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Posukh OL, Maslova EA, Danilchenko VY, Zytsar MV, Orishchenko KE. Functional Consequences of Pathogenic Variants of the GJB2 Gene (Cx26) Localized in Different Cx26 Domains. Biomolecules 2023; 13:1521. [PMID: 37892203 PMCID: PMC10604905 DOI: 10.3390/biom13101521] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2023] [Revised: 10/08/2023] [Accepted: 10/10/2023] [Indexed: 10/29/2023] Open
Abstract
One of the most common forms of genetic deafness has been predominantly associated with pathogenic variants in the GJB2 gene, encoding transmembrane protein connexin 26 (Cx26). The Cx26 molecule consists of an N-terminal domain (NT), four transmembrane domains (TM1-TM4), two extracellular loops (EL1 and EL2), a cytoplasmic loop, and a C-terminus (CT). Pathogenic variants in the GJB2 gene, resulting in amino acid substitutions scattered across the Cx26 domains, lead to a variety of clinical outcomes, including the most common non-syndromic autosomal recessive deafness (DFNB1A), autosomal dominant deafness (DFNA3A), as well as syndromic forms combining hearing loss and skin disorders. However, for rare and poorly documented variants, information on the mode of inheritance is often lacking. Numerous in vitro studies have been conducted to elucidate the functional consequences of pathogenic GJB2 variants leading to amino acid substitutions in different domains of Cx26 protein. In this work, we summarized all available data on a mode of inheritance of pathogenic GJB2 variants leading to amino acid substitutions and reviewed published information on their functional effects, with an emphasis on their localization in certain Cx26 domains.
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Affiliation(s)
- Olga L. Posukh
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
| | - Ekaterina A. Maslova
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
| | - Valeriia Yu. Danilchenko
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
| | - Marina V. Zytsar
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
| | - Konstantin E. Orishchenko
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia; (E.A.M.); (V.Y.D.); (M.V.Z.); (K.E.O.)
- Novosibirsk State University, 630090 Novosibirsk, Russia
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2
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Zong YJ, Liu XZ, Tu L, Sun Y. Cytomembrane Trafficking Pathways of Connexin 26, 30, and 43. Int J Mol Sci 2023; 24:10349. [PMID: 37373495 DOI: 10.3390/ijms241210349] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2023] [Revised: 06/12/2023] [Accepted: 06/13/2023] [Indexed: 06/29/2023] Open
Abstract
The connexin gene family is the most prevalent gene that contributes to hearing loss. Connexins 26 and 30, encoded by GJB2 and GJB6, respectively, are the most abundantly expressed connexins in the inner ear. Connexin 43, which is encoded by GJA1, appears to be widely expressed in various organs, including the heart, skin, the brain, and the inner ear. The mutations that arise in GJB2, GJB6, and GJA1 can all result in comprehensive or non-comprehensive genetic deafness in newborns. As it is predicted that connexins include at least 20 isoforms in humans, the biosynthesis, structural composition, and degradation of connexins must be precisely regulated so that the gap junctions can properly operate. Certain mutations result in connexins possessing a faulty subcellular localization, failing to transport to the cell membrane and preventing gap junction formation, ultimately leading to connexin dysfunction and hearing loss. In this review, we provide a discussion of the transport models for connexin 43, connexins 30 and 26, mutations affecting trafficking pathways of these connexins, the existing controversies in the trafficking pathways of connexins, and the molecules involved in connexin trafficking and their functions. This review can contribute to a new way of understanding the etiological principles of connexin mutations and finding therapeutic strategies for hereditary deafness.
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Affiliation(s)
- Yan-Jun Zong
- Department of Otorhinolaryngology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China
| | - Xiao-Zhou Liu
- Department of Otorhinolaryngology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China
| | - Lei Tu
- Division of Gastroenterology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China
| | - Yu Sun
- Department of Otorhinolaryngology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China
- Hubei Province Key Laboratory of Oral and Maxillofacial Development and Regeneration, Wuhan 430022, China
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Zhang Y, Xu Q, Sun Q, Kong R, Liu H, Yi X, Liang Z, Letcher RJ, Liu C. Ustiloxin A inhibits proliferation of renal tubular epithelial cells in vitro and induces renal injury in mice by disrupting structure and respiratory function of mitochondria. JOURNAL OF HAZARDOUS MATERIALS 2023; 448:130791. [PMID: 36706486 DOI: 10.1016/j.jhazmat.2023.130791] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/01/2022] [Revised: 01/08/2023] [Accepted: 01/12/2023] [Indexed: 06/18/2023]
Abstract
Recently, we found that Ustiloxin A (UA, a mycotoxin) was widely detected in paddy environment and rice samples from several countries, and was also detected in human urine samples from China. However, the current knowledge about the health risks of UA are limited. In this research, the cytotoxicity of UA in mice renal tubular epithelial cells (mRTECs) was evaluated, and the results indicated that UA arrested cell cycle in G2/M phase via altering cellular morphology and microtubule, and inhibited the proliferation and division of mRTECs. Furthermore, UA could inhibit mitochondrial respiration via binding to the CoQ-binding site in dihydro-orotate dehydrogenase (DHODH) protein, and resulted in mitochondrial damage. These adverse effects of UA on mitochondria might be responsible for the cytotoxicity observed in vitro. In vivo, UA at concentrations that were comparable to the realistic concentrations of human exposure induced renal insufficiency in mice, and this might be associated with the renal mitochondrial damage in mice. However, exposure to UA at those realistic concentrations did not promote the progression from renal insufficiency to renal fibrosis and chronic kidney disease was not observed in mice.
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Affiliation(s)
- Yongkang Zhang
- College of Fisheries, Huazhong Agricultural University, Wuhan 430070, China
| | - Qiaolin Xu
- College of Fisheries, Huazhong Agricultural University, Wuhan 430070, China
| | - Qian Sun
- College of Fisheries, Huazhong Agricultural University, Wuhan 430070, China
| | - Ren Kong
- School of Environmental Studies, China University of Geosciences, Wuhan 430074, China
| | - Hao Liu
- College of Plant Science & Technology, Huazhong Agricultural University, Wuhan 430070, China
| | - Xun'e Yi
- College of Fisheries, Huazhong Agricultural University, Wuhan 430070, China
| | - Zhengqi Liang
- College of Fisheries, Huazhong Agricultural University, Wuhan 430070, China
| | - Robert J Letcher
- Departments of Chemistry and Biology, Carleton University, Ottawa K1S 5B6, ON, Canada
| | - Chunsheng Liu
- School of Environmental Studies, China University of Geosciences, Wuhan 430074, China.
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Brotherton DH, Savva CG, Ragan TJ, Dale N, Cameron AD. Conformational changes and CO 2-induced channel gating in connexin26. Structure 2022; 30:697-706.e4. [PMID: 35276081 PMCID: PMC9592558 DOI: 10.1016/j.str.2022.02.010] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2022] [Revised: 01/24/2022] [Accepted: 02/14/2022] [Indexed: 12/22/2022]
Abstract
Connexins form large-pore channels that function either as dodecameric gap junctions or hexameric hemichannels to allow the regulated movement of small molecules and ions across cell membranes. Opening or closing of the channels is controlled by a variety of stimuli, and dysregulation leads to multiple diseases. An increase in the partial pressure of carbon dioxide (PCO2) has been shown to cause connexin26 (Cx26) gap junctions to close. Here, we use cryoelectron microscopy (cryo-EM) to determine the structure of human Cx26 gap junctions under increasing levels of PCO2. We show a correlation between the level of PCO2 and the size of the aperture of the pore, governed by the N-terminal helices that line the pore. This indicates that CO2 alone is sufficient to cause conformational changes in the protein. Analysis of the conformational states shows that movements at the N terminus are linked to both subunit rotation and flexing of the transmembrane helices. High-resolution cryo-EM structures of connexin26 at varying levels of PCO2 CO2 alone causes conformational changes in the protein under stable pH conditions The N-terminal helices regulate the aperture of the pore KID syndrome mutations affecting CO2 sensitivity map to flexion points of structure
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Affiliation(s)
- Deborah H Brotherton
- School of Life Sciences, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, UK
| | - Christos G Savva
- Leicester Institute of Structural and Chemical Biology, Department of Molecular and Cell Biology, University of Leicester, Lancaster Road, LE1 7HB Leicester, UK
| | - Timothy J Ragan
- Leicester Institute of Structural and Chemical Biology, Department of Molecular and Cell Biology, University of Leicester, Lancaster Road, LE1 7HB Leicester, UK
| | - Nicholas Dale
- School of Life Sciences, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, UK.
| | - Alexander D Cameron
- School of Life Sciences, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, UK.
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Maslova EA, Orishchenko KE, Posukh OL. Functional Evaluation of a Rare Variant c.516G>C (p.Trp172Cys) in the GJB2 (Connexin 26) Gene Associated with Nonsyndromic Hearing Loss. Biomolecules 2021; 11:61. [PMID: 33466560 PMCID: PMC7824951 DOI: 10.3390/biom11010061] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2020] [Revised: 01/03/2021] [Accepted: 01/04/2021] [Indexed: 01/05/2023] Open
Abstract
Mutations in the GJB2 gene encoding transmembrane protein connexin 26 (Cx26) are the most common cause for hearing loss worldwide. Cx26 plays a crucial role in the ionic and metabolic homeostasis in the inner ear, indispensable for normal hearing process. Different pathogenic mutations in the GJB2 gene can affect all stages of the Cx26 life cycle and result in nonsyndromic autosomal recessive (DFNB1) or dominant (DFNA3) deafness and syndromes associating hearing loss with skin disorders. This study aims to elucidate the functional consequences of a rare GJB2 variant c.516G>C (p.Trp172Cys) found with high frequency in deaf patients from indigenous populations of Southern Siberia (Russia). The substitution c.516G>C leads to the replacement of tryptophan at a conserved amino acid position 172 with cysteine (p.Trp172Cys) in the second extracellular loop of Cx26 protein. We analyzed the subcellular localization of mutant Cx26-p.Trp172Cys protein by immunocytochemistry and the hemichannels permeability by dye loading assay. The GJB2 knockout HeLa cell line has been generated using CRISPR/Cas9 genome editing tool. Subsequently, the HeLa transgenic cell lines stably expressing different GJB2 variants (wild type and mutations associated with hearing loss) were established based on knockout cells and used for comparative functional analysis. The impaired trafficking of mutant Cx26-p.Trp172Cys protein to the plasma membrane and reduced hemichannels permeability support the pathogenic effect of the c.516G>C (p.Trp172Cys) variant and its association with nonsyndromic hearing loss. Our data contribute to a better understanding of the role of mutations in the second extracellular loop of Cx26 protein in pathogenesis of deafness.
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Affiliation(s)
- Ekaterina A. Maslova
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia; (E.A.M.); (K.E.O.)
- Novosibirsk State University, Novosibirsk 630090, Russia
| | - Konstantin E. Orishchenko
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia; (E.A.M.); (K.E.O.)
- Novosibirsk State University, Novosibirsk 630090, Russia
| | - Olga L. Posukh
- Federal Research Center Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia; (E.A.M.); (K.E.O.)
- Novosibirsk State University, Novosibirsk 630090, Russia
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6
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Calligari P, Torsello M, Bortoli M, Orian L, Polimeno A. Modelling of Ca2+-promoted structural effects in wild type and post-translationally modified Connexin26. MOLECULAR SIMULATION 2019. [DOI: 10.1080/08927022.2019.1690653] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Affiliation(s)
- Paolo Calligari
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy
| | - Mauro Torsello
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy
| | - Marco Bortoli
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy
| | - Laura Orian
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy
| | - Antonino Polimeno
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy
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Potential of cryo-EM for high-resolution structural analysis of gap junction channels. Curr Opin Struct Biol 2019; 54:78-85. [PMID: 30797124 DOI: 10.1016/j.sbi.2019.01.005] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2018] [Revised: 12/03/2018] [Accepted: 01/09/2019] [Indexed: 11/20/2022]
Abstract
Gap junction family proteins form conduits connecting the cytoplasm of adjacent cells, thereby enabling electrical and chemical coupling to maintain physiological homeostasis. Gap junction proteins comprise two gene families, connexins in chordates and innexins in pre-chordates. Their channel structures have been analyzed by electron or X-ray crystallography, but only a few atomic structures have been reported. Recent advances in single-particle cryo-electron microscopy (cryo-EM) will help to elucidate these structures further. Here the structural biology of gap junction channels utilizing crystallography and single-particle cryo-EM is overviewed to shed light on the functional mechanisms of cell-cell communication that are essential for multicellular organisms.
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8
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Abrams CK, Peinado A, Mahmoud R, Bocarsly M, Zhang H, Chang P, Botello-Smith WM, Freidin MM, Luo Y. Alterations at Arg 76 of human connexin 46, a residue associated with cataract formation, cause loss of gap junction formation but preserve hemichannel function. Am J Physiol Cell Physiol 2018; 315:C623-C635. [PMID: 30044662 DOI: 10.1152/ajpcell.00157.2018] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The connexins are members of a family of integral membrane proteins that form gap junction channels between apposed cells and/or hemichannels across the plasma membranes. The importance of the arginine at position 76 (Arg76) in the structure and/or function of connexin 46 (Cx46) is highlighted by its conservation across the entire connexin family and the occurrence of pathogenic mutations at this (or the corresponding homologous) residue in a number of human diseases. Two mutations at Arg76 in Cx46 are associated with cataracts in humans, highlighting the importance of this residue. We examined the expression levels and macroscopic and single-channel properties of human Cx46 and compared them with those for two pathogenic mutants, namely R76H and R76G. To gain further insight into the role of charge at this position, we generated two additional nonnaturally occurring mutants, R76K (charge conserving) and R76E (charge inverting). We found that, when expressed exogenously in Neuro2a cells, all four mutants formed membrane hemichannels, inducing membrane permeability at levels comparable to those recorded in cells expressing the wild-type Cx46. In contrast, the number of gap-junction plaques and the magnitude of junctional coupling were reduced by all four mutations. To gain further insight into the role of Arg76 in the function of Cx46, we performed homology modeling of Cx46 and in silico mutagenesis of Arg76 to Gly, His, or Glu. Our studies suggest that the loss of interprotomeric interactions has a significant effect on the extracellular domain conformation and dynamics, thus affecting the hemichannel docking required for formation of cell-cell channels.
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Affiliation(s)
- Charles K Abrams
- Department of Neurology and Rehabilitation, University of Illinois at Chicago College of Medicine , Chicago, Illinois
- Department of Neurology State University of New York Downstate Medical Center , Brooklyn New York
| | - Alejandro Peinado
- Department of Neurology and Rehabilitation, University of Illinois at Chicago College of Medicine , Chicago, Illinois
| | - Rola Mahmoud
- Department of Neurology State University of New York Downstate Medical Center , Brooklyn New York
| | - Matan Bocarsly
- Department of Neurology State University of New York Downstate Medical Center , Brooklyn New York
| | - Han Zhang
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences , Pomona, California
| | - Paul Chang
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences , Pomona, California
| | - Wesley M Botello-Smith
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences , Pomona, California
| | - Mona M Freidin
- Department of Neurology and Rehabilitation, University of Illinois at Chicago College of Medicine , Chicago, Illinois
- Department of Neurology State University of New York Downstate Medical Center , Brooklyn New York
| | - Yun Luo
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences , Pomona, California
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Abstract
Mutations in gap junction proteins encoding beta connexions are believed to be a major cause for congenital hearing loss. The purpose of this study was to do comparative analyses of frequencies of most prominent mutations responsible for congenital deafness. Using fluorescence PCR method, the entire coding region of GJB2 gene, GJB3 gene, and SLC26A4 was analyzed. Direct DNA sequencing was used to analyze mutations in these genes among unrelated 2,674 cases of newborns. Also, 12S rRNA mutation was also studied in these cases. In 2,674 cases of newborns from June 2013 to June 2014, found deafness mutation in 137 cases (5.12 % of carrier rate), carrying GJB2 mutations in 68 cases (2.54 % of carry rate), GJB3 mutations in 10 cases (0.37 % of carry rate), SLC26A4 mutations in 54 cases (2.02 % of carry rate), and mitochondrial 12S rRNA mutations in five cases (0.19 % of carry rate). The study concludes that GJB2 gene mutation is the most common and mitochondrial 12S rRNA mutations are the least common mutation for congenital hearing loss in Chinese newborns.
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Dalamón VK, Buonfiglio P, Larralde M, Craig P, Lotersztein V, Choate K, Pallares N, Diamante V, Elgoyhen AB. Connexin 26 (GJB2) mutation in an Argentinean patient with keratitis-ichthyosis-deafness (KID) syndrome: a case report. BMC MEDICAL GENETICS 2016; 17:37. [PMID: 27141831 PMCID: PMC4855445 DOI: 10.1186/s12881-016-0298-y] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/02/2015] [Accepted: 04/22/2016] [Indexed: 11/20/2022]
Abstract
Background Keratitis-Ichthyosis-Deafness (KID) syndrome is a rare condition characterized by pre-lingual sensorineural deafness with skin hyperkeratinization. The primary cause of the disease is a loss-of-function mutation in the GJB2 gene. Mutations in Argentinean patients have not been described. Case presentation We studied a 2 year-old boy with bilateral congenital sensorineural deafness with dry skin over the entire body, hypotrichosis of the scalp, thin and light-blond hair. Analysis of the GJB2 gene nucleotide sequence revealed the substitution of guanine-148 by adenine predicted to result in an Asp50Asn amino acid substitution. Conclusion This is the first KID report in a patient from Argentina. This de novo mutation proved to be the cause of keratitis-ichthyosis-deafness syndrome (KID-syndrome) in the patient, and has implications in medical genetic practice.
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Affiliation(s)
- Viviana Karina Dalamón
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor Torres"-(INGEBI), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Vuelta de Obligado 2490. (1428) Ciudad Autónoma de Buenos Aires, Buenos Aires, Argentina.
| | - Paula Buonfiglio
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor Torres"-(INGEBI), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Vuelta de Obligado 2490. (1428) Ciudad Autónoma de Buenos Aires, Buenos Aires, Argentina
| | - Margarita Larralde
- Servicio de Dermatología Pediátrica, Hospital Ramos Mejía, Ciudad Autonoma de Buenos Aires, Argentina.,Departamento de Dermatología, Hospital Alemán, Ciudad Autonoma de Buenos Aires, Argentina
| | - Patricio Craig
- Departamento de Química Biológica e Instituto de Química y Fisicoquímica Biológicas, Universidad de Buenos Aires, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Ciudad Autonoma de Buenos Aires, Argentina
| | - Vanesa Lotersztein
- Servicio de Genética, Hospital de Clínicas "José de San Martín", Ciudad Autonoma de Buenos Aires, Argentina
| | - Keith Choate
- Dermatology Yale University School of Medicine, New Haven, USA
| | - Norma Pallares
- Instituto Superior de Implantes Cocleares "Dr. Vicente Diamante", Ciudad Autonoma de Buenos Aires, Argentina
| | - Vicente Diamante
- Instituto Superior de Implantes Cocleares "Dr. Vicente Diamante", Ciudad Autonoma de Buenos Aires, Argentina
| | - Ana Belén Elgoyhen
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor Torres"-(INGEBI), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Vuelta de Obligado 2490. (1428) Ciudad Autónoma de Buenos Aires, Buenos Aires, Argentina.,Departamento de Farmacología, Facultad de Medicina, Universidad de Buenos Aires, Ciudad Autonoma de Buenos Aires, Argentina
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Oshima A, Matsuzawa T, Murata K, Tani K, Fujiyoshi Y. Hexadecameric structure of an invertebrate gap junction channel. J Mol Biol 2016; 428:1227-1236. [DOI: 10.1016/j.jmb.2016.02.011] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2015] [Revised: 02/05/2016] [Accepted: 02/08/2016] [Indexed: 12/12/2022]
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12
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Abstract
Gap-junction channels (GJCs) communicate the cytoplasm of adjacent cells and are formed by head-to-head association of two hemichannels (HCs), one from each of the neighbouring cells. GJCs mediate electrical and chemical communication between cells, whereas undocked HCs participate in paracrine signalling because of their permeability to molecules such as ATP. Sustained opening of HCs under pathological conditions results in water and solute fluxes that cannot be compensated by membrane transport and therefore lead to cell damage. Mutations of Cx26 (connexin 26) are the most frequent cause of genetic deafness and it is therefore important to understand the structure–function relationship of wild-type and deafness-associated mutants. Currently available connexin HC expression systems severely limit the pace of structural studies and there is no simple high-throughput HC functional assay. The Escherichia coli-based expression system presented in the present study yields milligram amounts of purified Cx26 HCs suitable for functional and structural studies. We also show evidence of functional activity of recombinant Cx26 HCs in intact bacteria using a new growth complementation assay. The E. coli-based expression system has high potential for structural studies and high-throughput functional screening of HCs. Human connexin 26 (Cx26) hemichannels (HCs) can be expressed in bacteria in a functional form. This new expression system yields milligram amounts of purified HCs and has high potential to advance studies of connexin HCs.
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13
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Zonta F, Girotto G, Buratto D, Crispino G, Morgan A, Abdulhadi K, Alkowari M, Badii R, Gasparini P, Mammano F. The p.Cys169Tyr variant of connexin 26 is not a polymorphism. Hum Mol Genet 2015; 24:2641-8. [PMID: 25628337 PMCID: PMC4383868 DOI: 10.1093/hmg/ddv026] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2014] [Accepted: 01/21/2015] [Indexed: 12/31/2022] Open
Abstract
Mutations in the GJB2 gene, which encodes the gap junction protein connexin 26 (Cx26), are the primary cause of hereditary prelingual hearing impairment. Here, the p.Cys169Tyr missense mutation of Cx26 (Cx26C169Y), previously classified as a polymorphism, has been identified as causative of severe hearing loss in two Qatari families. We have analyzed the effect of this mutation using a combination of confocal immunofluorescence microscopy and molecular dynamics simulations. At the cellular level, our results show that the mutant protein fails to form junctional channels in HeLa transfectants despite being correctly targeted to the plasma membrane. At the molecular level, this effect can be accounted for by disruption of the disulfide bridge that Cys169 forms with Cys64 in the wild-type structure (Cx26WT). The lack of the disulfide bridge in the Cx26C169Y protein causes a spatial rearrangement of two important residues, Asn176 and Thr177. In the Cx26WT protein, these residues play a crucial role in the intra-molecular interactions that permit the formation of an intercellular channel by the head-to-head docking of two opposing hemichannels resident in the plasma membrane of adjacent cells. Our results elucidate the molecular pathogenesis of hereditary hearing loss due to the connexin mutation and facilitate the understanding of its role in both healthy and affected individuals.
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Affiliation(s)
- Francesco Zonta
- Dipartimento di Fisica e Astronomia 'G. Galilei', Università di Padova, 35131 Padova, Italy
| | - Giorgia Girotto
- Department of Medical, Surgical and Health Sciences, University of Trieste, 34100 Trieste, Italy
| | - Damiano Buratto
- Dipartimento di Fisica e Astronomia 'G. Galilei', Università di Padova, 35131 Padova, Italy
| | - Giulia Crispino
- Dipartimento di Fisica e Astronomia 'G. Galilei', Università di Padova, 35131 Padova, Italy, Istituto Veneto di Medicina Molecolare, Fondazione per la Ricerca Biomedica Avanzata, 35129 Padova, Italy
| | - Anna Morgan
- Department of Medical, Surgical and Health Sciences, University of Trieste, 34100 Trieste, Italy
| | - Khalid Abdulhadi
- Audiology and Balance Unit, National Program for Early Detection of Hearing Loss, WH, Hamad Medical Corporation (HMC) Doha, Doha, Qatar
| | - Moza Alkowari
- Molecular Genetics Laboratory, Department of Laboratory of Medicine and Pathology, Hamad Medical Corporation (HMC), Doha, Qatar
| | - Ramin Badii
- Molecular Genetics Laboratory, Department of Laboratory of Medicine and Pathology, Hamad Medical Corporation (HMC), Doha, Qatar
| | - Paolo Gasparini
- Department of Medical, Surgical and Health Sciences, University of Trieste, 34100 Trieste, Italy, Medical Genetics, Institute for Maternal and Child Health-IRCCS 'Burlo Garofolo', Trieste, Italy and
| | - Fabio Mammano
- Dipartimento di Fisica e Astronomia 'G. Galilei', Università di Padova, 35131 Padova, Italy, Istituto Veneto di Medicina Molecolare, Fondazione per la Ricerca Biomedica Avanzata, 35129 Padova, Italy, CNR Institute of Cell Biology and Neurobiology, 00015 Monterotondo, Rome, Italy
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Jagger DJ, Forge A. Connexins and gap junctions in the inner ear--it's not just about K⁺ recycling. Cell Tissue Res 2014; 360:633-44. [PMID: 25381570 PMCID: PMC4452565 DOI: 10.1007/s00441-014-2029-z] [Citation(s) in RCA: 71] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Accepted: 10/06/2014] [Indexed: 12/19/2022]
Abstract
Normal development, function and repair of the sensory epithelia in the inner ear are all dependent on gap junctional intercellular communication. Mutations in the connexin genes GJB2 and GJB6 (encoding CX26 and CX30) result in syndromic and non-syndromic deafness via various mechanisms. Clinical vestibular defects, however, are harder to connect with connexin dysfunction. Cx26 and Cx30 proteins are widely expressed in the epithelial and connective tissues of the cochlea, where they may form homomeric or heteromeric gap junction channels in a cell-specific and spatiotemporally complex fashion. Despite the study of mutant channels and animal models for both recessive and dominant autosomal deafness, it is still unclear why gap junctions are essential for auditory function, and why Cx26 and Cx30 do not compensate for each other in vivo. Cx26 appears to be essential for normal development of the auditory sensory epithelium, but may be dispensable during normal hearing. Cx30 appears to be essential for normal repair following sensory cell loss. The specific modes of intercellular signalling mediated by inner ear gap junction channels remain undetermined, but they are hypothesised to play essential roles in the maintenance of ionic and metabolic homeostasis in the inner ear. Recent studies have highlighted involvement of gap junctions in the transfer of essential second messengers between the non-sensory cells, and have proposed roles for hemichannels in normal hearing. Here, we summarise the current knowledge about the molecular and functional properties of inner ear gap junctions, and about tissue pathologies associated with connexin mutations.
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Affiliation(s)
- Daniel J Jagger
- UCL Ear Institute, University College London, 332 Gray's Inn Road, London, WC1X 8EE, UK,
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15
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R75Q de novo dominant mutation of GJB2 in a Chinese family with hearing loss and palmoplantar keratoderma. Int J Pediatr Otorhinolaryngol 2014; 78:1461-6. [PMID: 24975403 DOI: 10.1016/j.ijporl.2014.06.008] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/19/2014] [Revised: 06/01/2014] [Accepted: 06/05/2014] [Indexed: 12/21/2022]
Abstract
OBJECTIVES Mutations in the GJB2 gene encoding connexin 26 (Cx26) are major causes of hereditary deafness. This study aimed to characterize the mutation profiles of the GJB2 gene in a Chinese family with sensorineural hearing loss. METHODS A Chinese family that included three individuals with sensorineural hearing loss and palmoplantar keratoderma underwent complete physical examinations, audiological examinations including pure tone audiometry and auditory brainstem response, skin pathological examination, and temporal CT scans. The entire coding region of GJB2, GJB3, GJB6, and the coding exons (exon7+8 and 19) of SLC26A4, mitochondrial 12SrRNA, and tRNA Ser (UCN) were sequenced. Structural analysis was performed to detect the effects of mutation on the tertiary structure of Cx26. RESULTS A dominant GJB2 mutation, c.224G>A (p.Arg75Gln, p.R75Q), was detected in the family. No other mutation was identified in GJB2, GJB3, GJB6, or the coding exons (exon7+8 and 19) of SLC26A4, mitochondrial 12SrRNA, and tRNA Ser (UCN). Structural analysis revealed that the p.R75Q mutation likely affects the structural stability and permeation properties of the Cx26 gap junction channel. CONCLUSION Our findings provide further evidence of a correlation between the p.R75Q mutation in Cx26 and a syndromic hearing impairment with palmoplantar keratoderma.
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16
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Martines F, Salvago P, Bartolotta C, Cocuzza S, Fabiano C, Ferrara S, La Mattina E, Mucia M, Sammarco P, Sireci F, Martines E. A genotype-phenotype correlation in Sicilian patients with GJB2 biallelic mutations. Eur Arch Otorhinolaryngol 2014; 272:1857-65. [PMID: 24627074 DOI: 10.1007/s00405-014-2970-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2013] [Accepted: 02/20/2014] [Indexed: 11/29/2022]
Abstract
The aim of this work was to study the genotype distribution of Sicilian patients with biallelic GJB2 mutations; to correlate genotype classes and/or specific mutations of GJB2 gene (35delG-non-35delG) with audiologic profiles. A total of 10 different mutations and 11 different genotypes were evidenced in 73 SNHL subjects; 35delG (90.36 % of cases) and IVS1+1 (13.69 %) were the most common mutations found in the cohort with a significant difference in the distribution between North and South Sicily. Audiological evaluation revealed a severe (16/73) to profound (47/73) hearing loss (HL) in 86.13 % of cases without significant difference between the degree of HL and the province of origin of the subjects (P = 0.727). The homozygous truncating (T/T) genotype was the most widespread (89.04 % of cases), with a severe-to-profound hearing impairment in 90.36 % of T/T class with respect to truncating/non-truncating (T/NT) and non-truncating/non-truncating (NT/NT) genotypes (P = 0.012). From the comparison of homozygous 35delG and 35delG/non-35delG genotypes, a more profound HL in the homozygous 35delG than in compound heterozygous 35delG/non-35delG (p < 0.0001) resulted. This study confirms that 35delG is the most common mutation in the Mediterranean area with a heterogeneous distribution of the genotypes between North and South Sicily; probands homozygotes for 35delG or presenting a T/T genotype are more apt to have a severe-to-profound HL.
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Affiliation(s)
- Francesco Martines
- Dipartimento di Biomedicina Sperimentale e Neuroscienze Cliniche (BioNeC), Sezione di Otorinolaringoiatria, Università degli Studi di Palermo, Via del Vespro, 129, 90127, Palermo, Italy,
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17
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Zonta F, Buratto D, Cassini C, Bortolozzi M, Mammano F. Molecular dynamics simulations highlight structural and functional alterations in deafness-related M34T mutation of connexin 26. Front Physiol 2014; 5:85. [PMID: 24624091 PMCID: PMC3941013 DOI: 10.3389/fphys.2014.00085] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2013] [Accepted: 02/12/2014] [Indexed: 11/22/2022] Open
Abstract
Mutations of the GJB2 gene encoding the connexin 26 (Cx26) gap junction protein, which is widely expressed in the inner ear, are the primary cause of hereditary non-syndromic hearing loss in several populations. The deafness–associated single amino acid substitution of methionine 34 (M34) in the first transmembrane helix (TM1) with a threonine (T) ensues in the production of mutant Cx26M34T channels that are correctly synthesized and assembled in the plasma membrane. However, mutant channels overexpressed in HeLa cells retain only 11% of the wild type unitary conductance. Here we extend and rationalize those findings by comparing wild type Cx26 (Cx26WT) and Cx26M34T mutant channels in silico, using molecular dynamics simulations. Our results indicate that the quaternary structure of the Cx26M34T hemichannel is altered at the level of the pore funnel due to the disruption of the hydrophobic interaction between M34 and tryptophan 3 (W3) in the N–terminal helix (NTH). Our simulations also show that external force stimuli applied to the NTHs can detach them from the inner wall of the pore more readily in the mutant than in the wild type hemichannel. These structural alterations significantly increase the free energy barrier encountered by permeating ions, correspondingly decreasing the unitary conductance of the Cx26M34T hemichannel. Our results accord with the proposal that the mutant resides most of the time in a low conductance state. However, the small displacement of the NTHs in our Cx26M34T hemichannel model is not compatible with the formation of a pore plug as in the related Cx26M34A mutant.
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Affiliation(s)
- Francesco Zonta
- Dipartimento di Fisica e Astronomia "G. Galilei", Università degli Studi di Padova Padova, Italy
| | - Damiano Buratto
- Dipartimento di Fisica e Astronomia "G. Galilei", Università degli Studi di Padova Padova, Italy
| | - Chiara Cassini
- Dipartimento di Fisica e Astronomia "G. Galilei", Università degli Studi di Padova Padova, Italy
| | - Mario Bortolozzi
- Dipartimento di Fisica e Astronomia "G. Galilei", Università degli Studi di Padova Padova, Italy ; Istituto Veneto di Medicina Molecolare, Fondazione per la Ricerca Biomedica Avanzata Padova, Italy
| | - Fabio Mammano
- Dipartimento di Fisica e Astronomia "G. Galilei", Università degli Studi di Padova Padova, Italy ; Istituto Veneto di Medicina Molecolare, Fondazione per la Ricerca Biomedica Avanzata Padova, Italy ; Istituto di Neuroscienze, Consiglio Nazionale delle Ricerche Padova, Italy
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18
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Fiori MC, Reuss L, Cuello LG, Altenberg GA. Functional analysis and regulation of purified connexin hemichannels. Front Physiol 2014; 5:71. [PMID: 24611052 PMCID: PMC3933781 DOI: 10.3389/fphys.2014.00071] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2013] [Accepted: 02/06/2014] [Indexed: 01/08/2023] Open
Abstract
Gap-junction channels (GJCs) are aqueous channels that communicate adjacent cells. They are formed by head-to-head association of two hemichannels (HCs), one from each of the adjacent cells. Functional HCs are connexin hexamers composed of one or more connexin isoforms. Deafness is the most frequent sensineural disorder, and mutations of Cx26 are the most common cause of genetic deafness. Cx43 is the most ubiquitous connexin, expressed in many organs, tissues, and cell types, including heart, brain, and kidney. Alterations in its expression and function play important roles in the pathophysiology of very frequent medical problems such as those related to cardiac and brain ischemia. There is extensive information on the relationship between phosphorylation and Cx43 targeting, location, and function from experiments in cells and organs in normal and pathological conditions. However, the molecular mechanisms of Cx43 regulation by phosphorylation are hard to tackle in complex systems. Here, we present the use of purified HCs as a model for functional and structural studies. Cx26 and Cx43 are the only isoforms that have been purified, reconstituted, and subjected to functional and structural analysis. Purified Cx26 and Cx43 HCs have properties compatible with those demonstrated in cells, and present methodologies for the functional analysis of purified HCs reconstituted in liposomes. We show that phosphorylation of serine 368 by PKC produces a partial closure of the Cx43 HCs, changing solute selectivity. We also present evidence that the effect of phosphorylation is highly cooperative, requiring modification of several connexin subunits, and that phosphorylation of serine 368 elicits conformational changes in the purified HCs. The use of purified HCs is starting to provide critical data to understand the regulation of HCs at the molecular level.
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Affiliation(s)
- Mariana C Fiori
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center Lubbock, TX, USA
| | - Luis Reuss
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center Lubbock, TX, USA
| | - Luis G Cuello
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center Lubbock, TX, USA
| | - Guillermo A Altenberg
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center Lubbock, TX, USA
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19
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Oshima A. Structure and closure of connexin gap junction channels. FEBS Lett 2014; 588:1230-7. [PMID: 24492007 DOI: 10.1016/j.febslet.2014.01.042] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2013] [Revised: 01/24/2014] [Accepted: 01/24/2014] [Indexed: 11/29/2022]
Abstract
Connexin gap junctions comprise assembled channels penetrating two plasma membranes for which gating regulation is associated with a variety of factors, including voltage, pH, Ca(2+), and phosphorylation. Functional studies have established that various parts of the connexin peptides are related to channel closure and electrophysiology studies have provided several working models for channel gating. The corresponding structural models supporting these findings, however, are not sufficient because only small numbers of closed connexin structures have been reported. To fully understand the gating mechanisms, the channels should be visualized in both the open and closed states. Electron crystallography and X-ray crystallography studies recently revealed three-dimensional structures of connexin channels in a couple of states in which the main difference is the conformation of the N-terminal domain, which have helped to clarify the structure in regard to channel closure. Here the closure models for connexin gap junction channels inferred from structural and functional studies are described in the context of each domain of the connexin protein associated with gating modulation.
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Affiliation(s)
- Atsunori Oshima
- Cellular and Structural Physiology Institute (CeSPI), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
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20
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Role of gamma carboxylated Glu47 in connexin 26 hemichannel regulation by extracellular Ca²⁺: insight from a local quantum chemistry study. Biochem Biophys Res Commun 2014; 445:10-5. [PMID: 24468086 PMCID: PMC3969289 DOI: 10.1016/j.bbrc.2014.01.063] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/26/2013] [Accepted: 01/16/2014] [Indexed: 11/24/2022]
Abstract
QM calculations show that Ca2+ binds to γGlu47 in connexin hemichannels. Molecular models of increasing size are employed in hybrid DFT calculations. Ca2+ binding affects the interaction between γGlu47 and Arg75, Arg184. Ca2+ binding alters the structure in a critical region of connexin hemichannels.
Connexin hemichannels are regulated by several gating mechanisms, some of which depend critically on the extracellular Ca2+ concentration ([Ca2+]e). It is well established that hemichannel activity is inhibited at normal (∼1 mM) [Ca2+]e, whereas lowering [Ca2+]e to micromolar levels fosters hemichannel opening. Atomic force microscopy imaging shows significant and reversible changes of pore diameter at the extracellular mouth of Cx26 hemichannels exposed to different [Ca2+]e, however, the underlying molecular mechanisms are not fully elucidated. Analysis of the crystal structure of connexin 26 (Cx26) gap junction channels, corroborated by molecular dynamics (MD) simulations, suggests that several negatively charged amino acids create a favorable environment for low-affinity Ca2+ binding within the extracellular vestibule of the Cx26 hemichannel. In particular a highly conserved glutammic acid, found in position 47 in most connexins, is thought to undergo post translational gamma carboxylation (γGlu47), and is thus likely to play an important role in Ca2+ coordination. γGlu47 may also form salt bridges with two conserved arginines (Arg75 and Arg184 in Cx26), which are considered important in stabilizing the structure of the extracellular region. Using a combination of quantum chemistry methods, we analyzed the interaction between γGlu47, Arg75 and Arg184 in a Cx26 hemichannel model both in the absence and in the presence of Ca2+. We show that Ca2+ imparts significant local structural changes and speculate that these modifications may alter the structure of the extracellular loops in Cx26, and may thus account for the mechanism of hemichannel closure in the presence of mM [Ca2+]e.
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21
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A novel homozygous mutation in the EC1/EC2 interaction domain of the gap junction complex connexon 26 leads to profound hearing impairment. BIOMED RESEARCH INTERNATIONAL 2014; 2014:307976. [PMID: 24551843 PMCID: PMC3914288 DOI: 10.1155/2014/307976] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/29/2013] [Revised: 09/27/2013] [Accepted: 10/14/2013] [Indexed: 11/18/2022]
Abstract
To date, about 165 genetic loci or genes have been identified which are associated with nonsyndromal hearing impairment. In about half the cases, genetic defects in the GJB2 gene (connexin 26) are the most common cause of inner-ear deafness. The genes GJB2 and GJB6 are localized on chromosome 13q11-12 in tandem orientation. Connexins belong to the group of "gap junction" proteins, which form connexons, each consisting of six connexin molecules. These are responsible for the exchange of ions and smaller molecules between neighboring cells. Mutational analysis in genes GJB2 and GJB6 was brought by direct sequencing of the coding exons including the intron transitions. Here we show in the participating extended family a homozygous mutation c.506G>A, (TGC>TAC) p.Cys169Tyr, in the GJB2 gene, which could be proven for the first time and led to nonsyndromal severe hearing impairment in the afflicted patients. The mutation is located in the EC1/EC2 interaction complex of the gap junction connexon 26 complex and interrupts the K(+) circulation and therefore the ion homeostasis in the inner ear. The homozygous mutation p.Cys169Tyr identified here provides a novel insight into the structure-function relationship of the gap junction complex connexin/connexon 26.
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22
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de Zwart-Storm EA, Martin PE, van Steensel MAM. Gap junction diseases of the skin: novel insights from new mutations. ACTA ACUST UNITED AC 2014. [DOI: 10.1586/edm.09.47] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
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23
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Axelsen LN, Calloe K, Holstein-Rathlou NH, Nielsen MS. Managing the complexity of communication: regulation of gap junctions by post-translational modification. Front Pharmacol 2013; 4:130. [PMID: 24155720 PMCID: PMC3804956 DOI: 10.3389/fphar.2013.00130] [Citation(s) in RCA: 95] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2013] [Accepted: 09/30/2013] [Indexed: 12/21/2022] Open
Abstract
Gap junctions are comprised of connexins that form cell-to-cell channels which couple neighboring cells to accommodate the exchange of information. The need for communication does, however, change over time and therefore must be tightly controlled. Although the regulation of connexin protein expression by transcription and translation is of great importance, the trafficking, channel activity and degradation are also under tight control. The function of connexins can be regulated by several post translational modifications, which affect numerous parameters; including number of channels, open probability, single channel conductance or selectivity. The most extensively investigated post translational modifications are phosphorylations, which have been documented in all mammalian connexins. Besides phosphorylations, some connexins are known to be ubiquitinated, SUMOylated, nitrosylated, hydroxylated, acetylated, methylated, and γ-carboxyglutamated. The aim of the present review is to summarize our current knowledge of post translational regulation of the connexin family of proteins.
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Affiliation(s)
- Lene N Axelsen
- Department of Biomedical Sciences and The Danish National Research Foundation Centre for Cardiac Arrhythmia, Faculty of Health and Medical Sciences, University of Copenhagen Copenhagen, Denmark
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24
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Ambrosi C, Walker AE, DePriest AD, Cone AC, Lu C, Badger J, Skerrett IM, Sosinsky GE. Analysis of trafficking, stability and function of human connexin 26 gap junction channels with deafness-causing mutations in the fourth transmembrane helix. PLoS One 2013; 8:e70916. [PMID: 23967136 PMCID: PMC3744544 DOI: 10.1371/journal.pone.0070916] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2013] [Accepted: 06/25/2013] [Indexed: 01/24/2023] Open
Abstract
Human Connexin26 gene mutations cause hearing loss. These hereditary mutations are the leading cause of childhood deafness worldwide. Mutations in gap junction proteins (connexins) can impair intercellular communication by eliminating protein synthesis, mis-trafficking, or inducing channels that fail to dock or have aberrant function. We previously identified a new class of mutants that form non-functional gap junction channels and hemichannels (connexons) by disrupting packing and inter-helix interactions. Here we analyzed fourteen point mutations in the fourth transmembrane helix of connexin26 (Cx26) that cause non-syndromic hearing loss. Eight mutations caused mis-trafficking (K188R, F191L, V198M, S199F, G200R, I203K, L205P, T208P). Of the remaining six that formed gap junctions in mammalian cells, M195T and A197S formed stable hemichannels after isolation with a baculovirus/Sf9 protein purification system, while C202F, I203T, L205V and N206S formed hemichannels with varying degrees of instability. The function of all six gap junction-forming mutants was further assessed through measurement of dye coupling in mammalian cells and junctional conductance in paired Xenopus oocytes. Dye coupling between cell pairs was reduced by varying degrees for all six mutants. In homotypic oocyte pairings, only A197S induced measurable conductance. In heterotypic pairings with wild-type Cx26, five of the six mutants formed functional gap junction channels, albeit with reduced efficiency. None of the mutants displayed significant alterations in sensitivity to transjunctional voltage or induced conductive hemichannels in single oocytes. Intra-hemichannel interactions between mutant and wild-type proteins were assessed in rescue experiments using baculovirus expression in Sf9 insect cells. Of the four unstable mutations (C202F, I203T, L205V, N206S) only C202F and N206S formed stable hemichannels when co-expressed with wild-type Cx26. Stable M195T hemichannels displayed an increased tendency to aggregate. Thus, mutations in TM4 cause a range of phenotypes of dysfunctional gap junction channels that are discussed within the context of the X-ray crystallographic structure.
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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, United States of America
| | - Amy E. Walker
- National Center for Microscopy and Imaging Research, Center for Research in Biological Systems, University of California San Diego, La Jolla, California, United States of America
| | - Adam D. DePriest
- Biology Department, State University of New York Buffalo State, Buffalo, New York, United States of America
| | - Angela C. Cone
- National Center for Microscopy and Imaging Research, Center for Research in Biological Systems, University of California San Diego, La Jolla, California, United States of America
| | - Connie Lu
- National Center for Microscopy and Imaging Research, Center for Research in Biological Systems, University of California San Diego, La Jolla, California, United States of America
| | - John Badger
- DeltaG Technologies, San Diego, California, United States of America
| | - I. Martha Skerrett
- Biology Department, State University of New York Buffalo State, Buffalo, New York, United States of America
| | - Gina E. Sosinsky
- National Center for Microscopy and Imaging Research, Center for Research in Biological Systems, University of California San Diego, La Jolla, California, United States of America
- Department of Neurosciences, University of California San Diego, La Jolla, California, United States of America
- * E-mail:
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25
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Fasciani I, Temperán A, Pérez-Atencio LF, Escudero A, Martínez-Montero P, Molano J, Gómez-Hernández JM, Paino CL, González-Nieto D, Barrio LC. Regulation of connexin hemichannel activity by membrane potential and the extracellular calcium in health and disease. Neuropharmacology 2013; 75:479-90. [PMID: 23587648 DOI: 10.1016/j.neuropharm.2013.03.040] [Citation(s) in RCA: 64] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2012] [Revised: 03/26/2013] [Accepted: 03/27/2013] [Indexed: 10/27/2022]
Abstract
Connexins are thought to solely mediate cell-to-cell communication by forming gap junction channels composed of two membrane-spanning hemichannels positioned end-to-end. However, many if not all connexin isoforms also form functional hemichannels (i.e., the precursors of complete channels) that mediate the rapid exchange of ions, second messengers and metabolites between the cell interior and the interstitial space. Electrical and molecular signaling via connexin hemichannels is now widely recognized to be important in many physiological scenarios and pathological conditions. Indeed, mutations in connexins that alter hemichannel function have been implicated in several diseases. Here, we present a comprehensive overview of how hemichannel activity is tightly regulated by membrane potential and the external calcium concentration. In addition, we discuss the genetic mutations known to alter hemichannel function and their deleterious effects, of which a better understanding is necessary to develop novel therapeutic approaches for diseases caused by hemichannel dysfunction. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.
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Affiliation(s)
- Ilaria Fasciani
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain
| | - Ana Temperán
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain
| | - Leonel F Pérez-Atencio
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain
| | - Adela Escudero
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain; Unit of Molecular Genetics-INGEM, Hospital La Paz (IDIPAZ), Madrid, Spain
| | | | - Jesús Molano
- Unit of Molecular Genetics-INGEM, Hospital La Paz (IDIPAZ), Madrid, Spain
| | - Juan M Gómez-Hernández
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain
| | - Carlos L Paino
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain
| | - Daniel González-Nieto
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain; Center for Biomedical Technology, Universidad Politécnica de Madrid, Spain
| | - Luis C Barrio
- Unit of Experimental Neurology-Neurobiology, "Ramón y Cajal" Hospital (IRYCIS), Madrid, Spain.
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Oshima A, Matsuzawa T, Nishikawa K, Fujiyoshi Y. Oligomeric structure and functional characterization of Caenorhabditis elegans Innexin-6 gap junction protein. J Biol Chem 2013; 288:10513-21. [PMID: 23460640 DOI: 10.1074/jbc.m112.428383] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Innexin is the molecular component of invertebrate gap junctions. Here we successfully expressed and purified Caenorhabditis elegans innexin-6 (INX-6) gap junction channels and characterized the molecular dimensions and channel permeability using electron microscopy (EM) and microinjection of fluorescent dye tracers, respectively. Negative staining and thin-section EM of isolated INX-6 gap junction membranes revealed a loosely packed hexagonal lattice and a greater cross-sectional width than that of connexin26 and connexin43 (Cx43)-GFP. In gel filtration analysis, the elution profile of purified INX-6 channels in dodecyl maltoside solution exhibited a peak at ∼400 kDa that was shifted to ∼800 kDa in octyl glucose neopentyl glycol. We also obtained the class averages of purified INX-6 channels from these peak fractions by single particle analysis. The class average from the ∼800-kDa fraction showed features of the junction form with a longitudinal height of 220 Å, a channel diameter of 110 Å in the absence of detergent micelles, and an extracellular gap space of 60 Å, whereas the class averages from the ∼400-kDa fraction showed diameters of up to 140 Å in the presence of detergent micelles. These findings indicate that the purified INX-6 channels are predominantly hemichannels in dodecyl maltoside and docked junction channels in octyl glucose neopentyl glycol. Dye transfer experiments revealed that the INX-6-GFP-His channels are permeable to 3- and 10-kDa tracers, whereas no significant amounts of these tracers passed through the Cx43-GFP channels. Based on these findings, INX-6 channels have a larger overall structure and greater permeability than connexin channels.
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Affiliation(s)
- Atsunori Oshima
- Cellular and Structural Physiology Institute (CeSPI), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
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Abrams CK, Islam M, Mahmoud R, Kwon T, Bargiello TA, Freidin MM. Functional requirement for a highly conserved charged residue at position 75 in the gap junction protein connexin 32. J Biol Chem 2013; 288:3609-19. [PMID: 23209285 PMCID: PMC3561579 DOI: 10.1074/jbc.m112.392670] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2012] [Revised: 11/28/2012] [Indexed: 01/10/2023] Open
Abstract
Charcot Marie Tooth disease (CMT) is a group of inherited disorders characterized clinically by exclusively or predominantly peripheral nerve dysfunction. CMT1X, the most common form of X-linked CMT is caused by mutations in connexin 32 (Cx32). In this work, we used dual whole cell patch clamp recording to examine the functional effects of mutations at the Arg(75) position. This residue is highly conserved among members of the connexin family, and disease-causing mutations have been identified at this (or the corresponding) position in Cx26, Cx43, and Cx46. Thus, a better understanding of the effects of mutations of this position in Cx32 may have relevance to pathogenesis of a number of different human diseases. All three mutants associated with CMT1X (R75P, R75Q, and R75W) showed very low levels of coupling similar to those of the cells transfected with vector alone. Heterotypic pairing with Cx32 WT showed that the absence of coupling for these mutants in the homotypic configuration could be explained by shifts in their hemichannel G(j)-V(j) relations. Examination of the expression levels and gating characteristics of seven additional mutants (R75A, R75D, R75E, R75H, R75K, R75L, and R75V) at this position suggest that the positive charge at position 75 in Cx32 is required for normal channel function but not for gap junction assembly. Our studies also suggest that disease treatment strategies for CMT1X, which correct trafficking abnormalities in Cx32, may be ineffective for the group of mutations also conferring changes in gating properties of Cx32 channels.
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Affiliation(s)
- Charles K Abrams
- Department of Neurology, State University of New York, Downstate Medical Center, Brooklyn, New York 11203, USA.
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Dash-Wagh S, Jacob S, Lindberg S, Fridberger A, Langel U, Ulfendahl M. Intracellular Delivery of Short Interfering RNA in Rat Organ of Corti Using a Cell-penetrating Peptide PepFect6. MOLECULAR THERAPY. NUCLEIC ACIDS 2012; 1:e61. [PMID: 23232329 PMCID: PMC3528302 DOI: 10.1038/mtna.2012.50] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
RNA interference (RNAi) using short interfering RNA (siRNA) is an attractive therapeutic approach for treatment of dominant-negative mutations. Some rare missense dominant-negative mutations lead to congenital-hearing impairments. A variety of viral vectors have been tested with variable efficacy for modulating gene expression in inner ear. However, there is concern regarding their safety for clinical use. Here, we report a novel cell-penetrating peptide (CPP)-based nonviral approach for delivering siRNA into inner ear tissue using organotypic cultures as model system. PepFect6 (PF6), a variant of stearyl-TP10, was specially designed for improved delivery of siRNA by facilitating endosomal release. We show that PF6 was internalized by all cells without inducing cytotoxicity in cochlear cultures. PF6/siRNA nanoparticles lead to knockdown of target genes, a housekeeping gene and supporting cell-specific connexin 26. Interestingly, application of PF6/connexin 26 siRNA exhibited knockdown of both connexin 26 and 30 mRNA and their absence led to impaired intercellular communication as demonstrated by reduced transfer of calcein among the PF6/connexin 26-siRNA–treated cells. Thus, we conclude that PF6 is an efficient nonviral vector for delivery of siRNA, which can be applied as a tool for the development of siRNA-based therapeutic applications for hearing impairments.
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Affiliation(s)
- Suvarna Dash-Wagh
- Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden
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Fiori MC, Figueroa V, Zoghbi ME, Saéz JC, Reuss L, Altenberg GA. Permeation of calcium through purified connexin 26 hemichannels. J Biol Chem 2012; 287:40826-34. [PMID: 23048025 DOI: 10.1074/jbc.m112.383281] [Citation(s) in RCA: 73] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
BACKGROUND Indirect evidence suggests that connexin hemichannels are permeable to Ca(2+), but direct demonstration is lacking. RESULTS Calcium moves into liposomes containing purified Cx26 in response to a concentration gradient. CONCLUSION Cx26 hemichannels are permeable to Ca(2+). SIGNIFICANCE Cx26 hemichannels may play a role in Ca(2+) influx into cells under conditions that lead to hemichannel activation, such as ischemic damage. Gap junction channels communicate the cytoplasms of two cells and are formed by head to head association of two hemichannels, one from each of the cells. Gap junction channels and hemichannels are permeable to ions and hydrophilic molecules of up to M(r) 1,000, including second messengers and metabolites. Intercellular Ca(2+) signaling can occur by movement of a number of second messengers, including Ca(2+), through gap junction channels, or by a paracrine pathway that involves activation of purinergic receptors in neighboring cells following ATP release through hemichannels. Understanding Ca(2+) permeation through Cx26 hemichannels is important to assess the role of gap junction channels and hemichannels in health and disease. In this context, it is possible that increased Ca(2+) influx through hemichannels under ischemic conditions contributes to cell damage. Previous studies suggest Ca(2+) permeation through hemichannels, based on indirect arguments. Here, we demonstrate for the first time hemichannel permeability to Ca(2+) by measuring Ca(2+) transport through purified Cx26 hemichannels reconstituted in liposomes. We trapped the low affinity Ca(2+)-sensitive fluorescent probe Fluo-5N into the liposomes and followed the increases in intraliposomal [Ca(2+)] in response to an imposed [Ca(2+)] gradient. We show that Ca(2+) does move through Cx26 hemichannels and that the permeability of the hemichannels to Ca(2+) is high, similar to that for Na(+). We suggest that hemichannels can be a significant pathway for Ca(2+) influx into cells under conditions such as ischemia.
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Affiliation(s)
- Mariana C Fiori
- Department of Cell Physiology and Molecular Biophysics, Texas Tech Health Sciences Center, Lubbock, TX 79430-6551, USA
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30
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Iossa S, Marciano E, Franzé A. GJB2 Gene Mutations in Syndromic Skin Diseases with Sensorineural Hearing Loss. Curr Genomics 2012; 12:475-785. [PMID: 22547955 PMCID: PMC3219843 DOI: 10.2174/138920211797904098] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2011] [Revised: 07/27/2011] [Accepted: 08/10/2011] [Indexed: 01/10/2023] Open
Abstract
The GJB2 gene is located on chromosome 13q12 and it encodes the connexin 26, a transmembrane protein involved in cell-cell attachment of almost all tissues. GJB2 mutations cause autosomal recessive (DFNB1) and sometimes dominant (DFNA3) non-syndromic sensorineural hearing loss. Moreover, it has been demonstrated that connexins are involved in regulation of growth and differentiation of epidermis and, in fact, GJB2 mutations have also been identified in syndromic disorders with hearing loss associated with various skin disease phenotypes. GJB2 mutations associated with skin disease are, in general, transmitted with a dominant inheritance pattern. Nonsyndromic deafness is caused prevalently by a loss-of-function, while literature evidences suggest for syndromic deafness a mechanism based on gain-of-function. The spectrum of skin manifestations associated with some mutations seems to have a very high phenotypic variability. Why some mutations can lead to widely varying cutaneous manifestations is poorly understood and in particular, the reason why the skin disease-deafness phenotypes differ from each other thus remains unclear. This review provides an overview of recent findings concerning pathogenesis of syndromic deafness imputable to GJB2 mutations with an emphasis on relevant clinical genotype-phenotype correlations. After describing connexin 26 fundamental characteristics, the most relevant and recent information about its known mutations involved in the syndromic forms causing hearing loss and skin problems are summarized. The possible effects of the mutations on channel expression and function are discussed.
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Evaluation of the pathogenicity of GJB3 and GJB6 variants associated with nonsyndromic hearing loss. Biochim Biophys Acta Mol Basis Dis 2012; 1832:285-91. [PMID: 22617145 DOI: 10.1016/j.bbadis.2012.05.009] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2012] [Revised: 05/14/2012] [Accepted: 05/14/2012] [Indexed: 01/12/2023]
Abstract
A number of genes responsible for hearing loss are related to ion recycling and homeostasis in the inner ear. Connexins (Cx26 encoded by GJB2, Cx31 encoded by GJB3 and Cx30 encoded by GJB6) are core components of gap junctions in the inner ear. Gap junctions are intercellular communication channels and important factors that are associated with hearing loss. To date, a molecular genetics study of GJB3 and GJB6 as a causative gene for hearing loss has not been performed in Korea. This study was therefore performed to elucidate the genetic characteristics of Korean patients with nonsyndromic sensorineural hearing loss and to determine the pathological mechanism of hearing loss by analyzing the intercellular communication function of Cx30 and Cx31 variants. Sequencing analysis of the GJB3 and GJB6 genes in our population revealed a total of nine variants, including four novel variants in the two genes. Three of the novel variants (Cx31-p.V27M, Cx31-p.V43M and Cx-30-p.I248V) and two previously reported variants (Cx31-p.V84I and Cx30-p.A40V) were selected for functional studies using a pathogenicity prediction program and assessed for whether the mutations were located in a conserved region of the protein. The results of biochemical and ionic coupling tests showed that both the Cx31-p.V27M and Cx31-p.V84I variants did not function normally when each was expressed as a heterozygote with the wild-type Cx31. This study demonstrated that two variants of Cx31 were pathogenic mutations with deleterious effect. This information will be valuable in understanding the pathogenic role of GJB3 and GJB6 mutations associated with hearing loss.
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Ek-Vitorin JF, Burt JM. Structural basis for the selective permeability of channels made of communicating junction proteins. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:51-68. [PMID: 22342665 DOI: 10.1016/j.bbamem.2012.02.003] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/03/2011] [Revised: 01/24/2012] [Accepted: 02/01/2012] [Indexed: 01/08/2023]
Abstract
The open state(s) of gap junction channels is evident from their permeation by small ions in response to an applied intercellular (transjunctional/transchannel) voltage gradient. That an open channel allows variable amounts of current to transit from cell-to-cell in the face of a constant intercellular voltage difference indicates channel open/closing can be complete or partial. The physiological significance of such open state options is, arguably, the main concern of junctional regulation. Because gap junctions are permeable to many substances, it is sensible to inquire whether and how each open state influences the intercellular diffusion of molecules as valuable as, but less readily detected than current-carrying ions. Presumably, structural changes perceived as shifts in channel conductivity would significantly alter the transjunctional diffusion of molecules whose limiting diameter approximates the pore's limiting diameter. Moreover, changes in junctional permeability to some molecules might occur without evident changes in conductivity, either at macroscopic or single channel level. Open gap junction channels allow the exchange of cytoplasmic permeants between contacting cells by simple diffusion. The identity of such permeants, and the functional circumstances and consequences of their junctional exchange presently constitute the most urgent (and demanding) themes of the field. Here, we consider the necessity for regulating this exchange, the possible mechanism(s) and structural elements likely involved in such regulation, and how regulatory phenomena could be perceived as changes in chemical vs. electrical coupling; an overall reflection on our collective knowledge of junctional communication is then applied to suggest new avenues of research. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.
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Carnarius C, Kreir M, Krick M, Methfessel C, Moehrle V, Valerius O, Brüggemann A, Steinem C, Fertig N. Green fluorescent protein changes the conductance of connexin 43 (Cx43) hemichannels reconstituted in planar lipid bilayers. J Biol Chem 2011; 287:2877-86. [PMID: 22139870 DOI: 10.1074/jbc.m111.319871] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
In mammalian tissues, connexin 43 (Cx43) is the most prominent member of the connexin family. In a single lipid bilayer, six connexin subunits assemble into a hemichannel (connexon). Direct communication of apposing cells is realized by two adjacent hemichannels, which can form gap junction channels. Here, we established an expression system in Pichia pastoris to recombinantly produce and purify Cx43 as well as Cx43 fused to green fluorescent protein (GFP). Proteins were isolated from crude cell membrane fractions via affinity chromatography. Cx43 and Cx43-GFP hemichannels were reconstituted in giant unilamellar vesicles as proven by fluorescence microscopy, and their electrophysiological behavior was analyzed on the single channel level by planar patch clamping. Cx43 and Cx43-GFP both showed an ohmic behavior and a voltage-dependent open probability. Cx43 hemichannels exhibited one major mean conductance of 224 ± 26 picosiemens (pS). In addition, a subconductance state at 124 ± 5 pS was identified. In contrast, the analysis of Cx43-GFP single channels revealed 10 distinct conductance states in the range of 15 to 250 pS, with a larger open probability at 0 mV as compared with Cx43, which suggests that intermolecular interactions between the GFP molecules alter the electrophysiology of the protein.
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Affiliation(s)
- Christian Carnarius
- Institute of Organic and Biomolecular Chemistry, University of Göttingen, Tammannstrasse 2, 37077 Göttingen, Germany
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Two-dimensional kinetics of inter-connexin interactions from single-molecule force spectroscopy. J Mol Biol 2011; 412:72-9. [PMID: 21802430 DOI: 10.1016/j.jmb.2011.07.013] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2011] [Revised: 07/07/2011] [Accepted: 07/11/2011] [Indexed: 11/23/2022]
Abstract
Gap junction channels are intercellular channels that form by docking the extracellular loops of connexin protein subunits. While the structure and function of gap junctions as intercellular channels have been characterized using different techniques, the physics of the inter-connexin interaction remain unknown. Moreover, as far as we know, the capacity of gap junction channels to work as adhesion complexes supporting pulling forces has not yet been quantitatively addressed. We report the first quantitative characterization of the kinetics and binding strength of the interaction of a short peptide mimicking extracellular loop 2 of Cx26 with membrane-reconstituted Cx26, combining the imaging and force spectroscopy capabilities of atomic force microscopy. The fast dissociation rate inferred a dynamic bond, while the slow association rate reflected the reduced flexibility and small size of extracellular loops. Our results propose the gap junction channel as an adhesion complex that associates slowly and dissociates fast at low force but is able to support important pulling forces in its native, hexameric form.
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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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Luo K, Turnbull MW. Characterization of nonjunctional hemichannels in caterpillar cells. JOURNAL OF INSECT SCIENCE (ONLINE) 2011; 11:6. [PMID: 21521140 PMCID: PMC3281302 DOI: 10.1673/031.011.0106] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/24/2010] [Accepted: 12/08/2010] [Indexed: 05/30/2023]
Abstract
Recent studies have demonstrated that hemichannels, which form gap junctions when paired from apposing cells, may serve additional roles when unpaired including cell adhesion and paracrine communication. Hemichannels in mammals are formed by connexins or pannexins, while in insects they are formed by pannexin homologues termed innexins. The formation of functional gap junctions by insect innexins has been established, although their ability to form functional nonjunctional hemichannels has not been reported. Here the characteristics of nonjunctional hemichannels were examined in three lepidopteran cell types, two cell lines (High Five and Sf9) and explanted hemocytes from Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae). Selective fluorescent dye uptake by hemichannels was observed in a significant minority of cells, using fluorescence microscopy and flow cytometry. Carbenoxelone, an inhibitor of mammalian junctions, disrupted dye uptake, while flufenamic acid and mefloquine did not. The presence of Ca(2+) and Mg(2+) in the media increased hemichannel activity. Additionally, lipopolysaccharide, a stimulator of immune activity in lepidopterans, decreased dye uptake. These results demonstrate for the first time the activity of nonjunctional hemichannels in insect cells, as well as pharmacological tools to manipulate them. These results will facilitate the further examination of the role of innexins and nonjunctional hemichannels in insect cell biology, including paracrine signaling, and comparative studies of mammalian pannexins and insect innexins.
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Affiliation(s)
- Kaijun Luo
- Current Address: School of Life Sciences, Yunnan University, Kunming, Yunnan 650091, China
| | - Matthew W. Turnbull
- Department of Entomology, Soils, and Plant Sciences, Clemson University, Clemson, South Carolina 29634-03 15, USA
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Abstract
BACKGROUND Connexins are a family of transmembrane proteins that are widely expressed in the human body. Connexins play an important role in cell-cell communication and homeostasis in various tissues by forming gap junction channels, which enable a direct passage of ions or metabolites from one cell to another. Twenty-one different connexins are expressed in humans, each having distinct expression patterns and regulation properties. Knowledge on this family of proteins can be gained by making an inventory of mutations and associated diseases in human. DESIGN PubMed and other relevant databases were searched. In addition, key review articles were screened for relevant original publications. Sections of representative organs were photographed and annotated. RESULTS The crucial role of connexins is highlighted by the discovery of mutations in connexin genes which cause a variety of disorders such as myelin-related diseases, skin disorders, hearing loss, congenital cataract, or more complex syndromes such as the oculodendrodigital dysplasia. This review systematically addresses current knowledge on mutations in connexin genes and disease, focusing on the correlation between genetic defects, cellular phenotypes and clinical manifestations. CONCLUSIONS The review of diseases caused by mutations in connexin genes highlights the essential nature of connexin function and intercellular communication in tissue homeostasis.
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Affiliation(s)
- Anna Pfenniger
- Department of Pathology and Immunology, Faculty of Medicine, University of Geneva, Geneva, Switzerland
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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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Zhang J, Scherer SS, Yum SW. Dominant Cx26 mutants associated with hearing loss have dominant-negative effects on wild type Cx26. Mol Cell Neurosci 2010; 47:71-8. [PMID: 21040787 DOI: 10.1016/j.mcn.2010.10.002] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2010] [Revised: 09/23/2010] [Accepted: 10/15/2010] [Indexed: 11/19/2022] Open
Abstract
Mutations in GJB2, the gene encoding the human gap junction protein connexin26 (Cx26), cause either non-syndromic hearing loss or syndromes affecting both hearing and skin. We have investigated whether dominant Cx26 mutants can interact physically with wild type Cx26. HeLa cells stably expressing wild type Cx26 were transiently transfected to co-express nine individual dominant Cx26 mutants; six associated with non-syndromic hearing loss (W44C, W44S, R143Q, D179N, R184Q, and C202F) and three associated with hearing loss and palmoplantar keratoderma (G59A, R75Q, and R75W). All mutants co-localized and co-immunoprecipitated with wild type Cx26, indicating that they interact physically, likely by forming admixed heteromeric/heterotypic channels. Furthermore, all nine mutants inhibited the transfer of calcein in cells stably expressing Cx26, demonstrating that they each have dominant effects on wild type Cx26. Taken together, these results show that dominant-negative effects of these Cx26 mutants likely contribute to the pathogenesis of hearing loss.
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Affiliation(s)
- Junxian Zhang
- Department of Pediatrics, Division of Neurology, The Children's Hospital of Philadelphia, USA
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40
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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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Kamiya K, Tsumoto K, Arakawa S, Shimizu S, Morita I, Yoshimura T, Akiyoshi K. Preparation of connexin43-integrated giant Liposomes by a baculovirus expression-liposome fusion method. Biotechnol Bioeng 2010; 107:836-43. [DOI: 10.1002/bit.22845] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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42
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Nakagawa S, Maeda S, Tsukihara T. Structural and functional studies of gap junction channels. Curr Opin Struct Biol 2010; 20:423-30. [PMID: 20542681 DOI: 10.1016/j.sbi.2010.05.003] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2010] [Revised: 04/26/2010] [Accepted: 05/10/2010] [Indexed: 11/28/2022]
Abstract
X-ray analysis of the human connexin26 gap junction channel has provided structural details of its open state. The gap junction channel is formed by paired hemichannels on two adjacent cells; each hemichannel consists of six protomers, and exhibits a six-fold symmetry. The protomer folds in a typical four-helix bundle. The amino-terminal region folds in a short helix and is inserted into the lumen to form a funnel structure. The structure of the amino-terminal region could explain the channel's gating mechanism. Extensive interactions between two hemichannels allow the gap junction channel to tightly connect two adjacent cells. The gap junction, which consists of hundreds of gap junction channels, could both serve as an intracellular channel and contribute to cellular adhesion.
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Affiliation(s)
- So Nakagawa
- Institute for Protein Research, Osaka University, OLABB, 6-2-3 Furuedai, Suita 565-0874, Japan
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43
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Ambrosi C, Gassmann O, Pranskevich JN, Boassa D, Smock A, Wang J, Dahl G, Steinem C, Sosinsky GE. Pannexin1 and Pannexin2 channels show quaternary similarities to connexons and different oligomerization numbers from each other. J Biol Chem 2010; 285:24420-31. [PMID: 20516070 DOI: 10.1074/jbc.m110.115444] [Citation(s) in RCA: 128] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Pannexins are homologous to innexins, the invertebrate gap junction family. However, mammalian pannexin1 does not form canonical gap junctions, instead forming hexameric oligomers in single plasma membranes and intracellularly. Pannexin1 acts as an ATP release channel, whereas less is known about the function of Pannexin2. We purified cellular membranes isolated from MDCK cells stably expressing rat Pannexin1 or Pannexin2 and identified pannexin channels (pannexons) in single membranes by negative stain and immunogold labeling. Protein gel and Western blot analysis confirmed Pannexin1 (Panx1) or Pannexin2 (Panx2) as the channel-forming proteins. We expressed and purified Panx1 and Panx2 using a baculovirus Sf9 expression system and obtained doughnut-like structures similar to those seen previously in purified connexin hemichannels (connexons) and mammalian membranes. Purified pannexons were comparable in size and overall appearance to Connexin46 and Connexin50 connexons. Pannexons and connexons were further analyzed by single-particle averaging for oligomer and pore diameters. The oligomer diameter increased with increasing monomer molecular mass, and we found that the measured oligomeric pore diameter for Panxs was larger than for Connexin26. Panx1 and Panx2 formed active homomeric channels in Xenopus oocytes and in vitro vesicle assays. Cross-linking and native gels of purified homomeric full-length and a C-terminal Panx2 truncation mutant showed a banding pattern more consistent with an octamer. We purified Panx1/Panx2 heteromeric channels and found that they were unstable over time, possibly because Panx1 and Panx2 homomeric pannexons have different monomer sizes and oligomeric symmetry from each other.
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Affiliation(s)
- Cinzia Ambrosi
- National Center for Microscopy and Imaging Research, University of California San Diego, La Jolla, California 92093-0608, USA
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44
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Dominant connexin26 mutants associated with human hearing loss have trans-dominant effects on connexin30. Neurobiol Dis 2010; 38:226-36. [PMID: 20096356 DOI: 10.1016/j.nbd.2010.01.010] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2009] [Revised: 11/27/2009] [Accepted: 01/12/2010] [Indexed: 11/23/2022] Open
Abstract
Dominant mutations in GJB2, the gene encoding the human gap junction protein connexin26 (Cx26), cause hearing loss. We investigated whether dominant Cx26 mutants interact directly with Cx30. HeLa cells stably expressing nine dominant Cx26 mutants, six associated with non-syndromic hearing loss (W44C, W44S, R143Q, D179N, R184Q and C202F) and three associated with hearing loss and palmoplantar keratoderma (G59A, R75Q and R75W), individually or together with Cx30, were analyzed by immunocytochemistry, co-immunoprecipitation, and functional assays (scrape-loading and/or fluorescence recovery after photobleaching). When expressed alone, all mutants formed gap junction plaques, but with impaired intercellular dye transfer. When expressed with Cx30, all mutants co-localized and co-immunoprecipitated with Cx30, indicating they likely co-assembled into heteromers. Furthermore, 8/9 Cx26 mutants inhibited the transfer of neurobiotin or calcein, indicating that these Cx26 mutants have trans-dominant effects on Cx30, an effect that may contribute to the pathogenesis of hearing loss.
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45
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Gassmann O, Kreir M, Ambrosi C, Pranskevich J, Oshima A, Röling C, Sosinsky G, Fertig N, Steinem C. The M34A mutant of Connexin26 reveals active conductance states in pore-suspending membranes. J Struct Biol 2009; 168:168-76. [PMID: 19236918 PMCID: PMC2785080 DOI: 10.1016/j.jsb.2009.02.004] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2008] [Revised: 02/09/2009] [Accepted: 02/11/2009] [Indexed: 11/20/2022]
Abstract
Connexin26 (Cx26) is a member of the connexin family, the building blocks for gap junction intercellular channels. These dodecameric assemblies are involved in gap junction-mediated cell-cell communication allowing the passage of ions and small molecules between two neighboring cells. Mutations in Cx26 lead to the disruption of gap junction-mediated intercellular communication with consequences such as hearing loss and skin disorders. We show here that a mutant of Cx26, M34A, forms an active hemichannel in lipid bilayer experiments. A comparison with the Cx26 wild-type is presented. Two different techniques using micro/nano-structured substrates for the formation of pore-suspending lipid membranes are used. We reconstituted the Cx26 wild-type and Cx26M34A into artificial lipid bilayers and observed single channel activity for each technique, with conductance levels of around 35, 70 and 165 pS for the wild-type. The conductance levels of Cx26M34A were found at around 45 and 70 pS.
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Affiliation(s)
- Oliver Gassmann
- Institute for Organic and Biomolecular Chemistry, University of Göttingen, Germany
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46
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Kyle JW, Berthoud VM, Kurutz J, Minogue PJ, Greenspan M, Hanck DA, Beyer EC. The N terminus of connexin37 contains an alpha-helix that is required for channel function. J Biol Chem 2009; 284:20418-27. [PMID: 19478091 PMCID: PMC2740466 DOI: 10.1074/jbc.m109.016907] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2009] [Revised: 05/24/2009] [Indexed: 12/11/2022] Open
Abstract
The cytoplasmic N-terminal domain of connexins has been implicated in multiple aspects of gap junction function, including connexin trafficking/assembly and channel gating. A synthetic peptide corresponding to the first 23 amino acids of human connexin37 was prepared, and circular dichroism and nuclear magnetic resonance studies showed that this N-terminal peptide was predominantly alpha-helical between glycine 5 and glutamate 16. The importance of this structure for localization of the protein at appositional membranes and channel function was tested by expression of site-directed mutants of connexin37 in which amino acids leucine 10 and glutamine 15 were replaced with prolines or alanines. Wild type connexin37 and both substitution mutants localized to appositional membranes between transfected HeLa cells. The proline mutant did not allow intercellular transfer of microinjected neurobiotin; the alanine mutant allowed transfer, but less extensively than wild type connexin37. When expressed alone in Xenopus oocytes, wild type connexin37 produced hemichannel currents, but neither of the double substitution mutants produced detectable currents. The proline mutant (but not the alanine mutant) inhibited co-expressed wild type connexin37. Taken together, our data suggest that the alpha-helical structure of the connexin37 N terminus may be dispensable for protein localization, but it is required for channel and hemichannel function.
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Affiliation(s)
- John W. Kyle
- From the Departments of Medicine (Section of Cardiology)
| | | | - Josh Kurutz
- Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois 60637
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47
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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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48
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Hoang Dinh E, Ahmad S, Chang Q, Tang W, Stong B, Lin X. Diverse deafness mechanisms of connexin mutations revealed by studies using in vitro approaches and mouse models. Brain Res 2009; 1277:52-69. [PMID: 19230829 DOI: 10.1016/j.brainres.2009.02.008] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2009] [Revised: 02/01/2009] [Accepted: 02/04/2009] [Indexed: 12/12/2022]
Abstract
Mutations in connexins (Cxs), the constitutive protein subunits of gap junction (GJ) intercellular channels, are one of the most common human genetic defects that cause severe prelingual non-syndromic hearing impairments. Many subtypes of Cxs (e.g., Cxs 26, 29, 30, 31, 43) and pannexins (Panxs) are expressed in the cochlea where they contribute to the formation of a GJ-based intercellular communication network. Cx26 and Cx30 are the predominant cochlear Cxs and they co-assemble in most GJ plaques to form hybrid GJs. The cellular localization of specific Cx subtypes provides a basis for understanding the molecular structure of GJs and hemichannels in the cochlea. Information about the interactions among the various co-assembled Cx partners is critical to appreciate the functional consequences of various types of genetic mutations. In vitro studies of reconstituted GJs in cell lines have yielded surprisingly heterogeneous mechanisms of dysfunction caused by various Cx mutations. Availability of multiple lines of Cx-mutant mouse models has provided some insight into the pathogenesis processes in the cochlea of deaf mice. Here we summarize recent advances in understanding the structure and function of cochlear GJs and give a critical review of current findings obtained from both in vitro studies and mouse models on the mechanisms of Cx mutations that lead to cell death in the cochlea and hearing loss.
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Affiliation(s)
- Emilie Hoang Dinh
- Department of Otolaryngology, Emory University School of Medicine, 615 Michael Street, Atlanta, GA 30322-3030, USA
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49
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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: 110] [Impact Index Per Article: 7.3] [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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50
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Gap junctions and connexins in the inner ear: their roles in homeostasis and deafness. Curr Opin Otolaryngol Head Neck Surg 2009; 16:452-7. [PMID: 18797288 DOI: 10.1097/moo.0b013e32830e20b0] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
PURPOSE OF REVIEW Mutations in GJB2 and GJB6, the genes encoding the gap-junction proteins connexin 26 and connexin 30, are the most common cause of autosomal recessive nonsyndromic deafness in many populations across the world. In this review, we discuss current ideas about the roles of gap junctions in the inner ear and the implications of connexin mutations on auditory function. RECENT FINDINGS In recent years, a complex picture of the roles of gap junctions in cochlear physiology emerged. Rather than being mere conduits for the circulation of potassium ions in the inner ear, gap junctions have been implicated in intercellular signaling among nonsensory cells and may be involved in the maintenance of the endothelial barrier in the stria vascularis. Studies of mutant channels and mouse models for connexin-related deafness have provided valuable insights into some of the mechanisms by which connexin dysfunction causes cochlear degeneration. They have also identified potential therapeutic interventions for specific connexin mutations, such as the restoration of normal connexin 26 protein levels in GJB6-associated deafness. SUMMARY Despite recent advances, a better understanding of the complexity of gap-junctional communication in the inner ear and the structure-function relationships of connexin proteins is required for the development of mechanism-based treatments of connexin-associated hearing loss.
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