GIP receptor: The role of post-activation receptor behavior for the incretin effect

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The incretin hormones, GIP and GLP-1, are gut hormones secreted in response to nutrients in the chyme. They have key roles in setting postprandial metabolic tone. Pancreatic β cells are a principal target of the incretins. About 50% of glucose-stimulated insulin secretion is the result of incretin activity. Incretins have effects beyond β cells. For example, GIP enhances insulin-sensitivity of adipocytes, reinforcing a postprandial anabolic metabolism in those cells. Because of the “pro-insulin” activity of the incretins (i.e., enhanced insulin secretion and action), they are excellent drug targets for the treatment of insulin resistance. Some of that therapeutic promise has already been met, as drugs targeting incretins are currently in use. The effects of incretins on whole body physiology, though complex, are fairly well described. More significant gaps remain in our knowledge of incretin biology at the cellular level, which is especially true for GIP. The objective of this project is to address those gaps by a thorough examination of incretin receptor biology. Incretins signal by G protein-coupled receptors (GPCRs). It is well established that the post-activation trafficking of GPCRs contributes significantly to their biological effects. We have created tools to study the behavior of the GIP receptor (GIPR) in adipocytes, a physiologically relevant cell type. Using those tools, we have: i) defined the effect of GIP effect on cultured adipocytes; ii) established that GIPR trafficking does not conform to canonical GPCR behavior; iii) established that the intracellular trafficking of a GIPR coding variant (Q354E) is altered compared to the predominant human GIPR sequence. The Q354E variant is associated with human metabolic alterations. Therefore, those results provide a link between aberrant trafficking and GIPR function. Here I propose to build on that foundation to further define incretin biology at the cellular level and to extend that knowledge to whole animal studies. We will: 1) Characterize regulation of GIPR trafficking and control of GIPR enhancement of insulin action in adipocytes. 2) Define the trafficking of the incretin receptors (GIPR and GLP-1) in β cells, linking incretin receptor cell biology to physiologic activities of the receptor. These studies will also reveal how the behavior/activity of one incretin receptor influences the behavior/activity of the other. 3) Investigate the impact of the GIPR-Q354 variant on metabolism in studies of mice engineered to model the human GIPR-Q354 variant. These studies will provide unique insights into the physiological disruptions induced by altered post-activation trafficking of the GIPR. This project addresses significant gaps in knowledge of incretin biology, an enhanced understanding that will inform future efforts to harness the incretin system in the treatment of metabolic diseases.