1
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Zhekova HR, Ramirez Echemendía DP, Sejdiu BI, Pushkin A, Tieleman DP, Kurtz I. Molecular dynamics simulations of lipid-protein interactions in SLC4 proteins. Biophys J 2024; 123:1705-1721. [PMID: 38760929 PMCID: PMC11214021 DOI: 10.1016/j.bpj.2024.05.013] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2023] [Revised: 04/09/2024] [Accepted: 05/14/2024] [Indexed: 05/20/2024] Open
Abstract
The SLC4 family of secondary bicarbonate transporters is responsible for the transport of HCO3-, CO32-, Cl-, Na+, K+, NH3, and H+, which are necessary for regulation of pH and ion homeostasis. They are widely expressed in numerous tissues throughout the body and function in different cell types with different membrane properties. Potential lipid roles in SLC4 function have been reported in experimental studies, focusing mostly on two members of the family: AE1 (Cl-/HCO3- exchanger) and NBCe1 (Na+-CO32-cotransporter). Previous computational studies of the outward-facing state of AE1 with model lipid membranes revealed enhanced protein-lipid interactions between cholesterol (CHOL) and phosphatidylinositol bisphosphate (PIP2). However, the protein-lipid interactions in other members of the family and other conformation states are still poorly understood and this precludes the detailed studies of a potential regulatory role for lipids in the SLC4 family. In this work, we performed coarse-grained and atomistic molecular dynamics simulations on three members of the SLC4 family with different transport modes: AE1, NBCe1, and NDCBE (an Na+-CO32-/Cl- exchanger), in model HEK293 membranes consisting of CHOL, PIP2, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin. The recently resolved inward-facing state of AE1 was also included in the simulations. Lipid-protein contact analysis of the simulated trajectories was performed with the ProLint server, which provides a multitude of visualization tools for illustration of areas of enhanced lipid-protein contact and identification of putative lipid binding sites within the protein matrix. We observed enrichment of CHOL and PIP2 around all proteins with subtle differences in their distribution depending on the protein type and conformation state. Putative binding sites were identified for CHOL, PIP2, phosphatidylcholine, and sphingomyelin in the three studied proteins, and their potential roles in the SLC4 transport function, conformational transition, and protein dimerization are discussed.
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Affiliation(s)
- Hristina R Zhekova
- Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
| | - Daniel P Ramirez Echemendía
- Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
| | - Besian I Sejdiu
- Department of Structural Biology and Center of Excellence for Data Driven Discovery, St Jude Children's Research Hospital, Memphis, Tennessee
| | - Alexander Pushkin
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California
| | - D Peter Tieleman
- Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.
| | - Ira Kurtz
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California; Brain Research Institute, University of California, Los Angeles, Los Angeles, California.
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2
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Maroofian R, Zamani M, Kaiyrzhanov R, Liebmann L, Karimiani EG, Vona B, Huebner AK, Calame DG, Misra VK, Sadeghian S, Azizimalamiri R, Mohammadi MH, Zeighami J, Heydaran S, Toosi MB, Akhondian J, Babaei M, Hashemi N, Schnur RE, Suri M, Setzke J, Wagner M, Brunet T, Grochowski CM, Emrick L, Chung WK, Hellmich UA, Schmidts M, Lupski JR, Galehdari H, Severino M, Houlden H, Hübner CA. Biallelic variants in SLC4A10 encoding a sodium-dependent bicarbonate transporter lead to a neurodevelopmental disorder. Genet Med 2024; 26:101034. [PMID: 38054405 PMCID: PMC11157690 DOI: 10.1016/j.gim.2023.101034] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2023] [Revised: 11/21/2023] [Accepted: 11/24/2023] [Indexed: 12/07/2023] Open
Abstract
PURPOSE SLC4A10 encodes a plasma membrane-bound transporter, which mediates Na+-dependent HCO3- import, thus mediating net acid extrusion. Slc4a10 knockout mice show collapsed brain ventricles, an increased seizure threshold, mild behavioral abnormalities, impaired vision, and deafness. METHODS Utilizing exome/genome sequencing in families with undiagnosed neurodevelopmental disorders and international data sharing, 11 patients from 6 independent families with biallelic variants in SLC4A10 were identified. Clinico-radiological and dysmorphology assessments were conducted. A minigene assay, localization studies, intracellular pH recordings, and protein modeling were performed to study the possible functional consequences of the variant alleles. RESULTS The families harbor 8 segregating ultra-rare biallelic SLC4A10 variants (7 missense and 1 splicing). Phenotypically, patients present with global developmental delay/intellectual disability and central hypotonia, accompanied by variable speech delay, microcephaly, cerebellar ataxia, facial dysmorphism, and infrequently, epilepsy. Neuroimaging features range from some non-specific to distinct neuroradiological findings, including slit ventricles and a peculiar form of bilateral curvilinear nodular heterotopia. In silico analyses showed 6 of 7 missense variants affect evolutionarily conserved residues. Functional analyses supported the pathogenicity of 4 of 7 missense variants. CONCLUSION We provide evidence that pathogenic biallelic SLC4A10 variants can lead to neurodevelopmental disorders characterized by variable abnormalities of the central nervous system, including altered brain ventricles, thus resembling several features observed in knockout mice.
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Affiliation(s)
- Reza Maroofian
- Department of Neuromuscular Diseases, UCL Queen Square Institute of Neurology, London, United Kingdom.
| | - Mina Zamani
- Department of Biology, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran; Narges Medical Genetics and Prenatal Diagnosis Laboratory, Kianpars, Ahvaz, Iran
| | - Rauan Kaiyrzhanov
- Department of Neuromuscular Diseases, UCL Queen Square Institute of Neurology, London, United Kingdom
| | - Lutz Liebmann
- Institute of Human Genetics, Jena University Hospital, Friedrich Schiller Universität, Am Klinikum 1, Jena, Germany
| | - Ehsan Ghayoor Karimiani
- Molecular and Clinical Sciences Institute, St. George's, University of London, Cranmer Terrace, London, United Kingdom
| | - Barbara Vona
- Institute of Human Genetics, University Medical Center Göttingen, Göttingen, Germany; Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany
| | - Antje K Huebner
- Institute of Human Genetics, Jena University Hospital, Friedrich Schiller Universität, Am Klinikum 1, Jena, Germany
| | - Daniel G Calame
- Division of Neurology and Developmental Neuroscience, Department of Pediatrics, Baylor College of Medicine, Houston, TX; Texas Children's Hospital, Houston, TX; Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX
| | - Vinod K Misra
- Division of Genetic, Genomic & Metabolic Disorders, Discipline of Pediatrics, College of Medicine, Central Michigan University, Mount Pleasant, MI
| | - Saeid Sadeghian
- Department of Pediatric Neurology, Golestan Medical, Educational, and Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
| | - Reza Azizimalamiri
- Department of Pediatric Neurology, Golestan Medical, Educational, and Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
| | | | - Jawaher Zeighami
- Narges Medical Genetics and Prenatal Diagnosis Laboratory, Kianpars, Ahvaz, Iran
| | - Sogand Heydaran
- Department of Biology, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran
| | - Mehran Beiraghi Toosi
- Pediatric Neurology Department, Ghaem Hospital, Mashhad University of Medical Sciences, Mashhad, Iran; Neuroscience Research Center, Mashhad University of Medical Science, Mashhad, Iran
| | - Javad Akhondian
- Pediatric Neurology Department, Ghaem Hospital, Mashhad University of Medical Sciences, Mashhad, Iran
| | - Meisam Babaei
- Department of Pediatrics, North Khorasan University of Medical Sciences, Bojnurd, Iran
| | - Narges Hashemi
- Department of Pediatrics, School of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
| | | | - Mohnish Suri
- Clinical Genetics Service, Nottingham University Hospitals NHS Trust, Nottingham, United Kingdom
| | - Jonas Setzke
- Institute of Human Genetics, University Medical Center Göttingen, Göttingen, Germany; Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany
| | - Matias Wagner
- Institute of Human Genetics, Klinikum rechts der Isar, School of Medicine, Technical University of Munich, Munich, Germany; Institute of Neurogenomics, Helmholtz Zentrum München, Neuherberg, Germany; Department of Pediatric Neurology and Developmental Medicine and LMU Center for Children with Medical Complexity, Dr. von Hauner Children's Hospital, LMU Hospital, Ludwig-Maximilians-University, Munich, Germany
| | - Theresa Brunet
- Institute of Human Genetics, Klinikum rechts der Isar, School of Medicine, Technical University of Munich, Munich, Germany; Institute of Neurogenomics, Helmholtz Zentrum München, Neuherberg, Germany
| | | | - Lisa Emrick
- Division of Neurology and Developmental Neuroscience, Department of Pediatrics, Baylor College of Medicine, Houston, TX; Texas Children's Hospital, Houston, TX; Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX
| | - Wendy K Chung
- Department of Pediatrics, Boston Children's Hospital, Harvard Medical School, Boston, MA
| | - Ute A Hellmich
- Friedrich Schiller University Jena, Faculty of Chemistry and Earth Sciences, Institute of Organic Chemistry and Macromolecular Chemistry, Jena, Germany; Center for Biomolecular Magnetic Resonance (BMRZ), Goethe University, Frankfurt, Germany; Cluster of Excellence Balance of the Microverse, Friedrich Schiller University Jena, Jena, Germany
| | - Miriam Schmidts
- Pediatrics Genetics Division, Center for Pediatrics and Adolescent Medicine, Faculty of Medicine, Freiburg University, Freiburg, Germany; Genome Research Division, Human Genetics Department, Radboud University Medical Center, Nijmegen, The Netherlands; CIBSS-Centre for Integrative Biological Signalling Studies, University of Freiburg, Freiburg, Germany
| | - James R Lupski
- Texas Children's Hospital, Houston, TX; Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX; Human Genome Sequencing Center, Baylor College of Medicine, Houston, TX
| | - Hamid Galehdari
- Department of Biology, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran
| | | | - Henry Houlden
- Department of Neuromuscular Diseases, UCL Queen Square Institute of Neurology, London, United Kingdom
| | - Christian A Hübner
- Institute of Human Genetics, Jena University Hospital, Friedrich Schiller Universität, Am Klinikum 1, Jena, Germany; Center for Rare Diseases, Jena University Hospital, Jena, Germany
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3
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Zhekova HR, Ramirez-Echemendía DP, Sejdiu BI, Pushkin A, Tieleman DP, Kurtz I. Coarse-grained molecular dynamics simulations of lipid-protein interactions in SLC4 proteins. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.06.26.546592. [PMID: 37425774 PMCID: PMC10327080 DOI: 10.1101/2023.06.26.546592] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/11/2023]
Abstract
The SLC4 family of secondary bicarbonate transporters is responsible for the transport of HCO 3 -, CO 3 2- , Cl - , Na + , K + , NH 3 and H + necessary for regulation of pH and ion homeostasis. They are widely expressed in numerous tissues throughout the body and function in different cell types with different membrane properties. Potential lipid roles in SLC4 function have been reported in experimental studies, focusing mostly on two members of the family: AE1 (Cl - /HCO 3 - exchanger) and NBCe1 (Na + -CO 3 2- cotransporter). Previous computational studies of the outward facing (OF) state of AE1 with model lipid membranes revealed enhanced protein-lipid interactions between cholesterol (CHOL) and phosphatidylinositol bisphosphate (PIP2). However, the protein-lipid interactions in other members of the family and other conformation states are still poorly understood and this precludes the detailed studies of a potential regulatory role for lipids in the SLC4 family. In this work, we performed multiple 50 µs coarse-grained molecular dynamics simulations on three members of the SLC4 family with different transport modes: AE1, NBCe1 and NDCBE (a Na + -CO 3 2- /Cl - exchanger), in model HEK293 membranes consisting of CHOL, PIP2, phosphatidylcholine (POPC), phosphatidylethanolamine (POPE), phosphatidylserine (POPS), and sphingomyelin (POSM). The recently resolved inward-facing (IF) state of AE1 was also included in the simulations. Lipid-protein contact analysis of the simulated trajectories was performed with the ProLint server, which provides a multitude of visualization tools for illustration of areas of enhanced lipid-protein contact and identification of putative lipid binding sites within the protein matrix. We observed enrichment of CHOL and PIP2 around all proteins with subtle differences in their distribution depending on the protein type and conformation state. Putative binding sites were identified for CHOL, PIP2, POPC, and POSM in the three studied proteins and their potential roles in the SLC4 transport function, conformational transition and protein dimerization were discussed. Statement of significance The SLC4 protein family is involved in critical physiological processes like pH and blood pressure regulation and maintenance of ion homeostasis. Its members can be found in various tissues. A number of studies suggest possible lipid regulation of the SLC4 function. However, the protein-lipid interactions in the SLC4 family are still poorly understood. Here we make use of long coarse-grained molecular dynamics simulations to assess the protein-lipid interactions in three SLC4 proteins with different transport modes, AE1, NBCe1, and NDCBE. We identify putative lipid binding sites for several lipid types of potential mechanistic importance, discuss them in the framework of the known experimental data and provide a necessary basis for further studies on lipid regulation of SLC4 function.
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Brady CT, Marshall A, Zhang C, Parker MD. NBCe1-B/C-knockout mice exhibit an impaired respiratory response and an enhanced renal response to metabolic acidosis. Front Physiol 2023; 14:1201034. [PMID: 37405134 PMCID: PMC10315466 DOI: 10.3389/fphys.2023.1201034] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2023] [Accepted: 06/07/2023] [Indexed: 07/06/2023] Open
Abstract
The sodium-bicarbonate cotransporter (NBCe1) has three primary variants: NBCe1-A, -B and -C. NBCe1-A is expressed in renal proximal tubules in the cortical labyrinth, where it is essential for reclaiming filtered bicarbonate, such that NBCe1-A knockout mice are congenitally acidemic. NBCe1-B and -C variants are expressed in chemosensitive regions of the brainstem, while NBCe1-B is also expressed in renal proximal tubules located in the outer medulla. Although mice lacking NBCe1-B/C (KOb/c) exhibit a normal plasma pH at baseline, the distribution of NBCe1-B/C indicates that these variants could play a role in both the rapid respiratory and slower renal responses to metabolic acidosis (MAc). Therefore, in this study we used an integrative physiologic approach to investigate the response of KOb/c mice to MAc. By means of unanesthetized whole-body plethysmography and blood-gas analysis, we demonstrate that the respiratory response to MAc (increase in minute volume, decrease in pCO2) is impaired in KOb/c mice leading to a greater severity of acidemia after 1 day of MAc. Despite this respiratory impairment, the recovery of plasma pH after 3-days of MAc remained intact in KOb/c mice. Using data gathered from mice housed in metabolic cages we demonstrate a greater elevation of renal ammonium excretion and greater downregulation of the ammonia recycling enzyme glutamine synthetase in KOb/c mice on day 2 of MAc, consistent with greater renal acid-excretion. We conclude that KOb/c mice are ultimately able to defend plasma pH during MAc, but that the integrated response is disturbed such that the burden of work shifts from the respiratory system to the kidneys, delaying the recovery of pH.
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Affiliation(s)
- Clayton T. Brady
- Jacobs School of Medicine and Biomedical Sciences, Department of Physiology and Biophysics, The State University of New York: The University at Buffalo, Buffalo, NY, United States
| | - Aniko Marshall
- Jacobs School of Medicine and Biomedical Sciences, Department of Physiology and Biophysics, The State University of New York: The University at Buffalo, Buffalo, NY, United States
| | - Chen Zhang
- Jacobs School of Medicine and Biomedical Sciences, Department of Physiology and Biophysics, The State University of New York: The University at Buffalo, Buffalo, NY, United States
- Department of Biological Sciences, The State University of New York: The University at Buffalo, Buffalo, NY, United States
| | - Mark D. Parker
- Jacobs School of Medicine and Biomedical Sciences, Department of Physiology and Biophysics, The State University of New York: The University at Buffalo, Buffalo, NY, United States
- Jacobs School of Medicine and Biomedical Sciences, Department of Ophthalmology, The State University of New York: The University at Buffalo, Buffalo, NY, United States
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5
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Bakshi T, Pham D, Kaur R, Sun B. Hidden Relationships between N-Glycosylation and Disulfide Bonds in Individual Proteins. Int J Mol Sci 2022; 23:ijms23073742. [PMID: 35409101 PMCID: PMC8998389 DOI: 10.3390/ijms23073742] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2022] [Revised: 03/25/2022] [Accepted: 03/28/2022] [Indexed: 02/04/2023] Open
Abstract
N-Glycosylation (NG) and disulfide bonds (DBs) are two prevalent co/post-translational modifications (PTMs) that are often conserved and coexist in membrane and secreted proteins involved in a large number of diseases. Both in the past and in recent times, the enzymes and chaperones regulating these PTMs have been constantly discovered to directly interact with each other or colocalize in the ER. However, beyond a few model proteins, how such cooperation affects N-glycan modification and disulfide bonding at selective sites in individual proteins is largely unknown. Here, we reviewed the literature to discover the current status in understanding the relationships between NG and DBs in individual proteins. Our results showed that more than 2700 human proteins carry both PTMs, and fewer than 2% of them have been investigated in the associations between NG and DBs. We summarized both these proteins with the reported relationships in the two PTMs and the tools used to discover the relationships. We hope that, by exposing this largely understudied field, more investigations can be encouraged to unveil the hidden relationships of NG and DBs in the majority of membranes and secreted proteins for pathophysiological understanding and biotherapeutic development.
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Affiliation(s)
- Tania Bakshi
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada;
| | - David Pham
- Department of Computing Science, Simon Fraser University, Burnaby, BC V5A 1S6, Canada;
| | - Raminderjeet Kaur
- Faculty of Health Science, Simon Fraser University, Burnaby, BC V5A 1S6, Canada;
| | - Bingyun Sun
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada;
- Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada
- Correspondence:
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Wang W, Tsirulnikov K, Zhekova HR, Kayık G, Khan HM, Azimov R, Abuladze N, Kao L, Newman D, Noskov SY, Zhou ZH, Pushkin A, Kurtz I. Cryo-EM structure of the sodium-driven chloride/bicarbonate exchanger NDCBE. Nat Commun 2021; 12:5690. [PMID: 34584093 PMCID: PMC8478935 DOI: 10.1038/s41467-021-25998-2] [Citation(s) in RCA: 23] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2021] [Accepted: 09/08/2021] [Indexed: 02/08/2023] Open
Abstract
SLC4 transporters play significant roles in pH regulation and cellular sodium transport. The previously solved structures of the outward facing (OF) conformation for AE1 (SLC4A1) and NBCe1 (SLC4A4) transporters revealed an identical overall fold despite their different transport modes (chloride/bicarbonate exchange versus sodium-carbonate cotransport). However, the exact mechanism determining the different transport modes in the SLC4 family remains unknown. In this work, we report the cryo-EM 3.4 Å structure of the OF conformation of NDCBE (SLC4A8), which shares transport properties with both AE1 and NBCe1 by mediating the electroneutral exchange of sodium-carbonate with chloride. This structure features a fully resolved extracellular loop 3 and well-defined densities corresponding to sodium and carbonate ions in the tentative substrate binding pocket. Further, we combine computational modeling with functional studies to unravel the molecular determinants involved in NDCBE and SLC4 transport.
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Affiliation(s)
- Weiguang Wang
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA ,grid.509979.b0000 0004 7666 6191Electron Imaging Center for Nanomachines, California NanoSystems Institute, University of California, Los Angeles, CA USA
| | - Kirill Tsirulnikov
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA
| | - Hristina R. Zhekova
- grid.22072.350000 0004 1936 7697Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Gülru Kayık
- grid.22072.350000 0004 1936 7697Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Hanif Muhammad Khan
- grid.22072.350000 0004 1936 7697Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Rustam Azimov
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA
| | - Natalia Abuladze
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA
| | - Liyo Kao
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA
| | - Debbie Newman
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA
| | - Sergei Yu. Noskov
- grid.22072.350000 0004 1936 7697Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Z. Hong Zhou
- grid.509979.b0000 0004 7666 6191Electron Imaging Center for Nanomachines, California NanoSystems Institute, University of California, Los Angeles, CA USA ,grid.19006.3e0000 0000 9632 6718Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, CA USA
| | - Alexander Pushkin
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA
| | - Ira Kurtz
- grid.19006.3e0000 0000 9632 6718Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA USA ,grid.19006.3e0000 0000 9632 6718Brain Research Institute, University of California, Los Angeles, CA USA
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Su P, Wu H, Wang M, Cai L, Liu Y, Chen LM. IRBIT activates NBCe1-B by releasing the auto-inhibition module from the transmembrane domain. J Physiol 2020; 599:1151-1172. [PMID: 33237573 PMCID: PMC7898672 DOI: 10.1113/jp280578] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2020] [Accepted: 11/18/2020] [Indexed: 12/16/2022] Open
Abstract
Key points The electrogenic Na+/HCO3−cotransporter NBCe1‐B is widely expressed in many tissues, including pancreas, submandibular gland, brain, heart, etc. NBCe1‐B has very low activity under basal condition due to auto‐inhibition, but can be fully activated by protein interaction with the IP3R‐binding protein released with inositol 1,4,5‐trisphosphate (IRBIT). The structural components of the auto‐inhibition domain and the IRBIT‐binding domain of NBCe1‐B are finely characterized based on systematic mutations in the present study and data from previous studies. Reducing negative charges on the cytosol side of the transmembrane domain greatly decreases the magnitude of the auto‐inhibition of NBCe1‐B. We propose that the auto‐inhibition domain functions as a brake module that inactivates NBCe1‐B by binding to, via electrostatic attraction, the transmembrane domain; IRBIT activates NBCe1‐B by releasing the brake from the transmembrane domain via competitive binding to the auto‐inhibition domain.
Abstract The electrogenic Na+/HCO3− cotransporter NBCe1‐B is widely expressed in many tissues in the body. NBCe1‐B exhibits only basal activity due to the action of the auto‐inhibition domain (AID) in its unique amino‐terminus. However, NBCe1‐B can be activated by interaction with the IP3R‐binding protein released with inositol 1,4,5‐trisphosphate (IRBIT). Here, we investigate the molecular mechanism underlying the auto‐inhibition of NBCe1‐B and its activation by IRBIT. The IRBIT‐binding domain (IBD) of NBCe1‐B spans residues 1−52, essentially consisting of two arms, one negatively charged (residues 1−24) and the other positively charged (residues 40−52). The AID mainly spans residues 40−85, overlapping with the IBD in the positively charged arm. The magnitude of auto‐inhibition of NBCe1‐B is greatly decreased by manipulating the positively charged residues in the AID or by replacing a set of negatively charged residues with neutral ones in the transmembrane domain. The interaction between IRBIT and NBCe1‐B is abolished by mutating a set of negatively charged Asp/Glu residues (to Asn/Gln) plus a set of Ser/Thr residues (to Ala) in the PEST domain of IRBIT. However, this interaction is not affected by replacing the same set of Ser/Thr residues in the PEST domain with Asp. We propose that: (1) the AID, acting as a brake, binds to the transmembrane domain via electrostatic interaction to slow down NBCe1‐B; (2) IRBIT activates NBCe1‐B by releasing the brake from the transmembrane domain. The electrogenic Na+/HCO3−cotransporter NBCe1‐B is widely expressed in many tissues, including pancreas, submandibular gland, brain, heart, etc. NBCe1‐B has very low activity under basal condition due to auto‐inhibition, but can be fully activated by protein interaction with the IP3R‐binding protein released with inositol 1,4,5‐trisphosphate (IRBIT). The structural components of the auto‐inhibition domain and the IRBIT‐binding domain of NBCe1‐B are finely characterized based on systematic mutations in the present study and data from previous studies. Reducing negative charges on the cytosol side of the transmembrane domain greatly decreases the magnitude of the auto‐inhibition of NBCe1‐B. We propose that the auto‐inhibition domain functions as a brake module that inactivates NBCe1‐B by binding to, via electrostatic attraction, the transmembrane domain; IRBIT activates NBCe1‐B by releasing the brake from the transmembrane domain via competitive binding to the auto‐inhibition domain.
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Affiliation(s)
- Pan Su
- Key Laboratory of Molecular Biophysics of Ministry of Education, School of Life Science & Technology, Huazhong University of Science & Technology, Wuhan, Hubei, China
| | - Han Wu
- Key Laboratory of Molecular Biophysics of Ministry of Education, School of Life Science & Technology, Huazhong University of Science & Technology, Wuhan, Hubei, China
| | - Meng Wang
- Key Laboratory of Molecular Biophysics of Ministry of Education, School of Life Science & Technology, Huazhong University of Science & Technology, Wuhan, Hubei, China
| | - Lu Cai
- Key Laboratory of Molecular Biophysics of Ministry of Education, School of Life Science & Technology, Huazhong University of Science & Technology, Wuhan, Hubei, China
| | - Ying Liu
- Key Laboratory of Molecular Biophysics of Ministry of Education, School of Life Science & Technology, Huazhong University of Science & Technology, Wuhan, Hubei, China
| | - Li-Ming Chen
- Key Laboratory of Molecular Biophysics of Ministry of Education, School of Life Science & Technology, Huazhong University of Science & Technology, Wuhan, Hubei, China
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8
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Functional (un)cooperativity in elevator transport proteins. Biochem Soc Trans 2020; 48:1047-1055. [PMID: 32573703 DOI: 10.1042/bst20190970] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2020] [Revised: 05/26/2020] [Accepted: 05/28/2020] [Indexed: 11/17/2022]
Abstract
The activity of enzymes is subject to regulation at multiple levels. Cooperativity, the interconnected behavior of active sites within a protein complex, directly affects protein activity. Cooperativity is a mode of regulation that requires neither extrinsic factors nor protein modifications. Instead, it allows enzymes themselves to modulate reaction rates. Cooperativity is an important regulatory mechanism in soluble proteins, but also examples of cooperative membrane proteins have been described. In this review, we summarize the current knowledge on interprotomer cooperativity in elevator-type proteins, a class of membrane transporters characterized by large rigid-body movements perpendicular to the membrane, and highlight well-studied examples and experimental approaches.
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9
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Chang YN, Jaumann EA, Reichel K, Hartmann J, Oliver D, Hummer G, Joseph B, Geertsma ER. Structural basis for functional interactions in dimers of SLC26 transporters. Nat Commun 2019; 10:2032. [PMID: 31048734 PMCID: PMC6497670 DOI: 10.1038/s41467-019-10001-w] [Citation(s) in RCA: 39] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2018] [Accepted: 04/13/2019] [Indexed: 12/13/2022] Open
Abstract
The SLC26 family of transporters maintains anion equilibria in all kingdoms of life. The family shares a 7 + 7 transmembrane segments inverted repeat architecture with the SLC4 and SLC23 families, but holds a regulatory STAS domain in addition. While the only experimental SLC26 structure is monomeric, SLC26 proteins form structural and functional dimers in the lipid membrane. Here we resolve the structure of an SLC26 dimer embedded in a lipid membrane and characterize its functional relevance by combining PELDOR/DEER distance measurements and biochemical studies with MD simulations and spin-label ensemble refinement. Our structural model reveals a unique interface different from the SLC4 and SLC23 families. The functionally relevant STAS domain is no prerequisite for dimerization. Characterization of heterodimers indicates that protomers in the dimer functionally interact. The combined structural and functional data define the framework for a mechanistic understanding of functional cooperativity in SLC26 dimers.
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Affiliation(s)
- Yung-Ning Chang
- Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Str. 9, 60438, Frankfurt am Main, Germany
| | - Eva A Jaumann
- Institute of Physical and Theoretical Chemistry, Goethe University Frankfurt, Max-von-Laue Str. 7, 60438, Frankfurt am Main, Germany
| | - Katrin Reichel
- Department of Theoretical Biophysics, Max Planck Institute of Biophysics, Max-von-Laue Str. 3, 60438, Frankfurt am Main, Germany
| | - Julia Hartmann
- Department of Neurophysiology, Institute of Physiology and Pathophysiology, Philipps University, 35037, Marburg, Germany
| | - Dominik Oliver
- Department of Neurophysiology, Institute of Physiology and Pathophysiology, Philipps University, 35037, Marburg, Germany.,DFG Research Training Group, Membrane Plasticity in Tissue Development and Remodeling, Philipps University, GRK 2213, Philipps, Germany
| | - Gerhard Hummer
- Department of Theoretical Biophysics, Max Planck Institute of Biophysics, Max-von-Laue Str. 3, 60438, Frankfurt am Main, Germany. .,Institute of Biophysics, Goethe University Frankfurt, Max-von-Laue Str. 1, 60438, Frankfurt am Main, Germany.
| | - Benesh Joseph
- Institute of Physical and Theoretical Chemistry, Goethe University Frankfurt, Max-von-Laue Str. 7, 60438, Frankfurt am Main, Germany. .,Institute of Biophysics, Goethe University Frankfurt, Max-von-Laue Str. 1, 60438, Frankfurt am Main, Germany.
| | - Eric R Geertsma
- Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Str. 9, 60438, Frankfurt am Main, Germany.
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10
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Fukano K, Tsukuda S, Oshima M, Suzuki R, Aizaki H, Ohki M, Park SY, Muramatsu M, Wakita T, Sureau C, Ogasawara Y, Watashi K. Troglitazone Impedes the Oligomerization of Sodium Taurocholate Cotransporting Polypeptide and Entry of Hepatitis B Virus Into Hepatocytes. Front Microbiol 2019; 9:3257. [PMID: 30671048 PMCID: PMC6331526 DOI: 10.3389/fmicb.2018.03257] [Citation(s) in RCA: 33] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2018] [Accepted: 12/14/2018] [Indexed: 12/12/2022] Open
Abstract
Current anti-hepatitis B virus (HBV) agents, which include nucleos(t)ide analogs and interferons, can significantly suppress HBV infection. However, there are limitations in the therapeutic efficacy of these agents, indicating the need to develop anti-HBV agents with different modes of action. In this study, through a functional cell-based chemical screening, we found that a thiazolidinedione, troglitazone, inhibits HBV infection independently of the compound's ligand activity for peroxisome proliferator-activated receptor γ (PPARγ). Analog analysis suggested chemical moiety required for the anti-HBV activity and identified ciglitazone as an analog having higher anti-HBV potency. Whereas, most of the reported HBV entry inhibitors target viral attachment to the cell surface, troglitazone blocked a process subsequent to viral attachment, i.e., internalization of HBV preS1 and its receptor, sodium taurocholate cotransporting polypeptide (NTCP). We also found that NTCP was markedly oligomerized in the presence of HBV preS1, but such NTCP oligomerization was abrogated by treatment with troglitazone, but not with pioglitazone, correlating with inhibition activity to viral internalization. Also, competitive peptides that blocked NTCP oligomerization impeded viral internalization and infection. This work represents the first report identifying small molecules and peptides that specifically inhibit the internalization of HBV. This study is also significant in proposing a possible role for NTCP oligomerization in viral entry, which will shed a light on a new aspect of the cellular mechanisms regulating HBV infection.
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Affiliation(s)
- Kento Fukano
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan.,Department of Analytical Biochemistry, Meiji Pharmaceutical University, Kiyose, Japan
| | - Senko Tsukuda
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan.,Liver Cancer Prevention Research Unit, Center for Integrative Medical Sciences, RIKEN, Wako, Japan
| | - Mizuki Oshima
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan.,Department of Applied Biological Science, Tokyo University of Science, Noda, Japan
| | - Ryosuke Suzuki
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan
| | - Hideki Aizaki
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan
| | - Mio Ohki
- Drug Design Laboratory, Graduate School of Medical Life Science, Yokohama City University, Yokohama, Japan
| | - Sam-Yong Park
- Drug Design Laboratory, Graduate School of Medical Life Science, Yokohama City University, Yokohama, Japan
| | - Masamichi Muramatsu
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan
| | - Takaji Wakita
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan
| | - Camille Sureau
- Laboratoire de Virologie Moléculaire, Institut National de la Transfusion Sanguine, CNRS, INSERM U1134, Paris, France
| | - Yuki Ogasawara
- Department of Analytical Biochemistry, Meiji Pharmaceutical University, Kiyose, Japan
| | - Koichi Watashi
- Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan.,Department of Applied Biological Science, Tokyo University of Science, Noda, Japan.,JST CREST, Saitama, Japan
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11
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Huynh KW, Jiang J, Abuladze N, Tsirulnikov K, Kao L, Shao X, Newman D, Azimov R, Pushkin A, Zhou ZH, Kurtz I. CryoEM structure of the human SLC4A4 sodium-coupled acid-base transporter NBCe1. Nat Commun 2018; 9:900. [PMID: 29500354 PMCID: PMC5834491 DOI: 10.1038/s41467-018-03271-3] [Citation(s) in RCA: 68] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2017] [Accepted: 02/01/2018] [Indexed: 12/19/2022] Open
Abstract
Na+-coupled acid-base transporters play essential roles in human biology. Their dysfunction has been linked to cancer, heart, and brain disease. High-resolution structures of mammalian Na+-coupled acid-base transporters are not available. The sodium-bicarbonate cotransporter NBCe1 functions in multiple organs and its mutations cause blindness, abnormal growth and blood chemistry, migraines, and impaired cognitive function. Here, we have determined the structure of the membrane domain dimer of human NBCe1 at 3.9 Å resolution by cryo electron microscopy. Our atomic model and functional mutagenesis revealed the ion accessibility pathway and the ion coordination site, the latter containing residues involved in human disease-causing mutations. We identified a small number of residues within the ion coordination site whose modification transformed NBCe1 into an anion exchanger. Our data suggest that symporters and exchangers utilize comparable transport machinery and that subtle differences in their substrate-binding regions have very significant effects on their transport mode.
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Affiliation(s)
- Kevin W Huynh
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA
- California NanoSystems Institute, University of California, Los Angeles, CA, 90095, USA
| | - Jiansen Jiang
- California NanoSystems Institute, University of California, Los Angeles, CA, 90095, USA
- Department of Microbiology, Immunology & Molecular Genetics, University of California, Los Angeles, CA, 90095, USA
| | - Natalia Abuladze
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA
| | - Kirill Tsirulnikov
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA
| | - Liyo Kao
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA
| | - Xuesi Shao
- Department of Neurobiology, University of California, Los Angeles, CA, 90095, USA
| | - Debra Newman
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA
| | - Rustam Azimov
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA
| | - Alexander Pushkin
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA.
| | - Z Hong Zhou
- California NanoSystems Institute, University of California, Los Angeles, CA, 90095, USA.
- Department of Microbiology, Immunology & Molecular Genetics, University of California, Los Angeles, CA, 90095, USA.
| | - Ira Kurtz
- Department of Medicine, Division of Nephrology, David Geffen School of Medicine, University of California, Los Angeles, CA, 90095, USA.
- Brain Research Institute, University of California, Los Angeles, CA, 90095, USA.
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12
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The crystal structure of the regulatory domain of the human sodium-driven chloride/bicarbonate exchanger. Sci Rep 2017; 7:12131. [PMID: 28935959 PMCID: PMC5608694 DOI: 10.1038/s41598-017-12409-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2017] [Accepted: 09/06/2017] [Indexed: 12/12/2022] Open
Abstract
The sodium-driven chloride/bicarbonate exchanger (NDCBE) is essential for maintaining homeostatic pH in neurons. The crystal structure at 2.8 Å resolution of the regulatory N-terminal domain of human NDCBE represents the first crystal structure of an electroneutral sodium-bicarbonate cotransporter. The crystal structure forms an equivalent dimeric interface as observed for the cytoplasmic domain of Band 3, and thus establishes that the consensus motif VTVLP is the key minimal dimerization motif. The VTVLP motif is highly conserved and likely to be the physiologically relevant interface for all other members of the SLC4 family. A novel conserved Zn2+-binding motif present in the N-terminal domain of NDCBE is identified and characterized in vitro. Cellular studies confirm the Zn2+ dependent transport of two electroneutral bicarbonate transporters, NCBE and NBCn1. The Zn2+ site is mapped to a cluster of histidines close to the conserved ETARWLKFEE motif and likely plays a role in the regulation of this important motif. The combined structural and bioinformatics analysis provides a model that predicts with additional confidence the physiologically relevant interface between the cytoplasmic domain and the transmembrane domain.
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13
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Park HJ, Lee S, Ju E, Jones JA, Choi I. Alternative transcription of sodium/bicarbonate transporter SLC4A7 gene enhanced by single nucleotide polymorphisms. Physiol Genomics 2017; 49:167-176. [PMID: 28087757 PMCID: PMC5374452 DOI: 10.1152/physiolgenomics.00112.2016] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2016] [Revised: 01/11/2017] [Accepted: 01/11/2017] [Indexed: 12/31/2022] Open
Abstract
Genome-wide association studies have identified the single nucleotide polymorphism (SNP) rs3278 in the human SLC4A7 gene as one of the marker loci for addiction vulnerability. This marker is located in an intron of the gene, and its genomic role has been unknown. In this study, we examined rs3278 and three adjacent SNPs prevalent in alcoholics for their effects on an alternative promoter that would lead to the production of the NH2-terminally truncated protein NBCn1ΔN450, missing the first 450 amino acids. Analysis of the transcription start site database and a promoter prediction algorithm identified a cluster of three promoters in intron 7 and two short CpG-rich sites in intron 6. The promoter closest to rs3278 showed strong transcription activity in luciferase reporter gene assays. Major-to-minor allele substitution at rs3278 resulted in increased transcription activity. Equivalent substitutions at adjacent rs3772723 (intron 7) and rs13077400 (exon 8) had negligible effect; however, the substitution at nonsynonymous rs3755652 (exon 8) increased the activity by more than twofold. The concomitant substitution at rs3278/rs3755652 produced an additive effect. The rs3755652 had more profound effects on the promoter than the upstream regulatory CpG sites. The amino acid change E326K caused by rs3755652 had negligible effect on transporter function. In HEK 293 cells, NBCn1ΔN450 was expressed in plasma membranes, but at significantly lower levels than the nontruncated NBCn1-E. The pH change mediated by NBCn1ΔN450 was also low. We conclude that rs3278 and rs3755652 stimulate an alternative transcription of the SLC4A7 gene, increasing the production of a defective transporter.
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Affiliation(s)
- Hae Jeong Park
- Department of Pharmacology, Kyung Hee University School of Medicine, Seoul, South Korea; and
| | - Soojung Lee
- Department of Physiology, Emory University School of Medicine, Atlanta, Georgia
| | - Eunji Ju
- Department of Physiology, Emory University School of Medicine, Atlanta, Georgia
| | - Jayre A Jones
- Department of Physiology, Emory University School of Medicine, Atlanta, Georgia
| | - Inyeong Choi
- Department of Physiology, Emory University School of Medicine, Atlanta, Georgia
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14
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Thornell IM, Bevensee MO. Phosphatidylinositol 4,5-bisphosphate degradation inhibits the Na+/bicarbonate cotransporter NBCe1-B and -C variants expressed in Xenopus oocytes. J Physiol 2016; 593:541-58. [PMID: 25398525 DOI: 10.1113/jphysiol.2014.284307] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2014] [Accepted: 11/04/2014] [Indexed: 12/16/2022] Open
Abstract
KEY POINTS We previously reported that the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2 ) directly stimulates heterologously expressed electrogenic Na(+)/bicarbonate cotransporter NBCe1-A in an excised macropatch from the Xenopus oocyte, and indirectly stimulates NBCe1-B and -C in the intact oocyte primarily through inositol 1,4,5-trisphosphate/Ca(2+). In the current study, we expand on a previous observation that PIP2 may also directly stimulate NBCe1 in the intact oocyte. In this study on oocytes, we co-expressed either NBCe1-B or -C and a voltage-sensitive phosphatase (VSP), which depletes PIP2 without changing inositol 1,4,5-trisphosphate, and monitored NBCe1-mediated currents with the two-electrode voltage-clamp technique or pHi changes using Vm/pH-sensitive microelectrodes. Activating VSP inhibited NBCe1-B and -C outward currents and NBCe1-mediated pHi increases, and changes in NBCe1 activity paralleled changes in surface PIP2. This study is a quantitative assessment of PIP2 itself as a regulator of NBCe1-B and -C in the intact cell, and represents the first use of VSP to characterize the PIP2 sensitivity of a transporter. These data combined with our previous work demonstrate that NBCe1-B and -C are regulated by two PIP2-mediated signalling pathways. Specifically, a decrease in PIP2 per se can inhibit NBCe1, whereas hydrolysis of PIP2 to inositol 1,4,5-trisphosphate/Ca(2+) can stimulate the transporter. ABSTRACT The electrogenic Na(+)/bicarbonate cotransporter (NBCe1) of the Slc4 gene family is a powerful regulator of intracellular pH (pHi) and extracellular pH (pHo), and contributes to solute reabsorption and secretion in many epithelia. Using Xenopus laevis oocytes expressing NBCe1 variants, we have previously reported that the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) directly stimulates NBCe1-A in an excised macropatch, and indirectly stimulates NBCe1-B and -C in the intact oocyte primarily through inositol 1,4,5-trisphosphate (InsP3)/Ca(2+). In the current study, we used the two-electrode voltage-clamp technique alone or in combination with pH/voltage-sensitive microelectrodes or confocal fluorescence imaging of plasma membrane PIP2 to characterize the PIP2 sensitivity of NBCe1-B and -C in whole oocytes by co-expressing a voltage-sensitive phosphatase (VSP) that decreases PIP2 and bypasses the InsP3/Ca(2+) pathway. An oocyte depolarization that activated VSP only transiently stimulated the NBCe1-B/C current, consistent with an initial rapid depolarization-induced NBCe1 activation, and then a subsequent slower VSP-mediated NBCe1 inhibition. Upon repolarization, the NBCe1 current decreased, and then slowly recovered with an exponential time course that paralleled PIP2 resynthesis as measured with a PIP2-sensitive fluorophore and confocal imaging. A subthreshold depolarization that minimally activated VSP caused a more sustained increase in NBCe1 current, and did not lead to an exponential current recovery following repolarization. Similar results were obtained with oocytes expressing a catalytically dead VSP mutant at all depolarized potentials. Depleting endoplasmic reticulum Ca(2+) did not inhibit the NBCe1 current recovery following repolarization from VSP activation, demonstrating that changes in InsP3/Ca(2+) were not responsible. This study demonstrates for the first time that depleting PIP2 per se inhibits NBCe1 activity. The data in conjunction with previous findings implicate a dual PIP2 regulatory pathway for NBCe1 involving both PIP2 itself and generated InsP3/Ca(2+).
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Affiliation(s)
- Ian M Thornell
- Department of Cell, Developmental and Integrative Biology, University of Alabama at Birmingham, Birmingham, AL, 35294, USA
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15
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Liu Y, Yang J, Chen LM. Structure and Function of SLC4 Family [Formula: see text] Transporters. Front Physiol 2015; 6:355. [PMID: 26648873 PMCID: PMC4664831 DOI: 10.3389/fphys.2015.00355] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2015] [Accepted: 11/10/2015] [Indexed: 12/12/2022] Open
Abstract
The solute carrier SLC4 family consists of 10 members, nine of which are [Formula: see text] transporters, including three Na(+)-independent Cl(-)/[Formula: see text] exchangers AE1, AE2, and AE3, five Na(+)-coupled [Formula: see text] transporters NBCe1, NBCe2, NBCn1, NBCn2, and NDCBE, as well as "AE4" whose Na(+)-dependence remains controversial. The SLC4 [Formula: see text] transporters play critical roles in pH regulation and transepithelial movement of electrolytes with a broad range of demonstrated physiological relevances. Dysfunctions of these transporters are associated with a series of human diseases. During the past decades, tremendous amount of effort has been undertaken to investigate the topological organization of the SLC4 transporters in the plasma membrane. Based upon the proposed topology models, mutational and functional studies have identified important structural elements likely involved in the ion translocation by the SLC4 transporters. In the present article, we review the advances during the past decades in understanding the structure and function of the SLC4 transporters.
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Affiliation(s)
- Ying Liu
- Key Laboratory of Molecular Biophysics of Ministry of Education, Department of Biophysics and Molecular Physiology, School of Life Science and Technology, Huazhong University of Science and TechnologyWuhan, China
| | - Jichun Yang
- Department of Physiology and Pathophysiology, School of Basic Medical Sciences, Peking University Health Science CenterBeijing, China
| | - Li-Ming Chen
- Key Laboratory of Molecular Biophysics of Ministry of Education, Department of Biophysics and Molecular Physiology, School of Life Science and Technology, Huazhong University of Science and TechnologyWuhan, China
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16
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Abstract
The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.
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Affiliation(s)
- Ira Kurtz
- Division of Nephrology, David Geffen School of Medicine, Los Angeles, CA; Brain Research Institute, UCLA, Los Angeles, CA
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17
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Abstract
Cation-coupled HCO3(-) transport was initially identified in the mid-1970s when pioneering studies showed that acid extrusion from cells is stimulated by CO2/HCO3(-) and associated with Na(+) and Cl(-) movement. The first Na(+)-coupled bicarbonate transporter (NCBT) was expression-cloned in the late 1990s. There are currently five mammalian NCBTs in the SLC4-family: the electrogenic Na,HCO3-cotransporters NBCe1 and NBCe2 (SLC4A4 and SLC4A5 gene products); the electroneutral Na,HCO3-cotransporter NBCn1 (SLC4A7 gene product); the Na(+)-driven Cl,HCO3-exchanger NDCBE (SLC4A8 gene product); and NBCn2/NCBE (SLC4A10 gene product), which has been characterized as an electroneutral Na,HCO3-cotransporter or a Na(+)-driven Cl,HCO3-exchanger. Despite the similarity in amino acid sequence and predicted structure among the NCBTs of the SLC4-family, they exhibit distinct differences in ion dependency, transport function, pharmacological properties, and interactions with other proteins. In epithelia, NCBTs are involved in transcellular movement of acid-base equivalents and intracellular pH control. In nonepithelial tissues, NCBTs contribute to intracellular pH regulation; and hence, they are crucial for diverse tissue functions including neuronal discharge, sensory neuron development, performance of the heart, and vascular tone regulation. The function and expression levels of the NCBTs are generally sensitive to intracellular and systemic pH. Animal models have revealed pathophysiological roles of the transporters in disease states including metabolic acidosis, hypertension, visual defects, and epileptic seizures. Studies are being conducted to understand the physiological consequences of genetic polymorphisms in the SLC4-members, which are associated with cancer, hypertension, and drug addiction. Here, we describe the current knowledge regarding the function, structure, and regulation of the mammalian cation-coupled HCO3(-) transporters of the SLC4-family.
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Affiliation(s)
- Christian Aalkjaer
- Department of Biomedicine, and the Water and Salt Research Center, Aarhus University, Aarhus, Denmark; Department of Physiology, Emory University School of Medicine, Atlanta, USA
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18
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Effects of Nt-truncation and coexpression of isolated Nt domains on the membrane trafficking of electroneutral Na+/HCO3- cotransporters. Sci Rep 2015; 5:12241. [PMID: 26192895 PMCID: PMC4507446 DOI: 10.1038/srep12241] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2015] [Accepted: 06/22/2015] [Indexed: 12/21/2022] Open
Abstract
The SLC4 genes are all capable of producing multiple variants by alternative splicing or using alternative promoters. The physiological consequences of such diversity are of great interest to investigators. Here, we identified two novel variants of the electroneutral Na+/ cotransporter NBCn1, one full-length starting with “MIPL” and the other Nt-truncated starting with “MDEL”. Moreover, we identified a new promoter of Slc4a10 encoding NBCn2 and a novel type of Nt-truncated NBCn2 starting with “MHAN”. When heterologously expressed, the new NBCn1 variants were well localized to the plasma membrane and exhibited characteristic NBCn1 activity. However, MHAN-NBCn2 was poorly localized on the plasma membrane. By deletion mutations, we identified the Nt regions important for the surface localization of NBCn2. Interestingly, coexpressing the full-length NBCn2 greatly enhances the surface abundance of the Nt-truncated NBCn2. Co-immunoprecipitation and bimolecular fluorescence complementation studies showed that the full-length and Nt-truncated NBCn2 interact with each other to form heterodimers in neuro-2A cells. Finally, we showed that the isolated Nt domain interacts with and enhances the surface abundance of the Nt-truncated NBCn2. The present study expands our knowledge of the NBCn1 and NBCn2 transcriptome, and provides insights into how the Nt domain could affect transporter function by regulating its membrane trafficking.
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19
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Zhu Q, Kao L, Azimov R, Abuladze N, Newman D, Kurtz I. Interplay between disulfide bonding and N-glycosylation defines SLC4 Na+-coupled transporter extracellular topography. J Biol Chem 2015; 290:5391-404. [PMID: 25568315 DOI: 10.1074/jbc.m114.619320] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
The extracellular loop 3 (EL-3) of SLC4 Na(+)-coupled transporters contains 4 highly conserved cysteines and multiple N-glycosylation consensus sites. In the electrogenic Na(+)-HCO3(-) cotransporter NBCe1-A, EL-3 is the largest extracellular loop and is predicted to consist of 82 amino acids. To determine the structural-functional importance of the conserved cysteines and the N-glycosylation sites in NBCe1-A EL-3, we analyzed the potential interplay between EL-3 disulfide bonding and N-glycosylation and their roles in EL-3 topological folding. Our results demonstrate that the 4 highly conserved cysteines form two intramolecular disulfide bonds, Cys(583)-Cys(585) and Cys(617)-Cys(642), respectively, that constrain EL-3 in a folded conformation. The formation of the second disulfide bond is spontaneous and unaffected by the N-glycosylation state of EL-3 or the first disulfide bond, whereas formation of the first disulfide bond relies on the presence of the second disulfide bond and is affected by N-glycosylation. Importantly, EL-3 from each monomer is adjacently located at the NBCe1-A dimeric interface. When the two disulfide bonds are missing, EL-3 adopts an extended conformation highly accessible to protease digestion. This unique adjacent parallel location of two symmetrically folded EL-3 loops from each monomer resembles a domain-like structure that is potentially important for NBCe1-A function in vivo. Moreover, the formation of this unique structure is critically dependent on the finely tuned interplay between disulfide bonding and N-glycosylation in the membrane processed NBCe1-A dimer.
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Affiliation(s)
| | - Liyo Kao
- From the Department of Medicine and
| | | | | | | | - Ira Kurtz
- From the Department of Medicine and Brain Research Institute, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
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20
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Alka K, Casey JR. Bicarbonate transport in health and disease. IUBMB Life 2014; 66:596-615. [PMID: 25270914 DOI: 10.1002/iub.1315] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2014] [Accepted: 09/10/2014] [Indexed: 12/28/2022]
Abstract
Bicarbonate (HCO3(-)) has a central place in human physiology as the waste product of mitochondrial energy production and for its role in pH buffering throughout the body. Because bicarbonate is impermeable to membranes, bicarbonate transport proteins are necessary to enable control of bicarbonate levels across membranes. In humans, 14 bicarbonate transport proteins, members of the SLC4 and SLC26 families, function by differing transport mechanisms. In addition, some anion channels and ZIP metal transporters contribute to bicarbonate movement across membranes. Defective bicarbonate transport leads to diseases, including systemic acidosis, brain dysfunction, kidney stones, and hypertension. Altered expression levels of bicarbonate transporters in patients with breast, colon, and lung cancer suggest an important role of these transporters in cancer.
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Affiliation(s)
- Kumari Alka
- Department of Biochemistry, University of Alberta, Edmonton, AB, Canada
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21
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Structure, function, and regulation of the SLC4 NBCe1 transporter and its role in causing proximal renal tubular acidosis. Curr Opin Nephrol Hypertens 2014; 22:572-83. [PMID: 23917030 DOI: 10.1097/mnh.0b013e328363ff43] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
PURPOSE OF REVIEW There has been significant progress in our understanding of the structural and functional properties and regulation of the electrogenic sodium bicarbonate cotansporter NBCe1, a membrane transporter that plays a key role in renal acid-base physiology. The NBCe1 variant NBCe1-A mediates basolateral electrogenic sodium-base transport in the proximal tubule and is critically required for transepithelial bicarbonate absorption. Mutations in NBCe1 cause autosomal recessive proximal renal tubular acidosis (pRTA). The review summarizes recent advances in this area. RECENT FINDINGS A topological model of NBCe1 has been established that provides a foundation for future structure-functional studies of the transporter. Critical residues and regions have been identified in NBCe1 that play key roles in its structure, function (substrate transport, electrogenicity) and regulation. The mechanisms of how NBCe1 mutations cause pRTA have also recently been elucidated. SUMMARY Given the important role of proximal tubule transepithelial bicarbonate absorption in systemic acid-base balance, a clear understanding of the structure-functional properties of NBCe1 is a prerequisite for elucidating the mechanisms of defective transepithelial bicarbonate transport in pRTA.
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Kurtz I. NBCe1 as a model carrier for understanding the structure-function properties of Na⁺ -coupled SLC4 transporters in health and disease. Pflugers Arch 2014; 466:1501-16. [PMID: 24515290 DOI: 10.1007/s00424-014-1448-8] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2013] [Revised: 01/07/2014] [Accepted: 01/08/2014] [Indexed: 01/17/2023]
Abstract
SLC4 transporters are membrane proteins that in general mediate the coupled transport of bicarbonate (carbonate) and share amino acid sequence homology. These proteins differ as to whether they also transport Na(+) and/or Cl(-), in addition to their charge transport stoichiometry, membrane targeting, substrate affinities, developmental expression, regulatory motifs, and protein-protein interactions. These differences account in part for the fact that functionally, SLC4 transporters have various physiological roles in mammals including transepithelial bicarbonate transport, intracellular pH regulation, transport of Na(+) and/or Cl(-), and possibly water. Bicarbonate transport is not unique to the SLC4 family since the structurally unrelated SLC26 family has at least three proteins that mediate anion exchange. The present review focuses on the first of the sodium-dependent SLC4 transporters that was identified whose structure has been most extensively studied: the electrogenic Na(+)-base cotransporter NBCe1. Mutations in NBCe1 cause proximal renal tubular acidosis (pRTA) with neurologic and ophthalmologic extrarenal manifestations. Recent studies have characterized the important structure-function properties of the transporter and how they are perturbed as a result of mutations that cause pRTA. It has become increasingly apparent that the structure of NBCe1 differs in several key features from the SLC4 Cl(-)-HCO3 (-) exchanger AE1 whose structural properties have been well-studied. In this review, the structure-function properties and regulation of NBCe1 will be highlighted, and its role in health and disease will be reviewed in detail.
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Affiliation(s)
- Ira Kurtz
- Division of Nephrology, David Geffen School of Medicine, and Brain Research Institute, UCLA, Los Angeles, CA, USA,
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Chang MH, Chen AP, Romero MF. NBCe1A dimer assemble visualized by bimolecular fluorescence complementation. Am J Physiol Renal Physiol 2014; 306:F672-80. [PMID: 24477681 DOI: 10.1152/ajprenal.00284.2013] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Mutations in the electrogenic Na(+)/HCO3(-) cotransporter (NBCe1) that cause proximal renal tubular acidosis (pRTA), glaucoma, and cataracts in patients are recessive. Parents and siblings of these affected individuals seem asymptomatic although their tissues should make some mutant NBCe1 protein. Biochemical studies with AE1 and NBCe1 indicate that both, and probably all, Slc4 members form dimers. However, the physiologic implications of dimerization have not yet been fully explored. Here, human NBCe1A dimerization is demonstrated by biomolecular fluorescence complementation (BiFC). An enhanced yellow fluorescent protein (EYFP) fragment (1-158, EYFP(N)) or (159-238, EYFP(C)) was fused to the NH2 or COOH terminus of NBCe1A and mix-and-matched expressed in Xenopus oocyte. The EYFP fluorescent signal was observed only when both EYFP fragments are fused to the NH2 terminus of NBCe1A (EYFP(N)-N-NBCe1A w/ EYFP(C)-N-NBCe1A), and the electrophysiology data demonstrated this EYFP-NBCe1A coexpressed pair have wild-type transport function. These data suggest NBCe1A forms dimers and that NH2 termini from the two monomers are in close proximity, likely pair up, to form a functional unit. To explore the physiologic significance of NBCe1 dimerization, we chose two severe NBCe1 mutations (6.6 and 20% wild-type function individually): S427L (naturally occurring) and E91R (for NH2-terminal structure studies). When we coexpressed S427L and E91R, we measured 50% wild-type function, which can only occur if the S427L-E91R heterodimer is the functional unit. We hypothesize that the dominant negative effect of heterozygous NBCe1 carrier should be obvious if the mutated residues are structurally crucial to the dimer formation. The S427L-E91R heterodimer complex allows the monomers to structurally complement each other resulting in a dimer with wild-type like function.
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Affiliation(s)
- Min-Hwang Chang
- Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, 200 First St. SW, Rochester, MN 55905.
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Kurtz I, Zhu Q. Proximal renal tubular acidosis mediated by mutations in NBCe1-A: unraveling the transporter's structure-functional properties. Front Physiol 2013; 4:350. [PMID: 24391589 PMCID: PMC3867943 DOI: 10.3389/fphys.2013.00350] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2013] [Accepted: 11/13/2013] [Indexed: 12/20/2022] Open
Abstract
NBCe1 belongs to the SLC4 family of base transporting membrane proteins that plays a significant role in renal, extrarenal, and systemic acid-base homeostasis. Recent progress has been made in characterizing the structure-function properties of NBCe1 (encoded by the SLC4A4 gene), and those factors that regulate its function. In the kidney, the NBCe1-A variant that is expressed on the basolateral membrane of proximal tubule is the key transporter responsible for overall transepithelial bicarbonate absorption in this nephron segment. NBCe1 mutations impair transepithelial bicarbonate absorption causing the syndrome of proximal renal tubular acidosis (pRTA). Studies of naturally occurring NBCe1 mutant proteins in heterologous expression systems have been very helpful in elucidation the structure-functional properties of the transporter. NBCe1 mutations are now known to cause pRTA by various mechanisms including the alteration of the transporter function (substrate ion interaction, electrogenicity), abnormal processing to the plasma membrane, and a perturbation in its structural properties. The elucidation of how NBCe1 mutations cause pRTA in addition to the recent studies which have provided further insight into the topology of the transporter have played an important role in uncovering its critically important structural-function properties.
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Affiliation(s)
- Ira Kurtz
- Division of Nephrology, David Geffen School of Medicine, UCLA Los Angeles, CA, USA ; Brain Research Institute, UCLA Los Angeles, CA, USA
| | - Quansheng Zhu
- Division of Nephrology, David Geffen School of Medicine, UCLA Los Angeles, CA, USA
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Bjerregaard-Andersen K, Perdreau-Dahl H, Guldsten H, Praetorius J, Jensen JK, Morth JP. The N-terminal cytoplasmic region of NCBE displays features of an intrinsic disordered structure and represents a novel target for specific drug screening. Front Physiol 2013; 4:320. [PMID: 24223558 PMCID: PMC3819638 DOI: 10.3389/fphys.2013.00320] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2013] [Accepted: 10/15/2013] [Indexed: 01/18/2023] Open
Abstract
The sodium dependent bicarbonate transporter NCBE/NBCn2 is predominantly expressed in the central nervous system (CNS). The highest protein concentrations are found in the choroid plexus. The primary function of this integral plasma membrane transport protein is to regulate intracellular neuronal pH and also probably to maintain the pH homeostasis across the blood-cerebrospinal fluid barrier. NCBE is predicted to contain at least 10 transmembrane helices. The N- and C- termini are both cytoplasmic, with a large N-terminal domain (Nt-NCBE) and a relatively small C-terminal domain (Ct-NCBE). The Nt-NCBE is likely to be involved in bicarbonate recognition and transport and contains key areas of regulation involving pH sensing and protein-protein interactions. Intrinsic disordered protein regions (IDPRs) are defined as protein regions having no rigid three-dimensional structure under physiological conditions. They are believed to be involved in signaling networks in which specific, low affinity, protein-protein interactions play an important role. We predict that NCBE and other SoLute Carrier 4 (SLC4) family members have a high level of intrinsic disorder in their cytoplasmic regions. To provide biophysical evidence for the IDPRs predicted in Nt-NCBE, we produced pure (>99%), recombinant Nt-NCBE using E. coli as the expression host. The protein was used to perform differential scanning fluorescence spectroscopy (DSF), in order to search for small molecules that would induce secondary or tertiary structure in the IDPRs. We expect this to assist the development of selective pharmaceutical compounds against individual SLC4 family members. We have also determined a low resolution (4 Å) X-ray crystal structure of the N-terminal core domain. The N-terminal cytoplasmic domain (cdb3) of anion exchanger 1 (AE1) shares a similar fold with the N-terminal core domain of NCBE. Crystallization conditions for the full-length N-terminal domain have been sought, but only the core domain yields diffracting crystals.
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Affiliation(s)
- Kaare Bjerregaard-Andersen
- Norwegian Centre for Molecular Medicine, University of Oslo Oslo, Norway ; Danish Chinese Centre for Cancer and Proteases, Department for Molecular Biology and Genetics, University of Aarhus Aarhus, Denmark
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Seki G, Horita S, Suzuki M, Yamazaki O, Usui T, Nakamura M, Yamada H. Molecular mechanisms of renal and extrarenal manifestations caused by inactivation of the electrogenic Na(+)-HCO3 (-) cotransporter NBCe1. Front Physiol 2013; 4:270. [PMID: 24101904 PMCID: PMC3787273 DOI: 10.3389/fphys.2013.00270] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2013] [Accepted: 09/10/2013] [Indexed: 11/13/2022] Open
Abstract
The electrogenic Na(+)-HCO3 (-) cotransporter NBCe1 plays an essential role in bicarbonate absorption from renal proximal tubules, but also mediates the other biological processes in extrarenal tissues such as bicarbonate secretion from pancreatic ducts, maintenance of tissue homeostasis in eye, enamel maturation in teeth, or local pH regulation in synapses. Homozygous mutation in NBCe1 cause proximal renal tubular acidosis (pRTA) associated with extrarenal manifestations such as short stature, ocular abnormalities, enamel abnormalities, and migraine. Functional analyses of NBCe1 mutants using different expression systems suggest that at least a 50% reduction of the transport activity may be required to induce severe pRTA. In addition to functional impairments, some NBCe1 mutants show trafficking defects. Some of the pRTA-related NBCe1 mutants showing the cytoplasmic retention have been shown to exert a dominant negative effect through hetero-oligomer complexes with wild-type NBCe1 that may explain the occurrence of extrarenal manifestations in the heterozygous carries of NBCe1 mutations. Both NBCe1 knockout (KO) and W516X knockin (KI) mice showed very severe pRTA and reproduced most of the clinical manifestations observed in human pRTA patients. Functional analysis on isolated renal proximal tubules from W516X KI mice directly confirmed the indispensable role of NBCe1 in bicarbonate absorption from this nephron segment. In this review, we will focus on the molecular mechanisms underling the renal and extrarenal manifestations caused by NBCe1 inactivation.
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Affiliation(s)
- George Seki
- Department of Internal Medicine, School of Medicine, The University of Tokyo Tokyo, Japan
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Zhu Q, Shao XM, Kao L, Azimov R, Weinstein AM, Newman D, Liu W, Kurtz I. Missense mutation T485S alters NBCe1-A electrogenicity causing proximal renal tubular acidosis. Am J Physiol Cell Physiol 2013; 305:C392-405. [PMID: 23636456 DOI: 10.1152/ajpcell.00044.2013] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mutations in SLC4A4, the gene encoding the electrogenic Na(+)-HCO3(-) cotransporter NBCe1, cause severe proximal renal tubular acidosis (pRTA), growth retardation, decreased IQ, and eye and teeth abnormalities. Among the known NBCe1 mutations, the disease-causing mechanism of the T485S (NBCe1-A numbering) mutation is intriguing because the substituted amino acid, serine, is structurally and chemically similar to threonine. In this study, we performed intracellular pH and whole cell patch-clamp measurements to investigate the base transport and electrogenic properties of NBCe1-A-T485S in mammalian HEK 293 cells. Our results demonstrated that Ser substitution of Thr485 decreased base transport by ~50%, and importantly, converted NBCe1-A from an electrogenic to an electroneutral transporter. Aqueous accessibility analysis using sulfhydryl reactive reagents indicated that Thr485 likely resides in an NBCe1-A ion interaction site. This critical location is also supported by the finding that G486R (a pRTA causing mutation) alters the position of Thr485 in NBCe1-A thereby impairing its transport function. By using NO3(-) as a surrogate ion for CO3(2-), our result indicated that NBCe1-A mediates electrogenic Na(+)-CO3(2-) cotransport when functioning with a 1:2 charge transport stoichiometry. In contrast, electroneutral NBCe1-T485S is unable to transport NO3(-), compatible with the hypothesis that it mediates Na(+)-HCO3(-) cotransport. In patients, NBCe1-A-T485S is predicted to transport Na(+)-HCO3(-) in the reverse direction from blood into proximal tubule cells thereby impairing transepithelial HCO3(-) absorption, possibly representing a new pathogenic mechanism for generating human pRTA.
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Affiliation(s)
- Quansheng Zhu
- Division of Nephrology, Department of Medicine, University of California, Los Angeles, CA 90095-1689, USA.
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28
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Identification of dominant negative effect of L522P mutation in the electrogenic Na⁺-HCO₃⁻ cotransporter NBCe1. Pflugers Arch 2013; 465:1281-91. [PMID: 23559100 DOI: 10.1007/s00424-013-1277-1] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2012] [Revised: 03/06/2013] [Accepted: 03/25/2013] [Indexed: 12/19/2022]
Abstract
Homozygous mutations in the electrogenic Na(+)-HCO3 (-) cotransporter NBCe1 cause proximal renal tubular acidosis (pRTA) associated with extrarenal manifestations such as ocular abnormalities and migraine. Previously, the NBCe1 cytosolic mutant S982NfsX4 was shown to have a dominant negative effect by forming hetero-oligomer complexes with wild type (WT), which might be responsible for the occurrence of glaucoma and migraine in the heterozygous family members. In this study, we investigated whether the NBCe1 L522P mutant has a similar dominant negative effect. Functional analyses in Xenopus oocytes and HEK293 cells revealed that the L522P mutant had no transport activity due to defective membrane expression. Furthermore, when coexpressed with WT, L522P significantly reduced the transport activity of WT. In HEK293 cells, the cytosolic mutant L522P reduced the membrane expression of NBCe1 by forming hetero-oligomer complexes with WT. Among the artificial Leu(522) mutants, L522I showed proper membrane expression and normal transport activity. However, the other mutants L522R, L522K, L522D, and L522E showed a predominant cytosolic retention. Moreover, L522R had a dominant negative effect, when coexpressed with WT. These results indicate that Leu(522) plays an important role in the structure and trafficking of NBCe1. They also suggest that the NBCe1 mutants retaining in cytoplasm may have the dominant negative effect in common, which may induce some clinical manifestations.
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Islam ST, Lam JS. Topological mapping methods for α-helical bacterial membrane proteins--an update and a guide. Microbiologyopen 2013; 2:350-64. [PMID: 23408725 PMCID: PMC3633358 DOI: 10.1002/mbo3.72] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2012] [Revised: 01/04/2013] [Accepted: 01/10/2013] [Indexed: 12/19/2022] Open
Abstract
Integral membrane proteins with α-helical transmembrane segments (TMS) are known to play important and diverse roles in prokaryotic cell physiology. The net hydrophobicity of TMS directly corresponds to the observed difficulties in expressing and purifying these proteins, let alone producing sufficient yields for structural studies using two-/three-dimensional (2D/3D) crystallographic or nuclear magnetic resonance methods. To gain insight into the function of these integral membrane proteins, topological mapping has become an important tool to identify exposed and membrane-embedded protein domains. This approach has led to the discovery of protein tracts of functional importance and to the proposition of novel mechanistic hypotheses. In this review, we synthesize the various methods available for topological mapping of α-helical integral membrane proteins to provide investigators with a comprehensive reference for choosing techniques suited to their particular topological queries and available resources.
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Affiliation(s)
- Salim T Islam
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
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30
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Lee SK, Boron WF, Parker MD. Substrate specificity of the electrogenic sodium/bicarbonate cotransporter NBCe1-A (SLC4A4, variant A) from humans and rabbits. Am J Physiol Renal Physiol 2013; 304:F883-99. [PMID: 23324180 PMCID: PMC3625843 DOI: 10.1152/ajprenal.00612.2012] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Accepted: 01/10/2013] [Indexed: 11/22/2022] Open
Abstract
In the basolateral membrane of proximal-tubule cells, NBCe1-A (SLC4A4, variant A), operating with an apparent Na(+):HCO(3)(-) stoichiometry of 1:3, contributes to the reclamation of HCO(3)(-) from the glomerular filtrate, thereby preventing whole body acidosis. Others have reported that NBCe1-like activity in human, rabbit, and rat renal preparations is substantially influenced by lithium, sulfite, oxalate, and harmaline. These data were taken as evidence for the presence of distinct Na(+) and CO(3)(2-) binding sites in NBCe1-A, favoring a model of 1 Na(+):1 HCO(3)(-):1 CO(3)(2-). Here, we reexamine these findings by expressing human or rabbit NBCe1-A clones in Xenopus oocytes. In oocytes, NBCe1-A exhibits a 1:2 stoichiometry and could operate in one of five thermodynamically equivalent transport modes: 1) cotransport of Na(+) + 2 HCO(3)(-), 2) cotransport of Na(+) + CO(3)(2-), 3) transport of NaCO(3)(-), 4) exchange of Na(+) + HCO(3)(-) for H(+), or 5) HCO(3)(-)-activated exchange of Na(+) for 2 H(+). In contrast to the behavior of NBCe1-like activity in renal preparations, we find that cloned NBCe1-A is only slightly stimulated by Li(+), not at all influenced by sulfite or oxalate, and only weakly inhibited by harmaline. These negative data do not uniquely support any of the five models above. In addition, we find that NBCe1-A mediates a small amount of Na(+)-independent NO(3)(-) transport and that NBCe1-A is somewhat inhibited by extracellular benzamil. We suggest that the features of NBCe1-like activity in renal preparations are influenced by yet-to-be-identified renal factors. Thus the actual ionic substrates of NBCe1 remain to be identified.
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Affiliation(s)
- Seong-Ki Lee
- Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, OH 44106, USA
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Parker MD, Boron WF. The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters. Physiol Rev 2013; 93:803-959. [PMID: 23589833 PMCID: PMC3768104 DOI: 10.1152/physrev.00023.2012] [Citation(s) in RCA: 197] [Impact Index Per Article: 17.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.
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Affiliation(s)
- Mark D Parker
- Dept. of Physiology and Biophysics, Case Western Reserve University School of Medicine, 10900 Euclid Ave., Cleveland, OH 44106-4970, USA.
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Zhu Q, Liu W, Kao L, Azimov R, Newman D, Abuladze N, Kurtz I. Topology of NBCe1 protein transmembrane segment 1 and structural effect of proximal renal tubular acidosis (pRTA) S427L mutation. J Biol Chem 2013; 288:7894-7906. [PMID: 23362273 DOI: 10.1074/jbc.m112.404533] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In the kidney proximal tubule, NBCe1-A plays a critical role in absorbing HCO3(-) from cell to blood. NBCe1-A transmembrane segment 1 (TM1) is involved in forming part of the ion permeation pathway, and a missense mutation S427L in TM1 impairs ion transport, causing proximal renal tubular acidosis. In the present study, we examined the topology of NBCe1-A-TM1 in detail and its structural perturbation induced by S427L. We analyzed the N-terminal cytoplasmic region (Cys-389-Gln-424) of NBCe1-A-TM1 using the substituted cysteine scanning accessibility method combined with extensive chemical stripping, in situ chemical probing, and functional transport assays. NBCe1-A-TM1 was previously modeled on the anion exchanger 1 TM1 (AE1-TM1); however, our data demonstrated that the topology of AE1-TM1 differs significantly from NBCe1-A-TM1. Our findings revealed that NBCe1-A-TM1 is unusually long, consisting of 31 membrane-embedded amino acids (Phe-412 to Thr-442). The linker region (Arg-394-Pro-411) between the N terminus of TM1 and the cytoplasmic domain is minimally exposed to aqueous and is potentially folded in a helical structure that intimately interacts with the NBCe1-A cytoplasmic domain. In contrast, AE1-TM1 contains 25 amino acids connected to an aqueous-exposed cytoplasmic region. Based on our new NBCe1-A-TM1 model, Ser-427 resides in the middle of TM1. Leucine substitution at Ser-427 blocks the normal aqueous access to Thr-442, Ala-435, and Lys-404, implying a significant alteration of NBCe1-TM1 orientation. Our study provides novel structural insights into the pathogenic mechanism of S427L in mediating proximal renal tubular acidosis.
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Affiliation(s)
- Quansheng Zhu
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689.
| | - Weixin Liu
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
| | - Liyo Kao
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
| | - Rustam Azimov
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
| | - Debra Newman
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
| | - Natalia Abuladze
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
| | - Ira Kurtz
- Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689; Brain Research Institute, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1689
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Reimold FR, Stewart AK, Stolpe K, Heneghan JF, Shmukler BE, Alper SL. Substitution of transmembrane domain Cys residues alters pH(o)-sensitive anion transport by AE2/SLC4A2 anion exchanger. Pflugers Arch 2012; 465:839-51. [PMID: 23271450 DOI: 10.1007/s00424-012-1196-6] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2012] [Revised: 11/19/2012] [Accepted: 11/30/2012] [Indexed: 10/27/2022]
Abstract
AE2/SLC4A2 is the most widely expressed of the Na(+)-independent SLC4 Cl(-)/HCO3 (-) exchangers and is essential for postnatal survival, but its structure remains unknown. We have generated and expressed a mouse AE2 construct devoid of transmembrane domain cysteine (Cys) residues, mAE2Cys-less, to enhance the utility of Cys-substitution mutagenesis for structural and structure-function studies of mAE2. mAE2Cys-less expressed in Xenopus oocytes exhibited partial reduction of stilbene disulfonate-sensitive anion exchange activity. This activity was independent of the mAE2 N-terminal cytosolic domain and was accompanied by near-normal surface expression, without change in K 1/2 for extracellular Cl(-). mAE2Cys-less exhibited wildtype activation of anion exchange by hypertonicity and by NH4Cl, and wildtype inhibition of anion exchange by acidic intracellular pH (pHi) in the absence of NH4 (+). However, inhibition of anion exchange by extracellular pH (pHo) exhibited an alkaline shifted pHo(50) value of at least 0.6-0.7 pH units. Although SO4 (2-) transport by mAE2Cys-less resembled wildtype mAE2 in its stimulation by acidic pHo, the absence of transmembrane domain Cys residues abrogated activation of oxalate transport by acidic pHo. The contrasting enhancement of SO4 (2-) transport by alkaline pHo in the mAE1 anion translocation pathway mutant E699Q (Am J Physiol Cell Physiol 295: C302) was phenocopied by the corresponding mutant E1007Q in both AE2 and AE2Cys-less. However, the absence of transmembrane domain Cys residues exacerbated the reduced basal anion transport function exhibited by this and other missense substitutions at AE2 residue E1007. AE2Cys-less will be a valuable experimental tool for structure-function studies of the SLC4 gene family, but its utility for studies of AE2 regulation by extracellular pH must be evaluated in the context of its alkaline-shifted pHo sensitivity, resembling that of AE2 gastric parietal cell variant AE2c1.
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Affiliation(s)
- Fabian R Reimold
- Renal Division and Molecular and Vascular Medicine Division, Beth Israel Deaconess Medical Center, 99 Brookline Avenue, RN-380F, Boston, MA 02215, USA
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Lee S, Yang HS, Kim E, Ju EJ, Kwon MH, Dudley RK, Smith Y, Yun CC, Choi I. PSD-95 interacts with NBCn1 and enhances channel-like activity without affecting Na/HCO(3) cotransport. Cell Physiol Biochem 2012. [PMID: 23183381 DOI: 10.1159/000343332] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
BACKGROUND/AIMS The sodium/bicarbonate transporter NBCn1 plays an essential role in intracellular pH regulation and transepithelial HCO(3)(-) movement in the body. NBCn1 also has sodium channel-like activity uncoupled to Na/HCO(3) cotransport. We previously reported that NBCn1 interacts with the postsynaptic density protein PSD-95 in the brain. Here, we elucidated the structural determinant and functional consequence of NBCn1/PSD-95 interaction. METHODS RESULTS In rat hippocampal CA3 neurons, NBCn1 was localized to the postsynaptic membranes of both dendritic shafts and spines and occasionally to the presynaptic membranes. A GST/NBCn1 fusion protein containing the C-terminal 131 amino acids of NBCn1 pulled down PSD-95 from rat brain lysates, whereas GST/NBCn1-ΔETSL (deletion of the last four amino acids) and GST/NBCn2 (NCBE) lacking the same ETSL did not. NBCn1 and PSD-95 were coimmunoprecipitated in HEK 293 cells, and their interaction did not affect the efficacy of PSD-95 to bind to the NMDA receptor NR2A. PSD-95 has negligible effects on intracellular pH changes mediated by NBCn1 in HEK 293 cells and Xenopus oocytes. However, PSD-95 increased an ionic conductance produced by NBCn1 channel-like activity. This increase was abolished by NBCn1-ΔETSL or by the peptide containing the last 15 amino acids of NBCn1. CONCLUSION Our data suggest that PSD-95 interacts with NBCn1 and increases its channel-like activity while negligibly affecting Na/HCO(3) cotransport. The possibility that the channel-like activity occurs via an intermolecular cavity of multimeric NBCn1 proteins is discussed.
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Affiliation(s)
- Soojung Lee
- Department of Physiology, Emory University School of Medicine, Atlanta, GA 30322, USA
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Suzuki M, Seki G, Yamada H, Horita S, Fujita T. Functional Roles of Electrogenic Sodium Bicarbonate Cotransporter NBCe1 in Ocular Tissues. Open Ophthalmol J 2012; 6:36-41. [PMID: 22798968 PMCID: PMC3394102 DOI: 10.2174/1874364101206010036] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2012] [Revised: 05/22/2012] [Accepted: 05/24/2012] [Indexed: 12/04/2022] Open
Abstract
Electrogenic Na+-HCO3- cotransporter NBCe1 is expressed in several tissues such as kidney, eye, and brain, where it may mediate distinct biological processes. In particular, NBCe1 in renal proximal tubules is essential for the regulation of systemic acid/base balance. On the other hand, NBCe1 in eye may be indispensable for the maintenance of tissue homeostasis. Consistent with this view, homozygous mutations in NBCe1 cause severe proximal renal tubular acidosis associated with ocular abnormalities such as band keratopathy, glaucoma, and cataract. The widespread expression of NBCe1 in eye suggests that the inactivation of NBCe1 per se may be responsible for the occurrence of these ocular abnormalities. In this review, we discuss about physiological and pathological roles of NBCe1 in eye.
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Affiliation(s)
- Masashi Suzuki
- Department of Internal Medicine, Faculty of Medicine, University of Tokyo, Japan
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36
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Sergeev M, Godin AG, Kao L, Abuladze N, Wiseman PW, Kurtz I. Determination of membrane protein transporter oligomerization in native tissue using spatial fluorescence intensity fluctuation analysis. PLoS One 2012; 7:e36215. [PMID: 22558387 PMCID: PMC3338697 DOI: 10.1371/journal.pone.0036215] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2011] [Accepted: 04/02/2012] [Indexed: 11/18/2022] Open
Abstract
Membrane transporter proteins exist in a complex dynamic equilibrium between various oligomeric states that include monomers, dimers, dimer of dimers and higher order oligomers. Given their sub-optical microscopic resolution size, the oligomerization state of membrane transporters is difficult to quantify without requiring tissue disruption and indirect biochemical methods. Here we present the application of a fluorescence measurement technique which combines fluorescence image moment analysis and spatial intensity distribution analysis (SpIDA) to determine the oligomerization state of membrane proteins in situ. As a model system we analyzed the oligomeric state(s) of the electrogenic sodium bicarbonate cotransporter NBCe1-A in cultured cells and in rat kidney. The approaches that we describe offer for the first time the ability to investigate the oligomeric state of membrane transporter proteins in their native state.
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Affiliation(s)
- Mikhail Sergeev
- Department of Physics, McGill University, Montréal, Québec, Canada
| | - Antoine G. Godin
- Department of Physics, McGill University, Montréal, Québec, Canada
| | - Liyo Kao
- David Geffen School Medicine, University of California Los Angeles, Los Angeles, California, United States of America
| | - Natalia Abuladze
- David Geffen School Medicine, University of California Los Angeles, Los Angeles, California, United States of America
| | - Paul W. Wiseman
- Department of Physics, McGill University, Montréal, Québec, Canada
- Department of Chemistry, McGill University, Montréal, Québec, Canada
| | - Ira Kurtz
- David Geffen School Medicine, University of California Los Angeles, Los Angeles, California, United States of America
- Brain Research Institute, University of California Los Angeles, Los Angeles, California, United States of America
- * E-mail:
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De Giusti VC, Orlowski A, Villa-Abrille MC, de Cingolani GEC, Casey JR, Alvarez BV, Aiello EA. Antibodies against the cardiac sodium/bicarbonate co-transporter (NBCe1) as pharmacological tools. Br J Pharmacol 2012; 164:1976-89. [PMID: 21595652 DOI: 10.1111/j.1476-5381.2011.01496.x] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Abstract
BACKGROUND AND PURPOSE Na(+) /HCO(3) (-) co-transport (NBC) regulates intracellular pH (pH(i) ) in the heart. We have studied the electrogenic NBC isoform NBCe1 by examining the effect of functional antibodies to this protein. EXPERIMENTAL APPROACH We generated two antibodies against putative extracellular loop domains 3 (a-L3) and 4 (a-L4) of NBCe1 which recognized NBCe1 on immunoblots and immunostaining experiments. pH(i) was monitored using epi-fluorescence measurements in cat ventricular myocytes. Transport activity of total NBC and of NBCe1 in isolation were evaluated after an ammonium ion-induced acidosis (expressed as H(+) flux, J(H) , in mmol·L(-1) min(-1) at pH(i) 6.8) and during membrane depolarization with high extracellular potassium (potassium pulse, expressed as ΔpH(i) ) respectively. KEY RESULTS The potassium pulse produced a pH(i) increase of 0.18 ± 0.006 (n= 5), which was reduced by the a-L3 antibody (0.016 ± 0.019). The a-L-3 also decreased J(H) by 50%. Surprisingly, during the potassium pulse, a-L4 induced a higher pH(i) increase than control,(0.25 ± 0.018) whereas the recovery of pH(i) from acidosis was faster (J(H) was almost double the control value). In perforated-patch experiments, a-L3 prolonged and a-L4 shortened action potential duration, consistent with blockade and stimulation of NBCe1-carried anionic current respectively. CONCLUSIONS AND IMPLICATIONS Both antibodies recognized NBCe1, but they had opposing effects on the function of this transporter, as the a-L3 was inhibitory and the a-L4 was excitatory. These antibodies could be valuable in studies on the pathophysiology of NBCe1 in cardiac tissue, opening a path for their potential clinical use.
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Affiliation(s)
- Verónica C De Giusti
- Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, La Plata, Argentina
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Homo- and hetero-dimeric architecture of the human liver Na+-dependent taurocholate co-transporting protein. Biochem J 2012; 441:1007-15. [DOI: 10.1042/bj20111234] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
The NTCP (Na+–taurocholate co-transporting protein)/SLC10A [solute carrier family 10 (Na+/bile acid co-transporter family)] 1 is tightly controlled to ensure hepatic bile salt uptake while preventing toxic bile salt accumulation. Many transport proteins require oligomerization for their activity and regulation. This is not yet established for bile salt transporters. The present study was conducted to elucidate the oligomeric state of NTCP. Chemical cross-linking revealed the presence of NTCP dimers in rat liver membranes and U2OS cells stably expressing NTCP. Co-immunoprecipitation of tagged NTCP proteins revealed a physical interaction between subunits. The C-terminus of NTCP was not required for subunit interaction, but was essential for exit from the ER (endoplasmic reticulum). NTCP without its C-terminus (NTCP Y307X) retained full-length wtNTCP (wild-type NTCP) in the ER in a dominant fashion, suggesting that dimerization occurs early in the secretory pathway. FRET (fluorescence resonance energy transfer) using fluorescently labelled subunits further demonstrated that dimerization persists at the plasma membrane. NTCP belongs to the SLC10A protein family which consists of seven members. NTCP co-localized in U2OS cells with SLC10A4 and SLC10A6, but not with SLC10A3, SLC10A5 or SLC10A7. SLC10A4 and SLC10A6 co-immunoprecipitated with NTCP, demonstrating that heteromeric complexes can be formed between SLC10A family members in vitro. Expression of SLC10A4 and NTCP Y307X resulted in a reduction of NTCP abundance at the plasma membrane and NTCP-mediated taurocholate uptake, whereas expression of SLC10A6 or NTCP E257N, an inactive mutant, did not affect NTCP function. In conclusion, NTCP adopts a dimeric structure in which individual subunits are functional. Bile salt uptake is influenced by heterodimerization when this impairs NTCP plasma membrane trafficking.
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Abstract
SLC4A gene family proteins include bicarbonate transporters that move HCO(3)(-) across the plasma membrane and regulate intracellular pH and transepithelial movement of acid-base equivalents. These transporters are Cl/HCO(3) exchangers, electrogenic Na/HCO(3) cotransporters, electroneutral Na/HCO(3) cotransporters, and Na(+)-driven Cl/HCO(3) exchanger. Studies of the bicarbonate transporters in vitro and in vivo have demonstrated their physiological importance for acid-base homeostasis at the cellular and systemic levels. Recent advances in structure/function analysis have also provided valuable information on domains or motifs critical for regulation, ion translocation, and protein topology. This chapter focuses on the molecular mechanisms of ion transport along with associated structural aspects from mutagenesis of particular residues and from chimeric constructs. Structure/function studies have helped to understand the mechanism by which ion substrates are moved via the transporters. This chapter also describes some insights into the structure of SLC4A1 (AE1) and SLC4A4 (NBCe1) transporters. Finally, as some SLC4A transporters exist in concert with other proteins in the cells, the structural features associated with protein-protein interactions are briefly discussed.
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Affiliation(s)
- Inyeong Choi
- Department of Physiology, Emory University, Atlanta, Georgia, USA.
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40
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Vilas GL, Loganathan SK, Quon A, Sundaresan P, Vithana EN, Casey J. Oligomerization of SLC4A11 protein and the severity of FECD and CHED2 corneal dystrophies caused by SLC4A11 mutations. Hum Mutat 2011; 33:419-28. [PMID: 22072594 DOI: 10.1002/humu.21655] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2011] [Accepted: 10/31/2011] [Indexed: 11/06/2022]
Abstract
Mutations in the SLC4A11 gene, which encodes a plasma membrane borate transporter, cause recessive congenital hereditary endothelial corneal dystrophy type 2 (CHED2), corneal dystrophy and perceptive deafness (Harboyan syndrome), and dominant late-onset Fuchs endothelial corneal dystrophy (FECD). We analyzed missense SLC4A11 mutations identified in FECD and CHED2 patients and expressed in transfected HEK 293 cells. Chemical cross-linking and migration in nondenaturing gels showed that SLC4A11 exists as a dimer. Furthermore, co-immunoprecipitation of epitope-tagged proteins revealed heteromeric interactions between wild-type (WT) and mutant SLC4A11 proteins. When expressed alone, FECD- and CHED2-causing mutant SLC4A11 proteins are primarily retained intracellularly. Co-expression with WT SLC4A11 partially rescued the cell surface trafficking of CHED2 mutants, but not FECD mutants. CHED2 alleles of SLC4A11 did not affect cell surface processing of WT SLC4A11. In contrast, FECD mutants reduced WT cell surface processing efficiency, consistent with dominant inheritance of FECD. The reduction in movement of WT protein to the cell surface caused by FECD SLC4A11 helps to explain the dominant inheritance of this disorder. Similarly, the failure of CHED2 mutant SLC4A11 to affect the processing of WT protein, explains the lack of symptoms found in CHED2 carriers and the recessive inheritance of the disorder.
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Affiliation(s)
- Gonzalo L Vilas
- Membrane Protein Disease Research Group, Department of Physiology, School of Molecular and Systems Medicine, University of Alberta, Edmonton, Canada
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41
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Brast S, Grabner A, Sucic S, Sitte HH, Hermann E, Pavenstädt H, Schlatter E, Ciarimboli G. The cysteines of the extracellular loop are crucial for trafficking of human organic cation transporter 2 to the plasma membrane and are involved in oligomerization. FASEB J 2011; 26:976-86. [PMID: 22085643 DOI: 10.1096/fj.11-180679] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Human organic cation transporter 2 (hOCT2) is involved in transport of many endogenous and exogenous organic cations, mainly in kidney and brain cells. Because the quaternary structure of transmembrane proteins plays an essential role for their cellular trafficking and function, we investigated whether hOCT2 forms oligomeric complexes, and if so, which part of the transporter is involved in the oligomerization. A yeast 2-hybrid mating-based split-ubiquitin system (mbSUS), fluorescence resonance energy transfer, Western blot analysis, cross-linking experiments, immunofluorescence, and uptake measurements of the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium were applied to human embryonic kidney 293 (HEK293) cells transfected with hOCT2 and partly also to freshly isolated human proximal tubules. The role of cysteines for oligomerization and trafficking of the transporter to the plasma membranes was investigated in cysteine mutants of hOCT2. hOCT2 formed oligomers both in the HEK293 expression system and in native human kidneys. The cysteines of the large extracellular loop are important to enable correct folding, oligomeric assembly, and plasma membrane insertion of hOCT2. Mutation of the first and the last cysteines of the loop at positions 51 and 143 abolished oligomer formation. Thus, the cysteines of the extracellular loop are important for correct trafficking of the transporter to the plasma membrane and for its oligomerization.
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Affiliation(s)
- Sabine Brast
- Experimentelle Nephrologie, Medizinische Klinik und Poliklinik D, Domagkstrasse 3A, 48149 Münster, Germany
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Functional characterization of nonsynonymous single nucleotide polymorphisms in the electrogenic Na+-HCO3- cotransporter NBCe1A. Pflugers Arch 2011; 461:249-59. [PMID: 21234596 DOI: 10.1007/s00424-010-0918-x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2010] [Revised: 12/10/2010] [Accepted: 12/20/2010] [Indexed: 12/22/2022]
Abstract
The electrogenic Na(+)-HCO(3)(-) cotransporter NBCe1 encoded by SLC4A4 plays essential roles in the regulation of intracellular/extracellular pH. Homozygous mutations in NBCe1 cause proximal renal tubular acidosis associated with ocular abnormalities. In the present study, we tried to perform functional characterization of the four nonsynonymous single nucleotide polymorphisms (SNPs), E122G, S356Y, K558R, and N640I in NBCe1A. Functional analysis in Xenopus oocytes revealed that while the K558R variant had a significantly reduced transport activity corresponding to 47% of the wild-type activity, the remaining variants E122G, S356Y, and N640I did not change the NBCe1A activity. Apparent Na(+) affinity of K558R was not different from that of wild-type NBCe1A. Immunohistological analyses in HEK293 cells and MDCK cells indicated that none of these SNPs changed the trafficking behaviors of NBCe1A. Functional analysis in HEK293 cells also revealed that only the K558R variant had a reduced transport activity, corresponding to 41-47% of the wild-type activity. From these results, we conclude that among four SNPs, only the K558R variant, which is predicted to lie in transmembrane segment 5, significantly reduces the NBCe1A activity without changing the trafficking behavior or the apparent extracellular Na(+) affinity.
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Basu A, Mazor S, Casey JR. Distance Measurements within a Concatamer of the Plasma Membrane Cl−/HCO3− Exchanger, AE1. Biochemistry 2010; 49:9226-40. [DOI: 10.1021/bi101134h] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Affiliation(s)
- Arghya Basu
- Membrane Protein Research Group, Department of Physiology and Department of Biochemistry, School of Molecular and Systems Medicine, 721 Medical Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
| | - Shirley Mazor
- Membrane Protein Research Group, Department of Physiology and Department of Biochemistry, School of Molecular and Systems Medicine, 721 Medical Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
| | - Joseph R. Casey
- Membrane Protein Research Group, Department of Physiology and Department of Biochemistry, School of Molecular and Systems Medicine, 721 Medical Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
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Huc-Brandt S, Marcellin D, Graslin F, Averseng O, Bellanger L, Hivin P, Quemeneur E, Basquin C, Navarro V, Pourcher T, Darrouzet E. Characterisation of the purified human sodium/iodide symporter reveals that the protein is mainly present in a dimeric form and permits the detailed study of a native C-terminal fragment. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2010; 1808:65-77. [PMID: 20797386 DOI: 10.1016/j.bbamem.2010.08.013] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/29/2010] [Revised: 08/16/2010] [Accepted: 08/18/2010] [Indexed: 12/31/2022]
Abstract
The sodium/iodide symporter is an intrinsic membrane protein that actively transports iodide into thyroid follicular cells. It is a key element in thyroid hormone biosynthesis and in the radiotherapy of thyroid tumours and their metastases. Sodium/iodide symporter is a very hydrophobic protein that belongs to the family of sodium/solute symporters. As for many other membrane proteins, particularly mammalian ones, little is known about its biochemistry and structure. It is predicted to contain 13 transmembrane helices, with an N-terminus oriented extracellularly. The C-terminal, cytosolic domain contains approximately one hundred amino acid residues and bears most of the transporter's putative regulatory sites (phosphorylation, sumoylation, di-acide, di-leucine or PDZ-binding motifs). In this study, we report the establishment of eukaryotic cell lines stably expressing various human sodium/iodide symporter recombinant proteins, and the development of a purification protocol which allowed us to purify milligram quantities of the human transporter. The quaternary structure of membrane transporters is considered to be essential for their function and regulation. Here, the oligomeric state of human sodium/iodide symporter was analysed for the first time using purified protein, by size exclusion chromatography and light scattering spectroscopy, revealing that the protein exists mainly as a dimer which is stabilised by a disulfide bridge. In addition, the existence of a sodium/iodide symporter C-terminal fragment interacting with the protein was also highlighted. We have shown that this fragment exists in various species and cell types, and demonstrated that it contains the amino-acids [512-643] from the human sodium/iodide symporter protein and, therefore, the last predicted transmembrane helix. Expression of either the [1-512] truncated domain or the [512-643] domain alone, as well as co-expression of the two fragments, was performed, and revealed that co-expression of [1-512] with [512-643] allowed the reconstitution of a functional protein. These findings constitute an important step towards an understanding of some of the post-translational mechanisms that finely tune iodide accumulation through human sodium/iodide symporter regulation.
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Affiliation(s)
- Sylvaine Huc-Brandt
- CEA, iBEB, SBTN, Centre de Marcoule, Bat 170, BP17171, 30207 Bagnols sur Cèze, CEDEX, France.
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45
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Defective membrane expression of the Na(+)-HCO(3)(-) cotransporter NBCe1 is associated with familial migraine. Proc Natl Acad Sci U S A 2010; 107:15963-8. [PMID: 20798035 DOI: 10.1073/pnas.1008705107] [Citation(s) in RCA: 120] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Homozygous mutations in SLC4A4, encoding the electrogenic Na(+)-HCO(3)(-) cotransporter NBCe1, have been known to cause proximal renal tubular acidosis (pRTA) and ocular abnormalities. In this study, we report two sisters with pRTA, ocular abnormalities, and hemiplegic migraine. Genetic analysis ruled out pathological mutations in the known genes for familial hemiplegic migraine, but identified a homozygous 65-bp deletion (Delta65bp) in the C terminus of NBCe1, corresponding to the codon change S982NfsX4. Several heterozygous members of this family also presented glaucoma and migraine with or without aura. Despite the normal electrogenic activity in Xenopus oocytes, the Delta65bp mutant showed almost no transport activity due to a predominant cytosolic retention in mammalian cells. Furthermore, coexpression experiments uncovered a dominant negative effect of the mutant through hetero-oligomer formation with wild-type NBCe1. Among other pRTA pedigrees with different NBCe1 mutations, we identified four additional homozygous patients with migraine. The immunohistological and functional analyses of these mutants demonstrate that the near total loss of NBCe1 activity in astrocytes can cause migraine potentially through dysregulation of synaptic pH.
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46
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Zhu Q, Kao L, Azimov R, Newman D, Liu W, Pushkin A, Abuladze N, Kurtz I. Topological location and structural importance of the NBCe1-A residues mutated in proximal renal tubular acidosis. J Biol Chem 2010; 285:13416-26. [PMID: 20197274 DOI: 10.1074/jbc.m109.093286] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
NBCe1-A electrogenically cotransports Na(+) and HCO(3)(-) across the basolateral membrane of renal proximal tubule cells. Eight missense mutations and 3 nonsense mutations in NBCe1-A cause severe proximal renal tubular acidosis (pRTA). In this study, the topologic properties and structural importance of the 8 endogenous residues mutated in pRTA and the in situ topology of NBCe1-A were examined by the substituted cysteine accessibility method. Of the 55 analyzed individually introduced cysteines, 8 were labeled with both membrane permeant (biotin maleimide (BM)) and impermeant (2-((5(6)-tetramethylrhodamine)carboxylamino)ethyl methanethiosulfonate (MTS-TAMRA)) sulfhydryl reagents, 4 with only BM, and 3 with only MTS-TAMRA. The location of the labeled and unlabeled introduced cysteines clearly indicates that the transmembrane region of NBCe1-A contains 14 transmembrane segments (TMs). In this in situ based NBCe1-A topology, residues mutated in pRTA (pRTA residues) are assigned as: Ser(427), TM1; Thr(485) and Gly(486), TM3; Arg(510) and Leu(522), TM4; Ala(799), TM10; and Arg(881), TM12. Substitution of pRTA residues with cysteines impaired the membrane trafficking of R510C and R881C, the remaining membrane-processed constructs had various impaired transport function. Surprisingly, none of the membrane-processed constructs was accessible to labeling with BM and MTS-TAMRA, nor were they functionally sensitive to the inhibition by (2-aminoethyl)methanethiosulfonate. Functional analysis of Thr(485) with different amino acid substitutions indicated it resides in a unique region important for NBCe1-A function. Our findings demonstrate that the pRTA residues in NBCe1-A are buried in the protein complex/lipid bilayer where they perform important structural roles.
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Affiliation(s)
- Quansheng Zhu
- Division of Nephrology, David Geffen School of Medicine, UCLA, Los Angeles, California 90095, USA
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Park HJ, Rajbhandari I, Yang HS, Lee S, Cucoranu D, Cooper DS, Klein JD, Sands JM, Choi I. Neuronal expression of sodium/bicarbonate cotransporter NBCn1 (SLC4A7) and its response to chronic metabolic acidosis. Am J Physiol Cell Physiol 2010; 298:C1018-28. [PMID: 20147654 DOI: 10.1152/ajpcell.00492.2009] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
The sodium-bicarbonate cotransporter NBCn1 (SLC4A7) is an acid-base transporter that normally moves Na(+) and HCO(3)(-) into the cell. This membrane protein is sensitive to cellular and systemic pH changes. We examined NBCn1 expression and localization in the brain and its response to chronic metabolic acidosis. Two new NBCn1 antibodies were generated by immunizing a rabbit and a guinea pig. The antibodies stained neurons in a variety of rat brain regions, including hippocampal pyramidal neurons, dentate gyrus granular neurons, posterior cortical neurons, and cerebellar Purkinje neurons. Choroid plexus epithelia were also stained. Double immunofluorescence labeling showed that NBCn1 and the postsynaptic density protein PSD-95 were found in the same hippocampal CA3 neurons and partially colocalized in dendrites. PSD-95 was pulled down from rat brain lysates with the GST/NBCn1 fusion protein and was also coimmunoprecipitated with NBCn1. Chronic metabolic acidosis was induced by feeding rats with normal chow or 0.4 M HCl-containing chow for 7 days. Real-time PCR and immunoblot showed upregulation of NBCn1 mRNA and protein in the hippocampus of acidotic rats. NBCn1 immunostaining was enhanced in CA3 neurons, posterior cortical neurons, and cerebellar granular cells. Intraperitoneal administration of N-methyl-d-aspartate caused neuronal death determined by caspase-3 activity, and this effect was more severe in acidotic rats. Administering N-methyl-d-aspartate also inhibited NBCn1 upregulation in acidotic rats. We conclude that NBCn1 in neurons is upregulated by chronic acid loads, and this upregulation is associated with glutamate excitotoxicity.
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Affiliation(s)
- Hae Jeong Park
- Dept. of Physiology, Emory Univ., Atlanta, GA 30322, USA
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48
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Hou Z, Cherian C, Drews J, Wu J, Matherly LH. Identification of the minimal functional unit of the homo-oligomeric human reduced folate carrier. J Biol Chem 2009; 285:4732-40. [PMID: 20018840 DOI: 10.1074/jbc.m109.086033] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The reduced folate carrier (RFC) is the major transport system for folates in mammals. We previously demonstrated the existence of human RFC (hRFC) homo-oligomers and established the importance of these higher order structures to intracellular trafficking and carrier function. In this report, we examined the operational significance of hRFC oligomerization and the minimal functional unit for transport. In negative dominance experiments, multimeric transporters composed of different ratios of active (either wild type (WT) or cysteine-less (CLFL)) and inactive (either inherently inactive (Y281L and R373A) due to mutation, or resulting from inactivation of the Y126C mutant by (2-sulfonatoethyl) methanethiosulfonate (MTSES)) hRFC monomers were expressed in hRFC-null HeLa (R5) cells, and residual WT or CLFL activity was measured. In either case, residual transport activity with increasing levels of inactive mutant correlated linearly with the fraction of WT or CLFL hRFC in plasma membranes. When active covalent hRFC dimers, generated by fusing CLFL and Y126C monomers, were expressed in R5 cells and treated with MTSES, transport activity of the CLFL-CLFL dimer was unaffected, whereas Y126C-Y126C was potently (64%) inhibited; heterodimeric CLFL-Y126C and Y126C-CLFL were only partly (27 and 23%, respectively) inhibited by MTSES. In contrast to Y126C-Y126C, trans-stimulation of methotrexate uptake by intracellular folates for Y126C-CLFL and CLFL-Y126C was nominally affected by MTSES. Collectively, these results strongly support the notion that each hRFC monomer comprises a single translocation pathway for anionic folate substrates and functions independently of other monomers (i.e. despite an oligomeric structure, hRFC functions as a monomer).
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Affiliation(s)
- Zhanjun Hou
- Developmental Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan 48201, USA
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Boron WF, Chen L, Parker MD. Modular structure of sodium-coupled bicarbonate transporters. ACTA ACUST UNITED AC 2009; 212:1697-706. [PMID: 19448079 DOI: 10.1242/jeb.028563] [Citation(s) in RCA: 103] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Mammalian genomes contain 10 SLC4 genes that, between them, encode three Cl-HCO(3) exchangers, five Na(+)-coupled HCO(3) transporters (NCBTs), one reported borate transporter, and what is reported to be a fourth Cl-HCO(3) exchanger. The NCBTs are expressed throughout the body and play important roles in maintaining intracellular and whole-body pH, as well as contributing to transepithelial transport processes. The importance of NCBTs is underscored by the genetic association of dysfunctional NCBT genes with blindness, deafness, epilepsy, hypertension and metal retardation. Key to understanding the action and regulation of NCBTs is an appreciation of the diversity of NCBT gene products. The transmembrane domains of human NCBT paralogs are 50-84% identical to each other at the amino acid level, and are capable of a diverse range of actions, including electrogenic Na/HCO(3) cotransport (i.e. NBCe1 and NBCe2) and electroneutral Na/HCO(3) cotransport (i.e. NBCn1 and NBCn2), as well as Na(+)-dependent Cl-HCO(3) exchange (i.e. NDCBE). Furthermore, by the use of alternative promoters and alternative-splicing events, individual SLC4 genes have the potential to generate multiple splice variants (as many as 16 in the case of NBCn1), each of which could have unique temporal and spatial patterns of distribution, unitary transporter activity (i.e. flux mediated by one molecule), array of protein-binding partners, and complement of regulatory stimuli. In the first section of this review, we summarize our present knowledge of the function and distribution of mammalian NCBTs and their multiple variants. In the second section of this review we consider the molecular consequences of NCBT variation.
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Affiliation(s)
- Walter F Boron
- Department of Physiology, Case Western Reserve University Medical School, 10900 Euclid Avenue, Cleveland, OH 44106, USA
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Yang HS, Kim E, Lee S, Park HJ, Cooper DS, Rajbhandari I, Choi I. Mutation of Aspartate 555 of the Sodium/Bicarbonate Transporter SLC4A4/NBCe1 Induces Chloride Transport. J Biol Chem 2009; 284:15970-9. [PMID: 19336397 DOI: 10.1074/jbc.m109.001438] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To understand the mechanism for ion transport through the sodium/bicarbonate transporter SLC4A4 (NBCe1), we examined amino acid residues, within transmembrane domains, that are conserved among electrogenic Na/HCO(3) transporters but are substituted with residues at the corresponding site of all electroneutral Na/HCO(3) transporters. Point mutants were constructed and expressed in Xenopus oocytes to assess function using two-electrode voltage clamp. Among the mutants, D555E (charge-conserved substitution of the aspartate at position 555 with a glutamate) produced decreasing HCO(3)(-) currents at more positive membrane voltages. Immunohistochemistry showed D555E protein expression in oocyte membranes. D555E induced Na/HCO(3)-dependent pH recovery from a CO(2)-induced acidification. Current-voltage relationships revealed that D555E produced an outwardly rectifying current in the nominally CO(2)/HCO(3)(-)-free solution that was abolished by Cl(-) removal from the bath. In the presence of CO(2)/HCO(3)(-), however, the outward current produced by D555E decreased only slightly after Cl(-) removal. Starting from a Cl(-)-free condition, D555E produced dose-dependent outward currents in response to a series of chloride additions. The D555E-mediated chloride current decreased by 70% in the presence of CO(2)/HCO(3)(-). The substitution of Asp(555) with an asparagine also produced a Cl(-) current. Anion selectivity experiments revealed that D555E was broadly permissive to other anions including NO(3)(-). Fluorescence measurements of chloride transport were done with human embryonic kidney HEK 293 cells expressing NBCe1 and D555E. A marked increase in chloride transport was detected in cells expressing D555E. We conclude that Asp(555) plays a role in HCO(3)(-) selectivity.
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Affiliation(s)
- Han Soo Yang
- Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322, USA
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