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Type 1 diabetes develops when most insulin-producing β cells of the pancreas are killed by an autoimmune attack. The in vivo conditions modulating the sensitivity and resistance of β cells to this attack remain largely obscure. Here, we show that connexin 36 (Cx36), a trans-membrane protein that forms gap junctions between β cells in the pancreatic islets, protects mouse β cells against both cytotoxic drugs and cytokines that prevail in the islet environment at the onset of type 1 diabetes. We documented that this protection was at least partially dependent on intercellular communication, which Cx36 and other types of connexin channels establish within pancreatic islets. We further found that proinflammatory cytokines decreased expression of Cx36 and that experimental reduction or augmentation of Cx36 levels increased or decreased β cell apoptosis, respectively. Thus, we conclude that Cx36 is central to β cell protection from toxic insults.

Background— Formation of platelet plug initiates hemostasis after vascular injury and triggers thrombosis in ischemic disease. However, the mechanisms leading to the formation of a stable thrombus are poorly understood. Connexins comprise a family of proteins that form gap junctions enabling intercellular coordination of tissue activity, a process termed gap junctional intercellular communication . Methods and Results— In the present study, we show that megakaryocytes and platelets express connexin 37 (Cx37). Deletion of the Cx37 gene in mice shortens bleeding time and increases thrombus propensity. Aggregation is increased in murine Cx37 −/− platelets or in murine Cx37 +/+ and human platelets treated with gap junction blockers. Intracellular microinjection of neurobiotin, a Cx37-permeant tracer, revealed gap junctional intercellular communication in platelet aggregates, which was impaired in Cx37 −/− platelets and in human platelets exposed to gap junction blockers. Finally, healthy subjects homozygous for Cx37–1019C, a prognostic marker for atherosclerosis, display increased platelet responses compared with subjects carrying the Cx37–1019T allele. Expression of these polymorphic channels in communication-deficient cells revealed a decreased permeability of Cx37–1019C channels for neurobiotin. Conclusions— We propose that the establishment of gap junctional communication between Cx37-expressing platelets provides a mechanism to limit thrombus propensity. To our knowledge, these data provide the first evidence incriminating gap junctions in the pathogenesis of thrombosis.

The advent of multicellular organisms, some 800 million years ago, necessitated the development of mechanisms for cell-to-cell synchronization and for the spread of signals across increasingly large cell populations [168, 185]. Many structures and mechanisms have evolved to achieve such functions [4, 15]. Among these mechanisms, one which is prominent in both the invertebrate and the vertebrate world, across the entire phylogenetic scale, involves the transmembrane flux of large cytosolic and extracellular molecules [4, 15, 65, 66, 69–71, 121, 128, 129, 147, 154, 163]. These fluxes, in turn, are dependent on the formation of specific channels that in all animal classes are made by tetra-span integral membrane proteins [65, 66, 69–71, 121, 128, 129, 147, 154, 163] (Fig. 1). Fig. 1 Innexins, pannexins, and connexins form different types of gap junctional and “hemi-channels.” Invertebrates express many different innexins (purple; 25 isoforms in C. elegans) that may form either gap junction channels for cell-to-cell ... Keywords: Gap junctions, Connexons, Pannexons, Innexons, Membrane channels, Ca2+, ATP, Glutamate Three junctional protein families form membrane channels permeable to large molecules Early electrophysiological and electron microscopy studies converged in the realization that gap junctions, the membrane domains that concentrate intramembrane particles at sites of close membrane apposition, were the physical substrate of cell-to-cell communication in both invertebrate [59] and vertebrate tissues [139] (Fig. 1). The finding that similar drugs (the long-chain alcohols heptanol and octanol) and conditions (intracellular acidification) inhibited intercellular communication in both invertebrate and vertebrate systems [58, 85] was taken as further support that all gap junctions had a similar structure and function. Still, the different size of intramembrane particles, their different partition into the P- and the E-fracture faces of the cell membrane, and the different width of the gap space delineated by the two interacting membranes, suggested that the proteins making vertebrate gap junctions were different from those making the invertebrate structures [96]. These differences have functional consequences, as most elegantly demonstrated by co-culturing cell lines from different animal species. In these experiments, heterotypic coupling was shown between insect cells, as well as between different types of vertebrate cells, whereas virtually no coupling was observed between cells of phylogenetically distant species [48]. There is no longer any question that vertebrate gap junction channels are made by various combinations of different connexin proteins [66, 69–71]. The 20–21 isoforms of this family in rodents and man differ in size, but share a similar membrane topography. Thus, all connexins feature four transmembrane domains connected by two extracellular loops, each comprising three highly conserved Cys residues, a cytoplasmic loop, and both N and C termini in the cytosol (Fig. 2). The difference in size of the different connexin isoforms is essentially due to a different length of the cytoplasmic loop and/or the C terminus [39, 64, 65, 69–71, 109]. Current nomenclature (Table 1) designates connexin proteins as nCxZ, where n is the species (e.g., h, m, r for human, mouse, rat, respectively), and Z is the predicted molecular weight, in kDa. The genes encoding connexin proteins are named according to subgroups, in the order of discovery (e.g., Gjb1 encodes Cx32, Gjb2 encodes Cx26, Gja1 encodes Cx43, etc). Fig. 2 Membrane topography of the proteins forming gap junctions and “hemi-channels.” All invertebrate innexins, vertebrate ortholog pannexins, and nonhomologous vertebrate connexins feature four transmembrane domains connected by two extracellular ... Table 1 The family of human connexins Attempts to identify the proteins making invertebrate gap junctions initially revealed the proteins Ogre, Passover, Uncoordinated, and Shaking B in Drosophila and Caenorhabditis (hence the original OPUS acronym to name these proteins) with no primary sequence homology to connexins [128, 129, 195]. Since that time, more than 25 other junctional proteins revealing significant similarities have been identified in C. elegans, and it is clear that many other forms are expressed in other invertebrate species [75, 128, 129, 195] (Table 2). Collectively, these proteins have been named innexins to stress their invertebrate distribution and their role, analogous to that of connexins, in the formation of gap junctions (Fig. 1). Strikingly, innexins also share with connexins a similar structure and membrane topography (Fig. 2), even though the two sets of proteins have no homology in their primary amino acid sequence [75, 128, 129, 195]. Notably, connexins display three conserved Cys within each of the two extracellular loops, whereas innexins only carry two such residues. Thus, it is curious in retrospect that hydra development was reported to be blocked by an antibody prepared against Cx32 [53], that other antibodies have detected connexin-like proteins in anemone [112] and other marine invertebrates [5], and that junctional proteins isolated from hepatopancreas of crayfish and lobster were reported to have a sequence similar to that of rat liver connexins [51]. Table 2 A present view of the growing family of invertebrate innexins Sequencing of mammalian genomes has revealed a third family comprising only three genes that code for proteins with a primary sequence showing about 20% similarity to that of innexins [121, 154, 195]. On this basis, these proteins were thought to represent vertebrate homologs of the innexins, and were termed pannexins (Table 3) to encompass both invertebrate and vertebrate members [121, 154]. Like connexins and innexins, all three pannexins display N- and C-terminal domains within the cytoplasm, large extracellular and cytoplasmic loop domains, and four membrane spanning segments (Fig. 2). Like innexins but in contrast to connexins, pannexins contain two Cys residues in each extracellular loop [121, 154]. Furthermore, and in marked contrast with both innexins and connexins, pannexins display consensus sequences for glycosylation [13, 14, 125] (Fig. 2). The distribution of Pnx1 (Pnx1), the most studied form, is widespread and, in most types of cells and tissues, largely overlaps with that of connexins [17, 8]. While at least some phenotypes resulting from loss of specific connexin species are not compensated by pannexin changes [136, 146], suggesting a different function of the two protein families, in other cases, the effects of Pnx1 transfection mimicked that of Cx43, implicating a comparable/overlapping role of these two proteins [93]. Table 3 The family of human pannexins Connexins and innexins form cell-to-cell channels at gap junctional regions of the cell membrane Expression and deletion studies in a variety of systems have established that connexin hexamers, termed connexons, concentrate at gap junction domains of the cell membrane, where the intercellular space is reduced to a gap 2–3 nm wide. At these sites, the connexons of one cell align with, and strongly bind to the connexons of an adjacent cell, establishing a continuous intercellular hydrophilic pathway (Fig. 1) for the cell-to-cell exchange of multiple types of cytosolic molecules [64, 65, 69–71, 155]. The functional importance of this electrical and metabolic coupling is shown by a variety of striking and tissue-specific phenotypes that can be experimentally induced after overexpression or knock-out of individual connexin isoforms, as well as after the knock-in replacement of one isoform by another [92, 192]. It is further stressed by the identification of a number of diseases that are undoubtedly linked to connexin mutations [43, 57, 90, 94, 127, 143]. A variety of other diseases are thought to be due to altered amounts and/or function of these gap junction proteins [21, 108, 153]. Similarly, innexins oligomerize to form innexons that cluster at gap junctions of invertebrate cells (Fig. 1). Functional expression studies in paired Xenopus oocytes demonstrated that several, even though not all innexins also formed intercellular channels [8, 95, 166], and that at least some innexin mutations give rise to phenotypes expected for lack of gap junction-mediated intercellular communication [10, 25, 27, 37, 179]. In fact, it was on the basis of such dysfunctional phenotypes that the OPUS gene family was first identified. In contrast, and in spite of an initial report [18], pannexons appear unable to form sizable amounts of cell-to-cell channels under most conditions [76, 146, 147, 163] (Fig. 1). This lack of formation of functional gap junctions is likely due to the glycosylation of the extracellular loops of pannexins [13, 14, 125], which, as mentioned above, is not observed for either connexins or innexins. Still, forced expression of pannexins in paired Xenopus oocytes increased the conductance of the junctional cell membrane to current carrying ions in a way that can only be accounted for by the formation of pannexin cell-to-cell channels [13].

© 2015 Springer Basel. The different types of cells in the lung, from the conducting airway epithelium to the alveolar epithelium and the pulmonary vasculature, are interconnected by gap junctions. The specific profile of gap junction proteins, the connexins, expressed in these different cell types forms compartments of intercellular communication that can be further shaped by the release of extracellular nucleotides via pannexin1 channels. In this review, we focus on the physiology of connexins and pannexins and describe how this lung communication network modulates lung function and host defenses in conductive and respiratory airways.

The cystic fibrosis (CF) gene encodes a cAMP-gated Cl- channel (cystic fibrosis transmembrane conductance regulator [CFTR]) that mediates fluid transport across the luminal surfaces of a variety of epithelial cells. We have previously shown that gap junctional communication and Cl- secretion were concurrently regulated by cAMP in cells expressing CFTR. To determine whether intercellular communication and CFTR-dependent secretion are related, we have compared gap junctional coupling in a human pancreatic cell line harboring the DeltaF508 mutation in CFTR and in the same cell line in which the defect was corrected by transfection with wild-type CFTR. Both cell lines expressed connexin45 (Cx45), as evidenced by RT-PCR, immunocytochemistry, and dual patch-clamp recording. Exposure to agents that elevate intracellular cAMP or specifically activate protein kinase A evoked Cl- currents and markedly increased junctional conductance of CFTR-expressing pairs, but not in the parental cells. The latter effect, which was caused by an increase in single-channel activity but not in unitary conductance of Cx45 channels, was not prevented by exposing CFTR-expressing cells to a Cl- channel blocker. We conclude that expression of functional CFTR restored the cAMP-dependent regulation of junctional conductance in CF cells. Direct intercellular communication coordinates multicellular activity in tissues that are major targets of CF manifestations. Consequently, defective regulation of gap junction channels may contribute to the altered functions of tissues affected in CF.

Participation of connexin 40 (Cx40) in the regulation of renin secretion and in the tubuloglomerular feedback (TGF) component of renal autoregulation suggests that gap junctional coupling through Cx40 contributes to the function of the juxtaglomerular apparatus. In the present experiments, we determined the effect of targeted Cx40 deletion in C57BL/6 and FVB mice on TGF responsiveness. In C57BL/6 mice, stop-flow pressure (PSF) fell from 40.3 ± 2 to 34.5 ± 2 mmHg in wild-type (WT) and from 31 ± 1.06 to 26.6 ± 0.98 mmHg in Cx40−/− mice. PSF changes of 5.85 ± 0.67 mmHg in WT and of 4.3 ± 0.55 mmHg in Cx40−/− mice were not significantly different ( P = 0.08). In FVB mice, PSF fell from 37.4 ± 1.5 to 31.6 ± 1.5 mmHg in WT and from 28.1 ± 1.6 to 25.4 ± 1.7 mmHg in Cx40−/−, with mean TGF responses being significantly greater in WT than Cx40−/− (5.5 ± 0.55 vs. 2.7 ± 0.84 mmHg; P = 0.002). In both genetic backgrounds, PSF values were significantly lower in Cx40−/− than WT mice at all flow rates. Arterial blood pressure in the animals prepared for micropuncture was not different between WT and Cx40−/− mice. We conclude that the TGF response magnitude in superficial cortical nephrons is reduced by 30–50% in mice without Cx40, but that with the exception of a small number of nephrons, residual TGF activity is maintained. Thus gap junctional coupling appears to modulate TGF, perhaps by determining the kinetics of signal transmission.

Connexins are members of a large family of transmembrane proteins that oligomerize to form connexons or hemichannels, and connexons of adjacent cells dock to make gap junction channels. These channels allow the exchange of ions and small metabolites between the cytosol and extracellular space, or between the cytosols of neighbouring cells. Connexins are important in cardiovascular physiology; they support conducted vascular responses and allow for coordinated contraction of the heart. Four main connexins are expressed in the cardiovascular system: Cx37, Cx40, Cx43 and Cx45. Their expression pattern is not uniform and depends on intrinsic and environmental factors. Significant changes in the expression pattern, the cellular localization and the opening of connexin channels have been described during the development of atherosclerosis and after ischemia and reperfusion. In this review, we provide an overview of the roles of different connexins in these pathologies.

Connexins are a family of transmembrane proteins that are widely expressed in the human body. Connexins play an important role in cell-cell communication and homeostasis in various tissues by forming gap junction channels, which enable a direct passage of ions or metabolites from one cell to another. Twenty-one different connexins are expressed in humans, each having distinct expression patterns and regulation properties. Knowledge on this family of proteins can be gained by making an inventory of mutations and associated diseases in human.

Cellular interaction in blood vessels is maintained by multiple communication pathways, including gap junctions. They consist of intercellular channels ensuring direct interaction between endothelial and smooth muscle cells and the synchronization of their behavior along the vascular wall. Gap-junction channels arise from the docking of two hemichannels or connexons, formed by the assembly of six connexins, and achieve direct cellular communication by allowing the transport of small metabolites, second messengers, and ions between two adjacent cells. Physiologic variations in connexin expression are observed along the vascular tree, with most common connexins being Cx37, Cx40, and Cx43. Changes in the level of expression of connexins have been correlated to the development of vascular disease, such as hypertension, atherosclerosis, or restenosis. Recent studies on connexin-deficient mice highlighted key roles of these communication pathways in the development of these pathologies and confirmed the need for targeted pharmacologic approaches for their prevention and treatment. The aim of this issue is to review the current knowledge on the implication of gap junctions in vascular function and most common cardiovascular diseases. Antioxid. Redox Signal. 11, 267–282.

Electrical synapses continuously transfer signals bi-directionally from one cell to another, directly or indirectly via intermediate cells. Electrical synapses are common in many brain structures such as the inferior olive, the subcoeruleus nucleus and the neocortex, between neurons and between glial cells. In the cortex, interneurons have been shown to be electrically coupled and proposed to participate in large, continuous cortical syncytia, as opposed to smaller spatial domains of electrically coupled cells. However, to explore the significance of these findings it is imperative to map the electrical synaptic microcircuits, in analogy with in vitro studies on monosynaptic and disynaptic chemical coupling. Since "walking" from cell to cell over large distances with a glass pipette is challenging, microinjection of (fluorescent) dyes diffusing through gap-junctions remains so far the only method available to decipher such microcircuits even though technical limitations exist. Based on circuit theory, we derive analytical descriptions of the AC electrical coupling in networks of isopotential cells. We then suggest an operative electrophysiological protocol to distinguish between direct electrical connections and connections involving one or more intermediate cells. This method allows inferring the number of intermediate cells, generalizing the conventional coupling coefficient, which provides limited information. We validate our method through computer simulations, theoretical and numerical methods and electrophysiological paired recordings.

Atherosclerosis, the main cause of death and disability in adult populations of industrialized societies, is a multifactorial progressive process involving a variety of pathogenic mechanisms. Our current view on the pathogenesis of the disease implies complex patterns of interactions between a dysfunctional endothelium, leukocytes, and activated smooth muscle cells in which cytokines and growth factors are known to play a crucial role. Apart from paracrine cell-to-cell signalling, a role for gap junction-mediated intercellular communication in the development of the disease has been recently suggested. Gap junction channels result from the docking of two hemichannels or connexons, formed by the hexameric assembly of connexins, and directly connect the cytoplasm of adjacent cells. In this review, we summarize existing evidence implicating connexins in atherosclerosis. Indeed, the expression pattern of vascular connexins is altered during atherosclerotic plaque formation. In addition, changes in connexin expression or gap junctional communication have been observed in vascular cells in vitro by disturbances in blood flow, cholesterol, inflammatory cytokines, and growth factors. Furthermore, genetically modifying connexin expression affects the course of the atherosclerotic process in mouse models of the disease. Finally, the involvement of connexins in treatment of atherosclerotic disease will be discussed.

Local injury induces a complex orchestrated response to stimulate healing of injured tissues, cellular regeneration and phagocytosis. Practically, inflammation is defined as a defense process whereby fluid and white blood cells accumulate at a site of injury. The balance of cytokines, chemokines, and growth factors is likely to play a key role in regulating important cell functions such as migration, proliferation, and matrix synthesis during the process of inflammation. Hence, the initiation, maintenance, and resolution of innate responses depend upon cellular communication. A process similar to tissue repair and subsequent scarring is found in a variety of fibrotic diseases. This may occur in a single organ such as liver, kidneys, pancreas, lung, skin, and heart, but fibrosis may also have a more generalized distribution such as in atherosclerosis. The purpose of this review is to summarize recent advances on the contribution of gap junction-mediated intercellular communication in the modulation of the inflammatory response and tissue repair.

Tumor necrosis factor-alpha (TNF-alpha) signaling is central to the transmission of the innate immune response and subsequent activation of the adaptive immune system. The functioning of both systems is required for optimal clearance of pathogens from the airways. In cystic fibrosis (CF), dysfunction of the CF transmembrane conductance regulator (CFTR) is associated with recurrent pulmonary infections despite an intense inflammatory and immune response. We reported recently that TNF-alpha decreased gap junction connectivity in non-CF airway cells, a mechanism that was absent in CF cells expressing the DeltaPhe-508 mutant of CFTR. We have now identified the tyrosine kinase c-Src as a possible pathway between the mediators of inflammation and the gap junction protein connexin43 (Cx43). Indeed, TNF-alpha increased the proportion of activated c-Src in non-CF airway cells. Moreover, pharmacological antagonists and expression in non-CF cells of a dominant negative construct of c-Src prevented Cx43 channel closure by TNF-alpha. Finally, gap junction channel closure was prevented by expression of a Cx43 mutant lacking tyrosine phosphorylation sites for c-Src. Additional experiments showed that activation of c-Src was defective in CF airway cells but rescued in CFTR-corrected CF cells. These data suggest that CFTR dysfunction is associated with altered TNF-alpha signaling, resulting in the persistence of gap junction connectivity in CF airway cells. We propose that altered regulation of c-Src may contribute to the dysregulated inflammatory response that is characteristic of the CF phenotype.

Gap junction channels provide a pathway for coordinating multicellular activity. To evaluate the contribution of cell-to-cell communication in the function of epithelial cells, we studied the strength of gap junctional coupling in pancreatic acinar and duct cells exposed to agents known to elevate the intracellular concentration of Ca(2+) or cAMP. In acinar cells, we observed that maximal concentrations of acetylcholine evoked a biphasic increase in cytosolic Ca(2+) mobilization. The second sustained phase, which depends on Ca(2+) influx into the cell, was associated with the rapid closure of gap junction channels. In duct cells, stimulation of CFTR-dependent Cl(-) currents with cAMP analogs markedly increased gap junctional conductance in pairs of cells. Interestingly, cAMP had no effect on intercellular communication between cells harboring the DeltaF508 mutation of CFTR. An abnormal pattern of gap junctional coupling may contribute to the altered functions of tissues affected in cystic fibrosis.

To determine whether junctional communication between pancreatic acinar cells contributes to their secretory function in vivo, we have compared wild-type mice, which express the gap junctional proteins connexin32 (Cx32) and connexin26, to mice deficient for the Cx32 gene. Pancreatic acinar cells from Cx32 (−/−) mice failed to express Cx32 as evidenced by reverse transcription–PCR and immunolabeling and showed a marked reduction (4.8- and 25-fold, respectively) in the number and size of gap junctions. Dye transfer studies showed that the extent of intercellular communication was inhibited in Cx32 (−/−) acini. However, electrical coupling was detected by dual patch clamp recording in Cx32 (−/−) acinar cell pairs. Although wild-type and Cx32 (−/−) acini were similarly stimulated to release amylase by carbamylcholine, Cx32 (−/−) acini showed a twofold increase of their basal secretion. This effect was caused by an increase in the proportion of secreting acini, as detected with a reverse hemolytic plaque assay. Blood measurements further revealed that Cx32 (−/−) mice had elevated basal levels of circulating amylase. The results, which demonstrate an inverse relationship between the extent of acinar cell coupling and basal amylase secretion in vivo, support the view that the physiological recruitment of secretory acinar cells is regulated by gap junction mediated intercellular communication.