G Protein-coupled Receptor 55 Expression Research Articles

GPR55 activation improves anxiety- and depression-like behaviors of mice during methamphetamine withdrawal

Methamphetamine is a potent and highly addictive neurotoxic psychostimulant that triggers a spectrum of adverse emotional responses during withdrawal. G-protein coupled receptor 55 (GPR55), a novel endocannabinoid receptor, is closely associated with mood regulation. Herein, we developed a murine model of methamphetamine-induced anxiety- and depressive-like behavior during abstinence which showed a decreased GPR55 expression in the hippocampus. Activation of GPR55 mitigated these behavioral symptoms, concomitantly ameliorating impairments in hippocampal neurogenesis and reducing neuroinflammation. These findings underscore the pivotal role of GPR55 in mediating the neuropsychological consequences of methamphetamine withdrawal, potentially via mechanisms involving the modulation of hippocampal neurogenesis and inflammation.

The peripheral endocannabinoid system and its association with biomarkers of inflammation in untreated patients with multiple sclerosis.

The endocannabinoid system (ECS) has been found altered in patients with multiple sclerosis (MS). However, whether the ECS alteration is present in the early stage of MS remains unknown. First, we aimed to compare the ECS profile between newly diagnosed MS patients and healthy controls (HCs). Next, we explored the association of the ECS, biomarkers of inflammation, and clinical parameters in newly diagnosed MS patients. Whole blood gene expression of ECS components and levels of endocannabinoids in plasma were measured by real-time quantitative polymerase chain reaction and ultra-high-pressure liquid chromatography-mass spectrometry, respectively, in 66 untreated MS patients and 46 HCs. No differences were found in the gene expression or plasma levels of the selected ECS components between newly diagnosed MS patients and HCs. Interferon-γ, encoded by the gene IFNG, correlated positively (ρ = 0.60) with the expression of G protein-coupled receptor 55 (GPR55), and interleukin1β (IL1B) correlated negatively (ρ = -0.50) with cannabinoid receptor 2 (CNR2) in HCs. We found no alteration in the peripheral ECS between untreated patients with MS and HC. Furthermore, our results indicate that the ECS has a minor overall involvement in the early stage of MS on inflammatory markers and clinical parameters when compared with HCs.

The Expression of the Endocannabinoid Receptors CB2 and GPR55 Is Highly Increased during the Progression of Alzheimer's Disease in AppNL-G-F Knock-In Mice.

The endocannabinoid system (ECS) and associated lipid transmitter-based signaling systems play an important role in modulating brain neuroinflammation. ECS is affected in neurodegenerative disorders, such as Alzheimer's disease (AD). Here we have evaluated the non-psychotropic endocannabinoid receptor type 2 (CB2) and lysophosphatidylinositol G-protein-coupled receptor 55 (GPR55) localization and expression during Aβ-pathology progression. Hippocampal gene expression of CB2 and GPR55 was explored by qPCR analysis, and brain distribution was evaluated by immunofluorescence in the wild type (WT) and APP knock-in AppNL-G-F AD mouse model. Furthermore, the effects of Aβ42 on CB2 and GPR55 expression were assessed in primary cell cultures. CB2 and GPR55 mRNA levels were significantly upregulated in AppNL-G-F mice at 6 and 12 months of age, compared to WT. CB2 was highly expressed in the microglia and astrocytes surrounding the Aβ plaques. Differently, GPR55 staining was mainly detected in neurons and microglia but not in astrocytes. In vitro, Aβ42 treatment enhanced CB2 receptor expression mainly in astrocytes and microglia cells, whereas GPR55 expression was enhanced primarily in neurons. These data show that Aβ pathology progression, particularly Aβ42, plays a crucial role in increasing the expression of CB2 and GPR55 receptors, supporting CB2 and GPR55 implications in AD.

The neuroprotective effects of GPR55 against hippocampal neuroinflammation and impaired adult neurogenesis in CSDS mice

Depression is one of the most prevalent mental illnesses in the world today, and the onset of depression is usually accompanied by neuroinflammation and impaired adult neurogenesis. As a new potential member of the endocannabinoid (eCB) system, G protein coupled receptor 55 (GPR55) has been associated with mood regulation. However, the role of GPR55 in the pathophysiology of depression remains poorly understood. Thus, a 10-day chronic social defeat stress (CSDS) paradigm was utilized as an animal model of depression to explore the potential role of GPR55 in depression. After CSDS, the protein level of GPR55 decreased significantly, but the mRNA expression did not change significantly, highlighting that although the GPR55 protein was involved in the progression of the depression- and anxiety-like phenotypes, its mRNA was not. Additionally, depression- and anxiety-like behaviors were also accompanied by neuroinflammation and impaired adult neurogenesis in the hippocampus. Interestingly, O-1602, a GPR55 agonist, remarkably prevented the development of depression- and anxiety-like behaviors as well as hippocampal neuroinflammation and neurogenesis deficits induced by CSDS. However, after electroacupuncture (EA) alleviated depression- and anxiety-like behaviors induced by CSDS, treatment with a GPR55 antagonist (CID16020046) reversed this effect. Our research demonstrated that downregulation of GPR55 expression in the hippocampus might mediate CSDS-induced depression- and anxiety-like phenotypes, and activation and upregulation of GPR55, which might be correlated with its anti-inflammatory and subsequent neuroprotective effects, could be a potential treatment for depression.

Antagonism of G protein-coupled receptor 55 prevents lipopolysaccharide-induced damages in human dental pulp cells.

Pulpitis is a common oral inflammatory disease in dental pulp commonly associated with bacterial infection. G protein-coupled receptor 55 (GPR55) is a member of the G protein-coupled receptors family that has been found to regulate inflammatory response. However, its roles in dental pulp inflammation have not been investigated. In this study, we used lipopolysaccharide (LPS) to induce inflammation in human dental pulp cells (hDPCs) to simulate an in vitro model of pulpitis. We found that LPS markedly induced the GPR55 expression in hDPCs. Treatment with the GPR55 antagonist ML-193 ameliorated the LPS-caused decrease in cell viability and increase in lactate dehydrogenase release. The upregulated inflammatory cytokines, interleukin-6 (IL-6) and tumour necrosis factor α, in LPS-challenged hDPCs were also attenuated by ML-193. Treatment with ML-193 ameliorated LPS-induced production of the inflammatory mediators cyclooxygenase-2/prostaglandin E2 (COX-2/PGE2), and inducible nitric oxide synthase/nitric oxide (iNOS/NO) in hDPCs. ML-193 also inhibited the activation of Toll-like receptor 4-myeloid differentiation primary response 88-nuclear factor-κB (TLR4-Myd88-NF-κB) signaling in LPS-induced hDPCs via decreased expressions of TLR4, Myd88, and p-NF-κB 65. Our study provides evidence that the GPR55 antagonist ML-193 exhibited anti-inflammatory activity in LPS-stimulated hDPCs through inhibiting TLR4-Myd88-NF-κB signaling. The results imply that ML-193 might be a novel agent for pulpitis.

Gene Expression Data Mining Reveals the Involvement of GPR55 and Its Endogenous Ligands in Immune Response, Cancer, and Differentiation.

G protein-coupled receptor 55 (GPR55) is a recently deorphanized lipid- and peptide-sensing receptor. Its lipidic endogenous agonists belong to lysoglycerophospholipids, with lysophosphatidylinositol (LPI) being the most studied. Peptide agonists derive from fragmentation of pituitary adenylate cyclase-activating polypeptide (PACAP). Although GPR55 and its ligands were implicated in several physiological and pathological conditions, their biological function remains unclear. Thus, the aim of the study was to conduct a large-scale re-analysis of publicly available gene expression datasets to identify physiological and pathological conditions affecting the expression of GPR55 and the production of its ligands. The study revealed that regulation of GPR55 occurs predominantly in the context of immune activation pointing towards the role of the receptor in response to pathogens and in immune cell lineage determination. Additionally, it was revealed that there is almost no overlap between the experimental conditions affecting the expression of GPR55 and those modulating agonist production. The capacity to synthesize LPI was enhanced in various types of tumors, indicating that cancer cells can hijack the motility-related activity of GPR55 to increase aggressiveness. Conditions favoring accumulation of PACAP-derived peptides were different than those for LPI and were mainly related to differentiation. This indicates a different function of the two agonist classes and possibly the existence of a signaling bias.

Expression of Lysophosphatidylinositol Signaling-relevant Molecules in Colorectal Cancer.

Lysophosphatidylinositol (LPI) is a subspecies of the lysophospholipid mediators produced when phospholipase hydrolyzes membrane phosphatidylinositol. Previously, we used mass spectrometry-based lipidomics to demonstrate that LPI is selectively elevated in colorectal cancer (CRC) tissues. Here, we hypothesized that the expression levels of the LPI biosynthetic enzyme and LPI receptor - DDHD domain containing 1 (DDHD1) and G protein-coupled receptor 55 (GPR55), respectively - may be correlated with malignant potential, and we evaluated their roles in the context of CRC. Colorectal specimens from 92 CRC patients underwent DDHD1 and GPR55 immunolabeling. Correlation between protein expression levels and clinicopathological variables was examined. Depth of tumor invasion was positively correlated with DDHD1 expression. Regardless of the degree of invasion depth, GPR55 was highly expressed in CRC tissues. Neither DDHD1 nor GPR55 expression levels were associated with disease-free survival. DDHD1 expression is associated with depth of tumor invasion in CRC tissues and may be involved in tumor progression.

Effects of O-1602 and CBD on TNBS-induced colonic disturbances.

This study attempted to provide the effects and mechanisms of two cannabinoids, O-1602 and cannabidiol (CBD), on colonic motility of 2,4,6-trinitro-benzene sulfonic acid (TNBS) colitis. TNBS was used to induce the model of motility disorder. G protein-coupled receptor 55 (GPR55) expression was detected using real-time PCR and immunohistochemistry in colon. Pro-inflammatory cytokines and myeloperoxidase were also measured. The colonic motility was measured by upper GI transit in vivo and recorded using electrical stimulation organ bath technique in vitro. Freshly isolated smooth muscle from the rat colon were applied to determine the membrane potential and Ca2+ -ATPase activity, respectively. CBD or O-1602 separately improved inflammatory conditions significantly in TNBS-induced colitis rats. However, sole CBD pretreatment reduced GPR55 expression, which was up-regulated in TNBS colitis. O-1602 and CBD each lowered MPO and IL-6 levels remarkably in TNBS colitis, while TNF-α levels experienced no change. CBD rescued the downward colonic motility in TNBS colitis in vivo; however, it decreased the upward contraction of the smooth muscle strip under electrical stimulation in vitro. Pretreatment with CBD prevented against TNBS-induced changes of Ca2+ -ATPase activity of smooth muscle cells. However, membrane potential of the smooth muscle cells decreased by TNBS experienced no change after O-1602 or CBD import. The present study suggested that CBD participated in the regulation of colonic motility in rats, and the mechanisms may be involved in the regulation of inlammatory factors andCa2+ -ATPase activity through GPR55.

Cellular localization and regulation of receptors and enzymes of the endocannabinoid system in intestinal and systemic inflammation

Surveys suggest that Cannabis provides benefit for people with inflammatory bowel disease. However, mechanisms underlying beneficial effects are not clear. We performed in situ hybridization RNAscope® combined with immunohistochemistry to show cell-specific distribution and regulation of cannabinoid receptor 1 and 2 (CB1, CB2), G protein-coupled receptor 55 (GPR55), and monoacylglycerol lipase (MGL) mRNA in immune cells using murine models of intestinal and systemic inflammation. In healthy animals, the presence in enteric ganglia is high for CB1 mRNA, but low for CB2 and GPR55 mRNAs. MGL mRNA is predominant throughout the intestinal wall including myenteric neurons, epithelium, circular and longitudinal muscular layers, and the lamina propria. Within the immune system, B220+ cells exhibit high gene expression for CB2 while the expression of CB2 in F4/80+ and CD3+ cells is less prominent. In contrast, GPR55 mRNA is highly present in F4/80+ and CD3+ cells. qRT-PCR of total colonic segments shows that the expression of GPR55 and MGL genes drops during intestinal inflammation. Also at cellular levels, GPR55 and MGL gene expression is reduced in F4/80+, but not CD3+ cells. As to systemic inflammation, reduced gene expression of MGL is observed in ileum by qRT-PCR, while at cellular levels, altered gene expression is also seen for CB1 and GPR55 in CD3+ but not F4/80+ cells. In summary, our study reveals changes in gene expression of members of the endocannabinoid system in situ attesting particularly GPR55 and MGL a distinct cellular role in the regulation of the immune response to intestinal and systemic inflammation.

The LPI/GPR55 axis enhances human breast cancer cell migration via HBXIP and p-MLC signaling.

The G protein-coupled receptor 55 (GPR55) is expressed in multiple tissues, and has been implicated in cancer pathogenesis, but little is known about its role in the migratory behavior of cancer cells, particularly breast cancer cells. In this study we first showed that GPR55 expression levels in 38 metastatic lymph nodes of breast cancer patients were profoundly elevated, and were positively associated in human breast cancer cells with their migratory ability. Moreover, the plasma levels of GPR55 endogenous agonist L-a-lysophosphatidylinositol (LPI) were significantly increased in breast cancer patients compared with healthy individuals. In human breast cancer LM-MCF-7 and MDA-MB-231 cells, treatment with LPI (2.5 μmol/L) significantly increased filopodia formation and resulted in cell migration, which could be blocked either by the GPR55 antagonist CID16020046 or by siRNA-mediated GPR55 knockdown. Furthermore, dual-luciferase report gene assays showed that GPR55 upregulated HBXIP at the promoter; GPR55 expression levels were positively correlated with HBXIP expression levels in breast cancer tissues and 8 breast cancer cell lines. We also showed that the LPI/GPR55 axis promoted the migration of breast cancer cells via two mutually exclusive pathways - the HBXIP/p-ERK1/2/Capn4 and MLCK/MLC signaling pathways. In xenograft nude mouse model, loss of GPR55 mainly affected breast cancer cell metastasis and the formation of metastatic foci. Thus, GPR55 is involved in the migratory behavior of human breast cancer cells and could serve as a pharmacological target for preventing metastasis.

Mechanisms of l-alpha-lysophosphatidylinositol-induced relaxation in human pulmonary arteries

Aimsl-Alpha-lysophosphatidylinositol (LPI) is an endogenous agonist of G protein-coupled receptor 55 (GPR55) which relaxes mesenteric arteries on activation. The aim of the present study was to determine the influence and underlying mechanisms of LPI-induced relaxation in human pulmonary arteries (hPAs). Main methodsFunctional studies were performed in isolated hPAs using organ bath technique. The expression of GPR55 in hPAs and bronchioles was determined by real-time qPCR, Western blot analysis, and immunohistochemistry. Key findingsLPI induced a concentration-dependent vasorelaxation in endothelium-intact hPAs. This effect was attenuated by the GPR55 antagonist CID16020046, the peroxisome proliferator-activated receptor-γ (PPARγ) antagonist GW9662, the putative endothelial cannabinoid receptor (CBe) antagonist O-1918 and the inhibitor of nitric oxide (NO) synthase (L-NAME). In addition, vasorelaxation was also attenuated by the presence of a high KCl concentration, selective blockers of small (KCa2.3; UCL1684), intermediate (KCa3.1; TRAM-34) and large conductance (KCa1.1; iberiotoxin) calcium-activated potassium channels and by endothelium denudation. However, vasorelaxation was not attenuated by the cannabinoid CB1 receptor antagonist AM251 or by the cyclooxygenase inhibitor indomethacin. SignificanceThe study showed that the LPI-induced vasorelaxation was endothelium-dependent and mediated by GPR55, PPARγ and CBe receptors, occurred in a NO- and calcium-activated potassium channel-dependent manner in isolated hPAs. LPI seems to possess positive, hypotensive properties in pulmonary vascular bed.

Advances in the Physiology of GPR55 in the Central Nervous System.

Background:The G protein-coupled receptor 55 (GPR55) is a mammalian orphan receptor that awaits a formal classification. There are an increasing number of reports directed to know the physiology and pathophysiology of this receptor. Lamentably, its functions in the central nervous system (CNS) have been scarcely elucidated.Methods:A bibliographic search in PubMed database about GPR55 actions in the CNS was made. The information was grouped for brain structures to facilitate the interpretation. Finally, we constructed a schematic representation of the current knowledge about the potential participation of GPR55 in some physiological and pathophysiological events.Results:Seventy nine papers were included in the review. Only few of them showed data about GPR55 (mRNA/protein) expression in multiple brain areas. The rest showed findings in different preparations both in vitro and in vivo conditions that allowed us to speculate a potential activity of GPR55 in the different brain areas.Conclusion:GPR55 mRNA is expressed in several brain areas as the hippocampus, hypothalamus, frontal cortex and cerebellum; but due to the lack of information, only some speculative information about its function in these regions has been suggested. Therefore, this review provide relevant information to motivate further research about GPR55 physiology/pathophysiology in the CNS.

Lysophosphatidylinositol-induced activation of the cation channel TRPV2 triggers glucagon-like peptide-1 secretion in enteroendocrine L cells

The lysophosphatidylinositol (LPI) has crucial roles in multiple physiological processes, including insulin exocytosis from pancreatic islets. However, the role of LPI in secretion of glucagon-like peptide-1 (GLP-1), a hormone that enhances glucose-induced insulin secretion, is unclear. Here, we used the murine enteroendocrine L cell line GLUTag and primary murine small intestinal cells to elucidate the mechanism of LPI-induced GLP-1 secretion. Exogenous LPI addition increased intracellular Ca2+ concentrations ([Ca2+] i ) in GLUTag cells and induced GLP-1 secretion from both GLUTag and acutely prepared primary intestinal cells. The [Ca2+] i increase was suppressed by an antagonist for G protein-coupled receptor 55 (GPR55) and by silencing of GPR55 expression, indicating involvement of Gq and G12/13 signaling pathways in the LPI-induced increased [Ca2+] i levels and GLP-1 secretion. However, GPR55 agonists did not mimic many of the effects of LPI. We also found that phospholipase C inhibitor and Rho-associated kinase inhibitor suppressed the [Ca2+] i increase and that LPI increased the number of focal adhesions, indicating actin reorganization. Of note, blockage or silencing of transient receptor potential cation channel subfamily V member 2 (TRPV2) channels suppressed both the LPI-induced [Ca2+] i increase and GLP-1 secretion. Furthermore, LPI accelerated TRPV2 translocation to the plasma membrane, which was significantly suppressed by a GPR55 antagonist. These findings suggest that TRPV2 activation via actin reorganization induced by Gq and G12/13 signaling is involved in LPI-stimulated GLP-1 secretion in enteroendocrine L cells. Because GPR55 agonists largely failed to mimic the effects of LPI, its actions on L cells are at least partially independent of GPR55 activation.

G protein-coupled receptor 55 (GPR55) expresses differently in patients with Crohn’s disease and ulcerative colitis

Aim: To investigate the levels of G protein-coupled receptor 55 (GPR55) expression in colonic tissue of inflammatory bowel disease (IBD) patients and healthy controls, and its potential implication in IBD treatment.Methods: Fifty patients were enrolled in our prospective study: n = 21 with Crohn’s disease (CD) and n = 16 with ulcerative colitis (UC); 19 women and 18 men. Control consisted of 13 non-IBD patients. In each subject, two biopsies were taken from different colonic locations. In IBD patients, biopsies both from endoscopically inflamed and non-inflamed areas were drawn and the development of inflammation confirmed in histopathological examination. GPR55 mRNA and protein expression were measured using real-time PCR and Western blot, respectively.Results: GPR55 expression at mRNA and protein level was detected in all samples tested. The level of GPR55 mRNA expression in non-inflamed colonic areas was comparable in all analyzed groups (p = .2438). However, in the inflamed tissues GPR55 mRNA expression was statistically significantly (p < .0001) higher (6.9 fold) in CD patients compared to UC. Moreover, CD patients manifested higher (12.5 fold) GPR55 mRNA expression in inflamed compared with non-inflamed colonic tissues (p < .0001). Although no significant differences were stated, GPR55 protein level tends to decrease in IBD as compared to control.Conclusions: Different patterns of GPR55 expression at mRNA level were observed in IBD patients. We speculate that GPR55 is crucial for the mucosal inflammatory processes in IBD, particularly in CD and its expression may affect disease severity, and response to treatment. The GPR55 receptors may become an attractive target for novel therapeutic strategies in IBD.

A role for GPR55 in human placental venous endothelial cells.

Endocannabinoids and their G protein-coupled receptors have been suggested to play a key role in human pregnancy, by regulating important aspects such as implantation, decidualization, placentation and labor. G protein-coupled receptor 55 (GPR55) was previously postulated to be another cannabinoid receptor, since specific cannabinoids were shown to act independently of the classical cannabinoid receptors CB1 or CB2. Current knowledge about GPR55 expression and function in human placenta is very limited and motivated us to evaluate human placental GPR55 expression in relation to other human peripheral tissues and to analyze spatiotemporal GPR55 expression in human placenta. Gene expression analysis revealed low GPR55 levels in human placenta, when compared to spleen and lung, the organs showing highest GPR55 expression. Moreover, expression analysis showed 5.8 fold increased placental GPR55 expression at term compared to first trimester. Immunohistochemistry located GPR55 solely at the fetal endothelium of first trimester and term placentas. qPCR and immunocytochemistry consistently confirmed GPR55 expression in isolated primary placental arterial and venous endothelial cells. Incubation with L-α-lysophosphatidylinositol (LPI), the specific and functional ligand for GPR55, at a concentration of 1 µM, significantly enhanced migration of venous, but not arterial endothelial cells. LPI-enhanced migration was inhibited by the GPR55 antagonist O-1918, suggesting a role of the LPI-GPR55 axis in placental venous endothelium function.

Elevated cannabinoid receptor 1 and G protein-coupled receptor 55 expression in proximal tubule cells and whole kidney exposed to diabetic conditions.

Hyperglycaemia increases the risk of developing diabetic nephropathy, with primary targets in the glomerulus and proximal tubule. Importantly, glomerular damage in the kidney leads to elevated albumin levels in the filtrate, which contributes to tubular structural modifications that lead to dysfunction. Diabetes alters the endocannabinoid system in a number of target organs, with previous research characterizing tissue-specific changes in the expression of the cannabinoid receptor 1 (CB1 ) and G protein-coupled receptor 55 (GPR55), a putative cannabinoid receptor, in diabetes. Although these receptors have a functional role in the cannabinoid system in the kidney, there has been little investigation into changes in the expression of CB1 and GPR55 in the proximal tubule under diabetic conditions. In this study, CB1 and GPR55 messenger RNA and protein levels were quantified in cultured human kidney cells and then treated with either elevated glucose, elevated albumin, or a combination of glucose and albumin for 4, 6, 18, or 24h. In addition, CB1 and GPR55 protein expression was characterized in whole-kidney lysate from streptozotocin-induced diabetic Sprague-Dawley rats. In vitro exposure to elevated glucose and albumin increased CB1 and GPR55 messenger RNA and protein expression in proximal tubule cells in a time-dependant manner. In whole kidney of streptozotocin-induced diabetic rats, CB1 protein was upregulated, whereas GPR55 protein concentration was not altered. Thus, expression of CB1 and GPR55 in proximal tubules is altered in response to elevated levels of glucose and albumin. Further investigations should determine if these receptors are effective physiological targets for the treatment and prevention of diabetic nephropathy.

Co-Localization of Alpha1-Adrenoceptors and GPR55: A Novel Prostate Cancer Paradigm?

α1-adrenoceptors (α1-ARs) and “cannabinoid-like” G Protein Coupled Receptor 55 (GPR55) belong to the G-protein coupled receptor (GPCR) family and play a crucial role in regulating prostate function. Although physical and functional interactions between the cannabinoid and adrenergic systems have been reported, analysis of functional interactions between α1-AR and GPR55 in normal and neoplastic prostate has not been reported. Since GPR55 levels are high in rodent adrenal gland, we propose a function link between the adrenergic system and GPR55 receptor. Confocal Laser Scanning Microscopy (CLSM) was employed to examine the endogenous α1-AR and GPR55 expression and their co-localization, expressed as fluorescence, in vitro in human andro-gen-insensitive PC-3 and androgen-sensitive LNCaP prostatic carcinoma cell lines, using the fluo-rescent ligands—Syto 62 (nuclear stain), BODIPY FL-Prazosin (QAPB; fluorescent quinazoline α1-AR ligand) and Tocriflour (T1117; a novel fluorescent diarylpyrazole cannabinoid/GPR55 ligand). Fluorescent ligand binding in untreated PC-3 cells and LNCaP cells and spheroids showed hetero-geneous expression of both α1-ARs and GPR55. A small proportion of cells had both α1-ARs and GPR55 in relatively equal numbers indicating a degree of co-localization. Co-localization of fluo-rescent ligand binding exhibited a stronger correlation in LNCaP (0.87) as compared to PC-3 (0.63) cells. Upregulation of α1-AR was observed in PC-3 cells following chronic doxazosin incubation. Robust T1117 binding, suggestive of GPR55 upregulation, was also observed in these cells. The presence of subtype-rich cells with a degree of co-localization between α1-ARs and GPR55 indicates a possibility for dimerisation or functional interaction and a new paradigm for functional synergism in which interactions may be either between cells or involve converging intracellular signaling processes.

The differential characterization of GPR55 receptor in human peripheral blood reveals a distinctive expression in monocytes and NK cells and a proinflammatory role in these innate cells.

G protein-coupled receptor 55 (GPR55) is activated by endogenous, plant-derived and synthetic cannabinoids. Recent studies reported a broad tissue distribution for GPR55 and found prominent roles for this receptor in inflammatory pain, gut and bone physiology, as well as cancer. However, little is known about the expression and function of GPR55 in immune cells. To address this question, we performed a detailed characterization of GPR55 in different human innate and adaptive immune populations using polychromatic flow cytometry and we found that monocytes and NK cells expressed remarkable levels of this receptor compared to several cells of adaptive immunity. GPR55 activation by the specific agonist O-1602 boosted IL-12 and TNF-α production, and decreased endocytic activity, in LPS-activated monocytes. In addition, it increased CD69 activation marker expression, granzyme B and CD107a-dependent cytotoxicity and IFN-γ and TNF-α production in NK cells activated by both IL-2 and IL-12. These over-stimulatory effects of GPR55 were antagonized by its selective antagonist cannabidiol. Altogether, our data thus unveil a proinflammatory role for GPR55 in innate immunity that may be important for the design of new immune therapeutic strategies.

Lysophosphatidylinositol-GPR55 signaling in the regulation of metabolism and cardiometabolic disorders

G protein coupled receptor 55 (GPR55) was initially identified as the orphan receptor and then suggested to be the atypical non-CB1/non-CB2 cannabinoid receptor. However, later studies revealed that GPR55 is specific for lysophosphatidylinositol (LPI) synthesized from membrane phosphoinositides by phospholipases A1 or A2. Recent findings suggest that LPI-GPR55 signaling is involved in the pathogenesis of cardiometabolic disorders. GPR55 is expressed in adipose tissue, endothelial cells, inflammatory cells, and cardiomyocytes. Plasma LPI concentration and GPR55 expression in the adipose tissue are modified by nutritional status, leptin level and glucose tolerance. Ligand-activated GPR55 has anabolic effect in human adipose tissue, stimulates insulin secretion, induces hyperpolarization of endothelial cells, inhibits platelet aggregation, and increases Ca 2+ concentration in cultured cardiomyocytes. In this review the recent findings about role of GPR55 signaling in adipose tissue, metabolic homeostasis and cardiovascular system are summarized. Adipobiology 2013; 5: 23-31.

Rod Photoreceptors Express GPR55 in the Adult Vervet Monkey Retina

Cannabinoids exert their actions mainly through two receptors, the cannabinoid CB1 receptor (CB1R) and cannabinoid CB2 receptor (CB2R). In recent years, the G-protein coupled receptor 55 (GPR55) was suggested as a cannabinoid receptor based on its activation by anandamide and tetrahydrocannabinol. Yet, its formal classification is still a matter of debate. CB1R and CB2R expression patterns are well described for rodent and monkey retinas. In the monkey retina, CB1R has been localized in its neural (cone photoreceptor, horizontal, bipolar, amacrine and ganglion cells) and CB2R in glial components (Müller cells). The aim of this study was to determine the expression pattern of GPR55 in the monkey retina by using confocal microscopy. Our results show that GPR55 is strictly localized in the photoreceptor layer of the extrafoveal portion of the retina. Co-immunolabeling of GPR55 with rhodopsin, the photosensitive pigment in rods, revealed a clear overlap of expression throughout the rod structure with most prominent staining in the inner segments. Additionally, double-label of GPR55 with calbindin, a specific marker for cone photoreceptors in the primate retina, allowed us to exclude expression of GPR55 in cones. The labeling of GPR55 in rods was further assessed with a 3D visualization in the XZ and YZ planes thus confirming its exclusive expression in rods. These results provide data on the distribution of GPR55 in the monkey retina, different than CB1R and CB2R. The presence of GPR55 in rods suggests a function of this receptor in scotopic vision that needs to be demonstrated.