Calcium mobilization and ERK1/2 phosphorylation were quantified in a cell line stably expressing GPR18 (HEK293/GPR18 cells). In addition, using the DiscoveRx PathHunter® CHO-K1 GPR18 β-arrestin cell line, recruitment of β-arrestin was quantified. KEY RESULTSConcentration-dependent increases in intracellular calcium and ERK1/2 phosphorylation were observed in the presence of NAGly, abnormal cannabidiol (AbnCBD), O-1602, O-1918 and Δ 9 -tetrahydrocannabinol (Δ 9 -THC) in HEK293/GPR18 cells. The initial rise in intracellular calcium in the presence of NAGly, O1918 and THC was blocked by either Gαq or Gαi/o inhibition. The ERK1/2 phosphorylation was inhibited by Pertussis toxin and N-arachidonoyl-L-serine (NARAS). Recruitment of β-arrestin in the PathHunter CHO-K1 GPR18 cell line revealed a differential pattern of GPR18 activation; of all the ligands tested, only Δ 9 -THC produced a concentration-dependent response. The localization of GPR18 receptors within the HEK293/GPR18 cells is both intracellular, and on the plasma membrane. CONCLUSIONS AND IMPLICATIONSThese findings suggest that GPR18 activation involves several signal transduction pathways indicative of biased agonism, thereby providing a plausible explanation for the apparent discrepancies in GPR18 activation found in the literature. Additionally, the results presented herein provide further evidence for GPR18 as a candidate cannabinoid receptor. Abbreviations

No abstract

The role of cannabinoid receptors in inflammation has been the topic of many research endeavors. Despite this effort, to date the involvement of the endocannabinoid system (ECS) in inflammation remains obscure. The ambiguity of cannabinoid involvement may be explained by the existence of cannabinoid receptors, other than CB1 and CB2, or a consequence of interaction of endocannabinoids with other signaling systems. GPR55 has been proposed to be a cannabinoid receptor; however the interaction of the endocannabinoid system with GPR55 remains elusive. Consequently this study set about to examine the effects of the endocannabinoids, anandamide (AEA) and virodhamine, on GPR55 mediated signaling. Specifically, we assessed changes in β-arrestin2 (βarr2) distribution and GPR55 receptor internalization following activation by lysophosphatidylinositol (LPI), the synthetic cannabinoid ligand SR141716A, and new selective synthetic GPR55 agonists. Data obtained from the experiments presented herein demonstrate that AEA and virodhamine modulate agonist-mediated recruitment of βarr2. AEA and virodhamine act as partial agonists; enhancing the agonist effect at low concentrations and inhibiting it at high concentrations. Furthermore, both virodhamine and AEA significantly attenuated agonist-induced internalization of GPR55. These effects are attributed to the expression of GPR55, and not CB1 and CB2 receptors, as we have established negligible expression of CB1 and CB2 in these GPR55-transfected U2OS cells. The identification of select endocannabinoids as GPR55 modulators will aide in elucidating the function of GPR55 in the ECS.

The therapeutic and psychoactive properties of cannabinoids have long been recognized. The type 2 receptor for cannabinoids (CB2) has emerged as an important therapeutic target in several pathologies, as it mediates beneficial effects of cannabinoids while having little if any psychotropic activity. Difficulties associated with the development of CB2-based therapeutic agents have been related to its intricate pharmacology, including the species specificity and functional selectivity of the CB2-initiated responses. We postulated that a plasmalemmal or subcellular location of the receptor may contribute to the differential signaling pathways initiated by its activation. To differentiate between these two, we used extracellular and intracellular administration of CB2 ligands and concurrent calcium imaging in CB2-expressing U2OS cells. We found that extracellular administration of anandamide was ineffective, whereas 2-arachidonoyl glycerol (2-AG) and WIN55,212-2 triggered delayed, CB2-dependent Ca2+ responses that were Gq protein-mediated. When microinjected, all agonists elicited fast, transient, and dose-dependent elevations in intracellular Ca2+ concentration upon activation of Gq-coupled CB2 receptors. The CB2 dependency was confirmed by the sensitivity to AM630, a selective CB2 antagonist, and by the unresponsiveness of untransfected U2OS cells to 2-AG, anandamide, or WIN55,212-2. Moreover, we provide functional and morphological evidence that CB2 receptors are localized at the endolysosomes, while their activation releases Ca2+ from inositol 1,4,5-trisphosphate-sensitive- and acidic-like Ca2+ stores. Our results support the functionality of intracellular CB2 receptors and their ability to couple to Gq and elicit Ca2+ signaling. These findings add further complexity to CB2 receptor pharmacology and argue for careful consideration of receptor localization in the development of CB2-based therapeutic agents.

Emerging evidence indicates the involvement of GPR55 and its proposed endogenous ligand, lysophosphatidylinositol (LPI), in nociception, yet their role in central pain processing has not been explored. Using Ca 21 imaging, we show here that LPI elicits concentration-dependent and GPR55-mediated increases in intracellular Ca 21 levels in dissociated rat periaqueductal gray (PAG) neurons, which express GPR55 mRNA. This effect is mediated by Ca 21 release from the endoplasmic reticulum via inositol 1,4,5-trisphosphate receptors and by Ca 21 entry via P/Q-type of voltage-gated Ca 21 channels. Moreover, LPI depolarizes PAG neurons and upon intra-PAG microinjection, reduces nociceptive threshold in the hot-plate test. Both these effects are dependent on GPR55 activation, because they are abolished by pretreatment with antagonist. Thus, we provide the first pharmacological evidence that GPR55 activation at central levels is pronociceptive, suggesting that interfering with GPR55 signaling in the PAG may promote analgesia.