1
|
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.
Collapse
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
| |
Collapse
|
2
|
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.
Collapse
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
| |
Collapse
|
3
|
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.
Collapse
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
| |
Collapse
|
4
|
The bile acid-sensitive ion channel (BASIC) mediates bile acid-dependent currents in bile duct epithelial cells. Pflugers Arch 2021; 473:1841-1850. [PMID: 34549327 PMCID: PMC8599227 DOI: 10.1007/s00424-021-02622-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2021] [Revised: 09/07/2021] [Accepted: 09/08/2021] [Indexed: 11/17/2022]
Abstract
The bile acid-sensitive ion channel (BASIC) is a member of the Deg/ENaC family of ion channels that is activated by bile acids. Despite the identification of cholangiocytes in the liver and unipolar brush cells in the cerebellum as sites of expression, the physiological function of BASIC in these cell types is not yet understood. Here we used a cholangiocyte cell line, normal rat cholangiocytes (NRCs), which expresses BASIC to study the role of the channel in epithelial transport using Ussing chamber experiments. Apical application of bile acids induced robust and transient increases in transepithelial currents that were carried by Na+ and partly blocked by the BASIC inhibitor diminazene. Genetic ablation of the BASIC gene in NRC using a CRISPR-cas9 approach resulted in a decrease of the bile acid-mediated response that matched the diminazene-sensitive current in NRC WT cells, suggesting that cholangiocytes respond to bile acids with a BASIC-mediated Na+ influx. Taken together, we have identified BASIC as a component of the cholangiocyte transport machinery, which might mediate a bile acid-dependent modification of the bile and thus control bile flux and composition.
Collapse
|
5
|
Elkhatib W, Smith CL, Senatore A. A Na + leak channel cloned from Trichoplax adhaerens extends extracellular pH and Ca 2+ sensing for the DEG/ENaC family close to the base of Metazoa. J Biol Chem 2019; 294:16320-16336. [PMID: 31527080 PMCID: PMC6827283 DOI: 10.1074/jbc.ra119.010542] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2019] [Revised: 09/11/2019] [Indexed: 12/22/2022] Open
Abstract
Acid-sensitive ion channels belonging to the degenerin/epithelial sodium channel (DEG/ENaC) family activate in response to extracellular protons and are considered unique to deuterostomes. However, sensitivity to pH/protons is more widespread, where, for example, human ENaC Na+ leak channels are potentiated and mouse BASIC and Caenorhabditis elegans ACD-1 Na+ leak channels are blocked by extracellular protons. For many DEG/ENaC channels, extracellular Ca2+ ions modulate gating, and in some cases, the binding of protons and Ca2+ is interdependent. Here, we functionally characterize a DEG/ENaC channel from the early-diverging animal Trichoplax adhaerens, TadNaC6, that conducts Na+-selective leak currents in vitro sensitive to blockade by both extracellular protons and Ca2+. We determine that proton block is enhanced in low external Ca2+ concentration, whereas calcium block is enhanced in low external proton concentration, indicative of competitive binding of these two ligands to extracellular sites of the channel protein. TadNaC6 lacks most determinant residues for proton and Ca2+ sensitivity in other DEG/ENaC channels, and a mutation of one conserved residue (S353A) associated with Ca2+ block in rodent BASIC channels instead affected proton sensitivity, all indicative of independent evolution of H+ and Ca2+ sensitivity. Strikingly, TadNaC6 was potently activated by the general DEG/ENaC channel blocker amiloride, a rare feature only reported for the acid-activated channel ASIC3. The sequence and structural divergence of TadNaC6, coupled with its noncanonical functional features, provide unique opportunities for probing the proton, Ca2+, and amiloride regulation of DEG/ENaC channels and insight into the possible core-gating features of ancestral ion channels.
Collapse
Affiliation(s)
- Wassim Elkhatib
- Department of Biology, University of Toronto Mississauga, Mississauga, Ontario L5L 1C6, Canada
| | - Carolyn L Smith
- NINDS, National Institutes of Health, Bethesda, Maryland 20892
| | - Adriano Senatore
- Department of Biology, University of Toronto Mississauga, Mississauga, Ontario L5L 1C6, Canada
| |
Collapse
|
6
|
The Biosynthesis, Signaling, and Neurological Functions of Bile Acids. Biomolecules 2019; 9:biom9060232. [PMID: 31208099 PMCID: PMC6628048 DOI: 10.3390/biom9060232] [Citation(s) in RCA: 120] [Impact Index Per Article: 20.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2019] [Revised: 06/13/2019] [Accepted: 06/14/2019] [Indexed: 12/13/2022] Open
Abstract
Bile acids (BA) are amphipathic steroid acids synthesized from cholesterol in the liver. They act as detergents to expedite the digestion and absorption of dietary lipids and lipophilic vitamins. BA are also considered to be signaling molecules, being ligands of nuclear and cell-surface receptors, including farnesoid X receptor and Takeda G-protein receptor 5. Moreover, BA also activate ion channels, including the bile acid-sensitive ion channel and epithelial Na+ channel. BA regulate glucose and lipid metabolism by activating these receptors in peripheral tissues, such as the liver and brown and white adipose tissue. Recently, 20 different BA have been identified in the central nervous system. Furthermore, BA affect the function of neurotransmitter receptors, such as the muscarinic acetylcholine receptor and γ-aminobutyric acid receptor. BA are also known to be protective against neurodegeneration. Here, we review recent findings regarding the biosynthesis, signaling, and neurological functions of BA.
Collapse
|
7
|
Schmidt A, Joussen S, Hausmann R, Gründer S, Wiemuth D. Bile acids are potent inhibitors of rat P2X2 receptors. Purinergic Signal 2019; 15:213-221. [PMID: 31098843 DOI: 10.1007/s11302-019-09657-2] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2018] [Accepted: 04/17/2019] [Indexed: 01/06/2023] Open
Abstract
Extracellular adenosine triphosphate (ATP) regulates a broad variety of physiological functions in a number of tissues partly via ionotropic P2X receptors. Therefore, P2X receptors are promising targets for the development of therapeutically active molecules. Bile acids are cholesterol-derived amphiphilic molecules; their primary function is the facilitation of efficient nutrient fat digestion. However, bile acids have also been shown to serve as signaling molecules and as modulators of different membrane proteins and receptors including ion channels. In addition, some P2X receptors are sensitive to structurally related steroid hormones. In this study, we systematically analyzed whether rat P2X receptors are affected by micromolar concentrations of different bile acids. The taurine-conjugated bile acids TLCA, THDCA, and TCDCA potently inhibited P2X2, whereas other P2X receptors were only mildly affected. Furthermore, stoichiometry and species origin of the P2X receptors affected the modulation by bile acids: in comparison to rat P2X2, the heteromeric P2X2/3 receptor was less potently modulated and the human P2X2 receptor was potentiated by TLCA. In summary, bile acids are a new class of P2X receptor modulators, which might be of physiological relevance.
Collapse
Affiliation(s)
- Axel Schmidt
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany
| | - Sylvia Joussen
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany
| | - Ralf Hausmann
- Molecular Pharmacology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany
| | - Stefan Gründer
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany
| | - Dominik Wiemuth
- Institute of Physiology, RWTH Aachen University, Pauwelsstrasse 30, 52074, Aachen, Germany.
| |
Collapse
|