SPAK, OSR1 and Cab39/MO25 form an Interdependent Signaling system which Regulates Thiazide‐Sensitive Salt‐transport, Distal Tubule Mass and Blood Pressure

Abstract

STE20/SPS‐1‐related proline‐alanine‐rich protein kinase (SPAK) and, Oxidative Stress Related Kinase (OSR1), co‐localize at the apical membrane of the Thick ascending Limb (TAL) and Distal Convoluted Tubule (DCT) and both regulate the potassium‐dependent sodium‐chloride co‐transporter, NKCC2, thiazide‐sensitive sodium‐chloride cotransporter, NCC in vitro. Yet genetic ablation of SPAK in mice causes a salt‐wasting nephropathy that is restricted to the DCT, reminiscent of Gitelman's syndrome. Here, we explore why proper DCT function is especially SPAK‐dependent. In the TAL of SPAK−/− mice, OSR1 and Cab39/MO25, a newly described OSR1/SPAK regulatory protein, remain at the apical membrane where they function with a compensatory increase in the AMP‐activated kinase (AMPK) to hyper‐phosphorylate NKCC2. By contrast, the OSR1/SPAK/M025 signal transduction apparatus is completely disrupted in the DCT. OSR1 and MO25 become largely displaced from the thiazide‐sensitive sodium‐chloride cotransporter, NCC, and the apical membrane. OSR1 redistributes to dense punctate structures within the cytoplasm. These changes are paralleled by a dramatic decrease in NCC abundance and phosphorylation. Without SPAK and the proper localization of OSR1 and MO25, phosphorylation‐dependent regulation of NCC by dietary sodium restriction is lost. SPAK−/− mice also exhibit a decrease in the mass of the distal convoluted tubule, exclusive to DCT1. As a result of the interdependent nature of OSR1 and MO25 on SPAK in the DCT, SPAK−/− mice are highly sensitive to dietary salt‐restriction, displaying prolonged negative sodium balance and hypotension.