Gap26, a connexin mimetic peptide, inhibits currents carried by connexin43 hemichannels and gap junction channels

Vascular Endothelial Barrier Protection Prevents Atrial Fibrillation by Preserving Cardiac Nanostructure

BACKGROUND

Atrial fibrillation (AF), the most common cardiac arrhythmia, is widely associated with inflammation, vascular dysfunction, and elevated levels of the vascular leak-inducing cytokine, vascular endothelial growth factor (VEGF). Mechanisms underlying AF are poorly understood and current treatments only manage this progressive disease, rather than arresting the underlying pathology. The authors previously identified edema-induced disruption of sodium channel (NaV1.5)-rich intercalated disk nanodomains as a novel mechanism for AF initiation secondary to acute inflammation. Therefore, we hypothesized that protecting the vascular barrier can prevent vascular leak-induced atrial arrhythmias.

OBJECTIVES

In this study the authors tested the hypothesis that protecting the vascular barrier can prevent vascular leak-induced atrial arrhythmias. They identified 2 molecular targets for vascular barrier protection, connexin43 (Cx43) hemichannels and pannexin-1 (Panx1) channels, which have been implicated in cytokine-induced vascular leak.

METHODS

The authors undertook in vivo electrocardiography, electron microscopy, and super-resolution light microscopy studies in mice acutely treated with a clinically relevant level of VEGF.

RESULTS

AF incidence was increased in untreated mice exposed to VEGF relative to vehicle control subjects. VEGF also increased the average number of AF episodes. VEGF shifted NaV1.5 signal to longer distances from Cx43 gap junctions, measured by a distance transformation-based spatial analysis of 3-dimensional confocal images of intercalated disks. Similar effects were observed with NaV1.5 localized near mechanical junctions composed of neural cadherin. Blocking connexin43 hemichannels (αCT11 peptide) or Panx1 channels (PxIL2P peptide) significantly reduced the duration of AF episodes compared with VEGF alone with no treatment. Concurrently, both peptide therapies preserved NaV1.5 distance from gap junctions to control levels and reduced mechanical junction-adjacent intermembrane distance in these hearts. Notably, similar antiarrhythmic efficacy was also achieved with clinically-relevant small-molecule inhibitors of Cx43 and Panx1.

CONCLUSIONS

These results highlight vascular barrier protection as an antiarrhythmic strategy following inflammation-induced vascular leak.

Fibroblasts in heart scar tissue directly regulate cardiac excitability and arrhythmogenesis

After heart injury, dead heart muscle is replaced by scar tissue. Fibroblasts can electrically couple with myocytes, and changes in fibroblast membrane potential can lead to myocyte excitability, which suggests that fibroblast-myocyte coupling in scar tissue may be responsible for arrhythmogenesis. However, the physiologic relevance of electrical coupling of myocytes and fibroblasts and its impact on cardiac excitability in vivo have never been demonstrated. We genetically engineered a mouse that expresses the optogenetic cationic channel ChR2 (H134R) exclusively in cardiac fibroblasts. After myocardial infarction, optical stimulation of scar tissue elicited organ-wide cardiac excitation and induced arrhythmias in these animals. Complementing computational modeling with experimental approaches, we showed that gap junctional and ephaptic coupling, in a synergistic yet functionally redundant manner, excited myocytes coupled to fibroblasts.

Extracellular Vesicles and Cx43-Gap Junction Channels Are the Main Routes for Mitochondrial Transfer from Ultra-Purified Mesenchymal Stem Cells, RECs

Mitochondria are essential organelles for maintaining intracellular homeostasis. Their dysfunction can directly or indirectly affect cell functioning and is linked to multiple diseases. Donation of exogenous mitochondria is potentially a viable therapeutic strategy. For this, selecting appropriate donors of exogenous mitochondria is critical. We previously demonstrated that ultra-purified bone marrow-derived mesenchymal stem cells (RECs) have better stem cell properties and homogeneity than conventionally cultured bone marrow-derived mesenchymal stem cells. Here, we explored the effect of contact and noncontact systems on three possible mitochondrial transfer mechanisms involving tunneling nanotubes, connexin 43 (Cx43)-mediated gap junction channels (GJCs), and extracellular vesicles (Evs). We show that Evs and Cx43-GJCs provide the main mechanism for mitochondrial transfer from RECs. Through these two critical mitochondrial transfer pathways, RECs could transfer a greater number of mitochondria into mitochondria-deficient (ρ⁰) cells and could significantly restore mitochondrial functional parameters. Furthermore, we analyzed the effect of exosomes (EXO) on the rate of mitochondrial transfer from RECs and recovery of mitochondrial function. REC-derived EXO appeared to promote mitochondrial transfer and slightly improve the recovery of mtDNA content and oxidative phosphorylation in ρ⁰ cells. Thus, ultrapure, homogenous, and safe stem cell RECs could provide a potential therapeutic tool for diseases associated with mitochondrial dysfunction.

The roles of connexins and gap junctions in the progression of cancer

Gap junctions (GJs), which are composed of connexins (Cxs), provide channels for direct information exchange between cells. Cx expression has a strong spatial specificity; however, its influence on cell behavior and information exchange between cells cannot be ignored. A variety of factors in organisms can modulate Cxs and subsequently trigger a series of responses that have important effects on cellular behavior. The expression and function of Cxs and the number and function of GJs are in dynamic change. Cxs have been characterized as tumor suppressors in the past, but recent studies have highlighted the critical roles of Cxs and GJs in cancer pathogenesis. The complex mechanism underlying Cx and GJ involvement in cancer development is a major obstacle to the evolution of therapy targeting Cxs. In this paper, we review the post-translational modifications of Cxs, the interactions of Cxs with several chaperone proteins, and the effects of Cxs and GJs on cancer. Video Abstract.

Blocking Cx43 alleviates neuropathic pain in rats with chronic constriction injury via the P2X4 and P38/ERK-P65 pathways

Neuropathic pain is a growing concern in the medical community, and studies on new analgesic targets for neuropathic pain have become a new hot spot. Whether Connexin43 (Cx43) has a key role in neuropathic pain mediated by the purinergic 2X4 (P2X4) receptor in rats with chronic constriction injury (CCI) was explored in this study. Our experimental results show that blockade of Cx43 could attenuate neuropathic pain in rats suffering from CCI via the P2X4, p38, ERK, and NF-kB signalling pathways. These results suggest that Cx43 may be a promising therapeutic target for the development of novel pharmacological agents in the management of neuropathic pain.

Astroglial Connexins Inactivation Increases Relapse of Depressive-like Phenotype after Antidepressant Withdrawal

Studies suggest that astrocytic connexins (Cx) have an important role in the regulation of high brain functions through their ability to establish fine-tuned communication with neurons within the tripartite synapse. In light of these properties, growing evidence suggests a role of Cx in psychiatric disorders such as major depression but also in the therapeutic activity of antidepressant drugs. However, the real impact of Cx on treatment response and the underlying neurobiological mechanisms remain yet to be clarified. On this ground, the present study was designed to evaluate the functional activity of Cx in a mouse model of depression based on chronic corticosterone exposure and to determine to which extent their pharmacological inactivation influences the antidepressant-like activity of venlafaxine (VENLA). On the one hand, our results indicate that depressed mice have impaired Cx-based gap-junction and hemichannel activities. On the other hand, while VENLA exerts robust antidepressant-like activity in depressed mice; this effect is abolished by the pharmacological inhibition of Cx with carbenoxolone (CBX). Interestingly, the combination of VENLA and CBX is also associated with a higher rate of relapse after treatment withdrawal. To our knowledge, this study is one of the first to develop a model of relapse, and our results reveal that Cx-mediated dynamic neuroglial interactions play a critical role in the efficacy of monoaminergic antidepressant drugs, thus providing new targets for the treatment of depression.

Connexin 43: insights into candidate pathological mechanisms of depression and its implications in antidepressant therapy

Major depressive disorder (MDD), a chronic and recurrent disease characterized by anhedonia, pessimism or even suicidal thought, remains a major chronic mental concern worldwide. Connexin 43 (Cx43) is the most abundant connexin expressed in astrocytes and forms the gap junction channels (GJCs) between astrocytes, the most abundant and functional glial cells in the brain. Astrocytes regulate neurons' synaptic strength and function by expressing receptors and regulating various neurotransmitters. Astrocyte dysfunction causes synaptic abnormalities, which are related to various mood disorders, e.g., depression. Increasing evidence suggests a crucial role of Cx43 in the pathogenesis of depression. Depression down-regulates Cx43 expression in humans and rats, and dysfunction of Cx43 also induces depressive behaviors in rats and mice. Recently Cx43 has received considerable critical attention and is highly implicated in the onset of depression. However, the pathological mechanisms of depression-like behavior associated with Cx43 still remain ambiguous. In this review we summarize the recent progress regarding the underlying mechanisms of Cx43 in the etiology of depression-like behaviors including gliotransmission, metabolic disorders, and neuroinflammation. We also discuss the effects of antidepressants (monoamine antidepressants and ketamine) on Cx43. The clarity of the candidate pathological mechanisms of depression-like behaviors associated with Cx43 and its potential pharmacological roles for antidepressants will benefit the exploration of a novel antidepressant target.

Targeting gliovascular connexins prevents inflammatory blood-brain barrier leakage and astrogliosis

The blood-brain barrier is formed by capillary endothelial cells expressing Cx37, Cx40 and Cx43, and is joined by closely apposed astrocytes expressing Cx43 and Cx30. We investigated whether connexin-targeting peptides could limit barrier leakage triggered by LPS-induced systemic inflammation in mice. Intraperitoneal LPS increased endothelial and astrocytic Cx43 expression, elevated TNFα, IL1β, IFNγ and IL6 in plasma and IL6 in the brain, and induced barrier leakage recorded over 24h. Barrier leakage was largely prevented by global Cx43 knockdown and Cx43/Cx30 double-knockout in astrocytes, slightly diminished by endothelial Cx43 knockout and not protected by global Cx30 knockout. Intravenous administration of Gap27 or Tat-Gap19 just before LPS also prevented barrier leakage, and intravenous BAPTA-AM to chelate intracellular calcium was equally effective. Patch-clamp experiments demonstrated LPS-induced Cx43 hemichannel opening in endothelial cells, which was suppressed by Gap27, Gap19 and BAPTA. LPS additionally triggered astrogliosis that was prevented by intravenous Tat-Gap19 or BAPTA-AM. Cortically applied Tat-Gap19 or BAPTA-AM to primarily target astrocytes, also strongly diminished barrier leakage. In vivo dye uptake and in vitro patch-clamp showed Cx43 hemichannel opening in astrocytes that was induced by IL6 in a calcium-dependent manner. We conclude that targeting endothelial and astrocytic connexins is a powerful approach to limit barrier failure and astrogliosis.

Bioglass promotes wound healing by inhibiting endothelial cell pyroptosis through regulation of the connexin 43/reactive oxygen species (ROS) signaling pathway

Bioactive glass (BG) has recently shown great promise in soft tissue repair, especially in wound healing; however, the underlying mechanism remains unclear. Pyroptosis is a novel type of programmed cell death that is involved in various traumatic injury diseases. Here, we hypothesized that BG may promote wound healing through suppression of pyroptosis. To test this scenario, we investigated the possible effect of BG on pyroptosis in wound healing both in vivo and in vitro. This study showed that BG can accelerate wound closure, granulation formation, collagen deposition, and angiogenesis. Moreover, western blot analysis and immunofluorescence staining revealed that BG inhibited the expression of pyroptosis-related proteins in vivo and in vitro. In addition, while BG regulated the expression of connexin43 (Cx43), it inhibited reactive oxygen species (ROS) production. Cx43 activation and inhibition experiments further indicate that BG inhibited pyroptosis in endothelial cells by decreasing Cx43 expression and ROS levels. Taken together, these studies suggest that BG promotes wound healing by inhibiting pyroptosis via Cx43/ROS signaling pathway.

The resting membrane potential of hSC-CM in a syncytium is more hyperpolarised than that of isolated cells

Human-induced pluripotent stem cell (hiPSC) and stem cell (hSC) derived cardiomyocytes (CM) are gaining popularity as in vitro model for cardiology and pharmacology studies. A remaining flaw of these cells, as shown by single-cell electrophysiological characterization, is a more depolarized resting membrane potential (RMP) compared to native CM. Most reports attribute this to a lower expression of the Kir2.1 potassium channel that generates the IK1 current. However, most RMP recordings are obtained from isolated hSC/hiPSC-CMs whereas in a more native setting these cells are interconnected with neighboring cells by connexin-based gap junctions, forming a syncytium. Hereby, these cells are electrically connected and the total pool of IK1 increases. Therefore, the input resistance (Ri) of interconnected cells is lower than that of isolated cells. During patch clamp experiments pipettes need to be well attached or sealed to the cell, which is reflected in the seal resistance (Rs), because a nonspecific ionic current can leak through this pipette-cell contact or seal and balance out small currents within the cell such as IK1. By recording the action potential of isolated hSC-CMs and that of hSC-CMs cultured in small monolayers, we show that the RMP of hSC-CMs in monolayer is approximately -20 mV more hyperpolarized compared to isolated cells. Accordingly, adding carbenoxolone, a connexin channel blocker, isolates the cell that is patch clamped from its neighboring cells of the monolayer and depolarizes the RMP. The presented data show that the recorded RMP of hSC-CMs in a syncytium is more negative than that determined from isolated hSC/hiPSC-CMs, most likely because the active pool of Kir2.1 channels increased.

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.

Xentry-Gap19 inhibits Connexin43 hemichannel opening especially during hypoxic injury

Hypoxic injury results in cell death, tissue damage and activation of inflammatory pathways. This is mediated by pathological Connexin43 (Cx43) hemichannel (HC) opening resulting in osmotic and ionic imbalances as well as cytokine production perpetuating the inflammatory environment. Gap19 is an intracellularly acting Cx43 mimetic peptide that blocks HC opening and thus promotes cell survival. However, native Gap19, which must enter the cell in order to function, exhibits low cell permeability. In this study, Gap19 was conjugated to the cell-penetrating peptide, Xentry, to investigate if cellular uptake could be improved while maintaining peptide function. Cellular uptake of Xentry-Gap19 (XG19) was much greater than that of native Gap19 even under normal cell culture conditions. Peptide function was maintained post uptake as shown by reduced ethidium homodimer influx and ATP release due to Cx43 HC block. While XG19 blocked pathologic HC opening though, normal gap junction communication required for cell repair and survival mechanisms was not affected as shown in a dye scrape-load assay. Under hypoxic conditions, increased expression of Syndecan-4, a plasma membrane proteoglycan targeted by Xentry, enabled even greater XG19 uptake leading to higher inhibition of ATP release and greater cell survival. This suggests that XG19, which is targeted specifically to hypoxic cells, can efficiently and safely block Cx43 HC and could therefore be a novel treatment for hypoxic and inflammatory diseases.

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.

Neuromodulation of Astrocytic K+ Clearance

Potassium homeostasis is fundamental for brain function. Therefore, effective removal of excessive K+ from the synaptic cleft during neuronal activity is paramount. Astrocytes play a key role in K+ clearance from the extracellular milieu using various mechanisms, including uptake via Kir channels and the Na⁺-K⁺ ATPase, and spatial buffering through the astrocytic gap-junction coupled network. Recently we showed that alterations in the concentrations of extracellular potassium ([K⁺]_o) or impairments of the astrocytic clearance mechanism affect the resonance and oscillatory behavior of both the individual and networks of neurons. These results indicate that astrocytes have the potential to modulate neuronal network activity, however, the cellular effectors that may affect the astrocytic K+ clearance process are still unknown. In this study, we have investigated the impact of neuromodulators, which are known to mediate changes in network oscillatory behavior, on the astrocytic clearance process. Our results suggest that while some neuromodulators (5-HT; NA) might affect astrocytic spatial buffering via gap-junctions, others (DA; Histamine) primarily affect the uptake mechanism via Kir channels. These results suggest that neuromodulators can affect network oscillatory activity through parallel activation of both neurons and astrocytes, establishing a synergistic mechanism to maximize the synchronous network activity.

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation.

Astroglial connexins in epileptogenesis

The astroglial network connected through gap junctions assembling from connexins physiologically balances the concentrations of ions and neurotransmitters around neurons. Astrocytic dysfunction has been associated with many neurological disorders including epilepsy. Dissociated gap junctions result in the increased activity of connexin hemichannels which triggers brain pathophysiological changes. Previous studies in patients and animal models of epilepsy indicate that the reduced gap junction coupling from assembled connexin hemichannels in the astrocytes may play an important role in epileptogenesis. This abnormal cell-to-cell communication is now emerging as an important feature of brain pathologies and being considered as a novel therapeutic target for controlling epileptogenesis. In particular, candidate drugs with ability of inhibition of connexin hemichannel activity and enhancement of gap junction formation in astrocytes should be explored to prevent epileptogenesis and control epilepsy.

The Role of Connexin 43 in Lung Disease

The term lung disease describes a broad category of disorders that impair lung function. More than 35 million Americans have a preventable chronic lung disease with high mortality rates due to limited treatment efficacy. The recent increase in patients with lung disease highlights the need to increase our understanding of mechanisms driving lung inflammation. Connexins, gap junction proteins, and more specifically connexin 43 (Cx43), are abundantly expressed in the lung and are known to play a role in lung diseases. This review focuses on the role of Cx43 in pathology associated with acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) and asthma. Additionally, we discuss the role of Cx43 in preventing disease through the transfer of mitochondria between cells. We aim to highlight the need to better understand what cell types are expressing Cx43 and how this expression influences lung disease.

Synaptic Plasticity After Focal Cerebral Ischemia Was Attenuated by Gap26 but Enhanced by GAP-134

Objective: Synaptic plasticity is critical for neurorehabilitation after focal cerebral ischemia. Connexin 43 (Cx43), the main component of the gap junction, has been shown to be pivotal for synaptic plasticity. The objective of this study was to investigate the role of the Cx43 inhibitor (Gap26) and gap junction modifier (GAP-134) in neurorehabilitation and to study their contribution to synaptic plasticity after focal ischemia. Methods: Time course expression of both total and phosphorylated Cx43 (p-Cx43) were detected by western blotting at 3, 7, and 14 d after focal ischemia. Gap26 and GAP-134 were administered starting from 3 d post focal ischemia. Neurological performances were evaluated by balance beam walking test and Y-maze test at 1, 3, and 7 d. Golgi staining and transmission electron microscope (TEM) detection were conducted at 7 d for observing dendritic spine numbers and synaptic ultrastructure, respectively. Immunofluorescent staining was used at 7 d for detection of synaptic plasticity markers, including synaptophysin (SYN) and growth-associated protein-43 (GAP-43). Results: Expression levels of both total Cx43 and p-Cx43 were increased after focal cerebral ischemia, peaking at 7 d. Compared with the MCAO group, Gap26 worsened the neurological behavior and decreased the dendritic spine number while GAP-134 improved the neurobehavior and increased the number of dendritic spines. Moreover, Gap26 further destroyed the synaptic structure, concomitant with downregulated SYN and GAP-43, whereas GAP-134 alleviated synaptic destruction and upregulated SYN and GAP-43. Conclusion: These findings suggested that Cx43 or the gap junction was involved in synaptic plasticity, thereby promoting neural recovery after ischemic stroke. Treatments enhancing gap junctions may be potential promising therapeutic measures for neurorehabilitation after ischemic stroke.

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.

Light adaptation in the chick retina: Dopamine, nitric oxide, and gap-junction coupling modulate spatiotemporal contrast sensitivity

Adaptation to changes in ambient light intensity, in retinal cells and circuits, optimizes visual functions. In the retina, light-adaptation results in changes in light-sensitivity and spatiotemporal tuning of ganglion cells. Under light-adapted conditions, contrast sensitivity (CS) of ganglion cells is a bandpass function of spatial frequency; in contrast, dark-adaptation reduces CS, especially at higher spatial frequencies. In this work, we aimed to understand intrinsic neuromodulatory mechanisms that underlie retinal adaptation to changes in ambient light level. Specifically, we investigated how CS is affected by dopamine (DA), nitric oxide (NO), and modifiers of electrical coupling through gap junctions, under different conditions of adapting illumination. Using the optokinetic response as a behavioral readout of direction-selective ganglion cell activity, we characterized the spatial CS of chicks under high- and low-photopic conditions and how it was regulated by DA, NO, and gap-junction uncouplers. We observed that: (1) DA D2R-family agonists and a donor of NO increased CS tested in low-photopic illumination, as if observed in the high-photopic light; whereas (2) removing their effects using either DA antagonists or NO- synthase inhibitors mimicked low-photopic CS; (3) simulation of high-photopic CS by DA agonists was abolished by NO-synthase inhibitors; and (4) selectively blocking coupling via connexin 35/36-containing gap junctions, using a "designer" mimetic peptide, increased CS, as does strong illumination. We conclude that, in the chicken retina: (1) DA and NO induce changes in spatiotemporal processing, similar to those driven by increasing illumination, (2) DA possibly acts through stimulating NO synthesis, and (3) blockade of coupling via gap junctions containing connexin 35/36 also drives a change in retinal CS functions. As a noninvasive method, the optokinetic response can provide rapid, conditional, and reversible assessment of retinal functions when pharmacological reagents are injected into the vitreous humor. Finally, the chick's large eyes, and the many similarities between their adaptational circuit functions and those in mammals such as the mouse, make them a promising model for future retinal research.

Angiotensin II induces RAW264.7 macrophage polarization to the M1‑type through the connexin 43/NF‑κB pathway

Angiotensin II (AngII) serves an important inflammatory role in cardiovascular disease; it can induce macrophages to differentiate into the M1-type, produce inflammatory cytokines and resist pathogen invasion, and can cause a certain degree of damage to the body. Previous studies have reported that connexin 43 (Cx43) and NF-κB (p65) are involved in the AngII-induced inflammatory pathways of macrophages; however, the mechanisms underlying the effects of Cx43 and NF-κB (p65) on AngII-induced macrophage polarization have not been determined. Thus, the present study aimed to investigate the effects of Cx43 and NF-κB (p65) on the polarization process of AngII-induced macrophages. The macrophage polarization-related proteins and mRNAs were examined by flow cytometry, western blotting, immunofluorescence, ELISA and reverse transcription-quantitative PCR analyses. RAW264.7 macrophages were treated with AngII to simulate chronic inflammation and it was subsequently found that AngII promoted RAW 264.7 macrophage polarization towards the M1-type by such effects as the release of inducible nitric oxide synthase (iNOS), tumour necrosis factor (TNF)-α, IL-1β, the secretion of IL-6, and the expression of M1-type indicators, such as CD86. Simultaneously, compared with the control group, the protein expression levels of Cx43 and phosphorylated (p)-p65 were significantly increased following AngII treatment. The M1-related phenotypic indicators, iNOS, TNF-α, IL-1β, IL-6 and CD86, were inhibited by the NF-κB (p65) signalling pathway inhibitor BAY117082. Similarly, the Cx43 inhibitors, Gap26 and Gap19, also inhibited the expression of M1-related factors, and the protein expression levels of p-p65 in the Gap26/Gap19 groups were significantly decreased compared with the AngII group. Altogether, these findings suggested that AngII may induce the polarization of RAW264.7 macrophages to the M1-type through the Cx43/NF-κB (p65) signalling pathway.

A subset of taste receptor cells express biocytin-permeable channels activated by reducing extracellular Ca2+ concentration

Taste receptor cells (type II cells) transmit taste information to taste nerve fibres via ATP-permeable channels, including calcium homeostasis modulator (CALHM), connexin and/or pannexin1 channels, via the paracrine release of adenosine triphosphate (ATP) as a predominant transmitter. In the present study, we demonstrate that extracellular Ca²⁺ -dependent biocytin-permeable channels are present in a subset of type II cells in mouse fungiform taste buds using biocytin uptake, immunohistochemistry and in situ whole-cell recordings. Type II cells were labelled with biocytin in an extracellular Ca²⁺ concentration ([Ca²⁺ ]_out )-sensitive manner. We found that the ratio of biocytin-labelled type II cells to type II cells per taste bud was approximately 20% in 2 mM Ca²⁺ saline, and this ratio increased to approximately 50% in nominally Ca²⁺ -free saline. The addition of 300 µM GdCl₃ , which inhibits various channels including CALHM1 channels, significantly inhibited biocytin labelling in nominally Ca²⁺ -free saline, whereas the addition of 20 µM ruthenium red did not. Moreover, Cs⁺ -insensitive currents increased in nominally Ca²⁺ -free saline in approximately 40% of type II cells. These increased currents appeared at a potential of above -35 mV, reversed at approximately +10 mV and increased with depolarization. These results suggest that biocytin labels type II cells via ion channels activated by [Ca²⁺ ]_out reduction, probably "CALHM-like" channels, on the basolateral membrane and that taste receptor cells can be categorized into two groups based on differences in the expression levels of [Ca²⁺ ]_out -dependent biocytin-permeable channels. These data indicate electrophysiological and pharmacologically relevant properties of biocytin-permeable channels and suggest their contributions to taste signal transduction.

Inhibition of gap junction composed of Cx43 prevents against acute kidney injury following liver transplantation

Postoperative acute kidney injury (AKI) is a severe complication after liver transplantation (LT). Its deterioration and magnification lead to the increase in mortality. Connexin43 (Cx43) mediates direct transmission of intracellular signals between neighboring cells, always considered to be the potent biological basis of organ damage deterioration and magnification. Thus, we explored the effects of Cx43 on AKI following LT and its related possible mechanism. In this study, alternations of Cx43 expression were observed in 82 patients, receiving the first-time orthotopic LT. We built autologous orthotopic liver transplantation (AOLT) models with Sprague–Dawley (SD) rats in vivo, and hypoxia-reoxygenation (H/R) or lipopolysaccharide (LPS) pretreatment models with kidney tubular epithelial cells (NRK-52E) in vitro, both of which were the most important independent risk factors of AKI following LT. Then, different methods were used to alter the function of Cx43 channels to determine its protective effects on AKI. The results indicated that patients with AKI suffering from longer time of tracheal intubation or intensive care unit stay, importantly, had significantly lower survival rate at postoperative 30 days and 3 years. In rat AOLT models, as Cx43 was inhibited with heptanol, postoperative AKI was attenuated significantly. In vitro experiments, downregulation of Cx43 with selective inhibitors, or siRNA protected against post-hypoxic NRK-52E cell injuries caused by H/R and/or LPS, while upregulation of Cx43 exacerbated the above-mentioned cell injuries. Of note, alternation of Cx43 function regulated the content of reactive oxygen species (ROS), which not only mediated oxidative stress and inflammation reactions effectively, but also regulated necroptosis. Therefore, we concluded that Cx43 inhibition protected against AKI following LT through attenuating ROS transmission between the neighboring cells. ROS alternation depressed oxidative stress and inflammation reaction, which ultimately reduced necroptosis. This might offer new insights for targeted intervention for organ protection in LT, or even in other major surgeries.

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.

Connexin 26 and 43 play a role in regulating proinflammatory events in the epidermis

Dysregulation of Connexin (CX) expression and function is associated with a range of chronic inflammatory conditions including psoriasis and nonhealing wounds. To mimic a proinflammatory environment, HaCaT cells, a model human keratinocyte cell line, were challenged with 10 µg/ml peptidoglycan (PGN) isolated from Staphylococcus aureus for 15 min to 24 hr in the presence or absence of CX blockers and/or following CX26, CX43, PANX1 and TLR2 small interfering RNA (siRNA) knockdown (KD). Expression levels of IL-6, IL-8, CX26, CX43, PANX1, TLR2 and Ki67 were assessed by quantitative real-time polymerase chain reaction, western blot analysis and/or immunocytochemistry. Nuclear factor kappa β (NF-κβ) was blocked with BAY 11-7082, CX-channel function was determined by adenosine 5'-triphosphate (ATP) release assays. Enzyme-linked immunosorbent assay monitored IL6 release following PGN challenge in the presence or absence of siRNA or blockers of CX or purinergic signalling. Exposure to PGN induced IL-6, IL-8, CX26 and TLR2 gene expression but it did not influence CX43, PANX1 or Ki67 messenger RNA expression levels. CX43 protein levels were reduced following 24 hr PGN exposure. PGN-induced CX26 and IL-6 expression were also aborted by TLR2-KD and inhibition of NF-κβ. ATP and IL-6 release were stimulated following 15 min and 1-24 hr challenge with PGN, respectively. Release of both agents was inhibited by coincubation with CX-channel blockers, CX26-, CX43- and TLR2-KD. The IL-6 response was also reduced by purinergic blockers. CX-signalling plays a role in the innate immune response in the epidermis. PGN is detected by TLR2, which via NF-κβ, directly activates CX26 and IL-6 expression. CX43 and CX26 maintain proinflammatory signalling by permitting ATP release, however, PANX1 does not participate.

The role of connexin43 in neuropathic pain induced by spinal cord injury

Neuropathic pain is caused by the damage or dysfunction of the nervous system. In many neuropathic pain models, there is an increase in the number of gap junction (GJ) channels, especially the upregulation of the expression of connexin43 (Cx43), leading to the secretion of various types of cytokines and involvement in the formation of neuropathic pain. GJs are widely distributed in mammalian organs and tissues, and Cx43 is the most abundant connexin (Cx) in mammals. Astrocytes are the most abundant glial cell type in the central nervous system (CNS), which mainly express Cx43. More importantly, GJs play an important role in regulating cell metabolism, signaling, and function. Many existing literatures showed that Cx43 plays an important role in the nervous system, especially in the CNS under normal and pathological conditions. However, many internal mechanisms have not yet been thoroughly explored. In this review, we summarized the current understanding of the role and association of Cx and pannexin channels in neuropathic pain, especially after spinal cord injury, as well as some of our own insights and thoughts which suggest that Cx43 may become an emerging therapeutic target for future neuropathic pain, bringing new hope to patients.

The Role of Connexin-43 in the Inflammatory Process: A New Potential Therapy to Influence Keratitis

The studies outlined in this review highlight the relationship between inflammatory signaling molecules and connexin-43 (Cx43). Gap junction (GJ) channels and hemichannels (HCs) participate in the metabolic activity between intra- and extracellular space. Some ions and small molecules are exchanged from cell to cell or cell to extracellular space to affect the process of inflammation via GJ. We analyzed the effects of signaling molecules, such as innate immunity messengers, transcription factors, LPS, cytokine, inflammatory chemokines, and MMPs, on Cx43 expression during the inflammatory process. At the same time, we found that these signaling molecules play a critical role in the pathogenesis of keratitis. Thus, we assessed the function of Cx43 during inflammatory corneal disease. Corneal healing plays an essential role in the late stage of keratitis. We found that Cx43 is involved in wound healing. Studies have shown that the decrease of Cx43 can decrease the time of healing. We also report several Cx43 mimic peptides which can inhibit the activity of Cx43 Hc to mediate the releasing of adenosine triphosphate (ATP), which may in turn influence the inflammatory process.

Blocking connexin channels during vitrification of immature cat oocytes improves maturation capacity after warming

In the domestic cat, nuclear maturation and embryo development after vitrification of immature oocytes have been obtained but developmental competence after warming remains low. It has been reported that during folliculogenesis, the association and communication between the oocyte and the surrounding cumulus cells through connexin-based gap junctions is essential for normal oocyte and follicular development. Gap junctions result from the head-to-head interaction of two hemichannels; however, there is always a population of hemichannels not incorporated into gap junctions. These unopposed hemichannels are normally closed but may open under certain stress conditions, potentially also during vitrification and warming, turning them into toxic pores inducing cell injury and cell death. The aim of our study was to test whether inhibiting connexin 37 (Cx37) and connexin 43 (Cx43) channels with the connexin-targeting peptide Gap26 during vitrification and warming of cat immature cumulus-oocyte-complexes (COCs) could improve oocyte maturation and competence of resultant blastocysts derived by parthenogenetic activation. In the first experiment, our immunostainings confirmed the presence of Cx43 protein in the cytoplasm of immature cat oocytes and in the plasma membranes of cumulus cells. In the second experiment, COCs were randomly divided in three different groups: a control group (control), a group vitrified without Gap26 (vitrified) and a group vitrified with Gap26 (vitrified-peptide). The maturation rate was checked and oocytes from all three different experimental groups were parthenogenetically activated and cultured in vitro until day 8. After vitrification and warming, 49% of the oocytes in the control group matured, while this was 8% and 19% in the vitrified and vitrified-peptide groups, respectively. Compared to the vitrified group, oocytes in the vitrified-peptide group had significantly larger maturation rates. No blastocysts were detected at day 8 in the vitrified group, while 2% and 13% of the oocytes further developed to blastocyst at day 8 in the vitrified-peptide and control non-vitrified group, respectively. We conclude that the use of Gap26 in vitrification and warming media to vitrify immature cat oocytes improves maturation success and allows such oocytes to reach the blastocyst stage (2%) at day 8 after parthenogenetic activation.

Blocking connexin channels improves embryo development of vitrified bovine blastocysts

Connexins (Cxs) are required for normal embryo development and implantation. They form gap junctions (GJs) connecting the cytoplasm of adjacent cells and hemichannels (HCs), which are normally closed but open in response to stress conditions. Excessive HC opening is detrimental for cell function and may lead to cell death. We found that hatching of in vitro-produced bovine embryos, matured in serum-containing conditions, was significantly improved when vitrification/warming was done in the presence of Gap26 that targets GJA1 (Cx43) and GJA4 (Cx37). Further work showed that HCs from blastocysts produced after oocyte maturation in the presence of serum were open shortly after vitrification/warming, and this was prevented by Gap26. Gap26, applied for the exposure times used, inhibited Cx43 and Cx37 HCs while it did not have an effect on GJs. Interestingly, Gap26 had no effect on blastocyst degeneration or cell death. We conclude that blocking HCs protects embryos during vitrification and warming by a functional effect not linked to cell death.

Novel approach to temozolomide resistance in malignant glioma: connexin43-directed therapeutics

Resistance of malignant glioma, including glioblastoma (GBM), to the chemotherapeutic temozolomide (TMZ) remains a key obstacle in treatment strategies. The gap junction protein connexin43 (Cx43) has complex roles in the establishment, progression, and persistence of malignant glioma. Recent findings demonstrate that connexins play an important role in the microenvironment of malignant glioma and that Cx43 is capable of conferring chemotherapeutic resistance to GBM cells. Carboxyl-terminal Cx43 peptidomimetics show therapeutic promise in overcoming TMZ resistance via mechanisms that may include modulating junctional activity between tumor cells and peritumoral cells and/or downstream molecular signaling events mediated by Cx43 protein binding. High levels of intra-tumor and inter-tumor heterogeneity make it difficult to clearly define specific populations for Cx43-targeted therapy; hence, development of in vitro models that better mimic the microenvironment of malignant glioma, and the incorporation of patient-derived stem cells, could provide opportunities for patient-specific drug screening. This review summarizes recent advances in understanding the roles of Cx43 in malignant glioma, with a special focus on tumor microenvironment, TMZ resistance, and therapeutic opportunity offered by Cx43 peptidomimetics.

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.

A novel mechanism of depression: role for connexins

Major depressive disorder (MDD) is a chronic and debilitating illness that affects over 350 million people worldwide; however, current treatments have failed to cure or prevent the progress of depression. Increasing evidence suggests a crucial role for connexins in MDD. In this review, we have summarised recent accomplishments regarding the role of connexins, gap junctions, and hemichannels in the aetiology of MDD, and discussed the limitations of current research. A blockage of gap junctions or hemichannels induces depressive behaviour. Possible underlying mechanisms include the regulation of neurosecretory functions and synaptic activity by gap junctions and hemichannels. Gap junctions are functionally inhibited under stress conditions. Conversely, hemichannel permeability is increased. Antidepressants inhibit hemichannel permeability; however, they have contrasting effects on the function of gap junctions under normal conditions and can protect them against stress. In conclusion, the blockage of hemichannels concurrent with improvements in gap junction functionality might be potential targets for depression treatment.

The Potential for Connexin Hemichannels to Drive Breast Cancer Progression through Regulation of the Inflammatory Response

Over the past few decades, connexin hemichannels have become recognized as major players in modulating the inflammatory response. Chronic inflammation is documented to promote tumorigenesis and is a critical component of tumor progression. Furthermore, inflammation is strongly linked to angiogenesis, immunotolerance, invasiveness, metastasis, and resistance in breast cancers. In this review, the literature on the role of connexin hemichannels in inflammation is summarized, and the potential role for hemichannel-mediated inflammation in driving breast cancer progression is discussed. Lastly, the potential for connexin-based therapeutics to modulate the inflammatory component of the tumor microenvironment as an avenue for the treatment of breast cancer is also discussed.

Roles of astrocytic connexin-43, hemichannels, and gap junctions in oxygen-glucose deprivation/reperfusion injury induced neuroinflammation and the possible regulatory mechanisms of salvianolic acid B and carbenoxolone

Background

Glia-mediated neuroinflammation is related to brain injury exacerbation after cerebral ischemia/reperfusion (I/R) injury. Astrocytic hemichannels or gap junctions, which were mainly formed by connexin-43, have been implicated in I/R damage. However, the exact roles of astrocytic hemichannels and gap junction in neuroinflammatory responses induced by I/R injury remain unknown.

Methods

Primary cultured astrocytes were subjected to OGD/R injury, an in vitro model of I/R injury. Salvianolic acid B (SalB) or carbenoxolone (CBX) were applied for those astrocytes. Besides, Cx43 mimetic peptides Gap19 or Gap26 were also applied during OGD/R injury; Cx43 protein levels were determined by western blot and cytoimmunofluorescene staining, hemichannel activities by Ethidium bromide uptake and ATP concentration detection, and gap junction intercellular communication (GJIC) permeability by parachute assay. Further, astrocyte-conditioned medium (ACM) was collected and incubated with microglia. Meanwhile, ATP or apyrase were applied to explore the role of ATP during OGD/R injury. Microglial activation, M1/M2 phenotypes, and M1/M2-related cytokines were detected. Also, microglia-conditioned medium (MEM) was collected and incubated with astrocytes to further investigate its influence on astrocytic hemichannel activity and GJIC permeability. Lastly, effects of ACM and MCM on neuronal viability were detected by flow cytometry.

Results

We found that OGD/R induced abnormally opened hemichannels with increased ATP release and EtBr uptake but reduced GJIC permeability. WB tests showed decreased astrocytic plasma membrane’s Cx43, while showing an increase in cytoplasma. Treating OGD/R-injured microglia with ATP or OGD/R-ACM induced further microglial activation and secondary pro-inflammatory cytokine release, with the M1 phenotype predominating. Conversely, astrocytes incubated with OGD/R-MCM exhibited increased hemichannel opening but reduced GJIC coupling. Both SalB and CBX inhibited abnormal astrocytic hemichannel opening and ATP release and switched the activated microglial phenotype from M1 to M2, thus providing effective neuroprotection. Application of Gap19 or Gap26 showed similar results with CBX. We also found that OGD/R injury caused both plasma membrane p-Cx43(Ser265) and p-Src(Tyr416) significantly upregulated; application of SalB may be inhibiting Src kinase and attenuating Cx43 internalization. Meanwhile, CBX treatment induced obviously downregulation of p-Cx43(Ser368) and p-PKC(Ser729) protein levels in plasma membrane.

Conclusions

We propose a vicious cycle exists between astrocytic hemichannel and microglial activation after OGD/R injury, which would aggravate neuroinflammatory responses and neuronal damage. Astrocytic Cx43, hemichannels, and GJIC play critical roles in OGD/R injury-induced neuroinflammatory responses; treatment differentially targeting astrocytic Cx43, hemichannels, and GJIC may provide novel avenues for therapeutics during cerebral I/R injury.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1127-3) contains supplementary material, which is available to authorized users.

Connexin 43 Hemichannel as a Novel Mediator of Sterile and Infectious Inflammatory Diseases

Cytoplasmic membrane-bound connexin 43 (Cx43) proteins oligomerize into hexameric channels (hemichannels) that can sometimes dock with hemichannels on adjacent cells to form gap junctional (GJ) channels. However, the possible role of Cx43 hemichannels in sterile and infectious inflammatory diseases has not been adequately defined due to the lack of selective interventions. Here we report that a proinflammatory mediator, the serum amyloid A (SAA), resembled bacterial endotoxin by stimulating macrophages to up-regulate Cx43 expression and double-stranded RNA-activated protein kinase R (PKR) phosphorylation in a TLR4-dependent fashion. Two well-known Cx43 mimetic peptides, the GAP26 and TAT-GAP19, divergently affected macrophage hemichannel activities in vitro, and differentially altered the outcome of lethal sepsis in vivo. By screening a panel of Cx43 mimetic peptides, we discovered that one cysteine-containing peptide, P5 (ENVCYD), effectively attenuated hemichannel activities, and significantly suppressed endotoxin-induced release of ATP and HMGB1 in vitro. In vivo, the P5 peptide conferred a significant protection against hepatic ischemia/reperfusion injury and lethal microbial infection. Collectively, these findings have suggested a pathogenic role of Cx43 hemichannels in sterile injurious as well as infectious inflammatory diseases possibly through facilitating extracellular ATP efflux to trigger PKR phosphorylation/activation.

Role of Neuron-Glial Interaction Mediated by IL-1β in Ectopic Tooth Pain

Although many reports have demonstrated that ectopic pain develops in the orofacial region following tooth pulp inflammation, which often causes misdiagnosis and inappropriate treatment for patients with pulpitis, the precise mechanism remains unknown. In the present study, we hypothesized that the functional interaction between satellite glial cells and neurons mediated by interleukin 1β (IL-1β) in the trigeminal ganglion (TG) is involved in ectopic orofacial pain associated with tooth pulp inflammation. The digastric muscle electromyogram (D-EMG) activity elicited by capsaicin administration into the maxillary second molar tooth pulp was analyzed to evaluate the noxious reflex and was significantly increased in rats with inflammation of the maxillary first molar (M1) versus rats injected with saline. A significant increase in the expression of connexin43 (Cx43), a gap junction containing protein, was observed in activated satellite glial cells surrounding second molar-innervating neurons in the TG after M1 pulpitis. Daily administration of Gap26, a Cx43 mimetic peptide and inhibitor, in the TG significantly suppressed the enhancement of capsaicin-induced D-EMG activity and the percentage of Fluoro-Gold (FG)-labeled cells encircled by glial fibrillary acid protein-immunoreactive (IR) + Cx43-IR cells after M1 pulp inflammation ( P < 0.01). The percentage of FG-labeled cells encircled by glial fibrillary acid protein-IR + IL-1β-IR cells, IL-1 type I receptor-IR cells labeled with FG, and TRPV1-IR cells labeled with FG significantly increased after M1 pulp inflammation ( P < 0.01). Daily administration of IL-1ra, an IL-1 receptor antagonist, into the TG significantly reduced the enhancement of capsaicin-induced D-EMG activity and the percentage of TRPV1-IR neurons labeled with FG after M1 pulp inflammation ( P < 0.01). The present findings suggest that satellite glial cell is activated in the TG via activated gap junctions composed of Cx43 following tooth pulp inflammation, which leads to the hyperactivation of remote neurons via IL-1β mechanisms and results in ectopic tooth pulp pain in the adjacent tooth.

Acute connexin43 temporal and spatial expression in response to ischemic stroke

Connexin43 (Cx43) gap junctions expressed in astrocytes can significantly impact neuronal survival in stroke. However, little is known regarding Cx43 spatial and temporal expression during the initial stages of brain ischemia. Using immunohistochemistry and Western blot analysis, we examined Cx43 spatial and temporal expression as a function of neuronal injury within the first 24 h after permanent middle cerebral artery occlusion (pMCAO). Western blot analysis showed a significant increase in Cx43 protein expression in the core ischemic area at 2 and 3 h after pMCAO. However, after 6 h of pMCAO Cx43 levels were significantly reduced. This reduction was due to cell death and concomitant Cx43 degradation in the expanding focal ischemic region, while the peri-infarct zone revealed intense Cx43 staining. The neuronal cell-death marker Fluoro-Jade C labeled injured neurons faintly at 1 h post-pMCAO with a time-dependent increase in both intensity and size of punctate staining. In addition, decreased microtubule-associated protein 2 (MAP2) immunoreactivity and thionin staining similarly indicated cell damage beginning at 1 h after pMCAO. Taken together, Cx43 expression is sensitive to neuronal injury and can be detected as early as 2 h post-pMCAO. These findings underscore Cx43 gap junction as a potential early target for therapeutic intervention in ischemic stroke.

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.

Connexin hemichannel inhibition reduces acetaminophen-induced liver injury in mice

Historically, connexin hemichannels have been considered as structural precursors of gap junctions. However, accumulating evidence points to independent roles for connexin hemichannels in cellular signaling by connecting the intracellular compartment with the extracellular environment. Unlike gap junctions, connexin hemichannels seem to be mainly activated in pathological processes. The present study was set up to test the potential involvement of hemichannels composed of connexin32 and connexin43 in acute hepatotoxicity induced by acetaminophen. Prior to this, in vitro testing was performed to confirm the specificity and efficacy of TAT-Gap24 and TAT-Gap19 in blocking connexin32 and connexin43 hemichannels, respectively. Subsequently, mice were overdosed with acetaminophen followed by treatment with TAT-Gap24 or TAT-Gap19 or a combination of both after 1.5h. Sampling was performed 3, 6, 24 and 48h following acetaminophen administration. Evaluation of the effects of connexin hemichannel inhibition was based on a series of clinically relevant read-outs, measurement of inflammatory cytokines and oxidative stress. Subsequent treatment of acetaminophen-overdosed mice with TAT-Gap19 only marginally affected liver injury. In contrast, a significant reduction in serum alanine aminotransferase activity was found upon administration of TAT-Gap24 to intoxicated animals. Furthermore, co-treatment of acetaminophen-overdosed mice with both peptides revealed an additive effect as even lower serum alanine aminotransferase activity was observed. Blocking of connexin32 or connexin43 hemichannels individually was found to decrease serum quantities of pro-inflammatory cytokines, while no effects were observed on the occurrence of hepatic oxidative stress. This study shows for the first time a role for connexin hemichannels in acetaminophen-induced acute liver failure.

Contribution of Astroglial Cx43 Hemichannels to the Modulation of Glutamatergic Currents by D-Serine in the Mouse Prefrontal Cortex

Astrocytes interact dynamically with neurons by modifying synaptic activity and plasticity. This interplay occurs through a process named gliotransmission, meaning that neuroactive molecules are released by astrocytes. Acting as a gliotransmitter, D-serine, a co-agonist of the NMDA receptor at the glycine-binding site, can be released by astrocytes in a calcium [Ca²⁺]_i-dependent manner. A typical feature of astrocytes is their high expression level of connexin43 (Cx43), a protein forming gap junction channels and hemichannels associated with dynamic neuroglial interactions. Pharmacological and genetic inhibition of Cx43 hemichannel activity reduced the amplitude of NMDA EPSCs in mouse layer 5 prefrontal cortex pyramidal neurons without affecting AMPA EPSC currents. This reduction of NMDA EPSCs was rescued by addition of D-serine in the extracellular medium. LTP of NMDA and AMPA EPSCs after high-frequency stimulation was reduced by prior inhibition of Cx43 hemichannel activity. Inactivation of D-serine synthesis within the astroglial network resulted in the reduction of NMDA EPSCs, which was rescued by adding extracellular D-serine. We showed that the activity of Cx43 hemichannels recorded in cultured astrocytes was [Ca²⁺]_I dependent. Accordingly, in acute cortical slices, clamping [Ca²⁺]_i at a low level in astroglial network resulted in an inhibition of NMDA EPSC potentiation that was rescued by adding extracellular D-serine. This work demonstrates that astroglial Cx43 hemichannel activity is associated with D-serine release. This process, occurring by direct permeation of D-serine through hemichannels or indirectly by Ca²⁺ entry and activation of other [Ca²⁺]_i-dependent mechanisms results in the modulation of synaptic activity and plasticity.SIGNIFICANCE STATEMENT We recorded neuronal glutamatergic (NMDA and AMPA) responses in prefrontal cortex (PFC) neurons and used pharmacological and genetic interventions to block connexin-mediated hemichannel activity specifically in a glial cell population. For the first time in astrocytes, we demonstrated that hemichannel activity depends on the intracellular calcium concentration and is associated with D-serine release. Blocking hemichannel activity reduced the LTP of these excitatory synaptic currents triggered by high-frequency stimulation. These observations may be particularly relevant in the PFC, where D-serine and its converting enzyme are highly expressed.

Breakthrough Cancer Pain Is Associated with Spinal Gap Junction Activation via Regulation of Connexin 43 in a Mouse Model

Breakthrough cancer pain (BTcP) is a high-intensity, short-duration, unpredictable and uncontrollable pain. Recent studies have shown that activation of gap junction (GJ) in spinal cord plays an important role in the pathogenesis of BTcP. We examined the expressions of Glial fibrillary acidic protein (GFAP), connexin (Cx) 43 protein and phosphorylation of Cx43 (p-Cx43) in the spinal cord of mice. In addition, we investigated the effects of Gap26, a selective GJ blocker, on the expressions of GFAP, Cx43 and p-Cx43 in BTcP mice. We found that the expressions of GFAP and Cx43 proteins were significantly upregulated while p-Cx43 was down-regulated in the spinal cord in a mouse model of BTcP. The overexpression of Cx43 protein in the spinal cord increased GJ formation and enhanced BTcP. The variation of the ratio of p-Cx43/T-Cx43 (total Cx43) affected the function of GJ to induce BTcP. Furthermore, BTcP was alleviated by Gap26 via reducing pain hypersensitivity. The inhibition of Cx43 and p-Cx43 by Gap26 attenuated BTcP but the p/T ratio of Cx43 remained unchanged in BTcP mice. We reveal that the expression and phosphorylation of Cx43 affected BTcP and GJ activation facilitated BTcP via a Cx43-mediated signaling in the spinal cord. The finding may provide a scientific rationale for discovery and development of novel therapeutic targets for the treatment of BTcP clinically.

Connexin channel and its role in diabetic retinopathy

Diabetic retinopathy is the leading cause of blindness in the working age population. Unfortunately, there is no cure for this devastating ocular complication. The early stage of diabetic retinopathy is characterized by the loss of various cell types in the retina, namely endothelial cells and pericytes. As the disease progresses, vascular leakage, a clinical hallmark of diabetic retinopathy, becomes evident and may eventually lead to diabetic macular edema, the most common cause of vision loss in diabetic retinopathy. Substantial evidence indicates that the disruption of connexin-mediated cellular communication plays a critical role in the pathogenesis of diabetic retinopathy. Yet, it is unclear how altered communication via connexin channel mediated cell-to-cell and cell-to-extracellular microenvironment is linked to the development of diabetic retinopathy. Recent observations suggest the possibility that connexin hemichannels may play a role in the pathogenesis of diabetic retinopathy by allowing communication between cells and the microenvironment. Interestingly, recent studies suggest that connexin channels may be involved in regulating retinal vascular permeability. These cellular events are coordinated at least in part via connexin-mediated intercellular communication and the maintenance of retinal vascular homeostasis. This review highlights the effect of high glucose and diabetic condition on connexin channels and their impact on the development of diabetic retinopathy.

The Role of Connexins in Wound Healing and Repair: Novel Therapeutic Approaches

Gap junctions are intercellular proteins responsible for mediating both electrical and biochemical coupling through the exchange of ions, second messengers and small metabolites. They consist of two connexons, with (one) connexon supplied by each cell. A connexon is a hexamer of connexins and currently more than 20 connexin isoforms have been described in the literature thus far. Connexins have a short half-life, and therefore gap junction remodeling constantly occurs with a high turnover rate. Post-translational modification, such as phosphorylation, can modify their channel activities. In this article, the roles of connexins in wound healing and repair are reviewed. Novel strategies for modulating the function or expression of connexins, such as the use of antisense technology, synthetic mimetic peptides and bioactive materials for the treatment of skin wounds, diabetic and pressure ulcers as well as cornea wounds, are considered.

Astrocyte sigma-1 receptors modulate connexin 43 expression leading to the induction of below-level mechanical allodynia in spinal cord injured mice

We have previously shown using a spinal cord injury (SCI) model that gap junctions contribute to the early spread of astrocyte activation in the lumbar spinal cord and that this astrocyte communication plays critical role in the induction of central neuropathic pain. Sigma-1 receptors (Sig-1Rs) have been implicated in spinal astrocyte activation and the development of peripheral neuropathic pain, yet their contribution to central neuropathic pain remains unknown. Thus, we investigated whether SCI upregulates spinal Sig-1Rs, which in turn increase the expression of the astrocytic gap junction protein, connexin 43 (Cx43) leading to the induction of central neuropathic pain. A thoracic spinal cord hemisection significantly increased both astrocyte activation and Cx43 expression in lumbar dorsal horn. Sig-1Rs were also increased in lumbar dorsal horn astrocytes, but not neurons or microglia. Intrathecal injection of an astrocyte metabolic inhibitor (fluorocitrate); a gap junction/hemichannel blocker (carbenoxolone); or a Cx43 mimetic peptide (43Gap26) significantly reduced SCI-induced bilateral below-level mechanical allodynia. Blockade of Sig-1Rs with BD1047 during the induction phase of pain significantly suppressed the SCI-induced development of mechanical allodynia, astrocyte activation, increased expression of Cx43 in both total and membrane levels, and increased association of Cx43 with Sig-1R. However, SCI did not change the expression of oligodendrocyte (Cx32) or neuronal (Cx36) gap junction proteins. These findings demonstrate that SCI activates astrocyte Sig-1Rs leading to increases in the expression of the gap junction protein, Cx43 and astrocyte activation in the lumbar dorsal horn, and ultimately contribute to the induction of bilateral below-level mechanical allodynia.

Characterization of intercellular communication and mitochondrial donation by mesenchymal stromal cells derived from the human lung

Background

Bone marrow-derived mesenchymal stromal cells (BM-MSCs) are capable of repairing wounded lung epithelial cells by donating cytoplasmic material and mitochondria. Recently, we characterized two populations of human lung-derived mesenchymal stromal cells isolated from digested parenchymal lung tissue (LT-MSCs) from healthy individuals or from lung transplant recipients’ bronchoalveolar lavage fluid (BAL-MSCs). The aim of this study was to determine whether LT-MSCs and BAL-MSCs are also capable of donating cytoplasmic content and mitochondria to lung epithelial cells.

Methods

Cytoplasmic and mitochondrial transfer was assessed by co-culturing BEAS2B epithelial cells with Calcein AM or Mitotracker Green FM-labelled MSCs. Transfer was then measured by flow cytometry and validated by fluorescent microscopy. Molecular inhibitors were used to determine the contribution of microtubules/tunnelling nanotubes (TNTs, cytochalasin D), gap junctions (carbenoxolone), connexin-43 (gap26) and microvesicles (dynasore).

Results

F-actin microtubules/TNTs extending from BM-MSCs, LT-MSCs and BAL-MSCs to bronchial epithelial cells formed within 45 minutes of co-culturing cells. Each MSC population transferred a similar volume of cytoplasmic content to epithelial cells. Inhibiting microtubule/TNTs, gap junction formation and microvesicle endocytosis abrogated the transfer of cytoplasmic material from BM-MSCs, LT-MSCs and BAL-MSCs to epithelial cells. In contrast, blocking connexin-43 gap junction formation had no effect on cytoplasmic transfer. All MSC populations donated mitochondria to bronchial epithelial cells with similar efficiency. Mitochondrial transfer was reduced in all co-cultures after microtubule/TNT or endocytosis inhibition. Gap junction formation inhibition reduced mitochondrial transfer in BM-MSC and BAL-MSC co-cultures but had no effect on transfer in LT-MSC co-cultures. Connexin-43 inhibition did not impact mitochondrial transfer. Finally, bronchial epithelial cells were incapable of donating cytoplasmic content or mitochondria to any MSC population.

Conclusion

Similar to their bone marrow counterparts, LT-MSCs and BAL-MSCs can donate cytoplasmic content and mitochondria to bronchial epithelial cells via multiple mechanisms. Given that BM-MSCs utilize these mechanisms to mediate the repair of damaged bronchial epithelial cells, both LT-MSCs and BAL-MSCs will probably function similarly.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0354-8) contains supplementary material, which is available to authorized users.