Association with ZO-1 correlates with plasma membrane partitioning in truncated connexin45 mutants

Methylation of the Promoter Region of the Tight Junction Protein-1 by DNMT1 Induces EMT-like Features in Multiple Myeloma

The molecular alterations that initiate the development of multiple myeloma (MM) are not fully understood. Our results revealed that TJP1 was downregulated in MM and positively related to the overall survival of MM patients in The Cancer Genome Atlas (TCGA) database and patient samples. In parallel, cell adhesion capacity representing MM metastasis was decreased in MM patients compared with healthy samples, together with the significantly activated epithelial-to-mesenchymal transition (EMT) transcriptional-like patterns of MM cells. Further analyses demonstrated that TJP1 negatively regulated EMT and consequently positively regulated cell adhesion in MM from TCGA database and MM1s cells. Furthermore, the methylation level of each CpG site on the TJP1 promoter was negatively correlated with TJP1 expression levels. Quantitative real-time PCR and western blot assays demonstrated that methylase DNMT1 regulated the methylation of TJP1. Finally, treatment with a combination of the MM clinical medicine bortezomib, methylation inhibitor, or TJP1 overexpression significantly suppressed the viability and progression of tumor cells of MM orthotopic models. In summary, our results indicate that DNMT1 promotes the methylation of TJP1 promoter, thereby decreasing its expression and regulating the development of EMT-inhibited MM cell adhesion. Therefore, methylation of TJP1 is a potential therapeutic agent to prevent the progression of MM disease.

Expression patterns of ZO-1/2 and their effects on porcine oocyte in vitro maturation and early embryonic development

Zonula occludens (ZO)-1 and ZO-2 are involved in epithelial polarity maintenance, gene transcription, cell proliferation and tumor cell metastasis. Regulating ZO-1/2 expression influences the early embryonic development of mice, but whether they are involved in oocyte maturation is still poorly understood. In the present study, the expression patterns of ZO-1 and ZO-2 in porcine cumulus cells and oocytes matured in vitro and early embryos from parthenogenetic activation were detected by qRT-PCR or Western blot, and then their roles in porcine oocyte maturation and early embryo development were investigated by shRNA technology. ZO-1 and ZO-2 were found to be expressed in cumulus cells, oocytes and early embryos, while ZO-1α⁺ was expressed only in cumulus cells, morula and blastocysts. During in vitro maturation (IVM), the abundance of ZO-1 and ZO-2 in oocytes was significantly higher than that in cumulus cells at 0 h (P < 0.01), and their mRNA and protein levels displayed relatively higher expression at 0 and 18 h, respectively. Compared with the control groups, cumulus cell expansion, oocyte nucleus maturation, and subsequent cleavage were not influenced by treatment of the cumulus-oocyte complexes (COCs) with ZO-1-shRNA1, ZO-2-shRNA2 or combined ZO-1-shRNA1 and ZO-2-shRNA2 lentivirus (P > 0.05). However, the blastocyst rate was reduced by treatment of COCs with ZO-1-shRNA1 but not ZO-2-shRNA2. The total cell number of blastocysts was decreased by downregulation of ZO-1 and ZO-2 (P < 0.05). Downregulation of ZO-1 and ZO-2 also resulted in a significant decrease (P < 0.05) in the expression of Cx43, Cx45, PTX3 and PTGS2 in cumulus cells, Cx45, BMP15, ZP3 and C-KIT in MII oocytes, and Nanog in blastocysts, with the exception of HAS2 expression in cumulus cells and Oct4 expression in blastocysts (P > 0.05). Altogether, the above results indicate that ZO-1 and ZO-2 display similar expression patterns during porcine oocyte IVM and are critical to porcine oocyte maturation and early embryonic development.

Gap-134, a Connexin43 activator, prevents age-related development of ventricular fibrosis in Scn5a+/- mice

Down-regulation of Connexin43 (Cx43) has often been associated with the development of cardiac fibrosis. We showed previously that Scn5a heterozygous knockout mice (Scn5a^+/-), which mimic familial progressive cardiac conduction defect, exhibit an age-dependent decrease of Cx43 expression and phosphorylation concomitantly with activation of TGF-β pathway and fibrosis development in the myocardium between 45 and 60 weeks of age. The aim of this study was to investigate whether Gap-134 prevents Cx43 down-regulation with age and fibrosis development in Scn5a^+/- mice. We observed in 60-week-old Scn5a^+/- mouse heart a Cx43 expression and localization remodeling correlated with fibrosis. Chronic administration of a potent and selective gap junction modifier, Gap-134 (danegaptide), between 45 and 60 weeks, increased Cx43 expression and phosphorylation on serine 368 and prevented Cx43 delocalization. Furthermore, we found that Gap-134 prevented fibrosis despite the persistence of the conduction defects and the TGF-β canonical pathway activation. In conclusion, the present study demonstrates that the age-dependent decrease of Cx43 expression is involved in the ventricular fibrotic process occurring in Scn5a^+/- mice. Finally, our study suggests that gap junction modifier, such as Gap-134, could be an effective anti-fibrotic agent in the context of age-dependent fibrosis in progressive cardiac conduction disease.

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

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

Intramolecular signaling in a cardiac connexin: Role of cytoplasmic domain dimerization

Gap junctions, composed of connexins, mediate electrical coupling and impulse propagation in the working myocardium. In the human heart, the spatio-temporal regulation and distinct functional properties of the three dominant connexins (Cx43, Cx45, and Cx40) suggests non-redundant physiological roles for each isoform. There are substantial differences in gating properties, expression, and trafficking among these isoforms, however, little is known about the determinants of these different phenotypes. To gain insight regarding these determinants, we focused on the carboxyl-terminal (CT) domain because of its importance in channel regulation and large degree of sequence divergence among connexin family members. Using in vitro biophysical experiments, we identified a structural feature unique to Cx45: high affinity (K_D~100nM) dimerization between CT domains. In this study, we sought to determine if this dimerization occurs in cells and to identify the biological significance of the dimerization. Using a bimolecular fluorescence complementation assay, we demonstrate that the CT domains dimerize at the plasma membrane. By inhibiting CT dimerization with a mutant construct, we show that CT dimerization is necessary for proper Cx45 membrane localization, turnover, phosphorylation status, and binding to protein partners. Furthermore, CT dimerization is needed for normal intercellular communication and hemichannel activity. Altogether, our results demonstrate that CT dimerization is a structural feature important for correct Cx45 function. This study is significant because discovery of how interactions mediated by the CT domains can be modulated would open the door to strategies to ameliorate the pathological effects of altered connexin regulation in the failing heart.

The enhancement of Cx45 expression and function in renal interlobar artery of spontaneously hypertensive rats at different age

BACKGROUND/AIMS

This study was designed to investigate the expression and function of gap junction protein connexin 45 (Cx45) in renal interlobar artery (RIA) of spontaneously hypertensive rats (SHR), and the association between hypertension and enhanced vasoconstrictive response in SHR.

METHODS

Western blot analysis and pressure myography were used to examine the differences in expression and function of Cx45 in vascular smooth muscle cells (VSMCs) of RIA between SHR and normotensive Wistar-Kyoto (WKY) rats.

RESULTS

Our results demonstrated that 1) whole-cell patch clamp measurements showed that the membrane capacitance and conductance of in-situ RIA VSMCs of SHR were significantly greater than those of WKY rats (p<0.05, n=6), suggesting that the coupling of gap junction between VSMCs of RIA was enhanced in SHR; 2) the KCl or phenylephrine (PE)-stimulated RIA constriction was more pronounced in SHR than that in WKY rats (p<0.05, n=10). After applying a gap junction inhibitor 18β-glycyrrhetintic acid (18β-GA), the inhibitory effect of 18β-GA on KCl or PE-induced vasoconstriction was greater in SHR (p<0.05, n=10); and 3) the expression of Cx45 in RIA of SHR was greater than that in WKY rats (p<0.05, n=3) at 4, 12 and 48 wks of age.

CONCLUSIONS

The hypertension-induced elevation of Cx45 may affect communication between VSMCs and coupling between VSMCs and endothelium, which results in an increased vasoconstrictive response in renal artery and might contribute to the development of hypertension.

Characterization of the connexin45 carboxyl-terminal domain structure and interactions with molecular partners

Mechanisms underlying the initiation and persistence of lethal cardiac rhythms are of significant clinical and scientific interests. Gap junctions are principally involved in forming the electrical connections between myocytes, and changes in distribution, density, and properties are consistent characteristics in arrhythmic heart disease. Therefore, understanding the structure and function of gap junctions during normal and abnormal impulse propagation are essential in the control of arrhythmias. For example, Cx45 is predominately expressed in the specialized myocytes of the impulse generation and conduction system. In both ventricular and atrial human working myocytes, Cx45 is present in very low quantities. However, a reduction in Cx43 coupled with an increased Cx45 protein levels within the ventricles have been observed after myocardial infarction and end-stage heart failure. Cx45 may influence electrical and/or metabolic coupling as a result of pathophysiological overexpression. Our goal was to identify mechanisms that could cause cellular coupling to be different between the cardiac connexins. Based upon the conserved transmembrane and extracellular loop segments, our focus was on identifying features within the divergent cytoplasmic portions. Here, we biophysically characterize the carboxyl-terminal domain of Cx45 (Cx45CT). Purification revealed the possibility of oligomeric species, which was confirmed by analytical ultracentrifugation experiments. Sedimentation equilibrium and circular dichroism studies of different Cx45CT constructs identified one region of α-helical structure (A333-N361) that mediates CT dimerization through hydrophobic contacts. Interestingly, the binding affinity of Cx45CT dimerization is 1000-fold stronger than Cx43CT dimerization. Cx45CT resonance assignments were also used to identify the binding sites and affinities of molecular partners involved in the Cx45 regulation; although none disrupted dimerization, many of these proteins interacted within one intrinsically disordered region (P278-P285). This domain has similarities with other cardiac connexins, and we propose they constitute a master regulatory domain, which contains overlapping molecular partner binding, cis-trans proline isomerization, and phosphorylation sites.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Connexins and steroidogenesis in mouse Leydig cells

Connexin43 has been recognized as forming gap junctions in Leydig cells. However, previous work has shown that mouse Leydig cells lacking this connexin do not suffer any limitation of their ability to produce testosterone when stimulated with luteinizing hormone. The objective of this study was to identify additional connexins in mouse Leydig cells that could be required for steroidogenesis. A reverse transcription - polymerase chain reaction screen involving isolated adult Leydig cells identified connexin36 and connexin45 as expressed along with connexin43. Treatment of dissociated testes with carbenoxolone, a nonspecific blocker of gap junctional coupling, significantly reduced testosterone output as did treatment with quinine, which disrupts coupling provided by connexin36 and connexin45 gap junctions but not those composed of connexin43, indicating that either or both of connexins 36 and 45 could be involved in supporting Leydig cell steroidogenesis. Immunolabeling of adult mouse testis sections confirmed the localization of connexin36 along with connexin43 in Leydig cell gap junctions but not connexin45, which is distributed throughout the cells. It was concluded that connexin36, connexin43, and connexin45 are coexpressed in Leydig cells with connexins 36 and 43 contributing to gap junctions. The role of connexin45 remains to be elucidated.

Molecular connexin partner remodeling orchestrates connexin traffic: from physiology to pathophysiology

Connexins, through gap junctional intercellular communication, are known to regulate many physiological functions involved in developmental processes such as cell proliferation, differentiation, migration and apoptosis. Strikingly, alterations of connexin expression and trafficking are often, if not always, associated with human developmental diseases and carcinogenesis. In this respect, disrupted trafficking dynamics and aberrant intracytoplasmic localization of connexins are considered as typical features of functionality failure leading to the pathological state. Recent findings demonstrate that interactions of connexins with numerous protein partners, which take place throughout connexin trafficking, are essential for gap junction formation, membranous stabilization and degradation. In the present study, we give an overview of the physiological molecular machinery and of the specific interactions between connexins and their partners, which are involved in connexin trafficking, and we highlight their changes in pathological situations.

Gap junctional channels are parts of multiprotein complexes

Gap junctional channels are a class of membrane channels composed of transmembrane channel-forming integral membrane proteins termed connexins, innexins or pannexins that mediate direct cell-to-cell or cell-to extracellular medium communication in almost all animal tissues. The activity of these channels is tightly regulated, particularly by intramolecular modifications as phosphorylations of proteins and 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 signaling enzymes, substrates, and potential effectors (such as channels) into multiprotein signaling 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 regulations). The present review summarizes the recent progress regarding the proteins capable of interacting with junctional proteins and highlights the function of these protein-protein interactions in cell physiology and aberrant function in diseases. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and functions.

Spontaneous lung dysfunction and fibrosis in mice lacking connexin 40 and endothelial cell connexin 43

Gap junction proteins (connexins) facilitate intercellular communication and serve several roles in regulation of tissue function and remodeling. To examine the physiologic effects of depleting two prominent endothelial connexins, Cx40 and Cx43, transgenic mice were generated by breeding Cx40-deficient mice (Cx40(-/-)) with a vascular endothelial cell (VEC)-specific Cx43-deficient mouse strain (VEC Cx43(-/-)) to produce double-connexin knockout mice (VEC Cx43(-/-)/Cx40(-/-)). The life span in VEC Cx43(-/-)/Cx40(-/-) mice was dramatically shortened, which correlated with severe spontaneous lung abnormalities as the mice aged including increased fibrosis, aberrant alveolar remodeling, and increased lung fibroblast content. Moreover, VEC Cx43(-/-)/Cx40(-/-) mice exhibited cardiac hypertrophy and hypertension. Because VEC Cx43(-/-)/Cx40(-/-) mice demonstrated phenotypic hallmarks that were remarkably similar to those in mice deficient in caveolin-1, pulmonary caveolin expression was examined. Lungs from VEC Cx43(-/-)/Cx40(-/-) mice demonstrated significantly decreased expression of caveolin-1 and caveolin-2. This suggests that expression of caveolin-1 may be linked to expression of Cx40 and endothelial Cx43. Moreover, the phenotype of caveolin-1(-/-) mice and VEC Cx43(-/-)/Cx40(-/-) mice may arise via a common mechanism.

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.

Reciprocal influence of connexins and apical junction proteins on their expressions and functions

Membranes of adjacent cells form intercellular junctional complexes to mechanically anchor neighbour cells (anchoring junctions), to seal the paracellular space and to prevent diffusion of integral proteins within the plasma membrane (tight junctions) and to allow cell-to-cell diffusion of small ions and molecules (gap junctions). These different types of specialised plasma membrane microdomains, sharing common adaptor molecules, particularly zonula occludens proteins, frequently present intermingled relationships where the different proteins co-assemble into macromolecular complexes and their expressions are co-ordinately regulated. Proteins forming gap junction channels (connexins, particularly) and proteins fulfilling cell attachment or forming tight junction strands mutually influence expression and functions of one another.

Connexin hemichannel composition determines the FGF-1-induced membrane permeability and free [Ca2+]i responses

Cell surface hemichannels (HCs) composed of different connexin (Cx) types are present in diverse cells and their possible role on FGF-1-induced cellular responses remains unknown. Here, we show that FGF-1 transiently (4-14 h, maximal at 7 h) increases the membrane permeability through HCs in HeLa cells expressing Cx43 or Cx45 under physiological extracellular Ca(2+)/Mg(2+) concentrations. The effect does not occur in HeLa cells expressing HCs constituted of Cx26 or Cx43 with its C-terminus truncated at aa 257, or in parental nontransfected HeLa cells. The increase in membrane permeability is associated with a rise in HC levels at the cell surface and a proportional increase in HC unitary events. The response requires an early intracellular free Ca(2+) concentration increase, activation of a p38 MAP kinase-dependent pathway, and a regulatory site of Cx subunit C-terminus. The FGF-1-induced rise in membrane permeability is also associated with a late increase in intracellular free Ca(2+) concentration, suggesting that responsive HCs allow Ca(2+) influx. The cell density of Cx26 and Cx43 HeLa transfectants cultured in serum-free medium was differentially affected by FGF-1. Thus, the FGF-1-induced cell permeabilization and derived consequences depend on the Cx composition of HCs.

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.

Diminished zonula occludens-1 expression in the failing human heart

BACKGROUND

Reduced expression of the major gap junction protein connexin 43 (Cx43) in the failing human heart may lead to arrhythmias and sudden cardiac death. Cx43 interacts with the actin binding protein, zonula occludens-1 (ZO-1), and it has recently been demonstrated that ZO-1 regulates the formation and function of Cx43 gap junctions. We hypothesize that normal expression of ZO-1 and its interaction with Cx43 are required for appropriate assembly and function of Cx43 gap junctions in the heart. Here, we determined whether expression of ZO-1 is altered in patients with heart failure.

METHODS

We examined ventricular myocardium from hearts of patients in end-stage heart failure, obtained at transplant, for ZO-1 expression by immunohistochemistry. We also subjected lysates made from this tissue to immunoblotting to determine the level of ZO-1 expression.

RESULTS AND CONCLUSIONS

ZO-1 was found at 96% of the intercalated discs in nonfailing control human hearts, where it colocalized with Cx43. In contrast, there was ZO-1 immunostaining at 5% of intercalated discs in failing hearts, coincident with a reduction in Cx43 staining in intercalated discs. Immunoblotting analysis showed that there was a 95% reduction in ZO-1 expression in human heart failure. Loss of ZO-1 at intercalated discs in heart failure may play a critical role in remodeling of Cx43 gap junctions, which may contribute to abnormal impulse propagation and arrhythmogenesis, thereby predisposing patients in heart failure to sudden cardiac death.

Transgenic overexpression of connexin50 induces cataracts

To examine the effects of increased expression of Cx50 in the mouse lens, transgenic mice were generated using a DNA construct containing the human Cx50 coding region and a C-terminal FLAG epitope driven by the chicken betaB1-crystallin promoter. Expression of this protein in paired Xenopus oocytes induced gap junctional currents of similar magnitude to wild type human Cx50. Three lines of transgenic mice expressing the transgenic protein were analyzed. Lenses from transgenic mice were smaller than those from non-transgenic littermates, and had cataracts that were already visible at postnatal day 1. Expression of the transgene resulted in a 3- to 13-fold increase in Cx50 protein levels above those of non-transgenic animals. Light microscopy revealed alterations in epithelial cell differentiation, fiber cell structure, interactions between fiber cells and areas of liquefaction. Scanning electron microscopy showed fiber cells of varying widths with bulging areas along single fibers. Anti-Cx50 and anti-FLAG immunoreactivities were detected at appositional membranes and in intracellular vesicles in transgenic lenses. N-cadherin, Cx46, ZO-1 and aquaporin 0 localized mainly at the plasma membrane, although some N-cadherin and aquaporin 0 was associated with the intracellular vesicles. The abundance and solubility/integrity of alphaA-, alphaB-, beta- and gamma-crystallin were unaffected. These results demonstrate that transgenic expression of Cx50 in mice leads to cataracts associated with formation of cytoplasmic vesicles containing Cx50 and decreased or slowed epithelial differentiation without major alterations in the distribution of other integral membrane or membrane-associated proteins or the integrity/solubility of crystallins.