1
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Sarkar D, Galleano I, Heusser SA, Ou SY, Uzun GR, Khoo KK, van der Heden van Noort GJ, Harrison JS, Pless SA. Protein semisynthesis underscores the role of a conserved lysine in activation and desensitization of acid-sensing ion channels. Cell Chem Biol 2024; 31:1000-1010.e6. [PMID: 38113885 DOI: 10.1016/j.chembiol.2023.11.013] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2023] [Revised: 07/21/2023] [Accepted: 11/28/2023] [Indexed: 12/21/2023]
Abstract
Acid-sensing ion channels (ASICs) are trimeric ion channels that open a cation-conducting pore in response to proton binding. Excessive ASIC activation during prolonged acidosis in conditions such as inflammation and ischemia is linked to pain and stroke. A conserved lysine in the extracellular domain (Lys211 in mASIC1a) is suggested to play a key role in ASIC function. However, the precise contributions are difficult to dissect with conventional mutagenesis, as replacement of Lys211 with naturally occurring amino acids invariably changes multiple physico-chemical parameters. Here, we study the contribution of Lys211 to mASIC1a function using tandem protein trans-splicing (tPTS) to incorporate non-canonical lysine analogs. We conduct optimization efforts to improve splicing and functionally interrogate semisynthetic mASIC1a. In combination with molecular modeling, we show that Lys211 charge and side-chain length are crucial to activation and desensitization, thus emphasizing that tPTS can enable atomic-scale interrogations of membrane proteins in live cells.
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Affiliation(s)
- Debayan Sarkar
- Department of Drug Design and Pharmacology, University of Copenhagen, 2100 Copenhagen, Denmark
| | - Iacopo Galleano
- Department of Drug Design and Pharmacology, University of Copenhagen, 2100 Copenhagen, Denmark; Department of Cell and Chemical Biology, Leiden University Medical Centre, Leiden, the Netherlands
| | | | - Sofie Yuewei Ou
- Department of Drug Design and Pharmacology, University of Copenhagen, 2100 Copenhagen, Denmark
| | - Gül Refika Uzun
- Department of Drug Design and Pharmacology, University of Copenhagen, 2100 Copenhagen, Denmark
| | - Keith K Khoo
- Department of Drug Design and Pharmacology, University of Copenhagen, 2100 Copenhagen, Denmark
| | | | | | - Stephan Alexander Pless
- Department of Drug Design and Pharmacology, University of Copenhagen, 2100 Copenhagen, Denmark.
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2
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Zhang L, Wang X, Chen J, Sheng S, Kleyman TR. Extracellular intersubunit interactions modulate epithelial Na + channel gating. J Biol Chem 2023; 299:102914. [PMID: 36649907 PMCID: PMC9975279 DOI: 10.1016/j.jbc.2023.102914] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2022] [Revised: 12/13/2022] [Accepted: 01/12/2023] [Indexed: 01/15/2023] Open
Abstract
Epithelial Na+ channels (ENaCs) and related channels have large extracellular domains where specific factors interact and induce conformational changes, leading to altered channel activity. However, extracellular structural transitions associated with changes in ENaC activity are not well defined. Using crosslinking and two-electrode voltage clamp in Xenopus oocytes, we identified several pairs of functional intersubunit contacts where mouse ENaC activity was modulated by inducing or breaking a disulfide bond between introduced Cys residues. Specifically, crosslinking E499C in the β-subunit palm domain and N510C in the α-subunit palm domain activated ENaC, whereas crosslinking βE499C with αQ441C in the α-subunit thumb domain inhibited ENaC. We determined that bridging βE499C to αN510C or αQ441C altered the Na+ self-inhibition response via distinct mechanisms. Similar to bridging βE499C and αQ441C, we found that crosslinking palm domain αE557C with thumb domain γQ398C strongly inhibited ENaC activity. In conclusion, we propose that certain residues at specific subunit interfaces form microswitches that convey a conformational wave during ENaC gating and its regulation.
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Affiliation(s)
- Lei Zhang
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Department of Nephrology, Hunan Key Laboratory of Kidney Disease and Blood Purification, The Second Xiangya Hospital, Central South University, Changsha, Hunan, China; The Third Xiangya Hospital, Central South University, Changsha, Hunan, China
| | - Xueqi Wang
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Department of Nephrology, Hunan Key Laboratory of Kidney Disease and Blood Purification, The Second Xiangya Hospital, Central South University, Changsha, Hunan, China; The Third Xiangya Hospital, Central South University, Changsha, Hunan, China
| | - Jingxin Chen
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Shaohu Sheng
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
| | - Thomas R Kleyman
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
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3
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Kasahara Y, Narukawa M, Takeuchi A, Tominaga M, Abe K, Asakura T. Molecular logic of salt taste reception in special reference to transmembrane channel-like 4 (TMC4). J Physiol Sci 2022; 72:31. [DOI: 10.1186/s12576-022-00856-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2022] [Accepted: 11/13/2022] [Indexed: 12/05/2022]
Abstract
AbstractThe taste is biologically of intrinsic importance. It almost momentarily perceives environmental stimuli for better survival. In the early 2000s, research into taste reception was greatly developed with discovery of the receptors. However, the mechanism of salt taste reception is not fully elucidated yet and many questions still remain. At present, next-generation sequencing and genome-editing technologies are available which would become pivotal tools to elucidate the remaining issues. Here we review current mechanisms of salt taste reception in particular and characterize the properties of transmembrane channel-like 4 as a novel salt taste-related molecule that we found using these sophisticated tools.
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4
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Zhang L, Wang X, Chen J, Kleyman TR, Sheng S. Accessibility of ENaC extracellular domain central core residues. J Biol Chem 2022; 298:101860. [PMID: 35339489 PMCID: PMC9052164 DOI: 10.1016/j.jbc.2022.101860] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2022] [Revised: 03/16/2022] [Accepted: 03/21/2022] [Indexed: 11/20/2022] Open
Abstract
The epithelial Na+ channel (ENaC)/degenerin family has a similar extracellular architecture, where specific regulatory factors interact and alter channel gating behavior. The extracellular palm domain serves as a key link to the channel pore. In this study, we used cysteine-scanning mutagenesis to assess the functional effects of Cys-modifying reagents on palm domain β10 strand residues in mouse ENaC. Of the 13 ENaC α subunit mutants with Cys substitutions examined, only mutants at sites in the proximal region of β10 exhibited changes in channel activity in response to methanethiosulfonate reagents. Additionally, Cys substitutions at three proximal sites of β and γ subunit β10 strands also rendered mutant channels methanethiosulfonate-responsive. Moreover, multiple Cys mutants were activated by low concentrations of thiophilic Cd2+. Using the Na+ self-inhibition response to assess ENaC gating behavior, we identified four α, two β, and two γ subunit β10 strand mutations that changed the Na+ self-inhibition response. Our results suggest that the proximal regions of β10 strands in all three subunits are accessible to small aqueous compounds and Cd2+ and have a role in modulating ENaC gating. These results are consistent with a structural model of mouse ENaC that predicts the presence of aqueous tunnels adjacent to the proximal part of β10 and with previously resolved structures of a related family member where palm domain structural transitions were observed with channels in an open or closed state.
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Affiliation(s)
- Lei Zhang
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Department of Nephrology, Hunan Key Laboratory of Kidney Disease and Blood Purification, The Second Xiangya Hospital, Central South University, Changsha, Hunan, China; The Third Xiangya Hospital of Central South University, Changsha, Hunan, China
| | - Xueqi Wang
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Department of Nephrology, Hunan Key Laboratory of Kidney Disease and Blood Purification, The Second Xiangya Hospital, Central South University, Changsha, Hunan, China; The Third Xiangya Hospital of Central South University, Changsha, Hunan, China
| | - Jingxin Chen
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Thomas R Kleyman
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA; Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
| | - Shaohu Sheng
- Departments of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
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5
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Wichmann L, Althaus M. Evolution of epithelial sodium channels: current concepts and hypotheses. Am J Physiol Regul Integr Comp Physiol 2020; 319:R387-R400. [PMID: 32783689 DOI: 10.1152/ajpregu.00144.2020] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
The conquest of freshwater and terrestrial habitats was a key event during vertebrate evolution. Occupation of low-salinity and dry environments required significant osmoregulatory adaptations enabling stable ion and water homeostasis. Sodium is one of the most important ions within the extracellular liquid of vertebrates, and molecular machinery for urinary reabsorption of this electrolyte is critical for the maintenance of body osmoregulation. Key ion channels involved in the fine-tuning of sodium homeostasis in tetrapod vertebrates are epithelial sodium channels (ENaCs), which allow the selective influx of sodium ions across the apical membrane of epithelial cells lining the distal nephron or the colon. Furthermore, ENaC-mediated sodium absorption across tetrapod lung epithelia is crucial for the control of liquid volumes lining the pulmonary surfaces. ENaCs are vertebrate-specific members of the degenerin/ENaC family of cation channels; however, there is limited knowledge on the evolution of ENaC within this ion channel family. This review outlines current concepts and hypotheses on ENaC phylogeny and discusses the emergence of regulation-defining sequence motifs in the context of osmoregulatory adaptations during tetrapod terrestrialization. In light of the distinct regulation and expression of ENaC isoforms in tetrapod vertebrates, we discuss the potential significance of ENaC orthologs in osmoregulation of fishes as well as the putative fates of atypical channel isoforms in mammals. We hypothesize that ancestral proton-sensitive ENaC orthologs might have aided the osmoregulatory adaptation to freshwater environments whereas channel regulation by proteases evolved as a molecular adaptation to lung liquid homeostasis in terrestrial tetrapods.
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Affiliation(s)
- Lukas Wichmann
- Institute for Animal Physiology, Justus Liebig University, Giessen, Germany
| | - Mike Althaus
- Department of Natural Sciences, Bonn-Rhein-Sieg University of Applied Sciences, Rheinbach, Germany
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Noreng S, Posert R, Bharadwaj A, Houser A, Baconguis I. Molecular principles of assembly, activation, and inhibition in epithelial sodium channel. eLife 2020; 9:59038. [PMID: 32729833 PMCID: PMC7413742 DOI: 10.7554/elife.59038] [Citation(s) in RCA: 26] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2020] [Accepted: 07/29/2020] [Indexed: 12/11/2022] Open
Abstract
The molecular bases of heteromeric assembly and link between Na+ self-inhibition and protease-sensitivity in epithelial sodium channels (ENaCs) are not fully understood. Previously, we demonstrated that ENaC subunits – α, β, and γ – assemble in a counterclockwise configuration when viewed from outside the cell with the protease-sensitive GRIP domains in the periphery (Noreng et al., 2018). Here we describe the structure of ENaC resolved by cryo-electron microscopy at 3 Å. We find that a combination of precise domain arrangement and complementary hydrogen bonding network defines the subunit arrangement. Furthermore, we determined that the α subunit has a primary functional module consisting of the finger and GRIP domains. The module is bifurcated by the α2 helix dividing two distinct regulatory sites: Na+ and the inhibitory peptide. Removal of the inhibitory peptide perturbs the Na+ site via the α2 helix highlighting the critical role of the α2 helix in regulating ENaC function.
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Affiliation(s)
- Sigrid Noreng
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Richard Posert
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Arpita Bharadwaj
- Vollum Institute, Oregon Health & Science University, Portland, United States
| | - Alexandra Houser
- Neuroscience Graduate Program, Oregon Health & Science University, Portland, United States
| | - Isabelle Baconguis
- Vollum Institute, Oregon Health & Science University, Portland, United States
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7
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Abstract
Epithelial Na+ channels (ENaCs) are members of a family of cation channels that function as sensors of the extracellular environment. ENaCs are activated by specific proteases in the biosynthetic pathway and at the cell surface and remove embedded inhibitory tracts, which allows channels to transition to higher open-probability states. Resolved structures of ENaC and an acid-sensing ion channel revealed highly organized extracellular regions. Within the periphery of ENaC subunits are unique domains formed by antiparallel β-strands containing the inhibitory tracts and protease cleavage sites. ENaCs are inhibited by Na+ binding to specific extracellular site(s), which promotes channel transition to a lower open-probability state. Specific inositol phospholipids and channel modification by Cys-palmitoylation enhance channel open probability. How these regulatory factors interact in a concerted manner to influence channel open probability is an important question that has not been resolved. These various factors are reviewed, and the impact of specific factors on human disorders is discussed.
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Affiliation(s)
- Thomas R Kleyman
- Renal-Electrolyte Division, Department of Medicine, and Departments of Cell Biology and of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania
| | - Douglas C Eaton
- Division of Nephrology, Department of Medicine, Emory University, Atlanta, Georgia
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8
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Abstract
Cl- is the major extracellular (Cl-out) and intracellular (Cl-in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl-]in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl-. Cl- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl- and argues that Cl- is a bona fide signaling ion, a function deserving extensive exploration.
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Affiliation(s)
- Benjamin P Lüscher
- Epithelial Signaling and Transport Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, Maryland
| | - Laura Vachel
- Epithelial Signaling and Transport Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, Maryland
| | - Ehud Ohana
- Department of Clinical Biochemistry and Pharmacology, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
| | - Shmuel Muallem
- Epithelial Signaling and Transport Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, Maryland
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9
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Wang X, Chen J, Shi S, Sheng S, Kleyman TR. Analyses of epithelial Na + channel variants reveal that an extracellular β-ball domain critically regulates ENaC gating. J Biol Chem 2019; 294:16765-16775. [PMID: 31551351 DOI: 10.1074/jbc.ra119.010001] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2019] [Revised: 09/19/2019] [Indexed: 11/06/2022] Open
Abstract
Epithelial Na+ channel (ENaC)-mediated Na+ transport has a key role in the regulation of extracellular fluid volume, blood pressure, and extracellular [K+]. Among the thousands of human ENaC variants, only a few exist whose functional consequences have been experimentally tested. Here, we used the Xenopus oocyte expression system to investigate the functional roles of four nonsynonymous human ENaC variants located within the β7-strand and its adjacent loop of the α-subunit extracellular β-ball domain. αR350Wβγ and αG355Rβγ channels exhibited 2.5- and 1.8-fold greater amiloride-sensitive currents than WT αβγ human ENaCs, respectively, whereas αV351Aβγ channels conducted significantly less current than WT. Currents in αH354Rβγ-expressing oocytes were similar to those expressing WT. Surface expression levels of three mutants (αR350Wβγ, αV351Aβγ, and αG355Rβγ) were similar to that of WT. However, three mutant channels (αR350Wβγ, αH354Rβγ, and αG355Rβγ) exhibited a reduced Na+ self-inhibition response. Open probability of αR350Wβγ was significantly greater than that of WT. Moreover, other Arg-350 variants, including αR350G, αR350L, and αR350Q, also had significantly increased channel activity. A direct comparison of αR350W and two previously reported gain-of-function variants revealed that αR350W increases ENaC activity similarly to αW493R, but to a much greater degree than does αC479R. Our results indicate that αR350W along with αR350G, αR350L, and αR350Q, and αG355R are novel gain-of-function variants that function as gating modifiers. The location of these multiple functional variants suggests that the αENaC β-ball domain portion that interfaces with the palm domain of βENaC critically regulates ENaC gating.
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Affiliation(s)
- Xueqi Wang
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261.,Department of Nephrology, The Second Xiangya Hospital of Central South University, Changsha, Hunan 410011, China.,The Third Xiangya Hospital of Central South University, Changsha, Hunan 410013, China
| | - Jingxin Chen
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Shujie Shi
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Shaohu Sheng
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Thomas R Kleyman
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261.,Department of Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261.,Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
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10
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Mutchler SM, Kleyman TR. New insights regarding epithelial Na+ channel regulation and its role in the kidney, immune system and vasculature. Curr Opin Nephrol Hypertens 2019; 28:113-119. [PMID: 30585851 PMCID: PMC6349474 DOI: 10.1097/mnh.0000000000000479] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
PURPOSE OF REVIEW This review describes recent findings regarding the epithelial Na channel (ENaC) and its roles in physiologic and pathophysiologic states. We discuss new insights regarding ENaC's structure, its regulation by various factors, its potential role in hypertension and nephrotic syndrome, and its roles in the immune system and vasculature. RECENT FINDINGS A recently resolved structure of ENaC provides clues regarding mechanisms of ENaC activation by proteases. The use of amiloride in nephrotic syndrome, and associated complications are discussed. ENaC is expressed in dendritic cells and contributes to immune system activation and increases in blood pressure in response to NaCl. ENaC is expressed in endothelial ENaC and has a role in regulating vascular tone. SUMMARY New findings have emerged regarding ENaC and its role in the kidney, immune system, and vasculature.
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Affiliation(s)
- Stephanie M. Mutchler
- Department of Medicine, University of Pittsburgh, Pittsburgh, PA
- Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, PA
| | - Thomas R. Kleyman
- Department of Medicine, University of Pittsburgh, Pittsburgh, PA
- Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, PA
- Department of Cell Biology, University of Pittsburgh, Pittsburgh, PA
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Noreng S, Bharadwaj A, Posert R, Yoshioka C, Baconguis I. Structure of the human epithelial sodium channel by cryo-electron microscopy. eLife 2018; 7:39340. [PMID: 30251954 PMCID: PMC6197857 DOI: 10.7554/elife.39340] [Citation(s) in RCA: 105] [Impact Index Per Article: 17.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2018] [Accepted: 09/18/2018] [Indexed: 12/11/2022] Open
Abstract
The epithelial sodium channel (ENaC), a member of the ENaC/DEG superfamily, regulates Na+ and water homeostasis. ENaCs assemble as heterotrimeric channels that harbor protease-sensitive domains critical for gating the channel. Here, we present the structure of human ENaC in the uncleaved state determined by single-particle cryo-electron microscopy. The ion channel is composed of a large extracellular domain and a narrow transmembrane domain. The structure reveals that ENaC assembles with a 1:1:1 stoichiometry of α:β:γ subunits arranged in a counter-clockwise manner. The shape of each subunit is reminiscent of a hand with key gating domains of a 'finger' and a 'thumb.' Wedged between these domains is the elusive protease-sensitive inhibitory domain poised to regulate conformational changes of the 'finger' and 'thumb'; thus, the structure provides the first view of the architecture of inhibition of ENaC.
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Affiliation(s)
- Sigrid Noreng
- Department of Biochemistry & Molecular Biology, Oregon Health and Science University, Portland, United States
| | - Arpita Bharadwaj
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Richard Posert
- Department of Biochemistry & Molecular Biology, Oregon Health and Science University, Portland, United States
| | - Craig Yoshioka
- Department of Biomedical Engineering, Oregon Health and Science University, Portland, United States
| | - Isabelle Baconguis
- Vollum Institute, Oregon Health and Science University, Portland, United States
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12
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Webster MJ, Tarran R. Slippery When Wet: Airway Surface Liquid Homeostasis and Mucus Hydration. CURRENT TOPICS IN MEMBRANES 2018; 81:293-335. [PMID: 30243435 DOI: 10.1016/bs.ctm.2018.08.004] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The ability to regulate cell volume is crucial for normal physiology; equally the regulation of extracellular fluid homeostasis is of great importance. Alteration of normal extracellular fluid homeostasis contributes to the development of several diseases including cystic fibrosis. With regard to the airway surface liquid (ASL), which lies apically on top of airway epithelia, ion content, pH, mucin and protein abundance must be tightly regulated. Furthermore, airway epithelia must be able to switch from an absorptive to a secretory state as required. A heterogeneous population of airway epithelial cells regulate ASL solute and solvent composition, and directly secrete large mucin molecules, antimicrobials, proteases and soluble mediators into the airway lumen. This review focuses on how epithelial ion transport influences ASL hydration and ASL pH, with a specific focus on the roles of anion and cation channels and exchangers. The role of ions and pH in mucin expansion is also addressed. With regard to fluid volume regulation, we discuss the roles of nucleotides, adenosine and the short palate lung and nasal epithelial clone 1 (SPLUNC1) as soluble ASL mediators. Together, these mechanisms directly influence ciliary beating and in turn mucociliary clearance to maintain sterility and to detoxify the airways. Whilst all of these components are regulated in normal airways, defective ion transport and/or mucin secretion proves detrimental to lung homeostasis as such we address how defective ion and fluid transport, and a loss of homeostatic mechanisms, contributes to the development of pathophysiologies associated with cystic fibrosis.
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Affiliation(s)
- Megan J Webster
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States
| | - Robert Tarran
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States; Department of Cell Biology & Physiology, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States
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13
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Yoder N, Gouaux E. Divalent cation and chloride ion sites of chicken acid sensing ion channel 1a elucidated by x-ray crystallography. PLoS One 2018; 13:e0202134. [PMID: 30157194 PMCID: PMC6114778 DOI: 10.1371/journal.pone.0202134] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2018] [Accepted: 07/27/2018] [Indexed: 12/21/2022] Open
Abstract
Acid sensing ion channels (ASICs) are proton-gated ion channels that are members of the degenerin/epithelial sodium channel superfamily and are expressed throughout central and peripheral nervous systems. ASICs have been implicated in multiple physiological processes and are subject to numerous forms of endogenous and exogenous regulation that include modulation by Ca2+ and Cl- ions. However, the mapping of ion binding sites as well as a structure-based understanding of the mechanisms underlying ionic modulation of ASICs have remained elusive. Here we present ion binding sites of chicken ASIC1a in resting and desensitized states at high and low pH, respectively, determined by anomalous diffraction x-ray crystallography. The acidic pocket serves as a nexus for divalent cation binding at both low and high pH, while we observe divalent cation binding within the central vestibule on the resting channel at high pH only. Moreover, neutralization of residues positioned to coordinate divalent cations via individual and combined Glu to Gln substitutions reduced, but did not extinguish, modulation of proton-dependent gating by Ca2+. Additionally, we demonstrate that anion binding at the canonical thumb domain site is state-dependent and present a previously undetected anion site at the mouth of the extracellular fenestrations on the resting channel. Our results map anion and cation sites on ASICs across multiple functional states, informing possible mechanisms of modulation and providing a blueprint for the design of therapeutics targeting ASICs.
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Affiliation(s)
- Nate Yoder
- Vollum Institute, Oregon Health & Science University, Portland, Oregon, United States of America
| | - Eric Gouaux
- Vollum Institute, Oregon Health & Science University, Portland, Oregon, United States of America
- Howard Hughes Medical Institute, Oregon Health & Science University, Portland, Oregon, United States of America
- * E-mail:
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14
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Balchak DM, Thompson RN, Kashlan OB. The epithelial Na + channel γ subunit autoinhibitory tract suppresses channel activity by binding the γ subunit's finger-thumb domain interface. J Biol Chem 2018; 293:16217-16225. [PMID: 30131333 DOI: 10.1074/jbc.ra118.004362] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2018] [Revised: 08/21/2018] [Indexed: 01/11/2023] Open
Abstract
Epithelial Na+ channel (ENaC) maturation and activation require proteolysis of both the α and γ subunits. Cleavage at multiple sites in the finger domain of each subunit liberates their autoinhibitory tracts. Synthetic peptides derived from the proteolytically released fragments inhibit the channel, likely by reconstituting key interactions removed by the proteolysis. We previously showed that a peptide derived from the α subunit's autoinhibitory sequence (α-8) binds at the α subunit's finger-thumb domain interface. Despite low sequence similarity between the α and γ subunit finger domains, we hypothesized that a peptide derived from the γ subunit's autoinhibitory sequence (γ-11) inhibits the channel through an analogous mechanism. Using Xenopus oocytes, we found here that channels lacking a γ subunit thumb domain were no longer sensitive to γ-11, but remained sensitive to α-8. We identified finger domain sites in the γ subunit that dramatically reduced γ-11 inhibition. Using cysteines and sulfhydryl reactive cross-linkers introduced into both the peptide and the subunit, we also could cross-link γ-11 to both the finger domain and the thumb domain of the γ subunit. Our results suggest that α-8 and γ-11 occupy similar binding pockets within their respective subunits, and that proteolysis of the α and γ subunits activate the channel through analogous mechanisms.
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Affiliation(s)
| | | | - Ossama B Kashlan
- From the Department of Medicine, Renal-Electrolyte Division and .,the Department of Computational and Systems Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
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15
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Blobner BM, Wang XP, Kashlan OB. Conserved cysteines in the finger domain of the epithelial Na + channel α and γ subunits are proximal to the dynamic finger-thumb domain interface. J Biol Chem 2018; 293:4928-4939. [PMID: 29425099 DOI: 10.1074/jbc.m117.819367] [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: 09/21/2017] [Revised: 01/26/2018] [Indexed: 11/06/2022] Open
Abstract
The epithelial Na+ channel (ENaC) is a member of the ENaC/degenerin family of ion channels. In the structure of a related family member, the "thumb" domain's base interacts with the pore, and its tip interacts with the divergent "finger" domain. Between the base and tip, the thumb domain is characterized by a conserved five-rung disulfide ladder holding together two anti-parallel α helices. The ENaC α and γ subunits' finger domains harbor autoinhibitory tracts that can be proteolytically liberated to activate the channel and also host an ENaC-specific pair of cysteines. Using a crosslinking approach, we show that one of the finger domain cysteines in the α subunit (αCys-263) and both of the finger domain cysteines in the γ subunit (γCys-213 and γCys-220) lie near the dynamic finger-thumb domain interface. Our data suggest that the αCys-256/αCys-263 pair is not disulfide-bonded. In contrast, we found that the γCys-213/γCys-220 pair is disulfide-bonded. Our data also suggest that the γ subunit lacks the terminal rung in the thumb domain disulfide ladder, suggesting asymmetry between the subunits. We also observed functional asymmetry between the α and γ subunit finger-thumb domain interfaces; crosslinks bridging the α subunit finger-thumb interface only inhibited ENaC currents, whereas crosslinks bridging the γ subunit finger-thumb interface activated or inhibited currents dependent on the length of the crosslinker. Our data suggest that reactive cysteines lie at the dynamic finger-thumb interfaces of the α and γ subunits and may play a yet undefined role in channel regulation.
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Affiliation(s)
- Brandon M Blobner
- Departments of Medicine, Renal-Electrolyte Division, Pittsburgh, Pennsylvania 15261
| | - Xue-Ping Wang
- Departments of Medicine, Renal-Electrolyte Division, Pittsburgh, Pennsylvania 15261
| | - Ossama B Kashlan
- Departments of Medicine, Renal-Electrolyte Division, Pittsburgh, Pennsylvania 15261; Computational and Systems Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261.
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16
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Kleyman TR, Kashlan OB, Hughey RP. Epithelial Na + Channel Regulation by Extracellular and Intracellular Factors. Annu Rev Physiol 2017; 80:263-281. [PMID: 29120692 DOI: 10.1146/annurev-physiol-021317-121143] [Citation(s) in RCA: 59] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Epithelial Na+ channels (ENaCs) are members of the ENaC/degenerin family of ion channels that evolved to respond to extracellular factors. In addition to being expressed in the distal aspects of the nephron, where ENaCs couple the absorption of filtered Na+ to K+ secretion, these channels are found in other epithelia as well as nonepithelial tissues. This review addresses mechanisms by which ENaC activity is regulated by extracellular factors, including proteases, Na+, and shear stress. It also addresses other factors, including acidic phospholipids and modification of ENaC cytoplasmic cysteine residues by palmitoylation, which enhance channel activity by altering interactions of the channel with the plasma membrane.
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Affiliation(s)
- Thomas R Kleyman
- Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA; .,Department of Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.,Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA
| | - Ossama B Kashlan
- Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA; .,Department of Computational and Systems Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
| | - Rebecca P Hughey
- Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA; .,Department of Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.,Department of Microbiology and Molecular Genetics, University of Pittsburgh, Pittsburgh, Pennsylvania 15219, USA
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17
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Livraghi-Butrico A, Wilkinson KJ, Volmer AS, Gilmore RC, Rogers TD, Caldwell RA, Burns KA, Esther CR, Mall MA, Boucher RC, O'Neal WK, Grubb BR. Lung disease phenotypes caused by overexpression of combinations of α-, β-, and γ-subunits of the epithelial sodium channel in mouse airways. Am J Physiol Lung Cell Mol Physiol 2017; 314:L318-L331. [PMID: 29074490 DOI: 10.1152/ajplung.00382.2017] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The epithelial Na+ channel (ENaC) regulates airway surface hydration. In mouse airways, ENaC is composed of three subunits, α, β, and γ, which are differentially expressed (α > β > γ). Airway-targeted overexpression of the β subunit results in Na+ hyperabsorption, causing airway surface dehydration, hyperconcentrated mucus with delayed clearance, lung inflammation, and perinatal mortality. Notably, mice overexpressing the α- or γ-subunit do not exhibit airway Na+ hyperabsorption or lung pathology. To test whether overexpression of multiple ENaC subunits produced Na+ transport and disease severity exceeding that of βENaC-Tg mice, we generated double (αβ, αγ, βγ) and triple (αβγ) transgenic mice and characterized their lung phenotypes. Double αγENaC-Tg mice were indistinguishable from WT littermates. In contrast, double βγENaC-Tg mice exhibited airway Na+ absorption greater than that of βENaC-Tg mice, which was paralleled by worse survival, decreased mucociliary clearance, and more severe lung pathology. Double αβENaC-Tg mice exhibited Na+ transport rates comparable to those of βENaC-Tg littermates. However, αβENaC-Tg mice had poorer survival and developed severe parenchymal consolidation. In situ hybridization (RNAscope) analysis revealed both alveolar and airway αENaC-Tg overexpression. Triple αβγENaC-Tg mice were born in Mendelian proportions but died within the first day of life, and the small sample size prevented analyses of cause(s) of death. Cumulatively, these results indicate that overexpression of βENaC is rate limiting for generation of pathological airway surface dehydration. Notably, airway co-overexpression of β- and γENaC had additive effects on Na+ transport and disease severity, suggesting dose dependency of these two variables.
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Affiliation(s)
- Alessandra Livraghi-Butrico
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Kristen J Wilkinson
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Allison S Volmer
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Rodney C Gilmore
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Troy D Rogers
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | | | - Kimberlie A Burns
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Charles R Esther
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina.,Division of Pediatric Pulmonology, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Marcus A Mall
- Department of Translational Pulmonology, Translational Lung Research Center, Heidelberg, German Center for Lung Research, University of Heidelberg , Heidelberg , Germany.,Division of Pediatric Pulmonology and Allergy and Cystic Fibrosis Center, Dept. of Pediatrics, University of Heidelberg , Heidelberg , Germany
| | - Richard C Boucher
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Wanda K O'Neal
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
| | - Barbara R Grubb
- Marsico Lung Institute, University of North Carolina Cystic Fibrosis Center, School of Medicine, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina
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18
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Knoepp F, Bettmer J, Fronius M. Gadolinium released by the linear gadolinium-based contrast-agent Gd-DTPA decreases the activity of human epithelial Na + channels (ENaCs). BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:1040-1048. [PMID: 28257815 DOI: 10.1016/j.bbamem.2017.02.019] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/27/2016] [Revised: 02/16/2017] [Accepted: 02/27/2017] [Indexed: 02/05/2023]
Abstract
BACKGROUND Gadolinium-based-contrast-agents (GBCAs) are used for magnetic-resonance-imaging and associated with renal and cardiovascular adverse reactions caused by released Gd3+ ions. Gd3+ is also a modulator of mechano-gated ion channels, including the epithelial Na+ channel (ENaC) that is expressed in kidney epithelium and the vasculature. ENaC is important for salt-/water homeostasis and blood pressure regulation and a likely target of released Gd3+ from GBCAs causing the above-mentioned adverse reactions. Therefore this study examined the effect of Gd3+ and GBCAs on ENaC's activity. METHODS Human αβγENaC was expressed in Xenopus laevis oocytes and exposed to Gd3+, linear (Gd-DTPA, Magnevist) or cyclic (Dotarem) GBCAs. Transmembrane ion-currents (IM) were recorded by the two-electrode-voltage-clamp technique and Gd3+-release by Gd-DTPA was confirmed by inductively coupled plasma-mass spectrometry. RESULTS Gd3+ exerts biphasic effects on ENaC's activity: ≤0.3mmol/l decreased IM which was preventable by DEPC (modifies histidines). Strikingly Gd3+≥0.4mmol/l increased IM and this effect was prevented by cysteine-modifying MTSEA. Linear Gd-DTPA and Magnevist mimicked the effect of ≤0.3mmol/l Gd3+, whereas the chelator DTPA showed no effect. Gd3+ and Gd-DTPA increased the IC50 for amiloride, but did not affect ENaC's self-inhibition. Interestingly, cyclic Gd-DOTA (Dotarem) increased IM to a similar extent as its chelator DOTA, suggesting that the chelator rather than released Gd3+ is responsible for this effect. CONCLUSION These results confirm Gd3+-release from linear Gd-DTPA and indicate that the released Gd3+ amount is sufficient to interfere with ENaC's activity to provide putative explanations for GBCA-related adverse effects.
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Affiliation(s)
- Fenja Knoepp
- Excellence Cluster Cardio-Pulmonary System, Justus-Liebig-University Giessen, Aulweg 130, 35392 Giessen, Germany
| | - Joerg Bettmer
- Department of Physical and Analytical Chemistry, University of Oviedo, C/ Julián Clavería 8, 33006 Oviedo, Spain
| | - Martin Fronius
- Department of Physiology, University of Otago, PO Box 913, Dunedin 9054, New Zealand.
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19
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Lucas R, Yue Q, Alli A, Duke BJ, Al-Khalili O, Thai TL, Hamacher J, Sridhar S, Lebedyeva I, Su H, Tzotzos S, Fischer B, Gameiro AF, Loose M, Chakraborty T, Shabbir W, Aufy M, Lemmens-Gruber R, Eaton DC, Czikora I. The Lectin-like Domain of TNF Increases ENaC Open Probability through a Novel Site at the Interface between the Second Transmembrane and C-terminal Domains of the α-Subunit. J Biol Chem 2016; 291:23440-23451. [PMID: 27645999 DOI: 10.1074/jbc.m116.718163] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2016] [Indexed: 12/29/2022] Open
Abstract
Regulation of the epithelial sodium channel (ENaC), which regulates fluid homeostasis and blood pressure, is complex and remains incompletely understood. The TIP peptide, a mimic of the lectin-like domain of TNF, activates ENaC by binding to glycosylated residues in the extracellular loop of ENaC-α, as well as to a hitherto uncharacterized internal site. Molecular docking studies suggested three residues, Val567, Glu568, and Glu571, located at the interface between the second transmembrane and C-terminal domains of ENaC-α, as a critical site for binding of the TIP peptide. We generated Ala replacement mutants in this region of ENaC-α and examined its interaction with TIP peptide (3M, V567A/E568A/E571A; 2M, V567A/E568A; and 1M, E571A). 3M and 2M ENaC-α, but not 1M ENaC-α, displayed significantly reduced binding capacity to TIP peptide and to TNF. When overexpressed in H441 cells, 3M mutant ENaC-α formed functional channels with similar gating and density characteristics as the WT subunit and efficiently associated with the β and γ subunits in the plasma membrane. We subsequently assayed for increased open probability time and membrane expression, both of which define ENaC activity, following addition of TIP peptide. TIP peptide increased open probability time in H441 cells overexpressing wild type and 1M ENaC-α channels, but not 3M or 2M ENaC-α channels. On the other hand, TIP peptide-mediated reduction in ENaC ubiquitination was similar in cells overexpressing either WT or 3M ENaC-α subunits. In summary, this study has identified a novel site in ENaC-α that is crucial for activation of the open probability of the channel, but not membrane expression, by the lectin-like domain of TNF.
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Affiliation(s)
- Rudolf Lucas
- From the Vascular Biology Center, .,the Department of Pharmacology and Toxicology, and.,the Division of Pulmonary and Critical Care Medicine, Medical College of Georgia, Augusta, Georgia 30912
| | - Qiang Yue
- the Department of Physiology, Emory University, Atlanta, Georgia 30322
| | - Abdel Alli
- the Department of Physiology, Emory University, Atlanta, Georgia 30322.,the Department of Physiology and Functional Genomics, University of Florida College of Medicine, Gainesville, Florida 32610
| | | | - Otor Al-Khalili
- the Department of Physiology, Emory University, Atlanta, Georgia 30322
| | - Tiffany L Thai
- the Department of Physiology, Emory University, Atlanta, Georgia 30322
| | - Jürg Hamacher
- the Department of Pulmonology, Saarland University, D-66421 Homburg, Germany
| | | | - Iryna Lebedyeva
- the Department of Chemistry, Augusta University, Augusta, Georgia 30912
| | - Huabo Su
- From the Vascular Biology Center
| | - Susan Tzotzos
- Apeptico Research and Development, 1150 Vienna, Austria
| | | | | | - Maria Loose
- the Institute for Medical Microbiology, Justus-Liebig University, 35390 Giessen, Germany, and
| | - Trinad Chakraborty
- the Institute for Medical Microbiology, Justus-Liebig University, 35390 Giessen, Germany, and
| | - Waheed Shabbir
- the Department of Pharmacology and Toxicology, University Vienna, 1010 Vienna, Austria
| | - Mohammed Aufy
- the Department of Pharmacology and Toxicology, University Vienna, 1010 Vienna, Austria
| | - Rosa Lemmens-Gruber
- the Department of Pharmacology and Toxicology, University Vienna, 1010 Vienna, Austria
| | - Douglas C Eaton
- the Department of Physiology, Emory University, Atlanta, Georgia 30322,
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20
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The Epithelial Sodium Channel and the Processes of Wound Healing. BIOMED RESEARCH INTERNATIONAL 2016; 2016:5675047. [PMID: 27493961 PMCID: PMC4963570 DOI: 10.1155/2016/5675047] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/15/2016] [Accepted: 06/15/2016] [Indexed: 12/19/2022]
Abstract
The epithelial sodium channel (ENaC) mediates passive sodium transport across the apical membranes of sodium absorbing epithelia, like the distal nephron, the intestine, and the lung airways. Additionally, the channel has been involved in the transduction of mechanical stimuli, such as hydrostatic pressure, membrane stretch, and shear stress from fluid flow. Thus, in vascular endothelium, it participates in the control of the vascular tone via its activity both as a sodium channel and as a shear stress transducer. Rather recently, ENaC has been shown to participate in the processes of wound healing, a role that may also involve its activities as sodium transporter and as mechanotransducer. Its presence as the sole channel mediating sodium transport in many tissues and the diversity of its functions probably underlie the complexity of its regulation. This brief review describes some aspects of ENaC regulation, comments on evidence about ENaC participation in wound healing, and suggests possible regulatory mechanisms involved in this participation.
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21
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Hanukoglu I, Hanukoglu A. Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases. Gene 2016; 579:95-132. [PMID: 26772908 PMCID: PMC4756657 DOI: 10.1016/j.gene.2015.12.061] [Citation(s) in RCA: 238] [Impact Index Per Article: 29.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2015] [Revised: 12/20/2015] [Accepted: 12/22/2015] [Indexed: 01/24/2023]
Abstract
The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.
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Affiliation(s)
- Israel Hanukoglu
- Laboratory of Cell Biology, Faculty of Natural Sciences, Ariel University, Ariel, Israel.
| | - Aaron Hanukoglu
- Division of Pediatric Endocrinology, E. Wolfson Medical Center, Holon, Israel; Sackler School of Medicine, Tel-Aviv University, Tel Aviv, Israel
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22
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Chen J, Ray EC, Yates ME, Buck TM, Brodsky JL, Kinlough CL, Winarski KL, Hughey RP, Kleyman TR, Sheng S. Functional Roles of Clusters of Hydrophobic and Polar Residues in the Epithelial Na+ Channel Knuckle Domain. J Biol Chem 2015; 290:25140-50. [PMID: 26306034 DOI: 10.1074/jbc.m115.665398] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2015] [Indexed: 12/21/2022] Open
Abstract
The extracellular regions of epithelial Na(+) channel subunits are highly ordered structures composed of domains formed by α helices and β strands. Deletion of the peripheral knuckle domain of the α subunit in the αβγ trimer results in channel activation, reflecting an increase in channel open probability due to a loss of the inhibitory effect of external Na(+) (Na(+) self-inhibition). In contrast, deletion of either the β or γ subunit knuckle domain within the αβγ trimer dramatically reduces epithelial Na(+) channel function and surface expression, and impairs subunit maturation. We systematically mutated individual α subunit knuckle domain residues and assessed functional properties of these mutants. Cysteine substitutions at 14 of 28 residues significantly suppressed Na(+) self-inhibition. The side chains of a cluster of these residues are non-polar and are predicted to be directed toward the palm domain, whereas a group of polar residues are predicted to orient their side chains toward the space between the knuckle and finger domains. Among the mutants causing the greatest suppression of Na(+) self-inhibition were αP521C, αI529C, and αS534C. The introduction of Cys residues at homologous sites within either the β or γ subunit knuckle domain resulted in little or no change in Na(+) self-inhibition. Our results suggest that multiple residues in the α subunit knuckle domain contribute to the mechanism of Na(+) self-inhibition by interacting with palm and finger domain residues via two separate and chemically distinct motifs.
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Affiliation(s)
- Jingxin Chen
- From the Renal-Electrolyte Division, Department of Medicine
| | - Evan C Ray
- From the Renal-Electrolyte Division, Department of Medicine
| | | | | | | | | | | | - Rebecca P Hughey
- From the Renal-Electrolyte Division, Department of Medicine, the Department of Cell Biology, the Department of Microbiology and Molecular Genetics, and
| | - Thomas R Kleyman
- From the Renal-Electrolyte Division, Department of Medicine, the Department of Cell Biology, the Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Shaohu Sheng
- From the Renal-Electrolyte Division, Department of Medicine
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23
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Kashlan OB, Blobner BM, Zuzek Z, Tolino M, Kleyman TR. Na+ inhibits the epithelial Na+ channel by binding to a site in an extracellular acidic cleft. J Biol Chem 2014; 290:568-76. [PMID: 25389295 DOI: 10.1074/jbc.m114.606152] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
The epithelial Na(+) channel (ENaC) has a key role in the regulation of extracellular fluid volume and blood pressure. ENaC belongs to a family of ion channels that sense the external environment. These channels have large extracellular regions that are thought to interact with environmental cues, such as Na(+), Cl(-), protons, proteases, and shear stress, which modulate gating behavior. We sought to determine the molecular mechanism by which ENaC senses high external Na(+) concentrations, resulting in an inhibition of channel activity. Both our structural model of an ENaC α subunit and the resolved structure of an acid-sensing ion channel (ASIC1) have conserved acidic pockets in the periphery of the extracellular region of the channel. We hypothesized that these acidic pockets host inhibitory allosteric Na(+) binding sites. Through site-directed mutagenesis targeting the acidic pocket, we modified the inhibitory response to external Na(+). Mutations at selected sites altered the cation inhibitory preference to favor Li(+) or K(+) rather than Na(+). Channel activity was reduced in response to restraining movement within this region by cross-linking structures across the acidic pocket. Our results suggest that residues within the acidic pocket form an allosteric effector binding site for Na(+). Our study supports the hypothesis that an acidic cleft is a key ligand binding locus for ENaC and perhaps other members of the ENaC/degenerin family.
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Affiliation(s)
| | | | | | | | - Thomas R Kleyman
- From the Departments of Medicine and Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
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24
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Collier DM, Tomkovicz VR, Peterson ZJ, Benson CJ, Snyder PM. Intersubunit conformational changes mediate epithelial sodium channel gating. ACTA ACUST UNITED AC 2014; 144:337-48. [PMID: 25225551 PMCID: PMC4178938 DOI: 10.1085/jgp.201411208] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Residues forming interfaces between the three ENaC subunits participate in conformational changes required for transition between open and closed states. The epithelial Na+ channel (ENaC) functions as a pathway for Na+ absorption in the kidney and lung, where it is crucial for Na+ homeostasis and blood pressure regulation. However, the basic mechanisms that control ENaC gating are poorly understood. Here we define a role in gating for residues forming interfaces between the extracellular domains of the three ENaC subunits. Using cysteine substitution combined with chemical cross-linking, we determined that residues located at equivalent positions in the three subunits (αK477, βE446, and γE455) form interfaces with residues in adjacent subunits (βV85, γV87, and αL120, respectively). Cross-linking of these residues altered ENaC activity in a length-dependent manner; long cross-linkers increased ENaC current by increasing its open probability, whereas short cross-linkers reduced ENaC open probability. Cross-linking also disrupted ENaC gating responses to extracellular pH and Na+, signals which modulate ENaC activity during shifts in volume status. Introduction of charged side chains at the interfacing residues altered ENaC activity in a charge-dependent manner. Current increased when like charges were present at both interfacing residues, whereas opposing charges reduced current. Together, these data indicate that conformational changes at intersubunit interfaces participate in ENaC transitions between the open and closed states; movements that increase intersubunit distance favor the open state, whereas the closed state is favored when the distance is reduced. This provides a mechanism to modulate ENaC gating in response to changing extracellular conditions that threaten Na+ homeostasis.
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Affiliation(s)
- Daniel M Collier
- Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242 Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242
| | - Vivian R Tomkovicz
- Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242 Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242
| | - Zerubbabel J Peterson
- Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242 Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242
| | - Christopher J Benson
- Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242 Iowa City VA Health Care System, Iowa City, IA 52246
| | - Peter M Snyder
- Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242 Department of Internal Medicine and Department of Molecular Physiology and Biophysics, University of Iowa Carver College of Medicine, Iowa City, IA 52242 Iowa City VA Health Care System, Iowa City, IA 52246
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25
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Molecular genetics of Liddle's syndrome. Clin Chim Acta 2014; 436:202-6. [PMID: 24882431 DOI: 10.1016/j.cca.2014.05.015] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2014] [Revised: 05/11/2014] [Accepted: 05/15/2014] [Indexed: 01/04/2023]
Abstract
Liddle's syndrome, an autosomal dominant form of monogenic hypertension, is characterized by salt-sensitive hypertension with early penetrance, hypokalemia, metabolic alkalosis, suppression of plasma rennin activity and aldosterone secretion, and a clear-cut response to epithelial sodium channel (ENaC) blockers but not spironolactone therapy. Our understanding of ENaCs and Na(+) transport defects has expanded greatly over the past two decades and provides detailed insight into the molecular basis of Liddle's syndrome. In this review, we offer an overview of recent advances in understanding the molecular genetics of Liddle's syndrome, involving mutation analysis, molecular mechanisms and genetic testing. The ENaC in the distal nephron is composed of α, β and γ subunits that share similar structures. Mutations associated with Liddle's syndrome are positioned in either β or γ subunits and disturb or truncate a conserved proline-rich sequence (i.e., PY motif), leading to constitutive activation of the ENaC. Genetic testing has made it possible to make accurate diagnoses and develop tailored therapies for mutation carriers.
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26
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Edelheit O, Ben-Shahar R, Dascal N, Hanukoglu A, Hanukoglu I. Conserved charged residues at the surface and interface of epithelial sodium channel subunits--roles in cell surface expression and the sodium self-inhibition response. FEBS J 2014; 281:2097-111. [PMID: 24571549 DOI: 10.1111/febs.12765] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2013] [Revised: 02/12/2014] [Accepted: 02/24/2014] [Indexed: 01/18/2023]
Abstract
The epithelial sodium channel (ENaC) is composed of three homologous subunits that form a triangular pyramid-shaped funnel, anchored in the membrane with a stem of six transmembrane domains. We examined the structure-function relationships of 17 conserved charged residues on the surface of the ectodomain of human γ-ENaC subunit by alanine mutagenesis and co-expression with α- and β-ENaC subunits in Xenopus oocytes. The results showed that Na(+) conductance of cells expressing these mutants can be accounted for by two parameters: (a) the ENaC density on the cell surface as measured by the fluorescence of an α-EnaC-yellow fluorescent protein hybrid and (b) the sodium self-inhibition (SSI) response that reflects the open probability of the channel (Po). Overall, the activity of all 17 mutants was correlated with surface levels of ENaC. There was no significant correlation between these parameters measured for α- and γ-ENaC subunit mutants at nine homologous positions. Thus, the functions of most of the homologous surface residues examined differ between the two subunits. Only four mutants (K328, D510, R514 and E518) significantly reduced the SSI response. The α-ENaC homologs of three of these (R350, E530 and E538) also severely affected the SSI response. The cASIC1 homologs of these (K247, E417, Q421) are located at the interface between subunits, on or about the ion pathway at the rotational symmetry axis in the center of the trimer. Thus, it is likely that these residues are involved in conformational changes that lead to channel constriction and the SSI response upon Na(+) ion flooding.
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Affiliation(s)
- Oded Edelheit
- Sackler School of Medicine, Tel-Aviv University, Israel
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Chen J, Kleyman TR, Sheng S. Deletion of α-subunit exon 11 of the epithelial Na+ channel reveals a regulatory module. Am J Physiol Renal Physiol 2014; 306:F561-7. [PMID: 24402098 DOI: 10.1152/ajprenal.00587.2013] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Epithelial Na(+) channel (ENaC) subunits (α, β, and γ) found in functional complexes are translated from mature mRNAs that are similarly processed by the inclusion of 13 canonical exons. We examined whether individual exons 3-12, encoding the large extracellular domain, are required for functional channel expression. Human ENaCs with an in-frame deletion of a single α-subunit exon were expressed in Xenopus oocytes, and their functional properties were examined by two-electrode voltage clamp. With the exception of exon 11, deletion of an individual exon eliminated channel activity. Channels lacking α-subunit exon 11 were hyperactive. Oocytes expressing this mutant exhibited fourfold greater amiloride-sensitive whole cell currents than cells expressing wild-type channels. A parallel fivefold increase in channel open probability was observed with channels lacking α-subunit exon 11. These mutant channels also exhibited a lost of Na(+) self-inhibition, whereas we found similar levels of surface expression of mutant and wild-type channels. In contrast, in-frame deletions of exon 11 from either the β- or γ-subunit led to a significant loss of channel activity, in association with a marked decrease in surface expression. Our results suggest that exon 11 within the three human ENaC genes encodes structurally homologous yet functionally diverse domains and that exon 11 in the α-subunit encodes a module that regulates channel gating.
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Affiliation(s)
- Jingxin Chen
- Renal-Electrolyte Div., Univ. of Pittsburgh, 3550 Terrace St., Pittsburgh, PA 15261.
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Zhou R, Tomkovicz VR, Butler PL, Ochoa LA, Peterson ZJ, Snyder PM. Ubiquitin-specific peptidase 8 (USP8) regulates endosomal trafficking of the epithelial Na+ channel. J Biol Chem 2013; 288:5389-97. [PMID: 23297398 DOI: 10.1074/jbc.m112.425272] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Ubiquitination plays a key role in trafficking of the epithelial Na(+) channel (ENaC). Previous work indicated that ubiquitination enhances ENaC endocytosis and sorting to lysosomes for degradation. Moreover, a defect in ubiquitination causes Liddle syndrome, an inherited form of hypertension. In this work, we identified a role for USP8 in the control of ENaC ubiquitination and trafficking. USP8 increased ENaC current in Xenopus oocytes and collecting duct epithelia and enhanced ENaC abundance at the cell surface in HEK 293 cells. This resulted from altered endocytic sorting; USP8 abolished ENaC degradation in the endocytic pathway, but it had no effect on ENaC endocytosis. USP8 interacted with ENaC, as detected by co-immunoprecipitation, and it deubiquitinated ENaC. Consistent with a functional role for deubiquitination, mutation of the cytoplasmic lysines of ENaC reduced the effect of USP8 on ENaC cell surface abundance. In contrast to USP8, USP2-45 increased ENaC surface abundance by reducing endocytosis but not degradation. Thus, USP8 and USP2-45 selectively modulate ENaC trafficking at different steps in the endocytic pathway. Together with previous work, the data indicate that the ubiquitination state of ENaC is critical for the regulation of epithelial Na(+) absorption.
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Affiliation(s)
- Ruifeng Zhou
- Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, Iowa 52242, USA
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29
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Jacquillet G, Chichger H, Unwin RJ, Shirley DG. Protease stimulation of renal sodium reabsorption in vivo by activation of the collecting duct epithelial sodium channel (ENaC). Nephrol Dial Transplant 2012; 28:839-45. [DOI: 10.1093/ndt/gfs486] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Abstract
Among the compensatory mechanisms restoring circulating blood volume after severe haemorrhage, increased vasopressin secretion enhances water permeability of distal nephron segments and stimulates Na(+) reabsorption in cortical collecting tubules via epithelial sodium channels (ENaC). The ability of vasopressin to upregulate ENaC via a cAMP-dependent mechanism in the medium to long term is well established. This study addressed the acute regulatory effect of cAMP on human ENaC (hENaC) and thus the potential role of vasopressin in the initial compensatory responses to haemorrhagic shock. The effects of raising intracellular cAMP (using 5 mmol/L isobutylmethylxanthine (IBMX) and 50 μmol/L forskolin) on wild-type and Liddle-mutated hENaC activity expressed in Xenopus oocytes and hENaC localisation in oocyte membranes were evaluated by dual-electrode voltage clamping and immunohistochemistry, respectively. After 30 min, IBMX + forskolin had stimulated amiloride-sensitive Na(+) current by 52% and increased the membrane density of Na(+) channels in oocytes expressing wild-type hENaC. These responses were prevented by 5 μmol/L brefeldin A, which blocks antegrade vesicular transport. By contrast, IBMX + forskolin had no effects in oocytes expressing Liddle-mutated hENaC. cAMP stimulated rapid, exocytotic recruitment of wild-type hENaC into Xenopus oocyte membranes, but had no effect on constitutively over-expressed Liddle-mutated hENaC. Extrapolating these findings to the early cAMP-mediated effect of vasopressin on cortical collecting tubule cells, they suggest that vasopressin rapidly mobilises ENaC to the apical membrane of cortical collecting tubule cells, but does not enhance ENaC activity once inserted into the membrane. We speculate that this stimulatory effect on Na(+) reabsorption (and hence water absorption) may contribute to the early restoration of extracellular fluid volume following severe haemorrhage.
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Kusama N, Gautam M, Harding AMS, Snyder PM, Benson CJ. Acid-sensing ion channels (ASICs) are differentially modulated by anions dependent on their subunit composition. Am J Physiol Cell Physiol 2012; 304:C89-101. [PMID: 23135698 DOI: 10.1152/ajpcell.00216.2012] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Acid-sensing ion channels (ASICs) are sodium channels gated by extracellular protons. ASIC1a channels possess intersubunit Cl(-)-binding sites in the extracellular domain, which are highly conserved between ASIC subunits. We previously found that anions modulate ASIC1a gating via these sites. Here we investigated the effect of anion substitution on native ASICs in rat sensory neurons and heterologously expressed ASIC2a and ASIC3 channels by whole cell patch clamp. Similar to ASIC1a, anions modulated the kinetics of desensitization of other ASIC channels. However, unlike ASIC1a, anions also modulated the pH dependence of activation. Moreover, the order of efficacy of different anions to modulate ASIC2a and -3 was very different from that of ASIC1a. More surprising, mutations of conserved residues that form an intersubunit Cl(-)-binding site in ASIC1a only partially abrogated the effects of anion modulation of ASIC2a and had no effect on anion modulation of ASIC3. The effects of anions on native ASICs in rat dorsal root ganglion neurons mimicked those in heterologously expressed ASIC1a/3 heteromeric channels. Our data show that anions modulate a variety of ASIC properties and are dependent on the subunit composition, and the mechanism of modulation for ASIC2a and -3 is distinct from that of ASIC1a. We speculate that modulation of ASIC gating by Cl(-) is a novel mechanism to sense shifts in extracellular fluid composition.
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Affiliation(s)
- Nobuyoshi Kusama
- Dept. of Internal Medicine, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
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Collier DM, Peterson ZJ, Blokhin IO, Benson CJ, Snyder PM. Identification of extracellular domain residues required for epithelial Na+ channel activation by acidic pH. J Biol Chem 2012; 287:40907-14. [PMID: 23060445 DOI: 10.1074/jbc.m112.417519] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
A growing body of evidence suggests that the extracellular domain of the epithelial Na(+) channel (ENaC) functions as a sensor that fine tunes channel activity in response to changes in the extracellular environment. We previously found that acidic pH increases the activity of human ENaC, which results from a decrease in Na(+) self-inhibition. In the current work, we identified extracellular domain residues responsible for this regulation. We found that rat ENaC is less sensitive to pH than human ENaC, an effect mediated in part by the γ subunit. We identified a group of seven residues in the extracellular domain of γENaC (Asp-164, Gln-165, Asp-166, Glu-292, Asp-335, His-439, and Glu-455) that, when individually mutated to Ala, decreased proton activation of ENaC. γ(E455) is conserved in βENaC (Glu-446); mutation of this residue to neutral amino acids (Ala, Cys) reduced ENaC stimulation by acidic pH, whereas reintroduction of a negative charge (by MTSES modification of Cys) restored pH regulation. Combination of the seven γENaC mutations with β(E446A) generated a channel that was not activated by acidic pH, but inhibition by alkaline pH was intact. Moreover, these mutations reduced the effect of pH on Na(+) self-inhibition. Together, the data identify eight extracellular domain residues in human β- and γENaC that are required for regulation by acidic pH.
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Affiliation(s)
- Daniel M Collier
- Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, Iowa 52242, USA
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Chen J, Winarski KL, Myerburg MM, Pitt BR, Sheng S. Probing the structural basis of Zn2+ regulation of the epithelial Na+ channel. J Biol Chem 2012; 287:35589-35598. [PMID: 22930753 DOI: 10.1074/jbc.m112.394734] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Extracellular Zn(2+) activates the epithelial Na(+) channel (ENaC) by relieving Na(+) self-inhibition. However, a biphasic Zn(2+) dose response was observed, suggesting that Zn(2+) has dual effects on the channel (i.e. activating and inhibitory). To investigate the structural basis for this biphasic effect of Zn(2+), we examined the effects of mutating the 10 extracellular His residues of mouse γENaC. Four mutations within the finger subdomain (γH193A, γH200A, γH202A, and γH239A) significantly reduced the maximal Zn(2+) activation of the channel. Whereas γH193A, γH200A, and γH202A reduced the apparent affinity of the Zn(2+) activating site, γH239A diminished Na(+) self-inhibition and thus concealed the activating effects of Zn(2+). Mutation of a His residue within the palm subdomain (γH88A) abolished the low-affinity Zn(2+) inhibitory effect. Based on structural homology with acid-sensing ion channel 1, γAsp(516) was predicted to be in close proximity to γHis(88). Ala substitution of the residue (γD516A) blunted the inhibitory effect of Zn(2+). Our results suggest that external Zn(2+) regulates ENaC activity by binding to multiple extracellular sites within the γ-subunit, including (i) a high-affinity stimulatory site within the finger subdomain involving His(193), His(200), and His(202) and (ii) a low-affinity Zn(2+) inhibitory site within the palm subdomain that includes His(88) and Asp(516).
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Affiliation(s)
- Jingxin Chen
- Department of Medicine, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Katie L Winarski
- Department of Medicine, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Mike M Myerburg
- Department of Medicine, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Bruce R Pitt
- Department of Environmental and Occupational Health, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
| | - Shaohu Sheng
- Department of Medicine, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261.
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Kashlan OB, Blobner BM, Zuzek Z, Carattino MD, Kleyman TR. Inhibitory tract traps the epithelial Na+ channel in a low activity conformation. J Biol Chem 2012; 287:20720-6. [PMID: 22511777 DOI: 10.1074/jbc.m112.358218] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Proteolysis plays an important role in the maturation and activation of epithelial Na(+) channels (ENaCs). Non-cleaved channels are inactive at high extracellular Na(+) concentrations and fully cleaved channels are constitutively active. Cleavage of the α and γ subunits at multiple sites activates the channel through the release of imbedded inhibitory tracts. Peptides derived from these released tracts are also inhibitory, likely through binding at the inhibitory tract sites. We recently reported a model of the α subunit. We have now cross-linked Cys derivatives of the inhibitory peptide to the channel, using our model to predict sites at a domain interface of the α subunit that is in proximity to the N terminus of the peptide. Furthermore, peptide inhibition was mimicked in the absence of peptide by cross-linking the channel across the domain interface. Our results suggest a dynamic domain interface that can be exploited by inhibitory peptides and provides a mechanism for peptide inhibition and proteolytic activation.
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Affiliation(s)
- Ossama B Kashlan
- Department of Medicine, Renal-Electrolyte Division, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
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Sharotri V, Collier DM, Olson DR, Zhou R, Snyder PM. Regulation of epithelial sodium channel trafficking by proprotein convertase subtilisin/kexin type 9 (PCSK9). J Biol Chem 2012; 287:19266-74. [PMID: 22493497 DOI: 10.1074/jbc.m112.363382] [Citation(s) in RCA: 73] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
The epithelial Na(+) channel (ENaC) is critical for Na(+) homeostasis and blood pressure control. Defects in its regulation cause inherited forms of hypertension and hypotension. Previous work found that ENaC gating is regulated by proteases through cleavage of the extracellular domains of the α and γ subunits. Here we tested the hypothesis that ENaC is regulated by proprotein convertase subtilisin/kexin type 9 (PCSK9), a protease that modulates the risk of cardiovascular disease. PCSK9 reduced ENaC current in Xenopus oocytes and in epithelia. This occurred through a decrease in ENaC protein at the cell surface and in the total cellular pool, an effect that did not require the catalytic activity of PCSK9. PCSK9 interacted with all three ENaC subunits and decreased their trafficking to the cell surface by increasing proteasomal degradation. In contrast to its previously reported effects on the LDL receptor, PCSK9 did not alter ENaC endocytosis or degradation of the pool of ENaC at the cell surface. These results support a role for PCSK9 in the regulation of ENaC trafficking in the biosynthetic pathway, likely by increasing endoplasmic reticulum-associated degradation. By reducing ENaC channel number, PCSK9 could modulate epithelial Na(+) absorption, a major contributor to blood pressure control.
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Affiliation(s)
- Vikas Sharotri
- Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, Iowa 52242, USA
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Abstract
The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.
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Affiliation(s)
- Alexander Staruschenko
- Department of Physiology and Kidney Disease Center, Medical College of Wisconsin, Milwaukee, Wisconsin, USA.
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Kashlan OB, Kleyman TR. Epithelial Na(+) channel regulation by cytoplasmic and extracellular factors. Exp Cell Res 2012; 318:1011-9. [PMID: 22405998 DOI: 10.1016/j.yexcr.2012.02.024] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2012] [Accepted: 02/24/2012] [Indexed: 11/30/2022]
Abstract
Electrogenic Na(+) transport across high resistance epithelial is mediated by the epithelial Na(+) channel (ENaC). Our understanding of the mechanisms of ENaC regulation has continued to evolve over the two decades following the cloning of ENaC subunits. This review highlights many of the cellular and extracellular factors that regulate channel trafficking or gating.
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Affiliation(s)
- Ossama B Kashlan
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, PA, USA.
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Structural mechanisms underlying the function of epithelial sodium channel/acid-sensing ion channel. Curr Opin Nephrol Hypertens 2011; 20:555-60. [PMID: 21709553 DOI: 10.1097/mnh.0b013e328348bcac] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
PURPOSE OF REVIEW The epithelial sodium channel/degenerin family encompasses a group of cation-selective ion channels that are activated or modulated by a variety of extracellular stimuli. This review describes findings that provide new insights into the molecular mechanisms that control the function of these channels. RECENT FINDINGS Epithelial sodium channels facilitate Na⁺ reabsorption in the distal nephron and hence have a role in fluid volume homeostasis and arterial blood pressure regulation. Acid-sensing ion channels are broadly distributed in the nervous system where they contribute to the sensory processes. The atomic structure of acid-sensing ion channel 1 illustrates the complex trimeric architecture of these proteins. Each subunit has two transmembrane spanning helices, a highly organized ectodomain and intracellular N-terminus and C-terminus. Recent findings have begun to elucidate the structural elements that allow these channels to sense and respond to extracellular factors. This review emphasizes the roles of the extracellular domain in sensing changes in the extracellular milieu and of the residues in the extracellular-transmembrane domains interface in coupling extracellular changes to the pore of the channel. SUMMARY Epithelial sodium channels and acid-sensing ion channels have evolved to sense extracellular cues. Future research should be directed toward elucidating how changes triggered by extracellular factors translate into pore opening and closing events.
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Stewart AP, Haerteis S, Diakov A, Korbmacher C, Edwardson JM. Atomic force microscopy reveals the architecture of the epithelial sodium channel (ENaC). J Biol Chem 2011; 286:31944-52. [PMID: 21775436 DOI: 10.1074/jbc.m111.275289] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The epithelial sodium channel (ENaC) is a member of the ENaC/degenerin superfamily. ENaC is a heteromultimer containing three homologous subunits (α, β, and γ); however, the subunit stoichiometry is still controversial. Here, we addressed this issue using atomic force microscopy imaging of complexes between isolated ENaC and antibodies/Fab fragments directed against specific epitope tags on the α-, β- and γ-subunits. We show that for α-, β- and γ-ENaC alone, pairs of antibodies decorate the channel at an angle of 120°, indicating that the individual subunits assemble as homotrimers. A similar approach demonstrates that αβγ-ENaC assembles as a heterotrimer containing one copy of each subunit. Intriguingly, all four subunit combinations also produce higher-order structures containing two or three individual trimers. The trimer-of-trimers organization would account for earlier reports that ENaC contains eight to nine subunits.
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Affiliation(s)
- Andrew P Stewart
- Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, United Kingdom
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Kashlan OB, Kleyman TR. ENaC structure and function in the wake of a resolved structure of a family member. Am J Physiol Renal Physiol 2011; 301:F684-96. [PMID: 21753073 DOI: 10.1152/ajprenal.00259.2011] [Citation(s) in RCA: 107] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Our understanding of epithelial Na(+) channel (ENaC) structure and function has been profoundly impacted by the resolved structure of the homologous acid-sensing ion channel 1 (ASIC1). The structure of the extracellular and pore regions provide insight into channel assembly, processing, and the ability of these channels to sense the external environment. The absence of intracellular structures precludes insight into important interactions with intracellular factors that regulate trafficking and function. The primary sequences of ASIC1 and ENaC subunits are well conserved within the regions that are within or in close proximity to the plasma membrane, but poorly conserved in peripheral domains that may functionally differentiate family members. This review examines functional data, including ion selectivity, gating, and amiloride block, in light of the resolved ASIC1 structure.
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Affiliation(s)
- Ossama B Kashlan
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
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Chen J, Myerburg MM, Passero CJ, Winarski KL, Sheng S. External Cu2+ inhibits human epithelial Na+ channels by binding at a subunit interface of extracellular domains. J Biol Chem 2011; 286:27436-46. [PMID: 21659509 DOI: 10.1074/jbc.m111.232058] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Epithelial Na(+) channels (ENaCs) play an essential role in the regulation of body fluid homeostasis. Certain transition metals activate or inhibit the activity of ENaCs. In this study, we examined the effect of extracellular Cu(2+) on human ENaC expressed in Xenopus oocytes and investigated the structural basis for its effects. External Cu(2+) inhibited human αβγ ENaC with an estimated IC(50) of 0.3 μM. The slow time course and a lack of change in the current-voltage relationship were consistent with an allosteric (non pore-plugging) inhibition of human ENaC by Cu(2+). Experiments with mixed human and mouse ENaC subunits suggested that both the α and β subunits were primarily responsible for the inhibitory effect of Cu(2+) on human ENaC. Lowering bath solution pH diminished the inhibition by Cu(2+). Mutations of two α, two β, and two γ His residues within extracellular domains significantly reduced the inhibition of human ENaC by Cu(2+). We identified a pair of residues as potential Cu(2+)-binding sites at the subunit interface between thumb subdomain of αhENaC and palm subdomain of βhENaC, suggesting a counterclockwise arrangement of α, β, and γ ENaC subunits in a trimeric channel complex when viewed from above. We conclude that extracellular Cu(2+) is a potent inhibitor of human ENaC and binds to multiple sites within the extracellular domains including a subunit interface.
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Affiliation(s)
- Jingxin Chen
- Department of Medicine, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
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