Properties and regulation of gap junctional hemichannels in the plasma membranes of cultured cells

Calcium Regulation of Connexin Hemichannels

Connexin hemichannels (HCs) expressed at the plasma membrane of mammalian cells are of paramount importance for intercellular communication. In physiological conditions, HCs can form gap junction (GJ) channels, providing a direct diffusive path between neighbouring cells. In addition, unpaired HCs provide conduits for the exchange of solutes between the cytoplasm and the extracellular milieu, including messenger molecules involved in paracrine signalling. The synergistic action of membrane potential and Ca2+ ions controls the gating of the large and relatively unselective pore of connexin HCs. The four orders of magnitude difference in gating sensitivity to the extracellular ([Ca2+]e) and the cytosolic ([Ca2+]c) Ca2+ concentrations suggests that at least two different Ca2+ sensors may exist. While [Ca2+]e acts as a spatial modulator of the HC opening, which is most likely dependent on the cell layer, compartment, and organ, [Ca2+]c triggers HC opening and the release of extracellular bursts of messenger molecules. Such molecules include ATP, cAMP, glutamate, NAD+, glutathione, D-serine, and prostaglandins. Lost or abnormal HC regulation by Ca2+ has been associated with several diseases, including deafness, keratitis ichthyosis, palmoplantar keratoderma, Charcot-Marie-Tooth neuropathy, oculodentodigital dysplasia, and congenital cataracts. The fact that both an increased and a decreased Ca2+ sensitivity has been linked to pathological conditions suggests that Ca2+ in healthy cells finely tunes the normal HC function. Overall, further investigation is needed to clarify the structural and chemical modifications of connexin HCs during [Ca2+]e and [Ca2+]c variations. A molecular model that accounts for changes in both Ca2+ and the transmembrane voltage will undoubtedly enhance our interpretation of the experimental results and pave the way for developing therapeutic compounds targeting specific HC dysfunctions.

IP3RPEP6, a novel peptide inhibitor of IP3 receptor channels that does not affect connexin-43 hemichannels

AIM

Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are intracellular Ca2+ -release channels with crucial roles in cell function. Current IP3 R inhibitors suffer from off-target effects and poor selectivity towards the three distinct IP3 R subtypes. We developed a novel peptide inhibitor of IP3 Rs and determined its effect on connexin-43 (Cx43) hemichannels, which are co-activated by IP3 R stimulation.

METHODS

IP3RPEP6 was developed by in silico molecular docking studies and characterized by on-nucleus patch-clamp experiments of IP3 R2 channels and carbachol-induced IP3 -mediated Ca2+ responses in IP3 R1, 2 or 3 expressing cells, triple IP3 R KO cells and astrocytes. Cx43 hemichannels were studied by patch-clamp and ATP-release approaches, and by inhibition with Gap19 peptide. IP3RPEP6 interactions with IP3 Rs were verified by co-immunoprecipitation and affinity pull-down assays.

RESULTS

IP3RPEP6 concentration-dependently reduced the open probability of IP3 R2 channels and competitively inhibited IP3 Rs in an IC50 order of IP3 R2 (~3.9 μM) < IP3 R3 (~4.3 μM) < IP3 R1 (~9.0 μM), without affecting Cx43 hemichannels or ryanodine receptors. IP3RPEP6 co-immunoprecipitated with IP3 R2 but not with IP3 R1; interaction with IP3 R3 varied between cell types. The IC50 of IP3RPEP6 inhibition of carbachol-induced Ca2+ responses decreased with increasing cellular Cx43 expression. Moreover, Gap19-inhibition of Cx43 hemichannels significantly reduced the amplitude of the IP3 -Ca2+ responses and strongly increased the EC50 of these responses. Finally, we identified palmitoyl-8G-IP3RPEP6 as a membrane-permeable IP3RPEP6 version allowing extracellular application of the IP3 R-inhibiting peptide.

CONCLUSION

IP3RPEP6 inhibits IP3 R2/R3 at concentrations that have limited effects on IP3 R1. IP3 R activation triggers hemichannel opening, which strongly affects the amplitude and concentration-dependence of IP3 -triggered Ca2+ responses.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

Connexin 43: An Interface Connecting Neuroinflammation to Depression

Major depressive disorder (MDD) is a leading chronic mental illness worldwide, characterized by anhedonia, pessimism and even suicidal thoughts. Connexin 43 (Cx43), mainly distributed in astrocytes of the brain, is by far the most widely and ubiquitously expressed connexin in almost all vital organs. Cx43 forms gap junction channels in the brain, which mediate energy exchange and effectively maintain physiological homeostasis. Increasing evidence suggests the crucial role of Cx43 in the pathogenesis of MDD. Neuroinflammation is one of the most common pathological features of the central nervous system dysfunctions. Inflammatory factors are abnormally elevated in patients with depression and are closely related to nearly all links of depression. After activating the inflammatory pathway in the brain, the release and uptake of glutamate and adenosine triphosphate, through Cx43 in the synaptic cleft, would be affected. In this review, we have summarized the association between Cx43 and neuroinflammation, the cornerstones linking inflammation and depression, and Cx43 abnormalities in depression. We also discuss the significant association of Cx43 in inflammation and depression, which will help to explore new antidepressant drug targets.

Light-Activated Assembly of Connexon Nanopores in Synthetic Cells

During developmental processes and wound healing, activation of living cells occurs with spatiotemporal precision and leads to rapid release of soluble molecular signals, allowing communication and coordination between neighbors. Nonliving systems capable of similar responsive release hold great promise for information transfer in materials and site-specific drug delivery. One nonliving system that offers a tunable platform for programming release is synthetic cells. Encased in a lipid bilayer structure, synthetic cells can be outfitted with molecular conduits that span the bilayer and lead to material exchange. While previous work expressing membrane pore proteins in synthetic cells demonstrated content exchange, user-defined control over release has remained elusive. In mammalian cells, connexon nanopore structures drive content release and have garnered significant interest since they can direct material exchange through intercellular contacts. Here, we focus on connexon nanopores and present activated release of material from synthetic cells in a light-sensitive fashion. To do this, we re-engineer connexon nanopores to assemble after post-translational processing by a protease. By encapsulating proteases in light-sensitive liposomes, we show that assembly of nanopores can be triggered by illumination, resulting in rapid release of molecules encapsulated within synthetic cells. Controlling connexon nanopore activity provides an opportunity for initiating communication with extracellular signals and for transferring molecular agents to the cytoplasm of living cells in a rapid, light-guided manner.

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Synthesis and Biological Evaluation of Termini-Modified and Cyclic Variants of the Connexin43 Inhibitor Peptide5

Peptide5 is a 12–amino acid mimetic peptide that corresponds to a region of the extracellular loop 2 (EL2) of connexin43. Peptide5 regulates both cellular communication with the cytoplasm (hemichannels) and cell-to-cell communication (gap junctions), and both processes are implicated in neurological pathologies. To address the poor in vivo stability of native peptide5 and to improve its activity, twenty-five novel peptide5 mimetics were designed and synthesized. All the analogues underwent biological evaluation as a hemichannel blocker and as a gap junction disruptor, and several were assessed for stability in human serum. From this study, it was established that several acylations on the N-terminus were tolerated in the hemichannel assay. However, the replacement of the L-Lys with an N-methylated L-Lys to give H-VDCFLSRPTE-N-MeKT-OH showed good hemichannel and gap junction activity and was more stable in human serum. The cyclic peptide variants generally were not tolerated in either the hemichannel and gap junction assay although several possessed outstanding stability in human serum.

Connexins and Glucose Metabolism in Cancer

Connexins are a family of transmembrane proteins that regulate diverse cellular functions. Originally characterized for their ability to mediate direct intercellular communication through the formation of highly regulated membrane channels, their functions have been extended to the exchange of molecules with the extracellular environment, and the ability to modulate numerous channel-independent effects on processes such as motility and survival. Notably, connexins have been implicated in cancer biology for their context-dependent roles that can both promote or suppress cancer cell function. Moreover, connexins are able to mediate many aspects of cellular metabolism including the intercellular coupling of nutrients and signaling molecules. During cancer progression, changes to substrate utilization occur to support energy production and biomass accumulation. This results in metabolic plasticity that promotes cell survival and proliferation, and can impact therapeutic resistance. Significant progress has been made in our understanding of connexin and cancer biology, however, delineating the roles these multi-faceted proteins play in metabolic adaptation of cancer cells is just beginning. Glucose represents a major carbon substrate for energy production, nucleotide synthesis, carbohydrate modifications and generation of biosynthetic intermediates. While cancer cells often exhibit a dependence on glycolytic metabolism for survival, cellular reprogramming of metabolic pathways is common when blood perfusion is limited in growing tumors. These metabolic changes drive aggressive phenotypes through the acquisition of functional traits. Connections between glucose metabolism and connexin function in cancer cells and the surrounding stroma are now apparent, however much remains to be discovered regarding these relationships. This review discusses the existing evidence in this area and highlights directions for continued investigation.

Reduced nicotinamide adenine dinucleotide phosphate in redox balance and diseases: a friend or foe?

The nicotinamide adenine dinucleotide (NAD⁺/NADH) and nicotinamide adenine dinucleotide phosphate (NADP⁺/NADPH) redox couples function as cofactors or/and substrates for numerous enzymes to retain cellular redox balance and energy metabolism. Thus, maintaining cellular NADH and NADPH balance is critical for sustaining cellular homeostasis. The sources of NADPH generation might determine its biological effects. Newly-recognized biosynthetic enzymes and genetically encoded biosensors help us better understand how cells maintain biosynthesis and distribution of compartmentalized NAD(H) and NADP(H) pools. It is essential but challenging to distinguish how cells sustain redox couple pools to perform their integral functions and escape redox stress. However, it is still obscure whether NADPH is detrimental or beneficial as either deficiency or excess in cellular NADPH levels disturbs cellular redox state and metabolic homeostasis leading to redox stress, energy stress, and eventually, to the disease state. Additional study of the pathways and regulatory mechanisms of NADPH generation in different compartments, and the means by which NADPH plays a role in various diseases, will provide innovative insights into its roles in human health and may find a value of NADPH for the treatment of certain diseases including aging, Alzheimer's disease, Parkinson's disease, cardiovascular diseases, ischemic stroke, diabetes, obesity, cancer, etc.

Extracellular Cysteines Are Critical to Form Functional Cx46 Hemichannels

Connexin (Cxs) hemichannels participate in several physiological and pathological processes, but the molecular mechanisms that control their gating remain elusive. We aimed at determining the role of extracellular cysteines (Cys) in the gating and function of Cx46 hemichannels. We studied Cx46 and mutated all of its extracellular Cys to alanine (Ala) (one at a time) and studied the effects of the Cys mutations on Cx46 expression, localization, and hemichannel activity. Wild-type Cx46 and Cys mutants were expressed at comparable levels, with similar cellular localization. However, functional experiments showed that hemichannels formed by the Cys mutants did not open either in response to membrane depolarization or removal of extracellular divalent cations. Molecular-dynamics simulations showed that Cys mutants may show a possible alteration in the electrostatic potential of the hemichannel pore and an altered disposition of important residues that could contribute to the selectivity and voltage dependency in the hemichannels. Replacement of extracellular Cys resulted in "permanently closed hemichannels", which is congruent with the inhibition of the Cx46 hemichannel by lipid peroxides, through the oxidation of extracellular Cys. These results point to the modification of extracellular Cys as potential targets for the treatment of Cx46-hemichannel associated pathologies, such as cataracts and cancer, and may shed light into the gating mechanisms of other Cx hemichannels.

On the molecular nature of large-pore channels

Membrane transport is a fundamental means to control basic cellular processes such as apoptosis, inflammation, and neurodegeneration and is mediated by a number of transporters, pumps, and channels. Accumulating evidence over the last half century has shown that a type of so-called "large-pore channel" exists in various tissues and organs in gap-junctional and non-gap-junctional forms in order to flow not only ions but also metabolites such as ATP. They are formed by a number of protein families with little or no evolutionary linkages including connexin, innexin, pannexin, leucine-rich repeat-containing 8 (LRRC8), and calcium homeostasis modulator (CALHM). This review summarizes the history and concept of large-pore channels starting from connexin gap junction channels to the more recent developments in innexin, pannexin, LRRC8, and CALHM. We describe structural and functional features of large-pore channels that are crucial for their diverse functions on the basis of available structures.

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

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

Novel Tocolytic Strategy: Modulating Cx43 Activity by S-Nitrosation

Currently available tocolytics are ineffective at significantly delaying preterm birth. This is due in part to our failure to better understand the mechanisms that drive spontaneous preterm labor (sPTL). Cyclic nucleotides are not the primary contributors to myometrial quiescence, but instead nitric oxide (NO)-mediated protein S-nitrosation (SNO) is integral to the relaxation of the tissue. Connexin-43 (Cx43), a myometrial "contractile-associated protein" that functions as either a gap junction channel or an hemichannel (HC), was the focus of this study. Protein analysis determined that Cx43 is downregulated in sPTL myometrium. Furthermore, Cx43 is S-nitrosated by NO, which correlates with an increase of phosphorylated Cx43 at serine 368 (Cx43-pS368 -gap junction inhibition) as well as an increase in the HC open-state probability (quiescence). Pharmacologic inhibition of Cx43 with 18β-glycyrrhetinic acid (18β-GA) exhibits a negative inotropic effect on the myometrium in a dose-dependent manner, as does administration of nebivolol, an NO synthase activator that increases total protein SNOs. When 18β-GA and nebivolol were coadministered at their IC50 values, the effect on contractile dynamics was additive and all but eliminated contractions. The development of new tocolytics demands a better understanding of the underlying mechanisms of sPTL. Here it has been shown that 18β-GA and nebivolol leverage dysregulated pathways in the myometrium, resulting in a novel approach for the treatment of sPTL. SIGNIFICANCE STATEMENT: Although there are many known causes of preterm labor (PTL), the mechanisms of "spontaneous" PTL (sPTL) remain obfuscated, which is why treating this condition is so challenging. Here we have identified that connexin-43 (Cx43), an important contractile-associated protein, is dysregulated in sPTL myometrium and that the pharmacologic inhibition of Cx43 and its S-nitrosation with 18β-glycyrrhetinic acid and nebivolol, respectively, significantly blunts contraction in human myometrial tissue, presenting a novel approach to tocolysis that leverages maladjusted pathways in women who experience sPTL.

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 Multifaceted Role of Astrocyte Connexin 43 in Ischemic Stroke Through Forming Hemichannels and Gap Junctions

Ischemic stroke is a multi-factorial cerebrovascular disease with high worldwide morbidity and mortality. In the past few years, multiple studies have revealed the underlying mechanism of ischemia/reperfusion injury, including calcium overload, amino acid toxicity, oxidative stress, and inflammation. Connexin 43 (Cx43), the predominant connexin protein in astrocytes, has been recently proven to display non-substitutable roles in the pathology of ischemic stroke development and progression through forming gap junctions and hemichannels. Under normal conditions, astrocytic Cx43 could be found in hemichannels or in the coupling with other hemichannels on astrocytes, neurons, or oligodendrocytes to form the neuro-glial syncytium, which is involved in metabolites exchange between communicated cells, thus maintaining the homeostasis of the CNS environment. In ischemic stroke, the phosphorylation of Cx43 might cause the degradation of gap junctions and the opening of hemichannels, contributing to the release of inflammatory mediators. However, the remaining gap junctions could facilitate the exchange of protective and harmful metabolites between healthy and injured cells, protecting the injured cells to some extent or damaging the healthy cells depending on the balance of the exchange of protective and harmful metabolites. In this study, we review the changes in astrocytic Cx43 expression and distribution as well as the influence of these changes on the function of astrocytes and other cells in the CNS, providing new insight into the pathology of ischemic stroke injury; we also discuss the potential of astrocytic Cx43 as a target for the treatment of ischemic stroke.

Connexins: Key Players in the Control of Vascular Plasticity and Function

Of the 21 members of the connexin family, 4 (Cx37, Cx40, Cx43, and Cx45) are expressed in the endothelium and/or smooth muscle of intact blood vessels to a variable and dynamically regulated degree. Full-length connexins oligomerize and form channel structures connecting the cytosol of adjacent cells (gap junctions) or the cytosol with the extracellular space (hemichannels). The different connexins vary mainly with regard to length and sequence of their cytosolic COOH-terminal tails. These COOH-terminal parts, which in the case of Cx43 are also translated as independent short isoforms, are involved in various cellular signaling cascades and regulate cell functions. This review focuses on channel-dependent and -independent effects of connexins in vascular cells. Channels play an essential role in coordinating and synchronizing endothelial and smooth muscle activity and in their interplay, in the control of vasomotor actions of blood vessels including endothelial cell reactivity to agonist stimulation, nitric oxide-dependent dilation, and endothelial-derived hyperpolarizing factor-type responses. Further channel-dependent and -independent roles of connexins in blood vessel function range from basic processes of vascular remodeling and angiogenesis to vascular permeability and interactions with leukocytes with the vessel wall. Together, these connexin functions constitute an often underestimated basis for the enormous plasticity of vascular morphology and function enabling the required dynamic adaptation of the vascular system to varying tissue demands.

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.

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

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.

Inner Ear Connexin Channels: Roles in Development and Maintenance of Cochlear Function

Connexin 26 and connexin 30 are the prevailing isoforms in the epithelial and connective tissue gap junction systems of the developing and mature cochlea. The most frequently encountered variants of the genes that encode these connexins, which are transcriptionally coregulated, determine complete loss of protein function and are the predominant cause of prelingual hereditary deafness. Reducing connexin 26 expression by Cre/loxP recombination in the inner ear of adult mice results in a decreased endocochlear potential, increased hearing thresholds, and loss of >90% of outer hair cells, indicating that this connexin is essential for maintenance of cochlear function. In the developing cochlea, connexins are necessary for intercellular calcium signaling activity. Ribbon synapses and basolateral membrane currents fail to mature in inner hair cells of mice that are born with reduced connexin expression, even though hair cells do not express any connexin. In contrast, pannexin 1, an alternative mediator of intercellular signaling, is dispensable for hearing acquisition and auditory function.

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

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

Chronology of critical events in neonatal rat ventricular myocytes occurring during reperfusion after simulated ischemia

While an ischemic insult poses a lethal danger to myocardial cells, a significant proportion of cardiac myocytes remain viable throughout the ischemic episode and die, paradoxically, only after the blood flow is reinstated. Despite decades of research, the actual chronology of critical events leading to cardiomyocyte death during the reperfusion phase remains poorly understood. Arguably, identification of the pivotal event in this setting is necessary to design effective strategies aimed at salvaging the myocardium after an ischemic attack. Here we used neonatal rat ventricular myocytes (NRVMs) subjected to 20–30 min of simulated ischemia followed by 1 hour of “reperfusion”. Using different combinations of spectrally-compatible fluorescent indicators, we analyzed the relative timing of the following events: (1) abnormal increase in cytoplasmic [Ca²⁺] (T_CaCy); (2) abnormal increase in mitochondrial [Ca²⁺] (T_CaMi); (3) loss of mitochondrial inner membrane potential (ΔΨ_m) indicating mitochondrial permeability transitions (T_MPT); (4) sacrolemmal permeabilization (SP) to the normally impermeable small fluorophore TO-PRO3 (T_SP). In additional experiments we also analyzed the timing of abnormal uptake of Zn²⁺ into the cytoplasm (T_ZnCy) relative to T_CaCy and T_SP. We focused on those NRVMs which survived anoxia, as evidenced by at least 50% recovery of ΔΨ_m and the absence of detectable SP. In these cells, we found a consistent sequence of critical events in the order, from first to last, of T_CaCy, T_CaMi, T_MPT, T_SP. After detecting T_CaCy and T_CaMi, abrupt switches between 1.1 mM and nominally zero [Ca²⁺] in the perfusate quickly propagated to the cytoplasmic and mitochondrial [Ca²⁺]. Depletion of the sarcoplasmic reticulum with ryanodine (5 μM)/thapsigargin (1 μM) accelerated all events without changing their order. In the presence of ZnCl₂ (10–30 μM) in the perfusate we found a consistent timing sequence T_CaCy < T_Zn ≤ T_SP. In some cells ZnCl₂ interfered with Ca²⁺ uptake, causing “steps” or “gaps” in the [Ca²⁺]_Cy curve, a phenomenon never observed in the absence of ZnCl₂. Together, these findings suggest an evolving permeabilization of NRVM’s sarcolemma during reoxygenation, in which the expansion of the pore size determines the timing of critical events, including T_MPT.

Phosphorylation-Dependent Intra-Domain Interaction of the Cx37 Carboxyl-Terminus Controls Cell Survival

Differential phosphorylation of the carboxyl-terminus of connexin 37 (Cx37-CT) regulates phenotypic switching between cell growth phenotypes (cell death, cell cycle arrest, proliferation). The specific phosphorylation events in the Cx37-CT that are necessary for these growth regulatory effects are currently unknown. Through the combined use of deletion and site specific (de)phospho-mimetic Cx37-CT mutants, our data suggest a phosphorylation-dependent interaction between the mid-tail (aa 273⁻317) and end-tail (aa 318⁻333) portions of the Cx37-CT that regulates cell survival. As detected by mass spectrometry, Cx37 was phosphorylated at serines 275, 321, and 328; phosphomimetic mutations of these sites resulted in cell death when expressed in rat insulinoma cells. Alanine substitution at S328, but not at S275 or S321, also triggered cell death. Cx37-S275D uniquely induced the death of only low density, non-contact forming cells, but neither hemichannel open probability nor channel conductance distinguished death-inducing mutants. As channel function is necessary for cell death, together the data suggest that the phosphorylation state of the Cx37-CT controls an intra-domain interaction within the CT that modifies channel function and induces cell death.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Cx43 Channel Gating and Permeation: Multiple Phosphorylation-Dependent Roles of the Carboxyl Terminus

Connexin 43 (Cx43), a gap junction protein seemingly fit to support cardiac impulse propagation and synchronic contraction, is phosphorylated in normoxia by casein kinase 1 (CK1). However, during cardiac ischemia or pressure overload hypertrophy, this phosphorylation fades, Cx43 abundance decreases at intercalated disks and increases at myocytes' lateral borders, and the risk of arrhythmia rises. Studies in wild-type and transgenic mice indicate that enhanced CK1-phosphorylation of Cx43 protects from arrhythmia, while dephosphorylation precedes arrhythmia vulnerability. The mechanistic bases of these Cx43 (de)phosphoform-linked cardiac phenotypes are unknown. We used patch-clamp and dye injection techniques to study the channel function (gating, permeability) of Cx43 mutants wherein CK1-targeted serines were replaced by aspartate (Cx43-CK1-D) or alanine (Cx43-CK1-A) to emulate phosphorylation and dephosphorylation, respectively. Cx43-CK1-D, but not Cx43-CK1-A, displayed high Voltage-sensitivity and variable permselectivity. Both mutants showed multiple channel open states with overall increased conductivity, resistance to acidification-induced junctional uncoupling, and hemichannel openings in normal external calcium. Modest differences in the mutant channels' function and regulation imply the involvement of dissimilar structural conformations of the interacting domains of Cx43 in electrical and chemical gating that may contribute to the divergent phenotypes of CK1-(de)phospho-mimicking Cx43 transgenic mice and that may bear significance in arrhythmogenesis.

Cationic control of Panx1 channel function

The sequence and predicted membrane topology of pannexin1 (Panx1) places it in the family of gap junction proteins. However, rather than forming gap junction channels, Panx1 forms channels in the nonjunctional membrane. Panx1 operates in two distinct open states, depending on the mode of stimulation. The exclusively voltage-gated channel has a small conductance (<100 pS) and is highly selective for the flux of chloride ions. The Panx1 channel activated by various physiological stimuli or by increased concentrations of extracellular potassium ions has a large conductance (~500 pS, however, with multiple, long-lasting subconductance states) and is nonselectively permeable to small molecules, including ATP. To test whether the two open conformations also differ pharmacologically, the effects of di-and trivalent cations on the two Panx1 channel conformations were investigated. The rationale for this venture was that, under certain experimental conditions, ATP release from cells can be inhibited by multivalent cations, yet the literature indicates that the ATP release channel Panx1 is not affected by these ions. Consistent with previous reports, the Panx1 channel was not activated by removal of extracellular Ca²⁺ and the currents through the voltage-activated channel were not altered by Ca²⁺, Zn²⁺, Ba²⁺, or Gd³⁺. In contrast, the Panx1 channel activated to the large channel conformation by extracellular K⁺, osmotic stress, or low oxygen was inhibited by the multivalent cations in a dose-dependent way. Thus, monovalent cations activated the Panx1 channel from the closed state to the "large" conformation, while di- and trivalent cations exclusively inhibited this large channel conformation.

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.

Protein Kinase C Enhances Electrical Synaptic Transmission by Acting on Junctional and Postsynaptic Ca2+ Currents

By synchronizing neuronal activity, electrical transmission influences the coordination, pattern, and/or frequency of firing. In the hemaphroditic marine-snail, Aplysia calfornica, the neuroendocrine bag cell neurons use electrical synapses to synchronize a 30 min afterdischarge of action potentials for the release of reproductive hormone. During the afterdischarge, protein kinase C (PKC) is activated, although its impact on bag cell neuron electrical transmission is unknown. This was investigated here by monitoring electrical synapses between paired cultured bag cell neurons using dual whole-cell recording. Voltage clamp revealed a largely voltage-independent junctional current, which was enhanced by treating with a PKC activator, PMA, before recording. We also examined the transfer of presynaptic action potential-like waveforms (generated in voltage clamp) to the postsynaptic cell (measured in current clamp). For control pairs, the presynaptic spike-like waveforms mainly evoked electrotonic potentials; however, when PKC was triggered, these stimuli consistently produced postsynaptic action potentials. To assess whether this involved changes to postsynaptic responsiveness, single bag cell neurons were injected with junctional-like current mimicking that evoked by a presynaptic action potential. Unlike control neurons, which were less likely to spike, cells in PMA always fired action potentials to the junctional-like current. Furthermore, PKC activation increased a postsynaptic voltage-gated Ca²⁺ current, which was recruited even by modest depolarization associated with an electrotonic potential. Whereas PKC inhibits gap junctions in most systems, bag cell neurons are rather unique, as the kinase potentiates the electrical synapse; in turn, this synergizes with augmented postsynaptic Ca²⁺ current to promote synchronous firing.SIGNIFICANCE STATEMENT Electrical coupling is a fundamental form of communication. For the bag cell neurons of Aplysia, electrical synapses coordinate a prolonged burst of action potentials known as the afterdischarge. We looked at how protein kinase C, which is upregulated with the afterdischarge, influences information transfer across the synapse. The kinase activation increased junctional current, a remarkable finding given that this enzyme is largely considered inhibitory for gap junctions. There was also an augmentation in the ability of a presynaptic neuron to provoke postsynaptic action potentials. This increased excitability was, in part, due to enhanced postsynaptic voltage-dependent Ca²⁺ current. Thus, protein kinase C improves the fidelity of electrotonic transmission and promotes synchronous firing by modulating both junctional and membrane conductances.

Glia and hemichannels: key mediators of perinatal encephalopathy

Perinatal encephalopathy remains a major cause of disability, such as cerebral palsy. Therapeutic hypothermia is now well established to partially reduce risk of disability in late preterm/term infants. However, new and complementary therapeutic targets are needed to further improve outcomes. There is increasing evidence that glia play a key role in neural damage after hypoxia-ischemia and infection/inflammation. In this review, we discuss the role of astrocytic gap junction (connexin) hemichannels in the spread of neural injury after hypoxia-ischemia and/or infection/inflammation. Potential mechanisms of hemichannel mediated injury likely involve impaired intracellular calcium handling, loss of blood-brain barrier integrity and release of adenosine triphosphate (ATP) resulting in over-activation of purinergic receptors. We propose the hypothesis that inflammation-induced opening of connexin hemichannels is a key regulating event that initiates a vicious cycle of excessive ATP release, which in turn propagates activation of purinergic receptors on microglia and astrocytes. This suggests that developing new neuroprotective strategies for preterm infants will benefit from a detailed understanding of glial and connexin hemichannel responses.

Tonabersat Prevents Inflammatory Damage in the Central Nervous System by Blocking Connexin43 Hemichannels

The cis benzopyran compound tonabersat (SB-220453) has previously been reported to inhibit connexin26 expression in the brain by attenuating the p38-mitogen-activated protein kinase pathway. We show here that tonabersat directly inhibits connexin43 hemichannel opening. Connexin43 hemichannels have been called "pathological pores" based upon their role in secondary lesion spread, edema, inflammation, and neuronal loss following central nervous system injuries, as well as in chronic inflammatory disease. Both connexin43 hemichannels and pannexin channels released adenosine triphosphate (ATP) during ischemia in an in vitro ischemia model, but only connexin43 hemichannels contributed to ATP release during reperfusion. Tonabersat inhibited connexin43 hemichannel-mediated ATP release during both ischemia and reperfusion phases, with direct channel block confirmed using electrophysiology. Tonabersat also reduced connexin43 gap junction coupling in vitro, but only at higher concentrations, with junctional plaques internalized and degraded via the lysosomal pathway. Systemic delivery of tonabersat in a rat bright-light retinal damage model (a model for dry age-related macular degeneration) resulted in significantly improved functional outcomes assessed using electroretinography. Tonabersat also prevented thinning of the retina, especially the outer nuclear layer and choroid, assessed using optical coherence tomography. We conclude that tonabersat, already given orally to over 1000 humans in clinical trials (as a potential treatment for, and prophylactic treatment of, migraine because it was thought to inhibit cortical spreading depression), is a connexin hemichannel inhibitor and may have the potential to be a novel treatment of central nervous system injury and chronic neuroinflammatory disease.

Connexin hemichannel blockade improves survival of striatal GABA-ergic neurons after global cerebral ischaemia in term-equivalent fetal sheep

Basal ganglia injury at term remains a major cause of disability, such as cerebral palsy. In this study we tested the hypotheses that blockade of astrocytic connexin hemichannels with a mimetic peptide would improve survival of striatal phenotypic neurons after global cerebral ischaemia in term-equivalent fetal sheep, and that neuronal survival would be associated with electrophysiological recovery. Fetal sheep (0.85 gestation) were randomly assigned to receive a short or long (1 or 25 h) intracerebroventricular infusion of a mimetic peptide or vehicle, starting 90 minutes after 30 minutes of cerebral ischaemia. Sheep were killed 7 days after ischaemia. Cerebral ischaemia was associated with reduced numbers of calbindin-28k, calretinin, parvalbumin and GAD positive striatal neurons (P < 0.05 ischaemia + vehicle, n = 6 vs. sham ischaemia, n = 6) but not ChAT or nNOS positive neurons. Short infusion of peptide (n = 6) did not significantly improve survival of any striatal phenotype. Long infusion of peptide (n = 6) was associated with increased survival of calbindin-28k, calretinin, parvalbumin and GAD positive neurons (P < 0.05 vs. ischaemia + vehicle). Neurophysiological recovery was associated with improved survival of calbindin-28k, calretinin and parvalbumin positive striatal neurons (P < 0.05 for all). In conclusion, connexin hemichannel blockade after cerebral ischaemia in term-equivalent fetal sheep improves survival of striatal GABA-ergic neurons.

Inhibition of Gap Junction Elevates Glutamate Uptake in Cultured Astrocytes

Glutamate uptake is a main function of astrocytes to keep extracellular glutamate levels low and protect neurons against glutamate-induced excitotoxicity. On the other hand, astrocyte networks formed by gap junctions, which are consisted with connexins and connecting neighboring cells, are reported to play a critical role in maintaining the homeostasis in the brain. In the present study, we examined the effects of gap junction inhibitors on the glutamate uptake activity in cultured rat cortical astrocytes. At first, we confirmed the effects of gap junction inhibitors, 1-octanol and carbenoxolone, on cell-cell communication by the scrape-loading assay using a fluorescent dye Lucifer yellow. Both of 1-octanol and carbenoxolone treatments for 20 min in cultured astrocytes significantly suppressed the cell-cell communication assessed as the distance of dye-spreading. 1-octanol and carbenoxolone increased the glutamate uptake by astrocytes and glutamate aspartate transporter (GLAST) expression on the cell membrane. These results suggest that gap junction inhibitors increase the glutamate uptake activity through the increase of GLAST proteins located on the cell membrane. The regulation of gap junction in astrocytes might protect neurons against glutamate-induced excitotoxicity.

The role of connexin and pannexin containing channels in the innate and acquired immune response

Connexin (Cx) and pannexin (Panx) containing channels - gap junctions (GJs) and hemichannels (HCs) - are present in virtually all cells and tissues. Currently, the role of these channels under physiological conditions is well defined. However, their role in the immune response and pathological conditions has only recently been explored. Data from several laboratories demonstrates that infectious agents, including HIV, have evolved to take advantage of GJs and HCs to improve viral/bacterial replication, enhance inflammation, and help spread toxicity into neighboring areas. In the current review, we discuss the role of Cx and Panx containing channels in immune activation and the pathogenesis of several infectious diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Gap junctions and hemichannels: communicating cell death in neurodevelopment and disease

Gap junctions are unique membrane channels that play a significant role in intercellular communication in the developing and mature central nervous system (CNS). These channels are composed of connexin proteins that oligomerize into hexamers to form connexons or hemichannels. Many different connexins are expressed in the CNS, with some specificity with regard to the cell types in which distinct connexins are found, as well as the timepoints when they are expressed in the developing and mature CNS. Both the main neuronal Cx36 and glial Cx43 play critical roles in neurodevelopment. These connexins also mediate distinct aspects of the CNS response to pathological conditions. An imbalance in the expression, translation, trafficking and turnover of connexins, as well as mutations of connexins, can impact their function in the context of cell death in neurodevelopment and disease. With the ever-increasing understanding of connexins in the brain, therapeutic strategies could be developed to target these membrane channels in various neurological disorders.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Hemichannel-mediated release of lactate

In the central nervous system lactate contributes to the extracellular pool of readily available energy substrates and may also function as a signaling molecule which mediates communication between glial cells and neurons. Monocarboxylate transporters are believed to provide the main pathway for lactate transport across the membranes. Here we tested the hypothesis that lactate could also be released via opening of pannexin and/or functional connexin hemichannels. In acute slices prepared from the brainstem, hippocampus, hypothalamus and cortex of adult rats, enzymatic amperometric biosensors detected significant tonic lactate release inhibited by compounds, which block pannexin/connexin hemichannels and facilitated by lowering extracellular [Ca(2+)] or increased PCO2 Enhanced lactate release triggered by hypoxia was reduced by ∼50% by either connexin or monocarboxylate transporter blockers. Stimulation of Schaffer collateral fibers triggered lactate release in CA1 area of the hippocampus, which was facilitated in conditions of low extracellular [Ca(2+)], markedly reduced by blockade of connexin hemichannels and abolished by lactate dehydrogenase inhibitor oxamate. These results indicate that lactate transport across the membranes may occur via mechanisms other than monocarboxylate transporters. In the central nervous system, hemichannels may function as a conduit of lactate release, and this mechanism is recruited during hypoxia and periods of enhanced neuronal activity.

ATP as a Messenger in Astrocyte-Neuronal Communication

Astrocytes are activated during both excitatory and inhibitory synaptic transmission and respond with intracellular Ca2+i elevations. Ca2+i oscillations and waves in astrocytes now appear to represent the glial arm of a dynamic neuronal-glial signaling process. Advances within the last year have shown that stimuli that elevate Ca2+i in astrocytes have the potential to modulate synaptic function. Recent studies have shown that astrocytic calcium waves, initially believed to depend on the integrity of functional gap junction channels for the passage of intercellular signals, are actually mediated by release of ATP and subsequent activation of purinergic receptors on neighboring cells. ATP release is in turn regulated by the expression of gap junction proteins, establishing a novel dimension between gap junctions and extracellular-mediated signaling events. The role of ATP and its breakdown product, adenosine, on synaptic transmission are discussed.

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.

Regulation of gap junction channels and hemichannels by phosphorylation and redox changes: a revision

Post-translational modifications of connexins play an important role in the regulation of gap junction and hemichannel permeability. The prerequisite for the formation of functional gap junction channels is the assembly of connexin proteins into hemichannels and their insertion into the membrane. Hemichannels can affect cellular processes by enabling the passage of signaling molecules between the intracellular and extracellular space. For the intercellular communication hemichannels from one cell have to dock to its counterparts on the opposing membrane of an adjacent cell to allow the transmission of signals via gap junctions from one cell to the other. The controlled opening of hemichannels and gating properties of complete gap junctions can be regulated via post-translational modifications of connexins. Not only channel gating, but also connexin trafficking and assembly into hemichannels can be affected by post-translational changes. Recent investigations have shown that connexins can be modified by phosphorylation/dephosphorylation, redox-related changes including effects of nitric oxide (NO), hydrogen sulfide (H2S) or carbon monoxide (CO), acetylation, methylation or ubiquitination. Most of the connexin isoforms are known to be phosphorylated, e.g. Cx43, one of the most studied connexin at all, has 21 reported phosphorylation sites. In this review, we provide an overview about the current knowledge and relevant research of responsible kinases, connexin phosphorylation sites and reported effects on gap junction and hemichannel regulation. Regarding the effects of oxidants we discuss the role of NO in different cell types and tissues and recent studies about modifications of connexins by CO and H2S.

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.

Translating connexin biology into therapeutics

It is 45 years since gap junctions were first described. Universities face increasing commercial pressures and declining federal funding, with governments and funding foundations showing greater interest in gaining return on their investments. This review outlines approaches taken to translate gap junction research to clinical application and the challenges faced. The need for commercialisation is discussed and key concepts behind research patenting briefly described. Connexin channel roles in disease and injury are also discussed, as is identification of the connexin hemichannel as a therapeutic target which appears to play a role in both the start and perpetuation of the inflammasome pathway. Furthermore connexin hemichannel opening results in vascular dieback in acute injury and chronic disease. Translation to human indications is illustrated from the perspective of one connexin biotechnology company, CoDa Therapeutics, Inc.

A novel innexin2 forming membrane hemichannel exhibits immune responses and cell apoptosis in Scylla paramamosain

Innexins are a class of transmembrane proteins that are important for embryonic development, morphogenesis and electrical synapse formation. In the present study, a novel innexin2 gene from Scylla paramamosain was named Sp-inx2 and characterized. The complete cDNA and genomic DNA sequences of Sp-inx2 were revealed. Sp-inx2 mRNA transcripts were distributed in various tissues of S. paramamosain and were most abundant in the hemocytes. The Sp-inx2 was significantly upregulated in hemocyte, gill and hepatopancreas tissues with the challenge of either Vibrio alginolyticus, Vibrio parahaemolyticus or lipopolysaccharides (LPSs) when analyzed at 3 and 6 h using quantitative real-time PCR, suggesting that it could activate an immune response against the challenge of LPSs or Vibrio species. Using the chemical inhibitors carbenoxolone and probenecid, the absorption of the fluorescent dye Lucifer yellow decreased in the primary cultured hemocytes of crabs, thus confirming that hemichannels composed of Sp-inx2 existed in the crab hemocytes. With LPS stimulation, the level of mRNA transcripts and protein expression of Sp-inx2 in the same cultured hemocytes gradually increased from 6 to 48 h, while the activity of hemichannels was down-regulated at 6 and 12 h, demonstrating that LPSs could modulate the absorption activity of hemichannels in addition to its upregulation of Sp-inx2 gene expression. Furthermore, the dye uptake rate in HeLa cells in which Sp-inx2 was ectopically expressed increased dramatically but the increase was significantly down-regulated with the addition of 50 μg mL(-1) LPS, suggesting that the LPS stimulation could effectively reduce the activity of hemichannels. Interestingly, with the ectopic expression of Sp-inx2 in HeLa and EPC cells, apoptosis spontaneously occurred in both cultured cell lines when detected using TUNEL assay. In summary, a new Sp-inx2 gene was first characterized in a marine animal S. paramamosain and it had a function associated with immune response and cell apoptosis.