391-P: Targeted Deletion of Rho-Associated Coiled-Coil–Containing Protein Kinase 2 in Kidney Tubules Suppresses Expression and Function of Mineralocorticoid Receptor in Mice

The mineralocorticoid receptor (MR) is a steroid receptor responsible for fluid and electrolyte reabsorption in kidney tubules. Traditionally, MR has been considered a ligand-activated nuclear transcription factor primarily transactivated by aldosterone. Recent studies have indicated that MR is stimulated in an aldosterone-independent manner by Rac1, a member of the Rho small GTPase family. Rho and its effector Rho-associated coiled-coil-containing protein kinase 2 (ROCK2) plays important roles in various cellular functions such as cell adhesion and cytoskeletal rearrangement. However, the tissue-specific role of ROCK2 in kidney tubules is not fully understood. To investigate the role of ROCK2 signaling in tubules, we generated tubule-specific ROCK2 knockout mice (TR2KO) by crossing Ksp-Cre mice with mice containing loxP sites flanking in Exon 3 of ROCK2. These mice displayed no significant differences in body appearance or blood pressure compared to wild-type mice (WT). However, when sodium reabsorption was activated by low dietary potassium, the urinary excretion of sodium and chloride in TR2KO was significantly higher than that in WT. MR and its downstream target, epithelial sodium channel (EnaC), were significantly downregulated in TR2KO compared with WT at mRNA and protein levels. To identify factors related to the regulation of MR by ROCK2, we obtained several candidates from the Chip Atlas. Among these, the expression of STAT3 protein was significantly reduced in TR2KO. Using HEK293-MR cells, which stably expressed human MR, we found that siRNA-mediated STAT3 deletion led to the downregulation of MR at the mRNA level. These findings provide novel insights into the ROCK2 signaling in kidney tubules and establish a hitherto unidentified regulatory mechanism of MR in modulating electrolyte homeostasis.