Where are the gates in gap junction channels?

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

Mechanical stimulation-induced calcium wave propagation in cell monolayers: the example of bovine corneal endothelial cells

Intercellular communication is essential for the coordination of physiological processes between cells in a variety of organs and tissues, including the brain, liver, retina, cochlea and vasculature. In experimental settings, intercellular Ca(2+)-waves can be elicited by applying a mechanical stimulus to a single cell. This leads to the release of the intracellular signaling molecules IP3 and Ca(2+) that initiate the propagation of the Ca(2+)-wave concentrically from the mechanically stimulated cell to the neighboring cells. The main molecular pathways that control intercellular Ca(2+)-wave propagation are provided by gap junction channels through the direct transfer of IP3 and by hemichannels through the release of ATP. Identification and characterization of the properties and regulation of different connexin and pannexin isoforms as gap junction channels and hemichannels are allowed by the quantification of the spread of the intercellular Ca(2+)-wave, siRNA, and the use of inhibitors of gap junction channels and hemichannels. Here, we describe a method to measure intercellular Ca(2+)-wave in monolayers of primary corneal endothelial cells loaded with Fluo4-AM in response to a controlled and localized mechanical stimulus provoked by an acute, short-lasting deformation of the cell as a result of touching the cell membrane with a micromanipulator-controlled glass micropipette with a tip diameter of less than 1 μm. We also describe the isolation of primary bovine corneal endothelial cells and its use as model system to assess Cx43-hemichannel activity as the driven force for intercellular Ca(2+)-waves through the release of ATP. Finally, we discuss the use, advantages, limitations and alternatives of this method in the context of gap junction channel and hemichannel research.

Tryptophan scanning mutagenesis of the first transmembrane domain of the innexin Shaking-B(Lethal)

The channel proteins of gap junctions are encoded by two distinct gene families, connexins, which are exclusive to chordates, and innexins/pannexins, which are found throughout the animal kingdom. Although the relationship between the primary structure and function of the vertebrate connexins has been relatively well studied, there are, to our knowledge, no structure-function analyses of invertebrate innexins. In the first such study, we have used tryptophan scanning to probe the first transmembrane domain (M1) of the Drosophila innexin Shaking-B(Lethal), which is a component of rectifying electrical synapses in the Giant Fiber escape neural circuit. Tryptophan was substituted sequentially for 16 amino acids within M1 of Shaking-B(Lethal). Tryptophan insertion at every fourth residue (H27, T31, L35, and S39) disrupted gap junction function. The distribution of these sites is consistent with helical secondary structure and identifies the face of M1 involved in helix-helix interactions. Tryptophan substitution at several sites in M1 altered channel properties in a variety of ways. Changes in sensitivity to transjunctional voltage (Vj) were common and one mutation (S39W) induced sensitivity to transmembrane voltage (Vm). In addition, several mutations induced hemichannel activity. These changes are similar to those observed after substitutions within the transmembrane domains of connexins.

Connexin 43 mimetic peptide Gap26 confers protection to intact heart against myocardial ischemia injury

Unapposed connexin 43 hemichannels (Cx43Hc) are present on sarcolemma of cardiomyocytes. Whereas Cx43Hc remain closed during physiological conditions, their opening under ischemic stress contributes to irreversible tissue injury and cell death. To date, conventional blockers of connexin channels act unselectively on both gap junction channels and unapposed hemichannels. Here, we test the hypothesis that Gap26, a synthetic structural mimetic peptide deriving from the first extracellular loop of Cx43 and a presumed selective blocker of Cx43Hc, confers resistance to intact rat heart against ischemia injury. Langendorff-perfused intact rat hearts were utilized. Regional ischemia was induced by 40-min occlusion of the left anterior descendent coronary and followed by 180 min of reperfusion. Gap26 was applied either 10 min before or 30 min after the initiation of ischemia. Interestingly, myocardial infarct size was reduced by 48% and 55% in hearts treated with Gap26 before or during ischemia, respectively, compared to untreated hearts. Additionally, myocardial perfusate flow was increased in both groups during reperfusion by 37% and 32%, respectively. Application of Gap26 increased survival of isolated cardiomyocytes after simulated ischemia-reperfusion by nearly twofold compared to untreated cells. On the other hand, superfusion of tsA201 cells transiently expressing Cx43 with Gap26 caused 61% inhibition of Cx43Hc-mediated currents recorded using the patch clamp technique. In summary, we demonstrate for the first time that Cx43 mimetic peptide Gap26 confers protection to intact heart against ischemia-reperfusion injury whether administered before or after the occurrence of ischemia. In addition, we provide unequivocal evidence for the inhibitory effect of Gap26 on genuine Cx43Hc.

Pannexin: to gap or not to gap, is that a question?

Vertebrates express two families of gap junction proteins: the well characterized connexins and the recently discovered pannexins. The latter are related to invertebrate innexins. Here we present the hypothesis that pannexins, rather than providing a redundant system to gap junctions formed by connexins, exert a physiological role as nonjunctional membrane channels. Specifically, we propose that pannexins can serve as ATP release channels. This function presumptively is also performed by innexins in invertebrates, in addition to their traditional gap junction role.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Role of gap junctions in CO(2) chemoreception and respiratory control

Gap junctions are composed of connexins, which are organized into intercellular channels that form transmembrane pathways between neurons (cell-cell coupling), and in some cases, neurons and glia, for exchange of ions and small molecules (metabolic coupling) and ionic current (electrical coupling). Cell-cell coupling via gap junctions has been identified in brain stem neurons that function in CO(2)/H(+) chemoreception and respiratory rhythmogenesis; however, the exact roles of gap junctions in respiratory control are undetermined. Here we review the methods commonly used to study gap junctions in the mammalian brain stem under in vitro and in vivo conditions and briefly summarize the anatomical, pharmacological, and electrophysiological evidence to date supporting roles for cell-cell coupling in respiratory rhythmogenesis and central chemoreception. Specific research questions related to the role of gap junctions in respiratory control are suggested for future research.

Gap junctions in CO(2)-chemoreception and respiratory control

Recent evidence indicates that gap junctions play a more prominent role in normal functioning of the mammalian central nervous system (CNS) than was once believed. Accumulating evidence from both neonatal and adult rodents indicates that gap junctions participate in multiple aspects of respiratory control, including central CO(2) chemoreception, respiratory rhythmogenesis, and respiratory motoneuron output. This review provides an overview of gap junction neurobiology in the mammalian CNS and presents the anatomical and electrophysiological evidence for gap junctions in CO(2) chemoreception and respiratory control.

Modulation of gap and adherens junctional proteins in cultured neonatal pancreatic islets

Fetal and neonatal pancreatic islets have lower insulin secretory responses compared with adult islets. In culture conditions and after treatment with mammosomatotropic hormones, neonatal islets undergo maturation of the secretory machinery that might involve regulation of cell-cell contacts within the islet. This study is an investigation of the effect of prolonged culturing and in vitro treatment with prolactin on the expression of the gap junction-associated connexin 43 and the adherens junction-associated beta-catenin in cultured neonatal rat islets. Pancreatic islets from neonatal Wistar rats were cultured for 24 hours or 7 days, and the treated group was exposed to 2 microg/mL prolactin daily for 7 days. Connexin 43 and beta-catenin were barely detected at the cell-cell contacts in 24-hour-cultured islets, as revealed by immunocytochemical analysis. Nevertheless, both junctional proteins were well expressed at the junctional region in islet cells cultured for 7 days and showed even greater staining in islets after long-term prolactin treatment. In accordance with the morphologic data, neonatal islets cultured for 24 hours displayed a relatively low level of connexin 43, as determined by Western blot analysis. Culturing for 7 days or combined prolactin treatment induced a significant increase in connexin 43 expression; this was 40% greater in the prolactin-treated group than in the control group. Furthermore, an enhancement of the expression of beta-catenin and translocation of this protein to the cell-cell contact site was also observed in neonatal islets cultured for 7 days compared with those cultured for 24 hours. In vitro prolactin treatment induced even greater expression of beta-catenin in islet cells. A correlation was observed between the increased expression of these junctional proteins and an increase in insulin secretion in cultured neonatal islets. In conclusion, prolonged culturing and in vitro treatment with prolactin induce the modulation of gap and adherens junctional proteins in pancreatic islets, which may be an important event in the in vitro maturation process of neonatal islet cells.

IgG anti-melanocyte antibodies purified from patients with active vitiligo induce HLA-DR and intercellular adhesion molecule-1 expression and an increase in interleukin-8 release by melanocytes

An immunologic hypothesis is currently proposed as a possible pathogenesis of nonsegmental-type vitiligo. IgG antibodies against melanocyte surface antigens exist in the serum of patients with vitiligo vulgaris. IgG anti-melanocyte antibodies were reported to induce melanocyte damage in vitro by a complement-mediated mechanism and antibody-dependent cellular cytotoxicity. Perilesional melanocytes express major histocompatibility complex class II antigens and a higher intercellular adhesion molecule-1 compared with those in normal skin. The purpose of this study was to determine the role of IgG anti-melanocyte antibodies in the inappropriate expression of major histocompatibility complex class II antigens and intercellular adhesion molecule-1 on melanocytes. IgG anti-melanocyte antibody samples were purified from the individual serum of patients with active vitiligo. After incubation of IgG anti-melanocyte antibodies with cultured melanocytes, the results revealed: (i) IgG anti-melanocyte antibody stimulated HLA-DR expression on melanocytes; (ii) intercellular adhesion molecule-1 expression on melanocytes was significantly induced by IgG anti-melanocyte antibodies; and (iii) IgG anti-melanocyte antibodies induced an increase in interleukin-8 release from melanocytes. The major histocompatibility complex class II molecules expressed in melanocytes can present antigens to CD4 helper cells as antigen-presenting cells and elicit an immune response. Intercellular adhesion molecule-1 is an important adhesion molecule involved in leukocyte and parenchymal cell interaction and thus plays an essential part in immunologic and inflammatory reactions. It is reasonable to speculate that abnormal expressions of HLA-DR and intercellular adhesion molecule-1 on melanocytes by IgG anti-melanocyte antibodies would present vitiligo antigens and allow the antigen-specific immune effector cell attack that results in melanocytotoxicity.

Importance of gap junction in gastric mucosal restitution from acid-induced injury

Evidence is accumulating that gap junctional intercellular communication (GJIC) plays an important role in the gastric mucosal defense system. This study was conducted to determine whether GJIC mediates a restitution process in gastric mucosa. Male Sprague-Dawley rats were fasted and anesthetized. Gastric injury was induced by luminal perfusion with 0.2N HCl for 10 minutes. Mucosal integrity was continuously monitored by measuring the clearance of chromium 51-labeled ethylenediaminetetraacetic acid, which was used for analysis of recovery from the injury. Perfusion with 0.25% octanol (OCT; inhibitor of GJIC) was started after acid injury to assess its effect on restitution. The effect of irsogladine (IG; activator of GJIC) was also tested. Gastric mucosal GJIC was immunohistochemically evaluated with monoclonal antibody gap junction protein (connexin 32). Recovery from acid-induced mucosal injury occurred rapidly when acid perfusion was discontinued (within about 60 minutes). OCT, which didn't cause any injury to normal gastric mucosa, significantly inhibited the restitution. IG reversed this inhibition in a dose-dependent manner. In an immunohistochemical study, OCT-induced damage of gap junction was demonstrated, but not after IG pre-treatment. These findings suggest that GJIC may play a critical role in restitution in rat gastric mucosa and that gap junction function may be one of the important factors for the mucosal defense system.

Effect of membrane tension on gap junctional conductance of supporting cells in Corti's organ

The effects of turgor pressure-induced membrane tension on junctional coupling of Hensen cell isolates from the inner ear were evaluated by input capacitance or transjunctional conductance measurement techniques. Turgor pressure was altered by changing either pipette pressure or the osmolarities of extracellular solutions. Both positive pipette pressure and extracellular applications of hypotonic solutions, which caused cell size to concomitantly increase, uncoupled the cells as indicated by reduced input capacitance and transjunctional conductance. These changes were, in many cases, reversible and repeatable. Intracellular application of 50 microM H-7, a broad-based protein kinase inhibitor, and 10 mM BAPTA did not block the uncoupling effect of positive turgor pressure on inner ear gap junctions. The transjunctional conductance at a holding potential of -80 mV was 53.6 +/- 5.8 nS (mean +/- SEM, n = 9) and decreased approximately 40% at a turgor pressure of 1.41 +/- 0.05 kPa. Considering the coincident kinetics of cell deformation and uncoupling, we speculate that mechanical forces work directly on gap junctions of the inner ear. These results suggest that pathologies that induce imbalances in cochlear osmotic pressure regulation may compromise normal cochlear homeostasis.

Formation of the gap junction intercellular channel requires a 30 degree rotation for interdigitating two apposing connexons

Intercellular communication via gap junction membrane channels cannot occur until two apposing hemichannels (connexons) meet and dock to form a sealed cell-cell conduit. In particular, an important question is how does the structure at the extracellular surface influence the molecular recognition of the two connexons. In this study, cryoelectron microscopy and computer modeling provide evidence that the formation of the gap junction intercellular channel requires a 30 degree rotation between hemichannels for proper docking. With this amount of rotation, the peaks (protrusions) on one connexon fit into the valleys of the apposed connexon in the 3-D model, which would make for an ionically tight interface necessary for a functional cell-cell channel. Docking appears to be governed by a "lock and key" mechanism via a simple interdigitation of the six protrusions from each connexon. This interdigitation increases significantly the contact surface area and potential number of hydrogen bonds or hydrophobic interactions and/or other attractive interactions. Having a larger surface area than if the surfaces were flat would explain the biochemical requirements for conditions characterized previously for splitting of channels into hemichannels. The docked connexons were computationally fitted into two gap junction structures, which further confirmed the interdigitated manner of docking.