Inhibition of SGK1 Ameliorates Ventriculomegaly in a Genetic Rat Model Via Regulation of TRPV4 in the Choroid Plexus

Hydrocephalus is a clinical manifestation of cerebrospinal fluid (CSF) accumulation, leading to ventriculomegaly and increased intracranial pressure. Diuretics and other pharmacotherapies have failed to treat hydrocephalus in the human population; therefore, treatment options remain exclusively surgical. The need to develop pharmacotherapies for the treatment of hydrocephalus remains an unmet clinical need and paramount to this need is the study of brain fluid regulation. Previous studies in our laboratory identified the transient receptor potential vanilloid 4 (TRPV4) channel as a potential regulatory hub protein in the choroid plexus responsible, in part, for the regulation of CSF production. Antagonists of the TRPV4 channel were shown to ameliorate hydrocephalus in a genetic rat model. The current studies highlight a novel drug target: serum- and glucocorticoid-induced kinase 1 (SGK1) in the choroid plexus. A specific small molecular weight, membrane permeant inhibitor of SGK1, SI113, has shown promising results in cancer and metabolic research in recent years. In other tissues, SGK1 has been shown to phosphorylate and activate TRPV4 at the membrane, indicating SGK1 as an important upstream regulatory kinase. SI113 blocked TRPV4-mediated transepithelial ion flux and barrier conductance increases in a cell culture model of human choroid plexus epithelia (HIBCPP line), and ameliorated ventriculomegaly in the genetic rat model. In hydrocephalic animals, there is an increased apical membrane signal in the choroid plexus epithelial cells via immunofluorescent localization of p-SGK1 and TRPV4, but not of pan-SGK1. This is not accompanied by a significant increase in either protein or transcript abundance as measured by western blotting or qPCR, respectively. The change in localization is, however, attenuated by treatment with SI113. Animals treated with SI113 show no change in amounts of TRPV4 or total SGK1, but do show significantly reduced levels of p-SGK1. Thus, phosphorylation of SGK1 appears to be crucial for the activation of TRPV4 which is, in turn, a channel protein that is activated during the development of hydrocephalus. Like Trpv4-/-mice, Sgk1-/-mice do not show overt phenotypes, indicating pharmacological antagonism of the kinase would have minimal off-target effects. Overall, these data provide a strong preclinical basis for the use of SGK1 inhibitors in the treatment of hydrocephalus.