Cation permeation through connexin 43 hemichannels is cooperative, competitive and saturable with parameters depending on the permeant species

Connexin hemichannels drive lactation-induced osteocyte acidification and perilacunar-canalicular remodeling

The maternal skeleton experiences significant bone loss during lactation, followed by rapid restoration post weaning. Parathyroid-related protein (PTHrP)-induced acidification of the perilacunar matrix by osteocytes is crucial in this process, yet its mechanism remains unclear. Here, we identify Cx43 hemichannels (HCs) as key mediators of osteocyte acidification and perilacunar-canalicular remodeling (PLR). Utilizing transgenic mouse models expressing dominant-negative Cx43 mutants, we show that mice with impaired Cx43 HCs exhibit attenuated lactation-induced responses compared to wild-type and only gap junction-impaired groups, including lacunar enlargement, upregulation of PLR genes, and bone loss with compromised mechanical properties. Furthermore, inhibition of HCs by a Cx43 antibody blunts PTHrP-induced calcium influx and protein kinase A activation, followed by impaired osteocyte acidification. Additionally, impeded HCs suppress bone recovery during the post-lactation period. Our findings highlight the pivotal role of Cx43 HCs in orchestrating dynamic bone changes during lactation and recovery by regulating acidification and remodeling enzyme expression.

Ca2+ permeation through C-terminal cleaved, but not full-length human Pannexin1 hemichannels, mediates cell death

Pannexin1 hemichannels (Panx1 HCs) are found in the membrane of most mammalian cells and communicate the intracellular and extracellular spaces, enabling the passive transfer of ions and small molecules. They are involved in physiological and pathophysiological conditions. During apoptosis, the C-terminal tail of Panx1 is proteolytically cleaved, but the permeability features of hemichannels and their role in cell death remain elusive. To address these topics, HeLa cells transfected with full-length human Panx1 (fl-hPanx1) or C-terminal truncated hPanx1 (Δ371hPanx1) were exposed to alkaline extracellular saline solution, increasing the activity of Panx1 HCs. The Δ371hPanx1 HC was permeable to DAPI and Etd+, but not to propidium iodide, whereas fl-hPanx1 HC was only permeable to DAPI. Furthermore, the cytoplasmic Ca2+ signal increased only in Δ371hPanx1 cells, which was supported by bioinformatics approaches. The influx of Ca2+ through Δ371hPanx1 HCs was necessary to promote cell death up to about 95% of cells, whereas the exposure to alkaline saline solution without Ca2+ failed to induce cell death, and the Ca2+ ionophore A23187 promoted more than 80% cell death even in fl-hPanx1 transfectants. Moreover, cell death was prevented with carbenoxolone or 10Panx1 in Δ371hPanx1 cells, whereas it was undetectable in HeLa Panx1-/- cells. Pretreatment with Ferrostatin-1 and necrostatin-1 did not prevent cell death, suggesting that ferroptosis or necroptosis was not involved. In comparison, zVAD-FMK, a pancaspase inhibitor, reduced death by ~60%, suggesting the involvement of apoptosis. Therefore, alkaline pH increases the activity of Δ371hPanx1HCs, leading to a critical intracellular free-Ca2+ overload that promotes cell death.

Cx43 hemichannels and panx1 channels contribute to ethanol-induced astrocyte dysfunction and damage

BACKGROUND

Alcohol, a widely abused drug, significantly diminishes life quality, causing chronic diseases and psychiatric issues, with severe health, societal, and economic repercussions. Previously, we demonstrated that non-voluntary alcohol consumption increases the opening of Cx43 hemichannels and Panx1 channels in astrocytes from adolescent rats. However, whether ethanol directly affects astroglial hemichannels and, if so, how this impacts the function and survival of astrocytes remains to be elucidated.

RESULTS

Clinically relevant concentrations of ethanol boost the opening of Cx43 hemichannels and Panx1 channels in mouse cortical astrocytes, resulting in the release of ATP and glutamate. The activation of these large-pore channels is dependent on Toll-like receptor 4, P2X7 receptors, IL-1β and TNF-α signaling, p38 mitogen-activated protein kinase, and inducible nitric oxide (NO) synthase. Notably, the ethanol-induced opening of Cx43 hemichannels and Panx1 channels leads to alterations in cytokine secretion, NO production, gliotransmitter release, and astrocyte reactivity, ultimately impacting survival.

CONCLUSION

Our study reveals a new mechanism by which ethanol impairs astrocyte function, involving the sequential stimulation of inflammatory pathways that further increase the opening of Cx43 hemichannels and Panx1 channels. We hypothesize that targeting astroglial hemichannels could be a promising pharmacological approach to preserve astrocyte function and synaptic plasticity during the progression of various alcohol use disorders.

Evaluation of Connexin Hemichannel Activity In Vivo

Connexin hemichannels (Cx HCs) are hexameric structures at the cell plasma membrane, whose function as membrane transport proteins allows for the passive flow of small hydrophilic molecules and ions (≤1 kDa) between the cytosol and the extracellular environment. Activation of Cx HCs is highly dependent on pathological conditions. HC activity provokes changes in the microenvironment, inducing the dissemination of signaling molecules in both an autocrine and paracrine manner. Given the elicitation of a variety of signaling pathways, and assortment of Cx species and dispersion throughout the body, Cx HCs have been implicated in a range of processes such as cell proliferation, differentiation, cell death, and tissue modeling and remodeling. While studying the expression and localization of Cx HCs can be done using traditional laboratory techniques, such as immunoblot analysis, measuring the functionality/activity of the HCs requires a more explicit methodology and is essential for determining Cx-mediated physiological changes. The study of Cx HC function/activity has focused mainly on in vitro measurements through electrophysiological characterization or, more commonly, using HC-permeable dye uptake studies. Here, we describe the use of dye uptake to measure Cx HC activity in vivo using mechanically stimulated osteocytic Cx43 HCs with Evans blue dye as our model.

SARS-CoV-2 spike protein S1 activates Cx43 hemichannels and disturbs intracellular Ca2+ dynamics

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the ongoing coronavirus disease 2019 (COVID-19). An aspect of high uncertainty is whether the SARS-CoV-2 per se or the systemic inflammation induced by viral infection directly affects cellular function and survival in different tissues. It has been postulated that tissue dysfunction and damage observed in COVID-19 patients may rely on the direct effects of SARS-CoV-2 viral proteins. Previous evidence indicates that the human immunodeficiency virus and its envelope protein gp120 increase the activity of connexin 43 (Cx43) hemichannels with negative repercussions for cellular function and survival. Here, we evaluated whether the spike protein S1 of SARS-CoV-2 could impact the activity of Cx43 hemichannels.

RESULTS

We found that spike S1 time and dose-dependently increased the activity of Cx43 hemichannels in HeLa-Cx43 cells, as measured by dye uptake experiments. These responses were potentiated when the angiotensin-converting enzyme 2 (ACE2) was expressed in HeLa-Cx43 cells. Patch clamp experiments revealed that spike S1 increased unitary current events with conductances compatible with Cx43 hemichannels. In addition, Cx43 hemichannel opening evoked by spike S1 triggered the release of ATP and increased the [Ca2+]i dynamics elicited by ATP.

CONCLUSIONS

We hypothesize that Cx43 hemichannels could represent potential pharmacological targets for developing therapies to counteract SARS-CoV-2 infection and their long-term consequences.

Unnexin is a protein subunit of a large-pore channel expressed by unicellular organisms

Cells of vertebrate and invertebrate organisms express proteins specialized in membrane channel-based cell-cell communication that are absent in unicellular organisms. We recently described the prediction of some members of the large-pore channel family in kinetoplastids, consisting of proteins called unnexins, which share several structural features with innexin and pannexin proteins. Here, we demonstrated that the unnexin1 protein (Unx1) is delivered to the cell membrane, displaying a topology consisting of four transmembrane domains with C and N termini on the cytoplasmic side and form large-pore channels that are permeable to small molecules. Low extracellular Ca2+/Mg2+ levels or extracellular alkalinization, but not mechanical stretching, increases channel activity. The Unx1 channel mediates the influx of Ca2+ and does not form intercellular dye coupling between HeLa Unx1 transfected cells. Unx1 channel function was further evidenced by its ability to mediate ionic currents when expressed in Xenopus oocytes. Downregulation of Unx1 mRNA with morpholine contains Trypanosoma cruzi invasion. Phylogenetic analysis revealed the presence of Unx1 homologs in other protozoan parasites, suggesting a conserved function for these channel parasites in other protists. Our data demonstrate that Unx1 forms large-pore membrane channels, which may serve as a diffusional pathway for ions and small molecules that are likely to be metabolic substrates or waste products, and signaling autocrine and paracrine molecules that could be involved in cell invasion. As morpholinos-induced downregulation of Unx1 reduces the infectivity of trypomastigotes, the Unx1 channels might be an attractive target for developing trypanocide drugs.

Skeletal Muscle Atrophy Induced by Diabetes Is Mediated by Non-Selective Channels and Prevented by Boldine

Individuals with diabetes mellitus present a skeletal muscle myopathy characterized by atrophy. However, the mechanism underlying this muscular alteration remains elusive, which makes it difficult to design a rational treatment that could avoid the negative consequences in muscles due to diabetes. In the present work, the atrophy of skeletal myofibers from streptozotocin-induced diabetic rats was prevented with boldine, suggesting that non-selective channels inhibited by this alkaloid are involved in this process, as has previously shown for other muscular pathologies. Accordingly, we found a relevant increase in sarcolemma permeability of skeletal myofibers of diabetic animals in vivo and in vitro due to de novo expression of functional connexin hemichannels (Cx HCs) containing connexins (Cxs) 39, 43, and 45. These cells also expressed P2X7 receptors, and their inhibition in vitro drastically reduced sarcolemma permeability, suggesting their participation in the activation of Cx HCs. Notably, sarcolemma permeability of skeletal myofibers was prevented by boldine treatment that blocks Cx43 and Cx45 HCs, and now we demonstrated that it also blocks P2X7 receptors. In addition, the skeletal muscle alterations described above were not observed in diabetic mice with myofibers deficient in Cx43/Cx45 expression. Moreover, murine myofibers cultured for 24 h in high glucose presented a drastic increase in sarcolemma permeability and levels of NLRP3, a molecular member of the inflammasome, a response that was also prevented by boldine, suggesting that, in addition to the systemic inflammatory response found in diabetes, high glucose can promote the expression of functional Cx HCs and activation of the inflammasome in skeletal myofibers. Therefore, Cx43 and Cx45 HCs play a critical role in myofiber degeneration, and boldine could be considered a potential therapeutic agent to treat muscular complications due to diabetes.

Gap Junctional Coupling Between Retinal Astrocytes Exacerbates Neuronal Damage in Ischemia-Reperfusion Injury

Purpose

Retinal astrocytes abundantly express connexin 43 (Cx43), a transmembrane protein that forms gap junction (GJ) channels and unopposed hemichannels. While it is well established that Cx43 is upregulated in retinal injuries, it is unclear whether astrocytic Cx43 plays a role in retinal ganglion cell (RGC) loss associated with injury. Here, we investigated the effect of astrocyte-specific deletion of Cx43 (Cx43KO) and channel inhibitors on RGC loss in retinal ischemia/reperfusion (I/R) injury and assessed changes in expression and GJ channel and hemichannel function that occur in I/R injury. The effect of Cx43 deletion on neural function in the uninjured retina was also assessed.

Methods

Cx43 expression, astrocyte density and morphology, and RGC death in wild-type and Cx43KO mice after I/R injury were determined using immunohistochemistry and Western blotting. Visual function was assessed using ERG recordings. GJ coupling and hemichannel activity were evaluated using tracer coupling and uptake studies, respectively.

Results

Loss of RGCs in I/R injury was accompanied by an increase of Cx43 expression in astrocytes. Functional studies indicated that I/R injury augmented astrocytic GJ coupling but not Cx43 hemichannel activity. Importantly, deletion of astrocytic Cx43 improved neuronal survival in acute ischemia but did not affect RGC function in the absence of injury. In support, pharmacologic inhibition of GJ coupling provided neuroprotection in I/R injury.

Conclusions

The increase in Cx43 expression and GJ coupling during acute I/R injury exacerbates RGC loss. Inhibition of astrocytic Cx43 channels might represent a useful strategy to promote RGC survival in pathologic conditions.

A novel voltage-clamp/dye uptake assay reveals saturable transport of molecules through CALHM1 and connexin channels

Large-pore channels permeable to small molecules such as ATP, in addition to atomic ions, are emerging as important regulators in health and disease. Nonetheless, their mechanisms of molecular permeation and selectivity remain mostly unexplored. Combining fluorescence microscopy and electrophysiology, we developed a novel technique that allows kinetic analysis of molecular permeation through connexin and CALHM1 channels in Xenopus oocytes rendered translucent. Using this methodology, we found that (1) molecular flux through these channels saturates at low micromolar concentrations, (2) kinetic parameters of molecular transport are sensitive to modulators of channel gating, (3) molecular transport and ionic currents can be differentially affected by mutation and gating, and (4) N-terminal regions of these channels control transport kinetics and permselectivity. Our methodology allows analysis of how human disease-causing mutations affect kinetic properties and permselectivity of molecular signaling and enables the study of molecular mechanisms, including selectivity and saturability, of molecular transport in other large-pore channels.

A method for assessing ionic and molecular permeation in connexin hemichannels

Connexin hemichannels are permeable to both atomic ions and small molecules. Yet, they have different selectivity for ions and signaling molecules critical for biological functions. Activity of connexin hemichannels in living cells is commonly evaluated by methods that include electrophysiology and fluorescence-based approaches. Although less common, luminescence and radioactivity-based uptake/release assays have been also successfully used to determine selectivity and permeability to different molecules. The current methods, however, have important technical and quantitative limitations that make them unsuitable for simultaneously evaluating ionic and molecular permeability using different stimuli that control channel gating (e.g., voltage or extracellular Ca²⁺). To address this, we have recently designed a novel methodology that combines two-electrode voltage clamp (TEVC) and dye uptake assays in translucent Xenopus oocytes. This method allows for the evaluation of molecular transport kinetics in connexin hemichannels, and its utility can also be extended to other large pore channels, such as those formed by pannexin and CALHM. In this article, we describe step by step the protocol to perform the TEVC/Dye uptake assay.

Connexin Gap Junctions and Hemichannels Link Oxidative Stress to Skeletal Physiology and Pathology

PURPOSE OF REVIEW

The goal of this review is to provide an overview of the impact and underlying mechanism of oxidative stress on connexin channel function, and their roles in skeletal aging, estrogen deficiency, and glucocorticoid excess associated bone loss.

RECENT FINDINGS

Connexin hemichannel opening is increased under oxidative stress conditions, which confers a cell protective role against oxidative stress-induced cell death. Oxidative stress acts as a key contributor to aging, estrogen deficiency, and glucocorticoid excess-induced osteoporosis and impairs osteocytic network and connexin gap junction communication. This paper reviews the current knowledge for the role of oxidative stress and connexin channels in the pathogenesis of osteoporosis and physiological and pathological responses of connexin channels to oxidative stress. Oxidative stress decreases osteocyte viability and impairs the balance of anabolic and catabolic responses. Connexin 43 (Cx43) channels play a critical role in bone remodeling, mechanotransduction, and survival of osteocytes. Under oxidative stress conditions, there is a consistent reduction of Cx43 expression, while the opening of Cx43 hemichannels protects osteocytes against cell injury caused by oxidative stress.

Harnessing the therapeutic potential of antibodies targeting connexin hemichannels

BACKGROUND

Connexin hemichannels have been implicated in pathology-promoting conditions, including inflammation, numerous widespread human diseases, including cancer and diabetes, and several rare diseases linked to pathological point mutations.

METHODS

We analysed the literature focusing on antibodies capable of modulating hemichannel function, highlighting generation methods, applications to basic biomedical research and translational potential.

RESULTS

Anti-hemichannel antibodies generated over the past 3 decades targeted mostly connexin 43, with a focus on cancer treatment. A slow transition from relatively unselective polyclonal antibodies to more selective monoclonal antibodies resulted in few products with interesting characteristics that are under evaluation for clinical trials. Selection of antibodies from combinatorial phage-display libraries, has permitted to engineer a monoclonal antibody that binds to and blocks pathological hemichannels formed by connexin 26, 30 and 32.

CONCLUSIONS

All known antibodies that modulate connexin hemichannels target the two small extracellular loops of the connexin proteins. The extracellular region of different connexins is highly conserved, and few residues of each connexins are exposed. The search for new antibodies may develop an unprecedented potential for therapeutic applications, as it may benefit tremendously from novel whole-cell screening platforms that permit in situ selection of antibodies against membrane proteins in native state. The demonstrated efficacy of mAbs in reaching and modulating hemichannels in vivo, together with their relative specificity for connexins overlapping epitopes, should hopefully stimulate an interest for widening the scope of anti-hemichannel antibodies. There is no shortage of currently incurable diseases for which therapeutic intervention may benefit from anti-hemichannel antibodies capable of modulating hemichannel function selectively and specifically.

The role of connexin proteins and their channels in radiation-induced atherosclerosis

Radiotherapy is an effective treatment for breast cancer and other thoracic tumors. However, while high-energy radiotherapy treatment successfully kills cancer cells, radiation exposure of the heart and large arteries cannot always be avoided, resulting in secondary cardiovascular disease in cancer survivors. Radiation-induced changes in the cardiac vasculature may thereby lead to coronary artery atherosclerosis, which is a major cardiovascular complication nowadays in thoracic radiotherapy-treated patients. The underlying biological and molecular mechanisms of radiation-induced atherosclerosis are complex and still not fully understood, resulting in potentially improper radiation protection. Ionizing radiation (IR) exposure may damage the vascular endothelium by inducing DNA damage, oxidative stress, premature cellular senescence, cell death and inflammation, which act to promote the atherosclerotic process. Intercellular communication mediated by connexin (Cx)-based gap junctions and hemichannels may modulate IR-induced responses and thereby the atherosclerotic process. However, the role of endothelial Cxs and their channels in atherosclerotic development after IR exposure is still poorly defined. A better understanding of the underlying biological pathways involved in secondary cardiovascular toxicity after radiotherapy would facilitate the development of effective strategies that prevent or mitigate these adverse effects. Here, we review the possible roles of intercellular Cx driven signaling and communication in radiation-induced atherosclerosis.

Quantitative Automated Assays in Living Cells to Screen for Inhibitors of Hemichannel Function

In vertebrates, intercellular communication is largely mediated by connexins (Cx), a family of structurally related transmembrane proteins that assemble to form hemichannels (HCs) at the plasma membrane. HCs are upregulated in different brain disorders and represent innovative therapeutic targets. Identifying modulators of Cx-based HCs is of great interest to better understand their function and define new treatments. In this study, we developed automated versions of two different cell-based assays to identify new pharmacological modulators of Cx43-HCs. As HCs remain mostly closed under physiological conditions in cell culture, depletion of extracellular Ca²⁺ was used to increase the probability of opening of HCs. The first assay follows the incorporation of a fluorescent dye, Yo-Pro, by real-time imaging, while the second is based on the quenching of a fluorescent protein, YFP^QL, by iodide after iodide uptake. These assays were then used to screen a collection of 2242 approved drugs and compounds under development. This study led to the identification of 11 candidate hits blocking Cx43-HC, active in the two assays, with 5 drugs active on HC but not on gap junction (GJ) activities. To our knowledge, this is the first screening on HC activity and our results suggest the potential of a new use of already approved drugs in central nervous system disorders with HC impairments.

Apocynin Treatment Prevents Cardiac Connexin 43 Hemichannels Hyperactivity by Reducing Nitroso-Redox Stress in Mdx Mice

Duchenne muscular dystrophy (DMD) is a fatal disease that causes cardiomyopathy and is associated with oxidative stress. In the heart, oxidative stress interferes with the location of connexin 43 (Cx43) to the intercalated discs causing its lateralization to the plasma membrane where Cx43 forms hemichannels. We tested the hypothesis that in DMD cardiomyopathy, increased oxidative stress is associated with the formation and activation of Cx43 hemichannels. For this, we used mdx mice as a DMD model and evaluated cardiac function, nitroso-redox changes and Cx43 hemichannels permeability. Mdx hearts presented increased NADPH oxidase-derived oxidative stress and increased Cx43 S-nitrosylation compared to controls. These redox changes were associated with increased Cx43 lateralization, decreased cardiac contractility and increased arrhythmic events. Pharmacological inhibition of NADPH oxidase using apocynin (one month) reduced systemic oxidative stress and reversed the aforementioned changes towards normal, except Cx43 lateralization. Opening of Cx43 hemichannels was blocked by apocynin treatment and by acute hemichannel blockade with carbenoxolone. NADPH oxidase inhibition also prevented the occurrence of apoptosis in mdx hearts and reversed the ventricular remodeling. These results show that NADPH oxidase activity in DMD is associated with S-nitrosylation and opening of Cx43 hemichannels. These changes lead to apoptosis and cardiac dysfunction and were prevented by NADPH oxidase inhibition.

Vitamin E Blocks Connexin Hemichannels and Prevents Deleterious Effects of Glucocorticoid Treatment on Skeletal Muscles

Glucocorticoids are frequently used as anti-inflammatory and immunosuppressive agents. However, high doses and/or prolonged use induce undesired secondary effects such as muscular atrophy. Recently, de novo expression of connexin43 and connexin45 hemichannels (Cx43 HCs and Cx45 HCs, respectively) has been proposed to play a critical role in the mechanism underlying myofiber atrophy induced by dexamethasone (Dex: a synthetic glucocorticoid), but their involvement in specific muscle changes promoted by Dex remains poorly understood. Moreover, treatments that could prevent the undesired effects of glucocorticoids on skeletal muscles remain unknown. In the present work, a 7-day Dex treatment in adult mice was found to induce weight loss and skeletal muscle changes including expression of functional Cx43/Cx45 HCs, elevated atrogin immunoreactivity, atrophy, oxidative stress and mitochondrial dysfunction. All these undesired effects were absent in muscles of mice simultaneously treated with Dex and vitamin E (VitE). Moreover, VitE was found to rapidly inhibit the activity of Cx HCs in freshly isolated myofibers of Dex treated mice. Exposure to alkaline pH induced free radical generation only in HeLa cells expressing Cx43 or Cx45 where Ca²⁺ was present in the extracellular milieu, response that was prevented by VitE. Besides, VitE and two other anti-oxidant compounds, Tempol and Resveratrol, were found to inhibit Cx43 HCs in HeLa cells transfectants. Thus, we propose that in addition to their intrinsic anti-oxidant potency, some antioxidants could be used to reduce expression and/or opening of Cx HCs and consequently reduce the undesired effect of glucocorticoids on skeletal muscles.

Permeation of Molecules through Astroglial Connexin 43 Hemichannels Is Modulated by Cytokines with Parameters Depending on the Permeant Species

Recent studies indicate that connexin hemichannels do not act as freely permeable non-selective pores, but they select permeants in an isoform-specific manner with cooperative, competitive and saturable kinetics. The aim of this study was to investigate whether the treatment with a mixture of IL-1β plus TNF-α, a well-known pro-inflammatory condition that activates astroglial connexin 43 (Cx43) hemichannels, could alter their permeability to molecules. We found that IL-1β plus TNF-α left-shifted the dye uptake rate vs. dye concentration relationship for Etd and 2-NBDG, but the opposite took place for DAPI or YO-PRO-1, whereas no alterations were observed for Prd. The latter modifications were accompanied of changes in K_d (Etd, DAPI, YO-PRO-1 or 2-NBDG) and Hill coefficients (Etd and YO-PRO-1), but not in alterations of V_max. We speculate that IL-1β plus TNF-α may distinctively affect the binding sites to permeants in astroglial Cx43 hemichannels rather than their number in the cell surface. Alternatively, IL-1β plus TNF-α could induce the production of endogenous permeants that may favor or compete for in the pore-lining residues of Cx43 hemichannels. Future studies shall elucidate whether the differential ionic/molecule permeation of Cx43 hemichannels in astrocytes could impact their communication with neurons in the normal and inflamed nervous system.

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.

Connexin43 Hemichannel Targeting With TAT-Gap19 Alleviates Radiation-Induced Endothelial Cell Damage

BACKGROUND

Emerging evidence indicates an excess risk of late occurring cardiovascular diseases, especially atherosclerosis, after thoracic cancer radiotherapy. Ionizing radiation (IR) induces cellular effects which may induce endothelial cell dysfunction, an early marker for atherosclerosis. In addition, intercellular communication through channels composed of transmembrane connexin proteins (Cxs), i.e. Gap junctions (direct cell-cell coupling) and hemichannels (paracrine release/uptake pathway) can modulate radiation-induced responses and therefore the atherosclerotic process. However, the role of endothelial hemichannel in IR-induced atherosclerosis has never been described before.

MATERIALS AND METHODS

Telomerase-immortalized human Coronary Artery/Microvascular Endothelial cells (TICAE/TIME) were exposed to X-rays (0.1 and 5 Gy). Production of reactive oxygen species (ROS), DNA damage, cell death, inflammatory responses, and senescence were assessed with or without applying a Cx43 hemichannel blocker (TAT-Gap19).

RESULTS

We report here that IR induces an increase in oxidative stress, cell death, inflammatory responses (IL-8, IL-1β, VCAM-1, MCP-1, and Endothelin-1) and premature cellular senescence in TICAE and TIME cells. These effects are significantly reduced in the presence of the Cx43 hemichannel-targeting peptide TAT-Gap19.

CONCLUSION

Our findings suggest that endothelial Cx43 hemichannels contribute to various IR-induced processes, such as ROS, cell death, inflammation, and senescence, resulting in an increase in endothelial cell damage, which could be protected by blocking these hemichannels. Thus, targeting Cx43 hemichannels may potentially exert radioprotective effects.

Structural determinants underlying permeant discrimination of the Cx43 hemichannel

Connexin (Cx) gap junction channels comprise two hemichannels in neighboring cells, and their permeability is well-described, but permeabilities of the single Cx hemichannel remain largely unresolved. Moreover, determination of isoform-specific Cx hemichannel permeability is challenging because of concurrent expression of other channels with similar permeability profiles and inhibitor sensitivities. The mammalian Cx hemichannels Cx30 and Cx43 are gated by extracellular divalent cations, removal of which promotes fluorescent dye uptake in both channels but atomic ion conductance only through Cx30. To determine the molecular determinants of this difference, here we employed chimeras and mutagenesis of predicted pore-lining residues in Cx43. We expressed the mutated channels in Xenopus laevis oocytes to avoid background activity of alternative channels. Oocytes expressing a Cx43 hemichannel chimera containing the N terminus or the first extracellular loop from Cx30 displayed ethidium uptake and, unlike WT Cx43, ion conduction, an observation further supported by molecular dynamics simulations. Additional C-terminal truncation of the chimeric Cx43 hemichannel elicited an even greater ion conductance with a magnitude closer to that of Cx30. The inhibitory profile for the connexin hemichannels depended on the permeant, with conventional connexin hemichannel inhibitors having a higher potency toward the ion conductance pathway than toward fluorescent dye uptake. Our results demonstrate a permeant-dependent, isoform-specific inhibition of connexin hemichannels. They further reveal that the outer segments of the pore-lining region, including the N terminus and the first extracellular loop, together with the C terminus preclude ion conductance of the open Cx43 hemichannel.

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.

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.

ATP promotes the fast migration of dendritic cells through the activity of pannexin 1 channels and P2X7 receptors

Upon its release from injured cells, such as infected, transformed, inflamed, or necrotic cells, extracellular adenosine-5'-triphosphate (ATP) acts as a danger signal that recruits phagocytes, such as neutrophils, macrophages, and dendritic cells (DCs), to the site of injury. The sensing of extracellular ATP occurs through purinergic (P2) receptors. We investigated the cellular mechanisms linking purinergic signaling to DC motility. We found that ATP stimulated fast DC motility through an autocrine signaling loop, which was initiated by the activation of P2X₇ receptors and further amplified by pannexin 1 (Panx1) channels. Upon stimulation of the P2X₇ receptor by ATP, Panx1 contributed to fast DC motility by increasing the permeability of the plasma membrane, which resulted in supplementary ATP release. In the absence of Panx1, DCs failed to increase their speed of migration in response to ATP, despite exhibiting a normal P2X₇ receptor-mediated Ca²⁺ response. In addition to DC migration, Panx1 channel- and P2X₇ receptor-dependent signaling was further required to stimulate the reorganization of the actin cytoskeleton. In vivo, functional Panx1 channels were required for the homing of DCs to lymph nodes, although they were dispensable for DC maturation. These data suggest that P2X₇ receptors and Panx1 channels are crucial players in the regulation of DC migration to endogenous danger signals.

Permeant-specific gating of connexin 30 hemichannels

Gap junctions confer interconnectivity of the cytoplasm in neighboring cells via docking of two connexons expressed in each of the adjacent membranes. Undocked connexons, referred to as hemichannels, may open and connect the cytoplasm with the extracellular fluid. The hemichannel configuration of connexins (Cxs) displays isoform-specific permeability profiles that are not directly determined by the size and charge of the permeant. To further explore Ca²⁺-mediated gating and permeability features of connexin hemichannels, we heterologously expressed Cx30 hemichannels in Xenopus laevis oocytes. The sensitivity toward divalent cation-mediated gating differed between small atomic ions (current) and fluorescent dye permeants, indicating that these permeants are distinctly gated. Three aspartate residues in Cx30 (Asp-50, Asp-172, and Asp-179) have been implicated previously in the Ca²⁺ sensitivity of other hemichannel isoforms. Although the aspartate at position Asp-50 was indispensable for divalent cation-dependent gating of Cx30 hemichannels, substitutions of the two other residues had no significant effect on gating, illustrating differences in the gating mechanisms between connexin isoforms. Using the substituted cysteine accessibility method (SCAM), we evaluated the role of possible pore-lining residues in the permeation of ions and ethidium through Cx30 hemichannels. Of the cysteine-substituted residues, interaction of a proposed pore-lining cysteine at position 37 with the positively charged compound [2-(trimethylammonium)ethyl] methane thiosulfonate bromide (MTS-ET) increased Cx30-mediated currents with unperturbed ethidium permeability. In summary, our results demonstrate that the permeability of hemichannels is regulated in a permeant-specific manner and underscores that hemichannels are selective rather than non-discriminating and freely diffusable pores.

On Biophysical Properties and Sensitivity to Gap Junction Blockers of Connexin 39 Hemichannels Expressed in HeLa Cells

Although connexins (Cxs) are broadly expressed by cells of mammalian organisms, Cx39 has a very restricted pattern of expression and the biophysical properties of Cx39-based channels [hemichannels (HCs) and gap junction channels (GJCs)] remain largely unknown. Here, we used HeLa cells transfected with Cx39 (HeLa-Cx39 cells) in which intercellular electrical coupling was not detected, indicating the absence of GJCs. However, functional HCs were found on the surface of cells exposed to conditions known to increase the open probability of other Cx HCs (e.g., extracellular divalent cationic-free solution (DCFS), extracellular alkaline pH, mechanical stimulus and depolarization to positive membrane potentials). Cx39 HCs were blocked by some traditional Cx HC blockers, but not by others or a pannexin1 channel blocker. HeLa-Cx39 cells showed similar resting membrane potentials (RMPs) to those of parental cells, and exposure to DCFS reduced RMPs in Cx39 transfectants, but not in parental cells. Under these conditions, unitary events of ~75 pS were frequent in HeLa-Cx39 cells and absent in parental cells. Real-time cellular uptake experiments of dyes with different physicochemical features, as well as the application of a machine-learning approach revealed that Cx39 HCs are preferentially permeable to molecules characterized by six categories of descriptors, namely: (1) electronegativity, (2) ionization potential, (3) polarizability, (4) size and geometry, (5) topological flexibility and (6) valence. However, Cx39 HCs opened by mechanical stimulation or alkaline pH were impermeable to Ca²⁺. Molecular modeling of Cx39-based channels suggest that a constriction present at the intracellular portion of the para helix region co-localizes with an electronegative patch, imposing an energetic and steric barrier, which in the case of GJCs may hinder channel function. Results reported here demonstrate that Cx39 form HCs and add to our understanding of the functional roles of Cx39 HCs under physiological and pathological conditions in cells that express them.

Mathematical model of LsrR-binding and derepression in Escherichia coli K12

Quorum sensing (QS) enables bacterial communication and collective behavior in response to self-secreted signaling molecules. Unlocking its genetic regulation will provide insight towards understanding its influence on pathogenesis, formation of biofilms, and many other phenotypes. There are few datasets available that link QS-mediated gene expression to its regulatory components and even fewer mathematical models that incorporate known mechanistic detail. By integrating these data with annotated sequence information, mathematical inferences can be pieced together that shed light on regulatory structure. A first principles model, developed here for the E. coli QS system, builds on known mechanistic detail and is used to develop a working model of LuxS-regulated (Lsr) activity. That is, our model is meant to discriminate among hypothetical mechanisms governing lsr transcriptional regulation. Our simulations are in qualitative agreement with experimentally observed data. Importantly, our results point to the importance of transcriptional regulator, LsrR, cycling on genetic control. We also found several experimental observations in E. coli and homologous systems that were not explained by current mechanistic understanding. For example, by comparing simulations with reports of the integrating host factor in Aggrigatibacter actinomycetemcomitans, we conclude that additional transcriptional components are likely involved. An iterative process of simulation and experiment, therefore, is needed to inform new experiments and incorporate new model detail, the benefit of which will more rapidly validate mechanistic understanding.

Fast skeletal myofibers of mdx mouse, model of Duchenne muscular dystrophy, express connexin hemichannels that lead to apoptosis

Skeletal muscles of patients with Duchenne muscular dystrophy (DMD) show numerous alterations including inflammation, apoptosis, and necrosis of myofibers. However, the molecular mechanism that explains these changes remains largely unknown. Here, the involvement of hemichannels formed by connexins (Cx HCs) was evaluated in skeletal muscle of mdx mouse model of DMD. Fast myofibers of mdx mice were found to express three connexins (39, 43 and 45) and high sarcolemma permeability, which was absent in myofibers of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice (deficient in skeletal muscle Cx43/Cx45 expression). These myofibers did not show elevated basal intracellular free Ca(2+) levels, immunoreactivity to phosphorylated p65 (active NF-κB), eNOS and annexin V/active Caspase 3 (marker of apoptosis) but presented dystrophin immunoreactivity. Moreover, muscles of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice exhibited partial decrease of necrotic features (big cells and high creatine kinase levels). Accordingly, these muscles showed similar macrophage infiltration as control mdx muscles. Nonetheless, the hanging test performance of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice was significantly better than that of control mdx Cx43(fl/fl)Cx45(fl/fl) mice. All three Cxs found in skeletal muscles of mdx mice were also detected in fast myofibers of biopsy specimens from patients with muscular dystrophy. Thus, reduction of Cx expression and/or function of Cx HCs may be potential therapeutic approaches to abrogate myofiber apoptosis in DMD.

Integrin-mediated transactivation of P2X7R via hemichannel-dependent ATP release stimulates astrocyte migration

Our previous reports indicate that ligand-induced αVβ3 integrin and Syndecan-4 engagement increases focal adhesion formation and migration of astrocytes. Additionally, ligated integrins trigger ATP release through unknown mechanisms, activating P2X7 receptors (P2X7R), and the uptake of Ca(2+) to promote cell adhesion. However, whether the activation of P2X7R and ATP release are required for astrocyte migration and whether αVβ3 integrin and Syndecan-4 receptors communicate with P2X7R via ATP remains unknown. Here, cells were stimulated with Thy-1, a reported αVβ3 integrin and Syndecan-4 ligand. Results obtained indicate that ATP was released by Thy-1 upon integrin engagement and required the participation of phosphatidylinositol-3-kinase (PI3K), phospholipase-C gamma (PLCγ) and inositol trisphosphate (IP3) receptors (IP3R). IP3R activation leads to increased intracellular Ca(2+), hemichannel (Connexin-43 and Pannexin-1) opening, and ATP release. Moreover, silencing of the P2X7R or addition of hemichannel blockers precluded Thy-1-induced astrocyte migration. Finally, Thy-1 lacking the integrin-binding site did not stimulate ATP release, whereas Thy-1 mutated in the Syndecan-4-binding domain increased ATP release, albeit to a lesser extent and with delayed kinetics compared to wild-type Thy-1. Thus, hemichannels activated downstream of an αVβ3 integrin-PI3K-PLCγ-IP3R pathway are responsible for Thy-1-induced, hemichannel-mediated and Syndecan-4-modulated ATP release that transactivates P2X7Rs to induce Ca(2+) entry. These findings uncover a hitherto unrecognized role for hemichannels in the regulation of astrocyte migration via P2X7R transactivation induced by integrin-mediated ATP release.

Protein kinase C-dependent regulation of connexin43 gap junctions and hemichannels

Connexin43 (Cx43) generates intercellular gap junction channels involved in, among others, cardiac and brain function. Gap junctions are formed by the docking of two hemichannels from neighbouring cells. Undocked Cx43 hemichannels can upon different stimuli open towards the extracellular matrix and allow transport of molecules such as fluorescent dyes and ATP. A range of phosphorylated amino acids have been detected in the C-terminus of Cx43 and their physiological role has been intensively studied both in the gap junctional form of Cx43 and in its hemichannel configuration. We present the current knowledge of protein kinase C (PKC)-dependent regulation of Cx43 and discuss the divergent results.

Linoleic acid permeabilizes gastric epithelial cells by increasing connexin 43 levels in the cell membrane via a GPR40- and Akt-dependent mechanism

Linoleic acid (LA) is known to activate G-protein coupled receptors and connexin hemichannels (Cx HCs) but possible interlinks between these two responses remain unexplored. Here, we evaluated the mechanism of action of LA on the membrane permeability mediated by Cx HCs in MKN28 cells. These cells were found to express connexins, GPR40, GPR120, and CD36 receptors. The Cx HC activity of these cells increased after 5 min of treatment with LA or GW9508, an agonist of GPR40/GPR120; or exposure to extracellular divalent cation-free solution (DCFS), known to increase the open probability of Cx HCs, yields an immediate increase in Cx HC activity of similar intensity and additive with LA-induced change. Treatment with a CD36 blocker or transfection with siRNA-GPR120 maintains the LA-induced Cx HC activity. However, cells transfected with siRNA-GPR40 did not show LA-induced Cx HC activity but activity was increased upon exposure to DCFS, confirming the presence of activatable Cx HCs in the cell membrane. Treatment with AKTi (Akt inhibitor) abrogated the LA-induced Cx HC activity. In HeLa cells transfected with Cx43 (HeLa-Cx43), LA induced phosphorylation of surface Cx43 at serine 373 (S373), site for Akt phosphorylation. HeLa-Cx43 but not HeLa-Cx43 cells with a S373A mutation showed a LA-induced Cx HC activity directly related to an increase in cell surface Cx43 levels. Thus, the increase in membrane permeability induced by LA is mediated by an intracellular signaling pathway activated by GPR40 that leads to an increase in membrane levels of Cx43 phosphorylated at serine 373 via Akt.

Isoform-specific phosphorylation-dependent regulation of connexin hemichannels

Connexins form gap junction channels made up of two connexons (hemichannels) from adjacent cells. Unopposed hemichannels may open toward the extracellular space upon stimulation by, e.g., removal of divalent cations from the extracellular solution and allow isoform-specific transmembrane flux of fluorescent dyes and physiologically relevant molecules, such as ATP and ions. Connexin (Cx)43 and Cx30 are the major astrocytic connexins. Protein kinase C (PKC) regulates Cx43 in its cell-cell gap junction configuration and may also act to keep Cx43 hemichannels closed. In contrast, the regulation of Cx30 hemichannels by PKC is unexplored. To determine phosphorylation-dependent regulation of Cx30 and Cx43 hemichannels, these were heterologously expressed in Xenopus laevis oocytes and opened with divalent cation-free solution. Inhibition of PKC activity did not affect hemichannel opening of either connexin. PKC activation had no effect on Cx43-mediated hemichannel activity, whereas both dye uptake and current through Cx30 hemichannels were reduced. We detected no PKC-induced connexin internalization from the plasma membrane, indicating that PKC reduced Cx30 hemichannel activity by channel closure. In an attempt to resolve the PKC phosphorylation site(s) on Cx30, alanine mutations of putative cytoplasmic PKC consensus sites were created to prevent phosphorylation (T5A, T8A, T102A, S222A, S225A, S239A, and S258A). These Cx30 mutants responded to PKC activation, suggesting that Cx30 hemichannels are not regulated by phosphorylation of a single site. In conclusion, Cx30, but not Cx43, hemichannels close upon PKC activation, illustrating that connexin hemichannels display not only isoform-specific permeability profiles but also isoform-specific regulation by PKC.

Analysis of connexin expression during seizures induced by 4-aminopyridine in the rat hippocampus

Background

In epilepsy, seizures are generated by abnormal synchronous activity in neurons. In the rat hippocampus (HIP), epileptiform activity has been found to be associated with gap junctions (GJs). GJs are formed by the combination of two hemichannels, each composed of six connexins. At low doses, the convulsive drug 4-aminopyridine (4-AP) produces epileptiform activity without affecting glutamate levels; therefore, GJs could participate in its effect. Based on this argument, in this study, the expression of Cx 32, Cx 36 and Cx 43 protein and mRNA in the HIP of rats treated with 4-AP was evaluated. The evaluation of connexins was carried out by chemifluorescent immunoassay, semiquantitative RT-PCR and immunofluorescence to detect the amount and distribution of connexins and of cellular markers in the HIP and dentate gyrus (DG) of animals treated with NaCl and 4-AP in the right entorhinal cortex. In these animals, convulsive behavior and EEG signals were analyzed.

Results

The animals treated with 4-AP showed convulsive behavior and epileptiform activity 60 min after the administration. A significant increase in the protein expression of Cx 32, Cx 36 and Cx 43 was found in the HIP contralateral and ipsilateral to the site of 4-AP administration. A trend toward an increase in the mRNA of Cx 32 and Cx 43 was also found. An increase in the cellular density of Cx 32 and Cx 43 was found in the right HIP and DG, and an increase in the cellular density of oligodendrocytes in the DG and a decrease in the number of cells marked with NeuN were observed in the left HIP.

Conclusions

Cx 32 and Cx 43 associated with oligodendrocytes and astrocytes had an important role in the first stages of seizures induced by 4-AP, whereas Cx36 localized to neurons could be associated with later stages. Additionally, these results contribute to our understanding of the role of connexins in acute seizures and allow us to direct our efforts to other new anticonvulsant strategies for seizure treatment.

Activation, permeability, and inhibition of astrocytic and neuronal large pore (hemi)channels

Astrocytes and neurons express several large pore (hemi)channels that may open in response to various stimuli, allowing fluorescent dyes, ions, and cytoplasmic molecules such as ATP and glutamate to permeate. Several of these large pore (hemi)channels have similar characteristics with regard to activation, permeability, and inhibitor sensitivity. Consequently, their behaviors and roles in astrocytic and neuronal (patho)physiology remain undefined. We took advantage of the Xenopus laevis expression system to determine the individual characteristics of several large pore channels in isolation. Expression of connexins Cx26, Cx30, Cx36, or Cx43, the pannexins Px1 or Px2, or the purinergic receptor P2X7 yielded functional (hemi)channels with isoform-specific characteristics. Connexin hemichannels had distinct sensitivity to alterations of extracellular Ca(2+) and their permeability to dyes and small atomic ions (conductance) were not proportional. Px1 and Px2 exhibited conductance at positive membrane potentials, but only Px1 displayed detectable fluorescent dye uptake. P2X7, in the absence of Px1, was permeable to fluorescent dyes in an agonist-dependent manner. The large pore channels displayed overlapping sensitivity to the inhibitors Brilliant Blue, gadolinium, and carbenoxolone. These results demonstrated isoform-specific characteristics among the large pore membrane channels; an open (hemi)channel is not a nonselective channel. With these isoform-specific properties in mind, we characterized the divalent cation-sensitive permeation pathway in primary cultured astrocytes. We observed no activation of membrane conductance or Cx43-mediated dye uptake in astrocytes nor in Cx43-expressing C6 cells. Our data underscore that although Cx43-mediated transport is observed in overexpressing cell systems, such transport may not be detectable in native cells under comparable experimental conditions.

Roles of gap junctions, connexins, and pannexins in epilepsy

Enhanced gap junctional communication (GJC) between neurons is considered a major factor underlying the neuronal synchrony driving seizure activity. In addition, the hippocampal sharp wave ripple complexes, associated with learning and seizures, are diminished by GJC blocking agents. Although gap junctional blocking drugs inhibit experimental seizures, they all have other non-specific actions. Besides interneuronal GJC between dendrites, inter-axonal and inter-glial GJC is also considered important for seizure generation. Interestingly, in most studies of cerebral tissue from animal seizure models and from human patients with epilepsy, there is up-regulation of glial, but not neuronal gap junctional mRNA and protein. Significant changes in the expression and post-translational modification of the astrocytic connexin Cx43, and Panx1 were observed in an in vitro Co⁺⁺ seizure model, further supporting a role for glia in seizure-genesis, although the reasons for this remain unclear. Further suggesting an involvement of astrocytic GJC in epilepsy, is the fact that the expression of astrocytic Cx mRNAs (Cxs 30 and 43) is several fold higher than that of neuronal Cx mRNAs (Cxs 36 and 45), and the number of glial cells outnumber neuronal cells in mammalian hippocampal and cortical tissue. Pannexin expression is also increased in both animal and human epileptic tissues. Specific Cx43 mimetic peptides, Gap 27 and SLS, inhibit the docking of astrocytic connexin Cx43 proteins from forming intercellular gap junctions (GJs), diminishing spontaneous seizures. Besides GJs, Cx membrane hemichannels in glia and Panx membrane channels in neurons and glia are also inhibited by traditional gap junctional pharmacological blockers. Although there is no doubt that connexin-based GJs and hemichannels, and pannexin-based membrane channels are related to epilepsy, the specific details of how they are involved and how we can modulate their function for therapeutic purposes remain to be elucidated.

The dual face of connexin-based astroglial Ca(2+) communication: a key player in brain physiology and a prime target in pathology

For decades, studies have been focusing on the neuronal abnormalities that accompany neurodegenerative disorders. Yet, glial cells are emerging as important players in numerous neurological diseases. Astrocytes, the main type of glia in the central nervous system , form extensive networks that physically and functionally connect neuronal synapses with cerebral blood vessels. Normal brain functioning strictly depends on highly specialized cellular cross-talk between these different partners to which Ca(2+), as a signaling ion, largely contributes. Altered intracellular Ca(2+) levels are associated with neurodegenerative disorders and play a crucial role in the glial responses to injury. Intracellular Ca(2+) increases in single astrocytes can be propagated toward neighboring cells as intercellular Ca(2+) waves, thereby recruiting a larger group of cells. Intercellular Ca(2+) wave propagation depends on two, parallel, connexin (Cx) channel-based mechanisms: i) the diffusion of inositol 1,4,5-trisphosphate through gap junction channels that directly connect the cytoplasm of neighboring cells, and ii) the release of paracrine messengers such as glutamate and ATP through hemichannels ('half of a gap junction channel'). This review gives an overview of the current knowledge on Cx-mediated Ca(2+) communication among astrocytes as well as between astrocytes and other brain cell types in physiology and pathology, with a focus on the processes of neurodegeneration and reactive gliosis. Research on Cx-mediated astroglial Ca(2+) communication may ultimately shed light on the development of targeted therapies for neurodegenerative disorders in which astrocytes participate. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Hunting for connexin hemichannels

Connexin hemichannels (connexons) are building blocks of gap junctions but also function as free unapposed channels, which has become an active field of research. Defining functions of hemichannels and their involvement in any biological event requires ruling out possible participation of other channels that share biophysical and regulatory properties, for example pannexins, CALHM1 and P2X receptors. The lack of specific inhibitors for these channels has become an obstacle in elucidating the role of connexin hemichannels. Several experimental approaches are now available to identify hemichannels at the cell surface and to characterize their electrophysiological, permeability and regulatory properties. The use of connexin knockout/knockdown, and the development of peptides that target intracellular connexin domains and specific antibodies directed to extracellular domains have helped to dissect the role of hemichannels in endogenously expressing systems. Moreover, studies of connexin mutants in exogenous expression systems have provided convincing evidence on hemichannels in the pathogenesis of several human genetic diseases. We here present a brief overview of connexin hemichannels as functional channels and itemize a list of aspects to consider when concluding on their involvement.

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.

Distinct permeation profiles of the connexin 30 and 43 hemichannels

Connexin 43 (Cx43) hemichannels may form open channels in the plasma membrane when exposed to specific stimuli, e.g. reduced extracellular concentration of divalent cations, and allow passage of fluorescent molecules and presumably a range of smaller physiologically relevant molecules. However, the permeability profile of Cx43 hemichannels remains unresolved. Exposure of Cx43-expressing Xenopus laevis oocytes to divalent cation free solution induced a gadolinium-sensitive uptake of the fluorescent dye ethidium. In spite thereof, a range of biological molecules smaller than ethidium, such as glutamate, lactate, and glucose, did not permeate the pore whereas ATP did. In contrast, permeability of glutamate, glucose and ATP was observed in oocytes expressing Cx30. Exposure to divalent cation free solutions induced a robust membrane conductance in Cx30-expressing oocytes but none in Cx43-expressing oocytes. C-terminally truncated Cx43 (M257) displayed increased dye uptake and, unlike wild type Cx43 channels, conducted current. Neither Cx30 nor Cx43 acted as water channels in their hemichannel configuration. Our results demonstrate that connexin hemichannels have isoform-specific permeability profiles and that dye uptake cannot be equaled to permeability of smaller physiologically relevant molecules in given settings.

HIV increases the release of dickkopf-1 protein from human astrocytes by a Cx43 hemichannel-dependent mechanism

Human immunodeficiency virus-1 (HIV) is a public health issue and a major complication of the disease is NeuroAIDS. In vivo, microglia/macrophages are the main cells infected. However, a low but significant number of HIV-infected astrocytes has also been detected, but their role in the pathogenesis of NeuroAIDS is not well understood. Our previous data indicate that gap junction channels amplify toxicity from few HIV-infected into uninfected astrocytes. Now, we demonstrated that HIV infection of astrocytes results in the opening of connexin43 hemichannels (HCs). HIV-induced opening of connexin43 HCs resulted in dysregulated secretion of dickkopf-1 protein (DKK1, a soluble wnt pathway inhibitor). Treatment of mixed cultures of neurons and astrocytes with DKK1, in the absence of HIV infection, resulted in the collapse of neuronal processes. HIV infection of mixed cultures of human neurons and astrocytes also resulted in the collapse of neuronal processes through a DKK1-dependent mechanism. In addition, dysregulated DKK1 expression in astrocytes was observed in human brain tissue sections of individuals with HIV encephalitis as compared to tissue sections from uninfected individuals. Thus, we demonstrated that HIV infection of astrocytes induces dysregulation of DKK1 by a HC-dependent mechanism that contributes to the brain pathogenesis observed in HIV-infected individuals. Our studies demonstrated that HIV infection of astrocytes, despite minimal replication and a low number of infected cells, induces dysregulation of DKK1 secretion by a Cx43 hemichannel (HC)-dependent mechanism. Enhanced DKK1 secretion in response to HIV infection of glial cells compromised formation and stability of neuronal processes, similar to the synaptic compromise observed in HIV-infected individuals. In addition, analysis of human brain tissue sections obtained from encephalitic individuals also shows enhanced expression of DKK1 in astrocytes. Our data provide a novel mechanism by which HIV infection of glial cells participate in the pathogenesis of brain dysfunction observed in HIV-infected individuals. LRP5 = Low-density lipoprotein receptor-related protein 5.

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.

Connexin- and pannexin-based channels in normal skeletal muscles and their possible role in muscle atrophy

Precursor cells of skeletal muscles express connexins 39, 43 and 45 and pannexin1. In these cells, most connexins form two types of membrane channels, gap junction channels and hemichannels, whereas pannexin1 forms only hemichannels. All these channels are low-resistance pathways permeable to ions and small molecules that coordinate developmental events. During late stages of skeletal muscle differentiation, myofibers become innervated and stop expressing connexins but still express pannexin1 hemichannels that are potential pathways for the ATP release required for potentiation of the contraction response. Adult injured muscles undergo regeneration, and connexins are reexpressed and form membrane channels. In vivo, connexin reexpression occurs in undifferentiated cells that form new myofibers, favoring the healing process of injured muscle. However, differentiated myofibers maintained in culture for 48 h or treated with proinflammatory cytokines for less than 3 h also reexpress connexins and only form functional hemichannels at the cell surface. We propose that opening of these hemichannels contributes to drastic changes in electrochemical gradients, including reduction of membrane potential, increases in intracellular free Ca(2+) concentration and release of diverse metabolites (e.g., NAD(+) and ATP) to the extracellular milieu, contributing to multiple metabolic and physiologic alterations that characterize muscles undergoing atrophy in several acquired and genetic human diseases. Consequently, inhibition of connexin hemichannels expressed by injured or denervated skeletal muscles might reduce or prevent deleterious changes triggered by conditions that promote muscle atrophy.

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.

Glucose increases intracellular free Ca(2+) in tanycytes via ATP released through connexin 43 hemichannels

The ventromedial hypothalamus is involved in regulating feeding and satiety behavior, and its neurons interact with specialized ependymal-glial cells, termed tanycytes. The latter express glucose-sensing proteins, including glucose transporter 2, glucokinase, and ATP-sensitive K(+) (K(ATP) ) channels, suggesting their involvement in hypothalamic glucosensing. Here, the transduction mechanism involved in the glucose-induced rise of intracellular free Ca(2+) concentration ([Ca(2+) ](i) ) in cultured β-tanycytes was examined. Fura-2AM time-lapse fluorescence images revealed that glucose increases the intracellular Ca(2+) signal in a concentration-dependent manner. Glucose transportation, primarily via glucose transporters, and metabolism via anaerobic glycolysis increased connexin 43 (Cx43) hemichannel activity, evaluated by ethidium uptake and whole cell patch clamp recordings, through a K(ATP) channel-dependent pathway. Consequently, ATP export to the extracellular milieu was enhanced, resulting in activation of purinergic P2Y(1) receptors followed by inositol trisphosphate receptor activation and Ca(2+) release from intracellular stores. The present study identifies the mechanism by which glucose increases [Ca(2+) ](i) in tanycytes. It also establishes that Cx43 hemichannels can be rapidly activated under physiological conditions by the sequential activation of glucosensing proteins in normal tanycytes.

Connexin and pannexin as modulators of myocardial injury

Multicellular organisms have developed a variety of mechanisms that allow communication between their cells. Whereas some of these systems, as neurotransmission or hormones, make possible communication between remote areas, direct cell-to-cell communication through specific membrane channels keep in contact neighboring cells. Direct communication between the cytoplasm of adjacent cells is achieved in vertebrates by membrane channels formed by connexins. However, in addition to allowing exchange of ions and small metabolites between the cytoplasms of adjacent cells, connexin channels also communicate the cytosol with the extracellular space, thus enabling a completely different communication system, involving activation of extracellular receptors. Recently, the demonstration of connexin at the inner mitochondrial membrane of cardiomyocytes, probably forming hemichannels, has enlarged the list of actions of connexins. Some of these mechanisms are also shared by a different family of proteins, termed pannexins. Importantly, these systems allow not only communication between healthy cells, but also play an important role during different types of injury. The aim of this review is to discuss the role played by both connexin hemichannels and pannexin channels in cell communication and injury. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.