mTORC2 critically regulates renal potassium handling

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Abstract

The mTOR pathway orchestrates cellular homeostasis. The rapamycin-sensitive mTOR complex (mTORC1) in the kidney has been widely studied; however, mTORC2 function in renal tubules is poorly characterized. Here, we generated mice lacking mTORC2 in the distal tubule (Rictor^fl/fl Ksp-Cre mice), which were viable and had no obvious phenotype, except for a 2.5-fold increase in plasma aldosterone. Challenged with a low-Na⁺ diet, these mice adequately reduced Na⁺ excretion; however, Rictor^fl/fl Ksp-Cre mice rapidly developed hyperkalemia on a high-K⁺ diet, despite a 10-fold increase in serum aldosterone levels, implying that mTORC2 regulates kaliuresis. Phosphorylation of serum- and glucocorticoid-inducible kinase 1 (SGK1) and PKC-α was absent in Rictor^fl/fl Ksp-Cre mice, indicating a functional block in K⁺ secretion activation via ROMK channels. Indeed, patch-clamp experiments on split-open tubular segments from the transition zone of the late connecting tubule and early cortical collecting duct demonstrated that Ba²⁺-sensitive apical K⁺ currents were barely detectable in the majority of Rictor^fl/fl Ksp-Cre mice. Conversely, epithelial sodium channel (ENaC) activity was largely preserved, suggesting that the reduced ability to maintain K⁺ homeostasis is the result of impaired apical K⁺ conductance and not a reduced electrical driving force for K⁺ secretion. Thus, these data unravel a vital and nonredundant role of mTORC2 for distal tubular K⁺ handling.

Authors

Florian Grahammer, Viatcheslav Nesterov, Azaz Ahmed, Frederic Steinhardt, Lukas Sandner, Frederic Arnold, Tomke Cordts, Silvio Negrea, Marko Bertog, Marcus A. Ruegg, Michael N. Hall, Gerd Walz, Christoph Korbmacher, Ferruh Artunc, Tobias B. Huber