WNK1 is a central osmolality sensor for arginine vasopressin release and acts through the OSR1/SPAK kinase cascade

Abstract

Background: Maintaining internal osmolality constancy and water homeostasis is essential for life. The circumventricular organs (CVOs) of the brain, including the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), lack a blood-brain barrier. Neurons in OVLT and SFO detect increases in serum osmolality that stimulate the production of vasopressin (AVP) in paraventricular nuclei (PVN) to be released in the posterior pituitary. We have recently reported that WNK1 in sensory neurons in CVOs functions as an osmolality sensor for AVP release (Jin et al., JCI, 2023). WNK1 activates Kv3.1 to increase action potential, traveling down to PVN to increase AVP synthesis for release from the posterior pituitary. Here, we propose that OSR1/SPAK acts as the downstream kinase for WNK1 in regulating AVP release. We further investigated the regulation of vasopressin release by the WNK1-OSR1/SPAK cascade in response to extracellular hyperosmolality. Methods: Cre recombinase-carrying retrograde AAV virus was stereotaxically injected into PVN nuclei from normal mice and SPAK-null mice with a homozygous OSR1-floxed allele (SAPK−/−;OSR1flox/flox). Metabolic cage studies were performed on mice. Urine output, water intake, serum osmolality, serum AVP, and copeptin levels were measured. K+ currents were measured from the OVLT neurons by whole-cell patch clamp. Results: Increased osmolality, either by water restriction or mannitol injection, activated OSR1/SPAK in OVLT, as evident by increased serine-373 phospho-SPAK and serine-325 phospho-OSR. Like the WNK1 deletion, the double deletion of OSR/SPAK in OVLT caused polyuria with decreased urine osmolality that persisted in water restriction. Circulating levels of AVP and copeptin were increased by water restriction in control mice. In contrast, water restriction failed to increase the levels in OSR/SPAK-deletion mice. Knockdown of the Kv3.1 channel in OVLT by shRNA showed the central diabetes insipidus (DI) phenotype as well. Hypertonicity increased K+ currents and WNKs inhibitor prevented the increase in OVLT neurons. Conclusions: Mice with neuronal-specific OSR1/SPAK deletion have water homeostasis defects consistent with partial central DI; Evidence supporting that OSR1/SPAK acts downstream of WNK1 to regulate AVP release; WNK1-OSR1/SPAK activates Kv3.1 to increase firing frequency of neurons projecting from CVOs to PVN. Our findings reveal that an intracellular protein acts as a sensor for extracellular tonicity and provide fresh insights into the mechanisms by which the body maintains osmolality constancy. Future studies will investigate how OSR1/SPAK regulates Kv3.1 channels. NIDDK. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.