Quantitative analysis of muscarinic acetylcholine receptor homo- and heterodimerization in live cells: regulation of receptor down-regulation by heterodimerization

Although previous pharmacological and biochemical data support the notion that muscarinic acetylcholine receptors (mAChR) form homo- and heterodimers, the existence of mAChR oligomers in live cells is still a matter of controversy. Here we used bioluminescence resonance energy transfer to demonstrate that M(1), M(2), and M(3) mAChR can form constitutive homo- and heterodimers in living HEK 293 cells. Quantitative bioluminescence resonance energy transfer analysis has revealed that the cell receptor population in cells expressing a single subtype of M(1), M(2), or M(3) mAChR is predominantly composed of high affinity homodimers. Saturation curve analysis of cells expressing two receptor subtypes demonstrates the existence of high affinity M(1)/M(2), M(2)/M(3), and M(1)/M(3) mAChR heterodimers, although the relative affinity values were slightly lower than those for mAChR homodimers. Short term agonist treatment did not modify the oligomeric status of homo- and heterodimers. When expressed in JEG-3 cells, the M(2) receptor exhibits much higher susceptibility than the M(3) receptor to agonist-induced down-regulation. Coexpression of M(3) mAChR with increasing amounts of the M(2) subtype in JEG-3 cells resulted in an increased agonist-induced down-regulation of M(3), suggesting a novel role of heterodimerization in the mechanism of mAChR long term regulation.