51
|
Sánchez HA, Orellana JA, Verselis VK, Sáez JC. Metabolic inhibition increases activity of connexin-32 hemichannels permeable to Ca2+ in transfected HeLa cells. Am J Physiol Cell Physiol 2009; 297:C665-78. [PMID: 19587218 PMCID: PMC2740400 DOI: 10.1152/ajpcell.00200.2009] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2009] [Accepted: 07/01/2009] [Indexed: 02/08/2023]
Abstract
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.
Collapse
Affiliation(s)
- Helmuth A Sánchez
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile
| | | | | | | |
Collapse
|
52
|
Warren NJ, Tawhai MH, Crampin EJ. Mathematical modelling of calcium wave propagation in mammalian airway epithelium: evidence for regenerative ATP release. Exp Physiol 2009; 95:232-49. [PMID: 19700517 DOI: 10.1113/expphysiol.2009.049585] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Airway epithelium has been shown to exhibit intracellular calcium waves after mechanical stimulation. Two classes of mechanism have been proposed to explain calcium wave propagation: diffusion through gap junctions of the intracellular messenger inositol 1,4,5-trisphosphate (IP3), and diffusion of paracrine extracellular messengers such as ATP. We have used single cell recordings of airway epithelium to parameterize a model of an airway epithelial cell. This was then incorporated into a spatial model of a cell culture where both mechanisms for calcium wave propagation are possible. It is shown that a decreasing return on the radius of Ca2+ wave propagation is achieved as the amount of ATP released from the stimulated cell increases. It is therefore shown that for a Ca2+ wave to propagate large distances, a significant fraction of the intracellular ATP pool would be required to be released. Further to this, the radial distribution of maximal calcium response from the stimulated cell does not produce the same flat profile of maximal calcium response seen in experiential studies. This suggests that an additional mechanism is important in Ca2+ wave propagation, such as regenerative release of ATP from cells downstream of the stimulated cell.
Collapse
Affiliation(s)
- N J Warren
- Auckland Bioengineering Institute, Level 6, 70 Symonds Street, CBD, Auckland, New Zealand.
| | | | | |
Collapse
|
53
|
|
54
|
De Vuyst E, Wang N, Decrock E, De Bock M, Vinken M, Van Moorhem M, Lai C, Culot M, Rogiers V, Cecchelli R, Naus CC, Evans WH, Leybaert L. Ca(2+) regulation of connexin 43 hemichannels in C6 glioma and glial cells. Cell Calcium 2009; 46:176-87. [PMID: 19656565 DOI: 10.1016/j.ceca.2009.07.002] [Citation(s) in RCA: 163] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2009] [Revised: 06/01/2009] [Accepted: 07/08/2009] [Indexed: 10/20/2022]
Abstract
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.
Collapse
Affiliation(s)
- Elke De Vuyst
- Department of Basic Medical Sciences, Ghent University, Belgium
| | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
55
|
Schalper KA, Palacios-Prado N, Orellana JA, Sáez JC. Currently Used Methods for Identification and Characterization of Hemichannels. ACTA ACUST UNITED AC 2009; 15:207-18. [DOI: 10.1080/15419060802014198] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
|
56
|
O'Carroll SJ, Alkadhi M, Nicholson LFB, Green CR. Connexin43 Mimetic Peptides Reduce Swelling, Astrogliosis, and Neuronal Cell Death after Spinal Cord Injury. ACTA ACUST UNITED AC 2009; 15:27-42. [DOI: 10.1080/15419060802014164] [Citation(s) in RCA: 131] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
|
57
|
Ransford GA, Fregien N, Qiu F, Dahl G, Conner GE, Salathe M. Pannexin 1 contributes to ATP release in airway epithelia. Am J Respir Cell Mol Biol 2009; 41:525-34. [PMID: 19213873 DOI: 10.1165/rcmb.2008-0367oc] [Citation(s) in RCA: 179] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
ATP is a paracrine regulator of critical airway epithelial cell functions, but the mechanism of its release is poorly understood. Pannexin (Panx) proteins, related to invertebrate innexins, form channels (called pannexons) that are able to release ATP from several cell types. Thus, ATP release via pannexons was examined in airway epithelial cells. Quantitative RT-PCR showed Panx1 expression in normal human airway epithelial cells during redifferentiation at the air-liquid interface (ALI), at a level comparable to that of alveolar macrophages; Panx3 was not expressed. Immunohistochemistry showed Panx1 expression at the apical pole of airway epithelia. ALI cultures exposed to hypotonic stress released ATP to an estimated maximum of 255 (+/-64) nM within 1 minute after challenge (n = 6 cultures from three different lungs) or to approximately 1.5 (+/-0.4) microM, recalculated to a normal airway surface liquid volume. Using date- and culture-matched cells (each n > or = 16 from 4 different lungs), the pannexon inhibitors carbenoxolone (10 microM) and probenecid (1 mM), but not the connexon inhibitor flufenamic acid (100 microM), inhibited ATP release by approximately 60%. The drugs affected Panx1 currents in Xenopus oocytes expressing exogenous Panx1 correspondingly. In addition, suppression of Panx1 expression using lentivirus-mediated production of shRNA in differentiated airway epithelial cells inhibited ATP release upon hypotonic stress by approximately 60% as well. These data not only show that Panx1 is expressed apically in differentiated airway epithelial cells but also that it contributes to ATP release in these cells.
Collapse
Affiliation(s)
- George A Ransford
- Division of Pulmonary and Critical Care Medicine (R-47), University of Miami Miller School of Medicine, 1600 NW 10th Ave., RMSB 7058, Miami, FL 33136, USA
| | | | | | | | | | | |
Collapse
|
58
|
Romanov RA, Rogachevskaja OA, Khokhlov AA, Kolesnikov SS. Voltage dependence of ATP secretion in mammalian taste cells. ACTA ACUST UNITED AC 2009; 132:731-44. [PMID: 19029378 PMCID: PMC2585863 DOI: 10.1085/jgp.200810108] [Citation(s) in RCA: 77] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Mammalian type II taste cells release the afferent neurotransmitter adenosine triphosphate (ATP) through ATP-permeable ion channels, most likely to be connexin (Cx) and/or pannexin hemichannels. Here, we show that ion channels responsible for voltage-gated (VG) outward currents in type II cells are ATP permeable and demonstrate a strong correlation between the magnitude of the VG current and the intensity of ATP release. These findings suggest that slowly deactivating ion channels transporting the VG outward currents can also mediate ATP secretion in type II cells. In line with this inference, we studied a dependence of ATP secretion on membrane voltage with a cellular ATP sensor using different pulse protocols. These were designed on the basis of predictions of a model of voltage-dependent transient ATP efflux. Consistently with curves that were simulated for ATP release mediated by ATP-permeable channels deactivating slowly, the bell-like and Langmuir isotherm–like potential dependencies were characteristic of ATP secretion obtained for prolonged and short electrical stimulations of taste cells, respectively. These observations strongly support the idea that ATP is primarily released via slowly deactivating channels. Depolarizing voltage pulses produced negligible Ca2+ transients in the cytoplasm of cells releasing ATP, suggesting that ATP secretion is mainly governed by membrane voltage under our recording conditions. With the proviso that natural connexons and pannexons are kinetically similar to exogenously expressed hemichannels, our findings suggest that VG ATP release in type II cells is primarily mediated by Cx hemichannels.
Collapse
Affiliation(s)
- Roman A Romanov
- Institute of Cell Biophysics, Russian Academy of Sciences, Moscow 142290, Russia
| | | | | | | |
Collapse
|
59
|
Anselmi F, Hernandez VH, Crispino G, Seydel A, Ortolano S, Roper SD, Kessaris N, Richardson W, Rickheit G, Filippov MA, Monyer H, Mammano F. ATP release through connexin hemichannels and gap junction transfer of second messengers propagate Ca2+ signals across the inner ear. Proc Natl Acad Sci U S A 2008; 105:18770-5. [PMID: 19047635 PMCID: PMC2596208 DOI: 10.1073/pnas.0800793105] [Citation(s) in RCA: 262] [Impact Index Per Article: 16.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2008] [Indexed: 11/18/2022] Open
Abstract
Extracellular ATP controls various signaling systems including propagation of intercellular Ca(2+) signals (ICS). Connexin hemichannels, P2x7 receptors (P2x7Rs), pannexin channels, anion channels, vesicles, and transporters are putative conduits for ATP release, but their involvement in ICS remains controversial. We investigated ICS in cochlear organotypic cultures, in which ATP acts as an IP(3)-generating agonist and evokes Ca(2+) responses that have been linked to noise-induced hearing loss and development of hair cell-afferent synapses. Focal delivery of ATP or photostimulation with caged IP(3) elicited Ca(2+) responses that spread radially to several orders of unstimulated cells. Furthermore, we recorded robust Ca(2+) signals from an ATP biosensor apposed to supporting cells outside the photostimulated area in WT cultures. ICS propagated normally in cultures lacking either P2x7R or pannexin-1 (Px1), as well as in WT cultures exposed to blockers of anion channels. By contrast, Ca(2+) responses failed to propagate in cultures with defective expression of connexin 26 (Cx26) or Cx30. A companion paper demonstrates that, if expression of either Cx26 or Cx30 is blocked, expression of the other is markedly down-regulated in the outer sulcus. Lanthanum, a connexin hemichannel blocker that does not affect gap junction (GJ) channels when applied extracellularly, limited the propagation of Ca(2+) responses to cells adjacent to the photostimulated area. Our results demonstrate that these connexins play a dual crucial role in inner ear Ca(2+) signaling: as hemichannels, they promote ATP release, sustaining long-range ICS propagation; as GJ channels, they allow diffusion of Ca(2+)-mobilizing second messengers across coupled cells.
Collapse
Affiliation(s)
- Fabio Anselmi
- Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, 35129 Padua, Italy
| | - Victor H. Hernandez
- Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, 35129 Padua, Italy
| | - Giulia Crispino
- Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, 35129 Padua, Italy
| | - Anke Seydel
- Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, 35129 Padua, Italy
| | - Saida Ortolano
- Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, 35129 Padua, Italy
- Department of Physics “G. Galilei,” University of Padua, 35129 Padua, Italy
| | - Stephen D. Roper
- Department of Physiology and Biophysics and Program in Neurosciences, Miller School of Medicine, University of Miami, Miami, FL 33136
| | - Nicoletta Kessaris
- Wolfson Institute for Biomedical Research and Department of Biology, University College London, London WC1E 6BT, United Kingdom
| | - William Richardson
- Wolfson Institute for Biomedical Research and Department of Biology, University College London, London WC1E 6BT, United Kingdom
| | - Gesa Rickheit
- Leibniz-Institut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), D-13125 Berlin, Germany; and
| | - Mikhail A. Filippov
- Department of Clinical Neurobiology, University Hospital of Neurology, Heidelberg, Germany
| | - Hannah Monyer
- Department of Clinical Neurobiology, University Hospital of Neurology, Heidelberg, Germany
| | - Fabio Mammano
- Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, 35129 Padua, Italy
- Department of Physics “G. Galilei,” University of Padua, 35129 Padua, Italy
| |
Collapse
|
60
|
Abstract
Genetic studies have linked many nonsyndromic deafness patients to mutations in genes coding for gap junction proteins. To better understand molecular identities of gap junctions in the cochlea, we investigated the expression of pannexins (Panxs). Western blot and reverse transcription-PCR detected the expression of Panx1 and Panx2. Immunolabeling localized Panx1 to the inner and outer sulcus, as well as the Claudius cells. Both Panx1 and Panx2 were expressed in the spiral and Scarpa's ganglion neurons. These data for the first time showed expressions of Panxs in the cochlea, therefore adding a new family of gap junction proteins to those used to form intercellular transport pathways in the cochlea.
Collapse
|
61
|
Evans WH, Leybaert L. Mimetic peptides as blockers of connexin channel-facilitated intercellular communication. ACTA ACUST UNITED AC 2008; 14:265-73. [PMID: 18392994 DOI: 10.1080/15419060801891034] [Citation(s) in RCA: 114] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
There is a dearth of chemical inhibitors of connexin-mediated intercellular communication. The advent of short "designer" connexin mimetic peptides has provided new tools to inhibit connexin channels quickly and reversibly. This perspective describes the development of mimetic peptides, especially Gap 26 and 27 that are the most popular and correspond to specific sequences in the extracellular loops of connexins 37, 40 and 43. Initially they were used to inhibit gap-junctional coupling in a wide range of mammalian cells and tissues. Currently, they are also being examined as therapeutic agents that accelerate wound healing and in the early treatment of spinal cord injury. The mimetic peptides bind to connexin hemichannels, influencing channel properties as shown by lowering of electrical conductivity and potently blocking the entry of small reporter dyes and the release of ATP by cells. A mechanism is proposed to help explain the dual action of connexin mimetic peptides on connexin hemichannels and gap-junctional coupling.
Collapse
Affiliation(s)
- W Howard Evans
- Department of Medical Biochemistry and Immunology, Wales Heart Research Institute Cardiff University Medical School, Heath Park, Cardiff, UK.
| | | |
Collapse
|
62
|
Samoilova M, Wentlandt K, Adamchik Y, Velumian AA, Carlen PL. Connexin 43 mimetic peptides inhibit spontaneous epileptiform activity in organotypic hippocampal slice cultures. Exp Neurol 2008; 210:762-75. [DOI: 10.1016/j.expneurol.2008.01.005] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2007] [Revised: 01/02/2008] [Accepted: 01/07/2008] [Indexed: 01/12/2023]
|
63
|
De Vuyst E, De Bock M, Decrock E, Van Moorhem M, Naus C, Mabilde C, Leybaert L. In situ bipolar electroporation for localized cell loading with reporter dyes and investigating gap junctional coupling. Biophys J 2008; 94:469-79. [PMID: 17872956 PMCID: PMC2157259 DOI: 10.1529/biophysj.107.109470] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2007] [Accepted: 09/05/2007] [Indexed: 11/18/2022] Open
Abstract
Electroporation is generally used to transfect cells in suspension, but the technique can also be applied to load a defined zone of adherent cells with substances that normally do not permeate the plasma membrane. In this case a pulsed high-frequency oscillating electric field is applied over a small two-wire electrode positioned close to the cells. We compared unipolar with bipolar electroporation pulse protocols and found that the latter were ideally suited to efficiently load a narrow longitudinal strip of cells in monolayer cultures. We further explored this property to determine whether electroporation loading was useful to investigate the extent of dye spread between cells coupled by gap junctions, using wild-type and stably transfected C6 glioma cells expressing connexin 32 or 43. Our investigations show that the spatial spread of electroporation-loaded 6-carboxyfluorescein, as quantified by the standard deviation of Gaussian dye spread or the spatial constant of exponential dye spread, was a reliable approach to investigate the degree of cell-cell coupling. The spread of reporter dye between coupled cells was significantly larger with electroporation loading than with scrape loading, a widely used method for dye-coupling studies. We conclude that electroporation loading and dye transfer is a robust technique to investigate gap-junctional coupling that combines minimal cell damage with accurate probing of the degree of cell-cell communication.
Collapse
Affiliation(s)
- Elke De Vuyst
- Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, B-9000 Ghent, Belgium
| | | | | | | | | | | | | |
Collapse
|
64
|
Zhao Y, Migita K, Sato C, Usune S, Iwamoto T, Katsuragi T. Endoplasmic reticulum is a key organella in bradykinin-triggered ATP release from cultured smooth muscle cells. J Pharmacol Sci 2007; 105:57-65. [PMID: 17827868 DOI: 10.1254/jphs.fp0070865] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
Abstract
ATP has broad functions as an autocrine/paracrine molecule. The mode of ATP release and its intracellular source, however, are little understood. Here we show that bradykinin via B(2)-receptor stimulation induces the extracellular release of ATP via the inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)]-signaling pathway in cultured taenia coli smooth muscle cells. It was found that bradykinin also increased the production of Ins(1,4,5)P(3) and 2-APB-inhibitable [Ca(2+)](i). The evoked release of ATP was suppressed by the Ca(2+)-channel blockers, nifedipine, and verapamil. Moreover, the extracellular release of ATP was elicited by photoliberation of Ins(1,4,5)P(3). Bradykinin caused a quick and transient accumulation of intracellular ATP from cells treated with 1% perchloric acid solution (PCA), but not with the cell lysis buffer. Peak accumulation was prevented by 2-APB and thapsigargin, but not by nifedipine or verapamil, inhibitors of extracellular release of ATP. These findings suggest that bradykinin elicits the extracellular release of ATP that is mediated by the Ins(1,4,5)P(3)-induced Ca(2+) signaling and, finally, leads to a Ca(2+)-dependent export of ATP from the cells. Furthermore, the bradykinin-induced transient accumulation of ATP in the cells treated with PCA may imply a possible release of ATP from the endoplasmic reticulum.
Collapse
Affiliation(s)
- Yumei Zhao
- Department of Pharmacology, School of Medicine, Fukuoka University, Japan
| | | | | | | | | | | |
Collapse
|
65
|
Retamal MA, Schalper KA, Shoji KF, Orellana JA, Bennett MVL, Sáez JC. Possible involvement of different connexin43 domains in plasma membrane permeabilization induced by ischemia-reperfusion. J Membr Biol 2007; 218:49-63. [PMID: 17705051 DOI: 10.1007/s00232-007-9043-y] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2007] [Accepted: 06/15/2007] [Indexed: 01/04/2023]
Abstract
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.
Collapse
Affiliation(s)
- Mauricio A Retamal
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile
| | | | | | | | | | | |
Collapse
|
66
|
Romanov RA, Rogachevskaja OA, Bystrova MF, Jiang P, Margolskee RF, Kolesnikov SS. Afferent neurotransmission mediated by hemichannels in mammalian taste cells. EMBO J 2007; 26:657-67. [PMID: 17235286 PMCID: PMC1794384 DOI: 10.1038/sj.emboj.7601526] [Citation(s) in RCA: 265] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2006] [Accepted: 12/05/2006] [Indexed: 01/06/2023] Open
Abstract
In mammalian taste buds, ionotropic P2X receptors operate in gustatory nerve endings to mediate afferent inputs. Thus, ATP secretion represents a key aspect of taste transduction. Here, we characterized individual vallate taste cells electrophysiologically and assayed their secretion of ATP with a biosensor. Among electrophysiologically distinguishable taste cells, a population was found that released ATP in a manner that was Ca(2+) independent but voltage-dependent. Data from physiological and pharmacological experiments suggested that ATP was released from taste cells via specific channels, likely to be connexin or pannexin hemichannels. A small fraction of ATP-secreting taste cells responded to bitter compounds, indicating that they express taste receptors, their G-protein-coupled and downstream transduction elements. Single cell RT-PCR revealed that ATP-secreting taste cells expressed gustducin, TRPM5, PLCbeta2, multiple connexins and pannexin 1. Altogether, our data indicate that tastant-responsive taste cells release the neurotransmitter ATP via a non-exocytotic mechanism dependent upon the generation of an action potential.
Collapse
Affiliation(s)
- Roman A Romanov
- Institute of Cell Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, Russia
| | - Olga A Rogachevskaja
- Institute of Cell Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, Russia
| | - Marina F Bystrova
- Institute of Cell Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, Russia
| | - Peihua Jiang
- Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, NY, USA
| | - Robert F Margolskee
- Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, NY, USA
| | - Stanislav S Kolesnikov
- Institute of Cell Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, Russia
| |
Collapse
|
67
|
Davis KA, Samson SE, Best K, Mallhi KK, Szewczyk M, Wilson JX, Kwan CY, Grover AK. Ca2+-mediated ascorbate release from coronary artery endothelial cells. Br J Pharmacol 2007; 147:131-9. [PMID: 16331296 PMCID: PMC1615852 DOI: 10.1038/sj.bjp.0706492] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
1.--The addition of Ca(2+) ionophore A23187 or ATP to freshly isolated or cultured pig coronary artery endothelial cells (PCEC) potentiated the release of ascorbate (Asc). Cultured PCEC were used to characterize the Ca(2+)-mediated release. An increase in Ca(2+)-mediated Asc release was observed from PCEC preincubated with Asc, Asc-2-phosphate or dehydroascorbic acid (DHAA). 2.--The effects of various ATP analogs and inhibition by suramin were consistent with the ATP-induced release being mediated by P2Y2-like receptors. 3.--ATP-stimulated Asc release was Ca(2+)-mediated because (a) ATP analogs that increased Asc release also elevated cytosolic [Ca(2+)], (b) Ca(2+) ionophore A23187 and cyclopiazonic acid stimulated the Asc release, (c) removing extracellular Ca(2+) and chelating intracellular Ca(2+)inhibited the ATP-induced release, and (d) inositol-selective phospholipase C inhibitor U73122 also inhibited this release. 4.--Accumulation of Asc by PCEC was examined at Asc concentrations of 10 microM (Na(+)-Asc symporter not saturated) and 5 mM (Na(+)-Asc symporter saturated). At 10 microM Asc, A23187 and ATP caused an inhibition of Asc accumulation but at 5 mM Asc, both the agents caused a stimulation. Substituting gluconate for chloride did not affect the basal Asc uptake but it abolished the effects of A23187. 5.--PCEC but not pig coronary artery smooth muscle cells show a Ca(2+)- mediated Asc release pathway that may be activated by agents such as ATP.
Collapse
Affiliation(s)
- Kim A Davis
- Department of Medicine, McMaster University, Hamilton, Ontario, Canada
| | - Sue E Samson
- Department of Medicine, McMaster University, Hamilton, Ontario, Canada
| | - Kelly Best
- Department of Biology, McMaster University, Hamilton, Ontario, Canada
| | | | | | - John X Wilson
- Department of Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada
| | - Chiu-Yin Kwan
- Department of Medicine, McMaster University, Hamilton, Ontario, Canada
| | - Ashok K Grover
- Department of Medicine, McMaster University, Hamilton, Ontario, Canada
- Department of Biology, McMaster University, Hamilton, Ontario, Canada
- Author for correspondence:
| |
Collapse
|
68
|
De Vuyst E, Decrock E, De Bock M, Yamasaki H, Naus CC, Evans WH, Leybaert L. Connexin hemichannels and gap junction channels are differentially influenced by lipopolysaccharide and basic fibroblast growth factor. Mol Biol Cell 2006; 18:34-46. [PMID: 17079735 PMCID: PMC1751325 DOI: 10.1091/mbc.e06-03-0182] [Citation(s) in RCA: 156] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Gap junction (GJ) channels are formed by two hemichannels (connexons), each contributed by the cells taking part in this direct cell-cell communication conduit. Hemichannels that do not interact with their counterparts on neighboring cells feature as a release pathway for small paracrine messengers such as nucleotides, glutamate, and prostaglandins. Connexins are phosphorylated by various kinases, and we compared the effect of various kinase-activating stimuli on GJ channels and hemichannels. Using peptides identical to a short connexin (Cx) amino acid sequence to specifically block hemichannels, we found that protein kinase C, Src, and lysophosphatidic acid (LPA) inhibited GJs and hemichannel-mediated ATP release in Cx43-expressing C6 glioma cells (C6-Cx43). Lipopolysaccharide (LPS) and basic fibroblast growth factor (bFGF) inhibited GJs, but they stimulated ATP release via hemichannels in C6-Cx43. LPS and bFGF inhibited hemichannel-mediated ATP release in HeLa-Cx43 cells, but they stimulated it in HeLa-Cx43 with a truncated carboxy-terminal (CT) domain or in HeLa-Cx26, which has a very short CT. Hemichannel potentiation by LPS was inhibited by blockers of the arachidonic acid metabolism, and arachidonic acid had a potentiating effect like LPS and bFGF. We conclude that GJ channels and hemichannels display similar or oppositely directed responses to modulatory influences, depending on the balance between kinase activity and the activity of the arachidonic acid pathway. Distinctive hemichannel responses to pathological stimulation with LPS or bFGF may serve to optimize the cell response, directed at strictly controlling cellular ATP release, switching from direct GJ communication to indirect paracrine signaling, or maximizing cell-protective strategies.
Collapse
Affiliation(s)
- Elke De Vuyst
- *Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, B-9000 Ghent, Belgium
| | - Elke Decrock
- *Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, B-9000 Ghent, Belgium
| | - Marijke De Bock
- *Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, B-9000 Ghent, Belgium
| | - Hiroshi Yamasaki
- Department of Bioscience, School of Science and Technology, Kwansei Gakuin University, Gakuin, Sanda 669-13, Japan
| | - Christian C. Naus
- Department of Cellular and Physiological Sciences, Faculty of Medicine, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3; and
| | - W. Howard Evans
- Department of Medical Biochemistry and Immunology, Cardiff University School of Medicine, Cardiff CF14 4XN, United Kingdom
| | - Luc Leybaert
- *Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, B-9000 Ghent, Belgium
| |
Collapse
|
69
|
Gomes P, Srinivas SP, Vereecke J, Himpens B. Gap junctional intercellular communication in bovine corneal endothelial cells. Exp Eye Res 2006; 83:1225-37. [PMID: 16938292 DOI: 10.1016/j.exer.2006.06.012] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2006] [Revised: 05/18/2006] [Accepted: 06/26/2006] [Indexed: 11/15/2022]
Abstract
Gap junctions and/or paracrine mediators, such as ATP, mediate intercellular communication (IC) in non-excitable cells. This study investigates the contribution of gap junctions toward IC during propagation of Ca(2+) waves in cultured bovine corneal endothelial cells (BCEC) elicited by applying a point mechanical stimulus to a single cell in a confluent monolayer. Changes in [Ca(2+)](i) were visualized using the fluorescent dye Fluo-4. The area reached by the Ca(2+) wave, called the active area (AA), was determined as a measure of efficacy of IC. RT-PCR and Western blotting showed expression of Cx43, a major form of connexin, in BCEC. In scrape-loading (using lucifer yellow) and fluorescence recovery after photobleaching (FRAP; using carboxyfluorescein) protocols, significant dye transfer of the hydrophilic dyes was evident indicating functional gap junctional IC (GJIC) in BCEC. Gap27 (300 microM), a connexin mimetic peptide that blocks gap junctions formed by Cx43, reduced the fluorescence recovery in FRAP experiments by 19%. Gap27 also reduced the active area of the Ca(2+) wave induced by point mechanical stimulation from 73,689 microm(2) to 26,936 microm(2), implying that GJIC contribution to the spread of the wave is at least approximately 63%. Inhibitors of ATP-mediated paracrine IC (PIC), such as a combination of apyrase VI and apyrase VII (5U/ml each; exogenous ATPases), suramin (200 microM; P2Y antagonist), or Gap26 (300 microM; blocker of Cx43 hemichannels) reduced the active area by 91%, 67%, and 55%, respectively. Therefore, estimating the contribution of GJIC from the residual active area after PIC inhibition appears to suggest that GJIC contributes no more than approximately 9% towards the active area of the Ca(2+) wave. Gap27 did not affect the enhancement in active area induced by ARL-67156 (200 microM, ectonucleotidase inhibitor), ATP release induced by point mechanical stimulation, and zero [Ca(2+)](o)-induced lucifer yellow uptake, indicating that the peptide has no influence on PIC. Exposure to Gap27 in the presence of PIC inhibitors led to a significant further inhibition of the Ca(2+) wave. The finding that the residual active area after inhibition of PIC by apyrases was much smaller than the reduction of the active area by Gap27, provides evidence for interaction between GJIC and PIC. These findings together suggest that functional gap junctions are present in BCEC, that both GJIC and PIC contribute significantly to IC, and that the two pathways interact.
Collapse
Affiliation(s)
- Priya Gomes
- Laboratory of Physiology, KU Leuven, Campus Gasthuisberg O/N, Box 802, Herestraat 49, B-3000 Leuven, Belgium
| | | | | | | |
Collapse
|
70
|
Barbe MT, Monyer H, Bruzzone R. Cell-cell communication beyond connexins: the pannexin channels. Physiology (Bethesda) 2006; 21:103-14. [PMID: 16565476 DOI: 10.1152/physiol.00048.2005] [Citation(s) in RCA: 181] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Direct cell-to-cell communication through specialized intercellular channels is a characteristic feature of virtually all multi-cellular organisms. The remarkable functional conservation of cell-to-cell coupling throughout the animal kingdom, however, is not matched at the molecular level of the structural protein components. Thus protostomes (including nematodes and flies) and deuterostomes (including all vertebrates) utilize two unrelated families of gap-junction genes, innexins and connexins, respectively. The recent discovery that pannexins, a novel group of proteins expressed by several organisms, are able to form intercellular channels has started a quest to understand their evolutionary relationship and functional contribution to cell communication in vivo. There are three pannexin genes in mammals, two of which are co-expressed in the developing and adult brain. Of note, pannexin1 can also form Ca2+-activated hemichannels that open at physiological extracellular Ca2+ concentrations and exhibit distinct pharmacological properties.
Collapse
Affiliation(s)
- Michael T Barbe
- Department of Clinical Neurobiology and Interdisciplinary Center for Neuroscience, University of Heidelberg, Heidelberg, Germany
| | | | | |
Collapse
|
71
|
Evans WH, De Vuyst E, Leybaert L. The gap junction cellular internet: connexin hemichannels enter the signalling limelight. Biochem J 2006; 397:1-14. [PMID: 16761954 PMCID: PMC1479757 DOI: 10.1042/bj20060175] [Citation(s) in RCA: 331] [Impact Index Per Article: 18.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2006] [Revised: 03/22/2006] [Accepted: 03/23/2006] [Indexed: 02/07/2023]
Abstract
Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.
Collapse
Affiliation(s)
- W Howard Evans
- Department of Medical Biochemistry and Immunology and the Wales Heart Research Institute, Cardiff University Medical School, Cardiff CF14 4XN, Wales, UK.
| | | | | |
Collapse
|
72
|
De Vuyst E, Decrock E, Cabooter L, Dubyak GR, Naus CC, Evans WH, Leybaert L. Intracellular calcium changes trigger connexin 32 hemichannel opening. EMBO J 2006; 25:34-44. [PMID: 16341088 PMCID: PMC1356351 DOI: 10.1038/sj.emboj.7600908] [Citation(s) in RCA: 200] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2005] [Accepted: 11/17/2005] [Indexed: 11/09/2022] Open
Abstract
Connexin hemichannels have been proposed as a diffusion pathway for the release of extracellular messengers like ATP and others, based on connexin expression models and inhibition by gap junction blockers. Hemichannels are opened by various experimental stimuli, but the physiological intracellular triggers are currently not known. We investigated the hypothesis that an increase of cytoplasmic calcium concentration ([Ca2+]i) triggers hemichannel opening, making use of peptides that are identical to a short amino-acid sequence on the connexin subunit to specifically block hemichannels, but not gap junction channels. Our work performed on connexin 32 (Cx32)-expressing cells showed that an increase in [Ca2+]i triggers ATP release and dye uptake that is dependent on Cx32 expression, blocked by Cx32 (but not Cx43) mimetic peptides and a calmodulin antagonist, and critically dependent on [Ca2+]i elevation within a window situated around 500 nM. Our results indicate that [Ca2+]i elevation triggers hemichannel opening, and suggest that these channels are under physiological control.
Collapse
Affiliation(s)
- Elke De Vuyst
- Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium
| | - Elke Decrock
- Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium
| | - Liesbet Cabooter
- Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium
| | - George R Dubyak
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, USA
| | - Christian C Naus
- Department of Cellular and Physiological Sciences, Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada
| | - W Howard Evans
- Department of Medical Biochemistry and Immunology, Cardiff University School of Medicine, Cardiff, UK
| | - Luc Leybaert
- Department of Physiology and Pathophysiology, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium
| |
Collapse
|
73
|
Ebong EE, Kim S, DePaola N. Flow regulates intercellular communication in HAEC by assembling functional Cx40 and Cx37 gap junctional channels. Am J Physiol Heart Circ Physiol 2005; 290:H2015-23. [PMID: 16361362 DOI: 10.1152/ajpheart.00204.2005] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Direct cell-to-cell transfer of ions and small signaling molecules via gap junctions plays a key role in vessel wall homeostasis. Vascular endothelial gap junctional channels are formed by the connexin (Cx) proteins Cx37, Cx40, and Cx43. The mechanisms regulating connexin expression and assembly into functional channels have not been fully identified. We investigated the dynamic regulation of endothelial gap junctional intercellular communication (GJIC) by fluid flow and the participation of each vascular connexin in functional human endothelial gap junctions in vitro. Human aortic endothelial cells (HAEC) were exposed for 5, 16, and 24 h to physiological flows in a parallel-plate flow chamber. Connexin protein expression and localization were evaluated by immunocytochemistry, and functional GJIC was evaluated by dye injection. Connexin-mimetic peptide inhibitors were used to assess the specific connexin composition of functional channels. HAEC monolayers in culture exhibited baseline functional communication at a striking low level despite abundant expression of Cx43 and Cx40 localized at cell-to-cell appositions. Upon exposure to flow, GJIC by dye spread demonstrated a significant time-dependent increase from baseline levels, reaching 7.5-fold in 24 h. Inhibition studies revealed that this response was mediated primarily by Cx40, with lesser contributions of the other two vascular connexins assembled into functional homotypic and/or heterotypic channels. This is the first study to demonstrate that flow simultaneously and differentially regulates expression of the Cx37, Cx40, and Cx43 proteins and their involvement in the augmentation of intercellular communication by dye transfer in human endothelial cells in vitro.
Collapse
Affiliation(s)
- Eno Essien Ebong
- Department of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
| | | | | |
Collapse
|
74
|
Sabirov RZ, Okada Y. ATP release via anion channels. Purinergic Signal 2005; 1:311-28. [PMID: 18404516 PMCID: PMC2096548 DOI: 10.1007/s11302-005-1557-0] [Citation(s) in RCA: 135] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2005] [Revised: 07/19/2005] [Accepted: 07/26/2005] [Indexed: 11/30/2022] Open
Abstract
ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.
Collapse
Affiliation(s)
- Ravshan Z. Sabirov
- Department of Cell Physiology, National Institute for Physiological Sciences, Okazaki, 444-8585 Japan
| | - Yasunobu Okada
- Department of Cell Physiology, National Institute for Physiological Sciences, Okazaki, 444-8585 Japan
| |
Collapse
|
75
|
Krysko DV, Leybaert L, Vandenabeele P, D'Herde K. Gap junctions and the propagation of cell survival and cell death signals. Apoptosis 2005; 10:459-69. [PMID: 15909108 DOI: 10.1007/s10495-005-1875-2] [Citation(s) in RCA: 123] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
Gap junctions are a unique type of intercellular channels that connect the cytoplasm of adjoining cells. Each gap junction channel is comprised of two hemichannels or connexons and each connexon is formed by the aggregation of six protein subunits known as connexins. Gap junction channels allow the intercellular passage of small (< 1.5 kDa) molecules and regulate essential processes during development and differentiation. However, their role in cell survival and cell death is poorly understood. We review experimental data that support the hypothesis that gap junction channels may propagate cell death and survival modulating signals. In addition, we explore the hypothesis that hemichannels (or unapposed connexons) might be used as a paracrine conduit to spread factors that modulate the fate of the surrounding cells. Finally, direct signal transduction activity of connexins in cell death and survival pathways is addressed.
Collapse
Affiliation(s)
- D V Krysko
- Department of Human Anatomy, Embryology, Histology and Medical Physics, Ghent University, 9000 Ghent, Belgium.
| | | | | | | |
Collapse
|
76
|
Abstract
Emerging studies indicate that connexins have activities completely unrelated to gap junctions and, conversely, that non-connexin proteins can form gap junction channels.
Collapse
Affiliation(s)
- Charles Stout
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA
| | | | | |
Collapse
|
77
|
Contreras JE, Sánchez HA, Véliz LP, Bukauskas FF, Bennett MV, Sáez JC. Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue. ACTA ACUST UNITED AC 2005; 47:290-303. [PMID: 15572178 PMCID: PMC3651737 DOI: 10.1016/j.brainresrev.2004.08.002] [Citation(s) in RCA: 184] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/27/2004] [Indexed: 01/24/2023]
Abstract
Gap junction channels and hemichannels formed of connexin subunits are found in most cell types in vertebrates. Gap junctions connect cells via channels not open to the extracellular space and permit the passage of ions and molecules of approximately 1 kDa. Single connexin hemichannels, which are connexin hexamers, are present in the surface membrane before docking with a hemichannel in an apposed membrane. Because of their high conductance and permeability in cell-cell channels, it had been thought that connexin hemichannels remained closed until docking to form a cell-cell channel. Now it is clear that at least some hemichannels can open to allow passage of molecules between the cytoplasm and extracellular space. Here we review evidence that gap junction channels may allow intercellular diffusion of necrotic or apoptotic signals, but may also allow diffusion of ions and substances from healthy to injured cells, thereby contributing to cell survival. Moreover, opening of gap junction hemichannels may exacerbate cell injury or mediate paracrine or autocrine signaling. In addition to the cell specific features of an ischemic insult, propagation of cell damage and death within affected tissues may be affected by expression and regulation of gap junction channels and hemichannels formed by connexins.
Collapse
Affiliation(s)
- Jorge E. Contreras
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Chile
| | - Helmuth A. Sánchez
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Chile
- Corresponding author. Tel.: +56 2 6862860; fax: +56 2 2225515. (H.A. Sánchez)
| | - Loreto P. Véliz
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Chile
| | | | - Michael V.L. Bennett
- Department of Neurosciences, Albert Einstein College of Medicine, Bronx, NY, USA
| | - Juan C. Sáez
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Chile
- Department of Neurosciences, Albert Einstein College of Medicine, Bronx, NY, USA
| |
Collapse
|
78
|
Sáez JC, Retamal MA, Basilio D, Bukauskas FF, Bennett MVL. Connexin-based gap junction hemichannels: gating mechanisms. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2005; 1711:215-24. [PMID: 15955306 PMCID: PMC3617572 DOI: 10.1016/j.bbamem.2005.01.014] [Citation(s) in RCA: 294] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/02/2004] [Revised: 01/20/2005] [Accepted: 01/26/2005] [Indexed: 01/16/2023]
Abstract
Connexins (Cxs) form hemichannels and gap junction channels. Each gap junction channel is composed of two hemichannels, also termed connexons, one from each of the coupled cells. Hemichannels are hexamers assembled in the ER, the Golgi, or a post Golgi compartment. They are transported to the cell surface in vesicles and inserted by vesicle fusion, and then dock with a hemichannel in an apposed membrane to form a cell-cell channel. It was thought that hemichannels should remain closed until docking with another hemichannel because of the leak they would provide if their permeability and conductance were like those of their corresponding cell-cell channels. Now it is clear that hemichannels formed by a number of different connexins can open in at least some cells with a finite if low probability, and that their opening can be modulated under various physiological and pathological conditions. Hemichannels open in different kinds of cells in culture with conductance and permeability properties predictable from those of the corresponding gap junction channels. Cx43 hemichannels are preferentially closed in cultured cells under resting conditions, but their open probability can be increased by the application of positive voltages and by changes in protein phosphorylation and/or redox state. In addition, increased activity can result from the recruitment of hemichannels to the plasma membrane as seen in metabolically inhibited astrocytes. Mutations of connexins that increase hemichannel open probability may explain cellular degeneration in several hereditary diseases. Taken together, the data indicate that hemichannels are gated by multiple mechanisms that independently or cooperatively affect their open probability under physiological as well as pathological conditions.
Collapse
Affiliation(s)
- Juan C Sáez
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Santiago, Chile.
| | | | | | | | | |
Collapse
|
79
|
Bruzzone R, Barbe MT, Jakob NJ, Monyer H. Pharmacological properties of homomeric and heteromeric pannexin hemichannels expressed in Xenopus oocytes. J Neurochem 2005; 92:1033-43. [PMID: 15715654 DOI: 10.1111/j.1471-4159.2004.02947.x] [Citation(s) in RCA: 368] [Impact Index Per Article: 19.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Several new findings have emphasized the role of neuron-specific gap junction proteins (connexins) and electrical synapses in processing sensory information and in synchronizing the activity of neuronal networks. We have recently shown that pannexins constitute an additional family of proteins that can form gap junction channels in a heterologous expression system and are also widely expressed in distinct neuronal populations in the brain, where they may represent a novel class of electrical synapses. In this study, we have exploited the hemichannel-forming properties of pannexins to investigate their sensitivity to well-known connexin blockers. By combining biochemical and electrophysiological approaches, we report here further evidence for the interaction of pannexin1 (Px1) with Px2 and demonstrate that the pharmacological sensitivity of heteromeric Px1/Px2 is similar to that of homomeric Px1 channels. In contrast to most connexins, both Px1 and Px1/Px2 hemichannels were not gated by external Ca2+. In addition, they exhibited a remarkable sensitivity to blockade by carbenoxolone (with an IC50 of approximately 5 microm), whereas flufenamic acid exerted only a modest inhibitory effect. The opposite was true in the case of connexin46 (Cx46), thus indicating that gap junction blockers are able to selectively modulate pannexin and connexin channels.
Collapse
|
80
|
Suadicani SO, Flores CE, Urban-Maldonado M, Beelitz M, Scemes E. Gap junction channels coordinate the propagation of intercellular Ca2+ signals generated by P2Y receptor activation. Glia 2005; 48:217-29. [PMID: 15390120 PMCID: PMC2586889 DOI: 10.1002/glia.20071] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
Astrocytes express gap junction proteins and multiple types of P2Y receptors (P2YRs) that contribute to the propagation of intercellular Ca(2+) waves (ICW). To gain access to the role played by gap junctional communication in ICW propagation generated by P2YR activation, we selectively expressed P2Y(1,2,4)R subtypes and Cx43 in the human 1321N1 astrocytoma cell line, which lacks endogenous P2 receptors. Fluorescence recovery after photobleaching revealed that 1321N1 cells are poorly dye-coupled and do not propagate ICW. Forced expression of Cx43 in 1321N1 cells (which did not show functional hemichannels) increased dye coupling and allowed short-range ICW transmission that was mainly mediated by intercellular diffusion of Ca(2+) generated in the stimulated cells. Astrocytoma clones expressing each of the P2YR subtypes were also able to propagate ICWs that were likely dependent on IP(3) generation. These waves exhibited properties particular to each P2YR subtype. Co-expression of eGFP-hCx43 and P2Y(1)R modified the properties of P2Y(1)R-generated ICW to those characteristics of P2Y(2)R. Increased coupling in P2Y(4)R clones induced by expression of eGFP-hCx43 abolished the ICWs observed in uncoupled P2Y(4)R clones. No changes in the behavior of ICWs generated in P2Y(2)R clones were observed after forced expression of Cx43. These data indicate that in 1321N1 cells gap junctional communication provides intercellular integration of Ca(2+) signals generated by P2YR activation, thus coordinating the propagation of intercellular calcium waves.
Collapse
Affiliation(s)
- S O Suadicani
- Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
| | | | | | | | | |
Collapse
|
81
|
Leybaert L, Braet K, Vandamme W, Cabooter L, Martin PEM, Evans WH. Connexin channels, connexin mimetic peptides and ATP release. ACTA ACUST UNITED AC 2004; 10:251-7. [PMID: 14681025 DOI: 10.1080/cac.10.4-6.251.257] [Citation(s) in RCA: 102] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
Abstract
Connexin hemichannels, that is, half gap junction channels (not connecting cells), have been implicated in the release of various messengers such as ATP and glutamate. We used connexin mimetic peptides, which are, small peptides mimicking a sequence on the connexin subunit, to investigate hemichannel functioning in endothelial cell lines. Short exposure (30 min) to synthetic peptides mimicking a sequence on the first or second extracellular loop of the connexin subunit strongly supressed ATP release and dye uptake triggered by either intracellular InsP(3) elevation or exposure to zero extracellular calcium, while gap junctional coupling was not affected under these conditions. The effect was dependent on the expression of connexin-43 in the cells. Connexin mimetic peptides thus appear to be interesting tools to distinguish connexin hemichannel from gap junction channel functioning. In addition, they are well suited to further explore the role of connexins in cellular release or uptake processes, to investigate hemichannel gating and to reveal new unknown functions of the large conductance hemichannel pathway between the cell and its environment. Work performed up to now with these peptides should be re-interpreted in terms of these new findings.
Collapse
Affiliation(s)
- Luc Leybaert
- Department of Physiology and Pathophysiology, Ghent University, Ghent, Belgium.
| | | | | | | | | | | |
Collapse
|
82
|
Katsuragi T, Migita K. [The mechanism of ATP release as an autocrine/paracrine molecule]. Nihon Yakurigaku Zasshi 2004; 123:382-8. [PMID: 15170077 DOI: 10.1254/fpj.123.382] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Many studies have been performed to clarify the underlying mechanisms of the release of ATP as an autocrine / paracrine signaling molecule. So far, there is a variety of findings on the mode of release of this nucleotide. This review focused on the possible mechanisms of ATP release. The ATP binding cassette, especially CFTR (cystic fibrosis transmembrane conductance regulator), is a strong candidate for a channel or a transporter for outward movement of ATP. CFTR, which is activated via phosphorylation by protein kinase A, causes an opening of channels for Cl(-) and ATP(4-), releasing ATP. However, the possible involvement of CFTR in ATP release is still under dispute. As another candidate of the membrane machinery, the hemichannel of gap junction has been raised. Mechanical stress and photoliberation of caged InsP(3) induce the release of ATP as a paracrine through the hemichannel accompanied with the increase of [Ca(2+)]i. These events result in the Ca(2+)wave as cell-to-cell communications. In conclusion, an authoritative view of the mechanism of ATP release remains to be made clear in future studies.
Collapse
|
83
|
Oike M, Droogmans G, Ito Y. [ATP release pathways in vascular endothelial cells]. Nihon Yakurigaku Zasshi 2004; 123:403-11. [PMID: 15170080 DOI: 10.1254/fpj.123.403] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Vascular endothelial cells regulate vascular tonus, growth, and angiogenesis in response to mechanical stresses. ATP release is one of well-known mechanosensitive responses in endothelial cells. Released ATP induces Ca(2+) responses and nitric oxide production in neighboring cells in an auto/paracrine manner. Mechanosensitive and agonist-induced ATP releases are also observed in other cell types, but the cellular mechanisms and pathways of ATP release are largely unknown. Reported candidates for ATP release pathways are ABC proteins including P-glycoprotein and CFTR, exocytosis of ATP-containing vesicles, and ATP-permeable anion channels. In vascular endothelium, vesicular exocytosis, volume-regulated anion channels (VRAC), and connexin hemichannels have been reported as candidates for ATP release pathways. We found that VRAC inhibitors suppressed hypotonic stress-induced ATP release in bovine aortic endothelial cells. Furthermore, extracellular ATP suppressed VRAC current in a voltage dependent manner, which could be fitted to the permeation-blocker model with a Kd(0) of 1 mM and delta value of 0.41. However, it should be noted that VRAC is probably not the only pathway for ATP release in the endothelium, because basal ATP release was not inhibited by VRAC inhibitors. Further investigations are definitely warranted to clarify the details and therapeutic significance of mechanosensitive ATP release in the endothelium.
Collapse
Affiliation(s)
- Masahiro Oike
- Department of Pharmacology, Graduate School of Medical Sciences, Kyushu University.
| | | | | |
Collapse
|
84
|
Braet K, Mabilde C, Cabooter L, Rapp G, Leybaert L. Electroporation loading and photoactivation of caged InsP3: tools to investigate the relation between cellular ATP release in response to intracellular InsP3 elevation. J Neurosci Methods 2004; 132:81-9. [PMID: 14687677 DOI: 10.1016/j.jneumeth.2003.09.003] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Photolytic liberation of InsP(3) in single cells triggers cell-to-cell propagating calcium changes that are communicated by a gap junctional and a paracrine purinergic pathway involving InsP(3)-triggered ATP release. We investigated the relation between the InsP(3) stimulus and the resulting ATP release in ECV304 cells using UV photolysis of caged compounds and bioluminescent ATP measurements. Careful consideration of all steps, starting from caged InsP(3) loading into the cells by electroporation, up to photoliberation upon UV exposure, allowed to derive a dose-response relation that revealed a first part with a flattening ATP release response in the below 10microM InsP(3) concentration range and a second phase of steeply increasing ATP release in response to above 10microM InsP(3) stimulation. ATP release triggered by below 10microM InsP(3) concentrations attained a level in the order of 30% above baseline ATP release, while the steeply increasing response to high InsP(3) concentrations attained values in the order of 150% above baseline. Our data indicate the involvement of low affinity InsP(3) receptor sites in the pathway leading to triggered ATP release, with activation of these receptors causing the release of 1-2% of the total cellular ATP pool.
Collapse
Affiliation(s)
- Katleen Braet
- Physiology and Pathophysiology, Ghent University, De Pintelaan 185 (Block B), B-9000 Ghent, Belgium
| | | | | | | | | |
Collapse
|
85
|
Oviedo-Orta E, Howard Evans W. Gap junctions and connexin-mediated communication in the immune system. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2004; 1662:102-12. [PMID: 15033582 DOI: 10.1016/j.bbamem.2003.10.021] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2003] [Revised: 10/20/2003] [Accepted: 10/20/2003] [Indexed: 10/26/2022]
Abstract
Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.
Collapse
Affiliation(s)
- Ernesto Oviedo-Orta
- Bristol Heart Institute, Bristol Royal Infirmary, Upper Maudlin Street, Bristol BS2 8HW, UK
| | | |
Collapse
|
86
|
Abstract
Propagation of interastrocyte Ca2+ waves is mediated by diffusion of extracellular adenosine triphosphate (ATP), and may require regenerative release of ATP. The ability of ATP to initiate release of intracellular ATP was assessed by labeling adenine nucleotide pools in astrocyte cultures with 14C-adenine. The 14C-purines released during exposure to ATP were then identified by thin-layer chromatography. ATP treatment caused a five-fold increase in release of 14C-ATP but not 14C-ADP or 14C-AMP, indicating selectivity for release of ATP. Other P2 receptor agonists also caused significant 14C-ATP release, and the P2 receptor antagonists suramin, reactive blue-2 and pyridoxalphosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) inhibited ATP-induced 14C-ATP release to varying degrees, suggesting the involvement of a P2 receptor. ATP-induced 14C-ATP release was not affected by chelation of intracellular Ca2+ with BAPTA-AM, or by blockers of Ca2+ release from intracellular stores or of extracellular Ca2+ influx, suggesting a Ca2+-independent response. ATP-induced 14C-ATP release was significantly inhibited by non-selective anion channel blockers but not by blockers of ATP-binding cassette proteins, gap junction hemichannels, or vesicular exocytosis. Release of adenine nucleotides induced by 0 Ca2+ was, in contrast, not selective for ATP, and was susceptible to inhibition by gap junction blockers. These findings indicate that astrocytes are capable of ATP-induced ATP release and support a role for regenerative ATP release in glial Ca2+ wave propagation.
Collapse
Affiliation(s)
- Christopher M Anderson
- Department of Neurology, University of California, San Francisco and Department of Veterans Affairs Medical Center, San Francisco, California 94121, USA.
| | | | | |
Collapse
|
87
|
Vandamme W, Braet K, Cabooter L, Leybaert L. Tumour necrosis factor alpha inhibits purinergic calcium signalling in blood-brain barrier endothelial cells. J Neurochem 2003; 88:411-21. [PMID: 14690529 DOI: 10.1046/j.1471-4159.2003.02163.x] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The breaching of the blood-brain barrier is an essential aspect in the pathogenesis of neuroinflammatory diseases, in which tumour necrosis factor alpha (TNF-alpha) as well as endothelial calcium ions play a key role. We investigated whether TNF-alpha could influence the communication of calcium signals between brain endothelial cells (GP8 and RBE4). Intercellular calcium waves triggered by mechanical stimulation or photoliberation of InsP3 in single cells were significantly reduced in size after TNF-alpha exposure (1000 U/mL, 2 and 24 h). Calcium signals are communicated between cells by means of gap junctional and paracrine purinergic signalling. TNF-alpha significantly inhibited gap junctional coupling, stimulated the basal release of ATP, and dose-dependently blocked the triggered component of ATP release. The cytokine displayed similar effects on the uptake of a fluorescent reporter dye into the cells. Previous work with connexin mimetic peptides demonstrated that the triggered ATP release in these cells is connexin-related; these peptides did, however, not influence the elevated basal ATP release caused by TNF-alpha. We conclude that TNF-alpha depresses calcium signal communication in blood-brain barrier endothelial cells, by reducing gap junctional coupling and by inhibiting triggered ATP release. The cytokine thus inhibits connexin-related communication pathways like gap junctions and connexin hemichannels.
Collapse
Affiliation(s)
- Wouter Vandamme
- Department of Physiology and Pathophysiology, Ghent University, Ghent, Belgium
| | | | | | | |
Collapse
|