mTORC2 critically regulates renal potassium handling

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 (Rictorfl/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, Rictorfl/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 Rictorfl/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 Rictorfl/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.

Potassium depletion stimulates Na-Cl cotransporter via phosphorylation and inactivation of the ubiquitin ligase Kelch-like 3

“…Recent studies demonstrated the role of PKCα in regulating electrolyte handling in the distal nephron [26]. We next performed Western blotting using phospho-specific antibody that recognizes active PKCα/β (bearing phosphorylation at Thr638/641).…”

Potassium acts through mTOR to regulate its own secretion

“…In addition to ENaC, it is also important to note that a role for regulation of K + channels themselves, in particular ROMK, cannot be ruled out (46). Indeed, one recent report found that genetic disruption of mTORC2 (via constitutive Rictor knockout) caused hyperkalemia by disrupting both ENaC and ROMK (9). Although this may play a role in chronic regulation, the bulk of evidence supports the idea that acute regulation proceeds predominantly through ENaC-dependent control of the electrical gradient that drives K + secretion (22).…”

Mitochondrial energetics in the kidney

“…mTORC1, which is a complex of mTOR, regulatory-associated protein of mTOR (Raptor) and several other proteins, regulates cell growth and proliferation and inhibits autophagy by stimulating anabolic processes. mTORC2, which is a complex of mTOR, rapamycin- insensitive companion of mTOR (Rictor) and several other proteins, is thought to regulate potassium and sodium levels in the kidney 48,49 . mTORC1 is considered a nutrient sensor because it can be activated by growth factors, nutrients such as amino acids and glucose, and oxidative stress, triggering pathways that lead to protein synthesis, nucleotide synthesis, lipid synthesis and mitochondrial biogenesis by activating the transcriptional repressor yin and yang 1 (YY1) 46,50 .…”