S-nitrosylation and permeation through connexin 43 hemichannels in astrocytes: induction by oxidant stress and reversal by reducing agents

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

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

Peptides and peptide-derived molecules targeting the intracellular domains of Cx43: gap junctions versus hemichannels

About a decade ago, the molecular determinants controlling the opening and closing of Cx43 gap junction channels have been identified. Advanced biophysical approaches revealed a critical role for structural rearrangements in the cytoplasmic loop and dimerization of the C-terminal tail, resulting in binding of the C-terminal tail to the cytoplasmic loop and Cx43 gap junction channel closure during cellular acidosis. This has spurred the development of Cx43-mimetic peptides and peptidomimetics that interfere with these loop/tail interactions, thereby preventing the closure of Cx43 gap junctions, e.g. in the heart upon ischemia. Recently, we found that loop/tail interactions control Cx43-hemichannel activity but with an opposite effect. Binding of the C-terminal tail to the cytoplasmic loop is a requisite for the opening of Cx43 hemichannels in response to different stimuli, like decreased extracellular [Ca2+], increased intracellular [Ca2+], positive membrane potentials or ischemia. Strikingly, peptides that favor the open state of Cx43 gap junctions like the L2 peptide inhibit Cx43-hemichannel opening. These tools now provide unprecedented opportunities to selectively inhibit Cx43 hemichannels while maintaining Cx43 gap junction communication, impossible to achieve with siRNA or knockdown approaches both affecting gap junctions and hemichannels. These tools not only are very helpful to unravel the role of Cx43 hemichannels in complex biological systems, but also hold therapeutic potential to counteract excessive Cx43-hemichannel activity like in ischemia/reperfusion in the brain and the heart or to prevent Cx43 hemichannel-mediated gliotransmitter release in the basal amygdala during memory consolidation in response to emotional events. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Protective effects of carbenoxolone are associated with attenuation of oxidative stress in ischemic brain injury

Accumulating evidence has suggested that the gap junction plays an important role in the determination of cerebral ischemia, but the underlying mechanisms remain to be elucidated. In this study, we assessed the effect of a gap-junction blocker, carbenoxolone (CBX), on ischemia/reperfusion-induced brain injury and the possible mechanisms. By using the transient cerebral ischemia model induced by occlusion of the middle cerebral artery for 30 min followed by reperfusion for 24 h, we found that pre-administration of CBX (25 mg/kg, intracerebroventricular injection, 30 min before cerebral ischemic surgery) diminished the infarction size in rats. And this was associated with a decrease of reactive oxygen species generation and inhibition of the activation of astrocytes and microglia. In PC12 cells, H2O2 treatment induced more coupling and apoptosis, while CBX partly inhibited the opening of gap junctions and improved the cell viability. These results suggest that cerebral ischemia enhances the opening of gap junctions. Blocking the gap junction with CBX may attenuate the brain injury after cerebral ischemia/reperfusion by partially contributing to amelioration of the oxidative stress and apoptosis.

Regulation of connexin hemichannel activity by membrane potential and the extracellular calcium in health and disease

Connexins are thought to solely mediate cell-to-cell communication by forming gap junction channels composed of two membrane-spanning hemichannels positioned end-to-end. However, many if not all connexin isoforms also form functional hemichannels (i.e., the precursors of complete channels) that mediate the rapid exchange of ions, second messengers and metabolites between the cell interior and the interstitial space. Electrical and molecular signaling via connexin hemichannels is now widely recognized to be important in many physiological scenarios and pathological conditions. Indeed, mutations in connexins that alter hemichannel function have been implicated in several diseases. Here, we present a comprehensive overview of how hemichannel activity is tightly regulated by membrane potential and the external calcium concentration. In addition, we discuss the genetic mutations known to alter hemichannel function and their deleterious effects, of which a better understanding is necessary to develop novel therapeutic approaches for diseases caused by hemichannel dysfunction. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Antibodies targeting extracellular domain of connexins for studies of hemichannels

Hemichannels are transmembrane channels composed of either a connexin or pannexin hexamer. The docking of the extracellular domains of connexin hemichannels contributed by neighboring cells forms a gap junction channel that joins the cytoplasm of adjacent cells. Connexins are expressed ubiquitously in different organs, but some subtypes are expressed exclusively in certain tissues and tumors. Both gap junction channels and hemichannels participate in diverse physiological and pathological responses. However, the lack of specific reagents that inhibit only gap junction channels or hemichannels is a challenge that makes it different to discern the specific roles of either channel. Fortunately, the available information regarding the connexin sequence, secondary and tertiary structure, and their biochemical and physiological properties permits the development of strategies to block exclusively the hemichannel activity exclusively, with no effect on gap junction activity. This task is accomplished through the use of specifics antibodies that target the extracellular sites of desired connexin subtype. However, the underlying mechanism of how antibodies targeting extracellular connexin epitopes actually inhibit hemichannels remains unknown. Although these antibodies are being used for detecting and blocking of hemichannels in normal and tumor cells, they can also be potentially used for tissue-specific treatment and drug delivery in clinical applications. In this article, we will first review the literature concerning the structure of connexins and the unique properties of extracellular loop domains of the connexins. Furthermore, we will discuss briefly the development of connexin (Cx) 43(E2) antibody, a specific antibody which detects the second extracellular loop of Cx43 and specifically prevents the opening of Cx43 hemichannels. We will then summarize the reported studies of specific reagents used for the inhibition of connexin hemichannels including antibodies developed against extracellular loop domains. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Diffusion of nitric oxide across cell membranes of the vascular wall requires specific connexin-based channels

NO is generated within cells and frequently must be transferred to responsive neighboring cells, as occurs in the endothelium-dependent relaxation of smooth muscle cells observed in blood vessels. It is thought that NO diffuses freely across cell membranes, but it may also permeate through low resistant membrane pathways. Here, we describe the participation of connexin (Cx)-formed channels in the NO transport across cell membranes and between endothelial and smooth muscle cells. We used a water-soluble NO donor of high molecular weight (S-nitrosylated albumin, BSA-NO) that does not permeate through cell membranes or Cx-based channels and the NO-sensitive dye 4,5-diaminofluorescein diacetate to detect changes of intracellular NO concentration. We found that NO generated in the extracellular space was not detected intracellularly in Cx-deficient HeLa cells, suggesting that cell membrane represents a significant diffusion barrier for NO transfer. However, Cx-based channels provide efficient pathways for NO signaling because NO opened and permeated hemichannels expressed in HeLa cells transfected with Cx43, Cx40, or Cx37. In contrast, NO closed hemichannels of HeLa-Cx32 cells, which otherwise are permeable to NO if are opened by a divalent cation-free extracellular solution. Consistent with this, blockade of Cx-based channels abolished the myoendothelial NO transfer and associated NO-dependent vasodilation induced by acethylcholine. These results indicate that Cx-based channels play a key role in the NO-dependent tonic control of vascular function and may direct the NO signal to specific targets, which provides a novel mechanistic basis for the critical role of Cxs in cell-cell communication in the vessel wall. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Gap junction channels and hemichannels in the CNS: regulation by signaling molecules

Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Human cell responses to ionizing radiation are differentially affected by the expressed connexins

In multicellular organisms, intercellular communication is essential for homeostatic functions and has a major role in tissue responses to stress. Here, we describe the effects of expression of different connexins, which form gap junction channels with different permeabilities, on the responses of human cells to ionizing radiation. Exposure of confluent HeLa cell cultures to (137)Cs γ rays, 3.7 MeV α particles, 1000 MeV protons or 1000 MeV/u iron ions resulted in distinct effects when the cells expressed gap junction channels composed of either connexin26 (Cx26) or connexin32 (Cx32). Irradiated HeLa cells expressing Cx26 generally showed decreased clonogenic survival and reduced metabolic activity relative to parental cells lacking gap junction communication. In contrast, irradiated HeLa cells expressing Cx32 generally showed enhanced survival and greater metabolic activity relative to the control cells. The effects on clonogenic survival correlated more strongly with effects on metabolic activity than with DNA damage as assessed by micronucleus formation. The data also showed that the ability of a connexin to affect clonogenic survival following ionizing radiation can depend on the specific type of radiation. Together, these findings show that specific types of connexin channels are targets that may be exploited to enhance radiotherapeutic efficacy and to formulate countermeasures to the harmful effects of specific types of ionizing radiation.

Connexin43 mimetic peptide is neuroprotective and improves function following spinal cord injury

Connexin43 (Cx43) is a gap junction protein up-regulated after spinal cord injury and is involved in the on-going spread of secondary tissue damage. To test whether a connexin43 mimetic peptide (Peptide5) reduces inflammation and tissue damage and improves function in an in vivo model of spinal cord injury, rats were subjected to a 10g, 12.5mm weight drop injury at the vertebral level T10 using a MASCIS impactor. Vehicle or connexin43 mimetic peptide was delivered directly to the lesion via intrathecal catheter and osmotic mini-pump for up to 24h after injury. Treatment with Peptide5 led to significant improvements in hindlimb function as assessed using the Basso-Beattie-Bresnahan scale. Peptide5 caused a reduction in Cx43 protein, increased Cx43 phosphorylation and decreased levels of TNF-α and IL-1β as assessed by Western blotting. Immunohistochemistry of tissue sections 5 weeks after injury showed reductions in astrocytosis and activated microglia as well as an increase in motor neuron survival. These results show that administration of a connexin mimetic peptide reduces secondary tissue damage after spinal cord injury by reducing gliosis and cytokine release and indicate the clinical potential for mimetic peptides in the treatment of spinal cord patients.

IP3, a small molecule with a powerful message

Research conducted over the past two decades has provided convincing evidence that cell death, and more specifically apoptosis, can exceed single cell boundaries and can be strongly influenced by intercellular communication networks. We recently reported that gap junctions (i.e. channels directly connecting the cytoplasm of neighboring cells) composed of connexin43 or connexin26 provide a direct pathway to promote and expand cell death, and that inositol 1,4,5-trisphosphate (IP3) diffusion via these channels is crucial to provoke apoptosis in adjacent healthy cells. However, IP3 itself is not sufficient to induce cell death and additional factors appear to be necessary to create conditions in which IP3 will exert proapoptotic effects. Although IP3-evoked Ca(2+) signaling is known to be required for normal cell survival, it is also actively involved in apoptosis induction and progression. As such, it is evident that an accurate fine-tuning of this signaling mechanism is crucial for normal cell physiology, while a malfunction can lead to cell death. Here, we review the role of IP3 as an intracellular and intercellular cell death messenger, focusing on the endoplasmic reticulum-mitochondrial synapse, followed by a discussion of plausible elements that can convert IP3 from a physiological molecule to a killer substance. Finally, we highlight several pathological conditions in which anomalous intercellular IP3/Ca(2+) signaling might play a role. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Boldine prevents renal alterations in diabetic rats

Diabetic nephropathy alters both structure and function of the kidney. These alterations are associated with increased levels of reactive oxygen species, matrix proteins, and proinflammatory molecules. Inflammation decreases gap junctional communication and increases hemichannel activity leading to increased membrane permeability and altering tissue homeostasis. Since current treatments for diabetic nephropathy do not prevent renal damage, we postulated an alternative treatment with boldine, an alkaloid obtained from boldo with antioxidant, anti-inflammatory, and hypoglycemic effects. Streptozotocin-induced diabetic and control rats were treated or not treated with boldine (50 mg/Kg/day) for ten weeks. In addition, mesangial cells were cultured under control conditions or in high glucose concentration plus proinflammatory cytokines, with or without boldine (100 µmol/L). Boldine treatment in diabetic animals prevented the increase in glycemia, blood pressure, renal thiobarbituric acid reactive substances and the urinary protein/creatinine ratio. Boldine also reduced alterations in matrix proteins and markers of renal damage. In mesangial cells, boldine prevented the increase in oxidative stress, the decrease in gap junctional communication, and the increase in cell permeability due to connexin hemichannel activity induced by high glucose and proinflammatory cytokines but did not block gap junction channels. Thus boldine prevented both renal and cellular alterations and could be useful for preventing tissue damage in diabetic subjects.

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

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

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

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

S-nitrosylation inhibits pannexin 1 channel function

S-nitrosylation is a post-translational modification on cysteine(s) that can regulate protein function, and pannexin 1 (Panx1) channels are present in the vasculature, a tissue rich in nitric oxide (NO) species. Therefore, we investigated whether Panx1 can be S-nitrosylated and whether this modification can affect channel activity. Using the biotin switch assay, we found that application of the NO donor S-nitrosoglutathione (GSNO) or diethylammonium (Z)-1-1(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA NONOate) to human embryonic kidney (HEK) 293T cells expressing wild type (WT) Panx1 and mouse aortic endothelial cells induced Panx1 S-nitrosylation. Functionally, GSNO and DEA NONOate attenuated Panx1 currents; consistent with a role for S-nitrosylation, current inhibition was reversed by the reducing agent dithiothreitol and unaffected by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a blocker of guanylate cyclase activity. In addition, ATP release was significantly inhibited by treatment with both NO donors. To identify which cysteine residue(s) was S-nitrosylated, we made single cysteine-to-alanine substitutions in Panx1 (Panx1(C40A), Panx1(C346A), and Panx1(C426A)). Mutation of these single cysteines did not prevent Panx1 S-nitrosylation; however, mutation of either Cys-40 or Cys-346 prevented Panx1 current inhibition and ATP release by GSNO. This observation suggested that multiple cysteines may be S-nitrosylated to regulate Panx1 channel function. Indeed, we found that mutation of both Cys-40 and Cys-346 (Panx1(C40A/C346A)) prevented Panx1 S-nitrosylation by GSNO as well as the GSNO-mediated inhibition of Panx1 current and ATP release. Taken together, these results indicate that S-nitrosylation of Panx1 at Cys-40 and Cys-346 inhibits Panx1 channel currents and ATP release.

Intercellular Ca(2+) waves: mechanisms and function

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

Connexin and pannexin hemichannels in inflammatory responses of glia and neurons

Mammals express ∼20 different connexins, the main gap junction forming proteins in mammals, and 3 pannexins, homologs of innexins, the main gap junction forming proteins in invertebrates. In both classes of gap junction, each channel is formed by two hemichannels, one contributed by each of the coupled cells. There is now general, if not universal, agreement that hemichannels of both classes can open in response to various physiological and pathological stimuli when they are not apposed to another hemichannels and face the external milieu. Connexin (and likely pannexin) hemichannel permeability is consistent with that of the cell-cell channels and open hemichannels can be a release site for relatively large molecules such as ATP and glutamate, which can serve as transmitters between cells. Here we describe three experimental paradigms in which connexin and pannexin hemichannel signaling occurs. (1) In cultures of spinal astrocytes FGF-1 causes the release of ATP, and ATP causes opening of pannexin hemichannels, which then release further ATP. Subsequently, several hours later, connexin hemichannels are also opened by an unknown mechanism. Release of ATP appears to become self sustaining through action of P2X7 receptors to open pannexin hemichannels and then connexin hemichannels, both of which are ATP permeable. (2) Spinal cord injury by dropping a small weight on the exposed cord is followed by release of ATP in the region surrounding the primary lesion. This release is greatly reduced in a mouse in which Cx43 is knocked down in the astrocytes. Application of FGF-1 causes a similar release of ATP in the uninjured spinal cord, and an inhibitor of the FGF-1 receptor, PD173074, inhibits both FGF-1 and injury-induced release. Reduction in ATP release is associated with reduced inflammation and less secondary expansion of the lesion. (3) Cortical astrocytes in culture are permeabilized by hypoxia, and this effect is increased by high or zero glucose. The mechanism of permeabilization is opening of Cx43 hemichannels, which can lead to cell death. Activated microglia secrete TNF-α and IL-1β, which open connexin hemichannels in astrocytes. Astrocytes release ATP and glutamate which can kill neurons in co-culture through activation of neuronal pannexin hemichannels. These studies implicate two kinds of gap junction hemichannel in inflammatory responses and cell death. This article is part of a Special Issue entitled Electrical Synapses.

The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system

Gap junctions (GJs) are expressed in most cell types of the nervous system, including neuronal stem cells, neurons, astrocytes, oligodendrocytes, cells of the blood brain barrier (endothelial cells and astrocytes) and under inflammatory conditions in microglia/macrophages. GJs connect cells by the docking of two hemichannels, one from each cell with each hemichannel being formed by 6 proteins named connexins (Cx). Unapposed hemichannels (uHC) also can be open on the surface of the cells allowing the release of different intracellular factors to the extracellular space. GJs provide a mechanism of cell-to-cell communication between adjacent cells that enables the direct exchange of intracellular messengers, such as calcium, nucleotides, IP(3), and diverse metabolites, as well as electrical signals that ultimately coordinate tissue homeostasis, proliferation, differentiation, metabolism, cell survival and death. Despite their essential functions in physiological conditions, relatively little is known about the role of GJs and uHC in human diseases, especially within the nervous system. The focus of this review is to summarize recent findings related to the role of GJs and uHC in physiologic and pathologic conditions of the central nervous system.

Manipulating connexin communication channels: use of peptidomimetics and the translational outputs

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

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

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.

Posttranslational modifications in connexins and pannexins

Posttranslational modification is a common cellular process that is used by cells to ensure a particular protein function. This can happen in a variety of ways, e.g., from the addition of phosphates or sugar residues to a particular amino acid, ensuring proper protein life cycle and function. In this review, we assess the evidence for ubiquitination, glycosylation, phosphorylation, S-nitrosylation as well as other modifications in connexins and pannexin proteins. Based on the literature, we find that posttranslational modifications are an important component of connexin and pannexin regulation.

Pathogenic connexin-31 forms constitutively active hemichannels to promote necrotic cell death

Mutations in Connexin-31 (Cx31) are associated with multiple human diseases including erythrokeratodermia variabilis (EKV). The molecular action of Cx31 pathogenic mutants remains largely elusive. We report here that expression of EKV pathogenic mutant Cx31R42P induces cell death with necrotic characteristics. Inhibition of hemichannel activity by a connexin hemichannel inhibitor or high extracellular calcium suppresses Cx31R42P-induced cell death. Expression of Cx31R42P induces ER stress resulting in reactive oxygen species (ROS) production, in turn, to regulate gating of Cx31R42P hemichannels and Cx31R42P induced cell death. Moreover, Cx31R42P hemichannels play an important role in mediating ATP release from the cell. In contrast, no hemichannel activity was detected with cells expressing wildtype Cx31. Together, the results suggest that Cx31R42P forms constitutively active hemichannels to promote necrotic cell death. The Cx31R42P active hemichannels are likely resulted by an ER stress mediated ROS overproduction. The study identifies a mechanism of EKV pathogenesis induced by a Cx31 mutant and provides a new avenue for potential treatment strategy of the disease.

Replacement of a single cysteine in the fourth transmembrane region of zebrafish pannexin 1 alters hemichannel gating behavior

Pannexin1 (Panx1) is a novel candidate for an electrical synapse protein in the retina. At present Panx1 is considered to function as a hemichannel. Since information about the gating properties of Panx1 channels to date rely on blocker pharmacology, we have begun to establish a structural context of channel function starting with site directed mutagenesis of cysteine residues in transmembrane domains of Panx1. Dye uptake and whole cell voltage clamp recordings of transfected N2a cells demonstrate that zfPanx1 forms voltage activated hemichannels with a large unitary conductance in vitro. The function of this channel was significantly reduced following mutation of a single cysteine residue (C282W) in the fourth transmembrane region. This result suggests a role of this domain in gating of the Panx1 hemichannel.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

Hemichannels: permeants and their effect on development, physiology and death

Hemichannels, which are one half of the gap junction channels, have independent physiological roles. Although hemichannels consisting of connexins are more widely documented, hemichannels of pannexins, proteins homologous to invertebrate gap junction proteins also have been studied. There are at least 21 different connexin and three pannexin isotypes. This variety in isotypes results in tissue-specific hemichannels, which have been implicated in varied events ranging from development, cell survival, to cell death. Hemichannel function varies with its spatio-temporal opening, thus demanding a refined degree of regulation. This review discusses the activity of hemichannels and the molecules released in different physiological states and their impact on tissue functioning.

Connexin-based channels in astrocytes: how to study their properties

A typical feature of astrocytes is their high level of connexin expression. These membrane proteins constitute the molecular basis of two types of channels: gap junction channels that allow direct cytoplasm-to-cytoplasm communication and hemichannels that provide a pathway for exchanges between the intra- and extracellular media. An unusual property of these channels is their permeability for ions but also for small signaling molecules. They support intercellular communication that contribute to dynamic neuroglial interaction and interplay with neuronal activity and survival. Here, we describe multiple techniques based either on electrophysiological approaches or the monitoring of dye intercellular diffusion and uptake that permits an investigation of the properties of gap junction channels and hemichannels, respectively. These techniques are applied in astrocyte studies using in vitro models, mainly primary cultures and acute brain slices.

Nitric oxide signaling in the retina: what have we learned in two decades?

Two decades after its first detection in the retina, nitric oxide (NO) continues to puzzle visual neuroscientists. While its liberation by photoreceptors remains controversial, recent evidence supports three subtypes of amacrine cells as main sources of NO in the inner retina. NO synthesis was shown to depend on light stimulation, and mounting evidence suggests that NO is a regulator of visual adaptation at different signal processing levels. NO modulates light responses in all retinal neuron classes, and specific ion conductances are activated by NO in rods, cones, bipolar and ganglion cells. Light-dependent gap junction coupling in the inner and outer plexiform layers is also affected by NO. The vast majority of these effects were shown to be mediated by activation of the NO receptor soluble guanylate cyclase and resultant cGMP elevation. This review analyzes the current state of knowledge on physiological NO signaling in the retina.

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.

Single cysteines in the extracellular and transmembrane regions modulate pannexin 1 channel function

Pannexins form high-conductance ion channels in the membranes of many vertebrate cells. Functionally, they have been associated with multiple functional pathways like the propagation of calcium waves, ATP release, responses to ischemic conditions and apoptosis. In contrast to accumulating details which uncovered their functions, the molecular mechanisms for pannexin channel regulation and activation are hardly understood. To further elucidate regulatory mechanisms, we substituted cysteine residues, expected key elements for channel function, in extracellular and transmembrane regions of Pannexin 1 (Panx1). Most apparently, substitution of the transmembrane cysteine C40 resulted in constitutively open channels with profoundly increased activity. Hence, Xenopus laevis oocytes injected with corresponding cRNA showed strongly impaired viability, anomalous dye uptake and greatly increased whole-cell conductivity. All changes induced by C40 substitution were significantly reduced by the Panx1 channel blocker carbenoxolone, indicating that channel activity of the mutated Panx1 had been affected. In contrast, no changes occurred after substitution of the two other transmembrane cysteines, C215 and C227, in terms of channel conductivity. Finally, substitution of any of the four extracellular cysteines resulted in complete loss of channel function in both X. laevis oocytes and transfected N2A cells. From this, we conclude that cysteine residues of Panx1 reveal differential functional profiles for channel activation and drug sensitivity.

Functional gap junctions accumulate at the immunological synapse and contribute to T cell activation

Gap junction (GJ) mediates intercellular communication through linked hemichannels from each of two adjacent cells. Using human and mouse models, we show that connexin 43 (Cx43), the main GJ protein in the immune system, was recruited to the immunological synapse during T cell priming as both GJs and stand-alone hemichannels. Cx43 accumulation at the synapse was Ag specific and time dependent, and required an intact actin cytoskeleton. Fluorescence recovery after photobleaching and Cx43-specific inhibitors were used to prove that intercellular communication between T cells and dendritic cells is bidirectional and specifically mediated by Cx43. Moreover, this intercellular cross talk contributed to T cell activation as silencing of Cx43 with an antisense or inhibition of GJ docking impaired intracellular Ca(2+) responses and cytokine release by T cells. These findings identify Cx43 as an important functional component of the immunological synapse and reveal a crucial role for GJs and hemichannels as coordinators of the dendritic cell-T cell signaling machinery that regulates T cell activation.

Pathological hemichannels associated with human Cx26 mutations causing Keratitis-Ichthyosis-Deafness syndrome

Connexin (Cx) proteins form intercellular gap junction channels by first assembling into single membrane hemichannels that then dock to connect the cytoplasm of two adjacent cells. Gap junctions are highly specialized structures that allow the direct passage of small molecules between cells to maintain tissue homeostasis. Functional activity of nonjunctional hemichannels has now been shown in several experimental systems. Hemichannels may constitute an important diffusional exchange pathway with the extracellular space, but the extent of their normal physiological role is currently unknown. Aberrant hemichannel activity has been linked to mutations of connexin proteins involved in genetic diseases. Here, we review a proposed role for hemichannels in the pathogenesis of Keratitis-Ichthyosis-Deafness (KID) syndrome associated with connexin26 (Cx26) mutations. Continued functional evaluation of mutated hemichannels linked to human hereditary disorders may provide additional insights into the mechanisms governing their regulation in normal physiology and dysregulation in disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

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.

Connexin43 hemichannels contribute to cadmium-induced oxidative stress and cell injury

We investigated the potential involvement of connexin hemichannels in cadmium ions (Cd(2+))-elicited cell injury. Transfection of LLC-PK1 cells with a wild-type connexin43 (Cx43) sensitized them to Cd(2+)-elicited cell injury. The cell susceptibility to Cd(2+) was increased by depletion of glutathione (GSH) with DL-buthionine-[S,R]-sulfoximine, and decreased by N-acetyl-cysteine or glutathione reduced ethyl ester. Fibroblasts derived from Cx43 wild-type (Cx43+/+) and knockout (Cx43-/-) fetal littermates displayed different susceptibility to Cd(2+). Cd(2+) induced a higher concentration of reactive oxygen species, a stronger activation c-Jun N-terminal kinase, and significantly more severe cell injury in Cx43+/+ fibroblasts, as compared with Cx43-/- fibroblasts. Cd(2+) caused a reduction in intracellular GSH, whereas it elevated extracellular GSH. This effect of Cd(2+) was more dramatic in Cx43+/+ than Cx43-/- fibroblasts. Treatment of Cx43+/+ fibroblasts with Cd(2+) caused a Cx43 hemichannel-dependent influx of Lucifer Yellow and efflux of ATP. Collectively, our study demonstrates that Cx43 sensitizes cells to Cd(2+)-initiated cytotoxicity, possibly through hemichannel-mediated effects on intracellular oxidative status.

Calcium and connexin-based intercellular communication, a deadly catch?

Ca(2+) is known as a universal messenger mediating a wide variety of cellular processes, including cell death. In fact, this ion has been proposed as the 'cell death master', not only at the intracellular but also at the intercellular level. The most direct form of intercellular spread of cell death is mediated by gap junction channels. These channels have been shown to propagate cell death as well as cell survival signals between the cytoplasm of neighbouring cells, reflecting the dual role of Ca(2+) signals, i.e. cell death versus survival. Its precursor, the unopposed hemichannel (half of a gap junction channel), has recently joined in as a toxic pore connecting the intracellular with the extracellular environment and allowing the passage of a range of substances. The biochemical nature of the so-called intercellular cell death molecule, transferred through gap junctions or released/taken up via hemichannels, remains elusive but several studies pinpoint Ca(2+) itself or its messenger inositol trisphosphate as the responsible masters in crime. Although direct evidence is still lacking, indirect data including Ca(2+) involvement in intercellular communication and cell death, and effects of intercellular communication on intracellular Ca(2+) homeostasis, support this hypothesis. In addition, hemichannels and their molecular building blocks, connexin or pannexin proteins, may exert their effects on Ca(2+)-dependent cell death at the intracellular level, independently from their channel functions. This review provides a cutting edge overview of the current knowledge and underscores the intimate connection between intercellular communication, Ca(2+) signalling and cell death.

Reduced gap junctional communication among astrocytes in experimental diabetes: contributions of altered connexin protein levels and oxidative-nitrosative modifications

Experimental diabetes increases production of reactive oxygen-nitrogen species and inhibits astrocytic gap junctional communication in tissue culture and brain slices from streptozotocin (STZ)-diabetic rats by unidentified mechanisms. Relative connexin (Cx) protein levels were assessed by Western blotting using extracts from cultured astrocytes grown in high (25 mmol/liter) or low (5.5 mmol/liter) glucose for 2-3 weeks and STZ-diabetic rat brain. Chemiluminescent signals for diabetic samples were normalized to those of controls on the same blot and same protein load. Growth in high glucose did not alter relative Cx26 level, whereas Cx30 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were reduced by ∼30%, and Cx43 increased ∼1.9-fold. In the inferior colliculus of STZ-diabetic rats, Cx30 and Cx43 levels in three of four rats were half those of controls, whereas GAPDH and actin were unaffected. Diabetes did not affect levels of Cx30, Cx43, or GAPDH in cerebral cortex, but actin level rose 24%. Cx43 was predominantly phosphorylated in control and diabetic samples, so the reduced dye transfer is not due to overall dephosphorylation of Cx43. Astrocytic growth in high glucose reduced the dye-labeled area by 75%, but 10 min of treatment with dithiothreitol restored normal dye transfer. In contrast, nitric oxide donors inhibited dye transfer among astrocytes grown in low glucose by 50-65% within 1 hr. Thus, modifications arising from oxidative-nitrosative stress, not altered connexin levels, may underlie the reduced dye transfer among severely hyperglycemic cultured astrocytes, whereas both oxidative-nitrosative stress and regionally selective down-regulation of connexin protein content may affect gap junctional communication in the brains of STZ-diabetic rats.

Direct coupling through gap junctions is not involved in urethral neurotransmission

Interstitial cells of Cajal (ICC) are believed to participate in urethral neurotransmission and it was proposed that direct coupling of ICC and smooth muscle cells (SMC) through gap junctions (GJ) is involved, although this still remains unclear. Hence, we investigated the distribution of different connexins (Cx 43, Cx40, and Cx37) in the sheep and rat urethra, as well as their possible role in neurotransmission. Conventional PCR confirmed that three Cxs are expressed in the urethra. Moreover, both Cx43 and Cx37-immunoreactivity (-ir) were present in SMC, ICC, and the urothelium, although Cx37-ir was significantly weaker and Cx40-ir was limited to the endothelium. While these results indicate that GJ intercellular communication could occur between SMC and ICC, neither the contractile (noradrenergic) nor the relaxant (nitrergic) responses of the rat and sheep urethra to electrical field stimulation were significantly modified by two different GJ inhibitors: 18α-glycyrrhetinic acid and a cocktail of Cx mimetic peptides (Cx43Gap 26,Cx37, Cx43Gap 27, andCx40Gap 27). By contrast, contractions induced by high K+were effectively reduced by both blockers, evidence that they effectively inhibit intercellular communication. These results indicate that GJ are not implicated in urethral neurotransmission, although the question of whether ICC modulate neurotransmission through some other mechanism remains to be determined.

Regulation of cellular function by connexin hemichannels

Gap junctions and hemichannels are formed by a family of proteins called connexins. Till date up to twenty one different connexins have been characterized and their expression was observed to be spatio-temporally regulated. Gap junctions and hemichannels are involved in transfer of a variety of less than 1 kDa small molecules such as, ions, small metabolites, cAMP, ATP, IP3, prostaglandins, etc. Post-translational modifications of connexins and their interaction with other proteins are reported to be the key regulators of channel functions. Studies during the past decade or so, suggest the physiological important of connexin hemichannels mediating the communication between the cell and its environment. Molecules conveyed through the hemichannels elicit a variety of signaling pathways and influence cellular functions such as, cell cycle, tissue homeostasis, migration, mechanotransduction, oxidative stress. The purpose of the current review is to compile the reported studies so far and provide a general overview in our understanding how the molecular transfer through hemichannels regulates cellular signaling and functions.

Nε-lysine acetylation determines dissociation from GAP junctions and lateralization of connexin 43 in normal and dystrophic heart

Wanting to explore the epigenetic basis of Duchenne cardiomyopathy, we found that global histone acetylase activity was abnormally elevated and the acetylase P300/CBP-associated factor (PCAF) coimmunoprecipitated with connexin 43 (Cx43), which was N(ε)-lysine acetylated and lateralized in mdx heart. This observation was paralleled by Cx43 dissociation from N-cadherin and zonula occludens 1, whereas pp60-c-Src association was unaltered. In vivo treatment of mdx with the pan-histone acetylase inhibitor anacardic acid significantly reduced Cx43 N(ε)-lysine acetylation and restored its association to GAP junctions (GJs) at intercalated discs. Noteworthy, in normal as well as mdx mice, the class IIa histone deacetylases 4 and 5 constitutively colocalized with Cx43 either at GJs or in the lateralized compartments. The class I histone deacetylase 3 was also part of the complex. Treatment of normal controls with the histone deacetylase pan-inhibitor suberoylanilide hydroxamic acid (MC1568) or the class IIa-selective inhibitor 3-{4-[3-(3-fluorophenyl)-3-oxo-1-propen-1-yl]-1-methyl-1H-pyrrol-2-yl}-N-hydroxy-2-propenamide (MC1568) determined Cx43 hyperacetylation, dissociation from GJs, and distribution along the long axis of ventricular cardiomyocytes. Consistently, the histone acetylase activator pentadecylidenemalonate 1b (SPV106) hyperacetylated cardiac proteins, including Cx43, which assumed a lateralized position that partly reproduced the dystrophic phenotype. In the presence of suberoylanilide hydroxamic acid, cell to cell permeability was significantly diminished, which is in agreement with a Cx43 close conformation in the consequence of hyperacetylation. Additional experiments, performed with Cx43 acetylation mutants, revealed, for the acetylated form of the molecule, a significant reduction in plasma membrane localization and a tendency to nuclear accumulation. These results suggest that Cx43 N(ε)-lysine acetylation may have physiopathological consequences for cell to cell coupling and cardiac function.

FGF-1 induces ATP release from spinal astrocytes in culture and opens pannexin and connexin hemichannels

Spinal astrocytes are coupled by connexin (Cx) gap junctions and express pannexin 1 (Px1) and purinergic receptors. Fibroblast growth factor 1 (FGF-1), which is released in spinal cord injury, activated spinal astrocytes in culture, induced secretion of ATP, and permeabilized them to relatively large fluorescent tracers [ethidium (Etd) and lucifer yellow (LY)] through "hemichannels" (HCs). HCs can be formed by connexins or pannexins; they can open to extracellular space or can form gap junction (GJ) channels, one HC from each cell. (Pannexins may not form gap junctions in mammalian tissues, but they do in invertebrates). HC types were differentiated pharmacologically and by Px1 knockdown with siRNA and by use of astrocytes from Cx43 knockout mice. Permeabilization was reduced by apyrase (APY), an ATPase, and by P2X(7) receptor antagonists, implicating secretion of ATP and autocrine and/or paracrine action. Increased permeability of cells exposed to FGF-1 or ATP for 2 h was mediated largely by Px1 HCs activated by P2X(7) receptors. After a 7-h treatment, the permeability was mediated by both Cx43 and Px1 HCs. FGF-1 also caused reduction in gap junctional communication. Botulinum neurotoxin A, a blocker of vesicular release, reduced permeabilization when given 30 min before FGF-1 application, but not when given 1 h after FGF-1. We infer that ATP is initially released from vesicles and then it mediates continued release by action on P2X(7) receptors and opening of HCs. These changes in HCs and gap junction channels may promote inflammation and deprive neurons of astrocyte-mediated protection in spinal cord trauma and neurodegenerative disease.

Compartmentalized connexin 43 s-nitrosylation/denitrosylation regulates heterocellular communication in the vessel wall

OBJECTIVE

To determine whether S-nitrosylation of connexins (Cxs) modulates gap junction communication between endothelium and smooth muscle.

METHODS AND RESULTS

Heterocellular communication is essential for endothelium control of smooth muscle constriction; however, the exact mechanism governing this action remains unknown. Cxs and NO have been implicated in regulating heterocellular communication in the vessel wall. The myoendothelial junction serves as a conduit to facilitate gap junction communication between endothelial cells and vascular smooth muscle cells within the resistance vasculature. By using isolated vessels and a vascular cell coculture, we found that Cx43 is constitutively S-nitrosylated on cysteine 271 because of active endothelial NO synthase compartmentalized at the myoendothelial junction. Conversely, we found that stimulation of smooth muscle cells with the constrictor phenylephrine caused Cx43 to become denitrosylated because of compartmentalized S-nitrosoglutathione reductase, which attenuated channel permeability. We measured S-nitrosoglutathione breakdown and NO(x) concentrations at the myoendothelial junction and found S-nitrosoglutathione reductase activity to precede NO release.

CONCLUSIONS

This study provides evidence for compartmentalized S-nitrosylation/denitrosylation in the regulation of smooth muscle cell to endothelial cell communication.

Inhibition of cytokine-induced connexin43 hemichannel activity in astrocytes is neuroprotective

Astrocytes express high levels of connexin43, a protein that forms two types of channels: gap junction channels for direct intercellular communication, and hemichannels for exchanges with the extracellular space. Inflammation induces connexin43 hemichannel activation, which has been proposed to be involved in neuroglial interactions. Here, we investigated the contribution of connexin43 to NMDA-induced excitotoxicity in neuron/astrocyte co-cultures, after treatment with a pro-inflammatory cytokine mixture, containing TNF-alpha and IL1-beta (Mix), that stimulated astroglial connexin43 hemichannel activity. Interestingly, NMDA treatment induced a higher amount of neurotoxicity in Mix-treated co-cultures than in untreated ones, whereas this extent of neurotoxicity was absent in enriched neuron cultures or in co-cultures with connexin43 knock-out astrocytes. Furthermore, application of connexin43 hemichannel blockers or a synthetic cannabinoid prevented the Mix-induced potentiated NMDA neurotoxicity. Altogether, these data demonstrate that inflammation-induced astroglial hemichannel activation plays a critical role in neuronal death and suggest a neuroprotective role of connexin43 hemichannel blockade.

Connexin 43 hemichannels mediate the Ca2+ influx induced by extracellular alkalinization

Although alkaline pH is known to trigger Ca(2+) influx in diverse cells, no pH-sensitive Ca(2+) channel has been identified. Here, we report that extracellular alkalinization induces opening of connexin 43 hemichannels (Cx43 HCs). Increasing extracellular pH from 7.4 to 8.5, in the presence of physiological Ca(2+)/Mg(2+) concentrations, rapidly increased the ethidium uptake rate and open probability of HCs in Cx43 and Cx43EGFP HeLa transfectants (HeLa-Cx3 and HeLa-Cx43EGFP, respectively) but not in parental HeLa cells (HeLa-parental) lacking Cx43 HCs. The increase in ethidium uptake induced by pH 8.5 was not affected by raising the extracellular Ca(2+) concentration from 1.8 to 10 mM but was inhibited by a connexin HC inhibitor (La(3+)). Probenecid, a pannexin HC blocker, had no effect. Extracellular alkalinization increased the intracellular Ca(2+) levels only in cells expressing HCs. The above changes induced by extracellular alkalinization did not change the cellular distribution of Cx43, suggesting that HC activation occurs through a gating mechanism. Experiments on cells expressing a COOH-terminal truncated Cx43 mutant indicated that the effects of alkalinization on intracellular Ca(2+) and ethidium uptake did not depend on the Cx43 C terminus. Moreover, purified dephosphorylated Cx43 HCs reconstituted in liposomes were Ca(2+) permeable, suggesting that Ca(2+) influx through Cx43 HCs could account for the elevation in intracellular Ca(2+) elicited by extracellular alkalinization. These studies identify a membrane pathway for Ca(2+) influx and provide a potential explanation for the activation of cellular events induced by extracellular alkalinization.

Intracellular cysteine 346 is essentially involved in regulating Panx1 channel activity

Pannexins constitute a family of proteins exhibiting predominantly hemichannel activity. Pannexin channels have been suggested to participate in a wide spectrum of biological functions such as propagation of calcium waves, release of IL-1β, and responses to ischemic conditions. At present, the molecular mechanisms regulating pannexin hemichannel activity are essentially unknown. Because cysteines have been shown to constitute key elements in regulating hemichannel properties of the connexin-type we performed site-directed mutagenesis of intracellular cysteine residues of Panx1. Cysteine to serine exchange (Cys → Ser) at the C-terminal position amino acid 346 led to a constitutively leaky hemichannel and subsequently to cell death. Increased channel activity was demonstrated by dye uptake and electrophysiological profiling in injected Xenopus laevis oocytes and transfected N2A cells. Mutations of the remaining intracellular cysteines did not result in major changes of Panx1 channel properties. From these data we conclude that the Cys-346 residue is important for proper functioning of the Panx1 channel.

CO2-dependent opening of connexin 26 and related β connexins

We have previously shown connexin mediated CO(2)-dependent ATP release from the surface of the medulla oblongata. Given the localization of connexin 26 (Cx26) to the chemosensing areas of the medulla, we have tested in a heterologous expression system (HeLa cells) whether Cx26 may be sensitive to changes in PCO2. Cx26 responded to an increase in PCO2 at constant extracellular pH by opening and to a decrease in PCO2 by closing. Furthermore, Cx26 was partially activated at a physiological PCO2 of around 40 mmHg. Cx26 in isolated patches responded to changes in PCO2, suggesting direct CO(2) sensitivity of the hemichannel to CO(2). Heterologous expression of Cx26 in HeLa cells was sufficient to endow them with the capacity to release ATP in a CO(2)-sensitive manner. We have examined other heterologously expressed connexins for their ability to respond to changes in PCO2. The closely related β connexins Cx30 and Cx32 also displayed sensitivity to changes in PCO2, but with slightly different characteristics from Cx26. The more distant Cx43 exhibited CO(2)-dependent closing (possibly mediated through intracellular acidification), while Cx36 displayed no CO(2) sensitivity. These surprising findings suggest that connexins may play a hitherto unappreciated variety of signalling roles, and that Cx26 and related β connexins may impart direct sensitivity to CO(2) throughout the brain.