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Cheung G, Chever O, Rouach N. Connexons and pannexons: newcomers in neurophysiology. Front Cell Neurosci 2014; 8:348. [PMID: 25408635 PMCID: PMC4219455 DOI: 10.3389/fncel.2014.00348] [Citation(s) in RCA: 58] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2014] [Accepted: 10/06/2014] [Indexed: 11/14/2022] Open
Abstract
Connexin hemichannels are single membrane channels which have been traditionally thought to work in pairs to form gap junction channels across two opposing cells. In astrocytes, gap junction channels allow direct intercellular communication and greatly facilitate the transmission of signals. Recently, there has been growing evidence demonstrating that connexin hemichannels, as well as pannexin channels, on their own are open in various conditions. They allow bidirectional flow of ions and signaling molecules and act as release sites for transmitters like ATP and glutamate into the extracellular space. While much attention has focused on the function of connexin hemichannels and pannexons during pathological situations like epilepsy, inflammation, neurodegeneration or ischemia, their potential roles in physiology is often ignored. In order to fully understand the dynamic properties and roles of connexin hemichannels and pannexons in the brain, it is essential to decipher whether they also have some physiological functions and contribute to normal cerebral processes. Here, we present recent studies in the CNS suggesting emerging physiological functions of connexin hemichannels and pannexons in normal neuronal activity and behavior. We also discuss how these pioneer studies pave the way for future research to extend the physiological relevance of connexons and pannexons, and some fundamental issues yet to be addressed.
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Affiliation(s)
- Giselle Cheung
- Neuroglial Interactions in Cerebral Physiopathology, Center for Interdisciplinary Research in Biology, Collège de France, CNRS UMR 7241, INSERM U1050, Labex Memolife, PSL Research University Paris, France
| | - Oana Chever
- Neuroglial Interactions in Cerebral Physiopathology, Center for Interdisciplinary Research in Biology, Collège de France, CNRS UMR 7241, INSERM U1050, Labex Memolife, PSL Research University Paris, France
| | - Nathalie Rouach
- Neuroglial Interactions in Cerebral Physiopathology, Center for Interdisciplinary Research in Biology, Collège de France, CNRS UMR 7241, INSERM U1050, Labex Memolife, PSL Research University Paris, France
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Hansen DB, Ye ZC, Calloe K, Braunstein TH, Hofgaard JP, Ransom BR, Nielsen MS, MacAulay N. Activation, permeability, and inhibition of astrocytic and neuronal large pore (hemi)channels. J Biol Chem 2014; 289:26058-26073. [PMID: 25086040 DOI: 10.1074/jbc.m114.582155] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Astrocytes and neurons express several large pore (hemi)channels that may open in response to various stimuli, allowing fluorescent dyes, ions, and cytoplasmic molecules such as ATP and glutamate to permeate. Several of these large pore (hemi)channels have similar characteristics with regard to activation, permeability, and inhibitor sensitivity. Consequently, their behaviors and roles in astrocytic and neuronal (patho)physiology remain undefined. We took advantage of the Xenopus laevis expression system to determine the individual characteristics of several large pore channels in isolation. Expression of connexins Cx26, Cx30, Cx36, or Cx43, the pannexins Px1 or Px2, or the purinergic receptor P2X7 yielded functional (hemi)channels with isoform-specific characteristics. Connexin hemichannels had distinct sensitivity to alterations of extracellular Ca(2+) and their permeability to dyes and small atomic ions (conductance) were not proportional. Px1 and Px2 exhibited conductance at positive membrane potentials, but only Px1 displayed detectable fluorescent dye uptake. P2X7, in the absence of Px1, was permeable to fluorescent dyes in an agonist-dependent manner. The large pore channels displayed overlapping sensitivity to the inhibitors Brilliant Blue, gadolinium, and carbenoxolone. These results demonstrated isoform-specific characteristics among the large pore membrane channels; an open (hemi)channel is not a nonselective channel. With these isoform-specific properties in mind, we characterized the divalent cation-sensitive permeation pathway in primary cultured astrocytes. We observed no activation of membrane conductance or Cx43-mediated dye uptake in astrocytes nor in Cx43-expressing C6 cells. Our data underscore that although Cx43-mediated transport is observed in overexpressing cell systems, such transport may not be detectable in native cells under comparable experimental conditions.
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Affiliation(s)
- Daniel Bloch Hansen
- Department of Cellular and Molecular Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen N, Denmark
| | - Zu-Cheng Ye
- Department of Neurology, University of Washington, Seattle, Washington 98195
| | - Kirstine Calloe
- Department of Veterinary Clinical and Animal Science, Faculty of Health and Medical Sciences, University of Copenhagen, 1870 Frederiksberg, Denmark, and
| | - Thomas Hartig Braunstein
- Danish National Research Foundation Centre for Cardiac Arrhythmia and Department of Biomedicine, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen, Denmark
| | - Johannes Pauli Hofgaard
- Danish National Research Foundation Centre for Cardiac Arrhythmia and Department of Biomedicine, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen, Denmark
| | - Bruce R Ransom
- Department of Neurology, University of Washington, Seattle, Washington 98195
| | - Morten Schak Nielsen
- Danish National Research Foundation Centre for Cardiac Arrhythmia and Department of Biomedicine, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen, Denmark
| | - Nanna MacAulay
- Department of Cellular and Molecular Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen N, Denmark,.
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