Étude des voies de signalisation en aval du récepteur FFA1/GPR40 dans la cellule bêta pancréatique

Type 2 diabetes is characterized by the pancreatic beta cells’ incapacity to secrete sufficient insulin to maintain normal physiological glycaemia. The G-protein coupled-receptor GPR40 (GPR40) is a potential pharmaceutical target to treat type 2 diabetes since its activation by medium to long chain fatty acids potentiates glucose-induced insulin secretion. However, the effector molecules downstream GPR40’s signalling pathway that are responsible for the potentiation of insulin secretion remain unknown. The aim of this thesis is therefore to identify these effector molecules and determine how they influence insulin secretion. Many mechanisms are involved in the transport and the exocytosis of insulin granules in response to glucose. An important step during insulin secretion is actin filament remodeling which facilitates the recruitment of insulin granules at the membrane. A decrease in expression of a single molecule involved in actin remodeling is sufficient to significantly reduce insulin secretion in response to glucose. We observed that the protein kinase D1 (PKD1) plays an essential role for the actin remodeling resulting from GPR40’s activation by fatty acids. We also demonstrated that PKD1 is essential for the potentiation of insulin secretion by fatty acids without affecting insulin secretion induced by glucose alone. Our results have also shown that diacylglycerol acts as an important second messenger downstream of GPR40, and the GPR40-mediated potentiation mainly results from increasing the second phase of insulin secretion.  We also investigated the role of p21-activated kinase 4 (PAK4) in the potentiation of insulin secretion by fatty acids since PAK4 is known to be a PKD1 substrate in other cell models. We observed that PAK4 is phosphorylated in response to fatty acids in human Langerhans islets and that its phosphorylation in response to fatty acids depends on GPR40’s presence in mice islets. Similarly to PKD1, a reduction in PAK4 protein significantly alters the potentiation of insulin secretion by fatty acids without affecting insulin secretion in response to glucose itself. As GPR40’s activation is essential for the compensatory increase in insulin secretion during high-fat feeding in mice, we investigated the role of PKD1 in Langerhans islets when mice are fed a high-fat diet. PKD1’s absence in pancreatic beta cells accelerated the apparition of high-fat diet induced hyperglycemia and hyperinsulinemia. We also noticed following a high-fat diet that the insulin secretion in response to glucose during a hyperglycemic clamp is significantly reduced in mice deficient for PKD1 in pancreatic beta cells compared to control mice. We therefore concluded PKD1 is implicated in the compensatory increase in insulin secretion during high-fat feeding in mice. The results presented in this thesis therefore elucidate essential components involved in the potentiation of insulin secretion by fatty acids. This thesis also explores the impact of pancreatic beta cell dysfunction on the compensatory increase in insulin secretion during a high-fat diet and also discusses the potential of GPR40 as a drug target for type 2 diabetes following the unsuccessful clinical trials of GPR40 agonists.

Keyphrases: diabetes, Langerhans islets, pancreatic beta cells, insulin secretion, GPR40, protein kinase D1, p21-activated kinase 4, actin filament remodeling