Effect of Cholesterol on the Structural Dynamics of Metabotropic Glutamate Receptor (mGluR1): A Molecular Dynamics Study

Abstract

Metabotropic glutamate receptors (mGluRs) are class C G protein-coupled receptors (GPCRs) which regulate neurotransmission and are prevalently expressed in the central nervous system. The mGluR subfamily has eight members divided into three subgroups (I (mGluR1,5), II (mGluR2,3) and III (mGluR4,6,7,8)) based on their sequence homology, agonist selectivity and signaling function. The group I mGluRs (mGluR1 and mGluR5) are useful for treating diseases such as cancer, pain, schizophrenia, Alzheimer's disease, anxiety, and autism. Unlike most GPCRs which have the orthosteric binding pocket located in the transmembrane helices, the extracellular VFT domain of the mGluRs serves as the orthosteric ligand binding site while the 7-transmembrane (7TM) domain has allosteric binding sites. Using all-atom equilibrium MD simulations, we have studied the effect of different amounts of membrane cholesterol on the local structural dynamics of the mGlur1 7TM region in the inactive state. In addition, we have also studied the effect of thermostabilized apocytochrome b562RIL (BRIL) on the 7TM region. We observed that BRIL and cholesterol had contrasting effects on the 7TM domain of mGluR1. While the presence of BRIL was found to maintain the inactive conformation of the protein, increasing the amount of membrane cholesterol was found to trigger a conformational transition towards the active state. A salt bridge (E783-K834) present in the inactive state was found to break when membrane cholesterol was increased to 25%. We also observed similar protein fluctuations in the absence of BRIL and with the increased membrane cholesterol. This study thus sheds light on conformational changes observed in mGluR1 and the effect of cholesterol within the 7TM region, providing a framework for determining the activation process of mGluRs.