title: Rho GTPases dictate the mobility of the Na/H exchanger NHE3 in epithelia: role in apical retention and targeting.
abstract: Proximal tubular reabsorption of filtered sodium by the sodium/hydrogen exchanger isoform 3 (NHE3), located on the apical membrane, is fundamental to the maintenance of systemic volume and pH homeostasis. NHE3 is finely regulated by a variety of hormones and by changes in ionic composition and volume, likely requiring redistribution of the exchangers. We analyzed the subcellular distribution and dynamics of the exchangers by generating an epithelial line expressing NHE3 tagged with an exofacial epitope, which enabled us to monitor exchanger mobility and traffic in intact cells. Using determinations of fluorescence recovery after photobleaching in combination with dynamic measurements of subcellular distribution, we found that, in renal epithelial cells, NHE3 exists in four distinct subcompartments: a virtually immobile subpopulation that is retained on the apical membrane by interaction with the actin cytoskeleton in a manner that depends on the sustained activity of Rho GTPases; a mobile subpopulation on the apical membrane, which can be readily internalized; and two intracellular compartments that can be differentiated by their rate of exchange with the apical pool of NHE3. We provide evidence that detachment of the immobile fraction from its cytoskeletal anchorage leads to rapid internalization. These observations suggest that modulation of the mobile fraction of NHE3 on the apical membrane can alter the number of functional exchangers on the cell surface and, consequently, the rate of transepithelial ion transport. Regulation of the interaction of NHE3 with the actin cytoskeleton can therefore provide a new mode of regulation of sodium and hydrogen transport.
authors.affiliation: Department of Pediatrics and Program in Cell Biology, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8.
authors.forename: R Todd
authors.initials: RT
authors.lastname: Alexander
authors.forename: Wendy
authors.initials: W
authors.lastname: Furuya
authors.forename: Katalin
authors.initials: K
authors.lastname: Szászi
authors.forename: John
authors.initials: J
authors.lastname: Orlowski
authors.forename: Sergio
authors.initials: S
authors.lastname: Grinstein
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D003599:Cytoskeleton / Q000378:metabolism; D004285:Dogs; D004847:Epithelial Cells / Q000378:metabolism* / Q000648:ultrastructure; D036681:Fluorescence Recovery After Photobleaching; D006863:Hydrogen-Ion Concentration; D007684:Kidney Tubules / Q000166:cytology* / Q000378:metabolism; D008854:Microscopy, Electron; D008958:Models, Molecular; D021381:Protein Transport / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D020741:rho GTP-Binding Proteins / Q000378:metabolism*
chemical_list: D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D012964:Sodium; D020741:rho GTP-Binding Proteins; C039614:ethylisopropylamiloride