Molecular mechanisms underlying divergent incretin receptor responses in alpha versus beta cells

Diabetes is a disease that has reached epidemic proportions, with millions of people dying or suffering from a myriad of associated complications. Amongst the current treatments for the disease are incretin therapies, which are based on mimicking the function of incretin hormones, secreted from the gut in response to food intake and able to stimulate the release of insulin from the pancreas by binding specifically to proteins at the surface of cells in the pancreas called receptors. The incretins can also activate their target receptors in the brain to promote fullness and hunger reduction which is an added benefit for diabetic patients who require weight loss. There are two incretins, GLP-1 and GIP, and each one binds to its own receptor. Most incretin therapies to date have been directed at the GLP-1 receptor because the GIP receptor does not work at all in diabetic patients. However, we now know that if blood sugar levels are brought under control by other means, the GIP receptor will start working again, and as this is a very strong receptor that can work even better than the GLP-1 receptor in normal conditions, there is a good case for developing therapies that target both receptors at the same time. This has recently been achieved with a new medicine called tirzepatide, giving superior effects in controlling blood sugar levels as well as promoting weight loss. However, the way that these two receptors work in the pancreas is not completely clear. While they both promote insulin release from pancreatic beta cells, the GLP-1 receptor inhibits the release of another hormone, glucagon, from another cell type in the pancreas called alpha, which balances the effect of insulin, while the GIP receptor promotes glucagon release from these alpha cells. It is also not clear if glucagon is good or bad for diabetes, as on the one hand it promotes blood sugar levels to rise because of its effects in the liver, while on the other hand it makes beta cells in the pancreas release more insulin, so latest investigations suggest that it is actually an overall beneficial hormone for diabetic patients. In addition, the two receptors are not equally distributed amongst alpha and beta cells. In fact, there is considerably more GLP-1 receptor than GIP receptor in beta cells and much less GLP-1 receptor than GIP receptor in alpha cells. Despite this, the GIP receptor appears to function better than the GLP-1 receptor in healthy beta cells, while the GLP-1 receptor works almost as well as the GIP receptor in alpha cells in spite of its very low expression level. It is therefore likely that these receptors will work in different ways in these two cell types, as the amount of signal they send does not directly correlate with the amount of receptor there is in each one, and in alpha cells they are actually acting in opposite ways. In this project, we plan to clarify how exactly each receptor works in alpha versus beta cells by investigating how they move across the cell, the signals they send in each cell type, the partners they interact with and how much they depend on the action of an important known regulator of their function called beta-arrestin 2. Finally, we will also analyse how natural genetic variants of each receptor that exist in the general population can change the way these receptors function in the alpha versus beta cells of the pancreas. Overall, this project will help us understand why these receptors work differently to each other, why they have different strengths making them work better in some cases despite being present at much lower levels, and how genetic variation across individuals can modify the way they work in the two pancreatic cell types studied. This study will help us predict how well incretin therapies targeting each receptor will work for specific individuals, and give us new ideas about how best to target each receptor in alpha and beta cells to design newly improved therapies in the fight against type 2 diabetes.

The incretins, GLP-1 and GIP, are gut-derived insulinotropic hormones that bind to and activate their cognate receptors, GPCRs GLP-1R and GIPR, primarily expressed in pancreatic islets and the CNS, as well as other peripheral organs. Both incretin receptors are T2D targets, with most treatments to date targeting the GLP-1R, but novel therapies recently developed to concomitantly target the GIPR. The incretin receptors are not uniformly expressed across different islet endocrine cell types, with GLP-1R enriched vs GIPR in beta cells and GIPR expression much higher than GLP-1R in alpha cells. Despite these differences, signalling outputs from both receptors do not correlate well with gene expression patterns, with more pronounced GIPR effects on beta cells despite being significantly less expressed than GLP-1R, and opposite outputs in alpha cells, with GLP-1R inhibiting but GIPR potentiating glucagon secretion despite shared coupling to GalphaS, with unexplored mechanisms of action for either receptor in these cells and unclear repercussions from their effects on glucagon secretion on the overall islet function. Preliminary results from our lab indicate that GLP-1R and GIPR display profound differences in trafficking and spatiotemporal signalling, at least in beta cells, and that they can both signal to a similar extent from alpha cells despite the much lower expression of GLP-1R. In this project, we will establish the molecular mechanisms that lead to divergent GLP-1R vs GIPR responses in alpha vs beta cells and explore the reasons behind the disconnect between receptor levels and outputs, both in alpha and beta cell lines and in primary islets with specific expression of alpha vs beta cell biosensors. We will additionally study the effects of biasing each receptor for each cell type towards G protein over beta-arrestin signalling, as well as the impact of selected natural coding variants for each receptor on alpha vs beta cell signalling and overall islet responses.