Opposing effects of protein kinase C and protein kinase A on metabotropic glutamate receptor signaling: Selective desensitization of the inositol trisphosphate/Ca21 pathway by phosphorylation of the receptor-G protein-coupling domain

Signaling by the metabotropic glutamate receptor 1a (mGluR1a) can lead to the accumulation of inositol 1,4,5-trisphosphate (InsP3) and cAMP and to the modulation of K1 and Ca21 channel opening. At present, very little is known about how these different actions are integrated and eventually turned off. Unraveling the molecular mechanisms underlying these functions is crucial for understanding mGluR-mediated regulation of synaptic transmission. It has been shown that receptor-induced activation of the InsP3 pathway is subject to feedback inhibition mediated by protein kinase C (PKC). In this study, we provide evidence for a differential regulation by PKC and protein kinase A of two distinct mGluR1adependent signaling pathways. PKC activation selectively inhibits agonist-dependent stimulation of the InsP3 pathway but does not affect receptor signaling via cAMP. In contrast, protein kinase A potentiates agonist-independent signaling of the receptor via InsP3. Furthermore, we demonstrate that the selectivity of PKC action on receptor signaling rests on phosphorylation of a threonine residue located in the G protein-interacting domain of the receptor. Modification at Thr695 selectively disrupts mGluR1a–Gq/11 interaction without affecting signaling through Gs. Together, these data provide insight on the mechanisms by which selective downregulation of a specific receptor-dependent signaling pathway can be achieved and on how cross-talk between different second messenger cascades may contribute to fine-tune shortand longterm receptor activity.