Sage Journals: Discover world-class research

Abstract

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 cannabinoid-related GPCRs that have been reported thus far. We consider the physiological relevance of these molecular targets in modulating the ECS.

Introduction

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 Pharmacology.²

Herein, we intend to provide an overview of the 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 proliferator-activated 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 G_α13,^8,10 G_αq/11,¹¹ G_α12,¹¹ or G_α12/13^8,12 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 Δ⁹-tetrahydrocannabinol (Δ⁹-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 G_αi/o; however, subsequent findings suggested the participation of the G_αq/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, and −12).^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

FIG. 1.

Phylogenetic tree of cannabinoid receptors and the closely related Class A GPCRs (S1PR family and the orphan receptors GPR3, GPR6, and GPR12). Data were obtained from GPCRdb.org. CB1, cannabinoid receptor type 1; CB2, cannabinoid receptor type 2; GPCR, G-protein-coupled receptor; S1P, sphingosine-1-phosphate.

GPR3, GPR6, and GPR12, which share over 60% of sequence similarity, were first cloned in 1995.^47,48 These receptors constitutively activate adenylate cyclase by coupling to G_αs proteins. In fact, different groups have reported that when expressed in diverse cell lines, they can stimulate adenylate cyclase to levels similar in amplitude to agonist-activated GPCRs.^47,49,50 In addition to G_αs, GPR6 and GPR12 have also been suggested to couple to G_αi/o,^51,52 but further investigations are required to confirm this G-protein promiscuity.

GPR3, GPR6, and GPR12 are predominantly expressed in the brain and the reproductive system.⁴⁹ This family of constitutively active GPCRs is involved in neuronal differentiation and growth, as well as in the formation of synaptic contacts.⁴⁹ Therefore, their role in different neurological processes such as neurite outgrowth,⁴⁹ Alzheimer's disease,^53–57 development of cerebellar granule neurons,^58,59 neuropathic pain,⁶⁰ early phases of cocaine reinforcement,⁶¹ emotional-like responses,⁶² instrumental learning,⁶³ or Parkinson's disease^64,65 has been studied. Other pathophysiological conditions such as oocyte maturation,^66,67 dyslipidemia,⁶⁸ and cell proliferation⁶⁹ may also be impacted by the modulation of some of these receptors.

The bioactive lipids, sphingosine-1-phosphate^50,52 and/or sphingosylphosphorylcholine,⁵¹ have been proposed as endogenous ligands of these receptors (Fig. 2). However, other groups were not able to confirm this claim, and consequently, GPR3, GPR6, and GPR12 are still categorized as orphans.^30,70,71 Interestingly, among the very few ligands discovered so far for these receptors, the nonpsychoactive phytocannabinoid cannabidiol (CBD) stands out as being able to target GPR3 and GPR6,⁷² acting as a β-arrestin2 inverse agonist of both receptors. This functionality is of high interest in the GPR3 field because β-arrestin2 signaling at GPR3 has been directly linked to the manufacture of beta-amyloid plaque (Aβ_1–40 and Aβ_1–42) in Alzheimer's disease through complex formation with γ-secretase.^56,57 Because CBD is an inverse agonist of this signaling pathway at GPR3, it may represent a potential tool for the reduction of amyloid pathology. Other phytocannabinoids and several endocannabinoids were also tested but so far none of them were found to modulate this family of orphan receptors.^30,72

FIG. 2.

Structures of the putative GPR3, GPR6, and GPR12 endogenous ligands S1P and SPC and the GPR3 and GPR6 inverse agonist CBD. SPC, sphingosylphosphorylcholine.

So, a relationship between the cannabinoids and the orphan receptors GPR3, GPR6, and GPR12 has been evidenced. Nonetheless, extensive research and more pharmacological tools are needed to extract significant conclusions about the association of these receptors with the ECS and its ligands.

Alkylindole-Sensitive Receptors

As reported by different research groups, the well-known aminoalkylindole cannabinoid agonist WIN55,212-2 (Fig. 3) displays pharmacological functions independent of the cannabinoid receptors CB₁ and CB₂.^73–75 This fact led to the identification of novel targets commonly referred to as the alkylindole (AI)-sensitive receptors.^74,76,77 These cannabinoid-related receptors are modulated by AI derivatives, but not by other classes of cannabinoid ligands.⁷⁶ Diverse evidence suggests that the AI-sensitive receptors are G_αs-protein coupled receptors that are mainly expressed in microglia and astrocytomas.^76–79 However, their biological functions, pharmacology, and therapeutic value remain to be unraveled due to the lack of selective pharmacological tools.

FIG. 3.

Alkylindole derivatives WIN55,212–2, ST-11, ST-23, ST-25, and ST-48.

Recent studies from Stella and coworkers revealed the role of AI-sensitive receptors in the modulation of microglial cell migration and proliferation highlighting their potential in the treatment of gliomas.^77,78 Moreover, these authors have identified a series of naphthoyl AI derivatives, ST-11, ST-23, ST-25, and ST-48 (Fig. 3) among them, that bind to the AI-sensitive receptors.⁷⁸ These compounds display affinities in the nanomolar range when competing with [³H]WIN55,212-2 in DBT (Delayed Brain Tumor) cells which endogenously express AI-sensitive receptors, while lacking CB₁ and CB₂ receptors.⁸⁰ Compound ST-11 stands out from this study because of its potency at AI-sensitive receptors, while not interacting with CB₁ and CB₂ receptors. In addition, in vitro assays revealed that this compound inhibits cell migration and proliferation in the aforementioned mouse glioma cell line, DBT. Further studies revealed that ST-11 can reduce glioblastoma growth in a syngeneic mouse model.⁸¹

Even though extensive research is clearly needed to understand the pathophysiological function of these receptors, reported data suggest that AI-sensitive receptor agonists could represent a novel class of potential brain cancer antitumor drugs.

Cannabinoid-Related Oligomers

Numerous studies have shown that GPCRs, cannabinoid receptors among them, can exist and function as dimers or complexes of higher order.^82–85 This oligomerization may affect receptor signaling, receptor trafficking, and ligand binding. The physiological relevance of such dimerization has not yet been fully established for the cannabinoid receptors; nonetheless, the presence of cannabinoid homo- and heterodimers in specific tissues has been intensely reported over the last years.

For the CB₁ receptor, heteromers have been suggested to exist under certain physiological conditions with serotonin,⁸⁶ angiotensin,⁸⁷ opioid,^88–90 GPR55,⁹¹ somatostatin,⁹² orexin,^93,94 dopamine,^95–97 and adenosine⁹⁸ receptors among others (Table 1). Although CB₂ has been less investigated, recent research revealed that it may form heterodimers with CB₁,⁹⁹ with GPR55,^100,101 with the serotonin receptor 5HT_1A,¹⁰² or with the chemokine receptor CXCR4.¹⁰³ The expression of these heterodimers has been associated with different pathologies. For instance, the CB₂−CXCR4 and the CB₂−GPR55 dimers have been associated with cancer progression, while the CB₁−A_2A and the CB₁−D2 heteromers have been suggested to have physiological implications in neurodegenerative disorders such as Huntington's or Parkinson's diseases. All these data suggest that the ECS interacts in a significant manner with several other endogenous systems.

Table 1.

Cannabinoid-Related G-Protein-Coupled Receptor Dimers Reported So Far

Heterodimers		Homodimers
CB₁−D2	^95,97	CB₁−CB₁	^104–106
CB₁−A_2A	⁹⁸	CB₂−CB₂	^107–109
CB₁−5HT_2A	⁸⁶
CB₁−AT₁	⁸⁷
CB₁−GPR55	⁹¹
CB₁−SST5	⁹²
CB₁−OX1	⁹⁴
CB₁−OX2	⁹³
CB₁−μOR	⁹⁰
CB₁−δOR	⁸⁸
CB₁−CB₂	⁹⁹
CB₂−GPR55	^100,101
CB₂−5HT_1A	¹⁰²
CB₂−CXCR4	¹⁰³

CB₁, cannabinoid receptor type 1; CB₂, cannabinoid receptor type 2.

With regard to cannabinoid receptor homodimerization, more data have been published on CB₁ homodimers than on their CB₂ counterparts. The presence of CB₁ receptor homodimers has been reported in different biological tissues,^104–106 but their functional role has not been determined. In contrast, CB₂ homodimers have been evidenced,^107–109 but their pharmacological potential has not been explored yet.

In this field, bivalent ligands have emerged as promising new pharmacological entities and potential tools for the biological study of their respective dimeric receptors.^110–113 Despite their poor pharmacokinetic properties,¹¹⁴ bivalent ligands can exhibit enhanced activity and selectivity over their respective corresponding parent ligands offering unique pharmacological strategies. Bivalent ligands have been synthesized and evaluated for several GPCRs. Opioid,^115,116 dopamine,^117,118 and histamine¹¹⁹ are some of the receptors for which a bivalent compound provided higher activity than their monomer counterparts. CB₁ homobivalent^120–122 and heterobivalent^123–125 ligands have been also reported and explored. However, currently available receptor structural information challenges the fact that bivalent ligands can simultaneously bind to both receptors within the dimer, especially in the case of lipid receptors as the cannabinoids.¹²⁶ Therefore, novel drug design approaches to target dimers, as well as new techniques to determine bivalent binding, remain to be explored.

Homo- and heterodimerization likely influences the manner in which the ECS responds to ligands. Nevertheless, unambiguous data about their physical association in native tissues, as well as their pharmacology, are needed to clearly identify what biological functions are impacted by cannabinoid dimers.

Well-Established GPCRs Related to the Cannabinoids

Certain endo-, phyto-, and synthetic cannabinoid ligands have been shown to modulate well-known GPCRs. These GPCRs include members of established families such as the opioid, serotonin, muscarinic, dopamine, and adenosine families. For instance, the endocannabinoid anandamide has been shown to act at the adenosine receptor A₃,¹²⁷ the muscarinic acetylcholine receptors M1 and M4,¹²⁸ and the serotonin receptors 5-HT_1A and 5-HT_2A¹²⁹ among others. In addition, phytocannabinoids such as Δ⁹-THC and CBD have been shown to modulate the μ− and δ−opioid receptors,¹³⁰ while other plant-derived compounds such as CBG (cannabigerol) and Δ⁹-THCV (tetrahydrocannabivarin) display activity at the 5-HT_1A receptor.^131,132 Likewise, synthetic cannabinoids, such as the CB₁ inverse agonists taranabant (MK-0364) and rimonabant (SR141716), have also displayed activity in well-established targets. These include the adenosine A₃ and the tachykinin NK2 receptors.¹³³

Some of these cannabinoid ligands have been proposed to interact allosterically with the aforementioned targets. It is worth mentioning that the efficacy that most of these cannabinoids exhibit toward these GPCRs is lower than the one displayed at the CB₁ and/or CB₂ receptors. Therefore, there is no evidence indicating a necessary recategorization of these receptors.

Other GPCRs

Because of their ability to recognize lipids and their relatively close phylogenetic relationship with CB₁ and CB₂, several other Class A orphan or recently deorphanized GPCRs such as GPR40, GPR43, GPR41, GPR120 (currently classified as free fatty acid receptors FFA1, FFA2, FFA3, and FFA4, respectively), GPR23, GPR92 (recently categorized as lysophosphatidic acid receptors LPA4 and LPA5), GPR84, GPR119, or GPR35 have been postulated as possible cannabinoid receptor candidates.⁴ However, there is no available evidence since they do not meet some of the criteria established by the International Union of Pharmacology.^4,70

Conclusions

Two cannabinoid receptors, CB₁ and CB₂, have been validated at the molecular level as the main targets of the ECS. These two GPCRs have been widely explored in the development of numerous pathophysiological processes, and their therapeutic potential for the treatment of different diseases has been extensively confirmed.¹ Great efforts are being made to structurally understand these receptors; in fact, CB₁ in its inactive^134,135 and active¹³⁶ states has been recently crystallized. Despite possible crystallization artifacts, these structures will help shedding light into the complex pharmacology of the cannabinoid receptors.

Growing evidence suggests that other cannabinoid or cannabinoid-like receptors remain to be identified as important players of the ECS. Different endogenous, phytogenic, and/or synthetic cannabinoid ligands have been reported to modulate GPCRs such as GPR18, GPR55, GPR3, GPR6, or the AI-sensitive receptors, among others. Pharmacological discrepancies and the lack of selective ligands for these receptors are delaying the characterization of their relationship with the ECS. Consequently, no CB₃ receptor has yet been confirmed.²

Adding more complexity to the ECS scenario, molecular interactions of the cannabinoid receptors with other GPCRs have been reported. Co-localization or co-immunoprecipitation data suggest the presence of cannabinoid homo- and heterodimers in specific native tissues. Cannabinoid receptor dimerization may not only influence the pharmacology of these receptors but also it may provide new signaling pathways through the interacting protomers. However, due to the lack of appropriate tools, there is still limited in vivo information about the expression of cannabinoid dimers. Hence, it remains a challenge to elucidate their therapeutic relevance under specific physiological conditions.

Currently, appropriate characterization of cannabinoid ligands should take into account the activity at the aforementioned GPCRs. Possible biased agonism of ligands, allosterism, or cross-talk signaling could be determining the intricate GPCR pharmacology. In addition, differential coupling and regulation of G-proteins or the formation of oligomers are among GPCR intrinsic properties that might be delaying the validation of novel potential cannabinoid targets. Therefore, further research is needed to fully understand the physiopathological role of these non-CB₁, non-CB₂ GPCRs in the modulation of the ECS.

Footnotes

Acknowledgments

The authors acknowledge research support from NIH/NIDA grants R01 DA003934 and K05 DA021358 (P.H.R.).

Author Disclosure Statement

No competing financial interests exist.

Cite this article as: Morales P, Reggio PH (2017) An update on non-CB₁, non-CB₂ cannabinoid related G-protein-coupled receptors, Cannabis and Cannabinoid Research 2:1, 265–273, DOI: 10.1089/can.2017.0036.

Abbreviations Used

References

Pertwee

. Targeting the endocannabinoid system with cannabinoid receptor agonists: pharmacological strategies and therapeutic possibilities. Philos Trans R Soc B, 2012; 367:3353–3363.

IUPHAR BPS Guide to PHARMACOLOGY. Cannabinoid receptors. www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=13 (accessed June 7, 2017).

Morales

, Hurst

, Reggio

. In: Molecular targets of the phytocannabinoids: a complex picture. In: Kinghorn

, Gibbons

, eds. Phytocannabinoids: Unraveling the Complex Chemistry and Pharmacology of Cannabis sativa. Cham, Switzerland: Springer International Publishing, 2017; Vol. 103, pp. 103–131.

Pertwee

, Howlett

, Abood

, et al. International union of basic and clinical pharmacology. LXXIX. Cannabinoid receptors and their ligands: beyond CB1 and CB2. Pharmacol Rev, 2010; 62:588–631.

IUPHAR BPS Guide to PHARMACOLOGY. GPR18, GPR55 and GPR119. www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=114 (accessed June 7, 2017).

Ross

. The enigmatic pharmacology of GPR55. Trends Pharmacol Sci, 2009; 30:156–163.

Balenga

NAB

, Henstridge

, Kargl

, et al. Pharmacology, signaling and physiological relevance of the G protein-coupled receptor 55. In: Neubig

, ed. Pharmacology of G Protein Coupled Receptors. San Diego, CA: Advances in Pharmacology, Volume 62, Elsevier Inc., 2011, pp. 251–277.

Henstridge

, Balenga

NAB

, Ford

, et al. The GPR55 ligand L-alpha-lysophosphatidylinositol promotes RhoA-dependent Ca2+ signaling and NFAT activation. FASEB J, 2009; 23:183–193.

Sawzdargo

, Nguyen

, Lee

, et al. Identification and cloning of three novel human G protein-coupled receptor genes GPR52, PsiGPR53 and GPR55: GPR55 is extensively expressed in human brain. Brain research. Mol Brain Res, 1999; 64:193–198.

10.

Ryberg

, Larsson

, Sjögren

, et al. The orphan receptor GPR55 is a novel cannabinoid receptor. Br J Pharmacol, 2007; 152:1092–1101.

11.

Lauckner

, Jensen

, Chen

H-Y

, et al. GPR55 is a cannabinoid receptor that increases intracellular calcium and inhibits M current. Proc Natl Acad Sci U S A, 2008; 105:2699–2704.

12.

Henstridge

, Balenga

, Schröder

, et al. GPR55 ligands promote receptor coupling to multiple signalling pathways. Br J Pharmacol, 2010; 160:604–614.

13.

Andradas

, Blasco-Benito

, Castillo-Lluva

, et al. Activation of the orphan receptor GPR55 by lysophosphatidylinositol promotes metastasis in triple-negative breast cancer. Oncotarget, 2016; 7:47565–47575.

14.

Hofmann

, Yang

, Trauger

, et al. The GPR55 agonist, L-α-lysophosphatidylinositol, mediates ovarian carcinoma cell-induced angiogenesis. Br J Pharmacol, 2015; 172:4107–4118.

15.

Kargl

, Andersen

, Hasenöhrl

, et al. GPR55 promotes migration and adhesion of colon cancer cells indicating a role in metastasis. Br J Pharmacol, 2016; 173:142–154.

16.

Carey

, Gutierrez

, Deng

, et al. Inflammatory and neuropathic nociception is preserved in GPR55 knockout mice. Sci Rep, 2017; 7:94–4.

17.

Staton

, Hatcher

, Walker

, et al. The putative cannabinoid receptor GPR55 plays a role in mechanical hyperalgesia associated with inflammatory and neuropathic pain. Pain, 2008; 139:225–236.

18.

Tudurí

, Imbernon

, Hernández-Bautista

. GPR55: a new promising target for metabolism?. J Mol Endocrinol, 2017; 58:R191–R202.

19.

Tudurí

, López

, Diéguez

, et al. GPR55 and the regulation of glucose homeostasis. Int J Biochem Cell Biol, 2017; 88:204–207.

20.

AlSuleimani

, Hiley

. The GPR55 agonist lysophosphatidylinositol relaxes rat mesenteric resistance artery and induces calcium release in rat mesenteric artery endothelial cells. Br J Pharmacol, 2015; 172:3043–3057.

21.

Bondarenko

, Montecucco

, Panasiuk

, et al. GPR55 agonist lysophosphatidylinositol and lysophosphatidylcholine inhibit endothelial cell hyperpolarization via GPR-independent suppression of Na+-Ca2+ exchanger and endoplasmic reticulum Ca2+ refilling. Vascul Pharmacol, 2017; 89:39–48.

22.

Whyte

, Ryberg

, Sims

, et al. The putative cannabinoid receptor GPR55 affects osteoclast function in vitro and bone mass in vivo. Proc Natl Acad Sci U S A, 2009; 106:16511–16516.

23.

C-S

, Chen

, Sun

, et al. GPR55, a G-protein coupled receptor for lysophosphatidylinositol, plays a role in motor coordination. PLoS One, 2013; 8:e6031–4.

24.

Oka

, Nakajima

, Yamashita

, et al. Identification of GPR55 as a lysophosphatidylinositol receptor. Biochem Biophys Res Commun, 2007; 362:928–934.

25.

Oka

, Toshida

, Maruyama

, et al. 2-Arachidonoyl-sn-glycero-3-phosphoinositol: a possible natural ligand for GPR55. J Biochem, 2009; 145:13–20.

26.

Kihara

, Maceyka

, Spiegel

, et al. Lysophospholipid receptor nomenclature review: IUPHAR review 8. Br J Pharmacol, 2014; 171:3575–3594.

27.

Moriconi

, Cerbara

, Maccarrone

, et al. GPR55: current knowledge and future perspectives of a purported “Type-3” cannabinoid receptor. Curr Med Chem, 2010; 17:1411–1429.

28.

Nevalainen

, Irving

. GPR55, a Lysophosphatidylinositol receptor with cannabinoid sensitivity?. Curr Top Med Chem, 2010; 1:799–813.

29.

Sharir

, Abood

. Pharmacological characterization of GPR55, a putative cannabinoid receptor. Pharmacol Ther, 2010; 126:301–313.

30.

Yin

, Chu

, Li

, et al. Lipid G protein-coupled receptor ligand identification using beta-arrestin PathHunter assay. J Biol Chem, 2009; 284:12328–12338.

31.

Morales

, Jagerovic

. Advances towards the discovery of GPR55 ligands. Curr Med Chem, 2016; 23:2087–2100.

32.

McHugh

, Hu

SSJ

, Rimmerman

, et al. N-arachidonoyl glycine, an abundant endogenous lipid, potently drives directed cellular migration through GPR18, the putative abnormal cannabidiol receptor. BMC Neurosci, 2010; 11:4–4.

33.

Gantz

, Muraoka

, Yang

, et al. Cloning and chromosomal localization of a gene (GPR18) encoding a novel seven transmembrane receptor highly expressed in spleen and testis. Genomics, 1997; 42:462–466.

34.

Kohno

, Hasegawa

, Inoue

, et al. Identification of N-arachidonylglycine as the endogenous ligand for orphan G-protein-coupled receptor GPR18. Biochem Biophys Res Commun, 2006; 347:827–832.

35.

Console-Bram

, Brailoiu

, et al. Activation of GPR18 by cannabinoid compounds: a tale of biased agonism. Br J Pharmacol, 2014; 171:3908–3917.

36.

Takenouchi

, Inoue

, Kambe

, et al. N-arachidonoyl glycine induces macrophage apoptosis via GPR18. Biochem Biophys Res Commun, 2012; 418:366–371.

37.

Miller

, Leishman

, Oehler

, et al. Evidence for a GPR18 role in diurnal regulation of intraocular pressure. Invest Ophthalmol Vis Sci, 2016; 57:6419–6426.

38.

Qin

, Verdegaal

EME

, Siderius

, et al. Quantitative expression profiling of G-protein-coupled receptors (GPCRs) in metastatic melanoma: the constitutively active orphan GPCR GPR18 as novel drug target. Pigment Cell Melanoma Res, 2011; 24:207–218.

39.

Rajaraman

, Simcocks

, Hryciw

, et al. G protein coupled receptor 18: a potential role for endocannabinoid signaling in metabolic dysfunction. Mol Nutr Food Res, 2016; 60:92–102.

40.

, Puhl

, Ikeda

. N-arachidonyl glycine does not activate G protein–coupled receptor 18 signaling via canonical pathways. Mol Pharmacol, 2013; 83:267–282.

41.

Chiang

, Dalli

, Colas

, et al. Identification of resolvin D2 receptor mediating resolution of infections and organ protection. J Exp Med, 2015; 212:1203–1217.

42.

McHugh

. GPR18 in microglia: implications for the CNS and endocannabinoid system signalling. Br J Pharmacol, 2012; 167:1575–1582.

43.

Lingerfelt

, Zhao

, Sharir

, et al. Identification of crucial amino acid residues involved in agonist signaling at the GPR55 receptor. Biochemistry, 2017; 56:473–486.

44.

Fredriksson

, Schio

. The repertoire of G-protein–coupled receptors in fully. Mol Pharmacol, 2005; 67:1414–1425.

45.

Fredriksson

, Lagerström

, Lundin

L-G

, et al. The G-protein-coupled receptors in the human genome form five main families. Phylogenetic analysis, paralogon groups, and fingerprints. Mol Pharmacol, 2003; 63:1256–1272.

46.

Morales

, Hurst

, Reggio

. Methods for the development of in silico GPCR models. In: Reggio

, ed. Cannabinoids and Their Receptors. London: Methods in Enzymology, Volume 593, Elsevier Inc., 2017, pp. 405–448.

47.

Eggerickx

, Denef

, Labbe

, et al. Molecular cloning of an orphan G-protein-coupled receptor that constitutively activates adenylate cyclase. Biochem J, 1995; 309:837–843.

48.

Song

Z-H

, Modi

, Bonner

. Molecular cloning and chromosomal localization of human genes encoding three closely related G protein-coupled receptors. Genomics, 1995; 28:347–349.

49.

Tanaka

, Ishii

, Kasai

, et al. Neural expression of G protein-coupled receptors GPR3, GPR6, and GPR12 up-regulates cyclic AMP levels and promotes neurite outgrowth. J Biol Chem, 2007; 282:10506–10515.

50.

Uhlenbrock

, Gassenhuber

, Kostenis

. Sphingosine 1-phosphate is a ligand of the human gpr3, gpr6 and gpr12 family of constitutively active G protein-coupled receptors. Cell Signal, 2002; 14:941–953.

51.

Ignatov

, Lintzel

, Hermans-Borgmeyer

, et al. Role of the G-protein-coupled receptor GPR12 as high-affinity receptor for sphingosylphosphorylcholine and its expression and function in brain development. J Neurosci, 2003; 23:907–914.

52.

Ignatov

, Lintzel

, Kreienkamp

, et al. Sphingosine-1-phosphate is a high-affinity ligand for the G protein-coupled receptor GPR6 from mouse and induces intracellular Ca2+ release by activating the sphingosine-kinase pathway. Biochem Biophys Res Commun, 2003; 311:329–336.

53.

Benoit

, Hernandez

, Dinh

, et al. C1q-induced LRP1B and GPR6 proteins expressed early in Alzheimer disease mouse models, are essential for the C1q-mediated protection against amyloid neurotoxicity. J Biol Chem, 2013; 288:654–665.

54.

Huang

, Skwarek-Maruszewska

, Horré

, et al. Loss of GPR3 reduces the amyloid plaque burden and improves memory in Alzheimer's disease mouse models. Sci Transl Med, 2015; 7:309ra16–4.

55.

Nelson

, Sheng

. Gpr3 stimulates Aβ production via interactions with APP and β-arrestin2. PLoS One, 2013; 8:e7468–0.

56.

Thathiah

, Horré

, Snellinx

, et al. β-arrestin 2 regulates Aβ generation and γ-secretase activity in Alzheimer's disease. Nat Med, 2013; 19:43–49.

57.

Thathiah

, Spittaels

, Hoffmann

, et al. The orphan G protein-coupled receptor 3 modulates amyloid-beta peptide generation in neurons. Science, 2009; 323:946–951.

58.

Tanaka

, Shaikh

, Chiocca

, et al. The Gs-linked receptor GPR3 inhibits the proliferation of cerebellar granule cells during postnatal development. PLoS One, 2009; 4:1–12.

59.

Tanaka

, Miyagi

, Dohi

, et al. Developmental expression of GPR3 in rodent cerebellar granule neurons is associated with cell survival and protects neurons from various apoptotic stimuli. Neurobiol Dis, 2014; 68:215–227.

60.

Ruiz-Medina

, Ledent

, Valverde

. GPR3 orphan receptor is involved in neuropathic pain after peripheral nerve injury and regulates morphine-induced antinociception. Neuropharmacology, 2011; 61:43–50.

61.

Tourino

, Valjent

, Ruiz-Medina

, et al. The orphan receptor GPR3 modulates the early phases of cocaine reinforcement. Br J Pharmacol, 2012; 167:892–904.

62.

Valverde

, Celerier

, Baranyi

, et al. GPR3 receptor, a novel actor in the emotional-like responses. PLoS One, 2009; 4:e470–4.

63.

Lobo

, Cui

, Ostlund

, et al. Genetic control of instrumental conditioning by striatopallidal neuron-specific S1P receptor Gpr6. Nat Neurosci, 2007; 10:1395–1397.

64.

Oeckl

, Hengerer

, Ferger

. G-protein coupled receptor 6 deficiency alters striatal dopamine and cAMP concentrations and reduces dyskinesia in a mouse model of Parkinson's disease. Exp Neurol, 2014; 257:1–9.

65.

Oeckl

, Ferger

. Increased susceptibility of G-protein coupled receptor 6 deficient mice to MPTP neurotoxicity. Neuroscience, 2016; 337:218–223.

66.

Hinckley

, Vaccari

, Horner

, et al. The G-protein-coupled receptors GPR3 and GPR12 are involved in cAMP signaling and maintenance of meiotic arrest in rodent oocytes. Dev Biol, 2005; 287:249–261.

67.

Mehlmann

, Saeki

, Tanaka

, et al. The Gs-linked receptor GPR3 maintains meiotic arrest in mammalian oocytes. Science, 2004; 306:1947–1950.

68.

Bjursell

, Gerdin

, Jönsson

, et al. G protein-coupled receptor 12 deficiency results in dyslipidemia and obesity in mice. Biochem Biophys Res Commun, 2006; 348:359–366.

69.

, Zhang

, Meng

, et al. Involvement of GPR12 in the regulation of cell proliferation and survival. Mol Cell Biochem, 2012; 366:101–110.

70.

Davenport

, Alexander

SPH

, Sharman

, et al. International Union of Basic and Clinical Pharmacology. LXXXVIII. G protein-coupled receptor list: recommendations for new pairings with cognate ligands. Pharmacol Rev, 2013; 65:967–986.

71.

Southern

, Cook

, Neetoo-Isseljee

, et al. Screening beta-arrestin recruitment for the identification of natural ligands for orphan G-protein-coupled receptors. J Biomol Screen, 2013; 18:599–609.

72.

Laun

, Song

Z-H

. GPR3 and GPR6, novel molecular targets for cannabidiol. Biochem Biophys Res Commun, 2017; 490:17–21.

73.

Abood

, Sorensen

, Stella

. Endocannabinoids: actions at non-CB1/CB2 cannabinoid receptors. Springer: New York, 2013.

74.

Hájos

, Ledent

, Freund

. Novel cannabinoid-sensitive receptor mediates inhibition of glutamatergic synaptic transmission in the hippocampus. Neuroscience, 2001; 106:1–4.

75.

Monory

, Tzavara

, Lexime

, et al. Novel, not adenylyl cyclase-coupled cannabinoid binding site in cerebellum of mice. Biochem Biophys Res Commun, 2002; 292:231–235.

76.

Breivogel

, Griffin

, Di Marzo

, et al. Evidence for a new G protein-coupled cannabinoid receptor in mouse brain. Mol Pharmacol, 2001; 60:155–163.

77.

Fung

, Cherry

, Xu

, et al. Alkylindole-sensitive receptors modulate microglial cell migration and proliferation. Glia, 2015; 63:1797–1808.

78.

Fung

, Xu

, Hamel

, et al. Novel indole-based compounds that differentiate alkylindole-sensitive receptors from cannabinoid receptors and microtubules: characterization of their activity on glioma cell migration. Pharmacol Res, 2017; 115:233–241.

79.

Stella

. Cannabinoid and cannabinoid-like receptors in microglia, astrocytes, and astrocytomas. Glia, 2010; 58:1017–1030.

80.

Cudaback

, Marrs

, Moeller

, et al. The expression level of CB1 and CB2 receptors determines their efficacy at inducing apoptosis in astrocytomas. PLoS One, 2010; 5:e870–2.

81.

Cherry

, Haas

, Naydenov

, et al. ST-11: a new brain-penetrant microtubule-destabilizing agent with therapeutic potential for glioblastoma multiforme. Mol Cancer Ther, 2016; 15:2018–2029.

82.

Farran

. An Update on the physiological and therapeutic relevance of GPCR oligomers. Pharmacol Res, 2017; 117:303–327.

83.

Franco

, Martínez-Pinilla

, Lanciego

, et al. Basic pharmacological and structural evidence for class A G-protein-coupled receptor heteromerization. Front Pharmacol, 2016; 7:1–10.

84.

Scarselli

, Annibale

, McCormick

, et al. Revealing G-protein-coupled receptor oligomerization at the single-molecule level through a nanoscopic lens: methods, dynamics and biological function. FEBS J, 2016; 283:1197–1217.

85.

Vischer

, Castro

, Pin

J-P

. G protein-coupled receptor multimers: a question still open despite the use of novel approaches. Mol Pharmacol, 2015; 88:561–571.

86.

Viñals

, Moreno

, Lanfumey

, et al. Cognitive impairment induced by Delta9-tetrahydrocannabinol occurs through heteromers between cannabinoid CB1 and serotonin 5-HT2A receptors. PLoS Biol, 2015; 13:e100219–4.

87.

Rozenfeld

, Gupta

, Gagnidze

, et al. AT1R-CB1R heteromerization reveals a new mechanism for the pathogenic properties of angiotensin II. EMBO J, 2011; 30:2350–2363.

88.

Bushlin

, Gupta

, Stockton

, et al. Dimerization with cannabinoid receptors allosterically modulates delta opioid receptor activity during neuropathic pain. PLoS One, 2012; 7:e4978–9.

89.

Fujita

, Gomes

, Devi

. Revolution in GPCR signalling: opioid receptor heteromers as novel therapeutic targets: IUPHAR review 10. Br J Pharmacol, 2014; 171:4155–4176.

90.

Hojo

, Sudo

, Ando

, et al. mu-Opioid receptor forms a functional heterodimer with cannabinoid CB1 receptor: electrophysiological and FRET assay analysis. J Pharmacol Sci, 2008; 108:308–319.

91.

Martínez-Pinilla

, Reyes-Resina

, Oñatibia-Astibia

, et al. CB1 and GPR55 receptors are co-expressed and form heteromers in rat and monkey striatum. Exp Neurol, 2014; 261:44–52.

92.

Zou

, Somvanshi

, Kumar

. Somatostatin receptor 5 is a prominent regulator of signaling pathways in cells with coexpression of cannabinoid receptors 1. Neuroscience, 2017; 340:218–231.

93.

Jäntti

, Mandrika

, Kukkonen

. Human orexin/hypocretin receptors form constitutive homo- and heteromeric complexes with each other and with human CB1 cannabinoid receptors. Biochem Biophys Res Commun, 2014; 445:486–490.

94.

Ward

, Pediani

, Milligan

. Heteromultimerization of cannabinoid CB(1) receptor and orexin OX(1) receptor generates a unique complex in which both protomers are regulated by orexin A. J Biol Chem, 2011; 286:37414–37428.

95.

Glass

, Felder

. Concurrent stimulation of cannabinoid CB1 and dopamine D2 receptors augments cAMP accumulation in striatal neurons: evidence for a Gs linkage to the CB1 receptor. J Neurosci, 1997; 17:5327–5333.

96.

Kearn

, Blake-Palmer

, Daniel

, et al. Concurrent stimulation of cannabinoid CB1 and dopamine D2 receptors enhances heterodimer formation: a mechanism for receptor cross-talk?. Mol Pharmacol, 2005; 67:1697–1704.

97.

Przybyla

, Watts

. Ligand-induced regulation and localization of cannabinoid CB1 and dopamine D2L receptor heterodimers. J Pharmacol Exp Ther, 2010; 332:710–719.

98.

Moreno

, Chiarlone

, Medrano

, et al. Singular location and signaling profile of adenosine A2A-cannabinoid CB1 receptor heteromers in the dorsal striatum. Neuropsychopharmacology, 2017 [Epub ahead of print]; DOI: 10.1038/npp.2017.

99.

Callén

, Moreno

, Barroso-Chinea

, et al. Cannabinoid receptors CB1 and CB2 form functional heteromers in brain. J Biol Chem, 2012; 287:20851–20865.

100.

Balenga

, Martínez-Pinilla

, Kargl

, et al. Heteromerization of GPR55 and cannabinoid CB2 receptors modulates signaling. Br J Pharmacol, 2014; 171:5387–5406.

101.

Moreno

, Andradas

, Medrano

, et al. Targeting CB2-GPR55 receptor heteromers modulates cancer cell signaling. J Biol Chem, 2014; 289:21960–21972.

102.

Pazos

, Mohammed

, Lafuente

, et al. Mechanisms of cannabidiol neuroprotection in hypoxic-ischemic newborn pigs: role of 5HT(1A) and CB2 receptors. Neuropharmacology, 2013; 71:282–291.

103.

Coke

, Scarlett

, Chetram

, et al. Simultaneous activation of induced heterodimerization between CXCR4 chemokine receptor and cannabinoid receptor 2 (CB2) reveals a mechanism for regulation of tumor progression. J Biol Chem, 2016; 291:9991–10005.

104.

Mackie

. Cannabinoid receptor homo- and heterodimerization. Life Sci, 2005; 77:1667–1673.

105.

Wager-Miller

, Westenbroek

, Mackie

. Dimerization of G protein-coupled receptors: CB1 cannabinoid receptors as an example. Chem Phys Lipids, 2002; 121:83–89.

106.

, Filppula

, Mercier

, et al. Purification and mass spectroscopic analysis of human CB1 cannabinoid receptor functionally expressed using the baculovirus system. J Pept Res, 2005; 66:138–150.

107.

Filppula

, Yaddanapudi

, Mercier

, et al. Purification and mass spectroscopic analysis of human CB2 cannabinoid receptor functionally expressed using the baculovirus system. J Pept Res, 2004; 64:225–236.

108.

Singh

, Song

Z-H

, Reggio

. Structure of a cannabinoid receptor subtype 2 homodimer determined by cysteine and homobifunctional crosslinking experiments combined with computational studies. Biophys J, 2012; 102:244–a.

109.

Zvonok

, Yaddanapudi

, Williams

, et al. Comprehensive proteomic mass spectrometric characterization of human cannabinoid CB2 receptor. J Proteome Res, 2007; 6:2068–2079.

110.

Ferré

, Casadó

, Devi

, et al. G protein-coupled receptor oligomerization revisited: functional and pharmacological perspectives. Pharmacol Rev, 2014; 66:413–434.

111.

Franco

, Martínez-Pinilla

, Ricobaraza

, et al. Challenges in the development of heteromer-GPCR-based drugs. Prog Mol Biol Transl Sci, 2013; 117:143–162.

112.

Hiller

, Kühhorn

, Gmeiner

. Class A G-protein-coupled receptor (GPCR) dimers and bivalent ligands. J Med Chem, 2013; 56:6542–6559.

113.

Hübner

, Schellhorn

, Gienger

, et al. Structure-guided development of heterodimer-selective GPCR ligands. Nat Commun, 2016; 7:1229–8.

114.

Shonberg

, Scammells

, Capuano

. Design strategies for bivalent ligands targeting GPCRs. ChemMedChem, 2011; 6:963–974.

115.

Fulton

, Knapp

, Bidlack

, et al. Effect of linker substitution on the binding of butorphan univalent and bivalent ligands to opioid receptors. Bioorg Med Chem Lett, 2010; 20:1507–1509.

116.

Zhang

, Zhang

, Sromek

, et al. Synthesis and binding affinity of novel mono- and bivalent morphinan ligands for κ, μ, and δ opioid receptors. Bioorg Med Chem, 2011; 19:2808–2816.

117.

Gogoi

, Biswas

, Modi

, et al. Novel bivalent ligands for D2/D3 dopamine receptors: significant cooperative gain in D2 affinity and potency. ACS Med Chem Lett, 2012; 3:991–996.

118.

Huber

, Löber

, Hübner

, et al. Bivalent molecular probes for dopamine D2-like receptors. Bioorg Med Chem, 2012; 20:455–466.

119.

Birnkammer

, Spickenreither

, Brunskole

, et al. The bivalent ligand approach leads to highly potent and selective acylguanidine-type histamine H₂ receptor agonists. J Med Chem, 2012; 55:1147–1160.

120.

Fernández-Fernández

, Decara

, Bermúdez-Silva

, et al. Description of a bivalent cannabinoid ligand with hypophagic properties. Arch Pharm, 2013; 346:171–179.

121.

Huang

, Pemp

, Stadtmüller

, et al. Design, synthesis and in vitro evaluation of novel uni- and bivalent ligands for the cannabinoid receptor type 1 with variation of spacer length and structure. Bioorg Med Chem Lett, 2014; 24:4209–4214.

122.

Zhang

, Gilliam

, Maitra

, et al. Synthesis and biological evaluation of bivalent ligands for the cannabinoid 1 receptor. J Med Chem, 2010; 53:7048–7060.

123.

Fernández-Fernández

, Callado

, Girón

, et al. Combining rimonabant and fentanyl in a single entity: preparation and pharmacological results. Drug Design Devel Ther, 2014; 8:263–277.

124.

Le Naour

, Akgün

, Yekkirala

, et al. Bivalent ligands that target μ opioid (MOP) and cannabinoid1 (CB1) receptors are potent analgesics devoid of tolerance. J Med Chem, 2013; 56:5505–5513.

125.

Perrey

, Gilmour

, Thomas

, et al. Toward the development of bivalent ligand probes of cannabinoid CB1 and Orexin OX1 receptor heterodimers. ACS Med Chem Lett, 2014; 5:634–638.

126.

Glass

, Govindpani

, Furkert

, et al. One for the price of two…are bivalent ligands targeting cannabinoid receptor dimers capable of simultaneously binding to both receptors?. Trends Pharmacol Sci, 2016; xx:1–11.

127.

Lane

, Beukers

, Mulder-Krieger

, et al. The endocannabinoid 2-arachidonylglycerol is a negative allosteric modulator of the human A3 adenosine receptor. Biochem Pharmacol, 2010; 79:48–56.

128.

Christopoulos

, Wilson

. Interaction of anandamide with the M1 and M4 muscarinic acetylcholine receptors. Brain Res, 2001; 915:70–78.

129.

Boger

, Patterson

, Jin

. Structural requirements for 5-HT2A and 5-HT1A serotonin receptor potentiation by the biologically active lipid oleamide. Proc Natl Acad Sci U S A, 1998; 95:4102–4107.

130.

Kathmann

, Flau

, Redmer

, et al. Cannabidiol is an allosteric modulator at mu- and delta-opioid receptors. Naunyn Schmiedebergs Arch Pharmacol, 2006; 372:354–361.

131.

Cascio

, Gauson

, Stevenson

, et al. Evidence that the plant cannabinoid cannabigerol is a highly potent α2-adrenoceptor agonist and moderately potent 5HT1A receptor antagonist. Br J Pharmacol, 2010; 159:129–141.

132.

Cascio

, Zamberletti

, Marini

, et al. The phytocannabinoid, Δ9-tetrahydrocannabivarin, can act through 5-HT1A receptors to produce antipsychotic effects. Br J Pharmacol, 2015; 172:1305–1318.

133.

Fong

, Shearman

, Stribling

, et al. Pharmacological efficacy and safety profile of taranabant in preclinical species. Drug Dev Res, 2009; 70:349–362.

134.

Hua

, Vemuri

, Pu

, et al. Crystal structure of the human cannabinoid CB1. Cell, 2016; 167:750–762.

135.

Shao

, Yin

, Chapman

, et al. High-resolution crystal structure of the human CB1 cannabinoid receptor. Nature, 2016; 540:602–606.

136.

Hua

, Vemuri

, Nikas

, et al. Crystal structures of agonist-bound human cannabinoid receptor CB1. Nature, 2017; 547:468–471.

An Update on Non-CB 1,Non-CB 2 Cannabinoid Related G-Protein-Coupled Receptors

Abstract

Abstract

Introduction

GPR55 and GPR18

GPR3, GPR6, and GPR12

Alkylindole-Sensitive Receptors

Cannabinoid-Related Oligomers

Well-Established GPCRs Related to the Cannabinoids

Other GPCRs

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

Abbreviations Used

References