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Gaete PS, Kumar D, Fernandez CI, Valdez Capuccino JM, Bhatt A, Jiang W, Lin YC, Liu Y, Harris AL, Luo YL, Contreras JE. Large-pore connexin hemichannels function like molecule transporters independent of ion conduction. Proc Natl Acad Sci U S A 2024; 121:e2403903121. [PMID: 39116127 PMCID: PMC11331127 DOI: 10.1073/pnas.2403903121] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2024] [Accepted: 07/05/2024] [Indexed: 08/10/2024] Open
Abstract
Connexin hemichannels were identified as the first members of the eukaryotic large-pore channel family that mediate permeation of both atomic ions and small molecules between the intracellular and extracellular environments. The conventional view is that their pore is a large passive conduit through which both ions and molecules diffuse in a similar manner. In stark contrast to this notion, we demonstrate that the permeation of ions and of molecules in connexin hemichannels can be uncoupled and differentially regulated. We find that human connexin mutations that produce pathologies and were previously thought to be loss-of-function mutations due to the lack of ionic currents are still capable of mediating the passive transport of molecules with kinetics close to those of wild-type channels. This molecular transport displays saturability in the micromolar range, selectivity, and competitive inhibition, properties that are tuned by specific interactions between the permeating molecules and the N-terminal domain that lies within the pore-a general feature of large-pore channels. We propose that connexin hemichannels and, likely, other large-pore channels, are hybrid channel/transporter-like proteins that might switch between these two modes to promote selective ion conduction or autocrine/paracrine molecular signaling in health and disease processes.
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Affiliation(s)
- Pablo S. Gaete
- Department of Physiology and Membrane Biology, School of Medicine, University of California, Davis, CA95616
| | - Deepak Kumar
- Department of Biotechnology and Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, CA91766
| | - Cynthia I. Fernandez
- Department of Physiology and Membrane Biology, School of Medicine, University of California, Davis, CA95616
| | - Juan M. Valdez Capuccino
- Department of Pharmacology, Physiology, and Neuroscience, New Jersey Medical School, Rutgers, The State University of New Jersey, Newark, NJ07103
| | - Aashish Bhatt
- Department of Biotechnology and Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, CA91766
| | - Wenjuan Jiang
- Department of Biotechnology and Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, CA91766
| | - Yi-Chun Lin
- Department of Biotechnology and Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, CA91766
| | - Yu Liu
- Department of Pharmacology, Physiology, and Neuroscience, New Jersey Medical School, Rutgers, The State University of New Jersey, Newark, NJ07103
| | - Andrew L. Harris
- Department of Pharmacology, Physiology, and Neuroscience, New Jersey Medical School, Rutgers, The State University of New Jersey, Newark, NJ07103
| | - Yun L. Luo
- Department of Biotechnology and Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, CA91766
| | - Jorge E. Contreras
- Department of Physiology and Membrane Biology, School of Medicine, University of California, Davis, CA95616
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Jagielnicki M, Kucharska I, Bennett BC, Harris AL, Yeager M. Connexin Gap Junction Channels and Hemichannels: Insights from High-Resolution Structures. BIOLOGY 2024; 13:298. [PMID: 38785780 PMCID: PMC11117596 DOI: 10.3390/biology13050298] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/26/2024] [Revised: 03/29/2024] [Accepted: 04/08/2024] [Indexed: 05/25/2024]
Abstract
Connexins (Cxs) are a family of integral membrane proteins, which function as both hexameric hemichannels (HCs) and dodecameric gap junction channels (GJCs), behaving as conduits for the electrical and molecular communication between cells and between cells and the extracellular environment, respectively. Their proper functioning is crucial for many processes, including development, physiology, and response to disease and trauma. Abnormal GJC and HC communication can lead to numerous pathological states including inflammation, skin diseases, deafness, nervous system disorders, and cardiac arrhythmias. Over the last 15 years, high-resolution X-ray and electron cryomicroscopy (cryoEM) structures for seven Cx isoforms have revealed conservation in the four-helix transmembrane (TM) bundle of each subunit; an αβ fold in the disulfide-bonded extracellular loops and inter-subunit hydrogen bonding across the extracellular gap that mediates end-to-end docking to form a tight seal between hexamers in the GJC. Tissue injury is associated with cellular Ca2+ overload. Surprisingly, the binding of 12 Ca2+ ions in the Cx26 GJC results in a novel electrostatic gating mechanism that blocks cation permeation. In contrast, acidic pH during tissue injury elicits association of the N-terminal (NT) domains that sterically blocks the pore in a "ball-and-chain" fashion. The NT domains under physiologic conditions display multiple conformational states, stabilized by protein-protein and protein-lipid interactions, which may relate to gating mechanisms. The cryoEM maps also revealed putative lipid densities within the pore, intercalated among transmembrane α-helices and between protomers, the functions of which are unknown. For the future, time-resolved cryoEM of isolated Cx channels as well as cryotomography of GJCs and HCs in cells and tissues will yield a deeper insight into the mechanisms for channel regulation. The cytoplasmic loop (CL) and C-terminal (CT) domains are divergent in sequence and length, are likely involved in channel regulation, but are not visualized in the high-resolution X-ray and cryoEM maps presumably due to conformational flexibility. We expect that the integrated use of synergistic physicochemical, spectroscopic, biophysical, and computational methods will reveal conformational dynamics relevant to functional states. We anticipate that such a wealth of results under different pathologic conditions will accelerate drug discovery related to Cx channel modulation.
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Affiliation(s)
- Maciej Jagielnicki
- The Phillip and Patricia Frost Institute for Chemistry and Molecular Science, Department of Chemistry, University of Miami, 1201 Memorial Drive, Miami, FL 33146, USA; (M.J.); (I.K.)
| | - Iga Kucharska
- The Phillip and Patricia Frost Institute for Chemistry and Molecular Science, Department of Chemistry, University of Miami, 1201 Memorial Drive, Miami, FL 33146, USA; (M.J.); (I.K.)
| | - Brad C. Bennett
- Department of Biological and Environmental Sciences, Howard College of Arts and Sciences, Samford University, Birmingham, AL 35229, USA;
| | - Andrew L. Harris
- Rutgers New Jersey Medical School, Department of Pharmacology, Physiology and Neuroscience, Newark, NJ 07103, USA;
| | - Mark Yeager
- The Phillip and Patricia Frost Institute for Chemistry and Molecular Science, Department of Chemistry, University of Miami, 1201 Memorial Drive, Miami, FL 33146, USA; (M.J.); (I.K.)
- The Phillip and Patricia Frost Institute for Chemistry and Molecular Science, Department of Biochemistry and Molecular Biology, University of Miami, Miami, FL 33146, USA
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Posukh OL, Maslova EA, Danilchenko VY, Zytsar MV, Orishchenko KE. Functional Consequences of Pathogenic Variants of the GJB2 Gene (Cx26) Localized in Different Cx26 Domains. Biomolecules 2023; 13:1521. [PMID: 37892203 PMCID: PMC10604905 DOI: 10.3390/biom13101521] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2023] [Revised: 10/08/2023] [Accepted: 10/10/2023] [Indexed: 10/29/2023] Open
Abstract
One of the most common forms of genetic deafness has been predominantly associated with pathogenic variants in the GJB2 gene, encoding transmembrane protein connexin 26 (Cx26). The Cx26 molecule consists of an N-terminal domain (NT), four transmembrane domains (TM1-TM4), two extracellular loops (EL1 and EL2), a cytoplasmic loop, and a C-terminus (CT). Pathogenic variants in the GJB2 gene, resulting in amino acid substitutions scattered across the Cx26 domains, lead to a variety of clinical outcomes, including the most common non-syndromic autosomal recessive deafness (DFNB1A), autosomal dominant deafness (DFNA3A), as well as syndromic forms combining hearing loss and skin disorders. However, for rare and poorly documented variants, information on the mode of inheritance is often lacking. Numerous in vitro studies have been conducted to elucidate the functional consequences of pathogenic GJB2 variants leading to amino acid substitutions in different domains of Cx26 protein. In this work, we summarized all available data on a mode of inheritance of pathogenic GJB2 variants leading to amino acid substitutions and reviewed published information on their functional effects, with an emphasis on their localization in certain Cx26 domains.
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Affiliation(s)
- Olga L. Posukh
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
| | - Ekaterina A. Maslova
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
| | - Valeriia Yu. Danilchenko
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
| | - Marina V. Zytsar
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
| | - Konstantin E. Orishchenko
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
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Qi C, Lavriha P, Bayraktar E, Vaithia A, Schuster D, Pannella M, Sala V, Picotti P, Bortolozzi M, Korkhov VM. Structures of wild-type and selected CMT1X mutant connexin 32 gap junction channels and hemichannels. SCIENCE ADVANCES 2023; 9:eadh4890. [PMID: 37647412 PMCID: PMC10468125 DOI: 10.1126/sciadv.adh4890] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/07/2023] [Accepted: 07/27/2023] [Indexed: 09/01/2023]
Abstract
In myelinating Schwann cells, connection between myelin layers is mediated by gap junction channels (GJCs) formed by docked connexin 32 (Cx32) hemichannels (HCs). Mutations in Cx32 cause the X-linked Charcot-Marie-Tooth disease (CMT1X), a degenerative neuropathy without a cure. A molecular link between Cx32 dysfunction and CMT1X pathogenesis is still missing. Here, we describe the high-resolution cryo-electron cryo-myography (cryo-EM) structures of the Cx32 GJC and HC, along with two CMT1X-linked mutants, W3S and R22G. While the structures of wild-type and mutant GJCs are virtually identical, the HCs show a major difference: In the W3S and R22G mutant HCs, the amino-terminal gating helix partially occludes the pore, consistent with a diminished HC activity. Our results suggest that HC dysfunction may be involved in the pathogenesis of CMT1X.
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Affiliation(s)
- Chao Qi
- Institute of Molecular Biology and Biophysics, ETH Zurich, Zurich, Switzerland
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen, Switzerland
| | - Pia Lavriha
- Institute of Molecular Biology and Biophysics, ETH Zurich, Zurich, Switzerland
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen, Switzerland
| | - Erva Bayraktar
- Veneto Institute of Molecular Medicine (VIMM), Padua, Italy
- Department of Physics and Astronomy “G. Galilei”, University of Padua, Padua, Italy
| | - Anand Vaithia
- Institute of Molecular Biology and Biophysics, ETH Zurich, Zurich, Switzerland
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen, Switzerland
| | - Dina Schuster
- Institute of Molecular Biology and Biophysics, ETH Zurich, Zurich, Switzerland
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen, Switzerland
- Institute of Molecular Systems Biology, ETH Zurich, Zurich, Switzerland
| | - Micaela Pannella
- Veneto Institute of Molecular Medicine (VIMM), Padua, Italy
- Department of Physics and Astronomy “G. Galilei”, University of Padua, Padua, Italy
| | - Valentina Sala
- Veneto Institute of Molecular Medicine (VIMM), Padua, Italy
| | - Paola Picotti
- Institute of Molecular Systems Biology, ETH Zurich, Zurich, Switzerland
| | - Mario Bortolozzi
- Veneto Institute of Molecular Medicine (VIMM), Padua, Italy
- Department of Physics and Astronomy “G. Galilei”, University of Padua, Padua, Italy
| | - Volodymyr M. Korkhov
- Institute of Molecular Biology and Biophysics, ETH Zurich, Zurich, Switzerland
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen, Switzerland
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Oliveira MC, Verswyvel H, Smits E, Cordeiro RM, Bogaerts A, Lin A. The pro- and anti-tumoral properties of gap junctions in cancer and their role in therapeutic strategies. Redox Biol 2022; 57:102503. [PMID: 36228438 PMCID: PMC9557036 DOI: 10.1016/j.redox.2022.102503] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2022] [Revised: 09/06/2022] [Accepted: 10/06/2022] [Indexed: 11/24/2022] Open
Abstract
Gap junctions (GJs), essential structures for cell-cell communication, are made of two hemichannels (commonly called connexons), one on each adjacent cell. Found in almost all cells, GJs play a pivotal role in many physiological and cellular processes, and have even been linked to the progression of diseases, such as cancer. Modulation of GJs is under investigation as a therapeutic strategy to kill tumor cells. Furthermore, GJs have also been studied for their key role in activating anti-cancer immunity and propagating radiation- and oxidative stress-induced cell death to neighboring cells, a process known as the bystander effect. While, gap junction (GJ)-based therapeutic strategies are being developed, one major challenge has been the paradoxical role of GJs in both tumor progression and suppression, based on GJ composition, cancer factors, and tumoral context. Therefore, understanding the mechanisms of action, regulation, and the dual characteristics of GJs in cancer is critical for developing effective therapeutics. In this review, we provide an overview of the current understanding of GJs structure, function, and paradoxical pro- and anti-tumoral role in cancer. We also discuss the treatment strategies to target these GJs properties for anti-cancer responses, via modulation of GJ function.
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Affiliation(s)
- Maria C Oliveira
- Plasma Lab for Applications in Sustainability and Medicine-Antwerp (PLASMANT), Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium; Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Avenida dos Estados 5001, CEP 09210-580, Santo André, SP, Brazil.
| | - Hanne Verswyvel
- Plasma Lab for Applications in Sustainability and Medicine-Antwerp (PLASMANT), Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium; Center for Oncological Research (CORE), Integrated Personalized and Precision Oncology Network (IPPON), University of Antwerp, Universiteitsplein 1, B-2610, Antwerp, Belgium
| | - Evelien Smits
- Center for Oncological Research (CORE), Integrated Personalized and Precision Oncology Network (IPPON), University of Antwerp, Universiteitsplein 1, B-2610, Antwerp, Belgium
| | - Rodrigo M Cordeiro
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Avenida dos Estados 5001, CEP 09210-580, Santo André, SP, Brazil
| | - Annemie Bogaerts
- Plasma Lab for Applications in Sustainability and Medicine-Antwerp (PLASMANT), Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium
| | - Abraham Lin
- Plasma Lab for Applications in Sustainability and Medicine-Antwerp (PLASMANT), Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium; Center for Oncological Research (CORE), Integrated Personalized and Precision Oncology Network (IPPON), University of Antwerp, Universiteitsplein 1, B-2610, Antwerp, Belgium
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Kraujalis T, Gudaitis L, Kraujaliene L, Snipas M, Palacios-Prado N, Verselis VK. The Amino Terminal Domain and Modulation of Connexin36 Gap Junction Channels by Intracellular Magnesium Ions. Front Physiol 2022; 13:839223. [PMID: 35264979 PMCID: PMC8899287 DOI: 10.3389/fphys.2022.839223] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2021] [Accepted: 01/28/2022] [Indexed: 11/13/2022] Open
Abstract
Electrical synapses between neurons in the mammalian CNS are predominantly formed of the connexin36 (Cx36) gap junction (GJ) channel protein. Unique among GJs formed of a number of other members of the Cx gene family, Cx36 GJs possess a high sensitivity to intracellular Mg2+ that can robustly act to modulate the strength of electrical synaptic transmission. Although a putative Mg2+ binding site was previously identified to reside in the aqueous pore in the first extracellular (E1) loop domain, the involvement of the N-terminal (NT) domain in the atypical response of Cx36 GJs to pH was shown to depend on intracellular levels of Mg2+. In this study, we examined the impact of amino acid substitutions in the NT domain on Mg2+ modulation of Cx36 GJs, focusing on positions predicted to line the pore funnel, which constitutes the cytoplasmic entrance of the channel pore. We find that charge substitutions at the 8th, 13th, and 18th positions had pronounced effects on Mg2+ sensitivity, particularly at position 13 at which an A13K substitution completely abolished sensitivity to Mg2+. To assess potential mechanisms of Mg2+ action, we constructed and tested a series of mathematical models that took into account gating of the component hemichannels in a Cx36 GJ channel as well as Mg2+ binding to each hemichannel in open and/or closed states. Simultaneous model fitting of measurements of junctional conductance, gj, and transjunctional Mg2+ fluxes using a fluorescent Mg2+ indicator suggested that the most viable mechanism for Cx36 regulation by Mg2+ entails the binding of Mg2+ to and subsequent stabilization of the closed state in each hemichannel. Reduced permeability to Mg2+ was also evident, particularly for the A13K substitution, but homology modeling of all charge-substituted NT variants showed only a moderate correlation between a reduction in the negative electrostatic potential and a reduction in the permeability to Mg2+ ions. Given the reported role of the E1 domain in Mg2+ binding together with the impact of NT substitutions on gating and the apparent state-dependence of Mg2+ binding, this study suggests that the NT domain can be an integral part of Mg2+ modulation of Cx36 GJs likely through the coupling of conformational changes between NT and E1 domains.
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Affiliation(s)
- Tadas Kraujalis
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
- Department of Applied Informatics, Kaunas University of Technology, Kaunas, Lithuania
- *Correspondence: Tadas Kraujalis,
| | - Lukas Gudaitis
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
| | - Lina Kraujaliene
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
| | - Mindaugas Snipas
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
- Department of Mathematical Modelling, Kaunas University of Technology, Kaunas, Lithuania
| | - Nicolás Palacios-Prado
- Department of Physiology, Pontificia Universidad Católica de Chile, Santiago, Chile
- Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaiso, Valparaíso, Chile
| | - Vytas K. Verselis
- Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, New York, NY, United States
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The Complex and Critical Role of Glycine 12 (G12) in Beta-Connexins of Human Skin. Int J Mol Sci 2021; 22:ijms22052615. [PMID: 33807656 PMCID: PMC7961983 DOI: 10.3390/ijms22052615] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2021] [Revised: 03/02/2021] [Accepted: 03/03/2021] [Indexed: 11/28/2022] Open
Abstract
Glycine is an amino acid with unique properties because its side chain is composed of a single hydrogen atom. It confers conformational flexibility to proteins and conserved glycines are often indicative of protein domains involving tight turns or bends. All six beta-type connexins expressed in human epidermis (Cx26, Cx30, Cx30.3, Cx31, Cx31.1 and Cx32) contain a glycine at position 12 (G12). G12 is located about halfway through the cytoplasmic amino terminus and substitutions alter connexin function in a variety of ways, in some cases altering protein interactions and leading to cell death. There is also evidence that alteration of G12 changes the structure of the amino terminus in connexin- and amino acid- specific ways. This review integrates structural, functional and physiological information about the role of G12 in connexins, focusing on beta-connexins expressed in human epidermis. The importance of G12 substitutions in these beta-connexins is revealed in two hereditary skin disorders, keratitis ichthyosis and erythrokeratodermia variabilis, both of which result from missense mutations affecting G12.
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Lee HJ, Jeong H, Hyun J, Ryu B, Park K, Lim HH, Yoo J, Woo JS. Cryo-EM structure of human Cx31.3/GJC3 connexin hemichannel. SCIENCE ADVANCES 2020; 6:eaba4996. [PMID: 32923625 PMCID: PMC7455182 DOI: 10.1126/sciadv.aba4996] [Citation(s) in RCA: 29] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/09/2019] [Accepted: 07/15/2020] [Indexed: 05/28/2023]
Abstract
Connexin family proteins assemble into hexameric channels called hemichannels/connexons, which function as transmembrane channels or dock together to form gap junction intercellular channels (GJIChs). We determined the cryo-electron microscopy structures of human connexin 31.3 (Cx31.3)/GJC3 hemichannels in the presence and absence of calcium ions and with a hearing-loss mutation R15G at 2.3-, 2.5-, and 2.6-Å resolutions, respectively. Compared with available structures of GJICh in open conformation, Cx31.3 hemichannel shows substantial structural changes of highly conserved regions in the connexin family, including opening of calcium ion-binding tunnels, reorganization of salt-bridge networks, exposure of lipid-binding sites, and collocation of amino-terminal helices at the cytoplasmic entrance. We also found that the hemichannel has a pore with a diameter of ~8 Å and selectively transports chloride ions. Our study provides structural insights into the permeant selectivity of Cx31.3 hemichannel.
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Affiliation(s)
- Hyuk-Joon Lee
- Department of Life Sciences, Korea University, Seoul 02841, Republic of Korea
| | - Hyeongseop Jeong
- Department of Life Sciences, Korea University, Seoul 02841, Republic of Korea
- Korea Basic Science Institute, Chungcheongbuk-do 28119, Republic of Korea
| | - Jaekyung Hyun
- Korea Basic Science Institute, Chungcheongbuk-do 28119, Republic of Korea
- Molecular Cryo-electron Microscopy Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa 904-0496, Japan
| | - Bumhan Ryu
- Department of Life Sciences, Korea University, Seoul 02841, Republic of Korea
| | - Kunwoong Park
- Neurovascular Unit Research Group, Korea Brain Research Institute (KBRI), 41062 Daegu, Republic of Korea
| | - Hyun-Ho Lim
- Neurovascular Unit Research Group, Korea Brain Research Institute (KBRI), 41062 Daegu, Republic of Korea
| | - Jejoong Yoo
- Department of Physics, Sungkyunkwan University, Suwon, Republic of Korea
- Center for Self-assembly and Complexity (CSC), Institute for Basic Science (IBS), Pohang 37673, Republic of Korea
| | - Jae-Sung Woo
- Department of Life Sciences, Korea University, Seoul 02841, Republic of Korea
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Shui Y, Liu P, Zhan H, Chen B, Wang ZW. Molecular basis of junctional current rectification at an electrical synapse. SCIENCE ADVANCES 2020; 6:eabb3076. [PMID: 32923588 PMCID: PMC7455501 DOI: 10.1126/sciadv.abb3076] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/14/2020] [Accepted: 05/20/2020] [Indexed: 06/11/2023]
Abstract
Rectifying electrical synapses (RESs) exist across animal species, but their rectification mechanism is largely unknown. We investigated why RESs between AVA premotor interneurons and A-type cholinergic motoneurons (A-MNs) in Caenorhabditis elegans escape circuit conduct junctional currents (I j) only in the antidromic direction. These RESs consist of UNC-7 innexin in AVA and UNC-9 innexin in A-MNs. UNC-7 has multiple isoforms differing in the length and sequence of the amino terminus. In a heterologous expression system, only one UNC-7 isoform, UNC-7b, can form heterotypic gap junctions (GJs) with UNC-9 that strongly favor I j in the UNC-9 to UNC-7 direction. Knockout of unc-7b alone almost eliminated the I j, whereas AVA-specific expression of UNC-7b substantially rescued the coupling defect of unc-7 mutant. Neutralizing charged residues in UNC-7b amino terminus abolished the rectification property of UNC-7b/UNC-9 GJs. Our results suggest that the rectification property results from electrostatic interactions between charged residues in UNC-7b amino terminus.
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Affiliation(s)
- Yuan Shui
- Department of Neuroscience, University of Connecticut School of Medicine, Farmington, CT 06030, USA
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Santos-Miranda A, Chen H, Chen RC, Odoko-Ishimoto M, Aoyama H, Bai D. The amino terminal domain plays an important role in transjunctional voltage-dependent gating kinetics of Cx45 gap junctions. J Mol Cell Cardiol 2020; 143:71-84. [DOI: 10.1016/j.yjmcc.2020.04.004] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/07/2019] [Revised: 03/04/2020] [Accepted: 04/03/2020] [Indexed: 01/24/2023]
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Berry V, Ionides A, Pontikos N, Moghul I, Moore AT, Quinlan RA, Michaelides M. Whole Exome Sequencing Reveals Novel and Recurrent Disease-Causing Variants in Lens Specific Gap Junctional Protein Encoding Genes Causing Congenital Cataract. Genes (Basel) 2020; 11:genes11050512. [PMID: 32384692 PMCID: PMC7288463 DOI: 10.3390/genes11050512] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2020] [Revised: 04/20/2020] [Accepted: 05/04/2020] [Indexed: 11/16/2022] Open
Abstract
Pediatric cataract is clinically and genetically heterogeneous and is the most common cause of childhood blindness worldwide. In this study, we aimed to identify disease-causing variants in three large British families and one isolated case with autosomal dominant congenital cataract, using whole exome sequencing. We identified four different heterozygous variants, three in the large families and one in the isolated case. Family A, with a novel missense variant (c.178G>C, p.Gly60Arg) in GJA8 with lamellar cataract; family B, with a recurrent variant in GJA8 (c.262C>T, p.Pro88Ser) associated with nuclear cataract; and family C, with a novel variant in GJA3 (c.771dupC, p.Ser258GlnfsTer68) causing a lamellar phenotype. Individual D had a novel variant in GJA3 (c.82G>T, p.Val28Leu) associated with congenital cataract. Each sequence variant was found to co-segregate with disease. Here, we report three novel and one recurrent disease-causing sequence variant in the gap junctional protein encoding genes causing autosomal dominant congenital cataract. Our study further extends the mutation spectrum of these genes and further facilitates clinical diagnosis. A recurrent p.P88S variant in GJA8 causing isolated nuclear cataract provides evidence of further phenotypic heterogeneity associated with this variant.
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Affiliation(s)
- Vanita Berry
- UCL Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, UK;
- Moorfields Eye Hospital NHS Foundation Trust, London EC1V 2PD, UK; (A.I.); (A.T.M.)
- Correspondence: (V.B.); (M.M.); Tel.: +44-207-608-4041 (V.B.); +44-207-608-6864 (M.M.); Fax: +44-207-608-6863 (V.B.); +44-207-608-6903 (M.M.)
| | - Alex Ionides
- Moorfields Eye Hospital NHS Foundation Trust, London EC1V 2PD, UK; (A.I.); (A.T.M.)
| | - Nikolas Pontikos
- UCL Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, UK;
- Moorfields Eye Hospital NHS Foundation Trust, London EC1V 2PD, UK; (A.I.); (A.T.M.)
| | - Ismail Moghul
- UCL Cancer Institute, University College London, London WC1E 6BT, UK;
| | - Anthony T. Moore
- Moorfields Eye Hospital NHS Foundation Trust, London EC1V 2PD, UK; (A.I.); (A.T.M.)
- Ophthalmology Department, University of California School of Medicine, San Francisco, CA 94158, USA
| | - Roy A. Quinlan
- Department of Biosciences, University of Durham, Upper Mountjoy Science Site, Durham DH1 3LE, UK;
| | - Michel Michaelides
- UCL Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, UK;
- Moorfields Eye Hospital NHS Foundation Trust, London EC1V 2PD, UK; (A.I.); (A.T.M.)
- Correspondence: (V.B.); (M.M.); Tel.: +44-207-608-4041 (V.B.); +44-207-608-6864 (M.M.); Fax: +44-207-608-6863 (V.B.); +44-207-608-6903 (M.M.)
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12
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Structural insights into gap junction channels boosted by cryo-EM. Curr Opin Struct Biol 2020; 63:42-48. [PMID: 32339861 DOI: 10.1016/j.sbi.2020.03.008] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2019] [Revised: 03/15/2020] [Accepted: 03/15/2020] [Indexed: 11/23/2022]
Abstract
Regulating intercellular communication is essential for multicellular organisms. Gap junction channels are the major components mediating this function, but the molecular mechanisms underlying their opening and closing remain unclear. Single-particle cryo-electron microscopy (cryo-EM) is a powerful tool for investigating high-resolution protein structures that are difficult to crystallize, such as gap junction channels. Membrane protein structures are often determined in a detergent solubilized form, but lipid bilayers provide a near native environment for structural analysis. This review focuses on recent reports of gap junction channel structures visualized by cryo-EM. An overview of the differences observed in gap junction channel structures in the presence and absence of lipids is described, which may contribute to elucidating the regulation mechanisms of gap junction channel function.
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Khan AK, Jagielnicki M, McIntire WE, Purdy MD, Dharmarajan V, Griffin PR, Yeager M. A Steric “Ball-and-Chain” Mechanism for pH-Mediated Regulation of Gap Junction Channels. Cell Rep 2020; 31:107482. [PMID: 32320665 DOI: 10.1016/j.celrep.2020.03.046] [Citation(s) in RCA: 27] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2019] [Revised: 09/10/2019] [Accepted: 03/13/2020] [Indexed: 12/21/2022] Open
Abstract
Gap junction channels (GJCs) mediate intercellular communication and are gated by numerous conditions such as pH. The electron cryomicroscopy (cryo-EM) structure of Cx26 GJC at physiological pH recapitulates previous GJC structures in lipid bilayers. At pH 6.4, we identify two conformational states, one resembling the open physiological-pH structure and a closed conformation that displays six threads of density, that join to form a pore-occluding density. Crosslinking and hydrogen-deuterium exchange mass spectrometry reveal closer association between the N-terminal (NT) domains and the cytoplasmic loops (CL) at acidic pH. Previous electrophysiologic studies suggest an association between NT residue N14 and H100 near M2, which may trigger the observed movement of M2 toward M1 in our cryo-EM maps, thereby accounting for additional NT-CL crosslinks at acidic pH. We propose that these pH-induced interactions and conformational changes result in extension, ordering, and association of the acetylated NT domains to form a hexameric "ball-and-chain" gating particle.
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Wu X, Zhang W, Li Y, Lin X. Structure and Function of Cochlear Gap Junctions and Implications for the Translation of Cochlear Gene Therapies. Front Cell Neurosci 2019; 13:529. [PMID: 31827424 PMCID: PMC6892400 DOI: 10.3389/fncel.2019.00529] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2019] [Accepted: 11/13/2019] [Indexed: 12/23/2022] Open
Abstract
Connexins (Cxs) are ubiquitous membrane proteins that are found throughout vertebrate organs, acting as building blocks of the gap junctions (GJs) known to play vital roles in the normal function of many organs. Mutations in Cx genes (particularly GJB2, which encodes Cx26) cause approximately half of all cases of congenital hearing loss in newborns. Great progress has been made in understanding GJ function and the molecular mechanisms for the role of Cxs in the cochlea. Data reveal that multiple types of Cxs work together to ensure normal development and function of the cochlea. These findings include many aspects not proposed in the classic K+ recycling theory, such as the formation of normal cochlear morphology (e.g., the opening of the tunnel of Corti), the fine-tuning of the innervation of nerve fibers to the hair cells (HCs), the maturation of the ribbon synapses, and the initiation of the endocochlear potential (EP). New data, especially those collected from targeted modification of major Cx genes in the mouse cochlea, have demonstrated that Cx26 plays an essential role in the postnatal maturation of the cochlea. Studies also show that Cx26 and Cx30 assume very different roles in the EP generation, given that only Cx26 is required for normal hearing. This article will review our current understanding of the molecular structure, cellular distribution, and major functions of cochlear GJs. Potential implications of the knowledge of cochlear GJs on the design and implementation of translational studies of cochlear gene therapies for Cx mutations are also discussed.
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Affiliation(s)
- Xuewen Wu
- Department of Otolaryngology, Head-Neck and Surgery, Xiangya Hospital of Central South University, Changsha, China
- Department of Otolaryngology, Emory University School of Medicine, Atlanta, GA, United States
| | - Wenjuan Zhang
- Department of Otolaryngology, Wuhan Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China
| | - Yihui Li
- Department of Pharmacy, Changsha Hospital of Traditional Medicine, Changsha, China
| | - Xi Lin
- Department of Otolaryngology, Emory University School of Medicine, Atlanta, GA, United States
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15
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Potential of cryo-EM for high-resolution structural analysis of gap junction channels. Curr Opin Struct Biol 2019; 54:78-85. [PMID: 30797124 DOI: 10.1016/j.sbi.2019.01.005] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2018] [Revised: 12/03/2018] [Accepted: 01/09/2019] [Indexed: 11/20/2022]
Abstract
Gap junction family proteins form conduits connecting the cytoplasm of adjacent cells, thereby enabling electrical and chemical coupling to maintain physiological homeostasis. Gap junction proteins comprise two gene families, connexins in chordates and innexins in pre-chordates. Their channel structures have been analyzed by electron or X-ray crystallography, but only a few atomic structures have been reported. Recent advances in single-particle cryo-electron microscopy (cryo-EM) will help to elucidate these structures further. Here the structural biology of gap junction channels utilizing crystallography and single-particle cryo-EM is overviewed to shed light on the functional mechanisms of cell-cell communication that are essential for multicellular organisms.
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16
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Berry V, Ionides ACW, Pontikos N, Moghul I, Moore AT, Cheetham ME, Michaelides M. Whole-genome sequencing reveals a recurrent missense mutation in the Connexin 46 (GJA3) gene causing autosomal-dominant lamellar cataract. Eye (Lond) 2018; 32:1661-1668. [PMID: 29934635 PMCID: PMC6189195 DOI: 10.1038/s41433-018-0154-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2018] [Revised: 04/24/2018] [Accepted: 05/03/2018] [Indexed: 11/09/2022] Open
Abstract
PURPOSE Congenital cataract, opacification of the ocular lens, is clinically and genetically a heterogeneous childhood disease. In this study we aimed to identify the underlying genetic cause of isolated autosomal-dominant lamellar cataract in a multi-generation English family. METHODS Whole-genome sequencing (WGS) was undertaken in two affected subjects and one unaffected individual. Segregation analysis was performed and a known cataract-causing mutation was identified. Segregation was further validated by sanger sequencing in the entire pedigree. RESULTS A heterozygous mutation c.7 G > T; p.D3Y was identified in an NH2-terminal region of the gap junction protein GJA3 and found to co-segregate with disease. CONCLUSION We have identified a recurrent mutation in GJA3 in a large British pedigree causing the novel phenotype of autosomal-dominant congenital lamellar cataract. Previously, p.D3Y was found in a Hispanic family causing pulverulent cataract. WGS proved an efficient method to find the underlying molecular cause in this large family, which could not be mapped due to uninformative markers.
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Affiliation(s)
- Vanita Berry
- UCL Institute of Ophthalmology, 11-43 Bath Street, London, EC1V 9EL, UK.
| | | | - Nikolas Pontikos
- UCL Institute of Ophthalmology, 11-43 Bath Street, London, EC1V 9EL, UK
- UCL Genetics Institute, University College London, London, WC1E 6BT, UK
| | - Ismail Moghul
- UCL Cancer Institute, University College London, London, WC1E 6DD, UK
| | - Anthony T Moore
- Moorfields Eye Hospital, London, EC1V 2PD, UK
- Ophthalmology Department, University of California School of Medicine, San Francisco, CA, 94158, USA
| | | | - Michel Michaelides
- UCL Institute of Ophthalmology, 11-43 Bath Street, London, EC1V 9EL, UK.
- Moorfields Eye Hospital, London, EC1V 2PD, UK.
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17
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Rimkute L, Kraujalis T, Snipas M, Palacios-Prado N, Jotautis V, Skeberdis VA, Bukauskas FF. Modulation of Connexin-36 Gap Junction Channels by Intracellular pH and Magnesium Ions. Front Physiol 2018; 9:362. [PMID: 29706896 PMCID: PMC5906587 DOI: 10.3389/fphys.2018.00362] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2018] [Accepted: 03/23/2018] [Indexed: 11/13/2022] Open
Abstract
Connexin-36 (Cx36) protein forms gap junction (GJ) channels in pancreatic beta cells and is also the main Cx isoform forming electrical synapses in the adult mammalian brain. Cx36 GJs can be regulated by intracellular pH (pHi) and cytosolic magnesium ion concentration ([Mg2+]i), which can vary significantly under various physiological and pathological conditions. However, the combined effect and relationship of these two factors over Cx36-dependent coupling have not been previously studied in detail. Our experimental results in HeLa cells expressing Cx36 show that changes in both pHi and [Mg2+]i affect junctional conductance (gj) in an interdependent manner; in other words, intracellular acidification cause increase or decay in gj depending on whether [Mg2+]i is high or low, respectively, and intracellular alkalization cause reduction in gj independently of [Mg2+]i. Our experimental and modelling data support the hypothesis that Cx36 GJ channels contain two separate gating mechanisms, and both are differentially sensitive to changes in pHi and [Mg2+]i. Using recombinant Cx36 we found that two glutamate residues in the N-terminus could be partly responsible for the observed interrelated effect of pHi and [Mg2+]i. Mutation of glutamate at position 8 attenuated the stimulatory effect of intracellular acidification at high [Mg2+]i, while mutation at position 12 and double mutation at both positions reversed stimulatory effect to inhibition. Moreover, Cx36*E8Q lost the initial increase of gj at low [Mg2+]i and double mutation lost the sensitivity to high [Mg2+]i. These results suggest that E8 and E12 are involved in regulation of Cx36 GJ channels by Mg2+ and H+ ions.
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Affiliation(s)
- Lina Rimkute
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
| | - Tadas Kraujalis
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
- Department of Applied Informatics, Kaunas University of Technology, Kaunas, Lithuania
| | - Mindaugas Snipas
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
- Department of Mathematical Modelling, Kaunas University of Technology, Kaunas, Lithuania
| | - Nicolas Palacios-Prado
- Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso, Chile
- Department of Physiology, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Vaidas Jotautis
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
| | - Vytenis A. Skeberdis
- Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania
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18
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Sanchez HA, Slavi N, Srinivas M, Verselis VK. Syndromic deafness mutations at Asn 14 differentially alter the open stability of Cx26 hemichannels. J Gen Physiol 2017; 148:25-42. [PMID: 27353444 PMCID: PMC4924935 DOI: 10.1085/jgp.201611585] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2016] [Accepted: 06/10/2016] [Indexed: 01/23/2023] Open
Abstract
Connexin 26 (Cx26) is a transmembrane protein that forms hexameric hemichannels that can function when unopposed or dock to form intercellular gap junction channels. Aberrantly functioning unopposed hemichannels are a common feature of syndromic deafness associated with mutations in Cx26. In this study, we examine two different mutations at the same position in the N-terminal domain of Cx26, N14K and N14Y, which have been reported to produce different phenotypes in patients. We find that both N14K and N14Y, when expressed alone or together with wild-type (WT) Cx26, result in functional hemichannels with widely disparate functional properties. N14K currents are robust, whereas N14Y currents are small. The two mutants also exhibit opposite shifts in voltage-dependent loop gating, such that activation of N14K and N14Y is shifted in the hyperpolarizing and depolarizing directions, respectively. Deactivation kinetics suggests that N14K stabilizes and N14Y destabilizes the open state. Single N14K hemichannel recordings in low extracellular Ca(2+) show no evidence of stable closing transitions associated with loop gating, and N14K hemichannels are insensitive to pH. Together, these properties cause N14K hemichannels to be particularly refractory to closing. Although we find that the unitary conductance of N14K is indistinguishable from WT Cx26, mutagenesis and substituted cysteine accessibility studies suggest that the N14 residue is exposed to the pore and that the differential properties of N14K and N14Y hemichannels likely result from altered electrostatic interactions between the N terminus and the cytoplasmic extension of TM2 in the adjacent subunit. The combined effects that we observe on loop gating and pH regulation may explain the unusual buccal cutaneous manifestations in patients carrying the N14K mutation. Our work also provides new considerations regarding the underlying molecular mechanism of loop gating, which controls hemichannel opening in the plasma membrane.
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Affiliation(s)
- Helmuth A Sanchez
- Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461
| | - Nefeli Slavi
- Department of Biological Sciences, SUNY College of Optometry, New York, NY 10036
| | - Miduturu Srinivas
- Department of Biological Sciences, SUNY College of Optometry, New York, NY 10036
| | - Vytas K Verselis
- Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461
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19
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Bargiello TA, Oh S, Tang Q, Bargiello NK, Dowd TL, Kwon T. Gating of Connexin Channels by transjunctional-voltage: Conformations and models of open and closed states. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1860:22-39. [PMID: 28476631 DOI: 10.1016/j.bbamem.2017.04.028] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/01/2017] [Revised: 04/26/2017] [Accepted: 04/28/2017] [Indexed: 11/18/2022]
Abstract
Voltage is an important physiologic regulator of channels formed by the connexin gene family. Connexins are unique among ion channels in that both plasma membrane inserted hemichannels (undocked hemichannels) and intercellular channels (aggregates of which form gap junctions) have important physiological roles. The hemichannel is the fundamental unit of gap junction voltage-gating. Each hemichannel displays two distinct voltage-gating mechanisms that are primarily sensitive to a voltage gradient formed along the length of the channel pore (the transjunctional voltage) rather than sensitivity to the absolute membrane potential (Vm or Vi-o). These transjunctional voltage dependent processes have been termed Vj- or fast-gating and loop- or slow-gating. Understanding the mechanism of voltage-gating, defined as the sequence of voltage-driven transitions that connect open and closed states, first and foremost requires atomic resolution models of the end states. Although ion channels formed by connexins were among the first to be characterized structurally by electron microscopy and x-ray diffraction in the early 1980's, subsequent progress has been slow. Much of the current understanding of the structure-function relations of connexin channels is based on two crystal structures of Cx26 gap junction channels. Refinement of crystal structure by all-atom molecular dynamics and incorporation of charge changing protein modifications has resulted in an atomic model of the open state that arguably corresponds to the physiologic open state. Obtaining validated atomic models of voltage-dependent closed states is more challenging, as there are currently no methods to solve protein structure while a stable voltage gradient is applied across the length of an oriented channel. It is widely believed that the best approach to solve the atomic structure of a voltage-gated closed ion channel is to apply different but complementary experimental and computational methods and to use the resulting information to derive a consensus atomic structure that is then subjected to rigorous validation. In this paper, we summarize our efforts to obtain and validate atomic models of the open and voltage-driven closed states of undocked connexin hemichannels. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.
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Affiliation(s)
- Thaddeus A Bargiello
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, United States.
| | - Seunghoon Oh
- Department of Physiology, College of Medicine, Dankook University, Cheonan, Republic of Korea
| | - Qingxiu Tang
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, United States
| | - Nicholas K Bargiello
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, United States
| | - Terry L Dowd
- Department of Chemistry, Brooklyn College, Brooklyn, NY 11210, United States
| | - Taekyung Kwon
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, United States
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20
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Abstract
Gap junction channels facilitate the intercellular exchange of ions and small molecules, a process that is critical for the function of many different kinds of cells and tissues. Recent crystal structures of channels formed by one connexin isoform (connexin26) have been determined, and they have been subjected to molecular modeling. These studies have provided high-resolution models to gain insights into the mechanisms of channel conductance, molecular permeability, and gating. The models share similarities, but there are some differences in the conclusions reached by these studies. Many unanswered questions remain to allow an atomic-level understanding of intercellular communication mediated by connexin26. Because some domains of the connexin polypeptides are highly conserved (like the transmembrane regions), it is likely that some features of the connexin26 structure will apply to other members of the family of gap junction proteins. However, determination of high-resolution structures and modeling of other connexin channels will be required to account for the diverse biophysical properties and regulation conferred by the differences in their sequences.
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Affiliation(s)
- Eric C Beyer
- Department of Pediatrics, University of Chicago, Chicago, IL, 60637, USA
| | - Viviana M Berthoud
- Department of Pediatrics, University of Chicago, Chicago, IL, 60637, USA
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21
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Villanelo F, Escalona Y, Pareja-Barrueto C, Garate JA, Skerrett IM, Perez-Acle T. Accessing gap-junction channel structure-function relationships through molecular modeling and simulations. BMC Cell Biol 2017; 18:5. [PMID: 28124624 PMCID: PMC5267332 DOI: 10.1186/s12860-016-0121-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
Background Gap junction channels (GJCs) are massive protein channels connecting the cytoplasm of adjacent cells. These channels allow intercellular transfer of molecules up to ~1 kDa, including water, ions and other metabolites. Unveiling structure-function relationships coded into the molecular architecture of these channels is necessary to gain insight on their vast biological function including electrical synapse, inflammation, development and tissular homeostasis. From early works, computational methods have been critical to analyze and interpret experimental observations. Upon the availability of crystallographic structures, molecular modeling and simulations have become a valuable tool to assess structure-function relationships in GJCs. Modeling different connexin isoforms, simulating the transport process, and exploring molecular variants, have provided new hypotheses and out-of-the-box approaches to the study of these important channels. Methods Here, we review foundational structural studies and recent developments on GJCs using molecular modeling and simulation techniques, highlighting the methods and the cross-talk with experimental evidence. Results and discussion By comparing results obtained by molecular modeling and simulations techniques with structural and functional information obtained from both recent literature and structural databases, we provide a critical assesment of structure-function relationships that can be obtained from the junction between theoretical and experimental evidence.
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Affiliation(s)
- F Villanelo
- Computational Biology Lab. Fundación Ciencia & Vida, Santiago, Chile
| | - Y Escalona
- Computational Biology Lab. Fundación Ciencia & Vida, Santiago, Chile
| | - C Pareja-Barrueto
- Computational Biology Lab. Fundación Ciencia & Vida, Santiago, Chile
| | - J A Garate
- Computational Biology Lab. Fundación Ciencia & Vida, Santiago, Chile.,Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Playa Ancha, Valparaíso, Chile
| | - I M Skerrett
- State University of New York (SUNY) Buffalo State, Buffalo, NY, 14222, USA
| | - T Perez-Acle
- Computational Biology Lab. Fundación Ciencia & Vida, Santiago, Chile. .,Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Playa Ancha, Valparaíso, Chile.
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22
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Xu Q, Lin X, Matiukas A, Zhang X, Veenstra RD. Specificity of the connexin W3/4 locus for functional gap junction formation. Channels (Austin) 2016; 10:453-65. [PMID: 27304225 PMCID: PMC5034775 DOI: 10.1080/19336950.2016.1200775] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2016] [Revised: 05/23/2016] [Accepted: 06/08/2016] [Indexed: 10/21/2022] Open
Abstract
The N-terminal (NT) domain of the connexins forms an essential transjunctional voltage (Vj) sensor and pore-forming domain that when truncated, tagged, or mutated often leads to formation of a nonfunctional channel. The NT domain is relatively conserved among the connexins though the α- and δ-group connexins possess a G2 residue not found in the β- and γ-group connexins. Deletion of the connexin40 G2 residue (Cx40G2Δ) affected the Vj gating, increased the single channel conductance (γj), and decreased the relative K(+)/Cl(-) permeability (PK/PCl) ratio of the Cx40 gap junction channel. The conserved α/β-group connexin D2/3 and W3/4 loci are postulated to anchor the NT domain within the pore via hydrophilic and hydrophobic interactions with adjacent connexin T5 and M34 residues. Cx40D3N and D3R mutations produced limited function with progressive reductions in Vj gating and noisy low γj gap junction channels that reduced the γj of wild-type Cx40 channels from 150 pS to < 50 pS when coexpressed. Surprisingly, hydrophobic Cx40 W4F and W4Y substitution mutations were not compatible with function despite their ability to form gap junction plaques. These data are consistent with minor and major contributions of the G2 and D3 residues to the Cx40 channel pore structure, but not with the postulated hydrophobic W4 intermolecular interactions. Our results indicate an absolute requirement for an amphipathic W3/4 residue that is conserved among all α/β/δ/γ-group connexins. We alternatively hypothesize that the connexin D2/3-W3/4 locus interacts with the highly conserved FIFR M1 motif to stabilize the NT domain within the pore.
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Affiliation(s)
- Qin Xu
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, NY, USA
| | - Xianming Lin
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, NY, USA
| | - Arvydas Matiukas
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, NY, USA
| | - Xian Zhang
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, NY, USA
| | - Richard D. Veenstra
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, NY, USA
- Department of Cell and Developmental Biology, SUNY Upstate Medical University, Syracuse, NY, USA
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23
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Batir Y, Bargiello TA, Dowd TL. Structural studies of N-terminal mutants of Connexin 26 and Connexin 32 using (1)H NMR spectroscopy. Arch Biochem Biophys 2016; 608:8-19. [PMID: 27378082 PMCID: PMC5051353 DOI: 10.1016/j.abb.2016.06.019] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2016] [Revised: 06/27/2016] [Accepted: 06/28/2016] [Indexed: 12/25/2022]
Abstract
Alterations in gap junctions underlie the etiologies of syndromic deafness (KID) and Charcot-Marie Tooth disease (CMTX). Functional gap junctions are composed of connexin molecules with N-termini containing a flexible turn around G12, inserting the N-termini into the channel pore allowing voltage gating. The loss of this turn correlates with loss of Connexin 32 (Cx32) function by impaired trafficking to the cell membrane. Using (1)H NMR we show the N-terminus of a syndromic deafness mutation Cx26G12R, producing "leaky channels", contains a turn around G12 which is less structured and more flexible than wild-type. In contrast, the N-terminal structure of the same mutation in Cx32 chimera, Cx32*43E1G12R shows a larger constricted turn and no membrane current expression but forms membrane inserted hemichannels. Their function was rescued by formation of heteromeric channels with wild type subunits. We suggest the inflexible Cx32G12R N-terminus blocks ion conduction in homomeric channels and this channel block is relieved by incorporation of wild type subunits. In contrast, the increased open probability of Cx26G12R hemichannels is likely due to the addition of positive charge in the channel pore changing pore electrostatics and impairing hemichannel regulation by Ca(2+). These results provide mechanistic information on aberrant channel activity observed in disease.
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Affiliation(s)
- Yuksel Batir
- Department of Chemistry, Brooklyn College, Brooklyn, NY 11210, United States
| | - Thaddeus A Bargiello
- Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, United States
| | - Terry L Dowd
- Department of Chemistry, Brooklyn College, Brooklyn, NY 11210, United States; Ph.D. Program in Chemistry and Biochemistry, The Graduate Center of the City University of New York, New York, NY, 10016, United States.
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Rodriguez-Paris J, Waldhaus J, Gordhandas JA, Pique L, Schrijver I. Comparative functional characterization of novel non-syndromic GJB2 gene variant p.Gly45Arg and lethal syndromic variant p.Gly45Glu. PeerJ 2016; 4:e2494. [PMID: 27761313 PMCID: PMC5068369 DOI: 10.7717/peerj.2494] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2016] [Accepted: 08/26/2016] [Indexed: 11/20/2022] Open
Abstract
We characterized a novel GJB2 missense variant, c.133G>A, p.Gly45Arg, and compared it with the only other variant at the same amino acid position of the connexin 26 protein (Cx26) reported to date: c.134G>A, p.Gly45Glu. Whereas both variants are associated with hearing loss and are dominantly inherited, p.Gly45Glu has been implicated in the rare fatal keratitis-ichthyosis-deafness (KID) syndrome, which results in cutaneous infections and septicemia with premature demise in the first year of life. In contrast, p.Gly45Arg appears to be non-syndromic. Subcellular localization experiments in transiently co-transfected HeLa cells demonstrated that Cx26-WT (wild-type) and p.Gly45Arg form gap junctions, whereas Cx26-WT with p.Gly45Glu protein does not. The substitution of a nonpolar amino acid glycine in wildtype Cx26 at position 45 with a negatively charged glutamic acid (acidic) has previously been shown to interfere with Ca2+ regulation of hemichannel gating and to inhibit the formation of gap junctions, resulting in cell death. The novel variant p.Gly45Arg, however, changes this glycine to a positively charged arginine (basic), resulting in the formation of dysfunctional gap junctions that selectively affect the permeation of negatively charged inositol 1,4,5-trisphosphate (IP3) and contribute to hearing loss. Cx26 p.Gly45Arg transfected cells, unlike cells transfected with p.Gly45Glu, thrived at physiologic Ca2+ concentrations, suggesting that Ca2+ regulation of hemichannel gating is unaffected in Cx26 p.Gly45Arg transfected cells. Thus, the two oppositely charged amino acids that replace the highly conserved uncharged glycine in p.Gly45Glu and p.Gly45Arg, respectively, produce strikingly different effects on the structure and function of the Cx26 protein.
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Affiliation(s)
- Juan Rodriguez-Paris
- Department of Pathology, Stanford University, Stanford, CA, United States of America
| | - Jörg Waldhaus
- Department of Otolaryngology, Head and Neck Surgery, Stanford University, Stanford, CA, United States of America
| | - Jeenal A Gordhandas
- Department of Pathology, Stanford University, Stanford, CA, United States of America
| | - Lynn Pique
- Department of Pathology, Stanford University, Stanford, CA, United States of America
| | - Iris Schrijver
- Department of Pathology, Stanford University, Stanford, CA, United States of America.,Department of Pediatrics, Stanford University, Stanford, CA, United States of America
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25
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Ek Vitorín JF, Pontifex TK, Burt JM. Determinants of Cx43 Channel Gating and Permeation: The Amino Terminus. Biophys J 2016; 110:127-40. [PMID: 26745416 DOI: 10.1016/j.bpj.2015.10.054] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2015] [Revised: 10/11/2015] [Accepted: 10/30/2015] [Indexed: 11/30/2022] Open
Abstract
Separate connexin domains partake in proposed gating mechanisms of gap junction channels. The amino-terminus (NT) domains, which contribute to voltage sensing, may line the channel's cytoplasmic-facing funnel surface, stabilize the channel's overall structure through interactions with the transmembrane domains and each other, and integrate to form a compound particle to gate the channel closed. Interactions of the carboxyl-terminus (CT) and cytoplasmic loop (CL) domains underlie voltage- and low pH-triggered channel closure. To elucidate potential cooperation of these gating mechanisms, we replaced the Cx43NT with the Cx37NT (chimera Cx43(∗)NT37), leaving the remainder of the Cx43 sequence, including the CT and CL, unchanged. Compared to wild-type Cx43 (Cx43WT), Cx43(∗)NT37 junctions exhibited several functional alterations: extreme resistance to halothane- and acidification-induced uncoupling, absence of voltage-dependent fast inactivation, longer channel open times, larger unitary channel conductances, low junctional dye permeability/permselectivity, and an overall cation selectivity more typical of Cx37WT than Cx43WT junctions. Together, these results suggest a cohesive model of channel function wherein: 1) channel conductance and size selectivity are largely determined by pore diameter, whereas charge selectivity results from the NT domains, and 2) transition between fully open and (multiple) closed states involves global changes in structure of the pore-forming domains transduced by interactions of the pore-forming domains with either the NT, CT, or both, with the NT domains forming the gate of the completely closed channel.
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Affiliation(s)
| | - Tasha K Pontifex
- Department of Physiology, University of Arizona, Tucson, Arizona
| | - Janis M Burt
- Department of Physiology, University of Arizona, Tucson, Arizona
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26
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Bosone C, Andreu A, Echevarria D. GAP junctional communication in brain secondary organizers. Dev Growth Differ 2016; 58:446-55. [PMID: 27273333 DOI: 10.1111/dgd.12297] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2016] [Revised: 05/05/2016] [Accepted: 05/05/2016] [Indexed: 11/28/2022]
Abstract
Gap junctions (GJs) are integral membrane proteins that enable the direct cytoplasmic exchange of ions and low molecular weight metabolites between adjacent cells. They are formed by the apposition of two connexons belonging to adjacent cells. Each connexon is formed by six proteins, named connexins (Cxs). Current evidence suggests that gap junctions play an important part in ensuring normal embryo development. Mutations in connexin genes have been linked to a variety of human diseases, although the precise role and the cell biological mechanisms of their action remain almost unknown. Among the big family of Cxs, several are expressed in nervous tissue but just a few are expressed in the anterior neural tube of vertebrates. Many efforts have been made to elucidate the molecular bases of Cxs cell biology and how they influence the morphogenetic signal activity produced by brain signaling centers. These centers, orchestrated by transcription factors and morphogenes determine the axial patterning of the mammalian brain during its specification and regionalization. The present review revisits the findings of GJ composed by Cx43 and Cx36 in neural tube patterning and discuss Cx43 putative enrollment in the control of Fgf8 signal activity coming from the well known secondary organizer, the isthmic organizer.
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Affiliation(s)
- Camilla Bosone
- Instituto de Neurociencias, Universidad Miguel Hernández & Consejo Superior de Investigaciones Científicas, 03550, Sant Joan d'Alacant, Spain
| | - Abraham Andreu
- Institut de Biologie Paris-Seine (IBPS), Developmental Biology Laboratory, University Pierre and Marie Curie, Paris, France
| | - Diego Echevarria
- Instituto de Neurociencias, Universidad Miguel Hernández & Consejo Superior de Investigaciones Científicas, 03550, Sant Joan d'Alacant, Spain
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27
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García IE, Prado P, Pupo A, Jara O, Rojas-Gómez D, Mujica P, Flores-Muñoz C, González-Casanova J, Soto-Riveros C, Pinto BI, Retamal MA, González C, Martínez AD. Connexinopathies: a structural and functional glimpse. BMC Cell Biol 2016; 17 Suppl 1:17. [PMID: 27228968 PMCID: PMC4896260 DOI: 10.1186/s12860-016-0092-x] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
Abstract
Mutations in human connexin (Cx) genes have been related to diseases, which we termed connexinopathies. Such hereditary disorders include nonsyndromic or syndromic deafness (Cx26, Cx30), Charcot Marie Tooth disease (Cx32), occulodentodigital dysplasia and cardiopathies (Cx43), and cataracts (Cx46, Cx50). Despite the clinical phenotypes of connexinopathies have been well documented, their pathogenic molecular determinants remain elusive. The purpose of this work is to identify common/uncommon patterns in channels function among Cx mutations linked to human diseases. To this end, we compiled and discussed the effect of mutations associated to Cx26, Cx32, Cx43, and Cx50 over gap junction channels and hemichannels, highlighting the function of the structural channel domains in which mutations are located and their possible role affecting oligomerization, gating and perm/selectivity processes.
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Affiliation(s)
- Isaac E García
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Pavel Prado
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Amaury Pupo
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Oscar Jara
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Diana Rojas-Gómez
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Paula Mujica
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Carolina Flores-Muñoz
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Jorge González-Casanova
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Carolina Soto-Riveros
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Bernardo I Pinto
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Mauricio A Retamal
- Centro de Fisiología Celular e Integrativa, Facultad de Medicina, Clínica Alemana Universidad del Desarrollo, Santiago, Chile
| | - Carlos González
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Agustín D Martínez
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile.
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28
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Retamal MA, García IE, Pinto BI, Pupo A, Báez D, Stehberg J, Del Rio R, González C. Extracellular Cysteine in Connexins: Role as Redox Sensors. Front Physiol 2016; 7:1. [PMID: 26858649 PMCID: PMC4729916 DOI: 10.3389/fphys.2016.00001] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2015] [Accepted: 01/05/2016] [Indexed: 12/11/2022] Open
Abstract
Connexin-based channels comprise hemichannels and gap junction channels. The opening of hemichannels allow for the flux of ions and molecules from the extracellular space into the cell and vice versa. Similarly, the opening of gap junction channels permits the diffusional exchange of ions and molecules between the cytoplasm and contacting cells. The controlled opening of hemichannels has been associated with several physiological cellular processes; thereby unregulated hemichannel activity may induce loss of cellular homeostasis and cell death. Hemichannel activity can be regulated through several mechanisms, such as phosphorylation, divalent cations and changes in membrane potential. Additionally, it was recently postulated that redox molecules could modify hemichannels properties in vitro. However, the molecular mechanism by which redox molecules interact with hemichannels is poorly understood. In this work, we discuss the current knowledge on connexin redox regulation and we propose the hypothesis that extracellular cysteines could be important for sensing changes in redox potential. Future studies on this topic will offer new insight into hemichannel function, thereby expanding the understanding of the contribution of hemichannels to disease progression.
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Affiliation(s)
- Mauricio A Retamal
- Facultad de Medicina, Centro de Fisiología Celular e Integrativa, Clínica Alemana Universidad del Desarrollo Santiago, Chile
| | - Isaac E García
- Facultad de Ciencias, Centro Interdisciplinario de Neurociencias de Valparaíso, Instituto de Neurociencias, Universidad de Valparaíso Valparaíso, Chile
| | - Bernardo I Pinto
- Facultad de Ciencias, Centro Interdisciplinario de Neurociencias de Valparaíso, Instituto de Neurociencias, Universidad de Valparaíso Valparaíso, Chile
| | - Amaury Pupo
- Facultad de Ciencias, Centro Interdisciplinario de Neurociencias de Valparaíso, Instituto de Neurociencias, Universidad de Valparaíso Valparaíso, Chile
| | - David Báez
- Facultad de Ciencias, Centro Interdisciplinario de Neurociencias de Valparaíso, Instituto de Neurociencias, Universidad de Valparaíso Valparaíso, Chile
| | - Jimmy Stehberg
- Laboratorio de Neurobiología, Centro de Investigaciones Biomédicas, Universidad Andres Bello Santiago, Chile
| | - Rodrigo Del Rio
- Laboratory of Cardiorespiratory Control, Center for Biomedical Research, Universidad Autónoma de ChileSantiago, Chile; Dirección de Investigación, Universidad Científica del SurLima, Perú
| | - Carlos González
- Facultad de Ciencias, Centro Interdisciplinario de Neurociencias de Valparaíso, Instituto de Neurociencias, Universidad de Valparaíso Valparaíso, Chile
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29
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García IE, Bosen F, Mujica P, Pupo A, Flores-Muñoz C, Jara O, González C, Willecke K, Martínez AD. From Hyperactive Connexin26 Hemichannels to Impairments in Epidermal Calcium Gradient and Permeability Barrier in the Keratitis-Ichthyosis-Deafness Syndrome. J Invest Dermatol 2016; 136:574-583. [PMID: 26777423 DOI: 10.1016/j.jid.2015.11.017] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2015] [Revised: 11/05/2015] [Accepted: 11/06/2015] [Indexed: 01/19/2023]
Abstract
The keratitis-ichthyosis-deafness (KID) syndrome is characterized by corneal, skin, and hearing abnormalities. KID has been linked to heterozygous dominant missense mutations in the GJB2 and GJB6 genes, encoding connexin26 and 30, respectively. In vitro evidence indicates that KID mutations lead to hyperactive (open) hemichannels, which in some cases is accompanied by abnormal function of gap junction channels. Transgenic mouse models expressing connexin26 KID mutations reproduce human phenotypes and present impaired epidermal calcium homeostasis and abnormal lipid composition of the stratum corneum affecting the water barrier. Here we have compiled relevant data regarding the KID syndrome and propose a mechanism for the epidermal aspects of the disease.
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Affiliation(s)
- Isaac E García
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Felicitas Bosen
- LIMES (Life and Medical Sciences) Institute, University of Bonn, Bonn, Germany
| | - Paula Mujica
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Amaury Pupo
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Carolina Flores-Muñoz
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Oscar Jara
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Carlos González
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Klaus Willecke
- LIMES (Life and Medical Sciences) Institute, University of Bonn, Bonn, Germany.
| | - Agustín D Martínez
- Centro Interdisciplinario de Neurociencia de Valparaíso, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile.
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30
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Bai D. Structural analysis of key gap junction domains--Lessons from genome data and disease-linked mutants. Semin Cell Dev Biol 2015; 50:74-82. [PMID: 26658099 DOI: 10.1016/j.semcdb.2015.11.015] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2015] [Accepted: 11/26/2015] [Indexed: 12/27/2022]
Abstract
A gap junction (GJ) channel is formed by docking of two GJ hemichannels and each of these hemichannels is a hexamer of connexins. All connexin genes have been identified in human, mouse, and rat genomes and their homologous genes in many other vertebrates are available in public databases. The protein sequences of these connexins align well with high sequence identity in the same connexin across different species. Domains in closely related connexins and several residues in all known connexins are also well-conserved. These conserved residues form signatures (also known as sequence logos) in these domains and are likely to play important biological functions. In this review, the sequence logos of individual connexins, groups of connexins with common ancestors, and all connexins are analyzed to visualize natural evolutionary variations and the hot spots for human disease-linked mutations. Several gap junction domains are homologous, likely forming similar structures essential for their function. The availability of a high resolution Cx26 GJ structure and the subsequently-derived homology structure models for other connexin GJ channels elevated our understanding of sequence logos at the three-dimensional GJ structure level, thus facilitating the understanding of how disease-linked connexin mutants might impair GJ structure and function. This knowledge will enable the design of complementary variants to rescue disease-linked mutants.
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Affiliation(s)
- Donglin Bai
- Department of Physiology and Pharmacology, The University of Western Ontario, London, ON, Canada N6A 5C1.
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31
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Tong JJ, Minogue PJ, Kobeszko M, Beyer EC, Berthoud VM, Ebihara L. The connexin46 mutant, Cx46T19M, causes loss of gap junction function and alters hemi-channel gating. J Membr Biol 2015; 248:145-55. [PMID: 25404239 PMCID: PMC4300453 DOI: 10.1007/s00232-014-9752-y] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2014] [Accepted: 11/07/2014] [Indexed: 10/24/2022]
Abstract
An N-terminal mutant of connexin46 (T19M) alters a highly conserved threonine and has been linked to autosomal dominant cataracts. To study the cellular and functional consequences of substitution of this amino acid, T19M was expressed in Xenopus oocytes and in HeLa cells. Unlike wild-type Cx46, T19M did not induce intercellular conductances in Xenopus oocytes. In transfected HeLa cells, T19M was largely localized within the cytoplasm, with drastically reduced formation of gap junction plaques. Expression of rat T19M was cytotoxic, as evidenced by an almost complete loss of viable cells expressing the mutant protein by 48-72 h following transfection. When incubated in medium containing physiological concentrations of divalent cations, T19M-expressing cells showed increased uptake of DAPI as compared with cells expressing wild-type Cx46, suggesting aberrant connexin hemi-channel activity. Time-lapse and dye uptake studies suggested that T19M hemi-channels had reduced sensitivity to Ca(2+). Whole cell patch clamp studies of single transfected HeLa cells demonstrated that rat T19M formed functional hemi-channels with altered voltage-dependent gating. These data suggest that T19M causes cataracts by loss of gap junctional channel function and abnormally increased hemi-channel activity. Furthermore, they implicate this conserved threonine in both gap junction plaque formation and channel/hemi-channel gating in Cx46.
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Affiliation(s)
- Jun-Jie Tong
- Department of Physiology and Biophysics, Chicago Medical School, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064 USA
| | - Peter J. Minogue
- Department of Pediatrics, University of Chicago, 900 E. 57th Street, Chicago, IL 60637 USA
| | - Matthew Kobeszko
- Department of Physiology and Biophysics, Chicago Medical School, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064 USA
| | - Eric C. Beyer
- Department of Pediatrics, University of Chicago, 900 E. 57th Street, Chicago, IL 60637 USA
| | - Viviana M. Berthoud
- Department of Pediatrics, University of Chicago, 900 E. 57th Street, Chicago, IL 60637 USA
| | - Lisa Ebihara
- Department of Physiology and Biophysics, Chicago Medical School, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064 USA
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Abstract
Voltage is an important parameter that regulates the conductance of both intercellular and plasma membrane channels (undocked hemichannels) formed by the 21 members of the mammalian connexin gene family. Connexin channels display two forms of voltage-dependence, rectification of ionic currents and voltage-dependent gating. Ionic rectification results either from asymmetries in the distribution of fixed charges due to heterotypic pairing of different hemichannels, or by channel block, arising from differences in the concentrations of divalent cations on opposite sides of the junctional plaque. This rectification likely underpins the electrical rectification observed in some electrical synapses. Both intercellular and undocked hemichannels also display two distinct forms of voltage-dependent gating, termed Vj (fast)-gating and loop (slow)-gating. This review summarizes our current understanding of the molecular determinants and mechanisms underlying these conformational changes derived from experimental, molecular-genetic, structural, and computational approaches.
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Affiliation(s)
- Seunghoon Oh
- Department of Physiology, College of Medicine, Dankook University, Cheonan, Korea
| | - Thaddeus A Bargiello
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY, USA.
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33
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Xiao D, Chen S, Shao Q, Chen J, Bijian K, Laird DW, Alaoui-Jamali MA. Dynamin 2 interacts with connexin 26 to regulate its degradation and function in gap junction formation. Int J Biochem Cell Biol 2014; 55:288-97. [DOI: 10.1016/j.biocel.2014.09.021] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2014] [Revised: 09/09/2014] [Accepted: 09/19/2014] [Indexed: 11/16/2022]
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Bai D. Atrial fibrillation-linked GJA5/connexin40 mutants impaired gap junctions via different mechanisms. FEBS Lett 2014; 588:1238-43. [PMID: 24656738 DOI: 10.1016/j.febslet.2014.02.064] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2013] [Revised: 02/05/2014] [Accepted: 02/06/2014] [Indexed: 01/08/2023]
Abstract
The gap junctions (GJs) formed by Cx40 and Cx43 provide a low resistance passage allowing for rapid propagation of action potentials. Sporadic somatic mutations in GJA5 (encoding Cx40) have been identified in lone atrial fibrillation (AF) patients. More recently germline autosomal dominantly inherited mutations in GJA5 have been found in early onset lone AF patients in several families over generations. Characterizations of these AF-linked Cx40 mutants in model cells and in patient tissues revealed that some of the mutants reduced GJ channel function due to an impaired trafficking or channel formation. While others showed a gain-of-function in hemichannels. These functional alterations in GJs or hemichannel may play an important role in the pathogenesis of AF in the mutant carriers.
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Affiliation(s)
- Donglin Bai
- Department of Physiology and Pharmacology, Western University, London, Ontario N6A 5C1, Canada.
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35
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Harris AL, Contreras JE. Motifs in the permeation pathway of connexin channels mediate voltage and Ca (2+) sensing. Front Physiol 2014; 5:113. [PMID: 24744733 PMCID: PMC3978323 DOI: 10.3389/fphys.2014.00113] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2014] [Accepted: 03/06/2014] [Indexed: 11/13/2022] Open
Abstract
Connexin channels mediate electrical coupling, intercellular molecular signaling, and extracellular release of signaling molecules. Connexin proteins assemble intracellularly as hexamers to form plasma membrane hemichannels. The docking of two hemichannels in apposed cells forms a gap junction channel that allows direct electrical and selective cytoplasmic communication between adjacent cells. Hemichannels and junctional channels are gated by voltage, but extracellular Ca (2+) also gates unpaired plasma membrane hemichannels. Unlike other ion channels, connexin channels do not contain discrete voltage- or Ca (2+)-sensing modules linked to a separate pore-forming module. All studies to date indicate that voltage and Ca (2+) sensing are predominantly mediated by motifs that lie within or are exposed to the pore lumen. The sensors appear to be integral components of the gates, imposing an obligatory structural linkage between sensing and gating not commonly present in other ion channels, in which the sensors are semi-independent domains distinct from the pore. Because of this, the structural and electrostatic features that define connexin channel gating also define pore permeability properties, and vice versa; analysis/mutagenesis of gating and of permeability properties are linked. This offers unique challenges and opportunities for elucidating mechanisms of ligand and voltage-driven gating.
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Affiliation(s)
- Andrew L Harris
- Department of Pharmacology and Physiology, New Jersey Medical School, Rutgers University Newark, NJ, USA
| | - Jorge E Contreras
- Department of Pharmacology and Physiology, New Jersey Medical School, Rutgers University Newark, NJ, USA
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36
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Oshima A. Structure and closure of connexin gap junction channels. FEBS Lett 2014; 588:1230-7. [PMID: 24492007 DOI: 10.1016/j.febslet.2014.01.042] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2013] [Revised: 01/24/2014] [Accepted: 01/24/2014] [Indexed: 11/29/2022]
Abstract
Connexin gap junctions comprise assembled channels penetrating two plasma membranes for which gating regulation is associated with a variety of factors, including voltage, pH, Ca(2+), and phosphorylation. Functional studies have established that various parts of the connexin peptides are related to channel closure and electrophysiology studies have provided several working models for channel gating. The corresponding structural models supporting these findings, however, are not sufficient because only small numbers of closed connexin structures have been reported. To fully understand the gating mechanisms, the channels should be visualized in both the open and closed states. Electron crystallography and X-ray crystallography studies recently revealed three-dimensional structures of connexin channels in a couple of states in which the main difference is the conformation of the N-terminal domain, which have helped to clarify the structure in regard to channel closure. Here the closure models for connexin gap junction channels inferred from structural and functional studies are described in the context of each domain of the connexin protein associated with gating modulation.
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Affiliation(s)
- Atsunori Oshima
- Cellular and Structural Physiology Institute (CeSPI), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
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37
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Zou J, Salarian M, Chen Y, Veenstra R, Louis CF, Yang JJ. Gap junction regulation by calmodulin. FEBS Lett 2014; 588:1430-8. [PMID: 24440348 DOI: 10.1016/j.febslet.2014.01.003] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2013] [Revised: 01/09/2014] [Accepted: 01/10/2014] [Indexed: 10/25/2022]
Abstract
Intracellular Ca(2+) activated calmodulin (CaM) inhibits gap junction channels in the low nanomolar to high micromolar range of [Ca(2+)]i. This regulation plays an essential role in numerous cellular processes that include hearing, lens transparency, and synchronized contractions of the heart. Previous studies have indicated that gap junction mediated cell-to-cell communication was inhibited by CaM antagonists. More recent evidence indicates a direct role of CaM in regulating several members of the connexin family. Since the intracellular loop and carboxyl termini of connexins are largely "invisible" in electron microscopy and X-ray crystallographic structures due to disorder in these domains, peptide models encompassing the putative CaM binding sites of several intracellular domains of connexins have been used to identify the Ca(2+)-dependent CaM binding sites of these proteins. This approach has been used to determine the CaM binding affinities of peptides derived from a number of different connexin-subfamilies.
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Affiliation(s)
- Juan Zou
- Department of Chemistry, Center for Diagnostics and Therapeutics, Georgia State University, Atlanta, GA 30303, United States
| | - Mani Salarian
- Department of Chemistry, Center for Diagnostics and Therapeutics, Georgia State University, Atlanta, GA 30303, United States
| | - Yanyi Chen
- Department of Chemistry, Center for Diagnostics and Therapeutics, Georgia State University, Atlanta, GA 30303, United States
| | - Richard Veenstra
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, NY 13210, United States
| | - Charles F Louis
- Department of Cell Biology and Neuroscience, University of California, Riverside, CA 92521, United States.
| | - Jenny J Yang
- Department of Chemistry, Center for Diagnostics and Therapeutics, Georgia State University, Atlanta, GA 30303, United States.
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38
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Thévenin AF, Kowal TJ, Fong JT, Kells RM, Fisher CG, Falk MM. Proteins and mechanisms regulating gap-junction assembly, internalization, and degradation. Physiology (Bethesda) 2014; 28:93-116. [PMID: 23455769 DOI: 10.1152/physiol.00038.2012] [Citation(s) in RCA: 93] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Gap junctions (GJs) are the only known cellular structures that allow a direct cell-to-cell transfer of signaling molecules by forming densely packed arrays or "plaques" of hydrophilic channels that bridge the apposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for all aspects of multicellular life, including coordination of development, tissue function, and cell homeostasis, has been well documented. Assembly and degradation of these membrane channels is a complex process that includes biosynthesis of the connexin (Cx) subunit proteins (innexins in invertebrates) on endoplasmic reticulum (ER) membranes, oligomerization of compatible subunits into hexameric hemichannels (connexons), delivery of the connexons to the plasma membrane (PM), head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic spatially and temporally organized GJ channel plaques, as well as internalization of GJs into the cytoplasm followed by their degradation. Clearly, precise modulation of GJIC, biosynthesis, and degradation are crucial for accurate function, and much research currently addresses how these fundamental processes are regulated. Here, we review posttranslational protein modifications (e.g., phosphorylation and ubiquitination) and the binding of protein partners (e.g., the scaffolding protein ZO-1) known to regulate GJ biosynthesis, internalization, and degradation. We also look closely at the atomic resolution structure of a GJ channel, since the structure harbors vital cues relevant to GJ biosynthesis and turnover.
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Affiliation(s)
- Anastasia F Thévenin
- Department of Biological Sciences, Lehigh University, Bethlehem, Pennsylvania, USA
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39
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D'hondt C, Iyyathurai J, Vinken M, Rogiers V, Leybaert L, Himpens B, Bultynck G. Regulation of connexin- and pannexin-based channels by post-translational modifications. Biol Cell 2013; 105:373-98. [PMID: 23718186 DOI: 10.1111/boc.201200096] [Citation(s) in RCA: 52] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2013] [Accepted: 05/24/2013] [Indexed: 12/28/2022]
Abstract
Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi-protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post-translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N-acetylation, S-nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx-channel activity and localisation.
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Affiliation(s)
- Catheleyne D'hondt
- Laboratory of Molecular and Cellular Signalling, Department Cellular and Molecular Medicine, KU Leuven, Campus Gasthuisberg O/N 1, BE-3000, Leuven, Belgium.
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40
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Beyer EC, Lin X, Veenstra RD. Interfering amino terminal peptides and functional implications for heteromeric gap junction formation. Front Pharmacol 2013; 4:67. [PMID: 23734129 PMCID: PMC3659311 DOI: 10.3389/fphar.2013.00067] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2013] [Accepted: 04/29/2013] [Indexed: 02/05/2023] Open
Abstract
Connexin43 (Cx43) is widely expressed in many different tissues of the human body. In cells of some organs, Cx43 is co-expressed with other connexins (Cx), including Cx46 and Cx50 in lens, Cx40 in atrium, Purkinje fibers, and the blood vessel wall, Cx45 in heart, and Cx37 in the ovary. Interactions with the co-expressed connexins may have profound functional implications. The abilities of Cx37, Cx45, Cx46, and Cx50 to function in heteromeric gap junction combinations with Cx43 are well documented. Different studies disagree regarding the ability of Cx43 and Cx40 to produce functional heteromeric gap junctions with each other. We review previous studies regarding the heteromeric interactions of Cx43. The possibility of negative functional interactions between the cytoplasmic pore-forming amino-terminal (NT) domains of these connexins was assessed using pentameric connexin sequence-specific NT domain [interfering NT (iNT)] peptides applied to cells expressing homomeric Cx40, Cx37, Cx45, Cx46, and Cx50 gap junctions. A Cx43 iNT peptide corresponding to amino acids 9–13 (Ac-KLLDK-NH2) specifically inhibited the electrical coupling of Cx40 gap junctions in a transjunctional voltage (Vj)-dependent manner without affecting the function of homologous Cx37, Cx46, Cx50, and Cx45 gap junctions. A Cx40 iNT (Ac-EFLEE-OH) peptide counteracted the Vj-dependent block of Cx40 gap junctions, whereas a similarly charged Cx50 iNT (Ac-EEVNE-OH) peptide did not, suggesting that these NT domain interactions are not solely based on electrostatics. These data are consistent with functional Cx43 heteromeric gap junction formation with Cx37, Cx45, Cx46, and Cx50 and suggest that Cx40 uniquely experiences functional suppressive interactions with a Cx43 NT domain sequence. These findings present unique functional implications about the heteromeric interactions between Cx43 and Cx40 that may influence cardiac conduction in atrial myocardium and the specialized conduction system.
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Affiliation(s)
- Eric C Beyer
- Department of Pediatrics, University of Chicago Chicago, IL, USA
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41
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Tong JJ, Sohn BCH, Lam A, Walters DE, Vertel BM, Ebihara L. Properties of two cataract-associated mutations located in the NH2 terminus of connexin 46. Am J Physiol Cell Physiol 2013; 304:C823-32. [PMID: 23302783 PMCID: PMC3651606 DOI: 10.1152/ajpcell.00344.2012] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2012] [Accepted: 01/08/2013] [Indexed: 11/22/2022]
Abstract
Mutations in connexin 46 are associated with congenital cataracts. The purpose of this project was to characterize cellular and functional properties of two congenital cataract-associated mutations located in the NH2 terminus of connexin 46: Cx46D3Y and Cx46L11S, which we found localized to gap junctional plaques like wild-type Cx46 in transfected HeLa cells. Dual two-microelectrode-voltage-clamp studies of Xenopus oocyte pairs injected with wild-type or mutant rat Cx46 showed that oocyte pairs injected with D3Y or L11S cRNA failed to induce gap junctional coupling, whereas oocyte pairs injected with Cx46 showed high levels of coupling. D3Y, but not L11S, functionally paired with wild-type Cx46. To determine whether coexpression of D3Y or L11S affected the junctional conductance produced by wild-type lens connexins, we studied pairs of oocytes coinjected with equal amounts of mutant and wild-type connexin cRNA. Expression of D3Y or L11S almost completely abolished gap junctional coupling induced by Cx46. In contrast, expression of D3Y or L11S failed to inhibit junctional conductance induced by Cx50. To examine effects of the D3Y and L11S mutations on hemichannel activity, hemichannel currents were measured in connexin cRNA-injected oocytes. Oocytes expressing D3Y exhibited reduced hemichannel activity as well as alterations in voltage gating and charge selectivity while oocytes expressing L11S showed no hemichannel activity. Moreover, coexpression of mutant with wild-type Cx50 or Cx46 gave rise to hemichannels with distinct electrophysiological properties, suggesting that the mutant connexins were forming heteromeric channels with wild-type connexins. These data suggest D3Y and L11S cause cataracts by similar but not identical mechanisms.
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Affiliation(s)
- Jun-Jie Tong
- Department of Physiology and Biophysics, Rosalind Franklin University of Medicine and Science, North Chicago, IL 60064, USA
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42
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Grosely R, Sorgen PL. A history of gap junction structure: hexagonal arrays to atomic resolution. ACTA ACUST UNITED AC 2013; 20:11-20. [PMID: 23469928 DOI: 10.3109/15419061.2013.775256] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
Gap junctions are specialized membrane structures that provide an intercellular pathway for the propagation and/or amplification of signaling cascades responsible for impulse propagation, cell growth, and development. Prior to the identification of the proteins that comprise gap junctions, elucidation of channel structure began with initial observations of a hexagonal nexus connecting apposed cellular membranes. Concomitant with technological advancements spanning over 50 years, atomic resolution structures are now available detailing channel architecture and the cytoplasmic domains that have helped to define mechanisms governing the regulation of gap junctions. Highlighted in this review are the seminal structural studies that have led to our current understanding of gap junction biology.
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Affiliation(s)
- Rosslyn Grosely
- Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, Omaha, NE 68198-5870, USA
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43
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Su CC, Li SY, Yen YC, Nian JH, Liang WG, Yang JJ. Mechanism of Two Novel Human GJC3 Missense Mutations in Causing Non-Syndromic Hearing Loss. Cell Biochem Biophys 2012. [DOI: 10.1007/s12013-012-9481-8] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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44
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Zonta F, Polles G, Zanotti G, Mammano F. Permeation pathway of homomeric connexin 26 and connexin 30 channels investigated by molecular dynamics. J Biomol Struct Dyn 2012; 29:985-98. [PMID: 22292956 DOI: 10.1080/073911012010525027] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Mutations in the genes GJB2 and GJB6 encoding human connnexin26 (hCx26) and connexin30 (hCx30), respectively, are the leading cause of non-syndromic prelingual deafness in several human populations. In this work, we exploited the high degree (77%) of sequence similarity shared by hCx26 and hCx30 to create atomistic models of homomeric hCx26 and hCx30 connexons starting from the X-ray crystallographic structure of an intercellular channel formed by hCx26 protomers at 3.5-å resolution. The equilibrium dynamics of the two protein complexes was followed for 40 ns each by Molecular Dynamics (MD) simulations. Our results indicate that, in hCx26, positively charged Lys41 residues establish a potential barrier within the fully open channel, hindering ion diffusion in the absence of an electrochemical gradient. A similar role is played, in hCx30, by negatively charged Glu49 residues. The different position and charge of these two ion sieves account for the differences in unitary conductance observed experimentally. Our results are discussed in terms of present models of voltage gating in connexin channels.
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Affiliation(s)
- Francesco Zonta
- Department of Physics and Astronomy G.Galilei, University of Padua, 35129 Padua, Italy
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45
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Kalmatsky DB, Batir Y, Bargiello TA, Dowd T. Structural studies of N-terminal mutants of connexin 32 using (1)H NMR spectroscopy. Arch Biochem Biophys 2012; 526:1-8. [PMID: 22705201 PMCID: PMC3938314 DOI: 10.1016/j.abb.2012.05.027] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2012] [Revised: 05/02/2012] [Accepted: 05/22/2012] [Indexed: 12/11/2022]
Abstract
The amino terminus of gap junction proteins, connexins, plays a fundamental role in voltage gating and ion permeation. We have previously shown with (1)H NMR that the structure of the N-terminus of functional connexin molecules contains a flexible turn around G12 (Arch. Biochem. Biophys.490:9,2009) allowing the N-terminus to form a portion of the channel pore near the cytoplasmic entrance. The mutants of nonfunctional connexin molecules G12S and G12Y were found to prevent this turn. Previous functional studies of loci at which Cx32 mutations cause a peripheral neuropathy, Charcot-Marie-Tooth disease, have shown that G12S is not plasma membrane inserted. Presently, we solve the structure of nonfunctional Connexin 32 mutants W3D and Y7D which do not appear to be membrane inserted. Using 2D (1)H NMR, we report that similar to G12S and G12Y, alterations in hydrophobic sidechain interactions disrupt (Y7D) or constrain (W3D) the flexible turn around G12. The alteration in the open turn around residue 12, observed in all nonfunctional mutants G12S, G12Y, W3D and Y7D correlates with loss of function. We propose that loss of the open turn causes the N-terminus to extend out of the channel pore and this misfolding may target mutants for destruction in the endoplasmic reticulum.
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Affiliation(s)
- D. B. Kalmatsky
- Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn, New York, 11210
| | - Y. Batir
- Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn, New York, 11210
| | - T. A. Bargiello
- Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461
| | - T.L. Dowd
- Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn, New York, 11210
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46
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Schlingmann B, Schadzek P, Busko S, Heisterkamp A, Ngezahayo A. Cataract-associated D3Y mutation of human connexin46 (hCx46) increases the dye coupling of gap junction channels and suppresses the voltage sensitivity of hemichannels. J Bioenerg Biomembr 2012; 44:607-14. [PMID: 22843197 DOI: 10.1007/s10863-012-9461-0] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2012] [Accepted: 07/08/2012] [Indexed: 10/28/2022]
Abstract
Connexin46 (Cx46), together with Cx50, forms gap junction channels between lens fibers and participates in the lens pump-leak system, which is essential for the homeostasis of this avascular organ. Mutations in Cx50 and Cx46 correlate with cataracts, but the functional relationship between the mutations and cataract formation is not always clear. Recently, it was found that a mutation at the third position of hCx46 that substituted an aspartic acid residue with a tyrosine residue (hCx46D3Y) caused an autosomal dominant zonular pulverulent cataract. We expressed EGFP-labeled hCx46wt and hCx46D3Y in HeLa cells and found that the mutation did not affect the formation of gap junction plaques. Dye transfer experiments using Lucifer Yellow (LY) and ethidium bromide (EthBr) showed an increased degree of dye coupling between the cell pairs expressing hCx46D3Y in comparison to the cell pairs expressing hCx46wt. In Xenopus oocytes, two-electrode voltage-clamp experiments revealed that hCx46wt formed voltage-sensitive hemichannels. This was not observed in the oocytes expressing hCx46D3Y. The replacement of the aspartic acid residue at the third position by another negatively charged residue, glutamic acid, to generate the mutant hCx46D3E, restored the voltage sensitivity of the resultant hemichannels. Moreover, HeLa cell pairs expressing hCx46D3E and hCx46wt showed a similar degree of dye coupling. These results indicate that the negatively charged aspartic acid residue at the third position of the N-terminus of hCx46 could be involved in the determination of the degree of metabolite cell-to-cell coupling and is essential for the voltage sensitivity of the hCx46 hemichannels.
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47
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Bargiello TA, Tang Q, Oh S, Kwon T. Voltage-dependent conformational changes in connexin channels. BIOCHIMICA ET BIOPHYSICA ACTA 2012; 1818:1807-22. [PMID: 21978595 PMCID: PMC3367129 DOI: 10.1016/j.bbamem.2011.09.019] [Citation(s) in RCA: 52] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/19/2011] [Revised: 09/09/2011] [Accepted: 09/17/2011] [Indexed: 10/17/2022]
Abstract
Channels formed by connexins display two distinct types of voltage-dependent gating, termed V(j)- or fast-gating and loop- or slow-gating. Recent studies, using metal bridge formation and chemical cross-linking have identified a region within the channel pore that contributes to the formation of the loop-gate permeability barrier. The conformational changes are remarkably large, reducing the channel pore diameter from 15 to 20Å to less than 4Å. Surprisingly, the largest conformational change occurs in the most stable region of the channel pore, the 3(10) or parahelix formed by amino acids in the 42-51 segment. The data provide a set of positional constraints that can be used to model the structure of the loop-gate closed state. Less is known about the conformation of the V(j)-gate closed state. There appear to be two different mechanisms; one in which conformational changes in channel structure are linked to a voltage sensor contained in the N-terminus of Cx26 and Cx32 and a second in which the C-terminus of Cx43 and Cx40 may act either as a gating particle to block the channel pore or alternatively to stabilize the closed state. The later mechanism utilizes the same domains as implicated in effecting pH gating of Cx43 channels. It is unclear if the two V(j)-gating mechanisms are related or if they represent different gating mechanisms that operate separately in different subsets of connexin channels. A model of the V(j)-closed state of Cx26 hemichannel that is based on the X-ray structure of Cx26 and electron crystallographic structures of a Cx26 mutation suggests that the permeability barrier for V(j)-gating is formed exclusively by the N-terminus, but recent information suggests that this conformation may not represent a voltage-closed state. Closed state models are considered from a thermodynamic perspective based on information from the 3.5Å Cx26 crystal structure and molecular dynamics (MD) simulations. The applications of computational and experimental methods to define the path of allosteric molecular transitions that link the open and closed states are discussed. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.
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Affiliation(s)
- Thaddeus A Bargiello
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY, USA.
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48
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Polyamine sensitivity of gap junctions is required for skin pattern formation in zebrafish. Sci Rep 2012; 2:473. [PMID: 22737406 PMCID: PMC3382735 DOI: 10.1038/srep00473] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2012] [Accepted: 06/11/2012] [Indexed: 12/23/2022] Open
Abstract
Gap junctions allow the direct and bidirectional transfer of small molecules between cells. Polyamine sensitivity, which has been observed for a certain gap junction in vitro, confers rectification property to gap junction. Here we report that the polyamine sensitivity of gap junctions in vivo is crucial for skin pattern formation in zebrafish. Transgenic experiments have revealed that several connexin genes were able to rescue the spot phenotype of mutant zebrafish. Mutational analyses of the N-terminal region of connexins revealed that the ExxxE motif, a hypothetical polyamine-binding site, was important for connexin's role in pattern formation. Ectopic expression of spermidine/spermine N(1)-acetyltransferase (SSAT), a polyamine metabolic enzyme, also caused stripe pattern changes, which further indicates that the polyamine sensitivity of gap junctions is crucial. This is the first report to show that polyamine sensitivity has a physiologically relevant function and is related to skin pattern formation in animals.
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49
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Ek-Vitorin JF, Burt JM. Structural basis for the selective permeability of channels made of communicating junction proteins. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:51-68. [PMID: 22342665 DOI: 10.1016/j.bbamem.2012.02.003] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/03/2011] [Revised: 01/24/2012] [Accepted: 02/01/2012] [Indexed: 01/08/2023]
Abstract
The open state(s) of gap junction channels is evident from their permeation by small ions in response to an applied intercellular (transjunctional/transchannel) voltage gradient. That an open channel allows variable amounts of current to transit from cell-to-cell in the face of a constant intercellular voltage difference indicates channel open/closing can be complete or partial. The physiological significance of such open state options is, arguably, the main concern of junctional regulation. Because gap junctions are permeable to many substances, it is sensible to inquire whether and how each open state influences the intercellular diffusion of molecules as valuable as, but less readily detected than current-carrying ions. Presumably, structural changes perceived as shifts in channel conductivity would significantly alter the transjunctional diffusion of molecules whose limiting diameter approximates the pore's limiting diameter. Moreover, changes in junctional permeability to some molecules might occur without evident changes in conductivity, either at macroscopic or single channel level. Open gap junction channels allow the exchange of cytoplasmic permeants between contacting cells by simple diffusion. The identity of such permeants, and the functional circumstances and consequences of their junctional exchange presently constitute the most urgent (and demanding) themes of the field. Here, we consider the necessity for regulating this exchange, the possible mechanism(s) and structural elements likely involved in such regulation, and how regulatory phenomena could be perceived as changes in chemical vs. electrical coupling; an overall reflection on our collective knowledge of junctional communication is then applied to suggest new avenues of research. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.
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50
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Wang X, Xu X, Ma M, Zhou W, Wang Y, Yang L. pH-dependent channel gating in connexin26 hemichannels involves conformational changes in N-terminus. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1818:1148-57. [PMID: 22285739 DOI: 10.1016/j.bbamem.2011.12.027] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/19/2011] [Revised: 12/23/2011] [Accepted: 12/27/2011] [Indexed: 10/14/2022]
Abstract
Connexin (Cx) hemichannels controlling an exchange of ions and metabolites between the cytoplasm and extracellular milieu can be modulated by the variation of intracellular pH during physiological and pathological conditions. To address the mechanism by which the pH exerts its effect on hemichannels, we have performed two 100-ns molecular dynamics simulations of the Cx26 channel in both acidic and neutral states. The results show that: 1) transmembrane domains undergo clockwise motions around the pore axis under both acidic and neutral conditions, while extracellular segments keep stable. 2) Under neutral condition, Cx26 has a tightly closed configuration that occurs through the assembly of N-terminal helix (NTH) region. This shows a constriction formed by the interhelical interactions of Asp2 and Met1 from neighboring NTH, which shapes the narrowest segment (pore radius<2Å) of the pore, preventing the passage of ions from the extracellular side. This indicates that Asp2 may act as a channel gate. 3) Under the acidic condition, the constriction is relieved by the protonation of Asp2 causing interruption of interhelical interactions, Cx26 has a flexibly opening pore (pore radius>4.5Å) around NTH region, allowing the passage of chloride ions unimpeded by the side-chain Asp2. While in the extracellular part two chloride ions interact with the side-chain Lys41 from three subunits. Finally, we provide a plausible mechanism of pH-dependent gating of hemichannel that involves protonation of the aspartic residues, suggesting that the pH sensitivity of hemichannel permeability is a sophisticated mechanism for cell regulating ion permeation.
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Affiliation(s)
- Xia Wang
- Center of Bioinformatics, Northwest A&F University, Yangling, Shaanxi, 712100, China
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