1
|
Mikami Y, Grubb BR, Rogers TD, Dang H, Asakura T, Kota P, Gilmore RC, Okuda K, Morton LC, Sun L, Chen G, Wykoff JA, Ehre C, Vilar J, van Heusden C, Livraghi-Butrico A, Gentzsch M, Button B, Stutts MJ, Randell SH, O’Neal WK, Boucher RC. Chronic airway epithelial hypoxia exacerbates injury in muco-obstructive lung disease through mucus hyperconcentration. Sci Transl Med 2023; 15:eabo7728. [PMID: 37285404 PMCID: PMC10664029 DOI: 10.1126/scitranslmed.abo7728] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2022] [Accepted: 05/17/2023] [Indexed: 06/09/2023]
Abstract
Unlike solid organs, human airway epithelia derive their oxygen from inspired air rather than the vasculature. Many pulmonary diseases are associated with intraluminal airway obstruction caused by aspirated foreign bodies, virus infection, tumors, or mucus plugs intrinsic to airway disease, including cystic fibrosis (CF). Consistent with requirements for luminal O2, airway epithelia surrounding mucus plugs in chronic obstructive pulmonary disease (COPD) lungs are hypoxic. Despite these observations, the effects of chronic hypoxia (CH) on airway epithelial host defense functions relevant to pulmonary disease have not been investigated. Molecular characterization of resected human lungs from individuals with a spectrum of muco-obstructive lung diseases (MOLDs) or COVID-19 identified molecular features of chronic hypoxia, including increased EGLN3 expression, in epithelia lining mucus-obstructed airways. In vitro experiments using cultured chronically hypoxic airway epithelia revealed conversion to a glycolytic metabolic state with maintenance of cellular architecture. Chronically hypoxic airway epithelia unexpectedly exhibited increased MUC5B mucin production and increased transepithelial Na+ and fluid absorption mediated by HIF1α/HIF2α-dependent up-regulation of β and γENaC (epithelial Na+ channel) subunit expression. The combination of increased Na+ absorption and MUC5B production generated hyperconcentrated mucus predicted to perpetuate obstruction. Single-cell and bulk RNA sequencing analyses of chronically hypoxic cultured airway epithelia revealed transcriptional changes involved in airway wall remodeling, destruction, and angiogenesis. These results were confirmed by RNA-in situ hybridization studies of lungs from individuals with MOLD. Our data suggest that chronic airway epithelial hypoxia may be central to the pathogenesis of persistent mucus accumulation in MOLDs and associated airway wall damage.
Collapse
Affiliation(s)
- Yu Mikami
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Barbara R. Grubb
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Troy D. Rogers
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Hong Dang
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Takanori Asakura
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Pradeep Kota
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Rodney C. Gilmore
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Kenichi Okuda
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Lisa C. Morton
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Ling Sun
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Gang Chen
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Jason A. Wykoff
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Camille Ehre
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
- Department of Pediatrics, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
- Department of Microbiology and Immunology, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Juan Vilar
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Catharina van Heusden
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | | | - Martina Gentzsch
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Brian Button
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - M. Jackson Stutts
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Scott H. Randell
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Wanda K. O’Neal
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Richard C. Boucher
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA
| |
Collapse
|
2
|
Garcia-Caballero A, Gandini MA, Huang S, Chen L, Souza IA, Dang YL, Stutts MJ, Zamponi GW. Cav3.2 calcium channel interactions with the epithelial sodium channel ENaC. Mol Brain 2019; 12:12. [PMID: 30736831 PMCID: PMC6368719 DOI: 10.1186/s13041-019-0433-8] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2018] [Accepted: 02/04/2019] [Indexed: 11/10/2022] Open
Abstract
This study describes the functional interaction between Cav3.2 calcium channels and the Epithelial Sodium Channel (ENaC). β-ENaC subunits showed overlapping expression with endogenous Cav3.2 calcium channels in the thalamus and hypothalamus as detected by immunostaining. Moreover, β- and γ-ENaC subunits could be co-immunoprecipitated with Cav3.2 calcium channels from brain lysates, dorsal horn and lumbar dorsal root ganglia. Mutation of a cluster of lysines present in the intracellular N-terminus region of β-ENaC (K4R/ K5R/ K9R/ K16R/ K23R) reduced interactions with Cav3.2 calcium channels. Αβγ-ENaC channels enhanced Cav3.2 calcium channel trafficking to the plasma membrane in tsA-201 cells. This effect was reciprocal such that Cav3.2 channel expression also enhanced β-ENaC trafficking to the cell surface. T-type current density was increased when fully assembled αβγ-ENaC channels were transiently expressed in CAD cells, a neuronal derived cell line. Altogether, these findings reveal ENaC as an interactor and potential regulator of Cav3.2 calcium channels expressed in neuronal tissues.
Collapse
Affiliation(s)
- Agustin Garcia-Caballero
- Molecular Neuroscience, Department of Physiology and Pharmacology, Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Cumming School of Medicine, University of Calgary, 3330 Hospital Dr. NW, Calgary, T2N 4N1, Canada
| | - Maria A Gandini
- Molecular Neuroscience, Department of Physiology and Pharmacology, Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Cumming School of Medicine, University of Calgary, 3330 Hospital Dr. NW, Calgary, T2N 4N1, Canada
| | - Shuo Huang
- Molecular Neuroscience, Department of Physiology and Pharmacology, Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Cumming School of Medicine, University of Calgary, 3330 Hospital Dr. NW, Calgary, T2N 4N1, Canada
| | - Lina Chen
- Molecular Neuroscience, Department of Physiology and Pharmacology, Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Cumming School of Medicine, University of Calgary, 3330 Hospital Dr. NW, Calgary, T2N 4N1, Canada
| | - Ivana A Souza
- Molecular Neuroscience, Department of Physiology and Pharmacology, Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Cumming School of Medicine, University of Calgary, 3330 Hospital Dr. NW, Calgary, T2N 4N1, Canada
| | - Yan L Dang
- Cystic Fibrosis Center, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
| | - M Jackson Stutts
- Cystic Fibrosis Center, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
| | - Gerald W Zamponi
- Molecular Neuroscience, Department of Physiology and Pharmacology, Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Cumming School of Medicine, University of Calgary, 3330 Hospital Dr. NW, Calgary, T2N 4N1, Canada.
| |
Collapse
|
3
|
Kota P, Gentzsch M, Dang YL, Boucher RC, Stutts MJ. The N terminus of α-ENaC mediates ENaC cleavage and activation by furin. J Gen Physiol 2018; 150:1179-1187. [PMID: 29980634 PMCID: PMC6080898 DOI: 10.1085/jgp.201711860] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2017] [Revised: 03/05/2018] [Accepted: 05/28/2018] [Indexed: 12/23/2022] Open
Abstract
Epithelial Na+ channels comprise three homologous subunits (α, β, and γ) that are regulated by alternative splicing and proteolytic cleavage. Here, we determine the basis of the reduced Na+ current (INa) that results from expression of a previously identified, naturally occurring splice variant of the α subunit (α-ENaC), in which residues 34-82 are deleted (αΔ34-82). αΔ34-82-ENaC expression with WT β and γ subunits in Xenopus oocytes produces reduced basal INa, which can largely be recovered by exogenous trypsin. With this αΔ34-82-containing ENaC, both α and γ subunits display decreased cleavage fragments, consistent with reduced processing by furin or furin-like convertases. Data using MTSET modification of a cysteine, introduced into the degenerin locus in β-ENaC, suggest that the reduced INa of αΔ34-82-ENaC arises from an increased population of uncleaved, near-silent ENaC, rather than from a reduced open probability spread uniformly across all channels. After treatment with brefeldin A to disrupt anterograde trafficking of channel subunits, INa in oocytes expressing αΔ34-82-ENaC is reestablished more slowly than that in oocytes expressing WT ENaC. Overnight or acute incubation of oocytes expressing WT ENaC in the pore blocker amiloride increases basal ENaC proteolytic stimulation, consistent with relief of Na+ feedback inhibition. These responses are reduced in oocytes expressing αΔ34-82-ENaC. We conclude that the α-ENaC N terminus mediates interactions that govern the delivery of cleaved and uncleaved ENaC populations to the oocyte membrane.
Collapse
Affiliation(s)
- Pradeep Kota
- Marsico Lung Institute, Cystic Fibrosis Center and Department of Medicine, University of North Carolina, Chapel Hill, NC
| | - Martina Gentzsch
- Marsico Lung Institute, Cystic Fibrosis Center and Department of Medicine, University of North Carolina, Chapel Hill, NC
- Department of Cell Biology and Physiology, University of North Carolina, Chapel Hill, NC
| | - Yan L Dang
- Marsico Lung Institute, Cystic Fibrosis Center and Department of Medicine, University of North Carolina, Chapel Hill, NC
| | - Richard C Boucher
- Marsico Lung Institute, Cystic Fibrosis Center and Department of Medicine, University of North Carolina, Chapel Hill, NC
| | - M Jackson Stutts
- Marsico Lung Institute, Cystic Fibrosis Center and Department of Medicine, University of North Carolina, Chapel Hill, NC
| |
Collapse
|
4
|
Shobair M, Popov KI, Dang YL, He H, Stutts MJ, Dokholyan NV. Mapping allosteric linkage to channel gating by extracellular domains in the human epithelial sodium channel. J Biol Chem 2018; 293:3675-3684. [PMID: 29358325 DOI: 10.1074/jbc.ra117.000604] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2017] [Revised: 01/08/2018] [Indexed: 11/06/2022] Open
Abstract
The epithelial sodium channel (ENaC) mediates sodium absorption in lung, kidney, and colon epithelia. Channels in the ENaC/degenerin family possess an extracellular region that senses physicochemical changes in the extracellular milieu and allosterically regulates the channel opening. Proteolytic cleavage activates the ENaC opening, by the removal of specific segments in the finger domains of the α- and γ ENaC-subunits. Cleavage causes perturbations in the extracellular region that propagate to the channel gate. However, it is not known how the channel structure mediates the propagation of activation signals through the extracellular sensing domains. Here, to identify the structure-function determinants that mediate allosteric ENaC activation, we performed MD simulations, thiol modification of residues substituted by cysteine, and voltage-clamp electrophysiology recordings. Our simulations of an ENaC heterotetramer, α1βα2γ, in the proteolytically cleaved and uncleaved states revealed structural pathways in the α-subunit that are responsible for ENaC proteolytic activation. To validate these findings, we performed site-directed mutagenesis to introduce cysteine substitutions in the extracellular domains of the α-, β-, and γ ENaC-subunits. Insertion of a cysteine at the α-subunit Glu557 site, predicted to stabilize a closed state of ENaC, inhibited ENaC basal activity and retarded the kinetics of proteolytic activation by 2-fold. Our results suggest that the lower palm domain of αENaC is essential for ENaC activation. In conclusion, our integrated computational and experimental approach suggests key structure-function determinants for ENaC proteolytic activation and points toward a mechanistic model for the allosteric communication in the extracellular domains of the ENaC/degenerin family channels.
Collapse
Affiliation(s)
- Mahmoud Shobair
- From the Program in Molecular and Cellular Biophysics.,Curriculum in Bioinformatics and Computational Biology.,Department of Biochemistry and Biophysics, and.,Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | | | - Yan L Dang
- Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - Hong He
- Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - M Jackson Stutts
- Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - Nikolay V Dokholyan
- From the Program in Molecular and Cellular Biophysics, .,Curriculum in Bioinformatics and Computational Biology.,Department of Biochemistry and Biophysics, and.,Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| |
Collapse
|
5
|
Reihill JA, Walker B, Hamilton RA, Ferguson TEG, Elborn JS, Stutts MJ, Harvey BJ, Saint-Criq V, Hendrick SM, Martin SL. Inhibition of Protease-Epithelial Sodium Channel Signaling Improves Mucociliary Function in Cystic Fibrosis Airways. Am J Respir Crit Care Med 2017; 194:701-10. [PMID: 27014936 DOI: 10.1164/rccm.201511-2216oc] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
RATIONALE In cystic fibrosis (CF) a reduction in airway surface liquid (ASL) height compromises mucociliary clearance, favoring mucus plugging and chronic bacterial infection. Inhibitors of the epithelial sodium channel (ENaC) have therapeutic potential in CF airways to reduce hyperstimulated sodium and fluid absorption to levels that can restore airway hydration. OBJECTIVES To determine whether a novel compound (QUB-TL1) designed to inhibit protease/ENaC signaling in CF airways restores ASL volume and mucociliary function. METHODS Protease activity was measured using fluorogenic activity assays. Differentiated primary airway epithelial cell cultures (F508del homozygotes) were used to determined ENaC activity (Ussing chamber recordings), ASL height (confocal microscopy), and mucociliary function (by tracking the surface flow of apically applied microbeads). Cell toxicity was measured using a lactate dehydrogenase assay. MEASUREMENTS AND MAIN RESULTS QUB-TL1 inhibits extracellularly located channel activating proteases (CAPs), including prostasin, matriptase, and furin, the activities of which are observed at excessive levels at the apical surface of CF airway epithelial cells. QUB-TL1-mediated CAP inhibition results in diminished ENaC-mediated Na(+) absorption in CF airway epithelial cells caused by internalization of a prominent pool of cleaved (active) ENaCγ from the cell surface. Importantly, diminished ENaC activity correlates with improved airway hydration status and mucociliary clearance. We further demonstrate QUB-TL1-mediated furin inhibition, which is in contrast to other serine protease inhibitors (camostat mesylate and aprotinin), affords protection against neutrophil elastase-mediated ENaC activation and Pseudomonas aeruginosa exotoxin A-induced cell death. CONCLUSIONS QUB-TL1 corrects aberrant CAP activities, providing a mechanism to delay or prevent the development of CF lung disease in a manner independent of CF transmembrane conductance regulator mutation.
Collapse
Affiliation(s)
- James A Reihill
- 1 Biomolecular Sciences Research Group, School of Pharmacy, and
| | - Brian Walker
- 1 Biomolecular Sciences Research Group, School of Pharmacy, and
| | | | | | - J Stuart Elborn
- 2 School of Medicine, Dentistry & Biomedical Sciences, Queen's University, Belfast, Northern Ireland, United Kingdom
| | - M Jackson Stutts
- 3 Marsico Lung Institute and Cystic Fibrosis Center, University of North Carolina, Chapel Hill, North Carolina; and
| | - Brian J Harvey
- 4 Department of Molecular Medicine, Royal College of Surgeons in Ireland, RCSI-ERC Beaumont Hospital, Dublin, Ireland
| | - Vinciane Saint-Criq
- 4 Department of Molecular Medicine, Royal College of Surgeons in Ireland, RCSI-ERC Beaumont Hospital, Dublin, Ireland
| | - Siobhan M Hendrick
- 4 Department of Molecular Medicine, Royal College of Surgeons in Ireland, RCSI-ERC Beaumont Hospital, Dublin, Ireland
| | | |
Collapse
|
6
|
Shobair M, Dagliyan O, Kota P, Dang YL, He H, Stutts MJ, Dokholyan NV. Gain-of-Function Mutation W493R in the Epithelial Sodium Channel Allosterically Reconfigures Intersubunit Coupling. J Biol Chem 2015; 291:3682-92. [PMID: 26668308 DOI: 10.1074/jbc.m115.678052] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2015] [Indexed: 12/21/2022] Open
Abstract
Sodium absorption in epithelial cells is rate-limited by the epithelial sodium channel (ENaC) activity in lung, kidney, and the distal colon. Pathophysiological conditions, such as cystic fibrosis and Liddle syndrome, result from water-electrolyte imbalance partly due to malfunction of ENaC regulation. Because the quaternary structure of ENaC is yet undetermined, the bases of pathologically linked mutations in ENaC subunits α, β, and γ are largely unknown. Here, we present a structural model of heterotetrameric ENaC α1βα2γ that is consistent with previous cross-linking results and site-directed mutagenesis experiments. By using this model, we show that the disease-causing mutation αW493R rewires structural dynamics of the intersubunit interfaces α1β and α2γ. Changes in dynamics can allosterically propagate to the channel gate. We demonstrate that cleavage of the γ-subunit, which is critical for full channel activation, does not mediate activation of ENaC by αW493R. Our molecular dynamics simulations led us to identify a channel-activating electrostatic interaction between α2Arg-493 and γGlu-348 at the α2γ interface. By neutralizing a sodium-binding acidic patch at the α1β interface, we reduced ENaC activation of αW493R by more than 2-fold. By combining homology modeling, molecular dynamics, cysteine cross-linking, and voltage clamp experiments, we propose a dynamics-driven model for the gain-of-function in ENaC by αW493R. Our integrated computational and experimental approach advances our understanding of structure, dynamics, and function of ENaC in its disease-causing state.
Collapse
Affiliation(s)
- Mahmoud Shobair
- From the Program in Molecular and Cellular Biophysics, Curriculum in Bioinformatics and Computational Biology, Department of Biochemistry and Biophysics, and
| | - Onur Dagliyan
- From the Program in Molecular and Cellular Biophysics, Department of Biochemistry and Biophysics, and
| | - Pradeep Kota
- From the Program in Molecular and Cellular Biophysics, Department of Biochemistry and Biophysics, and
| | - Yan L Dang
- Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - Hong He
- Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - M Jackson Stutts
- Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - Nikolay V Dokholyan
- From the Program in Molecular and Cellular Biophysics, Curriculum in Bioinformatics and Computational Biology, Department of Biochemistry and Biophysics, and Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| |
Collapse
|
7
|
Tan CD, Hobbs C, Sameni M, Sloane BF, Stutts MJ, Tarran R. Cathepsin B contributes to Na+ hyperabsorption in cystic fibrosis airway epithelial cultures. J Physiol 2014; 592:5251-68. [PMID: 25260629 DOI: 10.1113/jphysiol.2013.267286] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
In cystic fibrosis (CF) lung disease, the absence of functional CF transmembrane conductance regulator results in Cl(-)/HCO3 (-) hyposecretion and triggers Na(+) hyperabsorption through the epithelial Na(+) channel (ENaC), which contribute to reduced airway surface liquid (ASL) pH and volume. Prostasin, a membrane-anchored serine protease with trypsin-like substrate specificity has previously been shown to activate ENaC in CF airways. However, prostasin is typically inactive below pH 7.0, suggesting that it may be less relevant in acidic CF airways. Cathepsin B (CTSB) is present in both normal and CF epithelia and is secreted into ASL, but little is known about its function in the airways. We hypothesized that the acidic ASL seen in CF airways may stimulate CTSB to activate ENaC, contributing to Na(+) hyperabsorption and depletion of CF ASL volume. In Xenopus laevis oocytes, CTSB triggered α- and γENaC cleavage and induced an increase in ENaC activity. In bronchial epithelia from both normal and CF donor lungs, CTSB localized to the apical membrane. In normal and CF human bronchial epithelial cultures, CTSB was detected at the apical plasma membrane and in the ASL. CTSB activity was significantly elevated in acidic ASL, which correlated with increased abundance of ENaC in the plasma membrane and a reduction in ASL volume. This acid/CTSB-dependent activation of ENaC was ameliorated with the cell impermeable, CTSB-selective inhibitor CA074, suggesting that CTSB inhibition may have therapeutic relevance. Taken together, our data suggest that CTSB is a pathophysiologically relevant protease that activates ENaC in CF airways.
Collapse
Affiliation(s)
- Chong Da Tan
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC, USA
| | - Carey Hobbs
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC, USA
| | - Mansoureh Sameni
- Department of Pharmacology, Wayne State University, School of Medicine, Detroit, MI, USA
| | - Bonnie F Sloane
- Department of Pharmacology, Wayne State University, School of Medicine, Detroit, MI, USA
| | - M Jackson Stutts
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC, USA
| | - Robert Tarran
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC, USA
| |
Collapse
|
8
|
Kota P, Buchner G, Chakraborty H, Dang YL, He H, Garcia GJM, Kubelka J, Gentzsch M, Stutts MJ, Dokholyan NV. The N-terminal domain allosterically regulates cleavage and activation of the epithelial sodium channel. J Biol Chem 2014; 289:23029-23042. [PMID: 24973914 DOI: 10.1074/jbc.m114.570952] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
The epithelial sodium channel (ENaC) is activated upon endoproteolytic cleavage of specific segments in the extracellular domains of the α- and γ-subunits. Cleavage is accomplished by intracellular proteases prior to membrane insertion and by surface-expressed or extracellular soluble proteases once ENaC resides at the cell surface. These cleavage events are partially regulated by intracellular signaling through an unknown allosteric mechanism. Here, using a combination of computational and experimental techniques, we show that the intracellular N terminus of γ-ENaC undergoes secondary structural transitions upon interaction with phosphoinositides. From ab initio folding simulations of the N termini in the presence and absence of phosphatidylinositol 4,5-bisphosphate (PIP2), we found that PIP2 increases α-helical propensity in the N terminus of γ-ENaC. Electrophysiology and mutation experiments revealed that a highly conserved cluster of lysines in the γ-ENaC N terminus regulates accessibility of extracellular cleavage sites in γ-ENaC. We also show that conditions that decrease PIP2 or enhance ubiquitination sharply limit access of the γ-ENaC extracellular domain to proteases. Further, the efficiency of allosteric control of ENaC proteolysis is dependent on Tyr(370) in γ-ENaC. Our findings provide an allosteric mechanism for ENaC activation regulated by the N termini and sheds light on a potential general mechanism of channel and receptor activation.
Collapse
Affiliation(s)
- Pradeep Kota
- Departments of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599; Departments of Molecular and Cellular Biophysics, and University of North Carolina, Chapel Hill, North Carolina 27599
| | - Ginka Buchner
- Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, and
| | - Hirak Chakraborty
- Departments of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599
| | - Yan L Dang
- Departments of Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599
| | - Hong He
- Departments of Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599
| | - Guilherme J M Garcia
- Biotechnology & Bioengineering Center, Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, Milwaukee, Wisconsin 53226
| | - Jan Kubelka
- Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, and
| | - Martina Gentzsch
- Departments of Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599,; Departments of Cell Biology and Physiology and University of North Carolina, Chapel Hill, North Carolina 27599
| | - M Jackson Stutts
- Departments of Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599
| | - Nikolay V Dokholyan
- Departments of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599; Departments of Molecular and Cellular Biophysics, and University of North Carolina, Chapel Hill, North Carolina 27599; Departments of Cystic Fibrosis and Pulmonary Diseases Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599,.
| |
Collapse
|
9
|
Hobbs CA, Blanchard MG, Alijevic O, Tan CD, Kellenberger S, Bencharit S, Cao R, Kesimer M, Walton WG, Henderson AG, Redinbo MR, Stutts MJ, Tarran R. Identification of the SPLUNC1 ENaC-inhibitory domain yields novel strategies to treat sodium hyperabsorption in cystic fibrosis airway epithelial cultures. Am J Physiol Lung Cell Mol Physiol 2013; 305:L990-L1001. [PMID: 24124190 DOI: 10.1152/ajplung.00103.2013] [Citation(s) in RCA: 64] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The epithelial sodium channel (ENaC) is responsible for Na(+) and fluid absorption across colon, kidney, and airway epithelia. Short palate lung and nasal epithelial clone 1 (SPLUNC1) is a secreted, innate defense protein and an autocrine inhibitor of ENaC that is highly expressed in airway epithelia. While SPLUNC1 has a bactericidal permeability-increasing protein (BPI)-type structure, its NH2-terminal region lacks structure. Here we found that an 18 amino acid peptide, S18, which corresponded to residues G22-A39 of the SPLUNC1 NH2 terminus inhibited ENaC activity to a similar degree as full-length SPLUNC1 (∼2.5 fold), while SPLUNC1 protein lacking this region was without effect. S18 did not inhibit the structurally related acid-sensing ion channels, indicating specificity for ENaC. However, S18 preferentially bound to the βENaC subunit in a glycosylation-dependent manner. ENaC hyperactivity is contributory to cystic fibrosis (CF) lung disease. Unlike control, CF human bronchial epithelial cultures (HBECs) where airway surface liquid (ASL) height was abnormally low (4.2 ± 0.6 μm), addition of S18 prevented ENaC-led ASL hyperabsorption and maintained CF ASL height at 7.9 ± 0.6 μm, even in the presence of neutrophil elastase, which is comparable to heights seen in normal HBECs. Our data also indicate that the ENaC inhibitory domain of SPLUNC1 may be cleaved away from the main molecule by neutrophil elastase, suggesting that it may still be active during inflammation or neutrophilia. Furthermore, the robust inhibition of ENaC by the S18 peptide suggests that this peptide may be suitable for treating CF lung disease.
Collapse
Affiliation(s)
- Carey A Hobbs
- Cystic Fibrosis/Pulmonary Research and Treatment Center, 7125 Thurston Bowles Bldg., UNC, Chapel Hill, NC 27599-7248.
| | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
10
|
Tarran R, Sabater JR, Clarke TC, Tan CD, Davies CM, Liu J, Yeung A, Garland AL, Stutts MJ, Abraham WM, Phillips G, Baker WR, Wright CD, Wilbert S. Nonantibiotic macrolides prevent human neutrophil elastase-induced mucus stasis and airway surface liquid volume depletion. Am J Physiol Lung Cell Mol Physiol 2013; 304:L746-56. [PMID: 23542952 DOI: 10.1152/ajplung.00292.2012] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Mucus clearance is an important component of the lung's innate defense system. A failure of this system brought on by mucus dehydration is common to both cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). Mucus clearance rates are regulated by the volume of airway surface liquid (ASL) and by ciliary beat frequency (CBF). Chronic treatment with macrolide antibiotics is known to be beneficial to both CF and COPD patients. However, chronic macrolide usage may induce bacterial resistance. We have developed a novel macrolide, 2'-desoxy-9-(S)-erythromycylamine (GS-459755), that has significantly diminished antibiotic activity against Staphylococcus aureus, Streptococcus pneumonia, Moraxella catarrhalis, and Haemophilus influenzae. Since neutrophilia frequently occurs in chronic lung disease and human neutrophil elastase (HNE) induces mucus stasis by activating the epithelial sodium channel (ENaC), we tested the ability of GS-459755 to protect against HNE-induced mucus stasis. GS-459755 had no effect on HNE activity. However, GS-459755 pretreatment protected against HNE-induced ASL volume depletion in human bronchial epithelial cells (HBECs). The effect of GS-459755 on ASL volume was dose dependent (IC₅₀ ~3.9 μM) and comparable to the antibacterial macrolide azithromycin (IC₅₀ ~2.4 μM). Macrolides had no significant effect on CBF or on transepithelial water permeability. However, the amiloride-sensitive transepithelial voltage, a marker of ENaC activity, was diminished by macrolide pretreatment. We conclude that GS-459755 may limit HNE-induced activation of ENaC and may be useful for the treatment of mucus dehydration in CF and COPD without inducing bacterial resistance.
Collapse
Affiliation(s)
- Robert Tarran
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27516, USA.
| | | | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
11
|
Abstract
Salt and fluid absorption is a shared function of many of the body's epithelia, but its use is highly adapted to the varied physiological roles of epithelia-lined organs. These functions vary from control of hydration of outward-facing epithelial surfaces to conservation and regulation of total body volume. In the most general context, salt and fluid absorption is driven by active Na(+) absorption. Cl(-) is absorbed passively through various available paths in response to the electrical driving force that results from active Na(+) absorption. Absorption of salt creates a concentration gradient that causes water to be absorbed passively, provided the epithelium is water permeable. Key differences notwithstanding, the transport elements used for salt and fluid absorption are broadly similar in diverse epithelia, but the regulation of these elements enables salt absorption to be tailored to very different physiological needs. Here we focus on salt absorption by exocrine glands and airway epithelia. In cystic fibrosis, salt and fluid absorption by gland duct epithelia is effectively prevented by the loss of cystic fibrosis transmembrane conductance regulator (CFTR). In airway epithelia, salt and fluid absorption persists, in the absence of CFTR-mediated Cl(-) secretion. The contrast of these tissue-specific changes in CF tissues is illustrative of how salt and fluid absorption is differentially regulated to accomplish tissue-specific physiological objectives.
Collapse
Affiliation(s)
- M M Reddy
- Department of Pediatrics, UCSD School of Medicine, University of California, San Diego, La Jolla, CA 92093, USA
| | | |
Collapse
|
12
|
Yu D, Thelin WR, Rogers TD, Stutts MJ, Randell SH, Grubb BR, Boucher RC. Regional differences in rat conjunctival ion transport activities. Am J Physiol Cell Physiol 2012; 303:C767-80. [PMID: 22814399 DOI: 10.1152/ajpcell.00195.2012] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Active ion transport and coupled osmotic water flow are essential to maintain ocular surface health. We investigated regional differences in the ion transport activities of the rat conjunctivas and compared these activities with those of cornea and lacrimal gland. The epithelial sodium channel (ENaC), sodium/glucose cotransporter 1 (Slc5a1), transmembrane protein 16 (Tmem16a, b, f, and g), cystic fibrosis transmembrane conductance regulator (Cftr), and mucin (Muc4, 5ac, and 5b) mRNA expression was characterized by RT-PCR. ENaC proteins were measured by Western blot. Prespecified regions (palpebral, fornical, and bulbar) of freshly isolated conjunctival tissues and cell cultures were studied electrophysiologically with Ussing chambers. The transepithelial electrical potential difference (PD) of the ocular surface was also measured in vivo. The effect of amiloride and UTP on the tear volume was evaluated in lacrimal gland excised rats. All selected genes were detected but with different expression patterns. We detected αENaC protein in all tissues, βENaC in palpebral and fornical conjunctiva, and γENaC in all tissues except lacrimal glands. Electrophysiological studies of conjunctival tissues and cell cultures identified functional ENaC, SLC5A1, CFTR, and TMEM16. Fornical conjunctiva exhibited the most active ion transport under basal conditions amongst conjunctival regions. PD measurements confirmed functional ENaC-mediated Na(+) transport on the ocular surface. Amiloride and UTP increased tear volume in lacrimal gland excised rats. This study demonstrated that the different regions of the conjunctiva exhibited a spectrum of ion transport activities. Understanding the specific functions of distinct regions of the conjunctiva may foster a better understanding of the physiology maintaining hydration of the ocular surface.
Collapse
Affiliation(s)
- Dongfang Yu
- Cystic Fibrosis/Pulmonary Research and Treatment Center. Univ. of North Carolina, Chapel Hill, Chapel Hill, NC 27599, USA
| | | | | | | | | | | | | |
Collapse
|
13
|
Hobbs CA, Blanchard MG, Kellenberger S, Bencharit S, Cao R, Kesimer M, Walton WG, Redinbo MR, Stutts MJ, Tarran R. Identification of SPLUNC1's ENaC-inhibitory domain yields novel strategies to treat sodium hyperabsorption in cystic fibrosis airways. FASEB J 2012; 26:4348-59. [PMID: 22798424 DOI: 10.1096/fj.12-207431] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
The epithelial sodium channel (ENaC) is responsible for Na+ and fluid absorption across colon, kidney, and airway epithelia. We have previously identified SPLUNC1 as an autocrine inhibitor of ENaC. We have now located the ENaC inhibitory domain of SPLUNC1 to SPLUNC1's N terminus, and a peptide corresponding to this domain, G22-A39, inhibited ENaC activity to a similar degree as full-length SPLUNC1 (∼2.5 fold). However, G22-A39 had no effect on the structurally related acid-sensing ion channels, indicating specificity for ENaC. G22-A39 preferentially bound to the β-ENaC subunit in a glycosylation-dependent manner. ENaC hyperactivity is contributory to cystic fibrosis (CF) lung disease. Addition of G22-A39 to CF human bronchial epithelial cultures (HBECs) resulted in an increase in airway surface liquid height from 4.2±0.6 to 7.9±0.6 μm, comparable to heights seen in normal HBECs, even in the presence of neutrophil elastase. Our data also indicate that the ENaC inhibitory domain of SPLUNC1 may be cleaved away from the main molecule by neutrophil elastase, which suggests that it may still be active during inflammation or neutrophilia. Furthermore, the robust inhibition of ENaC by the G22-A39 peptide suggests that this peptide may be suitable for treating CF lung disease.
Collapse
Affiliation(s)
- Carey A Hobbs
- Cystic Fibrosis/Pulmonary Research and Treatment Center, Department of Prosthodontics, University of North Carolina, Chapel Hill, 7125 Thurston Bowles Bldg., Chapel Hill, NC 27599-7248, USA
| | | | | | | | | | | | | | | | | | | |
Collapse
|
14
|
Kota P, García-Caballero A, Dang H, Gentzsch M, Stutts MJ, Dokholyan NV. Energetic and structural basis for activation of the epithelial sodium channel by matriptase. Biochemistry 2012; 51:3460-9. [PMID: 22471557 DOI: 10.1021/bi2014773] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Limited proteolysis, accomplished by endopeptidases, is a ubiquitous phenomenon underlying the regulation and activation of many enzymes, receptors, and other proteins synthesized as inactive precursors. Serine proteases make up one of the largest and most conserved families of endopeptidases involved in diverse cellular activities, including wound healing, blood coagulation, and immune responses. Heteromeric α,β,γ-epithelial sodium channels (ENaC) associated with diseases like cystic fibrosis and Liddle's syndrome are irreversibly stimulated by membrane-anchored proteases (MAPs) and furin-like convertases. Matriptase/channel activating protease-3 (CAP3) is one of the several MAPs that potently activate ENaC. Despite identification of protease cleavage sites, the basis for the enhanced susceptibility of α- and γ-ENaC to proteases remains elusive. Here, we elucidate the energetic and structural bases for activation of ENaC by CAP3. We find a region near the γ-ENaC furin site that has previously not been identified as a critical cleavage site for CAP3-mediated stimulation. We also report that CAP3 mediates cleavage of ENaC at basic residues downstream of the furin site. Our results indicate that surface proteases alone are sufficient to fully activate uncleaved ENaC and explain how ENaC in epithelia expressing surface-active proteases can appear refractory to soluble proteases. Our results support a model in which proteases prime ENaC for activation by cleaving at the furin site, and cleavage at downstream sites is accomplished by membrane surface proteases or extracellular soluble proteases. On the basis of our results, we propose a dynamics-driven "anglerfish" mechanism that explains less stringent sequence requirements for substrate recognition and cleavage by matriptase than by furin.
Collapse
Affiliation(s)
- Pradeep Kota
- Program in Cellular and Molecular Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7260, USA
| | | | | | | | | | | |
Collapse
|
15
|
Garcia-Caballero A, Ishmael SS, Dang Y, Gillie D, Bond JS, Milgram SL, Stutts MJ. Activation of the epithelial sodium channel by the metalloprotease meprin β subunit. Channels (Austin) 2011; 5:14-22. [PMID: 20953144 DOI: 10.4161/chan.5.1.13759] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
The Epithelial Na(+) Channel (ENaC) is an apical heteromeric channel that mediates Na(+) entry into epithelial cells from the luminal cell surface. ENaC is activated by proteases that interact with the channel during biosynthesis or at the extracellular surface. Meprins are cell surface and secreted metalloproteinases of the kidney and intestine. We discovered by affinity chromatography that meprins bind γ-ENaC, a subunit of the ENaC hetero-oligomer. The physical interaction involves NH(2)-terminal cytoplasmic residues 37-54 of γ-ENaC, containing a critical gating domain immediately before the first transmembrane domain, and the cytoplasmic COOH-terminal tail of meprin β (residues 679-704). This potential association was confirmed by co-expression and co-immunoprecipitation studies. Functional assays revealed that meprins stimulate ENaC expressed exogenously in Xenopus oocytes and endogenously in epithelial cells. Co-expression of ENaC subunits and meprin β or α/β in Xenopus oocytes increased amiloride-sensitive Na(+) currents approximately two-fold. This increase was blocked by preincubation with an inhibitor of meprin activity, actinonin. The meprin-mediated increase in ENaC currents in oocytes and epithelial cell monolayers required meprin β, but not the α subunit. Meprin β promoted cleavage of α and γ-ENaC subunits at sites close to the second transmembrane domain in the extracellular domain of each channel subunit. Thus, meprin β regulates the activity of ENaC in a metalloprotease-dependent fashion.
Collapse
Affiliation(s)
- Agustin Garcia-Caballero
- Cystic Fibrosis/Pulmonary Research and Treatment Center, Pennsylvania State University College of Medicine, Hershey, USA.
| | | | | | | | | | | | | |
Collapse
|
16
|
Gentzsch M, Dang H, Dang Y, Garcia-Caballero A, Suchindran H, Boucher RC, Stutts MJ. The cystic fibrosis transmembrane conductance regulator impedes proteolytic stimulation of the epithelial Na+ channel. J Biol Chem 2010; 285:32227-32. [PMID: 20709758 DOI: 10.1074/jbc.m110.155259] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) that prevent its proper folding and trafficking to the apical membrane of epithelial cells. Absence of cAMP-mediated Cl(-) secretion in CF airways causes poorly hydrated airway surfaces in CF patients, and this condition is exacerbated by excessive Na(+) absorption. The mechanistic link between missing CFTR and increased Na(+) absorption in airway epithelia has remained elusive, although substantial evidence implicates hyperactivity of the epithelial Na(+) channel (ENaC). ENaC is known to be activated by selective endoproteolysis of the extracellular domains of its α- and γ-subunits, and it was recently reported that ENaC and CFTR physically associate in mammalian cells. We confirmed this interaction in oocytes by co-immunoprecipitation and found that ENaC associated with wild-type CFTR was protected from proteolytic cleavage and stimulation of open probability. In contrast, ΔF508 CFTR, the most common mutant protein in CF patients, failed to protect ENaC from proteolytic cleavage and stimulation. In normal airway epithelial cells, ENaC was contained in the anti-CFTR immunoprecipitate. In CF airway epithelial cultures, the proportion of full-length to total α-ENaC protein signal was consistently reduced compared with normal cultures. Our results identify limiting proteolytic cleavage of ENaC as a mechanism by which CFTR down-regulates Na(+) absorption.
Collapse
Affiliation(s)
- Martina Gentzsch
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599, USA.
| | | | | | | | | | | | | |
Collapse
|
17
|
Rollins BM, Garcia-Caballero A, Stutts MJ, Tarran R. SPLUNC1 expression reduces surface levels of the epithelial sodium channel (ENaC) in Xenopus laevis oocytes. Channels (Austin) 2010; 4:255-9. [PMID: 20519934 DOI: 10.4161/chan.4.4.12255] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Throughout the body, the epithelial Na(+) channel (ENaC) plays a critical role in salt and liquid homeostasis. In cystic fibrosis airways, for instance, improper regulation of ENaC results in hyperabsorption of sodium that causes dehydration of airway surface liquid. This dysregulation then contributes to mucus stasis and chronic lung infections. ENaC is known to undergo proteolytic cleavage, which is required for its ability to conduct Na(+) ions. We have previously shown that the short, palate lung and nasal epithelial clone (SPLUNC1) binds to and inhibits ENaC in both airway epithelia and in Xenopus laevis oocytes. In this study, we found that SPLUNC1 was more potent at inhibiting ENaC than either SPLUNC2 or long PLUNC1 (LPLUNC1), two other PLUNC family proteins that are also expressed in airway epithelia. Furthermore, we were able to shed light on the potential mechanism of SPLUNC1's inhibition of ENaC. While SPLUNC1 did not inhibit proteolytic activity of trypsin, it significantly reduced ENaC currents by reducing the number of ENaCs in the plasma membrane. A better understanding of ENaC's regulation by endogenous inhibitors may aid in the development of novel therapies designed to inhibit hyperactive ENaC in cystic fibrosis epithelia.
Collapse
Affiliation(s)
- Brett M Rollins
- Cystic Fibrosis/Pulmonary Research and Treatment Center, The University of North Carolina, Chapel Hill, NC, USA
| | | | | | | |
Collapse
|
18
|
Abstract
The study of human monogenic diseases [pseudohypoaldosteronism type 1 (PHA-1) and Liddle's syndrome] as well as mouse models mimicking the salt-losing syndrome (PHA-1) or salt-sensitive hypertension (Liddle's syndrome) have established the epithelial sodium channel ENaC as a limiting factor in vivo in the control of ionic composition of the extracellular fluid, regulation of blood volume and blood pressure, lung alveolar clearance, and airway mucociliary clearance. In this review, we discuss more specifically the activation of ENaC by serine proteases. Recent in vitro and in vivo experiments indicate that membrane-bound serine proteases are of critical importance in the activation of ENaC in different organs, such as the kidney, the lung, or the cochlea. Progress in understanding the basic mechanism of proteolytic activation of ENaC is accelerating, but uncertainty about the most fundamental aspects persists, leaving numerous still-unanswered questions.
Collapse
Affiliation(s)
- Bernard C Rossier
- Department of Pharmacology and Toxicology, University of Lausanne, CH-1005 Lausanne, Switzerland.
| | | |
Collapse
|
19
|
Stutts MJ, Gatzy JT. Water and Electrolytes of Rat Lung: Effects of Fluid-Filling, Perfusion, and Metabolic Inhibitor Treatment. Exp Lung Res 2009. [DOI: 10.3109/01902148009069650] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
|
20
|
Abstract
Epithelial sodium channels (ENaCs) perform diverse physiological roles by mediating Na+ absorption across epithelial surfaces throughout the body. Excessive Na+ absorption in kidney and colon elevates blood pressure and in the airways disrupts mucociliary clearance. Potential therapies for disorders of Na+ absorption require better understanding of ENaC regulation. Recent work has established partial and selective proteolysis of ENaCs as an important means of channel activation. In particular, channel-activating transmembrane serine proteases (CAPs) and cognate inhibitors may be important in tissue-specific regulation of ENaCs. Although CAP2 (TMPRSS4) requires catalytic activity to activate ENaCs, there is not yet evidence of ENaC fragments produced by this serine protease and/or identification of the site(s) where CAP2 cleaves ENaCs. Here, we report that CAP2 cleaves at multiple sites in all three ENaC subunits, including cleavage at a conserved basic residue located in the vicinity of the degenerin site (α-K561, β-R503, and γ-R515). Sites in α-ENaC at K149/R164/K169/R177 and furin-consensus sites in α-ENaC (R205/R231) and γ-ENaC (R138) are responsible for ENaC fragments observed in oocytes coexpressing CAP2. However, the only one of these demonstrated cleavage events that is relevant for the channel activation by CAP2 takes place in γ-ENaC at position R138, the previously identified furin-consensus cleavage site. Replacement of arginine by alanine or glutamine (α,β,γR138A/Q) completely abolished both the Na+ current (INa) and a 75-kD γ-ENaC fragment at the cell surface stimulated by CAP2. Replacement of γ-ENaC R138 with a conserved basic residue, lysine, preserved both the CAP2-induced INa and the 75-kD γ-ENaC fragment. These data strongly support a model where CAP2 activates ENaCs by cleaving at R138 in γ-ENaC.
Collapse
Affiliation(s)
- Agustín García-Caballero
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599
| | - Yan Dang
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599
| | - Hong He
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599
| | - M. Jackson Stutts
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599
| |
Collapse
|
21
|
Harris M, Garcia-Caballero A, Stutts MJ, Firsov D, Rossier BC. Preferential assembly of epithelial sodium channel (ENaC) subunits in Xenopus oocytes: role of furin-mediated endogenous proteolysis. J Biol Chem 2008; 283:7455-63. [PMID: 18195015 DOI: 10.1074/jbc.m707399200] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The epithelial sodium channel (ENaC) is preferentially assembled into heteromeric alphabetagamma complexes. The alpha and gamma (not beta) subunits undergo proteolytic cleavage by endogenous furin-like activity correlating with increased ENaC function. We identified full-length subunits and their fragments at the cell surface, as well as in the intracellular pool, for all homo- and heteromeric combinations (alpha, beta, gamma, alphabeta, alphagamma, betagamma, and alphabetagamma). We assayed corresponding channel function as amiloride-sensitive sodium transport (I(Na)). We varied furin-mediated proteolysis by mutating the P1 site in alpha and/or gamma subunit furin consensus cleavage sites (alpha(mut) and gamma(mut)). Our findings were as follows. (i) The beta subunit alone is not transported to the cell surface nor cleaved upon assembly with the alpha and/or gamma subunits. (ii) The alpha subunit alone (or in combination with beta and/or gamma) is efficiently transported to the cell surface; a surface-expressed 65-kDa alpha ENaC fragment is undetected in alpha(mut)betagamma, and I(Na) is decreased by 60%. (iii) The gamma subunit alone does not appear at the cell surface; gamma co-expressed with alpha reaches the surface but is not detectably cleaved; and gamma in alphabetagamma complexes appears mainly as a 76-kDa species in the surface pool. Although basal I(Na) of alphabetagamma(mut) was similar to alphabetagamma, gamma(mut) was not detectably cleaved at the cell surface. Thus, furin-mediated cleavage is not essential for participation of alpha and gamma in alphabetagamma heteromers. Basal I(Na) is reduced by preventing furin-mediated cleavage of the alpha, but not gamma, subunits. Residual current in the absence of furin-mediated proteolysis may be due to non-furin endogenous proteases.
Collapse
Affiliation(s)
- Michael Harris
- Département de Pharmacologie et Toxicologie, Université de Lausanne, CH-1005 Lausanne, Switzerland
| | | | | | | | | |
Collapse
|
22
|
Thelin WR, Chen Y, Gentzsch M, Kreda SM, Sallee JL, Scarlett CO, Borchers CH, Jacobson K, Stutts MJ, Milgram SL. Direct interaction with filamins modulates the stability and plasma membrane expression of CFTR. J Clin Invest 2007; 117:364-74. [PMID: 17235394 PMCID: PMC1765518 DOI: 10.1172/jci30376] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2006] [Accepted: 11/21/2006] [Indexed: 12/15/2022] Open
Abstract
The role of the cystic fibrosis transmembrane conductance regulator (CFTR) as a cAMP-dependent chloride channel on the apical membrane of epithelia is well established. However, the processes by which CFTR is regulated on the cell surface are not clear. Here we report the identification of a protein-protein interaction between CFTR and the cytoskeletal filamin proteins. Using proteomic approaches, we identified filamins as proteins that associate with the extreme CFTR N terminus. Furthermore, we identified a disease-causing missense mutation in CFTR, serine 13 to phenylalanine (S13F), which disrupted this interaction. In cells, filamins tethered plasma membrane CFTR to the underlying actin network. This interaction stabilized CFTR at the cell surface and regulated the plasma membrane dynamics and confinement of the channel. In the absence of filamin binding, CFTR was internalized from the cell surface, where it prematurely accumulated in lysosomes and was ultimately degraded. Our data demonstrate what we believe to be a previously unrecognized role for the CFTR N terminus in the regulation of the plasma membrane stability and metabolic stability of CFTR. In addition, we elucidate the molecular defect associated with the S13F mutation.
Collapse
Affiliation(s)
- William R Thelin
- Department of Cell and Developmental Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA
| | | | | | | | | | | | | | | | | | | |
Collapse
|
23
|
Harris M, Firsov D, Vuagniaux G, Stutts MJ, Rossier BC. A Novel Neutrophil Elastase Inhibitor Prevents Elastase Activation and Surface Cleavage of the Epithelial Sodium Channel Expressed in Xenopus laevis Oocytes. J Biol Chem 2007; 282:58-64. [PMID: 17090546 DOI: 10.1074/jbc.m605125200] [Citation(s) in RCA: 85] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The amiloride-sensitive epithelial sodium channel (ENaC) constitutes a limiting step in sodium reabsorption across distal airway epithelium and controlling mucociliary clearance. ENaC is activated by serine proteases secreted in the extracellular milieu. In cystic fibrosis lungs, high concentrations of secreted neutrophil elastase (NE) are observed. hNE could activate ENaC and contribute to further decreased mucociliary clearance. The aims of this study were (i) to test the ability of an engineered human neutrophil elastase inhibitor (EPI-hNE4) to specifically inhibit the elastase activation of ENaC-mediated amiloride-sensitive currents (I(Na)) and (ii) to examine the effect of elastase on cell surface expression of ENaC and its cleavage pattern (exogenous proteolysis). Oocytes were exposed to hNE (10-100 microg/ml) and/or trypsin (10 microg/ml) for 2-5 min in the presence or absence of EPI-hNE4 (0.7 microm). hNE activated I(Na) 3.6-fold (p < 0.001) relative to non-treated hENaC-injected oocytes. EPI-hNE4 fully inhibited hNE-activated I(Na) but had no effect on trypsin- or prostasin-activated I(Na). The co-activation of I(Na) by hNE and trypsin was not additive. Biotinylation experiments revealed that cell surface gamma ENaC (but not alpha or beta ENaC) exposed to hNE for 2 min was cleaved (as a 67-kDa fragment) and correlated with increased I(Na). The elastase-induced exogenous proteolysis pattern is distinct from the endogenous proteolysis pattern induced upon preferential assembly, suggesting a causal relationship between gamma ENaC cleavage and ENaC activation, taking place at the plasma membrane.
Collapse
Affiliation(s)
- Michael Harris
- Département de Pharmacologie et de Toxicologie, Université de Lausanne, CH-1005 Lausanne, Switzerland
| | | | | | | | | |
Collapse
|
24
|
Dejima K, Randell SH, Stutts MJ, Senior BA, Boucher RC. Potential role of abnormal ion transport in the pathogenesis of chronic sinusitis. Arch Otolaryngol Head Neck Surg 2006; 132:1352-62. [PMID: 17178948 DOI: 10.1001/archotol.132.12.1352] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
OBJECTIVES To create well-differentiated cultures of normal and chronic sinusitis paranasal sinus epithelial cells and to compare their electrophysiologic properties. DESIGN In vitro investigation using primary sinus epithelial cells, initially cultured on plastic tissue culture dishes. Cells were characterized by means of immunocytochemical analysis and then passaged to air-liquid interface culture conditions. The morphologic features of air-liquid interface cultures were assessed using light and electron microscopy. Epithelial Na(+) channel, Na(+)-K(+)-2Cl(-) cotransporter, cystic fibrosis transmembrane conductance regulator, and Ca(2+)-activated Cl(-) channel function were investigated in Ussing chambers. SUBJECTS Specimens were obtained from 15 patients undergoing transsphenoidal pituitary procedures, tumor removal, or trauma repair and from 9 patients with chronic sinusitis. RESULTS After culture at an air-liquid interface for 21 days, the epithelium was pseudostratified and contained basal, mucous secretory, and ciliated cells. There were no detectable morphologic differences between normal and chronic sinusitis cells. In cultures of normal cells, median basal short circuit current was 4.7 microA/cm(2), and Na(+) transport, defined as the amiloride hydrochloride-sensitive component, was approximately 20% of the total. Basal and amiloride-sensitive short circuit currents were greater in cultures of chronic sinusitis cells. Basal short circuit currents in both types of cultures were insensitive to the Cl(-) transport inhibitor bumetanide, but all responded to forskolin or uridine triphosphate. After amiloride pretreatment, forskolin and uridine triphosphate responses were greater in chronic sinusitis cells. CONCLUSIONS We established methods for well-differentiated sinus epithelial cultures. The cells exhibited Na(+) absorption and Cl(-) secretion, and elevated rates of ion transport may be pathophysiologically relevant in chronic sinusitis.
Collapse
Affiliation(s)
- Kenji Dejima
- Department of Otorhinolaryngology, Kyoto Prefectural University of Medicine, Kyoto, Japan
| | | | | | | | | |
Collapse
|
25
|
Thelin WR, Kesimer M, Tarran R, Kreda SM, Grubb BR, Sheehan JK, Stutts MJ, Milgram SL. The Cystic Fibrosis Transmembrane Conductance Regulator Is Regulated by a Direct Interaction with the Protein Phosphatase 2A. J Biol Chem 2005; 280:41512-20. [PMID: 16239222 DOI: 10.1074/jbc.m507308200] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated chloride channel expressed at the apical surface of epithelia. Although the regulation of CFTR by protein kinases is well documented, channel deactivation by phosphatases is not well understood. We find that the serine/threonine phosphatase PP2A can physically associate with the CFTR COOH terminus. PP2A is a heterotrimeric phosphatase composed of a catalytic subunit and two divergent regulatory subunits (A and B). The cellular localization and substrate specificity of PP2A is determined by the unique combination of A and B regulatory subunits, which can give rise to at least 75 different enzymes. By mass spectrometry, we identified the exact PP2A regulatory subunits associated with CFTR as Aalpha and B'epsilon and find that the B'epsilon subunit binds CFTR directly. PP2A subunits localize to the apical surface of airway epithelia and PP2A phosphatase activity co-purifies with CFTR in Calu-3 cells. In functional assays, inhibitors of PP2A block rundown of basal CFTR currents and increase channel activity in excised patches of airway epithelia and in intact mouse jejunum. Moreover, PP2A inhibition in well differentiated human bronchial epithelial cells results in a CFTR-dependent increase in the airway surface liquid. Our data demonstrate that PP2A is a relevant CFTR phosphatase in epithelial tissues. Our results may help reconcile differences in phosphatase-mediated channel regulation observed for different tissues and cells. Furthermore, PP2A may be a clinically relevant drug target for CF, which should be considered in future studies.
Collapse
Affiliation(s)
- William R Thelin
- Department of Cell and Developmental Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA
| | | | | | | | | | | | | | | |
Collapse
|
26
|
Huang P, Gilmore E, Kultgen P, Barnes P, Milgram S, Stutts MJ. Local regulation of cystic fibrosis transmembrane regulator and epithelial sodium channel in airway epithelium. Ann Am Thorac Soc 2005; 1:33-7. [PMID: 16113409 DOI: 10.1513/pats.2306012] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Regulation of cystic fibrosis transmembrane regulator (CFTR) and epithelial sodium channel (ENaC) in airway epithelia strongly influences the rate of mucociliary clearance (MCC) by determining the volume of airway surface liquid. MCC increases in response to stimuli originating on the airway surface, and CFTR and ENaC in airway epithelia appear to be regulated by local rather than systemic signaling. Although all signals that regulate CFTR and ENaC in airways have not been identified, the release of nucleotides from airway epithelial cells exposed to physical stimuli initiates a series of events that coordinately favor increased MCC. These events include activation of adenosine A2B receptors that stimulate CFTR and P2Y2 receptors that inhibit ENaC. Together these actions result in an increased volume of airway surface liquid and increased MCC rates. Stimulation of CFTR by A(2B)AR uses protein kinase (PK) A signaling elements that are localized within the apical/subapical compartment, including G proteins, adenylyl cyclase, PKA-II, A-kinase anchoring proteins, and phosphodiesterases. Inhibition of ENaC by P2Y2 receptors appears to be mediated by phospholipase C-beta3, possibly through an effect on the levels of phosphatidylinositol 4,5-bisphosphonate in the apical membrane.
Collapse
Affiliation(s)
- Pingbo Huang
- Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
| | | | | | | | | | | |
Collapse
|
27
|
Barnes AP, Livera G, Huang P, Sun C, O'Neal WK, Conti M, Stutts MJ, Milgram SL. Phosphodiesterase 4D Forms a cAMP Diffusion Barrier at the Apical Membrane of the Airway Epithelium. J Biol Chem 2005; 280:7997-8003. [PMID: 15611099 DOI: 10.1074/jbc.m407521200] [Citation(s) in RCA: 89] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We demonstrated previously that Calu-3 airway epithelial cells sense adenosine on their luminal surface through adenosine A2B receptors coupled to adenylyl cyclase. Occupancy of these receptors leads to activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel through protein kinase A (PKA) anchored at the apical membrane. Because luminal A2B receptor activation does not raise total cellular cAMP levels, we hypothesized that activation of phosphodiesterases (PDEs) confines cAMP generated by apical A2B receptors to a microdomain that includes the CFTR channel. Using reverse transcription-PCR, Western blotting, and activity measurements, PDE4D was identified as the major PDE species in airway epithelia. Consistent with these results, inhibitors of PDE4, but not PDE3, selectively abolished the lateral confinement of cAMP signaling in apical membrane patches during cell-attached recordings. Furthermore, stimulation of the CFTR in excised apical patches by rolipram and RS25344 indicated that PDE4 is localized in close proximity to the CFTR channel. Indeed, immunohistochemistry of human airway sections revealed that PDE4D is localized in the apical domain of the cell. PDE4 was activated after luminal adenosine exposure in a PKA-dependent manner. Because PDE4 activity is positively regulated by PKA, our results support a model whereby the PDE diffusion barrier is proportional to the degree of receptor stimulation. These findings underscore the concept that subcellular localization of individual PDE isozymes is an important mechanism for confining cAMP signaling to functional domains within cells.
Collapse
Affiliation(s)
- Anthony P Barnes
- Department of Cell and Developmental Biology and Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA
| | | | | | | | | | | | | | | |
Collapse
|
28
|
Caldwell RA, Boucher RC, Stutts MJ. Neutrophil elastase activates near-silent epithelial Na+ channels and increases airway epithelial Na+ transport. Am J Physiol Lung Cell Mol Physiol 2005; 288:L813-9. [PMID: 15640288 DOI: 10.1152/ajplung.00435.2004] [Citation(s) in RCA: 186] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Neutrophil elastase is a serine protease that is abundant in the airways of individuals with cystic fibrosis (CF), a genetic disease manifested by excessive airway Na(+) absorption and consequent depletion of the airway surface liquid layer. Although endogenous epithelium-derived serine proteases regulate epithelial Na(+) transport, the effects of neutrophil elastase on epithelial Na(+) transport and epithelial Na(+) channel (ENaC) activity are unknown. Low micromolar concentrations of human neutrophil elastase (hNE) applied to the apical surface of a human bronchial cell line (16HBE14o-/beta gamma) increased Na(+) transport about twofold. Similar effects were observed with trypsin, also a serine protease. Proteolytic inhibitors of hNE or trypsin selectively abolished the enzyme-induced increase of epithelial Na(+) transport. At the level of the single channel, submicromolar concentrations of hNE increased activity of near-silent ENaC approximately 108-fold in patches from NIH-3T3 cells expressing rat alpha-, beta-, and gamma-ENaC subunits. However, no enzyme effects were observed on basally active ENaCs. Trypsin exposure following hNE revealed no additional increase in amiloride-sensitive short-circuit current or in ENaC activity, suggesting these enzymes share a common mode of action for increasing Na(+) transport, likely through proteolytic activation of ENaC. The hNE-induced increase of near-silent ENaC activity in CF airways could contribute to Na(+) hyperabsorption, reduced airway surface liquid height, and dehydrated mucus culminating in inefficient mucociliary clearance.
Collapse
Affiliation(s)
- Ray A Caldwell
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599-7248, USA.
| | | | | |
Collapse
|
29
|
Abstract
The regulation of epithelial Na+ channel (ENaC) function is critical for normal salt and water balance. This regulation is achieved through cell surface insertion/retrieval of channels, by changes in channel open probability (Po), or through a combination of these processes. Epithelium-derived serine proteases, including channel activating protease (CAP) and prostasin, regulate epithelial Na+ transport, but the molecular mechanism is unknown. We tested the hypothesis that extracellular serine proteases activate a near-silent ENaC population resident in the plasma membrane. Single-channel events were recorded in outside-out patches from fibroblasts (NIH/3T3) stably expressing rat alpha-, beta-, and gamma-subunits (rENaC), before and during exposure to trypsin, a serine protease homologous to CAP and prostasin. Under baseline conditions, near-silent patches were defined as having rENaC activity (NPo) < 0.03, where N is the number of channels. Within 1-5 min of 3 microg/ml bath trypsin superfusion, NPo increased approximately 66-fold (n = 7). In patches observed to contain a single functional channel, trypsin increased Po from 0.02 +/- 0.01 to 0.57 +/- 0.03 (n = 3, mean +/- SE), resulting from the combination of an increased channel open time and decreased channel closed time. Catalytic activity was required for activation of near-silent ENaC. Channel conductance and the Na+/Li+ current ratio with trypsin were similar to control values. Modulation of ENaC Po by endogenous epithelial serine proteases is a potentially important regulator of epithelial Na+ transport, distinct from the regulation achieved by hormone-induced plasma membrane insertion of channels.
Collapse
Affiliation(s)
- Ray A Caldwell
- Cystic Fibrosis/Pulmonary Research and Treatment Center, CB#7248, University of North Carolina, Chapel Hill, NC 27599-7248, USA.
| | | | | |
Collapse
|
30
|
Abstract
The epithelial Na+ channel (ENaC), composed of three subunits (alphabetagamma), is expressed in various Na(+)-absorbing epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. By using patch clamp techniques, we have examined the effect of cytosolic ATP on the activity of the rat alphabetagammaENaC (rENaC) stably expressed in NIH-3T3 cells and in Madin-Darby canine kidney epithelial cells. The inward whole-cell current attributable to rENaC activity ran down when these cells were dialyzed with an ATP-free pipette solution in the conventional whole-cell voltage-clamping technique. This run down was prevented by 2 mM ATP (but not by AMP or ADP) in the pipette solution or by the poorly or non-hydrolyzable analogues of ATP (adenosine 5'-O-(thiotriphosphate) and adenosine 5'-(beta,gamma-imino)triphosphate) in both cell lines, suggesting that protection from run down was mediated through non-hydrolytic nucleotide binding. Accordingly, we demonstrate binding of ATP (but not AMP) to alpharENaC expressed in Madin-Darby canine kidney cells, which was inhibited upon mutation of the two putative nucleotide-binding motifs of alpharENaC. Single channel analyses indicated that the run down of currents observed in the whole-cell recording was attributable to run down of channel activity, defined as NPo (the product of the number of channels and open probability). We propose that this novel ATP regulation of ENaC may be, at least in part, involved in the fine-tuning of ENaC activity under physiologic and pathophysiologic conditions.
Collapse
Affiliation(s)
- Toru Ishikawa
- Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818, Japan.
| | | | | | | | | |
Collapse
|
31
|
Mohler PJ, Kultgen PL, Stutts MJ, Milgram SL. Biochemical assays for studying indirect interactions between CFTR and the cytoskeleton. Methods Mol Med 2002; 70:383-94. [PMID: 11917538 DOI: 10.1385/1-59259-187-6:383] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/24/2023]
Affiliation(s)
- Peter J Mohler
- Department of Cell and Molecular Physiology, University of North Carolina, Chapel Hill, NC, USA
| | | | | | | |
Collapse
|
32
|
Affiliation(s)
- Scott H Donaldson
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC, USA
| | | | | |
Collapse
|
33
|
Yerxa BR, Sabater JR, Davis CW, Stutts MJ, Lang-Furr M, Picher M, Jones AC, Cowlen M, Dougherty R, Boyer J, Abraham WM, Boucher RC. Pharmacology of INS37217 [P(1)-(uridine 5')-P(4)- (2'-deoxycytidine 5')tetraphosphate, tetrasodium salt], a next-generation P2Y(2) receptor agonist for the treatment of cystic fibrosis. J Pharmacol Exp Ther 2002; 302:871-80. [PMID: 12183642 DOI: 10.1124/jpet.102.035485] [Citation(s) in RCA: 128] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
INS37217 [P(1)-(uridine 5')-P(4)-(2'-deoxycytidine 5')tetraphosphate, tetrasodium salt] is a deoxycytidine-uridine dinucleotide with agonist activity at the P2Y(2) receptor. In primate lung tissues, the P2Y(2) receptor mRNA was located by in situ hybridization predominantly in epithelial cells and not in smooth muscle or stromal tissue. The pharmacologic profile of INS37217 parallels that of UTP, leading to increased chloride and water secretion, increased cilia beat frequency, and increased mucin release. The combined effect of these actions was confirmed in an animal model of tracheal mucus velocity that showed that a single administration of INS37217 significantly enhanced mucus transport for at least 8 h after dosing. This extended duration of action is consistent with the ability of INS37217 to resist metabolism by airway cells and sputum enzymes. The enhanced metabolic stability and resultant increased duration of improved mucociliary clearance may confer significant advantages to INS37217 over other P2Y(2) agonists in the treatment of diseases such as cystic fibrosis.
Collapse
Affiliation(s)
- B R Yerxa
- Inspire Pharmaceuticals, Inc., 4222 Emperor Boulevard, Suite 470, Durham, NC 27703, USA.
| | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
34
|
Huang P, Lazarowski ER, Tarran R, Milgram SL, Boucher RC, Stutts MJ. Compartmentalized autocrine signaling to cystic fibrosis transmembrane conductance regulator at the apical membrane of airway epithelial cells. Proc Natl Acad Sci U S A 2001; 98:14120-5. [PMID: 11707576 PMCID: PMC61178 DOI: 10.1073/pnas.241318498] [Citation(s) in RCA: 176] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Physical stimulation of airway surfaces evokes liquid secretion, but the events that mediate this vital protective function are not understood. When cystic fibrosis transmembrane conductance regulator (CFTR) channel activity was used as a functional readout, we found signaling elements compartmentalized at both extracellular and intracellular surfaces of the apical cell membrane that activate apical Cl(-) conductance in Calu-3 cells. At the outer surface, ATP was released by physical stimuli, locally converted to adenosine, and sensed by A(2B) adenosine receptors. These receptors couple to G proteins, adenylyl cyclase, and protein kinase A, at the intracellular face of the apical membrane to activate colocalized CFTR. Thus, airways have evolved highly efficient mechanisms to "flush" noxious stimuli from airway surfaces by selective activation of apical membrane signal transduction and effector systems.
Collapse
Affiliation(s)
- P Huang
- Cystic Fibrosis Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599-7248, USA.
| | | | | | | | | | | |
Collapse
|
35
|
Abstract
The epithelial Na(+) channel (ENaC) is implicated in the pathogenesis of salt-sensitive hypertension. Recent evidence from animal models suggests that the vasoactive peptide, endothelin (ET-1), may be an important negative regulator of ENaC in vivo. We investigated the signaling pathway involved in endothelin-mediated ENaC inhibition. Experiments were performed in NIH 3T3 cells stably expressing genes for the three (alpha, beta, and gamma) ENaC subunits. In whole cell patch clamp experiments, we found that ET-1 treatment induced a dose-dependent decrease in amiloride-sensitive currents. Using receptor-specific antagonists, we determined that the effects of ET-1 were attributed to activation of the ET(B) receptor. Moreover, the inhibitory effect of ET-1 on ENaC could be completely blocked when cells were pretreated with the selective Src family kinase inhibitor, PP2. Further studies revealed that basal Src family kinase activity strongly regulates ENaC whole cell currents and single channel gating. These results suggest that Src family kinases lie in a signaling pathway activated by ET-1 and are components of a novel negative regulatory cascade resulting in ENaC inhibition.
Collapse
Affiliation(s)
- E S Gilmore
- Cystic Fibrosis/Pulmonary Research and Treatment Center and Department of Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA
| | | | | |
Collapse
|
36
|
Donaldson SH, Lazarowski ER, Picher M, Knowles MR, Stutts MJ, Boucher RC. Basal nucleotide levels, release, and metabolism in normal and cystic fibrosis airways. Mol Med 2000; 6:969-82. [PMID: 11147574 PMCID: PMC1949922] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/18/2023] Open
Abstract
BACKGROUND Cystic fibrosis (CF) is a syndrome caused by mutations in the cystic fibrosis transmembrane regulator (CFTR) gene. Despite advances in our understanding of the molecular pathogenesis of CF, the link between CFTR gene mutations and the pathogenesis of CF lung disease remains poorly defined. CFTR has been assigned a number of putative functions that may contribute to innate airway defense, including the regulation of adenosine 5'-triphosphate (ATP) release into the extracellular environment. Because extracellular ATP and uridine 5'-triphosphate (UTP) may regulate airway mucociliary clearance via interaction with luminal P2Y2 receptors, the loss of CFTR-mediated nucleotide release could explain the defect in CF airway defense. MATERIALS AND METHODS We tested the physiologic importance of CFTR-mediated nucleotide release in vivo by directly measuring levels of ATP and UTP in nasal airway surface liquid from normal and CF subjects. Because these basal nucleotide levels reflect the net activities of nucleotide release and metabolic pathways, we also measured constitutive rates of nucleotide release and metabolism on well-differentiated normal and CF airway cultures in vitro. The measurement of ATP release rates were paralleled by in vivo studies employing continuous nasal perfusion in normal and CF subjects. Finally, the regulation of ATP release by isoproterenol and methacholine-stimulated submucosal gland secretion was tested. RESULTS These studies revealed that steady-state ATP and UTP levels were similar in normal (470 +/- 131 nM and 37 +/- 7 nM, respectively) and CF (911 +/- 199 nM and 33 +/- 12 nM, respectively) subjects. The rates of both ATP release and metabolism were also similar in normal and CF airway epithelia both in vitro and in vivo. Airway submucosal glands did not secrete nucleotides, but rather, secreted a soluble nucleotidase in response to cholinergic stimuli. CONCLUSION The concentration of ATP in airway surface liquid is in a range that is relevant for the activation of airway nucleotide receptors. However, despite this finding that suggests endogenous nucleotides may be important for the regulation of mucociliary clearance, our data do not support a role for CFTR in regulating extracellular nucleotide concentrations on airway surfaces.
Collapse
Affiliation(s)
- S H Donaldson
- Cystic Fibrosis Research and Treatment Center, University of North Carolina, Chapel Hill 27599, USA.
| | | | | | | | | | | |
Collapse
|
37
|
Abstract
Cystic fibrosis transmembrane regulator (CFTR) is reported to be preferentially regulated by membrane-bound protein kinase A (PKAII). We tested for close physical and functional association of PKA with CFTR in inside-out membrane patches excised from Calu-3 cells. In the presence of MgATP, 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP) increased the product of CFTR channel number and open probability (from 0.36 +/- 0.12 to 1.23 +/- 0.57, n = 20, P < 0.0025), and this stimulation was abolished by PKI. Thus Calu-3 membrane isolated from cells retains PKA holoenzyme that is functionally coupled to CFTR. PKAII is anchored at specific subcellular sites by A kinase anchoring proteins (AKAPs). Exposure of excised patches to HT-31, a peptide that disrupts the association of PKAII and AKAPs, prevented CPT-cAMP stimulation of CFTR. Therefore, PKA holoenzyme in isolated membrane patches is bound to AKAPs. In whole cell voltage-clamp studies, intracellular dialysis of Calu-3 cells with HT-31 blocked the activation of CFTR by extracellular adenosine. These results suggest that AKAPs mediate PKA compartmentalization with CFTR and are required for activation of CFTR by physiological regulators.
Collapse
Affiliation(s)
- P Huang
- Departments of Medicine and CF/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27599, USA.
| | | | | | | | | |
Collapse
|
38
|
Trotter KW, Fraser ID, Scott GK, Stutts MJ, Scott JD, Milgram SL. Alternative splicing regulates the subcellular localization of A-kinase anchoring protein 18 isoforms. J Cell Biol 1999; 147:1481-92. [PMID: 10613906 PMCID: PMC2174236 DOI: 10.1083/jcb.147.7.1481] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The cAMP-dependent protein kinase (PKA) is localized to specific subcellular compartments by association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of functionally related proteins that bind the regulatory (R) subunit of PKA with high affinity and target the kinase to specific subcellular organelles. Recently, AKAP18, a low molecular weight plasma membrane AKAP that facilitates PKA-mediated phosphorylation of the L-type Ca(2+) channel, was cloned. We now report the cloning of two additional isoforms of AKAP18, which we have designated AKAP18beta and AKAP18gamma, that arise from alternative mRNA splicing. The AKAP18 isoforms share a common R subunit binding site, but have distinct targeting domains. The original AKAP18 (renamed AKAP18alpha) and AKAP18beta target the plasma membrane when expressed in HEK-293 cells, while AKAP18gamma is cytosolic. When expressed in epithelial cells, AKAP18alpha is targeted to lateral membranes, whereas AKAP18beta is accumulated at the apical membrane. A 23-amino acid insert, following the plasma membrane targeting domain, facilitates the association of AKAP18beta with the apical membrane. The data suggest that AKAP18 isoforms are differentially targeted to modulate distinct intracellular signaling events. Furthermore, the data suggest that plasma membrane AKAPs may be targeted to subdomains of the cell surface, adding additional specificity in intracellular signaling.
Collapse
Affiliation(s)
- Kevin W. Trotter
- Department of Cell and Molecular Physiology, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - Iain D.C. Fraser
- Howard Hughes Medical Institute, Vollum Institute, Portland, OR 97201
| | - Gregory K. Scott
- Howard Hughes Medical Institute, Vollum Institute, Portland, OR 97201
| | - M. Jackson Stutts
- Cystic Fibrosis/Pulmonary Research and Treatment Center, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| | - John D. Scott
- Howard Hughes Medical Institute, Vollum Institute, Portland, OR 97201
| | - Sharon L. Milgram
- Department of Cell and Molecular Physiology, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
| |
Collapse
|
39
|
Mohler PJ, Kreda SM, Boucher RC, Sudol M, Stutts MJ, Milgram SL. Yes-associated protein 65 localizes p62(c-Yes) to the apical compartment of airway epithelia by association with EBP50. J Cell Biol 1999; 147:879-90. [PMID: 10562288 PMCID: PMC2156157 DOI: 10.1083/jcb.147.4.879] [Citation(s) in RCA: 147] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/1999] [Accepted: 10/07/1999] [Indexed: 12/28/2022] Open
Abstract
We recently showed that the COOH terminus of the cystic fibrosis transmembrane conductance regulator associates with the submembranous scaffolding protein EBP50 (ERM-binding phosphoprotein 50 kD; also called Na(+)/H(+) exchanger regulatory factor). Since EBP50 associates with ezrin, this interaction links the cystic fibrosis transmembrane conductance regulator (CFTR) to the cortical actin cytoskeleton. EBP50 has two PDZ domains, and CFTR binds with high affinity to the first PDZ domain. Here, we report that Yes-associated protein 65 (YAP65) binds with high affinity to the second EBP50 PDZ domain. YAP65 is concentrated at the apical membrane in airway epithelia and interacts with EBP50 in cells. The COOH terminus of YAP65 is necessary and sufficient to mediate association with EBP50. The EBP50-YAP65 interaction is involved in the compartmentalization of YAP65 at the apical membrane since mutant YAP65 proteins lacking the EBP50 interaction motif are mislocalized when expressed in airway epithelial cells. In addition, we show that the nonreceptor tyrosine kinase c-Yes is contained within EBP50 protein complexes by association with YAP65. Subapical EBP50 protein complexes, containing the nonreceptor tyrosine kinase c-Yes, may regulate apical signal transduction pathways leading to changes in ion transport, cytoskeletal organization, or gene expression in epithelial cells.
Collapse
Affiliation(s)
- Peter J. Mohler
- Department of Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
| | - Silvia M. Kreda
- The Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
| | - Richard C. Boucher
- The Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
| | - Marius Sudol
- Department of Biochemistry, Mount Sinai Medical School, New York University, New York, NY 10029
| | - M. Jackson Stutts
- The Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
| | - Sharon L. Milgram
- Department of Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
| |
Collapse
|
40
|
Abstract
CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.
Collapse
Affiliation(s)
- E M Schwiebert
- Department of Physiology and Biophysics, Gregory Fleming James CF Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | | | | | | | | |
Collapse
|
41
|
Short DB, Trotter KW, Reczek D, Kreda SM, Bretscher A, Boucher RC, Stutts MJ, Milgram SL. An apical PDZ protein anchors the cystic fibrosis transmembrane conductance regulator to the cytoskeleton. J Biol Chem 1998; 273:19797-801. [PMID: 9677412 DOI: 10.1074/jbc.273.31.19797] [Citation(s) in RCA: 358] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
The function of the cystic fibrosis transmembrane conductance regulator (CFTR) as a Cl- channel in the apical membrane of epithelial cells is extensively documented. However, less is known about the molecular determinants of CFTR residence in the apical membrane, basal regulation of its Cl- channel activity, and its reported effects on the function of other transporters. These aspects of CFTR function likely require specific interactions between CFTR and unknown proteins in the apical compartment of epithelial cells. Here we report that CFTR interacts with the recently discovered protein, EBP50 (ERM-binding phosphoprotein 50). EBP50 is concentrated at the apical membrane in human airway epithelial cells, in vivo, and CFTR and EBP50 associate in in vitro binding assays. The CFTR-EBP50 interaction requires the COOH-terminal DTRL sequence of CFTR and utilizes either PDZ1 or PDZ2 of EBP50, although binding to PDZ1 is of greater affinity. Through formation of a complex, the interaction between CFTR and EBP50 may influence the stability and/or regulation of CFTR Cl- channel function in the cell membrane and provides a potential mechanism through which CFTR can affect the activity of other apical membrane proteins.
Collapse
Affiliation(s)
- D B Short
- Department of Physiology, University of North Carolina, Chapel Hill, North Carolina 27599, USA
| | | | | | | | | | | | | | | |
Collapse
|
42
|
Hofmann T, Stutts MJ, Ziersch A, Rückes C, Weber WM, Knowles MR, Lindemann H, Boucher RC. Effects of topically delivered benzamil and amiloride on nasal potential difference in cystic fibrosis. Am J Respir Crit Care Med 1998; 157:1844-9. [PMID: 9620916 DOI: 10.1164/ajrccm.157.6.9709043] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
The raised nasal transepithelial potential difference (PD) in cystic fibrosis (CF) reflects accelerated active transport of Na+, and is inhibited by topical administration of the Na+ channel blocker, amiloride. The aim of this study was to investigate the dose-effect and time course of topically administered Na+ conductance inhibitors to inhibit nasal PD, including benzamil, an analog of amiloride. We measured the magnitude of drug inhibition of Na+ transport [percent inhibition of baseline PD (DeltaPD%)] and duration of inhibition of PD, defined as the time when drug inhibition of PD had recovered by 50% (effective time = ET50). Amiloride [10(-)3 M (n = 16), 3 x 10(-)3 M (n = 9), 6 x 10(-)3 M (n = 7), 10(-)2 M (n = 3)] or benzamil [1.7 x 10(-)3 M (n = 7), and 7 x 10(-)3 M (n = 5)] were administered to the nasal surface via an aerosol generated by a jet nebulizer and a nasal mask. The concentration-dependent magnitude (DeltaPD%) of inhibition was similar for amiloride and benzamil ( approximately 67- 77%), whereas the duration of inhibition (ET50) was about two-and-a-half times longer after benzamil administration as compared with equivalent concentrations of amiloride [1.6 +/- 0. 06 versus 4.5 +/- 0.6 h (ET50 +/- SEM), at 6-7 x 10(-)3 M]. In vitro studies of cultured normal nasal epithelia demonstrated directly that benzamil induced an approximately 2-fold more prolonged inhibition of active Na+ transport than amiloride. These data suggest aerosolized benzamil is a candidate long-duration Na+ channel blocker for CF.
Collapse
Affiliation(s)
- T Hofmann
- Cystic Fibrosis Working Group and Selbst. Funktionsbereich Allergologie und Pneumologie der Justus Liebig Universitäts-Kinderklinik, Giessen, Germany
| | | | | | | | | | | | | | | |
Collapse
|
43
|
Stutts MJ, Rossier BC, Boucher RC. Cystic fibrosis transmembrane conductance regulator inverts protein kinase A-mediated regulation of epithelial sodium channel single channel kinetics. J Biol Chem 1997; 272:14037-40. [PMID: 9162024 DOI: 10.1074/jbc.272.22.14037] [Citation(s) in RCA: 156] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Abnormal regulation of ion channels by members of the ABC transport protein superfamily has been implicated in hyperinsulinemic hypoglycemia and in excessive Na+ absorption by airway epithelia in cystic fibrosis (CF). How ABC proteins regulate ion conductances is unknown, but must generally involve either the number or activity of specific ion channels. Here we report that the cystic fibrosis transmembrane conductance regulator (CFTR), which is defective in CF, reverses the regulation of the activity of single epithelial sodium channels (ENaC) by cAMP. ENaC expressed alone in fibroblasts responded to activation of cAMP-dependent protein kinase with increased open probability (Po) and mean open time, whereas ENaC co-expressed with CFTR exhibited decreased Po and mean open time under conditions optimal for PKA-mediated protein phosphorylation. Thus, CFTR regulates ENaC at the level of single channel gating, by switching the response of single channel Po to cAMP from an increase to a decrease.
Collapse
Affiliation(s)
- M J Stutts
- Department of Medicine, Cystic Fibrosis/Pulmonary Research and Treatment Center, The University of North Carolina, Chapel Hill, North Carolina 27599-7248, USA.
| | | | | |
Collapse
|
44
|
Chinet TC, Gabriel SE, Penland CM, Sato M, Stutts MJ, Boucher RC, Van Scott MR. CFTR-like chloride channels in non-ciliated bronchiolar epithelial (Clara) cells. Biochem Biophys Res Commun 1997; 230:470-5. [PMID: 9016805 DOI: 10.1006/bbrc.1996.5939] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Distal airways are believed to be a major site of disease in cystic fibrosis. The product of the CF gene, CFTR, is expressed in non-ciliated bronchiolar epithelial (Clara) cells. Clara cells in primary culture are capable of Cl- secretion which can be stimulated by cAMP and extracellular nucleotides. We used the patch clamp technique to look for Cl- channels in the apical membrane of rabbit Clara cells. In cell-attached patches, we recorded Cl- channels with a conductance for outward currents of 7.5 +/- 0.4 pS (n = 10) and for inward currents of 3.2 +/- 0.5 pS (n = 10); these channels typically exhibited slow kinetics and were not inhibited by the Cl- channel blockers NPPB and DIDS. Channel activity was not noticeably dependent on pipette potential. Addition of chlorophenylthio-cyclic AMP (cpt-cAMP) to the bath increased the percentage of cell-attached patches with active channels (33.8% vs 56.7%; p< 0.05) and the channel open probability (0.49 +/- 0.03 vs 0.84 +/- 0.02; p< 0.05). Extracellular ATP increased the percentage of cell-attached patches with active channels (28.7% to 50.0%; p< 0.05) but had no significant effect on the channel open probability (0.62 +/- 0.07 vs 0.60 +/- 0.06). In conclusion, rabbit non-ciliated bronchiolar epithelial cells express low-conductance Cl- channels that share many similarities with the CFTR-related Cl- channel and are regulated by cAMP and extracellular ATP.
Collapse
|
45
|
Larsen EH, Price EM, Gabriel SE, Stutts MJ, Boucher RC. Clusters of Cl- channels in CFTR-expressing Sf9 cells switch spontaneously between slow and fast gating modes. Pflugers Arch 1996; 432:528-37. [PMID: 8766014 DOI: 10.1007/s004240050166] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The Sf9 insect Spodoptora frugiperda cell line was used for heterologous expression of the cloned human cystic fibrosis transmembrane conductance regulator (CFTR) cDNA, or the cloned beta-galactosidase gene, using the baculovirus Autographa califonica as the infection vector. Using application of the patch-clamp technique, evidence for functional expression of CFTR was obtained according to the following three criteria. Firstly, whole-cell currents recorded 2 days after infection with CFTR revealed a statistically significant increase of membrane conductance, approximately 25 times above that of mock-infected control cells, with the reversal potential of the major current component being governed by the chloride equilibrium potential (ECl). Secondly, in contrast to uninfected cells and cells infected with beta-galactosidase, the membrane conductance to chloride of CFTR-injected cells was stimulated by cytosolic adenosine 3',5'-cyclic monophosphate (cAMP), which was raised by exposing the cells to 10 microM forskolin. Thirdly, recordings of currents through single channels in excised outside-out membrane patches of CFTR-infected cells revealed channels which were clearly different from the native insect chloride channel. Excised outside-out patches of CFTR-infected and forskolin-stimulated cells exhibited wave-like gating kinetics of well-resolved current transitions. All-point Gaussian distributions revealed contributions from several (five to nine) identical channels. Such channels, in excised outside-out patches, studied with a pipette [Cl-] = 40 mM and a bath [Cl-] = 150 mM, rectified the current in agreement with simple electrodiffusion and with a single-channel Goldman-Hodgkin-Katz permeability, PCl = 1. 34 x 10(-14) +/- 0.23 x 10(-14 )cm3/s (n = 5), corresponding to a physiological single-channel conductance of 2.8 +/- 0.5 pS (VM = ECl) and a limiting conductance, gamma150/150, = 7.7 +/- 1.3 pS ([Cl-]Bath = [Cl-]Cell = 150 mM). Currents recorded from multichannel excised outside-out patches could shift from the above mode of resolvable unitary conductance transitions to one which was too fast to reveal the dwell-times of closed and open states. During periods characterized by noisy currents, the variance (sigma2) of current fluctuations about their stationary mean value depicted a U-shaped function of membrane potential, with a minimum value at a pipette potential where the chloride current was shown to be zero. Thus, it can be concluded that the current fluctuations are caused by fast gating of channels specific for chloride ions. Switching back and forth between the two gating modes of clusters of chloride channels occurred from moment to moment in excised patches when the membrane potential was held at a constant value indicating cooperative gating as a result of interaction between neighbouring chloride channels.
Collapse
Affiliation(s)
- E H Larsen
- Cystic Fibrosis/Pulmonary Research and Treatment Center, School of Medicine, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7248, USA
| | | | | | | | | |
Collapse
|
46
|
Larsen EH, Gabriei SE, Stutts MJ, Fullton J, Price EM, Boucher RC. Endogenous chloride channels of insect sf9 cells. Evidence for coordinated activity of small elementary channel units. J Gen Physiol 1996; 107:695-714. [PMID: 8783071 PMCID: PMC2219394 DOI: 10.1085/jgp.107.6.695] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The endogenous Cl- conductance of Spodoptera frugiperda (Sf9) cells was studied 20-35 h after plating out of either uninfected cells or cells infected by a baculovirus vector carrying the cloned beta-galactosidase gene (beta-Gal cells). With the cation Tris+ in the pipette and Na+ in the bath, the reversal potential of whole-cell currents was governed by the prevailing Cl- equilibrium potential and could be fitted by the Goldman-Hodgkin-Katz equation with similar permeabilities for uninfected and beta-Gal cells. In the frequency range 0.12 < f < 300 Hz, the power density spectrum of whole-cell Cl- currents could be fitted by three Lorentzians. Independent of membrane potential, >50% of the total variance of whole-cell current fluctuations was accounted for by the low frequency Lorentzian (fc = 0.40 +/- 0.03 Hz, n = 6). Single-Cl- channels showed complex gating kinetics with long lasting (seconds) openings interrupted by similar long closures. In the open state, channels exhibited fast burst-like closures. Since the patches normally contained more than a single channel, it was not possible to measure open and closed dwell-time distributions for comparing single-Cl- channel activity with the kinetic features of whole-cell currents. However, the power density spectrum of Cl- currents of cell-attached and excised outside-out patches contained both high and low frequency Lorentzian components, with the corner frequency of the slow component (fc = 0.40 +/- 0.02 Hz, n = 4) similar to that of whole-cell current fluctuations. Chloride channels exhibited multiple conductance states with similar Goldman-Hodgkin-Katz-type rectification. Single-channel permeabilities covered the range from approximately 0.6.10(-14) cm5/s to approximately 6.10(-14) cm3/s, corresponding to a limiting conductance (gamma 150/150) of approximately 3.5 pS and approximately 35 pS, respectively. All states reversed near the same membrane potential, and they exhibited similar halide ion selectivity, P1 > PCl approximately PBr. Accordingly, Cl- current amplitudes larger than current flow through the smallest channel unit resolved seem to result from simultaneous open/shut events of two or more channel units.
Collapse
Affiliation(s)
- E H Larsen
- Department of Medicine, University of North Carolina at Chapel Hill 27599-7020, USA.
| | | | | | | | | | | |
Collapse
|
47
|
Lazarowski ER, Watt WC, Stutts MJ, Brown HA, Boucher RC, Harden TK. Enzymatic synthesis of UTP gamma S, a potent hydrolysis resistant agonist of P2U-purinoceptors. Br J Pharmacol 1996; 117:203-9. [PMID: 8825364 PMCID: PMC1909382 DOI: 10.1111/j.1476-5381.1996.tb15175.x] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
1. The defective Cl- secretion characteristic of cystic fibrosis airway epithelial cells can be bypassed by an alternative Ca2+ dependent Cl- secretory pathway that is activated by extracellular nucleotides, e.g. uridine-5'triphosphate (UTP), acting on P2U purinoceptors. Since UTP is susceptible to hydrolysis by nucleotidases and phosphatases present in the airways, the identification of stable P2U-purinoceptor agonists would be of therapeutic relevance. 2. Uridine-5'-O-(3-thiotriphosphate) (UTP gamma S) was synthesized by nucleoside diphosphate kinase-catalyzed transfer of the gamma-phosphorothioate from guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) or adenosine-5' = O-(3-thiotriphosphate) (ATP gamma S) to UDP. Formation of UTP gamma S was illustrated by observation of transfer of 35S from [35S]-GTP gamma S and transfer of 3H from [3H]-UDP. The chemical identity of high performance liquid chromatography (h.p.l.c.)-purified UTP gamma S was confirmed by nuclear magnetic resonance analysis. 3. Human 1321N1 astrocytoma cells stably expressing the phospholipase C-coupled human P2U-purinoceptor were utilized to test the activity of UTP gamma S. UTP gamma S (EC50 = 240 nM) was essentially equipotent to UTP and ATP for stimulation of inositol phosphate formation. 4. Unlike [3H]-UTP, [3H]-UTP gamma S was not hydrolyzed by alkaline phosphatase, acid phosphatase, or apyrase. Moreover, no hydrolysis was detected during a 1 h incubation with human nasal epithelial cells. 5. UTP gamma S was equally potent and efficacious with UTP for stimulation of Cl- secretion by human nasal epithelium from both normal donors and cystic fibrosis patients. Based on its high potency and resistance to hydrolysis, UTP gamma S represents a promising compound for treatment of cystic fibrosis.
Collapse
Affiliation(s)
- E R Lazarowski
- Department of Pharmacology, University of North Carolina, Chapel Hill 27599, USA
| | | | | | | | | | | |
Collapse
|
48
|
Lazarowski ER, Watt WC, Stutts MJ, Boucher RC, Harden TK. Pharmacological selectivity of the cloned human P2U-purinoceptor: potent activation by diadenosine tetraphosphate. Br J Pharmacol 1995; 116:1619-27. [PMID: 8564228 PMCID: PMC1908898 DOI: 10.1111/j.1476-5381.1995.tb16382.x] [Citation(s) in RCA: 199] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
1. The human P2U-purinoceptor was stably expressed in 1321N1 human astrocytoma cells and the pharmacological selectivity of the expressed receptor was studied by measurement of inositol lipid hydrolysis. 2. High basal levels of inositol phosphates occurred in P2U-purinoceptor-expressing cells. This phenomenon was shown to be due to release of large amounts of ATP from 1321N1 cells, and could be circumvented by adoption of an assay protocol that did not involve medium changes. 3. UTP, ATP and ATP gamma S were full and potent agonists for activation of phospholipase C with EC50 values of 140 nM, 230 nM, and 1.72 microM, respectively. 5BrUTP, 2C1ATP and 8BrATP were also full agonists although less potent than their natural congeners. Little or no effect was observed with the selective P2Y-, P2X-, and P2T-purinoceptor agonists, 2MeSATP, alpha,beta-MeATP, and 2MeSADP, respectively. 4. Diadenosine tetraphosphate, Ap4A, was a surprisingly potent agonist at the expressed P2U-purinoceptor with an EC50 (720 nM) in the range of the most potent P2U-purinoceptor agonists. Ap4A may be a physiologically important activator of P2U-purinoceptors.
Collapse
Affiliation(s)
- E R Lazarowski
- Department of Pharmacology, University of North Carolina, School of Medicine, Chapel Hill 27599, USA
| | | | | | | | | |
Collapse
|
49
|
Wei LY, Stutts MJ, Hoffman MM, Roepe PD. Overexpression of the cystic fibrosis transmembrane conductance regulator in NIH 3T3 cells lowers membrane potential and intracellular pH and confers a multidrug resistance phenotype. Biophys J 1995; 69:883-95. [PMID: 8519988 PMCID: PMC1236317 DOI: 10.1016/s0006-3495(95)79962-3] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Because of the similarities between the cystic fibrosis transmembrane conductance regulator (CFTR) and multidrug resistance (MDR) proteins, recent observations of decreased plasma membrane electrical potential (delta psi) in cells overexpressing either MDR protein or the CFTR, and the effects of delta psi on passive diffusion of chemotherapeutic drugs, we have analyzed chemotherapeutic drug resistance for NIH 3T3 cells overexpressing different levels of functional CFTR. Three separate clones not previously exposed to chemotherapeutic drugs exhibit resistance to doxorubicin, vincristine, and colchicine that is similar to MDR transfectants not previously exposed to chemotherapeutic drugs. Two other clones expressing lower levels of CFTR are less resistant. As shown previously these clones exhibit decreased plasma membrane delta psi similar to MDR transfectants, but four of five exhibit mildly acidified intracellular pH in contrast to MDR transfectants, which are in general alkaline. Thus the MDR protein and CFTR-mediated MDR phenotypes are distinctly different. Selection of two separate CFTR clones on either doxorubicin or vincristine substantially increases the observed MDR and leads to increased CFTR (but not measurable MDR or MRP) mRNA expression. CFTR overexpressors also exhibit a decreased rate of 3H -vinblastine uptake. These data reveal a new and previously unrecognized consequence of CFTR expression, and are consistent with the hypothesis that membrane depolarization is an important determinant of tumor cell MDR.
Collapse
Affiliation(s)
- L Y Wei
- Program in Molecular Pharmacology and Therapeutics, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA
| | | | | | | |
Collapse
|
50
|
Abstract
Cystic fibrosis transmembrane regulator (CFTR), the gene product that is mutated in cystic fibrosis (CF) patients, has a well-recognized function as a cyclic adenosine 3',5'-monophosphate (cAMP)-regulated chloride channel, but this property does not account for the abnormally high basal rate and cAMP sensitivity of sodium ion absorption in CF airway epithelia. Expression of complementary DNAs for rat epithelial Na+ channel (rENaC) alone in Madin Darby canine kidney (MDCK) epithelial cells generated large amiloride-sensitive sodium currents that were stimulated by cAMP, whereas coexpression of human CFTR with rENaC generated smaller basal sodium currents that were inhibited by cAMP. Parallel studies that measured regulation of sodium permeability in fibroblasts showed similar results. In CF airway epithelia, the absence of this second function of CFTR as a cAMP-dependent regulator likely accounts for abnormal sodium transport.
Collapse
Affiliation(s)
- M J Stutts
- Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill 27599-7020, USA
| | | | | | | | | | | | | |
Collapse
|