Functional characterization of three mouse formyl peptide receptors

Immune signature of gene expression pattern shared by autism spectrum disorder and Huntington's disease

Autism spectrum disorder (ASD) and Huntington's disease (HD) are complex neurological conditions with unclear causes and limited treatments, affecting individuals, families, and society. Despite ASD and HD representing two opposing stages of neuronal development and degeneration, they share similar clinical-pathological features in motor function. In this study, we leveraged transcriptomic data from the prefrontal cortex available in public databases to identify shared transcriptional characteristics of ASD and HD. Differential expression analysis revealed that the majority of differentially expressed genes (DEGs) were up-regulated in ASD carriers, whereas most DEGs were down-regulated in HD carriers. Among the DEGs shared between both diseases, three out of seven protein-coding genes were related to the immune system. Furthermore, we identified two enriched pathways shared between ASD and HD DEGs. The gene interaction network analysis unveiled four hub genes shared by both diseases, all of which are associated with immune functions. The findings suggest a shared gene expression pattern in the prefrontal cortex of people with ASD and HD, closely linked to the immune system. These findings will contribute to exploring the biological mechanisms underlying the shared phenotypes of these two diseases from an immunological perspective.

Tumor-derived mitochondrial formyl peptides suppress tumor immunity through modification of the tumor microenvironment

Mitochondrial N-formylpeptides are released from damaged or dead cells to the extracellular spaces and cause inflammatory responses. The role of mitochondrial N-formylpeptides in aseptic systemic inflammatory response syndromes induced by trauma or cardiac surgery has been well investigated. However, there are no reports regarding the role of mitochondrial N-formylpeptides in cancer. In this study, we investigated the role of tumor cell-derived mitochondrial N-formylpeptides in anti-tumor immunity using knockout murine tumor cells of mitochondrial methionyl-tRNA formyltransferase (MTFMT), which catalyze N-formylation of mitochondrial DNA-encoded proteins. There was no apparent difference among the wild-type and MTFMT-knockout clones of E.G7-OVA cells with respect to morphology, mitochondrial dynamics, glycolysis and oxidative phosphorylation, oxygen consumption rate, or in vitro cell growth. In contrast, in vivo tumor growth of MTFMT-knockout cells was slower than that of wild-type cells. A reduced number of myeloid-derived suppressor cells and an increase of cytotoxic T-lymphocytes in the tumor tissues were observed in the MTFMT-knockout tumors. These results suggested that tumor cell-derived mitochondrial N-formylpeptides had a negative role in the host anti-tumor immunity through modification of the tumor microenvironment.

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.

The formyl peptide receptors FPR1 and FPR2 as targets for inflammatory disorders: recent advances in the development of small-molecule agonists

Formyl peptide receptors (FPRs) comprise a class of chemoattractant pattern recognition receptors, for which several physiological functions like host-defences, as well as the regulation of inflammatory responses, have been ascribed. With accumulating evidence that agonism of FPR1/FPR2 can confer pro-resolution of inflammation, increased attention from academia and industry has led to the discovery of new and interesting small-molecule FPR1/FPR2 agonists. Focused attention on the development of appropriate physicochemical and pharmacokinetic profiles is yielding synthesis of new compounds with promising in vivo readouts. This review presents an overview of small-molecule FPR1/FPR2 agonist medicinal chemistry developed over the past 20 years, with a particular emphasis on interrogation in the increasingly sophisticated bioassays which have been developed.

Macrophage-driven cardiac inflammation and healing: insights from homeostasis and myocardial infarction

Early and prompt reperfusion therapy has markedly improved the survival rates among patients enduring myocardial infarction (MI). Nonetheless, the resulting adverse remodeling and the subsequent onset of heart failure remain formidable clinical management challenges and represent a primary cause of disability in MI patients worldwide. Macrophages play a crucial role in immune system regulation and wield a profound influence over the inflammatory repair process following MI, thereby dictating the degree of myocardial injury and the subsequent pathological remodeling. Despite numerous previous biological studies that established the classical polarization model for macrophages, classifying them as either M1 pro-inflammatory or M2 pro-reparative macrophages, this simplistic categorization falls short of meeting the precision medicine standards, hindering the translational advancement of clinical research. Recently, advances in single-cell sequencing technology have facilitated a more profound exploration of macrophage heterogeneity and plasticity, opening avenues for the development of targeted interventions to address macrophage-related factors in the aftermath of MI. In this review, we provide a summary of macrophage origins, tissue distribution, classification, and surface markers. Furthermore, we delve into the multifaceted roles of macrophages in maintaining cardiac homeostasis and regulating inflammation during the post-MI period.

Developmental and homeostatic signaling transmitted by the G-protein coupled receptor FPR2

Formyl peptide receptor 2 (FPR2) and its mouse counterpart Fpr2 are the members of the G protein-coupled receptor (GPCR) family. FPR2 is the only member of the FPRs that interacts with ligands from different sources. FPR2 is expressed in myeloid cells as well as epithelial cells, endothelial cells, neurons, and hepatocytes. During the past years, some unusual properties of FPR2 have attracted intense attention because FPR2 appears to possess dual functions by activating or inhibiting intracellular signal pathways based on the nature, concentration of the ligands, and the temporal and spatial settings of the microenvironment in vivo, the cell types it interacts with. Therefore, FPR2 controls an abundant array of developmental and homeostatic signaling cascades, in addition to its "classical" capacity to mediate the migration of hematopoietic and non-hematopoietic cells including malignant cells. In this review, we summarize recent development in FPR2 research, particularly in its role in diseases, therefore helping to establish FPR2 as a potential target for therapeutic intervention.

A fluorescent photoaffinity probe for formyl peptide receptor 1 labelling in living cells

Fluorescent ligands for G-protein coupled receptors (GPCRs) are valuable tools for studying the expression, pharmacology and modulation of these therapeutically important proteins in living cells. Here we report a fluorescent photoaffinity probe for Formyl peptide receptor 1 (FPR1), a critical component of the innate immune response to bacterial infection and a promising target in inflammatory diseases. We demonstrate that the probe binds and covalently crosslinks to FPR1 with good specificity at nanomolar concentrations in living cells and is a useful tool for visualisation and characterisation of this receptor.

AZ2158 is a more potent formyl peptide receptor 1 inhibitor than the commonly used peptide antagonists in abolishing neutrophil chemotaxis

Formyl peptide receptor 1 (FPR1), a G protein-coupled receptor expressed in phagocytes, recognizes short N-formylated peptides originating from proteins synthesized by bacteria and mitochondria. Such FPR1 agonists are important regulators of neutrophil functions and by that, determinants of inflammatory reactions. As FPR1 is implicated in promoting both pro-inflammatory and pro-resolving responses associated with inflammatory diseases, characterization of ligands that potently and selectively modulate FPR1 induced functions might be of high relevance. Accordingly, a number of FPR1 specific antagonists have been identified and shown to inhibit agonist binding or receptor down-stream signaling as well as neutrophil functions such as granule secretion and NADPH oxidase activity. The inhibitory effect on neutrophil chemotaxis induced by FPR1 agonists has generally not been part of basic antagonist characterization. In this study we show that the inhibitory effects on neutrophil chemotaxis of established FPR1 antagonists (i.e., cyclosporin H, BOC1 and BOC2) are limited. Our data demonstrate that the recently described small molecule AZ2158 is a potent and selective FPR1 antagonist in human neutrophils. In contrast to the already established FPR1 antagonists, AZ2158 also potently inhibits chemotaxis. Whereas the cyclosporin H inhibition was agonist selective, AZ2158 inhibited the FPR1 response induced by both a balanced and a biased FPR1 agonist equally well. In accordance with the species specificity described for many FPR1 ligands, AZ2158 was not recognized by the mouse orthologue of FPR1. Our data demonstrate that AZ2158 may serve as an excellent tool compound for further mechanistic studies of human FPR1 mediated activities.

Promiscuous Receptors and Neuroinflammation: The Formyl Peptide Class

Formyl peptide receptors, abbreviated as FPRs in humans, are G-protein coupled receptors (GPCRs) mainly found in mammalian leukocytes. However, they are also expressed in cell types crucial for homeostatic brain regulation, including microglia and blood-brain barrier endothelial cells. Thus, the roles of these immune-associated receptors are extensive, from governing cellular adhesion and directed migration through chemotaxis, to granule release and superoxide formation, to phagocytosis and efferocytosis. In this review, we will describe the similarities and differences between the two principal pro-inflammatory and anti-inflammatory FPRs, FPR1 and FPR2, and the evidence for their importance in the development of neuroinflammatory disease, alongside their potential as therapeutic targets.

Formylpeptide receptor 2: Nomenclature, structure, signalling and translational perspectives: IUPHAR review 35

We discuss the fascinating pharmacology of formylpeptide receptor 2 (FPR2; often referred to as FPR2/ALX since it binds lipoxin A₄ ). Initially identified as a low-affinity 'relative' of FPR1, FPR2 presents complex and diverse biology. For instance, it is activated by several classes of agonists (from peptides to proteins and lipid mediators) and displays diverse expression patterns on myeloid cells as well as epithelial cells and endothelial cells, to name a few. Over the last decade, the pharmacology of FPR2 has progressed from being considered a weak chemotactic receptor to a master-regulator of the resolution of inflammation, the second phase of the acute inflammatory response. We propose that exploitation of the biology of FPR2 offers innovative ways to rectify chronic inflammatory states and represents a viable avenue to develop novel therapies. Recent elucidation of FPR2 structure will facilitate development of the anti-inflammatory and pro-resolving drugs of next decade.

Therapeutic potential of WKYMVm in diseases

The synthetic hexapeptide WKYMVm, screened from a synthetic peptide library, has been identified as an agonist of FPRs with the strongest activating effect on FPR2. WKYMVm plays an anti-inflammatory role in most inflammatory diseases by increasing the chemotaxis of phagocytes and regulating the secretion of inflammatory factors. WKYMVm can inhibit or promote the progression of different types of tumors, which depends on the regulation of WKYMVm on various components such as immune cells, inflammatory factors, chemokines, and tumor epithelial cells. Another major function of WKYMVm is to promote angiogenesis, which is reflected in its therapeutic value in ischemic diseases, wound healing and bone repair. In addition to the above functions, this paper also reviews the effects of WKYMVm on fibrosis, insulin resistance, osteolytic diseases and neurodegenerative diseases. By summarizing related studies, this review can increase people’s comprehensive understanding of WKYMVm, promote its broad and in-depth research, and help to exert its therapeutic value as soon as possible.

Dysfunction of resolution receptor triggers cardiomyopathy of obesity and signs of non-resolving inflammation in heart failure

Heart failure with preserved ejection fraction (HFpEF) has been an emerging type of cardiac disease since the pseudo-left ventricle function is preserved; therefore, challenges in finding the target and treatment. Damage and pathogen-associated molecular patterns (DAMPs and PAMPs) are widely investigated in acute and chronic inflammation in heart failure; however, lifestyle-associated molecular patterns (LAMPs: diet, sleep, exercise), particularly in obesity, remains of interest due to the enormous increase of HFpEF patients. In this review, we covered obesity-related cardiomyopathy, LAMPs, and resolution receptor dysfunction in the context of heart failure with preserved ejection fraction.

Therapeutic evaluation of immunomodulators in reducing surgical wound infection

Despite many advances in infection control practices, including prophylactic antibiotics, surgical site infections (SSIs) remain a significant cause of morbidity, prolonged hospitalization, and death worldwide. Our innate immune system possesses a multitude of powerful antimicrobial strategies which make it highly effective in combating bacterial, fungal, and viral infections. However, pathogens use various stealth mechanisms to avoid the innate immune system, which in turn buy them time to colonize wounds and damage tissues at surgical sites. We hypothesized that immunomodulators that can jumpstart and activate innate immune responses at surgical sites, would likely reduce infection at surgical sites. We used three immunomodulators; fMLP (formyl-Methionine-Lysine-Proline), CCL3 (MIP-1α), and LPS (Lipopolysaccharide), based on their documented ability to elicit strong inflammatory responses; in a surgical wound infection model with Pseudomonas aeruginosa to evaluate our hypothesis. Our data indicate that one-time topical treatment with these immunomodulators at low doses significantly increased proinflammatory responses in infected and uninfected surgical wounds and were as effective, (or even better), than a potent prophylactic antibiotic (Tobramycin) in reducing P. aeruginosa infection in wounds. Our data further show that immunomodulators did not have adverse effects on tissue repair and wound healing processes. Rather, they enhanced healing in both infected and uninfected wounds. Collectively, our data demonstrate that harnessing the power of the innate immune system by immunomodulators can significantly boost infection control and potentially stimulate healing. We propose that topical treatment with these immunomodulators at the time of surgery may have therapeutic potential in combating SSI, alone or in combination with prophylactic antibiotics.

MAP kinases in regulation of NOX activity stimulated through two types of formyl peptide receptors in murine bone marrow granulocytes

The functional activity of the phagocytes, as well as the development and resolution of the inflammation, is determined by formylpeptide receptors (FPRs) signaling. There is a growing data on the signaling pathways from two major types of formylpeptide receptors, FPR1 and FPR2, which could be activated by different sets of ligands to provide certain defense functions. Generation of reactive oxygen species (ROS) by the membrane enzyme NADPH oxidase is the most important among them. One of the most studied and significant mechanism for the regulation of activity of NADPH oxidase is phosphorylation by a variety of kinases, including MAP kinases. The question arose whether the role of MAPKs differ in the activation of NADPH oxidase through FPR1 and FPR2. We have studied Fpr1- and Fpr2-induced phosphorylation of p38, ERK, and JNK kinases and their role in the activation of the respiratory burst in isolated mice bone marrow granulocytes. Data has shown distinct patterns of MAP kinase activity for Fpr1 and Fpr2: JNK was involved in both Fpr1 and Fpr2 mediated activation of ROS production, while p38 MAPK and ERK were involved in Fpr1 induced ROS generation only.

Role of Mitochondria-Derived Danger Signals Released After Injury in Systemic Inflammation and Sepsis

Significance: Sepsis is a major public health concern, with high mortality and morbidity, especially among patients undergoing trauma. It is characterized by a systemic inflammatory response syndrome (SIRS) occurring in response to infection. Although classically associated with pathogens, many patients with SIRS do not have infection. The variability of the disease course cannot be fully explained by our current understanding of its pathogenesis. Thus, other factors are likely to play key roles in the development and progression of SIRS/sepsis. Recent Advances: Circulating levels of damage-associated molecular patterns (DAMPs) seem to correlate with SIRS/sepsis morbidity and mortality. Of the known DAMPs, those of mitochondrial (mt) origin have been of particular interest, since their DNA (mtDNA) and formyl peptides (mtFPs) resemble bacterial DNA and peptides, and hence, when released, may be recognized as "danger signals." Critical Issues: mtDAMPs released after tissue injury trigger immune responses similar to those induced by pathogens. Thus, they can result in systemic inflammation and organ damage, similar to that observed in SIRS/sepsis. We will discuss recent findings on the roles of mtDAMPs, particularly regarding the less recognized mtFPs, in the activation of inflammatory responses and development of SIRS/sepsis. Future Directions: There are no established methods to predict the course of SIRS/sepsis, but clinical studies reveal that plasma levels of mtDAMPs may correlate with the outcome of the disease. We propose that non-pathogen-initiated, mtDAMPs-induced SIRS/sepsis events need further studies aimed at early clinical recognition and better treatment of this disease.

Analysis of the vomeronasal organ transcriptome reveals variable gene expression depending on age and function in rabbits

The vomeronasal organ (VNO) is a chemosensory organ specialized in pheromone detection that shows a broad morphofunctional and genomic diversity among mammals. However, its expression patterns have only been well-characterized in mice. Here, we provide the first comprehensive RNA sequencing study of the rabbit VNO across gender and sexual maturation stages. We characterized the VNO transcriptome, updating the number and expression of the two main vomeronasal receptor families, including 128 V1Rs and 67 V2Rs. Further, we defined the expression of formyl-peptide receptor and transient receptor potential channel families, both known to have specific roles in the VNO. Several sex hormone-related pathways were consistently enriched in the VNO, highlighting the relevance of this organ in reproduction. Moreover, whereas juvenile and adult VNOs showed significant transcriptome differences, male and female did not. Overall, these results contribute to understand the genomic basis of behavioural responses mediated by the VNO in a non-rodent model.

Formyl Peptide Receptor (FPR)1 Modulation by Resveratrol in an LPS-Induced Neuroinflammatory Animal Model

Among therapeutic approaches that have been investigated, targeting of receptors implicated in managing neuroinflammation has been described. One such family of receptors comprises the formyl peptide receptors (FPRs) whose ligands could play a role in host defense. The murine FPR gene family includes at least six members while in humans there are only three. The two most important members are the Fpr1 and Fpr2. Fpr1encodes murine FPR1, which is considered the murine orthologue of human FPR. Resveratrol, a non-flavonoid polyphenol rich in red wine and grapes, apart from its beneficial health effects and anti-inflammatory properties, has been reported to reduce neuroinflammation in different neurodegenerative disease models. Resveratrol anti-inflammatory responses involve the activation of the protein deacetylase sirtuin 1 (SIRT1) gene. In this work we have investigated in an LPS-based murine model of neuroinflammation the role of FPR1, examining not only if this receptor undergoes a reduction of its expression during neuroinflammation, but also whether treatment with resveratrol was able to modulate its expression leading to an amelioration of neuroinflammatory picture in a murine model of neuroinflammation. Results of this work showed that FPR1 together with SIRT1 resulted upregulated by resveratrol treatment and that this increase is associated with an amelioration of the neuroinflammatory picture, as demonstrated by the induction of IL-10 and IL1-RA expression and the downregulation of proinflammatory mediators, such as TNF-α and IL-1β. The expression and the modulation of FPR1 by resveratrol may be evaluated in order to propose a novel anti-inflammatory and pro-resolving therapeutic approach for the reduction of the detrimental effects associated with neuro-inflammation based neurodegenerative diseases and also as a promising strategy to promote human health by a diet rich in antioxidative bioactive compounds.

Applanmerotic acids A and B, two meroterpenoid dimers with an unprecedented polycyclic skeleton from Ganoderma applanatum that inhibit formyl peptide receptor 2

Applanmerotic acids A and B (1 and 2) with a polycyclic skeleton isolated from Ganoderma applantum showed anti-inflammatory activity via inhibiting the activation of FPR2.

The Contribution of Formyl Peptide Receptor Dysfunction to the Course of Neuroinflammation: A Potential Role in the Brain Pathology

Chronic inflammatory processes within the central nervous system (CNS) are in part responsible for the development of neurodegenerative and psychiatric diseases. These processes are associated with, among other things, the increased and disturbed activation of microglia and the elevated production of proinflammatory factors. Recent studies indicated that the disruption of the process of resolution of inflammation (RoI) may be the cause of CNS disorders. It is shown that the RoI is regulated by endogenous molecules called specialized pro-resolving mediators (SPMs), which interact with specific membrane receptors. Some SPMs activate formyl peptide receptors (FPRs), which belong to the family of seven-transmembrane G protein-coupled receptors. These receptors take part not only in the proinflammatory response but also in the resolution of the inflammation process. Therefore, the activation of FPRs might have complex consequences.

This review discusses the potential role of FPRs, and in particular the role of FPR2 subtype, in the brain under physiological and pathological conditions and their involvement in processes underlying neurodegenerative and psychiatric disorders as well as ischemia, the pathogenesis of which involves the dysfunction of inflammatory processes.

The N-formyl peptide receptors: contemporary roles in neuronal function and dysfunction

N-formyl peptide receptors (FPRs) were first identified upon phagocytic leukocytes, but more than four decades of research has unearthed a plethora of non-myeloid roles for this receptor family. FPRs are expressed within neuronal tissues and markedly in the central nervous system, where FPR interactions with endogenous ligands have been implicated in the pathophysiology of several neurodegenerative diseases including Alzheimer's disease and Parkinson's disease, as well as neurological cancers such as neuroblastoma. Whilst the homeostatic function of FPRs in the nervous system is currently undefined, a variety of novel physiological roles for this receptor family in the neuronal context have been posited in both human and animal settings. Rapid developments in recent years have implicated FPRs in the process of neurogenesis and neuronal differentiation which, upon greater characterisation, could represent a novel pharmacological target for neuronal regeneration therapies that may be used in the treatment of brain/spinal cord injury, stroke and neurodegeneration. This review aims to summarize the recent progress made to determine the physiological role of FPRs in a neuronal setting, and to put forward a case for FPRs as a novel pharmacological target for conditions of the nervous system, and for their potential to open the door to novel neuronal regeneration therapies.

Bam32/DAPP1-Dependent Neutrophil Reactive Oxygen Species in WKYMVm-Induced Microvascular Hyperpermeability

B cell adaptor molecule of 32 kDa (Bam32), known as dual adapter for phosphotyrosine and 3-phosphoinositides 1 (DAPP1), has been implicated in regulating lymphocyte proliferation and recruitment during inflammation. However, its role in neutrophils during inflammation remains unknown. Using intravital microscopy, we examined the role of Bam32 in formyl peptide receptor agonist WKYMVm-induced permeability changes in post-capillary venules and assessed simultaneously neutrophil adhesion and emigration in cremaster muscles of Bam32-deficient (Bam32^−/−) and wild-type (WT) control mice. We observed significantly reduced WKYMVm-induced microvascular hyperpermeability accompanied by markedly decreased neutrophil emigration in Bam32^−/− mice. The Bam32-specific decrease in WKYMVm-induced hyperpermeability was neutrophil-dependent as this was verified in bone marrow transplanted chimeric mice. We discovered that Bam32 was critically required for WKYMVm-induced intracellular and extracellular production of reactive oxygen species (ROS) in neutrophils. Pharmacological scavenging of ROS eliminated the differences in WKYMVm-induced hyperpermeability between Bam32^−/− and WT mice. Deficiency of Bam32 decreased WKYMVm-induced ERK1/2 but not p38 or JNK phosphorylation in neutrophils. Inhibition of ERK1/2 signaling cascade suppressed WKYMVm-induced ROS generation in WT neutrophils and microvascular hyperpermeability in WT mice. In conclusion, our study reveals that Bam32-dependent, ERK1/2-involving ROS generation in neutrophils is critical in WKYMVm-induced microvascular hyperpermeability during neutrophil recruitment.

A Ganoderma-Derived Compound Exerts Inhibitory Effect Through Formyl Peptide Receptor 2

Formyl peptide receptors (FPRs) are G protein-coupled receptors (GPCRs) widely expressed in neutrophils and other phagocytes. FPRs play important roles in host defense, inflammation, and the pathogenesis of infectious and inflammatory diseases. Because of these functions, FPRs are potential targets for anti-inflammatory therapies. In order to search for potentially novel anti-inflammatory agents, we examined Ganoderma (Lingzhi), a Chinese medicinal herbs known for its anti-inflammatory effects, and found that compound 18 (C18) derived from Ganoderma cochlear could limit the inflammatory response through FPR-related signaling pathways. Further studies showed that C18 could bind to FPR2 and induce conformation change of the receptor that differed from the conformational change induced by the pan-agonist, WKYMVm. C18 inhibited at the receptor level and blocked WKYMVm signaling through FPR2, resulting in reduced superoxide production and compromised cell chemotaxis. These results identified for the first time that a Ganoderma-derived component with inhibitory effects that acts through a G protein-coupled receptor FPR2. Considering its less than optimal IC₅₀ value, further optimization of C18 would be necessary for future applications.

FPR-1 is an important regulator of neutrophil recruitment and a tissue-specific driver of pulmonary fibrosis

Neutrophils are the most abundant inflammatory cells at the earliest stages of wound healing and play important roles in wound repair and fibrosis. Formyl peptide receptor 1 (FPR-1) is abundantly expressed on neutrophils and has been shown to regulate their function, yet the importance of FPR-1 in fibrosis remains ill defined. FPR-1-deficient (fpr1-/-) mice were protected from bleomycin-induced pulmonary fibrosis but developed renal and hepatic fibrosis normally. Mechanistically, we observed a failure to effectively recruit neutrophils to the lungs of fpr1-/- mice, whereas neutrophil recruitment was unaffected in the liver and kidney. Using an adoptive transfer model we demonstrated that the defect in neutrophil recruitment to the lung was intrinsic to the fpr1-/- neutrophils, as C57BL/6 neutrophils were recruited normally to the damaged lung in fpr1-/- mice. Finally, C57BL/6 mice in which neutrophils had been depleted were protected from pulmonary fibrosis. In conclusion, FPR-1 and FPR-1 ligands are required for effective neutrophil recruitment to the damaged lung. Failure to recruit neutrophils or depletion of neutrophils protects from pulmonary fibrosis.

MTBHsp70-exFPR1-pulsed Dendritic Cells Enhance the Immune Response against Cervical Cancer

Cervical cancer is the most common malignancy of the female reproductive system. Dendritic cell (DC)-based immunological therapy is a novel treatment for this cancer. DCs are specialized antigen-presenting cells (APCs) in the human immune system, and they can activate the T cells used in tumor immunological therapy. In this study, we developed a novel immunotherapeutic peptide by linking the Mycobacterium tuberculosis (MTB) heat shock protein 70 (Hsp70) functional peptide to the extracellular domain of FPR1, a protein overexpressed in cervical cancer, to obtain an MTBHsp70-exFPR1 fusion protein. Our experiments confirmed that the MTBHsp70-exFPR1 protein could promote DC maturation and induce the secretion of IL-12p70, IL-1β, and TNF-α. The antitumor effect of human cytotoxic T lymphocytes (CTLs) activated by autologous DCs was assessed in NOG mice. These results indicate that DCs pulsed with MTBHsp70-exFPR1 can enhance antitumor immunity against cervical cancer, providing a novel immune therapeutic strategy.

The annexin A1/FPR2 signaling axis expands alveolar macrophages, limits viral replication, and attenuates pathogenesis in the murine influenza A virus infection model

Pattern recognition receptors (PRRs) are key elements in the innate immune response. Formyl peptide receptor (FPR) 2 is a PRR that, in addition to proinflammatory, pathogen-derived compounds, also recognizes the anti-inflammatory endogenous ligand annexin A1 (AnxA1). Because the contribution of this signaling axis in viral infections is undefined, we investigated AnxA1-mediated FPR2 activation on influenza A virus (IAV) infection in the murine model. AnxA1-treated mice displayed significantly attenuated pathology upon a subsequent IAV infection with significantly improved survival, impaired viral replication in the respiratory tract, and less severe lung damage. The AnxA1-mediated protection against IAV infection was not caused by priming of the type I IFN response but was associated with an increase in the number of alveolar macrophages (AMs) and enhanced pulmonary expression of the AM-regulating cytokine granulocyte-M-CSF (GM-CSF). Both AnxA1-mediated increase in AM levels and GM-CSF production were abrogated when mouse (m)FPR2 signaling was antagonized but remained up-regulated in mice genetically deleted for mFPR1, an mFPR2 isoform also serving as AnxA1 receptor. Our results indicate a novel protective function of the AnxA1-FPR2 signaling axis in IAV pathology via GM-CSF–associated maintenance of AMs, expanding knowledge on the potential use of proresolving mediators in host defense against pathogens.—Schloer, S., Hübel, N., Masemann, D., Pajonczyk, D., Brunotte, L., Ehrhardt, C., Brandenburg, L.-O., Ludwig, S., Gerke, V., Rescher, U. The annexin A1/FPR2 signaling axis expands alveolar macrophages, limits viral replication, and attenuates pathogenesis in the murine influenza A virus infection model.

Critical role for PI3Kγ-dependent neutrophil reactive oxygen species in WKYMVm-induced microvascular hyperpermeability

PI3K has been indicated in regulating microvascular permeability changes during inflammation. However, its role in neutrophil-driven microvascular leakage in acute inflammation remains unclear. Using intravital microscopy in mice, we examined the role of PI3Kγ and PI3Kδ in formyl peptide WKYMVm- and chemokine CXCL2-induced permeability changes and assessed simultaneously neutrophil adhesion and emigration in post-capillary venules of murine cremaster muscle. We found a PI3Kγ-specific mechanism in WKYMVm-induced but not CXCL2-induced microvascular hyperpermeability. The increased microvascular permeability triggered by WKYMVm was not entirely due to neutrophil adhesion and emigration in cremasteric microvasculature in different PI3K transgenic mouse strains. The PI3Kγ-specific hyperpermeability was neutrophil-mediated as this was reduced after depletion of neutrophils in mouse circulation. Chimeric mice with PI3Kγ-deficient neutrophils but wild-type endothelium also showed reduced hyperpermeability. Furthermore, we found that the catalytic function of PI3Kγ was required for reactive oxygen species (ROS) generation in neutrophils stimulated with WKYMVm. Pharmacological scavenging PI3Kγ-dependent ROS in the tissue eliminated the discrepancy in hyperpermeability between different PI3K transgenic mice and alleviated WKYMVm-induced microvascular leakage in all mouse strains tested. In conclusion, our study uncovers the critical role for PI3Kγ-dependent ROS generation by neutrophils in formyl peptide-induced microvascular hyperpermeability during neutrophil recruitment.

Inhibition of FPR2 impaired leukocytes recruitment and elicited non-resolving inflammation in acute heart failure

Lifestyle or age-related risk factors over-activate the inflammation that triggers acute heart failure (HF)-related mortality following myocardial infarction (MI). Post-MI activated leukocytes express formyl peptide receptor 2 (FPR2) that is essential for inflammation-resolution and in cardiac healing. However, the role of FPR2 in acute HF is incomplete and remain of interest. Here, we aimed to determine whether pharmacological inhibition of FPR2 perturb leukocyte trafficking in acute HF. Male C57BL/6 (8-12 weeks) mice were subjected to acute HF (MI-d1) using permanent coronary artery ligation that develops irreversible acute and chronic heart failure. FPR2 antagonist WRW4 (1 μg/kg/day) was subcutaneously injected 3 h post-MI maintaining saline-injected MI-controls. Leukocytes were quantitated using flow cytometry, and acute decompensated HF was confirmed using echocardiography and histology. FPR2 inhibition decreased the expression of FPR2 in the LV and spleen tissues. Administration of WRW4 inhibitor to mice primed immature and inactive neutrophils infiltration Ly6G^int and intensified the Ccl2 expression compared to MI-control in the infarcted LV post-MI. Leukocyte profiling revealed an overall decrease in monocytes (23.3 ± 2%) in WRW4-injected mice compared with MI-control (49.1 ± 2%) in infarcted LV. FPR2 inhibition increased F4/80⁺/Ly6C^hi pro-inflammatory macrophages (14.8 ± 2%) compared with MI-control (10 ± 1%) with increased transcripts of pro-inflammatory markers TNF-α and IL-1β, and decreased Arg-1 expression in the infarcted LV compared to MI-controls is suggestive of the impaired acute inflammatory response. Inhibition of FPR2 using WRW4 also disturbed splenocardiac leukocytes recruitment by priming immature neutrophils leading to the onset of incomplete resolution signaling in acute decompensated HF post-MI.

The formyl peptide receptor agonist FPRa14 induces differentiation of Neuro2a mouse neuroblastoma cells into multiple distinct morphologies which can be specifically inhibited with FPR antagonists and FPR knockdown using siRNA

The N-formyl peptide receptors (FPRs) have been identified within neuronal tissues and may serve as yet undetermined functions within the nervous system. The FPRs have been implicated in the progression and invasiveness of neuroblastoma and other cancers. In this study the effects of the synthetic FPR agonist FPRa14, FPR antagonists and FPR knockdown using siRNA on mouse neuroblastoma neuro2a (N2a) cell differentiation plus toxicity were examined. The FPRa14 (1-10μM) was found to induce a significant dose-dependent differentiation response in mouse neuroblastoma N2a cells. Interestingly, three distinct differentiated morphologies were observed, with two non-archetypal forms observed at the higher FPRa14 concentrations. These three forms were also observed in the human neuroblastoma cell-lines IMR-32 and SH-SY5Y when exposed to 100μM FPRa14. In N2a cells combined knockdown of FPR1 and FPR2 using siRNA inhibited the differentiation response to FPRa14, suggesting involvement of both receptor subtypes. Pre-incubating N2a cultures with the FPR1 antagonists Boc-MLF and cyclosporin H significantly reduced FPRa14-induced differentiation to near baseline levels. Meanwhile, the FPR2 antagonist WRW4 had no significant effect on FPRa14-induced N2a differentiation. These results suggest that the N2a differentiation response observed has an FPR1-dependent component. Toxicity of FPRa14 was only observed at higher concentrations. All three antagonists used blocked FPRa14-induced toxicity, whilst only siRNA knockdown of FPR2 reduced toxicity. This suggests that the toxicity and differentiation involve different mechanisms. The demonstration of neuronal differentiation mediated via FPRs in this study represents a significant finding and suggests a role for FPRs in the CNS. This finding could potentially lead to novel therapies for a range of neurological conditions including neuroblastoma, Alzheimer's disease, Parkinson's disease and neuropathic pain. Furthermore, this could represent a potential avenue for neuronal regeneration therapies.

Targeted Delivery of a Ligand-Drug Conjugate via Formyl Peptide Receptor 1 through Cholesterol-Dependent Endocytosis

G protein-coupled receptors (GPCRs) undergo ligand-induced internalization that carries the cognate ligands into intracellular compartments. The present study explores this property for the use of formyl peptide receptor 1 (FPR1), a class A GPCR that binds formylated peptides, as a potential target for drug delivery. A pH-sensitive peptide-drug conjugate consisting of doxorubicin (DOX), N-ε-maleimidocaproic acid hydrazide (EMCH), and the formyl peptide fMet-Leu-Phe-Cys (abbreviated as DEF) was prepared. DEF retained pharmacological activities of formyl peptides in binding to FPR1 and mobilization of Ca²⁺ from intracellular stores. However, the conjugated DOX was no longer cell membrane-permeable and relied on FPR1 for cellular entry. DOX was released from DEF into acidic compartments labeled with fluorescent trackers for endosomes. Treatment of cells with pharmacological inhibitors that block clathrin- or caveolae-mediated endocytosis did not abrogate FPR1-dependent DEF internalization, nor did inhibition of macropinocytosis and phagocytosis. In contrast, cholesterol depletion abrogated DEF internalization through FPR1, suggesting characteristics of cholesterol-dependent uptake mediated by a cell surface receptor. These results demonstrate the possibility of using FPR1 for targeted drug delivery.

Formyl peptide receptor 1 signalling promotes experimental colitis in mice

Inflammatory bowel disease is characterised by intricate immune cell interactions with tissue cells and such cross-talks can become deregulated. The formyl peptide receptor 1 (Fpr1) is expressed by both immune and stromal cells including epithelial cells. We evaluated the development of the physiopathology of the DNBS induced colitis in Fpr1 KO mice on the C57BL/6 genetic background compared to C57BL/6 genetic background animals. We have assessed both macroscopic and histological markers of the diseased, together with the immunohistochemical and molecular changes. DNBS-treated Fpr1 KO mice showed a i) reduction in weight loss, ii) lower extent of colon injury and iii) an increase in MPO activity. Molecular analyses indicated that in absence of Fpr1 there was reduced NF-κB translocation into the nucleus, cytokines levels, FOXP3 and GATA3, CD4, CD8 and CD45 expression as well as a dysregulation of TGF-β signalling. In addition, the colon of DNBS-injected Fpr1 KO mice displayed a lower degree of expression of Bax and higher expression of Bcl-2 compared correspondent WT mice. Finally, intravital microscopy investigation of the microcirculation post-DNBS instillation revealed a lower degree of neutrophil-endothelial cell rolling and adhesion - mediated by P-selectin and ICAM-1 - in Fpr1 KO mice. All the main outcome in the study have a P-value, statistical significance of evidence, less than 0.05. We provide evidence for an important pathogenic role of mouse Fpr1 in experimental colitis, an outcome effected through modulation of immune cell recruitment together with a modulation of local cellular activation and survival.

Absence of formyl peptide receptor 1 causes endometriotic lesion regression in a mouse model of surgically-induced endometriosis

Endometriosis is a female disease in which endometrial tissues grows outside the uterus. Patients showed alterations in endocrine and immune systems. Endometriotic lesions are characterized by deregulated interaction between immune cells and tissue cells. The formyl peptide receptor 1 (Fpr1) is expressed by both immune and stromal cells including epithelial cells. We investigated the development of the physiopathology of the surgically-induced endometriosis in Fpr1 KO mice compared to WT animals. Operated Fpr1 KO mice showed lower duration of uterine pain behaviors, lower size of developed cysts and reduced mast cell numbers. Immunohistochemical analyses indicated changes in NGF, VEGF and ICAM-1 expression associated with the pathology, which were reduced in absence of the Fpr1 gene. Molecular analyses indicated that in absence of Fpr1 there was reduced neutrophils accumulation and nitrosative stress formation, NF-κB translocation into the nucleus as well as NRLP3 inflammasome signalling. Fpr1 gene deletion caused reduction of resistance to the apoptosis, assessed by TUNEL assay. We underline the pathogenic role of Fpr1 in experimental endometriosis, which is the result of modulation of immune cell recruitment, suggesting it as a new target to control the pathologic features of endometriotic lesions.

Formyl peptide derived lipopeptides disclose differences between the receptors in mouse and men and call the pepducin concept in question

A pepducin is a lipopeptide containing a peptide sequence that is identical to one of the intracellular domains of the G-protein coupled receptor (GPCR) assumed to be the target. Neutrophils express two closely related formyl peptide receptors belonging to the family of GPCRs; FPR1 and FPR2 in human and their respective orthologue Fpr1 and Fpr2 in mouse. By applying the pepducin concept, we have earlier identified FPR2 activating pepducins generated from the third intracellular loop of FPR2. The third intracellular loop of FPR2 differs in two amino acids from that of FPR1, seven from Fpr2 and three from Fpr1. Despite this, we found that pepducins generated from FPR1, FPR2, Fpr1 and Fpr2 all targeted FPR2 in human neutrophils and Fpr2 in mouse, but with different modulating outcomes. Whereas the FPR1/Fpr1 derived pepducins inhibited the FPR2 function in human neutrophils, they activated Fpr2 in mouse. The FPR2 derived pepducin activated FPR2/Fpr2, whereas the pepducin generated from Fpr2 inhibited both FPR2 and Fpr2. In summary, our data demonstrate that pepducins generated from the third intracellular loop of human FPR1/2 and mouse Fpr1/2, all targeted FPR2 in human and Fpr2 in mouse. With respect to the modulating outcomes, pepducin inhibitors identified for FPR2 are in fact activators for Fpr2 in mouse neutrophils. Our data thus questions the validity of pepducin concept regarding their receptor selectivity but supports the notion that FPR2/Fpr2 may recognize a lipopeptide molecular pattern, and highlight the differences in ligand recognition profile between FPR2 and its mouse orthologue Fpr2.

Novel ureidopropanamide based N-formyl peptide receptor 2 (FPR2) agonists with potential application for central nervous system disorders characterized by neuroinflammation

Formyl peptide receptor2 (FPR2) is a G-protein coupled receptor that plays critical roles in inflammatory reactions. FPR2-specific interaction can be possibly used to facilitate the resolution of pathological inflammatory responses by enhancing endogenous anti-inflammation systems. Starting from our lead agonist 5, we designed new ureidopropanamides derivatives able to activate FPR2 in transfected cells and human neutrophils. The new FPR2 agonists showed good stability towards oxidative metabolism in vitro. Moreover, selected compounds showed anti-inflammatory properties in LPS-stimulated rat primary microglial cells. (S)-3-(4-Cyanophenyl)-N-[[1-(3-chloro-4-fluorophenyl)cyclopropyl]methyl]-2-[3-(4-fluorophenyl)ureido]propanamide ((S)-17) emerged as prospective pharmacological tool to study the effects of FPR2 activation in the central nervous system (CNS) being able to reduce IL-1β and TNF-α levels in LPS-stimulated microglial cells and showing good permeation rate in hCMEC/D3 cells, an in vitro model of blood brain barrier. These results are very promising and can open new therapeutic perspectives in the treatment of CNS disorders characterized by neuroinflammation.

FPR2: A Novel Promising Target for the Treatment of Influenza

The Formyl-peptide receptor-2 (FPR2) is a seven transmembrane G protein-coupled receptor, which plays an important role in sensing of bacteria and modulation of immune responses. FPR2 is also used by viruses for their own profit. Annexin A1, one of the multiple ligands of FPR2, is incorporated in the budding virus membrane of influenza A viruses (IAV). Thereby, once IAV infect a host cell, FPR2 is activated. FPR2-signaling leads to an increase in viral replication, a dysregulation of the host immune response and a severe disease. Conversely, experiments using FPR2 antagonists in a preclinical model of IAV infections in mice showed that blocking FPR2 protects animals from lethal infections. Thus, FPR2 represents a very attractive host target against influenza. In this review we will give an overview on the pathogenesis of influenza with a focus on the role of FPR2 and we will discuss the advantages of using FPR2 antagonists to treat the flu.

The Formyl Peptide Receptors: Diversity of Ligands and Mechanism for Recognition

The formyl peptide receptors (FPRs) are G protein-coupled receptors that transduce chemotactic signals in phagocytes and mediate host-defense as well as inflammatory responses including cell adhesion, directed migration, granule release and superoxide production. In recent years, the cellular distribution and biological functions of FPRs have expanded to include additional roles in homeostasis of organ functions and modulation of inflammation. In a prototype, FPRs recognize peptides containing N-formylated methionine such as those produced in bacteria and mitochondria, thereby serving as pattern recognition receptors. The repertoire of FPR ligands, however, has expanded rapidly to include not only N-formyl peptides from microbes but also non-formyl peptides of microbial and host origins, synthetic small molecules and an eicosanoid. How these chemically diverse ligands are recognized by the three human FPRs (FPR1, FPR2 and FPR3) and their murine equivalents is largely unclear. In the absence of crystal structures for the FPRs, site-directed mutagenesis, computer-aided ligand docking and structural simulation have led to the identification of amino acids within FPR1 and FPR2 that interact with several formyl peptides. This review article summarizes the progress made in the understanding of FPR ligand diversity as well as ligand recognition mechanisms used by these receptors.

Formylated MHC Class Ib Binding Peptides Activate Both Human and Mouse Neutrophils Primarily through Formyl Peptide Receptor 1

Two different immune recognition systems have evolved in parallel to recognize peptides starting with an N-formylated methionine, and recognition similarities/differences between these two systems have been investigated. A number of peptides earlier characterized in relation to the H2-M3 complex that presents N-formylated peptides to cytotoxic T cells, have been characterized in relation to the formyl peptide receptors expressed by phagocytic neutrophils in both men (FPRs) and mice (Fprs). FPR1/Fpr1 was identified as the preferred receptor for all fMet-containing peptides examined, but there was no direct correlation between H2-M3 binding and the neutrophil activation potencies. Similarly, there was no direct correlation between the activities induced by the different peptides in human and mouse neutrophils, respectively. The formyl group was important in both H2-M3 binding and FPR activation, but FPR2 was the preferred receptor for the non-formylated peptide. The structural requirements differed between the H2-M3 and FPR/Fpr recognition systems and these data suggest that the two recognition systems have different evolutionary traits.

The peptidomimetic Lau-(Lys-βNSpe)6-NH2 antagonizes formyl peptide receptor 2 expressed in mouse neutrophils

The formyl peptide receptor (FPR) gene family has a complex evolutionary history and comprises eight murine members but only three human representatives. To enable translation of results obtained in mouse models of human diseases, more comprehensive knowledge of the pharmacological similarities/differences between the human and murine FPR family members is required. Compared to FPR1 and FPR2 expressed by human neutrophils, very little is known about agonist/antagonist recognition patterns for their murine orthologues, but now we have identified two potent and selective formylated peptide agonists (fMIFL and PSMα2) for Fpr1 and Fpr2, respectively. These peptides were used to determine the inhibition profile of a set of antagonists with known specificities for the two FPRs in relation to the corresponding murine receptors. Some of the most potent and selective antagonists for the human receptors proved to be devoid of effect on their murine orthologues as determined by their inability to inhibit superoxide release from murine neutrophils upon stimulation with receptor-specific agonists. The Boc-FLFLF peptide was found to be a selective antagonist for Fpr1, whereas the lipidated peptidomimetic Lau-(Lys-βNSpe)₆-NH₂ and the hexapeptide WRW₄ were identified as Fpr2-selective antagonists.

FITC Conjugation Markedly Enhances Hepatic Clearance of N-Formyl Peptides

In both septic and aseptic inflammation, N-formyl peptides may enter the circulation and induce a systemic inflammatory response syndrome similar to that observed during septic shock. The inflammatory response is brought about by the binding of N-formyl peptide to formyl peptide receptors (FPRs), specific signaling receptors expressed on myeloid as well as non-myeloid cells involved in the inflammatory process. N-formyl peptides conjugated with fluorochromes, such as fluorescein isothiocyanate (FITC) are increasingly experimentally used to identify tissues involved in inflammation. Hypothesizing that the process of FITC-conjugation may transfer formyl peptide to a ligand that is efficiently cleared from the circulation by the natural powerful hepatic scavenging regime we studied the biodistribution of intravenously administered FITC-fNLPNTL (Fluorescein-isothiocyanate- N-Formyl-Nle-Leu-Phe-Nle-Tyr-Lys) in mice. Our findings can be summarized as follows: i) In contrast to unconjugated fNLPNTL, FITC-fNLPNTL was rapidly taken up in the liver; ii) Mouse and human liver sinusoidal endothelial cells (LSECs) and hepatocytes express formyl peptide receptor 1 (FRP1) on both mRNA (PCR) and protein (Western blot) levels; iii) Immunohistochemistry showed that mouse and human liver sections expressed FRP1 in LSECs and hepatocytes; and iv) Uptake of FITC-fNLPNTL could be largely blocked in mouse and human hepatocytes by surplus-unconjugated fNLPNTL, thereby suggesting that the hepatocytes in both species recognized FITC-fNLPNTL and fNLPNTL as indistinguishable ligands. This was in contrast to the mouse and human LSECs, in which the uptake of FITC-fNLPNTL was mediated by both FRP1 and a scavenger receptor, specifically expressed on LSECs. Based on these results we conclude that a significant proportion of FITC-fNLPNTL is taken up in LSECs via a scavenger receptor naturally expressed in these cells. This calls for great caution when using FITC-fNLPNTL and other chromogen-conjugated formyl peptides as a probe to identify cells in a liver engaged in inflammation. Moreover, our finding emphasizes the role of the liver as an important neutralizer of otherwise strong inflammatory signals such as formyl peptides.

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.

Gliadin Induces Neutrophil Migration via Engagement of the Formyl Peptide Receptor, FPR1

Background

Gliadin, the immunogenic component within gluten and trigger of celiac disease, is known to induce the production of Interleukin-8, a potent neutrophil-activating and chemoattractant chemokine. We sought to study the involvement of neutrophils in the early immunological changes following gliadin exposure.

Methods

Utilizing immunofluorescence microscopy and flow cytometry, the redistribution of major tight junction protein, Zonula occludens (ZO)-1, and neutrophil recruitment were assessed in duodenal tissues of gliadin-gavaged C57BL/6 wild-type and Lys-GFP reporter mice, respectively. Intravital microscopy with Lys-GFP mice allowed monitoring of neutrophil recruitment in response to luminal gliadin exposure in real time. In vitro chemotaxis assays were used to study murine and human neutrophil chemotaxis to gliadin, synthetic alpha-gliadin peptides and the neutrophil chemoattractant, fMet-Leu-Phe, in the presence or absence of a specific inhibitor of the fMet-Leu-Phe receptor-1 (FPR1), cyclosporine H. An irrelevant protein, zein, served as a control.

Results

Redistribution of ZO-1 and an influx of CD11b⁺Lys6G⁺ cells in the lamina propria of the small intestine were observed upon oral gavage of gliadin. In vivo intravital microscopy revealed a slowing down of GFP⁺ cells within the vessels and influx in the mucosal tissue within 2 hours after challenge. In vitro chemotaxis assays showed that gliadin strongly induced neutrophil migration, similar to fMet-Leu-Phe. We identified thirteen synthetic gliadin peptide motifs that induced cell migration. Blocking of FPR1 completely abrogated the fMet-Leu-Phe-, gliadin- and synthetic peptide-induced migration.

Conclusions

Gliadin possesses neutrophil chemoattractant properties similar to the classical neutrophil chemoattractant, fMet-Leu-Phe, and likewise uses FPR1 in the process.

The role of formylated peptides and formyl peptide receptor 1 in governing neutrophil function during acute inflammation

Neutrophil migration to sites of inflammation and the subsequent execution of multiple functions are designed to contain and kill invading pathogens. These highly regulated and orchestrated processes are controlled by interactions between numerous receptors and their cognate ligands. Unraveling and identifying those that are central to inflammatory processes may represent novel therapeutic targets for the treatment of neutrophil-dominant inflammatory disorders in which dysregulated neutrophil recruitment, function, and elimination serve to potentiate rather than resolve an initial inflammatory insult. The first G protein-coupled receptor to be described on human neutrophils, formyl peptide receptor 1 (FPR1), is one such receptor that plays a significant role in the execution of these functions through multiple intracellular signaling pathways. Recent work has highlighted important observations with regard to both receptor function and the importance and functional relevance of FPR1 in the pathogenesis of a range of both sterile and infective inflammatory conditions. In this review, we explore the multiple components of neutrophil migration and function in both health and disease, with a focus on the role of FPR1 in these processes. The current understanding of FPR1 structure, function, and signaling is examined, alongside discussion of the potential importance of FPR1 in inflammatory diseases suggesting that FPR1 is a key regulator of the inflammatory environment.

Adaptive evolution of formyl peptide receptors in mammals

The formyl peptide receptors (FPRs) are a family of chemoattractant receptors with important roles in host defense and the regulation of inflammatory reactions. In humans, three FPR paralogs have been identified (FPR1, FPR2, and FPR3) and may have functionally diversified by gene duplication and adaptive evolution. However, the evolutionary mechanisms operating in the diversification of FPR family genes and the changes in selection pressures have not been characterized to date. Here, we have made a comprehensive evolutionary analysis of FPR genes from mammalian species. Phylogenetic analysis showed that an early duplication was responsible for FPR1 and FPR2/FPR3 splitting, and FPR3 originated from the latest duplication event near the origin of primates. Codon-based tests of positive selection reveal interesting patterns in FPR1 and FPR2 versus FPR3, with the first two genes showing clear evidence of positive selection at some sites while the majority of them evolve under strong negative selection. In contrast, our results suggest that the selective pressure may be relaxed in the FPR3 lineage. Of the six amino acid sites inferred to evolve under positive selection in FPR1 and FPR2, four sites were located in extracellular loops of the protein. The electrostatic potential of the extracellular surface of FPR might be affected more frequently with amino acid substitutions in positively selected sites. Thus, positive selection of FPRs among mammals may reflect a link between changes in the sequence and surface structure of the proteins and is likely to be important in the host's defense against invading pathogens.

Development of small molecule non-peptide formyl peptide receptor (FPR) ligands and molecular modeling of their recognition

Formyl peptide receptors (FPRs) are G protein-coupled receptors (GPCRs) expressed on a variety of cell types. These receptors play an important role in the regulation of inflammatory reactions and sensing cellular damage. They have also been implicated in the pathogenesis of various diseases, including neurodegenerative diseases, cataract formation, and atherogenesis. Thus, FPR ligands, both agonists and antagonists, may represent novel therapeutics for modulating host defense and innate immunity. A variety of molecules have been identified as receptor subtype-selective and mixed FPR agonists with potential therapeutic value during last decade. This review describes our efforts along with recent advances in the identification, optimization, biological evaluation, and structure-activity relationship (SAR) analysis of small molecule non-peptide FPR agonists and antagonists, including chiral molecules. Questions regarding the interaction at the molecular level of benzimidazoles, pyrazolones, pyridazin-3(2H)-ones, N-phenylureas and other derivatives with FPR1 and FPR2 are discussed. Application of computational models for virtual screening and design of FPR ligands is also considered.

Lack of formyl peptide receptor 1 and 2 leads to more severe inflammation and higher mortality in mice with of pneumococcal meningitis

Bacterial meningitis is, despite progress in research and the development of new treatment strategies, still a cause of severe neuronal sequelae. The brain is protected from penetrating pathogens by both the blood-brain barrier and the innate immune system. The invading pathogens are recognized by pattern recognition receptors including the G-protein coupled formyl peptide receptors (FPRs), which are expressed by immune cells of the central nervous system. The expression of FPRs is up-regulated during bacterial meningitis, but the consequence on the progression of inflammation and impact on mortality are far from clear. Therefore, we used mFPR1 and mFPR2-deficient mice to investigate the effects on inflammation, bacterial growth and mortality in a mouse model of pneumococcal meningitis. Our results revealed increased bacterial burden, increased neutrophil infiltration and higher mortality in mFPR1/2-deficient mice in comparison to wild-type mice. The mFPR1- or mFPR2-deficient mice also showed significantly increased glial cell density, whereas the immune responses including the expression of anti-inflammatory cytokines and antimicrobial peptides were decreased in bacterial meningitis. Taken together, the results suggest that FPR1 and FPR2 play an important role in the innate immune responses against Streptococcus pneumoniae within the central nervous system and the lack of the receptors leads to a dysregulation of the inflammatory response compared with wild-type mice.

The non-peptide GLP-1 receptor agonist WB4-24 blocks inflammatory nociception by stimulating β-endorphin release from spinal microglia

BACKGROUND AND PURPOSE

Two peptide agonists of the glucagon-like peptide-1 (GLP-1) receptor, exenatide and GLP-1 itself, exert anti-hypersensitive effects in neuropathic, cancer and diabetic pain. In this study, we have assessed the anti-allodynic and anti-hyperalgesic effects of the non-peptide agonist WB4-24 in inflammatory nociception and the possible involvement of microglial β-endorphin and pro-inflammatory cytokines.

EXPERIMENTAL APPROACH

We used rat models of inflammatory nociception induced by formalin, carrageenan or complete Freund's adjuvant (CFA), to test mechanical allodynia and thermal hyperalgesia. Expression of β-endorphin and pro-inflammatory cytokines was measured using real-time quantitative PCR and fluorescent immunoassays.

KEY RESULTS

WB4-24 displaced the specific binding of exendin (9-39) in microglia. Single intrathecal injection of WB4-24 (0.3, 1, 3, 10, 30 and 100 μg) exerted dose-dependent, specific, anti-hypersensitive effects in acute and chronic inflammatory nociception induced by formalin, carrageenan and CFA, with a maximal inhibition of 60-80%. Spinal WB4-24 was not effective in altering nociceptive pain. Subcutaneous injection of WB4-24 was also antinociceptive in CFA-treated rats. WB4-24 evoked β-endorphin release but did not inhibit expression of pro-inflammatory cytokines in either the spinal cord of CFA-treated rats or cultured microglia stimulated by LPS. WB4-24 anti-allodynia was prevented by a microglial inhibitor, β-endorphin antiserum and a μ-opioid receptor antagonist.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that WB4-24 inhibits inflammatory nociception by releasing analgesic β-endorphin rather than inhibiting the expression of proalgesic pro-inflammatory cytokines in spinal microglia, and that the spinal GLP-1 receptor is a potential target molecule for the treatment of pain hypersensitivity including inflammatory nociception.

Physiological characterization of formyl peptide receptor expressing cells in the mouse vomeronasal organ

The mouse vomeronasal organ (VNO) is a chemosensory structure that detects both hetero- and conspecific social cues. Based on largely monogenic expression of either type 1 or 2 vomeronasal receptors (V1Rs/V2Rs) or members of the formyl peptide receptor (FPR) family, the vomeronasal sensory epithelium harbors at least three neuronal subpopulations. While various neurophysiological properties of both V1R- and V2R-expressing neurons have been described using genetically engineered mouse models, the basic biophysical characteristics of the more recently identified FPR-expressing vomeronasal neurons have not been studied. Here, we employ a transgenic mouse strain that coexpresses an enhanced variant of yellow fluorescent protein together with FPR-rs3 allowing to identify and analyze FPR-rs3-expressing neurons in acute VNO tissue slices. Single neuron electrophysiological recordings allow comparative characterization of the biophysical properties inherent to a prototypical member of the FPR-expressing subpopulation of VNO neurons. In this study, we provide an in-depth analysis of both passive and active membrane properties, including detailed characterization of several types of voltage-activated conductances and action potential discharge patterns, in fluorescently labeled vs. unmarked vomeronasal neurons. Our results reveal striking similarities in the basic (electro) physiological architecture of both transgene-expressing and non-expressing neurons, confirming the suitability of this genetically engineered mouse model for future studies addressing more specialized issues in vomeronasal FPR neurobiology.

Design, synthesis and characterization of fMLF-mimicking AApeptides

The tripeptide N-formyl-Met-Leu-Phe (fMLF) is a potent neutrophil chemoattractant and the reference agonist for the G protein-coupled N-formylpeptide receptor (FPR). As it plays a very important role in host defense and inflammation, there has been considerable interest in the development of fMLF analogues in the hope of identifying potential therapeutic agents. Herein we report the design, synthesis, and evaluation of AApeptides that mimic the structure and function of fMLF. The lead AApeptides induced calcium mobilization and mitogen-activated protein kinase (MAPK) signal transduction pathways in FPR-transfected rat basophilic leukemic (RBL) cells. More intriguingly, at high concentrations, certain AApeptides were more effective than fMLF in the induction of calcium mobilization. Their agonistic activity is further supported by their ability to stimulate chemotaxis and the production of superoxide in HL-60 cells. Similarly to fMLF, these AApeptides are much more selective towards FPR1 than FPR2. These results suggest that the fMLF-mimicking AApeptides might emerge as a new class of therapeutic agents that target FPRs.