potential substrate for abnormal rhythms and reentry.

The role of connexins in controlling cell growth and gene expression

The purpose of this paper is to provide a brief overview of current thinking on the role of connexins, in particular Cx43, in growth regulation, and a more detailed discussion as to potential mechanisms involved with an emphasis on gene expression. While the precise molecular mechanism by which connexins can affect the growth of normal or tumor cells remains elusive, a number of exciting reports have expanded our understanding and are presented in some detail. Thus, we will discuss (Section 2): the role of protein-protein interactions in integrating connexins into multiple signal transduction pathways; phosphorylation at specific sites and reversal of growth inhibition; the role of the carboxy-terminal regulatory domain as a signaling molecule. Some of our latest work on the potential functions of endogenously produced carboxy-terminal fragments of Cx43 are also presented (Section 3). Finally, Section 4 will pay tribute to the rapidly emerging realization that connexins such as Cx43 and Cx32 exert important and extensive effects on gene expression, particularly those genes linked to growth regulation.

Change in permeant size selectivity by phosphorylation of connexin 43 gap-junctional hemichannels by PKC

Gap-junctional channels, permeable to large hydrophilic solutes of up to M(r) approximately 1,000, are responsible for cell-to-cell communication. Phosphorylation of connexin 43 (Cx43) by PKC abolishes the permeability of gap-junctional channels and hemichannels to large hydrophilic solutes, but not to small inorganic ions. Here, we report on a methodology to produce purified hemichannels of controlled subunit composition and apply it to the generation of hemichannels with variable number of PKC-phosphorylated subunits. The subunit composition was determined by luminescence resonance energy transfer. We show that all Cx43 subunits in the hemichannel hexamer have to be phosphorylated to abolish sucrose (M(r) 342) permeability. We also show that the hemichannel pores with all subunits phosphorylated by PKC have a sizable diameter, allowing for permeation of the small hydrophilic solute ethyleneglycol (M(r) 62). These results indicate that phosphorylation of Cx43 by PKC alters the hemichannel size selectivity and explain why PKC activity affects dye transfer between cells without consistent effects on electrical communication.

Cloning of GJA1 (connexin43) and its expression in canine ovarian follicles throughout the estrous cycle

GJA1 (also known as connexin43 or Cx43) is the most abundant gap junction protein in mammalian tissues including the ovary. Here, it facilitates intercellular communication among granulosa cells and growing oocytes, thereby connecting the developing gamete to the hormonal axis as well as to the essential network of supporting granulosa cells. To date, the pattern of follicular GJA1 expression has not yet been defined for canines, a species with unique reproductive physiology including delays in follicle development, ovulation, oocyte maturation and fertilization. Here, we report the complete mRNA sequence for canine GJA1 and identify not only increases (P<0.05) in GJA1 mRNA expression in follicles at the secondary stage and larger, but also differences in expression levels between estrous cycle stages in both secondary and antral stage follicles. Expression of GJA1 mRNA in secondary follicles during proestrus was higher than in anestrus or estrus (P<0.01), and at diestrus (P<0.10). Antral follicles obtained during estrus expressed lower levels of GJA1 mRNA than any other cycle stage (P<0.01). GJA1 mRNA expression in primary and large antral follicles was similar across the estrous cycle. Despite the extensive length of the canine estrous cycle as compared with that of other mammals, the GJA1 mRNA and protein expression profiles are not significantly different from those reported for other species and suggests that mechanisms regulating GJA1 transcription are not likely to contribute to the extended delays in follicle and oocyte development in the dog.

18β-glycyrrhetinic acid promotes src interaction with connexin43 in rat cardiomyocytes

The mechanism by which 18beta-glycyrrhetinic acid regulates gap junction intercellular communication (GJIC) remains poorly understood. In this study, treatment of cultured rat neonatal cardiomyocytes with 18beta-glycyrrhetinic acid resulted in dose-dependent inhibition of GJIC as assessed by fluorescent dye transfer analysis. 18beta-Glycyrrhetinic acid induced time-dependent serine/threonine dephosphorylation and redistribution of connexin43 (Cx43) in cardiomyocytes and the induced Cx43 dephosphorylation was prevented by the protein phosphatase inhibitor, calyculin A. However, functional analyses showed that the inhibitory effect of 18beta-glycyrrhetinic acid on dye spreading among cardiomyocytes was not blocked by calyculin A, but was blocked by the Src-selective tyrosine kinase inhibitor, PP2. 18beta-Glycyrrhetinic acid also induced an increase in the levels of phosphorylated Src, and this effect was prevented by PP2. Immunoprecipitation using anti-Cx43 and anti-p-Src antibodies showed that 18beta-glycyrrhetinic acid increased the association between p-Src and Cx43 and induced tyrosine phosphorylation of Cx43. We conclude that the inhibitory effect of 18beta-glycyrrhetinic acid on GJIC in cardiomyocytes involves Src-mediated tyrosine phosphorylation of Cx43.

Herpes simplex virus-type 2 infectivity and agents that block gap junctional intercellular communication

In rat liver epithelial (WB) cells, the protein kinase C inhibitor H7 blocked gap junctional intercellular communication (GJIC) and reduced virus infectivity. Octanol, 18-beta-glycyrrhetinic acid, and staurosporine, agents that reduce GJIC, had no effect upon virus infectivity. Previous studies demonstrated that herpes simplex virus-type 2 (HSV-2) infection was accompanied by attenuated GJIC. Of agents tested, only H7 reduced plaque forming unit (pfu) ability in a dose-dependent manner with 100% plaque reduction at 40 microM without evidence of cytotoxicity. Dye transfer indicated that H7 decreased GJIC, although Western blotting revealed that it did not alter phosphorylation of the gap junction protein, connexin 43 (Cx43). Using indirect immunofluorescence, Cx43 was found to localize in membrane plaques in uninfected cells and H7 did not alter this distribution. However, Cx43 was lost from the membrane at 24h in both H7-treated and untreated cells infected with HSV-2. Viral infection increased serine phosphorylation, particularly in the nuclear region, and this effect was reduced following H7 treatment. Thus, H7 attenuated both GJIC and infectivity of HSV-2 in WB cells but the anti-viral effects were due to reduced nuclear protein phosphorylation rather than alterations in phosphorylation or localization of Cx43.

ZO-1 is required for protein kinase C gamma-driven disassembly of connexin 43

We have previously reported that protein kinase C gamma (PKC-gamma) is activated by phorbol-12-myristate-13-acetate (TPA) and that this causes PKC-gamma translocation to membranes and phosphorylation of the gap junction protein, connexin 43 (Cx43). This phosphorylation, on S368 of Cx43, causes disassembly of Cx43 out of cell junctional plaques resulting in the inhibition of dye transfer. The purpose of this study is to identify the specific role of zonula occludens protein-1 (ZO-1), a tight junction protein with recently established effects on gap junctions, in this PKC-gamma-driven Cx43 disassembly. For this purpose, ZO-1 levels in lens epithelial cells in culture were decreased by up to 70% using specific siRNA. The down-regulation of ZO-1 caused a stable interaction of PKC-gamma with Cx43 even without normal enzyme activation by TPA. However, after TPA activation of the PKC-gamma, the Cx43 did not disassemble out of plaques even though the PKC-gamma enzyme was activated and the Cx43 was phosphorylated on S368. Confocal microscopy demonstrated that the siRNA treatment caused a loss of ZO-1 from borders of large junctional Cx43 cell-to-cell plaques and resulted in the accumulation of Cx43 aggregates inside of cells. Loss of the specific "plaquetosome" arrangement of large Cx43 plaques surrounded by ZO-1 was accompanied by a complete loss of functional dye transfer. These results suggest that ZO-1 is required for Cx43 control, both for dye transfer, and, for the PKC-gamma-driven disassembly response.

Protein kinase A-mediated phosphorylation of connexin36 in mouse retina results in decreased gap junctional communication between AII amacrine cells

Gap junctions in AII amacrine cells of mammalian retina participate in the coordination of the rod and cone signaling pathway involved in visual adaptation. Upon stimulation by light, released dopamine binds to D(1) receptors on AII amacrine cells leading to increased intracellular cAMP (cyclic adenosine monophosphate) levels. AII amacrine cells express the gap junctional protein connexin36 (Cx36). Phosphorylation of Cx36 has been hypothesized to regulate gap junctional activity of AII amacrine cells. However, until now in vivo phosphorylation of Cx36 has not been reported. Indeed, it had been concluded that Cx36 in bovine retina is not phosphorylated, but in vitro phosphorylation for Cx35, the bass ortholog of Cx36, had been shown. To clarify this experimental discrepancy, we examined protein kinase A (PKA)-induced phosphorylation of Cx36 in mouse retina as a possible mechanism to modulate the extent of gap junctional coupling. The cytoplasmic domains of Cx36 and the total Cx36 protein were phosphorylated in vitro by PKA. Mass spectroscopy revealed that all four possible PKA consensus motifs were phosphorylated; however, domains point mutated at the sites in question showed a prevalent usage of Ser-110 and Ser-293. Additionally, we demonstrated that Cx36 was phosphorylated in cultured mouse retina. Furthermore, activation of PKA increased the level of phosphorylation of Cx36. cAMP-stimulated, PKA-mediated phosphorylation of Cx36 protein was accompanied by a decrease of tracer coupling between AII amacrine cells. Our results link increased phosphorylation of Cx36 to down-regulation of permeability through gap junction channels mediating light adaptation in the retina.

Reduction and redistribution of gap and adherens junction proteins after ischemia and reperfusion

BACKGROUND

Previous studies have demonstrated that alterations in myocardial structure, consistent with tissue and sarcomere disruption as well as myofibril dissociation, occur after myocardial ischemia and reperfusion. In this study we determine the onset of these structural changes and their contribution to electrical conduction.

METHODS

Langendorff perfused rabbit hearts (n = 47) were subjected to 0, 5, 10, 15, 20, 25, and 30 minutes global ischemia, followed by 120 minutes reperfusion. Hemodynamics were recorded and tissue samples were collected for histochemical and immunohistochemical studies. Orthogonal epicardial conduction velocities were measured, with temperature controlled, in a separate group of 10 hearts subjected to 0 or 30 minutes of global ischemia, followed by 120 minutes of reperfusion.

RESULTS

Histochemical and quantitative light microscopy spatial analysis showed significantly increased longitudinal and transverse interfibrillar separation after 15 minutes or more of ischemia (p < 0.05 versus control). Confocal immunohistochemistry and Western blot analysis demonstrated significant reductions (p < .05 versus control) of the intercellular adherens junction protein, N-cadherin, and the active phosphorylated isoform of the principal gap junction protein, connexin 43 at more than 15 minutes of ischemia. Cellular redistribution of connexin 43 was also evidenced on immunohistochemistry. No change in integrin-beta1, an extracellular matrix attachment protein, or in epicardial conduction velocity anisotropy was observed.

CONCLUSIONS

These data indicate that there are significant alterations in the structural integrity of the myocardium as well as gap and adherens junction protein expression with increasing global ischemia time. The changes occur coincident with previously observed significant decreases in postischemic functional recovery, but are not associated with altered expression of matrix binding proteins or electrical anisotropic conduction.

Mathematical model of morphogen electrophoresis through gap junctions

Gap junctional communication is important for embryonic morphogenesis. However, the factors regulating the spatial properties of small molecule signal flows through gap junctions remain poorly understood. Recent data on gap junctions, ion transporters, and serotonin during left-right patterning suggest a specific model: the net unidirectional transfer of small molecules through long-range gap junctional paths driven by an electrophoretic mechanism. However, this concept has only been discussed qualitatively, and it is not known whether such a mechanism can actually establish a gradient within physiological constraints. We review the existing functional data and develop a mathematical model of the flow of serotonin through the early Xenopus embryo under an electrophoretic force generated by ion pumps. Through computer simulation of this process using realistic parameters, we explored quantitatively the dynamics of morphogen movement through gap junctions, confirming the plausibility of the proposed electrophoretic mechanism, which generates a considerable gradient in the available time frame. The model made several testable predictions and revealed properties of robustness, cellular gradients of serotonin, and the dependence of the gradient on several developmental constants. This work quantitatively supports the plausibility of electrophoretic control of morphogen movement through gap junctions during early left-right patterning. This conceptual framework for modeling gap junctional signaling -- an epigenetic patterning mechanism of wide relevance in biological regulation -- suggests numerous experimental approaches in other patterning systems.

Oxidized phospholipids alter vascular connexin expression, phosphorylation, and heterocellular communication

OBJECTIVE

In endothelial cells (EC) and vascular smooth muscle cells (VSMC) from atherosclerotic mice, connexin (Cx) expression becomes distorted. Lipoprotein-derived phospholipid oxidation products (OxPAPC) play a critical role in atherosclerosis, and we hypothesized that they may act as trigger molecules causing the changes in connexin expression.

METHODS AND RESULTS

We applied OxPAPC to murine carotid arteries in vivo and vascular cell cocultures. OxPAPC applied to carotids induced an upregulation of both Cx37 and Cx43 in the VSMC. In EC, Cx43 was upregulated and Cx37 was downregulated, whereas Cx40 in EC remained constant. In the vascular cell coculture, OxPAPC caused similar changes in Cx37 and Cx43 but caused a decrease in Cx40 in EC and an elevation of Cx40 in VSMC. In the coculture model, OxPAPC treatment led to the selective disappearance of Cx40 at the myoendothelial junction. Biocytin dye transfer between EC and VSMC coupling was dramatically reduced by OxPAPC. The decrease in dye transfer after OxPAPC treatment was correlated with an increase in tyrosine 265 phosphorylation of Cx43, especially at the in vitro myoendothelial junction.

CONCLUSIONS

We conclude that OxPAPC could be responsible for the changes in connexin expression previously reported in atherosclerosis.

Transition from preinvasive carcinoma in situ to seminoma is accompanied by a reduction of connexin 43 expression in Sertoli cells and germ cells

Carcinoma in situ (CIS) represents the preinvasive stage of human germ cell tumors, but the mechanism leading to pubertal proliferation and invasive malignancy remains unknown. Among testicular gap junctional proteins, connexin 43 (Cx43) represents the predominant Cx, and, previously, an inverse correlation between synthesis of Cx43 protein and progression of tumor development was detected. In the present study, using cDNA microarray analysis, in situ hybridization, semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) from tissue homogenates, RT-PCR from microdissected tubules with normal spermatogenesis and CIS, and seminoma cells from invasive seminoma, we asked whether reduction of Cx43 protein is accompanied by a change of Cx43 transcripts. We detected a significant downregulation of Cx43 at mRNA level in Sertoli and germ cells starting in seminiferous tubules infiltrated with CIS and resulting in a complete loss in seminoma cells. It was demonstrated, that downregulation of Cx43 expression in neoplastic human testis takes place at the transcriptional level and starts in CIS. This reduction of Cx43 expression further suggests that early intratubular derangement in Cx43 gene expression and disruption of intercellular communication between Sertoli cells and/or Sertoli and preinvasive tumor cells may play a role in the progression phase of human seminoma development.

A mouse strain difference in tumorigenesis induced by biodegradable polymers

The use of poly-L-lactic acid (PLLA) surgical implants for repair of bone fractures has gained popularity in the past decade. The aim of this study was to evaluate the in vivo effect of PLLA plates on subcutaneous tissue in two mouse strains, BALB/cJ and SJL/J, which have higher and lower tumorigenicity, respectively. Gap-junctional intercellular communication and protein expression of connexin 43 were significantly suppressed, whereas secretion of transforming growth factor-beta1 and expression of extracellular matrix, insulin-like growth factor binding protein 3, and cysteine-rich intestinal protein 2 were significantly increased in PLLA-implanted BALB/cJ mice when compared with BALB/cJ controls. Finally, tumors were formed after implantation of cultured cells from the more-tumorigenic BALB/cJ, but not SJL/J, mice into nude mice.

Administration of FGF-2 to the heart stimulates connexin-43 phosphorylation at protein kinase C target sites

Fibroblast growth factor-2 (FGF-2) confers acute, preconditioning-like cardiac resistance to ischemic injury in a protein kinase C (PKC)-dependent fashion. One of the downstream targets of PKC is the gap junction protein connexin-43 (Cx43). We thus examined the effects of FGF-2 on Cx43 phosphorylation at specific PKC sites in the adult heart. Rat hearts perfused ex vivo for 20 min with an FGF-2-containing solution displayed increased levels of phosphorylated 44-45 kDa Cx43, assessed by western blotting. In addition, FGF-2 significantly upregulated phosphorylation of the PKC target serines 262 and 368 on Cx43 at intercalated disks, assessed using phosphospecific antibodies in immunolocalization and western blotting assays. Our data show that FGF-2, administered by perfusion, can alter the phosphorylation status of Cx43 at cardiomyocyte intercalated disks, and suggest a link between phosphorylation of Cx43 at specific PKC sites and FGF-2 cardioprotection.

Life cycle of connexins in health and disease

Evaluation of the human genome suggests that all members of the connexin family of gap-junction proteins have now been successfully identified. This large and diverse family of proteins facilitates a number of vital cellular functions coupled with their roles, which range from the intercellular propagation of electrical signals to the selective intercellular passage of small regulatory molecules. Importantly, the extent of gap-junctional intercellular communication is under the direct control of regulatory events associated with channel assembly and turnover, as the vast majority of connexins have remarkably short half-lives of only a few hours. Since most cell types express multiple members of the connexin family, compensatory mechanisms exist to salvage tissue function in cases when one connexin is mutated or lost. However, numerous studies of the last decade have revealed that mutations in connexin genes can also lead to severe and debilitating diseases. In many cases, single point mutations lead to dramatic effects on connexin trafficking, assembly and channel function. This review will assess the current understanding of wild-type and selected disease-linked mutant connexin transport through the secretory pathway, gap-junction assembly at the cell surface, internalization and degradation.

Acetylcholine inhibits the hypoxia-induced reduction of connexin43 protein in rat cardiomyocytes

In a recent study, we demonstrated that vagal stimulation increases the survival of rats with myocardial infarction by inhibiting lethal arrhythmia through regulation of connexin43 (Cx43). However, the precise mechanisms for this effect remain to be elucidated. To investigate these mechanisms and the signal transduction for gap junction regulation, we investigated the effect of acetylcholine (ACh), a parasympathetic nerve system neurotransmitter, on the gap junction component Cx43 using H9c2 cells. When cells were subjected to hypoxia, the total Cx43 protein level was decreased. In contrast, pretreatment with ACh inhibited this effect. To investigate the signal transduction, cells were pretreated with L-NAME, a nitric oxide synthase inhibitor, followed by ACh and hypoxia. L-NAME was found to suppress the ACh effect. However, a NO donor, SNAP, partially inhibited the hypoxia-induced reduction in Cx43. To delineate the mechanisms of the decrease in Cx43 under hypoxia, cells were pretreated with MG132, a proteasome inhibitor. Proteasome inhibition produced a striking recovery of the decrease in the total Cx43 protein level under hypoxia. However, cotreatment with MG132 and ACh did not produce any further increase in the total Cx43 protein level. Functional studies using ACh or okadaic acid, a phosphatase inhibitor, revealed that both reagents inhibited the decrease in the dye transfer induced by hypoxia. These results suggest that ACh is responsible for restoring the decrease in the Cx43 protein level, resulting in functional activation of gap junctions.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

Electrical properties and cell-to-cell communication of the salivary gland cells of the snail, Helix pomatia

The aim of the present study was to assess the cellular mechanism of secretion in the salivary gland of the snail, Helix pomatia, using electrophysiological, electron microscopic and immunohistochemical techniques. A homogeneously distributed membrane potential (-56.6 +/- 9.8 mV) was determined mainly by a K+ -electrochemical gradient and partly by the contribution of the electrogenic Na+ -pump and Cl- conductance. Low resistance electrical coupling sites were identified physiologically. Transmission electron microscopy and innexin 2 antibody revealed the presence of gap-junction-like membrane structures between gland cells. It is suggested that gap-junctions are sites of electrotonic intercellular communication, which integrate the gland cells into a synchronized functional unit in the acinus. Stimulation of the salivary nerve elicited secretory potentials (depolarization) which could be mimicked by local application of acetylcholine, dopamine or serotonin. In voltage-clamp experiments four major conductances were identified: a delayed rectifier (IK), a transient (IA) and a Ca2+ -activated outward K+ current (IK(Ca)) and Ca2+ -inward currents (ICa). It is suggested that one or more of these conductances may give rise to a stimulus activated secretory potential leading to excitation-secretion coupling and subsequent the release of the mucus from the gland cells.

Chlorpromazine reduces the intercellular communication via gap junctions in mammalian cells

In the work presented herein, we evaluated the effect of chlorpromazine (CPZ) on gap junctions expressed by two mammalian cell types; Gn-11 cells (cell line derived from mouse LHRH neurons) and rat cortical astrocytes maintained in culture. We also attempted to elucidate possible mechanisms of action of CPZ effects on gap junctions. CPZ, in concentrations comparable with doses used to treat human diseases, was found to reduce the intercellular communication via gap junctions as evaluated with measurements of dye coupling (Lucifer yellow). In both cell types, maximal inhibition of functional gap junctions was reached within about 1 h of treatment with CPZ, an recovery was almost complete at about 5 h after CPZ wash out. In both cell types, CPZ treatment increased the phosphorylation state of connexin43 (Cx43), a gap junction protein subunit. Moreover, CPZ reduced the reactivity of Cx43 (immunofluorescence) at cell interfaces and concomitantly increased its reactivity in intracellular vesicles, suggesting an increased retrieval from and/or reduced insertion into the plasma membrane. CPZ also caused cellular retraction reducing cell-cell contacts in a reversible manner. The reduction in contact area might destabilize existing gap junctions and abrogate formation of new ones. Moreover, the CPZ-induced reduction in gap junctional communication may depend on the connexins (Cxs) forming the junctions. If Cx43 were the only connexin expressed, MAPK-dependent phosphorylation of this connexin would induce closure of gap junction channels.

Regulation of Connexin-43-Mediated Growth Inhibition by a Phosphorylatable Amino-Acid is Independent of Gap Junction-Forming Ability

The ability of the gap junction phosphoprotein connexin-43 (Cx43) to inhibit DNA synthesis in primary cardiomyocytes is regulated by serine (S) 262, a protein kinase C phosphorylation site that also affects metabolic coupling. We have now examined if the S262-regulated growth suppression is operating in transformed cells and if so whether it depends on gap junction channel forming ability. Serine 262 became phosphorylated in response to protein kinase C stimulation in HEK293 cells transiently expressing either Cx43 or the non-channel-forming carboxy-terminal tail of Cx43 (Cx43CT). Expression of either wild type Cx43 or Cx43CT inhibited DNA synthesis, as did their mutated versions simulating lack of phosphorylation by carrying an S262-to-alanine substitution. The ability to inhibit DNA synthesis was eliminated when expressing mutated versions of either Cx43 or Cx43CT simulating constitutive phosphorylation by carrying an S262-to-aspartate substitution. We conclude that S262 phosphorylation cancels growth inhibition by Cx43 independently of channel-forming ability.

Altered expression of connexin43 in the inhibition of gap junctional intercellular communication by chlorohydroxyfuranones in WB-F344 rat liver epithelial cells

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), 3,4-dichloro-5-hydroxy-2(5H)-furanone (MCA), and 3-chloro-4-methyl-5-hydroxy-2(5H)-furanone (MCF) promote foci formation in the two-stage cell transformation assay in vitro. These chlorohydroxyfuranones (CHFs) and their structural congener 3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone (CMCF) inhibit gap junctional intercellular communication (GJIC) in Balb/c 3T3 mouse fibroblast cells. In the present study, the effects of MX, MCA, CMCF, and MCF on GJIC were evaluated in liver cells (WB-F344 rat liver epithelial cells), the target cells of MX-induced carcinogenicity, using the scrape-loading dye transfer technique. The CHFs inhibited GJIC after 1 h exposure in a concentration-dependent fashion. The order of potency was MX>CMCF approximately MCA>MCF. In terms of the lowest observed effective concentrations, the difference in the potency was about 27-fold (MX 1.875 microM, MCF 50 microM). After a prolonged exposure period (12 h), the inhibition of GJIC by MX and CMCF remained stable, but MCA and MCF exhibited increasing inhibitory effects. After removal of the CHFs, the GJIC slowly recovered. At the transcriptional level, CHFs caused essentially no change in the level of connexin43 (Cx43) mRNA. Preincubation of cells with the protein kinase C (PKC) inhibitor did not modify the response, but the specific MEK 1 inhibitor PD98059 decreased substantially the inhibition of GJIC by all four CHFs. Activation of the mitogen-activated protein kinases (MAPKs) signaling pathway was necessary for inhibition of GJIC. CHFs did not increase the basal phosphorylation state of the Cx43 protein, but all CHFs caused a concentration-dependent degradation of the Cx43 protein. The results indicate that all the studied CHFs inhibit GJIC in WB-F344 cells by altering Cx43 expression.