GPR35 is a novel lysophosphatidic acid receptor

Biased constitutive signaling of the G protein-coupled receptor GPR35 suppresses gut barrier permeability

Agonist-independent, or constitutive, activity is an integral feature of G protein-coupled receptors, but its relevance in pathophysiological settings is generally poorly explored. GPR35 is a therapeutic target in inflammatory diseases of the lower gut. In colonic organoids from a human GPR35a-expressing transgenic mouse line, the GPR35 inverse agonist CID-2745687 increased barrier permeability substantially, indicating that constitutive receptor activity contributes to maintaining epithelial barrier integrity. High constitutive activity of GPR35 was also observed in both HT-29 and HEPG2 cells that express GPR35 endogenously. Mechanistic investigations in recombinant in vitro systems revealed that the constitutive activity of GPR35a was biased and not equivalent across signaling pathways. Hence, no constitutive interactions of the receptor with arrestin-adaptor proteins or activation of Gαo-containing G protein heterotrimers were detected while, even at low GPR35a expression levels, substantial constitutive activation of heterotrimers containing either Gα12 or Gα13 was observed. Similar biased constitutive activity was observed for the human GPR35b isoform. The extent of constitutive and agonist-mediated activity was dependent on receptor expression level. At high receptor levels, constitutive activation of Gα12 or Gα13 masked any agonist-induced effects while low expression levels with low constitutive activity allowed measurement of agonist-induced responses. These results highlight roles, selectivity, and the extent of constitutive activity of GPR35 in cells and tissues that express this receptor endogenously and highlight the contribution of its constitutive activity to maintaining the colonic epithelial barrier, potentially limiting the development of inflammatory bowel diseases.

The power of many: Multilevel targeting of representative chemokine and metabolite GPCRs in personalized cancer therapy

G protein-coupled receptors (GPCRs) are vital cell surface receptors that govern a myriad of physiological functions. Despite their crucial role in regulating antitumor immunity and tumorigenesis, therapeutic applications targeting GPCRs in oncology are currently limited. This review offers a focused examination of selected protumorigenic chemokine and metabolite-sensing GPCRs. Specifically, the review highlights five GPCRs able to orchestrate tumor immunobiology at three main levels: tumor immunity, cancer cell expansion, and blood vessel development. The review culminates by illuminating emerging therapies and discussing innovative strategies to harness the full potential of GPCR-targeted treatments, by applying a multireceptor and patient-specific logic.

Lysophosphatidic Acid Signaling in the Gastrointestinal System

The intestinal epithelium undergoes continuous homeostatic renewal to conduct the digestion and absorption of nutrients. At the same time, the intestinal epithelial barrier separates the host from the intestinal lumen, preventing systemic infection from enteric pathogens. To maintain homeostasis and epithelial functionality, stem cells, which reside in the base of intestinal crypts, generate progenitor cells that ultimately differentiate to produce an array of secretory and absorptive cells. Intestinal regeneration is regulated by niche signaling pathways, specifically, Wnt, bone morphogenetic protein, Notch, and epidermal growth factor. In addition, growth factors and other peptides have emerged as potential modulators of intestinal repair and inflammation through their roles in cellular proliferation, differentiation, migration, and survival. Lysophosphatidic acid (LPA) is such a factor that modulates the proliferation, survival, and migration of epithelial cells while also regulating trafficking of immune cells, both of which are important for tissue homeostasis. Perturbation of LPA signaling, however, has been shown to promote cancer and inflammation. This review focuses on the recent advances in LPA-mediated signaling that contribute to physiological and pathophysiological regulation of the gastrointestinal system.

From orphan to oncogene: The role of GPR35 in cancer and immune modulation

G protein-coupled receptors (GPCRs) are well-studied and the most traceable cell surface receptors for drug discovery. One of the intriguing members of this family is G protein-coupled receptors 35 (GPR35), which belongs to the class A rhodopsin-like family of GPCRs identified over two decades ago. GPR35 presents interesting features such as ubiquitous expression and distinct isoforms. Moreover, functional and genome-wide association studies on its widespread expression have linked GPR35 with pathophysiological disease progression. Various pieces of evidence have been accumulated regarding the independent or endogenous ligand-dependent role of GPR35 in cancer progression and metastasis. In the current scenario, the relationship of this versatile receptor and its putative endogenous ligands for the activation of oncogenic signal transduction pathways at the cellular level is an active area of research. These intriguing features offered by GPR35 make it an oncological target, justifying its uniqueness at the physiological and pathophysiological levels concerning other GPCRs. For pharmacologically targeting receptor-induced signaling, few potential competitive antagonists have been discovered that offer high selectivity at a human level. In addition to its fascinating features, targeting GPR35 at rodent and human orthologue levels is distinct, thus contributing to the sub-species selectivity. Strategies to modulate these issues will help us understand and truly target GPR35 at the therapeutic level. In this article, we have provided prospects on each topic mentioned above and suggestions to overcome the challenges. This review discusses the molecular mechanism and signal transduction pathways activated by endogenous ligands or spontaneous auto-activation of GPR35 that contributes towards disease progression. Furthermore, we have highlighted the GPR35 structure, ubiquitous expression, its role in immunomodulation, and at the pathophysiological level, especially in cancer, indicating its status as a versatile receptor. Subsequently, we discussed the various proposed ligands and their mechanism of interaction with GPR35. Additionally, we have summarized the GPR35 antagonist that provides insights into the opportunities for therapeutically targeting this receptor.

The Emerging Role of LPA as an Oncometabolite

Lysophosphatidic acid (LPA) is a phospholipid that displays potent signalling activities that are regulated in both an autocrine and paracrine manner. It can be found both extra- and intracellularly, where it interacts with different receptors to activate signalling pathways that regulate a plethora of cellular processes, including mitosis, proliferation and migration. LPA metabolism is complex, and its biosynthesis and catabolism are under tight control to ensure proper LPA levels in the body. In cancer patient specimens, LPA levels are frequently higher compared to those of healthy individuals and often correlate with poor responses and more aggressive disease. Accordingly, LPA, through promoting cancer cell migration and invasion, enhances the metastasis and dissemination of tumour cells. In this review, we summarise the role of LPA in the regulation of critical aspects of tumour biology and further discuss the available pre-clinical and clinical evidence regarding the feasibility and efficacy of targeting LPA metabolism for effective anticancer therapy.

Eriodictyol and thymonin act as GPR35 agonists

Although herbs and spices have been used in traditional medicine for more than a century owing to their health benefits, the associated underlying mechanism is still not clear. Since the G protein-coupled receptor 35 (GPR35) has been linked to exert various antioxidant and anti-inflammatory effects, we screened 19 different herbs and spices for possible GPR35 agonist(s) to understand the GPR35-dependent functions of herbs and spices. Among the screened extracts, the ethyl acetate extract of thyme exhibited a remarkable GPR35 agonistic activity. Activity-guided separations allowed us to identify 2 polyphenolic phytochemicals, eriodictyol and thymonin, acting as GPR35 agonists. Both eriodictyol and thymonin showed a potent and specific agonist activity toward GPR35 with half maximal effective concentration values of 5.48 and 8.41 µm, respectively. These findings indicate that these phytochemicals may have beneficial health effects upon GPR35 activation.

GPR35 acts a dual role and therapeutic target in inflammation

GPR35 is a G protein-coupled receptor with notable involvement in modulating inflammatory responses. Although the precise role of GPR35 in inflammation is not yet fully understood, studies have suggested that it may have both pro- and anti-inflammatory effects depending on the specific cellular environment. Some studies have shown that GPR35 activation can stimulate the production of pro-inflammatory cytokines and facilitate the movement of immune cells towards inflammatory tissues or infected areas. Conversely, other investigations have suggested that GPR35 may possess anti-inflammatory properties in the gastrointestinal tract, liver and certain other tissues by curbing the generation of inflammatory mediators and endorsing the differentiation of regulatory T cells. The intricate role of GPR35 in inflammation underscores the requirement for more in-depth research to thoroughly comprehend its functional mechanisms and its potential significance as a therapeutic target for inflammatory diseases. The purpose of this review is to concurrently investigate the pro-inflammatory and anti-inflammatory roles of GPR35, thus illuminating both facets of this complex issue.

Kynurenic Acid: A Novel Player in Cardioprotection against Myocardial Ischemia/Reperfusion Injuries

BACKGROUND

Myocardial infarction is one of the leading causes of mortality worldwide; hence, there is an urgent need to discover novel cardioprotective strategies. Kynurenic acid (KYNA), a metabolite of the kynurenine pathway, has been previously reported to have cardioprotective effects. However, the mechanisms by which KYNA may be protective are still unclear. The current study addressed this issue by investigating KYNA's cardioprotective effect in the context of myocardial ischemia/reperfusion.

METHODS

H9C2 cells and rats were exposed to hypoxia/reoxygenation or myocardial infarction, respectively, in the presence or absence of KYNA. In vitro, cell death was quantified using flow cytometry analysis of propidium iodide staining. In vivo, TTC-Evans Blue staining was performed to evaluate infarct size. Mitochondrial respiratory chain complex activities were measured using spectrophotometry. Protein expression was evaluated by Western blot, and mRNA levels by RT-qPCR.

RESULTS

KYNA treatment significantly reduced H9C2-relative cell death as well as infarct size. KYNA did not exhibit any effect on the mitochondrial respiratory chain complex activity. SOD2 mRNA levels were increased by KYNA. A decrease in p62 protein levels together with a trend of increase in PARK2 may mark a stimulation of mitophagy. Additionally, ERK1/2, Akt, and FOXO3α phosphorylation levels were significantly reduced after the KYNA treatment. Altogether, KYNA significantly reduced myocardial ischemia/reperfusion injuries in both in vitro and in vivo models.

CONCLUSION

Here we show that KYNA-mediated cardioprotection was associated with enhanced mitophagy and antioxidant defense. A deeper understanding of KYNA's cardioprotective mechanisms is necessary to identify promising novel therapeutic targets and their translation into the clinical arena.

GPR35 and mediators from platelets and mast cells in neutrophil migration and inflammation

Neutrophil recruitment from circulation to sites of inflammation is guided by multiple chemoattractant cues emanating from tissue cells, immune cells, and platelets. Here, we focus on the function of one G-protein coupled receptor, GPR35, in neutrophil recruitment. GPR35 has been challenging to study due the description of multiple ligands and G-protein couplings. Recently, we found that GPR35-expressing hematopoietic cells respond to the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). We discuss distinct response profiles of GPR35 to 5-HIAA compared to other ligands. To place the functions of 5-HIAA in context, we summarize the actions of serotonin in vascular biology and leukocyte recruitment. Important sources of serotonin and 5-HIAA are platelets and mast cells. We discuss the dynamics of cell migration into inflamed tissues and how multiple platelet and mast cell-derived mediators, including 5-HIAA, cooperate to promote neutrophil recruitment. Additional actions of GPR35 in tissue physiology are reviewed. Finally, we discuss how clinically approved drugs that modulate serotonin uptake and metabolism may influence 5-HIAA-GPR35 function, and we speculate about broader influences of the GPR35 ligand-receptor system in immunity and disease.

Recent advances in GPR35 pharmacology; 5-HIAA serotonin metabolite becomes a ligand

GPR35, an orphan receptor, has been waiting for its ligand since its cloning in 1998. Many endogenous and exogenous molecules have been suggested to act as agonists of GPR35 including kynurenic acid, zaprinast, lysophosphatidic acid, and CXCL17. However, complex and controversial responses to ligands among species have become a huge hurdle in the development of therapeutics in addition to the orphan state. Recently, a serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), is reported to be a high potency ligand for GPR35 by investigating the increased expression of GPR35 in neutrophils. In addition, a transgenic knock-in mouse line is developed, in which GPR35 was replaced with a human ortholog, making it possible not only to overcome the different selectivity of agonists among species but also to conduct therapeutic experiments on human GPR35 in mouse models. In the present article, I review the recent advances and prospective therapeutic directions in GPR35 research. Especially, I'd like to draw attention of readers to the finding of 5-HIAA as a ligand of GPR35 and lead to apply the 5-HIAA and human GPR35 knock-in mice to their research fields in a variety of pathophysiological conditions.

What defines a chemokine? - The curious case of CXCL17

Chemotactic cytokines (chemokines) are a group of around 40 small proteins which share a similar protein fold and are well known for their ability to direct the migration of leukocytes to a variety of tissue locations. CXCL17 was the last member of the chemokine family to be assigned and was admitted to the family based on theoretical modelling of the CXCL17 structure and chemotactic activity for monocytes and dendritic cells. Of Interest, CXCL17 expression appears to be restricted to mucosal tissues such as the tongue, stomach and lung, suggestive of specific roles at these locations. A putative CXCL17 receptor, GPR35 was reportedly identified and mice deficient in CXCL17 were generated and characterised. More recently, however, some apparent contradictions regarding aspects of CXCL17 biology have been raised by ourselves and others. Notably, GPR35 appears to be a receptor for the serotonin metabolite 5-hydroxyindoleacetic acid rather than for CXCL17 and modelling of CXCL17 using a variety of platforms fails to identify a chemokine-like fold. In this article, we summarize the discovery of CXCL17 and discuss key papers describing the subsequent characterisation of this protein. Ultimately, we pose the question, 'What defines a chemokine?' (185 words).

GPR35: from enigma to therapeutic target

The orphan G-protein-coupled receptor 35 (GPR35), although poorly characterised, is attracting considerable interest as a therapeutic target. Marked differences in pharmacology between human and rodent orthologues of the receptor and a dearth of antagonists with affinity for mouse and rat GPR35 have previously restricted the use of preclinical disease models. The development of improved ligands, novel transgenic knock-in mouse lines, and detailed analysis of the disease relevance of single-nucleotide polymorphisms (SNPs) have greatly enhanced understanding of the key roles of GPR35 and have stimulated efforts towards disease-targeted proof-of-concept studies. In this opinion article, new information on the biology of the receptor is considered, whilst insight into how GPR35 is currently being assessed for therapeutic utility - in areas ranging from inflammatory bowel diseases to nonalcoholic steatohepatitis and various cancers - is also provided.

Pharmacognosy and Effects of Cannabinoids in the Vascular System

Understanding the pharmacodynamics of cannabinoids is an essential subject due to the recent increasing global acceptance of cannabis and its derivation for recreational and therapeutic purposes. Elucidating the interaction between cannabinoids and the vascular system is critical to exploring cannabinoids as a prospective therapeutic agent for treating vascular-associated clinical conditions. This review aims to examine the effect of cannabinoids on the vascular system and further discuss the fundamental pharmacological properties and mechanisms of action of cannabinoids in the vascular system. Data from literature revealed a substantial interaction between endocannabinoids, phytocannabinoids, and synthetic cannabinoids within the vasculature of both humans and animal models. However, the mechanisms and the ensuing functional response is blood vessels and species-dependent. The current understanding of classical cannabinoid receptor subtypes and the recently discovered atypical cannabinoid receptors and the development of new synthetic analogs have further enhanced the pharmacological characterization of the vascular cannabinoid receptors. Compelling evidence also suggest that cannabinoids represent a formidable therapeutic candidate for vascular-associated conditions. Nonetheless, explanations of the mechanisms underlining these processes are complex and paradoxical based on the heterogeneity of receptors and signaling pathways. Further insight from studies that uncover the mechanisms underlining the therapeutic effect of cannabinoids in the treatment of vascular-associated conditions is required to determine whether the known benefits of cannabinoids thus currently outweigh the known/unknown risks.

Lysophosphatidylinositol Induced Morphological Changes and Stress Fiber Formation through the GPR55-RhoA-ROCK Pathway

We previously reported that lysophosphatidylinositol (LPI) functions as an endogenous agonist of GPR55, a novel cannabinoid receptor. However, the physiological roles of LPI-GPR55 have not yet been elucidated in detail. In the present study, we found that LPI induced morphological changes in GPR55-expressing HEK293 cells. LPI induced the cell rounding of GPR55-expressing HEK293 cells but not of empty-vector-transfected cells. LPI also induced the activation of small GTP-binding protein RhoA and increased stress fiber formation in GPR55-expressing HEK293 cells. The inhibition of RhoA and Rho kinase ROCK by the C3 exoenzyme and the ROCK inhibitor reduced LPI-induced cell rounding and stress fiber formation. These results clearly indicated that the LPI-induced morphological changes and the assembly of the cytoskeletons were mediated through the GPR55-RhoA-ROCK pathway.

GPR35 promotes neutrophil recruitment in response to serotonin metabolite 5-HIAA

Rapid neutrophil recruitment to sites of inflammation is crucial for innate immune responses. Here, we reveal that the G-protein-coupled receptor GPR35 is upregulated in activated neutrophils, and it promotes their migration. GPR35-deficient neutrophils are less recruited from blood vessels into inflamed tissue, and the mice are less efficient in clearing peritoneal bacteria. Using a bioassay, we find that serum and activated platelet supernatant stimulate GPR35, and we identify the platelet-derived serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) as a GPR35 ligand. GPR35 function in neutrophil recruitment is strongly dependent on platelets, with the receptor promoting transmigration across platelet-coated endothelium. Mast cells also attract GPR35+ cells via 5-HIAA. Mice deficient in 5-HIAA show a loss of GPR35-mediated neutrophil recruitment to inflamed tissue. These findings identify 5-HIAA as a GPR35 ligand and neutrophil chemoattractant and establish a role for platelet- and mast cell-produced 5-HIAA in cell recruitment to the sites of inflammation and bacterial clearance.

GPR35 in Intestinal Diseases: From Risk Gene to Function

Diet and gut microbial metabolites mediate host immune responses and are central to the maintenance of intestinal health. The metabolite-sensing G-protein coupled receptors (GPCRs) bind metabolites and trigger signals that are important for the host cell function, survival, proliferation and expansion. On the contrary, inadequate signaling of these metabolite-sensing GPCRs most likely participate to the development of diseases including inflammatory bowel diseases (IBD). In the intestine, metabolite-sensing GPCRs are highly expressed by epithelial cells and by specific subsets of immune cells. Such receptors provide an important link between immune system, gut microbiota and metabolic system. Member of these receptors, GPR35, a class A rhodopsin-like GPCR, has been shown to be activated by the metabolites tryptophan-derived kynurenic acid (KYNA), the chemokine CXCL17 and phospholipid derivate lysophosphatidic acid (LPA) species. There have been studies on GPR35 in the context of intestinal diseases since its identification as a risk gene for IBD. In this review, we discuss the pharmacology of GPR35 including its proposed endogenous and synthetic ligands as well as its antagonists. We elaborate on the risk variants of GPR35 implicated in gut-related diseases and the mechanisms by which GPR35 contribute to intestinal homeostasis.

Novel Role of GPR35 (G-Protein-Coupled Receptor 35) in the Regulation of Endothelial Cell Function and Blood Pressure

Supplemental Digital Content is available in the text.

GPR35 (G-protein–coupled receptor 35) is a poorly characterized receptor that has garnered increased interest as a therapeutic target through its implications in a range of inflammatory and cardiovascular diseases, but its biological functions stay largely unknown. The current study evaluated the effect of GPR35 on endothelial cell (EC) functions and hemodynamic homeostasis. In primary human aortic ECs, the expression of GPR35 was manipulated by transfections of adenovirus carrying either GPR35 cDNA or shRNA against GPR35, using adenovirus carrying β-gal as control. Mouse aortic ECs were isolated and cultured from GPR35 knockout and wild-type control mice. Our results indicated that genetic inhibition of GPR35 in human and mouse ECs significantly promoted cell proliferation, migration, and tube formation in vitro. The GCH1 (guanosine triphosphate cyclohydrolase I)-mediated biosynthesis of tetrahydrobiopterin was enhanced, reducing intracellular superoxide. Knocking down GCH1 or eNOS (endothelial nitric oxide synthase) significantly blunted the robust angiogenesis induced by GPR35 suppression. Male GPR35 knockout mice demonstrated reduced basal arterial blood pressure and an attenuated onset of hypertension in deoxycorticosterone acetate-salt induced hypertensive model compared with male GPR35 wild-type control mice in vivo, with concomitant improved endothelium-dependent vasodilation and decreased superoxide in isolated aortas. The difference in arterial blood pressure was absent between female GPR35 wild-type control and female GPR35 knockout mice. Our study provides novel insights into the roles of GPR35 in endothelial function and vascular tone modulation that critically contribute to the pathophysiology of blood pressure elevation. Antagonizing GPR35 activity might represent a potentially effective therapeutic approach to restore EC function and hemodynamic homeostasis.

Lysophosphatidic Acid-Mediated GPR35 Signaling in CX3CR1+ Macrophages Regulates Intestinal Homeostasis

Single-nucleotide polymorphisms in the gene encoding G protein-coupled receptor 35 (GPR35) are associated with increased risk of inflammatory bowel disease. However, the mechanisms by which GPR35 modulates intestinal immune homeostasis remain undefined. Here, integrating zebrafish and mouse experimental models, we demonstrate that intestinal Gpr35 expression is microbiota dependent and enhanced upon inflammation. Moreover, murine GPR35⁺ colonic macrophages are characterized by enhanced production of pro-inflammatory cytokines. We identify lysophosphatidic acid (LPA) as a potential endogenous ligand produced during intestinal inflammation, acting through GPR35 to induce tumor necrosis factor (Tnf) expression in macrophages. Mice lacking Gpr35 in CX3CR1⁺ macrophages aggravate colitis when exposed to dextran sodium sulfate, which is associated with decreased transcript levels of the corticosterone-generating gene Cyp11b1 and macrophage-derived Tnf. Administration of TNF in these mice restores Cyp11b1 expression and intestinal corticosterone production and ameliorates DSS-induced colitis. Our findings indicate that LPA signals through GPR35 in CX3CR1⁺ macrophages to maintain TNF-mediated intestinal homeostasis.

Metabolic Functions of G Protein-Coupled Receptors and β-Arrestin-Mediated Signaling Pathways in the Pathophysiology of Type 2 Diabetes and Obesity

Seven transmembrane receptors (7TMRs), often termed G protein-coupled receptors (GPCRs), are the most common target of therapeutic drugs used today. Many studies suggest that distinct members of the GPCR superfamily represent potential targets for the treatment of various metabolic disorders including obesity and type 2 diabetes (T2D). GPCRs typically activate different classes of heterotrimeric G proteins, which can be subgrouped into four major functional types: G_αs, G_αi, G_αq/11, and G_12/13, in response to agonist binding. Accumulating evidence suggests that GPCRs can also initiate β-arrestin-dependent, G protein-independent signaling. Thus, the physiological outcome of activating a certain GPCR in a particular tissue may also be modulated by β-arrestin-dependent, but G protein-independent signaling pathways. In this review, we will focus on the role of G protein- and β-arrestin-dependent signaling pathways in the development of obesity and T2D-related metabolic disorders.

Therapeutic Opportunities and Challenges in Targeting the Orphan G Protein-Coupled Receptor GPR35

GPR35 is a class A, rhodopsin-like G protein-coupled receptor (GPCR) first identified more than 20 years ago. In the intervening period, identification of strong expression in the lower intestine and colon, in a variety of immune cells including monocytes and a variety of dendritic cells, and in dorsal root ganglia has suggested potential therapeutic opportunities in targeting this receptor in a range of conditions. GPR35 is, however, unusual in a variety of ways that challenge routes to translation. These include the following: (i) Although a substantial range and diversity of endogenous ligands have been suggested as agonist partners for this receptor, it officially remains defined as an "orphan" GPCR. (ii) Humans express two distinct protein isoform sequences, while rodents express only a single form. (iii) The pharmacologies of the human and rodent orthologues of GPR35 are very distinct, with variation between rat and mouse GPR35 being as marked as that between either of these species and the human forms. Herein we provide perspectives on each of the topics above as well as suggesting ways to overcome the challenges currently hindering potential translation. These include a better understanding of the extent and molecular basis for species selective GPR35 pharmacology and the production of novel mouse models in which both "on-target" and "off-target" effects of presumptive GPR35 ligands can be better defined, as well as a clear understanding of the human isoform expression profile and its significance at both tissue and individual cell levels.

Druggable Lipid GPCRs: Past, Present, and Prospects

G protein-coupled receptors (GPCRs) have seven transmembrane spanning domains and comprise the largest superfamily with ~800 receptors in humans. GPCRs are attractive targets for drug discovery because they transduce intracellular signaling in response to endogenous ligands via heterotrimeric G proteins or arrestins, resulting in a wide variety of physiological and pathophysiological responses. The endogenous ligands for GPCRs are highly chemically diverse and include ions, biogenic amines, nucleotides, peptides, and lipids. In this review, we follow the KonMari method to better understand druggable lipid GPCRs. First, we have a comprehensive tidying up of lipid GPCRs including receptors for prostanoids, leukotrienes, specialized pro-resolving mediators (SPMs), lysophospholipids, sphingosine 1-phosphate (S1P), cannabinoids, platelet-activating factor (PAF), free fatty acids (FFAs), and sterols. This tidying up consolidates 46 lipid GPCRs and declutters several perplexing lipid GPCRs. Then, we further tidy up the lipid GPCR-directed drugs from the literature and databases, which identified 24 clinical drugs targeting 16 unique lipid GPCRs available in the market and 44 drugs under evaluation in more than 100 clinical trials as of 2019. Finally, we introduce drug designs for GPCRs that spark joy, such as positive or negative allosteric modulators (PAM or NAM), biased agonism, functional antagonism like fingolimod, and monoclonal antibodies (MAbs). These strategic drug designs may increase the efficacy and specificity of drugs and reduce side effects. Technological advances will help to discover more endogenous lipid ligands from the vast number of remaining orphan GPCRs and will also lead to the development novel lipid GPCR drugs to treat various diseases.

Prognostic value of CXCL17 and CXCR8 expression in patients with colon cancer

C-X-C motif chemokine ligand 17 (CXCL17) is a mucous chemokine and its expression is highly correlated with that of G protein-coupled receptor 35 (GPR35), which has been confirmed as its receptor and named C-X-C motif chemokine receptor 8 (CXCR8). CXCL17 is upregulated in several types of cancer. However, the biological role of this chemokine in colon cancer remains unknown. In the present study, the expression levels of CXCL17 and CXCR8 were examined using immunohistochemistry in 101 colon cancer tissues and 79 healthy tumour-adjacent tissues. CXCL17 and CXCR8 expression levels were increased in the colon cancer samples compared with tumour-adjacent samples. Patients with high CXCL17 expression had longer overall survival (OS) compared with patients with low expression of CXCL17 (log-rank test; P=0.027). However, CXCR8 expression, but not CXCL17, was an independent prognostic factor for OS in patients with colon cancer. The expression of CXCR8 correlated positively with that of CXCL17 in colon cancer samples (ρ=0.295; P=0.003). Furthermore, the combined high expression of CXCL17 and CXCR8 was a significant independent prognostic factor for OS in patients with colon cancer (P=0.001). In subgroups with a TNM stage of I–II, the patients with combined high expression of CXCL17 and CXCR8 had a longer survival compared with those without combined high expression (P=0.001). However, this difference was not observed in subgroups with a TNM stage of III–IV. Collectively, these findings suggest that CXCL17/CXCR8 signalling may be involved in colon cancer and contribute to improved patient outcomes.

Role of the autotaxin-lysophosphatidate axis in the development of resistance to cancer therapy

Autotaxin (ATX) is a secreted enzyme that hydrolyzes lysophosphatidylcholine to produce lysophosphatidate (LPA), which signals through six G-protein coupled receptors (GPCRs). Signaling through LPA is terminated by its degradation by a family of three lipid phosphate phosphatases (LPPs). LPP1 also attenuates signaling downstream of the activation of LPA receptors and some other GPCRs. The ATX-LPA axis mediates a plethora of activities such as cell proliferation, survival, migration, angiogenesis and inflammation, which perform an important role in facilitating wound healing. This wound healing response is hijacked by cancers where there is decreased expression of LPP1 and LPP3 and increased expression of ATX. This maladaptive regulation of LPA signaling also causes chronic inflammation, which has been recognized as one of the hallmarks in cancer. The increased LPA signaling promotes cell survival and migration and attenuates apoptosis, which stimulates tumor growth and metastasis. The wound healing functions of increased LPA signaling also protect cancer cells from effects of chemotherapy and radiotherapy. In this review, we will summarize knowledge of the ATX-LPA axis and its role in the development of resistance to chemotherapy and radiotherapy. We will also offer insights for developing strategies of targeting ATX-LPA axis as a novel part of cancer treatment. This article is part of a Special Issue entitled Lysophospholipids and their receptors: New data and new insights into their function edited by Susan Smyth, Viswanathan Natarajan and Colleen McMullen.

Identification of novel phytocannabinoids from Ganoderma by label-free dynamic mass redistribution assay

ETHNOPHARMACOLOGICAL RELEVANCE

Located throughout the body, cannabinoid receptors (CB1 and CB2) are therapeutic targets for obesity/metabolic diseases, neurological/mental disorders, and immune modulation. Phytocannabinoids are greatly important for the development of new medicines with high efficacy and/or minor side effects. Plants and fungi are used in traditional medicine for beneficial effects to mental and immune system. The current research studied five fungi from the genus Ganoderma and five plants: Ganoderma hainanense J.D. Zhao, L.W. Hsu & X.Q. Zhang; Ganoderma capense (Lloyd) Teng, Zhong Guo De Zhen Jun; Ganoderma cochlear (Blume & T. Nees) Bres., Hedwigia; Ganoderma resinaceum Boud.; Ganoderma applanatum (Pers.) Pat.; Carthamus tinctorius L. (Compositae); Cynanchum otophyllum C. K. Schneid. (Asclepiadaceae); Coffea arabica L. (Rubiaceae); Prinsepia utilis Royle (Rosaceae); Lepidium meyenii Walp. (Brassicaceae). They show immunoregulation, promotion of longevity and maintenance of vitality, stimulant effects on the central nervous system, hormone balance and other beneficial effects. However, it remains unclear whether cannabinoid receptors are involved in these effects.

AIM OF THE STUDY

This work aimed to identify components working on CB1 and CB2 from the above plants and fungi, as novel phytocannabinoids, and to investigate mechanisms of how these compounds affected the cells. By analyzing the structure-activity relationship, we could identify the core structure for future development.

MATERIALS AND METHODS

Eighty-two natural compounds were screened on stably transfected Chinese hamster ovary (CHO) cell lines, CHO-CB1 and CHO-CB2, with application of a label-free dynamic mass redistribution (DMR) technology that measured cellular responses to compounds. CP55,940 and WIN55,212-2 were agonist probe molecules, and SR141716A and SR144528 were antagonist probes. Pertussis toxin, cholera toxin, LY294002 and U73122 were signaling pathway inhibitors. The DMR data were acquired by Epic Imager software (Corning, NY), processed by Imager Beta 3.7 (Corning), and analyzed by GraphPad Prism 6 (GraphPad Software, San Diego, CA).

RESULTS

Transfected CHO-CB1 and CHO-CB2 cell lines were established and characterized. Seven compounds induced responses/activities in the cells. Among the seven compounds, four were purified from two Ganoderma species with potencies between 20 and 35 μM. Three antagonists: Kfb68 antagonized both receptors with a better desensitizing effect on CB2 to WIN55,212-2 over CP55,940. Kga1 and Kfb28 were antagonists selective to CB1 and CB2, respectively. Kfb77 was a special agonist and it stimulated CB1 in a mechanism different from that of CP55,940. Another three active compounds, derived from the Lepidium meyenii Walp. (Brassicaceae), were also identified but their effects were mediated through mechanisms much related to the signaling transduction pathways, especially through the stimulatory G_s protein.

CONCLUSIONS

We identified four natural cannabinoids that exhibited structural and functional diversities. Our work confirms the presence of active ingredients in the Ganoderma species to CB1 and CB2, and this finding establishes connections between the fungi and the cannabinoid receptors, which will serve as a starting point to connect their beneficial effects to the endocannabinoid system. This research will also enrich the inventory of cannabinoids and phytocannabinoids from fungi. Yet due to some limitations, further structure-activity relationship studies and mechanism investigation are warranted in future.

Chemical Proteomic Profiling of Lysophosphatidic Acid-Binding Proteins

Lysophosphatidic acid (LPA) is an endogenous cell signaling molecule, and dysregulation of LPA signaling pathways is accompanied by several types of cancer. Herein, we developed a chemical proteomic method for the proteome-wide identification of LPA-binding proteins. The method involves the synthesis of a desthiobiotin-conjugated LPA acyl phosphate probe for the covalent labeling, enrichment, and subsequent LC-MS/MS identification of LPA-binding proteins at the proteome-wide level. By conducting labeling reactions at two different probe concentrations (10 and 100 μM) in conjunction with an SILAC (stable isotope labeling by amino acids in cell culture)-based workflow, we characterized the LPA-binding capabilities of these proteins at the entire proteome scale, which led to the identification of 86 candidate LPA-binding proteins in HEK293T cells. Moreover, we validated that two of these proteins, annexin A5 and phosphoglycerate kinase 1, can bind directly with LPA. Together, we developed a novel LPA probe for the identification and characterizations of LPA-binding proteins from the entire human proteome. The method should be adaptable for the identification of other lipid-binding proteins.

Lysophosphatidic acid up-regulates IL-10 production to inhibit TNF-α synthesis in Mϕs stimulated with LPS

Bacterial LPS strongly induces pro-inflammatory responses of Mϕs after binding to CD14 protein and the TLR4/MD-2 receptor complex. The LPS-triggered signaling can be modulated by extracellular lysophosphatidic acid (LPA), which is of substantial importance for Mϕ functioning under specific pathophysiological conditions, such as atherosclerosis. The molecular mechanisms of the crosstalk between the LPS- and LPA-induced signaling, and the LPA receptors involved, are poorly known. In this report, we show that LPA strongly inhibits the LPS-induced TNF-α production at the mRNA and protein levels in primary Mϕs and Mϕ-like J774 cells. The decreased TNF-α production in LPA/LPS-stimulated cells is to high extent independent of NF-κB but is preceded by enhanced expression and secretion of the anti-inflammatory cytokine IL-10. The IL-10 elevation and TNF-α reduction are both abrogated upon depletion of the LPA₅ and LPA₆ receptors in J774 cells and can be linked with LPA-mediated activation of p38. We propose that the binding of LPA to LPA₅ and LPA₆ fine-tunes the LPS-induced inflammatory response by activating p38, and up-regulating IL-10 and down-regulating TNF-α production.

Lysophosphatidic Acid and Autotaxin-associated Effects on the Initiation and Progression of Colorectal Cancer

The intestinal epithelium interacts dynamically with the immune system to maintain its barrier function to protect the host, while performing the physiological roles in absorption of nutrients, electrolytes, water and minerals. The importance of lysophosphatidic acid (LPA) and its receptors in the gut has been progressively appreciated. LPA signaling modulates cell proliferation, invasion, adhesion, angiogenesis, and survival that can promote cancer growth and metastasis. These effects are equally important for the maintenance of the epithelial barrier in the gut, which forms the first line of defense against the milieu of potentially pathogenic stimuli. This review focuses on the LPA-mediated signaling that potentially contributes to inflammation and tumor formation in the gastrointestinal tract.

Lysophosphatidic Acid Signaling in Diabetic Nephropathy

Lysophosphatidic acid (LPA) is a bioactive phospholipid present in most tissues and body fluids. LPA acts through specific LPA receptors (LPAR1 to LPAR6) coupled with G protein. LPA binds to receptors and activates multiple cellular signaling pathways, subsequently exerting various biological functions, such as cell proliferation, migration, and apoptosis. LPA also induces cell damage through complex overlapping pathways, including the generation of reactive oxygen species, inflammatory cytokines, and fibrosis. Several reports indicate that the LPA–LPAR axis plays an important role in various diseases, including kidney disease, lung fibrosis, and cancer. Diabetic nephropathy (DN) is one of the most common diabetic complications and the main risk factor for chronic kidney diseases, which mostly progress to end-stage renal disease. There is also growing evidence indicating that the LPA–LPAR axis also plays an important role in inducing pathological alterations of cell structure and function in the kidneys. In this review, we will discuss key mediators or signaling pathways activated by LPA and summarize recent research findings associated with DN.

Effects of the LPA1 Receptor Deficiency and Stress on the Hippocampal LPA Species in Mice

Lysophosphatidic acid (LPA) is an important bioactive lipid species that functions in intracellular signaling through six characterized G protein-coupled receptors (LPA_1-6). Among these receptors, LPA₁ is a strong candidate to mediate the central effects of LPA on emotion and may be involved in promoting normal emotional behaviors. Alterations in this receptor may induce vulnerability to stress and predispose an individual to a psychopathological disease. In fact, mice lacking the LPA₁ receptor exhibit emotional dysregulation and cognitive alterations in hippocampus-dependent tasks. Moreover, the loss of this receptor results in a phenotype of low resilience with dysfunctional coping in response to stress and induces anxiety and several behavioral and neurobiological changes that are strongly correlated with mood disorders. In fact, our group proposes that maLPA1-null mice represent an animal model of anxious depression. However, despite the key role of the LPA-LPA₁-pathway in emotion and stress coping behaviors, the available information describing the mechanisms by which the LPA-LPA₁-pathway regulates emotion is currently insufficient. Because activation of LPA₁ requires LPA, here, we used a Matrix-Assisted Laser Desorption/ Ionization mass spectrometry-based approach to evaluate the effects of an LPA₁ receptor deficiency on the hippocampal levels of LPA species. Additionally, the impact of stress on the LPA profile was also examined in both wild-type (WT) and the Malaga variant of LPA1-null mice (maLPA₁-null mice). Mice lacking LPA₁ did not exhibit gross perturbations in the hippocampal LPA species, but the LPA profile was modified, showing an altered relative abundance of 18:0 LPA. Regardless of the genotype, restraint stress produced profound changes in all LPA species examined, revealing that hippocampal LPA species are a key target of stress. Finally, the relationship between the hippocampal levels of LPA species and performance in the elevated plus maze was established. To our knowledge, this study is the first to detect, identify and profile LPA species in the hippocampus of both LPA₁-receptor null mice and WT mice at baseline and after acute stress, as well as to link these LPA species with anxiety-like behaviors. In conclusion, the hippocampal LPA species are a key target of stress and may be involved in psychopathological conditions.

Metabolite-Sensing G Protein-Coupled Receptors Connect the Diet-Microbiota-Metabolites Axis to Inflammatory Bowel Disease

Increasing evidence has indicated that diet and metabolites, including bacteria- and host-derived metabolites, orchestrate host pathophysiology by regulating metabolism, immune system and inflammation. Indeed, autoimmune diseases such as inflammatory bowel disease (IBD) are associated with the modulation of host response to diets. One crucial mechanism by which the microbiota affects the host is signaling through G protein-coupled receptors (GPCRs) termed metabolite-sensing GPCRs. In the gut, both immune and nonimmune cells express GPCRs and their activation generally provide anti-inflammatory signals through regulation of both the immune system functions and the epithelial integrity. Members of GPCR family serve as a link between microbiota, immune system and intestinal epithelium by which all these components crucially participate to maintain the gut homeostasis. Conversely, impaired GPCR signaling is associated with IBD and other diseases, including hepatic steatosis, diabetes, cardiovascular disease, and asthma. In this review, we first outline the signaling, function, expression and the physiological role of several groups of metabolite-sensing GPCRs. We then discuss recent findings on their role in the regulation of the inflammation, their existing endogenous and synthetic ligands and innovative approaches to therapeutically target inflammatory bowel disease.

LPA1 , LPA2 , LPA4 , and LPA6 receptor expression during mouse brain development

Background

LPA is a small bioactive phospholipid that acts as an extracellular signaling molecule and is involved in cellular processes, including cell proliferation, migration, and differentiation. LPA acts by binding and activating at least six known G protein–coupled receptors: LPA_1–6. In recent years, LPA has been suggested to play an important role both in normal neuronal development and under pathological conditions in the nervous system.

Results

We show the expression pattern of LPA receptors during mouse brain development by using qRT‐PCR, in situ hybridization, and immunocytochemistry. Only LPA₁, LPA_2,LPA_4, and LPA₆ mRNA transcripts were detected throughout development stages from embryonic day 16 until postnatal day 30 of hippocampus, neocortex, cerebellum, and bulbus olfactorius in our experiments, while expression of LPA₃ and LPA₅ genes was below detection level. In addition to our qRT‐PCR results, we also analyzed the cellular protein expression of endogenous LPA receptors, with focus on LPA₁ and LPA₂ within postnatal brain slices and primary neuron differentiation with and without cytoskeleton stabilization and destabilization.

Conclusions

The expression of LPA receptors changes depends on the developmental stage in mouse brain and in cultured hippocampal primary neurons. Interestingly, we found that commercially available antibodies for LPA receptors are largely unspecific.

LPA1, ‐2, ‐4, and ‐6 genes are dynamically expressed during postnatal brain development.

LPA1, ‐2, ‐4, and ‐6 genes are differently expressed in the hippocampus, neocortex, cerebellum, and bulbus olfactorius.

LPA1 and ‐2 gene expression alters during neuronal differentiation.

LPA1, ‐2, ‐3, ‐4, and ‐6 genes are expressed in glia cells, but differed in gene expression levels.

The weight of nutrients: kynurenine metabolites in obesity and exercise

Obesity ultimately results from an imbalance between energy intake and expenditure. However, in addition to their bioenergetic value, nutrients and their metabolites can function as important signalling molecules in energy homeostasis. Indeed, macronutrients and their metabolites can be direct regulators of metabolism through their actions on different organs. In turn, target organs can decide to use, store or transform the incoming nutrients depending on their physiological context and in coordination with other cell types. Tryptophan-kynurenine metabolites are an example of a family of compounds that can serve as systemic integrators of energy metabolism by signalling to different cell types. These include adipocytes, immune cells and muscle fibres, in addition to the well-known effects of kynurenine metabolites on the central nervous system. In the context of energy metabolism, several of the effects elicited by kynurenic acid are mediated by the G-protein-coupled receptor, GPR35. As GPR35 is expressed in tissues such as the adipose tissue, immune cells and the gastrointestinal tract, this receptor could be a potential therapeutic target for the treatment of obesity, diabetes and other metabolic diseases. In addition, metabolic disorders often coincide with states of chronic inflammation, which further highlights GPR35 as an integration node in conditions where inflammation skews metabolism. Defining the molecular interplay between different tissues in the regulation of energy homeostasis can help us understand interindividual variability in the response to nutrient intake and develop safe and efficient therapies to fight obesity and metabolic disease.

Disruption of GPR35 Exacerbates Dextran Sulfate Sodium-Induced Colitis in Mice

BACKGROUND

G protein-coupled receptor 35 (GPR35) is an orphan receptor and is vastly expressed in immune cells and gastrointestinal cells, suggesting the potential physiological importance of GPR35 in these cells. Here, we tested the hypothesis that the lack of GPR35 expression in the colon mucosa exacerbates the severity of dextran sulfate sodium (DSS)-induced experimental colitis in mice.

METHODS

Colitis was induced in GPR35 wild-type (GPR35+/+) and GPR35 knockout (GPR35-/-) mice through the administration of DSS in drinking water for 5 days followed by regular facility water for 1 day. Induction of colitis was evaluated by measuring relative body weight loss, clinical illness scores, and morphological changes in the colon. Abolition of Gpr35 gene expression in the colon mucosa of GPR35-/- mice was confirmed by quantitative real-time PCR (qPCR). Gene expressions of inflammatory and tissue remodeling cytokines were detected by qPCR. Human colorectal epithelial Caco cells were transfected with siRNA against GPR35 before treated with 1% DSS in vitro. Protein expressions were measured using Western blot.

RESULTS

GPR35-/- mice receiving DSS showed a significantly worsened colitis disease with profound loss of body weight and a considerable amount of severe clinical illness compared to GPR35+/+ mice that received DSS. The histology of colon sections from GPR35-/- mice showed extensive pathological changes including submucosal edema, diffuse ulcerations, and evidence of complete loss of crypts compared to wild-type mice. The mean histopathological score was significantly higher in GPR35-/- mice as compared to GPR35+/+ mice. The qPCR data revealed significant expression of pro-inflammatory and tissue remodeling cytokines in GPR35-/- colon mucosa, including IL-1β, CXCL1, CXCL2, CCL2, HMGB1, TGFβ1, TGFβ3, MMP1/9/12. The protein expressions of Zonula occludens-1, E-cadherin, Claudin1 were decreased upon knocking down GPR35 with or without 1% DSS treatment.

CONCLUSIONS

Our experimental data suggest that lack of GPR35 resulted in worsened disease outcome in DSS-induced experimental colitis, indicating that GPR35 could play a crucial role in protecting from colonic inflammation and serve as a therapeutic target.

Evidence for the Existence of a CXCL17 Receptor Distinct from GPR35

The chemokine CXCL17 is associated with the innate response in mucosal tissues but is poorly characterized. Similarly, the G protein–coupled receptor GPR35, expressed by monocytes and mast cells, has been implicated in the immune response, although its precise role is ill-defined. A recent manuscript reported that GPR35 was able to signal in response to CXCL17, which we set out to confirm in this study. GPR35 was readily expressed using transfection systems but failed to signal in response to CXCL17 in assays of β-arrestin recruitment, inositol phosphate production, calcium flux, and receptor endocytosis. Similarly, in chemotaxis assays, GPR35 did not confirm sensitivity to a range of CXCL17 concentrations above that observed in the parental cell line. We subsequently employed a real time chemotaxis assay (TAXIScan) to investigate the migratory responses of human monocytes and the monocytic cell line THP-1 to a gradient of CXCL17. Freshly isolated human monocytes displayed no obvious migration to CXCL17. Resting THP-1 cells showed a trend toward directional migration along a CXCL17 gradient, which was significantly enhanced by overnight incubation with PGE₂. However, pretreatment of PGE₂-treated THP-1 cells with the well-characterized GPR35 antagonist ML145 did not significantly impair their migratory responses to CXCL17 gradient. CXCL17 was susceptible to cleavage with chymase, although this had little effect its ability to recruit THP-1 cells. We therefore conclude that GPR35 is unlikely to be a bona fide receptor for CXCL17 and that THP-1 cells express an as yet unidentified receptor for CXCL17.

SAR Studies of N-[2-(1H-Tetrazol-5-yl)phenyl]benzamide Derivatives as Potent G Protein-Coupled Receptor-35 Agonists

G protein-coupled receptor-35 (GPR35) has emerged as a potential target in the treatment of pain and inflammatory and metabolic diseases. We have discovered a series of potent GPR35 agonists based on a coumarin scaffold and found that the introduction of a 1H-tetrazol-5-yl group significantly increased their potency. We designed and synthesized a new series of N-[2-(1H-tetrazol-5-yl)phenyl]benzamide derivatives through a two-step synthetic approach, and characterized their agonistic activities against GPR35 using a dynamic mass redistribution (DMR) assay. N-(5-bromo-2-(1H-tetrazol-5-yl)phenyl)-4-methoxybenzamide (56) and N-(5-bromo-2-(1H-tetrazol-5-yl)phenyl)-2-fluoro-4-methoxybenzamide (63) displayed the highest agonistic potency agonist GPR35 with an EC₅₀ of 0.059 μM and 0.041 μM, respectively. The physicochemical properties of selected compounds were calculated to evaluate their druglikeness, suggesting that compounds 56 and 63 have good druglike properties. Together, N-[2-(1H-tetrazol-5-yl)phenyl]benzamide derivatives are potentially great candidates for developing potent GPR35 agonists.

Coming of Age for Autotaxin and Lysophosphatidate Signaling: Clinical Applications for Preventing, Detecting and Targeting Tumor-Promoting Inflammation

A quarter-century after the discovery of autotaxin in cell culture, the autotaxin-lysophosphatidate (LPA)-lipid phosphate phosphatase axis is now a promising clinical target for treating chronic inflammatory conditions, mitigating fibrosis progression, and improving the efficacy of existing cancer chemotherapies and radiotherapy. Nearly half of the literature on this axis has been published during the last five years. In cancer biology, LPA signaling is increasingly being recognized as a central mediator of the progression of chronic inflammation in the establishment of a tumor microenvironment which promotes cancer growth, immune evasion, metastasis, and treatment resistance. In this review, we will summarize recent advances made in understanding LPA signaling with respect to chronic inflammation and cancer. We will also provide perspectives on the applications of inhibitors of LPA signaling in preventing cancer initiation, as adjuncts extending the efficacy of current cancer treatments by blocking inflammation caused by either the cancer or the cancer therapy itself, and by disruption of the tumor microenvironment. Overall, LPA, a simple molecule that mediates a plethora of biological effects, can be targeted at its levels of production by autotaxin, LPA receptors or through LPA degradation by lipid phosphate phosphatases. Drugs for these applications will soon be entering clinical practice.

Stress, Depression, Resilience and Ageing: A Role for the LPA-LPA1 Pathway

BACKGROUND

Chronic stress affects health and the quality of life, with its effects being particularly relevant in ageing due to the psychobiological characteristics of this population. However, while some people develop psychiatric disorders, especially depression, others seem very capable of dealing with adversity. There is no doubt that along with the identification of neurobiological mechanisms involved in developing depression, discovering which factors are involved in positive adaptation under circumstances of extreme difficulty will be crucial for promoting resilience.

METHODS

Here, we review recent work in our laboratory, using an animal model lacking the LPA1 receptor, together with pharmacological studies and clinical evidence for the possible participation of the LPA1 receptor in mood and resilience to stress.

RESULTS

Substantial evidence has shown that the LPA1 receptor is involved in emotional regulation and in coping responses to chronic stress, which, if dysfunctional, may induce vulnerability to stress and predisposition to the development of depression. Given that there is commonality of mechanisms between those involved in negative consequences of stress and in ageing, this is not surprising, considering that the LPA1 receptor may be involved in coping with adversity during ageing.

CONCLUSION

Alterations in this receptor may be a susceptibility factor for the presence of depression and cognitive deficits in the elderly population. However, because this is only a promising hypothesis based on previous data, future studies should focus on the involvement of the LPA-LPA1 pathway in coping with stress and resilience in ageing.

Metabolite-Sensing G Protein-Coupled Receptors-Facilitators of Diet-Related Immune Regulation

Nutrition and the gut microbiome regulate many systems, including the immune, metabolic, and nervous systems. We propose that the host responds to deficiency (or sufficiency) of dietary and bacterial metabolites in a dynamic way, to optimize responses and survival. A family of G protein-coupled receptors (GPCRs) termed the metabolite-sensing GPCRs bind to various metabolites and transmit signals that are important for proper immune and metabolic functions. Members of this family include GPR43, GPR41, GPR109A, GPR120, GPR40, GPR84, GPR35, and GPR91. In addition, bile acid receptors such as GPR131 (TGR5) and proton-sensing receptors such as GPR65 show similar features. A consistent feature of this family of GPCRs is that they provide anti-inflammatory signals; many also regulate metabolism and gut homeostasis. These receptors represent one of the main mechanisms whereby the gut microbiome affects vertebrate physiology, and they also provide a link between the immune and metabolic systems. Insufficient signaling through one or more of these metabolite-sensing GPCRs likely contributes to human diseases such as asthma, food allergies, type 1 and type 2 diabetes, hepatic steatosis, cardiovascular disease, and inflammatory bowel diseases.

GPR35 mediates lodoxamide-induced migration inhibitory response but not CXCL17-induced migration stimulatory response in THP-1 cells; is GPR35 a receptor for CXCL17?

BACKGROUND AND PURPOSE

GPR35 has long been considered an orphan GPCR, because no endogenous ligand of GPR35 has been discovered. CXCL17 (a chemokine) has been reported to be an endogenous ligand of GPR35, and it has even been suggested that it be called CXCR8. However, at present there is no supporting evidence that CXCL17 does interact with GPR35.

EXPERIMENTAL APPROACH

We applied two assay systems to explore the relationship between CXCL17 and GPR35. An AP-TGF-α shedding assay in GPR35 over-expressing HEK293 cells was used as a gain-of-function assay. GPR35 knock-down by siRNA transfection was performed in endogenously GPR35-expressing THP-1 cells.

KEY RESULTS

In the AP-TGF-α shedding assay, lodoxamide, a well-known synthetic GPR35 agonist, was confirmed to be the most potent agonist among other reported agonists. However, neither human nor mouse CXCL17 had an effect on GPR35. Consistent with previous findings, G proteins Gαi/o and Gα12/13 were found to couple with GPR35. Furthermore, lodoxamide-induced activation of GPR35 was concentration-dependently inhibited by CID2745687 (a selective GPR35 antagonist). In endogenously GPR35-expressing THP-1 cells, lodoxamide concentration-dependently inhibited migration and this inhibitory effect was blocked by CID2745687 treatment or GPR35 siRNA transfection. However, even though CXCL17 stimulated the migration of THP-1 cells, which is consistent with a previous report, this stimulatory effect of CXCL17 was not blocked by CID2745687 or GPR35 siRNA.

CONCLUSIONS AND IMPLICATIONS

The present findings suggest that GPR35 functions as a migration inhibitory receptor, but CXCL17-stimulated migration of THP-1 cells is not dependent on GPR35.

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.

Interactions between the Kynurenine and the Endocannabinoid System with Special Emphasis on Migraine

Both the kynurenine and the endocannabinoid systems are involved in several neurological disorders, such as migraine and there are increasing number of reports demonstrating that there are interactions of two systems. Although their cooperation has not yet been implicated in migraine, there are reports suggesting this possibility. Additionally, the individual role of the endocannabinoid and kynurenine system in migraine is reviewed here first, focusing on endocannabinoids, kynurenine metabolites, in particular kynurenic acid. Finally, the function of NMDA and cannabinoid receptors in the trigeminal system-which has a crucial role in the pathomechanisms of migraine-will also be discussed. The interaction of the endocannabinoid and kynurenine system has been demonstrated to be therapeutically relevant in a number of pathological conditions, such as cannabis addiction, psychosis, schizophrenia and epilepsy. Accordingly, the cross-talk of these two systems may imply potential mechanisms related to migraine, and may offer new approaches to manage the treatment of this neurological disorder.

G protein-coupled receptor 35 contributes to mucosal repair in mice via migration of colonic epithelial cells

G protein-coupled receptor 35 (GPR35), a receptor for lysophosphatidic acid, is highly expressed in the gastrointestinal tract. Recently, GPR35 has been implicated in the onset of inflammatory bowel disease (IBD), but its role in physiological and pathological processes in the colon remains undefined. In this study, we investigated the contribution of GPR35-mediated signalling to mucosal repair of colonic epithelium in IBD. GPR35 function was examined in a wound healing model, using young adult mouse colon epithelium (YAMC) cells, and in a dextran sulphate sodium (DSS)-induced mouse model of colitis. Cell proliferation, mRNA expression, extracellular signal-regulated kinase (ERK) activation, and protein localization were determined by MTT assay, quantitative RT-PCR, western blotting, and immunohistochemistry, respectively. GPR35 agonists (YE120, zaprinast, and pamoic acid) promoted wound repair in a concentration-dependent manner independently of cell proliferation, whereas a specific GPR35 antagonist CID2745687, forskolin, and pertussis toxin reversed the YE120-induced effect. YE120 increased the mRNA expression of fibronectin and its receptor integrin α5, and ERK1/2 phosphorylation, but these responses were attenuated by CID2745687 and forskolin. Furthermore, the severity of DSS-induced colitis was significantly reduced by daily injections of pamoic acid via upregulation of fibronectin and integrin α5 in the colonic epithelium. GPR35 signalling promotes mucosal repair by inducing fibronectin and integrin α5 expression, coupling to Gi protein, and activating ERK1/2 in colonic epithelial cells. These findings define GPR35 as a candidate therapeutic target in IBD.

Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors

BACKGROUND

In the central nervous system (CNS), G protein-coupled receptors (GPCRs) are the most fruitful targets for neuropsychopharmacological drug development. Rhodopsin (class A) is the most studied class of GPCR and includes orphan receptors for which the endogenous ligand is not known or is unclear. Characterization of orphan GPCRs has proven to be challenging, and the production pace of GPCR-based drugs has been incredibly slow.

OBJECTIVE

Determination of the functions of these receptors may provide unexpected insight into physiological and neuropathological processes. Advances in various methods and techniques to investigate orphan receptors including in situ hybridization and knockdown/knockout (KD/KO) showed extensive expression of these receptors in the mammalian brain and unmasked their physiological and neuropathological roles. Due to these rapid progress and development, orphan GPCRs are rising as a new and promising class of drug targets for neurodegenerative diseases and psychiatric disorders.

CONCLUSION

This review presents a neuropsychopharmacological perspective of 26 orphan receptors of rhodopsin (class A) family, namely GPR3, GPR6, GPR12, GPR17, GPR26, GPR35, GPR39, GPR48, GPR49, GPR50, GPR52, GPR55, GPR61, GPR62, GPR63, GPR68, GPR75, GPR78, GPR83, GPR84, GPR85, GPR88, GPR153, GPR162, GPR171, and TAAR6. We discussed the expression of these receptors in mammalian brain and their physiological roles. Furthermore, we have briefly highlighted their roles in neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuroinflammation, inflammatory pain, bipolar and schizophrenic disorders, epilepsy, anxiety, and depression.

Development and validation of a specific and sensitive HPLC-ESI-MS method for quantification of lysophosphatidylinositols and evaluation of their levels in mice tissues

Increasing evidence suggests that lysophosphatidylinositols (LPIs), a subspecies of lysophospholipids, are important endogenous mediators. Although LPIs long remained among the less studied lysophospholipids, the identification of GPR55 as their molecular target sparked a renewed interest in the study of these bioactive lipids. Furthermore, increasing evidence points towards a role for LPIs in cancer development. However, a better understanding of the role and functions of LPIs in physiology and disease requires methods that allow for the quantification of LPI levels in cells and tissues. Because dedicated efficient methods for quantifying LPIs were missing, we decided to develop and validate an HPLC-ESI-MS method for the quantification of LPI species from tissues. LPIs are extracted from tissues by liquid/liquid extraction, pre-purified by solid-phase extraction, and finally analyzed by HPLC-ESI-MS. We determined the method's specificity and selectivity, we established calibration curves, determined the carry over (< 2%), LOD and LLOQ (between 0.116-7.82 and 4.62-92.5pmol on column, respectively), linearity (0.988<R(2)<0.997), repeatability (CV<20%), accuracy (> 80%), intermediate precision (CV<20%) as well as the recovery from tissues. We then applied the method to determine the relative abundance of the LPI species in 15 different mouse tissues. Finally, we quantified the absolute LPI levels in six different mouse tissues. We found that while 18:0 LPI represents more than 60% of all the LPI species in the periphery (e.g. liver, gastrointestinal tract, lungs, spleen) it is much less abundant in the central nervous system where the levels of 20:4 LPI are significantly higher. Thus this validated HPLC-ESI-MS method for quantifying LPIs represents a powerful tool that will facilitate the comprehension of the pathophysiological roles of LPIs.

G-Protein-Coupled Receptor 35 Mediates Human Saphenous Vein Vascular Smooth Muscle Cell Migration and Endothelial Cell Proliferation

Vascular smooth muscle cell (VSMC) migration and proliferation is central to neointima formation in vein graft failure following coronary artery bypass. However, there are currently no pharmacological interventions that prevent vein graft failure through intimal occlusion. It is hence a therapeutic target. Here, we investigated the contribution of GPR35 to human VSMC and endothelial cell (EC) migration, using a scratch-wound assay, and also the contribution to proliferation, using MTS and BrdU assays, in in vitro models using recently characterized human GPR35 ortholog-selective small-molecule agonists and antagonists. Real-time PCR studies showed GPR35 to be robustly expressed in human VSMCs and ECs. Stimulation of GPR35, with either the human-selective agonist pamoic acid or the reference agonist zaprinast, promoted VSMC migration in the scratch-wound assay. These effects were blocked by coincubation with either of the human GPR35-specific antagonists, CID-2745687 or ML-145. These GPR35-mediated effects were produced by inducing alterations in the actin cytoskeleton via the Rho A/Rho kinase signaling axis. Additionally, the agonist ligands stimulated a proliferative response in ECs. These studies highlight the potential that small molecules that stimulate or block GPR35 activity can modulate vascular proliferation and migration. These data propose GPR35 as a translational therapeutic target in vascular remodeling.