Human formyl peptide receptor 1 (FPR1), a G protein-coupled receptor on phagocytes, recognizes N-formylated peptides that originate either from bacteria or from mitochondria released during tissue damage. Gröper et al. (2020) tested whether biased agonism — ligand-selective activation of particular signaling pathways — lets FPR1 encode the source of the danger signal, and found no source-dependent pathway bias in a heterologous HeLa-FPR1 system: bacterial and mitochondria-derived agonists were all biased toward inhibition of cAMP formation, differing only in potency and efficacy. The questions the source paper left open are: (1) whether higher-order FPR complexes (homo- or hetero-oligomeric receptor assemblies) can decode ligand-source information beyond the pathway amplitudes and logistic parameters captured by monomeric-receptor assays, and (2) whether the observed signaling behavior generalizes to native immune cells, given the system bias of heterologous expression systems. A direct follow-up from the same group (still a preprint) shows the cellular environment profoundly reshapes FPR signaling — neutrophils lack the Gi-sensitive adenylyl cyclases ADCY5/ADCY6, so the cAMP-inhibition arm measured in HeLa cells does not even exist in native neutrophils — and studies through 2026 across eleven ligands at FPR1/FPR2 find little evidence of pathway bias. Meanwhile, a 2025 review states that the functional significance of FPR homo- and heterodimerization is still not understood, and no study has tested whether FPR oligomeric state changes signaling output in a ligand-source-dependent way. The problem — whether any mechanism, oligomeric or otherwise, lets FPR1 signaling in native phagocytes discriminate bacterial from mitochondrial formylated danger signals — remains open.

Background

Formyl peptide receptor 1 (FPR1) is a G protein-coupled receptor (GPCR) — a cell-surface receptor that signals via heterotrimeric G proteins — expressed abundantly on neutrophils and other phagocytes. Because mitochondria retain the bacterial practice of initiating protein synthesis with N-formylmethionine, N-formylated peptides arise both from invading bacteria (pathogen-associated molecular patterns, PAMPs) and from mitochondria released by damaged tissue (damage-associated molecular patterns, DAMPs). FPR1 binds both classes, which raises a signal-processing question: can an innate immune cell tell from FPR1 signaling alone whether the formylated peptide signals an infection or sterile tissue injury? Biased agonism denotes the ability of different ligands to stabilize distinct receptor conformations that preferentially activate some pathways over others; in the operational framework of ligand bias it is quantified by comparing, across pathways, the logarithm of the ratio of affinity and efficacy (transduction coefficient) of each agonist to a common reference agonist (yielding Δlog⁡R\Delta\log R and, between pathways, ΔΔlog⁡R\Delta\Delta\log R), with differences in potency (EC50EC_{50}, the half-maximally effective concentration) and maximal effect (EmaxE_{max}) reported separately. One recognized caveat is system bias: the pathway weighting observed in one cellular background need not hold in another, because cells differ in their effector and second-messenger machinery. Gröper et al. (2020) expressed human FPR1 in HeLa cells and compared agonist-evoked responses across three prototypical GPCR readouts — inhibition of cAMP (cyclic adenosine monophosphate, a second messenger) formation via the Gi protein, receptor internalization, and phosphorylation of the ERK1/2 (extracellular signal-regulated kinase) cascade — using matched bacterial (fMLF from E. coli, fMIVIL from Listeria monocytogenes), mitochondria-derived (the N-termini of MT-ND2, MT-ND6, and cytochrome b), viral, and host-derived peptides. Bacterial agonists showed higher potencies and efficacies, but no source-dependent pathway selectivity appeared: all ligand groups were similarly biased toward inhibition of cAMP formation, so the source of the signal was reflected only in the logistic parameters, not in the pathway choice. The authors noted that GPCR signaling can be further diversified by homo- and hetero-oligomerization of receptors: single-molecule imaging had established that the N-formyl peptide receptor undergoes a dynamic monomer–dimer equilibrium at the cell surface (Kasai et al. 2011), and ligand-specific conformational changes of the related receptor FPR2 were shown to depend on its dimerization state and to determine pro-resolving responses (Cooray et al. 2013). Whether such supramolecular FPR complexes can decode additional information — for example, the source of a formylated ligand — was posed as an open line of research, together with the question of whether the HeLa-FPR1 findings transfer to native immune cells, since the heterologous system may impose system bias. Subsequent structural work defined the monomeric side of the problem: cryo-EM structures of FPR1 in complex with the G protein Gi and various formyl peptides and agonists (Chen et al. 2022; Zhuang et al. 2022) showed how N-formyl peptides are recognized at the level of a single receptor protomer and how diverse agonists stabilize distinct conformations, but none of these structures addresses oligomeric assembly or ligand-source decoding.

Problem Statement

Determine whether FPR1 signaling can encode and decode the source of N-formylated danger ligands — distinguishing bacterial (PAMP) from mitochondria-derived (DAMP) formylated peptides — through mechanisms beyond the potency (EC50EC_{50}) and maximal-effect (EmaxE_{max}) differences already captured by monomeric-receptor assays in heterologous cells. This comprises two linked questions, exactly as posed by the source work: (1) whether higher-order FPR assemblies — FPR1 homodimers, FPR1/FPR2 heterodimers, or larger oligomeric receptor complexes — engage qualitatively different signaling outputs depending on ligand source, i.e., decode information that a monomeric receptor cannot; and (2) whether any source-discriminating signaling exists in native innate immune cells (such as neutrophils and monocytes) once the system bias of heterologous expression systems is removed, given that native cells lack part of the effector machinery (for example the Gi-sensitive adenylyl cyclases) assumed in those systems. A complete answer addresses both (1) and (2): it either identifies the molecular mechanism of source decoding (oligomerization state, ligand-specific receptor conformations, or cell-type-specific signaling machinery) with experimental evidence in native phagocytes using matched bacterial and mitochondrial N-formyl peptide panels under rigorous bias analysis, or demonstrates systematically that no such decoding exists in either setting — that ligand source is represented only in ligand potency and efficacy at a monomeric receptor.

References

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