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

Intramolecular loop/tail interactions are essential for connexin 43-hemichannel activity

Connexin-assembled gap junctions (GJs) and hemichannels coordinate intercellular signaling processes. Although the regulation of connexins in GJs has been well characterized, the molecular determinants controlling connexin-hemichannel activity are unresolved. Here we investigated the regulation of Cx43-hemichannel activity by actomyosin contractility and intracellular [Ca(2+)] ([Ca(2+)](i)) using plasma membrane-permeable TAT peptides (100 μM) designed to interfere with interactions between the cytoplasmic loop (CL) and carboxy-terminal (CT) in primary bovine corneal endothelial cells and HeLa, C6 glioma, and Xenopus oocytes ectopically expressing Cx43. Peptides corresponding to the last 10 CT aa (TAT-Cx43CT) prevented the inhibition of Cx43-hemichannel activity by contractility/high [Ca(2+)](i), whereas a reverse peptide (TAT-Cx43CTrev) did not. These effects were independent of zonula occludens-1, a cytoskeletal-associated Cx43-binding protein. In contrast, peptides corresponding to CL (TAT-L2) inhibited Cx43-hemichannel responses, whereas a mutant peptide (TAT-L2(H126K/I130N)) did not inhibit. In these assays, TAT-Cx43CT acted as a scaffold for TAT-L2 and vice versa, a finding supported by surface plasmon resonance measurements. Loop/tail interactions appeared essential for Cx43-hemichannel activity, because TAT-Cx43CT restored the activity of nonfunctional hemichannels, consisting of either Cx43 lacking the C-terminal tail (Cx43(M239)) or intact Cx43 ectopically expressed in Xenopus oocytes. We conclude that intramolecular loop/tail interactions control Cx43-hemichannel activity, laying the basis for developing hemichannel-specific blockers.

Phosphorylation of extrajunctional Cx43 in ischemic-preconditioned rat hearts

Ischemic preconditioning (IP) has been found to intensify ischemia-induced spreading of connexin 43 (Cx43) into the extrajunctional plasma membrane of the rat heart. Since ischemia is known to induce connexin dephosphorylation and plasmalemmal permeabilization, we considered whether IP might delay dephosphorylation of also extrajunctional connexin and thus impede myocyte permeabilization. Regional myocardial ischemia of both 15 and 45 min duration with and without IP, respectively, was induced in anesthetized rats. Sections of the hearts were immunostained for phosphorylated and dephosphorylated Cx43. The ratios of immunofluorescence in gap junctions and extrajunctional membranes, respectively, versus myocyte interiors were determined as relative fluorescence units (RFU). IP, however, was not found to reduce gap junctional dephosphorylated Cx43 (normal perfusion: 1.23 +/- 0.06 RFU; 15 min ischemia without and with IP: 1.80 +/- 0.09 and 2.10 +/- 0.24 RFU; 45 min ischemia without and with IP: 3.12 +/- 0.55 and 2.57 +/- 0.33 RFU, respectively). Extrajunctionally, only dephosphorylated Cx43 was found. Its occurrence was not prevented by IP, it was rather intensified by IP (normal perfusion: 1.02 +/- 0.01 RFU, 15 min ischemia without and with IP: 1.06 +/- 0.03 and 1.15 +/- 0.05 RFU (P < 0.02), and 45 min ischemia without and with IP: 1.30 +/- 0.07 and 1.27 +/- 0.07 RFU, respectively). Although under ischemia, junctional and extrajunctional Cx43 predominated in the dephosphorylated state, only a small fraction of myocytes were found permeabilized to propidium iodide. In conclusion, IP did not prevent ischemia-induced Cx43 dephosphorylation in gap junctions and extrajunctional plasma membranes. It is thus more likely that changes in non-channel functions or other localizations of Cx43 are involved in IP-induced myocardial protection.

Hypoxia in high glucose followed by reoxygenation in normal glucose reduces the viability of cortical astrocytes through increased permeability of connexin 43 hemichannels

Brain ischemia causes more extensive injury in hyperglycemic than normoglycemic subjects, and the increased damage is to astroglia as well as neurons. In the present work, we found that in cortical astrocytes from rat or mouse, reoxygenation after hypoxia in a medium mimicking interstitial fluid during ischemia increases hemichannel activity and decreases cell-cell communication via gap junctions as indicated by dye uptake and dye coupling, respectively. These effects were potentiated by high glucose during the hypoxia in a concentration-dependent manner (and by zero glucose) and were not observed in connexin 43(-/-) astrocytes. The responses were transient and persistent after short and long periods of hypoxia, respectively. The persistent responses were associated with a progressive reduction in cell viability that was prevented by La(3+) or peptides that block connexin 43 (Cx43) hemichannels or by inhibition of p38 MAP kinase prior to hypoxia-reoxygenation but not by treatments that block pannexin hemichannels. Block of Cx43 hemichannels did not affect the reduction in gap junction mediated dye coupling observed during reoxygenation. Cx43 hemichannels may be a novel therapeutic target to reduce cell death following stroke, particularly in hyperglycemic conditions.

Non-junction functions of pannexin-1 channels

Pannexins are large-pore ion channels with broad expression in the central nervous system (CNS). The channels function by releasing large signaling molecules, such ATP and arachidonic acid derivatives, from neurons and possibly astrocytes. They might also contribute to novel forms of non-synaptic communication in the CNS, thereby affecting synaptic function, astrocytic Ca(2+) wave propagation and possibly regulation of vascular tone in the brain. Panx1 activation in various in vitro pathological conditions implicates these channels in ischemic, excitotoxic and ATP-dependent cell death, whereas Panx coupling with purinergic receptors triggers the inflammasome. Novel functions for the pannexin channels are likely to be discovered as current understanding of how they are regulated in physiological and pathological situations improves.

Voltage-dependent facilitation of Cx46 hemichannels

Gap junction channels are formed by two hemichannels in series (one from each neighboring cell), which are in turn connexin hexamers. Under normal conditions, hemichannels at the plasma membrane are mostly closed but can be opened by changes in membrane voltage, extracellular divalent ion concentration, phosphorylation, pH, and redox potential. Recently, interactions between channels have been found to modulate the activity of several ion channels, including gap junction channels. Here, we studied whether connexin46 (Cx46) hemichannels display such behavior. We studied the response of the Cx46 hemichannels expressed in Xenopus laevis oocytes to consecutive depolarization pulses. Hemichannels formed by wild-type Cx46 and a COOH-terminal domain truncation mutant (Cx46DeltaCT) were activated by voltage pulses. When the hemichannels were depolarized repeatedly from -60 mV to +80 mV, the amplitude of the outward and tail currents increased progressively with successive pulses. This phenomenon ("current facilitation") depended on the amplitude of the depolarization, reaching a maximum at approximately +60 mV in oocytes expressing Cx46, and on the interval between pulses, disappearing with intervals longer than about 20 s. The current facilitation was also present in oocytes expressing Cx46DeltaCT, ruling out a primary role of this domain in the facilitation. Nominal removal of divalent cations from the extracellular side caused maximal current activation of Cx46 and Cx46DeltaCT hemichannels and prevented facilitation. The results suggest that Cx46 hemichannels show a cooperative activation independent of their COOH-terminal domain.

Pannexins, distant relatives of the connexin family with specific cellular functions?

Intercellular communication (IC) is mediated by gap junctions (GJs) and hemichannels, which consist of proteins. This has been particularly well documented for the connexin (Cx) family. Initially, Cxs were thought to be the only proteins capable of GJ formation in vertebrates. About 10 years ago, however, a new GJ-forming protein family related to invertebrate innexins (Inxs) was discovered in vertebrates, and named the pannexin (Panx) family. Panxs, which are structurally similar to Cxs, but evolutionarily distinct, have been shown to be co-expressed with Cxs in vertebrates. Both protein families show distinct properties and have their own particular function. Identification of the mechanisms that control Panx channel gating is a major challenge for future work. In this review, we focus on the specific properties and role of Panxs in normal and pathological conditions.

Metabolic inhibition increases activity of connexin-32 hemichannels permeable to Ca2+ in transfected HeLa cells

Numerous cell types express functional connexin (Cx) hemichannels (HCs), and membrane depolarization and/or exposure to a divalent cation-free bathing solution (DCFS) have been shown to promote HC opening. However, little is known about conditions that can promote HC opening in the absence of strong depolarization and when extracellular divalent cation concentrations remain at physiological levels. Here the effects of metabolic inhibition (MI), an in vitro model of ischemia, on the activity of mouse Cx32 HCs were examined. In HeLa cells stably transfected with mouse Cx32 (HeLa-Cx32), MI induced an increase in cellular permeability to ethidium (Etd). The increase in Etd uptake was directly related to an increase in levels of Cx32 HCs present at the cell surface. Moreover, MI increased membrane currents in HeLa-Cx32 cells. Underlying these currents were channels exhibiting a unitary conductance of approximately 90 pS, consistent with Cx32 HCs. These currents and Etd uptake were blocked by HC inhibitors. The increase in Cx32 HC activity was preceded by a rapid reduction in mitochondrial membrane potential and a rise in free intracellular Ca(2+) concentration ([Ca(2+)](i)). The increase in free [Ca(2+)](i) was prevented by HC blockade or exposure to extracellular DCFS and was virtually absent in parental HeLa cells. Moreover, inhibition of Cx32 HCs expressed by HeLa cells in low-confluence cultures drastically reduced cell death induced by oxygen-glucose deprivation, which is a more physiological model of ischemia-reperfusion. Thus HC blockade could reduce the increase in free [Ca(2+)](i) and cell death induced by ischemia-like conditions in cells expressing Cx32 HCs.

Ca(2+) regulation of connexin 43 hemichannels in C6 glioma and glial cells

Connexin hemichannels have a low open probability under normal conditions but open in response to various stimuli, forming a release pathway for small paracrine messengers. We investigated hemichannel-mediated ATP responses triggered by changes of intracellular Ca(2+) ([Ca(2+)](i)) in Cx43 expressing glioma cells and primary glial cells. The involvement of hemichannels was confirmed with gja1 gene-silencing and exclusion of other release mechanisms. Hemichannel responses were triggered when [Ca(2+)](i) was in the 500nM range but the responses disappeared with larger [Ca(2+)](i) transients. Ca(2+)-triggered responses induced by A23187 and glutamate activated a signaling cascade that involved calmodulin (CaM), CaM-dependent kinase II, p38 mitogen activated kinase, phospholipase A2, arachidonic acid (AA), lipoxygenases, cyclo-oxygenases, reactive oxygen species, nitric oxide and depolarization. Hemichannel responses were also triggered by activation of CaM with a Ca(2+)-like peptide or exogenous application of AA, and the cascade was furthermore operational in primary glial cells isolated from rat cortex. In addition, several positive feed-back loops contributed to amplify the responses. We conclude that an elevation of [Ca(2+)](i) triggers hemichannel opening, not by a direct action of Ca(2+) on hemichannels but via multiple intermediate signaling steps that are adjoined by distinct signaling mechanisms activated by high [Ca(2+)](i) and acting to restrain cellular ATP loss.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

Reduction of electrical coupling between microvascular endothelial cells by NO depends on connexin37

We have previously shown that increased nitric oxide (NO) production in sepsis impairs arteriolar-conducted vasoconstriction cGMP independently and that the gap junction protein connexin (Cx) 37 is required for this conducted response. In the present study, we hypothesized that NO impairs interendothelial electrical coupling in sepsis by targeting Cx37. We examined the effect of exogenous NO on coupling in monolayers of cultured microvascular endothelial cells derived from the hindlimb skeletal muscle of wild-type (WT), Cx37 null, Cx40 null, and Cx43(G60S) (nonfunctional mutant) mice. To assess coupling, we measured the spread of electrical current injected in the monolayer and calculated the monolayer intercellular resistance (inverse measure of coupling). The NO donor 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (DETA) rapidly and reversibly reduced coupling in cells from WT mice, cGMP independently. NO scavenger HbO(2) did not affect baseline coupling, but it eliminated DETA-induced reduction in coupling. Reduced coupling in response to DETA was also seen in cells from Cx40 null and Cx43(G60S) mice, but not in cells from Cx37 null mice. DETA did not alter the expression of Cx37, Cx40, and Cx43 in WT cells analyzed by immunoblotting and immunofluorescence. Furthermore, neither the peroxynitrite scavenger 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron (III), superoxide scavenger Mn(III)tetrakis(4-benzoic acid)porphyrin chloride, nor preloading of WT cells with the antioxidant ascorbate affected this reduction. We conclude that NO-induced reduction of electrical coupling between microvascular endothelial cells depends on Cx37 and propose that NO in sepsis impairs arteriolar-conducted vasoconstriction by targeting Cx37 within the arteriolar wall.

Modulation of Cx46 hemichannels by nitric oxide

Gap-junction hemichannels are composed of six protein subunits (connexins). Undocked hemichannels contribute to physiological autocrine/paracrine cell signaling, including release of signaling molecules, cell-volume regulation, and glucose uptake. In addition, hemichannels may be pathologically activated by dephosphorylation and cell-membrane depolarization. Such hemichannel opening may induce and/or accelerate cell death. It has been suggested that connexin43 (Cx43) hemichannels are sensitive to redox potential changes and that one or more intracellular cysteines is/are important for this process. Cx46 is expressed in the lens, and its dysfunction induces cataract formation. It contains six cysteines in the extracellular loops, one in the fourth transmembrane helix, and two in the COOH-terminal domain. The latter may be susceptible to oxidation by nitric oxide (NO), which could be involved in cataract formation through cysteine S-nitrosylation. Here we report studies of the effects of the NO donor S-nitrosoglutathione (GSNO) on the electrical properties and fluorescent-dye permeability of wild-type Cx46 and mutant hemichannels expressed in Xenopus laevis oocytes. GSNO enhanced hemichannel voltage sensitivity, increased tail-current amplitude, and changed activation and closing kinetics in Cx46 and Cx46-CT43 (Cx46 mutant in which the COOH terminus was replaced with that of Cx43), but not in Cx46-C3A (Cx46 in which the intracellular and transmembrane helix 4 cysteines were mutated to alanine). We conclude that Cx46 hemichannels are sensitive to NO and that the NO effects are mediated by modification of one or more intracellular cysteines. However, it is unlikely that NO induces cataract formation due to the hemichannel activation, because at normal resting potential, NO had no major effects on Cx46 hemichannel permeability.

Regulation of intracellular and mitochondrial sodium in health and disease

The transmembrane sodium gradient is essential for both excitability of the cardiac cell and the regulation of the cytoplasmic concentrations of Ca and protons. In addition, movements of Na across the mitochondrial membrane affect matrix protons and calcium. In the first part of the review, we discuss the most important pathways responsible for sarcolemmal and mitochondrial sodium movements. The bulk of the review considers the changes of intracellular Na concentration ([Na(+)](i)) that occur in disease, specifically, ischemia, reperfusion, and heart failure. We review evidence implicating the increase of intracellular sodium to either increased influx of sodium (via either sodium channels or sodium/hydrogen exchange) or, alternatively, to decreased efflux on the Na/K pump. Although much has been learned about sodium regulation in the heart, there are still many unanswered questions, particularly concerning mitochondrial Na regulation.

Connexins: a myriad of functions extending beyond assembly of gap junction channels

Connexins constitute a large family of trans-membrane proteins that allow intercellular communication and the transfer of ions and small signaling molecules between cells. Recent studies have revealed complex translational and post-translational mechanisms that regulate connexin synthesis, maturation, membrane transport and degradation that in turn modulate gap junction intercellular communication. With the growing myriad of connexin interacting proteins, including cytoskeletal elements, junctional proteins, and enzymes, gap junctions are now perceived, not only as channels between neighboring cells, but as signaling complexes that regulate cell function and transformation. Connexins have also been shown to form functional hemichannels and have roles altogether independent of channel functions, where they exert their effects on proliferation and other aspects of life and death of the cell through mostly-undefined mechanisms. This review provides an updated overview of current knowledge of connexins and their interacting proteins, and it describes connexin modulation in disease and tumorigenesis.

Connexin-related signaling in cell death: to live or let die?

Evidence is accumulating that some forms of cell death, like apoptosis, are not only governed by the complex interplay between extracellular and intracellular signals but are also strongly influenced by intercellular communicative networks. The latter is provided by arrays of channels consisting of connexin proteins, with gap junctions directly connecting the cytoplasm of neighboring cells and hemichannels positioned as pores that link the cytoplasm to the extracellular environment. The role of gap junctions in cell death communication has received considerable interest and recently hemichannels have joined in as potentially toxic pores adding their part to the cell death process. However, despite a large body of existing evidence, especially for gap junctions, the exact contribution of the connexin channel family still remains controversial, as both gap junctions and hemichannels may furnish cell death as well as cell survival signals. An additional layer of complexity is formed by the fact that connexin proteins as such, beyond their channel function, may influence the cell death process. We here review the current knowledge on connexins and their channels in cell death and specifically address the molecular mechanisms that underlie connexin-related signaling. We also briefly focus on pannexins, a novel set of connexin-like proteins that have been implicated in cellular responses to pathological insults.

Oxidative stress, lens gap junctions, and cataracts

The eye lens is constantly subjected to oxidative stress from radiation and other sources. The lens has several mechanisms to protect its components from oxidative stress and to maintain its redox state, including enzymatic pathways and high concentrations of ascorbate and reduced glutathione. With aging, accumulation of oxidized lens components and decreased efficiency of repair mechanisms can contribute to the development of lens opacities or cataracts. Maintenance of transparency and homeostasis of the avascular lens depend on an extensive network of gap junctions. Communication through gap junction channels allows intercellular passage of molecules (up to 1 kDa) including antioxidants. Lens gap junctions and their constituent proteins, connexins (Cx43, Cx46, and Cx50), are also subject to the effects of oxidative stress. These observations suggest that oxidative stress-induced damage to connexins (and consequent altered intercellular communication) may contribute to cataract formation.

Modulation of brain hemichannels and gap junction channels by pro-inflammatory agents and their possible role in neurodegeneration

In normal brain, neurons, astrocytes, and oligodendrocytes, the most abundant and active cells express pannexins and connexins, protein subunits of two families forming membrane channels. Most available evidence indicates that in mammals endogenously expressed pannexins form only hemichannels and connexins form both gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activity, hemichannels communicate the intra- and extracellular compartments and serve as a diffusional pathway for ions and small molecules. A subthreshold stimulation by acute pathological threatening conditions (e.g., global ischemia subthreshold for cell death) enhances neuronal Cx36 and glial Cx43 hemichannel activity, favoring ATP release and generation of preconditioning. If the stimulus is sufficiently deleterious, microglia become overactivated and release bioactive molecules that increase the activity of hemichannels and reduce gap junctional communication in astroglial networks, depriving neurons of astrocytic protective functions, and further reducing neuronal viability. Continuous glial activation triggered by low levels of anomalous proteins expressed in several neurodegenerative diseases induce glial hemichannel and gap junction channel disorders similar to those of acute inflammatory responses triggered by ischemia or infectious diseases. These changes are likely to occur in diverse cell types of the CNS and contribute to neurodegeneration during inflammatory process.

Gap junctions in the control of vascular function

Direct intercellular communication via gap junctions is critical in the control and coordination of vascular function. In the cardiovascular system, gap junctions are made up of one or more of four connexin proteins: Cx37, Cx40, Cx43, and Cx45. The expression of more than one gap-junction protein in the vasculature is not redundant. Rather, vascular connexins work in concert, first during the development of the cardiovascular system, and then in integrating smooth muscle and endothelial cell function, and in coordinating cell function along the length of the vessel wall. In addition, connexin-based channels have emerged as an important signaling pathway in the astrocyte-mediated neurovascular coupling. Direct electrical communication between endothelial cells and vascular smooth muscle cells via gap junctions is thought to play a relevant role in the control of vasomotor tone, providing the signaling pathway known as endothelium-derived hyperpolarizing factor (EDHF). Consistent with the importance of gap junctions in the regulation of vasomotor tone and arterial blood pressure, the expression of connexins is altered in diseases associated with vascular complications. In this review, we discuss the participation of connexin-based channels in the control of vascular function in physiologic and pathologic conditions, with a special emphasis on hypertension and diabetes.

Zinc modulation of hemi-gap-junction channel currents in retinal horizontal cells

Hemi-gap-junction (HGJ) channels of retinal horizontal cells (HCs) function as transmembrane ion channels that are modulated by voltage and calcium. As an endogenous retinal neuromodulator, zinc, which is coreleased with glutamate at photoreceptor synapses, plays an important role in shaping visual signals by acting on postsynaptic HCs in vivo. To understand more fully the regulation and function of HC HGJ channels, we examined the effect of Zn(2+) on HGJ channel currents in bass retinal HCs. Hemichannel currents elicited by depolarization in Ca(2+)-free medium and in 1 mM Ca(2+) medium were significantly inhibited by extracellular Zn(2+). The inhibition by Zn(2+) of hemichannel currents was dose dependent with a half-maximum inhibitory concentration of 37 microM. Compared with other divalent cations, Zn(2+) exhibited higher inhibitory potency, with the order being Zn(2+) > Cd(2+) approximately Co(2+) > Ca(2+) > Ba(2+) > Mg(2+). Zn(2+) and Ca(2+) were found to modulate HGJ channels independently in additivity experiments. Modification of histidine residues with N-bromosuccinimide suppressed the inhibitory action of Zn(2+), whereas modification of cysteine residues had no significant effect on Zn(2+) inhibition. Taken together, these results suggest that zinc acts on HGJ channels in a calcium-independent way and that histidine residues on the extracellular domain of HGJ channels mediate the inhibitory action of zinc.

Biological and biophysical properties of vascular connexin channels

Intercellular channels formed by connexin proteins play a pivotal role in the direct movement of ions and larger cytoplasmic solutes between vascular endothelial cells, between vascular smooth muscle cells, and between endothelial and smooth muscle cells. Multiple genetic and epigenetic factors modulate connexin expression levels and/or channel function, including cell-type-independent and cell-type-specific transcription factors, posttranslational modifications, and localized membrane targeting. Additionally, differences in protein-protein interactions, including those between connexins, significantly contribute to both vascular homeostasis and disease progression. The biophysical properties of the connexin channels identified in the vasculature, those formed by Cx37, Cx40, Cx43 and/or Cx45 proteins, are discussed in this chapter in the physiological and pathophysiological context of vessel function.

Loop gating of connexin hemichannels involves movement of pore-lining residues in the first extracellular loop domain

Unapposed connexin hemichannels exhibit robust closure in response to membrane hyperpolarization and extracellular calcium. This form of gating, termed "loop gating," is largely responsible for regulating hemichannel opening, thereby preventing cell damage through excessive flux of ions and metabolites. The molecular components and structural rearrangements underlying loop gating remain unknown. Here, using cysteine mutagenesis in Cx50, we demonstrate that residues at the TM1/E1 border undergo movement during loop gating. Replacement of Phe(43) in Cx50 with a cysteine resulted in small or no appreciable membrane currents. Bath application of dithiothreitol or TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl) ethylenediamine), reagents that exhibit strong transition metal chelating activity, led to robust currents indicating that the F43C substitution impaired hemichannel function, producing "lock-up" in a closed or poorly functional state due to formation of metal bridges. In support, Cd(2+) at submicromolar concentrations (50-100 nm) enhanced lock-up of F43C hemichannels. Moreover, lock-up occurred under conditions that favored closure, indicating that the sulfhydryl groups come close enough to each other or to other residues to coordinate metal ions with high affinity. In addition to F43C, metal binding was also found for G46C, and to a lesser extent, D51C substitutions, positions found to be pore-lining in the open state using the substituted-cysteine accessibility method, but not for A40C and A41C substitutions, which were not found to reside in the open pore. These results indicate that metal ions access the cysteine side chains through the open pore and that closure of the loop gate involves movement of the TM1/E1 region that results in local narrowing of the large aqueous connexin pore.

Role for nitric oxide in permeability of hippocampal neuronal hemichannels during oxygen glucose deprivation

Increased hemichannel opening induced by oxygen glucose deprivation (OGD) was reported in the hippocampal pyramidal neuron. It was suggested that the pannexin1 hemichannel opening could mediate ionic flux dysregulation, anoxic depolarization, and energy-depleting efflux of glucose and ATP for ischemic neurons. However, the regulatory mechanisms of pannexin1 hemichannel opening have been poorly understood. Here we showed that excessive generation of nitric oxide (NO) during ischemia could induce the calcein leakage from neurons, which was markedly reduced by NO synthase inhibitor. The calcein leakage from neurons during OGD was also attenuated by the application of N-ethylmaleimide (NEM), an SH-alkylating agent, and dithiothreitol (DTT), a reducer of oxidized sulfhydryl groups. However, the soluble guanylyl cyclase (sGC) inhibitor had a minor effect on the calcein leakage during OGD. Furthermore, the elevated intracellular but not extracellular levels of glutathione could also inhibit the calcein leakage during OGD. Similar results were observed in metabolic inhibition (MI), which is another ischemic-like condition. Finally, immunocytochemical and immunoblotting analysis revealed that, after 1 hr of OGD stimulation, the distribution and expression of pannexin1 showed no significant difference compared with control. However, the pannexin1 mRNA expression was elevated after 1 hr of OGD and a sustained increase was maintained during reperfusion. These results implied that the reactive oxygen species (ROS), especially NO, might be involved in the enhanced pannexin1 hemichannel opening and that the S-nitrosylation but not the NO/cGMP pathway played a more important role in this event.

Connexin hemichannel composition determines the FGF-1-induced membrane permeability and free [Ca2+]i responses

Cell surface hemichannels (HCs) composed of different connexin (Cx) types are present in diverse cells and their possible role on FGF-1-induced cellular responses remains unknown. Here, we show that FGF-1 transiently (4-14 h, maximal at 7 h) increases the membrane permeability through HCs in HeLa cells expressing Cx43 or Cx45 under physiological extracellular Ca(2+)/Mg(2+) concentrations. The effect does not occur in HeLa cells expressing HCs constituted of Cx26 or Cx43 with its C-terminus truncated at aa 257, or in parental nontransfected HeLa cells. The increase in membrane permeability is associated with a rise in HC levels at the cell surface and a proportional increase in HC unitary events. The response requires an early intracellular free Ca(2+) concentration increase, activation of a p38 MAP kinase-dependent pathway, and a regulatory site of Cx subunit C-terminus. The FGF-1-induced rise in membrane permeability is also associated with a late increase in intracellular free Ca(2+) concentration, suggesting that responsive HCs allow Ca(2+) influx. The cell density of Cx26 and Cx43 HeLa transfectants cultured in serum-free medium was differentially affected by FGF-1. Thus, the FGF-1-induced cell permeabilization and derived consequences depend on the Cx composition of HCs.

Stimulated efflux of amino acids and glutathione from cultured hippocampal slices by omission of extracellular calcium: likely involvement of connexin hemichannels

Omission of extracellular Ca(2+) for 15 min from the incubation medium of cultured hippocampal slices stimulated the efflux of glutathione, phosphoethanolamine, hypotaurine, and taurine. The efflux was reduced by several blockers of gap junctions, i.e. carbenoxolone, flufenamic acid, and endothelin-1, and by the connexin43 hemichannel blocking peptide Gap26 but was unchanged by the P2X(7) receptor inhibitor oxidized ATP, a pannexin1 hemichannel blocking peptide and an inactive analogue of carbenoxolone. Pretreatment of the slices with the neurotoxin N-methyl-d -aspartate left the efflux by Ca(2+) omission unchanged, indicating that the stimulated efflux primarily originated from glia. Elevated glutamate efflux was detected when Ca(2+) omission was combined with the glutamate uptake blocker l-trans-pyrrolidine-2,4-dicarboxylate and when both Ca(2+) and Mg(2+) were omitted from the medium. Omission of Ca(2+) for 15 min alone did not induce delayed toxicity, but in combination with blocked glutamate uptake, significant cell death was observed 24 h later. Our results indicate that omission of extracellular Ca(2+) stimulates efflux of glutathione and specific amino acids including glutamate via opening of glial hemichannels. This type of efflux may have protective functions via glutathione efflux but can aggravate toxicity in situations when glutamate reuptake is impaired, such as following a stroke.

Cx43 hemichannels and gap junction channels in astrocytes are regulated oppositely by proinflammatory cytokines released from activated microglia

Astrocytes have a role in maintaining normal neuronal functions, some of which depend on connexins, protein subunits of gap junction channels and hemichannels. Under inflammatory conditions, microglia release cytokines, including interleukin-1beta and tumor necrosis factor-alpha, that reduce intercellular communication via gap junctions. Now, we demonstrate that either conditioned medium harvested from activated microglia or a mixture of these two cytokines enhances the cellular exchange with the extracellular milieu via Cx43 hemichannels. These changes in membrane permeability were not detected in astrocytes cultured from Cx43 knock-out mice and were abrogated by connexin hemichannel blockers, including La3+, mimetic peptides, and niflumic acid. Both the reduction in gap junctional communication and the increase in membrane permeability were mediated by a p38 mitogen-activated protein kinase-dependent pathway. However, the increase in membrane permeability, but not the gap junction inhibition, was rapidly reversed by the sulfhydryl reducing agent dithiothreitol, indicating that final regulatory mechanisms are different. Treatment with proinflammatory cytokines reduced the total and cell surface Cx43 levels, suggesting that the increase in membrane permeability was attributable to an increase in hemichannels activity. Indeed, unitary events of approximately 220 pS corresponding to Cx43 hemichannels were much more frequent in astrocytes treated with microglia conditioned medium than under control conditions. Finally, the effect of cytokines enhanced the uptake and reduced the intercellular diffusion of glucose, which might explain changes in the metabolic status of astrocytes under inflammatory conditions. Accordingly, this opposite regulation may affect glucose trafficking and certainly will modify the metabolic status of astrocytes involved in brain inflammation.

Activated macrophages inhibit enterocyte gap junctions via the release of nitric oxide

Enterocytes exist in close association with tissue macrophages, whose activation during inflammatory processes leads to the release of nitric oxide (NO). Repair from mucosal injury requires the migration of enterocytes into the mucosal defect, a process that requires connexin43 (Cx43)-mediated gap junction communication between adjacent enterocytes. Enterocyte migration is inhibited during inflammatory conditions including necrotizing enterocolitis, in part, through impaired gap junction communication. We now hypothesize that activated macrophages inhibit gap junctions of adjacent enterocytes and seek to determine whether NO release from macrophages was involved. Using a coculture system of enterocytes and macrophages, we now demonstrate that "activation" of macrophages with lipopolysaccharide and interferon reduces the phosphorylation of Cx43 in adjacent enterocytes, an event known to inhibit gap junction communication. The effects of macrophages on enterocyte gap junctions could be reversed by treatment of macrophages with the inducible nitric oxide synthase (iNOS) inhibitor l-Lysine omega-acetamidine hydrochloride (l-NIL) and by incubation with macrophages from iNOS(-/-) mice, implicating NO in the process. Activated macrophages also caused a NO-dependent redistribution of connexin43 in adjacent enterocytes from the cell surface to an intracellular location, further suggesting NO release may inhibit gap junction function. Treatment of enterocytes with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) markedly inhibited gap junction communication as determined using single cell microinjection of the gap junction tracer Lucifer yellow. Strikingly, activated macrophages inhibited enterocyte migration into a scraped wound, which was reversed by l-NIL pretreatment. These results implicate enterocyte gap junctions as a target of the NO-mediated effects of macrophages during intestinal inflammation, particularly where enterocyte migration is impaired.

Human spermatozoa contain multiple targets for protein S-nitrosylation: an alternative mechanism of the modulation of sperm function by nitric oxide?

Nitric oxide (NO) enhances human sperm motility and capacitation associated with increased protein phosphorylation. NO activates soluble guanylyl cyclase, but can also modify protein function covalently via S-nitrosylation of cysteine. Remarkably, this mechanism remains unexplored in sperm although they depend on post-translational protein modification to achieve changes in function required for fertilisation. Our objective was to identify targets for S-nitrosylation in human sperm. Spermatozoa were incubated with NO donors and S-nitrosylated proteins were identified using the biotin switch assay and a proteomic approach using MS/MS. 240 S-nitrosylated proteins were detected in sperm incubated with S-nitroso-glutathione. Minimal levels were observed in glutathione or untreated samples. Proteins identified consistently based on multiple peptides included established targets for S-nitrosylation in other cells e.g. tubulin, GST and HSPs but also novel targets including A-kinase anchoring protein (AKAP) types 3 and 4, voltage-dependent anion-selective channel protein 3 and semenogelin 1 and 2. In situ localisation revealed S-nitrosylated targets on the postacrosomal region of the head and throughout the flagellum. Potential targets for S-nitrosylation in human sperm include physiologically significant proteins not previously reported in other cells. Their identification will provide novel insight into the mechanism of action of NO in spermatozoa.

Possible involvement of different connexin43 domains in plasma membrane permeabilization induced by ischemia-reperfusion

In vitro and in vivo studies support the involvement of connexin 43-based cell-cell channels and hemichannels in cell death propagation induced by ischemia-reperfusion. In this context, open connexin hemichannels in the plasma membrane have been proposed to act as accelerators of cell death. Progress on the mechanisms underlying the cell permeabilization induced by ischemia-reperfusion reveals the involvement of several factors leading to an augmented open probability and increased number of hemichannels on the cell surface. While open probability can be increased by a reduction in extracellular concentration of divalent cations and changes in covalent modifications of connexin 43 (oxidation and phosphorylation), increase in number of hemichannels requires an elevation of the intracellular free Ca(2+) concentration. Reversal of connexin 43 redox changes and membrane permeabilization can be induced by intracellular, but not extracellular, reducing agents, suggesting a cytoplasmic localization of the redox sensor(s). In agreement, hemichannels formed by connexin 45, which lacks cytoplasmic cysteines, or by connexin 43 with its C-terminal domain truncated to remove its cysteines are insensitive to reducing agents. Although further studies are required for a precise localization of the redox sensor of connexin 43 hemichannels, modulation of the redox potential is proposed as a target for the design of pharmacological tools to reduce cell death induced by ischemia-reperfusion in connexin 43-expressing cells.

Functional gap junctions facilitate melanoma antigen transfer and cross-presentation between human dendritic cells

Previously, we found that human dendritic cells (hDCs) pulsed with a melanoma cell lysate (MCL) and stimulated with TNF-alpha (MCL/TNF) acquire a mature phenotype in vitro and are able to trigger tumor-specific immune responses when they are used in melanoma immunotherapy in patients. In this study, we describe that MCL/TNF induces gap junction (GJ)-mediated intercellular communications and promotes melanoma Ag transfer between ex vivo produced hDCs from melanoma patients. hDCs also exhibit increased expression of the GJ-related protein connexin 43, which contributes to GJ plaque formation after MCL/TNF stimulation. The addition of GJ inhibitors suppresses intercellular tumor Ag transfer between hDCs, thus reducing melanoma-specific T cell activation. In summary, we demonstrate that MCL/TNF-stimulated hDCs can establish functional GJ channels that participate in melanoma Ag transfer, facilitating Ag cross-presentation and an effective dendritic cell-mediated melanoma-specific T cell response. These results suggest that GJs formed between hDCs used in cancer vaccination protocols could be essentials for the establishment of a more efficient antitumor response.

Opening of connexin 43 hemichannels is increased by lowering intracellular redox potential

Nonjunctional membrane in many cells contains connexin gap junction hemichannels (or connexons) that can open to allow permeation of small molecules. Opening of Cx43 hemichannels is infrequent in normal extracellular Ca(2+) and enhanced by low Ca(2+), positive membrane potentials, and dephosphorylation of critical residues. Here we report that lowering intracellular redox potential increases Cx43 hemichannel open probability under otherwise normal conditions. We studied dye uptake and single-channel activity in HeLa cells transfected with wild-type Cx43, Cx43 with enhanced GFP attached to its C terminus (Cx43-EGFP), and Cx43 with enhanced GFP attached to its N terminus (EGFP-Cx43). Dithiothreitol [(DTT) 10 mM], a membrane permeant-reducing agent, increased the rate of dye uptake by cells expressing Cx43 and Cx43-EGFP, but not by parental cells or cells expressing EGFP-Cx43. Induced dye uptake was blocked by La(3+), by a peptide gap junction and hemichannel blocker (gap 26), and by flufenamic acid. DTT increased Cx43-EGFP hemichannel opening at positive voltages. Bath application of reduced glutathione, a membrane impermeant-reducing agent, did not increase dye uptake, but glutathione in the recording pipette increased hemichannel opening at positive voltages, suggesting that it acted intracellularly. DTT caused little change in levels of surface Cx43 or Cx43-EGFP, or in intracellular pH. These findings suggest that lowering intracellular redox potential increases the opening of Cx43 and Cx43-EGFP hemichannels, possibly by action on cytoplasmic cysteine residues in the connexin C terminus.

The modulatory effects of connexin 43 on cell death/survival beyond cell coupling

Connexins form a diverse and ubiquitous family of integral membrane proteins. Characteristically, connexins are assembled into intercellular channels that aggregate into discrete cell-cell contact areas termed gap junctions (GJ), allowing intercellular chemical communication, and are essential for propagation of electrical impulses in excitable tissues, including, prominently, myocardium, where connexin 43 (Cx43) is the most important isoform. Previous studies have shown that GJ-mediated communication has an important role in the cellular response to stress or ischemia. However, recent evidence suggests that connexins, and in particular Cx43, may have additional effects that may be important in cell death and survival by mechanisms independent of cell to cell communication. Connexin hemichannels, located at the plasma membrane, may be important in paracrine signaling that could influence intracellular calcium and cell survival by releasing intracellular mediators as ATP, NAD(+), or glutamate. In addition, recent studies have shown the presence of connexins in cell structures other than the plasma membrane, including the cell nucleus, where it has been suggested that Cx43 influences cell growth and differentiation. In addition, translocation of Cx43 to mitochondria appears to be important for certain forms of cardioprotection. These findings open a new field of research of previously unsuspected roles of Cx43 intracellular signaling.

Change in permeant size selectivity by phosphorylation of connexin 43 gap-junctional hemichannels by PKC

Gap-junctional channels, permeable to large hydrophilic solutes of up to M(r) approximately 1,000, are responsible for cell-to-cell communication. Phosphorylation of connexin 43 (Cx43) by PKC abolishes the permeability of gap-junctional channels and hemichannels to large hydrophilic solutes, but not to small inorganic ions. Here, we report on a methodology to produce purified hemichannels of controlled subunit composition and apply it to the generation of hemichannels with variable number of PKC-phosphorylated subunits. The subunit composition was determined by luminescence resonance energy transfer. We show that all Cx43 subunits in the hemichannel hexamer have to be phosphorylated to abolish sucrose (M(r) 342) permeability. We also show that the hemichannel pores with all subunits phosphorylated by PKC have a sizable diameter, allowing for permeation of the small hydrophilic solute ethyleneglycol (M(r) 62). These results indicate that phosphorylation of Cx43 by PKC alters the hemichannel size selectivity and explain why PKC activity affects dye transfer between cells without consistent effects on electrical communication.

Roles of gap junctions and hemichannels in bone cell functions and in signal transmission of mechanical stress

Gap junctions formed by connexins (Cx) play an important role in transmitting signals between bone cells such as osteoblasts and osteoclasts, cells responsible for bone formation and bone remodeling, respectively. Gap junction intercellular communication (GJIC) has been demonstrated to mediate the process of osteoblast differentiation and bone formation. Furthermore, GJIC propagates Ca2+ signaling, conveys anabolic effects of hormones and growth factors, and regulates gene transcription of osteoblast differentiation markers. GJIC is also implicated to regulate osteoclast formation, survival and apoptosis. Compared with other bone cells, the most abundant type are osteocytes, which express large amounts of connexins. Mechanosensing osteocytes connect and form gap junctions with themselves and other cells only through the tips of their dendritic processes, a relatively small percent of the total cell surface area compared to other cells. Recent studies show that in addition to gap junctions, osteoblasts and osteocytes express functional hemichannels, the un-opposed halves of gap junction channels. Hemichannels are localized at the cell surface and function independently of gap junctions. Hemichannels in osteocytes mediate the immediate release of prostaglandins in response to mechanical stress. The major challenges remaining in the field are how the functions of these two types of channels are coordinated in bone cells and what the asserted, distinct effects of these channels are on bone formation and remodeling processes, and on conveying signals elicited by mechanical loading.

Functional connexin "hemichannels": a critical appraisal

"Hemichannels" are defined as the halves of gap junction channels (also termed connexons) that are contributed by one cell; "hemichannels" are considered to be functional if they are open in nonjunctional membranes in the absence of pairing with partners from adjacent cells. Several recent reviews have summarized the blossoming literature regarding functional "hemichannels", in some cases encyclopedically. However, most of these previous reviews have been written with the assumption that all data reporting "hemichannel" involvement really have studied phenomena in which connexons actually form the permeability or conductance pathway. In this review, we have taken a slightly different approach. We review the concept of "hemichannels", summarize properties that might be expected of half gap junctions and evaluate the extent to which the properties of presumptive "hemichannels" match expectations. Then we consider functions attributed to hemichannels, provide an overview of other channel types that might fulfill similar roles and provide sets of criteria that might be applied to verify involvement of connexin hemichannels in cell and tissue function. One firm conclusion is reached. The study of hemichannels is technically challenging and fraught with opportunities for misinterpretation, so that future studies must apply rigorous standards for detection of hemichannel expression and function. At the same time there are reasons to expect surprises, including the possibility that some time honored techniques for studying gap junctions may prove unsuitable for detecting hemichannels. We advise hemichannel researchers to proceed with caution and an open mind.

Extracellular S-nitrosoglutathione, but not S-nitrosocysteine or N(2)O(3), mediates protein S-nitrosation in rat spinal cord slices

There is evidence that protein S-nitrosothiols (PrSNOs) accumulate in inflammatory demyelinating disorders like multiple sclerosis and experimental allergic encephalomyelitis. However, very little is known regarding the mechanism by which PrSNOs are formed in target cells. The present study compares the ability of potential intercellular mediators of nitrosative damage including S-nitrosoglutathione (GSNO), S-nitrosocysteine and N(2)O(3) to induce protein S-nitros(yl)ation in the spinal cord, a CNS region that is commonly affected in multiple sclerosis and experimental allergic encephalomyelitis. The results clearly demonstrate that while all three NO-donors cause S-nitrosation of proteins in cell-free systems, only GSNO is a viable S-nitrosating agent in rat spinal cord slices. Generation of PrSNOs with GSNO occurs by S-transnitrosation as the process was not inhibited by either the NO-scavenger rutin or the N(2)O(3)-scavenger azide. Contrary to other cell types, nerve cells incorporate intact GSNO and neither functional l-amino acid transporters nor cell-surface thiols are required. We also found that there is a restricted number of proteins available for S-nitrosation, even at high, non-physiological concentrations of GSNO. These proteins are highly concentrated in mitochondria and mitochondria-rich subcellular compartments. This study is relevant to those CNS disorders characterized by excessive nitric oxide production.