Regulation of WNK signaling by potassium and Mo25: structure, function and physiology

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Mutations in human WNK (With No Lysine) kinases are associated with hyperkalemia, hypertension and chronic kidney disease. However, despite extensive characterization of (patho)physiological roles of WNKs in the kidney and extrarenal tissues, there is surprisingly little understanding of how WNKs themselves are regulated. WNKs regulate epithelial ion transport in the mammalian kidney. The applicants’ long-term goal is to achieve mechanistic understanding of epithelial ion transport mechanisms relevant to human kidney function. Chloride ion is known to regulate WNKs. The overall objective of this application is to define and understand new mechanisms of WNK regulation. This renewal application builds on three significant advances from the currently funded grant. First, potassium inhibits Drosophila and mammalian WNKs through chloride-independent mechanisms. Second, the scaffold protein Mo25 (Mouse protein 25/Cab39) is an important regulator of WNK signaling, and activates WNKs independent of its known effects on SPAK (Ste20-related proline alanine rich kinase) and OSR1 (oxidative stress response) kinases. Third, potassium and Mo25 have differential effects on the kidney-expressed mammalian WNKs 1, 3 and 4. The central hypothesis is that potassium and Mo25 directly regulate WNK kinase activity, with differential effects on mammalian WNK isoforms. Guided by strong preliminary data, the central hypothesis will be tested by pursuing three specific aims: 1) Determine the mechanism and physiological consequences of WNK regulation by potassium; 2) Elucidate novel mechanisms of Mo25 regulation of WNK signaling; and 3) Determine the molecular basis for differential WNK isoform regulation by potassium and Mo25. The approach is innovative by leveraging insights from biophysical and structural studies to determine molecular mechanisms of epithelial ion transport regulation, using a unique platform, the Drosophila Malpighian tubule, that has powerful molecular genetic tools and tractable physiologic readouts. Assays have been established, and demonstrated feasible in the investigators’ hands, to: 1) identify WNK potassium binding sites, generate potassium-insensitive WNK mutants, and test their effects on transepithelial ion transport; 2) determine how Mo25 regulates WNK activity in vitro and in the tubule, and probe the interactions between Mo25, potassium and chloride in WNK regulation; and 3) test differences in potassium and Mo25 regulation of WNKs 1, 3 and 4. Successful completion of the proposed studies will elucidate the most comprehensive mechanistic understanding of WNK regulation achieved to date. This is significant, because delineation of the importance of these WNK regulatory mechanisms in various (patho)physiological contexts, together with the molecular insights gained from the studies proposed here, will allow the development of targeted approaches to therapeutically modulate WNK signaling. This has translational impact in a broad range of conditions, including the treatment of dyskalemias, salt-sensitive hypertension, cardio- and cerebrovascular disease, and kidney disease.