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.