Molecular Dynamics Analysis of Early Activation Events in the Cannabinoid Type 2 Receptor Reveals a Putative Water-Mediated Allosteric Pathway

The cannabinoid receptor type 2 (CB2R) is a therapeutically relevant class A G protein-coupled receptor implicated in neuroinflammation, pain modulation, and immune regulation. However, the molecular events by which agonist binding promotes CB2R activation remain incompletely understood. Here, we investigated the conformational dynamics of AM12033-bound CB2R using four independent 1 μs all-atom molecular dynamics simulation replicas initiated from the agonist-bound inactive-like crystal structure. Principal component analysis and free-energy landscape reconstruction showed that CB2R samples active-like conformational states characterized by partial outward displacement of TM6, disruption of the TM3–TM7 interaction, and rearrangement of the NPxxY microswitch. A free-energy minimum displayed close structural similarity to the experimentally determined active CB2R–Gi complex, with a Cα RMSD of 1.71 Å. However, the persistence of the internal DRY interaction and the incomplete transition of the Trp2586.48 toggle switch indicate that the receptor predominantly populates a preactive or partially activated ensemble rather than a fully active G protein-bound state. A water-mediated Trp2586.48–water–water–Asp802.50 bridge formed reproducibly in three of four independent replicas after the activation-related transition and was associated with increased receptor-core hydration, activation-related microswitch populations, and shortened or rewired allosteric communication pathways from the toggle-switch region. These findings suggest that agonist-bound CB2R can sample active-like conformations before G protein engagement and support a putative role for a water-mediated allosteric pathway in early CB2R activation.