1
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Kolen B, Kortzak D, Franzen A, Fahlke C. An amino-terminal point mutation increases EAAT2 anion currents without affecting glutamate transport rates. J Biol Chem 2020; 295:14936-14947. [PMID: 32820048 DOI: 10.1074/jbc.ra120.013704] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2020] [Revised: 08/03/2020] [Indexed: 12/16/2022] Open
Abstract
Excitatory amino acid transporters (EAATs) are prototypical dual function proteins that function as coupled glutamate/Na+/H+/K+ transporters and as anion-selective channels. Both transport functions are intimately intertwined at the structural level: Secondary active glutamate transport is based on elevator-like movements of the mobile transport domain across the membrane, and the lateral movement of this domain results in anion channel opening. This particular anion channel gating mechanism predicts the existence of mutant transporters with changed anion channel properties, but without alteration in glutamate transport. We here report that the L46P mutation in the human EAAT2 transporter fulfills this prediction. L46 is a pore-forming residue of the EAAT2 anion channels at the cytoplasmic entrance into the ion conduction pathway. In whole-cell patch clamp recordings, we observed larger macroscopic anion current amplitudes for L46P than for WT EAAT2. Rapid l-glutamate application under forward transport conditions demonstrated that L46P does not reduce the transport rate of individual transporters. In contrast, changes in selectivity made gluconate permeant in L46P EAAT2, and nonstationary noise analysis revealed slightly increased unitary current amplitudes in mutant EAAT2 anion channels. We used unitary current amplitudes and individual transport rates to quantify absolute open probabilities of EAAT2 anion channels from ratios of anion currents by glutamate uptake currents. This analysis revealed up to 7-fold increased absolute open probability of L46P EAAT2 anion channels. Our results reveal an important determinant of the diameter of EAAT2 anion pore and demonstrate the existence of anion channel gating processes outside the EAAT uptake cycle.
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Affiliation(s)
- Bettina Kolen
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany
| | - Daniel Kortzak
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany
| | - Arne Franzen
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany
| | - Christoph Fahlke
- Molekular- und Zellphysiologie (IBI-1), Institute of Biological Information Processing, Forschungszentrum Jülich, Jülich, Germany.
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2
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Bignon Y, Sakhi I, Bitam S, Bakouh N, Keck M, Frachon N, Paulais M, Planelles G, Teulon J, Andrini O. Analysis of CLCNKB mutations at dimer-interface, calcium-binding site, and pore reveals a variety of functional alterations in ClC-Kb channel leading to Bartter syndrome. Hum Mutat 2019; 41:774-785. [PMID: 31803959 DOI: 10.1002/humu.23962] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2019] [Revised: 11/14/2019] [Accepted: 11/29/2019] [Indexed: 12/12/2022]
Abstract
Pathological missense mutations in CLCNKB gene give a wide spectrum of clinical phenotypes in Bartter syndrome type III patients. Molecular analysis of the mutated ClC-Kb channels can be helpful to classify the mutations according to their functional alteration. We investigated the functional consequences of nine mutations in the CLCNKB gene causing Bartter syndrome. We first established that all tested mutations lead to decreased ClC-Kb currents. Combining electrophysiological and biochemical methods in Xenopus laevis oocytes and in MDCKII cells, we identified three classes of mutations. One class is characterized by altered channel trafficking. p.A210V, p.P216L, p.G424R, and p.G437R are totally or partially retained in the endoplasmic reticulum. p.S218N is characterized by reduced channel insertion at the plasma membrane and altered pH-sensitivity; thus, it falls in the second class of mutations. Finally, we found a novel class of functionally inactivated mutants normally present at the plasma membrane. Indeed, we found that p.A204T alters the pH-sensitivity, p.A254V abolishes the calcium-sensitivity. p.G219C and p.G465R are probably partially inactive at the plasma membrane. In conclusion, most pathogenic mutants accumulate partly or totally in intracellular compartments, but some mutants are normally present at the membrane surface and simultaneously show a large range of altered channel gating properties.
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Affiliation(s)
- Yohan Bignon
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Imene Sakhi
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Sara Bitam
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Naziha Bakouh
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Mathilde Keck
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | | | - Marc Paulais
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Gabrielle Planelles
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Jacques Teulon
- Centre de Recherche des Cordeliers, INSERM, Sorbonne Université, Université de Paris, Laboratoire Physiologie Rénale et Tubulopathies, Paris, France.,CNRS ERL8228, Paris, France
| | - Olga Andrini
- Institut NeuroMyoGène, Univ Lyon, Université Claude Bernard Lyon 1, Lyon, France
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3
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Wojciechowski D, Kovalchuk E, Yu L, Tan H, Fahlke C, Stölting G, Alekov AK. Barttin Regulates the Subcellular Localization and Posttranslational Modification of Human Cl -/H + Antiporter ClC-5. Front Physiol 2018; 9:1490. [PMID: 30405442 PMCID: PMC6206076 DOI: 10.3389/fphys.2018.01490] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2018] [Accepted: 10/02/2018] [Indexed: 01/17/2023] Open
Abstract
Dent disease 1 (DD1) is a renal salt-wasting tubulopathy associated with mutations in the Cl-/H+ antiporter ClC-5. The disease typically manifests with proteinuria, hypercalciuria, nephrocalcinosis, and nephrolithiasis but is characterized by large phenotypic variability of no clear origin. Several DD1 cases have been reported lately with additional atypical hypokalemic metabolic alkalosis and hyperaldosteronism, symptoms usually associated with another renal disease termed Bartter syndrome (BS). Expression of the Bartter-like DD1 mutant ClC-5 G261E in HEK293T cells showed that it is retained in the ER and lacks the complex glycosylation typical for ClC-5 WT. Accordingly, the mutant abolished CLC ionic transport. Such phenotype is not unusual and is often observed also in DD1 ClC-5 mutants not associated with Bartter like phenotype. We noticed, therefore, that one type of BS is associated with mutations in the protein barttin that serves as an accessory subunit regulating the function and subcellular localization of ClC-K channels. The overlapping symptomatology of DD1 and BS, together with the homology between the proteins of the CLC family, led us to investigate whether barttin might also regulate ClC-5 transport. In HEK293T cells, we found that barttin cotransfection impairs the complex glycosylation and arrests ClC-5 in the endoplasmic reticulum. As barttin and ClC-5 are both expressed in the thin and thick ascending limbs of the Henle's loop and the collecting duct, interactions between the two proteins could potentially contribute to the phenotypic variability of DD1. Pathologic barttin mutants differentially regulated trafficking and processing of ClC-5, suggesting that the interaction between the two proteins might be relevant also for the pathophysiology of BS. Our findings show that barttin regulates the subcellular localization not only of kidney ClC-K channels but also of the ClC-5 transporter, and suggest that ClC-5 might potentially play a role not only in kidney proximal tubules but also in tubular kidney segments expressing barttin. In addition, they demonstrate that the spectrum of clinical, genetic and molecular pathophysiology investigation of DD1 should be extended.
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Affiliation(s)
| | - Elena Kovalchuk
- Institute for Neurophysiology, Hannover Medical School, Hanover, Germany
| | - Lan Yu
- Institute for Neurophysiology, Hannover Medical School, Hanover, Germany
| | - Hua Tan
- Institute of Complex Systems 4 (ICS-4) – Zelluläre Biophysik, Forschungszentrum Jülich, Jülich, Germany
| | - Christoph Fahlke
- Institute of Complex Systems 4 (ICS-4) – Zelluläre Biophysik, Forschungszentrum Jülich, Jülich, Germany
| | - Gabriel Stölting
- Institute of Complex Systems 4 (ICS-4) – Zelluläre Biophysik, Forschungszentrum Jülich, Jülich, Germany
| | - Alexi K. Alekov
- Institute for Neurophysiology, Hannover Medical School, Hanover, Germany
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4
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Wang C, Chen Y, Zheng B, Zhu M, Fan J, Wang J, Jia Z, Huang S, Zhang A. Novel compound heterozygous CLCNKB gene mutations (c.1755A>G/c.848_850delTCT) cause classic Bartter syndrome. Am J Physiol Renal Physiol 2018; 315:F844-F851. [PMID: 29442545 DOI: 10.1152/ajprenal.00077.2017] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Inactivated variants in CLCNKB gene encoding the basolateral chloride channel ClC-Kb cause classic Bartter syndrome characterized by hypokalemic metabolic alkalosis and hyperreninemic hyperaldosteronism. Here, we identified two cBS siblings presenting hypokalemia in a Chinese family due to novel compound heterozygous CLCNKB mutations (c.848_850delTCT/c.1755A>G). Compound heterozygosity was confirmed by amplifying and sequencing the patient's genomic DNA. The synonymous mutation c.1755A>G (Thr585Thr) was located at +2 bp from the 5' splice donor site in exon 15. Further transcript analysis demonstrated that this single nucleotide mutation causes exclusion of exon 15 in the cDNA from the proband and his mother. Furthermore, we investigated the expression and protein trafficking change of c.848_850delTCT (ΔTCT) and exon 15 deletion (ΔE15) mutation in vitro. The ΔE15 mutation markedly decreased the expression of ClC-Kb and resulted in a low-molecular-weight band (~55 kDa) trapping in the endoplasmic reticulum, while the ΔTCT mutant only decreased the total and plasma membrane ClC-Kb protein expression but did not affect the subcellular localization. Finally, we studied the physiological functions of mutations by using whole cell patch-clamp and found that the ΔE15 or ΔTCT mutation decreased the current of the ClC-Kb/barttin channel. These results suggested that the compound defective mutations of the CLCNKB gene are the molecular mechanism of the two cBS siblings.
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Affiliation(s)
- Chunli Wang
- Nanjing Key Laboratory of Pediatrics, Children's Hospital of Nanjing Medical University , Nanjing , China
| | - Ying Chen
- Department of Nephrology, Children's Hospital of Nanjing Medical University , Nanjing , China
| | - Bixia Zheng
- Nanjing Key Laboratory of Pediatrics, Children's Hospital of Nanjing Medical University , Nanjing , China
| | - Mengshu Zhu
- Nanjing Key Laboratory of Pediatrics, Children's Hospital of Nanjing Medical University , Nanjing , China
| | - Jia Fan
- Department of Physiology, Nanjing Medical University , Nanjing , China
| | - Juejin Wang
- Department of Physiology, Nanjing Medical University , Nanjing , China
| | - Zhanjun Jia
- Nanjing Key Laboratory of Pediatrics, Children's Hospital of Nanjing Medical University , Nanjing , China
| | - Songming Huang
- Department of Nephrology, Children's Hospital of Nanjing Medical University , Nanjing , China.,Jiangsu Key Laboratory of Pediatrics, Nanjing Medical University , Nanjing , China
| | - Aihua Zhang
- Department of Nephrology, Children's Hospital of Nanjing Medical University , Nanjing , China.,Jiangsu Key Laboratory of Pediatrics, Nanjing Medical University , Nanjing , China
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5
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Jentsch TJ, Pusch M. CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease. Physiol Rev 2018; 98:1493-1590. [DOI: 10.1152/physrev.00047.2017] [Citation(s) in RCA: 214] [Impact Index Per Article: 35.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.
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Affiliation(s)
- Thomas J. Jentsch
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany; and Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy
| | - Michael Pusch
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany; and Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy
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6
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Kovermann P, Hessel M, Kortzak D, Jen JC, Koch J, Fahlke C, Freilinger T. Impaired K + binding to glial glutamate transporter EAAT1 in migraine. Sci Rep 2017; 7:13913. [PMID: 29066757 PMCID: PMC5654970 DOI: 10.1038/s41598-017-14176-4] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2017] [Accepted: 10/06/2017] [Indexed: 12/28/2022] Open
Abstract
SLC1A3 encodes the glial glutamate transporter hEAAT1, which removes glutamate from the synaptic cleft via stoichiometrically coupled Na+-K+-H+-glutamate transport. In a young man with migraine with aura including hemiplegia, we identified a novel SLC1A3 mutation that predicts the substitution of a conserved threonine by proline at position 387 (T387P) in hEAAT1. To evaluate the functional effects of the novel variant, we expressed the wildtype or mutant hEAAT1 in mammalian cells and performed whole-cell patch clamp, fast substrate application, and biochemical analyses. T387P diminishes hEAAT1 glutamate uptake rates and reduces the number of hEAAT1 in the surface membrane. Whereas hEAAT1 anion currents display normal ligand and voltage dependence in cells internally dialyzed with Na+-based solution, no anion currents were observed with internal K+. Fast substrate application demonstrated that T387P abolishes K+-bound retranslocation. Our finding expands the phenotypic spectrum of genetic variation in SLC1A3 and highlights impaired K+ binding to hEAAT1 as a novel mechanism of glutamate transport dysfunction in human disease.
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Affiliation(s)
- Peter Kovermann
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Margarita Hessel
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Daniel Kortzak
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Joanna C Jen
- Departments of Neurology and Neurobiology, UCLA School of Medicine, Los Angeles, USA
| | - Johannes Koch
- Department of Paediatrics, Salzburger Universitätsklinikum, Salzburg, Austria
| | - Christoph Fahlke
- Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, Jülich, Germany
| | - Tobias Freilinger
- Department of Neurology and Epileptology, Hertie-Institute for Clinical Brain Research (HIH), Tübingen, Germany.
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7
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Guzman RE, Bungert-Plümke S, Franzen A, Fahlke C. Preferential association with ClC-3 permits sorting of ClC-4 into endosomal compartments. J Biol Chem 2017; 292:19055-19065. [PMID: 28972156 DOI: 10.1074/jbc.m117.801951] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2017] [Revised: 09/15/2017] [Indexed: 11/06/2022] Open
Abstract
ClC-4 is an intracellular Cl-/H+ exchanger that is highly expressed in the brain and whose dysfunction has been linked to intellectual disability and epilepsy. Here we studied the subcellular localization of human ClC-4 in heterologous expression systems. ClC-4 is retained in the endoplasmic reticulum (ER) upon overexpression in HEK293T cells. Co-expression with distinct ClC-3 splice variants targets ClC-4 to late endosome/lysosomes (ClC-3a and ClC-3b) or recycling endosome (ClC-3c). When expressed in cultured astrocytes, ClC-4 sorted to endocytic compartments in WT cells but was retained in the ER in Clcn3-/- cells. To understand the virtual absence of ER-localized ClC-4 in WT astrocytes, we performed association studies by high-resolution clear native gel electrophoresis. Although other CLC channels and transporters form stable dimers, ClC-4 was mostly observed as monomer, with ClC-3-ClC-4 heterodimers being more stable than ClC-4 homodimers. We conclude that unique oligomerization properties of ClC-4 permit regulated targeting of ClC-4 to various endosomal compartment systems via expression of different ClC-3 splice variants.
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Affiliation(s)
- Raul E Guzman
- From the Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, 52425 Jülich, Germany
| | | | | | - Christoph Fahlke
- From the Institute of Complex Systems, Zelluläre Biophysik (ICS-4), Forschungszentrum Jülich, 52425 Jülich, Germany
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8
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Yamada T, Krzeminski M, Bozoky Z, Forman-Kay JD, Strange K. Role of CBS and Bateman Domains in Phosphorylation-Dependent Regulation of a CLC Anion Channel. Biophys J 2017; 111:1876-1886. [PMID: 27806269 DOI: 10.1016/j.bpj.2016.09.036] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2016] [Revised: 08/30/2016] [Accepted: 09/23/2016] [Indexed: 12/13/2022] Open
Abstract
Eukaryotic CLC anion channels and transporters are homodimeric proteins composed of multiple α-helical membrane domains and large cytoplasmic C-termini containing two cystathionine-β-synthase domains (CBS1 and CBS2) that dimerize to form a Bateman domain. The Bateman domains of adjacent CLC subunits interact to form a Bateman domain dimer. The functions of CLC CBS and Bateman domains are poorly understood. We utilized the Caenorhabditis elegans CLC-1/2/Ka/Kb anion channel homolog CLH-3b to characterize the regulatory roles of CLC cytoplasmic domains. CLH-3b activity is reduced by phosphorylation or deletion of a 14-amino-acid activation domain (AD) located on the linker connecting CBS1 and CBS2. We demonstrate here that phosphorylation-dependent reductions in channel activity require an intact Bateman domain dimer and concomitant phosphorylation or deletion of both ADs. Regulation of a CLH-3b AD deletion mutant is reconstituted by intracellular perfusion with recombinant 14-amino-acid AD peptides. The sulfhydryl reactive reagent 2-(trimethylammonium)ethyl methanethiosulfonate bromide (MTSET) alters in a phosphorylation-dependent manner the activity of channels containing single cysteine residues that are engineered into the short intracellular loop connecting membrane α-helices H and I (H-I loop), the AD, CBS1, and CBS2. In contrast, MTSET has no effect on channels in which cysteine residues are engineered into intracellular regions that are dispensable for regulation. These studies together with our previous work suggest that binding and unbinding of the AD to the Bateman domain dimer induces conformational changes that are transduced to channel membrane domains via the H-I loop. Our findings provide new, to our knowledge, insights into the roles of CLC Bateman domains and the structure-function relationships that govern the regulation of CLC protein activity by diverse ligands and signaling pathways.
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Affiliation(s)
- Toshiki Yamada
- MDI Biological Laboratory, Kathryn W. Davis Center for Regenerative Biology and Medicine, Salisbury Cove, Maine
| | - Mickael Krzeminski
- Department of Molecular Structure and Function, Hospital for Sick Kids, Toronto, Ontario
| | - Zoltan Bozoky
- Department of Molecular Structure and Function, Hospital for Sick Kids, Toronto, Ontario
| | - Julie D Forman-Kay
- Department of Molecular Structure and Function, Hospital for Sick Kids, Toronto, Ontario
| | - Kevin Strange
- MDI Biological Laboratory, Kathryn W. Davis Center for Regenerative Biology and Medicine, Salisbury Cove, Maine.
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9
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Regulatory Conformational Coupling between CLC Anion Channel Membrane and Cytoplasmic Domains. Biophys J 2017; 111:1887-1896. [PMID: 27806270 DOI: 10.1016/j.bpj.2016.09.037] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2016] [Revised: 08/30/2016] [Accepted: 09/23/2016] [Indexed: 12/23/2022] Open
Abstract
CLC anion channels are homodimeric proteins. Each subunit is comprised of 18 α-helices designated "A-R" and an intracellular carboxy-terminus containing two cystathionine-β-synthase (CBS1 and CBS2) domains. Conformational coupling between membrane and intracellular domains via poorly understood mechanisms is required for CLC regulation. The activity of the C. elegans CLC channel CLH-3b is reduced by phosphorylation of a carboxy-terminus "activation domain," which disrupts its interaction with CBS domains. CBS2 interfaces with a short intracellular loop, the H-I loop, connecting membrane helices H and I. Alanine mutation of a conserved H-I loop tyrosine residue, Y232, prevents regulation demonstrating that the loop functions to couple phosphorylation-dependent CBS domain conformational changes to channel membrane domains. To gain further insight into the mechanisms of this coupling, we mutated conserved amino acid residues in membrane helices H and I. Only mutation of the H-helix valine residue V228 to leucine prevented phosphorylation-dependent channel regulation. Structural and functional studies of other CLC proteins suggest that V228 may interact with Y529, a conserved R-helix tyrosine residue that forms part of the CLC ion conduction pathway. Mutation of Y529 to alanine also prevented CLH-3b regulation. Intracellular application of the sulfhydryl reactive reagent MTSET using CLH-3b channels engineered with single-cysteine residues in CBS2 indicate that V228L, Y529A, and Y232A disrupt putative regulatory intracellular conformational changes. Extracellular Zn2+ inhibits CLH-3b and alters the effects of intracellular MTSET on channel activity. The effects of Zn2+ are disrupted by V228L, Y529A, and Y232A. Collectively, our findings indicate that there is conformational coupling between CBS domains and the H and R membrane helices mediated by the H-I loop. We propose a simple model by which conformational changes in H and R helices mediate CLH-3b regulation by activation domain phosphorylation.
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10
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Cheng CJ, Lo YF, Chen JC, Huang CL, Lin SH. Functional severity of CLCNKB mutations correlates with phenotypes in patients with classic Bartter's syndrome. J Physiol 2017; 595:5573-5586. [PMID: 28555925 PMCID: PMC5556149 DOI: 10.1113/jp274344] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2017] [Accepted: 05/08/2017] [Indexed: 12/28/2022] Open
Abstract
KEY POINTS The highly variable phenotypes observed in patients with classic Bartter's syndrome (BS) remain unsatisfactorily explained. The wide spectrum of functional severity of CLCNKB mutations may contribute to the phenotypic variability, and the genotype-phenotype association has not been established. Low-level expression of the human ClC-Kb channel in mammalian cells impedes the functional study of CLCNKB mutations, and the underlying cause is still unclear. The human ClC-Kb channel is highly degraded by proteasome in human embryonic kidney cells. The C-terminal in-frame green fluorescent protein fusion may slow down the proteasome-mediated proteolysis. Barttin co-expression necessarily improves the stability, membrane trafficking and gating of ClC-Kb. CLCNKB mutations in barttin-binding sites, dimer interface or selectivity filter often have severe functional consequences. The remaining chloride conductance of the ClC-Kb mutant channel significantly correlates with the phenotypes, such as age at diagnosis, plasma chloride concentration, and the degree of calciuria in patients with classic BS. ABSTRACT Mutations in the CLCNKB gene encoding the human voltage-gated chloride ClC-Kb (hClC-Kb) channel cause classic Bartter's syndrome (BS). In contrast to antenatal BS, classic BS manifests with highly variable phenotypes. The functional severity of the mutant channel has been proposed to explain this phenomenon. Due to difficulties in the expression of hClC-Kb in heterologous expression systems, the functional consequences of mutant channels have not been thoroughly examined, and the genotype-phenotype association has not been established. In this study, we found that hClC-Kb, when expressed in human embryonic kidney (HEK) cells, was unstable due to degradation by proteasome. In-frame fusion of green fluorescent protein (GFP) to the C-terminus of the channel may ameliorate proteasome degradation. Co-expression of barttin increased protein abundance and membrane trafficking of hClC-Kb and markedly increased functional chloride current. We then functionally characterized 18 missense mutations identified in our classic BS cohort and others using HEK cells expressing hClC-Kb-GFP. Most CLCNKB mutations resulted in marked reduction in protein abundance and chloride current, especially those residing at barttin binding sites, dimer interface and selectivity filter. We enrolled classic BS patients carrying homozygous missense mutations with well-described functional consequences and clinical presentations for genotype-phenotype analysis. We found significant correlations of mutant chloride current with the age at diagnosis, plasma chloride concentration and urine calcium excretion rate. In conclusion, hClC-Kb expression in HEK cells is susceptible to proteasome degradation, and fusion of GFP to the C-terminus of hClC-Kb improves protein expression. The functional severity of the CLCNKB mutation is an important determinant of the phenotype in classic BS.
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Affiliation(s)
- Chih-Jen Cheng
- Department of Medicine, Division of Nephrology, Tri-Service General Hospital, National Defense Medical Center, Taipei, 114, Taiwan
| | - Yi-Fen Lo
- Department of Medicine, Division of Nephrology, Tri-Service General Hospital, National Defense Medical Center, Taipei, 114, Taiwan
| | - Jen-Chi Chen
- Department of Medicine, Division of Nephrology, Tri-Service General Hospital, National Defense Medical Center, Taipei, 114, Taiwan
| | - Chou-Long Huang
- Department of Medicine, Division of Nephrology, University of Texas Southwestern Medical Center, Dallas, TX, 75390-8856, USA
| | - Shih-Hua Lin
- Department of Medicine, Division of Nephrology, Tri-Service General Hospital, National Defense Medical Center, Taipei, 114, Taiwan
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11
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Tan H, Bungert-Plümke S, Fahlke C, Stölting G. Reduced Membrane Insertion of CLC-K by V33L Barttin Results in Loss of Hearing, but Leaves Kidney Function Intact. Front Physiol 2017; 8:269. [PMID: 28555110 PMCID: PMC5430073 DOI: 10.3389/fphys.2017.00269] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2017] [Accepted: 04/12/2017] [Indexed: 12/18/2022] Open
Abstract
In the mammalian ear, transduction of sound stimuli is initiated by K+ entry through mechano-sensitive channels into inner hair cells. K+ entry is driven by a positive endocochlear potential that is maintained by the marginal cell layer of the stria vascularis. This process requires basolateral K+ import by NKCC1 Na+−2Cl−−K+ co-transporters as well as Cl− efflux through ClC-Ka/barttin or ClC-Kb/barttin channels. Multiple mutations in the gene encoding the obligatory CLC-K subunit barttin, BSND, have been identified in patients with Bartter syndrome type IV. These mutations reduce the endocochlear potential and cause deafness. As CLC-K/barttin channels are also expressed in the kidney, patients with Bartter syndrome IV typically also suffer from salt-wasting hyperuria and electrolyte imbalances. However, there was a single report on a BSND mutation that resulted only in deafness, but not kidney disease. We herein studied the functional consequences of another recently discovered BSND mutation that predicts exchange of valine at position 33 by leucine. We combined whole-cell patch clamp, confocal microscopy and protein biochemistry to analyze how V33L affects distinct functions of barttin. We found that V33L reduced membrane insertion of CLC-K/barttin complexes without altering unitary CLC-K channel function. Our findings support the hypothesis of a common pathophysiology for the selective loss of hearing due to an attenuation of the total chloride conductance in the stria vascularis while providing enough residual function to maintain normal kidney function.
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Affiliation(s)
- Hua Tan
- Institute of Complex Systems - Zelluläre Biophysik (ICS-4), Forschungszentrum JülichJülich, Germany
| | - Stefanie Bungert-Plümke
- Institute of Complex Systems - Zelluläre Biophysik (ICS-4), Forschungszentrum JülichJülich, Germany
| | - Christoph Fahlke
- Institute of Complex Systems - Zelluläre Biophysik (ICS-4), Forschungszentrum JülichJülich, Germany
| | - Gabriel Stölting
- Institute of Complex Systems - Zelluläre Biophysik (ICS-4), Forschungszentrum JülichJülich, Germany
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12
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Seys E, Andrini O, Keck M, Mansour-Hendili L, Courand PY, Simian C, Deschenes G, Kwon T, Bertholet-Thomas A, Bobrie G, Borde JS, Bourdat-Michel G, Decramer S, Cailliez M, Krug P, Cozette P, Delbet JD, Dubourg L, Chaveau D, Fila M, Jourde-Chiche N, Knebelmann B, Lavocat MP, Lemoine S, Djeddi D, Llanas B, Louillet F, Merieau E, Mileva M, Mota-Vieira L, Mousson C, Nobili F, Novo R, Roussey-Kesler G, Vrillon I, Walsh SB, Teulon J, Blanchard A, Vargas-Poussou R. Clinical and Genetic Spectrum of Bartter Syndrome Type 3. J Am Soc Nephrol 2017; 28:2540-2552. [PMID: 28381550 DOI: 10.1681/asn.2016101057] [Citation(s) in RCA: 64] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2016] [Accepted: 02/27/2017] [Indexed: 12/30/2022] Open
Abstract
Bartter syndrome type 3 is a clinically heterogeneous hereditary salt-losing tubulopathy caused by mutations of the chloride voltage-gated channel Kb gene (CLCNKB), which encodes the ClC-Kb chloride channel involved in NaCl reabsorption in the renal tubule. To study phenotype/genotype correlations, we performed genetic analyses by direct sequencing and multiplex ligation-dependent probe amplification and retrospectively analyzed medical charts for 115 patients with CLCNKB mutations. Functional analyses were performed in Xenopus laevis oocytes for eight missense and two nonsense mutations. We detected 60 mutations, including 27 previously unreported mutations. Among patients, 29.5% had a phenotype of ante/neonatal Bartter syndrome (polyhydramnios or diagnosis in the first month of life), 44.5% had classic Bartter syndrome (diagnosis during childhood, hypercalciuria, and/or polyuria), and 26.0% had Gitelman-like syndrome (fortuitous discovery of hypokalemia with hypomagnesemia and/or hypocalciuria in childhood or adulthood). Nine of the ten mutations expressed in vitro decreased or abolished chloride conductance. Severe (large deletions, frameshift, nonsense, and essential splicing) and missense mutations resulting in poor residual conductance were associated with younger age at diagnosis. Electrolyte supplements and indomethacin were used frequently to induce catch-up growth, with few adverse effects. After a median follow-up of 8 (range, 1-41) years in 77 patients, chronic renal failure was detected in 19 patients (25%): one required hemodialysis and four underwent renal transplant. In summary, we report a genotype/phenotype correlation for Bartter syndrome type 3: complete loss-of-function mutations associated with younger age at diagnosis, and CKD was observed in all phenotypes.
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Affiliation(s)
- Elsa Seys
- Pediatric Nephrology Unit, American Memorial Hospital, Reims University Hospital, Reims, France
| | - Olga Andrini
- Unité Mixte de Recherche en Santé 1138, Team 3, Université Pierre et Marie Curie, Paris, France.,Faculté de Médecine, Université Paris Descartes, Paris, France
| | - Mathilde Keck
- Unité Mixte de Recherche en Santé 1138, Team 3, Université Pierre et Marie Curie, Paris, France.,Institut National de la Santé et la Recherche Médicale, Unité Mixte de Recherche en Santé 872, Paris, France
| | | | - Pierre-Yves Courand
- Centre d'Investigation Clinique, Hôpital Européen Georges Pompidou, Assistance Publique-Hôpitaux de Paris, Paris, France.,Cardiology Department, Hôpital de la Croix-Rousse, Hospices Civils de Lyon, Lyon, France.,Centre de Recherche en Acquisition et Traitement de l'Image pour la Santé; Centre National de la Recherche Scientifique Unité Mixte de Recherche 5220; Institut National de la Santé et la Recherche Médicale, Unité 1044; Institut National de Sciences Appliquées-Lyon; Université Claude Bernard Lyon 1, France
| | | | - Georges Deschenes
- Pediatric Nephrology Unit, Hôpital Robert Debré, Assistance Publique-Hôpitaux de Paris, Paris, France.,Centre de Référence des Maladies Rénales Héréditaires de l'Enfant et de l'Adulte, Paris, France
| | - Theresa Kwon
- Pediatric Nephrology Unit, Hôpital Robert Debré, Assistance Publique-Hôpitaux de Paris, Paris, France.,Centre de Référence des Maladies Rénales Héréditaires de l'Enfant et de l'Adulte, Paris, France
| | - Aurélia Bertholet-Thomas
- Néphrogones, Centre de Référence des Maladies Rénales Rares, Pediatric Nephrology, Rhumatology and Dermatology Unit, Hôpital Femme-Mère-Enfant and
| | - Guillaume Bobrie
- Nephrology Unit, Clinique du Vert Galant, Tremblay-en-France, France
| | | | | | | | - Mathilde Cailliez
- Pediatric Nephrology Unit, Hôpital de la Timone, Assistance Publique des Hôpitaux de Marseille, Marseille, France
| | - Pauline Krug
- Centre de Référence des Maladies Rénales Héréditaires de l'Enfant et de l'Adulte, Paris, France.,Pediatric Nephrology Unit and
| | - Paul Cozette
- Nephrology Unit, Centre Hospitalier du Pays d'Aix, Aix-en-Provence, France
| | - Jean Daniel Delbet
- Pediatric Nephrology Unit, Hôpital Trousseau, Assistance Publique des Hôpitaux de Paris, Paris, France
| | - Laurence Dubourg
- Exploration Fonctionnelle Rénale et Métabolique, Groupement Hospitalier est Hôpital Edouard Herriot, Hospices Civils de Lyon, Lyon, France
| | - Dominique Chaveau
- Departement of Nephrology, Centre de Référence des Maladies Rénales Rares du Sud-Ouest, Hôpital de Toulouse, Université Paul Sabatier, Toulouse, France
| | - Marc Fila
- Pediatric Nephrology Unit, Centre Hospitalier Universitaire de Montpellier, Montpellier, France
| | - Noémie Jourde-Chiche
- Faculté de Médecine, Centre de Référence des Maladies Rénales Rares du Sud-Ouest, Aix-MarseilleUniversité-Vascular Research Center of Marseille, Marseille, France.,Nephrology Unit, Hôpital de la Conception, Assistance Publique des Hopitaux de Marseille, Marseille,France
| | - Bertrand Knebelmann
- Centre de Référence des Maladies Rénales Héréditaires de l'Enfant et de l'Adulte, Paris, France.,Department of Nephrology, Hôpital Necker-Enfants-malades, Assistance Publique des Hôpitaux de Paris, Paris, France
| | - Marie-Pierre Lavocat
- Departement of Pediatrics, Hôpital Nord, Centre Hospitalier Universitaire de Saint Etienne, Saint Etienne, France
| | - Sandrine Lemoine
- Exploration Fonctionnelle Rénale et Métabolique, Groupement Hospitalier est Hôpital Edouard Herriot, Hospices Civils de Lyon, Lyon, France
| | - Djamal Djeddi
- Department of Pediatrics and Adolescent Medicine, Centre Hospitalier Universitaire d'Amiens, Amiens, France
| | - Brigitte Llanas
- Service de Néphrologie Pédiatrique, Groupement Hospitalier Pellegrin, Centre Hospitalier Universitaire de Bordeaux, Centre de Référence des Maladies Rénales Rares du Sud-Ouest, Bordeaux, France
| | - Ferielle Louillet
- Department of Pediatrics, Centre Hospitalier Universitaire Charles Nicolle, Rouen, France
| | - Elodie Merieau
- Nephrology Unit,Centre Hospitalier Universitaire Tours, Tours, France
| | - Maria Mileva
- Department of Pediatrics, Centre Hospitalier Pierre Oudot de Bourgoin-Jallieu, Bourgoin-Jallieu, France
| | - Luisa Mota-Vieira
- Molecular Genetics Unit, Hospital do Divino Espírito Santo de Ponta Delgada, Entidade Pública Empresarial Regional, Açores, Portugal
| | - Christiane Mousson
- Nephrology Unit, Centre Hospitalier Universitaire de Dijon, Dijon, France
| | - François Nobili
- Pediatric Nephrology Unit, Centre Hospitalier Universitaire de Besançon, Besançon, France
| | - Robert Novo
- Pediatric Nephrology Unit, Hôpital Jeanne de Flandre, Centre Hospitalier Universitaire de Lille, Lille, France
| | | | - Isabelle Vrillon
- Pediatric Nephrology Unit, Hôpitaux de Brabois, Centre Hospitalier Universitaire de Nancy, Vandoeuvre Les Nancy, France
| | - Stephen B Walsh
- Centre for Nephrology, University College London, London, UK; and
| | - Jacques Teulon
- Unité Mixte de Recherche en Santé 1138, Team 3, Université Pierre et Marie Curie, Paris, France.,Institut National de la Santé et la Recherche Médicale, Unité Mixte de Recherche en Santé 872, Paris, France
| | - Anne Blanchard
- Faculté de Médecine, Université Paris Descartes, Paris, France.,Cardiology Department, Hôpital de la Croix-Rousse, Hospices Civils de Lyon, Lyon, France.,Centre de Référence des Maladies Rénales Héréditaires de l'Enfant et de l'Adulte, Paris, France.,Institut National de la Santé et la Recherche Médicale, Unité Mixte de Recherche en Santé 970, Paris-Cardiovascular Research Center, Paris, France
| | - Rosa Vargas-Poussou
- Department of Genetics and.,Centre de Référence des Maladies Rénales Héréditaires de l'Enfant et de l'Adulte, Paris, France.,Institut National de la Santé et la Recherche Médicale, Unité Mixte de Recherche en Santé 970, Paris-Cardiovascular Research Center, Paris, France
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13
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Stölting G, Fahlke C. Chloride channels in renal salt and water transport. Acta Physiol (Oxf) 2017; 219:11-13. [PMID: 27636562 DOI: 10.1111/apha.12802] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Affiliation(s)
- G. Stölting
- Institute of Complex Systems - Zelluläre Biophysik (ICS-4); Forschungszentrum Jülich; Jülich Germany
| | - C. Fahlke
- Institute of Complex Systems - Zelluläre Biophysik (ICS-4); Forschungszentrum Jülich; Jülich Germany
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