The M3 Muscarinic Acetylcholine Receptor Can Signal through Multiple G Protein Families.

The M3 muscarinic acetylcholine receptor (MR) is a G protein-coupled receptor (GPCR) that regulates important physiologic processes, including vascular tone, bronchoconstriction, and insulin secretion. It is expressed on a wide variety of cell types, including pancreatic beta, smooth muscle, neuronal, and immune cells. Agonist binding to the MR is thought to initiate intracellular signaling events primarily through the heterotrimeric G protein Gq. However, reports differ on the ability of MR to couple to other G proteins beyond Gq. Using members from the four primary G protein families (Gq, Gi, Gs, and G13) in radioligand binding, GTP turnover experiments, and cellular signaling assays, including live cell G protein dissociation and second messenger assessment of cAMP and inositol trisphosphate, we show that other G protein families, particularly Gi and Gs, can also interact with the human MR. We further show that these interactions are productive as assessed by amplification of classic second messenger signaling events. Our findings demonstrate that the MR is more promiscuous with respect to G protein interactions than previously appreciated. SIGNIFICANCE STATEMENT: The study reveals that the human M3 muscarinic acetylcholine receptor (MR), known for its pivotal roles in diverse physiological processes, not only activates intracellular signaling via Gq as previously known but also functionally interacts with other G protein families such as Gi and Gs, expanding our understanding of its versatility in mediating cellular responses. These findings signify a broader and more complex regulatory network governed by MR and have implications for therapeutic targeting.