Neurons are known to employ the endogenous cannabinoid system to communicate with other cells of the CNS. Endocannabioid signaling recruits microglia toward neurons by engaging cannabinoid CB2 and abnormal cannabidiol (Abn-CBD) receptors. The Abn-CBD receptor is a prominent atypical cannabinoid receptor that had been discriminated by means of various pharmacological and genetic tools but remained to be identified at the molecular level. We recently introduced N-arachidonoyl glycine (NAGly) signaling via GPR18 receptors as an important novel signaling mechanism in microglial-neuronal communication. NAGly is an endogenous, enzymatically oxygenated metabolite of the endocannabinoid N-arachidonoyl ethanolamide (AEA). Our recent studies support strongly two hypotheses; first that NAGly initiates directed microglial migration in the CNS through activation of GPR18, and second that GPR18 is the Abn-CBD receptor. Here we present siRNA knockdown data in further support of these hypotheses.
A GPR18-targetting siRNA pSUPER G418 GFP cDNA plasmid was created and transfected into BV-2 microglia. Successfully transfected GFP+ GPR18 siRNA BV-2 microglia displayed reduced GPR18 mRNA levels and immunocytochemical staining. Cell migration induced by 1 μM concentrations of NAGly, O-1602 and Abn-CBD were significantly attenuated in GFP+ cells.
Our data provide definitive evidence that these compounds, characteristic of Abn-CBD receptor pharmacology, are acting via GPR18 in BV-2 microglia. A fuller understanding of the hitherto unidentified cannabinoid receptors such as GPR18; their molecular interactions with endogenous ligands; and how phytocannabinoids influence their signaling is vital if we are to comprehensively assess the function of the endogenous cannabinoid signaling system in human health and disease.
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