N-Arachidonyl glycine does not activate G protein-coupled receptor 18 signaling via canonical pathways

Potent, Selective Agonists for the Cannabinoid-like Orphan G Protein-Coupled Receptor GPR18: A Promising Drug Target for Cancer and Immunity

The human orphan G protein-coupled receptor GPR18, activated by Δ9-tetrahydrocannabinol (THC), constitutes a promising drug target in immunology and cancer. However, studies on GPR18 are hampered by the lack of suitable tool compounds. In the present study, potent and selective GPR18 agonists were developed showing low nanomolar potency at human and mouse GPR18, determined in β-arrestin recruitment assays. Structure-activity relationships were analyzed, and selectivity versus cannabinoid (CB) and CB-like receptors was assessed. Compound 51 (PSB-KK1415, EC50 19.1 nM) was the most potent GPR18 agonist showing at least 25-fold selectivity versus CB receptors. The most selective GPR18 agonist 50 (PSB-KK1445, EC50 45.4 nM) displayed >200-fold selectivity versus both CB receptor subtypes, GPR55, and GPR183. The new GPR18 agonists showed minimal species differences, while THC acted as a weak partial agonist at the mouse receptor. The newly discovered compounds represent the most potent and selective GPR18 agonists reported to date.

Structure Prediction, Evaluation, and Validation of GPR18 Lipid Receptor Using Free Programs

The GPR18 receptor, often referred to as the N-arachidonylglycine receptor, although assigned (along with GPR55 and GPR119) to the new class A GPCR subfamily-lipid receptors, officially still has the status of a class A GPCR orphan. While its signaling pathways and biological significance have not yet been fully elucidated, increasing evidence points to the therapeutic potential of GPR18 in relation to immune, neurodegenerative, and cancer processes to name a few. Therefore, it is necessary to understand the interactions of potential ligands with the receptor and the influence of particular structural elements on their activity. Thus, given the lack of an experimentally solved structure, the goal of the present study was to obtain a homology model of the GPR18 receptor in the inactive state, meeting all requirements in terms of protein structure quality and recognition of active ligands. To increase the reliability and precision of the predictions, different contemporary protein structure prediction methods and software were used and compared herein. To test the usability of the resulting models, we optimized and compared the selected structures followed by the assessment of the ability to recognize known, active ligands. The stability of the predicted poses was then evaluated by means of molecular dynamics simulations. On the other hand, most of the best-ranking contemporary CADD software/platforms for its full usability require rather expensive licenses. To overcome this down-to-earth obstacle, the overarching goal of these studies was to test whether it is possible to perform the thorough CADD experiments with high scientific confidence while using only license-free/academic software and online platforms. The obtained results indicate that a wide range of freely available software and/or academic licenses allow us to carry out meaningful molecular modelling/docking studies.

Endocannabinoid signaling pathways: beyond CB1R and CB2R

The search for cannabinoid receptors other than CB1R and CB2R has been ongoing for over a decade. A number of orphan receptors have been proposed as potential cannabinoid receptors primarily based on phylogenic arguments and reactivity towards known endocannabinoids and phytocannabinoids. Seven putative cannabinoid receptors are described and discussed, and evidence for and against their inclusion in this category are presented.

Cannabinoid receptor CB1 and CB2 interacting proteins: Techniques, progress and perspectives

Cannabinoid receptors 1 and 2 (CB₁ and CB₂) are implicated in a range of physiological processes and have gained attention as promising therapeutic targets for a number of diseases. Protein-protein interactions play an integral role in modulating G protein-coupled receptor (GPCR) expression, subcellular distribution and signaling, and the identification and characterization of these will not only improve our understanding of GPCR function and biology, but may provide a novel avenue for therapeutic intervention. A variety of techniques are currently being used to investigate GPCR protein-protein interactions, including Förster/fluorescence and bioluminescence resonance energy transfer (FRET and BRET), proximity ligation assay (PLA), and bimolecular fluorescence complementation (BiFC). However, the reliable application of these methodologies is dependent on the use of appropriate controls and the consideration of the physiological context. Though not as extensively characterized as some other GPCRs, the investigation of CB₁ and CB₂ interacting proteins is a growing area of interest, and a range of interacting partners have been identified to date. This review summarizes the current state of the literature regarding the cannabinoid receptor interactome, provides commentary on the methodologies and techniques utilized, and discusses future perspectives.

Function and therapeutic potential of N-acyl amino acids

N-acyl amino acids (NAAs) are amphiphilic molecules, with different potential fatty acid and head group moieties. NAAs are the largest family of anandamide congener lipids discovered to date. In recent years, several NAAs have been identified as potential ligands, engaging novel binding sites and mechanisms for modulation of membrane proteins such as G-protein coupled receptors (GPRs), nuclear receptors, ion channels, and transporters. NAAs play a key role in a variety of physiological functions as lipid signaling molecules. Understanding the structure, function roles, and pharmacological potential of these NAAs is still in its infancy, and the biochemical roles are also mostly unknown. This review will provide a summary of the literature on NAAs and emphasize their therapeutic potential.

Phytocannabinoids Biosynthesis in Angiosperms, Fungi, and Liverworts and Their Versatile Role

Phytocannabinoids are a structurally diverse class of bioactive naturally occurring compounds found in angiosperms, fungi, and liverworts and produced in several plant organs such as the flower and glandular trichrome of Cannabis sativa, the scales in Rhododendron, and oil bodies of liverworts such as Radula species; they show a diverse role in humans and plants. Moreover, phytocannabinoids are prenylated polyketides, i.e., terpenophenolics, which are derived from isoprenoid and fatty acid precursors. Additionally, targeted productions of active phytocannabinoids have beneficial properties via the genes involved and their expression in a heterologous host. Bioactive compounds show a remarkable non-hallucinogenic biological property that is determined by the variable nature of the side chain and prenyl group defined by the enzymes involved in their biosynthesis. Phytocannabinoids possess therapeutic, antibacterial, and antimicrobial properties; thus, they are used in treating several human diseases. This review gives the latest knowledge on their role in the amelioration of abiotic (heat, cold, and radiation) stress in plants. It also aims to provide synthetic and biotechnological approaches based on combinatorial biochemical and protein engineering to synthesize phytocannabinoids with enhanced properties.

Biosynthesis of Nature-Inspired Unnatural Cannabinoids

Natural products make up a large proportion of medicine available today. Cannabinoids from the plant Cannabis sativa is one unique class of meroterpenoids that have shown a wide range of bioactivities and recently seen significant developments in their status as therapeutic agents for various indications. Their complex chemical structures make it difficult to chemically synthesize them in efficient yields. Synthetic biology has presented a solution to this through metabolic engineering in heterologous hosts. Through genetic manipulation, rare phytocannabinoids that are produced in low yields in the plant can now be synthesized in larger quantities for therapeutic and commercial use. Additionally, an exciting avenue of exploring new chemical spaces is made available as novel derivatized compounds can be produced and investigated for their bioactivities. In this review, we summarized the biosynthetic pathways of phytocannabinoids and synthetic biology efforts in producing them in heterologous hosts. Detailed mechanistic insights are discussed in each part of the pathway in order to explore strategies for creating novel cannabinoids. Lastly, we discussed studies conducted on biological targets such as CB1, CB2 and orphan receptors along with their affinities to these cannabinoid ligands with a view to inform upstream diversification efforts.

Effects of GPR18 Ligands on Body Weight and Metabolic Parameters in a Female Rat Model of Excessive Eating

GPR18 has been proposed to play a role in the progression of metabolic disease and obesity. Therefore, the aim of this study was to determine the effects of selective GRP18 ligands (the antagonists PSB-CB5 and PSB-CB27 and the agonist PSB-KK1415) on body mass and the development of metabolic disorders commonly accompanying obesity. Experiments were carried out on female Wistar rats. In order to determine the anorectic activity of the investigated ligands, their effect on food and water intake in a model of excessive eating was assessed. Lipid profile, glucose and insulin levels as well as alanine aminotransferase, aspartate aminotransferase, and γ-glutamyl transpeptidase activity in plasma were also evaluated. Potential side effects were examined in rat models of pica behavior and conditioned taste aversion. Animals treated with different ligands gained significantly less weight than rats from the obese control group. Effects of GPR18 antagonists on food intake and body weight were specific and unrelated to visceral illness, stress or changes in spontaneous activity. However, the GPR18 agonist is likely to affect body weight by inducing gastrointestinal disorders such as nausea. The presented preliminary data support the idea that the search for selective GPR18 antagonists for the treatment of obesity might be promising.

Novel selective agonist of GPR18, PSB-KK-1415 exerts potent anti-inflammatory and anti-nociceptive activities in animal models of intestinal inflammation and inflammatory pain

BACKGROUND

GPR18 is a recently deorphanized receptor which was reported to act with several endogenous cannabinoid ligands. Here, we aimed to describe the role of GPR18 in intestinal inflammation and inflammatory pain.

METHODS

The anti-inflammatory activity of selective GPR18 agonist, PSB-KK-1415, and antagonist, PSB-CB5, was characterized in semi-chronic and chronic mouse models of colitis induced by 2,4,6-trinitrobenzenesulfonic acid (TNBS). The extent of inflammation was evaluated based on the macroscopic and microscopic scores, quantification of myeloperoxidase (MPO) activity, and Western blot analyses of tumor necrosis factor-α (TNF-α) and interleukin-6 in colonic tissue. The expression of GPR18 in colonic samples from patients with Crohn's disease (CD) was quantified using real-time PCR. The anti-nociceptive potential of the agonist in intestinal inflammation was evaluated in the mouse model of inflammatory pain.

KEY RESULTS

In semi-chronic colitis, PSB-KK-1415 reduced macroscopic score (1.79 ± 0.22 vs. 2.61 ± 0.48), expression of TNF-α (1.89 ± 0.36 vs. 2.83 ± 0.64), and microscopic score (5.00 ± 0.33 vs. 6.45 ± 0.40), all compared to mice with colitis. In chronic colitis, PSB-KK-1415 decreased macroscopic score (3.33 ± 1.26 vs. 4.00 ± 1.32) and MPO activity (32.23 ± 8.51 vs. 41.33 ± 11.64) compared to inflamed mice. In the mouse model of inflammatory pain, PSB-KK-1415 decreased the number of pain-induced behaviors in both, controls (32.60 ± 2.54 vs. 58.00 ± 6.24) and inflamed mice (60.83 ± 2.85 vs. 85.00 ± 5.77) compared to animals without treatment with PSB-KK-1415 (P < 0.005 for both). Lastly, we showed an increased expression of GPR18 in CD patients compared to healthy controls (3.77 ± 1.46 vs. 2.38 ± 0.66, p = 0.87).

CONCLUSIONS & INFERENCES

We showed that GPR18 is worth considering as a potential treatment target in intestinal inflammation and inflammatory pain.

Discovery of Tricyclic Xanthines as Agonists of the Cannabinoid-Activated Orphan G-Protein-Coupled Receptor GPR18

GPR18 is a rhodopsin-like orphan G-protein-coupled receptor (GPCR) that is activated by the natural cannabinoid (CB) Δ⁹-tetrahydrocannabinol (THC). It is highly expressed in immune cells and represents a promising new drug target. However, THC is much more potent in activating CB receptors than GPR18, and several other proposed lipidic agonists for GPR18 have not been independently confirmed. Herein we describe the first non-lipid-like agonists for GPR18 based on a tricyclic xanthine-derived scaffold, along with initial structure-activity relationships. PSB-KD107 (5) and PSB-KD477 (16) displayed significantly higher potency and efficacy than THC, determined in a GPR18-dependent β-arrestin recruitment assay, and were found to be selective versus the CB-sensitive receptors CB₁, CB₂, and GPR55. Structure-activity relationships were steep, and indole substitution was crucial for biological activity. These first selective agonists, which are structurally distinct from the lipidic agonist(s), will allow target validation studies and may eventually contribute to the deorphanization of GPR18.

Therapeutic Exploitation of GPR18: Beyond the Cannabinoids?

GPR18 is a G-protein-coupled receptor that belongs to the orphan class A family. Even though it shares low sequence homology with the cannabinoid receptors CB₁R and CB₂R, a growing body of research suggests its relationship with the endocannabinoid system, not only because it is able to recognize cannabinoid ligands but also because of its expression and ability to heteromerize with CBRs. In this review, we aim to analyze the biological relevance, reported modulators, and structural features of GPR18. In order to guide future drug design in this field, highlights from molecular modeling of GPR18 will be provided.

Endocannabinoid metabolism and transport as targets to regulate intraocular pressure

Cannabinoids are part of an endogenous signaling system found throughout the body, including the eye. Hepler and Frank showed in the early 1970s that plant cannabinoids can lower intraocular pressure (IOP), an effect since shown to occur via cannabinoid CB1 and GPR18 receptors. Endocannabinoids are synthesized and metabolized enzymatically. Enzymes implicated in endocannabinoids breakdown include monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), but also ABHD12, NAAA, and COX-2. Inhibition of MAGL activity raises levels of the endocannabinoid 2-arachidonoyl glycerol and substantially lowers IOP. Blocking other cannabinoid metabolizing enzymes or cannabinoid transporters may similarly contribute to lowering IOP and so serve as therapeutic targets for treating glaucoma. We have tested blockers for several cannabinoid-metabolizing enzymes and transporters (FABP5 and membrane reuptake) for their ability to alter ocular pressure in a murine model of IOP. Of FAAH, ABHD12, NAAA, and COX2, only FAAH was seen to play a role in regulation of IOP. Only the FAAH blocker URB597 lowered IOP, but in a temporally, diurnally, and sex-specific manner. We also tested two blockers of cannabinoid transport (SBFI-26 and WOBE437), finding that each lowered IOP in a CB1-dependent manner. Though we see a modest, limited role for FAAH, our results suggest that MAGL is the primary cannabinoid-metabolizing enzyme in regulating ocular pressure, thus pointing towards a role of 2-arachidonoyl glycerol. Interestingly, inhibition of cannabinoid transport mechanisms independent of hydrolysis may prove to be an alternative strategy to lower ocular pressure.

Pharmacological data of cannabidiol- and cannabigerol-type phytocannabinoids acting on cannabinoid CB1, CB2 and CB1/CB2 heteromer receptors

BACKGROUND

Recent approved medicines whose active principles are Δ⁹Tetrahidrocannabinol (Δ⁹-THC) and/or cannabidiol (CBD) open novel perspectives for other phytocannabinoids also present in Cannabis sativa L. varieties. Furthermore, solid data on the potential benefits of acidic and varinic phytocannabinoids in a variety of diseases are already available. Mode of action of cannabigerol (CBG), cannabidiolic acid (CBDA), cannabigerolic acid (CBGA), cannabidivarin (CBDV) and cannabigerivarin (CBGV) is, to the very least, partial.

HYPOTHESIS/PURPOSE

Cannabinoid CB₁ or CB₂ receptors, which belong to the G-protein-coupled receptor (GPCR) family, are important mediators of the action of those cannabinoids. Pure CBG, CBDA, CBGA, CBDV and CBGV from Cannabis sativa L. are differentially acting on CB₁ or CB₂ cannabinoid receptors.

STUDY DESIGN

Determination of the affinity of phytocannabinoids for cannabinoid receptors and functional assessment of effects promoted by these compounds when interacting with cannabinoid receptors.

METHODS

A heterologous system expressing the human versions of CB₁ and/or CB₂ receptors was used. Binding to membranes was measured using radioligands and binding to living cells using a homogenous time resolved fluorescence resonance energy transfer (HTRF) assay. Four different functional outputs were assayed: determination of cAMP levels and of extracellular-signal-related-kinase phosphorylation, label-free dynamic mass redistribution (DMR) and ß-arrestin recruitment.

RESULTS

Affinity of cannabinoids depend on the ligand of reference and may be different in membranes and in living cells. All tested phytocannabinoids have agonist-like behavior but behaved as inverse-agonists in the presence of selective receptor agonists. CBGV displayed enhanced potency in many of the functional outputs. However, the most interesting result was a biased signaling that correlated with differential affinity, i.e. the overall results suggest that the binding mode of each ligand leads to specific receptor conformations underlying biased signaling outputs.

CONCLUSION

Results here reported and the recent elucidation of the three-dimensional structure of CB₁ and CB₂ receptors help understanding the mechanism of action that might be protective and the molecular drug-receptor interactions underlying biased signaling.

Computational Investigations on the Binding Mode of Ligands for the Cannabinoid-Activated G Protein-Coupled Receptor GPR18

GPR18 is an orphan G protein-coupled receptor (GPCR) expressed in cells of the immune system. It is activated by the cannabinoid receptor (CB) agonist ∆⁹-tetrahydrocannabinol (THC). Several further lipids have been proposed to act as GPR18 agonists, but these results still require unambiguous confirmation. In the present study, we constructed a homology model of the human GPR18 based on an ensemble of three GPCR crystal structures to investigate the binding modes of the agonist THC and the recently reported antagonists which feature an imidazothiazinone core to which a (substituted) phenyl ring is connected via a lipophilic linker. Docking and molecular dynamics simulation studies were performed. As a result, a hydrophobic binding pocket is predicted to accommodate the imidazothiazinone core, while the terminal phenyl ring projects towards an aromatic pocket. Hydrophobic interaction of Cys251 with substituents on the phenyl ring could explain the high potency of the most potent derivatives. Molecular dynamics simulation studies suggest that the binding of imidazothiazinone antagonists stabilizes transmembrane regions TM1, TM6 and TM7 of the receptor through a salt bridge between Asp118 and Lys133. The agonist THC is presumed to bind differently to GPR18 than to the distantly related CB receptors. This study provides insights into the binding mode of GPR18 agonists and antagonists which will facilitate future drug design for this promising potential drug target.

Inhibition of store-operated calcium channels by N-arachidonoyl glycine (NAGly): no evidence for the involvement of lipid-sensing G protein coupled receptors

N-arachidonoyl glycine (NAGly) is an endogenous lipid deriving from the endocannabinoid anandamide (AEA). Identified as a ligand of several G-protein coupled receptors (GPCRs), it can however exert biological responses independently of GPCRs. NAGly was recently shown to depress store-operated Ca²⁺ entry (SOCE) but its mechanism of action remains elusive. The major aim of this study was to gain a better knowledge on the NAGly-dependent impairment of SOCE in neurons of the central nervous system (CNS) from mice. First, we examined the expression of genes encoding for putative lipid sensing GPCRs using transcriptomic data publicly available. This analysis showed that the most abundant GPCRs transcripts present in the cerebral cortices of embryonic brains were coding for lysophosphatidic acid (LPA) and sphingosine-1 phosphate (S1P) receptors. Next, the presence of functional receptors was assessed with live-cell calcium imaging experiments. In primary cortical cells S1P and LPA mobilize Ca²⁺ from internal stores via a mechanism sensitive to the S1P and LPA receptor antagonists Ex26, H2L5186303, or Ki16425. However, none of these compounds prevented or attenuated the NAGly-dependent impairment of SOCE. We found no evidence for the requirement of lipid sensing GPCRs in this inhibitory process, indicating that NAGly is an endogenous modulator interfering with the core machinery of SOCE. Moreover, these data also raise the intriguing possibility that the depression of SOCE could play a role in the central effects of NAGly.

N-Acyl Amino Acids: Metabolism, Molecular Targets, and Role in Biological Processes

The lipid signal is becoming increasingly crowded as increasingly fatty acid amide derivatives are being identified and considered relevant therapeutic targets. The identification of N-arachidonoyl-ethanolamine as endogenous ligand of cannabinoid type-1 and type-2 receptors as well as the development of different–omics technologies have the merit to have led to the discovery of a huge number of naturally occurring N-acyl-amines. Among those mediators, N-acyl amino acids, chemically related to the endocannabinoids and belonging to the complex lipid signaling system now known as endocannabinoidome, have been rapidly growing for their therapeutic potential. Here, we review the current knowledge of the mechanisms for the biosynthesis and inactivation of the N-acyl amino acids, as well as the various molecular targets for some of the N-acyl amino acids described so far.

N-Palmitoylglycine and other N-acylamides activate the lipid receptor G2A/GPR132

The G-protein-coupled receptor GPR132, also known as G2A, is activated by 9-hydroxyoctadecadienoic acid (9-HODE) and other oxidized fatty acids. Other suggested GPR132 agonists including lysophosphatidylcholine (LPC) have not been readily reproduced. Here, we identify N-acylamides in particular N-acylglycines, as lipid activators of GPR132 with comparable activity to 9-HODE. The order-of-potency is N-palmitoylglycine > 9-HODE ≈ N-linoleoylglycine > linoleamide > N-oleoylglycine ≈ N-stereoylglycine > N-arachidonoylglycine > N-docosehexanoylglycine. Physiological concentrations of N-acylglycines in tissue are sufficient to activate GPR132. N-linoleoylglycine and 9-HODE also activate rat and mouse GPR132, despite limited sequence conservation to human. We describe pharmacological tools for GPR132, identified through drug screening. SKF-95667 is a novel GPR132 agonist. SB-583831 and SB-583355 are peptidomimetic molecules containing core amino acids (glycine and phenylalanine, respectively), and structurally related to previously described ligands. A telmisartan analog, GSK1820795A, antagonizes the actions of N-acylamides at GPR132. The synthetic cannabinoid CP-55 940 also activates GPR132. Molecular docking to a homology model suggested a site for lipid binding, predicting the acyl side-chain to extend into the membrane bilayer between TM4 and TM5 of GPR132. Small-molecule ligands are envisaged to occupy a "classical" site encapsulated in the 7TM bundle. Structure-directed mutagenesis indicates a critical role for arginine at position 203 in transmembrane domain 5 to mediate GPR132 activation by N-acylamides. Our data suggest distinct modes of binding for small-molecule and lipid agonists to the GPR132 receptor. Antagonists, such as those described here, will be vital to understand the physiological role of this long-studied target.

Towards A Molecular Understanding of The Cannabinoid Related Orphan Receptor GPR18: A Focus on Its Constitutive Activity

The orphan G-protein coupled receptor (GPCR), GPR18, has been recently proposed as a potential member of the cannabinoid family as it recognizes several endogenous, phytogenic, and synthetic cannabinoids. Potential therapeutic applications for GPR18 include intraocular pressure, metabolic disorders, and cancer. GPR18 has been reported to have high constitutive activity, i.e., activation/signaling occurs in the absence of an agonist. This activity can be reduced significantly by the A3.39N mutation. At the intracellular (IC) ends of (transmembrane helices) TMH3 and TMH6 in GPCRs, typically, a pair of oppositely charged amino acids form a salt bridge called the "ionic lock". Breaking of this salt bridge creates an IC opening for coupling with G protein. The GPR18 "ionic lock" residues (R3.50/S6.33) can form only a hydrogen bond. In this paper, we test the hypothesis that the high constitutive activity of GPR18 is due to the weakness of its "ionic lock" and that the A3.39N mutation strengthens this lock. To this end, we report molecular dynamics simulations of wild-type (WT) GPR18 and the A3.39N mutant in fully hydrated (POPC) phophatidylcholine lipid bilayers. Results suggest that in the A3.39N mutant, TMH6 rotates and brings R3.50 and S6.33 closer together, thus strengthening the GPR18 "ionic lock".

Modulation of the Endocannabinoid System as a Potential Anticancer Strategy

Currently, the involvement of the endocannabinoid system in cancer development and possible options for a cancer-regressive effect of cannabinoids are controversially discussed. In recent decades, a number of preclinical studies have shown that cannabinoids have an anticarcinogenic potential. Therefore, especially against the background of several legal simplifications with regard to the clinical application of cannabinoid-based drugs, an extended basic knowledge about the complex network of the individual components of the endocannabinoid system is required. The canonical endocannabinoid system consists of the endocannabinoids N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol as well as the G_i/o protein-coupled transmembrane cannabinoid receptors CB₁ and CB₂. As a result of extensive studies on the broader effect of these factors, other fatty acid derivatives, transmembrane and intracellular receptors, enzymes and lipid transporters have been identified that contribute to the effect of endocannabinoids when defined in the broad sense as “extended endocannabinoid system.” Among these additional components, the endocannabinoid-degrading enzymes fatty acid amide hydrolase and monoacylglycerol lipase, lipid transport proteins of the fatty acid-binding protein family, additional cannabinoid-activated G protein-coupled receptors such as GPR55, members of the transient receptor family, and peroxisome proliferator-activated receptors were identified as targets for possible strategies to combat cancer progression. Other endocannabinoid-related fatty acids such as 2-arachidonoyl glyceryl ether, O-arachidonoylethanolamine, N-arachidonoyldopamine and oleic acid amide showed an effect via cannabinoid receptors, while other compounds such as endocannabinoid-like substances exert a permissive action on endocannabinoid effects and act via alternative intracellular target structures. This review gives an overview of the modulation of the extended endocannabinoid system using the example of anticancer cannabinoid effects, which have been described in detail in preclinical studies.

Protective Effect of N-Arachidonoyl Glycine-GPR18 Signaling after Excitotoxical Lesion in Murine Organotypic Hippocampal Slice Cultures

N-arachidonoyl glycine (NAGly) is an endocannabinoid involved in the regulation of different immune cells. It was shown to activate the GPR18 receptor, which was postulated to switch macrophages from cytotoxic to reparative. To study GPR18 expression and neuroprotection after NAGly treatment we used excitotoxically lesioned organotypic hippocampal slice cultures (OHSC). The effect of NAGly was also tested in isolated microglia and astrocytes as these cells play a crucial role during neuronal injury. In the present study, the GPR18 receptor was found in OHSC at mRNA level and was downregulated after N-Methyl-D-aspartate (NMDA) treatment at a single time point. Furthermore, treatment with NAGly reduced neuronal damage and this effect was abolished by GPR18 and cannabinoid receptor (CB)₂ receptor antagonists. The activation but not motility of primary microglia and astrocytes was influenced when incubated with NAGly. However, NAGly alone reduced the phosphorylation of Akt but no changes in activation of the p44/42 and p38 MAPK and CREB pathways in BV2 cells could be observed. Given NAGly mediated actions we speculate that GPR18 and its ligand NAGly are modulators of glial and neuronal cells during neuronal damage.

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB₁ and CB₂) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB₁/CB₂ receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain.

Results: This work summarized novel data supporting that, besides cannabinoid CB₁ and CB₂ receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

Cannabinoids and Cardiovascular System

Cannabinoids influence cardiovascular variables in health and disease via multiple mechanisms. The chapter covers the impact of cannabinoids on cardiovascular function in physiology and pathology and presents a critical analysis of the proposed signalling pathways governing regulation of cardiovascular function by endogenously produced and exogenous cannabinoids. We know that endocannabinoid system is overactivated under pathological conditions and plays both a protective compensatory role, such as in some forms of hypertension, atherosclerosis and other inflammatory conditions, and a pathophysiological role, such as in disease states associated with excessive hypotension. This chapter focuses on the mechanisms affecting hemodynamics and vasomotor effects of cannabinoids in health and disease states, highlighting mismatches between some studies. The chapter will first review the effects of marijuana smoking on cardiovascular system and then describe the impact of exogenous cannabinoids on cardiovascular parameters in humans and experimental animals. This will be followed by analysis of the impact of cannabinoids on reactivity of isolated vessels. The article critically reviews current knowledge on cannabinoid induction of vascular relaxation by cannabinoid receptor-dependent and -independent mechanisms and dysregulation of vascular endocannabinoid signaling in disease states.

Modulation of the Oxidative Stress and Lipid Peroxidation by Endocannabinoids and Their Lipid Analogues

Growing evidence supports the pivotal role played by oxidative stress in tissue injury development, thus resulting in several pathologies including cardiovascular, renal, neuropsychiatric, and neurodegenerative disorders, all characterized by an altered oxidative status. Reactive oxygen and nitrogen species and lipid peroxidation-derived reactive aldehydes including acrolein, malondialdehyde, and 4-hydroxy-2-nonenal, among others, are the main responsible for cellular and tissue damages occurring in redox-dependent processes. In this scenario, a link between the endocannabinoid system (ECS) and redox homeostasis impairment appears to be crucial. Anandamide and 2-arachidonoylglycerol, the best characterized endocannabinoids, are able to modulate the activity of several antioxidant enzymes through targeting the cannabinoid receptors type 1 and 2 as well as additional receptors such as the transient receptor potential vanilloid 1, the peroxisome proliferator-activated receptor alpha, and the orphan G protein-coupled receptors 18 and 55. Moreover, the endocannabinoids lipid analogues N-acylethanolamines showed to protect cell damage and death from reactive aldehydes-induced oxidative stress by restoring the intracellular oxidants-antioxidants balance. In this review, we will provide a better understanding of the main mechanisms triggered by the cross-talk between the oxidative stress and the ECS, focusing also on the enzymatic and non-enzymatic antioxidants as scavengers of reactive aldehydes and their toxic bioactive adducts.

Molecular and functional interaction between GPR18 and cannabinoid CB2 G-protein-coupled receptors. Relevance in neurodegenerative diseases

GPR18, still considered an orphan receptor, may respond to endocannabinoids, whose canonical receptors are CB₁ and CB₂. GPR18 and CB₂ receptors share a role in peripheral immune response regulation and are co-expressed in microglia, which are immunocompetent cells in the central nervous system (CNS). We aimed at identifying heteroreceptor complexes formed by GPR18 and CB₁R or CB₂R in resting and activated microglia. Receptor-receptor interaction was assessed using energy-transfer approaches, and receptor function by determining cAMP levels and ERK1/2 phosphorylation in heterologous cells and primary cultures of microglia. Heteroreceptor identification in primary cultures of microglia was achieved by in situ proximity ligation assays. Energy transfer results showed interaction of GPR18 with CB₂R but not with CB₁R. CB₂-GPR18 heteroreceptor complexes displayed particular functional properties (heteromer prints) often consisting of negative cross-talk (activation of one receptor reduces signaling arising from the partner receptor) and cross-antagonism (the response of one of the receptors is blocked by a selective antagonist of the partner receptor). Activated microglia showed the heteromer print (negative cross-talk and bidirectional cross-antagonism) and increased expression of CB₂R and GPR18. Due to the important role of CB₂R in neuroprotection, we further investigated heteroreceptor occurrence in primary cultures of microglia from transgenic mice overexpressing human APP_Sw,Ind, an Alzheimer's disease model. Microglial cells from transgenic mice showed the heteromer print and functional interactions that were similar to those found in cells from wild-type animals that were activated by treatment with lipopolysaccharide and interferon-γ. Our results suggest that GPR18 and its heteromers may play important roles in neurodegenerative processes.

Structure-activity relationships of imidazothiazinones and analogs as antagonists of the cannabinoid-activated orphan G protein-coupled receptor GPR18

GPR18 is a cannabinoid-activated orphan G protein-coupled receptor (GPCR) that is selectively expressed on immune cells. Despite its significant potential as a drug target for inflammatory diseases and cancer immunotherapy, only very few GPR18 ligands have been described to date. In the present study we investigated the structure-activity relationships (SARs) of (Z)-2-(3-(4-chlorobenzyloxy)benzylidene)-6,7-dihydro-2H-imidazo[2,1-b][1,3]thiazin-3(5H)-one (PSB-CB5, 5), the most potent GPR18 antagonist described to date. Analogs were synthesized that exhibit broad modifications of the heterocyclic core and/or variation of substituents at the benzylidene moiety. The compounds were investigated in β-arrestin recruitment assays as inhibitors of human GPR18 activation by tetrahydrocannabinol (THC). Selectivity was assessed versus the cannabinoid receptors (CB₁ and CB₂) and versus GPR55, another orphan GPCR that interacts with cannabinoids. Phenyloxyalkyloxy-substituted benzylidenethiazinones with long alkyl chains (optimal length: hexamethylene) efficiently blocked GPR18 with similarly high potency as lead structure 5. (Z)-2-(3-(6-(4-Chlorophenoxy)hexyloxy)benzylidene)-6,7-dihydro-2H-imidazo[2,1-b][1,3]thiazin-3(5H)-one (PSB-CB-27, 23) exhibited the best profile: it displayed an IC₅₀ value of 650 nM at GPR18 and showed improved selectivity versus CB receptors as compared to lead structure 5. Importantly, in contrast to 5, which showed only partial inhibition (60%), 23 led to a complete blockade of THC-induced GPR18 activation and is thus a superior tool for target validation. In addition, several compounds, e.g. 18 and 22, were identified as dual GPR18/GPR55 antagonists with similar potency at both targets, and selectivity versus CB receptors.

The quest for endothelial atypical cannabinoid receptor: BKCa channels act as cellular sensors for cannabinoids in in vitro and in situ endothelial cells

Endothelium-dependent component of cannabinoid-induced vasodilation has been postulated to require G-protein-coupled non-CB1/CB2 endothelial cannabinoid (eCB) receptor. GPR18 was proposed as a candidate for eCBR. To address the hypothesis that the effects attributed to eCBR are mediated by G-protein-coupled receptor (GPCR)-independent targets, we studied the electrical responses in endothelial cells, focusing on BKCa channels. In patches excised from endothelial-derived EA.hy926 cells, N-arachidonoyl glycine (NAGly) and abnormal cannabidiol (abn-cbd), prototypical agonists for eCB receptor, stimulate single BKCa activity in a concentration- and Ca2+-dependent manner. The postulated eCB receptor inhibitors rimonabant and AM251 were found to inhibit basal and stimulated by NAGly- and abn-cbd BKCa activity in cell-free patches. In isolated mice aortas, abn-cbd and NAGly produced endothelial cell hyperpolarization that was sensitive to paxilline, a selective BKCa inhibitor, but not to GPR18 antibody, and mimicked by NS1619, a direct BKCa opener. In excised patches from mice aortic endothelium, single channel activity with characteristics similar to BKCa was established by the addition of abn-cbd and NAGly. We conclude that the two cannabinoids abn-cbd and NAGly initiate a GPR18-independent activation of BKCa channels in mice aortic endothelial cells that might contribute to vasodilation to cannabinoids.

The G protein-coupled receptors deorphanization landscape

G protein-coupled receptors (GPCRs) are usually highlighted as being both the largest family of membrane proteins and the most productive source of drug targets. However, most of the GPCRs are understudied and hence cannot be used immediately for innovative therapeutic strategies. Besides, there are still around 100 orphan receptors, with no described endogenous ligand and no clearly defined function. The race to discover new ligands for these elusive receptors seems to be less intense than before. Here, we present an update of the various strategies employed to assign a function to these receptors and to discover new ligands. We focus on the recent advances in the identification of endogenous ligands with a detailed description of newly deorphanized receptors. Replication being a key parameter in these endeavors, we also discuss the latest controversies about problematic ligand-receptor pairings. In this context, we propose several recommendations in order to strengthen the reporting of new ligand-receptor pairs.

Targeting the endocannabinoid system as a potential anticancer approach

The endocannabinoid system is currently under intense investigation due to the therapeutic potential of cannabinoid-based drugs as treatment options for a broad variety of diseases including cancer. Besides the canonical endocannabinoid system that includes the cannabinoid receptors CB₁ and CB₂ and the endocannabinoids N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol, recent investigations suggest that other fatty acid derivatives, receptors, enzymes, and lipid transporters likewise orchestrate this system as components of the endocannabinoid system when defined as an extended signaling network. As such, fatty acids acting at cannabinoid receptors (e.g. 2-arachidonoyl glyceryl ether [noladin ether], N-arachidonoyldopamine) as well as endocannabinoid-like substances that do not elicit cannabinoid receptor activation (e.g. N-palmitoylethanolamine, N-oleoylethanolamine) have raised interest as anticancerogenic substances. Furthermore, the endocannabinoid-degrading enzymes fatty acid amide hydrolase and monoacylglycerol lipase, lipid transport proteins of the fatty acid binding protein family, additional cannabinoid-activated G protein-coupled receptors, members of the transient receptor potential family as well as peroxisome proliferator-activated receptors have been considered as targets of antitumoral cannabinoid activity. Therefore, this review focused on the antitumorigenic effects induced upon modulation of this extended endocannabinoid network.

The Effect of Chronic Activation of the Novel Endocannabinoid Receptor GPR18 on Myocardial Function and Blood Pressure in Conscious Rats

Although acute activation of the novel endocannabinoid receptor GPR18 causes hypotension, there are no reports on GPR18 expression in the heart or its chronic modulation of cardiovascular function. In this study, after demonstrating GPR18 expression in the heart, we show that chronic (2 weeks) GPR18 activation with its agonist abnormal cannabidiol (abn-cbd; 100 µg·kg·d; i.p) produced hypotension, suppressed the cardiac sympathetic dominance, and improved left ventricular (LV) function (increased the contractility index dp/dtmax and reduced LV end-diastolic pressure, LVEDP) in conscious rats. Ex vivo studies revealed increased: (1) cardiac and plasma adiponectin (ADN) levels; (2) vascular (aortic) endothelial nitric oxide synthase (eNOS) expression, (3) vascular and serum nitric oxide (NO) levels; (4) myocardial and plasma cyclic guanosine monophosphate (cGMP) levels; (5) phosphorylation of myocardial protein kinase B (Akt) and extracellular signal regulated kinase 1/2 (ERK1/2) along with reduced myocardial reactive oxygen species (ROS) in abn-cbd treated rats. These biochemical responses contributed to the hemodynamic responses and were GPR18-mediated because concurrent treatment with the competitive GPR18 antagonist (O-1918) abrogated the abn-cbd-evoked hemodynamic and biochemical responses. The current findings present new evidence for a salutary cardiovascular role for GPR18, mediated, at least partly, via elevation in the levels of adiponectin.

N-Acyl Amino Acids (Elmiric Acids): Endogenous Signaling Molecules with Therapeutic Potential

The subject of N-acyl amino acid conjugates has been rapidly growing in recent years, especially with regard to their analgesic and anti-inflammatory actions. The field comprises a large family of lipid signaling molecules whose importance is only now being fully realized. The most widely studied member is N-arachidonoyl glycine (NAGly), which differs structurally from the endocannabinoid anandamide (N-arachidonoyl ethanolamide) by a single oxygen atom even as the two are metabolically related. Topics that are covered in this minireview are: biosynthetic pathways for N-acyl amino acids, receptors for N-acyl amino acids, physiologic actions of N-acyl amino acids, pharmacological effects of N-acyl amino acids, and molecular mechanisms believed to be responsible for their effects. On the subject of mechanisms, we propose several possibilities whose basis is the currently available information. Four putative pathways can be suggested: 1) inhibition of fatty acid amide hydrolase-induced increases in anandamide or 2-arachidonoyl glycerol (2-AG) levels, resulting in analgesic activity; 2) binding to GPR18, initiating the production of anti-inflammatory eicosanoids (specifically, the data suggest roles for 15-deoxy-Δ^12,14-prostaglandin-J₂ and lipoxin A₄, both of which are potent inflammation-resolving molecules); 3) inactivation of T-type Cav3 channels; and 4) inhibition of the GLYT2 glycine transporter. Each pathway would produce analgesic effects. Also, the N-acyl amino acids do not bind to either cannabinoid or opioid receptors, thus reducing adverse actions and making them good templates for novel drug candidate molecules.

Cannabinoids Modulate Neuronal Activity and Cancer by CB1 and CB2 Receptor-Independent Mechanisms

Cannabinoids include the active constituents of Cannabis or are molecules that mimic the structure and/or function of these Cannabis-derived molecules. Cannabinoids produce many of their cellular and organ system effects by interacting with the well-characterized CB1 and CB2 receptors. However, it has become clear that not all effects of cannabinoid drugs are attributable to their interaction with CB1 and CB2 receptors. Evidence now demonstrates that cannabinoid agents produce effects by modulating activity of the entire array of cellular macromolecules targeted by other drug classes, including: other receptor types; ion channels; transporters; enzymes, and protein- and non-protein cellular structures. This review summarizes evidence for these interactions in the CNS and in cancer, and is organized according to the cellular targets involved. The CNS represents a well-studied area and cancer is emerging in terms of understanding mechanisms by which cannabinoids modulate their activity. Considering the CNS and cancer together allow identification of non-cannabinoid receptor targets that are shared and divergent in both systems. This comparative approach allows the identified targets to be compared and contrasted, suggesting potential new areas of investigation. It also provides insight into the diverse sources of efficacy employed by this interesting class of drugs. Obtaining a comprehensive understanding of the diverse mechanisms of cannabinoid action may lead to the design and development of therapeutic agents with greater efficacy and specificity for their cellular targets.

An Update on Non-CB1, Non-CB2 Cannabinoid Related G-Protein-Coupled Receptors

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.

N-arachidonoyl glycine, another endogenous agonist of GPR55

Interest in lipoamino acids as endogenous modulators of G-protein coupled receptors has escalated due to their involvement in a variety of physiologic processes. In particular, a role for these amino acid conjugates has emerged in the endocannabinoid system. The study presented herein investigated the effects of N-arachidonoyl glycine (NAGly) on a candidate endocannabinoid receptor, GPR55. Our novel findings reveal that NAGly induces concentration dependent increases in calcium mobilization and mitogen-activated protein kinase activities in HAGPR55/CHO cells. These increases were attenuated by the selective GPR55 antagonist ML193 (N-[4-[[(3,4-Dimethyl-5-isoxazolyl)amino]sulfonyl]phenyl]-6,8-dimethyl-2-(2-pyridinyl)-4-quinolinecarboxamide), supporting receptor mediated signaling. To our knowledge this is the first report identifying GPR55 as a target of the endogenous lipoamino acid, NAGly.

Cannabinoid Receptor-Related Orphan G Protein-Coupled Receptors

Of the druggable group of G protein-coupled receptors in the human genome, a number remain which have yet to be paired with an endogenous ligand-orphan GPCRs. Among these 100 or so entities, 3 have been linked to the cannabinoid system. GPR18, GPR55, and GPR119 exhibit limited sequence homology with the established CB₁ and CB₂ cannabinoid receptors. However, the pharmacology of these orphan receptors displays overlap with CB₁ and CB₂ receptors, particularly for GPR18 and GPR55. The linking of GPR119 to the cannabinoid receptors is less convincing and emanates from structural similarities of endogenous ligands active at these GPCRs, but which do not cross-react. This review describes the evidence for describing these orphan GPCRs as cannabinoid receptor-like receptors.

Characterization of non-olfactory GPCRs in human sperm with a focus on GPR18

G protein-coupled receptors (GPCRs) transduce external chemical cues into intracellular signals and are involved in a plethora of physiological processes, but knowledge regarding the function of these receptors in spermatozoa is limited. In the present study, we performed RNA-Seq and analyzed the expression of the all GPCRs except olfactory receptors in human spermatozoa. We revealed the expression of up to 223 different GPCR transcripts in human spermatozoa (FPKM > 0.1) and identified GPR18, a newly described cannabinoid receptor, together with GPR137 and GPR135, as one of the three most highly expressed GPCRs. To date, the expression of GPR18 was completely unknown in human spermatozoa. We confirmed GPR18 expression using RT-PCR and immuncytochemistry experiments and localized the GPR18 protein in the midpiece of human spermatozoa. Stimulation of human spermatozoa with the GPR18 ligand N-arachidonoylglycine induced the phosphorylation of 12 protein kinases, some of them are for example known to be involved in the acrosome reaction. In line with this, N-arachidonoylglycine affected the cytoskeleton by changing levels of F-actin and inducing the acrosome reaction in human spermatozoa in a concentration-dependent manner. Our results indicate that GPR18 might be involved in physiological processes of human spermatozoa, suggesting GPR18 to be a potential player in sperm physiology.

GPR18 undergoes a high degree of constitutive trafficking but is unresponsive to N-Arachidonoyl Glycine

The orphan receptor GPR18 has become a research target following the discovery of a putative endogenous agonist, N-arachidonoyl glycine (NAGly). Chemical similarity between NAGly and the endocannabinoid anandamide suggested the hypothesis that GPR18 is a third cannabinoid receptor. GPR18-mediated cellular signalling through inhibition of cyclic adenosine monophosphate (cAMP) and phosphorylation of extracellular signal-regulated kinase (ERK), in addition to physiological consequences such as regulation of cellular migration and proliferation/apoptosis have been described in response to both NAGly and anandamide. However, discordant findings have also been reported. Here we sought to describe the functional consequences of GPR18 activation in heterologously-expressing HEK cells. GPR18 expression was predominantly intracellular in stably transfected cell lines, but moderate cell surface expression could be achieved in transiently transfected cells which also had higher overall expression. Assays were employed to characterise the ability of NAGly or anandamide to inhibit cAMP or induce ERK phosphorylation through GPR18, or induce receptor trafficking. Positive control experiments, which utilised cells expressing hCB1 receptors (hCB1R), were performed to validate assay design and performance. While these functional pathways in GPR18-expressing cells were not modified on treatment with a panel of putative GPR18 ligands, a constitutive phenotype was discovered for this receptor. Our data reveal that GPR18 undergoes rapid constitutive receptor membrane trafficking-several-fold faster than hCB1R, a highly constitutively active receptor. To enhance the likelihood of detecting agonist-mediated receptor signalling responses, we increased GPR18 protein expression (by tagging with a preprolactin signal sequence) and generated a putative constitutively inactive receptor by mutating the hGPR18 gene at amino acid site 108 (alanine to asparagine). This A108N mutant did cause an increase in surface receptor expression (which may argue for reduced constitutive activity), but no ligand-mediated effects were detected. Two glioblastoma multiforme cell lines (which endogenously express GPR18) were assayed for NAGly-induced pERK phosphorylation, with negative results. Despite a lack of ligand-mediated responses in all assays, the constitutive trafficking of GPR18 remains an interesting facet of receptor function and will have consequences for understanding the role of GPR18 in physiology.

The effect of O-1602, an atypical cannabinoid, on morphine-induced conditioned place preference and physical dependence

BACKGROUND

Previous studies show that some non-CB1/non-CB2 effects of cannabinoids are mediated through G protein coupled receptor 55 (GPR55). As this receptor is activated by some of cannabinoid receptor ligands and is involved in the modulation of pain, it was hypothesized that this receptor may also interact with opioids. This study examined the effect of atypical cannabinoid O-1602 as a GPR55 agonist on morphine-induced conditioned place preference (CPP) and physical dependence.

METHODS

We used a biased CPP model to evaluate the effect of O-1602 (0.2, 1 and 5mg/kg, intraperitoneal; ip) on the acquisition and expression of morphine-induced CPP in male mice. The locomotor activities of mice were also recorded. Moreover, repeated administration of morphine (50, 50 and 75mg/kg/day) for three days, induced physical dependence. The withdrawal signs such as jumps and diarrhea were precipitated by administration of naloxone (5mg/kg, ip). The effect of O-1602 on the development of morphine physical dependence was assessed by injection of O-1602 (0.2, 1 and 5mg/kg) before morphine administrations.

RESULTS

Morphine (40mg/kg, subcutaneous; sc), but not O-1602 (5mg/kg) elicited significant preference in the post-conditioning phase. O-1602 at the doses of 0.2 and 1mg/kg, but not 5mg/kg reduced acquisition of morphine CPP with an increase in locomotor activity at the dose of 5mg/kg. O-1602 at the doses of 0.2, 1 and 5mg/kg also reduced expression of morphine CPP with an increase in locomotor activity at the dose of 5mg/kg. O-1602 had a significant inhibitory effect on development of morphine-induced physical dependence at the dose of 5mg/kg by decreasing jumps and diarrhea during withdrawal syndrome.

CONCLUSIONS

The present results indicate that O-1602 decreased acquisition and expression of morphine CPP and inhibited development of morphine-induced physical dependence.

G protein coupled receptor 18: A potential role for endocannabinoid signaling in metabolic dysfunction

Endocannabinoids are products of dietary fatty acids that are modulated by an alteration in food intake levels. Overweight and obese individuals have substantially higher circulating levels of the arachidonic acid derived endocannabinoids, anandamide and 2-arachidonoyl glycerol, and show an altered pattern of cannabinoid receptor expression. These cannabinoid receptors are part of a large family of G protein coupled receptors (GPCRs). GPCRs are major therapeutic targets for various diseases within the cardiovascular, neurological, gastrointestinal, and endocrine systems, as well as metabolic disorders such as obesity and type 2 diabetes mellitus. Obesity is considered a state of chronic low-grade inflammation elicited by an immunological response. Interestingly, the newly deorphanized GPCR (GPR18), which is considered to be a putative cannabinoid receptor, is proposed to have an immunological function. In this review, the current scientific knowledge on GPR18 is explored including its localization, signaling pathways, and pharmacology. Importantly, the involvement of nutritional factors and potential dietary regulation of GPR18 and its (patho)physiological roles are described. Further research on this receptor and its regulation will enable a better understanding of the complex mechanisms of GPR18 and its potential as a novel therapeutic target for treating metabolic disorders.

GPR18 Controls Reconstitution of Mouse Small Intestine Intraepithelial Lymphocytes following Bone Marrow Transplantation

Specific G protein coupled receptors (GPRs) regulate the proper positioning, function, and development of immune lineage subsets. Here, we demonstrate that GPR18 regulates the reconstitution of intraepithelial lymphocytes (IELs) of the small intestine following bone marrow transplantation. Through analysis of transcriptional microarray data, we find that GPR18 is highly expressed in IELs, lymphoid progenitors, and mature follicular B cells. To establish the physiological role of this largely uncharacterized GPR, we generated Gpr18^-/- mice. Despite high levels of GPR18 expression in specific hematopoietic progenitors, Gpr18^-/- mice have no defects in lymphopoiesis or myelopoiesis. Moreover, antibody responses following immunization with hapten-protein conjugates or infection with West Nile virus are normal in Gpr18^-/- mice. Steady-state numbers of IELs are also normal in Gpr18^-/- mice. However, competitive bone marrow reconstitution experiments demonstrate that GPR18 is cell-intrinsically required for the optimal restoration of small intestine TCRγδ⁺ and TCRαβ⁺ CD8αα⁺ IELs. In contrast, GPR18 is dispensable for the reconstitution of large intestine IELs. Moreover, Gpr18^-/- bone marrow reconstitutes small intestine IELs similarly to controls in athymic recipients. Gpr18^-/- chimeras show no changes in susceptibility to intestinal insults such as Citrobacter rodentium infections or graft versus host disease. These data reveal highly specific requirements for GPR18 in the development and reconstitution of thymus-derived intestinal IEL subsets in the steady-state and after bone marrow transplantation.

Activation of GPR18 by cannabinoid compounds: a tale of biased agonism

BACKGROUND AND PURPOSE

GPR18 is a candidate cannabinoid receptor, but its classification as such is controversial. The rationale of the study presented herein was to consider the effects of N-arachidonoyl glycine (NAGly) and cannabinoids via differential G-protein coupled pathways, in addition to β-arrestin signalling. Cellular localization of GPR18 receptors was also examined.

EXPERIMENTAL APPROACH

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 RESULTS

Concentration-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 Gα(q) or Gα(i/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 IMPLICATIONS

These 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.

GPR55: from orphan to metabolic regulator?

GPR55 belongs to the class A family of G-protein coupled receptor (GPCRs) and its activity is regulated by a range of synthetic and endogenous cannabinoids, and by lipid-derived ligands. Cannabinoids are known to be important in controlling appetite and metabolic balance, and it is now emerging that GPR55 may have a role to play in energy homeostasis through the regulation of food intake, fuel storage in adipocytes, gut motility and insulin secretion. This review summarises our current knowledge of expression and function of GPR55 in tissues involved in metabolic regulation, the signalling cascades through which GPR55 is reported to act to exert its effects, and it comments on the difficulties in reaching firm conclusions when using GPR55 ligands of poor specificity. Understanding the role of GPR55 in energy homeostasis may provide a novel target for therapeutic intervention in obesity and type 2 diabetes mellitus.

Endothelial atypical cannabinoid receptor: do we have enough evidence?

Cannabinoids and their synthetic analogues affect a broad range of physiological functions, including cardiovascular variables. Although direct evidence is still missing, the relaxation of a vast range of vascular beds induced by cannabinoids is believed to involve a still unidentified non-CB1 , non-CB2 Gi/o protein-coupled receptor located on endothelial cells, the so called endothelial cannabinoid receptor (eCB receptor). Evidence for the presence of an eCB receptor comes mainly from vascular relaxation studies, which commonly employ pertussis toxin as an indicator for GPCR-mediated signalling. In addition, a pharmacological approach is widely used to attribute the relaxation to eCB receptors. Recent findings have indicated a number of GPCR-independent targets for both agonists and antagonists of the presumed eCB receptor, warranting further investigations and cautious interpretation of the vascular relaxation studies. This review will provide a brief historical overview on the proposed novel eCB receptor, drawing attention to the discrepancies between the studies on the pharmacological profile of the eCB receptor and highlighting the Gi/o protein-independent actions of the eCB receptor inhibitors widely used as selective compounds. As the eCB receptor represents an attractive pharmacological target for a number of cardiovascular abnormalities, defining its molecular identity and the extent of its regulation of vascular function will have important implications for drug discovery. This review highlights the need to re-evaluate this subject in a thoughtful and rigorous fashion. More studies are needed to differentiate Gi/o protein-dependent endothelial cannabinoid signalling from that involving the classical CB1 and CB2 receptors as well as its relevance for pathophysiological conditions.

N-Arachidonoyl glycine suppresses Na⁺/Ca²⁺ exchanger-mediated Ca²⁺ entry into endothelial cells and activates BK(Ca) channels independently of GPCRs

Background and Purpose

N-arachidonoyl glycine (NAGly) is a lipoamino acid with vasorelaxant properties. We aimed to explore the mechanisms of NAGly's action on unstimulated and agonist-stimulated endothelial cells.

Experimental Approach

The effects of NAGly on endothelial electrical signalling were studied in combination with vascular reactivity.

Key Results

In EA.hy926 cells, the sustained hyperpolarization to histamine was inhibited by the non-selective Na⁺/Ca²⁺ exchanger (NCX) inhibitor bepridil and by an inhibitor of reversed mode NCX, KB-R7943. In cells dialysed with Cs⁺-based Na⁺-containing solution, the outwardly rectifying current with typical characteristics of NCX was augmented following histamine exposure, further increased upon external Na⁺ withdrawal and inhibited by bepridil. NAGly (0.3–30 μM) suppressed NCX currents in a URB597- and guanosine 5′-O-(2-thiodiphosphate) (GDPβS)-insensitive manner, [Ca²⁺]_i elevation evoked by Na⁺ removal and the hyperpolarization to histamine. In rat aorta, NAGly opposed the endothelial hyperpolarization and relaxation response to ACh. In unstimulated EA.hy926 cells, NAGly potentiated the whole-cell current attributable to large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in a GDPβS-insensitive, paxilline-sensitive manner and produced a sustained hyperpolarization. In cell-free inside-out patches, NAGly stimulated single BK_Ca channel activity.

Conclusion and Implications

Our data showed that NCX is a Ca²⁺ entry pathway in endothelial cells and that NAGly is a potent G-protein-independent modulator of endothelial electrical signalling and has a dual effect on endothelial electrical responses. In agonist pre-stimulated cells, NAGly opposes hyperpolarization and relaxation via inhibition of NCX-mediated Ca²⁺ entry, while in unstimulated cells, it promotes hyperpolarization via receptor-independent activation of BK_Ca channels.

Neuronal nitric oxide synthase-dependent elevation in adiponectin in the rostral ventrolateral medulla underlies g protein-coupled receptor 18-mediated hypotension in conscious rats

Direct activation of the endocannabinoid receptor G protein-coupled receptor 18 (GPR18) in the rostral ventrolateral medulla (RVLM) of conscious rats by abnormal cannabidiol (Abn CBD; trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol) elevates local nitric oxide (NO) and adiponectin (ADN) levels and reduces oxidative stress and blood pressure (BP). However, the molecular mechanisms for GPR18-mediated neurochemical responses, including the nitric oxide synthase isoform that generates NO, and their potential causal link to the BP reduction are not known. We hypothesized that GPR18-mediated enhancement of Akt, extracellular signal-regulated kinase 1/2 (ERK1/2), and neuronal nitric oxide synthase (nNOS) phosphorylation, triggered by a reduction in cAMP, accounts for the NO/ADN-dependent reductions in RVLM oxidative stress and BP. Intra-RVLM GPR18 activation (Abn CBD; 0.4 μg) enhanced RVLM Akt, ERK1/2, and nNOS phosphorylation as well as ADN levels during the hypotensive response. Prior GPR18 blockade with O-1918 (1,3-dimethoxy-5-methyl-2-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]benzene) produced the opposite effects and abrogated Abn CBD-evoked neurochemical and BP responses. Pharmacological inhibition of RVLM phosphoinositide 3-kinase (PI3K)/Akt (wortmannin), ERK1/2 (PD98059 [2-​(2-​amino-​3-​methoxyphenyl)-​4H-​1-​benzopyran-​4-​one]), or nNOS (N(ω)-propyl-l-arginine), or activation of adenylyl cyclase (forskolin) virtually abolished intra-RVLM Abn CBD-evoked hypotension and the increases in Akt, ERK1/2, and nNOS phosphorylation and in ADN levels in the RVLM. Our pharmacological and neurochemical findings support a pivotal role for PI3K, Akt, ERK1/2, nNOS, and adenylyl cyclase, via modulation of NO, ADN, and cAMP levels, in GPR18 regulation of the RVLM redox state and BP in conscious rats.

N-Acyl amino acids and their impact on biological processes

Over the last two decades a large number of N-long-chain acyl amino acids have been identified in the mammalian body. The pharmacological activities of only a few of them have been investigated and some have been found to be of considerable interest. Thus arachidonoyl serine is vasodilatory and neuroprotective, arachidonoyl glycine is antinociceptive, and oleoyl serine rescues bone loss. However, the pathophysiological/biochemical roles of these amides are mostly unknown.

The cannabinoid acids, analogs and endogenous counterparts

The cannabinoid acids are a structurally heterogeneous group of compounds some of which are endogenous molecules and others that are metabolites of phytocannabinoids. The prototypic endogenous substance is N-arachidonoyl glycine (NAgly) that is closely related in structure to the cannabinoid agonist anandamide. The most studied phytocannabinoid is Δ(9)-THC-11-oic acid, the principal metabolite of Δ(9)-THC. Both types of acids have in common several biological actions such as low affinity for CB1 anti-inflammatory activity and analgesic properties. This suggests that there may be similarities in their mechanism of action, a point that is discussed in this review. Also presented are reports on analogs of the acids that provide opportunities for the development of novel therapeutic agents, such as ajulemic acid.

Intercellular Lipid Mediators and GPCR Drug Discovery

G-protein-coupled receptors (GPCR) are the largest superfamily of receptors responsible for signaling between cells and tissues, and because they play important physiological roles in homeostasis, they are major drug targets. New technologies have been developed for the identification of new ligands, new GPCR functions, and for drug discovery purposes. In particular, intercellular lipid mediators, such as, lysophosphatidic acid and sphingosine 1-phosphate have attracted much attention for drug discovery and this has resulted in the development of fingolimod (FTY-720) and AM095. The discovery of new intercellular lipid mediators and their GPCRs are discussed from the perspective of drug development. Lipid GPCRs for lysophospholipids, including lysophosphatidylserine, lysophosphatidylinositol, lysophosphatidylcholine, free fatty acids, fatty acid derivatives, and other lipid mediators are reviewed.

The endocannabinoid/endovanilloid N-arachidonoyl dopamine (NADA) and synthetic cannabinoid WIN55,212-2 abate the inflammatory activation of human endothelial cells

Although cannabinoids, such as Δ(9)-tetrahydrocannabinol, have been studied extensively for their psychoactive effects, it has become apparent that certain cannabinoids possess immunomodulatory activity. Endothelial cells (ECs) are centrally involved in the pathogenesis of organ injury in acute inflammatory disorders, such as sepsis, because they express cytokines and chemokines, which facilitate the trafficking of leukocytes to organs, and they modulate vascular barrier function. In this study, we find that primary human ECs from multiple organs express the cannabinoid receptors CB1R, GPR18, and GPR55, as well as the ion channel transient receptor potential cation channel vanilloid type 1. In contrast to leukocytes, CB2R is only minimally expressed in some EC populations. Furthermore, we show that ECs express all of the known endocannabinoid (eCB) metabolic enzymes. Examining a panel of cannabinoids, we demonstrate that the synthetic cannabinoid WIN55,212-2 and the eCB N-arachidonoyl dopamine (NADA), but neither anandamide nor 2-arachidonoylglycerol, reduce EC inflammatory responses induced by bacterial lipopeptide, LPS, and TNFα. We find that endothelial CB1R/CB2R are necessary for the effects of NADA, but not those of WIN55,212-2. Furthermore, transient receptor potential cation channel vanilloid type 1 appears to counter the anti-inflammatory properties of WIN55,212-2 and NADA, but conversely, in the absence of these cannabinoids, its inhibition exacerbates the inflammatory response in ECs activated with LPS. These data indicate that the eCB system can modulate inflammatory activation of the endothelium and may have important implications for a variety of acute inflammatory disorders that are characterized by EC activation.