Regulation of gap junctions by phosphorylation of connexins

Connexin and Pannexin hemichannels are regulated by redox potential

Connexins (Cxs) and Pannexins (Panxs) are two non-related protein families, having both the property to form hemichannels at the plasma membrane. There are 21 genes coding for different Cx based proteins and only 3 for Panx. Under physiological conditions, these hemichannels (Cxs and Panxs) present a low open probability, but when open, they allow the release of signaling molecules to the extracellular space. However, under pathological conditions, these hemichannels increase their open probability, inducing important lysis of metabolites, and ionic imbalance, which in turn induce the massive entry of Ca⁺² to the cell. Actually, it is well recognized that Cxs and Panxs based channels play an important role in several diseases and -in many cases- this is associated with an aberrant hemichannel opening. Hemichannel opening and closing are controlled by a plethora of signaling including changes of the voltage plasma membrane, protein-protein interactions, and several posttranslational modifications, including protein cleavage, phosphorylation, glycosylation, hydroxylation and S-nitrosylation, among others. In particular, it has been recently shown that the cellular redox status modulates the opening/closing and permeability of at least Cx43, Cx46, and Panx1 hemichannels. Thus, for example, the gaseous transmitter nitric oxide (NO) can induce the S-nitrosylation of these proteins modulating in turn several of their properties. The reason is that the redox status of a cell is fundamental to set their response to the environment and also plays an important role in several pathologies. In this review, I will discuss how NO and other molecules associated with redox signaling modulate Cxs and Panx hemichannels properties.

The role of oxidative DNA damage in radiation induced bystander effect

Ionizing radiation (IR) has been described as a double-edged sword, since it is used for diagnostic and therapeutic medical applications, and at the same time it is a well known human mutagen and carcinogen, causing wide-ranging chromosomal aberrations. It is nowadays accepted that the detrimental effects of IR are not restricted only in the irradiated cells, but also to non-irradiated bystander or even distant cells manifesting various biological effects. This review presents the role of oxidative stress in the induction of bystander effects referring to the types of the implicated oxidative DNA lesions, the contributing intercellular and intracellular stress mediators, the way they are transmitted from irradiated to bystander cells and finally, the complex role of the bystander effect in the therapeutic efficacy of radiation treatment of cancer.

Connexins-mediated glia networking impacts myelination and remyelination in the central nervous system

In the central nervous system (CNS), the glial gap junctions are established among astrocytes (ASTs), oligodendrocytes (OLs), and/or between ASTs and OLs due to the expression of membrane proteins called connexins (Cxs). Together, the glial cells form a network of communicating cells that is important for the homeostasis of brain function for its involvement in the intercellular calcium wave propagation, exchange of metabolic substrates, cell proliferation, migration, and differentiation. Alternatively, Cxs are also involved in hemichannel function and thus participate in gliotransmission. In recent years, pathologic changes of oligodendroglia or demyelination found in transgenic mice with different subsets of Cxs or pharmacological insults suggest that glial Cxs may participate in the regulation of the myelination or remyelination processes. However, little is known about the underlying mechanisms. In this review, we will mainly focus on the functions of Cx-mediated gap junction channels, as well as hemichannels, in brain glial cells and discuss the way by which they impact myelination and remyelination. These aspects will be considered at the light of recent genetic and non-genetic studies related to demyelination and remyelination.

Non-targeted radiation effects in vivo: a critical glance of the future in radiobiology

Radiation-induced bystander effects (RIBE), demonstrate the induction of biological non-targeted effects in cells which have not directly hit by radiation or by free radicals produced by ionization events. Although RIBE have been demonstrated using a variety of biological endpoints the mechanism(s) of this phenomenon still remain unclear. The controversial results of the in vitro RIBE and the evidence of non-targeted effects in various in vivo systems are discussed. The experimental evidence on RIBE, indicate that a more analytical and mechanistic in depth approach is needed to secure an answer to one of the most intriguing questions in radiobiology.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Managing the complexity of communication: regulation of gap junctions by post-translational modification

Gap junctions are comprised of connexins that form cell-to-cell channels which couple neighboring cells to accommodate the exchange of information. The need for communication does, however, change over time and therefore must be tightly controlled. Although the regulation of connexin protein expression by transcription and translation is of great importance, the trafficking, channel activity and degradation are also under tight control. The function of connexins can be regulated by several post translational modifications, which affect numerous parameters; including number of channels, open probability, single channel conductance or selectivity. The most extensively investigated post translational modifications are phosphorylations, which have been documented in all mammalian connexins. Besides phosphorylations, some connexins are known to be ubiquitinated, SUMOylated, nitrosylated, hydroxylated, acetylated, methylated, and γ-carboxyglutamated. The aim of the present review is to summarize our current knowledge of post translational regulation of the connexin family of proteins.

JNK is a novel regulator of intercellular adhesion

c-Jun N-terminal Kinase (JNK) is a family of protein kinases, which are activated by stress stimuli such as inflammation, heat stress and osmotic stress, and regulate diverse cellular processes including proliferation, survival and apoptosis. In this review, we focus on a recently discovered function of JNK as a regulator of intercellular adhesion. We summarize the existing knowledge regarding the role of JNK during the formation of cell-cell junctions. The potential mechanisms and implications for processes requiring dynamic formation and dissolution of cell-cell junctions including wound healing, migration, cancer metastasis and stem cell differentiation are also discussed.

MicroRNA-1 downregulation increases connexin 43 displacement and induces ventricular tachyarrhythmias in rodent hypertrophic hearts

Downregulation of the muscle-specific microRNA-1 (miR-1) mediates the induction of pathologic cardiac hypertrophy. Dysfunction of the gap junction protein connexin 43 (Cx43), an established miR-1 target, during cardiac hypertrophy leads to ventricular tachyarrhythmias (VT). However, it is still unknown whether miR-1 and Cx43 are interconnected in the pro-arrhythmic context of hypertrophy. Thus, in this study we investigated whether a reduction in the extent of cardiac hypertrophy could limit the pathological electrical remodeling of Cx43 and the onset of VT by modulating miR-1 levels. Wistar male rats underwent mechanical constriction of the ascending aorta to induce pathologic left ventricular hypertrophy (LVH) and afterwards were randomly assigned to receive 10mg/kg valsartan, VAL (LVH+VAL) delivered in the drinking water or placebo (LVH) for 12 weeks. Sham surgery was performed for control groups. Programmed ventricular stimulation reproducibly induced VT in LVH compared to LVH+VAL group. When compared to sham controls, rats from LVH group showed a significant decrease of miR-1 and an increase of Cx43 expression and its ERK1/2-dependent phosphorylation, which displaces Cx43 from the gap junction. Interestingly, VAL administration to rats with aortic banding significantly reduced cardiac hypertrophy and prevented miR-1 down-regulation and Cx43 up-regulation and phosphorylation. Gain- and loss-of-function experiments in neonatal cardiomyocytes (NCMs) in vitro confirmed that Cx43 is a direct target of miR-1. Accordingly, in vitro angiotensin II stimulation reduced miR-1 levels and increased Cx43 expression and phosphorylation compared to un-stimulated NCMs. Finally, in vivo miR-1 cardiac overexpression by an adenoviral vector intra-myocardial injection reduced Cx43 expression and phosphorylation in mice with isoproterenol-induced LVH. In conclusion, miR-1 regulates Cx43 expression and activity in hypertrophic cardiomyocytes in vitro and in vivo. Treatment of pressure overload-induced myocyte hypertrophy reduces the risk of life-threatening VT by normalizing miR-1 expression levels with the consequent stabilization of Cx43 expression and activity within the gap junction.

Regulation of connexin- and pannexin-based channels by post-translational modifications

Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi-protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post-translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N-acetylation, S-nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx-channel activity and localisation.

Effects of phosphorylation on the structure and backbone dynamics of the intrinsically disordered connexin43 C-terminal domain

Phosphorylation of the connexin43 C-terminal (Cx43CT) domain regulates gap junction intercellular communication. However, an understanding of the mechanisms by which phosphorylation exerts its effects is lacking. Here, we test the hypothesis that phosphorylation regulates Cx43 gap junction intercellular communication by mediating structural changes in the C-terminal domain. Circular dichroism and nuclear magnetic resonance were used to characterize the effects of phosphorylation on the secondary structure and backbone dynamics of soluble and membrane-tethered Cx43CT domains. Cx43CT phospho-mimetic isoforms, which have Asp substitutions at specific Ser/Tyr sites, revealed phosphorylation alters the α-helical content of the Cx43CT domain only when attached to the membrane. The changes in secondary structure are due to variations in the conformational preference and backbone flexibility of residues adjacent and distal to the site(s) of modification. In addition to the known direct effects of phosphorylation on molecular partner interactions, the data presented here suggest phosphorylation may also indirectly regulate binding affinity by altering the conformational preference of the Cx43CT domain.

Retinoic acid regulates gap junction intercellular communication in human endometrial stromal cells through modulation of the phosphorylation status of connexin 43

Previous studies revealed that gap junction intercellular communication (GJIC) among uterine stromal cells plays critical roles in modulating decidualization, neovasularization, and embryo implantation. Connexin (Cx) proteins are the major component of gap junctions and Cx43 is the most widely expressed connexin in endometrium. Phosphorylation of Cx43 was found to impair gap junction communication in this tissue. Using primary human endometrial stromal cells (ESCs) and a stable high telomerase-expressing ESC transfectant (T-HESC), we found that retinoic acid (RA) altered the phosphorylation status of Cx43 protein such that there was a decrease in the phosphorylated (P1 and P2) species accompanied by an increase in the non-phosphorylated (P0) form. This process is dependent on protein phosphatase 2A (PP2A) activity since selective PP2A inhibitors prevented the ability of RA to dephosphorylate Cx43. Although RA had no effect on total PP2A expression or activity, it significantly increased the intracellular association of Cx43 and PP2A. Inhibition of transcription and protein synthesis by actinomycin D and cycloheximide, respectively, had no effect on the RA-induced changes in the Cx43 phosphorylation pattern. Furthermore, BMS493, a potent antagonist of the classical RA-mediated transcriptional pathway, did not inhibit RA-induced Cx43 dephosphorylation. Our data indicate that RA stimulates physical association of PP2A with Cx43, resulting in the dephosphorylation of Cx43 and, as a consequence, up-regulation of GJIC in ESCs. This process is independent of new mRNA and protein synthesis and suggests a novel mechanism by which aberrant retinoid metabolism can explain certain reproductive disorders manifested by dysfunctional endometrial cell GJIC.

Connexin43 confers Temozolomide resistance in human glioma cells by modulating the mitochondrial apoptosis pathway

Glioblastoma multiforme (GBM) is the most aggressive astrocytoma, and therapeutic options are generally limited to surgical resection, radiotherapy, and Temozolomide (TMZ) chemotherapy. TMZ is a DNA alkylating agent that causes DNA damage and induces cell death. Unfortunately, glioma cells often develop resistance to TMZ treatment, with DNA de-methylation of the MGMT promoter identified as the primary reason. However, the contributions from proteins that normally protect cells against cytotoxic stress in TMZ-induced apoptosis have not been extensively explored. Here, we showed that increasing the level of the gap junction protein, Cx43, in human LN18 and LN229 glioma cells enhances resistance to TMZ treatment while knockdown of Cx43 in these same cells sensitizes them to TMZ treatment. By expressing a channel-dead or a C-terminal truncation mutant of Cx43, we show that Cx43-mediated TMZ resistance involves both channel dependent and independent functions. Expression of Cx43 in LN229 cells decreases TMZ-induced apoptosis, as determined by Annexin V staining. Cx43-mediated chemoresistance appears to be acting via a mitochondrial apoptosis pathway as manifested by the reduction in Bax/Bcl-2 ratio and the release of cytochrome C. Our findings highlight additional mechanisms and proteins that contribute to TMZ resistance, and raise the possibility of increasing TMZ efficiency by targeting Cx43 protein. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

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'.

Neuronal differentiation requires a biphasic modulation of gap junctional intercellular communication caused by dynamic changes of connexin43 expression

It was suggested that gap junctional intercellular communication (GJIC) and connexin (Cx) proteins play a crucial role in cell proliferation and differentiation. However, the mechanisms of cell coupling in regulating cell fate during embryonic development are poorly understood. To study the role of GJIC in proliferation and differentiation, we used a human neural progenitor cell line derived from the ventral mesencephalon. Fluorescence recovery after photobleaching (FRAP) showed that dye coupling was extensive in proliferating cells but diminished after the induction of differentiation, as indicated by a 2.5-fold increase of the half-time of fluorescence recovery. Notably, recovery half-time decreased strongly (five-fold) in the later stage of differentiation. Western blot analysis revealed a similar time-dependent expression profile of Cx43, acting as the main gap junction-forming protein. Interestingly, large amounts of cytoplasmic Cx43 were retained mainly in the Golgi network during proliferation but decreased when differentiation was induced. Furthermore, down-regulation of Cx43 by small interfering RNA reduced functional cell coupling, which in turn resulted in a 50% decrease of both the proliferation rate and neuronal differentiation. Our findings suggest a dual function of Cx43 and GJIC in the neural development of ReNcell VM197 human progenitor cells. GJIC accompanied by high Cx43 expression is necessary (1) to maintain cells in a proliferative state and (2) to complete neuronal differentiation, including the establishment of a neural network. However, uncoupling of cells is crucial in the early stage of differentiation during cell fate commitment.

Multilayered regulation of cardiac ion channels

Essential to beat-to-beat heart function is the ability for cardiomyocytes to propagate electrical excitation and generate contractile force. Both excitation and contractility depend on specific ventricular ion channels, which include the L-type calcium channel (LTCC) and the connexin 43 (Cx43) gap junction. Each of these two channels is localized to a distinct subdomain of the cardiomyocyte plasma membrane. In this review, we focus on regulatory mechanisms that govern the lifecycles of LTCC and Cx43, from their biogenesis in the nucleus to directed delivery to T-tubules and intercalated discs, respectively. We discuss recent findings on how alternative promoter usage, tissue-specific transcription, and alternative splicing determine precise ion channel expression levels within a cardiomyocyte. Moreover, recent work on microtubule and actin-dependent trafficking for Cx43 and LTCC are introduced. Lastly, we discuss how human cardiac disease phenotypes can be attributed to defects in distinct mechanisms of channel regulation at the level of gene expression and channel trafficking. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Theca-derived BMP4 and BMP7 down-regulate connexin43 expression and decrease gap junction intercellular communication activity in immortalized human granulosa cells

CONTEXT

Connexin43 (Cx43)-coupled gap junctions in granulosa cells play important roles in follicular and oocyte development and may be modulated by theca cell-derived bone morphogenic protein (BMP) 4 and BMP7.

OBJECTIVE

The aim of this study was to examine the effects of BMP4 and BMP7 on Cx43 expression in human granulosa cells and its potential mediation by the Smad-dependent pathway.

DESIGN

An immortalized human granulosa (SVOG) cell was used to investigate Cx43 expression and gap junction intercellular communication (GJIC) activity after exposure to BMP4 and BMP7. A BMP type I inhibitor, dorsomorphin, and small interfering RNAs targeting Smad4 were used to verify the specificity of the effects.

SETTING

The study was conducted in an academic center.

MAIN OUTCOME MEASURES

Extracts were prepared from cultured cells, the Cx43 mRNA levels were examined using RT-quantitative real-time PCR, and the levels of Cx43 protein and phosphorylated Smad1/5/8 were assayed using Western blot analyses. GJIC activities between SVOG cells were evaluated using a scrape loading and dye transfer assay.

RESULTS

Treatment with BMP4 and BMP7 significantly decreased Cx43 mRNA and protein levels, as well as GJIC activities. These suppressive effects were attenuated by cotreatment with the BMP type I receptor inhibitor dorsomorphin. Furthermore, Smad4 knockdown reversed the effects of BMP4 and BMP7 on Cx43 expression.

CONCLUSION

Theca cell-derived BMP4 and BMP7 down-regulate Cx43 expression and decrease GJIC activity in human granulosa cells. Our findings indicate that this biological effect is most likely mediated by a Smad-dependent pathway.

Antofine-induced connexin43 gap junction disassembly in rat astrocytes involves protein kinase Cβ

Antofine, a phenanthroindolizidine alkaloid derived from Cryptocaryachinensis and Ficusseptica in the Asclepiadaceae milkweed family, is cytotoxic for various cancer cell lines. In this study, we demonstrated that treatment of rat primary astrocytes with antofine induced dose-dependent inhibition of gap junction intercellular communication (GJIC), as assessed by scrape-loading 6-carboxyfluorescein dye transfer. Levels of Cx43 protein were also decreased in a dose- and time-dependent manner following antofine treatment. Double-labeling immunofluorescence microscopy showed that antofine (10ng/ml) induced endocytosis of surface gap junctions into the cytoplasm, where Cx43 was co-localized with the early endosome marker EEA1. Inhibition of lysosomes or proteasomes by co-treatment with antofine and their respective specific inhibitors, NH4Cl or MG132, partially inhibited the antofine-induced decrease in Cx43 protein levels, but did not inhibit the antofine-induced inhibition of GJIC. After 30min of treatment, antofine induced a rapid increase in the intracellular Ca(2+) concentration and activation of protein kinase C (PKC)α/βII, which was maintained for at least 6h. Co-treatment of astrocytes with antofine and the intracellular Ca(2+) chelator BAPTA-AM prevented downregulation of Cx43 and inhibition of GJIC. Moreover, co-treatment with antofine and a specific PKCβ inhibitor prevented endocytosis of gap junctions, downregulation of Cx43, and inhibition of GJIC. Taken together, these findings indicate that antofine induces Cx43 gap junction disassembly by the PKCβ signaling pathway. Inhibition of GJIC by antofine may undermine the neuroprotective effect of astrocytes in CNS.

Gap junctional coupling in the vertebrate retina: variations on one theme?

Gap junctions connect cells in the bodies of all multicellular organisms, forming either homologous or heterologous (i.e. established between identical or different cell types, respectively) cell-to-cell contacts by utilizing identical (homotypic) or different (heterotypic) connexin protein subunits. Gap junctions in the nervous system serve electrical signaling between neurons, thus they are also called electrical synapses. Such electrical synapses are particularly abundant in the vertebrate retina where they are specialized to form links between neurons as well as glial cells. In this article, we summarize recent findings on retinal cell-to-cell coupling in different vertebrates and identify general features in the light of the evergrowing body of data. In particular, we describe and discuss tracer coupling patterns, connexin proteins, junctional conductances and modulatory processes. This multispecies comparison serves to point out that most features are remarkably conserved across the vertebrate classes, including (i) the cell types connected via electrical synapses; (ii) the connexin makeup and the conductance of each cell-to-cell contact; (iii) the probable function of each gap junction in retinal circuitry; (iv) the fact that gap junctions underlie both electrical and/or tracer coupling between glial cells. These pan-vertebrate features thus demonstrate that retinal gap junctions have changed little during the over 500 million years of vertebrate evolution. Therefore, the fundamental architecture of electrically coupled retinal circuits seems as old as the retina itself, indicating that gap junctions deeply incorporated in retinal wiring from the very beginning of the eye formation of vertebrates. In addition to hard wiring provided by fast synaptic transmitter-releasing neurons and soft wiring contributed by peptidergic, aminergic and purinergic systems, electrical coupling may serve as the 'skeleton' of lateral processing, enabling important functions such as signal averaging and synchronization.

Designer gap junctions that prevent cardiac arrhythmias

Cardiac gap junctions are specialized membrane structures comprised of arrays of intercellular channels responsible for propagation of the cardiac impulse. These channels are formed by oligomerization of individual protein subunits known as connexins. In response to a broad array of pathologic stressors, gap junction expression is disturbed, resulting in aberrant cardiac conduction and increased propensity for rhythm disturbances. In this article, we review some of the recently identified molecular regulators of connexin assembly, membrane targeting, and degradation, focusing on the role of post-translational phosphorylation of connexin 43, the major gap junctional protein expressed in ventricular myocardium. We also describe efforts to engineer "designer" gap junctions that are resistant to pathologic remodeling.

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.

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

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

Adenosine-triphosphate-sensitive K+ channel (Kir6.1): a novel phosphospecific interaction partner of connexin 43 (Cx43)

Connexin 43 (Cx43) is a phosphoprotein expressed in a wide variety of cells. Cx43 and adenosine-triphosphate-sensitive K(+)channels [K(+)(ATP)] are part of same signaling pathway that regulates cell survival during stress and ischemia preconditioning. Molecular mechanism for their coordinated role in ischemia/hypoxia preconditioning is not well known. Using pull down, co-immunoprecipitation assays and co-localization studies we provide evidence, for the first time that Kir6.1, a K(+)(ATP) channel protein component, can interact with Cx43. Further we show that the interaction was phospho-specific such that Cx43 phosphorylated at serine 262 (S262) interacted with Kir6.1 in preference to the unphosphorylated form of Cx43. Introduction of phospho-deficient mutation at serine 262 (S262A) in Cx43 completely abolished the interaction. Our data provide an interesting lead about a possible partnership between Cx43 and Kir6.1, which will help in better understanding their role in ischemia/hypoxia preconditioning.

Phosphorylation of serine residues in the C-terminal cytoplasmic tail of connexin43 regulates proliferation of ovarian granulosa cells

Connexin43 (Cx43) forms gap junctions that couple the granulosa cells of ovarian follicles. In Cx43 knockout mice, follicle growth is restricted as a result of impaired granulosa cell proliferation. We have used these mice to examine the importance of specific Cx43 phosphorylation sites in follicle growth. Serines at residues 255, 262, 279, and 282 are MAP kinase substrates that, when phosphorylated, reduce junctional conductance. Mutant forms of Cx43 were constructed with these serines replaced with amino acids that cannot be phosphorylated. These mutants were transduced into Cx43 knockout ovarian somatic cells that were combined with wild-type oocytes and grafted into immunocompromised female mice permitting follicle growth in vivo. Despite residues 255 or 262 being mutated to prevent their being phosphorylated, recombinant ovaries constructed with these mutants were able to rescue the null phenotype, restoring complete folliculogenesis. In contrast, Cx43 with serine to alanine mutations at both residues 279 and 282 or at all four residues failed to rescue folliculogenesis; the mutant molecules were largely confined to intracellular sites, with few gap junctions. Using an in vitro proliferation assay, we confirmed a decrease in proliferation of granulosa cells expressing the double mutant construct. These results indicate that Cx43 phosphorylation by MAP kinase at serines 279 and 282 occurs in granulosa cells of early follicles and that this is involved in regulating follicle development.

Connexin43 cardiac gap junction remodeling: lessons from genetically engineered murine models

Sudden cardiac death is responsible for several hundred thousand deaths each year in the United States. Multiple lines of evidence suggest that perturbation of gap junction expression and function in the heart, or what has come to be known as cardiac gap junction remodeling, plays a key mechanistic role in the pathophysiology of clinically significant cardiac arrhythmias. Here we review recent studies from our laboratory using genetically engineered murine models to explore mechanisms implicated in pathologic gap junction remodeling and their proarrhythmic consequences, with a particular focus on aberrant posttranslational phosphorylation of connexin43.

Control of vascular smooth muscle cell growth by connexin 43

Connexin 43 (Cx43), the principal gap junction protein in vascular smooth muscle cells (VSMCs), regulates movement of ions and other signaling molecules through gap junction intercellular communication (GJIC) and plays important roles in maintaining normal vessel function; however, many of the signaling mechanisms controlling Cx43 in VSMCs are not clearly described. The goal of this study was to investigate mechanisms of Cx43 regulation with respect to VSMC proliferation. Treatment of rat primary VSMCs with the cAMP analog 8Br-cAMP, the soluble guanylate cyclase (sGC) stimulator BAY 41-2272 (BAY), or the Cx inducer diallyl disulfide (DADS) significantly reduced proliferation after 72 h compared with vehicle controls. Bromodeoxyuridine uptake revealed reduction (p < 0.05) in DNA synthesis after 6 h and flow cytometry showed reduced (40%) S-phase cell numbers after 16 h in DADS-treated cells compared with vehicle controls. Cx43 expression significantly increased after 270 min treatment with 8Br-cAMP, 8Br-cGMP, BAY or DADS. Inhibition of PKA, PKG or PKC reversed 8Br-cAMP-stimulated increases in Cx43 expression, whereas only PKG or PKC inhibition reversed 8Br-cGMP- and BAY-stimulated increases in total Cx43. Interestingly, stimulation of Cx43 expression by DADS was not dependent on PKA, PKG or PKC. Using fluorescence recovery after photobleaching, only 8Br-cAMP or DADS increased GJIC with 8Br-cAMP mediated by PKC and DADS mediated by PKG. Further, DADS significantly increased phosphorylation at MAPK-sensitive Serine (Ser)255 and Ser279, the cell cycle regulatory kinase-sensitive Ser262 and PKC-sensitive Ser368 after 30 min while 8Br-cAMP significantly increased phosphorylation only at Ser279 compared with controls. This study demonstrates that 8Br-cAMP- and DADS-enhanced GJIC rather than Cx43 expression and/or phosphorylation plays important roles in the regulation of VSMC proliferation and provides new insights into the growth-regulatory capacities of Cx43 in VSM.

Connexin 43 phosphorylation and degradation are required for adipogenesis

Connexin-43 (Cx43) is a membrane phosphoprotein that mediates direct inter-cellular communication by forming gap junctions. In this way Cx43 can influence gene expression, differentiation and growth. Its role in adipogenesis, however, is poorly understood. In this study, we established that Cx43 becomes highly phosphorylated in early adipocyte differentiation and translocates to the plasma membrane from the endoplasmic reticulum. As preadipocytes differentiate, Cx43 phosphorylation declines, the protein is displaced from the plasma membrane, and total cellular levels are reduced via proteosomal degradation. Notably, we show that inhibiting Cx43 degradation or constitutively over-expressing Cx43 blocks adipocyte differentiation. These data reveal that transient activation of Cx43 via phosphorylation followed by its degradation is vital for preadipocyte differentiation and maturation of functional adipocytes.

Gap junction proteins on the move: connexins, the cytoskeleton and migration

Connexin43 (Cx43) has roles in cell-cell communication as well as channel independent roles in regulating motility and migration. Loss of function approaches to decrease Cx43 protein levels in neural cells result in reduced migration of neurons during cortical development in mice and impaired glioma tumor cell migration. In other cell types, correlations between Cx43 expression and cell morphology, adhesion, motility and migration have been noted. In this review we will discuss the common themes that have been revealed by a detailed comparison of the published results of neuronal cells with that of other cell types. In brief, these comparisons clearly show differences in the stability and directionality of protrusions, polarity of movement, and migration, depending on whether a) residual Cx43 levels remain after siRNA or shRNA knockdown, b) Cx43 protein levels are not detectable as in cells from Cx43(-/-) knockout mice or in cells that normally have no endogenous Cx43 expression, c) gain-of-function approaches are used to express Cx43 in cells that have no endogenous Cx43 and, d) Cx43 is over-expressed in cells that already have low endogenous Cx43 protein levels. What is clear from our comparisons is that Cx43 expression influences the adhesiveness of cells and the directionality of cellular processes. These observations are discussed in light of the ability of cells to rearrange their cytoskeleton and move in an organized manner. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Sex differences in cardiomyocyte connexin43 expression

Decreases in cardiac connexin43 (Cx43) play a critical role in abnormal cell-to-cell communication and have been linked to the resistance of the female heart to arrhythmias. We therefore hypothesized that Cx43 expression would be greater in female cardiomyocytes than in male cardiomyocytes under pathologic conditions. Adult ventricular myocytes were isolated from male and female rats and treated with phenylephrine (PE), a well-established pathologic stimulus. Cx43 gene and protein expression was determined. The expression of micro-RNA-1 (miR-1), a micro-RNA known to control Cx43 protein expression in cardiomyocytes, was also determined. Cx43 mRNA and protein levels were significantly higher in the female cardiomyocytes than in the male cardiomyocytes (mRNA: 1.4-fold; Protein: 5-fold, both P < 0.05) under both basal and pathologic conditions. PE treatment increased Cx43 expression only in female cardiomyocytes. Cx43 phosphorylation, a marker of preserved Cx43 function, was also higher (P < 0.05), and The expression of miR-1 was lower (P < 0.05) in the female cardiomyocytes after PE treatment. The expression of miR-1 was unchanged by PE treatment in male cardiomyocytes. Thus, a sex difference in miR-1 may be responsible for the sex difference in Cx43 expression in cardiomyocytes under pathologic conditions. Taken together, our results demonstrate a sex difference in Cx43 expression and site-specific phosphorylation that favors cardioprotection in female cardiomyocytes.

The gap junction channel protein connexin 43 is covalently modified and regulated by SUMOylation

SUMOylation is a posttranslational modification in which a member of the small ubiquitin-like modifier (SUMO) family of proteins is conjugated to lysine residues in specific target proteins. Most known SUMOylation target proteins are located in the nucleus, but there is increasing evidence that SUMO may also be a key determinant of many extranuclear processes. Gap junctions consist of arrays of intercellular channels that provide direct transfer of ions and small molecules between adjacent cells. Gap junction channels are formed by integral membrane proteins called connexins, of which the best-studied isoform is connexin 43 (Cx43). Here we show that Cx43 is posttranslationally modified by SUMOylation. The data suggest that the SUMO system regulates the Cx43 protein level and the level of functional Cx43 gap junctions at the plasma membrane. Cx43 was found to be modified by SUMO-1, -2, and -3. Evidence is provided that the membrane-proximal lysines at positions 144 and 237, located in the Cx43 intracellular loop and C-terminal tail, respectively, act as SUMO conjugation sites. Mutations of lysine 144 or lysine 237 resulted in reduced Cx43 SUMOylation and reduced Cx43 protein and gap junction levels. Altogether, these data identify Cx43 as a SUMOylation target protein and represent the first evidence that gap junctions are regulated by the SUMO system.

Sufentanil limits the myocardial infarct size by preservation of the phosphorylated connexin 43

Sufentanil, with a potent analgesia effect, has been wildly used in anesthesia and analgesia, especially for the cardiovascular surgeries. The aim of the study was to evaluate whether sufentanil provides cardioprotection and the effect of connexin 43 on the cardiac infarct size reduction. Sufentanil post-conditioning (bolus injection at 0.1, 0.3, 1, 3, 10 μg/kg) or ischemic post-conditioning (3 cycles of a 10s reperfusion alternating with a 10s ischemia) was induced in an intact rat heart model of ischemia-reperfusion injury. Both ischemic and sufentanil post-conditioning reduced the myocardial infarct size compared with control group. The infarct size limitation of sufentanil was dose-dependent, 1 μg/kg has the optimal effect and increasing dosage could not afford further cardioprotection. Connexin 43 underwent dephosphorylation in response to ischemia-reperfusion measured by Western blot at the anterior myocardium tissues of left ventricle while sufentanil preserved the phosphorylation of connexin 43. The results demonstrated that sufentanil limits myocardial infarct size which is similar with ischemic post-conditioning at the dosage of 1 μg/kg. Preservation of phosphorylation of connexin 43 plays an important role in the cardioprotection of ischemic and sufentanil post-conditioning.

The molecular role of connexin 43 in human trophoblast cell fusion

Connexin expression and gap junctional intercellular communication (GJIC) mediated by connexin 43 (Cx43)/gap junction A1 (GJA1) are required for cytotrophoblast fusion into the syncytium, the outer functional layer of the human placenta. Cx43 also impacts intracellular signaling through protein-protein interactions. The transcription factor GCM1 and its downstream target ERVW-1/SYNCYTIN-1 are key players in trophoblast fusion and exert their actions through the ERVW-1 receptor SLC1A5/ASCT-2/RDR/ATB(0). To investigate the molecular role of the Cx43 protein and its interaction with this fusogenic pathway, we utilized stable Cx43-transfected cell lines established from the choriocarcinoma cell line Jeg3: wild-type Jeg3, alphahCG/Cx43 (constitutive Cx43 expression), JpUHD/Cx43 (doxycyclin-inducible Cx43 expression), or JpUHD/trCx43 (doxycyclin-inducible Cx43 carboxyterminal deleted). We hypothesized that truncation of Cx43 at its C-terminus would inhibit trophoblast fusion and protein interaction with either ERVW-1 or SLC1A5. In the alphahCG/Cx43 and JpUHD/Cx43 lines, stimulation with cAMP caused 1) increase in GJA1 mRNA levels, 2) increase in percentage of fused cells, and 3) downregulation of SLC1A5 expression. Cell fusion was inhibited by GJIC blockade using carbenoxylone. Neither Jeg3, which express low levels of Cx43, nor the JpUHD/trCx43 cell line demonstrated cell fusion or downregulation of SLC1A5. However, GCM1 and ERVW-1 mRNAs were upregulated by cAMP treatment in both Jeg3 and all Cx43 cell lines. Silencing of GCM1 prevented the induction of GJA1 mRNA by forskolin in BeWo choriocarcinoma cells, demonstrating that GCM1 is upstream of Cx43. All cell lines and first-trimester villous explants also demonstrated coimmunoprecipitation of SLC1A5 and phosphorylated Cx43. Importantly, SLC1A5 and Cx43 gap junction plaques colocalized in situ to areas of fusing cytotrophoblast, as demonstrated by the loss of E-cadherin staining in the plasma membrane in first-trimester placenta. We conclude that Cx43-mediated GJIC and SLC1A5 interaction play important functional roles in trophoblast cell fusion.

Connexin43 interacts with βarrestin: a pre-requisite for osteoblast survival induced by parathyroid hormone

Parathyroid hormone (PTH) promotes osteoblast survival through a mechanism that depends on cAMP-mediated signaling downstream of the G protein-coupled receptor PTHR1. We present evidence herein that PTH-induced survival signaling is impaired in cells lacking connexin43 (Cx43). Thus, expression of functional Cx43 dominant negative proteins or Cx43 knock-down abolished the expression of cAMP-target genes and anti-apoptosis induced by PTH in osteoblastic cells. In contrast, cells lacking Cx43 were still responsive to the stable cAMP analog dibutyril-cAMP. PTH survival signaling was rescued by transfecting wild type Cx43 or a truncated dominant negative mutant of βarrestin, a PTHR1-interacting molecule that limits cAMP signaling. On the other hand, Cx43 mutants lacking the cytoplasmic domain (Cx43(Δ245)) or unable to be phosphorylated at serine 368 (Cx43(S368A)), a residue crucial for Cx43 trafficking and function, failed to restore the anti-apoptotic effect of PTH in Cx43-deficient cells. In addition, overexpression of wild type βarrestin abrogated PTH survival signaling in Cx43-expressing cells. Moreover, βarrestin physically associated in vivo to wild type Cx43 and to a lesser extent to Cx43(S368A) ; and this association and the phosphorylation of Cx43 in serine 368 were reduced by PTH. Furthermore, induction of Cx43(S368) phosphorylation or overexpression of wild type Cx43, but not Cx43(Δ245) or Cx43(S368A) , reduced the interaction between βarrestin and the PTHR1. These studies demonstrate that βarrestin is a novel Cx43-interacting protein and suggest that, by sequestering βarrestin, Cx43 facilitates cAMP signaling, thereby exerting a permissive role on osteoblast survival induced by PTH.

Structural basis for the selective permeability of channels made of communicating junction proteins

The open state(s) of gap junction channels is evident from their permeation by small ions in response to an applied intercellular (transjunctional/transchannel) voltage gradient. That an open channel allows variable amounts of current to transit from cell-to-cell in the face of a constant intercellular voltage difference indicates channel open/closing can be complete or partial. The physiological significance of such open state options is, arguably, the main concern of junctional regulation. Because gap junctions are permeable to many substances, it is sensible to inquire whether and how each open state influences the intercellular diffusion of molecules as valuable as, but less readily detected than current-carrying ions. Presumably, structural changes perceived as shifts in channel conductivity would significantly alter the transjunctional diffusion of molecules whose limiting diameter approximates the pore's limiting diameter. Moreover, changes in junctional permeability to some molecules might occur without evident changes in conductivity, either at macroscopic or single channel level. Open gap junction channels allow the exchange of cytoplasmic permeants between contacting cells by simple diffusion. The identity of such permeants, and the functional circumstances and consequences of their junctional exchange presently constitute the most urgent (and demanding) themes of the field. Here, we consider the necessity for regulating this exchange, the possible mechanism(s) and structural elements likely involved in such regulation, and how regulatory phenomena could be perceived as changes in chemical vs. electrical coupling; an overall reflection on our collective knowledge of junctional communication is then applied to suggest new avenues of research. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Gap junction communication in myelinating glia

Gap junction communication is crucial for myelination and axonal survival in both the peripheral nervous system (PNS) and central nervous system (CNS). This review examines the different types of gap junctions in myelinating glia of the PNS and CNS (Schwann cells and oligodendrocytes respectively), including their functions and involvement in neurological disorders. Gap junctions mediate intercellular communication among Schwann cells in the PNS, and among oligodendrocytes and between oligodendrocytes and astrocytes in the CNS. Reflexive gap junctions mediating transfer between different regions of the same cell promote communication between cellular compartments of myelinating glia that are separated by layers of compact myelin. Gap junctions in myelinating glia regulate physiological processes such as cell growth, proliferation, calcium signaling, and participate in extracellular signaling via release of neurotransmitters from hemijunctions. In the CNS, gap junctions form a glial network between oligodendrocytes and astrocytes. This transcellular communication is hypothesized to maintain homeostasis by facilitating restoration of membrane potential after axonal activity via electrical coupling and the re-distribution of potassium ions released from axons. The generation of transgenic mice for different subsets of connexins has revealed the contribution of different connexins in gap junction formation and illuminated new subcellular mechanisms underlying demyelination and cognitive defects. Alterations in metabolic coupling have been reported in animal models of X-linked Charcot-Marie-Tooth disease (CMTX) and Pelizaeus-Merzbarcher-like disease (PMLD), which are caused by mutations in the genes encoding for connexin 32 and connexin 47 respectively. Future research identifying the expression and regulation of gap junctions in myelinating glia is likely to provide a better understanding of myelinating glia in nervous system function, plasticity, and disease. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Laminin-111 stimulates proliferation of mouse embryonic stem cells through a reduction of gap junctional intercellular communication via RhoA-mediated Cx43 phosphorylation and dissociation of Cx43/ZO-1/drebrin complex

Gap junctions within extracellular matrix (ECM)-defined boundaries ensure synchronous activity between cells destined to become functional mediators that regulate cell behavior. However, the role of ECM in connexin (Cx) function in mouse embryonic stem cells (mESCs) has not been elucidated. Therefore, we examined the role of laminin-111 in the control of Cx43 functions and related signal pathways in mESCs. ECM components (laminin-111, fibronectin, and collagen I) increased Cx43 phosphorylation and decreased Lucifer yellow (Ly) diffusion. In addition, laminin-111 increased the proliferation index through reduction of gap junctional intercellular communication (GJIC), which was confirmed by 18α-glycyrrhetinic acid (18α-GA). Laminin-111 increased phosphorylation of focal adhesion kinase (FAK)/Src and protein kinase C (PKC), which were inhibited by integrin β1 antibody (Ab) and laminin receptor-1 (LR-1) Ab, respectively. In addition, inhibition of both FAK/Src and PKC blocked Cx43 phosphorylation. Laminin-111 increased the Ras homolog gene family, member A (RhoA) activation, which was blocked by FAK/Src and PKC inhibitors, suggesting the existence of parallel pathways that merge at RhoA. Inhibition of RhoA reversed the laminin-111-induced increase of Cx43 phosphorylation and reduction of GJIC. Laminin-111 also stimulated the dissociation of Cx43/ZO-1 complex followed by disruption of Cx43/drebrin and Cx43/F-actin complexes, which were reversed by C3 (RhoA inhibitor). ZO-1 small interfering (si) RNA significantly decreased Ly diffusion. Moreover, laminin-111 decreased Cx43 labeling at the intercellular junction, whereas pretreatment with degradation inhibitors (lysosomal protease inhibitor, chloroquine; proteasome inhibitor, lactacystin) increased Cx43 expression, reversely. In conclusion, laminin-111 stimulated mESC proliferation through a reduction of GJIC via RhoA-mediated Cx43 phosphorylation and Cx43/ZO-1/drebrin complex instability-mediated Cx43 degradation.

Endothelial Ca2+ wavelets and the induction of myoendothelial feedback

When arteries constrict to agonists, the endothelium inversely responds, attenuating the initial vasomotor response. The basis of this feedback mechanism remains uncertain, although past studies suggest a key role for myoendothelial communication in the signaling process. The present study examined whether second messenger flux through myoendothelial gap junctions initiates a negative-feedback response in hamster retractor muscle feed arteries. We specifically hypothesized that when agonists elicit depolarization and a rise in second messenger concentration, inositol trisphosphate (IP(3)) flux activates a discrete pool of IP(3) receptors (IP(3)Rs), elicits localized endothelial Ca(2+) transients, and activates downstream effectors to moderate constriction. With use of integrated experimental techniques, this study provided three sets of supporting observations. Beginning at the functional level, we showed that blocking intermediate-conductance Ca(2+)-activated K(+) channels (IK) and Ca(2+) mobilization from the endoplasmic reticulum (ER) enhanced the contractile/electrical responsiveness of feed arteries to phenylephrine. Next, structural analysis confirmed that endothelial projections make contact with the overlying smooth muscle. These projections retained membranous ER networks, and IP(3)Rs and IK channels localized in or near this structure. Finally, Ca(2+) imaging revealed that phenylephrine induced discrete endothelial Ca(2+) events through IP(3)R activation. These events were termed recruitable Ca(2+) wavelets on the basis of their spatiotemporal characteristics. From these findings, we conclude that IP(3) flux across myoendothelial gap junctions is sufficient to induce focal Ca(2+) release from IP(3)Rs and activate a discrete pool of IK channels within or near endothelial projections. The resulting hyperpolarization feeds back on smooth muscle to moderate agonist-induced depolarization and constriction.

Effects of mechanical forces and stretch on intercellular gap junction coupling

Mechanical forces provide fundamental physiological stimulus in living organisms. Recent investigations demonstrated how various types of mechanical load, like strain, pressure, shear stress, or cyclic stretch can affect cell biology and gap junction intercellular communication (GJIC). Depending on the cell type, the type of mechanical load and on strength and duration of application, these forces can induce hypertrophic processes and modulate the expression and function of certain connexins such as Cx43, while others such as Cx37 or Cx40 are reported to be less mechanosensitive. In particular, not only expression but also subcellular localization of Cx43 is altered in cardiomyocytes submitted to cyclic mechanical stretch resulting in the typical elongated cell shape with an accentuation of Cx43 at the cell poles. In the heart both cardiomyocytes and fibroblasts can alter their GJIC in response to mechanical load. In the vasculature both endothelial cells and smooth muscle cells are subject to strain and cyclic stretch resulting from the pulsatile flow. In addition, vascular endothelial cells are mainly affected by shear stress resulting from the blood flow parallel to their surface. These mechanical forces lead to a regulation of GJIC in vascular tissue. In bones, osteocytes and osteoblasts are coupled via gap junctions, which also react to mechanical forces. Since gap junctions are involved in regulation of cell growth and differentiation, the mechanosensitivity of the regulation of these channels might open new perspectives to explain how cells can respond to mechanical load, and how stretch induces self-organization of a cell layer which might have implications for embryology and the development of organs. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Smad ubiquitination regulatory factor-2 controls gap junction intercellular communication by modulating endocytosis and degradation of connexin43

Gap junctions consist of arrays of intercellular channels that enable adjacent cells to communicate both electrically and metabolically. Gap junction channels are made of a family of integral membrane proteins called connexins, of which the best-studied member is connexin43. Gap junctions are dynamic plasma membrane domains, and connexin43 has a high turnover rate in most tissue types. However, the mechanisms involved in the regulation of connexin43 endocytosis and transport to lysosomes are still poorly understood. Here, we demonstrate by live-cell imaging analysis that treatment of cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) induces endocytosis of subdomains of connexin43 gap junctions. The internalized, connexin43-enriched vesicles were found to fuse with early endosomes, which was followed by transport of connexin43 to the lumen of early endosomes. The HECT E3 ubiquitin ligase smad ubiquitination regulatory factor-2 (Smurf2) was found to be recruited to connexin43 gap junctions in response to TPA treatment. Depletion of Smurf2 by small interfering RNA (siRNA) resulted in enhanced levels of connexin43 gap junctions between adjacent cells and increased gap junction intercellular communication. Smurf2 depletion also counteracted the TPA-induced endocytosis and degradation of connexin43. Collectively, these data identify Smurf2 as a novel regulator of connexin43 gap junctions.

Role of gap junctions in epilepsy

Epilepsy is a common neurological disorder characterized by periodic and unpredictable seizures. Gap junctions have recently been proposed to be involved in the generation, synchronization and maintenance of seizure events. The present review mainly summarizes recent reports concerning the contribution of gap junctions to the pathophysiology of epilepsy, together with the regulation of connexin after clinical and experimental seizure activity. The anticonvulsant effects of gap junction blockers both in vitro and in vivo suggest that the gap junction is a candidate target for the development of antiepileptic drugs. It is also of interest that the roles of neuronal and astrocytic gap junctions in epilepsy have been investigated independently, based on evidence from pharmacological manipulations and connexin-knockout mice. Further studies using more specific manipulations of gap junctions in different cell types and in human epileptic tissue are needed to fully uncover the role of gap junctions in epilepsy.

Subsets of ATP-sensitive potassium channel (KATP) inhibitors increase gap junctional intercellular communication in metastatic cancer cell lines independent of SUR expression

Gap junctional intercellular communication (GJIC) regulates cellular homeostasis by propagating signaling molecules, exchanging cellular metabolites, and coupling electrical signals. In cancer, cells exhibit altered rates of GJIC which may play a role in neoplastic progression. K(ATP) channels help maintain membrane polarity and linkages between K(ATP) channel activity and rates of GJIC have been established. The mechanistic relationship has not been fully elucidated. We report the effects of treatment with multiple K(ATP) antagonist compounds on GJIC in metastatic cell lines demonstrating an increase in communication rates following treatment with compounds possessing specificities towards the SUR2 subunit of K(ATP). These effects remained consistent using cell lines with different expression levels of SUR1 and SUR2, suggesting possible off target effects on GJIC by these compounds.

eNOS activation and NO function: pregnancy adaptive programming of capacitative entry responses alters nitric oxide (NO) output in vascular endothelium--new insights into eNOS regulation through adaptive cell signaling

In pregnancy, vascular nitric oxide (NO) production is increased in the systemic and more so in the uterine vasculature, thereby supporting maximal perfusion of the uterus. This high level of functionality is matched in the umbilical vein, and in corresponding disease states such as pre-eclampsia, reduced vascular responses are seen in both uterine artery and umbilical vein. In any endothelial cell, NO actually produced by endothelial NO synthase (eNOS) is determined by the maximum capacity of the cell (eNOS expression levels), eNOS phosphorylation state, and the intracellular [Ca(2+)](i) concentration in response to circulating hormones or physical forces. Herein, we discuss how pregnancy-specific reprogramming of NO output is determined as much by pregnancy adaptation of [Ca(2+)](i) signaling responses as it is by eNOS expression and phosphorylation. By examining the changes in [Ca(2+)](i) signaling responses from human hand vein endothelial cells, uterine artery endothelial cells, and human umbilical vein endothelial cells in (where appropriate) nonpregnant, normal pregnant, and pathological pregnant (pre-eclamptic) state, it is clear that pregnancy adaptation of NO output occurs at the level of sustained phase 'capacitative entry' [Ca(2+)](i) response, and the adapted response is lacking in pre-eclamptic pregnancies. Moreover, gap junction function is an essential permissive regulator of the capacitative response and impairment of NO output results from any inhibitor of gap junction function, or capacitative entry using TRPC channels. Identifying these [Ca(2+)](i) signaling mechanisms underlying normal pregnancy adaptation of NO output not only provides novel targets for future treatment of diseases of pregnancy but may also apply to other common forms of hypertension.

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.

Three-dimensional modeling and quantitative analysis of gap junction distributions in cardiac tissue

Gap junctions play a fundamental role in intercellular communication in cardiac tissue. Various types of heart disease including hypertrophy and ischemia are associated with alterations of the spatial arrangement of gap junctions. Previous studies applied two-dimensional optical and electron-microscopy to visualize gap junction arrangements. In normal cardiomyocytes, gap junctions were primarily found at cell ends, but can be found also in more central regions. In this study, we extended these approaches toward three-dimensional reconstruction of gap junction distributions based on high-resolution scanning confocal microscopy and image processing. We developed methods for quantitative characterization of gap junction distributions based on analysis of intensity profiles along the principal axes of myocytes. The analyses characterized gap junction polarization at cell ends and higher-order statistical image moments of intensity profiles. The methodology was tested in rat ventricular myocardium. Our analysis yielded novel quantitative data on gap junction distributions. In particular, the analysis demonstrated that the distributions exhibit significant variability with respect to polarization, skewness, and kurtosis. We suggest that this methodology provides a quantitative alternative to current approaches based on visual inspection, with applications in particular in characterization of engineered and diseased myocardium. Furthermore, we propose that these data provide improved input for computational modeling of cardiac conduction.