Evolutionary analyses reveal immune cell receptor GPR84 as a conserved receptor for bacteria-derived molecules

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.

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.

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.

Function and regulation of GPR84 in human neutrophils

Human neutrophils are components of the innate immune system and are the most abundant white blood cells in the circulation. They are professional phagocytes and express several G protein-coupled receptors (GPCRs), which are essential for proper neutrophil functions. So far, the two formyl peptide receptors, FPR1 and FPR2, have been the most extensively studied group of neutrophil GPCRs, but recently, a new group, the free fatty acid (FFA) receptors, has attracted growing attention. Neutrophils express two FFA receptors, GPR84 and FFA2, which sense medium- and short-chain fatty acids respectively, and display similar activation profiles. The exact pathophysiological role of GPR84 is not yet fully understood, but it is generally regarded as a pro-inflammatory receptor that mediates neutrophil activation. In this review, we summarize current knowledge of how GPR84 affects human neutrophil functions and discuss the regulatory mechanisms that control these responses, focusing on the similarities and differences in comparison to the two FPRs and FFA2. 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.

Structural and pharmacological characterization of a medium-chain fatty acid receptor GPR84 in common carp (Cyprinus carpio)

The medium-chain fatty acid receptor GPR84, a member of the G protein-coupled receptor family, is mainly expressed in macrophages and microglia, and is involved in the regulation of inflammatory responses and retinal development in mammals and amphibians. However, structure, tissue distribution, and pharmacology of this receptor have rarely been reported in fish. In this study, we cloned the coding sequence (CDS) of common carp GPR84 (ccGPR84), examined its tissue distribution, and explored its cellular signaling function. The results showed that the CDS of ccGPR84 is 1191 bp and encodes a putative protein with 396 amino acids. Phylogenetic and chromosomal synteny analyses revealed that ccGPR84 was evolutionarily conserved with Cyprinids. Real-time quantitative PCR (qPCR) indicated that ccGPR84 was predominantly expressed in the intestine and spleen. Luciferase reporter assay demonstrated that nonanoic acid, capric acid (decanoic acid), undecanoic acid and lauric acid could inhibit cAMP signaling pathway and activate MAPK/ERK signaling pathway, while the potencies of these four fatty acids on the two signaling pathways were different. Lauric acid has the highest inhibitory potency on cAMP signaling pathway, followed by undecanoic acid, nonanoic acid, and capric acid. While for MAPK/ERK signaling pathway, nonanoic acid has the highest activation potency, followed by undecanoic acid, capric acid, and lauric acid. These findings lay the foundation for revealing the roles of different medium-chain fatty acids in the inflammatory response of common carp.

Protocol to characterize Gi/o and Gs protein-coupled receptors in transiently transfected cells using ELISA and cAMP measurements

Activation of Gs or Gi/o protein-coupled receptors (GPCRs) leads to changes of intracellular cyclic adenosine monophosphate (cAMP) levels. This protocol describes steps for cloning HA- and FLAG-tagged GPCRs, transient transfection of CHO-K1 or HEK293-T cells, and determination of basal and ligand-induced changes in intracellular cAMP levels. We detail enzyme-linked immunosorbent assays to determine relative GPCR plasma membrane and total expression levels. For complete details on the use and execution of this protocol, please refer to Schulze et al. (2022).1.