Cannabinoid receptor trafficking in peripheral cells is dynamically regulated by a binary biochemical switch

Patent review of cannabinoid receptor type 2 (CB2R) modulators (2016-present)

INTRODUCTION

Cannabinoid receptor type 2 (CB2R), predominantly expressed in immune tissues, is believed to play a crucial role within the body's protective mechanisms. Its modulation holds immense therapeutic promise for addressing a wide spectrum of dysbiotic conditions, including cardiovascular, gastrointestinal, liver, kidney, neurodegenerative, psychiatric, bone, skin, and autoimmune diseases, as well as lung disorders, cancer, and pain management.

AREAS COVERED

This review is an account of patents from 2016 up to 2023 which describes novel CB2R ligands, therapeutic applications, synthesis, as well as formulations of CB2R modulators.

EXPERT OPINION

The patents cover a vast, structurally diverse chemical space. The focus of CB2R ligand development has shifted from unselective dual-cannabinoid receptor type 1 (CB1R) and 2 agonists toward agonists with high selectivity over CB1R, particularly for indications associated with inflammation and tissue injury. Currently, there are at least eight CB2R agonists and one antagonist in active clinical development. A better understanding of the endocannabinoid system (ECS) and in particular of CB2R pharmacology is required to unlock the receptor's full therapeutic potential.

Cannabinoid CB2 receptor orthologues; in vitro function and perspectives for preclinical to clinical translation

Cannabinoid CB2 receptor agonists are in development as therapeutic agents, including for immune modulation and pain relief. Despite promising results in rodent preclinical studies, efficacy in human clinical trials has been marginal to date. Fundamental differences in ligand engagement and signalling responses between the human CB2 receptor and preclinical model species orthologues may contribute to mismatches in functional outcomes. This is a tangible possibility for the CB2 receptor in that there is a relatively large degree of primary amino acid sequence divergence between human and rodent. Here, we summarise CB2 receptor gene and protein structure, assess comparative molecular pharmacology between CB2 receptor orthologues, and review the current status of preclinical to clinical translation for drugs targeted at the CB2 receptor, focusing on comparisons between human, mouse and rat receptors. We hope that raising wider awareness of, and proposing strategies to address, this additional challenge in drug development will assist in ongoing efforts toward successful therapeutic translation of drugs targeted at the CB2 receptor. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Uptake Pathway of Styrene Maleic Acid Copolymer-Coated Lipid Emulsions Under Acidic Tumor Microenvironment

The purpose of this study was to elucidate and compare styrene maleic acid copolymer (SMA)-coated lipid emulsions (SMA emulsions) uptake pathway in vascular endothelial cells and surrounding cancer cells under not only neutral but also acidic pH, which is often observed in tumor microenvironment. DiI-labeled SMA emulsions were prepared using 1-palmitoyl-2-oleoyl-sn‑glycero-3-phosphocholine and triolein. In murine melanoma B16-BL6 (B16) cells and human umbilical vein endothelial cells (HUVEC), DiI-labeled SMA emulsions uptake under near-neutral (pH 7.4) and acidic (pH 6.0) conditions was determined by fluorescent analysis. SMA emulsions were taken up more efficiently into HUVEC than B16 cells under acidic condition in a temperature-dependent manner. Uptake study using endocytosis inhibitors showed that SMA emulsions were taken up by macropinocytosis and clathrin-mediated endocytosis in B16 cells. In HUVEC, however, they were taken up by clathrin- and caveolae-independent, but dynamin-dependent pathway. SMA emulsions would be internalized efficiently into vascular endothelial cells as well as cancer cells under acidic microenvironment via different endocytosis pathways. SMA emulsions could be a promising drug delivery carrier for anti-angiogenic drugs.

Goods and Bads of the Endocannabinoid System as a Therapeutic Target: Lessons Learned after 30 Years

The cannabis derivative marijuana is the most widely used recreational drug in the Western world and is consumed by an estimated 83 million individuals (∼3% of the world population). In recent years, there has been a marked transformation in society regarding the risk perception of cannabis, driven by its legalization and medical use in many states in the United States and worldwide. Compelling research evidence and the Food and Drug Administration cannabis-derived cannabidiol approval for severe childhood epilepsy have confirmed the large therapeutic potential of cannabidiol itself, Δ9-tetrahydrocannabinol and other plant-derived cannabinoids (phytocannabinoids). Of note, our body has a complex endocannabinoid system (ECS)-made of receptors, metabolic enzymes, and transporters-that is also regulated by phytocannabinoids. The first endocannabinoid to be discovered 30 years ago was anandamide (N-arachidonoyl-ethanolamine); since then, distinct elements of the ECS have been the target of drug design programs aimed at curing (or at least slowing down) a number of human diseases, both in the central nervous system and at the periphery. Here a critical review of our knowledge of the goods and bads of the ECS as a therapeutic target is presented to define the benefits of ECS-active phytocannabinoids and ECS-oriented synthetic drugs for human health. SIGNIFICANCE STATEMENT: The endocannabinoid system plays important roles virtually everywhere in our body and is either involved in mediating key processes of central and peripheral diseases or represents a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of the components of this complex system, and in particular of key receptors (like cannabinoid receptors 1 and 2) and metabolic enzymes (like fatty acid amide hydrolase and monoacylglycerol lipase), will advance our understanding of endocannabinoid signaling and activity at molecular, cellular, and system levels, providing new opportunities to treat patients.

2-Arachidonoylglycerol Modulates CXCL12-Mediated Chemotaxis in Mantle Cell Lymphoma and Chronic Lymphocytic Leukemia

To survive chemotherapy, lymphoma cells can relocate to protective niches where they receive support from the non-malignant cells. The biolipid 2-arachidonoylglycerol (2-AG), an agonist for the cannabinoid receptors CB1 and CB2, is released by stromal cells in the bone marrow. To investigate the role of 2-AG in lymphoma, we analyzed the chemotactic response of primary B-cell lymphoma cells enriched from peripheral blood of twenty-two chronic lymphocytic leukemia (CLL) and five mantle cell lymphoma (MCL) patients towards 2-AG alone and/or to the chemokine CXCL12. The expression of cannabinoid receptors was quantified using qPCR and the protein levels visualized by immunofluorescence and Western blot. Surface expression of CXCR4, the main cognate receptor to CXCL12, was analyzed by flow cytometry. Phosphorylation of key downstream signaling pathways activated by 2-AG and CXCL12 were measured by Western blot in three MCL cell lines and two primary CLL samples. We report that 2-AG induces chemotaxis in 80% of the primary samples, as well as 2/3 MCL cell lines. 2-AG induced in a dose-dependent manner, the migration of JeKo-1 cell line via CB1 and CB2. 2-AG affected the CXCL12-mediated chemotaxis without impacting the expression or internalization of CXCR4. We further show that 2-AG modulated p38 and p44/42 MAPK activation. Our results suggest that 2-AG has a previously unrecognized role in the mobilization of lymphoma cells by effecting the CXCL12-induced migration and the CXCR4 signaling pathways, however, with different effects in MCL compared to CLL.

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.

Cannabinoid receptor type 2 ligands: an analysis of granted patents since 2010

The G-protein-coupled cannabinoid receptor type 2 (CB2R) is a key element of the endocannabinoid (EC) system. EC/CB2R signaling has significant therapeutic potential in major pathologies affecting humans such as allergies, neurodegenerative disorders, inflammation or ocular diseases. CB2R agonism exerts anti-inflammatory and tissue protective effects in preclinical animal models of cardiovascular, gastrointestinal, liver, kidney, lung and neurodegenerative disorders. Existing ligands can be subdivided into endocannabinoids, cannabinoid-like and synthetic CB2R ligands that possess various degrees of potency on and selectivity against the cannabinoid receptor type 1. This review is an account of granted CB2R ligand patents from 2010 up to the present, which were surveyed using Derwent Innovation®.

Cannabinoid Receptor 2 (CB2) Signals via G-alpha-s and Induces IL-6 and IL-10 Cytokine Secretion in Human Primary Leukocytes

Cannabinoid receptor 2 (CB₂) is a promising therapeutic target for immunological modulation. There is, however, a deficit of knowledge regarding CB₂ signaling and function in human primary immunocompetent cells. We applied an experimental paradigm which closely models the in situ state of human primary leukocytes (PBMC; peripheral blood mononuclear cells) to characterize activation of a number of signaling pathways in response to a CB₂-selective ligand (HU308). We observed a "lag" phase of unchanged cAMP concentration prior to development of classically expected Gα_i-mediated inhibition of cAMP synthesis. Application of G protein inhibitors revealed that this apparent lag was a result of counteraction of Gα_i effects by concurrent Gα_s activation. Monitoring downstream signaling events showed that activation of p38 was mediated by Gα_i, whereas ERK1/2 and Akt phosphorylation were mediated by Gα_i-coupled βγ. Activation of CREB integrated multiple components; Gα_s and βγ mediated ∼85% of the response, while ∼15% was attributed to Gα_i. Responses to HU308 had an important functional outcome-secretion of interleukins 6 (IL-6) and 10 (IL-10). IL-2, IL-4, IL-12, IL-13, IL-17A, MIP-1α, and TNF-α were unaffected. IL-6/IL-10 induction had a similar G protein coupling profile to CREB activation. All response potencies were consistent with that expected for HU308 acting via CB₂. Additionally, signaling and functional effects were completely blocked by a CB₂-selective inverse agonist, giving additional evidence for CB₂ involvement. This work expands the current paradigm regarding cannabinoid immunomodulation and reinforces the potential utility of CB₂ ligands as immunomodulatory therapeutics.

Chromenopyrazole-based High Affinity, Selective Fluorescent Ligands for Cannabinoid Type 2 Receptor

Cannabinoid type 2 receptor (CB₂R) is an attractive target for the treatment of pain and inflammatory disorders. Availability of a selective CB₂R fluorescent ligand to study CB₂R expression and localization in healthy and disease conditions would greatly contribute to improving our understanding of this receptor. Herein, we report a series of chromenopyrazole-based CB₂R fluorescent ligands. The highest affinity fluorescent ligand was Cy5-containing 24 (hCB₂R pK _i = 7.38 ± 0.05), which had 131-fold selectivity over CB₁R. In a cAMP BRET assay, 24 behaved as a potent CB₂R inverse agonist. Widefield imaging experiments showed that 24 binds to CB₂R in live cells with good selectivity and low levels of nonspecific fluorescence. The high affinity, selectivity, and suitable imaging properties of fluorescent ligand 24 make it a valuable tool for studying CB₂R.

Cannabinoid Receptor 2 Signalling Bias Elicited by 2,4,6-Trisubstituted 1,3,5-Triazines

Cannabinoid receptor 2 (CB2) is predominantly distributed in immune tissues and cells and is a promising therapeutic target for modulating inflammation. In this study we designed and synthesised a series of 2,4,6-trisubstituted 1,3,5-triazines with piperazinylalkyl or 1,2-diethoxyethane (PEG2) chains as CB2 agonists, all of which were predicted to be considerably more polar than typical cannabinoid ligands. In this series, we found that triazines containing an adamantanyl group were conducive to CB2 binding whereas those with a cyclopentyl group were not. Although the covalent attachment of a PEG2 linker to the adamantyl triazines resulted in a decrease in binding affinity, some of the ligands produced very interesting hCB2 signalling profiles. Six compounds with notable hCB2 orthosteric binding were functionally characterised in three pathways; internalisation, cyclic adenosine monophosphate (cAMP) and ERK phosphorylation (pERK). These were predominantly confirmed to be hCB2 agonists, and upon comparison to a reference ligand (CP 55,940), four compounds exhibited signalling bias. Triazines 14 (UOSD017) and 15 were biased towards internalisation over cAMP and pERK, and 7 was biased away from pERK activation relative to cAMP and internalisation. Intriguingly, the triazine with an amino-PEG2-piperazinyl linker (13 [UOSD008]) was identified to be a mixed agonist/inverse agonist, exhibiting apparent neutral antagonism in the internalisation pathway, transient inverse agonism in the cAMP pathway and weak partial agonism in the pERK pathway. Both the cAMP and pERK signalling were pertussis toxin (PTX) sensitive, implying that 13 is acting as both a weak agonist and inverse agonist at CB2 via Gαi/o. Compound 10 (UOSD015) acted as a balanced high intrinsic efficacy agonist with the potential to produce greater hCB2-mediated efficacy than reference ligand CP 55,940. As 10 includes a Boc-protected PEG2 moiety it is also a promising candidate for further modification, for example with a secondary reporter or fluorophore. The highest affinity compound in this set of relatively polar hCB2 ligands was compound 16, which acted as a slightly partial balanced agonist in comparison with CP 55,940. The ligands characterised here may therefore exhibit unique functional properties in vivo and have the potential to be valuable in the future development of CB2-directed therapeutics.

Selective Photoaffinity Probe That Enables Assessment of Cannabinoid CB2 Receptor Expression and Ligand Engagement in Human Cells

Chemical tools and methods that report on G protein-coupled receptor (GPCR) expression levels and receptor occupancy by small molecules are highly desirable. We report the development of LEI121 as a photoreactive probe to study the type 2 cannabinoid receptor (CB₂R), a promising GPCR to treat tissue injury and inflammatory diseases. LEI121 is the first CB₂R-selective bifunctional probe that covalently captures CB₂R upon photoactivation. An incorporated alkyne serves as ligation handle for the introduction of reporter groups. LEI121 enables target engagement studies and visualization of endogenously expressed CB₂R in HL-60 as well as primary human immune cells using flow cytometry. Our findings show that strategically functionalized probes allow monitoring of endogenous GPCR expression and engagement in human cells using tandem photoclick chemistry and hold promise as biomarkers in translational drug discovery.

The presence and distribution of cannabinoid type 1 and 2 receptors in the mandibular gland: The influence of different physical forms of diets on their expression in piglets

We explored the expression and cell type distribution of cannabinoid receptors type 1 (CB1) and cannabinoid receptors type 2 (CB2) in the mandibular glands of pigs in relation to different physical forms of the diet. Thirty-two crossbred growing pigs (ages 5-6 weeks) were randomly allotted to four experimental groups (eight pigs/group) and fed four different physical types of the same diet for 4 weeks: finely ground pellet (FP), coarsely ground meal (CM), coarsely ground pellet (CP) and coarsely ground extruded (CE) with dMEAN of 0.46, 0.88, 0.84 and 0.66 mm respectively. At the end of the feeding trial, the pigs were euthanized and the mandibular gland was collected after dissection. By immunohistochemistry, positive signals for CB1 were found in the cytoplasm of duct epithelial cells of pigs fed CP, FP and CE diets and in the serous cells of mixed acini in pigs fed the coarser CM diet. Positive signals for CB2 were detected in duct epithelial cells and in neurons of ganglia close to major secretory ducts of all pigs. The differential expression and localization of these receptors in response to variable chewing activity due to the type of diet suggest that endocannabinoids may influence the functional activity of the mandibular gland by modifying qualitative and/or quantitative aspects of salivary secretion.

Pepcan-12 (RVD-hemopressin) is a CB2 receptor positive allosteric modulator constitutively secreted by adrenals and in liver upon tissue damage

Pepcan-12 (RVD-hemopressin; RVDPVNFKLLSH) is the major peptide of a family of endogenous peptide endocannabinoids (pepcans) shown to act as negative allosteric modulators (NAM) of cannabinoid CB1 receptors. Noradrenergic neurons have been identified to be a specific site of pepcan production. However, it remains unknown whether pepcans occur in the periphery and interact with peripheral CB2 cannabinoid receptors. Here, it is shown that pepcan-12 acts as a potent (K_i value ~50 nM) hCB2 receptor positive allosteric modulator (PAM). It significantly potentiated the effects of CB2 receptor agonists, including the endocannabinoid 2-arachidonoyl glycerol (2-AG), for [35S]GTPγS binding and cAMP inhibition (5–10 fold). In mice, the putative precursor pepcan-23 (SALSDLHAHKLRVDPVNFKLLSH) was identified with pepcan-12 in brain, liver and kidney. Pepcan-12 was increased upon endotoxemia and ischemia reperfusion damage where CB2 receptors play a protective role. The adrenals are a major endocrine site of production/secretion of constitutive pepcan-12, as shown by its marked loss after adrenalectomy. However, upon I/R damage pepcan-12 was strongly increased in the liver (from ~100 pmol/g to ~500 pmol/g) independent of adrenals. The wide occurrence of this endogenous hormone-like CB2 receptor PAM, with unforeseen opposite allosteric effects on cannabinoid receptors, suggests its potential role in peripheral pathophysiological processes.

Cannabimimetic phytochemicals in the diet - an evolutionary link to food selection and metabolic stress adaptation?

The endocannabinoid system (ECS) is a major lipid signalling network that plays important pro-homeostatic (allostatic) roles not only in the nervous system but also in peripheral organs. There is increasing evidence that there is a dietary component in the modulation of the ECS. Cannabinoid receptors in hominids co-evolved with diet, and the ECS constitutes a feedback loop for food selection and energy metabolism. Here, it is postulated that the mismatch of ancient lipid genes of hunter-gatherers and pastoralists with the high-carbohydrate diet introduced by agriculture could be compensated for via dietary modulation of the ECS. In addition to the fatty acid precursors of endocannabinoids, the potential role of dietary cannabimimetic phytochemicals in agriculturist nutrition is discussed. Dietary secondary metabolites from vegetables and spices able to enhance the activity of cannabinoid-type 2 (CB₂ ) receptors may provide adaptive metabolic advantages and counteract inflammation. In contrast, chronic CB₁ receptor activation in hedonic obese individuals may enhance pathophysiological processes related to hyperlipidaemia, diabetes, hepatorenal inflammation and cardiometabolic risk. Food able to modulate the CB₁ /CB₂ receptor activation ratio may thus play a role in the nutrition transition of Western high-calorie diets. In this review, the interplay between diet and the ECS is highlighted from an evolutionary perspective. The emerging potential of cannabimimetic food as a nutraceutical strategy is critically discussed.

LINKED ARTICLES

This article is part of a themed section on Principles of Pharmacological Research of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.11/issuetoc.

Heparin exerts anti-apoptotic effects on uterine explants by targeting the endocannabinoid system

Miscarriage caused by Gram-negative bacteria infecting the female genital tract is one of the most common complications of human pregnancy. Intraperitoneal administration of LPS to 7-days pregnant mice induces embryo resorption after 24 h. Here, we show that LPS induced apoptosis on uterine explants from 7-days pregnant mice and that CB1 receptor was involved in this effect. On the other hand, heparin has been widely used for the prevention of pregnancy loss in women with frequent miscarriage with or without thrombophilia. Besides its anticoagulant properties, heparin exerts anti-inflammatory, immunomodulatory and anti-apoptotic effects. Here, we sought to investigate whether the administration of heparin prevented LPS-induced apoptosis in uterine explants from 7-days pregnant mice. We found that heparin enhanced cell survival in LPS-treated uterine explants and that this effect was mediated by increasing uterine FAAH activity. Taken together, our results point towards a novel mechanism involved in the protective effects of heparin.

Human lung-resident macrophages express CB1 and CB2 receptors whose activation inhibits the release of angiogenic and lymphangiogenic factors

Macrophages are pivotal effector cells in immune responses and tissue remodeling by producing a wide spectrum of mediators, including angiogenic and lymphangiogenic factors. Activation of cannabinoid receptor types 1 and 2 has been suggested as a new strategy to modulate angiogenesis in vitro and in vivo. We investigated whether human lung-resident macrophages express a complete endocannabinoid system by assessing their production of endocannabinoids and expression of cannabinoid receptors. Unstimulated human lung macrophage produce 2-arachidonoylglycerol,N-arachidonoyl-ethanolamine,N-palmitoyl-ethanolamine, and N-oleoyl-ethanolamine. On LPS stimulation, human lung macrophages selectively synthesize 2-arachidonoylglycerol in a calcium-dependent manner. Human lung macrophages express cannabinoid receptor types 1 and 2, and their activation induces ERK1/2 phosphorylation and reactive oxygen species generation. Cannabinoid receptor activation by the specific synthetic agonists ACEA and JWH-133 (but not the endogenous agonist 2-arachidonoylglycerol) markedly inhibits LPS-induced production of vascular endothelial growth factor-A, vascular endothelial growth factor-C, and angiopoietins and modestly affects IL-6 secretion. No significant modulation of TNF-α or IL-8/CXCL8 release was observed. The production of vascular endothelial growth factor-A by human monocyte-derived macrophages is not modulated by activation of cannabinoid receptor types 1 and 2. Given the prominent role of macrophage-assisted vascular remodeling in many tumors, we identified the expression of cannabinoid receptors in lung cancer-associated macrophages. Our results demonstrate that cannabinoid receptor activation selectively inhibits the release of angiogenic and lymphangiogenic factors from human lung macrophage but not from monocyte-derived macrophages. Activation of cannabinoid receptors on tissue-resident macrophages might be a novel strategy to modulate macrophage-assisted vascular remodeling in cancer and chronic inflammation.

Synthesis and pharmacological evaluation of new biphenylic derivatives as CB2 receptor ligands

Targeting type-2 cannabinoid receptor (CB2) is considered a feasible strategy to develop new drugs for the treatment of diseases like neuropathic pain, chronic inflammation, neurodegenerative disorders and cancer. Such drugs are devoid of the undesired central side effects that are typically mediated by the CB1 receptor. In this work we synthesized 18 biphenylic carboxamides as new CB2-selective ligands and evaluated their pharmacological profiles. The functional activity of these compounds is strongly influenced by the nature of the substituent at position 4' and 5 of the biphenyl scaffold. Position 5 seems to be responsible for the agonist or inverse agonist behaviour independently of the substituent in position 4', with the exception of the methoxyl group which transforms both full agonists and inverse agonists into neutral antagonists. This study provides a novel complete toolbox of CB2 functional modulators that derive from the same chemical scaffold. Such probes may be useful to investigate the biological role of CB2 receptors in cellular assays.

4'-O-methylhonokiol increases levels of 2-arachidonoyl glycerol in mouse brain via selective inhibition of its COX-2-mediated oxygenation

BACKGROUND AND PURPOSE

4'-O-methylhonokiol (MH) is a natural product showing anti-inflammatory, anti-osteoclastogenic, and neuroprotective effects. MH was reported to modulate cannabinoid CB2 receptors as an inverse agonist for cAMP production and an agonist for intracellular [Ca2+]. It was recently shown that MH inhibits cAMP formation via CB2 receptors. In this study, the exact modulation of MH on CB2 receptor activity was elucidated and its endocannabinoid substrate-specific inhibition (SSI) of cyclooxygenase-2 (COX-2) and CNS bioavailability are described for the first time.

METHODS

CB2 receptor modulation ([35S]GTPγS, cAMP, and β-arrestin) by MH was measured in hCB2-transfected CHO-K1 cells and native conditions (HL60 cells and mouse spleen). The COX-2 SSI was investigated in RAW264.7 cells and in Swiss albino mice by targeted metabolomics using LC-MS/MS.

RESULTS

MH is a CB2 receptor agonist and a potent COX-2 SSI. It induced partial agonism in both the [35S]GTPγS binding and β-arrestin recruitment assays while being a full agonist in the cAMP pathway. MH selectively inhibited PGE2 glycerol ester formation (over PGE2) in RAW264.7 cells and significantly increased the levels of 2-AG in mouse brain in a dose-dependent manner (3 to 20 mg kg(-1)) without affecting other metabolites. After 7 h from intraperitoneal (i.p.) injection, MH was quantified in significant amounts in the brain (corresponding to 200 to 300 nM).

CONCLUSIONS

LC-MS/MS quantification shows that MH is bioavailable to the brain and under condition of inflammation exerts significant indirect effects on 2-AG levels. The biphenyl scaffold might serve as valuable source of dual CB2 receptor modulators and COX-2 SSIs as demonstrated by additional MH analogs that show similar effects. The combination of CB2 agonism and COX-2 SSI offers a yet unexplored polypharmacology with expected synergistic effects in neuroinflammatory diseases, thus providing a rationale for the diverse neuroprotective effects reported for MH in animal models.

Endocannabinoid transport revisited

Endocannabinoids are arachidonic acid-derived endogenous lipids that activate the endocannabinoid system which plays a major role in health and disease. The primary endocannabinoids are anandamide (AEA, N-arachidonoylethanolamine) and 2-arachidonoyl glycerol. While their biosynthesis and metabolism have been studied in detail, it remains unclear how endocannabinoids are transported across the cell membrane. In this review, we critically discuss the different models of endocannabinoid trafficking, focusing on AEA cellular uptake which is best studied. The evolution of the current knowledge obtained with different AEA transport inhibitors is reviewed and the confusions caused by the lack of their specificity discussed. A comparative summary of the most important AEA uptake inhibitors and the studies involving their use is provided. Based on a comprehensive literature analysis, we propose a model of facilitated AEA membrane transport followed by intracellular shuttling and sequestration. We conclude that novel and more specific probes will be essential to identify the missing targets involved in endocannabinoid membrane transport.

Differential β-arrestin2 requirements for constitutive and agonist-induced internalization of the CB1 cannabinoid receptor

CB1 cannabinoid receptor (CB1R) undergoes both constitutive and agonist-induced internalization, but the underlying mechanisms of these processes and the role of β-arrestins in the regulation of CB1R function are not completely understood. In this study, we followed CB1R internalization using confocal microscopy and bioluminescence resonance energy transfer measurements in HeLa and Neuro-2a cells. We found that upon activation CB1R binds β-arrestin2 (β-arr2), but not β-arrestin1. Furthermore, both the expression of dominant-negative β-arr2 (β-arr2-V54D) and siRNA-mediated knock-down of β-arr2 impaired the agonist-induced internalization of CB1R. In contrast, neither β-arr2-V54D nor β-arr2-specific siRNA had a significant effect on the constitutive internalization of CB1R. However, both constitutive and agonist-induced internalization of CB1R were impaired by siRNA-mediated depletion of clathrin heavy chain. We conclude that although clathrin is required for both constitutive and agonist-stimulated internalization of CB1R, β-arr2 binding is only required for agonist-induced internalization of the receptor suggesting that the molecular mechanisms underlying constitutive and agonist-induced internalization of CB1R are different.

Modulating the endocannabinoid system in human health and disease--successes and failures

The discovery of the endocannabinoid system, comprising the G-protein coupled cannabinoid 1 and 2 receptors (CB1/2), their endogenous lipid ligands or endocannabinoids, and synthetic and metabolizing enzymes, has triggered an avalanche of experimental studies implicating the endocannabinoid system in a growing number of physiological/pathological functions. These studies have also suggested that modulating the activity of the endocannabinoid system holds therapeutic promise for a broad range of diseases, including neurodegenerative, cardiovascular and inflammatory disorders; obesity/metabolic syndrome; cachexia; chemotherapy-induced nausea and vomiting; and tissue injury and pain, amongst others. However, clinical trials with globally acting CB1 antagonists in obesity/metabolic syndrome, and other studies with peripherally-restricted CB1/2 agonists and inhibitors of the endocannabinoid metabolizing enzyme in pain, have introduced unexpected complexities, suggesting that a better understanding of the pathophysiological role of the endocannabinoid system is required to devise clinically successful treatment strategies.

Identification and quantification of a new family of peptide endocannabinoids (Pepcans) showing negative allosteric modulation at CB1 receptors

The α-hemoglobin-derived dodecapeptide RVD-hemopressin (RVDPVNFKLLSH) has been proposed to be an endogenous agonist for the cannabinoid receptor type 1 (CB(1)). To study this peptide, we have raised mAbs against its C-terminal part. Using an immunoaffinity mass spectrometry approach, a whole family of N-terminally extended peptides in addition to RVD-Hpα were identified in rodent brain extracts and human and mouse plasma. We designated these peptides Pepcan-12 (RVDPVNFKLLSH) to Pepcan-23 (SALSDLHAHKLRVDPVNFKLLSH), referring to peptide length. The most abundant Pepcans found in the brain were tested for CB(1) receptor binding. In the classical radioligand displacement assay, Pepcan-12 was the most efficacious ligand but only partially displaced both [(3)H]CP55,940 and [(3)H]WIN55,212-2. The data were fitted with the allosteric ternary complex model, revealing a cooperativity factor value α < 1, thus indicating a negative allosteric modulation. Dissociation kinetic studies of [(3)H]CP55,940 in the absence and presence of Pepcan-12 confirmed these results by showing increased dissociation rate constants induced by Pepcan-12. A fluorescently labeled Pepcan-12 analog was synthesized to investigate the binding to CB(1) receptors. Competition binding studies revealed K(i) values of several Pepcans in the nanomolar range. Accordingly, using competitive ELISA, we found low nanomolar concentrations of Pepcans in human plasma and ∼100 pmol/g in mouse brain. Surprisingly, Pepcan-12 exhibited potent negative allosteric modulation of the orthosteric agonist-induced cAMP accumulation, [(35)S]GTPγS binding, and CB(1) receptor internalization. Pepcans are the first endogenous allosteric modulators identified for CB(1) receptors. Given their abundance in the brain, Pepcans could play an important physiological role in modulating endocannabinoid signaling.

Cannabinoid receptors CB1 and CB2 form functional heteromers in brain

Exploring the role of cannabinoid CB(2) receptors in the brain, we present evidence of CB(2) receptor molecular and functional interaction with cannabinoid CB(1) receptors. Using biophysical and biochemical approaches, we discovered that CB(2) receptors can form heteromers with CB(1) receptors in transfected neuronal cells and in rat brain pineal gland, nucleus accumbens, and globus pallidus. Within CB(1)-CB(2) receptor heteromers expressed in a neuronal cell model, agonist co-activation of CB(1) and CB(2) receptors resulted in a negative cross-talk in Akt phosphorylation and neurite outgrowth. Moreover, one specific characteristic of CB(1)-CB(2) receptor heteromers consists of both the ability of CB(1) receptor antagonists to block the effect of CB(2) receptor agonists and, conversely, the ability of CB(2) receptor antagonists to block the effect of CB(1) receptor agonists, showing a bidirectional cross-antagonism phenomenon. Taken together, these data illuminate the mechanism by which CB(2) receptors can negatively modulate CB(1) receptor function.