Allosteric signaling through an mGlu2 and 5-HT _2A heteromeric receptor complex and its potential contribution to schizophrenia

Abstract

Heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCRs) can form multiprotein complexes (heteromers), which can alter the pharmacology and functions of the constituent receptors. Previous findings demonstrated that the Gq/11-coupled serotonin 5-HT2A receptor and the Gi/o-coupled metabotropic glutamate 2 (mGlu2) receptor—GPCRs that are involved in signaling alterations associated with psychosis—assemble into a heteromeric complex in the mammalian brain. In single-cell experiments with various mutant versions of the mGlu2 receptor, we showed that stimulation of cells expressing mGlu2–5-HT2A heteromers with an mGlu2 agonist led to activation of Gq/11 proteins by the 5-HT2A receptors. For this crosstalk to occur, one of the mGlu2 subunits had to couple to Gi/o proteins, and we determined the relative location of the Gi/o-contacting subunit within the mGlu2 homodimer of the heteromeric complex. Additionally, mGlu2-dependent activation of Gq/11, but not Gi/o, was reduced in the frontal cortex of 5-HT2A knockout mice and was reduced in the frontal cortex of postmortem brains from schizophrenic patients. These findings offer structural insights into this important target in molecular psychiatry.

Cross-signaling in metabotropic glutamate 2 and serotonin 2A receptor heteromers in mammalian cells

“…As was shown by Fribourg and colleagues in the Xenopus oocyte expression system [11] (also see [28]), the relative levels of mGlu 2 R and 2AR mRNA injected is a crucial factor of whether the formed heterocomplex would cross-signal or not in response to ligands targeting one or the other receptor. At the optimal mGlu 2 R/2AR mRNA ratio, maximal cross-signaling correlated with the most efficient presentation of each receptor’s protein at the cell surface.…”

“…Evidence associating mGlu 2 R with psychosis comes from the antipsychotic effects of strong agonists targeting group II metabotropic glutamate receptors [30,21] and from behavioral studies showing that mGlu 2 R, but not the closely related mGlu 3 R, mediates these antipsychotic-like effects in experimental models of psychosis [8,36,43]. The mGlu 2 and 2A receptors form heteromers in heterologous expression systems as well as in mouse and human frontal cortex [13,29,28]. The physiological significance of this complex is highlighted by behavioral experiments demonstrating that in experimental models of psychosis the antipsychotic-like effects of drugs targeting either receptor require the presence of the partner receptor [11] and that wild-type mGlu 2 R, but not a mutant unable to bind 2AR, can rescue the head-twitch behavior induced by the hallucinogenic 5-HT2 A receptor agonist DOI [29].…”

Reformulating a Pharmacophore for 5-HT2A Serotonin Receptor Antagonists

“…The N-terminally c-Myc-tagged form of wild type human 5-HT 2A receptor (pcDNA3.1-c-Myc-5-HT 2A ) has been described previously. 21 …”

Contribution of heteromerization to G protein-coupled receptor function

“…We [36,59,60] and others [12] have confirmed in HEK293 cells that at least a fraction of the populations of mGlu2 and 5-HT 2A receptors constitute part of the same protein complex as defined by co-immunoprecipitation assays (Figure 1D). It has also been demonstrated that these two receptors are expressed in close molecular proximity as defined independently by biophysical assays such as BRET [36], microscope-based FRET [36], flow cytometry-based-FRET [59,60], antibody-based time-resolved FRET [12], and combination of time-resolved FRET and the SNAP-tag approach [33].…”

“…These assays showed that labeling for both 5-HT 2A and mGlu2 receptors was observed in close sub-cellular proximity, particularly at or near synaptic junctions [59]. Using a sub-cellular fractionation approach to purify fractions enriched in presynaptic active zone (PAZ) and postsynaptic density (PSD) proteins, it has recently been confirmed that the 5-HT 2A receptor is detected mostly in the PSD, whereas the mGlu2 receptor is detected in both the PSD and the PAZ fractions [60]. In addition, 5-HT 2A and mGlu2 receptors can be co-immunoprecipitated from mouse [83] and human [36] frontal cortex plasma membrane preparations.…”