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Martí-Solans J, Børve A, Vevle L, Hejnol A, Lynagh T. Invertebrate Bile Acid-Sensitive Ion Channels and Their Emergence in Bilateria. FASEB J 2025; 39:e70526. [PMID: 40235278 DOI: 10.1096/fj.202403216r] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2024] [Revised: 03/12/2025] [Accepted: 03/27/2025] [Indexed: 04/17/2025]
Abstract
The broad Degenerin/epithelial sodium channel (DEG/ENaC) family includes a subfamily of bile acid-sensing ion channels (BASICs). While their biophysical properties are extensively studied in mammals, the presence and function of BASICs in invertebrates remain largely unexplored. Here, we present the first functional evidence of invertebrate BASICs, revealing conserved features and evolutionary adaptations across bilaterian species. Using electrophysiological and pharmacological approaches, we show that invertebrate BASICs exhibit species-specific bile acid sensitivity profiles and differing responses to channel blockers, amiloride, and diminazene, while retaining shared properties like inhibition by calcium ions and selective permeability of sodium ions. For example, the acorn worm Schizocardium californicum BASIC displays broad bile acid sensitivity similar to mammals, while the brachiopod Novocrania anomala BASIC is activated solely by ursodeoxycholic acid (UDCA) in our experiments. Mutagenesis of the conserved D444 residue in the pore-lining region confirms its critical role in gating. Combined functional and phylogenetic analysis suggests BASICs emerged early in bilaterian evolution, evolving from channels that were merely modulated by bile acids, like their acid-sensing ion channel cousins, into channels that are activated by bile acids. Tissue-specific expression patterns imply roles in bile acid-dependent sodium absorption or environmental sensing of bile acid-like compounds. Given the absence of endogenous bile acids in invertebrates, we propose that invertebrate BASICs may detect environmental compounds, contributing to ecological interactions. This study enhances our understanding of the evolutionary, functional, and ecological roles of BASICs, with implications for future research into their native ligands.
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Affiliation(s)
| | - Aina Børve
- Department of Biological Sciences, University of Bergen, Bergen, Norway
| | - Line Vevle
- Michael Sars Centre, University of Bergen, Bergen, Norway
| | - Andreas Hejnol
- Department of Biological Sciences, University of Bergen, Bergen, Norway
| | - Timothy Lynagh
- Michael Sars Centre, University of Bergen, Bergen, Norway
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2
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Freitas MM, Gouaux E. The bile acid-sensitive ion channel is gated by Ca 2+-dependent conformational changes in the transmembrane domain. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2025:2025.01.10.632481. [PMID: 39829759 PMCID: PMC11741473 DOI: 10.1101/2025.01.10.632481] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/22/2025]
Abstract
The bile acid-sensitive ion channel (BASIC) is the least understood member of the mammalian epithelial Na+ channel/degenerin (ENaC/DEG) superfamily of ion channels, which are involved in a variety of physiological processes. While some members of this superfamily, including BASIC, are inhibited by extracellular Ca2+ (Ca2+ o), the molecular mechanism underlying Ca2+ modulation remains unclear. Here, by determining the structure of human BASIC in the presence and absence of Ca2+ using single particle cryo-electron microscopy (cryo-EM), we reveal Ca2+-dependent conformational changes in the transmembrane domain and β-linkers. Electrophysiological experiments further show that a glutamate residue in the extracellular vestibule of the pore underpins the Ca2+-binding site, whose occupancy determines the conformation of the pore and therefore ion flow through the channel. These results reveal the molecular principles governing gating of BASIC and its regulation by Ca2+ ions, demonstrating that Ca2+ ions modulate BASIC function via changes in protein conformation rather than solely from pore-block, as proposed for other members of this superfamily.
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Affiliation(s)
- Makayla M. Freitas
- Vollum Institute, Oregon Health and Science University, 3232 SW Research Drive, Portland, OR, USA
| | - Eric Gouaux
- Vollum Institute, Oregon Health and Science University, 3232 SW Research Drive, Portland, OR, USA
- Howard Hughes Medical Institute, Oregon Health and Science University, 3232 SW Research Drive, Portland, OR, USA
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3
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Sivcev S, Kudova E, Zemkova H. Neurosteroids as positive and negative allosteric modulators of ligand-gated ion channels: P2X receptor perspective. Neuropharmacology 2023; 234:109542. [PMID: 37040816 DOI: 10.1016/j.neuropharm.2023.109542] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2022] [Revised: 03/06/2023] [Accepted: 04/07/2023] [Indexed: 04/13/2023]
Abstract
Neurosteroids are steroids synthesized de novo in the brain from cholesterol in an independent manner from peripheral steroid sources. The term "neuroactive steroid" includes all steroids independent of their origin, and newly synthesized analogs of neurosteroids that modify neuronal activities. In vivo application of neuroactive steroids induces potent anxiolytic, antidepressant, anticonvulsant, sedative, analgesic and amnesic effects, mainly through interaction with the γ-aminobutyric acid type-A receptor (GABAAR). However, neuroactive steroids also act as positive or negative allosteric regulators on several ligand-gated channels including N-methyl-d-aspartate receptors (NMDARs), nicotinic acetylcholine receptors (nAChRs) and ATP-gated purinergic P2X receptors. Seven different P2X subunits (P2X1-7) can assemble to form homotrimeric or heterotrimeric ion channels permeable for monovalent cations and calcium. Among them, P2X2, P2X4, and P2X7 are the most abundant within the brain and can be regulated by neurosteroids. Transmembrane domains are necessary for neurosteroid binding, however, no generic motif of amino acids can accurately predict the neurosteroid binding site for any of the ligand-gated ion channels including P2X. Here, we will review what is currently known about the modulation of rat and human P2X by neuroactive steroids and the possible structural determinants underlying neurosteroid-induced potentiation and inhibition of the P2X2 and P2X4 receptors.
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Affiliation(s)
- Sonja Sivcev
- Institute of Physiology, Czech Academy of Sciences, Prague, Czech Republic; Faculty of Science, Charles University, Prague, Czech Republic
| | - Eva Kudova
- Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Prague, Czech Republic
| | - Hana Zemkova
- Institute of Physiology, Czech Academy of Sciences, Prague, Czech Republic.
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4
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Diakov A, Nesterov V, Dahlmann A, Korbmacher C. Two adjacent phosphorylation sites in the C-terminus of the channel's α-subunit have opposing effects on epithelial sodium channel (ENaC) activity. Pflugers Arch 2022; 474:681-697. [PMID: 35525869 PMCID: PMC9192390 DOI: 10.1007/s00424-022-02693-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2022] [Accepted: 04/25/2022] [Indexed: 02/07/2023]
Abstract
How phosphorylation of the epithelial sodium channel (ENaC) contributes to its regulation is incompletely understood. Previously, we demonstrated that in outside-out patches ENaC activation by serum- and glucocorticoid-inducible kinase isoform 1 (SGK1) was abolished by mutating a serine residue in a putative SGK1 consensus motif RXRXX(S/T) in the channel’s α-subunit (S621 in rat). Interestingly, this serine residue is followed by a highly conserved proline residue rather than by a hydrophobic amino acid thought to be required for a functional SGK1 consensus motif according to invitro data. This suggests that this serine residue is a potential phosphorylation site for the dual-specificity tyrosine phosphorylated and regulated kinase 2 (DYRK2), a prototypical proline-directed kinase. Its phosphorylation may prime a highly conserved preceding serine residue (S617 in rat) to be phosphorylated by glycogen synthase kinase 3 β (GSK3β). Therefore, we investigated the effect of DYRK2 on ENaC activity in outside-out patches of Xenopus laevis oocytes heterologously expressing rat ENaC. DYRK2 included in the pipette solution significantly increased ENaC activity. In contrast, GSK3β had an inhibitory effect. Replacing S621 in αENaC with alanine (S621A) abolished the effects of both kinases. A S617A mutation reduced the inhibitory effect of GKS3β but did not prevent ENaC activation by DYRK2. Our findings suggest that phosphorylation of S621 activates ENaC and primes S617 for subsequent phosphorylation by GSK3β resulting in channel inhibition. In proof-of-concept experiments, we demonstrated that DYRK2 can also stimulate ENaC currents in microdissected mouse distal nephron, whereas GSK3β inhibits the currents.
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Affiliation(s)
- Alexei Diakov
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Waldstr, 6, 91054, Erlangen, Germany
| | - Viatcheslav Nesterov
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Waldstr, 6, 91054, Erlangen, Germany
| | - Anke Dahlmann
- Medizinische Klinik 4 - Nephrologie und Hypertensiologie, Universitätsklinikum Erlangen, Ulmenweg 18, 91054, Erlangen, Germany
| | - Christoph Korbmacher
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Waldstr, 6, 91054, Erlangen, Germany.
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5
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Ishimwe JA, Dola T, Ertuglu LA, Kirabo A. Bile acids and salt-sensitive hypertension: a role of the gut-liver axis. Am J Physiol Heart Circ Physiol 2022; 322:H636-H646. [PMID: 35245132 PMCID: PMC8957326 DOI: 10.1152/ajpheart.00027.2022] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/17/2022] [Revised: 03/01/2022] [Accepted: 03/02/2022] [Indexed: 12/22/2022]
Abstract
Salt-sensitivity of blood pressure (SSBP) affects 50% of the hypertensive and 25% of the normotensive populations. Importantly, SSBP is associated with increased risk for mortality in both populations independent of blood pressure. Despite its deleterious effects, the pathogenesis of SSBP is not fully understood. Emerging evidence suggests a novel role of bile acids in salt-sensitive hypertension and that they may play a crucial role in regulating inflammation and fluid volume homeostasis. Mechanistic evidence implicates alterations in the gut microbiome, the epithelial sodium channel (ENaC), the farnesoid X receptor, and the G protein-coupled bile acid receptor TGR5 in bile acid-mediated effects on cardiovascular function. The mechanistic interplay between excess dietary sodium-induced alterations in the gut microbiome and immune cell activation, bile acid signaling, and whether such interplay may contribute to the etiology of SSBP is still yet to be defined. The main goal of this review is to discuss the potential role of bile acids in the pathogenesis of cardiovascular disease with a focus on salt-sensitive hypertension.
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Affiliation(s)
- Jeanne A Ishimwe
- Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University Medical Center, Nashville, Tennessee
| | - Thanvi Dola
- Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University Medical Center, Nashville, Tennessee
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee
| | - Lale A Ertuglu
- Division of Nephrology, Department of Medicine, Vanderbilt University Medical Center, Nashville, Tennessee
| | - Annet Kirabo
- Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University Medical Center, Nashville, Tennessee
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee
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Keely SJ, Urso A, Ilyaskin AV, Korbmacher C, Bunnett NW, Poole DP, Carbone SE. Contributions of bile acids to gastrointestinal physiology as receptor agonists and modifiers of ion channels. Am J Physiol Gastrointest Liver Physiol 2022; 322:G201-G222. [PMID: 34755536 PMCID: PMC8782647 DOI: 10.1152/ajpgi.00125.2021] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/02/2021] [Revised: 10/28/2021] [Accepted: 11/08/2021] [Indexed: 02/03/2023]
Abstract
Bile acids (BAs) are known to be important regulators of intestinal motility and epithelial fluid and electrolyte transport. Over the past two decades, significant advances in identifying and characterizing the receptors, transporters, and ion channels targeted by BAs have led to exciting new insights into the molecular mechanisms involved in these processes. Our appreciation of BAs, their receptors, and BA-modulated ion channels as potential targets for the development of new approaches to treat intestinal motility and transport disorders is increasing. In the current review, we aim to summarize recent advances in our knowledge of the different BA receptors and BA-modulated ion channels present in the gastrointestinal system. We discuss how they regulate motility and epithelial transport, their roles in pathogenesis, and their therapeutic potential in a range of gastrointestinal diseases.
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Affiliation(s)
- Stephen J Keely
- Royal College of Surgeons in Ireland, Education and Research Centre, Beaumont Hospital, Dublin, Ireland
| | - Andreacarola Urso
- Department of Surgery, Vagelos College of Physicians and Surgeons, Columbia University, New York, New York
- Department of Pharmacology, Columbia University, New York, New York
| | - Alexandr V Ilyaskin
- Institute of Cellular and Molecular Physiology, Friedrich-Alexander University Erlangen-Nürnberg, Bavaria, Germany
| | - Christoph Korbmacher
- Institute of Cellular and Molecular Physiology, Friedrich-Alexander University Erlangen-Nürnberg, Bavaria, Germany
| | - Nigel W Bunnett
- Department of Molecular Pathobiology, Neuroscience Institute, New York University, New York, New York
- Department of Neuroscience and Physiology, Neuroscience Institute, New York University, New York, New York
| | - Daniel P Poole
- Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia
- Australian Research Council, Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia
| | - Simona E Carbone
- Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia
- Australian Research Council, Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia
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7
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Grosch M, Brunner K, Ilyaskin AV, Schober M, Staudner T, Schmied D, Stumpp T, Schmidt KN, Madej MG, Pessoa TD, Othmen H, Kubitza M, Osten L, de Vries U, Mair MM, Somlo S, Moser M, Kunzelmann K, Ziegler C, Haerteis S, Korbmacher C, Witzgall R. A polycystin-2 protein with modified channel properties leads to an increased diameter of renal tubules and to renal cysts. J Cell Sci 2021; 134:271186. [PMID: 34345895 PMCID: PMC8435292 DOI: 10.1242/jcs.259013] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2021] [Accepted: 07/22/2021] [Indexed: 01/14/2023] Open
Abstract
Mutations in the PKD2 gene cause autosomal-dominant polycystic kidney disease but the physiological role of polycystin-2, the protein product of PKD2, remains elusive. Polycystin-2 belongs to the transient receptor potential (TRP) family of non-selective cation channels. To test the hypothesis that altered ion channel properties of polycystin-2 compromise its putative role in a control circuit controlling lumen formation of renal tubular structures, we generated a mouse model in which we exchanged the pore loop of polycystin-2 with that of the closely related cation channel polycystin-2L1 (encoded by PKD2L1), thereby creating the protein polycystin-2poreL1. Functional characterization of this mutant channel in Xenopus laevis oocytes demonstrated that its electrophysiological properties differed from those of polycystin-2 and instead resembled the properties of polycystin-2L1, in particular regarding its permeability for Ca2+ ions. Homology modeling of the ion translocation pathway of polycystin-2poreL1 argues for a wider pore in polycystin-2poreL1 than in polycystin-2. In Pkd2poreL1 knock-in mice in which the endogenous polycystin-2 protein was replaced by polycystin-2poreL1 the diameter of collecting ducts was increased and collecting duct cysts developed in a strain-dependent fashion. Summary: Replacement of the pore region of polycystin-2 with that of polycystin-2L1 results in wider renal tubules and polycystic kidney disease, thus demonstrating the essential function of its ion channel properties.
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Affiliation(s)
- Melanie Grosch
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Katrin Brunner
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Alexandr V Ilyaskin
- Institute of Cellular and Molecular Physiology, Friedrich-Alexander University of Erlangen-Nürnberg, 91054 Erlangen, Germany
| | - Michael Schober
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Tobias Staudner
- Institute of Cellular and Molecular Physiology, Friedrich-Alexander University of Erlangen-Nürnberg, 91054 Erlangen, Germany
| | - Denise Schmied
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Tina Stumpp
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Kerstin N Schmidt
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - M Gregor Madej
- Department of Biophysics, University of Regensburg, 93053 Regensburg, Germany
| | - Thaissa D Pessoa
- Institute of Cellular and Molecular Physiology, Friedrich-Alexander University of Erlangen-Nürnberg, 91054 Erlangen, Germany
| | - Helga Othmen
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Marion Kubitza
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Larissa Osten
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Uwe de Vries
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Magdalena M Mair
- Faculty of Biology and Preclinical Medicine, University of Regensburg, 93053 Regensburg, Germany
| | - Stefan Somlo
- Departments of Medicine and Genetics, Yale University, New Haven, CT 06520, USA
| | - Markus Moser
- Institute of Experimental Hematology, Technical University of Munich, 81675 Munich, Germany
| | - Karl Kunzelmann
- Department of Physiology, University of Regensburg, 93053 Regensburg, Germany
| | - Christine Ziegler
- Department of Biophysics, University of Regensburg, 93053 Regensburg, Germany
| | - Silke Haerteis
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
| | - Christoph Korbmacher
- Institute of Cellular and Molecular Physiology, Friedrich-Alexander University of Erlangen-Nürnberg, 91054 Erlangen, Germany
| | - Ralph Witzgall
- Institute for Molecular and Cellular Anatomy, University of Regensburg, 93053 Regensburg, Germany
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8
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Ilyaskin AV, Sure F, Nesterov V, Haerteis S, Korbmacher C. Bile acids inhibit human purinergic receptor P2X4 in a heterologous expression system. J Gen Physiol 2019; 151:820-833. [PMID: 30988062 PMCID: PMC6572003 DOI: 10.1085/jgp.201812291] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2018] [Revised: 03/08/2019] [Accepted: 03/22/2019] [Indexed: 12/22/2022] Open
Abstract
We recently demonstrated that bile acids, especially tauro-deoxycholic acid (t-DCA), modify the function of the acid-sensing ion channel ASIC1a and other members of the epithelial sodium channel (ENaC)/degenerin (DEG) ion channel family. Surprisingly, ASIC1 shares a high degree of structural similarity with the purinergic receptor P2X4, a nonselective cation channel transiently activated by ATP. P2X4 is abundantly expressed in the apical membrane of bile duct epithelial cells and is therefore exposed to bile acids under physiological conditions. Here, we hypothesize that P2X4 may also be modulated by bile acids and investigate whether t-DCA and other common bile acids affect human P2X4 heterologously expressed in Xenopus laevis oocytes. We find that application of either t-DCA or unconjugated deoxycholic acid (DCA; 250 µM) causes a strong reduction (∼70%) of ATP-activated P2X4-mediated whole-cell currents. The inhibitory effect of 250 µM tauro-chenodeoxycholic acid is less pronounced (∼30%), and 250 µM chenodeoxycholic acid, cholic acid, or tauro-cholic acid did not significantly alter P2X4-mediated currents. t-DCA inhibits P2X4 in a concentration-dependent manner by reducing the efficacy of ATP without significantly changing its affinity. Single-channel patch-clamp recordings provide evidence that t-DCA inhibits P2X4 by stabilizing the channel's closed state. Using site-directed mutagenesis, we identifiy several amino acid residues within the transmembrane domains of P2X4 that are critically involved in mediating the inhibitory effect of t-DCA on P2X4. Importantly, a W46A mutation converts the inhibitory effect of t-DCA into a stimulatory effect. We conclude that t-DCA directly interacts with P2X4 and decreases ATP-activated P2X4 currents by stabilizing the closed conformation of the channel.
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Affiliation(s)
- Alexandr V Ilyaskin
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
| | - Florian Sure
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
| | - Viatcheslav Nesterov
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
| | - Silke Haerteis
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
| | - Christoph Korbmacher
- Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
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9
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Lenzig P, Wirtz M, Wiemuth D. Comparative electrophysiological analysis of the bile acid-sensitive ion channel (BASIC) from different species suggests similar physiological functions. Pflugers Arch 2018; 471:329-336. [PMID: 30353368 DOI: 10.1007/s00424-018-2223-z] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2018] [Revised: 10/11/2018] [Accepted: 10/12/2018] [Indexed: 12/29/2022]
Abstract
Despite the identification of cholangiocytes in the liver and unipolar brush cells in the cerebellum as sites of expression, the physiological function of the bile acid-sensitive ion channel (BASIC) remains unknown. Rat BASIC (rBASIC) and mouse BASIC (mBASIC) share 97% of their amino acid sequence but show strikingly different biophysical properties. rBASIC is inactive at rest while mBASIC is constitutively active, when expressed in Xenopus oocytes. This conundrum rendered the identification of the physiological function even more difficult. In this study, we investigated the electrophysiological and pharmacological properties of BASIC from rat, mouse, and human in Hek293 cells using the patch clamp technique. Surprisingly, in Hek293 cells, rBASIC and mBASIC showed almost completely identical properties. Both are blocked by extracellular Ca2+ and thus are inactive at rest; both are selective for Na+, show similar affinities for extracellular Ca2+, were inhibited by diminazene, and activated by various bile acids. This is in contrast to previous results derived from Xenopus oocytes as expression system and suggests that the cell type is important for shaping the biophysical properties of BASIC. Furthermore, we compared hBASIC with rBASIC and mBASIC and observed similar properties between these channels with one exception: the bile acid sensitivity profile of hBASIC is different from rBASIC and mBASIC; hBASIC is more sensitive to bile acids which are abundant in human bile but not in rodent bile. Taken together, these results suggest similar physiological roles for BASIC in different species.
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Affiliation(s)
- Pia Lenzig
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany
| | - Monika Wirtz
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany
| | - Dominik Wiemuth
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany.
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10
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The degenerin region of the human bile acid-sensitive ion channel (BASIC) is involved in channel inhibition by calcium and activation by bile acids. Pflugers Arch 2018; 470:1087-1102. [PMID: 29589117 DOI: 10.1007/s00424-018-2142-z] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2017] [Revised: 02/12/2018] [Accepted: 03/13/2018] [Indexed: 12/22/2022]
Abstract
The bile acid-sensitive ion channel (BASIC) is a member of the ENaC/degenerin family of ion channels. It is activated by bile acids and inhibited by extracellular Ca2+. The aim of this study was to explore the molecular mechanisms mediating these effects. The modulation of BASIC function by extracellular Ca2+ and tauro-deoxycholic acid (t-DCA) was studied in Xenopus laevis oocytes heterologously expressing human BASIC using the two-electrode voltage-clamp and outside-out patch-clamp techniques. Substitution of aspartate D444 to alanine or cysteine in the degenerin region of BASIC, a region known to be critically involved in channel gating, resulted in a substantial reduction of BASIC Ca2+ sensitivity. Moreover, mutating D444 or the neighboring alanine (A443) to cysteine significantly reduced the t-DCA-mediated BASIC stimulation. A combined molecular docking/simulation approach demonstrated that t-DCA may temporarily form hydrogen bonds with several amino acid residues including D444 in the outer vestibule of the BASIC pore or in the inter-subunit space. By these interactions, t-DCA may stabilize the open state of the channel. Indeed, single-channel recordings provided evidence that t-DCA activates BASIC by stabilizing the open state of the channel, whereas extracellular Ca2+ inhibits BASIC by stabilizing its closed state. In conclusion, our results highlight the potential role of the degenerin region as a critical regulatory site involved in the functional interaction of Ca2+ and t-DCA with BASIC.
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