Activation mechanism of the M5 muscarinic acetylcholine receptor.

Abstract

The muscarinic acetylcholine receptor family are class A G protein-coupled receptors (GPCRs) that help facilitate the actions of the neurotransmitter acetylcholine throughout the body. Because the M5 subtype (M5R) is exclusively expressed on dopaminergic neurons in the mesolimbic pathway, it is emerging as an attractive therapeutic target for treating addictive disorders. However, the lack of crystal structures on the intermediate and active conformations of the receptor has hampered the design of potent antagonist and agonists. Therefore, this study will explore the active, intermediate, and inactive conformations of an apo- M5R by running 300 ns Gaussian accelerated molecular dynamics (GaMD) simulations on the modelled active state of the receptor (PDB ID: 6OIJ). The active apo-M5R is expected to transition into the intermediate and inactive states because (1) muscarinic receptors are constitutively active but prefer the inactive state in the absence of agonists, and (2) GaMD simulations lower the energy barriers between protein conformations. Principal component analysis will be performed on the trajectories to identify the major movements of the protein during deactivation/activation, while the subregions involved in the transitions will be determined using community network analysis. The expected movements of the M5R include the outward displacement of TM6 and extension of the TM3 into the cytoplasm. Mutagenesis studies on the M5R also suggest that IL2 and IL3 are involved in G-protein coupling, so these loops are speculated to adopt distinct conformations in each conformation. The obtained intermediate and active conformations and the insights on the activation mechanism will aid further drug design initiatives to relieve addictive disorders.