Opposing effects of protein kinase C and protein kinase A on metabotropic glutamate receptor signaling: Selective desensitization of the inositol trisphosphate/Ca ²⁺ pathway by phosphorylation of the receptor-G protein-coupling domain

Abstract

Signaling by the metabotropic glutamate receptor 1␣ (mGluR1␣) can lead to the accumulation of inositol 1,4,5-trisphosphate (InsP 3) and cAMP and to the modulation of K ؉ and Ca 2؉ 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 InsP 3 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 mGluR1␣-dependent signaling pathways. PKC activation selectively inhibits agonist-dependent stimulation of the InsP 3 pathway but does not affect receptor signaling via cAMP. In contrast, protein kinase A potentiates agonist-independent signaling of the receptor via InsP 3. 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 Thr 695 selectively disrupts mGluR1␣-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 short-and longterm receptor activity.T he excitatory actions of glutamate in the central nervous system are mediated by two distinct classes of receptors: ionotropic and metabotropic. While ionotropic receptors drive fast neurotransmission, the stimulation of metabotropic glutamate receptors generates slower and longer lasting changes in the signaling cascades activated in neuronal and glial cells. Metabotropic glutamate receptors (mGluRs; mGluR1 through mGluR8) are classified into three distinct groups according to their sequence homology, pharmacological profile, and signaling properties; group I receptors are linked to phosphoinositide metabolism while groups II and III inhibit adenylyl cyclase (1, 2). Many functions have been ascribed to these receptors, such as regulation of neurotransmitter release, direct mediation of glutamatergic synaptic transmission, modulation of membrane excitability, and regulation of N-methyl-D-aspartate, ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and ␥-aminobutyric acid type A receptor activity (1). This vast array of effects underscores the role of mGluRs in modulating synaptic plasticity and their neurotoxic as well as neuroprotective action (1, 3).As for other G protein-coupled receptors (GPCRs), mGluRs induce diverse and long-lasting cellular responses; these actions depend on the effectors regulated by each second messenger molecule and on the cross-talk between signaling pathways. In the past several years, it has become increasingly clear that many GPCRs can i...

Group I Metabotropic Glutamate Receptors Bind to Protein Phosphatase 1C

“…PKC phosphorylation of mGluRs interacting with PP1␥1/2 was demonstrated (48) and caused desensitization of mGluR1a and mGluR5 isoforms (49,50). PKC phosphorylated Ser-881 in mGluR5, located in the PP1␥1/2 binding motifs identified in this study.…”

The Neurokinin A Receptor Activates Calcium and cAMP Responses through Distinct Conformational States

“…Role of Protein Kinases and Possible Physiological Implications-The overall effects of protein kinase effectors support the notion that phosphorylation of the receptor differentially stabilizes conformations of the receptor (12,50) by exhibiting conformation dependence in their specificity. Indeed, in agreement with the observed temporal correlation between rapid binding and activation of the calcium response, effectors of protein kinase C affect the intensity of the calcium response and the relative rapid/slow binding amplitudes (yet without changing the rates of rapid versus slow binding), whereas effectors of PKA (which have no effect on calcium responses) affect the rate of slow agonist binding but have no effect on the rate of rapid binding.…”

“…Conclusions-It is becoming increasingly well documented that, as in the case of conventional allosteric proteins (29,52), G protein-coupled receptors adopt several conformational states, among which more than one active state can be detected (1,50). Tachykinin NK2 receptors follow this scheme by adopting at least three (possibly four) distinct conformations corresponding to different structures with specific pharmacological and functional properties.…”

Distinct Functional Roles of the Metabotropic Glutamate Receptors 1 and 5 in the Rat Globus Pallidus

“…Classically, both mGluR1 and mGluR5 are known to activate phospholipase C via coupling to G q/11 -proteins, which leads to intracellular Ca 2ϩ release and activation of protein kinase C (PKC) (for review, see Conn and Patel, 1994). In turn, PKC can negatively feedback on the group I mGluR signaling by phosphorylation of mGluR1 (Francesconi and Duvoisin, 2000) and mGluR5 (Gereau and Heinemann, 1998), which leads to the receptor desensitization (Kawabata et al, 1996;Alagarsamy et al, 1999). Desensitization of group I mGluRs can also occur via proteins that regulate G-protein signaling, protein kinase A or G-protein-coupled receptor kinases (Sallese et al, 2000) (for review, see Alagarsamy et al, 2001;De Blasi et al, 2001).…”