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Mechanisms of Regulation of Epithelial Sodium Channel by SGK1 in A6 Cells

Mechanisms of Regulation of Epithelial Sodium Channel by SGK1 in A6 Cells The serum and glucocorticoid induced kinase 1 (SGK1) participates in the regulation of sodium reabsorption in the distal segment of the renal tubule, where it may modify the function of the epithelial sodium channel (ENaC). The molecular mechanism underlying SGK1 regulation of ENaC in renal epithelial cells remains controversial. We have addressed this issue in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible system. Expression of a constitutively active mutant of SGK1 (SGK1 T S425D ) induced a sixfold increase in amiloride-sensitive short-circuit current ( I sc ). Using noise analysis we demonstrate that SGK1 effect on I sc is due to a fourfold increase in the number of functional ENaCs in the membrane and a 43% increase in channel open probability. Impedance analysis indicated that SGK1 T S425D increased the absolute value of cell equivalent capacitance by an average of 13.7%. SGK1 T S425D also produced a 1.6–1.9-fold increase in total and plasma membrane subunit abundance, without changing the half-life of channels in the membrane. We conclude that in contrast to aldosterone, where stimulation of transport can be explained simply by an increase in channel synthesis, SGK1 effects are more complex and involve at least three actions: (1) increase of ENaC open probability; (2) increase of subunit abundance within apical membranes and intracellular compartments; and (3) activation of one or more pools of preexistent channels within the apical membranes and/or intracellular compartments. ENaC serum- and glucocorticoid-induced kinase sodium reabsorption open probability channel density Footnotes D. Alvarez de la Rosa's present address is Unidad de Farmacología, Facultad de Medicina, Universidad de La Laguna, 38071 La Laguna, Tenerife, Spain. Abbreviations used in this paper: C eq *, equivalent cell capacitance; CDPC, 6-chloro-3,5-diamino-pyrazine-2-carboxamide; E/C, experimental vs. control ratio; ENaC, epithelial sodium channel; f c , corner-frequency; I Na , blocker-sensitive sodium currents; i Na , single-channel current; I sc , short-circuit current; K B , blocker equilibrium constant; k ob , blocker on rate coefficient; k bo , blocker off rate coefficient; N o , open channel density; N T , functional channel density; PDS, power density spectra; PDK1, 3-phosphoinositide-dependent protein kinase-1; P o , open probability; R T , transepithelial electrical resistance; SGK1, serum and glucocorticoid-induced kinase 1; SGK1 T S425D , transfected constitutively active kinase SGK1; S o , low-frequency plateau value; TetO, tetracycline operator; TetR, tetracycline repressor; V oc , transepithelial open-circuit voltage; x ENaC, Xenopus laevis epithelial sodium channel; Z meas , measured impedance; Z T , transepithelial electrical impedance. Submitted: 7 June 2004 Accepted: 7 September 2004 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of General Physiology Rockefeller University Press

Mechanisms of Regulation of Epithelial Sodium Channel by SGK1 in A6 Cells

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References (43)

Publisher
Rockefeller University Press
Copyright
Copyright © 2004, by The Rockefeller University Press
ISSN
0022-1295
eISSN
1540-7748
DOI
10.1085/jgp.200409120
pmid
15452200
Publisher site
See Article on Publisher Site

Abstract

The serum and glucocorticoid induced kinase 1 (SGK1) participates in the regulation of sodium reabsorption in the distal segment of the renal tubule, where it may modify the function of the epithelial sodium channel (ENaC). The molecular mechanism underlying SGK1 regulation of ENaC in renal epithelial cells remains controversial. We have addressed this issue in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible system. Expression of a constitutively active mutant of SGK1 (SGK1 T S425D ) induced a sixfold increase in amiloride-sensitive short-circuit current ( I sc ). Using noise analysis we demonstrate that SGK1 effect on I sc is due to a fourfold increase in the number of functional ENaCs in the membrane and a 43% increase in channel open probability. Impedance analysis indicated that SGK1 T S425D increased the absolute value of cell equivalent capacitance by an average of 13.7%. SGK1 T S425D also produced a 1.6–1.9-fold increase in total and plasma membrane subunit abundance, without changing the half-life of channels in the membrane. We conclude that in contrast to aldosterone, where stimulation of transport can be explained simply by an increase in channel synthesis, SGK1 effects are more complex and involve at least three actions: (1) increase of ENaC open probability; (2) increase of subunit abundance within apical membranes and intracellular compartments; and (3) activation of one or more pools of preexistent channels within the apical membranes and/or intracellular compartments. ENaC serum- and glucocorticoid-induced kinase sodium reabsorption open probability channel density Footnotes D. Alvarez de la Rosa's present address is Unidad de Farmacología, Facultad de Medicina, Universidad de La Laguna, 38071 La Laguna, Tenerife, Spain. Abbreviations used in this paper: C eq *, equivalent cell capacitance; CDPC, 6-chloro-3,5-diamino-pyrazine-2-carboxamide; E/C, experimental vs. control ratio; ENaC, epithelial sodium channel; f c , corner-frequency; I Na , blocker-sensitive sodium currents; i Na , single-channel current; I sc , short-circuit current; K B , blocker equilibrium constant; k ob , blocker on rate coefficient; k bo , blocker off rate coefficient; N o , open channel density; N T , functional channel density; PDS, power density spectra; PDK1, 3-phosphoinositide-dependent protein kinase-1; P o , open probability; R T , transepithelial electrical resistance; SGK1, serum and glucocorticoid-induced kinase 1; SGK1 T S425D , transfected constitutively active kinase SGK1; S o , low-frequency plateau value; TetO, tetracycline operator; TetR, tetracycline repressor; V oc , transepithelial open-circuit voltage; x ENaC, Xenopus laevis epithelial sodium channel; Z meas , measured impedance; Z T , transepithelial electrical impedance. Submitted: 7 June 2004 Accepted: 7 September 2004

Journal

The Journal of General Physiology – Rockefeller University Press

Published: Oct 1, 2004

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