Extracellular Calcium Modulates Actions of Orthosteric and Allosteric Ligands on Metabotropic Glutamate Receptor 1αThis work was supported, in whole or in part, by National Institutes of Health Grants GM081749-01 (to J. J. Y.), DK078331 (to E. M. B.), and NS055179 (to R. A. H.). This work was also supported by a Brains and Behavior fellowship (to Y. J.)

• Published in: The Journal of biological chemistry Vol. 289; no. 3; pp. 1649 - 1661
• Main Authors: Jiang, Jason Y., Nagaraju, Mulpuri, Meyer, Rebecca C., Zhang, Li, Hamelberg, Donald, Hall, Randy A., Brown, Edward M., Conn, P. Jeffrey, Yang, Jenny J.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.01.2014
• Subjects: Allosteric Modulators; Calcium; Calcium-binding Proteins; Drug Discovery; G Protein-coupled Receptors (GPCR); Glutamate Receptors Metabotropic; Orthosteric Agonist; Orthosteric Antagonist; G Protein-coupled Receptors (GPCR); Allosteric Modulators; Calcium; Calcium-binding Proteins; Orthosteric Antagonist; Glutamate Receptors Metabotropic; Drug Discovery; Orthosteric Agonist
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M113.507665

Metabotropic glutamate receptor 1α (mGluR1α), a member of the family C G protein-coupled receptors, is emerging as a potential drug target for various disorders, including chronic neuronal degenerative diseases. In addition to being activated by glutamate, mGluR1α is also modulated by extracellular Ca2+. However, the underlying mechanism is unknown. Moreover, it has long been challenging to develop receptor-specific agonists due to homologies within the mGluR family, and the Ca2+-binding site(s) on mGluR1α may provide an opportunity for receptor-selective targeting by therapeutics. In the present study, we show that our previously predicted Ca2+-binding site in the hinge region of mGluR1α is adjacent to the site where orthosteric agonists and antagonists bind on the extracellular domain of the receptor. Moreover, we found that extracellular Ca2+ enhanced mGluR1α-mediated intracellular Ca2+ responses evoked by the orthosteric agonist l-quisqualate. Conversely, extracellular Ca2+ diminished the inhibitory effect of the mGluR1α orthosteric antagonist (S)-α-methyl-4-carboxyphenylglycine. In addition, selective positive (Ro 67-4853) and negative (7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester) allosteric modulators of mGluR1α potentiated and inhibited responses to extracellular Ca2+, respectively, in a manner similar to their effects on the response of mGluR1α to glutamate. Mutations at residues predicted to be involved in Ca2+ binding, including E325I, had significant effects on the modulation of responses to the orthosteric agonist l-quisqualate and the allosteric modulator Ro 67-4853 by extracellular Ca2+. These studies reveal that binding of extracellular Ca2+ to the predicted Ca2+-binding site in the extracellular domain of mGluR1α modulates not only glutamate-evoked signaling but also the actions of both orthosteric ligands and allosteric modulators on mGluR1α. Background: Extracellular Ca2+ alters mGluR1α activity but by an unknown mechanism. Results: Mutations in predicted Ca2+-binding sites modulated the potency of both orthosteric and allosteric modulators. Conclusion: Ca2+ binding exerts multiple types of effects on mGluR1α. Significance: Improved knowledge of the mechanisms underlying the actions of Ca2+ on mGluR1α activity could facilitate development of isoform-selective drugs and/or suggest ways to tune the actions of available drugs.