An Update on Non-CB1, Non-CB2 Cannabinoid Related G-Protein-Coupled Receptors
Paula Morales and Patricia H. Reggio
Cannabis and Cannabinoid Research, 2017, Volume 2.1, 265-273
The endocannabinoid system (ECS) has been shown to be of great importance in the regulation of numerous physiological and pathological processes. To date, two Class A G-protein-coupled receptors (GPCRs) have been discovered and validated as the main therapeutic targets of this system: the cannabinoid receptor type 1 (CB1), which is the most abundant neuromodulatory receptor in the brain, and the cannabinoid receptor type 2 (CB2), predominantly found in the immune system among other organs and tissues. Endogenous cannabinoid receptor ligands (endocannabinoids) and the enzymes involved in their synthesis, cell uptake, and degradation have also been identified as part of the ECS.
However, its complex pharmacology suggests that other GPCRs may also play physiologically relevant roles in this therapeutically promising system. In the last years, GPCRs such as GPR18 and GPR55 have emerged as possible missing members of the cannabinoid family. This categorization still stimulates strong debate due to the lack of pharmacological tools to validate it. Because of their close phylogenetic
relationship, the Class A orphan GPCRs, GPR3, GPR6, and GPR12, have also been associated with the cannabinoids.
Moreover, certain endo-, phyto-, and synthetic cannabinoid ligands have displayed activity at other well established GPCRs, including the opioid, adenosine, serotonin, and dopamine receptor families. In addition, the cannabinoid receptors have also been shown to form dimers with other GPCRs triggering cross-talk signaling under specific conditions. In this mini review, we aim to provide insight into the non-CB1, non-CB2 cannabinoidrelated GPCRs that have been reported thus far. We consider the physiological relevance of these molecular targets in modulating the ECS.
Keywords : cannabinoid receptors; endocannabinoid system; GPCRs; orphan receptors
The Class A G-protein-coupled receptors (GPCRs), cannabinoid receptor type 1 (CB1) and cannabinoid receptor type 2 (CB2), have been widely confirmed as cannabinoid targets. These receptors have been shown to be involved in numerous physiopathological processes, including pain, inflammation, cancer, metabolic syndromes, hypertension, and neurodegenerative disorders.1 Nonetheless, the complex pharmacology of the endocannabinoid system (ECS) and its wide implication in numerous biological functions suggest the existence of other receptors playing important physiological roles. Consequently, extensive research is currently focused on the identification of potential missing cannabinoid receptors.
Diverse Class A orphans or lately deorphanized GPCRs have been proposed and evaluated as possible ECS members. Nonetheless, the lack of selective ligands for these receptors along with their intricate signaling pathways is delaying a clear elucidation of their relationship with the ECS. Therefore, thus far no other GPCR has been categorized as the cannabinoid receptor type 3 by the International Union of
GPCRs that have been postulated as cannabinoid molecular targets and the current available evidence of
their relationship with the ECS. Non-GPCR targets of the cannabinoids such as the peroxisome proliferatoractivated receptors, ligand-gated ion channels, or transient receptor potential channels have been revised elsewhere and are beyond the scope of this review.3,4
GPR55 and GPR18
Several GPCRs have been postulated to be putative cannabinoid receptors, but so far, only GPR18 and
GPR55 have been demonstrated to be targets of a wide variety of endogenous, phytogenic, and synthetic cannabinoid ligands.4 Despite this fact, inconsistencies in pharmacological data in the literature are hampering their categorization.5,6
The cannabinoid-related class A GPCR GPR55 displays low sequence identity with CB1 and CB2 (*13% and 14%, respectively). GPR55 is widely expressed in the brain, as well as in the peripheral system, co-localizing with the cannabinoid receptors in diverse tissues.7–9 This receptor displays G-protein coupling promiscuity associating with Ga13,8,10 Gaq/11,11 Ga12,11 or Ga12/13 8,12 depending on the cell line or tissue. GPR55 has been implicated in different physiopathological conditions such as cancer,13–15 pain,11,16,17 metabolic disorders,18,19 vascular functions,20,21 bone physiology,22 and motor coordination.23
The phospholipid lysophosphatidylinositol (LPI) is considered the endogenous GPR55 ligand.8,24,25 In fact, GPR55 has also been named the LPI1 receptor.26 Numerous CB1 and CB2 ligands have also been reported to act as GPR55 modulators.6,27–29 However, significant pharmacological discrepancies have been found depending on the tested functional outcome.6 For instance, the well-known phytocannabinoid D9- tetrahydrocannabinol (D9-THC) displayed activation of GPR55 according to certain reports,10,11 while it was unable to exert any effect in other functional assays. 24,30 Cannabinoid ligands reported to be recognized by GPR55 and their intriguing pharmacology have been recently reviewed elsewhere.31
Although its sequence presents low identity with CB1 and CB2 (*13% and 8%), GPR18 has also been tightly associated with the ECS.4,32 High expression of GPR18 has been found in the lymphoid tissues, while it is moderately expressed in other organs such as lungs, brain, testis, or ovary.33,34 Initially, GPR18 was found to couple to Gai/o; however, subsequent findings suggested the participation of the Gaq/11 transduction pathway as well.34–36 Different reports have shown the therapeutic potential of this target in the treatment of pathologies such as intraocular pressure,37 cancer,38 or metabolic disorders39 among others.
N-arachidonoyl glycine (NAGly) has been suggested to be the endogenous GPR18 ligand by several research groups.32,34 However, other researchers were not able to confirm these data.40 Recent investigations point to the existence of another endogenous GPR18 activator: the polyunsaturated fatty acid metabolite, Resolvin D2 (RvD2), which is mainly involved in inflammatory processes. 41 In addition, and despite the pharmacological divergences observed among some reports, GPR18 has been shown to recognize an array of CB1 and/or CB2 ligands of endogenous, phytogenic, or synthetic
nature (reviewed by others).39,42
The pharmacological discrepancies on the appraisal of cannabinoids in these two receptors, as well as the lack of selective ligands targeting them, are delaying an insightful understanding of the relation of GPR55 and GPR18 with the ECS. These inconsistencies, which may rely on intrinsic properties of these GPCRs, or on the cell type or functional assay, need to be further studied. Intensive efforts are also focused on the structural understanding of these receptors,43 as well as the development of more potent and selective pharmacological tools for the study of these promising therapeutic targets.
GPR3, GPR6, and GPR12
GPR3, GPR6, and GPR12 are three orphan Class A GPCRs that exhibit a very close phylogenetic relationship with the cannabinoid receptors CB1 and CB2 (Fig. 1). Indeed, they belong to the same cluster of receptors, the so-called MECA cluster (which consists of the melanocortin receptors, the endothelial differentiation GPCRs, the cannabinoid receptors, the adenosine binding receptors, and the orphan receptor subset GPR3,6, and12).44,45 Because of their phylogenetic proximity, these orphan receptors share common conserved residues and unique sequence motifs with CB1 and CB2.46 According to Fredriksson et al. these orphan receptors may share a common ancestor with the cannabinoid receptors since they share the same chromosomal positions.45