Formyl peptide chemotactic receptor. Evidence for an active proteolytic fragment

Structural and conformational studies of biased agonism through formyl peptide receptors

G protein-coupled chemoattractant receptors are class A GPCRs that couple primarily to the Gi class of heterotrimeric G proteins. Initially identified for their abilities to mediate leukocyte chemotaxis, chemoattractant GPCRs such as the formyl peptide receptors (FPRs) have been known for their diverse cellular functions in response to a variety of agonists. Stimulation of FPR2, in particular, leads to ligand-dependent activation of pro-inflammatory signaling as well as anti-inflammatory and pro-resolving signaling. Recently, the structures of FPR2-Gi protein complexed with ligands of different compositions have been solved by crystallization and cryo-electron microscopy. Analysis of the structural data as well as molecular simulation have led to the findings that the FPR2 binding pocket is sufficiently large for accommodation of several different types of ligands, but in different poses. This mini-review focuses on the structural and conformational aspects of FPR2 for mechanisms underlying its biased agonism.

Identification of C-terminal phosphorylation sites of N-formyl peptide receptor-1 (FPR1) in human blood neutrophils

Accumulation, activation, and control of neutrophils at inflammation sites is partly driven by N-formyl peptide chemoattractant receptors (FPRs). Occupancy of these G-protein-coupled receptors by formyl peptides has been shown to induce regulatory phosphorylation of cytoplasmic serine/threonine amino acid residues in heterologously expressed recombinant receptors, but the biochemistry of these modifications in primary human neutrophils remains relatively unstudied. FPR1 and FPR2 were partially immunopurified using antibodies that recognize both receptors (NFPRa) or unphosphorylated FPR1 (NFPRb) in dodecylmaltoside extracts of unstimulated and N-formyl-Met-Leu-Phe (fMLF) + cytochalasin B-stimulated neutrophils or their membrane fractions. After deglycosylation and separation by SDS-PAGE, excised Coomassie Blue-staining bands (∼34,000 Mr) were tryptically digested, and FPR1, phospho-FPR1, and FPR2 content was confirmed by peptide mass spectrometry. C-terminal FPR1 peptides (Leu(312)-Arg(322) and Arg(323)-Lys(350)) and extracellular FPR1 peptide (Ile(191)-Arg(201)) as well as three similarly placed FPR2 peptides were identified in unstimulated and fMLF + cytochalasin B-stimulated samples. LC/MS/MS identified seven isoforms of Ala(323)-Lys(350) only in the fMLF + cytochalasin B-stimulated sample. These were individually phosphorylated at Thr(325), Ser(328), Thr(329), Thr(331), Ser(332), Thr(334), and Thr(339). No phospho-FPR2 peptides were detected. Cytochalasin B treatment of neutrophils decreased the sensitivity of fMLF-dependent NFPRb recognition 2-fold, from EC50 = 33 ± 8 to 74 ± 21 nM. Our results suggest that 1) partial immunopurification, deglycosylation, and SDS-PAGE separation of FPRs is sufficient to identify C-terminal FPR1 Ser/Thr phosphorylations by LC/MS/MS; 2) kinases/phosphatases activated in fMLF/cytochalasin B-stimulated neutrophils produce multiple C-terminal tail FPR1 Ser/Thr phosphorylations but have little effect on corresponding FPR2 sites; and 3) the extent of FPR1 phosphorylation can be monitored with C-terminal tail FPR1-phosphospecific antibodies.

Full characterization of GPCR monomer-dimer dynamic equilibrium by single molecule imaging

Receptor dimerization is important for many signaling pathways. However, the monomer-dimer equilibrium has never been fully characterized for any receptor with a 2D equilibrium constant as well as association/dissociation rate constants (termed super-quantification). Here, we determined the dynamic equilibrium for the N-formyl peptide receptor (FPR), a chemoattractant G protein-coupled receptor (GPCR), in live cells at 37°C by developing a single fluorescent-molecule imaging method. Both before and after liganding, the dimer-monomer 2D equilibrium is unchanged, giving an equilibrium constant of 3.6 copies/µm(2), with a dissociation and 2D association rate constant of 11.0 s(-1) and 3.1 copies/µm(2)s(-1), respectively. At physiological expression levels of ∼2.1 receptor copies/µm(2) (∼6,000 copies/cell), monomers continually convert into dimers every 150 ms, dimers dissociate into monomers in 91 ms, and at any moment, 2,500 and 3,500 receptor molecules participate in transient dimers and monomers, respectively. Not only do FPR dimers fall apart rapidly, but FPR monomers also convert into dimers very quickly.

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.

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.

Internalization of N-formyl peptide chemotactic receptor-ligand complex by human neutrophils. The role of the receptor's 2-kDa external domain and carbohydrates

We treated human neutrophils with papain to remove the external 2-kDa domain and along with it the two oligosaccharide side chains of the N-formyl peptide chemotactic receptor and investigated what effect their absence has on the ligand-receptor complex internalization. After prelabeling of the cells with 125I-hexapeptide for 5 min at 22 degrees C, about 95% of the bound radioactivity was located on the cell surface. During the first 5-min incubation at 37 degrees C both the control and papain-treated cells internalized 73% of the receptor-ligand complexes suggesting that internalization is very rapid in human neutrophils and that removal of the external domain and the carbohydrates of the receptor does not affect the rate. However, the truncated receptor-ligand complexes were degraded at a faster rate because the radioactivity released into the medium was significantly higher and correspondingly the acid-resistant radioactivity significantly lower in the papain-treated neutrophils than in control cells already at 5 min and all subsequent time points. The radioactivity accumulated in the medium of the control and papain-treated neutrophils represented inactivated 125I-hexapeptide as less than 5% of it at 5, 30 and 120 min were capable of rebinding. No receptor recycling was detected in either cells. These results indicate that removal of the 2-kDa external domain and the carbohydrates of the N-formyl chemotactic receptors has little effect on the internalization rate of the receptor-ligand complexes but accelerates markedly their intracellular degradation.

The superoxide-generating oxidase of phagocytic cells. Physiological, molecular and pathological aspects

Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O2-. The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were diassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O2- upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.

Neutral endopeptidase activity in the interaction of N-formyl-L-methionyl-L-leucyl-L-phenylalanine with human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMN) hydrolyze the synthetic chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe) at nanomolar concentrations in an autocatalytic-like manner that deviates from classical Michaelis-Menten kinetics [Yuli, I. & Snyderman, R. (1986) J. Biol. Chem. 261, 4902-4908]. By using inhibitors of distinct classes of endoproteases, this particular fMet-Leu-Phe degradation was attributed exclusively to an exoplasmic metalloendoprotease that matches the ubiquitous neutral endopeptidase (NEP). Membrane-bound NEP hydrolyzes non-chemotactic substrates according to a classic Michaelis-Menten mechanism. By competitive inhibition with non-chemotactic substrates, fMet-Leu-Phe was found to interact with membrane NEP through a single active site, in a non-cooperative mode with an apparent Km in the order of 1 mM. The discrepancy between the ordinary hydrolysis of the micromolar and millimolar concentrations of fMet-Leu-Phe, reported by others, and the particular degradation of the nanomolar fMet-Leu-Phe, could not be accounted for by any coherent correlation between NEP activity/inhibition and modulation of fMet-Leu-Phe binding to its receptor, and/or induction of fMet-Leu-Phe-receptor-mediated inflammatory responses. Based on these and previously reported results, a novel model is proposed in which the fMet-Leu-Phe-induced inflammatory stimulation of PMN involves both NEP and the fMet-Leu-Phe receptor. By this model, NEP and the fMet-Leu-Phe receptor are distinct membrane entities which can form dynamic binary and tertiary complexes; thus accounting for the unusual kinetic features of fMet-Leu-Phe degradation, as well as the two receptor states. The complex of NEP and the fMet-Leu-Phe receptor might be conceived as a chemotactic-perception mechanism that combines the high affinity of the receptor and the rapid turnover of NEP.

Effect of phorbol myristate acetate on processing of formyl peptide receptors by human neutrophils

We examined the effect of phorbol myristate acetate on the ability of human neutrophils to process formyl peptide receptors. The receptor was affinity-labeled and its extracellular localization assessed over time, by cleavage of extracellular labeled receptor with papain. Neutrophils were capable of internalizing (and/or recycling) affinity labeled formyl peptide receptor in the absence of extracellular calcium. This phenomenon was dependent upon stimulation with phorbol myristate acetate, suggesting a role for protein kinase C in this process.

Synthesis and use of a novel N-formyl peptide derivative to isolate a human N-formyl peptide receptor cDNA

N-formyl-methionyl peptides are powerful chemoattractants which bind to specific receptors on the neutrophil plasma membrane. A cDNA library from HL-60 cells, differentiated into granulocytes highly responsive to N-formyl-methionyl peptides, was constructed in the COS cell expression vector CDM8. A cDNA clone was isolated that conferred to COS cells the ability to bind a new and highly efficient hydrophilic derivative of N-formyl-Met-Leu-Phe-Lys. The transfected COS cells displayed two classes of binding sites with Kd values of 0.5-1 nM and 5-10 nM, respectively. The cDNA was 1.9 kb long with a 1050 bp open reading frame encoding a 350 residue protein. The hydropathy plot analysis revealed seven hydrophobic segments, a pattern quite similar to that of G protein-coupled receptors.

Dual Mg2+ control of formyl-peptide-receptor--G-protein interaction in HL 60 cells. Evidence that the low-agonist-affinity receptor interacts with and activates the G-protein

In neutrophils and several other phagocytic cell types, a pertussis- and cholera-toxin-sensitive form of the guanine-nucleotide-binding protein (G-protein) Gp couples receptors for N-formylmethionine-containing chemotactic peptides to stimulation of phospholipase C. Using membranes of myeloid differentiated HL 60 cells, we have examined the role of Mg2+ and guanine nucleotides in regulating (a) the interaction of the formyl-peptide receptor with the chemotactic agonist N-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe) and (b) the receptor-mediated activation of Gp. Mg2+ markedly enhanced the number of receptors with high affinity for the radiolabeled oligopeptide fMet-Leu-[3H]Phe. At the same time, Mg2+ largely increased the potency of guanosine-5'-(3-O-thio)triphosphate, but not of GDP or guanosine-5'-(2-O-thio)diphosphate, to inhibit binding of the peptide. Comparison of the potency of Mg2+ in eliciting these two effects and analysis of the specificities of the relevant divalent cation sites revealed that Mg2+ interacts with at least two independent sites on the receptor-Gp complex. One site is specific for Mg2+ and exhibits affinity in the micromolar range, the other site interacts with millimolar concentrations of several divalent cations in a non-selective fashion. It is suggested that the former site is located on Gp and that interaction of Mg2+ with this site is necessary for the receptor-mediated G-protein activation, whereas interaction of divalent cations with the latter site is necessary for high affinity agonist binding. The regulation of the formyl-peptide receptor binding properties by guanine nucleotides is independent of Gp activation, since inhibition of peptide binding is achieved by addition of both guanine nucleoside diphosphates and triphosphates and is readily seen both in the presence and in the absence of Mg2+. The latter finding, together with the observation that, at micromolar concentrations of Mg2+, high-affinity GTPase activity is stimulated by fMet-Leu-Phe primarily via low affinity receptors, suggests that, contrary to widely held opinions, (a) divalent cations are not required for a functional receptor--G-protein interaction and (b) high-affinity agonist binding is not a prerequisite for the receptor-mediated activation of the G-protein.

Identification of a human neutrophil protein of Mr 24 000 that binds N-formyl peptides: co-sedimentation with specific granules

In assaying subcellular fractions of human neutrophils for N-formyl peptide binding sites using the photoaffinity ligand FMLPL-SASD-125I (125I-labelled N-formylmethionylleucylphenylalanyl-N epsilon- (2-(p-azidosalicylamido)ethyl-1,3'-dithiopropionyl)-lysine) several molecular species were observed. We confirmed localization of the N-formyl peptide receptor of Mr 50 000-70 000 in the plasma membrane and specific granule fractions. A species of Mr 33 000-35 000 was detected in the light Golgi/endosomal fraction, whose size is consistent with the deglycosylated form of the receptor. In addition, a major binding species of Mr 24 000 was identified that co-localized on sucrose gradients with specific granule markers. This Mr 24 000 species, which was investigated further, was found to be released upon cell stimulation with phorbol myristate acetate or FMLP in the presence of dihydrocytochalasin B. It had an affinity for FMLPL-SASD of 145 nM (cf. 0.3 nM for the cell surface receptor). The specificity for the formyl group was lost as the nonformylated Met-Leu-Phe was as effective FMLPL in competing with FMLPL-SASD-125I for binding to th Mr 24 000 species. A structurally unrelated peptide, however, did not compete for the binding. The labelling of the Mr 24 000 species was dependent on the presence of Ca2+, as was its apparent Mr, which shifted from 24 000 to 50 000-70 000 in the presence of Ca2+. By incubating photoaffinity-labelled plasma membrane fractions with specific granule fractions, we could generate a receptor fragment of Mr 24 000, although the relationship to this fragment of the specific granule species is unknown at present. The N-terminal sequence of the Mr 24 000 species was determined and it appears to be a novel protein. Further work will allow its relationship to the receptor, if any, to be elucidated and allow assignment of a function to this potentially important molecule.

Activation of the neutrophil respiratory burst by chemoattractants: regulation of the N-formyl peptide receptor in the plasma membrane

The N-formyl peptide receptor mediates a number of host defensive responses of human neutrophils that result in chemotaxis, secretion of hydrolytic enzymes, and superoxide generation. Inappropriate activation or defective regulation of these responses can result in pathogenic states responsible for inflammatory disease. The receptor is a 50 to 70-kD, integral plasma membrane glycoprotein with intracellular and surface localization. Its abundance in the membrane is regulated by membrane flow and recycling processes. Cytoskeletal interactions are believed to control its organization in the plane of the membrane and interaction with other proteins. The receptor's most important interaction is with guanyl nucleotide binding proteins that serve as signal transduction partners ultimately leading to activation of effector responses. Because the interaction of the receptor with G proteins is necessary for transduction, control of this interaction may be at the root of understanding the molecular control of responses in these cells. This review briefly summarizes some of the molecular properties, dynamics, and interactions of this receptor system in human neutrophils and discusses how these characteristics may pertain to the activation and control of superoxide generation.

Effects of ibuprofen on chemotactic peptide-receptor binding and granulocyte response

Inhibition of complement-mediated granulocyte aggregation has been proposed recently as a mechanism of action of high-dose corticosteroids and ibuprofen in shock states. Such inhibition by corticosteroids may be effected through alteration of receptor function, and we have therefore examined the effect of ibuprofen on the extent and kinetics of binding of the synthetic chemotactic peptide formylmethionine-leucine-phenylalanine (FMLP) to its specific receptor on the granulocyte surface. Dose-dependent inhibition of binding was observed at ibuprofen concentrations paralleling plasma levels achieved with 30 mg/kg intravenous bolus therapy, and also at concentrations achieved with oral therapy. Ibuprofen did not affect the receptor number, but did decrease the association rate constant for the FMLP-receptor interaction (30% of normal for 0.125 mg/ml ibuprofen), leading to a decrease in receptor affinity for ligand. Dissociation kinetics, as determined by cold chase experiments, were unaltered by ibuprofen. We conclude that ibuprofen, like corticosteroids, can slow the rate of association of FMLP with its receptor on the granulocyte surface while allowing dissociation to proceed; altered kinetics of receptor-FMLP interaction may explain the inhibition of granulocyte aggregation. Blockade of granulocyte surface receptors for inflammatory stimuli may be important in the clinical effects of very high-dose corticosteroids and ibuprofen such as are administered in shock; such effects are seen at blood levels of ibuprofen that occur with oral therapy. Similar observations may hold for other physiologic stimuli.

Preparation and properties of an improved photoaffinity ligand for the N-formyl peptide receptor

A new superior photoaffinity ligand for the N-formyl peptide receptor was prepared by derivatization of N-formyl-Met-Leu-Phe-Lys with a commercially available heterobifunctional crosslinking agent. The product, N-formyl-Met-Leu-Phe-N epsilon-(2-(p-azidosalicylamido)ethyl-1,3'- dithiopropionyl)-Lys was readily synthesized and radiolabelled, and had increased specificity and stability as compared to previously used photoaffinity ligands. The ligand rapidly associated with the receptor with high affinity (Kd = 0.28 nM). Once bound, it was virtually non-dissociable (in the absence of photolysis) in an experimental time-frame (t1/2 (off) = 154 min). The covalent incorporation of the photoaffinity ligand into the receptor upon irradiation was complete within 5 min, minimizing cell damage, and the efficiency of covalent incorporation was approx. 40%. The derivative had enhanced biological activity, having an ED50 for superoxide anion production of 0.23 nM, 27-fold lower than its parent peptide. This derivative of the N-formyl peptide was useful for up to 3 months after radiolabelling, showing a progressive decline in specific activity during storage in the dark at 4 degrees C. The enhanced stability, reproducibility and solubility of the photoaffinity ligand is expected to aid in the purification of the N-formyl peptide receptor and will prove a useful tool for analysing receptor-mediated processes.

Studies on the structural requirements for synthetic formyl peptide chemoattractants

Seven small molecular weight new peptides related to the chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (CHO-Met-Leu-PheOH) have been prepared by classical peptide synthesis. Compounds were prepared to evaluate the requirements at the C-terminal Phe group/ Each analogue tested for its ability to induce lysosomal enzyme release from human neutrophils.

Leucocyte migration inhibition in vitro with inhibitors of aspartic and sulphhydryl proteinases

The human leucocyte migration in the leucocyte migration under agarose technique ( LMAT ) was investigated with specific inhibitors of aspartic, sulphhydryl , metallo- and serine proteinases. The general aspartic proteinase inhibitor pepstatin and the highly specific competitive cathepsin D inhibitor, N-gly-glu-gly-phe-leu-gly-D-phe-leu suppressed leucocyte migration at concentrations of 20-200 mumol/l and 30-59 mumol/l, respectively. The suphhydryl enzyme inactivator, N-ethylmaleimide suppressed leucocyte migration at concentrations of 100-200 mumol/l. Several inhibitors of serine- and metallo enzymes were tested, but none had any effect on leucocyte mobility, although the metal binding agent 8-hydroxyquinoline was strongly inhibitory to cell migration, the significance of which is discussed.