Multiple pathways in the trafficking and assembly of connexin 26, 32 and 43 into gap junction intercellular communication channels

Gap-junction-mediated bioelectric signaling required for slow muscle development and function in zebrafish

Bioelectric signaling, intercellular communication facilitated by membrane potential and electrochemical coupling, is emerging as a key regulator of animal development. Gap junction (GJ) channels can mediate bioelectric signaling by creating a fast, direct pathway between cells for the movement of ions and other small molecules. In vertebrates, GJ channels are formed by a highly conserved transmembrane protein family called the connexins. The connexin gene family is large and complex, creating challenges in identifying specific connexins that create channels within developing and mature tissues. Using the embryonic zebrafish neuromuscular system as a model, we identify a connexin conserved across vertebrate lineages, gjd4, which encodes the Cx46.8 protein, that mediates bioelectric signaling required for slow muscle development and function. Through mutant analysis and in vivo imaging, we show that gjd4/Cx46.8 creates GJ channels specifically in developing slow muscle cells. Using genetics, pharmacology, and calcium imaging, we find that spinal-cord-generated neural activity is transmitted to developing slow muscle cells, and synchronized activity spreads via gjd4/Cx46.8 GJ channels. Finally, we show that bioelectrical signal propagation within the developing neuromuscular system is required for appropriate myofiber organization and that disruption leads to defects in behavior. Our work reveals a molecular basis for GJ communication among developing muscle cells and reveals how perturbations to bioelectric signaling in the neuromuscular system may contribute to developmental myopathies. Moreover, this work underscores a critical motif of signal propagation between organ systems and highlights the pivotal role of GJ communication in coordinating bioelectric signaling during development.

Differential Regulation of Hemichannels and Gap Junction Channels by RhoA GTPase and Actin Cytoskeleton: A Comparative Analysis of Cx43 and Cx26

Connexins (Cxs) are transmembrane proteins that assemble into gap junction channels (GJCs) and hemichannels (HCs). Previous researches support the involvement of Rho GTPases and actin microfilaments in the trafficking of Cxs, formation of GJCs plaques, and regulation of channel activity. Nonetheless, it remains uncertain whether distinct types of Cxs HCs and GJCs respond differently to Rho GTPases or changes in actin polymerization/depolymerization dynamics. Our investigation revealed that inhibiting RhoA, a small GTPase that controls actin polymerization, or disrupting actin microfilaments with cytochalasin B (Cyto-B), resulted in reduced GJCs plaque size at appositional membranes and increased transport of HCs to non-appositional plasma membrane regions. Notably, these effects were consistent across different Cx types, since Cx26 and Cx43 exhibited similar responses, despite having distinct trafficking routes to the plasma membrane. Functional assessments showed that RhoA inhibition and actin depolymerization decreased the activity of Cx43 GJCs while significantly increasing HC activity. However, the functional status of GJCs and HCs composed of Cx26 remained unaffected. These results support the hypothesis that RhoA, through its control of the actin cytoskeleton, facilitates the transport of HCs to appositional cell membranes for GJCs formation while simultaneously limiting the positioning of free HCs at non-appositional cell membranes, independently of Cx type. This dynamic regulation promotes intercellular communications and reduces non-selective plasma membrane permeability through a Cx-type dependent mechanism, whereby the activity of Cx43 HCs and GJCs are differentially affected but Cx26 channels remain unchanged.

Spatiotemporal heterogeneity of LMOD1 expression summarizes two modes of cell communication in colorectal cancer

Cellular communication (CC) influences tumor development by mediating intercellular junctions between cells. However, the role and underlying mechanisms of CC in malignant transformation remain unknown. Here, we investigated the spatiotemporal heterogeneity of CC molecular expression during malignant transformation. It was found that although both tight junctions (TJs) and gap junctions (GJs) were involved in maintaining the tumor microenvironment (TME), they exhibited opposite characteristics. Mechanistically, for epithelial cells (parenchymal component), the expression of TJ molecules consistently decreased during normal-cancer transformation and is a potential oncogenic factor. For fibroblasts (mesenchymal component), the expression of GJs consistently increased during normal-cancer transformation and is a potential oncogenic factor. In addition, the molecular profiles of TJs and GJs were used to stratify colorectal cancer (CRC) patients, where subtypes characterized by high GJ levels and low TJ levels exhibited enhanced mesenchymal signals. Importantly, we propose that leiomodin 1 (LMOD1) is biphasic, with features of both TJs and GJs. LMOD1 not only promotes the activation of cancer-associated fibroblasts (CAFs) but also inhibits the Epithelial-mesenchymal transition (EMT) program in cancer cells. In conclusion, these findings demonstrate the molecular heterogeneity of CC and provide new insights into further understanding of TME heterogeneity.

The conducted vasomotor response and the principles of electrical communication in resistance arteries

Biological tissues are fed by arterial networks whose task is to set blood flow delivery in accordance with energetic demand. Coordinating vasomotor activity among hundreds of neighboring segments is an essential process, one dependent upon electrical information spreading among smooth muscle and endothelial cells. The "conducted vasomotor response" is a functional expression of electrical spread, and it is this process that lies at the heart of this critical review. Written in a narrative format, this review first highlights historical manuscripts and then characterizes the conducted response across a range of preparations. Trends are highlighted and used to guide subsequent sections, focused on cellular foundations, biophysical underpinnings, and regulation in health and disease. Key information has been tabulated; figures reinforce grounding concepts and reveal a framework within which theoretical and experimental work can be rationalized. This summative review highlights that despite 30 years of concerted experimentation, key aspects of the conducted response remain ill defined. Of note is the need to rationalize the regulation and deterioration of conduction in pathobiological settings. New quantitative tools, along with transgenic technology, are discussed as a means of propelling this investigative field forward.

Gap junction mediated bioelectric coordination is required for slow muscle development, organization, and function

Bioelectrical signaling, intercellular communication facilitated by membrane potential and electrochemical coupling, is emerging as a key regulator of animal development. Gap junction (GJ) channels can mediate bioelectric signaling by creating a fast, direct pathway between cells for the movement of ions and other small molecules. In vertebrates, GJ channels are formed by a highly conserved transmembrane protein family called the Connexins. The connexin gene family is large and complex, presenting a challenge in identifying the specific Connexins that create channels within developing and mature tissues. Using the embryonic zebrafish neuromuscular system as a model, we identify a connexin conserved across vertebrate lineages, gjd4, which encodes the Cx46.8 protein, that mediates bioelectric signaling required for appropriate slow muscle development and function. Through a combination of mutant analysis and in vivo imaging we show that gjd4/Cx46.8 creates GJ channels specifically in developing slow muscle cells. Using genetics, pharmacology, and calcium imaging we find that spinal cord generated neural activity is transmitted to developing slow muscle cells and synchronized activity spreads via gjd4/Cx46.8 GJ channels. Finally, we show that bioelectrical signal propagation within the developing neuromuscular system is required for appropriate myofiber organization, and that disruption leads to defects in behavior. Our work reveals the molecular basis for GJ communication among developing muscle cells and reveals how perturbations to bioelectric signaling in the neuromuscular system_may contribute to developmental myopathies. Moreover, this work underscores a critical motif of signal propagation between organ systems and highlights the pivotal role played by GJ communication in coordinating bioelectric signaling during development.

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.

Cytomembrane Trafficking Pathways of Connexin 26, 30, and 43

The connexin gene family is the most prevalent gene that contributes to hearing loss. Connexins 26 and 30, encoded by GJB2 and GJB6, respectively, are the most abundantly expressed connexins in the inner ear. Connexin 43, which is encoded by GJA1, appears to be widely expressed in various organs, including the heart, skin, the brain, and the inner ear. The mutations that arise in GJB2, GJB6, and GJA1 can all result in comprehensive or non-comprehensive genetic deafness in newborns. As it is predicted that connexins include at least 20 isoforms in humans, the biosynthesis, structural composition, and degradation of connexins must be precisely regulated so that the gap junctions can properly operate. Certain mutations result in connexins possessing a faulty subcellular localization, failing to transport to the cell membrane and preventing gap junction formation, ultimately leading to connexin dysfunction and hearing loss. In this review, we provide a discussion of the transport models for connexin 43, connexins 30 and 26, mutations affecting trafficking pathways of these connexins, the existing controversies in the trafficking pathways of connexins, and the molecules involved in connexin trafficking and their functions. This review can contribute to a new way of understanding the etiological principles of connexin mutations and finding therapeutic strategies for hereditary deafness.

The roles of connexins and gap junctions in the progression of cancer

Gap junctions (GJs), which are composed of connexins (Cxs), provide channels for direct information exchange between cells. Cx expression has a strong spatial specificity; however, its influence on cell behavior and information exchange between cells cannot be ignored. A variety of factors in organisms can modulate Cxs and subsequently trigger a series of responses that have important effects on cellular behavior. The expression and function of Cxs and the number and function of GJs are in dynamic change. Cxs have been characterized as tumor suppressors in the past, but recent studies have highlighted the critical roles of Cxs and GJs in cancer pathogenesis. The complex mechanism underlying Cx and GJ involvement in cancer development is a major obstacle to the evolution of therapy targeting Cxs. In this paper, we review the post-translational modifications of Cxs, the interactions of Cxs with several chaperone proteins, and the effects of Cxs and GJs on cancer. Video Abstract.

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.

Intracellular dynamics of polydnavirus innexin homologues

Polydnaviruses associated with ichneumonid parasitoid wasps (Ichnoviruses) encode large numbers of genes, often in multigene families. The Ichnovirus Vinnexin gene family, which is expressed in parasitized lepidopteran larvae, encodes homologues of Innexins, the structural components of insect gap junctions. Here, we have examined intracellular behaviours of the Campoletis sonorensis Ichnovirus (CsIV) Vinnexins, alone and in combination with a host Innexin orthologue, Innexin2 (Inx2). QRT-PCR verified that transcription of CsIV vinnexins occurs contemporaneously with inx2, implying co-occurrence of Vinnexin and Inx2 proteins. Confocal microscopy demonstrated that epitope-tagged VinnexinG (VnxG) and VinnexinQ2 (VnxQ2) exhibit similar subcellular localization as Spodoptera frugiperda Inx2 (Sf-Inx2). Surface biotinylation assays verified that all three proteins localize to the cell surface, and cytochalasin B and nocodazole that they rely on actin and microtubule cytoskeletal networks for localization. Immunomicroscopy following co-transfection of constructs indicates extensive co-localization of Vinnexins with each other and Sf-Inx2, and live-cell imaging of mCherry-labelled Inx2 supports that Vinnexins may affect Sf-Inx2 distribution in a Vinnexin-specific fashion. Our findings support that the Vinnexins may disrupt host cell physiology in a protein-specific manner through altering gap junctional intercellular channel communication, as well as indirectly by affecting multicellular junction characteristics.

Non-canonical roles of connexins

Gap junctions mediate cellular communication and homeostasis by controlling the intercellular exchange of small and hydrophilic molecules and ions. Gap junction channels are formed by the docking of 2 hemichannels of adjacent cells, which in turn are composed of 6 connexin subunits. Connexin proteins as such can also control the cellular life cycle independent of their channel activities. This has been most demonstrated in the context of cell growth and cell death. Different mechanisms are involved mainly related to direct interaction with cell growth or cell death regulators, but also implying effects on the expression of cell growth and cell death regulators. The present paper focuses on these atypical roles of connexin proteins.

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.

A New Look at the Functional Organization of the Golgi Ribbon

A characteristic feature of vertebrate cells is a Golgi ribbon consisting of multiple cisternal stacks connected into a single-copy organelle next to the centrosome. Despite numerous studies, the mechanisms that link the stacks together and the functional significance of ribbon formation remain poorly understood. Nevertheless, these questions are of considerable interest, since there is increasing evidence that Golgi fragmentation – the unlinking of the stacks in the ribbon – is intimately connected not only to normal physiological processes, such as cell division and migration, but also to pathological states, including neurodegeneration and cancer. Challenging a commonly held view that ribbon architecture involves the formation of homotypic tubular bridges between the Golgi stacks, we present an alternative model, based on direct interaction between the biosynthetic (pre-Golgi) and endocytic (post-Golgi) membrane networks and their connection with the centrosome. We propose that the central domains of these permanent pre- and post-Golgi networks function together in the biogenesis and maintenance of the more transient Golgi stacks, and thereby establish “linker compartments” that dynamically join the stacks together. This model provides insight into the reversible fragmentation of the Golgi ribbon that takes place in dividing and migrating cells and its regulation along a cell surface – Golgi – centrosome axis. Moreover, it helps to understand transport pathways that either traverse or bypass the Golgi stacks and the positioning of the Golgi apparatus in differentiated neuronal, epithelial, and muscle cells.

Architecture and Dynamics of the Neuronal Secretory Network

Regulated synthesis and movement of proteins between cellular organelles are central to diverse forms of biological adaptation and plasticity. In neurons, the repertoire of channel, receptor, and adhesion proteins displayed on the cell surface directly impacts cellular development, morphology, excitability, and synapse function. The immensity of the neuronal surface membrane and its division into distinct functional domains present a challenging landscape over which proteins must navigate to reach their appropriate functional domains. This problem becomes more complex considering that neuronal protein synthesis is continuously refined in space and time by neural activity. Here we review our current understanding of how integral membrane and secreted proteins important for neuronal function travel from their sites of synthesis to their functional destinations. We discuss how unique adaptations to the function and distribution of neuronal secretory organelles may facilitate local protein trafficking at remote sites in neuronal dendrites to support diverse forms of synaptic plasticity.

MiR-301a-3p in the pathogenesis of bacterial meningitis by targeting Cx43

Gap junction (GJ) is concerned with cell growth, differentiation, immune response, as well as many physiological and pathological processes. Cx43, as an important GJ protein, is associated with a variety of diseases. This study investigated the effect of miR-301a-3p in bacterial meningitis by targeting the Cx43 gene. The negative correlation between Cx43 and miR-301a-3p was because of the abnormal expression of related genes. MiR-301a-3p agomir was transfected into astrocytes for higher expression; CCK8 assay and flow cytometry showed that the high expression of miR-301a-3p would inhibit apoptosis and induces proliferation of astrocytes, whereas miR-301a-3p antagomir would inhibit proliferation and induce apoptosis. Bioinformatics analysis showed that Cx43 was the target gene of miR-301a-3p, and dual-luciferase assay and experiments repeated showed that miR-301a-3p regulated the expression of Cx43 on the 3'-untranslated region seed region. Therefore, miR-301a-3p played a biological role in the development of bacterial meningitis by regulating the expression of the target gene Cx43.

Emerging roles of gap junction proteins connexins in cancer metastasis, chemoresistance and clinical application

Connexin, a four-pass transmembrane protein, contributes to assembly of gap junctions among neighboring cells and thus facilitates gap junctional intercellular communication (GJIC). Traditionally, the roles of connexins were thought to mediate formation of hemichannels and GJIC assembly for transportation of ions and small molecules. Many studies have observed loss of GJIC, due to reduced expression or altered cytoplasmic localization of connexins, in primary tumor cells. Connexins are generally considered tumor-suppressive. However, recent studies of clinical samples suggested a different role of connexins in that expression levels and membrane localization of connexins, including Connexin 43 (Cx43, GJA1) and Connexin 26 (Cx26, GJB2), were found to be enhanced in metastatic lesions of cancer patients. Cx43- and Cx26-mediated GJIC was found to promote cancer cell migration and adhesion to the pulmonary endothelium. Regulatory circuits involved in the induction of connexins and their functional effects have also been reported in various types of cancer. Connexins expressed in stromal cells were correlated with metastasis and were implicated in regulating metastatic behaviors of cancer cells. Recent studies have revealed that connexins can contribute to cellular phenotypes via multiple ways, namely 1) GJIC, 2) C-terminal tail-mediated signaling, and 3) cell-cell adhesion during gap junction formation. Both expression levels and the subcellular localization could participate determining the functional roles of connexins in cancer. Compounds targeting connexins were thus tested as potential therapeutics intervening metastasis or chemoresistance. This review focuses on the recent findings in the correlation between the expression of connexins and patients’ prognosis, their roles in metastasis and chemoresistance, as well as the implications and concerns of using connexin-targeting drugs as anti-metastatic therapeutics. Overall, connexins may serve as biomarkers for cancer prognosis and as therapeutic targets for intervening metastasis and chemoresistance.

Intermediate compartment (IC): from pre-Golgi vacuoles to a semi-autonomous membrane system

Despite its discovery more than three decades ago and well-established role in protein sorting and trafficking in the early secretory pathway, the intermediate compartment (IC) has remained enigmatic. The prevailing view is that the IC evolved as a specialized organelle to mediate long-distance endoplasmic reticulum (ER)–Golgi communication in metazoan cells, but is lacking in other eukaryotes, such as plants and fungi. However, this distinction is difficult to reconcile with the high conservation of the core machineries that regulate early secretory trafficking from yeast to man. Also, it has remained unclear whether the pleiomorphic IC components—vacuoles, tubules and vesicles—represent transient transport carriers or building blocks of a permanent pre-Golgi organelle. Interestingly, recent studies have revealed that the IC maintains its compositional, structural and spatial properties throughout the cell cycle, supporting a model that combines the dynamic and stable aspects of the organelle. Moreover, the IC has been assigned novel functions, such as cell signaling, Golgi-independent trafficking and autophagy. The emerging permanent nature of the IC and its connections with the centrosome and the endocytic recycling system encourage reconsideration of its relationship with the Golgi ribbon, role in Golgi biogenesis and ubiquitous presence in eukaryotic cells.

Intrinsic Oncogenic Function of Intracellular Connexin26 Protein in Head and Neck Squamous Cell Carcinoma Cells

It has long been known that the gap junction is down-regulated in many tumours. One of the downregulation mechanisms is the translocation of connexin, a gap junction protein, from cell membrane into cytoplasm, nucleus, or Golgi apparatus. Interestingly, as tumours progress and reinforce their malignant phenotype, the amount of aberrantly-localised connexin increases in different malignant tumours including oesophageal squamous cell carcinoma, thus suggesting that such an aberrantly-localised connexin should be oncogenic, although gap junctional connexins are often tumour-suppressive. To define the dual roles of connexin in head and neck squamous cell carcinoma (HNSCC), we introduced the wild-type connexin26 (wtCx26) or the mutant Cx26 (icCx26) gene, the product of which carries the amino acid sequence AKKFF, an endoplasmic reticulum-Golgi retention signal, at the C-terminus and is not sorted to cell membrane, into the human FaDu hypopharyngeal cancer cell line that had severely impaired the expression of connexin during carcinogenesis. wtCx26 protein was trafficked to the cell membrane and formed gap junction, which successfully exerted cell-cell communication. On the other hand, the icCx26 protein was co-localised with a Golgi marker, as revealed by immunofluorescence, and thus was retained on the way to the cell membrane. While the forced expression of wtCx26 suppressed both cell proliferation in vitro and tumorigenicity in mice in vivo, icCx26 significantly enhanced both cell proliferation and tumorigenicity compared with the mock control clones, indicating that an excessive accumulation of connexin protein in intracellular domains should be involved in cancer progression and that restoration of proper subcellular sorting of connexin might be a therapeutic strategy to control HNSCC.

Unconventional protein secretion – new insights into the pathogenesis and therapeutic targets of human diseases

Most secretory proteins travel through a well-documented conventional secretion pathway involving the endoplasmic reticulum (ER) and the Golgi complex. However, recently, it has been shown that a significant number of proteins reach the plasma membrane or extracellular space via unconventional routes. Unconventional protein secretion (UPS) can be divided into two types: (i) the extracellular secretion of cytosolic proteins that do not bear a signal peptide (i.e. leaderless proteins) and (ii) the cell-surface trafficking of signal-peptide-containing transmembrane proteins via a route that bypasses the Golgi. Understanding the UPS pathways is not only important for elucidating the mechanisms of intracellular trafficking pathways but also has important ramifications for human health, because many of the proteins that are unconventionally secreted by mammalian cells and microorganisms are associated with human diseases, ranging from common inflammatory diseases to the lethal genetic disease of cystic fibrosis. Therefore, it is timely and appropriate to summarize and analyze the mechanisms of UPS involvement in disease pathogenesis, as they may be of use for the development of new therapeutic approaches. In this Review, we discuss the intracellular trafficking pathways of UPS cargos, particularly those related to human diseases. We also outline the disease mechanisms and the therapeutic potentials of new strategies for treating UPS-associated diseases.

Dileucine-like motifs in the C-terminal tail of connexin32 control its endocytosis and assembly into gap junctions

Defects in assembly of gap junction-forming proteins, called connexins (Cxs), are observed in a variety of cancers. Connexin32 (Cx32; also known as GJB1) is expressed by the polarized cells in epithelia. We discovered two dileucine-based motifs, which govern the intracellular sorting and endocytosis of transmembrane proteins, in the C-terminal tail of Cx32 and explored their role in regulating its endocytosis and gap junction-forming abilities in pancreatic and prostate cancer cells. One motif, designated as LI, was located near the juxtamembrane domain, whereas the other, designated as LL, was located distally. We also discovered a non-canonical motif, designated as LR, in the C-terminal tail. Our results showed that rendering these motifs non-functional had no effect on the intracellular sorting of Cx32. However, rendering the LL or LR motif nonfunctional enhanced the formation of gap junctions by inhibiting Cx32 endocytosis by the clathrin-mediated pathway. Rendering the LI motif nonfunctional inhibited gap junction formation by augmenting the endocytosis of Cx32 via the LL and LR motifs. Our studies have defined distinct roles of these motifs in regulating the endocytosis of Cx32 and its gap junction-forming ability.This article has an associated First Person interview with the first author of the paper.

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.

Golgi-independent secretory trafficking through recycling endosomes in neuronal dendrites and spines

Neurons face the challenge of regulating the abundance, distribution and repertoire of integral membrane proteins within their immense, architecturally complex dendritic arbors. While the endoplasmic reticulum (ER) supports dendritic translation, most dendrites lack the Golgi apparatus (GA), an essential organelle for conventional secretory trafficking. Thus, whether secretory cargo is locally trafficked in dendrites through a non-canonical pathway remains a fundamental question. Here we define the dendritic trafficking itinerary for key synaptic molecules in rat cortical neurons. Following ER exit, the AMPA-type glutamate receptor GluA1 and neuroligin 1 undergo spatially restricted entry into the dendritic secretory pathway and accumulate in recycling endosomes (REs) located in dendrites and spines before reaching the plasma membrane. Surprisingly, GluA1 surface delivery occurred even when GA function was disrupted. Thus, in addition to their canonical role in protein recycling, REs also mediate forward secretory trafficking in neuronal dendrites and spines through a specialized GA-independent trafficking network.

All cells must produce, sort and deliver molecular building blocks to the right places at the right time and in appropriate amounts. This is particularly important for neurons, which are the largest and most structurally complex cells in the body. A typical neuron consists of a cell body covered in branches called dendrites, plus a single cable-like structure known as an axon. Dendrites receive inputs from other neurons and relay the information to the cell body in the form of electrical signals. The cell body processes these electrical signals and the resulting signals then travel along the axon to terminals at the far-end. The axon terminals in turn pass the signals on to the dendrites of other neurons via junctions called synapses.

For synapses to work correctly, the membranes surrounding the dendrites need to contain receptor proteins that can detect incoming signals. These proteins must be continually replenished, raising the question of how newly made receptor molecules are shuttled to the appropriate locations within the dendrites.

A series of compartments called the Golgi complex play an important role in processing newly-made proteins in many different types of cells. As proteins pass through the Golgi, enzymes within the tunnel walls modify the proteins by adding or removing molecular groups. Therefore, it has been suggested that the route that the synapse receptor proteins take through the neuron to reach the dendrites always includes a visit to the Golgi. However, the Golgi complex in neurons is mostly confined to the cell body, raising the question of whether proteins that are locally produced within dendrites can make the journey to nearby synapses without visiting the Golgi complex.

Bowen et al. used a microscope to follow the movements of synapse receptor proteins through neurons grown in a dish. The experiments show that proteins destined for the dendrites make a number of stops after leaving the cell body. However, some synaptic proteins reach the dendrites without passing through the Golgi at all, suggesting neurons are much less dependent on the Golgi to process newly-made proteins than other types of cells.

Genetic mutations that prevent proteins from finding their way to their required destinations, or that disrupt the work of enzymes inside trafficking stations like the Golgi, cause numerous human diseases. Understanding how proteins travel to specific destinations inside healthy cells should also help reveal what happens when this process fails.

Microtubule-assisted altered trafficking of astrocytic gap junction protein connexin 43 is associated with depletion of connexin 47 during mouse hepatitis virus infection

Gap junctions (GJs) are important for maintenance of CNS homeostasis. GJ proteins, connexin 43 (Cx43) and connexin 47 (Cx47), play a crucial role in production and maintenance of CNS myelin. Cx43 is mainly expressed by astrocytes in the CNS and forms gap junction intercellular communications between astrocytes-astrocytes (Cx43-Cx43) and between astrocytes-oligodendrocytes (Cx43-Cx47). Mutations of these connexin (Cx) proteins cause dysmyelinating diseases in humans. Previously, it has been shown that Cx43 localization and expression is altered due to mouse hepatitis virus (MHV)-A59 infection both in vivo and in vitro; however, its mechanism and association with loss of myelin protein was not elaborated. Thus, we explored potential mechanisms by which MHV-A59 infection alters Cx43 localization and examined the effects of viral infection on Cx47 expression and its association with loss of the myelin marker proteolipid protein. Immunofluorescence and total internal reflection fluorescence microscopy confirmed that MHV-A59 used microtubules (MTs) as a conduit to reach the cell surface and restricted MT-mediated Cx43 delivery to the cell membrane. Co-immunoprecipitation experiments demonstrated that Cx43-β-tubulin molecular interaction was depleted due to protein-protein interaction between viral particles and MTs. During acute MHV-A59 infection, oligodendrocytic Cx47, which is mainly stabilized by Cx43 in vivo, was down-regulated, and its characteristic staining remained disrupted even at chronic phase. The loss of Cx47 was associated with loss of proteolipid protein at the chronic stage of MHV-A59 infection.

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.

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.

Cardiac Cx43, Cx40 and Cx45 co-assembling: involvement of connexins epitopes in formation of hemichannels and Gap junction channels

Background

This review comes after the International Gap Junction Conference (IGJC 2015) and describes the current knowledge on the function of the specific motifs of connexins in the regulation of the formation of gap junction channels. Moreover the review is complemented by a summarized description of the distinct contribution of gap junction channels in the electrical coupling.

Results

Complementary biochemical and functional characterization on cell models and primary cells have improved our understanding on the oligomerization of connexins and the formation and the electrical properties of gap junction channels. Studies mostly focused cardiac connexins Cx43 and Cx40 expressed in myocytes, while Cx45 and Cx30.2 have been less investigated, for which main findings are reviewed to highlight their critical contribution in the formation of gap junction channels for ensuring the orchestrated electrical impulse propagation and coordination of atrial and ventricular contraction and heart function, whereas connexin dysfunction and remodeling are pro-arrhythmic factors. Common and specific motifs of residues identified in different domain of each type of connexin determine the connexin homo- and hetero-oligomerization and the channels formation, which leads to specific electrical properties.

Conclusions

These motifs and the resulting formation of gap junction channels are keys to ensure the tissue homeostasis and function in each connexin expression pattern in various tissues of multicellular organisms. Altogether, the findings to date have significantly improved our understanding on the function of the different connexin expression patterns in healthy and diseased tissues, and promise further investigations on the contribution in the different types of connexin.

LRP6 acts as a scaffold protein in cardiac gap junction assembly

Low-density lipoprotein receptor-related protein 6 (LRP6) is a Wnt co-receptor in the canonical Wnt/β-catenin signalling. Here, we report the scaffold function of LRP6 in gap junction formation of cardiomyocytes. Cardiac LRP6 is spatially restricted to intercalated discs and binds to gap junction protein connexin 43 (Cx43). A deficiency in LRP6 disrupts Cx43 gap junction formation and thereby impairs the cell-to-cell coupling, which is independent of Wnt/β-catenin signalling. The defect in Cx43 gap junction resulting from LRP6 reduction is attributable to the defective traffic of de novo Cx43 proteins from the endoplasmic reticulum to the Golgi apparatus, leading to the lysosomal degradation of Cx43 proteins. Accordingly, the hearts of conditional cardiac-specific Lrp6-knockout mice consistently exhibit overt reduction of Cx43 gap junction plaques without any abnormality in Wnt signalling and are predisposed to lethal arrhythmias. These findings uncover a distinct role of LRP6 as a platform for intracellular protein trafficking.

LRP6 is known for its role as a Wnt co-receptor essential for the canonical Wnt/β-catenin signaling. Here, Li et al. show that LRP6 exerts a Wnt-independent scaffold function and regulates connexin 43 gap junction formation and coupling of cardiomyocytes in mouse hearts.

Inhibition of connexin 43 hemichannel-mediated ATP release attenuates early inflammation during the foreign body response

BACKGROUND

In the last 50 years, the use of medical implants has increased dramatically. Failure of implanted devices and biomaterials is a significant source of morbidity and increasing healthcare expenditures. An important cause of implant failure is the host inflammatory response. Recent evidence implicates extracellular ATP as an important inflammatory signaling molecule. A major pathway for release of cytoplasmic ATP into the extracellular space is through connexin hemichannels, which are the unpaired constituents of gap junction intercellular channels. Blockade of hemichannels of the connexin 43 (Cx43) isoform has been shown to reduce inflammation and improve healing. We have developed a Cx43 mimetic peptide (JM2) that targets the microtubule-binding domain of Cx43. The following report investigates the role of the Cx43 microtubule-binding domain in extracellular ATP release by Cx43 hemichannels and how this impacts early inflammatory events of the foreign body reaction.

METHODS

In vitro Cx43 hemichannel-mediated ATP release by cultured human microvascular endothelial cells subjected to hypocalcemic and normocalcemic conditions was measured after application of JM2 and the known hemichannel blocker, flufenamic acid. A submuscular silicone implant model was used to investigate in vivo ATP signaling during the early foreign body response. Implants were coated with control pluronic vehicle or pluronic carrying JM2, ATP, JM2+ATP, or known hemichannel blockers and harvested at 24 h for analysis.

RESULTS

JM2 significantly inhibited connexin hemichannel-mediated ATP release from cultured endothelial cells. Importantly, the early inflammatory response to submuscular silicone implants was inhibited by JM2. The reduction in inflammation by JM2 was reversed by the addition of exogenous ATP to the pluronic vehicle.

CONCLUSIONS

These data indicate that ATP released through Cx43 hemichannels into the vasculature is an important signal driving the early inflammatory response to implanted devices. A vital aspect of this work is that it demonstrates that targeted molecular therapeutics, such as JM2, provide the capacity to regulate inflammation in a clinically relevant system.

Connexins and skin disease: insights into the role of beta connexins in skin homeostasis

Cell-to-cell communication triggered by connexin channels plays a central role in maintaining epidermal homeostasis. Here, we discuss the role of the beta connexin subgroup, where site-specific mutations in at least 4 of these proteins lead to distinctive non-inflammatory and inflammatory hyperproliferative epidermal disorders. Recent advances in the molecular pathways evoked and correlation with clinical outcome are discussed. The latest data provide increasing evidence that connexins in the epidermis are sensors to environmental stress and that targeting aberrant hemichannel activity holds significant therapeutic potential for inflammatory skin disorders.

Cx30 exhibits unique characteristics including a long half-life when assembled into gap junctions

In the present study we investigated the life-cycle, trafficking, assembly and cell surface dynamics of a poorly characterized connexin family member, connexin 30 (Cx30), which plays a critical role in skin health and hearing. Unexpectedly, Cx30 localization at the cell surface and gap junctional intercellular communication was not affected by prolonged treatments with the ER-Golgi transport inhibitor brefeldin-A or the protein synthesis inhibitor cycloheximide, whereas Cx43 was rapidly cleared. Fluorescent recovery after photobleaching revealed that Cx30 plaques were rebuilt from the outer edges in keeping with older channels residing in the inner core of the plaque. Expression of a dominant-negative form of Sar1 GTPase led to the accumulation of Cx30 within the ER in contrast to a report that Cx30 traffics via a Golgi-independent pathway. Co-expression of Cx30 with Cx43 revealed that these connexins segregate into distinct domains within common gap junction plaques suggesting their assembly is governed by different mechanisms. In summary, Cx30 was found to be an unusually stable, long-lived connexin (half-life >12 hrs), which may underlie its specific role in the epidermis and cochlea.

Asymmetric expression of connexins between luminal epithelial- and myoepithelial- cells is essential for contractile function of the mammary gland

Intercellular communication is essential for glandular functions and tissue homeostasis. Gap junctions couple cells homotypically and heterotypically and co-ordinate reciprocal responses between the different cell types. Connexins (Cxs) are the main mammalian gap junction proteins, and the distribution of some Cx subtypes in the heterotypic gap junctions is not symmetrical; in the murine mammary gland, Cx26, Cx30 and Cx32 are expressed only in the luminal epithelial cells and Cx43 is expressed only in myoepithelial cells. Expression of all four Cxs peaks during late pregnancy and throughout lactation suggesting essential roles for these proteins in the functional secretory activity of the gland. Transgenic (Tg) mice over-expressing Cx26 driven by keratin 5 promoter had an unexpected mammary phenotype: the mothers were unable to feed their pups to weaning age leading to litter starvation and demise in early to mid-lactation. The mammary gland of K5-Cx26 female mice developed normally and produced normal levels of milk protein, suggesting a defect in delivery rather than milk production. Because the mammary gland of K5-Cx26 mothers contained excessive milk, we hypothesized that the defect may be in an inability to eject the milk. Using ex vivo three-dimensional mammary organoid cultures, we showed that tissues isolated from wild-type FVB females contracted upon treatment with oxytocin, whereas, organoids from Tg mice failed to do so. Unexpectedly, we found that ectopic expression of Cx26 in myoepithelial cells altered the expression of endogenous Cx43 resulting in impaired gap junction communication, demonstrated by defective dye coupling in mammary epithelial cells of Tg mice. Inhibition of gap junction communication or knock-down of Cx43 in organoids from wild-type mice impaired contraction in response to oxytocin, recapitulating the observations from the mammary glands of Tg mice. We conclude that Cx26 acts as a trans-dominant negative for Cx43 function in myoepithelial cells, highlighting the importance of cell type-specific expression of Cxs for optimal contractile function of the mammary myoepithelium.

Defective channels lead to an impaired skin barrier

Channels are integral membrane proteins that form a pore, allowing the passive movement of ions or molecules across a membrane (along a gradient), either between compartments within a cell, between intracellular and extracellular environments or between adjacent cells. The ability of cells to communicate with one another and with their environment is a crucial part of the normal physiology of a tissue that allows it to carry out its function. Cell communication is particularly important during keratinocyte differentiation and formation of the skin barrier. Keratinocytes in the skin epidermis undergo a programme of apoptosis-driven terminal differentiation, whereby proliferating keratinocytes in the basal (deepest) layer of the epidermis stop proliferating, exit the basal layer and move up through the spinous and granular layers of the epidermis to form the stratum corneum, the external barrier. Genes encoding different families of channel proteins have been found to harbour mutations linked to a variety of rare inherited monogenic skin diseases. In this Commentary, we discuss how human genetic findings in aquaporin (AQP) and transient receptor potential (TRP) channels reveal different mechanisms by which these channel proteins function to ensure the proper formation and maintenance of the skin barrier.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

Connexins: sensors of epidermal integrity that are therapeutic targets

Gap junction proteins (connexins) are differentially expressed throughout the multiple layers of the epidermis. A variety of skin conditions arise with aberrant connexin expression or function and suggest that maintaining the epidermal gap junction network has many important roles in preserving epidermal integrity and homeostasis. Mutations in a number of connexins lead to epidermal dysplasias giving rise to a range of dermatological disorders of differing severity. 'Gain of function' mutations reveal connexin-mediated roles in calcium signalling within the epidermis. Connexins are involved in epidermal innate immunity, inflammation control and in wound repair. The therapeutic potential of targeting connexins to improve wound healing responses is now clear. This review discusses the role of connexins in epidermal integrity, and examines the emerging evidence that connexins act as epidermal sensors to a variety of mechanical, temperature, pathogen-induced and chemical stimuli. Connexins thus act as an integral component of the skin's protective barrier.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

Pannexin1 channel proteins in the zebrafish retina have shared and unique properties

In mammals, a single pannexin1 gene (Panx1) is widely expressed in the CNS including the inner and outer retinae, forming large-pore voltage-gated membrane channels, which are involved in calcium and ATP signaling. Previously, we discovered that zebrafish lack Panx1 expression in the inner retina, with drPanx1a exclusively expressed in horizontal cells of the outer retina. Here, we characterize a second drPanx1 protein, drPanx1b, generated by whole-genome duplications during teleost evolution. Homology searches strongly support the presence of pannexin sequences in cartilaginous fish and provide evidence that pannexins evolved when urochordata and chordata evolution split. Further, we confirm Panx1 ohnologs being solely present in teleosts. A hallmark of differential expression of drPanx1a and drPanx1b in various zebrafish brain areas is the non-overlapping protein localization of drPanx1a in the outer and drPanx1b in the inner fish retina. A functional comparison of the evolutionary distant fish and mouse Panx1s revealed both, preserved and unique properties. Preserved functions are the capability to form channels opening at resting potential, which are sensitive to known gap junction and hemichannel blockers, intracellular calcium, extracellular ATP and pH changes. However, drPanx1b is unique due to its highly complex glycosylation pattern and distinct electrophysiological gating kinetics. The existence of two Panx1 proteins in zebrafish displaying distinct tissue distribution, protein modification and electrophysiological properties, suggests that both proteins fulfill different functions in vivo.

Nogo-A Represses Anatomical and Synaptic Plasticity in the Central Nervous System

Nogo-A was initially discovered as a myelin-associated growth inhibitory protein limiting axonal regeneration after central nervous system (CNS) injury. This review summarizes current knowledge on how myelin and neuronal Nogo-A and its receptors exert physiological functions ranging from the regulation of growth suppression to synaptic plasticity in the developing and adult intact CNS.

Gap junctions and blood-tissue barriers

Gap junction is a cell-cell communication junction type found in virtually all mammalian epithelia and endothelia and provides the necessary "signals" to coordinate physiological events to maintain the homeostasis of an epithelium and/or endothelium under normal physiological condition and following changes in the cellular environment (e.g., stimuli from stress, growth, development, inflammation, infection). Recent studies have illustrated the significance of this junction type in the maintenance of different blood-tissue barriers, most notably the blood-brain barrier and blood-testis barrier, which are dynamic ultrastructures, undergoing restructuring in response to stimuli from the environment. In this chapter, we highlight and summarize the latest findings in the field regarding how changes at the gap junction, such as the result of a knock-out, knock-down, knock-in, or gap junction inhibition and/or its activation via the use of inhibitors and/or activators, would affect the integrity or permeability of the blood-tissue barriers. These findings illustrate that much research is needed to delineate the role of gap junction in the blood-tissue barriers, most notably its likely physiological role in mediating or regulating the transport of therapeutic drugs across the blood-tissue barriers.

Connexin43 hemichannel-mediated regulation of connexin43

Background

Many signaling molecules and pathways that regulate gap junctions (GJs) protein expression and function are, in fact, also controlled by GJs. We, therefore, speculated an existence of the GJ channel-mediated self-regulation of GJs. Using a cell culture model in which nonjunctional connexin43 (Cx43) hemichannels were activated by cadmium (Cd²⁺), we tested this hypothesis.

Principal Findings

Incubation of Cx43-transfected LLC-PK1 cells with Cd²⁺ led to an increased expression of Cx43. This effect of Cd²⁺ was tightly associated with JNK activation. Inhibition of JNK abolished the elevation of Cx43. Further analysis revealed that the changes of JNK and Cx43 were controlled by GSH. Supplement of a membrane-permeable GSH analogue GSH ethyl ester or GSH precursor N-acetyl-cystein abrogated the effects of Cd²⁺ on JNK activation and Cx43 expression. Indeed, Cd²⁺ induced extracellular release of GSH. Blockade of Cx43 hemichannels with heptanol or Cx43 mimetic peptide Gap26 to prevent the efflux of GSH significantly attenuated the Cx43-elevating effects of Cd²⁺.

Conclusions

Collectively, our results thus indicate that Cd²⁺-induced upregulation of Cx43 is through activation of nonjunctional Cx43 hemichannels. Our findings thus support the existence of a hemichannel-mediated self-regulation of Cx43 and provide novel insights into the molecular mechanisms of Cx43 expression and function.

The role of the C-terminus in functional expression and internalization of rat connexin46 (rCx46)

The C-terminus (CT) of rCx46 consists of 186 residues (H230-I416). Recent studies showed that rCx46(28.2), truncated after H243, altered the formation of functional hemichannels when expressed in Xenopus oocytes, while rCx46(37.7), truncated after A333 formed gap junction hemichannels similarly to rCx46(wt). To analyze the role of the CT up to A333 in functional expression with cell imaging and dye-transfer techniques, different mutants were generated by C-terminal truncation between H243-A333, labeled with EGFP and expressed in HeLa cells. These rCx46 variants were characterized according to their compartmentalization in organelles, their presence in microscopic detectable vesicles and their ability to form gap junction plaques. rCx46 truncated after A311 (rCx46(35.3)) was compartmentalized, was found in vesicles and formed functional gap junction plaques similarly to rCx46(wt). With a truncation after P284 (rCx46(32.6)), the protein was not compartmentalized and the amount of vesicles containing the protein were reduced; however, functional gap junction plaque formation was not affected as compared to rCx46(35.3). rCx46(28.2) did not form functional gap junction plaques; it was not found in vesicles or in cellular compartments. Live-cell imaging and detection of annular junctions for rCx46(32.6) and rCx46(35.3) revealed that the truncation after P284 reduced the frequency of vesicle budding from gap junction plaques and the formation of annular junctions. These results suggest that the C-terminal region of rCx46 up to A311 (rCx46(35.3)) is necessary for its correct compartmentalization and internalization in the form of annular junctions, while the H230-P284 C-terminal region (rCx46(32.6)) is sufficient for the formation of dye coupled gap junction channels.