M3-muscarinic receptor mediates insulin release via receptor-phosphorylation dependent activation of protein kinase D1.

Despite the fact that it is known that G-protein coupled receptors are hyper-phosphorylated following agonist occupation in a process that regulates receptor signalling the physiological impact of this process remains poorly studied. We address this question here by investigating muscarinic receptor regulation of insulin release and glucose homeostasis in a transgenic knock-in mouse strain that expresses a phosphorylation-deficient mutant of the M3-muscarinic receptor. This mutant receptor is expressed normally in mouse tissues and couples to Gq/11-signalling pathways but is uncoupled from phosphorylation-dependent processes such as receptor internalisation and arrestin recruitment. Mutant mice showed a deficiency in glucose tolerance and insulin secretion indicating that M3¬-muscarinic receptor signalling that is independent of G-protein coupling is important in mediating muscarinic-regulation of glucose homeostasis. We show that the mechanism of action of pancreatic M3-muscarinic receptors centred on the activation of protein kinase D1 which in turn is regulated by the phosphorylation status of the receptor. We conclude that M3-muscarinic receptor augmentation of insulin release is independent of G-protein coupling but mediated by the phosphorylation/arrestin-dependent coupling of the receptor to protein kinase D1