Central Osmolality Sensing for Arginine Vasopressin Release Is Mediated by WNK1-OSR1/SPAK-Kv3.1 Cascade

Life depends on maintaining water homeostasis and internal osmolality constancy. In terrestrial animals, the release of the antidiuretic hormone arginine vasopressin (AVP) in response to variations of extracellular osmolality (tonicity) is crucial. We have reported that WNK1 kinase in the vascular-organ-of-lamina-terminalis (OVLT) nuclei of the brain mediates the hypertonicity-induced AVP release by activating the voltage-gated K⁺ channel Kv3.1 increasing action potential firing. The downstream mechanism for WNK1-mediated osmosensation is unknown. Here, we showed that the hypertonicity-induced increases in Kv3.1 currents in cultured cells required the oxidative stress-responsive-1 (OSR1) or STE20/SPS1-related proline/alanine-rich (SPAK) kinase. Both kinases were present in the mouse OVLT area. Hypertonicity induced by water restriction or mannitol injection increased the abundance of phosphorylated OSR1 and SPAK in the OVLT. Double deletion of Osr1 and Spak in the OVLT in mice caused polyuria with relative hypotonic urine that persisted in water restriction. The water restriction-induced AVP release was blunted in Osr1 and Spak-deleted mice. In brain slice recordings, the hypertonicity-induced increases in action potential firing in OVLT were blunted by Osr1 and Spak deletion. Deletion of the Kv3.1 channel in the OVLT showed a similar phenotype. Expression of the constitutively active OSR1 in the OVLT resulted in increased AVP release and inappropriate antidiuretic hormone secretion phenotype. In summary, OSR1/SPAK acts downstream of WNK1 to regulate AVP release in response to hypertonicity. In OVLT neurons OSR1/SPAK activates Kv3.1 to increase action potential firing. Thus, the WNK1-OSR1/SPAK-Kv3.1 cascade regulates water homeostasis and AVP release to control osmolality stability.