Analyzing phorbol ester effects on gap junctional communication: a dramatic inhibition of assembly

The role of gap junctions in the brain in health and disease

Gap junctions connect the cytosolic compartments of adjacent cells for direct electrotonic and metabolic cell-to-cell communication. Gap junctions between glial cells or neurons are ubiquitously expressed in the brain and play a role in brain development including cell differentiation, cell migration and survival, tissue homeostasis, as well as in human diseases including hearing loss, skin disease, neuropathies, epilepsy, brain trauma, and cardiovascular disease. Furthermore, gap junctions are involved in the synchronization and rhythmic oscillation of hippocampal and neocotical neuronal ensembles which might be important for memory formation and consolidation. In this review the accumulated evidence from mouse mutant and pharmacological studies using gap junction blockers is summarized and the progress made in dissecting the physiological, pathophysiological and behavioral roles of gap junction mediated intercellular communication in the brain is discussed.

Investigation of the reciprocal relationship between the expression of two gap junction connexin proteins, connexin46 and connexin43

Connexins are the transmembrane proteins that form gap junctions between adjacent cells. The function of the diverse connexin molecules is related to their tissue-specific expression and highly dynamic turnover. Although multiple connexins have been previously reported to compensate for each other's functions, little is known about how connexins influence their own expression or intracellular regulation. Of the three vertebrate lens connexins, two connexins, connexin43 (Cx43) and connexin46 (Cx46), show reciprocal expression and subsequent function in the lens and in lens cell culture. In this study, we investigate the reciprocal relationship between the expression of Cx43 and Cx46. Forced depletion of Cx43, by tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate, is associated with an up-regulation of Cx46 at both the protein and message level in human lens epithelial cells. An siRNA-mediated down-regulation of Cx43 results in an increase in the level of Cx46 protein, suggesting endogenous Cx43 is involved in the regulation of endogenous Cx46 turnover. Overexpression of Cx46, in turn, induces the depletion of Cx43 in rabbit lens epithelial cells. Cx46-induced Cx43 degradation is likely mediated by the ubiquitin-proteasome pathway, as (i) treatment with proteasome inhibitors restores the Cx43 protein level and (ii) there is an increase in Cx43 ubiquitin conjugation in Cx46-overexpressing cells. We also present data that shows that the C-terminal intracellular tail domain of Cx46 is essential to induce degradation of Cx43. Therefore, our study shows that Cx43 and Cx46 have novel functions in regulating each other's expression and turnover in a reciprocal manner in addition to their conventional roles as gap junction proteins in lens cells.

CX43 expression, phosphorylation, and distribution in the normal and autoimmune orchitic testis with a look at gap junctions joining germ cell to germ cell

Spermatogenesis requires connexin 43 (Cx43).This study examines normal gene transcription, translation, and phosphorylation of Cx43 to define its role on germ cell growth and Sertoli cell's differentiation, and identifies abnormalities arising from spontaneous autoimmune orchitis (AIO) in mink, a seasonal breeder and a natural model for autoimmunity. Northern blot analysis detected 2.8- and a 3.7-kb Cx43 mRNA bands in seminiferous tubule-enriched fractions. Cx43 mRNA increased in seminiferous tubule-enriched fractions throughout development and then seasonally with the completion of spermatogenesis. Cx43 protein levels increased transiently during the colonization of the tubules by the early-stage spermatocytes. Cx43 phosphorylated (PCx43) and nonphosphorylated (NPCx43) in Ser368 decreased during the periods of completion of meiosis and Sertoli cell differentiation, while Cx43 mRNA remained elevated throughout. PCx43 labeled chiefly the plasma membrane except by stage VII when vesicles were also labeled in Sertoli cells. Vesicles and lysosomes in Sertoli cells and the Golgi apparatus in the round spermatids were NPCx43 positive. A decrease in Cx43 gene expression was matched by a Cx43 protein increase in the early, not the late, phase of AIO. Total Cx43 and PCx43 decreased with the advance of orchitis. The study makes a novel finding of gap junctions connecting germ cells. The data indicate that Cx43 protein expression and phosphorylation in Ser368 are stage-specific events that may locally influence the acquisition of meiotic competence and the Sertoli cell differentiation in normal testis. AIO modifies Cx43 levels, suggesting changes in Cx43-mediated intercommunication and spermatogenic activity in response to cytokines imbalances in Sertoli cells.

Trafficking and recycling of the connexin43 gap junction protein during mitosis

During the cell cycle, gap junction communication, morphology and distribution of connexin43 (Cx43)-containing structures change dramatically. As cells round up in mitosis, Cx43 labeling is mostly intracellular and intercellular coupling is reduced. We investigated Cx43 distributions during mitosis both in endogenous and exogenous expressing cells using optical pulse-chase labeling, correlated light and electron microscopy, immunocytochemistry and biochemical analysis. Time-lapse imaging of green fluorescent protein (GFP)/tetracysteine tagged Cx43 (Cx43-GFP-4C) expressing cells revealed an early disappearance of gap junctions, progressive accumulation of Cx43 in cytoplasmic structures, and an unexpected subset pool of protein concentrated in the plasma membrane surrounding the midbody region in telophase followed by rapid reappearance of punctate plaques upon mitotic exit. These distributions were also observed in immuno-labeled endogenous Cx43-expressing cells. Photo-oxidation of ReAsH-labeled Cx43-GFP-4C cells in telophase confirmed that Cx43 is distributed in the plasma membrane surrounding the midbody as apparent connexons and in cytoplasmic vesicles. We performed optical pulse-chase labeling and single label time-lapse imaging of synchronized cells stably expressing Cx43 with internal tetracysteine domains through mitosis. In late telophase, older Cx43 is segregated mainly to the plasma membrane while newer Cx43 is intracellular. This older population nucleates new gap junctions permitting rapid resumption of communication upon mitotic exit.

Opposing and synergistic effects of cyclic mechanical stretch and α- or β-adrenergic stimulation on the cardiac gap junction protein Cx43

In the heart the most prominent cardiac gap junction protein is connexin43 (Cx43). Increased Cx43 expression has been identified in cardiac hypertrophy and may contribute to arrhythmias. Besides acute effects on gap junction channel function, chronic regulation of Cx43 expression can affect intercellular communication. Since both cyclic mechanical stretch (CMS) and catecholamines play an important role in cardiac physiology and pathophysiology, we wanted to elucidate whether a prolonged β- or α-adrenoceptor stimulation may modulate the effects of CMS on Cx43 expression. Neonatal rat cardiomyocytes were cultured on flexible 6-well plates. Thereafter, cells were kept static without any treatment or stimulated with 0.1μmol/L isoprenaline or phenylephrine for 24h without or with additional CMS (1Hz; 10% elongation). Isoprenaline and phenylephrine given alone significantly increased Cx43-protein and -mRNA level. Also CMS resulted in a significant Cx43-protein and -mRNA up-regulation. The combined treatment of the cells with either isoprenaline or phenylephrine and stretch also resulted in an up-regulation of Cx43-protein and -mRNA, which did not exceed those of stretch, isoprenaline or phenylephrine alone. However, while CMS reduced the Cx43-protein/mRNA ratio, adrenergic stimulation increased Cx43-protein/mRNA ratio. While isoprenaline and phenylephrine increased Cx43-phosphorylation, additional CMS significantly reduced P-Cx43/Cx43 ratio. For further investigation of the underlying signal transduction pathway, we examined the phosphorylated forms of ERK1/2, GSK3β and AKT and could demonstrate that these protein kinases are also significantly up-regulated following stretch or adrenoceptor stimulation. Again the combined treatment of cardiomyocytes with CMS and isoprenaline or phenylephrine had no additive effects. Thus, the combination of α- or β-adrenoceptor stimulation and CMS up-regulates Cx43 expression and leads to phosphorylation of ERK1/2 and AKT (=activation) and of GSK3β (=inactivation). There were no significant additive effects compared to CMS or adrenergic stimulation alone indicating a possible ceiling effect. However, CMS and adrenergic stimulation differentially affected Cx43-protein/mRNA ratio and Cx43-phosphorylation.

Phosphoproteome reveals an atlas of protein signaling networks during osteoblast adhesion

Cell adhesion on surfaces is a fundamental process in the emerging biomaterials field and developmental events as well. However, the mechanisms regulating this biological process in osteoblasts are not fully understood. Reversible phosphorylation catalyzed by kinases is probably the most important regulatory mechanism in eukaryotes. Therefore, the goal of this study is to assess osteoblast adhesion through a molecular prism under a peptide array technology, revealing essential signaling proteins governing adhesion-related events. First, we showed that there are main morphological changes on osteoblast shape during adhesion up to 3 h. Second, besides classical proteins activated upon integrin activation, our results showed a novel network involving signaling proteins such as Rap1A, PKA, PKC, and GSK3beta during osteoblast adhesion on polystyrene. Third, these proteins were grouped in different signaling cascades including focal adhesion establishment, cytoskeleton rearrangement, and cell-cycle arrest. We have thus provided evidence that a global phosphorylation screening is able to yield a systems-oriented look at osteoblast adhesion, providing new insights for understanding of bone formation and improvement of cell-substratum interactions. Altogether, these statements are necessary means for further intervention and development of new approaches for the progress of tissue engineering.

Implications and challenges of connexin connections to cancer

The idea that the gap junction family of proteins, connexins, are tumour suppressors has been widely supported through numerous cancer models. However, the paradigm that connexins and enhanced gap junctional intercellular communication is of universal benefit by restricting tumour growth has been challenged by more recent evidence that suggests a role for connexins in facilitating tumour progression and metastasis. Therefore, connexins might be better classified as conditional tumour suppressors that modulate cell proliferation, as well as adhesion and migration.

Modulatory effects of cAMP and PKC activation on gap junctional intercellular communication among thymic epithelial cells

BACKGROUND

We investigated the effects of the signaling molecules, cyclic AMP (cAMP) and protein-kinase C (PKC), on gap junctional intercellular communication (GJIC) between thymic epithelial cells (TEC).

RESULTS

Treatment with 8-Br-cAMP, a cAMP analog; or forskolin, which stimulates cAMP production, resulted in an increase in dye transfer between adjacent TEC, inducing a three-fold enhancement in the mean fluorescence of coupled cells, ascertained by flow cytometry after calcein transfer. These treatments also increased Cx43 mRNA expression, and stimulated Cx43 protein accumulation in regions of intercellular contacts. VIP, adenosine, and epinephrine which may also signal through cyclic nucleotides were tested. The first two molecules did not mimic the effects of 8-Br-cAMP, however epinephrine was able to increase GJIC suggesting that this molecule functions as an endogenous inter-TEC GJIC modulators. Stimulation of PKC by phorbol-myristate-acetate inhibited inter-TEC GJIC. Importantly, both the enhancing and the decreasing effects, respectively induced by cAMP and PKC, were observed in both mouse and human TEC preparations. Lastly, experiments using mouse thymocyte/TEC heterocellular co-cultures suggested that the presence of thymocytes does not affect the degree of inter-TEC GJIC.

CONCLUSIONS

Overall, our data indicate that cAMP and PKC intracellular pathways are involved in the homeostatic control of the gap junction-mediated communication in the thymic epithelium, exerting respectively a positive and negative role upon cell coupling. This control is phylogenetically conserved in the thymus, since it was seen in both mouse and human TEC preparations. Lastly, our work provides new clues for a better understanding of how the thymic epithelial network can work as a physiological syncytium.

Gap junctional intercellular communication as a target for liver toxicity and carcinogenicity

Direct communication between hepatocytes, mediated by gap junctions, constitutes a major regulatory platform in the control of liver homeostasis, ranging from hepatocellular proliferation to hepatocyte cell death. Inherent to this pivotal task, gap junction functionality is frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity and carcinogenicity. In the present paper, the deleterious effects of a number of chemical and biological toxic compounds on hepatic gap junctions are discussed, including environmental pollutants, biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. Particular attention is paid to the molecular mechanisms that underlie the abrogation of gap junction functionality. Since hepatic gap junctions are specifically targeted by tumor promoters and epigenetic carcinogens, both in vivo and in vitro, inhibition of gap junction functionality is considered as a suitable indicator for the detection of nongenotoxic hepatocarcinogenicity.

Ischemic Preconditioning Protects Against Gap Junctional Uncoupling in Cardiac Myofibroblasts

Ischemic preconditioning increases the heart's tolerance to a subsequent longer ischemic period. The purpose of this study was to investigate the role of gap junction communication in simulated preconditioning in cultured neonatal rat cardiac myofibroblasts. Gap junctional intercellular communication was assessed by Lucifer yellow dye transfer. Preconditioning preserved intercellular coupling after prolonged ischemia. An initial reduction in coupling in response to the preconditioning stimulus was also observed. This may protect neighboring cells from damaging substances produced during subsequent regional ischemia in vivo, and may preserve gap junctional communication required for enhanced functional recovery during subsequent reperfusion.

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.

The role of the cytoskeleton in the formation of gap junctions by Connexin 30

Mutations in the genes that encode Connexin 26 (GJB2) and Connexin 30 (GJB6) are the most common known cause of hereditary nonsyndromic sensorineural deafness. Cx26 and Cx30 share a similar protein structure, as well as the same expression distribution pattern in the cochlea. Cx26 has different intracellular trafficking properties compared to those of Cx43 and Cx32, whose trafficking manner is consistent with the classical membrane protein secretory pathway. Until now, however, the trafficking patterns of Cx30 have not been studied. By means of an immunofluorescence staining approach, we found that the targeting of Cx30 to gap junctions in transfected HeLa cells is not affected by brefeldin A, suggesting a Golgi-independent feature, similar to Cx26. Nocodazole had a minimal effect on assembly and distribution of Cx30 gap junctions. Cytochalasin B-induced actin filament depolymerization, however, affected both the pattern and the distribution of Cx30 gap junctions. Co-localization with and/or interaction between Cx30 and microtubules and cortical actin filaments, but not with the tight/adherens junction protein ZO-1, was confirmed by immunofluorescence and/or immunoprecipitation methods. The results suggest that the cytoskeleton, and especially actin filaments, are important components in the processes of assembly, trafficking and stabilization of Cx30 gap junctions.

Regulation of gap junctional intercellular communication by TCDD in HMEC and MCF-7 breast cancer cells

Previous studies suggest that many neoplastic tissues exhibit a decrease in gap junctional intercellular communication (GJIC). Many hydrocarbons and organochlorine compounds are environmental pollutants known to be carcinogenic. The effect of an organochlorine compound, TCDD, on GJIC in human breast cell lines has not been established. In the present study, we showed that TCDD causes an inhibition in the gap junctional activity in MCF-7 (breast cancer cells). In MCF-7 cells, an increase in the phosphorylated form of gap junctional protein, connexin 43 (Cx43), and PKC alpha was seen in the presence of TCDD. Gap junctional plaque formation was significantly decreased in MCF-7 cells in the presence of TCDD. Immunoprecipitation studies of PKC alpha showed that TCDD caused a significant 40% increase in the phosphorylated Cx43 in MCF-7 cells. TCDD also modulated the translocation of PKC alpha from the cytosol to the membrane and caused a 2-fold increase in the PKC alpha activity at 50 nM TCDD in MCF-7 cells. Calphostin C, an inhibitor of PKC alpha, showed a significant inhibition of PKC alpha activity in the presence of TCDD. Furthermore, TCDD also caused a decrease in the gap junctional activity and Cx43 protein in human mammary epithelial cells (HMEC). However, we observed a shift in the Cx43 plaques towards the perinuclear membrane in the presence of TCDD by confocal microscopy and Western blot. Overall, these results conclude that TCDD decreases GJIC by phosphorylating Cx43 via PKC alpha signaling pathway in MCF-7 cells; however, TCDD decreases the GJIC by affecting the localization of Cx43 in HMEC. These new findings elucidate the differential mode of effect of TCDD in the downregulation of GJIC in HMEC and MCF-7 cells.

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.

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

Regulation of gap junction intercellular communication in primary canine lens epithelial cells: role of protein kinase C

PURPOSE

Gap junction intercellular communication (GJIC) is important in maintaining lens epithelial cell homeostasis and reductions in GJIC may be associated with the development of cataract. Protein kinase C (PKC) activation can disrupt gap junction communication via phosphorylation of connexin 43 (C x 43) proteins that compose gap junction channels. This study examined the role of PKC activation in modulating GJIC in a primary canine lens epithelial cell (LEC) line.

METHODS

TPA (12-O-tetradecanoyl-phorbol-acetate), a potent PKC activator and inhibitor of GJIC, was utilized in the present study. Primary cultures of canine LEC were treated with TPA (0-1000 ng/ml) for 0.5 hr and GJIC was assessed by scrape loading/dye transfer (SL/DT), and immunoblotting to detect phosphorylation of C x 43 protein. Inhibition of general and calcium-dependent PKC activity was achieved by pretreatment of cells with GF109203X and Gö6976, respectively.

RESULTS

Treatment with TPA (1-1000 ng/ml) significantly decreased GJIC in canine LEC as assessed by SL/DT. Pretreatment with 10 and 100 ng/ml TPA decreased GJIC by 80% as compared to controls and increased Cx43 phosphorylation as assessed by immunoblotting. Pretreatment of cells with GF109203X and Gö6976, partially restored TPA-inhibited GJIC by 40% and 60%, respectively, and reduced C x 43 phosphorylation. Expression of calcium dependent PKC isoforms was detected in canine whole lens and LEC.

CONCLUSIONS

Treatment with TPA significantly reduces GJIC in canine LEC. These effects are mediated, in part, by activation of calcium-dependent PKC isoforms. Primary canine LEC are a useful model in the study of the molecular mechanisms involved in GJIC and cataractogenesis.

Connexin 35/36 is phosphorylated at regulatory sites in the retina

Connexin 35/36 is the most widespread neuronal gap junction protein in the retina and central nervous system. Electrical and/or tracer coupling in a number of neuronal circuits that express this connexin are regulated by light adaptation. In many cases, the regulation of coupling depends on signaling pathways that activate protein kinases such as PKA, and Cx35 has been shown to be regulated by PKA phosphorylation in cell culture systems. To examine whether phosphorylation might regulate Cx35/36 in the retina we developed phospho-specific polyclonal antibodies against the two regulatory phosphorylation sites of Cx35 and examined the phosphorylation state of this connexin in the retina. Western blot analysis with hybrid bass retinal membrane preparations showed Cx35 to be phosphorylated at both the Ser110 and Ser276 sites, and this labeling was eliminated by alkaline phosphatase digestion. The homologous sites of mouse and rabbit Cx36 were also phosphorylated in retinal membrane preparations. Quantitative confocal immunofluorescence analysis showed gap junctions identified with a monoclonal anti-Cx35 antibody to have variable levels of phosphorylation at both the Ser110 and Ser276 sites. Unusual gap junctions that could be identified by their large size (up to 32 microm2) and location in the IPL showed a prominent shift in phosphorylation state from heavily phosphorylated in nighttime, dark-adapted retina to weakly phosphorylated in daytime, light-adapted retina. Both Ser110 and Ser276 sites showed significant changes in this manner. Under both lighting conditions, other gap junctions varied from non-phosphorylated to heavily phosphorylated. We predict that changes in the phosphorylation states of these sites correlate with changes in the degree of coupling through Cx35/36 gap junctions. This leads to the conclusion that connexin phosphorylation mediates changes in coupling in some retinal networks. However, these changes are not global and likely occur in a cell type-specific or possibly a gap junction-specific manner.

Connexin hemichannels and gap junction channels are differentially influenced by lipopolysaccharide and basic fibroblast growth factor

Gap junction (GJ) channels are formed by two hemichannels (connexons), each contributed by the cells taking part in this direct cell-cell communication conduit. Hemichannels that do not interact with their counterparts on neighboring cells feature as a release pathway for small paracrine messengers such as nucleotides, glutamate, and prostaglandins. Connexins are phosphorylated by various kinases, and we compared the effect of various kinase-activating stimuli on GJ channels and hemichannels. Using peptides identical to a short connexin (Cx) amino acid sequence to specifically block hemichannels, we found that protein kinase C, Src, and lysophosphatidic acid (LPA) inhibited GJs and hemichannel-mediated ATP release in Cx43-expressing C6 glioma cells (C6-Cx43). Lipopolysaccharide (LPS) and basic fibroblast growth factor (bFGF) inhibited GJs, but they stimulated ATP release via hemichannels in C6-Cx43. LPS and bFGF inhibited hemichannel-mediated ATP release in HeLa-Cx43 cells, but they stimulated it in HeLa-Cx43 with a truncated carboxy-terminal (CT) domain or in HeLa-Cx26, which has a very short CT. Hemichannel potentiation by LPS was inhibited by blockers of the arachidonic acid metabolism, and arachidonic acid had a potentiating effect like LPS and bFGF. We conclude that GJ channels and hemichannels display similar or oppositely directed responses to modulatory influences, depending on the balance between kinase activity and the activity of the arachidonic acid pathway. Distinctive hemichannel responses to pathological stimulation with LPS or bFGF may serve to optimize the cell response, directed at strictly controlling cellular ATP release, switching from direct GJ communication to indirect paracrine signaling, or maximizing cell-protective strategies.

Electrical coupling of cardiomyocyte sheets occurs rapidly via functional gap junction formation

Previously, we have successfully created pulsatile myocardial tissue grafts using our novel technology, "cell sheet engineering", that layers cell sheets fabricated on temperature-responsive culture dishes to form three-dimensional (3-D) structures. Electrical coupling is established between layered neonatal rat cardiomyocyte sheets, resulting in the synchronized beating of 3-D myocardial tissues. However, the mechanism by which these layered cardiomyocyte sheets communicate electrically is not well-understood. In this study, we used a multiple-electrode extracellular recording system and demonstrated that bilayer cardiomyocyte sheets coupled electrically with slight delays 34+/-2 min (mean+/-SEM) after layering. These delays gradually decreased and the electrical actions of layered cell sheets were completely coupled 46+/-3 min (mean+/-SEM) after initial layering. Immunohistological analysis showed that connexin43, a gap junction (GJ)-related protein, existed not only at cell-to-cell interfaces but also on the free cell membrane in the cardiomyocyte sheet. Additionally, neither connexin40 nor connexin45, but only connexin43 was detected between bilayer cardiomyocyte sheets within 30 min after layering. Dye transfer assay demonstrated that the exchange of small molecules via GJs occurred within 30 min. The cell sheet manipulation technique using the temperature-responsive culture dishes has substantial advances and the exciting potential in the fields of cell and tissue physiology, as well as tissue engineering.

S-nitrosylation and permeation through connexin 43 hemichannels in astrocytes: induction by oxidant stress and reversal by reducing agents

Marked increase in cell permeability ascribed to open connexin (Cx)43 hemichannels is induced by metabolic inhibition (MI) of cortical astrocytes in culture, but the molecular mechanisms are not established. Dephosphorylation and/or oxidation of Cx43 hemichannels was proposed as a potential mechanism to increase their open probability. We now demonstrate that MI increases the number of hemichannels on the cell surface assayed by biotinylation and Western blot, and that this change is followed by increased dephosphorylation and S-nitrosylation. The increase in rate of dye uptake caused by MI is comparable to the increase in surface expression; thus, open probability and permeation per hemichannel may be unchanged. Reducing agents did not affect dephosphorylation of Cx43 hemichannels but reduced dye uptake and S-nitrosylation. Uptake was also reduced by elevated intracellular but not extracellular levels of reduced glutathione. Moreover, nitric oxide donors induced dye uptake and nitrosylation of surface Cx43 but did not affect its abundance or phosphorylation. Thus, permeability per channel is increased, presumably because of increase in open probability. We propose that increased dye uptake induced by MI is mediated by an increased number of Cx43 hemichannels in the surface and is associated with multiple molecular changes, among which nitrosylation of intracellular Cx43 cysteine residues may be a critical factor.

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.

Cardiomyocyte bridging between hearts and bioengineered myocardial tissues with mesenchymal transition of mesothelial cells

BACKGROUND

For the reconstruction of 3-dimensional (3D) tissues, we exploited an original method of tissue engineering that layers individual cell sheets harvested from temperature-responsive culture dishes. Stacked cardiomyocyte sheets demonstrated electrical and morphologic communication, resulting in synchronously beating myocardial tissue. When these bioengineered 3D tissue grafts are transplanted onto damaged hearts, gap junction communication between graft and host is likely critical for synchronized beating and functional improvement. In this study, these graft-to-heart morphologic communications were examined.

METHODS

Neonatal rat cardiomyocyte sheets were harvested from temperature-responsive culture dishes and layered to create 3D tissues. These constructs were then transplanted onto infarcted rat hearts. Histologic analyses and transmission electron microscopy (TEM) were performed to examine morphologic communications. The passage of small molecules through functional gap junctions was also detected using a dye-transfer assay.

RESULTS

Transplanted cardiomyocytes bridged between the grafts and hearts in intact areas. Connexin-43 staining and TEM revealed the existence of gap junctions and intercalated disks between the bridging cardiomyocytes. Furthermore, it was confirmed that a low-molecule fluorescent dye, calcein, was transferred from the grafts to the hearts via the bridging cardiomyocytes. Immunohistochemistry with anti-intercellular adhesion molecule-1 antibodies revealed that mesothelial cells in the epicardium scattered and transdifferentiated into mesenchymal cells between the graft and host.

CONCLUSIONS

The direct attachment of layered cardiomyocyte sheets on the heart surface promotes mesothelial cell transdifferentiation and cardiomyocyte bridging, leading to functional communication via gap junctions. These results indicate that these bioengineered myocardial tissues may improve damaged heart function via synchronized beating.

Temporal regulation of connexin phosphorylation in embryonic and adult tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in tissues and are important in development, tissue/cellular homeostasis, and carcinogenesis. Genome databases indicate that there are at least 20 connexins in the mouse and human. Connexin phosphorylation has been implicated in connexin assembly into gap junctions, gap junction turnover, and cell signaling events that occur in response to tumor promoters and oncogenes. Connexin43 (Cx43), the most widely expressed and abundant gap junction protein, can be phosphorylated at several different serine and tyrosine residues. Here, we focus on the dynamic regulation of Cx43 phosphorylation in tissue and how these regulatory events are affected during development, wound healing, and carcinogenesis. The activation of several kinases, including protein kinase A, protein kinase C, p34cdc2/cyclin B kinase, casein kinase 1, mitogen-activated protein kinase, and pp60src kinase, can lead to the phosphorylation of different residues in the C-terminal region of Cx43. The use of antibodies specific for phosphorylation at defined residues has allowed the examination of specific phosphorylation events both in tissue culture and in vivo. These new antibody tools and those under development will allow us to correlate specific phosphorylation events with changes in connexin function.

Nogo-66 Receptor at Cerebellar Cortical Glia Gap Junctions in the Rat

Nogo-A is a myelin inhibitor of neurite outgrowth that accounts for the difficulty in fiber regeneration in the central nervous system. Its 66-amino-acid extracellular domain (Nogo-66) contributes to the inhibitory activity of Nogo-A. The Nogo-66 receptor is widely distributed in neurons of the central nervous system, including the cerebellum. In our study on the distribution of Nogo-66 receptor in the cerebellar cortex in the rat, we unexpectedly found Nogo-66 receptor immunoreactivity in the glia cells, particularly abundant beneath the Purkinje cells. The presence of Nogo-66 receptor in glia cells has not been reported before. A detailed study was thus conducted. Immunoelectron microscopic investigation clearly demonstrated that the Nogo-66 receptor immunoreactivity could be ascertained at the gap junction between glia cells, indicating that the Nogo-66 receptor may modulate the communication between glia cells through gap junctions.

Up-regulation of gap junctional intercellular communication and connexin43 expression by retinoic acid in human endometrial stromal cells

CONTEXT

Gap junctions, made up of connexins (Cxs), play fundamental roles in coordinating a number of cellular processes through their ability to directly regulate cell-cell communication. Cx43 is the most widely expressed Cx in the endometrium and is known to be important in a variety of physiological and pathological processes in this tissue.

OBJECTIVE

In this study, we investigated the ability of the retinoid, all-trans-retinoic acid (RA), to regulate Cx43 expression in human endometrial stromal cells.

DESIGN

Primary endometrial stromal cells obtained from patients undegoing surgery for infertility workup were treated in vitro with RA and control compounds for different time periods, up to 48 h. Cx43 mRNA and protein levels, protein phosphorylation, and gap junctional intercellular communication (GJIC) were analyzed.

RESULTS

Treatment of the cells with RA showed a dose-dependent increase in Cx43 expression at both the mRNA and protein levels. In addition, RA induced a relative decrease in the phosphorylated species of Cx43 while causing a corresponding increase in the nonphosphorylated form. Concomitant with these changes, RA-treated cells demonstrated up to a 250% enhancement of GJIC as assessed by dye transfer experiments. Augmentation of GJIC and alterations of Cx43 expression were observed over the same range of RA concentrations. Treatment of cells with the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate increased the phosphorylated species of Cx43 and correspondingly inhibited GJIC.

CONCLUSIONS

Phosphorylation of Cx43 is inversely related to GJIC in endometrial stromal cells. Retinoids increase GJIC in endomentrial stromal cells through upregulation of Cx43 expression while inducing a decrease in the phosphorylated species of the protein. The data suggest a novel mechanism by which retinoids can influence endometrial cell biology.

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.

Structural organization of gap junction channels

Gap junctions were initially described morphologically, and identified as semi-crystalline arrays of channels linking two cells. This suggested that they may represent an amenable target for electron and X-ray crystallographic studies in much the same way that bacteriorhodopsin has. Over 30 years later, however, an atomic resolution structural solution of these unique intercellular pores is still lacking due to many challenges faced in obtaining high expression levels and purification of these structures. A variety of microscopic techniques, as well as NMR structure determination of fragments of the protein, have now provided clearer and correlated views of how these structures are assembled and function as intercellular conduits. As a complement to these structural approaches, a variety of mutagenic studies linking structure and function have now allowed molecular details to be superimposed on these lower resolution structures, so that a clearer image of pore architecture and its modes of regulation are beginning to emerge.

Mechanical strain opens connexin 43 hemichannels in osteocytes: a novel mechanism for the release of prostaglandin

Mechanosensing bone osteocytes express large amounts of connexin (Cx)43, the component of gap junctions; yet, gap junctions are only active at the small tips of their dendritic processes, suggesting another function for Cx43. Both primary osteocytes and the osteocyte-like MLO-Y4 cells respond to fluid flow shear stress by releasing intracellular prostaglandin E2 (PGE2). Cells plated at lower densities release more PGE2 than cells plated at higher densities. This response was significantly reduced by antisense to Cx43 and by the gap junction and hemichannel inhibitors 18 beta-glycyrrhetinic acid and carbenoxolone, even in cells without physical contact, suggesting the involvement of Cx43-hemichannels. Inhibitors of other channels, such as the purinergic receptor P2X7 and the prostaglandin transporter PGT, had no effect on PGE2 release. Cell surface biotinylation analysis showed that surface expression of Cx43 was increased by shear stress. Together, these results suggest fluid flow shear stress induces the translocation of Cx43 to the membrane surface and that unapposed hemichannels formed by Cx43 serve as a novel portal for the release of PGE2 in response to mechanical strain.

Protein kinase C spatially and temporally regulates gap junctional communication during human wound repair via phosphorylation of connexin43 on serine368

Phosphorylation of connexin43 (Cx43) on serine368 (S368) has been shown to decrease gap junctional communication via a reduction in unitary channel conductance. Examination of phosphoserine368 (pS368) in normal human skin tissue using a phosphorylation site–specific antibody showed relatively even distribution throughout the epidermal layers. However, 24 h after wounding, but not at 6 or 72 h, pS368 levels were dramatically increased in basal keratinocytes and essentially lost from suprabasal layers adjacent to the wound (i.e., within 200 μm of it). Scratch wounding of primary human keratinocytes caused a protein kinase C (PKC)-dependent increase in pS368 in cells adjacent to the scratch, with a time course similar to that found in the wounds. Keratinocytes at the edge of the scratch also transferred dye much less efficiently at 24 h, in a manner dependent on PKC. However, keratinocyte migration to fill the scratch required early (within <6 h) gap junctional communication. Our evidence indicates that PKC-dependent phosphorylation of Cx43 at S368 creates dynamic communication compartments that can temporally and spatially regulate wound healing.

The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.

Fusion of GFP to the carboxyl terminus of connexin43 increases gap junction size in HeLa cells

The pattern of gap junctional coupling between cells is thought to be important for the proper function of many types of tissues. At present, little is known about the molecular mechanisms that control the size and distribution of gap junctions. We addressed this issue by expressing connexin43 (Cx43) constructs in HeLa cells, a connexin-deficient cell line. HeLa cells expressing exogenously introduced wild-type Cx43 formed small, punctate gap junctions. By contrast, cells expressing Cx43-GFP formed large, sheet-like gap junctions. These results suggest that the GFP tag, which is fused to the carboxyl terminus of Cx43, alters gap junction size by masking the carboxyl terminal amino acids of Cx43 that comprise a zonula occludins-1 (ZO-1) binding site. We are currently testing this hypothesis using deletion and dominant-negative constructs that directly target the interaction between Cx43 and ZO-1.

Connexin phosphorylation as a regulatory event linked to gap junction channel assembly

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells and are important in development and maintenance of cell homeostasis. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the cell cycle and the connexin "lifecycle", such as trafficking, assembly/disassembly, degradation, as well as in the gating of "hemi" channels or intact gap junction channels. This review focuses on how phosphorylation can regulate the early stages of the connexin life cycle through assembly of functional gap junctional channels. The availability of sequences from the human genome databases has indicated that the number of connexins in the gene family is approximately 20, but we know mostly about how connexin43 (Cx43) is regulated. Recent technologies and investigations of interacting proteins have shown that activation of several kinases including protein kinase A, protein kinase C (PKC), p34(cdc2)/cyclin B kinase, casein kinase 1 (CK1), mitogen-activated protein kinase (MAPK) and pp60(src) kinase can lead to phosphorylation of the majority of the 21 serine and two of the tyrosine residues in the C-terminal region of Cx43. While many studies have correlated changes in kinase activity with changes in gap junctional communication, further research is needed to directly link specific phosphorylation events with changes in connexin oligomerization and gap junction assembly.

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.

Mechanism of oleic acid-induced gap junctional disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA) on gap junctions and identified the protein kinase C (PKC) isoforms involved in OA-mediated gap junction disassembly in cardiomyocytes. Control cardiomyocytes showed continuous staining of the plasma membrane at cell-cell contact areas using antibodies reacting with connexin 43 (Cx43). The spontaneous contraction rate of cultured cardiomyocytes was reduced in a time-dependent manner by OA. In addition, Cx43 expression at cell-cell junction decreased, suggesting the disassembly of gap junction. Staining for PKC and PKCalpha, which were shown to colocalize with Cx43, also decreased with increased duration of OA treatment. The effects of OA on these distributional changes at cell junctions were reversed by 24 h incubation in fresh culture medium devoid of OA. Immunoprecipitation assays confirmed the biochemical binding between Cx43 and PKC/PKCalpha, and this protein interaction was not affected by OA. This may provide the basis for simultaneous detachment of Cx and PKC/PKCalpha from the cell-cell junction to the cytosol upon OA stimulation. Western blot analysis showed that OA-induced Cx43 Ser368 phosphorylation, and that this effect could be blocked by cotreatment with the general PKC inhibitor, calphostin C, the PKC inhibitor, eV1-2, or the Src kinase inhibitor, PP1, but not by the PKCalpha inhibitor, Gö6976. eV1-2 also prevented the OA-induced disassembly of gap junctions. Taken together, these data suggest that OA-induced Cx43 Ser368 phosphorylation is mediated by activation of PKC and Src kinase and might be responsible for OA-induced gap junctional disassembly.

CCN3 (NOV) interacts with connexin43 in C6 glioma cells: possible mechanism of connexin-mediated growth suppression

Many tumor cells exhibit aberrant gap junctional intercellular communication, which can be restored by transfection with connexin genes. We have previously discovered that overexpression of connexin43 (Cx43) in C6 glioma cells not only reduces proliferation but also leads to production of soluble growth-inhibitory factors. We identified that several members of the CCN (Cyr61/connective tissue growth factor/nephroblastoma-overexpressed) family are up-regulated following Cx43 expression, including CCN3 (NOV). We now report evidence for an association between CCN3 and Cx43. Western blot analysis demonstrated that the 48-kDa full-length CCN3 protein was present in the lysate and conditioned medium of growth-suppressed C6-Cx43 cells, as well as primary astrocytes, but not in C6 parental and human glioma cells. Immunocytochemical examination of CCN3 revealed diffuse localization in parental C6 cells, whereas transfection of C6 cells with Cx43 (C6-Cx43) or with a modified Cx43 tagged to green fluorescent protein on its C terminus (Cx43-GFP) resulted in punctate staining, suggesting that CCN3 co-localizes with Cx43 in plaques at the plasma membrane. In cells expressing a C-terminal truncation of Cx43 (Cx43Delta244-382), this co-localization was lost. Glutathione S-transferase pull-down assay and co-immunoprecipitation demonstrated that CCN3 was able to physically interact with Cx43. In contrast, CCN3 was not found to associate with Cx43Delta244-382. Similar experiments revealed that CCN3 did not co-localize or associate with other connexins, including Cx40 or Cx32. Taken together, these data support an interaction of CCN3 with the C terminus of Cx43, which could play an important role in mediating growth control induced by specific gap junction proteins.

The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.

Regulation of connexin biosynthesis, assembly, gap junction formation, and removal

Gap junctions (GJs) are the only known cellular structures that allow a direct transfer of signaling molecules from cell-to-cell by forming hydrophilic channels that bridge the opposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for coordination of development, tissue function, and cell homeostasis is now well documented. In addition, recent findings have fueled the novel concepts that connexins, although redundant, have unique and specific functions, that GJIC may play a significant role in unstable, transient cell-cell contacts, and that GJ hemi-channels by themselves may function in intra-/extracellular signaling. Assembly of these channels is a complicated, highly regulated process that includes biosynthesis of the connexin subunit proteins on endoplasmic reticulum membranes, oligomerization of compatible subunits into hexameric hemi-channels (connexons), delivery of the connexons to the plasma membrane, head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic, spatially and temporally organized GJ channel aggregates (so-called plaques), and coordinated removal of channels into the cytoplasm followed by their degradation. Here we review the current knowledge of the processes that lead to GJ biosynthesis and degradation, draw comparisons to other membrane proteins, highlight novel findings, point out contradictory observations, and provide some provocative suggestive solutions.

Regulation of gap junctions by tyrosine protein kinases

Most of the gap junction proteins are regulated in part by post-translational phosphorylation. Phosphorylation has been shown to be important in gap junction assembly and turnover, and for channel function in the resting state. Connexin phosphorylation may be altered by the activation of intracellular signaling pathways in response to growth factors, tumor promoters, activated oncogenes, hormones and inflammatory mediators. In some instances altered phosphorylation has been associated with changes in connexin function and in other cases appears to be associated with changes in the levels of the connexin protein and/or mRNA. This review focuses on the role of tyrosine protein kinases in the regulation of gap junctions. The literature is most extensive for connexin43 and those studies are reviewed here. A great deal has been learned in recent years about how connexin43 is regulated by tyrosine kinase-dependent signaling pathways. These pathways are often complex and to some extent are cell type- and stimulus-dependent. Although considerable progress has been made in unraveling the cellular pathways that regulate connexin function, significant challenges remain to be addressed in identifying additional phosphorylation sites and determining the stoichiometries of the phosphorylation events that regulate connexin function and it's interaction with other cellular proteins.

Gap junction communication between cells aggregated on a cellulose-coated polystyrene: influence of connexin 43 phosphorylation

The appropriate functioning of tissues and organ systems depends on intercellular communication such as gap junctions formed by connexin (Cx) protein channels between adjacent cells. We have previously shown that Swiss 3T3 cells aggregated on hydrophilic cellulose substratum Cuprophan (CU) establish short linear gap junctions composed of Cx 43 in cell surface plaques. This phenomenon seems to depend on the high intracellular cyclic AMP (cAMP) concentration triggered by attachment of the cells to CU. We have now used a cellulose-coated polystyrene inducing the same cell behaviour to analyse the gap junction communication between aggregated cells. The transfer of the dye Lucifer Yellow (LY) between cells showed that cells aggregated on cellulose substratum rapidly (within 90 min) establish functional gap junctions. Inhibitors of cAMP protein kinase (PKI) or protein kinase C (GF109203X) both inhibited the diffusion of LY between neighbouring cells. Western blot analysis showed that this change in permeability was correlated with a decrease in Cx 43 phosphorylation. Thus, cellulose substrata seem to induce cell-cell communication through Cx 43 phosphorylation modulated by PKA and PKC. To understand the mechanisms by which a substratum regulates gap junctional communication is critically important for the emerging fields of tissue engineering and biohybrid devices.

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

Connexins are membrane-spanning proteins that form gap junction channels between adjacent cells. Connexin43 (Cx43), the most widely expressed member of the connexin family in tissues and cell lines, has a rapid turnover rate and its degradation involves both the lysosomal and ubiquitin-proteasome pathway. It was previously shown that the proteasome is involved in regulating the number of functional gap junctions at the plasma membrane. However, little is known about how proteasome-dependent turnover of Cx43 is controlled. Epidermal growth factor (EGF) induces hyperphosphorylation of Cx43 and a rapid, transient decrease in gap junctional intercellular communication. In this study, we show that, along with inhibition of gap junctional intercellular communication, EGF induces disorganization, internalization and degradation of Cx43 gap junction plaques in IAR20 rat liver epithelial cells. These EGF-induced modifications of Cx43 were counteracted by the MEK1 inhibitor PD98059, indicating that the effects were mediated by the mitogen-activated protein kinase pathway. The EGF-induced destruction of Cx43 was proteasome-dependent, because the loss of Cx43 protein was counteracted by the proteasome inhibitor MG132 but not the lysosomal inhibitor leupeptin. Furthermore, EGF induced ubiquitination of Cx43, which was associated with the Cx43 hyperphosphorylation. The EGF-induced Cx43 ubiquitination was counteracted by PD98059. The EGF-induced internalization of Cx43 was blocked by hypertonic sucrose treatment, indicating that EGF mediates internalization of Cx43 via a clathrin-dependent mechanism. Our results indicate that ubiquitination of Cx43 occurs at the plasma membrane before Cx43 internalization. Taken together, these data provide the first evidence that EGF-induced phosphorylation of Cx43 induces binding of ubiquitin and targets Cx43 for internalization and degradation in a proteasome-dependent manner.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Recovery of gap junctional intercellular communication after phorbol ester treatment requires proteasomal degradation of protein kinase C

Reversible down-regulation of gap junctional intercellular communication (GJIC) is proposed to be an important cellular mechanism in tumor promotion. Gap junction function is modified by a variety of tumor promoters, including the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Treatment of cells with TPA results in the activation and subsequent depletion of the TPA-responsive protein kinase C (PKC) isoforms. TPA-induced degradation of the PKC isoforms alpha, delta and epsilon was recently shown to occur via the ubiquitin-proteasome pathway. In the present study we investigated the role of the proteasome in the TPA-induced modification of GJIC in IAR20 rat liver epithelial cells. TPA exposure of IAR20 cells induced hyperphosphorylation of gap junction protein connexin43 and inhibition of GJIC. Prolonged TPA treatment induced down-regulation of PKCalpha, delta and epsilon and a reduction in the total PKC activity, which was associated with recovery of GJIC. Co-treatment of IAR20 cells with TPA and the proteasomal inhibitor MG132 suppressed down-regulation of PKCalpha, delta and epsilon and caused prolonged PKC activity. Under these conditions, the recovery of GJIC was blocked. The general PKC inhibitor GF109203X reversed the effect of MG132, indicating that the prolonged TPA-induced inhibition of GJIC caused by MG132 was due to the prolonged PKC activity. These results indicate that proteasomal degradation of PKC is one mechanism by which the recovery of GJIC after TPA treatment is regulated.