M3 muscarinic acetylcholine receptor-mediated signaling is regulated by distinct mechanisms.

We have used RNA interference previously to demonstrate that G protein-coupled receptor kinase 2 (GRK2) regulates endogenously expressed H1 histamine receptor in human embryonic kidney 293 cells. In this report, we investigate the regulation of endogenously expressed M(3) muscarinic acetylcholine receptor (M(3) mAChR). We show that knockdown of GRK2, GRK3, or GRK6, but not GRK5, significantly increased carbachol-mediated calcium mobilization. Stable expression of wild-type GRK2 or a kinase-dead mutant (GRK2-K220R) reduced calcium mobilization after receptor activation, whereas GRK2 mutants defective in Galpha(q) binding (GRK2-D110A, GRK2-R106A, and GRK2-R106A/K220R) had no effect on calcium signaling, suggesting that GRK2 primarily regulates G(q) after M(3) mAChR activation. The knockdown of arrestin-2 or arrestin-3 also significantly increased carbachol-mediated calcium mobilization. Knockdown of GRK2 and the arrestins also significantly enhanced carbachol-mediated activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), whereas prolonged ERK1/2 activation was only observed with GRK2 or arrestin-3 knockdown. We also investigated the role of casein kinase-1alpha (CK1alpha) and found that knockdown of CK1alpha increased calcium mobilization but not ERK activation. In summary, our data suggest that multiple proteins dynamically regulate M(3) mAChR-mediated calcium signaling, whereas GRK2 and arrestin-3 play the primary role in regulating ERK activation.