Human GPR17 Nonsynonymous Variants Identified in Individuals with Metabolic Diseases Have Distinct Functional Signaling Profiles

Abstract

GPR17 is a G protein-coupled receptor (GPCR) implicated in the regulation of glucose metabolism and energy homeostasis. Our genetic knockout studies in rodents suggest that GPR17 is a potential therapeutic target for the treatment of metabolic diseases. However, the contributions of GPR17 to human metabolism and metabolic deficits are not well understood. Here, we analyzed the human GPR17 coding sequences of individuals from control and metabolic disease cohorts that were comprised of patients with clinical phenotypes including severe insulin resistance, hypercholesterolemia, and obesity. Across cohorts, 18 nonsynonymous GPR17 variants were identified, including eight variants that were exclusive to the disease cohort. We characterized the protein expression levels, cellular localization, and downstream functional signaling profiles of nine human GPR17 variants (F43L, V96M, V103M, D105N, A131T, G136S, R248Q, R301H, and G354V). We found that the protein expression levels and subcellular localization for each of the nine GPR17 variants were similar to that of the wild type GPR17. As the endogenous GPR17 ligand is still elusive, we used a synthetic GPR17 agonist to quantitatively measure the functional signaling profiles of GPR17 variants. We found some of the variants had distinctly altered signaling profiles. GPR17-G136S lost agonist-mediated cAMP, Ca2+, and beta-arrestin signaling. GPR17-V96M retained cAMP inhibition similar to GPR17-WT but had impaired Ca2+ and beta-arrestin signaling. GPR17-D105N displayed impaired cAMP and Ca2+ signaling but enhanced agonist-stimulated beta-arrestin recruitment. Also, GPR17-G354V retained cAMP and Ca2+ signaling function but had attenuated beta-arrestin recruitment. The identification and functional profiling of naturally occurring human GPR17 variants from individuals with metabolic diseases revealed receptor variants with distinct signaling profiles, including differential signaling perturbations that resulted in receptor signaling bias. These results are expected to contribute to our understanding of the molecular signaling mechanisms underlying GPR17 in metabolic regulation.