In the first experiments described, the authors investigated the regulation and function of GPR55 signaling in atherosclerosis by feeding apolipoprotein E-deficient (Apoe-/-) mice Western diet (WD) for 4 or 16 weeks, and compared the effects to a global Gpr55 knockout (Apoe-/-Gpr55-/-) mouse model. At 4 weeks, Apoe-/-Gpr55-/- mice exhibited larger aortic plaques than Apoe-/- controls, suggesting that GPR55 signaling may counterbalance plaque development at the early disease stage. Although the difference in aortic lesion area was no longer observed at 16 weeks, the plaque burden was still found to be higher in the descending aorta of Apoe-/-Gpr55-/- mice. The authors note that these effects of Gpr55 deficiency on advanced plaque phenotype could be relevant to the complications that are observed in human atherosclerosis.
The authors also determined that Gpr55 expression was highest in B cells, followed by T cells, neutrophils, and monocytes. Additionally, the highest expression of Gpr55 was found in plasma cells, which was localized to the follicular and germinal center areas. To assess the functional GPR55 signaling response in B cells, the authors measured intracellular calcium levels in response to stimulation with the endogenous GPR55 ligand lysophosphatidylinositol (LPI). With a normal diet, increased intracellular calcium was only observed after LPI stimulation in Apoe-/- cells expressing Gpr55. Moreover, a dose-dependent calcium response to LPI was found in human peripheral blood B cells, indicating a functional LPI-GPR55 signaling pathway.