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mTOR Modulates Sodium and Potassium Conduction in Heterologous Expression System

The role of mammalian target of rapamycin (mTOR) complexes mTORC1 and mTORC2 in ion transport in kidney tubule has been an area of increasing interest in epithelial research. We and others have shown both in cultured cells and in vivo that mTORC2 is the hydrophobic motif kinase for serum and glucocorticoid kinase 1 (SGK1) and that its activity is required for epithelial Na+ channel (ENaC)‐dependent sodium reabsorption in the aldosterone‐sensitive distal nephron (ASDN). However, recent evidence has shown that mTORC2 can also modulate the renal outer medullary K+ channel (ROMK) activity directly. Electrogenic sodium reabsorption through ENaC generates the main driving force in ASDN for potassium secretion through ROMK, and is a key locus of acute regulation of K+ secretion. We suspected that differences observed in various studies may be due to acute vs. chronic regulation. We therefore compared the effects on ENaC and ROMK activity of chronic (through gene‐knockout) and acute (through a pharmacological approach) mTOR inhibition. We used CRISPR‐Cas9 to generate Sin1 (an essential component of mTORC2)‐deficient HEK‐293T cells, which were compared with wild type cells. The cells were transiently transfected with ENaC (alpha, beta gamma subunits) and ROMK1 channels. After recovery, the cells were treated with either vehicle or the mTOR inhibitor AZD8055 for 30 min and subjected to whole cell patch clamp to assess ENaC and ROMK currents. We found that in Sin1 KO cells (chronic loss of mTORC2) ENaC current was unaffected, while ROMK was markedly reduced. In contrast, acute mTOR inhibition with AZD8055 inhibited ENaC but not ROMK whole cell currents. These data provide initial support for the idea that acute pharmacologic inhibition of mTORC2 and chronic selective inhibition of mTORC2 have opposite effects on ENaC and ROMK activities. However, the results are also consistent with the possibility that global inhibition of mTOR (blocking mTORC1 and mTORC2) has distinct effects from selective inhibition of mTORC2. These observations may provide further insight into such diverse processes as aldosterone escape and the distinct plasma K+ concentrations seen in various types of hypertension.