Gap junctions are plasma membrane spatial microdomains constructed of assemblies of channel proteins called connexins in vertebrates and innexins in invertebrates. The channels provide direct intercellular communication pathways allowing rapid exchange of ions and metabolites up to approximately 1 kD in size. Approximately 20 connexins are identified in the human or mouse genome, and orthologues are increasingly characterized in other vertebrates. Most cell types express multiple connexin isoforms, making likely the construction of a spectrum of heteromeric hemichannels and heterotypic gap junctions that could provide a structural basis for the charge and size selectivity of these intercellular channels. The precise nature of the potential signalling information traversing junctions in physiologically defined situations remains elusive, but extensive progress has been made in elucidating how connexins are assembled into gap junctions. Also, participation of gap junction hemichannels in the propagation of calcium waves via an extracellular purinergic pathway is emerging. Connexin mutations have been identified in a number of genetically inherited channel communication-opathies. These are detected in connexin 32 in Charcot Marie Tooth-X linked disease, in connexins 26 and 30 in deafness and skin diseases, and in connexins 46 and 50 in hereditary cataracts. Biochemical approaches indicate that many of the mutated connexins are mistargeted to gap junctions and/or fail to oligomerize correctly into hemichannels. Genetic ablation approaches are helping to map out a connexin code and point to specific connexins being required for cell growth and differentiation as well as underwriting basic intercellular communication.

Gap junctions are membrane channels that mediate the direct passage of ions and molecules between adjacent cells. Recent tracer coupling and optical recording studies have revealed the presence of gap junction-mediated communication between neurons during neocortical development. We have visualized gap junctions in the developing rat cerebral cortex with electron microscopy and studied the pattern of expression and cellular localization of connexins 26, 32, and 43 that take part in their formation. We found that these connexins (Cxs) are expressed differentially during development, and their patterns of expression are correlated with important developmental events such as cell proliferation, migration, and formation of cortical neuronal circuits. Specifically, we observed that the developmental profile of Cx 26 during the first 3 weeks of postnatal life matched closely the development of neuronal coupling, suggesting that coupled neurons use this gap junction protein during circuit formation in the cortex. The subsequent diminution of Cx 26 was mirrored by an increase in Cx 32 immunoreactivity, which became pronounced at the late stages of cortical maturation. In contrast, Cx 43 was localized in the cortex throughout the period of development. Its localization in radial glial fibers closely associated with migrating neurons suggests that this Cx may be involved in neuronal migration.

Connexin hemichannels have been proposed as a diffusion pathway for the release of extracellular messengers like ATP and others, based on connexin expression models and inhibition by gap junction blockers. Hemichannels are opened by various experimental stimuli, but the physiological intracellular triggers are currently not known. We investigated the hypothesis that an increase of cytoplasmic calcium concentration ([Ca2+]i) triggers hemichannel opening, making use of peptides that are identical to a short amino-acid sequence on the connexin subunit to specifically block hemichannels, but not gap junction channels. Our work performed on connexin 32 (Cx32)-expressing cells showed that an increase in [Ca2+]i triggers ATP release and dye uptake that is dependent on Cx32 expression, blocked by Cx32 (but not Cx43) mimetic peptides and a calmodulin antagonist, and critically dependent on [Ca2+]i elevation within a window situated around 500 nM. Our results indicate that [Ca2+]i elevation triggers hemichannel opening, and suggest that these channels are under physiological control.

The contribution of gap junctions to endothelium‐dependent relaxation was investigated in isolated rabbit conduit artery preparations pre‐constricted by 10 μM phenylephrine (PhE). Acetylcholine (ACh) relaxed the thoracic aorta by ≈60 % and the superior mesenteric artery (SMA) by ≈90 %. A peptide possessing sequence homology with extracellular loop 2 of connexin 43 (Gap 27, 300 μM) inhibited relaxation by ≈40 % in both artery types. Gap 27 also attenuated the endothelium‐dependent component of the relaxation induced by ATP in thoracic aorta but did not modify force development in response to PhE. N G‐nitro‐L‐arginine methyl ester (L‐NAME, 300 μM), an inhibitor of NO synthase, attenuated ACh‐induced relaxation by ≈90 % in the aorta but only by ≈40 % in SMA (P < 0.05). Residual L‐NAME‐insensitive relaxations were almost abolished by 300 μM Gap 27 in aorta and inhibited in a concentration‐dependent fashion in SMA (≈50 % at 100 μM and ≈80 % at 10 mM). Gap 27 similarly attenuated the endothelium‐dependent component of L‐NAME‐insensitive relaxations to ATP in aorta. Responses to cyclopiazonic acid, which stimulates endothelium‐dependent relaxation through a receptor‐independent mechanism, were also attenuated by Gap 27, whereas this peptide exerted no effect on the NO‐mediated relaxation induced by sodium nitroprusside in preparations denuded of endothelium. ACh‐induced relaxation of ‘sandwich’ mounts of aorta or SMA were unaffected by Gap 27 but completely abolished by L‐NAME. We conclude that direct heterocellular communication between the endothelium and smooth muscle contributes to endothelium‐dependent relaxations evoked by both receptor‐dependent and ‐independent mechanisms. The inhibitory effects of Gap 27 peptide do not involve homocellular communication within the vessel wall or modulation of NO release or action.

The increasing use of nanoparticles in medicine has raised concerns over their ability to gain access to privileged sites in the body. Here, we show that cobalt-chromium nanoparticles (29.5 +/- 6.3 nm in diameter) can damage human fibroblast cells across an intact cellular barrier without having to cross the barrier. The damage is mediated by a novel mechanism involving transmission of purine nucleotides (such as ATP) and intercellular signalling within the barrier through connexin gap junctions or hemichannels and pannexin channels. The outcome, which includes DNA damage without significant cell death, is different from that observed in cells subjected to direct exposure to nanoparticles. Our results suggest the importance of indirect effects when evaluating the safety of nanoparticles. The potential damage to tissues located behind cellular barriers needs to be considered when using nanoparticles for targeting diseased states.

Phenylephrine (10 μM) evoked rises in tension in isolated rings of endothelium‐denuded rabbit superior mesenteric artery. These increases consisted of a tonic component with superimposed rhythmic activity, the frequency of which generally remained constant over time but whose amplitude exhibited cycle‐to‐cycle variability. The amplitude, but not the frequency, of the rhythmic activity was affected by a series of short peptides possessing sequence homology with extracellular loops 1 and 2 of connexin 43 (Cx43). Oscillatory behaviour was abolished at concentrations of 100–300 μM (IC50 of 20–30 μM), without change in average tone. No synergy was evident between peptides corresuponding to the extracellular loops, and cytoplasmic loop peptides were biologically inactive. The putative gap junction inhibitor heptanol mimicked the action of the extracellular loop peptides and abolished rhythmic activity at concentrations of 100–300 μM without effects on frequency. However, in marked contrast to the peptides, heptanol completely inhibited the contraction evoked by phenylephrine (IC50, 283 ± 28 μM). The presence of mRNA encoding Cx32, Cx40 and Cx43 was detected in the rabbit superior mesenteric artery by reverse transcriptase‐polymerase chain reaction. Western blot analysis showed that Cx43 was the major connexin in the endothelium‐denuded vessel wall. We conclude that intercellular communication between vascular smooth muscle cells via gap junctions is essential for synchronized rhythmic activity in isolated arterial tissue, whereas tonic force development appears to be independent of cell‐cell coupling. The molecular specificity of the peptide probes employed in the study suggests that the smooth muscle relaxant effects of heptanol may be non‐supecific and unrelated to inhibition of gap junctional communication.

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

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