Specific motifs in the external loops of connexin proteins can determine gap junction formation between chick heart myocytes

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

Retinoic Acid Influences connexin43 Expression During Joint Formation in the Regenerating Zebrafish Fin

Background

The regenerating zebrafish fin skeleton is comprised of multiple bony fin rays, each made of alternating bony segments and fin ray joints. This pattern is regulated by the gap junction protein Connexin43 (Cx43), which provides instructional cues to skeletal precursor cells (SPCs). Elevated Cx43 favors osteoblast differentiation and disfavors joint forming cell differentiation. The goal of this article is to test if retinoic acid (RA) contributes to the regulation of cx43 expression.

Materials and Methods

Functional studies inhibiting the RA-synthesizing enzyme Adh1a2 were evaluated using in situ hybridization to monitor gene expression and with measurements of the length of fin ray segments to monitor impacts on SPC differentiation and joint formation.

Results

Aldh1a2-knockdown leads to reduced expression of cx43 and increased expression of evx1, a gene required for joint formation. Additionally, inhibition of Aldh1a2 function leads to short fin ray segments. We also find evidence for synergy between aldh1a2 and cx43, suggesting that these genes function in a common molecular pathway to regulate joint formation.

Conclusions

The role of RA is to promote cx43 expression in the regenerating fin to regulate joint formation and the length of bony fin ray segments. We suggest that RA signaling must coordinate with additional pathways that also regulate cx43 transcription.

A novel mutation in GJC2 associated with hypomyelinating leukodystrophy type 2 disorder

Hypomyelinating leukodystrophy type 2 (HLD2), is an inherited genetic disease of the central nervous system caused by recessive mutations in the gap junction protein gamma 2 (GJC2/GJA12). HLD2 is characterized by nystagmus, developmental delay, motor impairments, ataxia, severe speech problem, and hypomyelination in the brain. The GJC2 sequence encodes connexin 47 protein (Cx47). Connexins are a group of membrane proteins that oligomerize to construct gap junctions protein. In the present study, a novel missense mutation gene c.760G>A (p.Val254Met) was identified in a patient with HLD2 by performing whole exome sequencing. Following the discovery of the new mutation in the proband, we used Sanger sequencing to analyze his affected sibling and parents. Sanger sequencing verified homozygosity of the mutation in the proband and his affected sibling. The autosomal recessive inheritance pattern was confirmed since Sanger sequencing revealed both healthy parents were heterozygous for the mutation. PolyPhen2, SIFT, PROVEAN, and CADD were used to evaluate the function prediction scores of detected mutations. Cx47 is essential for oligodendrocyte function, including adequate myelination and myelin maintenance in humans. Novel mutation p.Val254Met is located in the second extracellular domain of Cx47, both extracellular loops are highly conserved and probably induce intramolecular disulfide interactions. This novel mutation in the Cx47 gene causes oligodendrocyte dysfunction and HLD2 disorder.

Connexin 43: A Target for the Treatment of Inflammation in Secondary Complications of the Kidney and Eye in Diabetes

Of increasing prevalence, diabetes is characterised by elevated blood glucose and chronic inflammation that precedes the onset of multiple secondary complications, including those of the kidney and the eye. As the leading cause of end stage renal disease and blindness in the working population, more than ever is there a demand to develop clinical interventions which can both delay and prevent disease progression. Connexins are membrane bound proteins that can form pores (hemichannels) in the cell membrane. Gated by cellular stress and injury, they open under pathophysiological conditions and in doing so release 'danger signals' including adenosine triphosphate into the extracellular environment. Linked to sterile inflammation via activation of the nod-like receptor protein 3 inflammasome, targeting aberrant hemichannel activity and the release of these danger signals has met with favourable outcomes in multiple models of disease, including secondary complications of diabetes. In this review, we provide a comprehensive update on those studies which document a role for aberrant connexin hemichannel activity in the pathogenesis of both diabetic eye and kidney disease, ahead of evaluating the efficacy of blocking connexin-43 specific hemichannels in these target tissues on tissue health and function.

Development and physiological functions of the lymphatic system: insights from human genetic studies of primary lymphedema

Primary lymphedema is a long-term (chronic) condition characterized by tissue lymph retention and swelling that can affect any part of the body, although it usually develops in the arms or legs. Due to the relevant contribution of the lymphatic system to human physiology, while this review mainly focuses on the clinical and physiological aspects related to the regulation of fluid homeostasis and edema, clinicians need to know that the impact of lymphatic dysfunction with a genetic origin can be wide ranging. Lymphatic dysfunction can affect immune function so leading to infection; it can influence cancer development and spread, and it can determine fat transport so impacting on nutrition and obesity. Genetic studies and the development of imaging techniques for the assessment of lymphatic function have enabled the recognition of primary lymphedema as a heterogenic condition in terms of genetic causes and disease mechanisms. In this review, the known biological functions of several genes crucial to the development and function of the lymphatic system are used as a basis for understanding normal lymphatic biology. The disease conditions originating from mutations in these genes are discussed together with a detailed clinical description of the phenotype and the up-to-date knowledge in terms of disease mechanisms acquired from in vitro and in vivo research models.

Mechanisms of Connexin Regulating Peptides

Gap junctions (GJ) and connexins play integral roles in cellular physiology and have been found to be involved in multiple pathophysiological states from cancer to cardiovascular disease. Studies over the last 60 years have demonstrated the utility of altering GJ signaling pathways in experimental models, which has led to them being attractive targets for therapeutic intervention. A number of different mechanisms have been proposed to regulate GJ signaling, including channel blocking, enhancing channel open state, and disrupting protein-protein interactions. The primary mechanism for this has been through the design of numerous peptides as therapeutics, that are either currently in early development or are in various stages of clinical trials. Despite over 25 years of research into connexin targeting peptides, the overall mechanisms of action are still poorly understood. In this overview, we discuss published connexin targeting peptides, their reported mechanisms of action, and the potential for these molecules in the treatment of disease.

Impaired Cx43 gap junction endocytosis causes morphological and functional defects in zebrafish

Gap junctions mediate direct cell-to-cell communication by forming channels that physically couple cells, thereby linking their cytoplasm, permitting the exchange of molecules, ions, and electrical impulses. Gap junctions are assembled from connexin (Cx) proteins, with connexin 43 (Cx43) being the most ubiquitously expressed and best studied. While the molecular events that dictate the Cx43 life cycle have largely been characterized, the unusually short half-life of connexins of only 1-5 hours, resulting in constant endocytosis and biosynthetic replacement of gap junction channels has remained puzzling. The Cx43 C-terminal (CT) domain serves as the regulatory hub of the protein affecting all aspects of gap junction function. Here, deletion within the Cx43 CT (amino acids 256-289), a region known to encode key residues regulating gap junction turnover is employed to examine the effects of dysregulated Cx43 gap junction endocytosis using cultured cells (Cx43^∆256-289) and a zebrafish model (cx43^lh10). We report that this CT deletion causes defective gap junction endocytosis as well as increased gap junction intercellular communication (GJIC). Increased Cx43 protein content in cx43^lh10 zebrafish, specifically in the cardiac tissue, larger gap junction plaques and longer Cx43 protein half-lives coincide with severely impaired development. Our findings demonstrate for the first time that Cx43 gap junction endocytosis is an essential aspect of gap junction function and when impaired, gives rise to significant physiological problems as revealed here for cardiovascular development and function. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Over-activated hemichannels: A possible therapeutic target for human diseases

In our body, all the cells are constantly sharing chemical and electrical information with other cells. This intercellular communication allows them to respond in a concerted way to changes in the extracellular milieu. Connexins are transmembrane proteins that have the particularity of forming two types of channels; hemichannels and gap junction channels. Under normal conditions, hemichannels allow the controlled release of signaling molecules to the extracellular milieu. However, under certain pathological conditions, over-activated hemichannels can induce and/or exacerbate symptoms. In the last decade, great efforts have been put into developing new tools that can modulate these over-activated hemichannels. Small molecules, antibodies and mimetic peptides have shown a potential for the treatment of human diseases. In this review, we summarize recent findings in the field of hemichannel modulation via specific tools, and how these tools could improve patient outcome in certain pathological conditions.

Lymphatic Connexins and Pannexins in Health and Disease

This review highlights current knowledge on the expression and function of connexins and pannexins, transmembrane channel proteins that play an important role in intercellular communication, in both the developing and mature lymphatic vasculature. A particular focus is given to the involvement of these proteins in functions of the healthy lymphatic system. We describe their influence on the maintenance of extracellular fluid homeostasis, immune cell trafficking to draining lymph nodes and dietary nutrient absorption by intestinal villi. Moreover, new insights into connexin mutations in primary and secondary lymphedema as well as on the implication of lymphatic connexins and pannexins in acquired cardiovascular diseases are discussed, allowing for a better understanding of the role of these proteins in pathologies linked to dysfunctions in the lymphatic system.

Peptidic Connexin43 Therapeutics in Cardiac Reparative Medicine

Connexin (Cx43)-formed channels have been linked to cardiac arrhythmias and diseases of the heart associated with myocardial tissue loss and fibrosis. These pathologies include ischemic heart disease, ischemia-reperfusion injury, heart failure, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, and Duchenne muscular dystrophy. A number of Cx43 mimetic peptides have been reported as therapeutic candidates for targeting disease processes linked to Cx43, including some that have advanced to clinical testing in humans. These peptides include Cx43 sequences based on the extracellular loop domains (e.g., Gap26, Gap 27, and Peptide5), cytoplasmic-loop domain (Gap19 and L2), and cytoplasmic carboxyl-terminal domain (e.g., JM2, Cx43tat, CycliCX, and the alphaCT family of peptides) of this transmembrane protein. Additionally, RYYN peptides binding to the Cx43 carboxyl-terminus have been described. In this review, we survey preclinical and clinical data available on short mimetic peptides based on, or directly targeting, Cx43, with focus on their potential for treating heart disease. We also discuss problems that have caused reluctance within the pharmaceutical industry to translate peptidic therapeutics to the clinic, even when supporting preclinical data is strong. These issues include those associated with the administration, stability in vivo, and tissue penetration of peptide-based therapeutics. Finally, we discuss novel drug delivery technologies including nanoparticles, exosomes, and other nanovesicular carriers that could transform the clinical and commercial viability of Cx43-targeting peptides in treatment of heart disease, stroke, cancer, and other indications requiring oral or parenteral administration. Some of these newly emerging approaches to drug delivery may provide a path to overcoming pitfalls associated with the drugging of peptide therapeutics.

Connexins and the Epithelial Tissue Barrier: A Focus on Connexin 26

Simple Summary

Tissues that face the external environment are known as ‘epithelial tissue’ and form barriers between different body compartments. This includes the outer layer of the skin, linings of the intestine and airways that project into the lumen connecting with the external environment, and the cornea of the eye. These tissues do not have a direct blood supply and are dependent on exchange of regulatory molecules between cells to ensure co-ordination of tissue events. Proteins known as connexins form channels linking cells directly and permit exchange of small regulatory signals. A range of environmental stimuli can dysregulate the level of connexin proteins and or protein function within the epithelia, leading to pathologies including non-healing wounds. Mutations in these proteins are linked with hearing loss, skin and eye disorders of differing severity. As such, connexins emerge as prime therapeutic targets with several agents currently in clinical trials. This review outlines the role of connexins in epithelial tissue and how their dysregulation contributes to pathological pathways.

Abstract

Epithelial tissue responds rapidly to environmental triggers and is constantly renewed. This tissue is also highly accessible for therapeutic targeting. This review highlights the role of connexin mediated communication in avascular epithelial tissue. These proteins form communication conduits with the extracellular space (hemichannels) and between neighboring cells (gap junctions). Regulated exchange of small metabolites less than 1kDa aide the co-ordination of cellular activities and in spatial communication compartments segregating tissue networks. Dysregulation of connexin expression and function has profound impact on physiological processes in epithelial tissue including wound healing. Connexin 26, one of the smallest connexins, is expressed in diverse epithelial tissue and mutations in this protein are associated with hearing loss, skin and eye conditions of differing severity. The functional consequences of dysregulated connexin activity is discussed and the development of connexin targeted therapeutic strategies highlighted.

Peptide Binding Sites of Connexin Proteins

Intercellular gap junction (GJ) contacts formed by the coupling of connexin (Cx) hemichannels (HCs) embedded into the plasma membranes of neighboring cells play significant role in the development, signaling and malfunctions of mammalian tissues. Understanding and targeting GJ functions, however, calls for finding valid Cx subtype-specific inhibitors. We conjecture the lack of information about binding interactions between the GJ interface forming extracellular EL1 and EL2 loops and peptide mimetics designed to specifically inhibit Cx43HC coupling to Cx43GJ. Here, we explore active spots at the GJ interface using known peptide inhibitors that mimic various segments of EL1 and EL2. Binding interactions of these peptide inhibitors and the non-peptide inhibitor quinine has been modelled in combination with the use of blind docking molecular mechanics (MM). The neuron-specific Cx36HC and astrocyte-specific Cx43HC subtypes were modelled with a template derived from the high-resolution structure of Cx26GJ. GJ-coupled and free Cx36HC and Cx43HC models were obtained by dissection of GJs (GJ-coupled) followed by 50 ns molecular dynamics (free). Molecular mechanics (MM) calculations were performed by the docking of inhibitors, explicitly the designed Cx43 EL1 or EL2 loop sequence mimetics (GAP26, P5 or P180–195, GAP27, Peptide5, respectively) and the Cx36 subtype-specific quinine into the model structures. In order to explore specific binding interactions between inhibitors and CxHC subtypes, MM/Generalized Born Surface Area (MM/GBSA) ΔGbind values for representative conformers of peptide mimetics and quinine were evaluated by mapping the binding surface of Cx36HC and Cx43HC for all inhibitors. Quinine specifically contacts Cx36 EL1 residues V54-C55-N56-T57-L58, P60 and N63. Blocking the vestibule by the side of Cx36HC entry, quinine explicitly interacts with the non-conserved V54, L58, N63 residues of Cx36 EL1. In addition, our work challenges the predicted specificity of peptide mimetics, showing that the docking site of peptides is unrelated to the location of the sequence they mimic. Binding features, such as unaffected EL2 residues and the lack of Cx43 subtype-specificity of peptide mimetics, suggest critical roles for peptide stringency and dimension, possibly pertaining to the Cx subtype-specificity of peptide inhibitors.

Connexin 43 gap junctional intercellular communication inhibits evx1 expression and joint formation in regenerating fins

The gap junction protein Connexin 43 (Cx43) contributes to cell fate decisions that determine the location of fin ray joints during regeneration. Here, we provide insights into how Cx43, expressed medially, influences changes in gene expression in lateral skeletal precursor cells. Using the Gap27 peptide inhibitor specific to Cx43, we show that Cx43-gap junctional intercellular communication (GJIC) influences Cx43-dependent skeletal phenotypes, including segment length. We also demonstrate that Cx43-GJIC influences the expression of the Smp/β-catenin pathway in the lateral skeletal precursor cells, and does not influence the Sema3d pathway. Moreover, we show that the cx43^lh10 allele, which has increased Cx43 protein levels, exhibits increased regenerate length and segment length. These phenotypes are rescued by Gap27, suggesting that increased Cx43 is responsible for the observed Cx43 phenotypes. Finally, our findings suggest that inhibition of Cx43 hemichannel activity does not influence Cx43-dependent skeletal phenotypes. These data provide evidence that Cx43-GJIC is responsible for regulating cell fate decisions associated with appropriate joint formation in the regenerating fin.

Peptide-based targeting of connexins and pannexins for therapeutic purposes

INTRODUCTION

Connexin and pannexin (hemi)channels play an important role in paracrine and autocrine signaling pathways. The opening of these cellular pores is linked to a wide range of diseases. Therefore, pharmacological closing of connexin and pannexin (hemi)channels seems a promising therapeutic strategy. However, the currently available inhibitors cope with recurring problems concerning selectivity, specificity, stability and/or solubility.

AREAS COVERED

A number of peptides that mimic specific regions in the native sequence of connexins and pannexins have the potential to overcome some of these hurdles. In this paper, an overview is provided on these peptide-based inhibitors of connexin and pannexin (hemi)channels for therapeutic purposes. The authors also provide the reader with their expert perspectives on the future of these peptide-based inhibitors.

EXPERT OPINION

Peptide mimetics can become valuable tools in the treatment of connexin-related and pannexin-related diseases. This can be made possible provided that available peptides are optimized, and new peptide mimetics are designed based on knowledge of the mechanisms underlying the gating control of connexin and pannexin (hemi)channels.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

The lipidated connexin mimetic peptide SRPTEKT-Hdc is a potent inhibitor of Cx43 channels with specificity for the pS368 phospho-isoform

Connexin (Cx) mimetic peptides derived from extracellular loop II sequences (e.g., Gap27: SRPTEKTIFII; Peptide5: VDCFLSRPTEKT) have been used as reversible, Cx-specific blockers of hemichannel (HCh) and gap junction channel (GJCh) function. These blockers typically require high concentrations (~5 µM, <1 h for HCh; ~100 µM, >1 h for GJCh) to achieve inhibition. We have shown that addition of a hexadecyl (Hdc) lipid tail to the conserved SRPTEKT peptide sequence (SRPTEKT-Hdc) results in a novel, highly efficacious, and potent inhibitor of mechanically induced Ca2+-wave propagation (IC50 64.8 pM) and HCh-mediated dye uptake (IC50 45.0 pM) in Madin-Darby canine kidney cells expressing rat Cx43 (MDCK43). The lack of similar effect on dye coupling (NBD-MTMA) suggested channel conformation-specific inhibition. Here we report that SRPTEKT-Hdc inhibition of Ca2+-wave propagation, dye coupling, and dye uptake depended on the functional configuration of Cx43 as determined by phosphorylation at serine 368 (S368). Ca2+-wave propagation was enhanced in MDCK cells expressing single-site mutants of Cx43 that mimicked (MDCK43-S368D) or favored (MDCK43-S365A) phosphorylation at S368. Furthermore, SRPTEKT-Hdc potently inhibited GJCh-mediated Ca2+-wave propagation (IC50 230.4 pM), dye coupling, and HCh-mediated dye uptake in MDCK43-S368D and -S365A cells. In contrast, Ca2+-wave propagation, dye coupling, and dye uptake were largely unaffected (IC50 12.3 μM) by SRPTEKT-Hdc in MDCK43-S368A and -S365D cells, mutations that mimic or favor dephosphorylation at S368. Together, these data indicate that SRPTEKT-Hdc is a potent inhibitor of physiological Ca2+-wave signaling mediated specifically by the pS368 phosphorylated form of Cx43.

Therapeutic Targeting of Connexin Channels: New Views and Challenges

Connexins, in particular connexin 43 (Cx43), function as gap junction channels (GJCs) and hemichannels (HCs). Only recently, specific tools have been developed to study their pleiotropic functions. Based on various protein interaction sites, distinct connexin-mimetic peptides have been established that enable discrimination between the function of HCs and GJCs. Although the precise mechanism of action of most of these peptides is still a matter of debate, an increasing number of studies report on important effects of those compounds in disease models. In this review, we summarize the structure, life cycle, and the most important physiological and pathological functions of both connexin GJCs and HCs. We provide a critical overview on the use of connexin-targeting peptides, in particular targeting Cx43, with a special focus on the remaining questions and hurdles to be taken in the research field of connexin channels.

Autosomal Recessive Oculodentodigital Dysplasia: A Case Report and Review of the Literature

Oculodentodigital dysplasia (ODDD) is a rare condition characterized by a typical facial appearance and variable findings of the eyes, teeth, and fingers. ODDD is caused by mutations in the GJA1 gene in chromosome 6q22 and inherited in an autosomal dominant manner in the majority of the patients. However, in recent clinical reports, autosomal recessive ODDD cases due to by GJA1 mutations were also described. Here, we report on a 14-year-old boy with microphthalmia, microcornea, narrow nasal bridge, hypoplastic alae nasi, prominent columnella, hypodontia, dental caries, and partial syndactyly of the 2nd and 3rd toes. These clinical findings were concordant with the diagnosis of ODDD, and a novel homozygous mutation (c.442C>T, p.Arg148Ter) was determined in the GJA1 gene leading to a premature stop codon. His phenotypically normal parents were found to be carriers of the same mutation. This is the third family in the literature in which ODDD segregates in an autosomal recessive manner.

Lipidated connexin mimetic peptides potently inhibit gap junction-mediated Ca2+-wave propagation

Connexin (Cx) mimetic peptides (e.g., Gap27: SRPTEKTIFII; Peptide5: VDCFLSRPTEKT) reversibly inhibit hemichannel (HCh) and gap junction channel (GJCh) function in a concentration- and time-dependent manner (HCh: ~5 µM, <1 h; GJCh: ~100 µM, > 1 h). We hypothesized that addition of a hexadecyl tail to SRPTEKT (SRPTEKT- Hdc) would improve its ability to concentrate in the plasma membrane and consequently increase its inhibitory efficacy. We show that SRPTEKT- Hdc inhibited intercellular Ca²⁺-wave propagation in Cx43-expressing MDCK and rabbit tracheal epithelial cells in a time (61-75 min)- and concentration (IC₅₀: 66 pM)-dependent manner, a concentration efficacy five orders of magnitude lower than observed for the nonlipidated Gap27. HCh-mediated dye uptake was inhibited by SRPTEKT- Hdc with similar efficacy. Following peptide washout, HCh-mediated dye uptake was restored to control levels, whereas Ca²⁺-wave propagation was only partially restored. Scrambled and reverse sequence lipidated peptides had no detectable inhibitory effect on Ca²⁺-wave propagation or dye uptake. Cx43 expression was unchanged by SRPTEKT- Hdc incubation; however, Triton-insoluble Cx43 was reduced by SRPTEKT- Hdc exposure and reversed following washout. In summary, our results show that SRPTEKT- Hdc blocked HCh function and intercellular Ca²⁺ signaling at concentrations that minimally affected dye coupling. Selective inhibition of intercellular Ca²⁺ signaling, likely indicative of channel conformation-specific SRPTEKT- Hdc binding, could contribute significantly to the protective effects of these mimetic peptides in settings of injury. Our data also demonstrate that lipidation represents a paradigm for development of highly potent, efficacious, and selective mimetic peptide inhibitors of hemichannel and gap junction channel-mediated signaling.

Angiotensin II-Induced Mesangial Cell Damaged Is Preceded by Cell Membrane Permeabilization Due to Upregulation of Non-Selective Channels

Connexin43 (Cx43), pannexin1 (Panx1) and P2X₇ receptor (P2X₇R) are expressed in kidneys and are known to constitute a feedforward mechanism leading to inflammation in other tissues. However, the possible functional relationship between these membrane channels and their role in damaged renal cells remain unknown. In the present work, we found that MES-13 cells, from a cell line derived from mesangial cells, stimulated with angiotensin II (AngII) developed oxidative stress (OS, thiobarbituric acid reactive species (TBARS) and generated pro-inflammatory cytokines (ELISA; IL-1β and TNF-α). The membrane permeability increased progressively several hours before the latter outcome, which was a response prevented by Losartan, indicating the involvement of AT1 receptors. Western blot analysis showed that the amount of phosphorylated MYPT (a substrate of RhoA/ROCK) and Cx43 increased progressively and in parallel in cells treated with AngII, a response followed by an increase in the amount in Panx1 and P2X₇R. Greater membrane permeability was partially explained by opening of Cx43 hemichannels (Cx43 HCs) and Panx1 channels (Panx1 Chs), as well as P2X₇Rs activation by extracellular ATP, which was presumably released via Cx HCs and Panx1 Chs. Additionally, inhibition of RhoA/ROCK blocked the progressive increase in membrane permeability, and the remaining response was explained by the other non-selective channels. The rise of activity in the RhoA/ROCK-dependent pathway, as well as in Cx HCs, P2X₇R, and to a minor extent in Panx1 Chs led to higher amounts of TBARS and pro-inflammatory cytokines. We propose that AngII-induced mesangial cell damage could be effectively inhibited by concomitantly inhibiting the RhoA/ROCK-dependent pathway and one or more non-selective channel(s) activated through this pathway.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

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.

Cx47 fine-tunes the handling of serum lipids but is dispensable for lymphatic vascular function

Mutations in the gap junction protein connexin47 (Cx47) are associated with lymphedema. However, the role of Cx47 in lymphatic pathophysiology is unknown. We demonstrate that Cx47 is expressed in lymphatic endothelial cells by whole-mount immunostaining and qPCR. To determine if Cx47 plays a role in lymphatic vessel function we analysed Cx47^-/- mice. Cx47-deficiency did not affect lymphatic contractility (contractile amplitude or frequency) or lymphatic morphology (vessel diameter or number of valves). Interstitial fluid drainage or dendritic cell migration through lymphatic vessels was also not affected by Cx47-deficiency. Cx47 is dispensable for long-chain fatty acid absorption from the gut but rather promotes serum lipid handling as prolonged elevated triglyceride levels were observed in Cx47-deficient mice after oral lipid tolerance tests. When crossed with Apolipoprotein E-deficient (Apoe^-/-) mice, LDL-cholesterol was decreased in young Cx47^-/-Apoe^-/- adults as compared to Apoe^-/- mice, which was inverted later in life. Finally, advanced atherosclerotic plaques in thoracic-abdominal aortas of 15 months-old mice tended to be larger in Cx47^-/-Apoe^-/- mice. These plaques contained fewer macrophages but similar amounts of T lymphocytes, collagen and lipids than plaques of Apoe^-/- mice. In conclusion, Cx47 is expressed in lymphatic endothelium and seems modestly implicated in multiple aspects of lymphatic pathophysiology.

Characterizing the mode of action of extracellular Connexin43 channel blocking mimetic peptides in an in vitro ischemia injury model

BACKGROUND

Non-selective Connexin43 hemichannels contribute to secondary lesion spread. The hemichannel blocking peptidomimetic Peptide5, derived from the second extracellular loop of the human Connexin43 protein, prevents lesion spread and reduces vascular permeability in preclinical models of central nervous system injury. The molecular mode of action of Peptide5, however, was unknown and is described here.

METHODS

Human cerebral microvascular endothelial cells and APRE-19 cells were used. Scrape loading was used to assess gap junction function and hypoxic, acidic ion-shifted Ringer solution induced ATP release used to assess hemichannel function. Peptide modifications, including amino acid substitutions and truncations, and competition assays were used to demonstrate Peptide5 functional specificity and site of action respectively.

RESULTS

Peptide5 inhibits Connexin43 hemichannel-mediated ATP release by acting on extracellular loop two of Connexin43, adjacent to its matching sequence within the protein. Precise sequence specificity is important for hemichannel block, but less so for uncoupling of gap junction channels (seen only at high concentrations). The SRPTEKT motif is central to Peptide5 function but on its own is not sufficient to inhibit hemichannels. Both the SRPTEKT motif and Peptide5 reduce gap junction communication, but neither uncoupling below 50%.

CONCLUSIONS

Reduced gap junction coupling at high peptide concentrations appears to be relatively non-specific. However, Peptide5 at low concentrations acts upon extracellular loop two of Connexin43 to block hemichannels in a precise, sequence specific manner.

GENERAL SIGNIFICANCE

The concentration dependent and sequence specific action of Peptide5 supports its development for the treatment of retinal injury and chronic disease, as well as other central nervous system injury and disease conditions.

Decreases in Gap Junction Coupling Recovers Ca2+ and Insulin Secretion in Neonatal Diabetes Mellitus, Dependent on Beta Cell Heterogeneity and Noise

Diabetes is caused by dysfunction to β-cells in the islets of Langerhans, disrupting insulin secretion and glucose homeostasis. Gap junction-mediated electrical coupling between β-cells in the islet plays a major role in coordinating a pulsatile secretory response at elevated glucose and suppressing insulin secretion at basal glucose. Previously, we demonstrated that a critical number of inexcitable cells can rapidly suppress the overall islet response, as a result of gap junction coupling. This was demonstrated in a murine model of Neonatal Diabetes Mellitus (NDM) involving expression of ATP-insensitive K_ATP channels, and by a multi-cellular computational model of islet electrical activity. Here we examined the mechanisms by which gap junction coupling contributes to islet dysfunction in NDM. We first verified the computational model against [Ca²⁺] and insulin secretion measurements in islets expressing ATP-insensitive K_ATP channels under different levels of gap junction coupling. We then applied this model to predict how different K_ATP channel mutations found in NDM suppress [Ca²⁺], and the role of gap junction coupling in this suppression. We further extended the model to account for stochastic noise and insulin secretion dynamics. We found experimentally and in the islet model that reductions in gap junction coupling allow progressively greater glucose-stimulated [Ca²⁺] and insulin secretion following expression of ATP-insensitive K_ATP channels. The model demonstrated good correspondence between suppression of [Ca²⁺] and clinical presentation of different NDM mutations. Significant recoveries in [Ca²⁺] and insulin secretion were predicted for many mutations upon reductions in gap junction coupling, where stochastic noise played a significant role in the recoveries. These findings provide new understanding how the islet functions as a multicellular system and for the role of gap junction channels in exacerbating the effects of decreased cellular excitability. They further suggest novel therapeutic options for NDM and other monogenic forms of diabetes.

Diabetes is a disease reaching a global epidemic, which results from dysfunction to the islets of Langerhans in the pancreas and their ability to secrete the hormone insulin to regulate glucose homeostasis. Islets are multicellular structures that show extensive coupling between heterogeneous cellular units; and central to the causes of diabetes is a dysfunction to these cellular units and their interactions. Understanding the inter-relationship between structure and function is challenging in biological systems, but is crucial to the cause of disease and discovering therapeutic targets. With the goal of further characterizing the islet of Langerhans and its excitable behavior, we examined the role of important channels in the islet where dysfunction is linked to or causes diabetes. Advances in our ability to computationally model perturbations in physiological systems has allowed for the testing of hypothesis quickly, in systems that are not experimentally accessible. Using an experimentally validated model and modeling human mutations, we discover that monogenic forms of diabetes may be remedied by a reduction in electrical coupling between cells; either alone or in conjunction with pharmacological intervention. Knowledge of biological systems in general is also helped by these findings, in that small changes to cellular elements may lead to major disruptions in the overall system. This may then be overcome by allowing the system components to function independently in the presence of dysfunction to individual cells.

Connexin43 plays diverse roles in co-ordinating cell migration and wound closure events

Chronic wounds are not only debilitating to patients, but also impose a huge financial burden on healthcare providers, as current treatments are not particularly effective. Wound healing is a highly co-ordinated process involving a vast array of signalling molecules and different cell types, therefore a substantial amount of research has been carried out in the quest to develop new therapies. The gap junction (GJ) protein connexin43 (Cx43) is one of the many molecules whose expression has been found to be up-regulated in chronic wounds and as a result targeting it may have therapeutic potential. Two different approaches have been adopted to investigate this: knockdown of Cx43 using antisense oligonucleotides and connexin mimetic peptides (CMPs) which inhibit the function of Cx43 without affecting gene expression. These peptides are targeted to the C-terminal domain or the extracellular loops of Cx43 and thus are likely to function by different means. However, both block channel function and have been shown to enhance cell migration rates. In recent years, non-channel functions have emerged for Cx43, many of which are linked to cytoskeletal dynamics and the extracellular matrix (ECM), showing that Cx43 plays diverse roles in co-ordinating wound closure events. It is clear that both CMPs and antisense oligonucleotides hold therapeutic potential, however maintaining Cx43 expression may be beneficial to the cell by preserving other non-channel functions of Cx43. Recent data in the field will be discussed in this article.

Cell communication across gap junctions: a historical perspective and current developments

Collaborative communication lies at the centre of multicellular life. Gap junctions (GJs) are surface membrane structures that allow direct communication between cells. They were discovered in the 1960s following the convergence of the detection of low-resistance electrical interactions between cells and anatomical studies of intercellular contact points. GJs purified from liver plasma membranes contained a 27 kDa protein constituent; it was later named Cx32 (connexin 32) after its full sequence was determined by recombinant technology. Identification of Cx43 in heart and later by a further GJ protein, Cx26 followed. Cxs have a tetraspan organization in the membrane and oligomerize during intracellular transit to the plasma membrane; these were shown to be hexameric hemichannels (connexons) that could interact end-to-end to generate GJs at areas of cell-to-cell contact. The structure of the GJ was confirmed and refined by a combination of biochemical and structural approaches. Progress continues towards obtaining higher atomic 3D resolution of the GJ channel. Today, there are 20 and 21 highly conserved members of the Cx family in the human and mouse genomes respectively. Model organisms such as Xenopus oocytes and zebra fish are increasingly used to relate structure to function. Proteins that form similar large pore membrane channels in cells called pannexins have also been identified in chordates. Innexins form GJs in prechordates; these two other proteins, although functionally similar, are very different in amino acid sequence to the Cxs. A time line tracing the historical progression of wide ranging research in GJ biology over 60 years is mapped out. The molecular basis of channel dysfunctions in disease is becoming evident and progress towards addressing Cx channel-dependent pathologies, especially in ischaemia and tissue repair, continues.

The action of mimetic peptides on connexins protects fibroblasts from the negative effects of ischemia reperfusion

Connexins have been proposed as a target for therapeutic treatment of a variety of conditions. The main approaches have been by antisense or small peptides specific against connexins. Some of these peptides enhance communication while others interfere with connexin binding partners or bind to the intracellular and extracellular loops of connexins. Here, we explored the mechanism of action of a connexin mimetic peptide by evaluating its effect on gap junction channels, connexin protein levels and hemichannel activity in fibroblast cells under normal conditions and following ischemia reperfusion injury which elevates Cx43 levels, increases hemichannel activity and causes cell death. Our results showed that the effects of the mimetic peptide were concentration-dependent. High concentrations (100-300 μM) significantly reduced Cx43 protein levels and GJIC within 2 h, while these effects did not appear until 6 h when using lower concentrations (10-30 μM). Cell death can be reduced when hemichannel opening and GJIC were minimised.

Summary: Connexin mimetic peptides can reduce the levels of connexin proteins in cells and can prevent the spread of cell death that occurs following ischemia reperfusion injury, which has therapeutic potential.

Perturbation of chemical coupling by an endothelial Cx40 mutant attenuates endothelium-dependent vasodilation by KCa channels and elevates blood pressure in mice

Mutant forms of connexin40 (Cx40) exist in the human population and predispose carriers to atrial fibrillation. Since endothelial expression of Cx40 is important for electrical and chemical communication within the arterial wall, carriers of mutant Cx40 proteins may be predisposed to peripheral arterial dysfunction and dysregulation of blood pressure. We have therefore studied mice expressing either a chemically dysfunctional mutant, Cx40T202S, or wild-type Cx40, with native Cx40, specifically in the endothelium. Blood pressure was measured by telemetry under normal conditions and during cardiovascular stress induced by locomotor activity, phenylephrine or nitric oxide blockade (N(ɷ)-nitro-L-arginine methyl ester hydroxide, L-NAME). Blood pressure of Cx40T202STg mice was significantly elevated at night when compared with wild-type or Cx40Tg mice, without change in mean heart rate, pulse pressure or locomotor activity. Analysis over 24 h showed that blood pressure of Cx40T202STg mice was significantly elevated at rest and additionally during locomotor activity. In contrast, neither plasma renin concentration nor pressor responses to phenylephrine or L-NAME were altered, the latter indicating that nitric oxide bioavailability was normal. In isolated, pressurised mesenteric arteries, hyperpolarisation and vasodilation evoked by SKA-31, the selective modulator of SKCa and IKCa channels, was significantly reduced in Cx40T202STg mice, due to attenuation of the SKCa component. Acetylcholine-induced ascending vasodilation in vivo was also significantly attenuated in cremaster muscle arterioles of Cx40T202STg mice, compared to wild-type and Cx40Tg mice. We conclude that endothelial expression of the chemically dysfunctional Cx40T202S reduces peripheral vasodilator capacity mediated by SKCa-dependent hyperpolarisation and also increases blood pressure.

Connexin 43 is an emerging therapeutic target in ischemia/reperfusion injury, cardioprotection and neuroprotection

Connexins are widely distributed proteins in the body that are crucially important for heart and brain functions. Six connexin subunits form a connexon or hemichannel in the plasma membrane. Interactions between two hemichannels in a head-to-head arrangement result in the formation of a gap junction channel. Gap junctions are necessary to coordinate cell function by passing electrical current flow between heart and nerve cells or by allowing exchange of chemical signals and energy substrates. Apart from its localization at the sarcolemma of cardiomyocytes and brain cells, connexins are also found in the mitochondria where they are involved in the regulation of mitochondrial matrix ion fluxes and respiration. Connexin expression is affected by age and gender as well as several pathophysiological alterations such as hypertension, hypertrophy, diabetes, hypercholesterolemia, ischemia, post-myocardial infarction remodeling or heart failure, and post-translationally connexins are modified by phosphorylation/de-phosphorylation and nitros(yl)ation which can modulate channel activity. Using knockout/knockin technology as well as pharmacological approaches, one of the connexins, namely connexin 43, has been identified to be important for cardiac and brain ischemia/reperfusion injuries as well as protection from it. Therefore, the current review will focus on the importance of connexin 43 for irreversible injury of heart and brain tissues following ischemia/reperfusion and will highlight the importance of connexin 43 as an emerging therapeutic target in cardio- and neuroprotection.

Gap junction proteins and their role in spinal cord injury

Gap junctions are specialized intercellular communication channels that are formed by two hexameric connexin hemichannels, one provided by each of the two adjacent cells. Gap junctions and hemichannels play an important role in regulating cellular metabolism, signaling, and functions in both normal and pathological conditions. Following spinal cord injury (SCI), there is damage and disturbance to the neuronal elements of the spinal cord including severing of axon tracts and rapid cell death. The initial mechanical disruption is followed by multiple secondary cascades that cause further tissue loss and dysfunction. Recent studies have implicated connexin proteins as playing a critical role in the secondary phase of SCI by propagating death signals through extensive glial networks. In this review, we bring together past and current studies to outline the distribution, changes and roles of various connexins found in neurons and glial cells, before and in response to SCI. We discuss the contribution of pathologically activated connexin proteins, in particular connexin 43, to functional recovery and neuropathic pain, as well as providing an update on potential connexin specific pharmacological agents to treat SCI.

The connexin43 mimetic peptide Gap19 inhibits hemichannels without altering gap junctional communication in astrocytes

In the brain, astrocytes represent the cellular population that expresses the highest amount of connexins (Cxs). This family of membrane proteins is the molecular constituent of gap junction channels and hemichannels that provide pathways for direct cytoplasm-to-cytoplasm and inside-out exchange, respectively. Both types of Cx channels are permeable to ions and small signaling molecules allowing astrocytes to establish dynamic interactions with neurons. So far, most pharmacological approaches currently available do not distinguish between these two channel functions, stressing the need to develop new specific molecular tools. In astrocytes two major Cxs are expressed, Cx43 and Cx30, and there is now evidence indicating that at least Cx43 operates as a gap junction channel as well as a hemichannel in these cells. Based on studies in primary cultures as well as in acute hippocampal slices, we report here that Gap19, a nonapeptide derived from the cytoplasmic loop of Cx43, inhibits astroglial Cx43 hemichannels in a dose-dependent manner, without affecting gap junction channels. This peptide, which not only selectively inhibits hemichannels but is also specific for Cx43, can be delivered in vivo in mice as TAT-Gap19, and displays penetration into the brain parenchyma. As a result, Gap19 combined with other tools opens up new avenues to decipher the role of Cx43 hemichannels in interactions between astrocytes and neurons in physiological as well as pathological situations.

Extracellular domains play different roles in gap junction formation and docking compatibility

GJ (gap junction) channels mediate direct intercellular communication and play an important role in many physiological processes. Six connexins oligomerize to form a hemichannel and two hemichannels dock together end-to-end to form a GJ channel. Connexin extracellular domains (E1 and E2) have been shown to be important for the docking, but the molecular mechanisms behind the docking and formation of GJ channels are not clear. Recent developments in atomic GJ structure and functional studies on a series of connexin mutants revealed that E1 and E2 are likely to play different roles in the docking. Non-covalent interactions at the docking interface, including hydrogen bonds, are predicted to form between interdocked extracellular domains. Protein sequence alignment analysis on the docking compatible/incompatible connexins indicate that the E1 domain is important for the formation of the GJ channel and the E2 domain is important in the docking compatibility in heterotypic channels. Interestingly, the hydrogen-bond forming or equivalent residues in both E1 and E2 domains are mutational hot spots for connexin-linked human diseases. Understanding the molecular mechanisms of GJ docking can assist us to develop novel strategies in rescuing the disease-linked connexin mutants.

Differentiating connexin hemichannels and pannexin channels in cellular ATP release

Adenosine triphosphate (ATP) plays a fundamental role in cellular communication, with its extracellular accumulation triggering purinergic signaling cascades in a diversity of cell types. While the roles for purinergic signaling in health and disease have been well established, identification and differentiation of the specific mechanisms controlling cellular ATP release is less well understood. Multiple mechanisms have been proposed to regulate ATP release with connexin (Cx) hemichannels and pannexin (Panx) channels receiving major focus. However, segregating the specific roles of Panxs and Cxs in ATP release in a plethora of physiological and pathological contexts has remained enigmatic. This multifaceted problem has arisen from the selectivity of pharmacological inhibitors for Panxs and Cxs, methodological differences in assessing Panx and Cx function and the potential compensation by other isoforms in gene silencing and genetic knockout models. Consequently, there remains a void in the current understanding of specific contributions of Panxs and Cxs in releasing ATP during homeostasis and disease. Differentiating the distinct signaling pathways that regulate these two channels will advance our current knowledge of cellular communication and aid in the development of novel rationally-designed drugs for modulation of Panx and Cx activity, respectively.

Connexin and pannexin hemichannels in brain glial cells: properties, pharmacology, and roles

Functional interaction between neurons and glia is an exciting field that has expanded tremendously during the past decade. Such partnership has multiple impacts on neuronal activity and survival. Indeed, numerous findings indicate that glial cells interact tightly with neurons in physiological as well as pathological situations. One typical feature of glial cells is their high expression level of gap junction protein subunits, named connexins (Cxs), thus the membrane channels they form may contribute to neuroglial interaction that impacts neuronal activity and survival. While the participation of gap junction channels in neuroglial interactions has been regularly reviewed in the past, the other channel function of Cxs, i.e., hemichannels located at the cell surface, has only recently received attention. Gap junction channels provide the basis for a unique direct cell-to-cell communication, whereas Cx hemichannels allow the exchange of ions and signaling molecules between the cytoplasm and the extracellular medium, thus supporting autocrine and paracrine communication through a process referred to as “gliotransmission,” as well as uptake and release of metabolites. More recently, another family of proteins, termed pannexins (Panxs), has been identified. These proteins share similar membrane topology but no sequence homology with Cxs. They form multimeric membrane channels with pharmacology somewhat overlapping with that of Cx hemichannels. Such duality has led to several controversies in the literature concerning the identification of the molecular channel constituents (Cxs versus Panxs) in glia. In the present review, we update and discuss the knowledge of Cx hemichannels and Panx channels in glia, their properties and pharmacology, as well as the understanding of their contribution to neuroglial interactions in brain health and disease.

Endothelial control of vasodilation: integration of myoendothelial microdomain signalling and modulation by epoxyeicosatrienoic acids

Endothelium-derived epoxyeicosatrienoic acids (EETs) are fatty acid epoxides that play an important role in the control of vascular tone in selected coronary, renal, carotid, cerebral and skeletal muscle arteries. Vasodilation due to endothelium-dependent smooth muscle hyperpolarization (EDH) has been suggested to involve EETs as a transferable endothelium-derived hyperpolarizing factor. However, this activity may also be due to EETs interacting with the components of other primary EDH-mediated vasodilator mechanisms. Indeed, the transfer of hyperpolarization initiated in the endothelium to the adjacent smooth muscle via gap junction connexins occurs separately or synergistically with the release of K(+) ions at discrete myoendothelial microdomain signalling sites. The net effects of such activity are smooth muscle hyperpolarization, closure of voltage-dependent Ca(2+) channels, phospholipase C deactivation and vasodilation. The spatially localized and key components of the microdomain signalling complex are the inositol 1,4,5-trisphosphate receptor-mediated endoplasmic reticulum Ca(2+) store, Ca(2+)-activated K(+) (KCa), transient receptor potential (TRP) and inward-rectifying K(+) channels, gap junctions and the smooth muscle Na(+)/K(+)-ATPase. Of these, TRP channels and connexins are key endothelial effector targets modulated by EETs. In an integrated manner, endogenous EETs enhance extracellular Ca(2+) influx (thereby amplifying and prolonging KCa-mediated endothelial hyperpolarization) and also facilitate the conduction of this hyperpolarization to spatially remote vessel regions. The contribution of EETs and the receptor and channel subtypes involved in EDH-related microdomain signalling, as a candidate for a universal EDH-mediated vasodilator mechanism, vary with vascular bed, species, development and disease and thus represent potentially selective targets for modulating specific artery function.

Polymorphism in endothelial connexin40 enhances sensitivity to intraluminal pressure and increases arterial stiffness

OBJECTIVE

To determine whether impairment of endothelial connexin40 (Cx40), an effect that can occur in hypertension and aging, contributes to the arterial dysfunction and stiffening in these conditions.

APPROACH AND RESULTS

A new transgenic mouse strain, expressing a mutant Cx40, (Cx40T202S), specifically in the vascular endothelium, has been developed and characterized. This mutation produces nonfunctional hemichannels, whereas gap junctions containing the mutant are electrically, but not chemically, patent. Mesenteric resistance arteries from Cx40T202S mice showed increased sensitivity of the myogenic response to intraluminal pressure in vitro, compared with wild-type mice, whereas transgenic mice overexpressing native Cx40 (Cx40Tg) showed reduced sensitivity. In control and Cx40Tg mice, the sensitivity to pressure of myogenic constriction was modulated by both NO and endothelium-derived hyperpolarization; however, the endothelium-derived hyperpolarization component was absent in Cx40T202S arteries. Analysis of passive mechanical properties revealed that arterial stiffness was enhanced in vessels from Cx40T202S mice, but not in wild-type or Cx40Tg mice.

CONCLUSIONS

Introduction of a mutant form of Cx40 in the endogenous endothelial Cx40 population prevents endothelium-derived hyperpolarization activation during myogenic constriction, enhancing sensitivity to intraluminal pressure and increasing arterial stiffness. We conclude that genetic polymorphisms in endothelial Cx40 can contribute to the pathogenesis of arterial disease.

The bizarre pharmacology of the ATP release channel pannexin1

Pannexins were originally thought to represent a second and redundant family of gap junction proteins in addition to the well characterized connexins. However, it is now evident that pannexins function as unapposed membrane channels and the major role of Panx1 is that of an ATP release channel. Despite the contrasting functional roles, connexins, innexins and pannexins share pharmacological properties. Most gap junction blockers also attenuate the function of Panx1, including carbenoxolone, mefloquine and flufenamic acid. However, in contrast to connexin based gap junction channels, Panx1 channel activity can be attenuated by several groups of drugs hitherto considered very specific for other proteins. The drugs affecting Panx1 channels include several transport inhibitors, chloride channel blockers, mitochondrial inhibitors, P2X7 receptor ligands, inflammasome inhibitors and malaria drugs. These observations indicate that Panx1 may play an extended role in a wider spectrum of physiological functions. Alternatively, Panx1 may share structural domains with other proteins, not readily revealed by sequence alignments. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Selective inhibition of Cx43 hemichannels by Gap19 and its impact on myocardial ischemia/reperfusion injury

Connexin-43 (Cx43), a predominant cardiac connexin, forms gap junctions (GJs) that facilitate electrical cell-cell coupling and unapposed/nonjunctional hemichannels that provide a pathway for the exchange of ions and metabolites between cytoplasm and extracellular milieu. Uncontrolled opening of hemichannels in the plasma membrane may be deleterious for the myocardium and blocking hemichannels may confer cardioprotection by preventing ionic imbalance, cell swelling and loss of critical metabolites. Currently, all known hemichannel inhibitors also block GJ channels, thereby disturbing electrical cell-cell communication. Here we aimed to characterize a nonapeptide, called Gap19, derived from the cytoplasmic loop (CL) of Cx43 as a hemichannel blocker and examined its effect on hemichannel currents in cardiomyocytes and its influence in cardiac outcome after ischemia/reperfusion. We report that Gap 19 inhibits Cx43 hemichannels without blocking GJ channels or Cx40/pannexin-1 hemichannels. Hemichannel inhibition is due to the binding of Gap19 to the C-terminus (CT) thereby preventing intramolecular CT-CL interactions. The peptide inhibited Cx43 hemichannel unitary currents in both HeLa cells exogenously expressing Cx43 and acutely isolated pig ventricular cardiomyocytes. Treatment with Gap19 prevented metabolic inhibition-enhanced hemichannel openings, protected cardiomyocytes against volume overload and cell death following ischemia/reperfusion in vitro and modestly decreased the infarct size after myocardial ischemia/reperfusion in mice in vivo. We conclude that preventing Cx43 hemichannel opening with Gap19 confers limited protective effects against myocardial ischemia/reperfusion injury.

Connexin mimetic peptides inhibit Cx43 hemichannel opening triggered by voltage and intracellular Ca2+ elevation

Connexin mimetic peptides (CxMPs), such as Gap26 and Gap27, are known as inhibitors of gap junction channels but evidence is accruing that these peptides also inhibit unapposed/non-junctional hemichannels (HCs) residing in the plasma membrane. We used voltage clamp studies to investigate the effect of Gap26/27 at the single channel level. Such an approach allows unequivocal identification of HC currents by their single channel conductance that is typically ~220 pS for Cx43. In HeLa cells stably transfected with Cx43 (HeLa-Cx43), Gap26/27 peptides inhibited Cx43 HC unitary currents over minutes and increased the voltage threshold for HC opening. By contrast, an elevation of intracellular calcium ([Ca(2+)](i)) to 200-500 nM potentiated the unitary HC current activity and lowered the voltage threshold for HC opening. Interestingly, Gap26/27 inhibited the Ca(2+)-potentiated HC currents and prevented lowering of the voltage threshold for HC opening. Experiments on isolated pig ventricular cardiomyocytes, which display strong endogenous Cx43 expression, demonstrated voltage-activated unitary currents with biophysical properties of Cx43 HCs that were inhibited by small interfering RNA targeting Cx43. As observed in HeLa-Cx43 cells, HC current activity in ventricular cardiomyocytes was potentiated by [Ca(2+)](i) elevation to 500 nM and was inhibited by Gap26/27. Our results indicate that under pathological conditions, when [Ca(2+)](i) is elevated, Cx43 HC opening is promoted in cardiomyocytes and CxMPs counteract this effect.

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Manipulating Connexin Communication Channels: Use of Peptidomimetics and the Translational Outputs

Gap junctions are key components underpinning multicellularity. They provide cell to cell channel pathways that enable direct intercellular communication and cellular coordination in tissues and organs. The channels are constructed of a family of connexin (Cx) membrane proteins. They oligomerize inside the cell, generating hemichannels (connexons) composed of six subunits arranged around a central channel. After transfer to the plasma membrane, arrays of Cx hemichannels (CxHcs) interact and couple with partners in neighboring attached cells to generate gap junctions. Cx channels have been studied using a range of technical approaches. Short peptides corresponding to sequences in the extra- and intracellular regions of Cxs were used first to generate epitope-specific antibodies that helped studies on the organization and functions of gap junctions. Subsequently, the peptides themselves, especially Gap26 and -27, mimetic peptides derived from each of the two extracellular loops of connexin43 (Cx43), a widely distributed Cx, have been extensively applied to block Cx channels and probe the biology of cell communication. The development of a further series of short peptides mimicking sequences in the intracellular loop, especially the extremity of the intracellular carboxyl tail of Cx43, followed. The primary inhibitory action of the peptidomimetics occurs at CxHcs located at unapposed regions of the cell’s plasma membrane, followed by inhibition of cell coupling occurring across gap junctions. CxHcs respond to a range of environmental conditions by increasing their open probability. Peptidomimetics provide a way to block the actions of CxHcs with some selectivity. Furthermore, they are increasingly applied to address the pathological consequences of a range of environmental stresses that are thought to influence Cx channel operation. Cx peptidomimetics show promise as candidates in developing new therapeutic approaches for containing and reversing damage inflicted on CxHcs, especially in hypoxia and ischemia in the heart and in brain functions.

Paracrine signaling through plasma membrane hemichannels

Plasma membrane hemichannels composed of connexin (Cx) proteins are essential components of gap junction channels but accumulating evidence suggests functions of hemichannels beyond the communication provided by junctional channels. Hemichannels not incorporated into gap junctions, called unapposed hemichannels, can open in response to a variety of signals, electrical and chemical, thereby forming a conduit between the cell's interior and the extracellular milieu. Open hemichannels allow the bidirectional passage of ions and small metabolic or signaling molecules of below 1-2kDa molecular weight. In addition to connexins, hemichannels can also be formed by pannexin (Panx) proteins and current evidence suggests that Cx26, Cx32, Cx36, Cx43 and Panx1, form hemichannels that allow the diffusive release of paracrine messengers. In particular, the case is strong for ATP but substantial evidence is also available for other messengers like glutamate and prostaglandins or metabolic substances like NAD(+) or glutathione. While this field is clearly in expansion, evidence is still lacking at essential points of the paracrine signaling cascade that includes not only messenger release, but also downstream receptor signaling and consequent functional effects. The data available at this moment largely derives from in vitro experiments and still suffers from the difficulty of separating the functions of connexin-based hemichannels from gap junctions and from pannexin hemichannels. However, messengers like ATP or glutamate have universal roles in the body and further defining the contribution of hemichannels as a possible release pathway is expected to open novel avenues for better understanding their contribution to a variety of physiological and pathological processes. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Connexins in wound healing; perspectives in diabetic patients

Skin lesions are common events and we have evolved to rapidly heal them in order to maintain homeostasis and prevent infection and sepsis. Most acute wounds heal without issue, but as we get older our bodies become compromised by poor blood circulation and conditions such as diabetes, leading to slower healing. This can result in stalled or hard-to-heal chronic wounds. Currently about 2% of the Western population develop a chronic wound and this figure will rise as the population ages and diabetes becomes more prevalent [1]. Patient morbidity and quality of life are profoundly altered by chronic wounds [2]. Unfortunately a significant proportion of these chronic wounds fail to respond to conventional treatment and can result in amputation of the lower limb. Life quality and expectancy following amputation is severely reduced. These hard to heal wounds also represent a growing economic burden on Western society with published estimates of costs to healthcare services in the region of $25B annually [3]. There exists a growing need for specific and effective therapeutic agents to improve healing in these wounds. In recent years the gap junction protein Cx43 has been shown to play a pivotal role early on in the acute wound healing process at a number of different levels [4-7]. Conversely, abnormal expression of Cx43 in wound edge keratinocytes was shown to underlie the poor rate of healing in diabetic rats, and targeting its expression with an antisense gel restored normal healing rates [8]. The presence of Cx43 in the wound edge keratinocytes of human chronic wounds has also been reported [9]. Abnormal Cx43 biology may underlie the poor healing of human chronic wounds and be amenable therapeutic intervention [7]. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexins as therapeutic targets in lung disease

INTRODUCTION

The lung is a mechanically active system exposed to the external environment and is particularly sensitive to injury and inflammation. Studies have identified intercellular communication pathways that promote proper lung function in response to injury and disease. These pathways involve connexins (Cxs) and gap junctional intercellular communication (GJIC).

AREAS COVERED IN THIS REVIEW

The functional expression of Cxs in airway epithelium and vasculature, under normal and pathological conditions, is reviewed. Inhibition of GJIC and/or silencing of Cxs have been shown to modulate the course of disease development. Cx-based channels: i) coordinate ciliary beating and fluid transport to promote clearance of particulates, ii) regulate secretion of pulmonary surfactant, in response to deep inhalation by interconnecting type I and type II alveolar epithelial cells, and iii) are key mediators of pro- and anti-inflammatory signalling by the pulmonary endothelium, in order to modulate leukocyte recruitment from the circulation.

EXPERT OPINION

Cx-based channels play several central roles in promoting a regulated inflammatory response and facilitating lung repair, thus enabling the pulmonary epithelium and vasculature to behave as integrated systems. Several pathologies can disrupt the normal communication pathways required for proper lung function, including acute lung injury, asthma, cystic fibrosis, pulmonary fibrosis and cancer.