Coupling of metabotropic glutamate receptors 2 and 4 to G alpha 15, G alpha 16, and chimeric G alpha q/i proteins: characterization of new antagonists.

Together with the calcium-sensing receptor, the metabotropic glutamate receptors (mGluRs) share no sequence homology with the other G protein-coupled receptors (GPCRs) and therefore constitute a new family of receptors. Recently, it was reported that G alpha 15 and G alpha 16 subunits allow many GPCRs to activate phospholipase C (PLC). Furthermore, the exchange of a few carboxyl-terminal residues of G alpha q by those of G alpha 12 or G alpha o allows the resulting chimeric G alpha subunits (G alpha ql and G alpha qol respectively) to couple Gi-coupled receptors to PLC. We report that mGluR2 and mGluR4, two receptors negatively coupled to adenylyl cyclase, activate PLC when coexpressed with G alpha 15, G alpha ql or G alpha qo. This indicates that the carboxyl-terminal end of the G alpha subunit also plays an important role in the specific interaction between mGluRs and the G proteins. In addition, the measurement of PLC activation by Gi-coupled mGluRs coexpressed with these G alpha subunits constitutes an easy functional assay for the pharmacological characterization of these receptors. The rank order of potency of antagonists was found to be (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine approximately (R,S)- alpha-methyl-4-phosphonophenylglycine > (R,S)-alpha-methyl-4-sulfonophenylglycine > (R,S)-alpha-methyl-4-tetrazolylphenylglycine = (S)-2-amino-2-methyl-4-phosphonobutyrate for mGluR2 and to be (R,S)-alpha-methyl-4-phosphonophenylglycine > or = (S)-2-amino-2-methyl-4-phosphonobutyrate > > (R,S)-alpha-methyl-4-sulfonophenylglycine [(R,S)-alpha-methyl-4-tetrazolylphenylglycine and (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine being inactive at 1 mM] for mGluR4. Using this functional assay, (R,S)-alpha-methyl-4-phosphonophenylglycine was found to have a similar KB value for mGluR2 and mGluR4.