A particle-receptor model for the insulin-induced closure of connexin43 channels

From spikes to intercellular waves: Tuning intercellular calcium signaling dynamics modulates organ size control

Information flow within and between cells depends significantly on calcium (Ca²⁺) signaling dynamics. However, the biophysical mechanisms that govern emergent patterns of Ca²⁺ signaling dynamics at the organ level remain elusive. Recent experimental studies in developing Drosophila wing imaginal discs demonstrate the emergence of four distinct patterns of Ca²⁺ activity: Ca²⁺ spikes, intercellular Ca²⁺ transients, tissue-level Ca²⁺ waves, and a global “fluttering” state. Here, we used a combination of computational modeling and experimental approaches to identify two different populations of cells within tissues that are connected by gap junction proteins. We term these two subpopulations “initiator cells,” defined by elevated levels of Phospholipase C (PLC) activity, and “standby cells,” which exhibit baseline activity. We found that the type and strength of hormonal stimulation and extent of gap junctional communication jointly determine the predominate class of Ca²⁺ signaling activity. Further, single-cell Ca²⁺ spikes are stimulated by insulin, while intercellular Ca²⁺ waves depend on Gαq activity. Our computational model successfully reproduces how the dynamics of Ca²⁺ transients varies during organ growth. Phenotypic analysis of perturbations to Gαq and insulin signaling support an integrated model of cytoplasmic Ca²⁺ as a dynamic reporter of overall tissue growth. Further, we show that perturbations to Ca²⁺ signaling tune the final size of organs. This work provides a platform to further study how organ size regulation emerges from the crosstalk between biochemical growth signals and heterogeneous cell signaling states.

Calcium (Ca²⁺) is a universal second messenger that regulates a myriad of cellular processes such as cell division, cell proliferation and apoptosis. Multiple patterns of Ca²⁺ signaling including single-cell spikes, multicellular Ca²⁺ transients, large-scale Ca²⁺ waves, and global “fluttering” have been observed in epithelial systems during organ development. Key molecular players and biophysical mechanisms involved in formation of these patterns during organ development are not well understood. In this work, we developed a generalized multicellular model of Ca²⁺ that captures all the key categories of Ca²⁺ activity as a function of key hormonal signals. Integration of model predictions and experiments reveals two subclasses of cell populations and demonstrates that Ca²⁺ signaling activity at the organ scale is defined by a general decrease in gap junction communication as an organ grows. Our experiments also reveal that a “goldilocks zone” of optimal Ca²⁺ activity is required to achieve optimal growth at the organ level.

Role of the Connexin C-terminus in skin pattern formation of Zebrafish

Background

Zebrafish display a striped skin pattern on their body; two types of connexins, namely, Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), are involved in stripe pattern formation. Herein, we investigated the role of the C-terminal (CT) domains of Cx39.4 and Cx41.8 in vivo and in vitro.

Methods

To investigate the role of CT domains in vivo, we established transgenic zebrafish lines expressing the CT-domain-modified connexin series in pigmented cells and observed skin patterns in fish. To investigate the role of the CT domains in vitro, we expressed the CT-domain modified connexin series in Neuro-2a (N2a) cells and calculated the plaque formation frequency.

Results

The overexpression of Cx39.4 lacking a CT domain produced skin patterns similar to that produced by full-length Cx39.4 in the cx39.4 ^-/- mutant and in cx39.4 and cx41.8 double-knockout mutant zebrafish. Fluorescence-protein-fused CT-domain-modified Cx39.4 formed gap junction plaques between N2a cells. The overexpression of CT-truncated Cx41.8 rescued the mutant phenotype in the cx41.8 ^-/- mutant but did not function in the double knockout zebrafish. Fluorescence-protein-fused CT-truncated Cx41.8 hardly formed plaques between N2a cells without Cx39.4 but formed gap junction plaques when co-expressed with Cx39.4.

Conclusions

The CT domain of Cx39.4 is not required for protein function, at least in the pigment cells of zebrafish. However, the need for the CT domain of Cx41.8 depends on Cx39.4 expression.

General significance

These results provide evidence for the interactions between Cx39.4 and Cx41.8 in pigment cells of zebrafish and suggest that at least one connexin must have a CT domain.

Dispelling myths about connexins, pannexins and P2X7 in hypoxic-ischemic central nervous system

In membrane physiology, as in other fields, myths or speculations may be repeated so often and so widely that they are perceived as facts. To some extent, this has occurred with regard to gap junctions, hemichannels, pannexin channels and P2X7 (ionotropic receptors), especially concerning the interpretation of the individual role of these channels in hypoxic-ischemic CNS since these channels may be closed by the same pharmacological blockers. Significance of existing controversial data are highlighted and contradictory views from different groups are critically discussed herein.

Phosphorylation of Cx43 residue Y313 by Src contributes to blocking the interaction with Drebrin and disassembling gap junctions

Phosphorylation regulates connexin43 (Cx43) function from assembly/disassembly to coupling at the plaque. Src is a tyrosine kinase known to both phosphorylate Cx43 (residues Y247 and Y265) and affect gap junction intercellular communication. However, the Cx43 carboxyl-terminal (CT) domain contains additional tyrosine residues and proteomic discovery mass spectrometry data identified Y313 as a potential phosphorylation target. Based upon the study of Lin et al. (2001) J. Cell Biol., which still observed tyrosine phosphorylation by Src when using a Cx43 Y247/Y265F mutant, we addressed the possibility of Y313 phosphorylation (pY313) by Src. In vitro Src phosphorylation of purified Cx43CT followed by mass spectroscopy revealed that Src also phosphorylates Y313. This observation was confirmed by repeating the in vitro phosphorylation using different combinations of Cx43CT Y → F mutants and a general anti-pTyr antibody. Next, a phospho-specific antibody was generated to help characterize the importance of pY313. We established an in cyto experimental system by stably expressing Cx43 WT and mutants (Y247F, Y265F, Y313F, Y247/265F, Y247/313F, Y265/313F, or Y247/265/313F) in Cx43-deficient HeLa cells. Cx43 WT and mutants, in the absence of v-Src, localized to the plasma membrane and formed gap junctions. When v-Src was over-expressed, Cx43 WT localized intracellularly, while all of the single and double mutants remained able to form plaques and transfer dye, albeit variable in number and amount, respectively. Complete Src-resistance was only achieved with the Cx43 Y247/265/313F mutant. Furthermore, Cx43 Y265F inhibited the ability of v-Src to phosphorylate Y247 and Y313 as well as phosphorylation at both Y265 and Y313 was necessary to inhibit the Cx43 interaction with Drebrin. Finally, we observed in diseased cardiac tissue, in which Src is active, an increase in intercalated disc and intracellular localized Cx43 pY313.

Protein⁻Protein Interactions with Connexin 43: Regulation and Function

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

Cx32 hemichannel opening by cytosolic Ca2+ is inhibited by the R220X mutation that causes Charcot-Marie-Tooth disease

Mutations of the GJB1 gene encoding connexin 32 (Cx32) cause the X-linked form of Charcot-Marie-Tooth disease (CMTX1), a demyelinating peripheral neuropathy for which there is no cure. A growing body of evidence indicates that ATP release through Cx32 hemichannels in Schwann cells could be critical for nerve myelination, but it is unknown if CMTX1 mutations alter the cytosolic Ca2+-dependent gating mechanism that controls Cx32 hemichannel opening and ATP release. The current study uncovered that loss of the C-terminus in Cx32 (R220X mutation), which causes a severe CMTX1 phenotype, inhibits hemichannel opening during a canonical IP3-mediated increase in cytosolic Ca2+ in HeLa cells. Interestingly, the gating function of R220X hemichannels was completely restored by both the intracellular and extracellular application of a peptide that mimics the Cx32 cytoplasmic loop. All-atom molecular dynamics simulations suggest that loss of the C-terminus in the mutant hemichannel triggers abnormal fluctuations of the cytoplasmic loop which are prevented by binding to the mimetic peptide. Experiments that stimulated R220X hemichannel opening by cell depolarization displayed reduced voltage sensitivity with respect to wild-type hemichannels which was explained by loss of subconductance states at the single channel level. Finally, experiments of intercellular diffusion mediated by wild-type or R220X gap junction channels revealed similar unitary permeabilities to ions, signalling molecules (cAMP) or larger solutes (Lucifer yellow). Taken together, our findings support the hypothesis that paracrine signalling alteration due to Cx32 hemichannel dysfunction underlies CMTX1 pathogenesis and suggest a candidate molecule for novel studies investigating a therapeutic approach.

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.

Cysteine residues in the cytoplasmic carboxy terminus of connexins dictate gap junction plaque stability

Cysteine residues within the cytoplasmic carboxyl-terminus of gap junction–forming proteins are required to stabilize gap junction plaque organization. The stability of gap junction plaque organization can be modified. Gap junction stability may provide a stable supramolecular platform for modulation of gap junction functions.

Gap junctions are cellular contact sites composed of clustered connexin transmembrane proteins that act in dual capacities as channels for direct intercellular exchange of small molecules and as structural adhesion complexes known as gap junction nexuses. Depending on the connexin isoform, the cluster of channels (the gap junction plaque) can be stably or fluidly arranged. Here we used confocal microscopy and mutational analysis to identify the residues within the connexin proteins that determine gap junction plaque stability. We found that stability is altered by changing redox balance using a reducing agent—indicating gap junction nexus stability is modifiable. Stability of the arrangement of connexins is thought to regulate intercellular communication by establishing an ordered supramolecular platform. By identifying the residues that establish plaque stability, these studies lay the groundwork for exploration of mechanisms by which gap junction nexus stability modulates intercellular communication.

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.

Determinants of Cx43 Channel Gating and Permeation: The Amino Terminus

Separate connexin domains partake in proposed gating mechanisms of gap junction channels. The amino-terminus (NT) domains, which contribute to voltage sensing, may line the channel's cytoplasmic-facing funnel surface, stabilize the channel's overall structure through interactions with the transmembrane domains and each other, and integrate to form a compound particle to gate the channel closed. Interactions of the carboxyl-terminus (CT) and cytoplasmic loop (CL) domains underlie voltage- and low pH-triggered channel closure. To elucidate potential cooperation of these gating mechanisms, we replaced the Cx43NT with the Cx37NT (chimera Cx43(∗)NT37), leaving the remainder of the Cx43 sequence, including the CT and CL, unchanged. Compared to wild-type Cx43 (Cx43WT), Cx43(∗)NT37 junctions exhibited several functional alterations: extreme resistance to halothane- and acidification-induced uncoupling, absence of voltage-dependent fast inactivation, longer channel open times, larger unitary channel conductances, low junctional dye permeability/permselectivity, and an overall cation selectivity more typical of Cx37WT than Cx43WT junctions. Together, these results suggest a cohesive model of channel function wherein: 1) channel conductance and size selectivity are largely determined by pore diameter, whereas charge selectivity results from the NT domains, and 2) transition between fully open and (multiple) closed states involves global changes in structure of the pore-forming domains transduced by interactions of the pore-forming domains with either the NT, CT, or both, with the NT domains forming the gate of the completely closed channel.

Connexin43 Forms Supramolecular Complexes through Non-Overlapping Binding Sites for Drebrin, Tubulin, and ZO-1

Gap junctions are membrane specialization domains identified in most tissue types where cells abut each other. The connexin channels found in these membrane domains are conduits for direct cell-to-cell transfer of ions and molecules. Connexin43 (Cx43) is the most ubiquitous connexin, with critical roles in heart, skin, and brain. Several studies described the interaction between Cx43 and the cytoskeleton involving the actin binding proteins Zonula occludens (ZO-1) and drebrin, as well as with tubulin. However, a direct interaction has not been identified between drebrin and Cx43. In this study, co-IP and NMR experiments were used to demonstrate that the Cx43-CT directly interacts with the highly conserved N-terminus region of drebrin. Three Cx43-CT areas were found to be involved in drebrin binding, with residues 264–275 being critical for the interaction. Mimicking Src phosphorylation within this region (Y265) significantly disrupted the interaction between the Cx43-CT and drebrin. Immunofluorescence showed colocalization of Cx43, drebrin, and F-actin in astrocytes and Vero cells membrane, indicating that Cx43 forms a submembrane protein complex with cytoskeletal and scaffolding proteins. The co-IP data suggest that Cx43 indirectly interacts with F-actin through drebrin. Along with the known interaction of the Cx43-CT with ZO-1 and tubulin, the data presented here for drebrin indicate non-overlapping and separated binding sites for all three proteins for which simultaneous binding could be important in regulating cytoskeleton rearrangements, especially for neuronal migration during brain development.

Mutant connexin 50 (S276F) inhibits channel and hemichannel functions inducing cataract

This study was designed to detect the expression, detergent resistance, subcellular localization, and channel and hemichannel functions of mutant Cx50 to understand the forming mechanism for inducing congenital cataract by a novel mutation p.S276F in connexin 50 (Cx50) reported previously by us. HeLa and human lens epithelial (HLE) cells were transfected with wild-type Cx50 and mutant Cx50 (S276F). We examined the functional characteristics of mutant Cx50 (S276F) in comparison with those of wild-type Cx50 using immunoblot, confocal fluorescence microscopy, dye transfer analysis and dye uptake assay. The mutant and wild-type Cx50 were expressed in equal levels and could efficiently localize to the plasma membrane without transportation and assembly problems. Scrape loading dye transfer was significantly evident in cells transfected with wild-type Cx50 compared to those in cells transfected with mutant Cx50 and cotransfected with wild-type and mutant Cx50. The dye uptake was found to be significantly lower in cells transfected with mutant Cx50 than in cells transfected with wild- type Cx50 and cells cotransfected with wild-type and mutant Cx50. The transfected HeLa and HLE cell lines showed similar performance in all the experiments. These results indicated that the mutant Cx50 (S276F) might inhibit the function of gap junction channel in a dominant negative manner, but inhibit the hemichannel function in a recessive negative manner.

Diabetes Increases Cryoinjury Size with Associated Effects on Cx43 Gap Junction Function and Phosphorylation in the Mouse Heart

Diabetic patients develop larger myocardial infarctions and have an increased risk of death following a heart attack. The poor response to myocardial injury in the diabetic heart is likely related to the many metabolic derangements from diabetes that create a poor substrate in general for wound healing, response to injury and infection. Studies in rodents have implicated a role for the gap junction protein connexin 43 (Cx43) in regulating the injury response in diabetic skin wounds. In this study, we sought to determine whether diabetes alters Cx43 molecular interactions or intracellular communication in the cryoinjured STZ type I diabetic mouse heart. We found that epicardial cryoinjury size is increased in diabetic mice and this increase is prevented by preinjury insulin administration. Consistent with these findings, we found that intercellular coupling via gap junctions is decreased after insulin administration in diabetic and nondiabetic mice. This decrease in coupling is associated with a concomitant increase in phosphorylation of Cx43 at serine 368, a residue known to decrease channel conductance. Taken together, our results suggest that insulin regulates both gap junction-mediated intercellular communication and injury propagation in the mouse heart.

Gap junctional regulation of pressure, fluid force, and electrical fields in the epigenetics of cardiac morphogenesis and remodeling

Epigenetic factors of pressure load, fluid force, and electrical fields that occur during cardiac contraction affect cardiac development, morphology, function, and pathogenesis. These factors are orchestrated by intercellular communication mediated by gap junctions, which synchronize action potentials and second messengers. Misregulation of the gap junction protein connexin (Cx) alters cardiogenesis, and can be a pathogenic factor causing cardiac conduction disturbance, fatal arrhythmia, and cardiac remodeling in disease states such as hypertension and ischemia. Changes in Cx expression can occur even when the DNA sequence of the Cx gene itself is unaltered. Posttranslational modifications might reduce arrhythmogenic substrates, improve cardiac function, and promote remodeling in a diseased heart. In this review, we discuss the epigenetic features of gap junctions that regulate cardiac morphology and remodeling. We further discuss potential clinical applications of current knowledge of the structure and function of gap junctions.

Functional analysis and regulation of purified connexin hemichannels

Gap-junction channels (GJCs) are aqueous channels that communicate adjacent cells. They are formed by head-to-head association of two hemichannels (HCs), one from each of the adjacent cells. Functional HCs are connexin hexamers composed of one or more connexin isoforms. Deafness is the most frequent sensineural disorder, and mutations of Cx26 are the most common cause of genetic deafness. Cx43 is the most ubiquitous connexin, expressed in many organs, tissues, and cell types, including heart, brain, and kidney. Alterations in its expression and function play important roles in the pathophysiology of very frequent medical problems such as those related to cardiac and brain ischemia. There is extensive information on the relationship between phosphorylation and Cx43 targeting, location, and function from experiments in cells and organs in normal and pathological conditions. However, the molecular mechanisms of Cx43 regulation by phosphorylation are hard to tackle in complex systems. Here, we present the use of purified HCs as a model for functional and structural studies. Cx26 and Cx43 are the only isoforms that have been purified, reconstituted, and subjected to functional and structural analysis. Purified Cx26 and Cx43 HCs have properties compatible with those demonstrated in cells, and present methodologies for the functional analysis of purified HCs reconstituted in liposomes. We show that phosphorylation of serine 368 by PKC produces a partial closure of the Cx43 HCs, changing solute selectivity. We also present evidence that the effect of phosphorylation is highly cooperative, requiring modification of several connexin subunits, and that phosphorylation of serine 368 elicits conformational changes in the purified HCs. The use of purified HCs is starting to provide critical data to understand the regulation of HCs at the molecular level.

Relevance of gap junctions and large pore channels in traumatic brain injury

In case of traumatic brain injury (TBI), occurrence of central nervous tissue damage is frequently aligned with local modulations of neuronal and glial gap junction channel expression levels. The degree of gap junctional protein expression and intercellular coupling efficiency, as well as hemichannel function has substantially impact on the course of trauma recovery and outcome. During TBI, gap junctions are especially involved in the intercellular molecule trafficking on repair of blood vessels and the regulation of vasomotor tone. Furthermore, gliosis and astrocytic swelling due to mechanical strain injury point out the consequences of derailed gap junction communication. This review addresses the outstanding role of gap junction channels in TBI pathophysiology and links the current state of results to applied clinical procedures as well as perspectives in acute and long-term treatment options.

Peptides and peptide-derived molecules targeting the intracellular domains of Cx43: gap junctions versus hemichannels

About a decade ago, the molecular determinants controlling the opening and closing of Cx43 gap junction channels have been identified. Advanced biophysical approaches revealed a critical role for structural rearrangements in the cytoplasmic loop and dimerization of the C-terminal tail, resulting in binding of the C-terminal tail to the cytoplasmic loop and Cx43 gap junction channel closure during cellular acidosis. This has spurred the development of Cx43-mimetic peptides and peptidomimetics that interfere with these loop/tail interactions, thereby preventing the closure of Cx43 gap junctions, e.g. in the heart upon ischemia. Recently, we found that loop/tail interactions control Cx43-hemichannel activity but with an opposite effect. Binding of the C-terminal tail to the cytoplasmic loop is a requisite for the opening of Cx43 hemichannels in response to different stimuli, like decreased extracellular [Ca2+], increased intracellular [Ca2+], positive membrane potentials or ischemia. Strikingly, peptides that favor the open state of Cx43 gap junctions like the L2 peptide inhibit Cx43-hemichannel opening. These tools now provide unprecedented opportunities to selectively inhibit Cx43 hemichannels while maintaining Cx43 gap junction communication, impossible to achieve with siRNA or knockdown approaches both affecting gap junctions and hemichannels. These tools not only are very helpful to unravel the role of Cx43 hemichannels in complex biological systems, but also hold therapeutic potential to counteract excessive Cx43-hemichannel activity like in ischemia/reperfusion in the brain and the heart or to prevent Cx43 hemichannel-mediated gliotransmitter release in the basal amygdala during memory consolidation in response to emotional events. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Effective protection of rabbits' explosive brain injury through blocking gap junction communication

BACKGROUND

The gap junction plays an important role in spreading of apoptotic and necrotic signals from injured and stressed cells to the neighboring viable cells. The present study was performed to investigate the important role of gap junction communication on rabbits' explosive brain injury.

METHODS

Explosion of paper detonators was used to create explosive brain injury model in 60 rabbits, which was randomly divided into control group and experimental group. Octanol, an efficient blocker of gap junction, was injected in the left ventricle to block gap junction communication in the experimental group 2 hours before injury, while the same volume of saline was utilized in the control group.

RESULTS

Penumbra volume around the brain contusion in the experimental group was significantly less than that in the control group at 1d and 3d after brain damage. RT-PCR and Western blotting analysis indicated that the expression of connexin-43 (Cx43) and caspase-3 was significantly lower in the experimental group than that in the control group at all time points.

CONCLUSION

Rabbits' explosive brain injury can be efficiently attenuated through blocking the gap junction communication, which benefit for deeper understanding the mechanism of brain injury.

K ATP channel agonists preserve connexin43 protein in infarcted rats by a protein kinase C-dependent pathway

Downward remodelling of gap junctional proteins between myocytes may trigger ventricular arrhythmia after myocardial infarction. We have demonstrated that ATP-sensitive potassium (K_ATP) channel agonists attenuated post-infarction arrhythmias. However, the involved mechanisms remain unclear. The purpose of this study was to determine whether K_ATP channel agonists can attenuate arrhythmias through preserving protein kinase C (PKC)-–dependent connexin43 level after myocardial infarction. Male Wistar rats after ligating coronary artery were randomized to either vehicle, nicorandil, pinacidil, glibenclamide or a combination of nicorandil and glibenclamide or pinacidil and glibenclamide for 4 weeks. To elucidate the role of PKC in the modulation of connexin43 level, carbachol and myristoylated PKC V1–2 peptide were also assessed. Myocardial connexin43 level was significantly decreased in vehicle-treated infarcted rats compared with sham. Attenuated connexin43 level was blunted after administering K_ATP channel agonists, assessed by immunofluorescent analysis, Western blotting, and real-time quantitative reverse transcription-PCR of connexin43. Arrhythmic scores during programmed stimulation in the K_ATP channel agonists-treated rats were significantly lower than those treated with vehicle. The beneficial effects of K_ATP channel agonists were blocked by either glibenclamide or 5-hydroxydecanoate. Addition of the PKC activator, phorbol 12-myristate 13-acetate and the specific PKC agonist, carbachol, blocked the effects of nicorandil on connexin43 phosphorylation and dye permeability. The specific PKC antagonist, myristoylated PKC V1–2 peptide, did not have additional beneficial effects on connexin43 phosphorylation compared with rats treated with nicorandil alone. Chronic use of K_ATP channel agonists after infarction, resulting in enhanced connexin43 level through a PKC-dependent pathway, may attenuate the arrhythmogenic response to programmed electrical stimulation.

Coregulation of multiple signaling mechanisms in pp60v-Src-induced closure of Cx43 gap junction channels

Attenuation in gap junctional coupling has consistently been associated with induction of rapid or synchronous cell division in normal and pathological conditions. In the case of the v-src oncogene, gating of Cx43 gap junction channels has been linked to both direct phosphorylation of tyrosines (Y247 and 265) and phosphorylation of the serine targets of Erk1/2 (S255, 279 and 282) on the cytoplasmic C-terminal domain of Cx43. However, only the latter has been associated with acute, rather than chronic, gating of the channels immediately after v-src expression, a process that is mediated through a "ball-and-chain" mechanism. In this study we show that, while ERK1/2 is necessary for acute closure of gap junction channels, it is not sufficient. Rather, multiple pathways converge to regulate Cx43 coupling in response to expression of v-src, including parallel signaling through PKC and MEK1/2, with additional positive and negative regulatory effects mediated by PI3 kinase, distinguished by the involvement of Akt.

Gap junctional channels are parts of multiprotein complexes

Gap junctional channels are a class of membrane channels composed of transmembrane channel-forming integral membrane proteins termed connexins, innexins or pannexins that mediate direct cell-to-cell or cell-to extracellular medium communication in almost all animal tissues. The activity of these channels is tightly regulated, particularly by intramolecular modifications as phosphorylations of proteins and via the formation of multiprotein complexes where pore-forming subunits bind to auxiliary channel subunits and associate with scaffolding proteins that play essential roles in channel localization and activity. Scaffolding proteins link signaling enzymes, substrates, and potential effectors (such as channels) into multiprotein signaling complexes that may be anchored to the cytoskeleton. Protein-protein interactions play essential roles in channel localization and activity and, besides their cell-to-cell channel-forming functions, gap junctional proteins now appear involved in different cellular functions (e.g. transcriptional and cytoskeletal regulations). The present review summarizes the recent progress regarding the proteins capable of interacting with junctional proteins and highlights the function of these protein-protein interactions in cell physiology and aberrant function in diseases. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and functions.

Mechanism for modulation of gating of connexin26-containing channels by taurine

The mechanisms of action of endogenous modulatory ligands of connexin channels are largely unknown. Previous work showed that protonated aminosulfonates (AS), notably taurine, directly and reversibly inhibit homomeric and heteromeric channels that contain Cx26, a widely distributed connexin, but not homomeric Cx32 channels. The present study investigated the molecular mechanisms of connexin channel modulation by taurine, using hemichannels and junctional channels composed of Cx26 (homomeric) and Cx26/Cx32 (heteromeric). The addition of a 28-amino acid "tag" to the carboxyl-terminal domain (CT) of Cx26 (Cx26(T)) eliminated taurine sensitivity of homomeric and heteromeric hemichannels in cells and liposomes. Cleavage of all but four residues of the tag (Cx26(Tc)) resulted in taurine-induced pore narrowing in homomeric hemichannels, and restored taurine inhibition of heteromeric hemichannels (Cx26(Tc)/Cx32). Taurine actions on junctional channels were fully consistent with those on hemichannels. Taurine-induced inhibition of Cx26/Cx32(T) and nontagged Cx26 junctional channels was blocked by extracellular HEPES, a blocker of the taurine transporter, confirming that the taurine-sensitive site of Cx26 is cytoplasmic. Nuclear magnetic resonance of peptides corresponding to Cx26 cytoplasmic domains showed that taurine binds to the cytoplasmic loop (CL) and not the CT, and that the CT and CL directly interact. ELISA showed that taurine disrupts a pH-dependent interaction between the CT and the CT-proximal half of the CL. These studies reveal that AS disrupt a pH-driven cytoplasmic interdomain interaction in Cx26-containing channels, causing closure, and that the Cx26CT has a modulatory role in Cx26 function.

The connexin43 carboxyl terminus and cardiac gap junction organization

The precise spatial order of gap junctions at intercalated disks in adult ventricular myocardium is thought vital for maintaining cardiac synchrony. Breakdown or remodeling of this order is a hallmark of arrhythmic disease of the heart. The principal component of gap junction channels between ventricular cardiomyocytes is connexin43 (Cx43). Protein-protein interactions and modifications of the carboxyl-terminus of Cx43 are key determinants of gap junction function, size, distribution and organization during normal development and in disease processes. Here, we review data on the role of proteins interacting with the Cx43 carboxyl-terminus in the regulation of cardiac gap junction organization, with particular emphasis on Zonula Occludens-1. The rapid progress in this area suggests that in coming years we are likely to develop a fuller understanding of the molecular mechanisms causing pathologic remodeling of gap junctions. With these advances come the promise of novel approach to the treatment of arrhythmia and the prevention of sudden cardiac death. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The gap junction protein Cx43 regulates B-lymphocyte spreading and adhesion

The gap junction protein connexin43 (Cx43) is widely expressed in mammalian cells and forms intercellular channels for the transfer of small molecules between adjacent cells, as well as hemichannels that mediate bidirectional transport of molecules between the cell and the surrounding environment. Cx43 regulates cell adhesion and migration in neurons and glioma cells, and we now show that Cx43 influences BCR-, LFA-1- and CXCL12-mediated activation of the Rap1 GTPase. Using shRNA knockdown of Cx43 in WEHI 231 cells, we show that Cx43 is required for sustained Rap1 activation and BCR-mediated spreading. To determine the domains of Cx43 that are important for this effect, Cx43-null J558 μm3 B cells (which express a wild-type IgM BCR) were transfected with wild-type Cx43-GFP or a C-terminal-truncated Cx43 (Cx43ΔT-GFP). Expression of wild-type Cx43-GFP, but not Cx43ΔT-GFP, was sufficient to restore sustained, BCR-mediated Rap1 activation and cell spreading. Cx43, and specifically the C-terminal domain, was also important for LFA-1- and CXCL12-mediated Rap1 activation, spreading and adhesion to an endothelial cell monolayer. These data show that Cx43 has an important and previously unreported role in B-cell processes that are essential to normal B-cell development and immune responses.

Intramolecular loop/tail interactions are essential for connexin 43-hemichannel activity

Connexin-assembled gap junctions (GJs) and hemichannels coordinate intercellular signaling processes. Although the regulation of connexins in GJs has been well characterized, the molecular determinants controlling connexin-hemichannel activity are unresolved. Here we investigated the regulation of Cx43-hemichannel activity by actomyosin contractility and intracellular [Ca(2+)] ([Ca(2+)](i)) using plasma membrane-permeable TAT peptides (100 μM) designed to interfere with interactions between the cytoplasmic loop (CL) and carboxy-terminal (CT) in primary bovine corneal endothelial cells and HeLa, C6 glioma, and Xenopus oocytes ectopically expressing Cx43. Peptides corresponding to the last 10 CT aa (TAT-Cx43CT) prevented the inhibition of Cx43-hemichannel activity by contractility/high [Ca(2+)](i), whereas a reverse peptide (TAT-Cx43CTrev) did not. These effects were independent of zonula occludens-1, a cytoskeletal-associated Cx43-binding protein. In contrast, peptides corresponding to CL (TAT-L2) inhibited Cx43-hemichannel responses, whereas a mutant peptide (TAT-L2(H126K/I130N)) did not inhibit. In these assays, TAT-Cx43CT acted as a scaffold for TAT-L2 and vice versa, a finding supported by surface plasmon resonance measurements. Loop/tail interactions appeared essential for Cx43-hemichannel activity, because TAT-Cx43CT restored the activity of nonfunctional hemichannels, consisting of either Cx43 lacking the C-terminal tail (Cx43(M239)) or intact Cx43 ectopically expressed in Xenopus oocytes. We conclude that intramolecular loop/tail interactions control Cx43-hemichannel activity, laying the basis for developing hemichannel-specific blockers.

Mechanisms of gap junction traffic in health and disease

Gap junctions (GJs) allow direct communication between cells. In the heart, GJs mediate the electrical coupling of cardiomyocytes and as such dictate the speed and direction of cardiac conduction. A prominent feature of acquired structural heart disease is remodeling of GJ protein expression and localization concomitant with increased susceptibility to lethal arrhythmias, leading many to hypothesize that the two are causally linked. Detailed understanding of the cellular mechanisms that regulate GJ localization and function within cardiomyocytes may therefore uncover potential therapeutic strategies for a significant clinical problem. This review will outline our current understanding of GJ cell biology with the intent of highlighting cellular mechanisms responsible for GJ remodeling associated with cardiac disease.

Regulation of gap junctional charge selectivity in cells coexpressing connexin 40 and connexin 43

Expression of connexin 40 (Cx40) and Cx43 in cardiovascular tissues varies as a function of age, injury, and development with unknown consequences on the selectivity of junctional communication and its acute regulation. We investigated the PKC-dependent regulation of charge selectivity in junctions composed of Cx43, Cx40, or both by simultaneous assessment of junctional permeance rate constants (B(dye)) for dyes of similar size but opposite charge, N,N,N-trimethyl-2-[methyl-(7-nitro-2,1,3-benzoxadiol-4-yl)amino]ethanaminium (NBD-M-TMA; +1) and Alexa 350 (-1). The ratio of dye rate constants (B(NBD-M-TMA)/B(Alexa 350)) indicated that Cx40 junctions are cation selective (10.7 +/- 0.5), whereas Cx43 junction are nonselective (1.22 +/- 0.14). In coexpressing cells, a broad range of junctional selectivities was observed with mean cation selectivity increasing as the Cx40 to Cx43 expression ratio increased. PKC activation reduced or eliminated dye permeability of Cx43 junctions without altering their charge selectivity, had no effect on either permeability or charge selectivity of Cx40 junctions, and significantly increased the cation selectivity of junctions formed by coexpressing cells (approaching charge selectivity of Cx40 junctions). Junctions composed of Cx43 truncated at residue 257 (Cx43tr) were also not charge selective, but when Cx43tr was coexpressed with Cx40, a broad range of junctional selectivities that was unaffected by PKC activation was observed. Thus, whereas the charge selectivities of homomeric/homotypic Cx43 and Cx40 junctions appear invariant, the selectivities of junctions formed by cells coexpressing Cx40 and Cx43 vary considerably, reflecting both their relative expression levels and phosphorylation-dependent regulation. Such regulation could represent a mechanism by which coexpressing cells such as vascular endothelium and atrial cells regulate acutely the selective intercellular communication mediated by their gap junctions.

Effect of gap junction inhibition on intracerebral hemorrhage-induced brain injury in mice

It has been reported that gap junction contributes to ischemic brain injury and gap junction inhibitors improve neurological outcome in ischemic brain injury models. In the present study, we investigated the effects of gap junction inhibitor, carbenoxolone, on mortality, neurological deficits and brain edema in mice with intracerebral hemorrhage. A total of 80 male CD-1 mice were divided into two parts with two end-points for this study. In part one, animals were divided into four groups: sham, vehicle treatment following intracerebral hemorrhage induction, low-dose carbenoxolone (33 mg/kg) treatment 1 hour after intracerebral hemorrhage induction and high-dose carbenoxolone (100 mg/kg) treatment 1 hour after intracerebral hemorrhage induction groups. Animals were euthanized after 24 hours. In part two, animals were divided into four groups: sham, vehicle treatment 1 hour after intracerebral hemorrhage induction, single high-dose of carbenoxolone treatment at 1 hour after intracerebral hemorrhage induction and three high-doses of carbenoxolone treatment 1, 24 and 48 hours respectively after intracerebral hemorrhage induction. Animals were euthanized after 72 hours. Intracerebral hemorrhage was induced by collagenase injection. Neurological deficits were evaluated using modified Garcia's neurological test, wire hanging and beam balance tests. Brain edema was measured by brain water content. Our results showed that intracerebral hemorrhage produced brain edema and neurological deficits in mice. Carbenoxolone treatment failed to reduce brain edema and neurological deficits. In fact, the high dose of carbenoxolone aggravated neurological deficits and increased mortality 72 hours after the treatment. In conclusion, inhibition of gap junction has no short-term neuroprotective effect on intracerebral hemorrhage-induced brain injury. Further studies are required to assess the long-term effects of gap junction inhibitors in intracerebral hemorrhage models.

Mouse lens connexin23 (Gje1) does not form functional gap junction channels but causes enhanced ATP release from HeLa cells

In the mouse genome, 20 connexin genes have been detected that code for proteins of high sequence identity in the two extracellular loops, especially six conserved cysteine residues. The mouse connexin23 (Cx23) gene (Gje1) differs from all other connexin genes in vertebrates, since it codes for a protein that contains only 4 instead of 6 cysteine residues in the extracellular loops. Recently, two zebrafish connexin genes (Cx23a and Cx23b) have been identified, and a mouse mutant in the Gje1 gene has been described that exhibits a developmental defect in the lens. Here, we have compared the Cx23 gene in different mammalian species and found no transcripts in cDNA libraries of primates. Furthermore, all primate genomes analyzed contain stop codons in the Cx23 sequence, indicating inactivation of the orthologous primate GJE1 gene. No Cx23 mRNA was found in human eye. In order to analyze the properties of mouse Cx23 channels, we isolated HeLa cell clones stably expressing wild-type mCx23 or mCx23 fused to eGFP. Cells expressing Cx23-eGFP demonstrated its insertion in the plasma membrane but no punctate staining in contacting membranes characteristic for junctional plaques. In addition, we tested whether Cx23 forms functional gap junction channels electrophysiologically in cell pairs as well as by microinjection of neurobiotin and found that mouse Cx23 did not form gap junction channels in HeLa cells. However, there was a significant release of ATP from different Cx23 HeLa cell clones, even in the presence of normal culture medium with high calcium ion concentration, suggesting a hemichannel-based function of Cx23. Therefore, Cx23 seems to share functional properties with pannexin (hemi) channels rather than gap junction channels of other connexins.

Connexin 43 hemichannels are permeable to ATP

Astrocytes are electrically nonexcitable cells that communicate by means of Ca(2+) signaling. Long-distance intercellular Ca(2+) waves are initiated by release of ATP and activation of purinergic receptors on nearby cells. Previous studies have implicated connexin 43 (Cx43) in ATP release, but definitive proof that ATP exits through Cx43 hemichannels does not exist. Here, through several alternative approaches, we show that ATP anions can permeate through Cx43 hemichannels. First, openings of Cx43 hemichannels were detected in both cell-attached and inside-out patch recordings in C6 cells expressing Cx43, but not in C6 cells expressing Cx43-eGFP (enhanced green fluorescent protein) or a C-terminus truncation mutant of Cx43. Second, Cx43 hemichannel openings were inhibited by three structurally different gap-junction channel blockers, but not by the P2X(7) blocker Brilliant blue G. Third, bioluminescence imaging of ATP combined with single-channel recording in the inside-out patch configuration showed that ATP efflux coincided with channel openings and was absent when the Cx43 hemichannel was closed. Fourth, ion replacement experiments confirmed that Cx43 hemichannels are permeable to ATP. In summary, these observations provide the first direct evidence for efflux of ATP through Cx43 hemichannels. Furthermore, a putative Cx43 hemichannel with characteristics identical to the Cx43 hemichannel in C6 cells was identified in the membrane of hippocampal astrocytes in acutely prepared slices.

Structural changes in the carboxyl terminus of the gap junction protein connexin 40 caused by the interaction with c-Src and zonula occludens-1

c-Src can disrupt the connexin 43 (Cx43) and zonula occludens-1 (ZO-1) interaction, leading to down-regulation of gap junction intercellular communication. Previously, the authors characterized the interaction of domains from these proteins with the carboxyl terminus of Cx43 (Cx43CT) and found that binding of the c-Src SH3 domain to Cx43CT disrupted the Cx43CT/ZO-1 PDZ-2 domain complex. Because Cx43 and Cx40 form heteromeric connexons and display similar mechanisms of pH regulation, the authors addressed whether Cx40CT interacts with these domains in a similar manner as Cx43CT. Nuclear magnetic resonance (NMR) data indicate that Cx40CT is an intrinsically disordered protein. NMR titrations determined that PDZ-2 affected the last 28 Cx40CT residues and SH3 shifted numerous amino-terminal Cx40CT residues. Finally, the Cx40CT/PDZ-2 complex was unaffected by SH3 and both domains interacted simultaneously with Cx40CT. This result differs from when the same experiment was performed with Cx43CT, suggesting different mechanisms of regulation exist between connexin isoforms, even when involving the same molecular partners.

Connexin43 enhances glioma invasion by a mechanism involving the carboxy terminus

Gliomas are particularly difficult to cure owing largely to their invasive nature. The neoplastic changes of astrocytes which give rise to these tumors frequently include a reduction of connexin43 (Cx43), the most abundant connexin isoform expressed in astrocytes. Cx43 is a subunit of gap junctions (GJ), intercellular channels which directly link the cytosol of adjacent cells and allow the regulated passage of ions and small molecules. To examine the role of Cx43 in glioma motility, we identified two variant C6 cell lines which endogenously express high (C6-H) or low (C6-L) levels of Cx43. In wound healing and transwell assays, C6-H cells were more motile than C6-L cells. To deduce whether Cx43 mediated these differences, assays were conducted on C6-H cells retrovirally transduced with Cx43 shRNA. Coincident with the stable knockdown of endogenous Cx43, a decrease in motility and invasion was observed. Gap junctional intercellular communication was also decreased, however motility assays conducted in the presence of GJ inhibitors did not reveal significant differences in cell motility. C6 cells transfected with full length or C-terminal truncated Cx43 (Cx43DeltaCT) were subjected to the aforementioned motility assays to expose alternate mechanisms of Cx43-mediated motility. Cells expressing full length Cx43 exhibited increased motility while cells expressing Cx43DeltaCT did not. This report, the first in which RNAi has been employed to reduce Cx43 expression in gliomas, indicates that the downregulation of Cx43 decreases motility of C6 cells. Furthermore, it is the first report to suggest that the Cx43 CT plays an important role in glioma motility.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Gap junctional complexes: From partners to functions

Gap junctions (GJ), specialised membrane structures that mediate cell-to-cell communication in almost all animal tissues, are composed of intercellular channel-forming integral membrane proteins termed connexins (Cxs), innexins or pannexins. The activity of these channels is closely regulated, particularly by intramolecular modifications as phosphorylation of proteins, via the formation of multiprotein complexes where pore-forming subunits bind to auxiliary channel subunits and associate with scaffolding proteins that play essential roles in channel localization and activity. Scaffolding proteins link signalling enzymes, substrates, and potential effectors (such as channels) into multiprotein signalling complexes that may be anchored to the cytoskeleton. Protein-protein interactions play essential roles in channel localization and activity and, besides their cell-to-cell channel-forming functions, gap junctional proteins now appear involved in different cellular functions (e.g. transcriptional and cytoskeletal regulation). The present review summarizes the recent progress regarding the proteins capable of interacting with junctional proteins and their functional importance.

Effect of charge substitutions at residue his-142 on voltage gating of connexin43 channels

Previous studies indicate that the carboxyl terminal of connexin43 (Cx43CT) is involved in fast transjunctional voltage gating. Separate studies support the notion of an intramolecular association between Cx43CT and a region of the cytoplasmic loop (amino acids 119-144; referred to as "L2"). Structural analysis of L2 shows two alpha-helical domains, each with a histidine residue in its sequence (H126 and H142). Here, we determined the effect of H142 replacement by lysine, alanine, and glutamate on the voltage gating of Cx43 channels. Mutation H142E led to a significant reduction in the frequency of occurrence of the residual state and a prolongation of dwell open time. Macroscopically, there was a large reduction in the fast component of voltage gating. These results resembled those observed for a mutant lacking the carboxyl terminal (CT) domain. NMR experiments showed that mutation H142E significantly decreased the Cx43CT-L2 interaction and disrupted the secondary structure of L2. Overall, our data support the hypothesis that fast voltage gating involves an intramolecular particle-receptor interaction between CT and L2. Some of the structural constrains of fast voltage gating may be shared with those involved in the chemical gating of Cx43.

Role of intramolecular interaction in connexin50: mediating the Ca2+-dependent binding of calmodulin to gap junction

Gap junction channels formed by connexin50 (Cx50) are critical for maintenance of eye lens transparency. Cleavage of the carboxyl terminus (CT) of Cx50 to produce truncated Cx50 (Cx50trunc) occurred naturally during maturation of lens fiber cells. The mechanism of its altered properties is under confirmation. It has been suggested that calmodulin (CaM) participates in gating some kinds of gap junction. Here, we performed confocal colocalization and co-immunoprecipitation experiments to study the relationships between Cx50 and CaM. Results exhibited that the CaM could colocalize Ca2+ dependently with CT in the linear area of cell-to-cell contact formed by Cx50trunc, while it could not localize in the linear area without expression of CT. Further study indicated that the CT could interact Ca2+ independently with the cytoplasmic loop (CL) of Cx50. These data put forward the importance of Ca2+-independent intramolecular interaction between CT and CL of Cx50, which mediate the Ca2+-dependent binding of CaM to Cx50. These intra- and intermolecular interactions may further improve our understanding of biological significance of the Cx50 in the eye lens.

c-Src: bridging the gap between phosphorylation- and acidification-induced gap junction channel closure

Gap junctions are a unique type of intercellular junction that mediate the direct exchange of small molecules between neighboring cells and play critical roles in the normal function of numerous organs. Mutations in the connexin proteins that make up gap junctions have been implicated in numerous human skin and neurosensory disorders. The ability of gap junctions to transmit molecules between cells is regulated by intracellular pH, the phosphorylation state of connexin, and the interaction of connexin with other cellular proteins. This Perspective focuses on the novel and complex events initiated by intracellular acidification resulting from tissue ischemia or hypoxia that lead to the interruption of intercellular communication between astrocytes. These events include alterations in connexin43 (Cx43) phosphorylation, disruption of beta-actin binding to Cx43, and the induced interaction of Cx43 with the c-Src tyrosine kinase, extracellular signal-regulated kinase 1 and 2, and mitogen-activated protein kinase phosphatase 1.

Structural organization of gap junction channels

Gap junctions were initially described morphologically, and identified as semi-crystalline arrays of channels linking two cells. This suggested that they may represent an amenable target for electron and X-ray crystallographic studies in much the same way that bacteriorhodopsin has. Over 30 years later, however, an atomic resolution structural solution of these unique intercellular pores is still lacking due to many challenges faced in obtaining high expression levels and purification of these structures. A variety of microscopic techniques, as well as NMR structure determination of fragments of the protein, have now provided clearer and correlated views of how these structures are assembled and function as intercellular conduits. As a complement to these structural approaches, a variety of mutagenic studies linking structure and function have now allowed molecular details to be superimposed on these lower resolution structures, so that a clearer image of pore architecture and its modes of regulation are beginning to emerge.

Connexin phosphorylation as a regulatory event linked to channel gating

The main proteins required for functional gap junction channels are known as connexins and most of their isoforms indicate that they can become phosphorylated. Connexin phosphorylation has been reported to participate in modifying junctional communication and the mechanisms involved apparently depend on which kinase becomes involved. Although multiple reports have suggested a strong influence of phosphorylation on channel gating, not enough physiological studies have been performed to determine precisely the gating mechanisms implicated. Moreover, gap junction channels follow other various gating mechanisms, including voltage gating and chemical gating, where phosphorylation could act as a modulator. The quest for this chapter has been to discriminate those instances where phosphorylation acts directly as a gating trigger and where it acts indirectly or only as a modulator. Despite recent efforts, the mechanisms involved in all these cases are barely understood.

pH-dependent dimerization of the carboxyl terminal domain of Cx43

Previous studies have demonstrated that the carboxyl terminus of the gap junction protein Cx43 (Cx43CT) can act as an independent, regulatory domain that modulates intercellular communication in response to appropriate chemical stimuli. Here, we have used NMR, chemical cross-linking, and analytical ultracentrifugation to further characterize the biochemical and biophysical properties of the Connexin43 carboxyl terminal domain (S255-I382). NMR-diffusion experiments at pH 5.8 suggested that the Connexin43 carboxyl terminus (CX43CT) may have a molecular weight greater than that of a monomer. Sedimentation equilibrium and cross-linking data demonstrated a predominantly dimeric state for the Cx43CT at pH 5.8 and 6.5, with limited dimer formation at a more neutral pH. NMR-filtered nuclear Overhauser effect studies confirmed these observations and identified specific areas of parallel orientation within Cx43CT, likely corresponding to dimerization domains. These regions included a portion of the SH3 binding domain, as well as two fragments previously found to organize in alpha-helical structures. Together, these data show that acidification causes Cx43CT dimer formation in vitro. Whether dimer formation is an important structural component of the regulation of Connexin43 channels remains to be determined. Dimerization may alter the affinity of Cx43CT regions for specific molecular partners, thus modifying the regulation of gap junction channels.

Alterations in metabolism and gap junction expression may determine the role of astrocytes as ?good samaritans? or executioners

Our knowledge of astroglia and their physiological and pathophysiological role(s) in the central nervous system (CNS) has grown during the past decade, revealing a complex picture. It is becoming increasingly clear that glia play a significant role in the homeostasis and function of the CNS and that neurons should no longer be considered the only cell type that responds, both rapidly and slowly, to electrochemical activity. We discuss recent advances in the field with an emphasis on the impact of hypoxia and ischemia on astrocytic metabolism and the functional relationship between glucose metabolism and gap junctions in astrocytes. We also address the controversy over whether astrocytic gap junctions mediate protection or killing of neurons during or after hypoxic or ischemic insults.

Structural Changes in the Carboxyl Terminus of the Gap Junction Protein Connexin43 Indicates Signaling between Binding Domains for c-Src and Zonula Occludens-1

Regulation of cell-cell communication by the gap junction protein connexin43 can be modulated by a variety of connexin-associating proteins. In particular, c-Src can disrupt the connexin43 (Cx43)-zonula occludens-1 (ZO-1) interaction, leading to down-regulation of gap junction intercellular communication. The binding sites for ZO-1 and c-Src correspond to widely separated Cx43 domains (approximately 100 residues apart); however, little is known about the structural modifications that may allow information to be transferred over this distance. Here, we have characterized the structure of the connexin43 carboxyl-terminal domain (Cx43CT) to assess its ability to interact with domains from ZO-1 and c-Src. NMR data indicate that the Cx43CT exists primarily as an elongated random coil, with two regions of alpha-helical structure. NMR titration experiments determined that the ZO-1 PDZ-2 domain affected the last 19 Cx43CT residues, a region larger than that reported to be required for Cx43CT-ZO-1 binding. The c-Src SH3 domain affected Cx43CT residues Lys-264-Lys-287, Ser-306-Glu-316, His-331-Phe-337, Leu-356-Val-359, and Ala-367-Ser-372. Only region Lys-264-Lys-287 contains the residues previously reported to act as an SH3 binding domain. The specificity of these interactions was verified by peptide competition experiments. Finally, we demonstrated that the SH3 domain could partially displace the Cx43CT-PDZ-2 complex. These studies represent the first structural characterization of a connexin domain when integrated in a multimolecular complex. Furthermore, we demonstrate that the structural characteristics of a disordered Cx43CT are advantageous for signaling between different binding partners that may be important in describing the mechanism of channel closure or internalization in response to pathophysiological stimuli.

The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

Defective epidermal barrier in neonatal mice lacking the C-terminal region of connexin43

More than 97% of mice in which the C-terminal region of connexin43 (Cx43) was removed (designated as Cx43K258stop) die shortly after birth due to a defect of the epidermal barrier. The abnormal expression of Cx43K258stop protein in the uppermost layers of the epidermis seems to perturb terminal differentiation of keratinocytes. In contrast to Cx43-deficient mice, neonatal Cx43K258stop hearts show no lethal obstruction of the right ventricular outflow tract, but signs of dilatation. Electrocardiographies of neonatal hearts reveal repolarization abnormalities in 20% of homozygous Cx43K258stop animals. The very rare adult Cx43K258stop mice show a compensation of the epidermal barrier defect but persisting impairment of cardiac function in echocardiography. Female Cx43K258stop mice are infertile due to impaired folliculogenesis. Our results indicate that the C-terminally truncated Cx43K258stop mice lack essential functions of Cx43, although the truncated Cx43 protein can form open gap junctional channels.

Regulation of gap junctions by tyrosine protein kinases

Most of the gap junction proteins are regulated in part by post-translational phosphorylation. Phosphorylation has been shown to be important in gap junction assembly and turnover, and for channel function in the resting state. Connexin phosphorylation may be altered by the activation of intracellular signaling pathways in response to growth factors, tumor promoters, activated oncogenes, hormones and inflammatory mediators. In some instances altered phosphorylation has been associated with changes in connexin function and in other cases appears to be associated with changes in the levels of the connexin protein and/or mRNA. This review focuses on the role of tyrosine protein kinases in the regulation of gap junctions. The literature is most extensive for connexin43 and those studies are reviewed here. A great deal has been learned in recent years about how connexin43 is regulated by tyrosine kinase-dependent signaling pathways. These pathways are often complex and to some extent are cell type- and stimulus-dependent. Although considerable progress has been made in unraveling the cellular pathways that regulate connexin function, significant challenges remain to be addressed in identifying additional phosphorylation sites and determining the stoichiometries of the phosphorylation events that regulate connexin function and it's interaction with other cellular proteins.

Diversity in protein–protein interactions of connexins: emerging roles

Gap junctions, specialised membrane structures that mediate cell-to-cell communication in almost all tissues, are composed of channel-forming integral membrane proteins termed connexins. The activity of these intercellular channels is closely regulated, particularly by intramolecular modifications as phosphorylations of proteins by protein kinases, which appear to regulate the gap junction at several levels, including assembly of channels in the plasma membrane, connexin turnover as well as directly affecting the opening and closure ("gating") of channels. The regulation of membrane channels by protein phosphorylation/dephosphorylation processes commonly requires the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, catalytic and regulatory subunits), that play essential roles in channel localisation and activity, linking signalling enzymes, substrates and effectors into a structure frequently anchored to the cytoskeleton. The present review summarises the up-to-date progress regarding the proteins capable of interacting or at least of co-localising with connexins and their functional importance.

The regulatory role of the C-terminal domain of connexin43

The C-terminal (CT) domain of connexin43 (Cx43) is thought to be important in the control of gap junction function via: a.) CT phosphorylation-dependent control of gap junction assembly and gating, b.) interactions of CT with key regulatory binding partners. To more closely examine CT-dependent regulation, we have expressed a hemagglutinin-Cx43CT (amino acids 235-382) fusion protein in Normal Rat Kidney (NRK) cells under a tetracycline-responsive inducible promoter. Western blot analysis shows that Cx43CT expression is markedly induced by at least 48 h oftreatment with the tetracycline analogue, doxycycline. Furthermore, Cx43CT is modified within the cell, as several treatments/conditions that increase endogenous Cx43 phosphorylation induced a mobility shift in Cx43CT. Treatment with kinase activators, including epidermal growth factor (EGF) and the tumor promoting phorbol ester 12-O-tetradecanylphorbol-13-acetate (TPA), caused a shift in the mobility of the Cx43CT in a manner consistent with the mobility shift observed upon increased phosphorylation of endogenous Cx43. Similarly, Cx43CT in mitotic cells is extensively shifted, consistent with reports which show that Cx43 is phosphorylated to a unique phosphoisoform in mitotic cells. These results indicate that the Cx43CT can interact with at least some of the kinases that phosphorylate endogenous Cx43 in cells and possibly modulate the effects of kinase activation on gap junctional communication.

Bisphosphonate-induced, hemichannel-mediated, anti-apoptosis through the Src/ERK pathway: a gap junction-independent action of connexin43

Preservation of the mechanosensory function of osteocytes by inhibiting their apoptosis might contribute to the beneficial effects of bisphosphonates in bone. We report herein a mechanism by which connexin43 hemichannel opening by bisphosphonates triggers the activation of the kinases Src and ERKs and promotes cell survival. Bisphosphonate-induced anti-apoptosis requires connexin channel integrity, but not gap junctions. Osteocytic cells express functional hemichannels that are opened by bisphosphonates, as demonstrated by dye uptake, regulation by established agonists and antagonists, and cell surface biotinylation. The anti-apoptotic effect of bisphosphonates depends on connexin43 expression in mouse embryonic fibroblasts and osteoblastic cells. Transfection of connexin43, but not other connexins, into connexin43 naïve cells confers de novo responsiveness to the drugs. The signal transducing property of connexin43 requires the pore-forming, as well as the C-terminal domains of the protein, the interaction of connexin43 with Src. and the activation of both Src and ERK kinases. These studies establish a role for connexin43 hemichannels in bisphosphonate action, and a novel function of connexin43--beyond gap junction communication--in the regulation of survival signaling pathways.