OBJECTIVE To identify the mechanisms by which the formyl peptide receptor 2 (FPR2) mediates both inflammatory and anti-inflammatory signaling in an agonist-dependent manner. METHODS Cells expressing FPR2 were incubated with weak agonists, Aβ42 and Ac2-26, before stimulation with a strong agonist, WKYMVm. Calcium mobilization, cAMP inhibition and MAP kinase activation were measured. Intramolecular FRET were determined using FPR2 constructs with an ECFP attached to the C- terminus and a FlAsH binding motif embedded in the first or third intracellular loop (IL1 or IL3, respectively). RESULTS Aβ42 did not induce significant Ca2 + mobilization, but positively modulated WKYMVm-induced Ca2 + mobilization and cAMP reduction in a dose-variable manner within a narrow range of ligand concentrations. Treating FPR2-expressing cells with Ac2-26, a peptide with anti-inflam?matory activity, negatively modulated WKYMVm-induced Ca2 + mobilization and cAMP reduction. Intra?molecular FRET assay showed that stimulation of the receptor constructs with Aβ42 brought the C-terminal domain closer to IL1 but away from IL3. An opposite conformational change was induced by Ac2-26. The FPR2 conformation induced by Aβ42 corresponded to enhanced ERK phosphorylation and attenuated p38 MAPK phosphorylation, whereas Ac2-26 induced FPR2 conformational change corresponding to elevated p38 MAPK phosphorylation and reduced ERK phosphorylation. CONCLUSION Aβ42 and Ac2-26 induce different conformational changes in FPR2. These findings provide a structural basis for FPR2 mediation of inflammatory vs anti-inflammatory functions and identify a type of receptor modulation that differs from the classic positive and negative allosteric modulation.

OBJECTIVE The chemokine-like receptor 1 (CMKLR1, ChemR23) is a functional receptor for chemerin, the chemerin-derived nonapeptide (C9), and the amyloid β peptide 1-42 (Aβ42). Because these peptides share little sequence homology, studies were conducted to investigate their pharmaco?logical properties and regulation at CMKLR1. METHODS Cells expressing CMKLR1 were incubated with Aβ42 before stimulation with a strong agonist, the C9 peptide. Calcium mobilization, cAMP inhibition and MAP kinase activation were measured. Intramolecular FRET were determined using CMKLR1 constructs with an ECFP attached to the C- terminus and a FlAsH binding motif embedded in the first intracellular loop (IL1). RESULTS Binding of both Aβ42 and the C9 peptide induced CMKLR1 internal?ization, but only the Aβ42-induced receptor internalization involved clathrin-coated pits. Likewise, Aβ42 but not C9 stimulated β-arrestin 2 translocation to plasma membranes. A robust Ca2+ flux was observed following C9 stimulation, whereas Aβ42 was ineffective even at micromolar concentrations. Despite its low potency in calcium mobilization assay, Aβ42 was able to alter C9 -induced Ca2+ flux in dose-dependent manner: a potentiation effect at 100 pmol·L-1 of Aβ42 was followed by a suppression at 10 nmol·L-1 and further potentiation at 1 μmol·L-1. This unusual and biphasic modulatory effect was also seen in the C9-induced ERK phosphorylation but the dose curve was opposite to that of Ca2+ flux and cAMP inhibition, suggesting a reciprocal regulatory mechanism. Intramolecular FRET assay confirmed that Aβ42 modulates CMKLR1 rather than its downstream signaling pathways. CONCLUSION These findings suggest Aβ42 as an allosteric modulator that can both positively and negatively regulate the activation state of CMKLR1 in a manner that differs from existing allosteric modulatory mechanisms.

Since the Monod-Wyman-Changeux (MWC) model was initially proposed to explain the allosteric interactions between proteins and their ligands 50 years ago, there have been various models and hypotheses such as the induced-fit model on the interaction. These theoretical developments have been used broadly in the study of allosteric modulations of enzymes and receptors. In 1980, Lefkowitz and coworkers proposed a ternary complex model (TCM) for the regulatory mechanism of G protein-coupled receptors (GPCRs) that laid the theoretical foundation in the study of allosteric sites and ligands of GPCRs, the largest family of known receptors. The findings on how ligands interact with receptors to cause a functional response have significantly impacted the drug discovery field and accelerated the identification of allosteric modulators.