GPR84 belongs to a peculiar category of cell-surface receptors: it is an orphan G protein–coupled receptor (GPCR), meaning that although it is known to respond to medium-chain fatty acids released during metabolic and inflammatory stress, its physiological ligand repertoire remains only partially defined. Expressed predominantly on macrophages, neutrophils, and other cells of the innate immune system, GPR84 has for over a decade been recognized as a credible therapeutic target in conditions ranging from idiopathic pulmonary fibrosis to inflammatory bowel disease and severe asthma, precisely because its activation amplifies the inflammatory and phagocytic programs that these diseases exploit or dysregulate. Yet turning that recognition into a viable drug has proven difficult, largely because GPR84 does not signal through a single, clean pathway.
Like most class A GPCRs, GPR84 can engage two functionally distinct downstream branches upon activation: a G protein–mediated pathway, principally through Gi, and a β-arrestin–mediated pathway, which governs receptor desensitization, internalization, and a separate set of cellular outcomes including chemotaxis. Full agonists activate both branches indiscriminately, which is part of why early GPR84-targeted compounds have produced inconsistent clinical results, including the failure of the antagonist GLPG1205 in ulcerative colitis despite encouraging effects in fibrotic disease models. Biased agonism — the ability of a ligand to selectively favor one branch over the other — has therefore emerged as an attractive strategy, but until now the structural basis for why some GPR84 ligands are biased and others are not has remained essentially unknown.
A study published in July 2026 in Nature Communications by Suzuki, Fujiyoshi, and colleagues at the Institute of Science Tokyo addresses this gap directly. The team used cryo-electron microscopy to resolve two complementary structural states of GPR84: the receptor’s inactive conformation bound to the antagonist GLPG1205, and its active, Gi-coupled conformation bound to DL-175, a well-characterized G protein–biased agonist. By combining these structures with cellular signaling assays and molecular dynamics simulations, the researchers reconstructed the conformational landscape spanning inactivation and biased activation, rather than relying on a single static snapshot.
The mechanistic payoff is unusually precise: a localized steric clash between DL-175 and a single leucine residue, positioned at what structural biologists designate site 6.52 on the receptor’s sixth transmembrane helix, selectively obstructs the conformational rearrangement that β-arrestin recruitment requires, while leaving the movements needed for Gi activation untouched. In other words, one side chain acts as a molecular gatekeeper, deciding which downstream branch of the signaling network gets switched on.
This granularity is scientifically satisfying, but it also invites caution: reducing a multifactorial allosteric phenomenon to a single steric contact, however elegant, risks oversimplifying a process that in living tissue involves receptor phosphorylation patterns, membrane composition, and cell-type-specific arrestin isoforms that this cryo-EM and simulation-based study did not directly examine. The structures were solved in reconstituted, heterologous systems rather than native immune cells, and the therapeutic hypothesis they support — that steric tuning around this pocket could yield antagonists or partial agonists with more favorable selectivity than first-generation compounds such as GLPG1205 — still awaits validation in disease-relevant models. Unfortunately, no in vivo data accompany this structural work, leaving open how faithfully the isolated receptor-ligand geometry predicts behavior in inflamed or fibrotic tissue. Nevertheless, by converting an empirical observation about ligand bias into an atomic-resolution structural rationale, this work supplies medicinal chemists with a genuine design blueprint rather than a trial-and-error target, an advance whose significance will ultimately be measured by whether it yields anti-inflammatory and antifibrotic compounds capable of separating therapeutic benefit from the side effects that have hampered this receptor’s clinical history so far.
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