Agonists for G-protein-coupled receptor 84 (GPR84) alter cellular morphology and motility but do not induce pro-inflammatory responses in microglia

A comprehensive review of GPR84: A novel player in pathophysiology and treatment

G protein-coupled receptor 84 (GPR84), a member of the highly conserved rhodopsin-like superfamily, represents a promising target for therapeutic drug development. Its distinctive expression profiles in adipocytes, gut endocrine cells, and various myeloid immune cells underscore its critical roles in fundamental physiological processes, particularly in metabolic regulation and immune responses. Over the past two decades, emerging research has demonstrated that GPR84 regulates immune cell chemotaxis, phagocytosis, and inflammatory responses, playing a pivotal role in metabolic disorders, inflammatory diseases, and organ fibrosis. However, the precise molecular mechanisms by which GPR84 is involved in these diseases remain largely uncharacterized, highlighting a significant gap in our understanding. Medium-chain fatty acids (MCFAs) are considered potential endogenous ligands for GPR84. Furthermore, the development of synthetic agonists and antagonists have provided valuable pharmacological tools for analyzing the ligand-GPR84 complex structure and investigating the extensive biological functions of GPR84. Ongoing preclinical and clinical studies highlight the potential of targeting GPR84 in molecular therapies, although concerns regarding drug safety and specificity require further investigation.

Synthetic GPR84 Agonists in Colorectal Cancer: Effective in THP-1 Cells but Ineffective in BMDMs and MC38 Mouse Tumor Models

Tumor-associated macrophages (TAMs) in the colorectal cancer (CRC) microenvironment promote tumor progression but can be reprogrammed into a pro-inflammatory state with anti-cancer properties. Activation of the G protein-coupled receptor 84 (GPR84) is associated with pro-inflammatory macrophage polarization, making it a potential target for CRC therapy. This study evaluates the effects of the GPR84 agonists 6-OAU and ZQ-16 on macrophage activation and anti-cancer efficacy. GPR84 expression on THP-1 macrophages and murine BMDMs was analyzed using flow cytometry. Macrophages were treated with 6-OAU or ZQ-16, and pro-inflammatory cytokine levels, reactive oxygen species (ROS) production, and phagocytosis were assessed using qPCR and functional assays. Anti-cancer effects were tested in a subcutaneous MC38 tumor model, with oral or intraperitoneal agonist administration. Pharmacokinetics and compound stability were also evaluated. In THP-1 macrophages, 6-OAU increased pro-inflammatory cytokines and ROS production, with ZQ-16 showing similar effects. However, neither agonist induced pro-inflammatory responses, ROS production, or phagocytosis in murine macrophages. In vivo, both agonists failed to inhibit tumor growth in the MC38 model despite systemic exposure. Current GPR84 agonists lack efficacy in promoting anti-cancer macrophage activity, limiting their potential as CRC therapies.

Mycobacteria Exploit Host GPR84 to Dampen Pro-Inflammatory Responses and Promote Infection in Macrophages

Tuberculosis (TB) remains the major cause of mortality and morbidity, causing approximately 1.3 million deaths annually. As a highly successful pathogen, Mycobacterium tuberculosis (Mtb) has evolved numerous strategies to evade host immune responses, making it essential to understand the interactions between Mtb and host cells. G-protein-coupled receptor 84 (GPR84), a member of the G-protein-coupled receptor family, contributes to the regulation of pro-inflammatory reactions and the migration of innate immune cells, such as macrophages. Its role in mycobacterial infection, however, has not yet been explored. We found that GPR84 is induced in whole blood samples from tuberculosis patients and Mycobacterium marinum (Mm)-infected macrophage models. Using a Mm-wasabi infection model in mouse tails, we found that GPR84 is an important determinant of the extent of tissue damage. Furthermore, from our studies in an in vitro macrophage Mm infection model, it appears that GPR84 inhibits pro-inflammatory cytokines expression and increases intracellular lipid droplet (LD) accumulation, thereby promoting intracellular bacterial survival. Our findings suggest that GPR84 could be a potential therapeutic target for host-directed anti-TB therapeutics.

Investigating the causal effect of various metabolites on postherpetic neuralgia: a Mendelian randomization study

Background

Common side effect of Herpes Zoster, postherpetic neuralgia (PHN), causes persistent pain that seriously affects quality of life. Lack of dependable biomarkers makes the clinical diagnosis and treatment of PHN difficult, so complicating the assessment of therapeutic efficacy. Blood metabolites are becoming more and more well known as significant disease markers. With an aim to find possible biomarkers for diagnosis and treatment, this work investigates the causal link between blood metabolites and PHN using Mendelian randomization.

Methods

This work evaluated causal relationships between PHN and 1,091 plasma metabolites using Mendelian randomization (MR). Complementing MR-Egger and weighted median approaches, the main causality analysis was done using inverse variance weighted (IVW) and Wald ratio (WR) approaches. Robustness was checked using sensitivity analyses including CAUSE, Cochran's Q tests, leave-one-out analysis, MR-PRESSO, and MR-Egger intercept analysis. Reverse MR analysis and linkage disequilibrium score regression (LDSC) was used to assess significant correlations as well. Two-step MR analysis was also used to look at the mediating function of positively correlated metabolites in the causal pathway.

Results

The results of this study indicated a significant association between N-acetyl-aspartyl-glutamate (NAAG) and PHN, with an odds ratio (OR) of 0.83 (95% CI: 0.76-0.91, p = 2.68E-05). Moreover, five potential associated metabolites were identified: Gamma-glutamylthreonine (OR = 1.60, 95% CI: 1.16-2.20, p = 0.004), 3-hydroxyphenylacetoylglutamine (OR = 1.43, 95% CI: 1.00-2.05, p = 0.048), Caprate (10:0) (OR = 1.86, 95% CI: 1.11-3.12, p = 0.018), X-12013 (OR = 1.64, 95% CI: 1.03-2.60, p = 0.035), and X-17328 (OR = 1.50, 95% CI: 1.04-2.18, p = 0.032). Additionally, NAAG likely acts as a complete mediator between FOLH1(CGPII) and postherpetic neuralgia in the causal pathway.

Conclusion

The results of this study indicated a significant association between N-acetyl-aspartyl-glutamate (NAAG) and PHN, with an odds ratio (OR) of 0.83 (95% CI: 0.76-0.91, p = 2.68E-05). Furthermore five possible related metabolites were found: Glutamylthreonine gamma-wise (OR = 1.60, 95% CI: 1.16-2.20, p = 0.004), 3-hydroxyphenylacetoylglutamine (OR = 1.43, 95% CI: 1.00-2.05, p = 0.048), Caprate (10:0) (OR = 1.86, 95% CI: 1.11-3.12, p = 0.018), X-12013 (OR = 1.64, 95% CI: 1.03-2.60, p = 0.035), and X-17328 (OR = 1.50, 95% CI: 1.04-2.18, p = 0.032). Furthermore, in the causal pathway NAAG most certainly serves as a complete mediator between FOLH1(CGPII) and postherpetic neuralgia.

G-protein-coupled receptor 84 regulates acute inflammation in normal and diabetic skin wounds

Lipids have emerged as potent regulators of immune cell function. In the skin, adipocyte lipolysis increases the local pool of free fatty acids and is essential for coordinating early macrophage inflammation following injury. Here, we investigate G-protein-coupled receptor 84 (GPR84), a medium-chain fatty acid (MCFA) receptor, for its potential to propagate pro-inflammatory signaling after skin injury. GPR84 signaling was identified as a key component of regulating myeloid cell numbers and subsequent tissue repair through in vivo administration of a pharmacological antagonist and the MCFA decanoic acid. We found that impaired injury-induced dermal adipocyte lipolysis is a hallmark of diabetes, and lipidomic analysis demonstrated that MCFAs are significantly reduced in diabetic murine wounds. Furthermore, local administration of decanoic acid rescued myeloid cell numbers and tissue repair during diabetic wound healing. Thus, GPR84 is a readily targetable lipid signaling pathway for manipulating injury-induced tissue inflammation with beneficial effects on acute diabetic healing.

Exploring orphan GPCRs in neurodegenerative diseases

Neurodegenerative disorders represent a significant and growing health burden worldwide. Unfortunately, limited therapeutic options are currently available despite ongoing efforts. Over the past decades, research efforts have increasingly focused on understanding the molecular mechanisms underlying these devastating conditions. Orphan receptors, a class of receptors with no known endogenous ligands, emerge as promising druggable targets for diverse diseases. This review aims to direct attention to a subgroup of orphan GPCRs, in particular class A orphans that have roles in neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Multiple sclerosis. We highlight the diverse roles orphan receptors play in regulating critical cellular processes such as synaptic transmission, neuronal survival and neuro-inflammation. Moreover, we discuss the therapeutic potential of targeting orphan receptors for the treatment of neurodegenerative disorders, emphasizing recent advances in drug discovery and preclinical studies. Finally, we outline future directions and challenges in orphan receptor research.

Systematic characterization of multi-omics landscape between gut microbial metabolites and GPCRome in Alzheimer's disease

Shifts in the magnitude and nature of gut microbial metabolites have been implicated in Alzheimer's disease (AD), but the host receptors that sense and respond to these metabolites are largely unknown. Here, we develop a systems biology framework that integrates machine learning and multi-omics to identify molecular relationships of gut microbial metabolites with non-olfactory G-protein-coupled receptors (termed the "GPCRome"). We evaluate 1.09 million metabolite-protein pairs connecting 408 human GPCRs and 335 gut microbial metabolites. Using genetics-derived Mendelian randomization and integrative analyses of human brain transcriptomic and proteomic profiles, we identify orphan GPCRs (i.e., GPR84) as potential drug targets in AD and that triacanthine experimentally activates GPR84. We demonstrate that phenethylamine and agmatine significantly reduce tau hyperphosphorylation (p-tau181 and p-tau205) in AD patient induced pluripotent stem cell-derived neurons. This study demonstrates a systems biology framework to uncover the GPCR targets of human gut microbiota in AD and other complex diseases if broadly applied.

Regulation of the pro-inflammatory G protein-coupled receptor GPR84

GPR84 is an understudied rhodopsin-like class A G protein-coupled receptor, which is arousing particular interest from a therapeutic perspective. Not least this reflects that gpr84 expression is significantly up-regulated following acute inflammatory stimuli and in inflammatory diseases, and that receptor activation plays a role in regulating pro-inflammatory responses and migration of cells of the innate immune system such as neutrophils, monocytes, macrophages and microglia. Although most physiological responses of GPR84 reflect receptor coupling to Gαi/o-proteins, several studies indicate that agonist-activated GPR84 can recruit arrestin adaptor proteins and this regulates receptor internalisation and desensitisation. To date, little is known on the patterns of either basal or ligand regulated GPR84 phosphorylation and how these might control these processes. Here, we consider what is known about the regulation of GPR84 signalling with a focus on how G protein receptor kinase-mediated phosphorylation regulates arrestin protein recruitment and receptor function. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

GPR84 in physiology-Many functions in many tissues

Members of the GPCR superfamily have a wide variety of physiological roles and are therefore valuable targets for developing effective medicines. However, within this superfamily are receptors that are less well characterized and remain orphans, including GPR84. This receptor is stimulated by ligands derived from dietary nutrients, specifically medium chain fatty acids (C9-14), and novel synthetic agonists. There are data demonstrating the role of GPR84 in inflammatory pathways, in addition to emerging data suggesting a key role for GPR84 as a nutrient-sensing GPCR involved in metabolism by sensing energy load via nutrient exposure and subsequent signalling leading to modulation of food intake. Exploring GPR84 pharmacology, its localization and what drives its expression has revealed multiple roles for this receptor. Here, we will reflect on these various roles of GRP84 demonstrated thus far, primarily by exploring data from pre-clinical and clinical studies in various physiological systems, with a specific focus on the gastrointestinal tract. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

Biased agonists of GPR84 and insights into biological control

GPR84 was first identified as an open reading frame encoding an orphan Class A G protein coupled receptor in 2001. Gpr84 mRNA is expressed in a limited number of cell types with the highest levels of expression being in innate immune cells, M1 polarised macrophages and neutrophils. The first reported ligands for this receptor were medium chain fatty acids with chain lengths between 9 and 12 carbons. Subsequently, a series of synthetic agonists that signal via the GPR84 receptor were identified. Radioligand binding assays and molecular modelling with site-directed mutagenesis suggest the presence of three ligand binding sites on the receptor, but the physiological agonist(s) of the receptor remain unidentified. Here, we review the effects of GPR84 agonists on innate immune cells following a series of chemical discoveries since 2001. The development of highly biased agonists has helped to probe receptor function in vitro, and the remaining challenge is to follow the effects of biased signalling to the physiological functions of innate immune cell types. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

The role of orphan G protein-coupled receptors in pain

G protein-coupled receptors (GPCRs), which form the largest family of membrane protein receptors in humans, are highly complex signaling systems with intricate structures and dynamic conformations and locations. Among these receptors, a specific subset is referred to as orphan GPCRs (oGPCRs) and has garnered significant interest in pain research due to their role in both central and peripheral nervous system function. The diversity of GPCR functions is attributed to multiple factors, including allosteric modulators, signaling bias, oligomerization, constitutive signaling, and compartmentalized signaling. This review primarily focuses on the recent advances in oGPCR research on pain mechanisms, discussing the role of specific oGPCRs including GPR34, GPR37, GPR65, GPR83, GPR84, GPR85, GPR132, GPR151, GPR160, GPR171, GPR177, and GPR183. The orphan receptors among these receptors associated with central nervous system diseases are also briefly described. Understanding the functions of these oGPCRs can contribute not only to a deeper understanding of pain mechanisms but also offer a reference for discovering new targets for pain treatment.

Targeting metabolic sensing switch GPR84 on macrophages for cancer immunotherapy

INTRODUCTION

As one of the major components of the tumor microenvironment, tumor-associated macrophages (TAMs) possess profound inhibitory activity against T cells and facilitate tumor escape from immune checkpoint blockade therapy. Converting this pro-tumorigenic toward the anti-tumorigenic phenotype thus is an important strategy for enhancing adaptive immunity against cancer. However, a plethora of mechanisms have been described for pro-tumorigenic differentiation in cancer, metabolic switches to program the anti-tumorigenic property of TAMs are elusive.

MATERIALS AND METHODS

From an unbiased analysis of single-cell transcriptome data from multiple tumor models, we discovered that anti-tumorigenic TAMs uniquely express elevated levels of a specific fatty acid receptor, G-protein-coupled receptor 84 (GPR84). Genetic ablation of GPR84 in mice leads to impaired pro-inflammatory polarization of macrophages, while enhancing their anti-inflammatory phenotype. By contrast, GPR84 activation by its agonist, 6-n-octylaminouracil (6-OAU), potentiates pro-inflammatory phenotype via the enhanced STAT1 pathway. Moreover, 6-OAU treatment significantly retards tumor growth and increases the anti-tumor efficacy of anti-PD-1 therapy.

CONCLUSION

Overall, we report a previously unappreciated fatty acid receptor, GPR84, that serves as an important metabolic sensing switch for orchestrating anti-tumorigenic macrophage polarization. Pharmacological agonists of GPR84 hold promise to reshape and reverse the immunosuppressive TME, and thereby restore responsiveness of cancer to overcome resistance to immune checkpoint blockade.

Molecular Cloning, Tissue Distribution, and Pharmacological Characterization of GPR84 in Grass Carp (Ctenopharyngodon Idella)

The G-protein-coupled receptor GPR84, activated by medium-chain fatty acids, primarily expressed in macrophages and microglia, is involved in inflammatory responses and retinal development in mammals and amphibians. However, our understanding of its structure, function, tissue expression, and signaling pathways in fish is limited. In this study, we cloned and characterized the coding sequence of GPR84 (ciGPR84) in grass carp. A phylogenetic analysis revealed its close relationship with bony fishes. High expression levels of GPR84 were observed in the liver and spleen. The transfection of HEK293T cells with ciGPR84 demonstrated its responsiveness to medium-chain fatty acids and diindolylmethane (DIM). Capric acid, undecanoic acid, and lauric acid activated ERK and inhibited cAMP signaling. Lauric acid showed the highest efficiency in activating the ERK pathway, while capric acid was the most effective in inhibiting cAMP signaling. Notably, DIM did not activate GPR84 in grass carp, unlike in mammals. These findings provide valuable insights for mitigating chronic inflammation in grass carp farming and warrant further exploration of the role of medium-chain fatty acids in inflammation regulation in this species.

Pro-phagocytic function and structural basis of GPR84 signaling

GPR84 is a unique orphan G protein-coupled receptor (GPCR) that can be activated by endogenous medium-chain fatty acids (MCFAs). The signaling of GPR84 is largely pro-inflammatory, which can augment inflammatory response, and GPR84 also functions as a pro-phagocytic receptor to enhance phagocytic activities of macrophages. In this study, we show that the activation of GPR84 by the synthetic agonist 6-OAU can synergize with the blockade of CD47 on cancer cells to induce phagocytosis of cancer cells by macrophages. We also determine a high-resolution structure of the GPR84-Gi signaling complex with 6-OAU. This structure reveals an occluded binding pocket for 6-OAU, the molecular basis of receptor activation involving non-conserved structural motifs of GPR84, and an unusual Gi-coupling interface. Together with computational docking and simulations studies, this structure also suggests a mechanism for the high selectivity of GPR84 for MCFAs and a potential routes of ligand binding and dissociation. These results provide a framework for understanding GPR84 signaling and developing new drugs targeting GPR84.

Lipopolysaccharide Preconditioning Restricts Microglial Overactivation and Alleviates Inflammation-Induced Depressive-like Behavior in Mice

Overactive microglia and severe neuroinflammation play crucial roles in the development of major depressive disorder. Preconditioning with lipopolysaccharide (LPS) provides protection against severe neuroinflammation. However, administering high doses of LPS to mice triggers depressive symptoms. Therefore, the optimal dose of LPS preconditioning needs to be determined by further experiments. LPS preconditioning is an effective agent in anti-inflammation and neuroprotection, but the mechanism by which LPS preconditioning acts in depression remain unclear. This study finds that the anti-inflammation mechanism of low-dose LPS preconditioning is mainly dependent on G-protein-coupled receptor 84 (GPR84). We use low-dose LPS for preconditioning and re-challenged mice or BV2 microglia with high-dose LPS. In addition, RNA-seq is used to explore underlying changes with LPS preconditioning. Low-dose LPS preconditioning reduces the expression of pro-inflammatory mediators and inhibits microglial activation, as well as suppresses the depressive-like behavior when the mice are re-challenged with high-dose LPS. Further investigation reveals that the tolerance-like response in microglia is dependent on the GPR84. Here, we show that low-dose LPS preconditioning can exert anti-inflammation effects and alleviates inflammation-induced depressive-like behavior in mice. As a potential therapeutic target for depression, LPS preconditioning needs to be given further attention regarding its effectiveness and safety.

Pro-phagocytic function and structural basis of GPR84 signaling

GPR84 is a unique orphan G protein-coupled receptor (GPCR) that can be activated by endogenous medium-chain fatty acids (MCFAs). The signaling of GPR84 is largely pro-inflammatory, which can augment inflammatory response, and GPR84 also functions as a pro-phagocytic receptor to enhance the phagocytic activities of macrophages. In this study, we first showed that the activation of GPR84 by the synthetic agonist 6-OAU could synergize with the blockade of CD47 on cancer cells to induce phagocytosis of cancer cells by macrophages. Then, we determined a high-resolution structure of the GPR84-Gi signaling complex with 6-OAU. This structure revealed a completely occluded binding pocket for 6-OAU, the molecular basis of receptor activation involving non-conserved structural motifs of GPR84, and an unusual Gi-coupling interface. Together with computational docking and simulations studies, our structure also suggested the mechanism for the high selectivity of GPR84 for MCFAs and the potential routes of ligand binding and dissociation. Our results provide a framework for understanding GPR84 signaling and developing new drugs targeting GPR84.

Molecular characterization and functional exploration of GPR84 in Chinese Giant Salamander (Andrias davidianus)

The G protein-coupled receptor 84 (GPR84) is a putative medium-chain fatty acids (MCFAs) receptor involved in immune regulation and other metabolic processes. Most available studies focused on the GPR84 characterization from mammals, neglecting vital information that could be obtained from other levels of life, such as amphibians, necessary for an apt evolutionary understanding of the orphan GPR84. Hence, this study molecularly characterized and functionally explored the GPR84 from the Chinese Giant Salamander (Andrias davidianus). Therefore, we report that the Chinese Giant Salamander (CGS), one of the world's largest amphibians, expresses a GPR84 protein having 376 amino acids, with about 70% homologous to other amphibians and around 50% to human GPR84. Investigating the relative localized expression of gpr84 mRNA in CGS using quantitative PCR revealed the highest expression in the kidney and liver. Furthermore, four medium-chain fatty acids (MCFAs) at micromolar levels activated CGS-GPR84 transfected and expressed in HEK293 cells. In HEK293 cells, four different concentrations of MCFAs inhibited forskolin-induced cAMP accumulation and resulted in a dose-dependent increase in extracellular signal-regulated kinases 1 and 2 (ERK1/2). Interestingly, MCFAs activation of GPR84 concomitantly led to the upregulation of inflammatory mediators such as Nuclear Factor Kappa B (NF-κB) and IL-6. Conclusively, this study successfully elucidated the intriguing molecular and functional properties of CGS GPR84, particularly as an immune modulator, and has positioned the findings within the existing body of knowledge for a better overall understanding of GPR84, especially in amphibians.

Type 2 diabetes is associated with increased circulating levels of 3-hydroxydecanoate activating GPR84 and neutrophil migration

Obesity and diabetes are associated with inflammation and altered plasma levels of several metabolites, which may be involved in disease progression. Some metabolites can activate G protein-coupled receptors (GPCRs) expressed on immune cells where they can modulate metabolic inflammation. Here, we find that 3-hydroxydecanoate is enriched in the circulation of obese individuals with type 2 diabetes (T2D) compared with nondiabetic controls. Administration of 3-hydroxydecanoate to mice promotes immune cell recruitment to adipose tissue, which was associated with adipose inflammation and increased fasting insulin levels. Furthermore, we demonstrate that 3-hydroxydecanoate stimulates migration of primary human and mouse neutrophils, but not monocytes, through GPR84 and Gα_i signaling in vitro. Our findings indicate that 3-hydroxydecanoate is a T2D-associated metabolite that increases inflammatory responses and may contribute to the chronic inflammation observed in diabetes.

Dietary Medium-Chain Triglyceride Decanoate Affects Glucose Homeostasis Through GPR84-Mediated GLP-1 Secretion in Mice

Background

Dietary triglycerides are an important energy source; however, their excess intake causes metabolic diseases such as obesity and type 2 diabetes. Medium-chain triglycerides (MCTs) as triglyceride forms of medium-chain fatty acids (MCFAs) are applied to meet the energy demands of athletes, the elderly, and people with stunted growth, because MCFAs are efficiently converted into energy for immediate utilization by the organs and do not accumulate as fat. Although the intake of each MCT type (octanoate; C8:0, decanoate; C10:0, and dodecanoate; C12:0) exhibits beneficial metabolic effects, individual functional differences remain unclear.

Methods

MCTs or MCFAs were administrated to male GPR84-deficient mice with a C57BL/6J background and mouse enteroendocrine cell line STC-1, and the effects on glucose homeostasis and gut hormone GLP-1 secretion were evaluated.

Results

C10:0 intake improves glucose metabolism through the MCFA receptor GPR84-mediated GLP-1 secretion. Each MCT intake showed resistance to obesity and improved metabolic parameters compared with lard intake. Moreover, oral administration of MCTs enhanced glucose tolerance, especially C10:0 administration, which sufficiently increased plasma GLP-1 levels. Additionally, C10:0 stimulation promoted GLP-1 secretion via GPR84 in STC-1, enhanced glucose tolerance through GPR84-mediated GLP-1 secretion, and showed resistance to high-fat diet (HFD)-induced obesity in mice.

Conclusions

Dietary MCT (C10:0) intake efficiently may protect against obesity and improve insulin resistance via GLP-1 secretion.

Discovery and Characterization of Novel Antagonists of the Proinflammatory Orphan Receptor GPR84

GPR84 is a poorly characterized, nominally orphan, proinflammatory G protein-coupled receptor that can be activated by medium chain length fatty acids. It is attracting considerable interest as a potential therapeutic target for antagonist ligands in both inflammatory bowel diseases and idiopathic pulmonary fibrosis. Successful screening of more than 300 000 compounds from a small molecule library followed by detailed analysis of some 50 drug-like hits identified 3-((5,6-bis(4-methoxyphenyl)-1,2,4-triazin-3-yl)methyl)-1H-indole as a high affinity and highly selective competitive antagonist of human GPR84. Tritiation of a di-iodinated form of the core structure produced [³H]3-((5,6-diphenyl-1,2,4-triazin-3-yl)methyl)-1H-indole, which allowed effective measurement of receptor levels in both transfected cell lines and lipopolysaccharide-treated THP-1 monocyte/macrophage cells. Although this compound series lacks significant affinity at mouse GPR84, homology modeling and molecular dynamics simulations provided a potential rationale for this difference, and alteration of two residues in mouse GPR84 to the equivalent amino acids in the human orthologue, predicted to open the antagonist binding pocket, validated this model. Sequence alignment of other species orthologues further predicted binding of the compounds as high affinity antagonists at macaque, pig, and dog GPR84 but not at the rat orthologue, and pharmacological experiments confirmed these predictions. These studies provide a new class of GPR84 antagonists that display species selectivity defined via receptor modeling and mutagenesis.

Immune-metabolic receptor GPR84 surrogate and endogenous agonists, 6-OAU and lauric acid, alter Brucella abortus 544 infection in both in vitro and in vivo systems

Brucella abortus, one of the most important members of the genus Brucella responsible for human disease, is an intracellular pathogen capable of avoiding or interfering components of the host immune responses that are critical for its virulence. GPR84, on the other hand, is a seven-transmembrane GPCR involved in the inflammatory response and its induced expression was associated with B. abortus infection of RAW264.7 cells. Here we examined the effects of the reported GPR84 surrogate and endogenous agonists, namely 6-n-octylaminouracil (6-OAU) and lauric acid (LU), respectively in the progression of B. abortus infection in a cell and mouse models. The in vitro studies revealed the LU had bactericidal effect against Brucella starting at 24 h post-incubation. Adhesion of Brucella to RAW264.7 cells was attenuated in both 6-OAU and LU treatments. Brucella uptake was observed to be inhibited in a dose and time-dependent manner in 6-OAU but only at the highest non-cytotoxic concentration in LU-treated cells. However, survival of Brucella within the cells was reduced only in LU-treated cells. We also investigated the possible inhibitory effects of the agonist in other Gram-negative bacterium, Salmonella Typhimurium and we found that both adhesion and uptake were inhibited in 6-OAU treatment and only the intracellular survival for LU treatment. Furthermore, 6-OAU treatment reduced ERK phosphorylation and MCP-1 secretion during Brucella infection as well as reduced MALT1 protein expression and ROS production in cells without infection. LU treatment attenuated ERK and JNK phosphorylation, MCP-1 secretion and NO accumulation but increased ROS production during infection, and similar pattern with MALT1 protein expression. The in vivo studies showed that both treatments via oral route augmented resistance to Brucella infection but more pronounced with 6-AOU as observed with reduced bacterial proliferation in spleens and livers. At 7 d post-treatment and 14 d post-infection, 6-OAU-treated mice displayed reduced IFN-γ serum level. At 7 d post-infection, high serum level of MCP-1 was observed in both treatments with the addition of TNF-α in LU group. IL-6 was increased in both treatments at 14 d post-infection with higher TNF-α, MCP-1 and IL-10 in LU group. Taken together, 6-OAU and LU are potential candidates representing pharmaceutical strategy against brucellosis and possibly other intracellular pathogens or inflammatory diseases.

GPR84 Is Essential for the Taste of Medium Chain Saturated Fatty Acids

The ability of mammalian taste cells to respond to fatty acids (FAs) has garnered significant attention of late and has been proposed to represent a sixth primary taste. With few exceptions, studies on FA taste have centered exclusively on polyunsaturated FAs, most notably on linoleic acid. In the current study, we have identified an additional FA receptor, GPR84, in the gustatory system that responds to the medium-chain saturated FAs (MCFAs) in male mice. GPR84 ligands activate both Type II and Type III taste cells in calcium imaging and patch-clamp recording assays. MCFAs depolarize and lead to a rise in intracellular free [Ca2+] in mouse taste cells in a concentration-dependent fashion, and the relative ligand specificity in taste cells is consistent with the response profile of GPR84 expressed in a heterologous system. A systemic Gpr84-/- mouse model reveals a specific deficit in both the neural (via chorda tympani recording) and behavioral responses to administration of oral MCFAs compared with WT mice. Together, we show that the peripheral taste system can respond to an additional class of FAs, the saturated FAs, and that the cognate receptor necessary for this ability is GPR84.

Modulation of the G-Protein-Coupled Receptor 84 (GPR84) by Agonists and Antagonists

Since the discovery of medium-chain fatty acids as GPR84 ligands, significant advancements have been made in the development of GPR84 agonists and antagonists. Most agonists have lipid-like structures except for 3,3'-diindolylmethane (DIM), which acts as an allosteric agonist. GPR84 activation in macrophages leads to increased cytokine secretion, chemotaxis, and phagocytosis, revealing the proinflammatory role of GPR84 associated with various inflammatory responses. Three GPR84 antagonists (S)-2-((1,4-dioxan-2-yl)methoxy)-9-(cyclopropylethynyl)-6,7-dihydro-4H-pyrimido[6,1-a]isoquinolin-4-one (GLPG1205), sodium 2-(3-pentylphenyl)acetate (PBI-4050), and sodium 2-(3,5-dipentylphenyl)acetate (PBI-4547) have displayed therapeutic effects in animal models of several inflammatory and fibrotic diseases and are being evaluated in clinical studies. Although GLPG1205 has failed in a clinical trial for ulcerative colitis, it is undergoing another phase II clinical study for idiopathic pulmonary fibrosis. Further studies are needed to resolve the GPR84 structure, identify more endogenous ligands, elucidate their physiological and pathological roles, and fulfill the therapeutic potential of GPR84 antagonists and agonists.

Therapeutic validation of an orphan G protein-coupled receptor: The case of GPR84

Despite the importance of members of the GPCR superfamily as targets of a broad range of effective medicines many GPCRs remain poorly characterised. GPR84 is an example. Expression of GPR84 is strongly up regulated in immune cells in a range of pro-inflammatory settings and clinical trials to treat idiopathic pulmonary fibrosis are currently ongoing using ligands with differing levels of selectivity and affinity as GPR84 antagonists. Although blockade of GPR84 may potentially prove effective also in diseases associated with inflammation of the lower gut there is emerging interest in defining if agonists of GPR84 might find utility in conditions in which regulation of metabolism or energy sensing is compromised. Here, we consider the physiological and pathological expression profile of GPR84 and, in the absence of direct structural information, recent developments and use of GPR84 pharmacological tool compounds to study its broader role and biology.

Antioxidant Properties of Embelin in Cell Culture. Electrochemistry and Theoretical Mechanism of Scavenging. Potential Scavenging of Superoxide Radical through the Cell Membrane

Embelin, a plant natural product found in Lysimachia punctata (Primulaceae), and Embelia ribes Burm (Myrsinaceae) fruit, possesses interesting biological and pharmacological properties. It is a unique chemical species as it includes both quinone and hydroquinone functional groups plus a long hydrophobic tail. By using hydrodynamic voltammetry, which generates the superoxide radical in situ, we show an unusual scavenging capability by embelin. Embelin as a scavenger of superoxide is stronger than the common food additive antioxidant 2,6-bis(1,1-dimethylethyl)-4-20 methylphenol, (butylated hydroxytoluene, BHT). In fact, embelin is even able to completely abolish the superoxide radical in the voltaic cell. Computational results indicate that two different types of embelin scavenging actions may be involved, initially through π–π interaction and followed by proton capture in the cell. A related mechanism describes embelin’s ability to circumvent superoxide leaking by transforming the anion radical into molecular oxygen. In order to confirm its antioxidant properties, its biological activity was tested in a study carried out in THP-1 human leukemic monocytes and BV-2 mice microglia. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, proliferation curves and antioxidant activity by the use of a fluorescent probe showed good antioxidant properties at 24 h. This suggests that embelin’s long alkyl C10 tail may be useful for cell membrane insertion which stimulates the antioxidant defense system, and cytoprotection in microglia. In conclusion, embelin could be an interesting pharmacological tool able to decrease the damage associated with metabolic and neurodegenerative diseases.

The Expression of Glycoprotein Genes in the Inflammatory Process of Kawasaki Disease

Background: Kawasaki disease (KD) is the most common form of febrile coronary vasculitis disease to occur in children. Early diagnosis and proper therapy can prevent the complication of coronary artery lesions (CAL). The main pathogenesis of KD is an inflammatory process related to the host's genetic characteristics. In innate human immunity, the interaction of leukocytes and glycoprotein plays an important role against microbes. The purpose of our study was to understand the role of leukocytes' glycoprotein genes during the acute phase of KD. Materials and Methods: We enrolled a total of 97 subjects from a medical center. Of those, 24 subjects were healthy controls, and 24 subjects were fever controls; the other 49 subjects were KD patients who had had blood samples taken both before and after IVIG treatment. We collected the total RNA from leukocytes and performed a quantitative polymerase chain reaction for the HP, GRP84, and CLEC4D genes in real time. Results: Compared with both the healthy and fever controls, the upregulation of HP, GRP84, and CLEC4D genes was significant in peripheral leukocytes during acute-phase KD. The transcriptional level of these respective genes not only demonstrated a positive correlation with each other, but were also effective predictors for KD (all auROC >0.87) according to the ROC curve analysis. The hyper-expression of these three genes was significantly associated with IVIG resistance, but not CAL formation. Conclusions: Our study demonstrates that the expression of HP, GRP84, and CLEC4D genes of leukocytes play an important role in the pathogenesis and primary IVIG response during the acute inflammatory process of KD.

A Biased Agonist at Immunometabolic Receptor GPR84 Causes Distinct Functional Effects in Macrophages

GPR84 is an orphan G-protein-coupled receptor that is expressed on immune cells and implicated in several inflammatory diseases. The validation of GPR84 as a therapeutic target is hindered by the narrow range of available chemical tools and consequent poor understanding of GPR84 pathophysiology. Here we describe the discovery and characterization of DL-175, a potent, selective, and structurally novel GPR84 agonist and the first to display significantly biased signaling across GPR84-overexpressing cells, primary murine macrophages, and human U937 cells. By comparing DL-175 with reported GPR84 ligands, we show for the first time that biased GPR84 agonists have markedly different abilities to induce chemotaxis in human myeloid cells, while causing similar levels of phagocytosis enhancement. This work demonstrates that biased agonism at GPR84 enables the selective activation of functional responses in immune cells and delivers a high-quality chemical probe for further investigation.

Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84

Fatty acid receptors have been recognized as important players in glycaemic control. This study is the first to describe a role for the medium-chain fatty acid (MCFA) receptor G-protein-coupled receptor (Gpr) 84 in skeletal muscle mitochondrial function and insulin secretion. We are able to show that Gpr84 is highly expressed in skeletal muscle and adipose tissue. Mice with global deletion of Gpr84 [Gpr84 knockout (KO)] exhibit a mild impairment in glucose tolerance when fed a MCFA-enriched diet. Studies in mice and pancreatic islets suggest that glucose intolerance is accompanied by a defect in insulin secretion. MCFA-fed KO mice also exhibit a significant impairment in the intrinsic respiratory capacity of their skeletal muscle mitochondria, but at the same time also exhibit a substantial increase in mitochondrial content. Changes in canonical pathways of mitochondrial biogenesis and turnover are unable to explain these mitochondrial differences. Our results show that Gpr84 plays a crucial role in regulating mitochondrial function and quality control.-Montgomery, M. K., Osborne, B., Brandon, A. E., O'Reilly, L., Fiveash, C. E., Brown, S. H. J., Wilkins, B. P., Samsudeen, A., Yu, J., Devanapalli, B., Hertzog, A., Tolun, A. A., Kavanagh, T., Cooper, A. A., Mitchell, T. W., Biden, T. J., Smith, N. J., Cooney, G. J., Turner, N. Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84.

Fatty Acid Signaling Mechanisms in Neural Cells: Fatty Acid Receptors

Fatty acids (FAs) are typically associated with structural and metabolic roles, as they can be stored as triglycerides, degraded by β-oxidation or used in phospholipids’ synthesis, the main components of biological membranes. It has been shown that these lipids exhibit also regulatory functions in different cell types. FAs can serve as secondary messengers, as well as modulators of enzymatic activities and substrates for cytokines synthesis. More recently, it has been documented a direct activity of free FAs as ligands of membrane, cytosolic, and nuclear receptors, and cumulative evidence has emerged, demonstrating its participation in a wide range of physiological and pathological conditions. It has been long known that the central nervous system is enriched with poly-unsaturated FAs, such as arachidonic (C20:4ω-6) or docosohexaenoic (C22:6ω-3) acids. These lipids participate in the regulation of membrane fluidity, axonal growth, development, memory, and inflammatory response. Furthermore, a whole family of low molecular weight compounds derived from FAs has also gained special attention as the natural ligands for cannabinoid receptors or key cytokines involved in inflammation, largely expanding the role of FAs as precursors of signaling molecules. Nutritional deficiencies, and alterations in lipid metabolism and lipid signaling have been associated with developmental and cognitive problems, as well as with neurodegenerative diseases. The molecular mechanism behind these effects still remains elusive. But in the last two decades, different families of proteins have been characterized as receptors mediating FAs signaling. This review focuses on different receptors sensing and transducing free FAs signals in neural cells: (1) membrane receptors of the family of G Protein Coupled Receptors known as Free Fatty Acid Receptors (FFARs); (2) cytosolic transport Fatty Acid-Binding Proteins (FABPs); and (3) transcription factors Peroxisome Proliferator-Activated Receptors (PPARs). We discuss how these proteins modulate and mediate direct regulatory functions of free FAs in neural cells. Finally, we briefly discuss the advantages of evaluating them as potential targets for drug design in order to manipulate lipid signaling. A thorough characterization of lipid receptors of the nervous system could provide a framework for a better understanding of their roles in neurophysiology and, potentially, help for the development of novel drugs against aging and neurodegenerative processes.

GPR34 in spinal microglia exacerbates neuropathic pain in mice

Background

Neuropathic pain is caused by sensory nerve injury, but effective treatments are currently lacking. Microglia are activated in the spinal dorsal horn after sensory nerve injury and contribute to neuropathic pain. Accordingly, molecules expressed by these cells are considered potential targets for therapeutic strategies. Our previous gene screening study using a mouse model of motor nerve injury showed that the G-protein-coupled receptor 34 gene (GPR34) is induced by nerve injury. Because GPR34 is now considered a microglia-enriched gene, we explored the possibility that it might be involved in microglial activation in the dorsal horn in a mouse model of neuropathic pain.

Methods

mRNA expression of GPR34 and pro-inflammatory molecules was determined by quantitative real-time PCR in wild-type and GPR34-deficient mice with L4 spinal nerve injury. In situ hybridization was used to identify GPR34 expression in microglia, and immunohistochemistry with the microglial marker Iba1 was performed to examine microglial numbers and morphology. Mechanical sensitivity was evaluated by the von Frey hair test. Liquid chromatography–tandem mass spectrometry quantified expression of the ligand for GPR34, lysophosphatidylserine (LysoPS), in the dorsal horn, and a GPR34 antagonist was intrathecally administrated to examine the effect of inhibiting LysoPS-GPR34 signaling on mechanical sensitivity.

Results

GPR34 was predominantly expressed by microglia in the dorsal horn after L4 nerve injury. There were no histological differences in microglial numbers or morphology between WT and GPR34-deficient mice. However, nerve injury-induced pro-inflammatory cytokine expression levels in microglia and pain behaviors were significantly attenuated in GPR34-deficient mice. Furthermore, the intrathecal administration of the GPR34 antagonist reduced neuropathic pain.

Conclusions

Inhibition of GPR34-mediated signal by GPR34 gene deletion reduced nerve injury-induced neuropathic pain by suppressing pro-inflammatory responses of microglia without affecting their morphology. Therefore, the suppression of GPR34 activity may have therapeutic potential for alleviating neuropathic pain.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1458-8) contains supplementary material, which is available to authorized users.

On-target and off-target effects of novel orthosteric and allosteric activators of GPR84

Many members of the G protein-coupled receptor family, including examples with clear therapeutic potential, remain poorly characterised. This often reflects limited availability of suitable tool ligands with which to interrogate receptor function. In the case of GPR84, currently a target for the treatment of idiopathic pulmonary fibrosis, recent times have seen the description of novel orthosteric and allosteric agonists. Using 2-(hexylthiol)pyrimidine-4,6 diol (2-HTP) and di(5,7-difluoro-1H-indole-3-yl)methane (PSB-16671) as exemplars of each class, in cell lines transfected to express either human or mouse GPR84, both ligands acted as effective on-target activators and with high co-operativity in their interactions. This was also the case in lipopolysaccharide-activated model human and mouse immune cell lines. However in mouse bone-marrow-derived neutrophils, where expression of GPR84 is particularly high, the capacity of PSB-16671 but not of 2-HTP to promote G protein activation was predominantly off-target because it was not blocked by an antagonist of GPR84 and was preserved in neutrophils isolated from GPR84 deficient mice. These results illustrate the challenges of attempting to study and define functions of poorly characterised receptors using ligands that have been developed via medicinal chemistry programmes, but where assessed activity has been limited largely to the initially identified target.

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.

Activation of the Immune-Metabolic Receptor GPR84 Enhances Inflammation and Phagocytosis in Macrophages

GPR84 is a member of the metabolic G protein-coupled receptor family, and its expression has been described predominantly in immune cells. GPR84 activation is involved in the inflammatory response, but the mechanisms by which it modulates inflammation have been incompletely described. In this study, we investigated GPR84 expression, activation, and function in macrophages to establish the role of the receptor during the inflammatory response. We observed that GPR84 expression in murine tissues is increased by endotoxemia, hyperglycemia, and hypercholesterolemia. Ex vivo studies revealed that GPR84 mRNA expression is increased by LPS and other pro-inflammatory molecules in different murine and human macrophage populations. Likewise, high glucose concentrations and the presence of oxidized LDL increased GPR84 expression in macrophages. Activation of the GPR84 receptor with a selective agonist, 6-(octylamino) pyrimidine-2,4(1H,3H)-dione (6-n-octylaminouracil, 6-OAU), enhanced the expression of phosphorylated Akt, p-ERK, and p65 nuclear translocation under inflammatory conditions and elevated the expression levels of the inflammatory mediators TNFα, IL-6, IL-12B, CCL2, CCL5, and CXCL1. In addition, GPR84 activation triggered increased bacterial adhesion and phagocytosis in macrophages. The enhanced inflammatory response mediated by 6-OAU was not observed in GPR84-/- cells nor in macrophages treated with a selective GPR84 antagonist. Collectively, our results reveal that GPR84 functions as an enhancer of inflammatory signaling in macrophages once inflammation is established. Therefore, molecules that antagonize the GPR84 receptor may be potential therapeutic tools in inflammatory and metabolic diseases.

Embelin and its derivatives unravel the signaling, proinflammatory and antiatherogenic properties of GPR84 receptor

GPR84 is an orphan G-protein coupled receptor, expressed on monocytes, macrophages and neutrophils and is significantly upregulated by inflammatory stimuli. The physiological role of GPR84 remains largely unknown. Medium chain fatty acids (MCFA) activate the receptor and have been proposed to be its endogenous ligands, although the high concentrations of MCFAs required for receptor activation generally exceed normal physiological levels. We identified the natural product embelin as a highly potent and selective surrogate GPR84 agonist (originally disclosed in patent application WO2007027661A2, 2007) and synthesized close structural analogs with widely varying receptor activities. These tools were used to perform a comprehensive study of GPR84 signaling and function in recombinant cells and in primary human macrophages and neutrophils. Activation of recombinant GPR84 by embelin in HEK293 cells results in G_i/o as well as G12/13-Rho signaling. In human macrophages, GPR84 initiates PTX sensitive Erk1/2 and Akt phosphorylation, PI-3 kinase activation, calcium flux, and release of prostaglandin E2. In addition, GPR84 signaling in macrophages elicits G_i Gβγ-mediated augmentation of intracellular cAMP, rather than the decrease expected from G_iα engagement. GPR84 activation drives human neutrophil chemotaxis and primes them for amplification of oxidative burst induced by FMLP and C5A. Loss of GPR84 is associated with attenuated LPS-induced release of proinflammatory mediators IL-6, KC-GROα, VEGF, MIP-2 and NGAL from peritoneal exudates. While initiating numerous proinflammatory activities in macrophages and neutrophils, GPR84 also possesses GPR109A-like antiatherosclerotic properties in macrophages. Macrophage receptor activation leads to upregulation of cholesterol transporters ABCA1 and ABCG1 and stimulates reverse cholesterol transport. These data suggest that GPR84 may be a target of therapeutic value and that distinct modes of receptor modulation (inhibition vs. stimulation) may be required for inflammatory and atherosclerotic indications.