The M5 muscarinic acetylcholine receptor third intracellular loop regulates receptor function and oligomerization

Abstract Besides some pharmacological, biochemical and biophysical evidences support the contention that muscarinic acetylcholine receptors can form homo- and heterodimers, the existence of specific M 3 and M 5 muscarinic receptors oligomers in living cells is a new concept. Interestingly, this phenomenon might have relevance in lymphocytic cholinergic function since both T- and B-cells naturally express high levels of these two receptor subtypes. Here, by means of co-immunoprecipitation and bioluminescence resonance energy transfer methods we demonstrated that M 3 and M 5 muscarinic receptors could form constitutive homo- and heterodimers in transiently transfected HEK-293T cells. Interestingly, this receptor–receptor interaction was unaltered by carbachol treatment but it was affected by the expression of a peptide corresponding to a portion of the third intracellular loop of the M 5 muscarinic receptor. In addition, the same peptide was able to abrogate the carbachol-induced mitogen-activated protein kinase phosphorylation and the carbachol-enhanced PHA-induced IL-2 production in derived lymphocytic T cells. Overall, these results suggest that the third intracellular loop of the M 5 muscarinic receptor might play a regulatory role in receptor function and heteromerization, thus providing the molecular framework for a potential cholinergic-based therapeutic intervention of the immune system.