Epidermal growth factor regulates ubiquitination, internalization and proteasome-dependent degradation of connexin43

Sent to destroy: the ubiquitin proteasome system regulates cell signaling and protein quality control in cardiovascular development and disease

The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia/reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible because of their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative "specific" mechanisms. The cytosolic receptors p62, NBR, and histone deacetylase 6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of "selective autophagy." Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the crosstalk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.

Mechanisms of gap junction traffic in health and disease

Gap junctions (GJs) allow direct communication between cells. In the heart, GJs mediate the electrical coupling of cardiomyocytes and as such dictate the speed and direction of cardiac conduction. A prominent feature of acquired structural heart disease is remodeling of GJ protein expression and localization concomitant with increased susceptibility to lethal arrhythmias, leading many to hypothesize that the two are causally linked. Detailed understanding of the cellular mechanisms that regulate GJ localization and function within cardiomyocytes may therefore uncover potential therapeutic strategies for a significant clinical problem. This review will outline our current understanding of GJ cell biology with the intent of highlighting cellular mechanisms responsible for GJ remodeling associated with cardiac disease.

Seek and destroy: the ubiquitin----proteasome system in cardiac disease

The ubiquitin-proteasome system (UPS) is a major proteolytic system that regulates the degradation of intracellular proteins in the heart. The UPS regulates the turnover of misfolded and damaged proteins, in addition to numerous cellular processes, by affecting the stability of short-lived proteins such as transcription factors and cell signaling pathways. The UPS is tightly regulated by the specificity of ubiquitin ligases that recognize specific substrates and direct the addition of ubiquitin, targeting the substrates for degradation by the 26S proteasome. An increasing number of cardiac ubiquitin ligases have been identified, and the number of substrates each one is known to recognize also has increased, expanding their roles. Although mainly cardioprotective roles have been attributed to ubiquitin ligases, new studies have identified exceptions to this rule. This review discusses the mechanisms of cardiac ubiquitin ligases and identifies their role in common cardiac diseases including cardiac hypertrophy, cardiac atrophy, ischemic heart disease, and diabetic cardiomyopathy.

Eps15 interacts with ubiquitinated Cx43 and mediates its internalization

Gap junctions (GJ) are specialized cell-cell contacts that provide direct intercellular communication (IC) between eukaryotic cells. Regulation of GJIC by degradation of Cx43 has been a matter of debate over the last two decades and both the proteasome and the lysosome have been implicated. However, the underlying mechanism and molecular players involved remain elusive. In this paper we demonstrate, for the first time, that the ubiquitin ligase Nedd4 is involved in Cx43 ubiquitination. Indeed, depletion of Nedd4 with siRNA resulted in a decrease of the amount of ubiquitin attached to Cx43. Ubiquitinated membrane proteins are often recognized and targeted by endocytic adaptors containing ubiquitin-binding domains, such as Eps15. By coimmunoprecipitation and immunofluorescence we show interaction of Cx43 with Eps15 and colocalization of these proteins mainly at the plasma membrane. Moreover, depletion of Eps15 results in an accumulation of Cx43 at the plasma membrane. Furthermore, the interaction of Eps15 with Cx43 requires the ubiquitin-interacting motif of Eps15 suggesting that the interaction occurs through the ubiquitin attached to Cx43. Data presented in this manuscript are consistent with a new molecular model in which Nedd4-mediated ubiquitination of Cx43 is required to recruit Eps15, through its ubiquitin-interacting motif, and targets ubiquitinated Cx43 to the endocytic pathway. This provides the basis for future studies aiming at identifying the molecular players and mechanisms involved in Cx43 internalization and degradation.

Downregulation of gap junction expression and function by endoplasmic reticulum stress

Gap junctional intercellular communication (GJIC) plays a critical role in the control of multiple cell behavior as well as in the maintenance of tissue and organ homeostasis. However, mechanisms involved in the regulation of gap junctions (GJs) have not been fully understood. Given endoplasmic reticulum (ER) stress and dysfunction of GJs coexist in several pathological situations, we asked whether GJs could be regulated by ER stress. Incubation of mesangial cells with ER stress-inducing agents (thapsigargin, tunicamycin, and AB(5) subtilase cytotoxin) resulted in a decrease in connexin 43 (Cx43) expression at both protein and mRNA levels. This was accompanied by a loss of GJIC, as evidenced by the reduced numbers of dye-coupled cells after single cell microinjection or scrape loading dye transfer. Further studies demonstrated that ER stress significantly inhibited the promoter activity of the Cx43 gene, reduced [(35)S]-methionine incorporation into Cx43 protein and accelerated degradation of Cx43. ER stress also decreased the Cx43 protein levels in several different cell types, including human umbilical vein endothelial cells, mouse-derived renin-secreting cells and human hepatoma cells. Furthermore, induction of ER stress by hypoxic chemicals thenoyltrifluoroacetone and cobalt chloride was found to be associated with a reduction in Cx43. Our findings thus reveal a close link between ER stress and GJs. ER stress may represent a novel mechanism underlying the altered GJs in a variety of pathological situations.

Ubiquitylation of the gap junction protein connexin-43 signals its trafficking from early endosomes to lysosomes in a process mediated by Hrs and Tsg101

Gap junctions are dynamic plasma membrane domains, and their protein constituents, the connexins, have a high turnover rate in most tissue types. However, the molecular mechanisms involved in degradation of gap junctions have remained largely unknown. Here, we show that ubiquitin is strongly relocalized to connexin-43 (Cx43; also known as Gja1) gap junction plaques in response to activation of protein kinase C. Cx43 remained ubiquitylated during its transition to a Triton X-100-soluble state and along its trafficking to early endosomes. Following internalization, Cx43 partly colocalized with the ubiquitin-binding proteins Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate; also known as Hgs) and Tsg101 (tumor susceptibility gene 101). Depletion of Hrs or Tsg101 by small interfering RNA abrogated trafficking of Cx43 from early endosomes to lysosomes. Under these conditions, Cx43 was able to undergo dephosphorylation and deubiquitylation, locate to the plasma membrane and form functional gap junctions. Simultaneous depletion of Hrs and Tsg101 caused accumulation of a phosphorylated and ubiquitylated subpopulation of Cx43 in early endosomes and in hybrid organelles between partly degraded annular gap junctions and endosomes. Collectively, these data reveal a central role of early endosomes in sorting of ubiquitylated Cx43, and identify Hrs and Tsg101 as crucial regulators of trafficking of Cx43 to lysosomes.

Activator of G protein signaling 8 (AGS8) is required for hypoxia-induced apoptosis of cardiomyocytes: role of G betagamma and connexin 43 (CX43)

Ischemic injury of the heart is associated with activation of multiple signal transduction systems including the heterotrimeric G-protein system. Here, we report a role of the ischemia-inducible regulator of G betagamma subunit, AGS8, in survival of cardiomyocytes under hypoxia. Cultured rat neonatal cardiomyocytes (NCM) were exposed to hypoxia or hypoxia/reoxygenation following transfection of AGS8siRNA or pcDNA::AGS8. Hypoxia-induced apoptosis of NCM was completely blocked by AGS8siRNA, whereas overexpression of AGS8 increased apoptosis. AGS8 formed complexes with G-proteins and channel protein connexin 43 (CX43), which regulates the permeability of small molecules under hypoxic stress. AGS8 initiated CX43 phosphorylation in a G betagamma-dependent manner by providing a scaffold composed of G betagamma and CX43. AGS8siRNA blocked internalization of CX43 following exposure of NCM to repetitive hypoxia; however it did not influence epidermal growth factor-mediated internalization of CX43. The decreased dye flux through CX43 that occurred with hypoxic stress was also prevented by AGS8siRNA. Interestingly, the G betagamma inhibitor Gallein mimicked the effect of AGS8 knockdown on both the CX43 internalization and the changes in cell permeability elicited by hypoxic stress. These data indicate that AGS8 is required for hypoxia-induced apoptosis of NCM, and that AGS8-G betagamma signal input increased the sensitivity of cells to hypoxic stress by influencing CX43 regulation and associated cell permeability. Under hypoxic stress, this unrecognized response program plays a critical role in the fate of NCM.

Identification of novel genes and pathways regulating SREBP transcriptional activity

BACKGROUND

Lipid metabolism in mammals is orchestrated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs) that control the expression of genes required for the uptake and synthesis of cholesterol, fatty acids, and triglycerides. SREBPs are thus essential for insulin-induced lipogenesis and for cellular membrane homeostasis and biogenesis. Although multiple players have been identified that control the expression and activation of SREBPs, gaps remain in our understanding of how SREBPs are coordinated with other physiological pathways.

METHODOLOGY

To identify novel regulators of SREBPs, we performed a genome-wide cDNA over-expression screen to identify proteins that might modulate the transcription of a luciferase gene driven from an SREBP-specific promoter. The results were verified through secondary biological assays and expression data were analyzed by a novel application of the Gene Set Enrichment Analysis (GSEA) method.

CONCLUSIONS/SIGNIFICANCE

We screened 10,000 different cDNAs and identified a number of genes and pathways that have previously not been implicated in SREBP control and cellular cholesterol homeostasis. These findings further our understanding of lipid biology and should lead to new insights into lipid associated disorders.

Connexin43 phosphorylation: structural changes and biological effects

Vertebrate gap junctions, composed of proteins from the connexin gene family, play critical roles in embryonic development, co-ordinated contraction of excitable cells, tissue homoeostasis, normal cell growth and differentiation. Phosphorylation of connexin43, the most abundant and ubiquitously expressed connexin, has been implicated in the regulation of gap junctional communication at several stages of the connexin 'life cycle', including hemichannel oligomerization, export of the protein to the plasma membrane, hemichannel activity, gap junction assembly, gap junction channel gating and connexin degradation. Consistent with a short (1-5 h) protein half-life, connexin43 phosphorylation is dynamic and changes in response to activation of many different kinases. The present review assesses our current understanding of the effects of phosphorylation on connexin43 structure and function that in turn regulate gap junction biology, with an emphasis on events occurring in heart and skin.

Epiregulin can effectively mature isolated cumulus-oocyte complexes, but fails as a substitute for the hCG/epidermal growth factor stimulus on cultured follicles

Epiregulin mediates LH ovulatory effects in vitro. This study evaluated the use of epiregulin as an alternative to hCG/epidermal growth factor (EGF) stimulus upon cultured ovarian follicles in contrast to isolated cumulus-oocyte complexes (COCs). Pre-antral mouse ovarian follicles were cultured for 12 days and final maturation was induced by administration of 0.65 nM EGF or 100 nM epiregulin without or with 1.2 IU/ml hCG. Results showed that both EGF or epiregulin as sole stimulators are poor inducers of mucification/expansion of cumulus cells and oocyte meiotic reinitiation in follicle-enclosed COCs (25+/-17 and 22+/-16% GVBD respectively; versus 97+/-4 and 90+/-15% GVBD by control hCG/EGF and hCG/epiregulin respectively; mean+/-S.D). Furthermore, EGF or epiregulin did not induce follicle luteinisation: progesterone production was marginally increased and oestradiol was incompletely shut down. Supposing that the sub-normal progesterone secretion was a potential cause for incomplete meiosis in this model, effectiveness of progesterone supplementation and addition of a progesterone receptor inhibitor (RU486) were evaluated on meiotic resumption. Progesterone was not found to be a major regulator of meiosis in this mouse model. Epiregulin induced meiosis more effectively in COCs isolated from cultured preovulatory follicles in a secondary culture well. In conclusion, epiregulin has similar effects as EGF upon fully grown follicles. Used as a sole stimulator of periovulatory events in intact cultured follicles, both are poor inducers of follicle luteinisation and oocyte maturation. By contrast, epiregulin is as efficient as hCG/EGF, when used as meiotic stimulator for COCs isolated from the follicular environment (mural granulosa and theca cells; and conditioned medium).

Regulation of gap junctions in porcine cumulus-oocyte complexes: contributions of granulosa cell contact, gonadotropins, and lipid rafts

Gap-junctional communication (GJC) plays a central role in oocyte growth. However, little is known about the regulation of connexin 43 (Cx43)-based gap-junction channels in cumulus-oocyte complexes (COCs) during in vitro maturation. We show that rupture of COCs from mural granulosa cells up-regulates Cx43-mediated GJC and that gonadotropins signal GJC breakdown by recruiting Cx43 to lipid rafts when oocyte meiosis resumes. Oocyte calcein uptake through gap junctions increases during early in vitro oocyte maturation and remains high until 18 h, when it falls simultaneously with the oocyte germinal vesicle breakdown. Immunodetection of Cx43 and fluorescence recovery after photobleaching assays revealed that the increase of GJC is independent of gonadotropins but requires RNA transcription, RNA polyadenylation, and translation. GJC rupture, in contrast, is achieved by a gonadotropin-dependent mechanism involving recruitment of Cx43 to clustered lipid rafts. These results show that GJC up-regulation in COCs in in vitro culture is independent of gonadotropins and transcriptionally regulated. However, GJC breakdown is gonadotropin dependent and mediated by the clustering of Cx43 in lipid raft microdomains. In conclusion, this study supports a functional role of lipid raft clustering of Cx43 in GJC breakdown in the COCs during in vitro maturation.

The TSG101 protein binds to connexins and is involved in connexin degradation

Gap junctions mediate electrical and metabolic communication between cells in almost all tissues and are proposed to play important roles in cellular growth control, differentiation and embryonic development. Gap junctional communication and channel assembly were suggested to be regulated by interaction of connexins with different proteins including kinases and phosphatases. Here, we identified the tumor susceptibility gene 101 (TSG101) protein to bind to the carboxyterminal tail of connexin45 in a yeast two-hybrid protein interaction screen. Glutathione S-transferase pull down experiments and immunoprecipitation revealed that not only connexin45 but also connexin30.2, -36, and -43 carboxyterminal regions were associated with TSG101 protein in pull down analyses and that connexin31, -43 and -45 co-precipitate with endogenous TSG101 protein in lysates from HM1 embryonic stem cells. TSG101 has been shown to be involved in cell cycle control, transcriptional regulation and turnover of endocytosed proteins. Thus, we decided to study the functional role of this interaction. SiRNA mediated knock down of TSG101 in HM1 embryonic stem cells led to increased levels of connexin43 and -45, prolonged half life of these connexins and increased transfer of microinjected Lucifer yellow. Our results suggest that TSG101 is involved in the degradation of connexins via interaction with connexin proteins.

Indole-3-carbinol prevents H(2)O(2)-induced inhibition of gap junctional intercellular communication by inactivation of PKB/Akt

Indole-3-carbinol (I3C) is a phytochemical found in cruciferous vegetables and possesses a variety of biological and biochemical effects. Despite a wealth of data about the chemopreventive properties of I3C, its effects on gap junctional intercellular communication (GJIC), which is associated with the promotion and progression phases of the multi-stage process of carcinogenesis, has not been studied. In this study, we examined the ability of I3C to prevent H(2)O(2)-induced inhibition of GJIC in WB-F344 rat liver epithelial cells (WB cells). The cells were preincubated with I3C for 48 hr, and then treated with 1 mM H(2)O(2) for 1 hr. We found that I3C could prevent the H(2)O(2)-induced inhibition of GJIC through prevention of the phosphorylated state of gap junction protein connexin 43 (Cx43) phosphorylation. Prevention of GJIC by I3C was dependent upon inactivation of Akt, but not MAPK, although inhibition of GJIC by H(2)O(2) leads to activation of both. Similar to I3C, modulation of Akt activation through the phosphoinositide-3 kinase inhibitor, LY294002, could also prevent H(2)O(2)-induced inhibition of GJIC and phosphorylation of Cx43. Our results suggest that I3C might exert its dietary chemopreventive effects by interfering with the Akt signaling pathway, which appears to be linked to modulating GJIC, a cellular mechanisms regulating cell proliferation, differentiation and apoptosis.

The detergent resistance of Connexin43 is lost upon TPA or EGF treatment and is an early step in gap junction endocytosis

Gap junctions are plasma membrane domains containing channels that directly connect the cytosols of neighbouring cells. Gap junction channels are made of a family of transmembrane proteins called connexins, of which the best studied is Connexin43 (Cx43). MAP kinase-induced phosphorylation of Cx43 has previously been shown to cause inhibition of gap junction channel permeability and increased Cx43 endocytosis. As Cx43 assembles into gap junction plaques, Cx43 acquires detergent resistance. Here we report that the detergent resistance is lost after activation of MAP kinase. Treatment of IAR20 rat liver epithelial cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) or epidermal growth factor (EGF) caused a rapid increase in the solubility of Cx43 in Triton X-100. This process was mediated by MAP kinase and was initiated at the plasma membrane. The data suggest that loss of the detergent resistance of Cx43 is an early step in TPA- and EGF-induced endocytosis of gap junctions.

Short-term pacing in the mouse alters cardiac expression of connexin43

Background

Cardiac insults such as ischemia, infarction, hypertrophy and dilatation are often accompanied by altered abundance and/or localization of the connexin43 gap junction protein, which may predispose towards arrhythmic complications. Models of chronic dyssynchronous cardiac activation have also been shown to result in redistribution of connexin43 in cardiomyocytes. We hypothesized that alterations in connexin43 expression and localization in the mouse heart might be induced by ventricular pacing over a short period of time.

Results

The subdiaphragmatic approach was used to pace a series of wild type mice for six hours before the hearts were removed for analysis. Mice were paced at 10–15% above their average anesthetized sinus rate and monitored to ensure 1:1 capture. Short-term pacing resulted in a significant reduction in connexin43 mRNA abundance, a partial redistribution of connexin43 from the sarcolemma to a non-sarcolemmal fraction, and accumulation of ubiquitinated connexin43 without a significant change in overall connexin43 protein levels. These early pacing-induced changes in connexin43 expression were not accompanied by decreased cardiac function, prolonged refractoriness or increased inducibility into sustained arrhythmias.

Conclusion

Our data suggest that short-term pacing is associated with incipient changes in the expression of the connexin43 gap junction, possibly including decreased production and a slowed rate of degradation. This murine model may facilitate the study of early molecular changes induced by pacing and may ultimately assist in the development of strategies to prevent gap junction remodeling and the associated arrhythmic complications of cardiac disease.

A novel connexin43-interacting protein, CIP75, which belongs to the UbL-UBA protein family, regulates the turnover of connexin43

The degradation of connexin43 (Cx43) has been reported to involve both lysosomal and proteasomal degradation pathways; however, very little is known about the mechanisms regulating these Cx43 degradation pathways. Using yeast two-hybrid, glutathione S-transferase pull-down, and co-immunoprecipitation approaches, we have identified a novel Cx43-interacting protein of approximately 75 kDa, CIP75. Laser confocal microscopy showed that CIP75 is located primarily at the endoplasmic reticulum, as indicated by the calnexin marker, with Cx43 co-localization in this perinuclear region. CIP75 belongs to the UbL (ubiquitin-like)-UBA (ubiquitin-associated) domain-containing protein family with a N-terminal UbL domain and a C-terminal UBA domain. The UBA domain of CIP75 is the main element mediating the interaction with Cx43, whereas the CIP75-interacting region in Cx43 resides in the PY motif and multiphosphorylation sites located between Lys 264 and Asn 302. Interestingly, the UbL domain interacts with the S2/RPN1 and S5a/RPN10 protein subunits of the regulatory 19 S proteasome cap subunit of the 26 S proteasome complex. Overexpression experiments suggested that CIP75 is involved in the turnover of Cx43 as measured by a significant stimulation of Cx43 degradation and reduction in its half-life with the opposite effects on Cx43 degradation observed in small interference RNA knockdown experiments.

Modulation of osteoblast gap junction connectivity by serum, TNFalpha, and TRAIL

We studied the effects of serum growth factors and of TNF family proteins on osteoblast gap junction connectivity. Serum starvation of human MG63 osteosarcoma cells or nontransformed osteoblasts decreased connexin43 protein. TNFalpha or TRAIL reduced connexin43 further. Serum starvation redistributed gap junctions but did not reduce intercellular diffusion. In contrast, TNFalpha or TRAIL reduced gap junctions on cell processes and decreased intercellular diffusion. Effects of TNFs on connexin43 were mediated by lysosomal proteolysis. Activating analogs of cAMP increased connexin43 protein, but did not block effects of serum starvation, TNFalpha, or TRAIL on connexin43 protein. Connexin43 and connectivity recovered overnight if stimuli were withdrawn. Surprisingly, connexin43 mRNA increased in serum starvation and with TNFalpha or TRAIL. Since beta-catenin is a binding partner of connexin43, when connexin43 is degraded, beta-catenin activation may contribute to a reflexive increase in connexin43 transcription. We conclude that osteoblast connectivity is regulated by a multifactorial system that maintains intercellular connections. Serum starvation, TNFalpha and TRAIL augmented connexin43 degradation and connexin43 transcription. Cell-cell communication was maintained in serum starvation, which may model response to acute injury, but was sensitive to TNFs. These inflammatory agents mediated selective, reversible removal of connexin43 from cell processes.

Pressure induces loss of gap junction communication and redistribution of connexin 43 in astrocytes

Astrocytes, the major glia in the nonmyelinated optic nerve head (ONH), connect via gap junctions built of connexin-43 (Cx43) to form a functional syncytium allowing communication and control of ionic and metabolic homeostasis of retinal ganglion cells (RGCs) axon. We examined gap junction intercellular communication (GJIC) by scrape loading assays in human ONH astrocytes exposed to hydrostatic (HP) or ambient pressure (CP) in vitro. Immunostaining, immunoprecipitation, and immunoblots were used to detect Cx43 distribution and phosphorylation in astrocytes exposed to HP with/without EGF receptor (EGFR) tyrosine kinase inhibitors AG1478 and AG82 and MAPK inhibitors U0126, PD98059, and SB203580. The data indicates that upon exposure to HP, astrocytes decrease GJIC and exhibit altered cellular localization and phosphorylation of Cx43. Inhibition of EGFR blocked the effects of HP on GJIC and HP-induced Cx43 tyrosine phosphorylation. Inhibitors of MAPK- ERK1/2 and -p38 caused partial closure of GJIC under CP and HP, which was maintained for 6 h. Inhibition of Big Mitogen-Activated Kinase 1/ERK5 (BMK1/ERK5) caused partial closure under CP and HP followed by full recovery after 6 h. Inhibition of MAPK did not affect the HP-induced increase in Cx43 serine 279/282 phosphorylation. We conclude that activation of the EGFR pathway in response to HP leads to decrease of GJIC via tyrosine phosphorylation of Cx43 in ONH astrocytes. In glaucoma under conditions of elevated intraocular pressure (IOP), astrocytes may lose GJIC altering the homeostasis of RGC axons, adopting the reactive phenotype, contributing to glaucomatous neuropathy.

Ubiquitination of gap junction proteins

Gap junctions are plasma membrane domains containing arrays of channels that exchange ions and small molecules between neighboring cells. Gap junctional intercellular communication enables cells to directly cooperate both electrically and metabolically. Several lines of evidence indicate that gap junctions are important in regulating cell growth and differentiation and for maintaining tissue homeostasis. Gap junction channels consist of a family of transmembrane proteins called connexins. Gap junctions are dynamic structures, and connexins have a high turnover rate in most tissues. Connexin43 (Cx43), the best-studied connexin isoform, has a half-life of 1.5-5 h; and its degradation involves both the lysosomal and proteasomal systems. Increasing evidence suggests that ubiquitin is important in the regulation of Cx43 endocytosis. Ubiquitination of Cx43 is thought to occur at the plasma membrane and has been shown to be regulated by protein kinase C and the mitogen-activated protein kinase pathway. Cx43 binds to the E3 ubiquitin ligase Nedd4, in a process modulated by Cx43 phosphorylation. The interaction between Nedd4 and Cx43 is mediated by the WW domains of Nedd4 and involves a proline-rich sequence conforming to a PY (XPPXY) consensus motif in the C terminus of Cx43. In addition to the PY motif, an overlapping tyrosine-based sorting signal conforming to the consensus of an YXXphi motif is involved in Cx43 endocytosis, indicating that endocytosis of gap junctions involves both ubiquitin-dependent and -independent pathways. Here, we discuss current knowledge on the ubiquitination of connexins.

Ubiquitin protein ligase Nedd4 binds to connexin43 by a phosphorylation-modulated process

Connexin43 is degraded by the proteasomal as well as the lysosomal pathway with ubiquitin playing a role in both degradation pathways. So far, no ubiquitin protein ligase has been identified for any of the connexins. By using pull-down assays, here we show binding of a ubiquitin protein ligase, Nedd4, to the C-terminus of connexin43. This observation was confirmed in vivo by coimmunoprecipitation and immunofluorescence, showing colocalization of Nedd4 and connexin43. Binding of Nedd4 to its interaction partners is generally carried out by its WW domains. Our results indicate that the interaction with connexin43 occurs through all three WW domains of Nedd4. Furthermore, whereas WW1 and WW2 domains mainly interact with the unphosphorylated form of connexin43, WW3 binds phosphorylated and unphosphorylated forms equally. In addition, using the surface plasmon resonance approach we show that only the WW2 domain binds to the PY motif located at the C-terminus of connexin43. Suppression of Nedd4 expression with siRNA resulted in an accumulation of gap junction plaques at the plasma membrane, suggesting an involvement of the ubiquitin protein ligase Nedd4 in gap junction internalization.

Gap junctional complexes: From partners to functions

Gap junctions (GJ), specialised membrane structures that mediate cell-to-cell communication in almost all animal tissues, are composed of intercellular channel-forming integral membrane proteins termed connexins (Cxs), innexins or pannexins. The activity of these channels is closely regulated, particularly by intramolecular modifications as phosphorylation of proteins, via the formation of multiprotein complexes where pore-forming subunits bind to auxiliary channel subunits and associate with scaffolding proteins that play essential roles in channel localization and activity. Scaffolding proteins link signalling enzymes, substrates, and potential effectors (such as channels) into multiprotein signalling complexes that may be anchored to the cytoskeleton. Protein-protein interactions play essential roles in channel localization and activity and, besides their cell-to-cell channel-forming functions, gap junctional proteins now appear involved in different cellular functions (e.g. transcriptional and cytoskeletal regulation). The present review summarizes the recent progress regarding the proteins capable of interacting with junctional proteins and their functional importance.

Negative growth control of osteosarcoma cell by Bowman-Birk protease inhibitor from soybean; involvement of connexin 43

Bowman-Birk protease inhibitor (BBI) from soybean acts as a potential chemopreventive agent in several types of tumors. However, the mechanism is still unclear. The present study was undertaken to estimate a mechanism of BBI-dependent negative growth control of human osteosarcoma cell (U2OS cell). BBI had negative growth control of the cells via induction of connexin (Cx) 43, a tumor suppressor gene in U2OS cells. This negative growth control by BBI was abrogated under down-regulation of Cx43 induced by a Cx43 antisense nucleotide treatment. It was also found that the BBI-dependent induction of Cx43 was due to elevation of Cx43 mRNA and stabilization of Cx43 protein. Especially, BBI-dependent inhibition of chymotrypsin-like activity in proteasome contributed to stabilization of Cx43 protein. These results suggest that a major negative growth effect of BBI is based on the restoration of Cx43 expression in U2OS cells.

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.

Endocytic processing of connexin43 gap junctions: a morphological study

Gap junctions are plasma membrane areas enriched in channels that provide direct intercellular communication. Gap junctions have a high turnover rate; however, the mechanisms by which gap junctions are degraded are incompletely understood. In the present study, we show that in response to phorbol ester treatment, the gap junction channel protein Cx43 (connexin43) is redistributed from the plasma membrane to intracellular vesicles positive for markers for early and late endosomes and for the endolysosomal protease cathepsin D. Immunoelectron microscopy studies indicate that the double membranes of internalized gap junctions undergo separation and cutting, resulting in multivesicular endosomes enriched in Cx43 protein. Using preloading of BSA-gold conjugates to mark lysosomes, we provide evidence suggesting that the degradation process of the double-membrane structure of annular gap junctions occurs prior to transport of Cx43 to the lysosome. The results further suggest that bafilomycin A1, an inhibitor of vacuolar H+-ATPases, causes accumulation of Cx43 in early endosomes. Taken together, these findings indicate that internalized gap junctions undergo a maturation process from tightly sealed double-membrane vacuoles to connexin-enriched multivesicular endosomes with a single limiting membrane. The results further suggest that along with the processing of the double-membrane structure of annular gap junctions, connexins are trafficked via early and late endosomes, finally resulting in their endolysosomal degradation.

Nerve growth factor increases connexin43 phosphorylation and gap junctional intercellular communication

The function of gap junctions is regulated by the phosphorylation state of their connexin subunits. Numerous growth factors are known to regulate connexin phosphorylation; however, the effect of nerve growth factor on gap junction function is not understood. The phosphorylation of connexin subunits is a key event during many aspects of the lifecycle of a connexin, including open/close states, assembly/trafficking, and degradation, and thus affects the functionality of the channel. PC12 cells infected with connexin43 (Cx43) retrovirus were used as a neuronal model to characterize the signal transduction pathways activated by nerve growth factor (NGF) that potentially affect the functional state of Cx43. Immunoblot analysis demonstrated that Cx43 and the mitogen-activated protein kinase (MAPK), ERK-1/2, were phosphorylated in response to TrkA activation via NGF and that phosphorylation could be prevented by treatment with the MEK-1/2 inhibitor U0126. The effects of NGF on gap junction intercellular communication were examined by monitoring fluorescence recovery after photobleaching PC12-Cx43 cells preloaded with calcein. Fluorescence recovery in the photobleached area increased after NGF treatment and decreased when pretreated with the MEK-1/2 inhibitor U0126. These data are the first to show a direct signaling link between neurotrophins and the phosphorylation of connexin proteins through the MAPK pathway resulting in increased gap junctional intercellular communication. Neurotrophic regulation of connexin activity provides a novel mechanism of regulating intercellular communication between neurons during nervous system development and repair.

Ubiquitylation of Ion Channels

Ubiquitylation (i.e., covalent attachment of ubiquitin moieties to proteins) of ion channels allows regulation of their activity and fate. Nedd4/Nedd4-like ubiquitin-protein ligases bind to, ubiquitylate, and modulate the internalization of several channels bearing PY motifs, whereas endoplasmic reticulum-associated degradation (involving ubiquitylation) plays an important role in the biogenesis of normal and defective channels.

Connexin43 PDZ2 binding domain mutants create functional gap junctions and exhibit altered phosphorylation

Connexin43 (Cx43) is the most abundantly expressed gap junction protein. The C-terminal tail of Cx43 is important for regulation of gap junctions via phosphorylation of specific tyrosine and serine residues and through interactions with cellular proteins. The C-terminus of Cx43 has been shown to interact with the PDZ2 domain of the tight and adherens junction associated zona occludens 1 (ZO-1) protein. Analysis of the PDZ2 binding domain of Cx43 indicated that positions -3 and -2, and the final hydrophobic amino acid at the C-terminus, are critical for ZO-1 binding. In addition, the C-termini of connexins 40 and 45, but not Cx32, interacted with ZO-1. To evaluate the functional significance of the Cx43-ZO-1 interaction, Cx43 wild type (Cx43wt) and mutants lacking either the C-terminal hydrophobic isoleucine (Cx43deltaI382) or the last five amino acids (Cx43delta378-382), required for ZO-1 binding in vitro, were introduced into a Cx43-deficient MDCK cell line. In vitro binding studies and coimmunoprecipitation assays indicated that these Cx43 mutants failed to interact with ZO-1. Confocal and deconvolution microscopy revealed that a fraction of Cx43wt colocalized with ZO-1 at the plasma membrane. A similar colocalization pattern was observed for the Cx43deltaI382 and Cx43 delta378-382 mutants, which were translocated to the plasma membrane and formed functional gap junction channels. The wt and mutant Cx43 appeared to have similar turnover rates. However, the P2 and P3 phosphoisoforms of the Cx43 mutants were significantly reduced compared to Cx43wt. These studies indicated that the interaction of Cx43 with ZO-1 may contribute to the regulation of Cx43 phosphorylation.

Doxorubicin induces EGF receptor-dependent downregulation of gap junctional intercellular communication in rat liver epithelial cells

Exposure of rat liver epithelial cells to doxorubicin, an anthraquinone derivative widely employed in cancer chemotherapy, led to a dose-dependent decrease in gap junctional intercellular communication (GJC). Gap junctions are clusters of inter-cellular channels consisting of connexins, the major connexin in the cells used being connexin-43 (Cx43). Doxorubicin-induced loss of GJC was mediated by activation of extracellular signal-regulated kinase (ERK)-1 and ERK-2, as demonstrated using inhibitors of ERK activation. Furthermore, activation of the epidermal growth factor (EGF) receptor by doxorubicin was responsible for ERK activation and the subsequent attenuation of GJC. Inhibition of GJC, however, was not by direct phosphorylation of Cx43 by ERK-1/2, whereas menadione, a 1,4-naphthoquinone derivative that was previously demonstrated to activate the same EGF receptor-dependent pathway as doxorubicin, resulting in downregulation of GJC, caused strong phos-phorylation of Cx43 at serines 279 and 282. Thus, ERK-dependent downregulation of GJC upon exposure to quinones may occur both by direct phosphorylation of Cx43 and in a phosphorylation-independent manner.

E3 ubiquitin ligases as regulators of membrane protein trafficking and degradation

Ubiquitination is a regulated post-translational modification that conjugates ubiquitin (Ub) to lysine residues of target proteins and determines their intracellular fate. The canonical role of ubiquitination is to mediate degradation by the proteasome of short-lived cytoplasmic proteins that carry a single, polymeric chain of Ub on a specific lysine residue. However, protein modification by Ub has much broader and diverse functions involved in a myriad of cellular processes. Monoubiquitination, at one or multiple lysine residues of transmembrane proteins, influences their stability, protein-protein recognition, activity and intracellular localization. In these processes, Ub functions as an internalization signal that sends the modified substrate to the endocytic/sorting compartments, followed by recycling to the plasma membrane or degradation in the lysosome. E3 ligases play a pivotal role in ubiquitination, because they recognize the acceptor protein and hence dictate the high specificity of the reaction. The multitude of E3s present in nature suggests their nonredundant mode of action and the need for their controlled regulation. Here we give a short account of E3 ligases that specifically modify and regulate membrane proteins. We emphasize the intricate network of interacting proteins that contribute to the substrate-E3 recognition and determine the substrate's cellular fate.

Cardiomyocyte hypertrophy and degradation of connexin43 through spatially restricted autocrine/paracrine heparin-binding EGF

Growth factor signaling can affect tissue remodeling through autocrine/paracrine mechanisms. Recent evidence indicates that EGF receptor transactivation by heparin-binding EGF (HB-EGF) contributes to hypertrophic signaling in cardiomyocytes. Here, we show that HB-EGF operates in a spatially restricted circuit in the extracellular space within the myocardium, revealing the critical nature of the local microenvironment in intercellular signaling. This highly localized microenvironment of HB-EGF signaling was demonstrated with 3D morphology, consistent with predictions from a computational model of EGF signaling. HB-EGF secretion by a given cardiomyocyte in mouse left ventricles led to cellular hypertrophy and reduced expression of connexin43 in the overexpressing cell and in immediately adjacent cells but not in cells farther away. Thus, HB-EGF acts as an autocrine and local paracrine cardiac growth factor that leads to loss of gap junction proteins within a spatially confined microenvironment. These findings demonstrate how cells can coordinate remodeling with their immediate neighboring cells with highly localized extracellular EGF signaling.

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

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

Metabolites of benzene are potent inhibitors of gap-junction intercellular communication

Chronic exposure to benzene has been shown to lead to bone marrow depression and the development of leukemia. The mechanism underlying the carcinogenicity of benzene is unknown, although a number of genetic changes including chromosomal aberrations have been associated with benzene toxicity. Metabolism of benzene is required for the induced toxicological effects. We have investigated the effect of trans,trans-muconaldehyde (MUC), hydroquinone (HQ), and four MUC metabolites on gap-junction intercellular communication (GJIC). Inhibition of GJIC has been considered a possible predictor of tumor promoters and non-genotoxic carcinogens, and shown to result in perturbation of hematopoiesis. MUC was found to be a strong inhibitor of GJIC (EC50=12 micromol L(-1)) in rat liver epithelial cells IAR20, with potency similar to that of chlordane (EC50=7 micromol L(-1)). HQ inhibited GJIC with an EC50 of 25 micromolmol L(-1), and the metabolite OH/CHO with an EC50 of 58 micromol L(-1). The other MUC metabolites tested, CHO/COOH and OH/COOH were weak inhibitors of GJIC whereas COOH/COOH had no effect. Benzene itself had no effect on GJIC when tested in concentrations up to 20 micromol L(-1). The relative potency observed for the metabolites on GJIC is similar to their hematotoxic effects. The effect of MUC on GJIC was observed to take place concordant with a dramatic loss of connexin 43 (Cx43) from the cells as visualized by Western blotting. Substances with the ability to inhibit Cx43-dependent GJIC have previously been observed to interfere with normal hematopoietic development. The ability of benzene metabolites to interfere with gap-junction functionality, and especially the dramatic loss of Cx43 induced by MUC, should therefore be considered as a possible mechanism for benzene-induced hematotoxicity and development of leukemia.

Connexin43 synthesis, phosphorylation, and degradation in regulation of transient inhibition of gap junction intercellular communication by the phorbol ester TPA in rat liver epithelial cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces transient inhibition of gap junction intercellular communication (GJIC) in several cell types. The initial block in GJIC has been attributed to protein kinase C (PKC) mediated phosphorylation of connexin gap junction proteins, including connexin43 (Cx43). Restoration of GJIC, associated with normalization of the Cx43 phosphorylation status, has been ascribed to different events, including dephosphorylation of Cx43 and de novo synthesis of Cx43 or other, non-gap junctional, proteins. The data presented suggest that restoration of GJIC during continuous TPA exposure in normal and transformed rat liver epithelial cells is dependent on synthesis of Cx43 protein, as well as the transport of already synthesized Cx43 from intracellular pools to the plasma membrane. Reactivation of inactivated Cx43 by dephosphorylation does not appear to be involved in the recovery of GJIC. Both PKC and MAP kinase is involved in TPA-induced degradation of Cx43 and inhibition of GJIC. We show that coincubation of TPA with the protein synthesis inhibitor cycloheximide or the transcription inhibitor actinomycin D results in synergistic enhancement of the level of activated ERK1/2. Together, the present data highlight Cx43 degradation and synthesis as critical determinants in TPA-induced modifications of cell-cell communication via gap junctions.

Ubiquitination and down-regulation of gap junction protein connexin-43 in response to 12-O-tetradecanoylphorbol 13-acetate treatment

Gap junctions are specialized plasma membrane domains enriched in connexin proteins that form channels between adjacent cells. Gap junctions are highly dynamic, and modulation of the connexin turnover rate is considered to play an important role in the regulation of gap junctional intercellular communication. In the present study, we show that the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) induces ubiquitination of connexin-43 (Cx43) in IAR20 rat liver epithelial cells. The accelerated ubiquitination of Cx43 in response to TPA occurred concomitantly with Cx43 hyperphosphorylation and inhibition of cell-cell communication via gap junctions. The TPA-induced ubiquitination of Cx43 was mediated via protein kinase C and partly involved the mitogen-activated protein kinase pathway. Following ubiquitination, Cx43 was internalized and degraded. The loss of Cx43 protein was counteracted by ammonium chloride, indicating that acidification of internalized Cx43 gap junctions is a prerequisite for its degradation. Furthermore, the Cx43 degradation was partly counteracted by leupeptin, an inhibitor of cathepsin B, H, and L. Cx43 internalization and subsequent degradation were blocked by inhibitors of the proteasome. Evidence is provided that Cx43 is modified by multiple monoubiquitins rather than a polyubiquitin chain in response to TPA. Moreover, the TPA-induced ubiquitination of Cx43 was blocked by proteasomal inhibitors. Taken together, the data indicate that Cx43 ubiquitination is a highly regulated process. Moreover, the results suggest that the proteasome might play an indirect role in Cx43 degradation by affecting the level of monoubiquitin conjugation and trafficking of Cx43 to endosomal compartments.

Reversible connexin 43 dephosphorylation during hypoxia and reoxygenation is linked to cellular ATP levels

Altered gap junction coupling of cardiac myocytes during ischemia may contribute to development of lethal arrhythmias. The phosphoprotein connexin 43 (Cx43) is the major constituent of gap junctions. Dephosphorylation of Cx43 and uncoupling of gap junctions occur during ischemia, but the significance of Cx43 phosphorylation in this setting is unknown. Here we show that Cx43 dephosphorylation in synchronously contracting myocytes during ischemia is reversible, independent of hypoxia, and closely associated with cellular ATP levels. Cx43 became profoundly dephosphorylated during hypoxia only when glucose supplies were limited and was completely rephosphorylated within 30 minutes of reoxygenation. Similarly, direct reduction of ATP by various combinations of metabolic inhibitors and by ouabain was closely paralleled by loss of phosphoCx43 and recovery of phosphoCx43 accompanied restoration of ATP. Dephosphorylation of Cx43 could not be attributed to hypoxia, acid pH or secreted metabolites, or to AMP-activated protein kinase; moreover, the process was selective for Cx43 because levels of phospho-extracellular signal regulated kinase (ERK)1/2 were increased throughout. Rephosphorylation of Cx43 was not dependent on new protein synthesis, or on activation of protein kinases A or G, ERK1/2, p38 mitogen-activated protein kinase, or Jun kinase; however, broad-spectrum protein kinase C inhibitors prevented Cx43 rephosphorylation while also sensitizing myocytes to reoxygenation-mediated cell death. We conclude that Cx43 is reversibly dephosphorylated and rephosphorylated during hypoxia and reoxygenation by a novel mechanism that is sensitive to nonlethal fluctuations in cellular ATP. The role of this regulated phosphorylation in the adaptation to ischemia remains to be determined.