Endogenous lipid activated G protein-coupled receptors: Emerging structural features from crystallography and molecular dynamics simulations

Abstract

Class A G-protein coupled receptors (GPCRs) are thought to have a common topology that includes seven transmembrane alpha helices (TMHs) that are arranged to form a closed bundle. This bundle forms the ligand binding pocket into which ligands are commonly thought to enter via the extracellular milieu. This ligand approach direction makes sense for GPCRs that have small positively charged ligands, such as the beta-2-adrenergic or the dopamine D2 receptor. However, there is a growing sub-group of Class A GPCRs that bind lipid-derived endogenous ligands, such as the cannabinoid CB1 and CB2 receptors (with endogenous ligands, N-arachidonoylethanolamine (anandamide) and sn-2-arachidonylglycerol (2-AG)) and the S1P1-5 receptors (with endogenous ligand, sphingosine-1-phosphate). Even the widely studied Class A GPCR, rhodopsin, binds a highly lipophillic chromophore (11-cis-retinal). For these receptors, ligand approach from the extracellular milieu has seemed unlikely given that the ligands of these receptors readily partition into lipid or are actually synthesized in the lipid bilayer. The recent X-ray-crystal structure of the sub-type 1 sphingosine-1-phosphate receptor (S1P1) provides important information on the key structural variations that may be the hallmarks for a Class A GPCR that binds lipid-derived ligands. These include an extracellular domain that is closed off to the extracellular milieu and the existence of an opening between transmembrane helices that may serve as a portal for ligand entry via the lipid bilayer. This review examines structural aspects that the cannabinoid receptors may share with the S1P1 receptor based upon sequence homology. This review also examines experimental and simulation results that suggest ligand entry via a lipid portal is quite likely for this emerging sub-group.

Molecular-Interaction and Signaling Profiles of AM3677, a Novel Covalent Agonist Selective for the Cannabinoid 1 Receptor

“…22 Accordingly, WT hCB1R and hCB1R variants with conservative Cys-to-Ser mutations at each of the receptor’s five TMH Cys residues (out of 13 total) were expressed in Flp-In-293 cells. Receptor functional competency was evaluated in saturation-binding assays on membrane preparations from each Flp-In-293 cell line with [ 3 H]CP55,940, a nonclassical, high-affinity cannabinoid radioligand universally utilized for profiling CB1R/CB2R orthosteric ligand binding.…”

“…20 Attempts have thus been made by us 21,22 and others 23 to delineate the structural features of ligand-dependent CB1R activation using computational prediction methods, albeit with underlying assumptions regarding the states of apo- and holo-CB1R in situ . This laboratory has also utilized nuclear magnetic resonance spectroscopy (NMR) to characterize experimentally the structure and dynamics of discrete CB1R transmembrane helix (TMH) domains.…”

Molecular Targets of the Phytocannabinoids: A Complex Picture

“…In many ways the CB 1 and CB 2 receptors are atypical within the Class A GPCRs [86]. The endogenous ligands for these receptors, sn-2-arachidonoylglycerol (2-AG) (CB 1 and CB 2 ) [87, 88] and anandamide (CB 1 ) [89] are lipid-derived agonists that are made on demand from the lipid bilayer and degraded by membrane associated enzymes [90–92], negating the need for vesicle storage.…”

Synthesis, biological activity and molecular modeling of new biphenylic carboxamides as potent and selective CB2 receptor ligands

“…Basically, the sphingoshine-1-phosphate type 1 receptor [S1P 1 (PDB code: 3V2Y)] was used as template because of both, the sequence identity percentage (27.03% and 24.16% for CB1 and CB2, respectively) and the close evolutionary relationship [43] (see Material and Methods).…”