The long term goals of this project are to determine the regulation of ion channels in the kidney and to integrate our findings to kidney diseases. This proposal will determine how the renal epithelial sodium channel (ENaC) is regulated by ATP-binding cassette transporter A1 (ABCA1). The proposed in vivo and in vitro experiments will test a central hypothesis that deletion of ABCA1 elevates Cho in cortical collecting duct (CCD) principal cells, this elevated Cho stabilizes phosphatidylinositol-4,5-bisphosphate (PIP2) in apical microvilli, and this microvilli- located PIP2 increases ENaC activity to enhance sodium retention and cause hypertension. The hypothesis is based on previous studies and two key preliminary data showing that both blood pressure and ENaC activity are elevated in ABCA1 KO mice and that inhibition of Cho synthesis causes PIP2 diffusion out of microvilli and reduces ENaC activity. The proposal is clinically significant, because investigation of ABCA1-controlled membrane and intracellular Cho homeostasis may provide a rationale for using Cho biosynthesis inhibitors such as statins to treat hypertension. The project is innovative because it will provide the first evidence for the role of ABCA1 in regulating ENaC activity and PIP2 lateral movement between two specialized apical membrane domains (microvilli and planar regions). A variety of experimental approaches including scanning ion conductance microscopy and the high resolution scanning patch-clamp techniques will be used to test three hypotheses (1) that deletion of ABCA1 increases ENaC activity and Na+ absorption thereby causing hypertension;(2) that Cho increases ENaC activity by stabilizing PIP2 in microvilli;and (3) that PIP2 in planar regions promotes formation of endocytic pits containing inactive ENaC.
The project aims at investigating whether a cholesterol transporter regulates the renal epithelial sodium channel and thereby affecting blood pressure through a pathway associated with a localized membrane lipid.