Towards the Development of Small-Molecule MO25 Binders as Potential Indirect SPAK/OSR1 Kinase Inhibitors

The binding of the scaffolding protein MO25 to SPAK and OSR1 protein kinases, which regulate ion homeostasis, causes increases of up to 100-fold in their catalytic activity. Various animal models have shown that the inhibition of SPAK and OSR1 lowers blood pressure, and so here we present a new indirect approach to inhibiting SPAK and OSR1 kinases by targeting their protein partner MO25. To explore this approach, we developed a fluorescent polarisation assay and used it in screening of a small in-house library of ≈4000 compounds. This led to the identification of one compound—HK01—as the first small-molecule inhibitor of the MO25-dependent activation of SPAK and OSR1 in vitro. Our data confirm the feasibility of targeting this protein–protein interaction by small-molecule compounds and highlights their potential to modulate ion co-transporters and thus cellular electrolyte balance.