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

Besides some pharmacological, biochemical and biophysical evidences support the contention that muscarinic acetylcholine receptors can form homo- and heterodimers, the existence of specific M3 and M5 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 M3 and M5 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 M5 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 M5 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.