5-HT_2A receptor-dependent phosphorylation of mGlu₂ receptor at Serine 843 promotes mGlu₂ receptor-operated G_i/o signaling

The serotonin 5-HT_2A 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-HT_2A 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-HT_2A receptor. Likewise, administration of LY379268 increased mGlu₂ receptor phosphorylation at Ser⁸⁴³ in prefrontal cortex of wild-type mice but not 5-HT_2A^−/− mice. Exposure of HEK-293 cells co-expressing mGlu₂ and 5-HT_2A 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-HT_2A receptor stimulation was prevented by the mGlu₂ receptor antagonist LY341495, while the LY379268-induced effect was abolished by the 5-HT_2A receptor antagonist M100907. Mutation of Ser⁸⁴³ into alanine strongly reduced G_i/o signaling elicited by mGlu₂ or 5-HT_2A 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.