5-HT2A receptor-dependent phosphorylation of mGlu2 receptor at Serine 843 promotes mGlu2 receptor-operated Gi/o signaling

The serotonin 5-HT2A and glutamate mGlu2 receptors continue to attract particular attention, given their implication in psychosis associated with schizophrenia and the mechanism of action of atypical antipsychotics and a new class of antipsychotics, respectively. A large body of evidence indicates a functional crosstalk between both receptors in the brain, but the underlying mechanisms are not entirely elucidated. Here, we have explored the influence of 5-HT2A receptor upon the phosphorylation pattern of mGlu2 receptor in light of the importance of specific phosphorylation events in regulating G protein-coupled receptor signaling and physiological outcomes. Among the five mGlu2 receptor-phosphorylated residues identified in HEK-293 cells, the phosphorylation of Ser843 was enhanced upon mGlu2 receptor stimulation by the orthosteric agonist LY379268 only in cells co-expressing the 5-HT2A receptor. Likewise, administration of LY379268 increased mGlu2 receptor phosphorylation at Ser843 in prefrontal cortex of wild-type mice but not 5-HT2A−/− mice. Exposure of HEK-293 cells co-expressing mGlu2 and 5-HT2A receptors to 5-HT also increased Ser843 phosphorylation state to a magnitude similar to that measured in LY379268-treated cells. In both HEK-293 cells and prefrontal cortex, Ser843 phosphorylation elicited by 5-HT2A receptor stimulation was prevented by the mGlu2 receptor antagonist LY341495, while the LY379268-induced effect was abolished by the 5-HT2A receptor antagonist M100907. Mutation of Ser843 into alanine strongly reduced Gi/o signaling elicited by mGlu2 or 5-HT2A receptor stimulation in cells co-expressing both receptors. Collectively, these findings identify mGlu2 receptor phosphorylation at Ser843 as a key molecular event that underlies the functional crosstalk between both receptors.