Probing The Structural Basis Of Metabotropic Glutamate Receptor 5 (Mglu(5)) Activation And Positive Allosteric Modulation

The metabotropic glutamate receptor 5 (mGlu5) is a class C G protein-coupled receptor (GPCR) that has emerged as a promising drug target for multiple central nervous system disorders. Structurally, mGlu5 forms obligate dimers. Each protomer possesses a 7-transmembrane (7TM) domain which is connected through a cysteine-rich domain (CRD) to a large extracellular domain (ECD), encompassing the orthosteric ligand-binding Venus flytrap domain (VFD). Recent spectroscopic studies on mGlu5 signaling demonstrated large-scale conformational changes underlie signal transmission from the ECD to the G protein coupling 7TM domain. Although these studies reported the necessity of extracellular loops (ECL) for activation of mGlu5 by orthosteric agonists, specific residues related to transmission of receptor activation remain unknown. We used a combination of site-directed mutagenesis, Ca2+ mobilization assays and radioligand binding studies to identify residues within the ECLs that aid in the propagation of orthosteric agonist-induced conformational changes in the VFD to the 7TM. Multiple ECL2 residues markedly reduced activation by glutamate but not allosteric agonists. These same mutations had no effect on receptor expression and thus likely contribute to the mGlu­5 activation network. Select mutations that compromised glutamate activation could be rescued with positive allosteric modulators (PAMs). mGlu5 PAMs are promising potential new therapeutic strategies for schizophrenia and cognition, however, structure-activity relationships for mGlu5 PAMs are notoriously difficult to interpret. A deeper understanding of the structural basis of dynamic receptor activation and modulation processes will aid future rational structure-based drug discovery efforts