The Proximal Tubule Response to Low Potassium Is Mediated by mTOR/AKT Activation via Kir4.2

Background: The renal epithelium is highly sensitive to changes in blood potassium (K+) levels. We have a detailed understanding of distal K+-sensitive signaling pathways, yet proximal mechanisms are less well-defined. Results: Mice lacking the basolateral proximal tubule inwardly rectifying K+ channel, Kir4.2, (Kir4.2−/−) demonstrated normal blood and urine electrolytes at baseline. Upon removal of K+ from their diet, Kir4.2−/− animals decompensated as evidenced by increased urinary K+ excretion and development of a proximal renal tubular acidosis compared to control animals (Kir4.2+/+). Potassium wasting in Kir4.2−/− mice was not proximal in origin, but was dependent upon increased ENaC activity along the connecting segment. This was demonstrated by the correction of K+ wasting in knockouts treated with the ENaC inhibitor, amiloride. Using lithium clearance studies, we further demonstrated increased distal sodium delivery in Kir4.2−/− animals due to reduced proximal tubule sodium reabsorption. Optical clearing and three-dimensional confocal imaging reveal Kir4.2−/− mice failed to undergo proximal tubule hypertrophy when fed a low K+ diet, while the distal convoluted tubule response was exaggerated in knockouts compared to control animals. A phosphoproteomics mass spectrometry analysis identified increased mTOR/AKT signaling as mediating the proximal dietary response to K+ depletion in control animals, and changes to mTOR/AKT signaling was blunted in Kir4.2 knockouts. Lastly, we demonstrated in isolated renal tubules that AKT phosphorylation in response to low K+ depended upon mTORC2 activation by secondary reductions in intracellular Cl−. Conclusions: In this work we identify the basolateral inwardly rectifying K+ channel, Kir4.2, as a mediator of the proximal tubule response to K+ deficiency. Kir4.2 knockout mice increase their distal nephron transport burden to maintain electrolyte homeostasis in the face of deficient proximal tubule cell function. Data identify a conserved proximal role for cell Cl− which, as it does along the distal convoluted tubule, responds to extracellular K+ changes to activate intracellular kinase signaling. Along the proximal tubule, mTOR/AKT signaling is essential for the cell response to low K+. Proximal and distal nephron cell Cl− responses work in concert to preserve K+ homeostasis. These studies were supported by NIH grants DP5-OD033412 (AST), DK51265, DK95785, DK62794, DK7569, P30DK114809 (RCH, MZZ), and the Vanderbilt Center for Kidney Disease and Vanderbilt Diabetes Research and Training Center (DK020593) pilot and feasibility grant (JSD). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.