Preclinical Model of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) via Gain-of-Function of WNK1-SPAK-Kv3.1 Cascade

Background: Life depends on maintaining water homeostasis and internal osmolality stability. The brain's circumventricular organs, the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT), lack the blood-brain barrier. Neurons in OVLT and SFO detect changes in serum osmolality, conveying the signals in forms of action potentials to the paraventricular nuclei (PVN) to produce the arginine vasopressin (AVP), to be released in the posterior pituitary. We have recently reported that WNK1 in OVLT nuclei functions as an osmolality sensor for AVP release. WNK1 may do so by directly phosphorating downstream effectors, through the intermediate OSR1/SPAK kinases or other kinases such as PKA. Here, we test the hypothesis that OSR1/SPAK kinase cascade mediates WNK1 regulation of AVP release through Kv3.1. Methods: In-vitro/ex-vivo studies include electrophysiological (EP) recording on HEK cell, isolated OVLT neurons. Metabolic cage clearance studies on mice with region-specific gene deletion or activation by stereotaxic injection of Cre-recombinase or retrograde AAV virus into OVLT or PVN nuclei in mice homozygous for Osr1/Spak-floxed allele, Kv3.1-floxed allele, constitutive-active Osr1-floxed allele, and control mice. Results: EP recording on HEK and isolated OVLT neurons established WNK1-OSR1/SPAK cascade in activating Kv3.1 in response to hypertonicity. OVLT expressed both OSR1 and SPAK, with SPAK higher abundance. Injection of mannitol or water restriction in mice revealed activation of OSR1/SPAK in OVLT evidenced by phospho-SPAK/OSR1. Double deletion of OSR1/SPAK in OVLT caused polyuria with decreased urine osmolality that persisted in water restriction and blunted water restriction-induced AVP release. In the brain slice recording, hyperosmolality-induced increases in action potential firing in OVLT was blunted by OSR1/SPAK deletion. Deletion of the Kv3.1 channel in OVLT showed the central diabetes insipidus phenotype as well. Induced expression of the constitutively active OSR1 in OVLT resulted in increased AVP release and inappropriate antidiuretic hormone secretion phenotype. Conclusion: Hypertonicity stimulates WNK1 in OVLT to activate Kv3.1, increase AP frequency and AVP release through OSR1/SPAK kinase. Loss of function of the cascade manifests as partial central diabetes insipidus whereas GOF as SIADH Funding: NIDDK Support