Cross-Signaling between the Metabotropic Glutamate 2 Receptor and the Serotonin (5-Ht) 2A Receptor in Hek-293 Cells

Abstract

Atypical antipsychotic drugs targeting the Gq-coupled 5-HT2A receptor (2AR) are widely used in the treatment of schizophrenia and psychosis. Recent studies point to a new class of potential antipsychotic drugs targeting the Gi-coupled metabotropic glutamate 2 receptor (mGluR2). We recently reported that a heteromeric complex formed between these two GPCRs integrates the actions of serotonergic and glutamatergic drugs, modulating the balance between Gi and Gq signaling and allowing us to predict the psychoactive properties of these drugs (Fribourg et al. 2011, Cell. 147(5):1011-23). Our results uncover a unifying mechanism of action of two families of antipsychotic drugs and pave the road for the development of assays for high-throughput screening of serotonergic and glutamatergic drugs. While developing such an assay, we needed to address a controversy raised by a recent study, in which co-expression of the two receptors in HEK-293 cells had no significant effect on either Gi or Gq signaling in response to several serotonergic and glutamatergic drugs (Delille et al., 2012, Neuropharmacology 62(7):2184-91). Since in Xenopus oocytes the degree of functional crosstalk between the two receptors depended on their expression levels, we generated several clones of HEK-293 cells expressing different levels of the two receptors in the background of the G-protein inwardly rectifying GIRK1/GIRK4 channel, which can serve as a reporter for both Gi and Gq signaling. Using fluorescence indicator and electrophysiological assays we were able to identify clones showing various degrees of functional crosstalk between the two receptors in response to single ligands. Our results have confirmed functional crosstalk of the two receptors. Even in cases where a single ligand failed to show crosstalk, like in the Delille et al. study, a combination of ligands targeting both receptors could elicit functional crosstalk.