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Lissoni A, Wang N, Nezlobinskii T, De Smet M, Panfilov AV, Vandersickel N, Leybaert L, Witschas K. Gap19, a Cx43 Hemichannel Inhibitor, Acts as a Gating Modifier That Decreases Main State Opening While Increasing Substate Gating. Int J Mol Sci 2020; 21:ijms21197340. [PMID: 33027889 PMCID: PMC7583728 DOI: 10.3390/ijms21197340] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2020] [Revised: 09/12/2020] [Accepted: 09/25/2020] [Indexed: 02/05/2023] Open
Abstract
Cx43 hemichannels (HCs) are electrically and chemically gated transmembrane pores with low open probability and multiple conductance states, which makes kinetic studies of channel gating in large datasets challenging. Here, we developed open access software, named HemiGUI, to analyze HC gating transitions and investigated voltage-induced HC opening based on up to ≈4000 events recorded in HeLa-Cx43-overexpressing cells. We performed a detailed characterization of Cx43 HC gating profiles and specifically focused on the role of the C-terminal tail (CT) domain by recording the impact of adding an EGFP tag to the Cx43 CT end (Cx43-EGFP) or by supplying the Cx43 HC-inhibiting peptide Gap19 that interferes with CT interaction with the cytoplasmic loop (CL). We found that Gap19 not only decreased HC opening activity to the open state (≈217 pS) but also increased the propensity of subconductance (≈80 pS) transitions that additionally became slower as compared to the control. The work demonstrates that large sample transition analysis allows detailed investigations on Cx43 HC gating and shows that Gap19 acts as a HC gating modifier by interacting with the CT that forms a crucial gating element.
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Affiliation(s)
- Alessio Lissoni
- Department of Basic and Applied Medical Sciences—Physiology Group, Ghent University, 9000 Ghent, Belgium; (A.L.); (N.W.); (M.D.S.)
| | - Nan Wang
- Department of Basic and Applied Medical Sciences—Physiology Group, Ghent University, 9000 Ghent, Belgium; (A.L.); (N.W.); (M.D.S.)
| | - Timur Nezlobinskii
- Department of Physics and Astronomy, Ghent University, 9000 Ghent, Belgium; (T.N.); (A.V.P.); (N.V.)
| | - Maarten De Smet
- Department of Basic and Applied Medical Sciences—Physiology Group, Ghent University, 9000 Ghent, Belgium; (A.L.); (N.W.); (M.D.S.)
| | - Alexander V. Panfilov
- Department of Physics and Astronomy, Ghent University, 9000 Ghent, Belgium; (T.N.); (A.V.P.); (N.V.)
- Laboratory of Computational Biology and Medicine, Ural Federal University, 620075 Ekaterinburg, Russia
| | - Nele Vandersickel
- Department of Physics and Astronomy, Ghent University, 9000 Ghent, Belgium; (T.N.); (A.V.P.); (N.V.)
| | - Luc Leybaert
- Department of Basic and Applied Medical Sciences—Physiology Group, Ghent University, 9000 Ghent, Belgium; (A.L.); (N.W.); (M.D.S.)
- Correspondence: (L.L.); (K.W.); Tel.: +32-9-332-3366 (L.L.); +32-9-332-6944 (K.W.)
| | - Katja Witschas
- Department of Basic and Applied Medical Sciences—Physiology Group, Ghent University, 9000 Ghent, Belgium; (A.L.); (N.W.); (M.D.S.)
- Correspondence: (L.L.); (K.W.); Tel.: +32-9-332-3366 (L.L.); +32-9-332-6944 (K.W.)
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Abstract
Harris explores the development of our current understanding of electrical coupling between cells and the channels that mediate it, highlighting the contributions of the Journal of General Physiology. As the physiology of synapses began to be explored in the 1950s, it became clear that electrical communication between neurons could not always be explained by chemical transmission. Instead, careful studies pointed to a direct intercellular pathway of current flow and to the anatomical structure that was (eventually) called the gap junction. The mechanism of intercellular current flow was simple compared with chemical transmission, but the consequences of electrical signaling in excitable tissues were not. With the recognition that channels were a means of passive ion movement across membranes, the character and behavior of gap junction channels came under scrutiny. It became evident that these gated channels mediated intercellular transfer of small molecules as well as atomic ions, thereby mediating chemical, as well as electrical, signaling. Members of the responsible protein family in vertebrates—connexins—were cloned and their channels studied by many of the increasingly biophysical techniques that were being applied to other channels. As described here, much of the evolution of the field, from electrical coupling to channel structure–function, has appeared in the pages of the Journal of General Physiology.
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Affiliation(s)
- Andrew L Harris
- Department of Pharmacology, Physiology, and Neuroscience, Rutgers New Jersey Medical School, Newark, NJ
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Nielsen BS, Alstrom JS, Nicholson BJ, Nielsen MS, MacAulay N. Permeant-specific gating of connexin 30 hemichannels. J Biol Chem 2017; 292:19999-20009. [PMID: 28982982 DOI: 10.1074/jbc.m117.805986] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2017] [Revised: 09/28/2017] [Indexed: 11/06/2022] Open
Abstract
Gap junctions confer interconnectivity of the cytoplasm in neighboring cells via docking of two connexons expressed in each of the adjacent membranes. Undocked connexons, referred to as hemichannels, may open and connect the cytoplasm with the extracellular fluid. The hemichannel configuration of connexins (Cxs) displays isoform-specific permeability profiles that are not directly determined by the size and charge of the permeant. To further explore Ca2+-mediated gating and permeability features of connexin hemichannels, we heterologously expressed Cx30 hemichannels in Xenopus laevis oocytes. The sensitivity toward divalent cation-mediated gating differed between small atomic ions (current) and fluorescent dye permeants, indicating that these permeants are distinctly gated. Three aspartate residues in Cx30 (Asp-50, Asp-172, and Asp-179) have been implicated previously in the Ca2+ sensitivity of other hemichannel isoforms. Although the aspartate at position Asp-50 was indispensable for divalent cation-dependent gating of Cx30 hemichannels, substitutions of the two other residues had no significant effect on gating, illustrating differences in the gating mechanisms between connexin isoforms. Using the substituted cysteine accessibility method (SCAM), we evaluated the role of possible pore-lining residues in the permeation of ions and ethidium through Cx30 hemichannels. Of the cysteine-substituted residues, interaction of a proposed pore-lining cysteine at position 37 with the positively charged compound [2-(trimethylammonium)ethyl] methane thiosulfonate bromide (MTS-ET) increased Cx30-mediated currents with unperturbed ethidium permeability. In summary, our results demonstrate that the permeability of hemichannels is regulated in a permeant-specific manner and underscores that hemichannels are selective rather than non-discriminating and freely diffusable pores.
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Affiliation(s)
| | | | - Bruce J Nicholson
- Department of Biochemistry, School of Medicine, University of Texas Health Science Center, San Antonio, Texas 78229
| | - Morten Schak Nielsen
- Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, 2200 Copenhagen N, Denmark
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Laleh MA, Naseri M, Zonouzi AAP, Zonouzi AP, Masoudi M, Ahangari N, Shams L, Nejatizadeh A. Diverse pattern of gap junction beta-2 and gap junction beta-4 genes mutations and lack of contribution of DFNB21, DFNB24, DFNB29, and DFNB42 loci in autosomal recessive nonsyndromic hearing loss patients in Hormozgan, Iran. JOURNAL OF RESEARCH IN MEDICAL SCIENCES 2017; 22:99. [PMID: 28900455 PMCID: PMC5583625 DOI: 10.4103/jrms.jrms_976_16] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/09/2016] [Revised: 04/26/2017] [Accepted: 05/12/2017] [Indexed: 11/04/2022]
Abstract
BACKGROUND We aimed to determine the contribution of four DFNB loci and mutation analysis of gap junction beta-2 (GJB2) and GJB4 genes in autosomal recessive nonsyndromic hearing loss (ARNSHL) in South of Iran. MATERIALS AND METHODS A total of 36 large ARNSHL pedigrees with at least two affected subjects were enrolled in the current study. The GJB2 and GJB4 genes mutations were screened using direct sequencing method. The GJB2 and GJB4 negative families were analyzed for the linkage to DFNB21, DFNB24, DFNB29, and DFNB42 loci by genotyping the corresponding STR markers using polymerase chain reaction-PAGE method. RESULTS We found a homozygous nonsense mutation W77X and a homozygous missense mutation C169W in 5.55% of studied families in GJB2 and GJB4 genes, respectively. Five heterozygous mutations including V63G, A78T, and R127H in GJB2 gene, and R103C and R227W in GJB4 gene were detected. We identified two novel variations V63G in GJB2 and R227W in GJB4. In silico analysis predicted that both novel variations are deleterious mutations. We did not unveil any linkage between DFNB21, DFNB24, DFNB29, and DFNB42 loci and ARNSHL among studied families. CONCLUSION This is the first report of GJB2 and GJB4 mutations from Hormozgan population. According to the previous publications regarding GJB2 and GJB4 mutations, the distribution of the mutations is different from other parts of Iran that should be considered in primary health-care programs. Further investigations are needed to evaluate the contribution of other loci in ARNSHL subjects in South of Iran.
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Affiliation(s)
- Masoud Akbarzadeh Laleh
- Molecular Medicine Research Center, Hormozgan University of Medical Sciences, Bandar Abbas, Iran
| | - Marzieh Naseri
- Molecular Medicine Research Center, Hormozgan University of Medical Sciences, Bandar Abbas, Iran
| | | | | | - Marjan Masoudi
- Molecular Medicine Research Center, Hormozgan University of Medical Sciences, Bandar Abbas, Iran
| | - Najmeh Ahangari
- Molecular Medicine Research Center, Hormozgan University of Medical Sciences, Bandar Abbas, Iran
| | - Leila Shams
- Molecular Medicine Research Center, Hormozgan University of Medical Sciences, Bandar Abbas, Iran
| | - Azim Nejatizadeh
- Molecular Medicine Research Center, Hormozgan University of Medical Sciences, Bandar Abbas, Iran
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Skerrett IM, Williams JB. A structural and functional comparison of gap junction channels composed of connexins and innexins. Dev Neurobiol 2017; 77:522-547. [PMID: 27582044 PMCID: PMC5412853 DOI: 10.1002/dneu.22447] [Citation(s) in RCA: 55] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2016] [Revised: 08/12/2016] [Accepted: 08/30/2016] [Indexed: 02/03/2023]
Abstract
Methods such as electron microscopy and electrophysiology led to the understanding that gap junctions were dense arrays of channels connecting the intracellular environments within almost all animal tissues. The characteristics of gap junctions were remarkably similar in preparations from phylogenetically diverse animals such as cnidarians and chordates. Although few studies directly compared them, minor differences were noted between gap junctions of vertebrates and invertebrates. For instance, a slightly wider gap was noted between cells of invertebrates and the spacing between invertebrate channels was generally greater. Connexins were identified as the structural component of vertebrate junctions in the 1980s and innexins as the structural component of pre-chordate junctions in the 1990s. Despite a lack of similarity in gene sequence, connexins and innexins are remarkably similar. Innexins and connexins have the same membrane topology and form intercellular channels that play a variety of tissue- and temporally specific roles. Both protein types oligomerize to form large aqueous channels that allow the passage of ions and small metabolites and are regulated by factors such as pH, calcium, and voltage. Much more is currently known about the structure, function, and structure-function relationships of connexins. However, the innexin field is expanding. Greater knowledge of innexin channels will permit more detailed comparisons with their connexin-based counterparts, and provide insight into the ubiquitous yet specific roles of gap junctions. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 522-547, 2017.
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Affiliation(s)
- I Martha Skerrett
- Biology Department, SUNY Buffalo State, 1300 Elmwood Ave, Buffalo, New York, 14222
| | - Jamal B Williams
- Biology Department, SUNY Buffalo State, 1300 Elmwood Ave, Buffalo, New York, 14222
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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: 12] [Impact Index Per Article: 1.7] [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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Da Y, Wang W, Liu Z, Chen H, Di L, Previch L, Chen Z. Aberrant trafficking of a Leu89Pro connexin32 mutant associated with X-linked dominant Charcot-Marie-Tooth disease. Neurol Res 2016; 38:897-902. [PMID: 27367520 DOI: 10.1080/01616412.2016.1204494] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
OBJECTIVE To determine the functional abnormalities of the Leu89Pro mutation in connexin32 (CX32), which we have previously reported is present within an X-linked dominant Charcot-Marie-Tooth disease family. In this family, male patients were moderately to severely affected. METHODS We performed immunofluorescence to investigate whether the Leu89Pro CX32 protein was transported to the cell membrane in HeLa and Schwann cells. First, we constructed the eukaryotic express plasmids expressing CX32 (wild-type or Leu89Pro) and enhanced green fluorescent protein by the gene recombination technology. Then the recombinant plasmids were transiently transfected into communication-incompetent HeLa cells and human Schwann cells by the lipofectamine method. Later, we double-labeled cells for both CX32 and markers of the ER (calnexin) or the Golgi (58-kDa protein) at 24 h or 48 h. The images were collected using a Leica TCS SP5 II confocal microscope. RESULTS The mutant CX32 protein was localized in the endoplasmic reticulum and failed to reach the cell membrane to form gap junctions. CONCLUSION Our results indicated that the Leu89Pro substitution in the second transmembrane domain of CX32 disrupts the trafficking of the protein, inhibiting the assembly of CX32 gap junctions, which in turn may result in peripheral neuropathy. This functional abnormality may explain the moderate to severe phenotype seen in Leu89Pro patients, and as such represents a promising therapeutic target in the treatment of this subset of CMTX patients.
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Affiliation(s)
- Yuwei Da
- a Department of Neurology , Xuan Wu Hospital, Capital Medical University , Beijing , China
| | - Wei Wang
- a Department of Neurology , Xuan Wu Hospital, Capital Medical University , Beijing , China
| | - Zhongfeng Liu
- b Cell Therapy Center , Xuan Wu Hospital, Capital Medical University, and Key Laboratory of Neurodegeneration, Ministry of Education , Beijing , China
| | - Hai Chen
- a Department of Neurology , Xuan Wu Hospital, Capital Medical University , Beijing , China
| | - Li Di
- a Department of Neurology , Xuan Wu Hospital, Capital Medical University , Beijing , China
| | - Lauren Previch
- c Department of Neurological Surgery , Wayne State University School of Medicine , Taylor , MI, USA
| | - Zhiguo Chen
- b Cell Therapy Center , Xuan Wu Hospital, Capital Medical University, and Key Laboratory of Neurodegeneration, Ministry of Education , Beijing , China
- d Center of Neural Injury and Repair, Beijing Institute for Brain Disorders , Beijing , China
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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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Characterization of a novel water pocket inside the human Cx26 hemichannel structure. Biophys J 2015; 107:599-612. [PMID: 25099799 DOI: 10.1016/j.bpj.2014.05.037] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2013] [Revised: 05/12/2014] [Accepted: 05/14/2014] [Indexed: 01/09/2023] Open
Abstract
Connexins (Cxs) are a family of vertebrate proteins constituents of gap junction channels (GJCs) that connect the cytoplasm of adjacent cells by the end-to-end docking of two Cx hemichannels. The intercellular transfer through GJCs occurs by passive diffusion allowing the exchange of water, ions, and small molecules. Despite the broad interest to understand, at the molecular level, the functional state of Cx-based channels, there are still many unanswered questions regarding structure-function relationships, perm-selectivity, and gating mechanisms. In particular, the ordering, structure, and dynamics of water inside Cx GJCs and hemichannels remains largely unexplored. In this work, we describe the identification and characterization of a believed novel water pocket-termed the IC pocket-located in-between the four transmembrane helices of each human Cx26 (hCx26) monomer at the intracellular (IC) side. Using molecular dynamics (MD) simulations to characterize hCx26 internal water structure and dynamics, six IC pockets were identified per hemichannel. A detailed characterization of the dynamics and ordering of water including conformational variability of residues forming the IC pockets, together with multiple sequence alignments, allowed us to propose a functional role for this cavity. An in vitro assessment of tracer uptake suggests that the IC pocket residue Arg-143 plays an essential role on the modulation of the hCx26 hemichannel permeability.
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Brennan MJ, Karcz J, Vaughn NR, Woolwine-Cunningham Y, DePriest AD, Escalona Y, Perez-Acle T, Skerrett IM. Tryptophan Scanning Reveals Dense Packing of Connexin Transmembrane Domains in Gap Junction Channels Composed of Connexin32. J Biol Chem 2015; 290:17074-84. [PMID: 25969535 PMCID: PMC4498046 DOI: 10.1074/jbc.m115.650747] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2015] [Revised: 04/29/2015] [Indexed: 11/06/2022] Open
Abstract
Tryptophan was substituted for residues in all four transmembrane domains of connexin32. Function was assayed using dual cell two-electrode voltage clamp after expression in Xenopus oocytes. Tryptophan substitution was poorly tolerated in all domains, with the greatest impact in TM1 and TM4. For instance, in TM1, 15 substitutions were made, six abolished coupling and five others significantly reduced function. Only TM2 and TM3 included a distinct helical face that lacked sensitivity to tryptophan substitution. Results were visualized on a comparative model of Cx32 hemichannel. In this model, a region midway through the membrane appears highly sensitive to tryptophan substitution and includes residues Arg-32, Ile-33, Met-34, and Val-35. In the modeled channel, pore-facing regions of TM1 and TM2 were highly sensitive to tryptophan substitution, whereas the lipid-facing regions of TM3 and TM4 were variably tolerant. Residues facing a putative intracellular water pocket (the IC pocket) were also highly sensitive to tryptophan substitution. Although future studies will be required to separate trafficking-defective mutants from those that alter channel function, a subset of interactions important for voltage gating was identified. Interactions important for voltage gating occurred mainly in the mid-region of the channel and focused on TM1. To determine whether results could be extrapolated to other connexins, TM1 of Cx43 was scanned revealing similar but not identical sensitivity to TM1 of Cx32.
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Affiliation(s)
- Matthew J Brennan
- From the Biology Department, State University of New York Buffalo State, Buffalo, New York 14222
| | - Jennifer Karcz
- From the Biology Department, State University of New York Buffalo State, Buffalo, New York 14222
| | - Nicholas R Vaughn
- From the Biology Department, State University of New York Buffalo State, Buffalo, New York 14222
| | - Yvonne Woolwine-Cunningham
- the Clinical and Translational Research Center, State University of New York at Buffalo, Buffalo, New York 14214
| | - Adam D DePriest
- the Department of Cancer Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263
| | - Yerko Escalona
- the Computational Biology Lab, Fundación Ciencia and Vida, 7780344 Santiago, Chile, and the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, 2360102 Valparaíso, Chile
| | - Tomas Perez-Acle
- the Computational Biology Lab, Fundación Ciencia and Vida, 7780344 Santiago, Chile, and the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, 2360102 Valparaíso, Chile
| | - I Martha Skerrett
- From the Biology Department, State University of New York Buffalo State, Buffalo, New York 14222,
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Seki A, Nishii K, Hagiwara N. Gap junctional regulation of pressure, fluid force, and electrical fields in the epigenetics of cardiac morphogenesis and remodeling. Life Sci 2014; 129:27-34. [PMID: 25447447 DOI: 10.1016/j.lfs.2014.10.022] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2014] [Revised: 10/16/2014] [Accepted: 10/29/2014] [Indexed: 01/25/2023]
Abstract
Epigenetic factors of pressure load, fluid force, and electrical fields that occur during cardiac contraction affect cardiac development, morphology, function, and pathogenesis. These factors are orchestrated by intercellular communication mediated by gap junctions, which synchronize action potentials and second messengers. Misregulation of the gap junction protein connexin (Cx) alters cardiogenesis, and can be a pathogenic factor causing cardiac conduction disturbance, fatal arrhythmia, and cardiac remodeling in disease states such as hypertension and ischemia. Changes in Cx expression can occur even when the DNA sequence of the Cx gene itself is unaltered. Posttranslational modifications might reduce arrhythmogenic substrates, improve cardiac function, and promote remodeling in a diseased heart. In this review, we discuss the epigenetic features of gap junctions that regulate cardiac morphology and remodeling. We further discuss potential clinical applications of current knowledge of the structure and function of gap junctions.
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Affiliation(s)
- Akiko Seki
- Department of Cardiology, Tokyo Women's Medical University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan; Support Center for Women Health Care Professionals and Researchers, Tokyo Women's Medical University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan.
| | - Kiyomasa Nishii
- Department of Anatomy and Neurobiology, National Defense Medical College, 3-2 Namiki, Tokorozawa, Saitama 359-8513, Japan
| | - Nobuhisa Hagiwara
- Department of Cardiology, Tokyo Women's Medical University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan
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De Bock M, Kerrebrouck M, Wang N, Leybaert L. Neurological manifestations of oculodentodigital dysplasia: a Cx43 channelopathy of the central nervous system? Front Pharmacol 2013; 4:120. [PMID: 24133447 PMCID: PMC3783840 DOI: 10.3389/fphar.2013.00120] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/13/2013] [Accepted: 09/02/2013] [Indexed: 12/30/2022] Open
Abstract
The coordination of tissue function is mediated by gap junctions (GJs) that enable direct cell–cell transfer of metabolic and electric signals. GJs are formed by connexins of which Cx43 is most widespread in the human body. In the brain, Cx43 GJs are mostly found in astroglia where they coordinate the propagation of Ca2+ waves, spatial K+ buffering, and distribution of glucose. Beyond its role in direct intercellular communication, Cx43 also forms unapposed, non-junctional hemichannels in the plasma membrane of glial cells. These allow the passage of several neuro- and gliotransmitters that may, combined with downstream paracrine signaling, complement direct GJ communication among glial cells and sustain glial-neuronal signaling. Mutations in the GJA1 gene encoding Cx43 have been identified in a rare, mostly autosomal dominant syndrome called oculodentodigital dysplasia (ODDD). ODDD patients display a pleiotropic phenotype reflected by eye, hand, teeth, and foot abnormalities, as well as craniofacial and bone malformations. Remarkably, neurological symptoms such as dysarthria, neurogenic bladder (manifested as urinary incontinence), spasticity or muscle weakness, ataxia, and epilepsy are other prominent features observed in ODDD patients. Over 10 mutations detected in patients diagnosed with neurological disorders are associated with altered functionality of Cx43 GJs/hemichannels, but the link between ODDD-related abnormal channel activities and neurologic phenotype is still elusive. Here, we present an overview on the nature of the mutants conveying structural and functional changes of Cx43 channels and discuss available evidence for aberrant Cx43 GJ and hemichannel function. In a final step, we examine the possibilities of how channel dysfunction may lead to some of the neurological manifestations of ODDD.
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Affiliation(s)
- Marijke De Bock
- Physiology Group, Department of Basic Medical Sciences, Ghent University Ghent, Belgium
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13
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Riquelme MA, Kar R, Gu S, Jiang JX. Antibodies targeting extracellular domain of connexins for studies of hemichannels. Neuropharmacology 2013; 75:525-32. [PMID: 23499293 DOI: 10.1016/j.neuropharm.2013.02.021] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2012] [Revised: 02/25/2013] [Accepted: 02/27/2013] [Indexed: 01/11/2023]
Abstract
Hemichannels are transmembrane channels composed of either a connexin or pannexin hexamer. The docking of the extracellular domains of connexin hemichannels contributed by neighboring cells forms a gap junction channel that joins the cytoplasm of adjacent cells. Connexins are expressed ubiquitously in different organs, but some subtypes are expressed exclusively in certain tissues and tumors. Both gap junction channels and hemichannels participate in diverse physiological and pathological responses. However, the lack of specific reagents that inhibit only gap junction channels or hemichannels is a challenge that makes it different to discern the specific roles of either channel. Fortunately, the available information regarding the connexin sequence, secondary and tertiary structure, and their biochemical and physiological properties permits the development of strategies to block exclusively the hemichannel activity exclusively, with no effect on gap junction activity. This task is accomplished through the use of specifics antibodies that target the extracellular sites of desired connexin subtype. However, the underlying mechanism of how antibodies targeting extracellular connexin epitopes actually inhibit hemichannels remains unknown. Although these antibodies are being used for detecting and blocking of hemichannels in normal and tumor cells, they can also be potentially used for tissue-specific treatment and drug delivery in clinical applications. In this article, we will first review the literature concerning the structure of connexins and the unique properties of extracellular loop domains of the connexins. Furthermore, we will discuss briefly the development of connexin (Cx) 43(E2) antibody, a specific antibody which detects the second extracellular loop of Cx43 and specifically prevents the opening of Cx43 hemichannels. We will then summarize the reported studies of specific reagents used for the inhibition of connexin hemichannels including antibodies developed against extracellular loop domains. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.
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Affiliation(s)
- Manuel A Riquelme
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
| | - Rekha Kar
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
| | - Sumin Gu
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
| | - Jean X Jiang
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA.
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14
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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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15
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Kronengold J, Srinivas M, Verselis VK. The N-terminal half of the connexin protein contains the core elements of the pore and voltage gates. J Membr Biol 2012; 245:453-63. [PMID: 22825713 PMCID: PMC3735448 DOI: 10.1007/s00232-012-9457-z] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2012] [Accepted: 06/20/2012] [Indexed: 11/21/2022]
Abstract
Connexins form channels with large aqueous pores that mediate fluxes of inorganic ions and biological signaling molecules. Studies aimed at identifying the connexin pore now include a crystal structure that provides details of putative pore-lining residues that need to be verified using independent biophysical approaches. Here we extended our initial cysteine-scanning studies of the TM1/E1 region of Cx46 hemichannels to include TM2 and TM3 transmembrane segments. No evidence of reactivity was observed in either TM2 or TM3 probed with small or large thiol-modifying reagents. Several identified pore residues in E1 of Cx46 have been verified in different Cx isoforms. Use of variety of thiol reagents indicates that the connexin hemichannel pore is large and flexible enough, at least in the extracellular part of the pore funnel, to accommodate uncommonly large side chains. We also find that that gating characteristics are largely determined by the same domains that constitute the pore. These data indicate that biophysical and structural studies are converging towards a view that the N-terminal half of the Cx protein contains the principal components of the pore and gating elements, with NT, TM1 and E1 forming the pore funnel.
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Affiliation(s)
- Jack Kronengold
- Department of Pharmacology, Yale University School of Medicine, New Haven, CT 06520, USA
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16
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Xu J, Nicholson BJ. The role of connexins in ear and skin physiology - functional insights from disease-associated mutations. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:167-78. [PMID: 22796187 DOI: 10.1016/j.bbamem.2012.06.024] [Citation(s) in RCA: 83] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/04/2012] [Revised: 06/23/2012] [Accepted: 06/29/2012] [Indexed: 12/20/2022]
Abstract
Defects in several different connexins have been associated with several different diseases. The most common of these is deafness, where a few mutations in connexin (Cx) 26 have been found to contribute to over 50% of the incidence of non-syndromic deafness in different human populations. Other mutations in Cx26 or Cx30 have also been associated with various skin phenotypes linked to deafness (palmoplanta keratoderma, Bart-Pumphrey syndrome, Vohwinkel syndrome, keratitis-ichthyosis-deafness syndrome, etc.). The large array of disease mutants offers unique opportunities to gain insights into the underlying function of gap junction proteins and their channels in the normal and pathogenic physiologies of the cochlea and epidermis. This review focuses on those mutants where the impact on channel function has been assessed, and correlated with the disease phenotype, or organ function in knock-out mouse models. These approaches have provided evidence supporting a role of gap junctions and hemichannels in K(+) removal and recycling in the ear, as well as possible roles for nutrient passage, in the cochlea. In contrast, increases in hemichannel opening leading to increased cell death, were associated with several keratitis-ichthyosis-deafness syndrome skin disease/hearing mutants. In addition to providing clues for therapeutic strategies, these findings allow us to better understand the specific functions of connexin channels that are important for normal tissue function. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.
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Affiliation(s)
- Ji Xu
- Department of Physiology, University of California, Los Angeles, CA 90095, USA
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17
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Prochnow N, Hoffmann S, Dermietzel R, Zoidl G. Replacement of a single cysteine in the fourth transmembrane region of zebrafish pannexin 1 alters hemichannel gating behavior. Exp Brain Res 2012; 199:255-64. [PMID: 19701745 DOI: 10.1007/s00221-009-1957-4] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2008] [Accepted: 07/18/2009] [Indexed: 01/09/2023]
Abstract
Pannexin1 (Panx1) is a novel candidate for an electrical synapse protein in the retina. At present Panx1 is considered to function as a hemichannel. Since information about the gating properties of Panx1 channels to date rely on blocker pharmacology, we have begun to establish a structural context of channel function starting with site directed mutagenesis of cysteine residues in transmembrane domains of Panx1. Dye uptake and whole cell voltage clamp recordings of transfected N2a cells demonstrate that zfPanx1 forms voltage activated hemichannels with a large unitary conductance in vitro. The function of this channel was significantly reduced following mutation of a single cysteine residue (C282W) in the fourth transmembrane region. This result suggests a role of this domain in gating of the Panx1 hemichannel.
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Affiliation(s)
- Nora Prochnow
- Department of Neuroanatomy and Molecular Brain Research, Ruhr-University Bochum, Bochum 44780, Germany.
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18
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Depriest A, Phelan P, Martha Skerrett I. Tryptophan scanning mutagenesis of the first transmembrane domain of the innexin Shaking-B(Lethal). Biophys J 2011; 101:2408-16. [PMID: 22098739 DOI: 10.1016/j.bpj.2011.10.004] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2011] [Revised: 10/03/2011] [Accepted: 10/06/2011] [Indexed: 12/25/2022] Open
Abstract
The channel proteins of gap junctions are encoded by two distinct gene families, connexins, which are exclusive to chordates, and innexins/pannexins, which are found throughout the animal kingdom. Although the relationship between the primary structure and function of the vertebrate connexins has been relatively well studied, there are, to our knowledge, no structure-function analyses of invertebrate innexins. In the first such study, we have used tryptophan scanning to probe the first transmembrane domain (M1) of the Drosophila innexin Shaking-B(Lethal), which is a component of rectifying electrical synapses in the Giant Fiber escape neural circuit. Tryptophan was substituted sequentially for 16 amino acids within M1 of Shaking-B(Lethal). Tryptophan insertion at every fourth residue (H27, T31, L35, and S39) disrupted gap junction function. The distribution of these sites is consistent with helical secondary structure and identifies the face of M1 involved in helix-helix interactions. Tryptophan substitution at several sites in M1 altered channel properties in a variety of ways. Changes in sensitivity to transjunctional voltage (Vj) were common and one mutation (S39W) induced sensitivity to transmembrane voltage (Vm). In addition, several mutations induced hemichannel activity. These changes are similar to those observed after substitutions within the transmembrane domains of connexins.
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Affiliation(s)
- Adam Depriest
- Biology Department, Buffalo State College, Buffalo, New York, USA
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19
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Maeda S, Tsukihara T. Structure of the gap junction channel and its implications for its biological functions. Cell Mol Life Sci 2011; 68:1115-29. [PMID: 20960023 PMCID: PMC11114897 DOI: 10.1007/s00018-010-0551-z] [Citation(s) in RCA: 100] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2010] [Revised: 09/28/2010] [Accepted: 09/30/2010] [Indexed: 12/16/2022]
Abstract
Gap junctions consist of arrays of intercellular channels composed of integral membrane proteins called connexin in vertebrates. Gap junction channels regulate the passage of ions and biological molecules between adjacent cells and, therefore, are critically important in many biological activities, including development, differentiation, neural activity, and immune response. Mutations in connexin genes are associated with several human diseases, such as neurodegenerative disease, skin disease, deafness, and developmental abnormalities. The activity of gap junction channels is regulated by the membrane voltage, intracellular microenvironment, interaction with other proteins, and phosphorylation. Each connexin channel has its own property for conductance and molecular permeability. A number of studies have tried to reveal the molecular architecture of the channel pore that should confer the connexin-specific permeability/selectivity properties and molecular basis for the gating and regulation. In this review, we give an overview of structural studies and describe the structural and functional relationship of gap junction channels.
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Affiliation(s)
- Shoji Maeda
- Institute for Protein Research, Osaka University, OLABB, 6-2-3 Furuedai, Suita, 565-0874 Japan
- Department of Life Science, University of Hyogo, 3-2-1 Koto, Kamighori, Akoh, Hyogo 678-1297 Japan
- Present Address: Paul Scherrer Institut, Biology and Chemistry OFLG 101, 5232 Villigen, Switzerland
| | - Tomitake Tsukihara
- Institute for Protein Research, Osaka University, OLABB, 6-2-3 Furuedai, Suita, 565-0874 Japan
- Department of Life Science, University of Hyogo, 3-2-1 Koto, Kamighori, Akoh, Hyogo 678-1297 Japan
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20
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Oshima A, Tani K, Toloue MM, Hiroaki Y, Smock A, Inukai S, Cone A, Nicholson BJ, Sosinsky GE, Fujiyoshi Y. Asymmetric configurations and N-terminal rearrangements in connexin26 gap junction channels. J Mol Biol 2010; 405:724-35. [PMID: 21094651 DOI: 10.1016/j.jmb.2010.10.032] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2010] [Revised: 10/13/2010] [Accepted: 10/19/2010] [Indexed: 10/18/2022]
Abstract
Gap junction channels are unique in that they possess multiple mechanisms for channel closure, several of which involve the N terminus as a key component in gating, and possibly assembly. Here, we present electron crystallographic structures of a mutant human connexin26 (Cx26M34A) and an N-terminal deletion of this mutant (Cx26M34Adel2-7) at 6-Å and 10-Å resolutions, respectively. The three-dimensional map of Cx26M34A was improved by data from 60° tilt images and revealed a breakdown of the hexagonal symmetry in a connexin hemichannel, particularly in the cytoplasmic domain regions at the ends of the transmembrane helices. The Cx26M34A structure contained an asymmetric density in the channel vestibule ("plug") that was decreased in the Cx26M34Adel2-7 structure, indicating that the N terminus significantly contributes to form this plug feature. Functional analysis of the Cx26M34A channels revealed that these channels are predominantly closed, with the residual electrical conductance showing normal voltage gating. N-terminal deletion mutants with and without the M34A mutation showed no electrical activity in paired Xenopus oocytes and significantly decreased dye permeability in HeLa cells. Comparing this closed structure with the recently published X-ray structure of wild-type Cx26, which is proposed to be in an open state, revealed a radial outward shift in the transmembrane helices in the closed state, presumably to accommodate the N-terminal plug occluding the pore. Because both Cx26del2-7 and Cx26M34Adel2-7 channels are closed, the N terminus appears to have a prominent role in stabilizing the open configuration.
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Affiliation(s)
- Atsunori Oshima
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
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21
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Abstract
Vertebrates express two families of gap junction proteins: the well-characterized connexins and the pannexins. In contrast to connexins, pannexins do not appear to form gap junction channels but instead function as unpaired membrane channels. Pannexins have no sequence homology to connexins but are distantly related to the invertebrate gap junction proteins, innexins. Despite the sequence diversity, pannexins and connexins form channels with similar permeability properties and exhibit similar membrane topology, with two extracellular loops, four transmembrane (TM) segments, and cytoplasmic localization of amino and carboxy termini. To test whether the similarities extend to the pore structure of the channels, pannexin 1 (Panx1) was subjected to analysis with the substituted cysteine accessibility method (SCAM). The thiol reagents maleimidobutyryl-biocytin and 2-trimethylammonioethyl-methanethiosulfonate reacted with several cysteines positioned in the external portion of the first TM segment (TM1) and the first extracellular loop. These data suggest that portions of TM1 and the first extracellular loop line the outer part of the pore of Panx1 channels. In this aspect, the pore structures of Panx1 and connexin channels are similar. However, although the inner part of the pore is lined by amino-terminal amino acids in connexin channels, thiol modification was detected in carboxyterminal amino acids in Panx1 channels by SCAM analysis. Thus, it appears that the inner portion of the pores of Panx1 and connexin channels may be distinct.
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Affiliation(s)
- Junjie Wang
- Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, FL 33136, USA
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22
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Cell membrane permeabilization via connexin hemichannels in living and dying cells. Exp Cell Res 2010; 316:2377-89. [DOI: 10.1016/j.yexcr.2010.05.026] [Citation(s) in RCA: 143] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2010] [Revised: 05/20/2010] [Accepted: 05/21/2010] [Indexed: 12/31/2022]
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23
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Chi NC, Bussen M, Brand-Arzamendi K, Ding C, Olgin JE, Shaw RM, Martin GR, Stainier DYR. Cardiac conduction is required to preserve cardiac chamber morphology. Proc Natl Acad Sci U S A 2010; 107:14662-7. [PMID: 20675583 PMCID: PMC2930423 DOI: 10.1073/pnas.0909432107] [Citation(s) in RCA: 95] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
Electrical cardiac forces have been previously hypothesized to play a significant role in cardiac morphogenesis and remodeling. In response to electrical forces, cultured cardiomyocytes rearrange their cytoskeletal structure and modify their gene expression profile. To translate such in vitro data to the intact heart, we used a collection of zebrafish cardiac mutants and transgenics to investigate whether cardiac conduction could influence in vivo cardiac morphogenesis independent of contractile forces. We show that the cardiac mutant dco(s226) develops heart failure and interrupted cardiac morphogenesis following uncoordinated ventricular contraction. Using in vivo optical mapping/calcium imaging, we determined that the dco cardiac phenotype was primarily due to aberrant ventricular conduction. Because cardiac contraction and intracardiac hemodynamic forces can also influence cardiac development, we further analyzed the dco phenotype in noncontractile hearts and observed that disorganized ventricular conduction could affect cardiomyocyte morphology and subsequent heart morphogenesis in the absence of contraction or flow. By positional cloning, we found that dco encodes Gja3/Cx46, a gap junction protein not previously implicated in heart formation or function. Detailed analysis of the mouse Cx46 mutant revealed the presence of cardiac conduction defects frequently associated with human heart failure. Overall, these in vivo studies indicate that cardiac electrical forces are required to preserve cardiac chamber morphology and may act as a key epigenetic factor in cardiac remodeling.
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MESH Headings
- Amino Acid Sequence
- Animals
- Animals, Genetically Modified
- Connexins/classification
- Connexins/genetics
- Connexins/metabolism
- Electrocardiography
- Embryo, Mammalian/embryology
- Embryo, Mammalian/metabolism
- Embryo, Mammalian/physiology
- Embryo, Nonmammalian/embryology
- Embryo, Nonmammalian/metabolism
- Embryo, Nonmammalian/physiology
- Gene Knockdown Techniques
- Green Fluorescent Proteins/genetics
- Green Fluorescent Proteins/metabolism
- Heart/embryology
- Heart/physiology
- Heart Conduction System/physiology
- In Situ Hybridization
- Mice
- Mice, Knockout
- Microscopy, Confocal
- Molecular Sequence Data
- Mutation
- Myocardium/metabolism
- Phylogeny
- Sequence Homology, Amino Acid
- Zebrafish/embryology
- Zebrafish/genetics
- Zebrafish/metabolism
- Zebrafish Proteins/genetics
- Zebrafish Proteins/metabolism
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Affiliation(s)
- Neil C. Chi
- Departments of Biochemistry and Biophysics, Programs in Developmental Biology, Genetics and Human Genetics
- Medicine and
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158; and
- Department of Medicine, Division of Cardiology, University of California at San Diego, La Jolla, CA 92093-0613J
| | | | - Koroboshka Brand-Arzamendi
- Departments of Biochemistry and Biophysics, Programs in Developmental Biology, Genetics and Human Genetics
| | | | - Jeffrey E. Olgin
- Medicine and
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158; and
| | - Robin M. Shaw
- Medicine and
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158; and
| | - Gail R. Martin
- Anatomy, and
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158; and
| | - Didier Y. R. Stainier
- Departments of Biochemistry and Biophysics, Programs in Developmental Biology, Genetics and Human Genetics
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158; and
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24
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Ambrosi C, Boassa D, Pranskevich J, Smock A, Oshima A, Xu J, Nicholson BJ, Sosinsky GE. Analysis of four connexin26 mutant gap junctions and hemichannels reveals variations in hexamer stability. Biophys J 2010; 98:1809-19. [PMID: 20441744 DOI: 10.1016/j.bpj.2010.01.019] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2009] [Revised: 12/15/2009] [Accepted: 01/04/2010] [Indexed: 11/19/2022] Open
Abstract
Connexin26 is a ubiquitous gap junction protein that serves critical homeostatic functions. Four single-site mutations found in the transmembrane helices (M1-M4) cause different types of dysfunctional channels: 1), Cx26T135A in M3 produces a closed channel; 2), Cx26M34A in M1 severely decreases channel activity; 3), Cx26P87L in M2 has been implicated in defective channel gating; and 4), Cx26V84L in M2, a nonsyndromic deafness mutant, retains normal dye coupling and electrophysiological properties but is deficient in IP(3) transfer. These mutations do not affect Cx26 trafficking in mammalian cells, and make normal-appearing channels in baculovirus-infected Sf9 membranes when imaged by negative stain electron microscopy. Upon dodecylmaltoside solubilization of the membrane fraction, Cx26M34A and Cx26V84L are stable as hexamers or dodecamers, but Cx26T135A and Cx26P87L oligomers are not. This instability is also found in Cx26T135A and Cx26P87L hemichannels isolated from mammalian cells. In this work, coexpression of both wild-type Cx26 and Cx26P87L in Sf9 cells rescued P87L hexamer stability. Similarly, in paired Xenopus oocytes, coexpression with wild-type restored function. In contrast, the stability of Cx26T135A hemichannels could not be rescued by coexpression with WT. Thus, T135 and P87 residues are in positions that are important for oligomer stability and can affect gap junction gating.
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Affiliation(s)
- Cinzia Ambrosi
- National Center for Microscopy and Imaging Research, Center for Research in Biological Systems, University of California, San Diego, La Jolla, California, USA
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25
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Affiliation(s)
- Andrew L Harris
- Department of Pharmacology and Physiology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, NJ 07103, USA.
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26
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Tang Q, Dowd TL, Verselis VK, Bargiello TA. Conformational changes in a pore-forming region underlie voltage-dependent "loop gating" of an unapposed connexin hemichannel. J Gen Physiol 2009; 133:555-70. [PMID: 19468074 PMCID: PMC2713147 DOI: 10.1085/jgp.200910207] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2009] [Accepted: 03/26/2009] [Indexed: 12/27/2022] Open
Abstract
The structure of the pore is critical to understanding the molecular mechanisms underlying selective permeation and voltage-dependent gating of channels formed by the connexin gene family. Here, we describe a portion of the pore structure of unapposed hemichannels formed by a Cx32 chimera, Cx32*Cx43E1, in which the first extracellular loop (E1) of Cx32 is replaced with the E1 of Cx43. Cysteine substitutions of two residues, V38 and G45, located in the vicinity of the border of the first transmembrane (TM) domain (TM1) and E1 are shown to react with the thiol modification reagent, MTSEA-biotin-X, when the channel resides in the open state. Cysteine substitutions of flanking residues A40 and A43 do not react with MTSEA-biotin-X when the channel resides in the open state, but they react with dibromobimane when the unapposed hemichannels are closed by the voltage-dependent "loop-gating" mechanism. Cysteine substitutions of residues V37 and A39 do not appear to be modified in either state. Furthermore, we demonstrate that A43C channels form a high affinity Cd2+ site that locks the channel in the loop-gated closed state. Biochemical assays demonstrate that A43C can also form disulfide bonds when oocytes are cultured under conditions that favor channel closure. A40C channels are also sensitive to micromolar Cd2+ concentrations when closed by loop gating, but with substantially lower affinity than A43C. We propose that the voltage-dependent loop-gating mechanism for Cx32*Cx43E1 unapposed hemichannels involves a conformational change in the TM1/E1 region that involves a rotation of TM1 and an inward tilt of either each of the six connexin subunits or TM1 domains.
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Affiliation(s)
- Qingxiu Tang
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461
| | - Terry L. Dowd
- Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn, NY 11210
| | - Vytas K. Verselis
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461
| | - Thaddeus A. Bargiello
- Dominic P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461
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27
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Structure of the connexin 26 gap junction channel at 3.5 Å resolution. Nature 2009; 458:597-602. [DOI: 10.1038/nature07869] [Citation(s) in RCA: 559] [Impact Index Per Article: 37.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2008] [Accepted: 02/09/2009] [Indexed: 11/09/2022]
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28
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Martínez AD, Acuña R, Figueroa V, Maripillan J, Nicholson B. Gap-junction channels dysfunction in deafness and hearing loss. Antioxid Redox Signal 2009; 11:309-22. [PMID: 18837651 PMCID: PMC2673109 DOI: 10.1089/ars.2008.2138] [Citation(s) in RCA: 105] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Gap-junction channels connect the cytoplasm of adjacent cells, allowing the diffusion of ions and small metabolites. They are formed at the appositional plasma membranes by a family of related proteins named connexins. Mutations in connexins 26, 31, 30, 32, and 43 have been associated with nonsyndromic or syndromic deafness. The majority of these mutations are inherited in an autosomal recessive manner, but a few of them have been associated with dominantly inherited hearing loss. Mutations in the connexin26 gene (GJB2) are the most common cause of genetic deafness. This review summarizes the most relevant and recent information about different mutations in connexin genes found in human patients, with emphasis on GJB2. The possible effects of the mutations on channel expression and function are discussed, in addition to their possible physiologic consequences for inner ear physiology. Finally, we propose that connexin channels (gap junctions and hemichannels) may be targets for age-related hearing loss induced by oxidative damage.
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Affiliation(s)
- Agustín D Martínez
- Centro de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso, Chile.
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29
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Fenwick A, Richardson RJ, Butterworth J, Barron MJ, Dixon MJ. Novel mutations in GJA1 cause oculodentodigital syndrome. J Dent Res 2008; 87:1021-6. [PMID: 18946008 DOI: 10.1177/154405910808701108] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Oculodentodigital syndrome (ODD) is a rare, usually autosomal-dominant disorder that is characterized by developmental abnormalities of the face, eyes, teeth, and limbs. The most common clinical findings include a long, narrow nose, short palpebral fissures, type III syndactyly, and dental abnormalities including generalized microdontia and enamel hypoplasia. Recently, it has been shown that mutations in the gene GJA1, which encodes the gap junction protein connexin 43, underlie oculodentodigital syndrome. Gap junction communication between adjacent cells is known to be vital during embryogenesis and subsequently for normal tissue homeostasis. Here, we report 8 missense mutations in the coding region of GJA1, 6 of which have not been described previously, in ten unrelated families diagnosed with ODD. In addition, immunofluorescence analyses of a developmental series of mouse embryos and adult tissue demonstrates a strong correlation between the sites of connexin 43 expression and the clinical phenotype displayed by individuals affected by ODD.
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Affiliation(s)
- A Fenwick
- Faculty of Life Sciences and Dental School, Michael Smith Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK
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30
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Toloue MM, Woolwine Y, Karcz JA, Kasperek EM, Nicholson BJ, Skerrett IM. Site-directed mutagenesis reveals putative regions of protein interaction within the transmembrane domains of connexins. ACTA ACUST UNITED AC 2008; 15:95-105. [PMID: 18649182 DOI: 10.1080/15419060802013463] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Abstract
Through cysteine-scanning mutagenesis, the authors have compared sites within the transmembrane domains of two connexins, one from the alpha-class (Cx50) and one from the beta-class (Cx32), where amino acid substitution disrupts the function of gap junction channels. In Cx32, 11 sites resulted in no channel function, or an aberrant voltage gating phenotype referred to as "reverse gating," whereas in Cx50, 7 such sites were identified. In both connexins, the sites lie along specific faces of transmembrane helices, suggesting that these may be sites of transmembrane domain interactions. In Cx32, one broad face of the M1 transmembrane domain and a narrower, polar face of M3 were identified, including one site that was shown to come into close apposition with M4 in the closed state. In Cx50, the same face of M3 was identified, but sensitive sites in M1 differed from Cx32. Many fewer sites in M1 disrupted channel function in Cx50, and those that did were on a different helical face to the sensitive sites in Cx32. A more in depth study of two sites in M1 and M2 of Cx32 showed that side-chain length or branching are important for maintenance of normal channel behavior, consistent with this being a site of transmembrane domain interaction.
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Affiliation(s)
- M M Toloue
- Department of Biochemistry, University of Texas Health Science Center at San Antonio, San Antonio, Texas, USA
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31
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Pantano S, Zonta F, Mammano F. A fully atomistic model of the Cx32 connexon. PLoS One 2008; 3:e2614. [PMID: 18648547 PMCID: PMC2481295 DOI: 10.1371/journal.pone.0002614] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2008] [Accepted: 05/10/2008] [Indexed: 11/18/2022] Open
Abstract
Connexins are plasma membrane proteins that associate in hexameric complexes to form channels named connexons. Two connexons in neighboring cells may dock to form a "gap junction" channel, i.e. an intercellular conduit that permits the direct exchange of solutes between the cytoplasm of adjacent cells and thus mediate cell-cell ion and metabolic signaling. The lack of high resolution data for connexon structures has hampered so far the study of the structure-function relationships that link molecular effects of disease-causing mutations with their observed phenotypes. Here we present a combination of modeling techniques and molecular dynamics (MD) to infer side chain positions starting from low resolution structures containing only C alpha atoms. We validated this procedure on the structure of the KcsA potassium channel, which is solved at atomic resolution. We then produced a fully atomistic model of a homotypic Cx32 connexon starting from a published model of the C alpha carbons arrangement for the connexin transmembrane helices, to which we added extracellular and cytoplasmic loops. To achieve structural relaxation within a realistic environment, we used MD simulations inserted in an explicit solvent-membrane context and we subsequently checked predictions of putative side chain positions and interactions in the Cx32 connexon against a vast body of experimental reports. Our results provide new mechanistic insights into the effects of numerous spontaneous mutations and their implication in connexin-related pathologies. This model constitutes a step forward towards a structurally detailed description of the gap junction architecture and provides a structural platform to plan new biochemical and biophysical experiments aimed at elucidating the structure of connexin channels and hemichannels.
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Affiliation(s)
- Sergio Pantano
- Institut Pasteur of Montevideo, Montevideo, Uruguay
- Venetian Institute of Molecular Medicine (VIMM), Padova, Italy
- Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia (CNISM), Rome, Italy
| | - Francesco Zonta
- Venetian Institute of Molecular Medicine (VIMM), Padova, Italy
- Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia (CNISM), Rome, Italy
| | - Fabio Mammano
- Venetian Institute of Molecular Medicine (VIMM), Padova, Italy
- Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia (CNISM), Rome, Italy
- Dipartimento di Fisica “G.Galilei”, Università di Padova, Padova, Italy
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32
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Oh S, Verselis VK, Bargiello TA. Charges dispersed over the permeation pathway determine the charge selectivity and conductance of a Cx32 chimeric hemichannel. J Physiol 2008; 586:2445-61. [PMID: 18372303 PMCID: PMC2464352 DOI: 10.1113/jphysiol.2008.150805] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2008] [Accepted: 03/21/2008] [Indexed: 11/08/2022] Open
Abstract
Previous studies have shown that charge substitutions in the amino terminus of a chimeric connexin, Cx32*43E1, which forms unapposed hemichannels in Xenopus oocytes, can result in a threefold difference in unitary conductance and alter the direction and amount of open channel current rectification. Here, we determine the charge selectivity of Cx32*43E1 unapposed hemichannels containing negative and/or positive charge substitutions at the 2nd, 5th and 8th positions in the N-terminus. Unlike Cx32 intercellular channels, which are weakly anion selective, the Cx32*43E1 unapposed hemichannel is moderately cation selective. Cation selectivity is maximal when the extracellular surface of the channel is exposed to low ionic strength solutions implicating a region of negative charge in the first extracellular loop of Cx43 (Cx43E1) in influencing charge selectivity analogous to that reported. Negative charge substitutions at the 2nd, 5th and 8th positions in the intracellular N-terminus substantially increase the unitary conductance and cation selectivity of the chimeric hemichannel. Positive charge substitutions at the 5th position decrease unitary conductance and produce a non-selective channel while the presence of a positive charge at the 5th position and negative charge at the 2nd results in a channel with conductance similar to the parental channel but with greater preference for cations. We demonstrate that a cysteine substitution of the 8th residue in the N-terminus can be modified by a methanthiosulphonate reagent (MTSEA-biotin-X) indicating that this residue lines the aqueous pore at the intracellular entrance of the channel. The results indicate that charge selectivity of the Cx32*43E1 hemichannel can be determined by the combined actions of charges dispersed over the permeation pathway rather than by a defined region that acts as a charge selectivity filter.
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Affiliation(s)
- Seunghoon Oh
- Department of Neuroscience, Kennedy Center, Albert Einstein College of Medicine, 1410 Pelham Parkway South, Bronx, NY 10461, USA
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33
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Gap junction channel structure in the early 21st century: facts and fantasies. Curr Opin Cell Biol 2007; 19:521-8. [PMID: 17945477 DOI: 10.1016/j.ceb.2007.09.001] [Citation(s) in RCA: 97] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2007] [Accepted: 09/05/2007] [Indexed: 02/04/2023]
Abstract
Gap junction channels connect the cytoplasms of adjacent cells through the end-to-end docking of single-membrane structures called connexons, formed by a ring of six connexin monomers. Each monomer contains four transmembrane alpha-helices, for a total of 24 alpha-helices in a connexon. The fundamental structure of the connexon pore is probably similar in unpaired connexons and junctional channels, and for channels formed by different connexin isoforms. Nevertheless, variability in results from structurally focused mutagenesis and electrophysiological studies raise uncertainty about the specific assignments of the transmembrane helices. Mapping of human mutations onto a suggested C(alpha) model predicts that mutations that disrupt helix-helix packing impair channel function. An experimentally determined structure at atomic resolution will be essential to confirm and resolve these concepts.
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34
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Oshima A, Tani K, Hiroaki Y, Fujiyoshi Y, Sosinsky GE. Three-dimensional structure of a human connexin26 gap junction channel reveals a plug in the vestibule. Proc Natl Acad Sci U S A 2007; 104:10034-9. [PMID: 17551008 PMCID: PMC1886001 DOI: 10.1073/pnas.0703704104] [Citation(s) in RCA: 148] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Connexin molecules form intercellular membrane channels facilitating electronic coupling and the passage of small molecules between adjoining cells. Connexin26 (Cx26) is the second smallest member of the gap junction protein family, and mutations in Cx26 cause certain hereditary human diseases such as skin disorders and hearing loss. Here, we report the electron crystallographic structure of a human Cx26 mutant (M34A). Although crystallization trials used hemichannel preparations, the density map revealed that two hemichannels redocked at their extracellular surfaces into full intercellular channels. These orthorhombic crystals contained two sets of symmetry-related intercellular channels within three lipid bilayers. The 3D map shows a prominent density in the pore of each hemichannel. This density contacts the innermost helices of the surrounding connexin subunits at the bottom of the vestibule. The density map suggests that physical blocking may play an important role that underlies gap junction channel regulation. Our structure allows us to suggest that the two docked hemichannels can be independent and may regulate their activity autonomously with a plug in the vestibule.
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Affiliation(s)
- Atsunori Oshima
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
- Core Research for Evolution Science and Technology (CREST), Japan Science and Technology Agency (JST), Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan; and
| | - Kazutoshi Tani
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
| | - Yoko Hiroaki
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
- Core Research for Evolution Science and Technology (CREST), Japan Science and Technology Agency (JST), Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan; and
| | - Yoshinori Fujiyoshi
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
- Core Research for Evolution Science and Technology (CREST), Japan Science and Technology Agency (JST), Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan; and
- Japan Biological Information Research Center (JBIRC), National Institute of Advanced Industrial Science and Technology (AIST), 2-41-6, Aomi, Koto-ku, Tokyo 135-0064, Japan
- To whom correspondence may be addressed. E-mail: or
| | - Gina E. Sosinsky
- National Center for Microscopy and Imaging Research, Department of Neurosciences, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0608
- To whom correspondence may be addressed. E-mail: or
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35
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McLachlan E, Manias JL, Gong XQ, Lounsbury CS, Shao Q, Bernier SM, Bai D, Laird DW. Functional characterization of oculodentodigital dysplasia-associated Cx43 mutants. ACTA ACUST UNITED AC 2007; 12:279-92. [PMID: 16531323 DOI: 10.1080/15419060500514143] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Abstract
Oculodentodigital dysplasia (ODDD) is associated with at least 28 connexin43 (Cx43) mutations. We characterized four of these mutants; Q49K, L90V, R202H, and V216L. Populations of these GFP-tagged mutants were transported to the cell surface in Cx43-negative HeLa cells and Cx43-positive NRK cells. Dual patch-clamp functional analysis in N2A cells demonstrated that channels formed by each mutant have dramatically reduced conductance. Dye-coupling analysis revealed that each mutant exhibits a dominant-negative effect on wild-type Cx43. Since ODDD patients display skeletal abnormalities, we examined the effect of three other Cx43 mutants previously shown to exert dominant-negative effects on wild-type Cx43 (G21R, G138R, and G60S) in neonatal calvarial osteoblasts. Differentiation was unaltered by expression of these mutants as alkaline phosphatase activity and extent of culture mineralization were unchanged. This suggests that loss-of-function Cx43 mutants are insufficient to deter committed osteoblasts from their normal function in vitro. Thus, we hypothesize that the bone phenotype of ODDD patients may result from disrupted gap junctional intercellular communication earlier in development or during bone remodeling.
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Affiliation(s)
- Elizabeth McLachlan
- Department of Anatomy and Cell Biology, The University of Western Ontario, London, Ontario, Canada
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36
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González D, Gómez-Hernández JM, Barrio LC. Molecular basis of voltage dependence of connexin channels: An integrative appraisal. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2007; 94:66-106. [PMID: 17470374 DOI: 10.1016/j.pbiomolbio.2007.03.007] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.
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Affiliation(s)
- Daniel González
- Research Department, Unit of Experimental Neurology, Ramón y Cajal Hospital, Carretera de Colmenar Viejo km 9, Madrid, Spain
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37
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Kovacs JA, Baker KA, Altenberg GA, Abagyan R, Yeager M. Molecular modeling and mutagenesis of gap junction channels. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2007; 94:15-28. [PMID: 17524457 PMCID: PMC2819402 DOI: 10.1016/j.pbiomolbio.2007.03.013] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Gap junction channels connect the cytoplasms of adjacent cells through the end-to-end docking of hexameric hemichannels called connexons. Each connexon is formed by a ring of 24 alpha-helices that are staggered by 30 degrees with respect to those in the apposed connexon. Current evidence suggests that the two connexons are docked by interdigitated, anti-parallel beta strands across the extracellular gap. The second extracellular loop, E2, guides selectivity in docking between connexons formed by different isoforms. There is considerably more sequence variability of the N-terminal portion of E2, suggesting that this region dictates connexon coupling. Mutagenesis, biochemical, dye-transfer and electrophysiological data, combined with computational studies, have suggested possible assignments for the four transmembrane alpha-helices within each subunit. Most current models assign M3 as the major pore-lining helix. Mapping of human mutations onto a C(alpha) model suggested that native helix packing is important for the formation of fully functional channels. Nevertheless, a mutant in which the M4 helix has been replaced with polyalanine is functional, suggesting that M4 is located on the perimeter of the channel. In spite of this substantial progress in understanding the structural biology of gap junction channels, an experimentally determined structure at atomic resolution will be essential to confirm these concepts.
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Affiliation(s)
- Julio A Kovacs
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
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38
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Harris AL. Connexin channel permeability to cytoplasmic molecules. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2007; 94:120-43. [PMID: 17470375 PMCID: PMC1995164 DOI: 10.1016/j.pbiomolbio.2007.03.011] [Citation(s) in RCA: 359] [Impact Index Per Article: 21.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Connexin channels are known to be permeable to a variety of cytoplasmic molecules. The first observation of second messenger junctional permeability, made approximately 30 years ago, sparked broad interest in gap junction channels as mediators of intercellular molecular signaling. Since then, much has been learned about the diversity of connexin channels with regard to isoform diversity, tissue and developmental distribution, modes of channel regulation, assembly, expression, biochemical modification and permeability, all of which appear to be dynamically regulated. This information has expanded the potential roles of connexin channels in development, physiology and disease, and made their elucidation much more complex--30 years ago such an orchestra of junctional dynamics was unanticipated. Only recently, however, have investigators been able to directly address, in this more complex framework, the key issue: what specific biological molecules, second messengers and others, are able to permeate the various types of connexin channels, and how well? An important related issue, given the ever-growing list of connexin-related pathologies, is how these permeabilities are altered by disease-causing connexin mutations. Together, many studies show that a variety of cytoplasmic molecules can permeate the different types of connexin channels. A few studies reveal differences in permeation by different molecules through a particular type of connexin channel, and differences in permeation by a particular molecule through different types of connexin channels. This article describes and evaluates the various methods used to obtain these data, presents an annotated compilation of the results, and discusses the findings in the context of what can be inferred about mechanism of selectivity and potential relevance to signaling. The data strongly suggest that highly specific interactions take place between connexin pores and specific biological molecular permeants, and that those interactions determine which cytoplasmic molecules can permeate and how well. At this time, the nature of those interactions is unclear. One hopes that with more detailed permeability and structural information, the specific molecular mechanisms of the selectivity can be elucidated.
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Affiliation(s)
- Andrew L Harris
- Department of Pharmacology and Physiology, New Jersey Medical School of UMDNJ, Newark, NJ 07103, USA.
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39
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Fleishman SJ, Sabag AD, Ophir E, Avraham KB, Ben-Tal N. The structural context of disease-causing mutations in gap junctions. J Biol Chem 2006; 281:28958-63. [PMID: 16864573 DOI: 10.1074/jbc.m605764200] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Gap junctions form intercellular channels that mediate metabolic and electrical signaling between neighboring cells in a tissue. Lack of an atomic resolution structure of the gap junction has made it difficult to identify interactions that stabilize its transmembrane domain. Using a recently computed model of this domain, which specifies the locations of each amino acid, we postulated the existence of several interactions and tested them experimentally. We introduced mutations within the transmembrane domain of the gap junction-forming protein connexin that were previously implicated in genetic diseases and that apparently destabilized the gap junction, as evidenced here by the absence of the protein from the sites of cell-cell apposition. The model structure helped identify positions on adjacent helices where second-site mutations restored membrane localization, revealing possible interactions between residue pairs. We thus identified two putative salt bridges and one pair involved in packing interactions in which one disease-causing mutation suppressed the effects of another. These results seem to reveal some of the physical forces that underlie the structural stability of the gap junction transmembrane domain and suggest that abrogation of such interactions bring about some of the effects of disease-causing mutations.
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Affiliation(s)
- Sarel J Fleishman
- Department of Biochemistry, George S. Wise Faculty of Life Sciences, Sackler School of Medicine, Tel-Aviv University, 69978 Ramat Aviv, Israel
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40
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Dong L, Liu X, Li H, Vertel BM, Ebihara L. Role of the N-terminus in permeability of chicken connexin45.6 gap junctional channels. J Physiol 2006; 576:787-99. [PMID: 16931554 PMCID: PMC1890425 DOI: 10.1113/jphysiol.2006.113837] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Previous studies have shown that gap junctional channels formed from the lens connexins Cx50 (or its chicken orthologue, Cx45.6) and Cx43 exhibit marked differences in transjunctional voltage gating and unitary conductance. In the present study, we used the negatively charged dye, Lucifer Yellow (LY), to examine and compare quantitative differences in dye transfer between pairs of HeLa cells stably transfected with Cx45.6 or Cx43. Our results show that Cx45.6 gap junctional channels are three times less permeable to LY than Cx43 channels. Replacement of the N-terminus of Cx45.6 with the corresponding domain of Cx43 increased LY permeability, reduced the transjunctional voltage (V(j)) gating sensitivity, and reduced the unitary conductance of Cx45.6-43N gap junctional channels. Further experiments, using a series of Alexa probes that had similar net charge but varied in size showed that the Cx45.6-43N had a significantly higher permeability for the two largest Alexa dyes than Cx45.6. These data suggest that the N-terminus plays a critical role in determining many of biophysical properties of Cx45.6 gap junctional channels, including molecular permeability and voltage gating.
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Affiliation(s)
- Lixian Dong
- Department of Physiology & Biophysics,Rosalind Franklin University of Medicine and Science, North Chicago, IL 60064, USA
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41
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Hu X, Ma M, Dahl G. Conductance of connexin hemichannels segregates with the first transmembrane segment. Biophys J 2006; 90:140-50. [PMID: 16214855 PMCID: PMC1367013 DOI: 10.1529/biophysj.105.066373] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2005] [Accepted: 09/13/2005] [Indexed: 11/18/2022] Open
Abstract
Gap junction channels are intercellular channels that mediate the gated transfer of molecules between adjacent cells. To identify the domain determining channel conductance, the first transmembrane segment (M1) was reciprocally exchanged between Cx46 and Cx32E(1)43. The resulting chimeras exhibited conductances similar to that of the respective M1 donor. Furthermore, a chimera with the carboxy-terminal half of M1 in Cx46 replaced by that of Cx32 exhibited a conductance similar to that of Cx32E(1)43, whereas the chimera with only the amino-terminal half of M1 replaced retained the unitary conductance of wild-type Cx46. Extending the M1 domain swapping to other connexins by replacing the carboxy-terminal half of M1 in Cx46 with that of Cx37 yielded a chimera channel with increased unitary conductance close to that of Cx37. Furthermore, a point mutant of Cx46, with leucine substituted by glycine in position 35, displayed a conductance much larger than that of the wild type. Thus, the M1 segment, especially the second half, contains important determinants of conductance of the connexin channel.
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Affiliation(s)
- Xinge Hu
- Department of Physiology and Biophysics, University of Miami, School of Medicine, Miami, Florida 33101, USA
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42
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Ma M, Dahl G. Cosegregation of permeability and single-channel conductance in chimeric connexins. Biophys J 2006; 90:151-63. [PMID: 16214854 PMCID: PMC1367014 DOI: 10.1529/biophysj.105.066381] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2005] [Accepted: 09/13/2005] [Indexed: 11/18/2022] Open
Abstract
The physiological function of gap junction channels goes well beyond their initially discovered role in electrical synchronization of excitable cells. In most tissues, gap junction cells facilitate the exchange of second messengers and metabolites between cells. To test which parts of the channels formed by connexins determine the exclusion limit for the transit of molecules in the size range of second messengers and metabolites a domain exchange approach was used in combination with an accessibility assay for nonelectrolytes and flux measurements. The experimental results suggest that two open hemichannel forming connexins, Cx46 and Cx32E(1)43, differ in accessibility and permeability. Sucrose is at the exclusion limit for Cx46 channels whereas sorbitol is at the exclusion limit for Cx32E(1)43 channels. In chimeras between these connexins, where the first transmembrane segment M1 is exchanged, the exclusion limits correlate with those of the M1 donor. The same segregation was found in a separate study for the unitary conductance of the channels. Thus, conductance and permeability/accessibility of the channels cosegregate with M1.
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Affiliation(s)
- Meiyun Ma
- Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33101, USA
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43
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Beahm DL, Oshima A, Gaietta GM, Hand GM, Smock AE, Zucker SN, Toloue MM, Chandrasekhar A, Nicholson BJ, Sosinsky GE. Mutation of a conserved threonine in the third transmembrane helix of alpha- and beta-connexins creates a dominant-negative closed gap junction channel. J Biol Chem 2005; 281:7994-8009. [PMID: 16407179 DOI: 10.1074/jbc.m506533200] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Single site mutations in connexins have provided insights about the influence specific amino acids have on gap junction synthesis, assembly, trafficking, and functionality. We have discovered a single point mutation that eliminates functionality without interfering with gap junction formation. The mutation occurs at a threonine residue located near the cytoplasmic end of the third transmembrane helix. This threonine is strictly conserved among members of the alpha- and beta-connexin subgroups but not the gamma-subgroup. In HeLa cells, connexin43 and connexin26 mutants are synthesized, traffic to the plasma membrane, and make gap junctions with the same overall appearance as wild type. We have isolated connexin26T135A gap junctions both from HeLa cells and baculovirus-infected insect Sf9 cells. By using cryoelectron microscopy and correlation averaging, difference images revealed a small but significant size change within the pore region and a slight rearrangement of the subunits between mutant and wild-type connexons expressed in Sf9 cells. Purified, detergent-solubilized mutant connexons contain both hexameric and partially disassembled structures, although wild-type connexons are almost all hexameric, suggesting that the three-dimensional mutant connexon is unstable. Mammalian cells expressing gap junction plaques composed of either connexin43T154A or connexin26T135A showed an absence of dye coupling. When expressed in Xenopus oocytes, these mutants, as well as a cysteine substitution mutant of connexin50 (connexin50T157C), failed to produce electrical coupling in homotypic and heteromeric pairings with wild type in a dominant-negative effect. This mutant may be useful as a tool for knocking down or knocking out connexin function in vitro or in vivo.
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Affiliation(s)
- Derek L Beahm
- Department of Biological Sciences, State University of New York, Buffalo, New York 14260, USA
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44
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Maza J, Das Sarma J, Koval M. Defining a minimal motif required to prevent connexin oligomerization in the endoplasmic reticulum. J Biol Chem 2005; 280:21115-21. [PMID: 15817491 DOI: 10.1074/jbc.m412612200] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In contrast to most multimeric transmembrane complexes that oligomerize in the endoplasmic reticulum (ER), the gap junction protein connexin43 (Cx43) oligomerizes in an aspect of the Golgi apparatus. The mechanisms that prevent oligomerization of Cx43 and related connexins in the ER are not well understood. Also, some studies suggest that connexins can oligomerize in the ER. We used connexin constructs containing a C-terminal dilysine-based ER retention/retrieval signal (HKKSL) transfected into HeLa cells to study early events in connexin oligomerization. Using this approach, Cx43-HKKSL was retained in the ER and prevented from oligomerization. However, another ER-retained HKKSL-tagged connexin, Cx32-HKKSL, had the capacity to oligomerize. Because this suggested that Cx43 contains a motif that prevented oligomerization in the ER, a series of HKKSL-tagged and untagged Cx32/Cx43 chimeras was screened to define this motif. The minimal motif, which prevented ER oligomerization, consisted of the complete third transmembrane domain and the second extracellular loop from Cx43 on a Cx32 backbone. We propose that charged residues present in Cx43 and related connexins help prevent ER oligomerization by stabilizing the third transmembrane domain in the membrane bilayer.
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Affiliation(s)
- Jose Maza
- Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA
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45
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Sosinsky GE, Nicholson BJ. Structural organization of gap junction channels. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2005; 1711:99-125. [PMID: 15925321 DOI: 10.1016/j.bbamem.2005.04.001] [Citation(s) in RCA: 169] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/30/2004] [Revised: 03/22/2005] [Accepted: 04/02/2005] [Indexed: 11/16/2022]
Abstract
Gap junctions were initially described morphologically, and identified as semi-crystalline arrays of channels linking two cells. This suggested that they may represent an amenable target for electron and X-ray crystallographic studies in much the same way that bacteriorhodopsin has. Over 30 years later, however, an atomic resolution structural solution of these unique intercellular pores is still lacking due to many challenges faced in obtaining high expression levels and purification of these structures. A variety of microscopic techniques, as well as NMR structure determination of fragments of the protein, have now provided clearer and correlated views of how these structures are assembled and function as intercellular conduits. As a complement to these structural approaches, a variety of mutagenic studies linking structure and function have now allowed molecular details to be superimposed on these lower resolution structures, so that a clearer image of pore architecture and its modes of regulation are beginning to emerge.
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Affiliation(s)
- Gina E Sosinsky
- National Center for Microscopy and Imaging Research, Department of Neurosciences, University of California San Diego, La Jolla, CA 92093-0608, USA
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46
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Weber PA, Chang HC, Spaeth KE, Nitsche JM, Nicholson BJ. The permeability of gap junction channels to probes of different size is dependent on connexin composition and permeant-pore affinities. Biophys J 2005; 87:958-73. [PMID: 15298902 PMCID: PMC1304503 DOI: 10.1529/biophysj.103.036350] [Citation(s) in RCA: 193] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Gap junctions have traditionally been characterized as nonspecific pores between cells passing molecules up to 1 kDa in molecular mass. Nonetheless, it has become increasingly evident that different members of the connexin (Cx) family mediate quite distinct physiological processes and are often not interchangeable. Consistent with this observation, differences in permeability to natural metabolites have been reported for different connexins, although the physical basis for selectivity has not been established. Comparative studies of different members of the connexin family have provided evidence for ionic charge selectivity, but surprisingly little is known about how connexin composition affects the size of the pore. We have employed a series of Alexa dyes, which share similar structural characteristics but range in size from molecular weight 350 to 760, to probe the permeabilities and size limits of different connexin channels expressed in Xenopus oocytes. Correlated dye transfer and electrical measurements on each cell pair, in conjunction with a three-dimensional mathematical model of dye diffusion in the oocyte system, allowed us to obtain single channel permeabilities for all three dyes in six homotypic and four heterotypic channels. Cx43 and Cx32 channels passed all three dyes with similar efficiency, whereas Cx26, Cx40, and Cx45 channels showed a significant drop-off in permeability with the largest dye. Cx37 channels only showed significant permeability for the smaller two dyes, but at two- to sixfold lower levels than other connexins tested. In the heterotypic cases studied (Cx26/Cx32 and Cx43/Cx37), permeability characteristics were found to resemble the more restrictive parental homotypic channel. The most surprising finding of the study was that the absolute permeabilities calculated for all gap junctional channels in this study are, with one exception, at least 2 orders of magnitude greater than predicted purely on the basis of hindered pore diffusion. Consequently, affinity between the probes and the pore creating an energetically favorable in-pore environment, which would elevate permeant concentration within the pore and hence the flux, is strongly implicated.
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Affiliation(s)
- Paul A Weber
- Department of Biological Sciences and Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260, USA
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Fleishman SJ, Unger VM, Yeager M, Ben-Tal N. A Calpha model for the transmembrane alpha helices of gap junction intercellular channels. Mol Cell 2004; 15:879-88. [PMID: 15383278 DOI: 10.1016/j.molcel.2004.08.016] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2004] [Revised: 07/06/2004] [Accepted: 07/22/2004] [Indexed: 10/26/2022]
Abstract
Gap junction channels connect the cytoplasms of apposed cells via an intercellular conduit formed by the end-to-end docking of two hexameric hemichannels called connexons. We used electron cryomicroscopy to derive a three-dimensional density map at 5.7 angstroms in-plane and 19.8 angstroms vertical resolution, allowing us to identify the positions and tilt angles for the 24 alpha helices within each hemichannel. The four hydrophobic segments in connexin sequences were assigned to the alpha helices in the map based on biochemical and phylogenetic data. Analyses of evolutionary conservation and compensatory mutations in connexin evolution identified the packing interfaces between the helices. The final model, which specifies the coordinates of Calpha atoms in the transmembrane domain, provides a structural basis for understanding the different physiological effects of almost 30 mutations and polymorphisms in terms of structural deformations at the interfaces between helices, revealing an intimate connection between molecular structure and disease.
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Affiliation(s)
- Sarel J Fleishman
- Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat Aviv, 69978, Israel
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Seki A, Duffy HS, Coombs W, Spray DC, Taffet SM, Delmar M. Modifications in the biophysical properties of connexin43 channels by a peptide of the cytoplasmic loop region. Circ Res 2004; 95:e22-8. [PMID: 15284189 DOI: 10.1161/01.res.0000140737.62245.c5] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Connexin43 (Cx43) channels reside in at least 3 states: closed, open, or residual. It is hypothesized that the residual state results from the interaction of an intracellular "gating element" with structures at the vestibule of the pore. Recently, we showed in vitro that there is an intramolecular interaction of the carboxyl-terminal domain (referred to as "CT") with a region in the cytoplasmic loop of Cx43 (amino acids 119 to 144; referred to as "L2"). Here, we assessed whether the L2 region was able to interact with the gating particle in a functional channel. Cx43 channels were recorded in the presence of a peptide corresponding to the L2 region, delivered via the patch pipette. This manipulation did not modify unitary conductance, but decreased the frequency of transitions into the residual state, prolonged open time, and altered the voltage dependence of the channel in a manner analogous to that observed after truncation of the CT domain. The latter correlated with the ability of the peptide to bind to the CT domain, as determined by mirror resonance spectroscopy. Overall, we propose that the L2 acts as a "receptor" that interacts with a flexible intracellular gating element during channel gating. The full text of this article is available online at http://circres.ahajournals.org.
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Affiliation(s)
- Akiko Seki
- Department of Pharmacology, State University of New York Upstate Medical University, Syracuse 13210, USA
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49
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Seki A, Coombs W, Taffet SM, Delmar M. Loss of electrical communication, but not plaque formation, after mutations in the cytoplasmic loop of connexin43. Heart Rhythm 2004; 1:227-33. [PMID: 15851157 DOI: 10.1016/j.hrthm.2004.03.066] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/20/2004] [Accepted: 03/18/2004] [Indexed: 10/26/2022]
Abstract
OBJECTIVES The aim of this study was to determine if the structural integrity of a region in the cytoplasmic loop (amino acids 119-144; region "L2") of connexin43 (Cx43) is necessary to maintain normal channel function. BACKGROUND Cx43 is the most abundant gap junction protein in the heart. The ability of these channels to close under pathologic conditions such as ischemia may be a key substrate for cardiac arrhythmias. Previous studies have shown that Cx43 regulation involves the intramolecular interaction of its carboxyl terminal domain (a "gating particle") with a separate region of the molecule acting as a receptor. We recently proposed that a region in the cytoplasmic loop of Cx43 (amino acids 119-144; region "L2") might function as a receptor. METHODS Using site-directed mutagenesis and patch clamp analysis, as well as fluorescent microscopy, we examined gap junction plaque formation and channel properties of Cx43 L2 mutants. RESULTS Deletions of 5 to 6 amino acids within the L2 domain interfered with the formation of functional gap junction channels, although gap junction plaques were clearly visible. Selected point mutations in the region (including those present in patients with oculodentodigital dysplasia) caused modifications ranging from complete channel closure to changes in unitary conductance. CONCLUSIONS These results show that the L2 region is essential for maintenance of the normal architecture of the channel pore. This information is consistent with the notion that the L2 region could be a receptor for the carboxy terminal domain; the latter interaction would lead to channel closure under conditions such as myocardial ischemia and infarction.
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Affiliation(s)
- Akiko Seki
- Department of Pharmacology, SUNY Upstate Medical University, Syracuse, New York, USA
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50
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Affiliation(s)
- Bruce J Nicholson
- Department of Biological Sciences, University of Buffalo, SUNY, 619 Cooke Hall, NY 14260, USA.
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