Gpr116 Regulation of Renal Acid Excretion

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Project Summary G protein-coupled receptors (GPCRs) are a diverse family of integral membrane proteins that recognize an assortment of extracellular molecules including neurotransmitters, hormones, light and odors. Accordingly, they are important pharmaceutical targets, with over 1200 FDA approved drugs targeting GPCRs. However, many GPCRs have unknown biologic roles. Thus, uncovering the function and physiologic significance of understudied GPCRs represents a wealth of untapped therapeutic potential. Our lab previously reported that an adhesion-class GPCR (aGPCR), Gpr116, is among the most highly expressed GPCRs in the kidney. However, its role in renal physiology had not been investigated. Recently published data presented in the current proposal demonstrate localization of Gpr116 to the apical membrane of A-intercalated cells (AICs) in murine collecting ducts. Furthermore, deletion of Gpr116 from AICs using a targeted Cre-Lox recombination system revealed Gpr116 to be a critical regulator of renal acid excretion. Mice deficient for Gpr116 in the kidney have significantly reduced urine pH and are incapable of further acidifying their urine after induction of a metabolic acidosis, suggesting that loss of Gpr116 is sufficient to maximally acidify urine. Notably, the reduction in urine pH is accompanied with an increase in blood pH and a decrease in pCO2, an acid/base disorder we term “renal tubular alkalosis”. Moreover, Gpr116-null animals have significantly more surface expression of V-ATPase proton pumps in AICs. Although these findings are significant, there are still substantial gaps in our knowledge regarding the function of Gpr116 in the kidney. For example, the molecular mechanisms that cause urine acidification in the absence of Gpr116 in AICs are unknown, and the endogenous activator of the receptor has yet to be identified. Therefore, the overarching goal of this proposal is to address these critical components of Gpr116 renal physiology. Based on previous observations and strong preliminary data outlined in this proposal, I hypothesize that Gpr116 activation is facilitated by membrane-bound glycoproteins in the microvilli of stimulated AICs, leading to a Rho GTPase cascade and a retrieval of V-ATPase from the luminal membrane. These support my central hypothesis that Gpr116 acts as a negative-feedback element to inhibit excessive acid secretion through regulation of V-ATPase endocytosis. I have proposed the following specific aims to test this hypothesis: 1) Determine the Gpr116-activated pathways that inhibit V-ATPase surface expression; and 2) Identify interactions and luminal conditions that facilitate endogenous activation of Gpr116 in AICs. Completion of these studies will define the molecular pathways relevant to Gpr116 in the kidney for the first time and establish the therapeutic potential of Gpr116 as a targeted modulator of renal acid excretion.