Aldosterone mediates activation of the thiazide-sensitive Na-Cl cotransporter through an SGK1 and WNK4 signaling pathway
David J. Rozansky, Tonya Cornwall, Arohan R. Subramanya, Shaunessy Rogers, Yong-Feng Yang, Larry L. David, Xiaoman Zhu, Chao-Ling Yang, David H. Ellison
David J. Rozansky, Tonya Cornwall, Arohan R. Subramanya, Shaunessy Rogers, Yong-Feng Yang, Larry L. David, Xiaoman Zhu, Chao-Ling Yang, David H. Ellison
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Research Article Nephrology

Aldosterone mediates activation of the thiazide-sensitive Na-Cl cotransporter through an SGK1 and WNK4 signaling pathway

Abstract

Aldosterone regulates volume homeostasis and blood pressure by enhancing sodium reabsorption in the kidney’s distal nephron (DN). On the apical surface of these renal epithelia, aldosterone increases expression and activity of the thiazide-sensitive Na-Cl cotransporter (NCC) and the epithelial sodium channel (ENaC). While the cellular mechanisms by which aldosterone regulates ENaC have been well characterized, the molecular mechanisms that link aldosterone to NCC-mediated Na+/Cl– reabsorption remain elusive. The serine/threonine kinase with-no-lysine 4 (WNK4) has previously been shown to reduce cell surface expression of NCC. Here we measured sodium uptake in a Xenopus oocyte expression system and found that serum and glucocorticoid–induced kinase 1 (SGK1), an aldosterone-responsive gene expressed in the DN, attenuated the inhibitory effect of WNK4 on NCC activity. In addition, we showed — both in vitro and in a human kidney cell line — that SGK1 bound and phosphorylated WNK4. We found one serine located within an established SGK1 consensus target sequence, and the other within a motif that was, to our knowledge, previously uncharacterized. Mutation of these target serines to aspartate, in order to mimic phosphorylation, attenuated the effect of WNK4 on NCC activity in the Xenopus oocyte system. These data thus delineate what we believe to be a novel mechanism for aldosterone activation of NCC through SGK1 signaling of WNK4 kinase.

Authors

David J. Rozansky, Tonya Cornwall, Arohan R. Subramanya, Shaunessy Rogers, Yong-Feng Yang, Larry L. David, Xiaoman Zhu, Chao-Ling Yang, David H. Ellison

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Figure 8

WNK4, WNK1, SGK1, and aldosterone paradox hypothesis.

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WNK4, WNK1, SGK1, and aldosterone paradox hypothesis. WNK4 kinase inhibi...
WNK4 kinase inhibits NCC activity. Multiple secondary messengers modulate WNK4 action. L-WNK1 inhibits WNK4, but is suppressed by ks-WNK1. L-WNK1 also phosphorylates OSR1/SPAK, thereby activating NCC. Symbols are as in Figure 1; secondary messenger proteins and ion transporter sizes vary relative to their proposed activity. (A) Our results suggest that SGK1, induced transcriptionally by aldosterone and activated by PI3K, negatively regulates WNK4’s action on NCC by phosphorylating 2 C-terminal serines of WNK4. SGK1 also positively effects ENaC and ROMK activity in the cortical collecting duct (CCD) through regulatory pathways that include SGK1. (B) Schematic proposal of renal response to hypovolemia, through hyperreninemia and/or stimulation of angiotensin II and aldosterone. Here SGK1 and WNK1 target similar C-terminal WNK4 serines for phosphorylation, relieving the WNK4 inhibitory effect on NCC, increasing NCC-mediated Na+/Cl– cotransport (shown by arrow thickness) while diminishing the regulatory value of SGK1 and other mechanisms in the CCD that lead to K+ secretion. (C) Schematic proposal of renal response to aldosterone stimulation by hyperkalemia. In the DCT there is upregulation of ks-WNK1 activity, which — by squelching L-WNK1 — leads to heightened WNK4 activity, low SPAK/OSR1 activity, and reduced NCC-mediated NaCl transport. We surmise that SGK1 has a transient effect on WNK4 with downstream ENaC and ROMK activation by SGK1 and other aldosterone-sensitive mechanisms, restoring potassium balance without affecting extracellular volume.