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Fisher CG, Falk MM. Endocytosis and Endocytic Motifs across the Connexin Gene Family. Int J Mol Sci 2023; 24:12851. [PMID: 37629031 PMCID: PMC10454166 DOI: 10.3390/ijms241612851] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2023] [Revised: 08/11/2023] [Accepted: 08/13/2023] [Indexed: 08/27/2023] Open
Abstract
Proteins fated to be internalized by clathrin-mediated endocytosis require an endocytic motif, where AP-2 or another adaptor protein can bind and recruit clathrin. Tyrosine and di-leucine-based sorting signals are such canonical motifs. Connexin 43 (Cx43) has three canonical tyrosine-based endocytic motifs, two of which have been previously shown to recruit clathrin and mediate its endocytosis. In addition, di-leucine-based motifs have been characterized in the Cx32 C-terminal domain and shown to mediate its endocytosis. Here, we examined the amino acid sequences of all 21 human connexins to identify endocytic motifs across the connexin gene family. We find that although there is limited conservation of endocytic motifs between connexins, 14 of the 21 human connexins contain one or more canonical tyrosine or di-leucine-based endocytic motif in their C-terminal or intracellular loop domain. Three connexins contain non-canonical (modified) di-leucine motifs. However, four connexins (Cx25, Cx26, Cx31, and Cx40.1) do not harbor any recognizable endocytic motif. Interestingly, live cell time-lapse imaging of different GFP-tagged connexins that either contain or do not contain recognizable endocytic motifs readily undergo endocytosis, forming clearly identifiable annular gap junctions when expressed in HeLa cells. How connexins without defined endocytic motifs are endocytosed is currently not known. Our results demonstrate that an array of endocytic motifs exists in the connexin gene family. Further analysis will establish whether the sites we identified in this in silico analysis are legitimate endocytic motifs.
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Affiliation(s)
| | - Matthias M. Falk
- Department of Biological Sciences, Lehigh University, 111 Research Drive, Bethlehem, PA 18015, USA
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Schadzek P, Stahl Y, Preller M, Ngezahayo A. Analysis of the dominant mutation N188T of human connexin46 (hCx46) using concatenation and molecular dynamics simulation. FEBS Open Bio 2019; 9:840-850. [PMID: 31034164 PMCID: PMC6487695 DOI: 10.1002/2211-5463.12624] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2018] [Revised: 02/06/2019] [Accepted: 02/26/2019] [Indexed: 11/27/2022] Open
Abstract
Connexins (Cx) are proteins that form cell-to-cell gap junction channels. A mutation at position 188 in the second extracellular loop (E2) domain of hCx46 has been linked to an autosomal dominant zonular pulverulent cataract. As it is dominantly inherited, it is possible that the mutant variant affects the co-expressed wild-type Cx and/or its interaction with other cellular components. Here, we proposed to use concatenated hCx46wt-hCx46N188T and hCx46N188T-hCx46wt to analyze how hCx46N188T affected co-expressed hCx46wt to achieve a dominant inheritance. Heterodimer hCx46wt-hCx46N188T formed fewer gap junction plaques compared to homodimer hCx46wt-hCx46wt, while the hCx46N188T-hCx46N188T homodimer formed almost no gap junction plaques. Dye uptake experiments showed that hemichannels of concatenated variants were similar to hemichannels of monomers. Molecular dynamics simulations revealed that for docking, the N188 of a protomer was engaged in hydrogen bonds (HBs) with R180, N189, and D191 of the counterpart protomer of the adjacent hemichannel. T188 suppressed the formation of HBs between protomers. Molecular dynamics simulations of an equimolar hCx46wt/hCx46N188T gap junction channel revealed a reduced number of HBs between protomers, suggesting reduction of gap junction channels between lens fibers co-expressing the variants.
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Affiliation(s)
- Patrik Schadzek
- Institute of Cell Biology and BiophysicsDepartment of Cell Physiology and BiophysicsLeibniz University HannoverGermany
| | - Yannick Stahl
- Institute of Cell Biology and BiophysicsDepartment of Cell Physiology and BiophysicsLeibniz University HannoverGermany
| | - Matthias Preller
- Institute for Biophysical ChemistryHannover Medical School (MHH)Germany
- Centre for Structural Systems Biology, DESY‐CampusHamburgGermany
| | - Anaclet Ngezahayo
- Institute of Cell Biology and BiophysicsDepartment of Cell Physiology and BiophysicsLeibniz University HannoverGermany
- Center for System Neurosciences (ZSN)HannoverGermany
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Schadzek P, Hermes D, Stahl Y, Dilger N, Ngezahayo A. Concatenation of Human Connexin26 (hCx26) and Human Connexin46 (hCx46) for the Analysis of Heteromeric Gap Junction Hemichannels and Heterotypic Gap Junction Channels. Int J Mol Sci 2018; 19:E2742. [PMID: 30217016 PMCID: PMC6163895 DOI: 10.3390/ijms19092742] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2018] [Revised: 09/10/2018] [Accepted: 09/11/2018] [Indexed: 12/16/2022] Open
Abstract
Gap junction channels and hemichannels formed by concatenated connexins were analyzed. Monomeric (hCx26, hCx46), homodimeric (hCx46-hCx46, hCx26-hCx26), and heterodimeric (hCx26-hCx46, hCx46-hCx26) constructs, coupled to GFP, were expressed in HeLa cells. Confocal microscopy showed that the tandems formed gap junction plaques with a reduced plaque area compared to monomeric hCx26 or hCx46. Dye transfer experiments showed that concatenation allows metabolic transfer. Expressed in Xenopus oocytes, the inside-out patch-clamp configuration showed single channels with a conductance of about 46 pS and 39 pS for hemichannels composed of hCx46 and hCx26 monomers, respectively, when chloride was replaced by gluconate on both membrane sides. The conductance was reduced for hCx46-hCx46 and hCx26-hCx26 homodimers, probably due to the concatenation. Heteromerized hemichannels, depending on the connexin-order, were characterized by substates at 26 pS and 16 pS for hCx46-hCx26 and 31 pS and 20 pS for hCx26-hCx46. Because of the linker between the connexins, the properties of the formed hemichannels and gap junction channels (e.g., single channel conductance) may not represent the properties of hetero-oligomerized channels. However, should the removal of the linker be successful, this method could be used to analyze the electrical and metabolic selectivity of such channels and the physiological consequences for a tissue.
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Affiliation(s)
- Patrik Schadzek
- Institut für Biophysik, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany.
| | - Doris Hermes
- Institut für Biophysik, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany.
- Department of Clinical Neurophysiology, University of Göttingen, Robert-Koch Str. 40, D-37075 Göttingen, Germany.
| | - Yannick Stahl
- Institut für Biophysik, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany.
| | - Nadine Dilger
- Institut für Biophysik, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany.
| | - Anaclet Ngezahayo
- Institut für Biophysik, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany.
- Zentrum für Systemische Neurowissenschaften Stiftung Tierärztliche Hochschule Hannover, Bünteweg 2, 30559 Hannover, Germany.
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Aasen T, Johnstone S, Vidal-Brime L, Lynn KS, Koval M. Connexins: Synthesis, Post-Translational Modifications, and Trafficking in Health and Disease. Int J Mol Sci 2018; 19:ijms19051296. [PMID: 29701678 PMCID: PMC5983588 DOI: 10.3390/ijms19051296] [Citation(s) in RCA: 62] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2018] [Revised: 04/20/2018] [Accepted: 04/21/2018] [Indexed: 02/06/2023] Open
Abstract
Connexins are tetraspan transmembrane proteins that form gap junctions and facilitate direct intercellular communication, a critical feature for the development, function, and homeostasis of tissues and organs. In addition, a growing number of gap junction-independent functions are being ascribed to these proteins. The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional level, and undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function. Here, we summarize these key regulatory events, with emphasis on how these affect connexin multifunctionality in health and disease.
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Affiliation(s)
- Trond Aasen
- Translational Molecular Pathology, Vall d'Hebron Institute of Research (VHIR), Autonomous University of Barcelona, CIBERONC, 08035 Barcelona, Spain.
| | - Scott Johnstone
- Robert M. Berne Cardiovascular Research Center, University of Virginia School of Medicine, P.O. Box 801394, Charlottesville, VI 22908, USA.
- Institute of Cardiovascular and Medical Sciences, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8TT, UK.
| | - Laia Vidal-Brime
- Translational Molecular Pathology, Vall d'Hebron Institute of Research (VHIR), Autonomous University of Barcelona, CIBERONC, 08035 Barcelona, Spain.
| | - K Sabrina Lynn
- Division of Pulmonary, Allergy, Critical Care and Sleep Medicine, Department of Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA.
| | - Michael Koval
- Division of Pulmonary, Allergy, Critical Care and Sleep Medicine, Department of Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA.
- Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA.
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Chen L, Su D, Li S, Guan L, Shi C, Li D, Hu S, Ma X. The connexin 46 mutant (V44M) impairs gap junction function causing congenital cataract. J Genet 2017; 96:969-976. [PMID: 29321356 DOI: 10.1007/s12041-017-0861-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Connexin 46 (Cx46) is important for gap junction channels formation which plays crucial role in the preservation of lens homeostasis and transparency. Previously, we have identified a missense mutation (p.V44M) of Cx46 in a congenital cataract family. This study aims at dissecting the potential pathogenesis of the causative mutant of cataract. Plasmids carrying wild-type (wt) and mutant (V44M) of Cx46 were constructed and expressed in Hela cells respectively.Western blotting and fluorescence microscopy were applied to analyse the expression and subcellular localization of recombinant proteins, respectively. Scrape loading dye transfer experiment was performed to detect the transfer capability of gap junction channels among cells expressed V44Mmutant. The results demonstrated that in transfected Hela cells, both wt-Cx46 and Cx46 V44M were localized abundantly in the plasma membrane. No significant difference was found between the protein expressions of the two types of Cx46. The fluorescent localization assay revealed the plaque formation, significantly reduced in the cells expressing Cx46 V44M. Immunoblotting analysis demonstrated that formation of Triton X-100 insoluble complex decreased obviously in mutant Cx46. Additionally, the scrape-loading dye-transfer experiment showed a lower dye diffusion distance of Cx46 V44M cells, which indicates that the gap junction intercellular communication activity was aberrant. Human Cx46 V44M mutant causing cataracts result in abnormally decreased formation of gap junction plaques and impaired gap junction channel function.
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Affiliation(s)
- Lijuan Chen
- Department of Ophthalmology, Hongqi Hospital of Mudanjiang Medical College, Mudanjiang 157000, Heilongjiang Province, People's Republic of China.
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Abstract
The lens is an avascular organ composed of an anterior epithelial cell layer and fiber cells that form the bulk of the organ. The lens expresses connexin43 (Cx43), connexin46 (Cx46) and connexin50 (Cx50). Epithelial Cx50 has critical roles in cell proliferation and differentiation, likely involving growth factor-dependent signaling pathways. Both Cx46 and Cx50 are crucial for lens transparency; mutations in their genes have been linked to congenital and age-related cataracts. Congenital cataract-associated connexin mutants can affect protein trafficking, stability and/or function, and the functional effects may differ between gap junction channels and hemichannels. Dominantly inherited cataracts may result from effects of the connexin mutant on its wild type isotype, the other co-expressed wild type connexin and/or its interaction with other cellular components.
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Affiliation(s)
| | - Anaclet Ngezahayo
- Institute of Biophysics, Leibniz University Hannover, Hannover, Germany.
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Schadzek P, Schlingmann B, Schaarschmidt F, Lindner J, Koval M, Heisterkamp A, Preller M, Ngezahayo A. The cataract related mutation N188T in human connexin46 (hCx46) revealed a critical role for residue N188 in the docking process of gap junction channels. BIOCHIMICA ET BIOPHYSICA ACTA 2016; 1858:57-66. [PMID: 26449341 DOI: 10.1016/j.bbamem.2015.10.001] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/11/2015] [Revised: 09/28/2015] [Accepted: 10/01/2015] [Indexed: 12/16/2022]
Abstract
The mutation N188T in human connexin46 (hCx46) correlates with a congenital nuclear pulverulent cataract. This mutation is in the second extracellular loop, a domain involved in docking of gap junction hemichannels. To analyze the functional consequences of this mutation, we expressed hCx46N188T and the wild type (hCx46wt) in Xenopus oocytes and HeLa cells. In Xenopus oocytes, hemichannels formed by hCx46wt and hCx46N188T had similar electrical properties. Additionally, a Ca(2+) and La(3+) sensitive current was observed in HeLa cells expressing eGFP-labeled hCx46wt or eGFP-labeled hCx46N188T. These results suggest that the N188T mutation did not alter apparent expression and the membrane targeting of the protein. Cells expressing hCx46wt-eGFP formed gap junction plaques, but plaques formed by hCx46N188T were extremely rare. A reduced plaque formation was also found in cells cotransfected with hCx46N188T-eGFP and mCherry-labeled hCx46wt as well as in cocultured cells expressing hCx46N188T-eGFP and hCx46wt-mCherry. Dye transfer experiments in cells expressing hCx46N188T revealed a lower transfer rate than cells expressing hCx46wt. We postulate that the N188T mutation affects intercellular connexon docking. This hypothesis is supported by molecular modeling of hCx46 using the crystal structure of hCx26 as a template. The model indicated that N188 is important for hemichannel docking through formation of hydrogen bonds with the residues R180, T189 and D191 of the opposing hCx46. The results suggest that the N188T mutation hinders the docking of the connexons to form gap junction channels. Moreover, the finding that a glutamine substitution (hCx46N188Q) could not rescue the docking emphasizes the specific role of N188.
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Affiliation(s)
- Patrik Schadzek
- Institute of Biophysics, Leibniz University Hannover, Germany
| | - Barbara Schlingmann
- Institute of Biophysics, Leibniz University Hannover, Germany; Division of Pulmonary, Allergy and Critical Care and Sleep Medicine, Department of Medicine and Department of Cell Biology, Emory School of Medicine, Atlanta, GA, USA
| | | | - Julia Lindner
- Institute of Biophysics, Leibniz University Hannover, Germany
| | - Michael Koval
- Division of Pulmonary, Allergy and Critical Care and Sleep Medicine, Department of Medicine and Department of Cell Biology, Emory School of Medicine, Atlanta, GA, USA; Department of Cell Biology, Emory University, Atlanta, GA, USA
| | | | - Matthias Preller
- Institute for Biophysical Chemistry, Hannover Medical School (MHH), Hannover, Germany; Center for Structural Systems Biology, German Electron Synchrotron (DESY), Hamburg, Germany.
| | - Anaclet Ngezahayo
- Institute of Biophysics, Leibniz University Hannover, Germany; Center for System Neurosciences (ZSN), Hannover, Germany.
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Abstract
The normal function and survival of cells in the avascular lens is facilitated by intercellular communication through an extensive network of gap junctions formed predominantly by three connexins (Cx43, Cx46, and Cx50). In expression systems, these connexins can all induce hemichannel currents, but other lens proteins (e.g., pannexin1) can also induce similar currents. Hemichannel currents have been detected in isolated lens fiber cells. These hemichannels may make significant contributions to normal lens physiology and pathophysiology. Studies of some connexin mutants linked to congenital cataracts have implicated hemichannels with aberrant voltage-dependent gating or modulation by divalent cations in disease pathogenesis. Hemichannels may also contribute to age- and disease-related cataracts.
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Affiliation(s)
- Eric C Beyer
- Department of Pediatrics, University of Chicago Chicago, IL, USA
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Falk MM, Kells RM, Berthoud VM. Degradation of connexins and gap junctions. FEBS Lett 2014; 588:1221-9. [PMID: 24486527 DOI: 10.1016/j.febslet.2014.01.031] [Citation(s) in RCA: 68] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2013] [Revised: 01/21/2014] [Accepted: 01/22/2014] [Indexed: 12/21/2022]
Abstract
Connexin proteins are short-lived within the cell, whether present in the secretory pathway or in gap junction plaques. Their levels can be modulated by their rate of degradation. Connexins, at different stages of assembly, are degraded through the proteasomal, endo-/lysosomal, and phago-/lysosomal pathways. In this review, we summarize the current knowledge about connexin and gap junction degradation including the signals and protein-protein interactions that participate in their targeting for degradation.
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Affiliation(s)
- Matthias M Falk
- Department of Biological Sciences, Lehigh University, 111 Research Drive, Iacocca Hall, D-218, Bethlehem, PA 18015, USA.
| | - Rachael M Kells
- Department of Biological Sciences, Lehigh University, 111 Research Drive, Iacocca Hall, D-218, Bethlehem, PA 18015, USA
| | - Viviana M Berthoud
- Department of Pediatrics, University of Chicago, 900 East 57th St., KCBD, Room 5150, Chicago, IL 60637, USA.
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