We have previously reported (Badolato, R., J.M. Wang, W.J. Murphy, A.R. Lloyd, D.F. Michiel, L.L. Bausserman, D.J. Kelvin, and J.J. Oppenheim. 1994. J. Exp. Med. 180:203; Xu, L., R. Badolato, W.J. Murphy, D.L. Longo, M. Anver, S. Hale, J.J. Oppenheim, and J.M. Wang. 1995. J. Immunol. 155:1184.) that the acute phase protein serum amyloid A (SAA) is a potent chemoattractant for human leukocytes in vitro and mouse phagocytes in vivo. To identify the signaling mechanisms, we evaluated patterns of cross-desensitization between SAA and other leukocyte chemoattrctants. We found that the chemotactic bacterial peptide, N-formyl- methionyl-leucyl-phenylalanine (fMLP), was able to specifically attenuate Ca²⁺ mobilization in human phagocytes induced by SAA, but only at very high concentrations, suggesting that SAA uses a low affinity fMLP receptor. Here we demonstrate that SAA selectively induced Ca²⁺ mobilization and migration of HEK cells expressing FPRL1, a human seven-transmembrane domain phagocyte receptor with low affinity for fMLP, and high affinity for lipoxin A4. Furthermore, radiolabeled SAA specifically bound to human phagocytes and FPRL1-transfected 293 cells. In contrast, SAA was not a ligand or agonist for FPR, the high affinity fMLP receptor. Thus, SAA is the first chemotactic ligand identified for FPRL1. Our results suggest that FPRL1 mediates phagocyte migration in response to SAA.

Amyloid beta (Abeta) is a major contributor to the pathogenesis of Alzheimer's disease (AD). Although Abeta has been reported to be directly neurotoxic, it also causes indirect neuronal damage by activating mononuclear phagocytes (microglia) that accumulate in and around senile plaques. In this study, we show that the 42 amino acid form of beta amyloid peptide, Abeta(42), is a chemotactic agonist for a seven-transmembrane, G-protein-coupled receptor named FPR-Like-1 (FPRL1), which is expressed on human mononuclear phagocytes. Moreover, FPRL1 is expressed at high levels by inflammatory cells infiltrating senile plaques in brain tissues from AD patients. Thus, FPRL1 may mediate inflammation seen in AD and is a potential target for developing therapeutic agents.

Alzheimer’s disease (AD) is characterized by overproduction of β amyloid peptides in the brain with progressive loss of neuronal cells. The 42-aa form of the β amyloid peptide (Aβ₄₂) is implied as a major causative factor, because it is toxic to neurons and elicits inflammatory responses in the brain by activating microglial cells. Despite the overproduction of Aβ₄₂, AD brain tissue also generates protective factor(s) that may antagonize the neurodestructive effect of Aβ₄₂. Humanin is a gene cloned from an apparently normal region of an AD brain and encodes a 24-aa peptide. Both secreted and synthetic Humanin peptides protect neuronal cells from damage by Aβ₄₂, and the effect of Humanin may involve putative cellular receptor(s). To elucidate the molecular identity of such receptor(s), we examined the activity of synthetic Humanin on various cells and found that Humanin induced chemotaxis of mononuclear phagocytes by using a human G protein-coupled formylpeptide receptor-like-1 (FPRL1) and its murine counterpart FPR2. Coincidentally, FPRL1 and FPR2 are also functional receptors used by Aβ₄₂ to chemoattract and activate phagocytic cells. Humanin reduced the aggregation and fibrillary formation by suppressing the effect of Aβ₄₂ on mononuclear phagocytes. In neuroblast cells, Humanin and Aβ₄₂ both activated FPRL1; however, only Aβ₄₂ caused apoptotic death of the cells, and its cytopathic effect was blocked by Humanin. We conclude that Humanin shares human FPRL1 and mouse FPR2 with Aβ₄₂ and suggest that Humanin may exert its neuroprotective effects by competitively inhibiting the access of FPRL1 to Aβ₄₂.

Trp-Lys-Tyr-Val-d-Met (WKYMVm) is a synthetic leukocyte-activating peptide postulated to use seven-transmembrane, G protein-coupled receptor(s). In the study to characterize the receptor(s) for WKYMVm, we found that this peptide induced marked chemotaxis and calcium flux in human phagocytes. The signaling induced by WKYMVm in phagocytes was attenuated by high concentrations of the bacterial chemotactic peptide fMLP, suggesting that WKYMVm might use receptor(s) for fMLP. This hypothesis was tested by using cells over expressing genes encoding two seven-transmembrane receptors, formyl peptide receptor (FPR) and formyl peptide receptor-like 1 (FPRL1), which are with high and low affinity for fMLP, respectively. Both FPR- and FPRL1-expressing cells mobilized calcium in response to picomolar concentrations of WKYMVm. While FPRL1-expressing cells migrated to picomolar concentrations of WKYMVm, nanomolar concentrations of the peptide were required to induce migration of FPR-expressing cells. In contrast, fMLP elicited both calcium flux and chemotaxis only in FPR-expressing cells with an efficacy comparable with WKYMVm. Thus, WKYMVm uses both FPR and FPRL1 to stimulate phagocytes with a markedly higher efficacy for FPRL1. Our study suggests that FPR and FPRL1 in phagocytes react to a broad spectrum of agonists and WKYMVm as a remarkably potent agonist provides a valuable tool for studying leukocyte signaling via these receptors.