title: Rab4 GTP/GDP modulates amiloride-sensitive sodium channel (ENaC) function in colonic epithelia.
abstract: The sodium-selective amiloride-sensitive epithelial sodium channel (ENaC) mediates electrogenic sodium re-absorption in tight epithelia. ENaC expression at the plasma membrane requires regulated transport, processing, and macromolecular assembly of subunit proteins in a defined and highly compartmentalized manner. Ras-related Rab GTPases monitor these processes in a highly regulated sequence of events. In order to evaluate the role of Rab proteins in ENaC function, Rab4 wild-type (WT), the GTPase-deficient mutant Rab4Q67L, and the dominant negative GDP-locked mutant Rab4S22N were over-expressed in the colon cancer cell line, HT-29 and amiloride-sensitive currents were recorded. Rab4 over-expression inhibited amiloride-sensitive currents. The effect was reversed by introducing Rab4-neutralizing antibody and Rab4 specific SiRNA. The GDP-locked Rab4 mutant inhibited, while GTPase-deficient mutant moderately stimulated amiloride-sensitive currents. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. Immunoprecipitation and pull-down assay suggest protein-protein interaction between Rab4 and ENaC. In addition, the functional modulation coincides with concomitant changes in ENaC expression at the cell surface and in intracellular pool. We propose that Rab4 is a critical element that regulates ENaC function by mechanisms that include GTP-GDP status, recycling, and expression level. Our observations imply that channel expression in apical membranes of epithelial cell system incorporates RabGTPase as an essential determinant of channel function and adds an exciting paradigm to ENaC therapeutics.
authors.affiliation: Center for Cell and Molecular Biology, Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ 07030, USA. ssaxena@stevens.edu
authors.forename: Sunil K
authors.initials: SK
authors.lastname: Saxena
authors.forename: Madhurima
authors.initials: M
authors.lastname: Singh
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Shibata
authors.forename: Simarna
authors.initials: S
authors.lastname: Kaur
authors.forename: Constantine
authors.initials: C
authors.lastname: George
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D003106:Colon / Q000187:drug effects / Q000378:metabolism*; D015536:Down-Regulation; D053503:Epithelial Sodium Channels; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D019073:HT29 Cells; D006801:Humans; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000378:metabolism* / Q000502:physiology; D020695:rab4 GTP-Binding Proteins / Q000502:physiology*
chemical_list: D053503:Epithelial Sodium Channels; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D006153:Guanosine Diphosphate; D000584:Amiloride; D006160:Guanosine Triphosphate; D020695:rab4 GTP-Binding Proteins