br Endogenous and synthetic ligands of

Endogenous and synthetic ligands of GPR55 There are currently limited commercially available synthetic agonists and antagonists that are specific for GPR55. A large number of studies have investigated the pharmacology of GPR55 4, 23, 25, 30, 31, 32, 33, 34, 35, with contradictory findings reported in the pharmacology and signalling of this CU CPT 22 [35]. Despite these inconsistencies, there is general agreement that specific cannabinoid ligands interact with GPR55 with high affinity 4, 30, 32, which are different to those reported for CB1 or CB2 receptors. For example, HU210, a potent synthetic agonist of CB1 and CB2 receptors, and JWH015, a selective CB2 agonist, are both potent agonists of GPR55, but at different concentrations 4, 30. Conversely, WIN55212-2, a synthetic cannabinoid that is a more potent agonist of CB2 than of CB1 receptors, was ineffective in the modulation of GPR55 4, 30. A more recent study indicated that synthetic compounds AM251 and SR141716A, which are both antagonists for CB1, act as agonists for GPR55. Furthermore, AM281, another antagonist for CB1, is also an agonist for GPR55, albeit less potently than AM251 and SR141716A [25]. In addition to typical cannabinoids, such as delta 9-tetrahydrocannabinol (THC) [36], GPR55 is also activated by abnormal cannabinoids, specifically abnormal cannabidiol [32] and its synthetic analogue O-1602 [32]. Furthermore, GPR55 is inhibited by cannabidiol [4] and its synthetic analogue O-1918. O-1602 and O-1918 are not specific modulators of GPR55 because they can also act on GPR18 [37]. In addition, the structural analogues of AEA, namely the fatty acids palmitoylethanolamide (PEA) and oleoyl ethanolamide (OEA), activate GPR55 via GTPγS binding [4]. Importantly, PEA has low affinity and OEA has no affinity for CB1 and CB2, although both are shown to activate GPR55. Conversely, other researchers found that neither ligand had an affinity for GPR55 [23]. A study looking at high-potency selective ligands for GPR55 found three compounds that are selective agonists, namely CID1792197, CID1172084 and CID2440433 [38], because these compounds did not interact with CB1 or CB2. A more recent study used an analogue of CID2440433 and GSK494581A, GSK319197A (GlaxoSmithKline) as a selective agonist for GPR55 [39]. A selective antagonist for GPR55, CID16020046, was also recently used to determine the role of GPR55 in platelet and endothelial function [40], in which this compound blocked the GPR55 potent agonist l-α-lysophosphatidylinositol (LPI) by inhibiting the signalling of downstream targets, such as nuclear factor of activated T cells (NFAT), serum response element (SRE) and nuclear factor (NF)-κB, as well as GPR55 internalisation. In addition to GPR55 being activated by cannabinoids, LPI, which is a lipid that is a constituent of the plasma membrane and is involved as a signalling molecule in metabolic processes, also regulates the receptor and appears to be one of the most potent endogenous agonists for GPR55 30, 31, 32, 33, 34.
Signalling pathways for GPR55 Experiments using HEK298 cells overexpressing GPR55 have shown that GPR55-mediated signalling is facilitated by the coupling of the receptor to Gα13, which leads to the activation of small GTPases, including Ras homologue gene family member A (RhoA), cell division control protein 42 (Cdc42) and Ras-related C3 botulinum toxin substrate 1 (Rac1) [4]. This results in oscillatory release of calcium from the endoplasmic reticulum, [34] which triggers several metabolic pathways [25]. Activation of GPR55 using LPI triggers NFAT through the calcineurin–NFAT pathway [34], as well as rapid phosphorylation of p38 mitogen-activated protein kinase (MAPK) [41], phosphorylation of the extracellular signal regulated kinase 1/2 (ERK1/2) pathways [25] as well as activation of 3′–5′-cyclic adenosine monophosphate response binding protein (CREB), NF-κB [25] and SRE [39] (Fig. 1).