Molecular Dynamics Study of 2-Arachidonoyl-Sn-Glycero-3-Phosphoinositol (2-AGPI) and Lysophosphatidylinositol (LPI) in a POPC Bilayer and their GPR55 Docking Sites

Abstract

GPR55 is a Class A G protein-coupled receptor that has been shown to be activated by cannabinoids (Henstridge et al. FASEB J, 2008). The receptor is expressed in several mammalian tissues including several brain regions. It has been reported that GPR55 is also activated by LPI found in rat brain (Oka et al. BBRC, 2007), with 2-AGPI having the highest activity (Oka et al. J Biochem, 2009). Since 2-AGPI and LPI are lipids and are shown to interact with the membrane-embedded GPR55 receptor, we undertook a study of the location and conformations they can adopt in a phospholipid bilayer, as well as, of their interaction modes with GPR55. To this end, 2-AGPI and LPI were added to a fully hydrated, pre-equilibrated POPC bilayer (28 waters/lipid; 72 lipid molecules with 36 in each leaflet) and their behavior in POPC was studied using the NAMD2 molecular dynamics software package (NPAT ensemble; P = 1atm, T=310K) with the CHARMM27 parameter set including data for polyunsaturated lipids, and the TIP3P water model. The MD studies place the 2-AGPI and LPI headgroups in the water-lipid interface with the inositol moiety either upright and solvated in water or bent and buried in the POPC headgroups. Extensive ligand inter- and intramolecular hydrogen bonding contributes to the inositol location and conformation. 2-AGPI's acyl tail is extremely flexible and prefers compact to moderately extended conformations, whereas LPI's tail prefers more extended ones. Following these MD studies, the ligands were docked in a GPR55 model in the TMHs 1,2,3, 6 and7 region using K2.60 as the primary interaction site. [Support: NIH RO1 DA023204 (MEA) and KO5 DA021358 (PHR)]