The synthetic chemoattractant Trp-Lys-Tyr-Met-Val-DMet activates neutrophils preferentially through the lipoxin A4 receptor

FPR1: A critical gatekeeper of the heart and brain

G protein-coupled receptors (GPCRs) are currently the most widely focused drug targets in the clinic, exerting their biological functions by binding to chemicals and activating a series of intracellular signaling pathways. Formyl-peptide receptor 1 (FPR1) has a typical seven-transmembrane structure of GPCRs and can be stimulated by a large number of endogenous or exogenous ligands with different chemical properties, the first of which was identified as formyl-methionine-leucyl-phenylalanine (fMLF). Through receptor-ligand interactions, FPR1 is involved in inflammatory response, immune cell recruitment, and cellular signaling regulation in key cell types, including neutrophils, neural stem cells (NSCs), and microglia. This review outlines the critical roles of FPR1 in a variety of heart and brain diseases, including myocardial infarction (MI), ischemia/reperfusion (I/R) injury, neurodegenerative diseases, and neurological tumors, with particular emphasis on the milestones of FPR1 agonists and antagonists. Therefore, an in-depth study of FPR1 contributes to the research of innovative biomarkers, therapeutic targets for heart and brain diseases, and clinical applications.

High levels of short-chain fatty acids secreted by Candida albicans hyphae induce neutrophil chemotaxis via free fatty acid receptor 2

Candida albicans belongs to our commensal mucosal flora and in immune-competent individuals in the absence of epithelial damage, this fungus is well tolerated and controlled by our immune defense. However, C. albicans is an opportunistic microorganism that can cause different forms of infections, ranging from superficial to life-threatening systemic infections. C. albicans is polymorphic and switches between different phenotypes (e.g. from yeast form to hyphal form). C. albicans hyphae are invasive and can grow into tissues to eventually reach circulation. During fungal infections, neutrophils in particular play a critical role for the defense, but how neutrophils are directed toward the invasive forms of fungi is less well understood. We set out to investigate possible neutrophil chemoattractants released by C. albicans into culture supernatants. We found that cell-free culture supernatants from the hyphal form of C. albicans induced both neutrophil chemotaxis and concomitant intracellular calcium transients. Size separation and hydrophobic sorting of supernatants indicated small hydrophilic factors as responsible for the activity. Further analysis showed that the culture supernatants contained high levels of short-chain fatty acids with higher levels from hyphae as compared to yeast. Short-chain fatty acids are known neutrophil chemoattractants acting via the neutrophil free fatty acid receptor 2. In line with this, the calcium signaling in neutrophils induced by hyphae culture supernatants was blocked by a free fatty acid receptor 2 antagonist and potently increased in the presence of a positive allosteric modulator. Our data imply that short-chain fatty acids may act as a recruitment signal whereby neutrophils can detect C. albicans hyphae.

Lipoxin A4 yields an electrophilic 15-oxo metabolite that mediates FPR2 receptor-independent anti-inflammatory signaling

The enzymatic oxidation of arachidonic acid is proposed to yield trihydroxytetraene species (termed lipoxins) that resolve inflammation via ligand activation of the formyl peptide receptor, FPR2. While cell and murine models activate signaling responses to synthetic lipoxins, primarily 5S,6R,15S-trihydroxy-7E,9E,11Z,13E-eicosatetraenoic acid (lipoxin A4, LXA4), there are expanding concerns about the biological formation, detection and signaling mechanisms ascribed to LXA4 and related di- and tri-hydroxy ω-6 and ω-3 fatty acids. Herein, the generation and actions of LXA4 and its primary 15-oxo metabolite were assessed in control, LPS-activated and arachidonic acid supplemented RAW 264.7 macrophages. Despite protein expression of all enzymes required for LXA4 synthesis, both LXA4 and its 15-oxo-LXA4 metabolite were undetectable. Moreover, synthetic LXA4 and the membrane permeable 15-oxo-LXA4 methyl ester that is rapidly de-esterified to 15-oxo-LXA4, displayed no ligand activity for the putative LXA4 receptor FPR2, as opposed to the FPR2 ligand WKYMVm. Alternatively, 15-oxo-LXA4, an electrophilic α,β-unsaturated ketone, alkylates nucleophilic amino acids such as cysteine to modulate redox-sensitive transcriptional regulatory protein and enzyme function. 15-oxo-LXA4 activated nuclear factor (erythroid related factor 2)-like 2 (Nrf2)-regulated gene expression of anti-inflammatory and repair genes and inhibited nuclear factor (NF)-κB-regulated pro-inflammatory mediator expression. LXA4 did not impact these macrophage anti-inflammatory and repair responses. In summary, these data show an absence of macrophage LXA4 formation and receptor-mediated signaling actions. Rather, if LXA4 were present in sufficient concentrations, this, and other more abundant mono- and poly-hydroxylated unsaturated fatty acids can be readily oxidized to electrophilic α,β-unsaturated ketone products that modulate the redox-sensitive cysteine proteome via G-protein coupled receptor-independent mechanisms.

Acute-serum amyloid A and A-SAA-derived peptides as formyl peptide receptor (FPR) 2 ligands

Originally, it was thought that a single serum amyloid A (SAA) protein was involved in amyloid A amyloidosis, but in fact, SAA represents a four-membered family wherein SAA1 and SAA2 are acute phase proteins (A-SAA). SAA is highly conserved throughout evolution within a wide range of animal species suggestive of an important biological function. In fact, A-SAA has been linked to a number of divergent biological activities wherein a number of these functions are mediated via the G protein-coupled receptor (GPCR), formyl peptide receptor (FPR) 2. For instance, through the activation of FPR2, A-SAA has been described to regulate leukocyte activation, atherosclerosis, pathogen recognition, bone formation and cell survival. Moreover, A-SAA is subject to post-translational modification, primarily through proteolytic processing, generating a range of A-SAA-derived peptides. Although very little is known regarding the biological effect of A-SAA-derived peptides, they have been shown to promote neutrophil and monocyte migration through FPR2 activation via synergy with other GPCR ligands namely, the chemokines CXCL8 and CCL3, respectively. Within this review, we provide a detailed analysis of the FPR2-mediated functions of A-SAA. Moreover, we discuss the potential role of A-SAA-derived peptides as allosteric modulators of FPR2.

Structural Determinants in the Staphylococcus aureus-Derived Phenol-Soluble Modulin α2 Peptide Required for Neutrophil Formyl Peptide Receptor Activation

Highly pathogenic Staphylococcus aureus strains produce phenol-soluble modulins (PSMs), which are N-formylated peptides. Nanomolar concentrations of PSMα2 are recognized by formyl peptide receptor 2 (FPR2), but unlike the prototypic FPR2 agonist WKYMVM, PSMα2 is a biased signaling agonist. The truncated N-terminal PSMα2 variant, consisting of the five N-terminal residues, is no longer recognized by FPR2, showing that the C-terminal part of PSMα2 confers FPR2 selectivity, whereas the N-terminal part may interact with the FPR1 binding site. In the current study, a combined pharmacological and genetic approach involving primary human neutrophils and engineered FPR knock-in and knockout cells was used to gain molecular insights into FPR1 and FPR2 recognition of formyl peptides as well as the receptor downstream signaling induced by these peptides. In comparison with the full-length PSMα2, we show that the peptide in which the N-terminal part of PSMα2 was replaced by fMet-Ile-Phe-Leu (an FPR1-selective peptide agonist) potently activates both FPRs for production of superoxide anions and β-arrestin recruitment. A shortened analog of PSMα2 (PSMα2_1-12), lacking the nine C-terminal residues, activated both FPR1 and FPR2 to produce reactive oxygen species, whereas β-arrestin recruitment was only mediated through FPR1. However, a single amino acid replacement (Gly-2 to Ile-2) in PSMα2_1-12 was sufficient to alter FPR2 signaling to include β-arrestin recruitment, highlighting a key role of Gly-2 in conferring FPR2-biased signaling. In conclusion, we provide structural insights into FPR1 and FPR2 recognition as well as the signaling induced by interaction with formyl peptides derived from PSMα2, originating from S. aureus bacteria.

Data describing expression of formyl peptide receptor 2 in human articular chondrocytes

The formyl peptide receptor 2 (FPR2) belongs to the family of seven-transmembrane G protein-coupled receptors (GPCR) and are expressed by many different cells but mainly studied in immune cells. FPR2 is involved in host defense against bacterial infections and clearance of damaged cells through the oxidative burst and chemotaxis of neutrophils. In addition, FPR2 has also been implicated as an immunomodulator in sterile inflammations, e.g. inflammatory joint diseases. Here we present data regarding FPR2 expression in human articular chondrocytes, isolated from healthy individuals and osteoarthritic patients, on both mRNA and protein level using qPCR and Imagestream flow cytometry. We also present data after receptor stimulation and monitoring of production of nitric oxide, reactive oxygen species, IL-6, IL-8 and MMP-3. The presented data show that human articular chondrocytes from patients with osteoarthritis as well as from healthy individuals express FPR2 both at mRNA and protein level. The biological relevance of FPR2 expression in chondrocytes needs to be further investigated.

Structure of formylpeptide receptor 2-Gi complex reveals insights into ligand recognition and signaling

Formylpeptide receptors (FPRs) as G protein-coupled receptors (GPCRs) can recognize formylpeptides derived from pathogens or host cells to function in host defense and cell clearance. In addition, FPRs, especially FPR2, can also recognize other ligands with a large chemical diversity generated at different stages of inflammation to either promote or resolve inflammation in order to maintain a balanced inflammatory response. The mechanism underlying promiscuous ligand recognition and activation of FPRs is not clear. Here we report a cryo-EM structure of FPR2-Gi signaling complex with a peptide agonist. The structure reveals a widely open extracellular region with an amphiphilic environment for ligand binding. Together with computational docking and simulation, the structure suggests a molecular basis for the recognition of formylpeptides and a potential mechanism of receptor activation, and reveals conserved and divergent features in Gi coupling. Our results provide a basis for understanding the molecular mechanism of the functional promiscuity of FPRs.

Staphylococcal superantigen-like protein 13 activates neutrophils via formyl peptide receptor 2

Staphylococcal superantigen-like (SSL) proteins, one of the major virulence factor families produced by Staphylococcus aureus, were previously demonstrated to be immune evasion molecules that interfere with a variety of innate immune defences. However, in contrast to characterised SSLs, which inhibit immune functions, we show that SSL13 is a strong activator of neutrophils via the formyl peptide receptor 2 (FPR2). Moreover, our data show that SSL13 acts as a chemoattractant and induces degranulation and oxidative burst in neutrophils. As with many other staphylococcal immune evasion proteins, SSL13 shows a high degree of human specificity. SSL13 is not able to efficiently activate mouse neutrophils, hampering in vivo experiments. In conclusion, SSL13 is a neutrophil chemoattractant and activator that acts via FPR2. Therefore, SSL13 is a unique SSL member that does not belong to the immune evasion class but is a pathogen alarming molecule. Our study provides a new concept of SSLs; SSLs not only inhibit host immune processes but also recruit human neutrophils to the site of infection. This new insight allows us to better understand complex interactions between host and S. aureus pathological processes.

Chlamydia trachomatis paralyses neutrophils to evade the host innate immune response

Chlamydia trachomatis, an obligate intracellular human pathogen, is a major cause of sexually transmitted diseases. Infections often occur without symptoms, a feature that has been attributed to the ability of the pathogen to evade the host immune response. We show here that C. trachomatis paralyses the host immune system by preventing the activation of polymorphic nuclear leukocytes (PMNs). PMNs infected with Chlamydia fail to produce neutrophil extracellular traps and the bacteria are able to survive in PMNs for extended periods of time. We have identified the secreted chlamydial protease-like activating factor (CPAF) as an effector mediating the evasion of the innate immune response since CPAF-deficient Chlamydia activate PMNs and are subsequently efficiently killed. CPAF suppresses the oxidative burst and interferes with chemical-mediated activation of neutrophils. We identified formyl peptide receptor 2 (FPR2) as a target of CPAF. FPR2 is cleaved by CPAF and released from the surface of PMNs. In contrast to previously described subversion mechanisms that mainly act on already activated PMNs, we describe here details of how Chlamydia actively paralyses PMNs, including the formation of neutrophil extracellular traps, to evade the host's innate immune response.

fMLP-dependent activation of Akt and ERK1/2 through ROS/Rho A pathways is mediated through restricted activation of the FPRL1 (FPR2) receptor

OBJECTIVE AND DESIGN

The objective of this study is to uncover the signal transduction pathways of N-formyl methionyl-leucyl-phenylalanine (fMLP) in monocyte.

MATERIALS OR SUBJECTS

Freshly isolated human peripheral blood monocytes (PBMC) were used for in vitro assessment of signal transduction pathways activated by fMLP.

TREATMENT

Time-course and dose-response experiments were used to evaluate the effect of fMLP along with the specific inhibitors/stimulators on the activation of downstream signaling kinases.

METHODS

Freshly isolated human PBMC were stimulated with fMLP for the desired time. Western blot and siRNA analysis were used to evaluate the activated intracellular signaling kinases, and flow analysis was performed to assess the levels of CD11b. Furthermore, luminescence spectrometry was performed to measure the levels of released hydrogen peroxide in the media.

RESULTS

fMLP strongly stimulated the activation of AKT and ERK1/2 through a RhoA-GTPase-dependent manner and also induced H₂O₂ release by monocytes. Furthermore, fMLP mediated its effects through restricted activation of formylpeptide receptor-like 1 (FPRL1/FPR2), but independently of either EGFR transactivation or intracellular calcium release. In addition, NAC reversed fMLP- and H₂O₂-induced activation of Akt and RhoA-GTPase.

CONCLUSION

Collectively, these data suggested that fMLP-activated ERK1/2 and Akt pathways through specific activation of the FPRL1/ROS/RoA-GTPase pathway.

Formyl Peptide Receptors in Mice and Men: Similarities and Differences in Recognition of Conventional Ligands and Modulating Lipopeptides

The pattern recognition formyl peptide receptors (FPRs) belong to the class of G-protein-coupled receptors (GPCRs), the largest group of cell surface receptors involved in a range of physiological processes and pathologies. The FPRs have regulatory function in the initiation as well as resolution of inflammatory reactions, making them highly interesting as targets for drug development. Recent research in the GPCR/FPR fields has uncovered novel receptor biology concepts, including biased signalling/functional selectivity, allosteric modulation, receptor reactivation and receptor cross-talk. When it comes to allosteric modulators, 'tailor-made' lipopeptides (pepducins and lipopeptoids) represent a novel concept of GPCR/FPR regulation. This MiniReview is focused on the basis for recognition of conventional ligands and immunomodulating lipopeptides, novel allosteric modulators for the FPRs, receptors that are highly expressed by both human and mouse neutrophils. The FPRs play key roles in host defence against microbial infections, tissue homeostasis and the initiation as well as resolution of inflammation but there are both similarities and differences in ligand recognition between mice and men. Thus, identification and functional characterization of activating and inhibiting ligands should provide insights into future design of FPR-based animal models of human diseases and development of therapeutics for treating inflammatory diseases.

Applying label-free dynamic mass redistribution assay for studying endogenous FPR1 receptor signalling in human neutrophils

INTRODUCTION

The label-free dynamic mass redistribution-based assay (DMR) is a powerful method for studying signalling pathways of G protein-coupled receptors (GPCRs). Herein we present the label-free DMR assay as a robust readout for pharmacological characterization of formyl peptide receptors (FPRs) in human neutrophils.

METHODS

Neutrophils were isolated from fresh human blood and their responses to FPR1 and FPR2 agonists, i.e. compound 43, fMLF and WKYMVm were measured in a label-free DMR assay using Epic Benchtop System from Corning®. Obtained DMR traces were used to calculate agonist potencies.

RESULTS

The potencies (pEC₅₀) of fMLF, WKYMVm and compound 43, determined on human neutrophils using the label-free DMR assay were 8.63, 7.76 and 5.92, respectively. The DMR response to fMLF, but not WKYMVm and compound 43 could be blocked by the FPR1-specific antagonist cyclosporin H.

DISCUSSION

We conclude that the DMR assay can be used, and complements more traditional methods, to study the signalling and pharmacology of endogenous FPR receptors in human neutrophils.

Mast cells mediate early neutrophil recruitment and exhibit anti-inflammatory properties via the formyl peptide receptor 2/lipoxin A4 receptor

BACKGROUND AND PURPOSE

In recent years, studies have focused on the resolution of inflammation, which can be achieved by endogenous anti-inflammatory agonists such as Annexin A1 (AnxA1). Here, we investigated the effects of mast cells (MCs) on early LPS-induced neutrophil recruitment and the involvement of the AnxA1-formyl peptide receptor 2/ALX (FPR2/ALX or lipoxin A₄ receptor) pathway.

EXPERIMENTAL APPROACH

Intravital microscopy (IVM) was used to visualize and quantify the effects of LPS (10 μg per mouse i.p.) on murine mesenteric cellular interactions. Furthermore, the role that MCs play in these inflammatory responses was determined in vivo and in vitro, and effects of AnxA1 mimetic peptide Ac2-26 were assessed.

KEY RESULTS

LPS increased both neutrophil endothelial cell interactions within the mesenteric microcirculation and MC activation (determined by IVM and ruthenium red dye uptake), which in turn lead to the early stages of neutrophil recruitment. MC recruitment of neutrophils could be blocked by preventing the pro-inflammatory activation (using cromolyn sodium) or enhancing an anti-inflammatory phenotype (using Ac2-26) in MCs. Furthermore, MCs induced neutrophil migration in vitro, and MC stabilization enhanced the release of AnxA1 from neutrophils. Pharmacological approaches (such as the administration of FPR pan-antagonist Boc2, or the FPR2/ALX antagonist WRW4) revealed neutrophil FPR2/ALX to be important in this process.

CONCLUSIONS AND IMPLICATIONS

Data presented here provide evidence for a role of MCs, which are ideally positioned in close proximity to the vasculature, to act as sentinel cells in neutrophil extravasation and resolution of inflammation via the AnxA1-FPR2/ALX pathway.

Lipoxin A4 stimulates endothelial miR-126-5p expression and its transfer via microvesicles

The proresolution lipid mediator lipoxin (LX)A₄ bestows protective bioactions on endothelial cells. We examined the impact of LXA₄ on transcellular endothelial signaling via microRNA (miR)-containing microvesicles. We report LXA₄ inhibition of MV release by TNF-α-treated HUVECs, associated with the down-regulation of 18 miR in endothelial microvesicles (EMVs) and the up-regulation of miR-126-5p, both in HUVECs and in EMVs. LXA₄ up-regulated miR-126-5p by ∼5-fold in HUVECs and promoted a release of microvesicles (LXA₄-EMVs) that enhanced miR-126-5p by ∼7-fold in recipient HUVECs. In these cells, LXA₄-EMVs abrogated the up-regulation of VCAM-1, induced in recipient HUVECs by EMVs released by untreated or TNF-α-treated HUVECs. LXA₄-EMVs also reduced by ∼40% the expression of SPRED1, which we validated as an miR-126-5p target, whereas they stimulated monolayer repair in an in vitro wound assay. This effect was lost when the EMVs were depleted of miR-126-5p. These results provide evidence that changes in miR expression and microvesicle packaging and transfer represent a mechanism of action of LXA₄, which may be relevant in vascular biology and inflammation.-Codagnone, M., Recchiuti, A., Lanuti, P., Pierdomenico, A. M., Cianci, E., Patruno, S., Mari, V. C., Simiele, F., Di Tomo, P., Pandolfi, A., Romano, M. Lipoxin A₄ stimulates endothelial miR-126-5p expression and its transfer via microvesicles.

Data on the NADPH-oxidase activity induced by WKYMVm and galectin-3 in bone marrow derived and exudated neutrophils isolated from four different mouse strains

Neutrophils are the key players in inflammatory reactions and the release of superoxide through the NADPH-oxidase upon neutrophil activation contributes to bacterial clearance and surrounding tissue damage. Here we describe data on the mouse neutrophil NADPH-oxidase activation induced by the mouse formyl peptide receptor (Fpr) agonist WKYMVm and galectin-3. Neutrophils isolated from bone marrow, peritoneal exudated, and in vitro TNFα primed bone marrow neutrophils from four different laboratory strains (C57BL/6, DBA/1, BALB/c and NMRI) were used. Both Fpr agonist and galectin-3 activated neutrophils to release superoxide. No differences were observed in the amounts of superoxide released from neutrophils derived from four different strains.

The Neutrophil Response Induced by an Agonist for Free Fatty Acid Receptor 2 (GPR43) Is Primed by Tumor Necrosis Factor Alpha and by Receptor Uncoupling from the Cytoskeleton but Attenuated by Tissue Recruitment

Ligands with improved potency and selectivity for free fatty acid receptor 2 (FFA2R) have become available, and we here characterize the neutrophil responses induced by one such agonist (Cmp1) and one antagonist (CATPB). Cmp1 triggered an increase in the cytosolic concentration of Ca(2+), and the neutrophils were then desensitized to Cmp1 and to acetate, a naturally occurring FFA2R agonist. The antagonist CATPB selectively inhibited responses induced by Cmp1 or acetate. The activated FFA2R induced superoxide anion secretion at a low level in naive blood neutrophils. This response was largely increased by tumor necrosis factor alpha (TNF-α) in a process associated with a recruitment of easily mobilizable granules, but neutrophils recruited to an aseptic inflammation in vivo were nonresponding. Superoxide production induced by Cmp1 was increased in latrunculin A-treated neutrophils, but no reactivation of desensitized FFA2R was induced by this drug, suggesting that the cytoskeleton is not directly involved in terminating the response. The functional and regulatory differences between the receptors that recognize short-chain fatty acids and formylated peptides, respectively, imply different roles of these receptors in the orchestration of inflammation and confirm the usefulness of a selective FFA2R agonist and antagonist as tools for the exploration of the precise role of the FFA2R.

The Lipidated Peptidomimetic Lau-((S)-Aoc)-(Lys-βNphe)6-NH2 Is a Novel Formyl Peptide Receptor 2 Agonist That Activates Both Human and Mouse Neutrophil NADPH Oxidase

Neutrophils expressing formyl peptide receptor 2 (FPR2) play key roles in host defense, immune regulation, and resolution of inflammation. Consequently, the search for FPR2-specific modulators has attracted much attention due to its therapeutic potential. Earlier described agonists for this receptor display potent activity for the human receptor (FPR2) but low activity for the mouse receptor orthologue (Fpr2), rendering them inapplicable in murine models of human disease. Here we describe a novel FPR2 agonist, the proteolytically stable α-peptide/β-peptoid hybrid Lau-((S)-Aoc)-(Lys-βNphe)6-NH2 (F2M2), showing comparable potency in activating human and mouse neutrophils by inducing a rise in intracellular Ca(2+) concentration and assembly of the superoxide-generating NADPH oxidase. This FPR2/Fpr2 agonist contains a headgroup consisting of a 2-aminooctanoic acid (Aoc) residue acylated with lauric acid (C12 fatty acid), which is linked to a peptide/peptoid repeat ((Lys-βNphe)6-NH2). Both the fatty acid moiety and the (S)-Aoc residue were required for FPR2/Fpr2 activation. This type of proteolytically stable FPR2-specific peptidomimetics may serve as valuable tools for future analysis of FPR2 signaling as well as for development of prophylactic immunomodulatory therapy. This novel class of cross-species FPR2/Fpr2 agonists should enable translation of results obtained with mouse neutrophils (and disease models) into enhanced understanding of human inflammatory and immune diseases.

Formyl-Peptide Receptor Agonists and Amorphous SiO2-NPs Synergistically and Selectively Increase the Inflammatory Responses of Human Monocytes and PMNs

We tested whether amorphous SiO₂-NPs and formylpeptide receptor (FPRs) agonists synergistically activate human monocytes and neutrophil polymorphonuclear granulocytes (PMNs). Peptide ligands specifically binding to FPR1 (f-MLP) and to FPR2 (MMK-1, WKYMVM and WKYMVm) human isoforms did not modify the association of SiO₂-NPs to both cell types or their cytotoxic effects. Similarly, the extent of CD80, CD86, CD83, ICAM-1 and MHCII expression in monocytes treated with SiO₂-NPs was not significantly altered by any FPRs agonist. However, FPR1 stimulation with f-MLP strongly increased the secretion of IL-1β, IL-6 and IL-8 by human monocytes, and of IL-8 by PMNs in the presence of SiO₂-NPs, due to the synergic stimulation of gene transcription. FPR2 agonists also up-modulated the production of IL-1β induced by monocytes treated with SiO₂-NPs. In turn, SiO₂-NPs increased the chemotaxis of PMNs toward FPR1-specific ligands, but not toward FPR2-specific ones. Conversely, the chemotaxis of monocytes toward FPR2-specific peptides was inhibited by SiO₂-NPs. NADPH-oxidase activation triggered by FPR1- and FPR2-specific ligands in both cell types was not altered by SiO₂-NPs. Microbial and tissue danger signals sensed by FPRs selectively amplified the functional responses of monocytes and PMN_S to SiO₂-NPs, and should be carefully considered in the assessment of the risk associated with nanoparticle exposure.

Topical corticosteroids do not revert the activated phenotype of eosinophils in eosinophilic esophagitis but decrease surface levels of CD18 resulting in diminished adherence to ICAM-1, ICAM-2, and endothelial cells

Swallowed topical corticosteroids are the standard therapy for eosinophilic esophagitis (EoE) in adults. Eosinophils in the blood of untreated EoE patients have an activated phenotype. Our aim was to determine if corticosteroids restore the phenotype of eosinophils to a healthy phenotype and if certain cell-surface molecules on blood eosinophils correlate with eosinophilic infiltration of the esophagus. Levels of eight surface markers on eosinophils from treated and untreated EoE patients were determined by flow cytometry and analyzed using multivariate methods of pattern recognition. Corticosteroid-treated EoE patients' eosinophils had decreased levels of CD18 compared to both untreated patients and healthy controls, but maintained their activated phenotype. CD18 expression correlated positively with eosinophil numbers in the esophagus and promoted the adherence of eosinophils to ICAM-1, ICAM-2, and to endothelial cells. The diminished expression of CD18 may be one mechanism behind the reduced entry of eosinophils into the esophagus in corticosteroid-treated EoE patients.

Formyl peptide receptor suppresses melanoma development and promotes NK cell migration

In order to understand how tumor cells can escape immune surveillance mechanisms and thus develop antitumor therapies, it is critically important to investigate the mechanisms by which the immune system interacts with the tumor microenvironment. In our current study, wild-type mice were inoculated with melanoma cell line B16-F10 (1 × 10(6)/mouse) and treated with the formyl peptide receptor (FPR) agonist WKYMVm or the FPR antagonist WRW(4). Growth of melanoma cell line B16-F10 was significantly inhibited in WKYMVm-treated mice and markedly promoted in WRW(4)-treated mice compared with control. Decreased number of myeloid-derived suppressor cells (MDSCs) and increased NK cell infiltration in tumor tissues were detected from WKYMVm-treated mice. Next, we showed that depletion of NK cell significantly increased tumor development in B16 tumor-bearing mice compared with the control group, and the suppressed tumor-developing effect of WKYMVm in B16 melanoma was abrogated with NK cell depletion. We also found that WKYMVm stimulates chemotactic migration in NK cells via the FPR family, and this was dependent on extracellular signal-related kinase (ERK) activation. Moreover, in our further experiment, we showed that the increased infiltration of NK cell and promoted NK cell chemotaxis in B16 melanoma induced by WKYMVm were both abolished with ERK inhibitor PD98059 administration. In conclusion, the FPR family promoted NK cell migration through ERK activation and inhibited B16 melanoma growth in a murine model.

A neutrophil inhibitory pepducin derived from FPR1 expected to target FPR1 signaling hijacks the closely related FPR2 instead

Pepducins constitute a unique class of G-protein coupled receptor (GPCR) modulating lipopeptides. Pepducins with inhibitory effects on neutrophils could potentially be developed into anti-inflammatory pharmaceuticals. A pepducin with a peptide sequence identical to the third intracellular loop of FPR1 was found to inhibit neutrophil functions including granule mobilization and superoxide production. This FPR1-derived pepducin selectively inhibited signaling and cellular responses through FPR2, but not FPR1 as expected. Binding to the neutrophil surface of a conventional FPR2 agonist is also inhibited. The fatty acid is essential for inhibition and pepducins with shorter peptides lose in potency. In summary, a pepducin designed to target FPR1 was found to hijack FPR2 and potently inhibit neutrophil functions.

Peptide length and folding state govern the capacity of staphylococcal β-type phenol-soluble modulins to activate human formyl-peptide receptors 1 or 2

Most staphylococci produce short α-type PSMs and about twice as long β-type PSMs that are potent leukocyte attractants and toxins. PSMs are usually secreted with the N-terminal formyl group but are only weak agonists for the leukocyte FPR1. Instead, the FPR1-related FPR2 senses PSMs efficiently and is crucial for leukocyte recruitment in infection. Which structural features distinguish FPR1 from FPR2 ligands has remained elusive. To analyze which peptide properties may govern the capacities of β-type PSMs to activate FPRs, full-length and truncated variants of such peptides from Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus lugdunensis were synthesized. FPR2 activation was observed even for short N- or C-terminal β-type PSM variants once they were longer than 18 aa, and this activity increased with length. In contrast, the shortest tested peptides were potent FPR1 agonists, and this property declined with increasing peptide length. Whereas full-length β-type PSMs formed α-helices and exhibited no FPR1-specific activity, the truncated peptides had less-stable secondary structures, were weak agonists for FPR1, and required N-terminal formyl-methionine residues to be FPR2 agonists. Together, these data suggest that FPR1 and FPR2 have opposed ligand preferences. Short, flexible PSM structures may favor FPR1 but not FPR2 activation, whereas longer peptides with α-helical, amphipathic properties are strong FPR2 but only weak FPR1 agonists. These findings should help to unravel the ligand specificities of 2 critical human PRRs, and they may be important for new, anti-infective and anti-inflammatory strategies.

A pepducin derived from the third intracellular loop of FPR2 is a partial agonist for direct activation of this receptor in neutrophils but a full agonist for cross-talk triggered reactivation of FPR2

We recently described a novel receptor cross-talk mechanism in neutrophils, unique in that the signals generated by the PAF receptor (PAFR) and the ATP receptor (P2Y₂R) transfer formyl peptide receptor 1 (FPR1) from a desensitized (non-signaling) state back to an actively signaling state (Forsman H et al., PLoS One, 8:e60169, 2013; Önnheim K, et al., Exp Cell Res, 323∶209, 2014). In addition to the G-protein coupled FPR1, neutrophils also express the closely related receptor FPR2. In this study we used an FPR2 specific pepducin, proposed to work as an allosteric modulator at the cytosolic signaling interface, to determine whether the cross-talk pathway is utilized also by FPR2. The pepducin used contains a fatty acid linked to a peptide sequence derived from the third intracellular loop of FPR2, and it activates as well as desensensitizes this receptor. We now show that neutrophils desensitized with the FPR2-specific pepducin display increased cellular responses to stimulation with PAF or ATP. The secondary PAF/ATP induced response was sensitive to FPR2-specific inhibitors, disclosing a receptor cross-talk mechanism underlying FPR2 reactivation. The pepducin induced an activity in naïve cells similar to that of a conventional FPR2 agonist, but with lower potency (partial efficacy), meaning that the pepducin is a partial agonist. The PAF- or ATP-induced reactivation was, however, much more pronounced when neutrophils had been desensitized to the pepducin as compared to cells desensitized to conventional agonists. The pepducin should thus in this respect be classified as a full agonist. In summary, we demonstrate that desensitized FPR2 can be transferred back to an actively signaling state by receptor cross-talk signals generated through PAFR and P2Y₂R, and the difference in agonist potency with respect to pepducin-induced direct receptor activation and cross-talk reactivation of FPR2 puts the concept of functional selectivity in focus.

Insight into structure-function relationship in phenol-soluble modulins using an alanine screen of the phenol-soluble modulin (PSM) α3 peptide

Phenol-soluble modulins (PSMs) are a family of peptides with multiple functions in staphylococcal pathogenesis. To gain insight into the structural features affecting PSM functions, we analyzed an alanine substitution library of PSMα3, a strongly cytolytic and proinflammatory PSM of Staphylococcus aureus with a significant contribution to S. aureus virulence. Lysine residues were essential for both receptor-dependent proinflammatory and receptor-independent cytolytic activities. Both phenotypes also required additional structural features, with the C terminus being crucial for receptor activation. Biofilm formation was affected mostly by hydrophobic amino acid positions, suggesting that the capacity to disrupt hydrophobic interactions is responsible for the effect of PSMs on biofilm structure. Antimicrobial activity, absent from natural PSMα3, could be created by the exchange of large hydrophobic side chains, indicating that PSMα3 has evolved to exhibit cytolytic rather than antimicrobial activity. In addition to gaining insight into the structure-function relationship in PSMs, our study identifies nontoxic PSMα3 derivatives for active vaccination strategies and lays the foundation for future efforts aimed to understand the biological role of PSM recognition by innate host defense.

Lack of activity of 15-epi-lipoxin A₄ on FPR2/ALX and CysLT1 receptors in interleukin-8-driven human neutrophil function

Neutrophil recruitment and survival are important control points in the development and resolution of inflammatory processes. 15-epi-lipoxin (LX)A interaction with formyl peptide receptor 2 (FPR2)/ALX receptor is suggested to enhance anti-inflammatory neutrophil functions and mediate resolution of airway inflammation. However, it has been reported that 15-epi-LXA₄ analogues can also bind to cysteinyl leukotriene receptor 1 (CysLT1) and that the CysLT1 antagonist MK-571 binds to FPR2/ALX, so cross-reactivity between FPR2/ALX and CysLT1 ligands cannot be discarded. It is not well established whether the resolution properties reported for 15-epi-LXA4 are mediated through FPR2/ALX, or if other receptors such as CysLT1 may also be involved. Evaluation of specific FPR2/ALX ligands and CysLT1 antagonists in functional biochemical and cellular assays were performed to establish a role for both receptors in 15-epi-LXA₄-mediated signalling and function. In our study, a FPR2/ALX synthetic peptide (WKYMVm) and a small molecule FPR2/ALX agonist (compound 43) induced FPR2/ALX-mediated signalling, enhancing guanosine triphosphate-gamma (GTPγ) binding and decreasing cyclic adenosine monophosphate (cAMP) levels, whereas 15-epi-LXA₄ was inactive. Furthermore, 15-epi-LXA4 showed neither binding affinity nor signalling towards CysLT1. In neutrophils, 15-epi-LXA₄ showed a moderate reduction of interleukin (IL)-8-mediated neutrophil chemotaxis but no effect on neutrophil survival was observed. In addition, CysLT1 antagonists were inactive in FPR2/ALX signalling or neutrophil assays. In conclusion, 15-epi-LXA₄ is not a functional agonist or an antagonist of FPR2/ALX or CysLT1, shows no effect on IL-8-induced neutrophil survival and produces only moderate inhibition in IL-8-mediated neutrophil migration. Our data do not support an anti-inflammatory role of 15-epi-LXA₄- FPR2/ALX interaction in IL-8-induced neutrophil inflammation.

Further studies on 2-arylacetamide pyridazin-3(2H)-ones: design, synthesis and evaluation of 4,6-disubstituted analogs as formyl peptide receptors (FPRs) agonists

Formyl peptide receptors (FPRs) play an essential role in the regulation of endogenous inflammation and immunity. In the present studies, a large series of pyridazin-3(2H)-one derivatives bearing an arylacetamide chain at position 2 was synthesized and tested for FPR agonist activity. The pyridazin-3(2H)-one ring was confirmed to be an appropriate scaffold to support FPR agonist activity, and its modification at the 4 and 6 positions led to the identification of additional active agonists, which induced intracellular Ca(2+) flux in HL-60 cells transfected with either FPR1, FPR2, or FPR3. Seven formyl peptide receptor 1 (FPR1)-specific and several mixed FPR1/FPR2 dual agonists were identified with low micromolar EC50 values. Furthermore, these agonists also activated human neutrophils, inducing intracellular Ca(2+) flux and chemotaxis. Finally, molecular docking studies indicated that the most potent pyridazin-3(2H)-ones overlapped in their best docking poses with fMLF and WKYMVM peptides in the FPR1 and FPR2 ligand binding sites, respectively. Thus, pyridazinone-based compounds represent potential lead compounds for further development of selective and/or potent FPR agonists.

Distinct signaling cascades elicited by different formyl peptide receptor 2 (FPR2) agonists

The formyl peptide receptor 2 (FPR2) is a remarkably versatile transmembrane protein belonging to the G-protein coupled receptor (GPCR) family. FPR2 is activated by an array of ligands, which include structurally unrelated lipids and peptide/proteins agonists, resulting in different intracellular responses in a ligand-specific fashion. In addition to the anti-inflammatory lipid, lipoxin A4, several other endogenous agonists also bind FPR2, including serum amyloid A, glucocorticoid-induced annexin 1, urokinase and its receptor, suggesting that the activation of FPR2 may result in potent pro- or anti-inflammatory responses. Other endogenous ligands, also present in biological samples, include resolvins, amyloidogenic proteins, such as beta amyloid (Aβ)-42 and prion protein (Prp)_106–126, the neuroprotective peptide, humanin, antibacterial peptides, annexin 1-derived peptides, chemokine variants, the neuropeptides, vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating polypeptide (PACAP)-27, and mitochondrial peptides. Upon activation, intracellular domains of FPR2 mediate signaling to G-proteins, which trigger several agonist-dependent signal transduction pathways, including activation of phospholipase C (PLC), protein kinase C (PKC) isoforms, the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) pathway, the mitogen-activated protein kinase (MAPK) pathway, p38MAPK, as well as the phosphorylation of cytosolic tyrosine kinases, tyrosine kinase receptor transactivation, phosphorylation and nuclear translocation of regulatory transcriptional factors, release of calcium and production of oxidants. FPR2 is an attractive therapeutic target, because of its involvement in a range of normal physiological processes and pathological diseases. Here, we review and discuss the most significant findings on the intracellular pathways and on the cross-communication between FPR2 and tyrosine kinase receptors triggered by different FPR2 agonists.

Local production of serum amyloid a is implicated in the induction of macrophage chemoattractants in Schwann cells during wallerian degeneration of peripheral nerves

The elevation of serum levels of serum amyloid A (SAA) has been regarded as an acute reactive response following inflammation and various types of injuries. SAA from the liver and extrahepatic tissues plays an immunomodulatory role in a variety of pathophysiological conditions. Inflammatory cytokines in the peripheral nerves have been implicated in the Wallerian degeneration of peripheral nerves after injury and in certain types of inflammatory neuropathies. In the present study, we found that a sciatic nerve axotomy could induce an increase of SAA1 and SAA3 mRNA expression in sciatic nerves. Immunohistochemical staining showed that Schwann cells are the primary sources of SAA production after nerve injury. In addition, interleukin-6-null mice, but not tumor necrosis factor-α-null mice showed a defect in the production of SAA1 in sciatic nerve following injury. Dexamethasone treatment enhanced the expression and secretion of SAA1 and SAA3 in sciatic nerve explants cultures, suggesting that interleukin-6 and corticosteroids might be major regulators for SAA production in Schwann cells following injury. Moreover, the stimulation of Schwann cells with SAA1 elicited the production of the macrophage chemoattractants, Ccl2 and Ccl3, in part through a G-protein coupled receptor. Our findings suggest that locally produced SAA might play an important role in Wallerian degeneration after peripheral nerve injury.

Enterococcus faecium stimulates human neutrophils via the formyl-peptide receptor 2

The human formyl-peptide receptor 2 (FPR2/ALX) senses phenol-soluble modulin (PSM) peptide toxins produced by pathogenic staphylococcal species and plays a crucial role in directing neutrophil influx during staphylococcal infection. However, it has remained unclear if FPR2 responds also to molecules from other bacterial pathogens. Here we analyzed a variety of Gram-positive and Gram-negative pathogens and found that apart from staphylococci only certain enterococcal strains have the capacity to stimulate FPR2/ALX. Most of the analyzed Enterococcus faecium but only sporadic Enterococcus faecalis strains released FPR2/ALX-stimulating molecules leading to neutrophil calcium ion fluxes, chemotaxis, and complement receptor upregulation. Among ten test strains vancomycin-resistant E. faecium had a significantly higher capacity to stimulate FPR2/ALX than vancomycin-susceptible strains, suggesting an association of strong FPR2/ALX activation with health-care associated strains. The enterococcal FPR2/ALX agonists were found to be peptides or proteins, which appear, however, to be unrelated to staphylococcal PSMs in sequence and physicochemical properties. Enterococci are among the most frequent invasive bacterial pathogens but the basis of enterococcal virulence and immune activation has remained incompletely understood. Our study indicates that previously unrecognized proteinaceous agonists contribute to Enterococcus-host interaction and underscores the importance of FPR2/ALX in host defense against major endogenous bacterial pathogens.

Receptor-dependent and -independent immunomodulatory effects of phenol-soluble modulin peptides from Staphylococcus aureus on human neutrophils are abrogated through peptide inactivation by reactive oxygen species

The virulence and pathogenesis mechanisms of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strains depend on a newly described group of phenol-soluble modulin (PSM) peptides (the PSMα peptides) with cytolytic activity. These toxins are α-helical peptides with a formyl group at the N terminus, and they activate neutrophils through formyl peptide receptor 2 (FPR2), a function closely correlated to the capacity of staphylococcal species to cause invasive infections. The effects of two synthetic PSMα peptides were investigated, and we show that they utilize FPR2 and promote neutrophils to produce reactive oxygen species (ROS) which in turn trigger inactivation of the peptides. Independently of FPR2, the PSMα peptides also downregulate the neutrophil response to other stimuli and exert a cytolytic effect to which apoptotic neutrophils are more sensitive than viable cells. The novel immunomodulatory functions of the PSMα peptides were sensitive to ROS generated by the neutrophil myeloperoxidase (MPO)-H(2)O(2) system, suggesting a role for this enzyme system in counteracting bacterial virulence.

A non-peptide receptor inhibitor with selectivity for one of the neutrophil formyl peptide receptors, FPR 1

The neutrophil formyl peptide receptors (FPR1 and FPR2) are members of the G-protein coupled receptor family. The signals generated by occupied FPRs are both pro-inflammatory and anti-inflammatory. Accordingly, these receptors have become a therapeutic target for the development of novel drugs that may be used to reduce injuries in inflammatory diseases including asthma, rheumatoid arthritis, Alzheimer's disease and cardiovascular diseases. To support the basis for a future pharmacological characterization, we have identified a small molecular non-peptide inhibitor with selectivity for FPR1. We used the FPR1 and FPR2 specific ligands fMLF and WKYMVM, respectively, and an earlier described ratio technique, to determine inhibitory activity combined with selectivity. We show that the compound 3,5-dichloro-N-(2-chloro-5-methyl-phenyl)-2-hydroxy-benzamide (BVT173187) fulfills the criteria for an FPR1 inhibitor selective for FPR1 over FPR2, and it inhibits the same functional repertoire in neutrophils as earlier described peptide antagonists. Accordingly, the new inhibitor reduced neutrophil activation with FPR1 agonists, leading to mobilization of adhesion molecules (CR3) and the generation of superoxide anion from the neutrophil NADPH-oxidase. The effects of a number of structural analogs were determined but these were either without activity or less active/specific than BVT173187. The potency of the new inhibitor for reduction of FPR1 activity was the same as that of the earlier described FPR1 antagonist cyclosporine H, but signaling through the C5aR and CXCR (recognizing IL8) was also affected by BVT173187.

Airway activation of formyl peptide receptors inhibits Th1 and Th17 cell responses via inhibition of mediator release from immune and inflammatory cells and maturation of dendritic cells

Formyl peptide receptors (FPRs) are chemoattractant receptors that mediate inflammatory cell responses to infection. Recent evidence indicates that noneosinophilic asthma phenotypes can be developed by both Th1 and Th17 cell responses when exposed to LPS-containing allergens. In this study, we evaluated the effects of airway activation of FPRs by their synthetic agonist, Trp-Lys-Tyr-Met-Val-D-Met (W-peptide), on the development of Th1 and Th17 cell responses in a noneosinophilic asthma mouse model. A noneosinophilic asthma mouse model was generated by intranasal sensitization with 10 μg of LPS plus 75 μg of OVA on days 0, 1, 2, and 7. Mice were then challenged with 50 μg of OVA alone on days 14, 15, 21, and 22. W-peptide was administered during the sensitization period, and immune and inflammatory responses were evaluated after OVA challenge. Lung inflammation after OVA challenge was partly abolished by airway activation of FPRs during sensitization. Maturation of dendritic cells (DCs) and migration of DCs from the lung to lung-draining lymph nodes were inhibited by FPR activation. In addition, airway activation of FPRs inhibited allergen-specific T cell proliferation in the lymph nodes. Production of IL-12 and IL-6 (Th1- and Th17-polarizing cytokines) from lung DCs was decreased by airway activation of FPRs. This effect resulted in the inhibition of allergen-specific Th1 and Th17 cell responses. Airway activation of FPRs during sensitization effectively prevents the development of Th1 and Th17 cell responses induced by LPS-containing allergens via multiple mechanisms, such as inhibition of DC maturation and migration and the production of Th1- and Th7-polarizing cytokines.

What formyl peptide receptors, if any, are triggered by compound 43 and lipoxin A4?

In this study, we determined receptor preferences for compound 43, a nitrosylated pyrazolone derivative, and the eicosanoid lipoxin A(4) (LXA(4)), potent anti-inflammatory mediators in many experimental in vivo models. Their effects have been suggested to be mediated through binding to formyl peptide receptor (FPR)2 [earlier known as formyl peptide receptor-like 1 or the lipoxin A(4) receptor (ALXR)], one of the two members of the FPR family expressed in neutrophils. Compound 43 activates all neutrophil functions investigated, whereas LXA(4) induces a unique inhibiting pathway suggested to involve β-arrestin binding as an early signalling step, but not a transient rise in intracellular Ca(2+). We show that compound 43 can activate not only FPR2 but also FPR1, the other neutrophil receptor in the FPR family, and FPR1 is actually the preferred receptor in human neutrophils and possibly also in the murine equivalent. LXA(4) analogues from two commercial sources were used, and neither of these induced any translocation of β-arrestin as measured in an enzyme fragment complementation assay. The conclusions drawn from these experiments are that neither compound 43 nor LXA(4) works as FPR2 agonists in neutrophils, findings of importance for a proper interpretation of results obtained with these compounds as regulators of inflammation.

N-formyl peptide receptor 3 (FPR3) departs from the homologous FPR2/ALX receptor with regard to the major processes governing chemoattractant receptor regulation, expression at the cell surface, and phosphorylation

Among human N-formyl peptide chemoattractant receptors, FPR2/ALX and FPR3 share the highest degree of amino acid identity (83%), and trigger similar cell responses upon ligand binding. Although FPR2/ALX is a promiscuous receptor, FPR3 has only one specific high affinity ligand, F2L, and a more restricted tissue/cell distribution. In this study, we showed that FPR2/ALX behaved as the prototypical receptor FPR1. The agonist-dependent phosphorylation used a hierarchical mechanism with a prominent role of Ser(329), Thr(332), and Thr(335). Phosphorylation of FPR2/ALX was essential for its desensitization but the lack of phosphorylation did not result in enhanced or sustained responses. In contrast, resting FPR3 displayed a marked level of phosphorylation, which was only slightly increased upon agonist stimulation. Another noticeable difference between the two receptors was their subcellular distribution in unstimulated cells. Although FPR2/ALX was evenly distributed at the plasma membrane FPR3 was localized in small intracellular vesicles. By swapping domains between FPR2/ALX and FPR3, we uncovered the determinants involved in the basal phosphorylation of FPR3. Experiments aimed at monitoring receptor-bound antibody uptake showed that the intracellular distribution of FPR3 resulted from a constitutive internalization that was independent of C terminus phosphorylation. Unexpectedly, exchanging residues 1 to 53, which encompass the N-terminal extracellular region and the first transmembrane domain, between FPR2/ALX and FPR3 switched localization of the receptors from the plasma membrane to intracellular vesicles and vice versa. A clathrin-independent, possibly caveolae-dependent, mechanism was involved in FPR3 constitutive internalization. The peculiar behavior of FPR3 most probably serves distinct physiological functions that remain largely unknown.

Neutrophil responses to staphylococcal pathogens and commensals via the formyl peptide receptor 2 relates to phenol-soluble modulin release and virulence

The mechanisms used by the immune system to discriminate between pathogenic and commensal bacteria have remained largely unclear. Recently, we have shown that virulence of Staphylococcus aureus depends on secretion of phenol-soluble modulin (PSM) peptides that disrupt neutrophils at micromolar concentrations. Moreover, all S. aureus PSMs stimulate and attract neutrophils at nanomolar concentrations via interaction with the formyl-peptide receptor 2 (FPR2). Here, we demonstrate that FPR2 allows neutrophils to adjust their responses in relation to the aggressiveness of staphylococcal species, which differ largely in their capacity to infect or colonize humans and animals. PSM-related peptides were detected in all human and animal pathogenic staphylococci, but were absent from most commensal species. Three PSMβ-like peptides produced by the serious human pathogen Staphylococcus lugdunensis were identified as the previously described S. lugdunensis-synergistic hemolysins (SLUSHs). SLUSHs attracted and stimulated human leukocytes in a FPR2-dependent manner, indicating that FPR2 is a general receptor for all PSM-like peptide toxins. Remarkably, the release of PSMs correlated closely with the apparent capacity of staphylococcal species to cause invasive infections and with their ability to activate FPR2. These findings suggest that the innate immune system may be able to respond in different ways to pathogenic or innocuous staphylococci by monitoring the presence of PSMs via FPR2.

Pyruvate formate lyase acts as a formate supplier for metabolic processes during anaerobiosis in Staphylococcus aureus

Previous studies demonstrated an upregulation of pyruvate formate lyase (Pfl) and NAD-dependent formate dehydrogenase (Fdh) in Staphylococcus aureus biofilms. To investigate their physiological role, we constructed fdh and pfl deletion mutants (Δfdh and Δpfl). Although formate dehydrogenase activity in the fdh mutant was lost, it showed little phenotypic alterations under oxygen-limited conditions. In contrast, the pfl mutant displayed pleiotropic effects and revealed the importance of formate production for anabolic metabolism. In the pfl mutant, no formate was produced, glucose consumption was delayed, and ethanol production was decreased, whereas acetate and lactate production were unaffected. All metabolic alterations could be restored by addition of formate or complementation of the Δpfl mutant. In compensation reactions, serine and threonine were consumed better by the Δpfl mutant than by the wild type, suggesting that their catabolism contributes to the refilling of formyl-tetrahydrofolate, which acts as a donor of formyl groups in, e.g., purine and protein biosynthesis. This notion was supported by reduced production of formylated peptides by the Δpfl mutant compared to that of the parental strain, as demonstrated by weaker formyl-peptide receptor 1 (FPR1)-mediated activation of leukocytes with the mutant. FPR1 stimulation could also be restored either by addition of formate or by complementation of the mutation. Furthermore, arginine consumption and arc operon transcription were increased in the Δpfl mutant. Unlike what occurred with the investigated anaerobic conditions, a biofilm is distinguished by nutrient, oxygen, and pH gradients, and we thus assume that Pfl plays a significant role in the anaerobic layer of a biofilm. Fdh might be critical in (micro)aerobic layers, as formate oxidation is correlated with the generation of NADH/H(+), whose regeneration requires respiration.

Intracellular generation of superoxide by the phagocyte NADPH oxidase: how, where, and what for?

Professional phagocytes increase their consumption of molecular oxygen during the phagocytosis of microbes or when encountering a variety of nonparticulate stimuli. In these circumstances, oxygen is reduced by the phagocyte NADPH oxidase, and reactive oxygen species (ROS), which are important for the microbicidal activity of the cells, are generated. The structure and function of the NADPH oxidase have been resolved in part by studying cells from patients with chronic granulomatous disease (CGD), a condition characterized by the inability of phagocytes to assemble a functional NADPH oxidase and thus to produce ROS. As a result, patients with CGD have a predisposition to infections as well as a variety of inflammatory symptoms. A long-standing paradigm has been that NADPH oxidase assembly occurs exclusively in the plasma membrane or invaginations thereof (phagosomes). A growing body of evidence points to the possibility that phagocytes are capable of NADPH oxidase assembly in nonphagosomal intracellular membranes, resulting in ROS generation within intracellular organelles also in the absence of phagocytosis. The exact nature of these ROS-producing organelles is yet to be determined, but granules are prime suspects. Recent clinical findings indicate that the generation of intracellular ROS by NADPH oxidase activation is important for limiting inflammatory reactions and that intracellular and extracellular ROS production are regulated differently. Here we discuss the accumulating knowledge of intracellular ROS production in phagocytes and speculate on the precise role of these oxidants in regulating the inflammatory process.

The FPR2-induced rise in cytosolic calcium in human neutrophils relies on an emptying of intracellular calcium stores and is inhibited by a gelsolin-derived PIP2-binding peptide

BACKGROUND

The molecular basis for neutrophil recognition of chemotactic peptides is their binding to specific G-protein-coupled cell surface receptors (GPCRs). Human neutrophils express two pattern recognition GPCRs, FPR1 and FPR2, which belong to the family of formyl peptide receptors. The high degree of homology between these two receptors suggests that they share many functional and signal transduction properties, although they exhibit some differences with respect to signaling. The aims of this study were to determine whether FPR2 triggers a unique signal that allows direct influx of extracellular calcium without the emptying of intracellular calcium stores, and whether the gelsolin-derived PIP2-binding peptide, PBP10, selectively inhibits FPR2-mediated transient rise in intracellular Ca2+.

RESULTS

The transient rise in intracellular Ca2+ induced by agonists for FPR1 or FPR2 in human neutrophils occurred also in the presence of a chelator of Ca2+ (EGTA). PBP10 inhibited not only FPR2-induced oxidase activity, but also the transient rise in intracellular Ca2+.

CONCLUSIONS

Ca2+ signaling mediated via FPR2 follows the same route as FPR1, which involves initial emptying of the intracellular stores. PBP10 inhibits selectively the signals generated by FPR2, both with respect to NADPH-oxidase activity and the transient rise in intracellular Ca2+ induced by agonist exposure.

Human formyl peptide receptor 2 senses highly pathogenic Staphylococcus aureus

Virulence of emerging community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) and other highly pathogenic S. aureus strains depends on their production of phenol-soluble modulin (PSM) peptide toxins, which combine the capacities to attract and lyse neutrophils. The molecular basis of PSM-stimulated neutrophil recruitment has remained unclear. Here, we demonstrate that the human formyl peptide receptor 2 (FPR2/ALX), which has previously been implicated in control of endogenous inflammatory processes, senses PSMs at nanomolar concentrations and initiates proinflammatory neutrophil responses to CA-MRSA. Specific blocking of FPR2/ALX or deletion of PSM genes in CA-MRSA severely diminished neutrophil detection of CA-MRSA. Furthermore, a specific inhibitor of FPR2/ALX and of its functional mouse counterpart blocked PSM-mediated leukocyte infiltration in vivo in a mouse model. Thus, the innate immune system uses a distinct FPR2/ALX-dependent mechanism to specifically sense bacterial peptide toxins and detect highly virulent bacterial pathogens. FPR2/ALX represents an attractive target for new anti-infective or anti-inflammatory strategies.

The annexin I sequence gln(9)-ala(10)-trp(11)-phe(12) is a core structure for interaction with the formyl peptide receptor 1

The N-terminal part of the calcium-regulated and phospholipid-binding protein annexin AI contains peptide sequences with pro- and anti-inflammatory activities. We have earlier shown that a proinflammatory signal triggered by one of these peptides, Gln(9)-Lys(25), is mediated by FPR1, a member of the formyl peptide receptor family expressed in human neutrophils. To determine the core structure in Gln(9)-Lys(25), smaller peptides were generated, and their capacity to activate neutrophils was determined. A peptide spanning from amino acid Glu(14) to Lys(25) was inactive, whereas the activity was retained in the Gln(9)-Tyr(20) peptide. Removal of amino acids from the C and N terminus of Gln(9)-Tyr(20) revealed that the first amino acid (Gln(9)) was of the utmost importance for activity. The core structure that activated the neutrophil NADPH oxidase to release superoxide anions was Gln(9)-Ala(10)-Trp(11)-Phe(12). This peptide also inhibited the activity induced by N-formyl-Met-Leu-Phe and WKYMVM. A structural model of the peptide agonist-FPR1 complex suggests that the transmembrane part of the binding pocket of the receptor binds optimally to a tetrapeptide. According to the model and the results presented, the N-terminal amino acid glutamine in Gln(9)-Phe(12) is located close to the bottom of the binding cleft, leaving for steric reasons insufficient space to extend the peptide at the N terminus. The addition of amino acids at the C terminus will not affect binding. The model presented may be helpful in developing specific FPR1 ligands.

Formyl peptide receptor-mediated proinflammatory consequences of peptide deformylase inhibition in Staphylococcus aureus

The biosynthesis of proteins with N-terminal formylated methionine residues and subsequent protein deformylation are unique and invariant bacterial processes. They are exploited by the capacity of the human innate immune system to sense formylated peptides (FPs) and targeted by the deformylation-blocking antibiotic actinonin. We show that human polymorphonuclear leukocytes respond via the formyl peptide receptor (FPR) with increased calcium ion fluxes, chemotactic migration, IL-8 release, and CD11b upregulation to the human pathogen Staphylococcus aureus upon actinonin treatment. These data underscore the crucial role of bacterial FPs in innate immunity and indicate that deformylase inhibition may have considerable proinflammatory consequences.

Multiple functional targets of the immunoregulatory activity of galectin-1: Control of immune cell trafficking, dendritic cell physiology, and T-cell fate

In the postgenomic era, the study of the glycome-the whole repertoire of saccharides in cells and tissues-has enabled the association of unique glycan structures with specific physiological and pathological processes. The responsibility for deciphering this biological information belongs to endogenous glycan-binding proteins or lectins. Galectin-1, a prototypic member of a family of structurally related proteins, has demonstrated selective antiinflammatory and immunoregulatory effects either by controlling immune cell trafficking, "fine-tuning" dendritic cell physiology and regulating T-cell fate. These regulatory functions mediated by an endogenous glycan-binding protein may contribute to fulfill the needs for immune cell homeostasis, including preservation of fetomaternal tolerance and prevention of collateral damage as a result of microbial invasion or autoimmune pathology. We will discuss here the conceptual framework which led to the study of galectin-glycan lattices as a novel paradigm of immune cell communication in physiological and pathological processes and will highlight selected methods and experimental strategies which have contributed to the study of the immunoregulatory activities of this multifaceted glycan-binding protein both in in vitro and in vivo biological settings.