Positive Allosteric Modulators Induced Conformational Changes in the Metabotropic Glutamate Receptor 2 - In Silico Predictions and Experimental Tests

Abstract

Background: Metabotropic glutamate type 2 receptor (mGlu2R) is a Class C G-protein coupled receptor (GPCR), widely distributed in the brain where it mainly functions to limit excess glutamate release. mGlu2R positive allosteric modulators (PAMs) provide a novel way to regulate glutamatergic function since they bind to the transmembrane domain (TMD) and enhance responses of mGlu2R to extracellular orthosteric ligands via modulation of affinity and/or efficacy. Compared to orthosteric agonists, PAMs have the unique ability to modulate glutamate release in a ‘state-dependent’ manner that helps fine-tune physiological responses.Methods: We docked different PAMs to a mGlu2R homology model based on the mGlu1R X-ray structure and performed 300 ns molecular dynamics (MD) simulations on receptor-ligand complexes. Mutagenesis and voltage-clamp experiments are being used to test the model's predictions on the effects of PAMs on wild-type and mutant mGlu2R expressed in the Xenopus laevis oocyte heterologous expression system.Results and Conclusions: Preliminary computational results indicate that PAMs can induce allosteric conformational changes in mGlu2R TMD helices and intracellular loop regions that accompany G-protein binding and/or activation, and thus enhance formation of an active mGlu2R conformation. In addition, our simulation results also show that PAM binding can affect the interaction between the pre-TMH1 and the extracellular loop 1, which may influence the orthosteric ligand binding in the extracellular Venus flytrap domain. These predictions are being validated experimentally. Our simulations suggest that mGlu2R PAM binding alone produces a unique PAM-bound conformation that may resemble an intermediate conformation formed on the pathway to full receptor activation. These results provide insight into the mechanisms of allosteric modulation of mGlu2R, a potential therapeutic target for a variety of neuropsychiatric disorders.