Potassium acts through mTOR to regulate its own secretion
Mads Vaarby Sørensen, Bidisha Saha, Iben Skov Jensen, Peng Wu, Niklas Ayasse, Catherine E. Gleason, Samuel Levi Svendsen, Wen-Hui Wang, David Pearce
Mads Vaarby Sørensen, Bidisha Saha, Iben Skov Jensen, Peng Wu, Niklas Ayasse, Catherine E. Gleason, Samuel Levi Svendsen, Wen-Hui Wang, David Pearce
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Research Article Cell biology Nephrology

Potassium acts through mTOR to regulate its own secretion

Abstract

Potassium (K+) secretion by kidney tubule cells is central to electrolyte homeostasis in mammals. In the K+-secreting principal cells of the distal nephron, electrogenic Na+ transport by the epithelial sodium channel (ENaC) generates the electrical driving force for K+ transport across the apical membrane. Regulation of this process is attributable in part to aldosterone, which stimulates the gene transcription of the ENaC-regulatory kinase, SGK1. However, a wide range of evidence supports the conclusion that an unidentified aldosterone-independent pathway exists. We show here that in principal cells, K+ itself acts through the type 2 mTOR complex (mTORC2) to activate SGK1, which stimulates ENaC to enhance K+ excretion. The effect depends on changes in K+ concentration on the blood side of the cells, and requires basolateral membrane K+-channel activity. However, it does not depend on changes in aldosterone, or on enhanced distal delivery of Na+ from upstream nephron segments. These data strongly support the idea that K+ is sensed directly by principal cells to stimulate its own secretion by activating the mTORC2/SGK1 signaling module, and stimulate ENaC. We propose that this local effect acts in concert with aldosterone and increased Na+ delivery from upstream nephron segments to sustain K+ homeostasis.

Authors

Mads Vaarby Sørensen, Bidisha Saha, Iben Skov Jensen, Peng Wu, Niklas Ayasse, Catherine E. Gleason, Samuel Levi Svendsen, Wen-Hui Wang, David Pearce

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Figure 5

mTORC2 activity is required for the effects of extracellular [K+] on ENaC.

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mTORC2 activity is required for the effects of extracellular [K+] on ENa...
(A) mTOR inhibition blocks K+-induced ENaC current in native CCD. Mouse collecting tubules were dissected in 5 mM [K+] buffer and subjected to apical membrane patch clamp, as in Figure 3A. The mTOR inhibitor AZD8055 was added, after which the bath [K+] was either changed to 1 mM or maintained at 5 mM [K+]. Whole-cell measurements were made at a holding potential of –60 mV. n = 5 or 3 mice for each group as indicated in Supplemental Table 1. *P < 0.05, **P < 0.01, ****P < 0.0001 by 1-way ANOVA: the differences are significant between 1 and 5 mM K+ and between AZD8055 treatments in comparison to their corresponding controls. pA, picoamperes. (B) mTOR inhibition blocks K+-induced ENaC current in mpkCCD cells. Cells were preincubated in 1 mM [K+], as in Figure 3B. AZD8055 or vehicle was added as indicated, after which [K+] was shifted to 5 mM where shown. Summary graph shown represents effect of AZD8055 on amiloride-sensitive Na+ current. Data are mean ± SEM from 3 independent experiments. ****P < 0.0001 by 1-way ANOVA. μA, microamperes. (C) AZD8055 blocks SGK1 S422 phosphorylation in mpkCCD cells. Cells from Figure 5B were lysed and prepared for Western blot, and stained with antibodies as indicated. Left panel: Western blot images showing blots stained with anti–phospho-SGK1 S422, and total SGK1 as labeled. Right panel: Quantitation of phospho-SGK1/total SGK1 (as described in Methods). Data are mean ± SEM from 3 independent experiments. **P < 0.01, ***P < 0.001, ****P < 0.0001 by 1-way ANOVA.