The formyl peptide chemoattractant receptor is encoded by a 2 kilobase messenger RNA. Expression in Xenopus oocytes

International Union of Basic and Clinical Pharmacology. LXXIII. Nomenclature for the formyl peptide receptor (FPR) family

Formyl peptide receptors (FPRs) are a small group of seven-transmembrane domain, G protein-coupled receptors that are expressed mainly by mammalian phagocytic leukocytes and are known to be important in host defense and inflammation. The three human FPRs (FPR1, FPR2/ALX, and FPR3) share significant sequence homology and are encoded by clustered genes. Collectively, these receptors bind an extraordinarily numerous and structurally diverse group of agonistic ligands, including N-formyl and nonformyl peptides of different composition, that chemoattract and activate phagocytes. N-formyl peptides, which are encoded in nature only by bacterial and mitochondrial genes and result from obligatory initiation of bacterial and mitochondrial protein synthesis with N-formylmethionine, is the only ligand class common to all three human receptors. Surprisingly, the endogenous anti-inflammatory peptide annexin 1 and its N-terminal fragments also bind human FPR1 and FPR2/ALX, and the anti-inflammatory eicosanoid lipoxin A4 is an agonist at FPR2/ALX. In comparison, fewer agonists have been identified for FPR3, the third member in this receptor family. Structural and functional studies of the FPRs have produced important information for understanding the general pharmacological principles governing all leukocyte chemoattractant receptors. This article aims to provide an overview of the discovery and pharmacological characterization of FPRs, to introduce an International Union of Basic and Clinical Pharmacology (IUPHAR)-recommended nomenclature, and to discuss unmet challenges, including the mechanisms used by these receptors to bind diverse ligands and mediate different biological functions.

Calcium flux assay in Xenopus oocytes

Many G protein-coupled receptors of interest to neuroscientists induce transient increases in [Ca(2+)](i), which can be used as a convenient measure of receptor activation in a variety of applications. This unit describes a simple calcium flux assay applied to Xenopus oocytes.

Identification of a ligand binding site in the human neutrophil formyl peptide receptor using a site-specific fluorescent photoaffinity label and mass spectrometry

A novel fluorescent photoaffinity cross-linking probe, formyl-Met-p-benzoyl-L-phenylalanine-Phe-Tyr-Lys-epsilon-N-fluorescei n (fMBpaFYK-fl), was synthesized and used to identify binding site residues in recombinant human phagocyte chemoattractant formyl peptide receptor (FPR). After photoactivation, fluorescein-labeled membranes from Chinese hamster ovary cells were solubilized in octylglucoside and separated by tandem anion exchange and gel filtration chromatography. A single peak of fluorescence was observed in extracts of FPR-expressing cells that was absent in extracts from wild type controls. Photolabeled Chinese hamster ovary membranes were cleaved with CNBr, and the fluorescent fragments were isolated on an antifluorescein immunoaffinity matrix. Matrix-assisted laser desorption ionization mass spectrometry identified a major species with mass = 1754, consistent with the CNBr fragment of fMBpaFYK-fl cross-linked to Val-Arg-Lys-Ala-Hse (an expected CNBr fragment of FPR, residues 83-87). This peptide was further cleaved with trypsin, repurified by antifluorescein immunoaffinity, and subjected to matrix-assisted laser desorption ionization mass spectrometry. A tryptic fragment with mass = 1582 was observed, which is the mass of fMBpaFYK-fl cross-linked to Val-Arg-Lys (FPR residues 83-85), an expected trypsin cleavage product of Val-Arg-Lys-Ala-Hse. Residues 83-85 lie within the putative second transmembrane-spanning region of FPR near the extracellular surface. A 3D model of FPR is presented, which accounts for intramembrane, site-directed mutagenesis results (Miettinen, H. M., Mills, J., Gripentrog, J., Dratz, E. A., Granger, B. L., and Jesaitis, A. J. (1997) J. Immunol. 159, 4045-4054) and the photochemical cross-linking data.

The N-formyl peptide receptor: a model for the study of chemoattractant receptor structure and function

N-formyl peptides, such as fMet-Leu-Phe, are one of the most potent chemoattractants for phagocytic leukocytes. The interaction of N-formyl peptides with their specific cell surface receptors has been studied extensively and used as a model system for the characterization of G-protein-coupled signal transduction in phagocytes. The cloning of the N-formyl peptide receptor cDNA from several species and the identification of homologous genes have allowed detailed studies of structural and functional aspects of the receptor. Recent findings that the receptor is expressed in nonhematopoietic cells and that nonformylated peptides can activate the receptor suggest potentially novel functions and the existence of additional ligands for this receptor.

Complementation of formyl peptide receptor-mediated signal transduction in Xenopus laevis oocytes

Formyl-methionine-containing peptides (e.g. fMet-Leu-Phe) stimulate a variety of neutrophil functions by interacting with specific cell surface receptors which are coupled via G-proteins to stimulation of phospholipase C. Two markedly distinct cDNAs coding for formyl peptide receptors have recently been isolated from a rabbit and a human cDNA library respectively. To examine the hitherto unknown signal transduction properties of the formyl peptide receptor encoded by the human cDNA, we have used the PCR to clone this cDNA from poly(A)+ RNA of myeloid differentiated human leukaemia (HL-60) cells, and have injected the cDNA-derived receptor cRNA into Xenopus laevis oocytes. Receptor activity was determined electrophysiologically by measuring the agonist-dependent opening of intracellular Ca2+ concentration ([Ca2+]i)-independent Cl- channels. Injection of pure formyl peptide receptor cRNA did not lead to peptide-dependent changes in membrane current. In contrast, marked alterations of membrane current were observed in response to formyl peptides when the receptor cRNA was supplemented with poly(A)+ RNA isolated from undifferentiated HL-60 cells. Injection of the latter RNA did not lead to formyl-peptide-dependent alterations of membrane current. Binding studies using a radioiodinated formyl peptide revealed that injection of formyl peptide receptor cRNA alone led to expression of the formyl peptide receptor on the oocyte surface, and that co-injection of poly(A)+ RNA from undifferentiated HL-60 cells did not alter the level of receptor expression. Size fractionation of poly(A)+ RNA from undifferentiated HL-60 cells showed that the mRNA required to complement formyl-peptide-dependent signal transduction in oocytes had a size of approx. 3-3.5 kb. These results strongly suggest that the human formyl peptide receptor requires a specific cofactor(s), which is lacking in Xenopus oocytes but is present in undifferentiated HL-60 cells, to activate the second messenger pathway in oocytes. Identification of this factor will provide important information about the molecular mechanisms by which G-protein-coupled granulocyte-activating receptors stimulate phospholipase C.

Chemosensory behaviour of Tetrahymena

Free swimming cells of the ciliated protozoan Tetrahymena are attracted to certain chemicals by chemokinesis. However, a special type of chemotaxis in response to a chemical gradient is found in cells gliding very slowly in semisolid media. In contrast to classical chemotaxis by leukocytes, which is solely positive towards chemo-attractants, the oriented chemokinesis by gliding Tetrahymena involves both positive and negative elements. The major chemo-attractants are peptides and/or proteins, and they may be compounds which signal the presence of food in the natural environment of this freshwater phagotroph.

Effects of iodothyronines on chemotactic peptide-receptor binding and superoxide production of human neutrophils

We studied the action of iodinated thyronines on the superoxide (O2-) production of human neutrophils stimulated with a chemotactic peptide N-formylmethionylleucylphenylalanine (FMLP) in vitro. L-Thyroxine and L-triiodothyronine elicited dose dependently a potent inhibitory action on the FMLP-induced O2- production with IC50 values of about 10(-6) M and 7.10(-6) M, respectively, but L-diiodothyronine did not. No difference in the inhibition was observed between the L-form and the D-form of the compounds. Inhibition of the O2- production by L-thyroxine was restored by the washing of the cells. L-Thyroxine did not affect the O2- production stimulated with either the fifth component of the complement (C5a) or phorbol 12-myristate 13-acetate. L-Thyroxine and L-triiodothyronine were found to block [3H]FMLP binding to its own receptor with IC50 values similar to those for the inhibition of the O2- production by changing the affinity for the peptide but not the number of the receptors. These results suggest that thyroxine and triiodothyronine interfere with the binding of FMLP to the receptors, leading to the inhibition of neutrophil functions, such as O2- production, and that the inhibitory effects result from extranuclear actions rather than nuclear receptor-mediated ones.

Isolation and partial characterization of the formyl peptide receptor components on human neutrophils

The receptor for formylated peptides such as FMLP has been reported to consist of glycoprotein components ranging from 24-95 kDa, and to exhibit both high and low affinity for ligand. Controversy exists on the molecular size and number of these components, and whether the different affinities represent distinct ligand binding sites. In this study, the receptor was found to be comprised of components, of 94, 68, and approximately 40 kDa molecular size. Competitive binding inhibition experiments showed that FMLP bound to the components in the following order from highest to lowest affinity: 68 kDa greater than approximately 40 kDa greater than 94 kDa. Our findings suggest that the FMLP receptor of human neutrophils contains at least three components, and that each component has a different affinity for FMLP.