Differential regulation of PI hydrolysis and adenylyl cyclase by muscarinic receptor subtypes

Muscarinic acetylcholine receptors (mAChRs), like many other neurotransmitter and hormone receptors, transduce agonist signals by activating G proteins to regulate ion channel activity and the generation of second messengers via the phosphoinositide (PI) and adenylyl cyclase systems. Human mAChRs are a family of at least four gene products which have distinct primary structures, ligand-binding properties and patterns of tissue-specific expression. To examine the question of whether functional differences exist between multiple receptor subtypes, we have investigated the ability of each subtype to regulate PI hydrolysis and adenylyl cyclase when expressed individually in a cell lacking endogenous mAChRs. We show that the HM2 and HM3 mAChRs efficiently inhibit adenylyl cyclase activity but poorly activate PI hydrolysis. In contrast, the HM1 and HM4 mAChRs strongly activate PI hydrolysis, but do not inhibit adenylyl cyclase, and in fact can substantially elevate cAMP levels. Interestingly, the subtypes that we find to be functionally similar are also more similar in sequence. Our results indicate that the different receptor subtypes are functionally specialized.