“Endocannabinoids and their G protein-coupled receptors have been suggested to play a key role in human pregnancy, by regulating important aspects such as implantation, decidualization, placentation and labor.

G protein-coupled receptor 55 (GPR55) was previously postulated to be another cannabinoid receptor, since specific cannabinoids were shown to act independently of the classical cannabinoid receptors CB1 or CB2.

Current knowledge about GPR55 expression and function in human placenta is very limited and motivated us to evaluate human placental GPR55 expression in relation to other human peripheral tissues and to analyze spatiotemporal GPR55 expression in human placenta.

Gene expression analysis revealed low GPR55 levels in human placenta, when compared to spleen and lung, the organs showing highest GPR55 expression.

Moreover, expression analysis showed 5.8 fold increased placental GPR55 expression at term compared to first trimester. Immunohistochemistry located GPR55 solely at the fetal endothelium of first trimester and term placentas. qPCR and immunocytochemistry consistently confirmed GPR55 expression in isolated primary placental arterial and venous endothelial cells. Incubation with L-α-lysophosphatidylinositol (LPI), the specific and functional ligand for GPR55, at a concentration of 1 µM, significantly enhanced migration of venous, but not arterial endothelial cells.

LPI-enhanced migration was inhibited by the GPR55 antagonist O-1918, suggesting a role of the LPI-GPR55 axis in placental venous endothelium function.”