40-OR: Effect of Naturally Occurring Genetic Variations in Incretin Receptors on Glucose Homeostasis

Type 2 diabetes is characterized by disruption of glucose homeostasis. Insulin is the primary driver of glucose homeostasis and released by pancreatic β cells in response to nutritional and endocrine signals. Insulin release is also stimulated by incretin hormones including Glucagon-Like peptide 1 (GLP-1) which acts on the GLP-1 receptor (GLP1-R) in islet producing the “Incretin effect.” Common genetic variations in GLP-1R alter this incretin effect. Specifically, the R131Q variant increases while the G168S variant decreases the effect of GLP-1. We have found that these genetic variations do not affect the affinity of GLP-1 nor its efficacy for G protein signaling. Taken together, these findings present a “pharmacological mystery” as to how genetic variation at GLP-1R affects incretin function. In this study, we assessed whether genetic variation in GLP-1R alters receptor trafficking resulting in differential incretin response. HEK293 cells stably expressing wild type (WT), R131Q, and G168S variants were used to measure receptor endocytosis and post-endocytic sorting to the lysosome. We found that WT GLP-1R degrades after endocytosis via its interaction with the GPCR-associated sorting protein-1 (GASP1), a protein highly expressed in islets. Importantly, we found that GLP-1R variant R131Q recycled following endocytosis while variant G168S was more rapidly degraded compared to the WT GLP-1R. To assess whether GASP1 mediated GLP-1R degradation affects incretin function, we used CRISPR to generate rat INS-1 cells with a knockout (KO) of GASP1. Deletion of GASP1 had no effect on acute GLP-1R signaling. However, prolong agonist pretreatment produced a loss of the incretin response in WT INS-1 cells, while GASP1 KO cells showed preservation of the incretin effect. These results suggest GASP1 mediates the post-endocytic degradation of GLP-1R in β cells and that altered trafficking of the GLP-1R variants may underlie differential incretin response in humans who carry these variants.