Historically, ligands for GPCRs have been identified before their receptor counterparts. With the cloning revolution, several unidentified receptors have been found and were labelled as “orphan” for their endogenous ligands. Orphan GPCRs have been shown to play key roles in various physiological functions, such as sensory perception, reproduction, development, growth, metabolism, and are also linked to major diseases, such as neuroinflammatory, metabolic and autoimmune diseases. Therefore, matching a ligand to an orphan GPCRs, the process of de-orphanizing, is of great importance in order to better understanding human physiology as well as to dissect the molecular mechanism governing the involvement of these receptors in human pathology.
GPR84 is an example of an orphan GPCR (Sharman et al., 2011), although it is widely accepted that medium‐chain fatty acids (MCFAs) can bind to and activate this receptor with modest potency. GPR84 is a Gi‐coupled class A GPCR mainly expressed in immune cells and microglia in the brain (Wojciechowicz & Ma'ayan, 2020). GPR84 has been shown to be an attractive target in pro‐inflammatory conditions (Gagnon et al., 2018; Suzuki et al., 2013; Vermeire et al., 2017; Wojciechowicz & Ma'ayan, 2020) and efforts have been made to discover GPR84 antagonists.
In this study Marsango et al. address two key questions in GPR84 biology and pharmacology: 1. how GPR84 expression profile correlates with physiological and pathological conditions? and 2. which ligands can be used as tool compounds to study the function and biology of this receptor?
Regarding the first question, GPR84 overexpression in immune cells in a range of pro‐inflammatory disorders renders it a promising target in inflammatory and fibrotic conditions, including neuroinflammation (Audoy‐Remus et al., 2015), with ongoing clinical trials in idiopathic pulmonary fibrosis (Labéguère et al., 2014). GPR84 has been additionally proposed to be a potential biomarker in different inflammatory diseases (Arijs et al., 2011; Planell et al., 2017). Some studies have also reported GPR84 involvement in pain, atherosclerosis, and even metabolic disorders (Nicol et al., 2015, Audoy‐Remus et al., 2015, Du Toit et al., 2018).
Regarding the second question, there is still a lot to be done in respect to tool compounds to study the function of this receptor towards clinical validation, as well as radiopharmaceuticals, including potential PET ligands, and suitable antibodies. Recent work has shown distinct functional outcomes of agonist ligands (Pillaiyar et al., 2018) with biased properties which can help to better elucidate the molecular pharmacology of this receptor. In addition, several GPR84 ligands have been described as well as GPR84 knockout mice. Among these ligands are orthosteric agonists such as alkylpyrimidine‐4,6‐diol derivatives (Liu et al., 2016; Zhang et al., 2016) and embelin (2,5‐dihydroxy‐3‐undecyl‐1,4‐benzoquinone) which is a natural product derived from the plant Embelia ribes (Gaidarov et al., 2018) which agonizes GPR84 but, interestingly, blocks the chemokine receptor CXCR2 and the adenosine A3 receptor (Gaidarov et al., 2018). IM (3,3′‐methylenebis‐1H‐indole) has been identified as a positive allosteric modulator of GPR84, a metabolite produced in vivo from indole‐3‐carbinol, which is present at high levels in some vegetables including broccoli and kale (Wang, Schoene, Milner, & Kim, 2012, Köse et al., 2020). GPR84 antagonists include a series of dihydropyrimidinoisoquinolinones (Labéguère et al., 2014), which behave as non‐competitive antagonists of GPR84 (Labéguère et al., 2020). From these series of compounds, GLPG1205 progressed into clinical development for the potential treatment of ulcerative colitis although it did not demonstrate sufficient efficacy (Labéguère et al., 2020).
Overall, GPR84 is a promising target to exploit and the investment in better tools to study its function in both disease and physiological settings will likely potentiate drug discovery campaigns against this orphan GPCR.
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