Multisubunit assembly of an integral plasma membrane channel protein, gap junction connexin43, occurs after exit from the ER

Intralumenal docking of connexin 36 channels in the ER isolates mistrafficked protein

The intracellular domains of connexins are essential for the assembly of gap junctions. For connexin 36 (Cx36), the major neuronal connexin, it has been shown that a dysfunctional PDZ-binding motif interferes with electrical synapse formation. However, it is still unknown how this motif coordinates the transport of Cx36. In the present study, we characterize a phenotype of Cx36 mutants that lack a functional PDZ-binding motif using HEK293T cells as an expression system. We provide evidence that an intact PDZ-binding motif is critical for proper endoplasmic reticulum (ER) export of Cx36. Removing the PDZ-binding motif of Cx36 results in ER retention and the formation of multimembrane vesicles containing gap junction-like connexin aggregates. Using a combination of site-directed mutagenesis and electron micrographs, we reveal that these vesicles consist of Cx36 channels that docked prematurely in the ER. Our data suggest a model in which ER-retained Cx36 channels reshape the ER membrane into concentric whorls that are released into the cytoplasm.

Functional Consequences of Pathogenic Variants of the GJB2 Gene (Cx26) Localized in Different Cx26 Domains

One of the most common forms of genetic deafness has been predominantly associated with pathogenic variants in the GJB2 gene, encoding transmembrane protein connexin 26 (Cx26). The Cx26 molecule consists of an N-terminal domain (NT), four transmembrane domains (TM1-TM4), two extracellular loops (EL1 and EL2), a cytoplasmic loop, and a C-terminus (CT). Pathogenic variants in the GJB2 gene, resulting in amino acid substitutions scattered across the Cx26 domains, lead to a variety of clinical outcomes, including the most common non-syndromic autosomal recessive deafness (DFNB1A), autosomal dominant deafness (DFNA3A), as well as syndromic forms combining hearing loss and skin disorders. However, for rare and poorly documented variants, information on the mode of inheritance is often lacking. Numerous in vitro studies have been conducted to elucidate the functional consequences of pathogenic GJB2 variants leading to amino acid substitutions in different domains of Cx26 protein. In this work, we summarized all available data on a mode of inheritance of pathogenic GJB2 variants leading to amino acid substitutions and reviewed published information on their functional effects, with an emphasis on their localization in certain Cx26 domains.

Endocytic trafficking of connexins in cancer pathogenesis

Gap junctions are specialized regions of the plasma membrane containing clusters of channels that provide for the diffusion of ions and small molecules between adjacent cells. A fundamental role of gap junctions is to coordinate the functions of cells in tissues. Cancer pathogenesis is usually associated with loss of intercellular communication mediated by gap junctions, which may affect tumor growth and the response to radio- and chemotherapy. Gap junction channels consist of integral membrane proteins termed connexins. In addition to their canonical roles in cell-cell communication, connexins modulate a range of signal transduction pathways via interactions with proteins such as β-catenin, c-Src, and PTEN. Consequently, connexins can regulate cellular processes such as cell growth, migration, and differentiation through both channel-dependent and independent mechanisms. Gap junctions are dynamic plasma membrane entities, and by modulating the rate at which connexins undergo endocytosis and sorting to lysosomes for degradation, cells rapidly adjust the level of gap junctions in response to alterations in the intracellular or extracellular milieu. Current experimental evidence indicates that aberrant trafficking of connexins in the endocytic system is intrinsically involved in mediating the loss of gap junctions during carcinogenesis. This review highlights the role played by the endocytic system in controlling connexin degradation, and consequently gap junction levels, and discusses how dysregulation of these processes contributes to the loss of gap junctions during cancer development. We also discuss the therapeutic implications of aberrant endocytic trafficking of connexins in cancer cells.

Pattern Distribution of Connexins in the Ortho- and Parakeratinized Epithelium of the Lingual Mucosa in Birds

Connexins are important proteins involved in cell-to-cell communication and cytodifferentiation during renewal and cornification of the multilayered epithelia. So far, there is a lack of reports on this subject in birds' structurally different ortho- and parakeratinized epithelium of the tongue. The study aims to describe the distribution and expression profiles of the α-connexins (Cx40 and 43) and β-connexins (Cx26, 30, and 31) in those epithelia in duck, goose, and domestic turkey. Research revealed the presence of the mentioned connexins and the occurrence of interspecies differences. Connexins form gap junctions in the cell membrane or are in the cytoplasm of keratinocytes. Differences in connexin expression were noted between the basal and intermediate layers, which may determine the proliferation of keratinocytes. Cx40, 43, and Cx30 in the gap junction of the keratinocytes of the intermediate layer are related to the synchronization of the cornification process. Because of the exfoliation of cornified plaques, a lack of connexins was observed in the cornified layer of orthokeratinized epithelium. However, in parakeratinized epithelium, connexins were present in the cell membrane of keratinocytes and thus maintained cellular integrity in gradually desquamating cells. The current studies will be useful in further comparative analyses of normal and pathological epithelia of the oral cavity in birds.

The human discs large protein 1 interacts with and maintains connexin 43 at the plasma membrane in keratinocytes

Gap junction channels, composed of connexins, allow direct cell-to-cell communication. Connexin 43 (Cx43; also known as GJA1) is widely expressed in tissues, including the epidermis. In a previous study of human papillomavirus-positive cervical epithelial tumour cells, we identified Cx43 as a binding partner of the human homologue of Drosophila Discs large (Dlg1; also known as SAP97). Dlg1 is a member of the membrane associated-guanylate kinase (MAGUK) scaffolding protein family, which is known to control cell shape and polarity. Here, we show that Cx43 also interacts with Dlg1 in uninfected keratinocytes in vitro and in keratinocytes, dermal cells and adipocytes in normal human epidermis in vivo. Depletion of Dlg1 in keratinocytes did not alter Cx43 transcription but was associated with a reduction in Cx43 protein levels. Reduced Dlg1 levels in keratinocytes resulted in a reduction in Cx43 at the plasma membrane with a concomitant reduction in gap junctional intercellular communication and relocation of Cx43 to the Golgi compartment. Our data suggest a key role for Dlg1 in maintaining Cx43 at the plasma membrane in keratinocytes.

Potential Role of Fenestrated Septa in Axonal Transport of Golgi Cisternae and Gap Junction Formation/Function

Crayfish axons contain a system of parallel membranous cisternae spaced by ~2 μm and oriented perpendicularly to the axon’s long axis. Each cisterna is composed of two roughly parallel membranes, separated by a 150–400 Å wide space. The cisternae are interrupted by 500–600 Å pores, each occupied by a microtubule. Significantly, filaments, likely made of kinesin, often bridge the gap between the microtubule and the edge of the pore. Neighboring cisternae are linked by longitudinal membranous tubules. In small axons, the cisternae seem to be continuous across the axon, while in large axons they are intact only at the axon’s periphery. Due to the presence of pores, we have named these structures “Fenestrated Septa” (FS). Similar structures are also present in vertebrates, including mammals, proving that they are widely expressed in the animal kingdom. We propose that FS are components of the “anterograde transport” mechanism that moves cisternae of the Golgi apparatus (GA) toward the nerve ending by means of motor proteins, likely to be kinesins. In crayfish lateral giant axons, we believe that vesicles that bud off FS at the nerve ending contain gap junction hemichannels (innexons) for gap junction channel and hemichannel formation and function.

Regulatory role of endoplasmic reticulum resident chaperone protein ERp29 in anti-murine β-coronavirus host cell response

Gap junctional intercellular communication (GJIC) involving astrocytes is important for proper CNS homeostasis. As determined in our previous studies, trafficking of the predominant astrocyte GJ protein, Connexin43 (Cx43), is disrupted in response to infection with a neurotropic murine β-coronavirus (MHV-A59). However, how host factors are involved in Cx43 trafficking and the infection response is not clear. Here, we show that Cx43 retention due to MHV-A59 infection was associated with increased ER stress and reduced expression of chaperone protein ERp29. Treatment of MHV-A59-infected astrocytes with the chemical chaperone 4-sodium phenylbutyrate increased ERp29 expression, rescued Cx43 transport to the cell surface, increased GJIC, and reduced ER stress. We obtained similar results using an astrocytoma cell line (delayed brain tumor) upon MHV-A59 infection. Critically, delayed brain tumor cells transfected to express exogenous ERp29 were less susceptible to MHV-A59 infection and showed increased Cx43-mediated GJIC. Treatment with Cx43 mimetic peptides inhibited GJIC and increased viral susceptibility, demonstrating a role for intercellular communication in reducing MHV-A59 infectivity. Taken together, these results support a therapeutically targetable ERp29-dependent mechanism where β-coronavirus infectivity is modulated by reducing ER stress and rescuing Cx43 trafficking and function.

Connexins and Glucose Metabolism in Cancer

Connexins are a family of transmembrane proteins that regulate diverse cellular functions. Originally characterized for their ability to mediate direct intercellular communication through the formation of highly regulated membrane channels, their functions have been extended to the exchange of molecules with the extracellular environment, and the ability to modulate numerous channel-independent effects on processes such as motility and survival. Notably, connexins have been implicated in cancer biology for their context-dependent roles that can both promote or suppress cancer cell function. Moreover, connexins are able to mediate many aspects of cellular metabolism including the intercellular coupling of nutrients and signaling molecules. During cancer progression, changes to substrate utilization occur to support energy production and biomass accumulation. This results in metabolic plasticity that promotes cell survival and proliferation, and can impact therapeutic resistance. Significant progress has been made in our understanding of connexin and cancer biology, however, delineating the roles these multi-faceted proteins play in metabolic adaptation of cancer cells is just beginning. Glucose represents a major carbon substrate for energy production, nucleotide synthesis, carbohydrate modifications and generation of biosynthetic intermediates. While cancer cells often exhibit a dependence on glycolytic metabolism for survival, cellular reprogramming of metabolic pathways is common when blood perfusion is limited in growing tumors. These metabolic changes drive aggressive phenotypes through the acquisition of functional traits. Connections between glucose metabolism and connexin function in cancer cells and the surrounding stroma are now apparent, however much remains to be discovered regarding these relationships. This review discusses the existing evidence in this area and highlights directions for continued investigation.

PDGF-AA promotes gap junction intercellular communication in chondrocytes via the PI3K/Akt pathway

BACKGROUND

Gap junction intercellular communication (GJIC) plays an important role in cell growth, development and homeostasis. Connexin 43 (Cx43) is an important half-channel protein responsible for gap junction formation. Platelet-derived growth factor AA (PDGF-AA) regulates the proliferation, migration, metabolism, apoptosis and cell cycle of chondrocytes. However, the role of PDGF-AA in gap junction intercellular communication in chondrocytes is not fully understood. In the current study, we performed experiments to explore the effect of PDGF-AA on GJIC and its underlying biomechanical mechanism.

METHODS

qPCR was performed to determine the expression of PDGF, PDGFR and connexin family genes in chondrocytes and/or cartilage. A scrape loading/dye transfer assay was used to determine GJIC. Western blot analysis was applied to detect the expression of Cx43 and PI3K/Akt signaling pathway proteins. Immunofluorescence staining was utilized to examine protein distribution. Scanning electron microscopy was used to delineate the morphology of chondrocytes.

RESULTS

Expression of PDGF-A mRNA was highest among the PDGF family in chondrocytes and cartilage tissues. PDGF-AA promoted functional GJIC formation in chondrocytes by upregulating the expression of Cx43. Enhanced functional GJIC formation in chondrocytes induced by PDGF-AA occurred through the activation of PI3K/Akt signaling and its nuclear accumulation.

CONCLUSION

For the first time, this study provides evidence demonstrating the role of PDGF-AA in cell-to-cell communication in chondrocytes through mediating Cx43 expression.

The Structural and the Functional Aspects of Intercellular Communication in iPSC-Cardiomyocytes

Recent advances in the technology of producing novel cardiomyocytes from induced pluripotent stem cells (iPSC-cardiomyocytes) fuel new hope for future clinical applications. The use of iPSC-cardiomyocytes is particularly promising for the therapy of cardiac diseases such as myocardial infarction, where these cells could replace scar tissue and restore the functionality of the heart. Despite successful cardiogenic differentiation, medical applications of iPSC-cardiomyocytes are currently limited by their pronounced immature structural and functional phenotype. This review focuses on gap junction function in iPSC-cardiomyocytes and portrays our current understanding around the structural and the functional limitations of intercellular coupling and viable cardiac graft formation involving these novel cardiac muscle cells. We further highlight the role of the gap junction protein connexin 43 as a potential target for improving cell–cell communication and electrical signal propagation across cardiac tissue engineered from iPSC-cardiomyocytes. Better insight into the mechanisms that promote functional intercellular coupling is the foundation that will allow the development of novel strategies to combat the immaturity of iPSC-cardiomyocytes and pave the way toward cardiac tissue regeneration.

Cx43 hemichannels contribute to astrocyte-mediated toxicity in sporadic and familial ALS

Our results demonstrate that connexin 43 hemichannels are the conduits for amyotrophic lateral sclerosis (ALS) astrocyte-mediated motor neuron toxicity and disease spread, acting as a common mechanism that can target both familial ALS and sporadic ALS populations. Furthermore, our present work provides proof of principle that tonabersat, as a drug already studied in clinical trials for other indications, could serve as a potential ALS therapeutic.

Connexin 43 (Cx43) gap junctions and hemichannels mediate astrocyte intercellular communication in the central nervous system under normal conditions and contribute to astrocyte-mediated neurotoxicity in amyotrophic lateral sclerosis (ALS). Here, we show that astrocyte-specific knockout of Cx43 in a mouse model of ALS slows disease progression both spatially and temporally, provides motor neuron (MN) protection, and improves survival. In addition, Cx43 expression is up-regulated in human postmortem tissue and cerebrospinal fluid from ALS patients. Using human induced pluripotent stem cell–derived astrocytes (hiPSC-A) from both familial and sporadic ALS, we establish that Cx43 is up-regulated and that Cx43-hemichannels are enriched at the astrocyte membrane. We also demonstrate that the pharmacological blockade of Cx43-hemichannels in ALS astrocytes using GAP 19, a mimetic peptide blocker, and tonabersat, a clinically tested small molecule, provides neuroprotection of hiPSC-MN and reduces ALS astrocyte-mediated neuronal hyperexcitability. Extending the in vitro application of tonabersat with chronic administration to SOD1^G93A mice results in MN protection with a reduction in reactive astrocytosis and microgliosis. Taking these data together, our studies identify Cx43 hemichannels as conduits of astrocyte-mediated disease progression and a pharmacological target for disease-modifying ALS therapies.

GJA1-20k and Mitochondrial Dynamics

Connexin 43 (Cx43) is the primary gap junction protein of mammalian heart ventricles and is encoded by the gene Gja1 which has a single coding exon and therefore cannot be spliced. We previously identified that Gja1 mRNA undergoes endogenous internal translation initiated at one of several internal AUG (M) start codons, generating N-terminal truncated protein isoforms that retain the C-terminus distal to the start site. GJA1-20k, whose translation initiates at mRNA M213, is usually the most abundant isoform in cells and greatly increases after ischemic and metabolic stress. GJA1-20k consists of a small segment of the last transmembrane domain and the complete C-terminus tail of Cx43, with a total size of about 20 kDa. The original role identified for GJA1-20k is as an essential subunit that facilitates the trafficking of full-length Cx43 hexameric hemichannels to cell-cell contacts, generating traditional gap junctions between adjacent cells facilitating, in cardiac muscle, efficient spread of electrical excitation. GJA1-20k deficient mice (generated by a M213L substitution in Gja1) suffer poor electrical coupling between cardiomycytes and arrhythmogenic sudden death two to 4 weeks after their birth. We recently identified that exogenous GJA1-20k expression also mimics the effect of ischemic preconditioning in mouse heart. Furthermore, GJA1-20k localizes to the mitochondrial outer membrane and induces a protective and DRP1 independent form of mitochondrial fission, preserving ATP production and generating less reactive oxygen species (ROS) under metabolic stress, providing powerful protection of myocardium to ischemic insult. In this manuscript, we focus on the detailed roles of GJA1-20k in mitochondria, and its interaction with the actin cytoskeleton.

Gap Junction-Dependent and -Independent Functions of Connexin43 in Biology

For the first time in animal evolution, the emergence of gap junctions allowed direct exchanges of cellular substances for communication between two cells. Innexin proteins constituted primordial gap junctions until the connexin protein emerged in deuterostomes and took over the gap junction function. After hundreds of millions of years of gene duplication, the connexin gene family now comprises 21 members in the human genome. Notably, GJA1, which encodes the Connexin43 protein, is one of the most widely expressed and commonly studied connexin genes. The loss of Gja1 in mice leads to swelling and a blockage of the right ventricular outflow tract and death of the embryos at birth, suggesting a vital role of Connexin43 gap junction in heart development. Since then, the importance of Connexin43-mediated gap junction function has been constantly expanded to other types of cells. Other than forming gap junctions, Connexin43 can also form hemichannels to release or uptake small molecules from the environment or even mediate many physiological processes in a gap junction-independent manner on plasma membranes. Surprisingly, Connexin43 also localizes to mitochondria in the cell, playing important roles in mitochondrial potassium import and respiration. At the molecular level, Connexin43 mRNA and protein are processed with very distinct mechanisms to yield carboxyl-terminal fragments with different sizes, which have their unique subcellular localization and distinct biological activities. Due to many exciting advancements in Connexin43 research, this review aims to start with a brief introduction of Connexin43 and then focuses on updating our knowledge of its gap junction-independent functions.

The Role of Connexin Hemichannels in Inflammatory Diseases

The connexin protein family consists of approximately 20 members, and is well recognized as the structural unit of the gap junction channels that perforate the plasma membranes of coupled cells and, thereby, mediate intercellular communication. Gap junctions are assembled by two preexisting hemichannels on the membranes of apposing cells. Non-junctional connexin hemichannels (CxHC) provide a conduit between the cell interior and the extracellular milieu, and are believed to be in a protectively closed state under physiological conditions. The development and characterization of the peptide mimetics of the amino acid sequences of connexins have resulted in the development of a panel of blockers with a higher selectivity for CxHC, which have become important tools for defining the role of CxHC in various biological processes. It is increasingly clear that CxHC can be induced to open by pathogen-associated molecular patterns. The opening of CxHC facilitates the release of damage-associated molecular patterns, a class of endogenous molecules that are critical for the pathogenesis of inflammatory diseases. The blockade of CxHC leads to attenuated inflammation, reduced tissue injury and improved organ function in human and animal models of about thirty inflammatory diseases and disorders. These findings demonstrate that CxHC may contribute to the intensification of inflammation, and serve as a common target in the treatments of various inflammatory diseases. In this review, we provide an update on the progress in the understanding of CxHC, with a focus on the role of these channels in inflammatory diseases.

Connexin hemichannel inhibition ameliorates epidermal pathology in a mouse model of keratitis ichthyosis deafness syndrome

Mutations in five different genes encoding connexin channels cause eleven clinically defined human skin diseases. Keratitis ichthyosis deafness (KID) syndrome is caused by point mutations in the GJB2 gene encoding Connexin 26 (Cx26) which result in aberrant activation of connexin hemichannels. KID syndrome has no cure and is associated with bilateral hearing loss, blinding keratitis, palmoplantar keratoderma, ichthyosiform erythroderma and a high incidence of childhood mortality. Here, we have tested whether a topically applied hemichhanel inhibitor (flufenamic acid, FFA) could ameliorate the skin pathology associated with KID syndrome in a transgenic mouse model expressing the lethal Cx26-G45E mutation. We found that FFA blocked the hemichannel activity of Cx26-G45E in vitro, and substantially reduced epidermal pathology in vivo, compared to untreated, or vehicle treated control animals. FFA did not reduce the expression of mutant connexin hemichannel protein, and cessation of FFA treatment allowed disease progression to continue. These results suggested that aberrant hemichannel activity is a major driver of skin disease in KID syndrome, and that the inhibition of mutant hemichannel activity could provide an attractive target to develop novel therapeutic interventions to treat this incurable disease.

Remodeling of Cardiac Gap Junctional Cell-Cell Coupling

The heart works as a functional syncytium, which is realized via cell-cell coupling maintained by gap junction channels. These channels connect two adjacent cells, so that action potentials can be transferred. Each cell contributes a hexameric hemichannel (=connexon), formed by protein subuntis named connexins. These hemichannels dock to each other and form the gap junction channel. This channel works as a low ohmic resistor also allowing the passage of small molecules up to 1000 Dalton. Connexins are a protein family comprising of 21 isoforms in humans. In the heart, the main isoforms are Cx43 (the 43 kDa connexin; ubiquitous), Cx40 (mostly in atrium and specific conduction system), and Cx45 (in early developmental states, in the conduction system, and between fibroblasts and cardiomyocytes). These gap junction channels are mainly located at the polar region of the cardiomyocytes and thus contribute to the anisotropic pattern of cardiac electrical conductivity. While in the beginning the cell–cell coupling was considered to be static, similar to an anatomically defined structure, we have learned in the past decades that gap junctions are also subject to cardiac remodeling processes in cardiac disease such as atrial fibrillation, myocardial infarction, or cardiomyopathy. The underlying remodeling processes include the modulation of connexin expression by e.g., angiotensin, endothelin, or catecholamines, as well as the modulation of the localization of the gap junctions e.g., by the direction and strength of local mechanical forces. A reduction in connexin expression can result in a reduced conduction velocity. The alteration of gap junction localization has been shown to result in altered pathways of conduction and altered anisotropy. In particular, it can produce or contribute to non-uniformity of anisotropy, and thereby can pre-form an arrhythmogenic substrate. Interestingly, these remodeling processes seem to be susceptible to certain pharmacological treatment.

Cysteine residues in the C-terminal tail of connexin32 regulate its trafficking

Gap junctions (GJs) are formed by the assembly of constituent transmembrane proteins called connexins (Cxs). Aberrations in this assembly of Cxs are observed in several genetic diseases as well as in cancers. Hence it becomes imperative to understand the molecular mechanisms underlying such assembly defect. The polarized cells in the epithelia express Connexin32 (Cx32). The C-terminal tail (CT) of Cx32 orchestrates several aspects of GJ dynamics, function and growth. The study here was aimed at determining if post-translational modifications, specifically, palmitoylation of cysteine residues, present in the CT of Cx32, has any effect on GJ assembly. The CT of Cx32 was found to harbor three cysteine residues, which are likely to be modified by palmitoylation. The study here has revealed for the first time that Cx32 is palmitoylated at cysteine 217 (C217) in cell line derived from prostate tumors. However, it was found that mutating C217 to alanine affected neither the trafficking nor the ability of Cx32 to assemble into GJs. Intriguingly, it was discovered that mutating cysteine 280 and 283, only in combination, blocked the trafficking of Cx32 from the trans-Golgi network to the cell surface. The mutants showed reduced stability due to enhanced lysosomal degradation. Overall, the findings reveal the importance of the two C-terminal cysteine residues of Cx32 in regulating its trafficking and stability and hence its ability to assemble into GJs.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Connexin hemichannel inhibitors with a focus on aminoglycosides

Connexins are membrane proteins involved directly in cell-to-cell communication through the formation of gap-junctional channels. These channels result from the head-to-head docking of two hemichannels, one from each of two adjacent cells. Undocked hemichannels are also present at the plasma membrane where they mediate the efflux of molecules that participate in autocrine and paracrine signaling, but abnormal increase in hemichannel activity can lead to cell damage in disorders such as cardiac infarct, stroke, deafness, cataracts, and skin diseases. For this reason, connexin hemichannels have emerged as a valid therapeutic target. Know small molecule hemichannel inhibitors are not ideal leads for the development of better drugs for clinical use because they are not specific and/or have toxic effects. Newer inhibitors are more selective and include connexin mimetic peptides, anti-connexin antibodies and drugs that reduce connexin expression such as antisense oligonucleotides. Re-purposed drugs and their derivatives are also promising because of the significant experience with their clinical use. Among these, aminoglycoside antibiotics have been identified as inhibitors of connexin hemichannels that do not inhibit gap-junctional channels. In this review, we discuss connexin hemichannels and their inhibitors, with a focus on aminoglycoside antibiotics and derivatives of kanamycin A that inhibit connexin hemichannels, but do not have antibiotic effect.

Src Regulation of Cx43 Phosphorylation and Gap Junction Turnover

The gap junction protein Connexin43 (Cx43) is highly regulated by phosphorylation at over a dozen sites by probably at least as many kinases. This Cx43 “kinome” plays an important role in gap junction assembly and turnover. We sought to gain a better understanding of the interrelationship of these phosphorylation events particularly related to src activation and Cx43 turnover. Using state-of-the-art live imaging methods, specific inhibitors and many phosphorylation-status specific antibodies, we found phospho-specific domains in gap junction plaques and show evidence that multiple pathways of disassembly exist and can be regulated at the cellular and subcellular level. We found Src activation promotes formation of connexisomes (internalized gap junctions) in a process involving ERK-mediated phosphorylation of S279/282. Proteasome inhibition dramatically and rapidly restored gap junctions in the presence of Src and led to dramatic changes in the Cx43 phospho-profile including to increased Y247, Y265, S279/282, S365, and S373 phosphorylation. Lysosomal inhibition, on the other hand, nearly eliminated phosphorylation on Y247 and Y265 and reduced S368 and S373 while increasing S279/282 phosphorylation levels. We present a model of gap junction disassembly where multiple modes of disassembly are regulated by phosphorylation and can have differential effects on cellular signaling.

The Probable, Possible, and Novel Functions of ERp29

The luminal endoplasmic reticulum (ER) protein of 29 kDa (ERp29) is a ubiquitously expressed cellular agent with multiple critical roles. ERp29 regulates the biosynthesis and trafficking of several transmembrane and secretory proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial sodium channel (ENaC), thyroglobulin, connexin 43 hemichannels, and proinsulin. ERp29 is hypothesized to promote ER to cis-Golgi cargo protein transport via COP II machinery through its interactions with the KDEL receptor; this interaction may facilitate the loading of ERp29 clients into COP II vesicles. ERp29 also plays a role in ER stress (ERS) and the unfolded protein response (UPR) and is implicated in oncogenesis. Here, we review the vast array of ERp29’s clients, its role as an ER to Golgi escort protein, and further suggest ERp29 as a potential target for therapies related to diseases of protein misfolding and mistrafficking.

Amyloid-β regulates gap junction protein connexin 43 trafficking in cultured primary astrocytes

Altered expression and function of astroglial gap junction protein connexin 43 (Cx43) has increasingly been associated to neurotoxicity in Alzheimer disease (AD). Although earlier studies have examined the effect of increased β-amyloid (Aβ) on Cx43 expression and function leading to neuronal damage, underlying mechanisms by which Aβ modulates Cx43 in astrocytes remain elusive. Here, using mouse primary astrocyte cultures, we have examined the cellular processes by which Aβ can alter Cx43 gap junctions. We show that Aβ_25-35 impairs functional gap junction coupling yet increases hemichannel activity. Interestingly, Aβ_25-35 increased the intracellular pool of Cx43 with a parallel decrease in gap junction assembly at the surface. Intracellular Cx43 was found to be partly retained in the endoplasmic reticulum-associated cell compartments. However, forward trafficking of the newly synthesized Cx43 that already reached the Golgi was not affected in Aβ_25-35-exposed astrocytes. Supporting this, treatment with 4-phenylbutyrate, a well-known chemical chaperone that improves trafficking of several transmembrane proteins, restored Aβ-induced impaired gap junction coupling between astrocytes. We further show that interruption of Cx43 endocytosis in Aβ_25-35-exposed astrocytes resulted in their retention at the cell surface in the form of functional gap junctions indicating that Aβ_25-35 causes rapid internalization of Cx43 gap junctions. Additionally, in silico molecular docking suggests that Aβ can bind favorably to Cx43. Our study thus provides novel insights into the cellular mechanisms by which Aβ modulates Cx43 function in astrocytes, the basic understanding of which is vital for the development of alternative therapeutic strategy targeting connexin channels in AD.

Regulation of gap junction intercellular communication by connexin ubiquitination: physiological and pathophysiological implications

Gap junctions consist of arrays of intercellular channels that enable adjacent cells to communicate both electrically and metabolically. Gap junctions have a wide diversity of physiological functions, playing critical roles in both excitable and non-excitable tissues. Gap junction channels are formed by integral membrane proteins called connexins. Inherited or acquired alterations in connexins are associated with numerous diseases, including heart failure, neuropathologies, deafness, skin disorders, cataracts and cancer. Gap junctions are highly dynamic structures and by modulating the turnover rate of connexins, cells can rapidly alter the number of gap junction channels at the plasma membrane in response to extracellular or intracellular cues. Increasing evidence suggests that ubiquitination has important roles in the regulation of endoplasmic reticulum-associated degradation of connexins as well as in the modulation of gap junction endocytosis and post-endocytic sorting of connexins to lysosomes. In recent years, researchers have also started to provide insights into the physiological roles of connexin ubiquitination in specific tissue types. This review provides an overview of the advances made in understanding the roles of connexin ubiquitination in the regulation of gap junction intercellular communication and discusses the emerging physiological and pathophysiological implications of these processes.

Gap Junctions in the Bone Marrow Lympho-Hematopoietic Stem Cell Niche, Leukemia Progression, and Chemoresistance

Abstract: The crosstalk between hematopoietic stem cells (HSC) and bone marrow (BM) microenvironment is critical for homeostasis and hematopoietic regeneration in response to blood formation emergencies after injury, and has been associated with leukemia transformation and progression. Intercellular signals by the BM stromal cells in the form of cell-bound or secreted factors, or by physical interaction, regulate HSC localization, maintenance, and differentiation within increasingly defined BM HSC niches. Gap junctions (GJ) are comprised of arrays of membrane embedded channels formed by connexin proteins, and control crucial signaling functions, including the transfer of ions, small metabolites, and organelles to adjacent cells which affect intracellular mechanisms of signaling and autophagy. This review will discuss the role of GJ in both normal and leukemic hematopoiesis, and highlight some of the most novel approaches that may improve the efficacy of cytotoxic drugs. Connexin GJ channels exert both cell-intrinsic and cell-extrinsic effects on HSC and BM stromal cells, involved in regenerative hematopoiesis after myelosuppression, and represent an alternative system of cell communication through a combination of electrical and metabolic coupling as well as organelle transfer in the HSC niche. GJ intercellular communication (GJIC) in the HSC niche improves cellular bioenergetics, and rejuvenates damaged recipient cells. Unfortunately, they can also support leukemia proliferation and survival by creating leukemic niches that provide GJIC dependent energy sources and facilitate chemoresistance and relapse. The emergence of new strategies to disrupt self-reinforcing malignant niches and intercellular organelle exchange in leukemic niches, while at the same time conserving normal hematopoietic GJIC function, could synergize the effect of chemotherapy drugs in eradicating minimal residual disease. An improved understanding of the molecular basis of connexin regulation in normal and leukemic hematopoiesis is warranted for the re-establishment of normal hematopoiesis after chemotherapy.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Concatenation of Human Connexin26 (hCx26) and Human Connexin46 (hCx46) for the Analysis of Heteromeric Gap Junction Hemichannels and Heterotypic Gap Junction Channels

Gap junction channels and hemichannels formed by concatenated connexins were analyzed. Monomeric (hCx26, hCx46), homodimeric (hCx46-hCx46, hCx26-hCx26), and heterodimeric (hCx26-hCx46, hCx46-hCx26) constructs, coupled to GFP, were expressed in HeLa cells. Confocal microscopy showed that the tandems formed gap junction plaques with a reduced plaque area compared to monomeric hCx26 or hCx46. Dye transfer experiments showed that concatenation allows metabolic transfer. Expressed in Xenopus oocytes, the inside-out patch-clamp configuration showed single channels with a conductance of about 46 pS and 39 pS for hemichannels composed of hCx46 and hCx26 monomers, respectively, when chloride was replaced by gluconate on both membrane sides. The conductance was reduced for hCx46-hCx46 and hCx26-hCx26 homodimers, probably due to the concatenation. Heteromerized hemichannels, depending on the connexin-order, were characterized by substates at 26 pS and 16 pS for hCx46-hCx26 and 31 pS and 20 pS for hCx26-hCx46. Because of the linker between the connexins, the properties of the formed hemichannels and gap junction channels (e.g., single channel conductance) may not represent the properties of hetero-oligomerized channels. However, should the removal of the linker be successful, this method could be used to analyze the electrical and metabolic selectivity of such channels and the physiological consequences for a tissue.

The interplay between electrical and chemical synaptogenesis

Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons are critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both of these classes of synapses. In addition, electrical and chemical synapses can mutually coregulate each other’s formation. Electrical activity, generated spontaneously by the nervous system or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.

Interaction of arsenic with gap junction protein connexin 43 alters gap junctional intercellular communication

Chronic exposure to Arsenic pollution in ground water is one of the largest environmental health disasters in the world. The toxicity of trivalent Arsenicals primarily happens due to its interaction with sulfhydryl groups in proteins. Arsenic binding to the protein can change the conformation of the protein and alter its interactions with other proteins leading to tissue damage. Therefore, much importance has been given to the studies of Arsenic bound proteins, for the purpose of understanding the origins of toxicity and to explore therapeutics. Here we study the dynamic effect of Arsenic on Connexin 43 (Cx43), a protein that forms the gap junctions, whose alteration deeply perturbs the cell-to-cell communication vital for maintaining tissue homeostasis. In silico molecular modelling and in vitro studies comparing Arsenic treated and untreated conditions show distinct results. Gap junction communication is severely disrupted by Arsenic due to reduced availability of unaltered Cx43 in the membrane bound form. In silico and Inductively Coupled Plasma Mass Spectrometry studies revealed the interaction of Arsenic to the Cx43 preferably occurs through surface exposed cysteines, thereby capping the thiol groups that form disulfide bonds in the tertiary structure. This leads to disruption of Cx43 oligomerization, and altered Cx43 is incompetent for transportation to the membrane surface, often forming aggregates primarily localizing in the endoplasmic reticulum. Loss of functional Cx43 on the cell surface have a deleterious effect on cellular homeostasis leading to selective vulnerability to cell death and tissue damage.

Connexin43 Carboxyl-Terminal Domain Directly Interacts with β-Catenin

Activation of Wnt signaling induces Connexin43 (Cx43) expression via the transcriptional activity of β-catenin, and results in the enhanced accumulation of the Cx43 protein and the formation of gap junction channels. In response to Wnt signaling, β-catenin co-localizes with the Cx43 protein itself as part of a complex at the gap junction plaque. Work from several labs have also shown indirect evidence of this interaction via reciprocal co-immunoprecipitation. Our goal for the current study was to identify whether β-catenin directly interacts with Cx43, and if so, the location of that direct interaction. Identifying residues involved in direct protein⁻protein interaction is of importance when they are correlated to the phosphorylation of Cx43, as phosphorylation can modify the binding affinities of Cx43 regulatory protein partners. Therefore, combining the location of a protein partner interaction on Cx43 along with the phosphorylation pattern under different homeostatic and pathological conditions will be crucial information for any potential therapeutic intervention. Here, we identified that β-catenin directly interacts with the Cx43 carboxyl-terminal domain, and that this interaction would be inhibited by the Src phosphorylation of Cx43CT residues Y265 and Y313.

Protein⁻Protein Interactions with Connexin 43: Regulation and Function

Connexins are integral membrane building blocks that form gap junctions, enabling direct cytoplasmic exchange of ions and low-molecular-mass metabolites between adjacent cells. In the heart, gap junctions mediate the propagation of cardiac action potentials and the maintenance of a regular beating rhythm. A number of connexin interacting proteins have been described and are known gap junction regulators either through direct effects (e.g., kinases) or the formation of larger multifunctional complexes (e.g., cytoskeleton scaffold proteins). Most connexin partners can be categorized as either proteins promoting coupling by stimulating forward trafficking and channel opening or inhibiting coupling by inducing channel closure, internalization, and degradation. While some interactions have only been implied through co-localization using immunohistochemistry, others have been confirmed by biophysical methods that allow detection of a direct interaction. Our understanding of these interactions is, by far, most well developed for connexin 43 (Cx43) and the scope of this review is to summarize our current knowledge of their functional and regulatory roles. The significance of these interactions is further exemplified by demonstrating their importance at the intercalated disc, a major hub for Cx43 regulation and Cx43 mediated effects.

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.

Altered translation initiation of Gja1 limits gap junction formation during epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is activated during development, wound healing, and pathologies including fibrosis and cancer metastasis. Hallmarks of EMT are remodeling of intercellular junctions and adhesion proteins, including gap junctions. The GJA1 mRNA transcript encoding the gap junction protein connexin43 (Cx43) has been demonstrated to undergo internal translation initiation, yielding truncated isoforms that modulate gap junctions. The PI3K/Akt/mTOR pathway is central to translation regulation and is activated during EMT, leading us to hypothesize that altered translation initiation would contribute to gap junction loss. Using TGF-β-induced EMT as a model, we find reductions in Cx43 gap junctions despite increased transcription and stabilization of Cx43 protein. Biochemical experiments reveal suppression of the internally translated Cx43 isoform, GJA1-20k in a Smad3 and ERK-dependent manner. Ectopic expression of GJA1-20k does not halt EMT, but is sufficient to rescue gap junction formation. GJA1-20k localizes to the Golgi apparatus, and using superresolution localization microscopy we find retention of GJA1-43k at the Golgi in mesenchymal cells lacking GJA1-20k. NativePAGE demonstrates that levels of GJA1-20k regulate GJA1-43k hexamer oligomerization, a limiting step in Cx43 trafficking. These findings reveal alterations in translation initiation as an unexplored mechanism by which the cell regulates Cx43 gap junction formation during EMT.

Progesterone Via its Type-A Receptor Promotes Myometrial Gap Junction Coupling

Effective labour contractions require synchronization of myometrial cells through gap junctions (GJs). Clasically, progesterone (P4) is known to inhibit the expression of connexin-43 (Cx43, major component of GJs) and GJ formation in myometrium. Our current study is based on a striking observation that challenges this dogma. We observed conspicuous differences in the intracellular localization of Cx43 protein in PRA versus PRB expressing myocytes. Thus in P4 stimulated PRA cells Cx43 protein forms GJs, whereas in PRB cells the forward trafficking of Cx43 and GJ formation is inhibited even when Cx43 is overexpressed. We found that P4, via PRA/B, differentially regulates Cx43 translation to generate a Cx43-20 K isoform, which facilitates the transport of full length Cx43 to plasma membrane. The P4 mediated regulation of Cx43 trafficking and GJ formation occurs via non-genomic pathway and involves the regulation of mTOR signaling since inhibition of this pathway restored the Cx43 trafficking defect in PRB cells. We propose that PRA is a master regulator of Cx43 expression, GJ formation and myocyte connectivity/synchronization for labour.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

GJA1-20k Arranges Actin to Guide Cx43 Delivery to Cardiac Intercalated Discs

RATIONALE

Delivery of Cx43 (connexin 43) to the intercalated disc is a continuous and rapid process critical for intercellular coupling. By a pathway of targeted delivery involving microtubule highways, vesicles of Cx43 hemichannels are efficiently trafficked to adherens junctions at intercalated discs. It has also been identified that actin provides rest stops for Cx43 forward trafficking and that Cx43 has a 20 kDa internally translated small C terminus isoform, GJA1-20k (Gap Junction Protein Alpha 1- 20 kDa), which is required for full-length Cx43 trafficking, but by an unknown mechanism.

OBJECTIVE

We explored the mechanism by which the GJA1-20k isoform is required for full-length Cx43 forward trafficking to intercalated discs.

METHODS AND RESULTS

Using an in vivo Adeno-associated virus serotype 9-mediated gene transfer system, we confirmed in whole animal that GJA1-20k markedly increases endogenous myocardial Cx43 gap junction plaque size at the intercalated discs. In micropatterned cell pairing systems, we found that exogenous GJA1-20k expression stabilizes filamentous actin without affecting actin protein expression and that GJA1-20k complexes with both actin and tubulin. We also found that filamentous actin regulates microtubule organization as inhibition of actin polymerization with a low dose of latrunculin A disrupts the targeting of microtubules to cell-cell junctions. GJA1-20k protects actin filament from latrunculin A disruption, preserving microtubule trajectory to the cell-cell border. For therapeutic implications, we found that prior in vivo Adeno-associated virus serotype 9-mediated gene delivery of GJA1-20k to the heart protects Cx43 localization to the intercalated discs against acute ischemic injury.

CONCLUSIONS

The internally translated GJA1-20k isoform stabilizes actin filaments, which guides growth trajectories of the Cx43 microtubule trafficking machinery, increasing delivery of Cx43 hemichannels to cardiac intercalated discs. Exogenous GJA1-20k helps to maintain cell-cell coupling in instances of anticipated myocardial ischemia.

Biological responses of osteocytic connexin 43 hemichannels to simulated microgravity

Connexin 43 (Cx43) hemichannels and gap junctions in osteocytes are responsive to mechanical loading, which is important for bone formation and remodeling. However, the mechanism of these Cx43-forming channels in the process of mechanical unloading is still not very clear. In this study, unloading caused by weightlessness was simulated by using a random position machine (RPM). Osteocytic MLO-Y4 cells were subjected to 2 h of RPM treatment, and levels of Cx43 mRNA and total and cell surface expressed protein were determined by quantitative real-time PCR, western blotting, and biotinylation analysis. Although mRNA was elevated by RPM, total protein level of Cx43 was not altered; however, surface biotinylated Cx43 was significantly reduced. Interestingly, RPM promoted the retention of Cx43 in the Golgi apparatus detected by co-immunofluorescence with antibodies against Cx43 and 58 K Golgi marker protein. Dye uptake assay showed that hemichannels were induced open after RPM for 2 h. Consistently, prostaglandin E₂ release was increased and this increase was completely attenuated with the treatment of a Cx43 hemichannel blocking antibody. Together, this study demonstrates increased activity of Cx43 hemichannels to RPM, and active Cx43 hemichannels with prostaglandin E₂ release are likely to module biological function under simulated weightless conditions. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1195-1202, 2017.

The connexin 43 C-terminus: A tail of many tales

Connexins are chordate gap junction channel proteins that, by enabling direct communication between the cytosols of adjacent cells, create a unique cell signalling network. Gap junctional intercellular communication (GJIC) has important roles in controlling cell growth and differentiation and in tissue development and homeostasis. Moreover, several non-canonical connexin functions unrelated to GJIC have been discovered. Of the 21 members of the human connexin family, connexin 43 (Cx43) is the most widely expressed and studied. The long cytosolic C-terminus (CT) of Cx43 is subject to extensive post-translational modifications that modulate its intracellular trafficking and gap junction channel gating. Moreover, the Cx43 CT contains multiple domains involved in protein interactions that permit crosstalk between Cx43 and cytoskeletal and regulatory proteins. These domains endow Cx43 with the capacity to affect cell growth and differentiation independently of GJIC. Here, we review the current understanding of the regulation and unique functions of the Cx43 CT, both as an essential component of full-length Cx43 and as an independent signalling hub. We highlight the complex regulatory and signalling networks controlled by the Cx43 CT, including the extensive protein interactome that underlies both gap junction channel-dependent and -independent functions. We discuss these data in relation to the recent discovery of the direct translation of specific truncated forms of Cx43. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Immunofluorescence reveals unusual patterns of labelling for connexin43 localized to calbindin-D28K-positive interstitial cells in the pineal gland

Gap junctions between cells in the pineal gland have been described ultrastructurally, but their connexin constituents have not been fully characterized. We used immunofluorescence in combination with markers of pineal cells to document the cellular localization of connexin43 (Cx43). Immunofluorescence labelling of Cx43 with several different antibodies was widely distributed throughout the pineal, whereas another connexin examined, connexin26, was not found in pineal but only in surrounding leptomeninges. Labelling apparently associated with plasma membranes was visualized either as fine Cx43-puncta (1-2 μm) or as unusually large pools of Cx43 ranging up to 4-7 μm in diameter or length. These puncta and pools were highly concentrated in perivascular spaces, where they were associated with numerous cells devoid of labelling for markers of pinealocytes (e.g. tryptophan hydroxylase and serotonin), and where they were minimally associated with blood vessels and lacked association with resident macrophages. Astrocytes labelled for glial fibrillary acidic protein were largely restricted to the anterior pole of the pineal gland, where they displayed only fine and sparse Cx43-puncta along their processes. Labelling for Cx43 was localized largely though not exclusively to the somata and long processes of a subpopulation of perivascular interstitial cells that were immunopositive for calbindin-D28K. These cells were often located among dense bundles or termination areas of sympathetic fibres labelled for tyrosine hydroxylase or serotonin. The results indicate that interstitial cells form abundant gap junctions composed of Cx43, and suggest that gap junction-mediated intracellular communication by these cells supports the activities of pinealocytes.

Spatio-temporal regulation of connexin43 phosphorylation and gap junction dynamics

Gap junctions are specialized membrane domains containing tens to thousands of intercellular channels. These channels permit exchange of small molecules (<1000Da) including ions, amino acids, nucleotides, metabolites and secondary messengers (e.g., calcium, glucose, cAMP, cGMP, IP₃) between cells. The common reductionist view of these structures is that they are composed entirely of integral membrane proteins encoded by the 21 member connexin human gene family. However, it is clear that the normal physiological function of this structure requires interaction and regulation by a variety of proteins, especially kinases. Phosphorylation is capable of directly modulating connexin channel function but the most dramatic effects on gap junction activity occur via the organization of the gap junction structures themselves. This is a direct result of the short half-life of the primary gap junction protein, connexin, which requires them to be constantly assembled, remodeled and turned over. The biological consequences of this remodeling are well illustrated during cardiac ischemia, a process wherein gap junctions are disassembled and remodeled resulting in arrhythmia and ultimately heart failure. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

The electrical synapse: Molecular complexities at the gap and beyond

Gap junctions underlie electrical synaptic transmission between neurons. Generally perceived as simple intercellular channels, "electrical synapses" have demonstrated to be more functionally sophisticated and structurally complex than initially anticipated. Electrical synapses represent an assembly of multiple molecules, consisting of channels, adhesion complexes, scaffolds, regulatory machinery, and trafficking proteins, all required for their proper function and plasticity. Additionally, while electrical synapses are often viewed as strictly symmetric structures, emerging evidence has shown that some components forming electrical synapses can be differentially distributed at each side of the junction. We propose that the molecular complexity and asymmetric distribution of proteins at the electrical synapse provides rich potential for functional diversity. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 562-574, 2017.

Myosin VI facilitates connexin 43 gap junction accretion

In this study, we demonstrate myosin VI enrichment at Cx43 (also known as GJA1)-containing gap junctions (GJs) in heart tissue, primary cardiomyocytes and cell culture models. In primary cardiac tissue and in fibroblasts from the myosin VI-null mouse as well as in tissue culture cells transfected with siRNA against myosin VI, we observe reduced GJ plaque size with a concomitant reduction in intercellular communication, as shown by fluorescence recovery after photobleaching (FRAP) and a new method of selective calcein administration. Analysis of the molecular role of myosin VI in Cx43 trafficking indicates that myosin VI is dispensable for the delivery of Cx43 to the cell surface and connexon movement in the plasma membrane. Furthermore, we cannot corroborate clathrin or Dab2 localization at gap junctions and we do not observe a function for the myosin-VI-Dab2 complex in clathrin-dependent endocytosis of annular gap junctions. Instead, we found that myosin VI was localized at the edge of Cx43 plaques by using total internal reflection fluorescence (TIRF) microscopy and use FRAP to identify a plaque accretion defect as the primary manifestation of myosin VI loss in Cx43 homeostasis. A fuller understanding of this derangement may explain the cardiomyopathy or gliosis associated with the loss of myosin VI.

Structural and Functional Consequences of Connexin 36 (Cx36) Interaction with Calmodulin

Functional plasticity of neuronal gap junctions involves the interaction of the neuronal connexin36 with calcium/calmodulin-dependent kinase II (CaMKII). The important relationship between Cx36 and CaMKII must also be considered in the context of another protein partner, Ca2+ loaded calmodulin, binding an overlapping site in the carboxy-terminus of Cx36. We demonstrate that CaM and CaMKII binding to Cx36 is calcium-dependent, with Cx36 able to engage with CaM outside of the gap junction plaque. Furthermore, Ca2+ loaded calmodulin activates Cx36 channels, which is different to other connexins. The NMR solution structure demonstrates that CaM binds Cx36 in its characteristic compact state with major hydrophobic contributions arising from W277 at anchor position 1 and V284 at position 8 of Cx36. Our results establish Cx36 as a hub binding Ca2+ loaded CaM and they identify this interaction as a critical step with implications for functions preceding the initiation of CaMKII mediated plasticity at electrical synapses.

Clinical and biophysical characterization of 19 GJB1 mutations

Objective

Charcot–Marie–Tooth disease type X1 (CMTX1), which is caused by mutations in the gap junction (GJ) protein beta‐1 gene (GJB1), is the second most common form of Charcot–Marie–Tooth disease (CMT). GJB1 encodes the GJ beta‐1 protein (GJB1), which forms GJs within the myelin sheaths of peripheral nerves. The process by which GJB1 mutants cause neuropathy has not been fully elucidated. This study evaluated the biophysical characteristics of GJB1 mutants and their correlations with the clinical features of CMTX1 patients.

Methods

All patients with a validated GJB1 mutation were assessed using the Charcot–Marie–Tooth disease neuropathy score version 2 (CMTNS). The impacts of the mutations on the biophysical functions of GJB1 were characterized by assessing intracellular localization, expression patterns, and GJ Ca²⁺ permeability.

Result

Nineteen GJB1 mutations were identified in 24 patients with a clinical diagnosis of CMT. Six are novel mutations: p.L6S, p.I20F, p.I101Rfs*8, p.F153L, p.R215P, and p.D278V. Diverse pathological effects of the mutations were demonstrated, including reduced GJB1 expression, intracellular mislocalization, and altered GJ functions. GJB1 mutations that caused a complete loss of GJ Ca²⁺ permeability appeared to be associated with an earlier disease onset, whereas those resulting in preservation of GJ permeability and with predominant cell membrane expression tended to have a later onset and a milder phenotype.

Interpretation

This study demonstrated that the degree of loss of GJ function caused by the GJB1 mutations might contribute to the onset and severity of neuropathic symptoms in CMTX1.

Systemic inflammation disrupts oligodendrocyte gap junctions and induces ER stress in a model of CNS manifestations of X-linked Charcot-Marie-Tooth disease

X-linked Charcot-Marie-Tooth disease (CMT1X) is a common form of inherited neuropathy resulting from different mutations affecting the gap junction (GJ) protein connexin32 (Cx32). A subset of CMT1X patients may additionally present with acute fulminant CNS dysfunction, typically triggered by conditions of systemic inflammation and metabolic stress. To clarify the underlying mechanisms of CNS phenotypes in CMT1X we studied a mouse model of systemic inflammation induced by lipopolysaccharide (LPS) injection to compare wild type (WT), connexin32 (Cx32) knockout (KO), and KO T55I mice expressing the T55I Cx32 mutation associated with CNS phenotypes. Following a single intraperitoneal LPS or saline (controls) injection at the age of 40-60 days systemic inflammatory response was documented by elevated TNF-α and IL-6 levels in peripheral blood and mice were evaluated 1 week after injection. Behavioral analysis showed graded impairment of motor performance in LPS treated mice, worse in KO T55I than in Cx32 KO and in Cx32 KO worse than WT. Iba1 immunostaining revealed widespread inflammation in LPS treated mice with diffusely activated microglia throughout the CNS. Immunostaining for the remaining major oligodendrocyte connexin Cx47 and for its astrocytic partner Cx43 revealed widely reduced expression of Cx43 and loss of Cx47 GJs in oligodendrocytes. Real-time PCR and immunoblot analysis indicated primarily a down regulation of Cx43 expression with secondary loss of Cx47 membrane localization. Inflammatory changes and connexin alterations were most severe in the KO T55I group. To examine why the presence of the T55I mutant exacerbates pathology even more than in Cx32 KO mice, we analyzed the expression of ER-stress markers BiP, Fas and CHOP by immunostaining, immunoblot and Real-time PCR. All markers were increased in LPS treated KO T55I mice more than in other genotypes. In conclusion, LPS induced neuroinflammation causes disruption of the main astrocyte-oligodendrocyte GJs, which may contribute to the increased sensitivity of Cx32 KO mice to LPS and of patients with CMT1X to various stressors. Moreover the presence of an intracellularly retained, misfolded CMT1X mutant such as T55I induces ER stress under inflammatory conditions, further exacerbating oligodendrocyte dysfunction and pathological changes in the CNS.

Regulation of gap junction channels and hemichannels by phosphorylation and redox changes: a revision

Post-translational modifications of connexins play an important role in the regulation of gap junction and hemichannel permeability. The prerequisite for the formation of functional gap junction channels is the assembly of connexin proteins into hemichannels and their insertion into the membrane. Hemichannels can affect cellular processes by enabling the passage of signaling molecules between the intracellular and extracellular space. For the intercellular communication hemichannels from one cell have to dock to its counterparts on the opposing membrane of an adjacent cell to allow the transmission of signals via gap junctions from one cell to the other. The controlled opening of hemichannels and gating properties of complete gap junctions can be regulated via post-translational modifications of connexins. Not only channel gating, but also connexin trafficking and assembly into hemichannels can be affected by post-translational changes. Recent investigations have shown that connexins can be modified by phosphorylation/dephosphorylation, redox-related changes including effects of nitric oxide (NO), hydrogen sulfide (H2S) or carbon monoxide (CO), acetylation, methylation or ubiquitination. Most of the connexin isoforms are known to be phosphorylated, e.g. Cx43, one of the most studied connexin at all, has 21 reported phosphorylation sites. In this review, we provide an overview about the current knowledge and relevant research of responsible kinases, connexin phosphorylation sites and reported effects on gap junction and hemichannel regulation. Regarding the effects of oxidants we discuss the role of NO in different cell types and tissues and recent studies about modifications of connexins by CO and H2S.

Molecular mechanisms regulating formation, trafficking and processing of annular gap junctions

Internalization of gap junction plaques results in the formation of annular gap junction vesicles. The factors that regulate the coordinated internalization of the gap junction plaques to form annular gap junction vesicles, and the subsequent events involved in annular gap junction processing have only relatively recently been investigated in detail. However it is becoming clear that while annular gap junction vesicles have been demonstrated to be degraded by autophagosomal and endo-lysosomal pathways, they undergo a number of additional processing events. Here, we characterize the morphology of the annular gap junction vesicle and review the current knowledge of the processes involved in their formation, fission, fusion, and degradation. In addition, we address the possibility for connexin protein recycling back to the plasma membrane to contribute to gap junction formation and intercellular communication. Information on gap junction plaque removal from the plasma membrane and the subsequent processing of annular gap junction vesicles is critical to our understanding of cell-cell communication as it relates to events regulating development, cell homeostasis, unstable proliferation of cancer cells, wound healing, changes in the ischemic heart, and many other physiological and pathological cellular phenomena.