Differential expansion of the N-formylpeptide receptor gene cluster in human and mouse

The Formyl Peptid Receptor Ligand Ac2-26 Improves the Integrity of the Blood-Brain Barrier in the Course of Pneumococcal Meningitis

BACKGROUND

The brain is protected from invading pathogens by the blood-brain barrier (BBB) and the innate immune system. Pattern recognition receptors play a crucial role in detecting bacteria and initiating the innate immune response. Among these are G-protein-coupled formyl peptide receptors (FPR), which are expressed by immune cells in the central nervous system. In this study, we investigated the influence of the FPR ligand Ac2-26 on the integrity of the BBB during pneumococcal meningitis.

METHODS

Wild-type (WT) and Fpr1- and Fpr2-deficient mice were intrathecally infected with Streptococcus pneumoniae. Subsequently, different groups of mice were treated with intraperitoneal injections of Ac2-26. The integrity of the BBB was analyzed using various markers through immunohistochemistry and immunofluorescence.

RESULTS

The results showed reduced BBB integrity during the course of bacterial meningitis. Treatment with Ac2-26 in WT mice significantly prolonged the maintenance of BBB integrity. However, this effect was not observed in Fpr2-deficient mice.

CONCLUSIONS

This study extends previous findings on the anti-inflammatory properties of Ac2-26 by demonstrating that Ac2-26 positively affects BBB integrity via FPR2 during pneumococcal meningitis. These findings suggest that further investigation of Ac2-26 and other FPR modulators as potential therapies for Streptococcus pneumoniae-induced meningitis is warranted.

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.

Role of Formyl Peptide Receptors and β-Arrestin-1 in suPAR Signal Transduction in Mouse Podocytes: Interactions with αVβ3-Integrin

The soluble urokinase plasminogen activator receptor (suPAR) has been implicated in a wide range of pathological conditions including primary nephrotic syndromes and acute kidney injuries. suPAR can trigger transduction cascades in podocytes by outside-in activation of αVβ3-integrin, but there is evidence that the functional cell surface response element is actually a complex of different types of receptors, which may also include the receptor for advanced glycation end-products (RAGE) and formyl peptide receptors (FPRs). Here we observed that ROS accumulation and Src activation could be evoked by continuous 24 h exposure to either suPAR or the FPR agonist fMLF. Responses to suPAR and fMLF were completely blocked by either the FPR antagonist WRW4 or by the αV-integrin inhibitor cilengitide. Moreover, endogenous podocyte mouse Fpr1 co-immunoprecipitates with β3-integrin, suggesting that these receptors occur as a complex on the cell surface. suPAR- and fMLF-evoked activation of Src and ROS differed in time course. Thus, robust pertussis toxin (PTX)-sensitive responses were evoked by 60 min exposures to fMLF but not to suPAR. By contrast, responses to 24 h exposures to either suPAR or fMLF were PTX-resistant and were instead abolished by knockdown of β-arrestin-1 (BAR1). FPRs, integrins, and RAGE (along with various Toll-like receptors) can all function as pattern-recognition receptors that respond to "danger signals" associated with infections and tissue injury. The fact that podocytes express such a wide array of pattern-recognition receptors suggests that the glomerular filter is designed to change its function under certain conditions, possibly to facilitate clearance of toxic macromolecules.

From odor to oncology: non-canonical odorant receptors in cancer

Odorant receptors, traditionally associated with olfaction as chemoreceptors, have been increasingly recognized for their presence and diverse functions in various non-nasal tissues throughout the body. Beyond their roles in sensory perception, emerging evidence suggests a compelling interplay between odorant receptors and cancer progression as well. Alongside the canonical GPCR odorant receptors, dysregulation of non-canonical odorant receptors such as trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and membrane-spanning 4A family (MS4As) has been observed in various cancer types, suggesting their contributions to cancer progression. The roles of these non-canonical chemoreceptors in cancer are complex, with some receptors promoting tumorigenesis and others acting as tumor-suppressing factors upon activation, depending on the cancer type. These findings shed light on the potential of non-canonical odorant receptors as therapeutic targets and prognostic markers in cancer, inviting further exploration to unravel their precise mechanisms of action and implications in cancer biology. In this review, we provide a comprehensive overview of the intricate relationships between these chemoreceptors and various types of cancer, potentially paving the way for innovative odor-based therapeutics. Ultimately, this review discusses the potential development of novel therapeutic strategies targeting these non-canonical chemoreceptors.

Human neutrophils ≠ murine neutrophils: Does it matter?

Human and murine neutrophils differ with respect to representation in blood, receptors, nuclear morphology, signaling pathways, granule proteins, NADPH oxidase regulation, magnitude of oxidant and hypochlorous acid production, and their repertoire of secreted molecules. These differences often matter and can undermine extrapolations from murine studies to clinical care, as illustrated by several failed therapeutic interventions based on mouse models. Likewise, coevolution of host and pathogen undercuts fidelity of murine models of neutrophil-predominant human infections. However, murine systems that accurately model the human condition can yield insights into human biology difficult to obtain otherwise. The challenge for investigators who employ murine systems is to distinguish models from pretenders and to know when the mouse provides biologically accurate insights. Testing with human neutrophils observations made in murine systems would provide a safeguard but is not always possible. At a minimum, studies that use exclusively murine neutrophils should have accurate titles supported by data and restrict conclusions to murine neutrophils and not encompass all neutrophils. For now, the integration of evidence from studies of neutrophil biology performed using valid murine models coupled with testing in vitro of human neutrophils combines the best of both approaches to elucidate the mysteries of human neutrophil biology.

Formyl peptide receptor 2 is an emerging modulator of inflammation in the liver

Formyl peptide receptors (FPRs), which are seven-membrane G-protein coupled receptors, recognize chemotactic signals to protect hosts from pathogenic infections and mediate inflammatory responses in the body. There are three isoforms of FPRs in humans-FPR1, FPR2, and FPR3-and they bind to N-formyl peptides, except FPR3, and to various endogenous agonists. Among FPR family members, FPR2 has a lower affinity for N-formyl peptides than FPR1 and binds with a wide range of endogenous or exogenous agonists. Thus, FPR2 is considered the most ambiguous member. Accumulating evidence has shown that FPR2 is involved in the host's defense against bacterial infection and inflammation in liver diseases, such as nonalcoholic fatty liver disease, liver fibrosis, and liver cancer, suggesting the pathophysiological relevance of FPR2 to the liver. However, FPR2 has been shown to promote or suppress inflammation, depending on the type of FPR2-expressing cell and FPR2-bound ligands in the liver. Therefore, it is important to understand FPR2's function per se and to elucidate the mechanism underlying immunomodulation initiated by ligand-activated FPR2 before suggesting FPR2 as a novel therapeutic agent for liver diseases. In this review, up-to-date knowledge of FPR2, with general information on the FPR family, is provided. We shed light on the dual action of FPR2 in the liver and discuss the hepatoprotective roles of FPR2 itself and FPR2 agonists in mediating anti-inflammatory responses.

WKYMVm Works by Targeting Immune Cells

WKYMVm (Trp-Lys-Tyr-Met-Val-D-Met) is a synthetic hexapeptide identified as a potent agonist of FPRs. FPRs are widely expressed on the cell membrane of immune cells. Therefore, WKYMVm participates in the regulation of immune cells by activating FPRs, and plays a therapeutic role in infections, tumors, autoimmune diseases and so on. WKYMVm can promote the chemotactic migration, increase the bactericidal activity of neutrophils and monocytes. WKYMVm also regulates the number and polarization of macrophages, affects the maturation of DCs and the differentiation of T cells, and promotes the activation and chemotaxis of NK cells. These functions make WKYMVm a candidate drug for immunotherapy. In this paper, we summarize the regulatory effects and underlying mechanisms of WKYMVm on six immune cells (neutrophils, monocytes, macrophages, DCs, T cells and NK cells) to increase comprehensive understanding and promote further research on WKYMVm.

The Hidden Role of Non-Canonical Amyloid β Isoforms in Alzheimer's Disease

Recent advances have placed the pro-inflammatory activity of amyloid β (Aβ) on microglia cells as the focus of research on Alzheimer's Disease (AD). Researchers are confronted with an astonishing spectrum of over 100 different Aβ variants with variable length and chemical modifications. With the exception of Aβ1-42 and Aβ1-40, the biological significance of most peptides for AD is as yet insufficiently understood. We therefore aim to provide a comprehensive overview of the contributions of these neglected Aβ variants to microglia activation. First, the impact of Aβ receptors, signaling cascades, scavenger mechanisms, and genetic variations on the physiological responses towards various Aβ species is described. Furthermore, we discuss the importance of different types of amyloid precursor protein processing for the generation of these Aβ variants in microglia, astrocytes, oligodendrocytes, and neurons, and highlight how alterations in secondary structures and oligomerization affect Aβ neurotoxicity. In sum, the data indicate that gene polymorphisms in Aβ-driven signaling pathways in combination with the production and activity of different Aβ variants might be crucial factors for the initiation and progression of different forms of AD. A deeper assessment of their interplay with glial cells may pave the way towards novel therapeutic strategies for individualized medicine.

Amyloid beta and its naturally occurring N-terminal variants are potent activators of human and mouse formyl peptide receptor 1

Formyl peptide receptors (FPRs) may contribute to inflammation in Alzheimer's disease through interactions with neuropathological Amyloid beta (Aβ) peptides. Previous studies reported activation of FPR2 by Aβ_1-42, but further investigation of other FPRs and Aβ variants is needed. This study provides a comprehensive overview of the interactions of mouse and human FPRs with different physiologically relevant Aβ-peptides using transiently transfected cells in combination with calcium imaging. We observed that, in addition to hFPR2, all other hFPRs also responded to Aβ_1-42, Aβ_1-40, and the naturally occurring variants Aβ_11-40 and Aβ_17-40. Notably, Aβ_11-40 and Aβ_17-40 are very potent activators of mouse and human FPR1, acting at nanomolar concentrations. Buffer composition and aggregation state are extremely crucial factors that critically affect the interaction of Aβ with different FPR subtypes. To investigate the physiological relevance of these findings, we examined the effects of Aβ_11-40 and Aβ_17-40 on the human glial cell line U87. Both peptides induced a strong calcium flux at concentrations that are very similar to those obtained in experiments for hFPR1 in HEK cells. Further immunocytochemistry, qPCR, and pharmacological experiments verified that these responses were primarily mediated through hFPR1. Chemotaxis experiments revealed that Aβ_11-40 but not Aβ_17-40 evoked cell migration, which argues for a functional selectivity of different Aβ peptides. Together, these findings provide the first evidence that not only hFPR2 but also hFPR1 and hFPR3 may contribute to neuroinflammation in Alzheimer's disease through an interaction with different Aβ variants.

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.

Effects of Formyl Peptide Receptor Agonists Ac9-12 and WKYMV in In Vivo and In Vitro Acute Inflammatory Experimental Models

Formyl peptide receptors (Fprs) are a G-protein-coupled receptor family mainly expressed on leukocytes. The activation of Fpr1 and Fpr2 triggers a cascade of signaling events, leading to leukocyte migration, cytokine release, and increased phagocytosis. In this study, we evaluate the effects of the Fpr1 and Fpr2 agonists Ac_9-12 and WKYMV, respectively, in carrageenan-induced acute peritonitis and LPS-stimulated macrophages. Peritonitis was induced in male C57BL/6 mice through the intraperitoneal injection of 1 mL of 3% carrageenan solution or saline (control). Pre-treatments with Ac_9-12 and WKYMV reduced leukocyte influx to the peritoneal cavity, particularly neutrophils and monocytes, and the release of IL-1β. The addition of the Fpr2 antagonist WRW4 reversed only the anti-inflammatory actions of WKYMV. In vitro, the administration of Boc2 and WRW4 reversed the effects of Ac_9-12 and WKYMV, respectively, in the production of IL-6 by LPS-stimulated macrophages. These biological effects of peptides were differently regulated by ERK and p38 signaling pathways. Lipidomic analysis evidenced that Ac_9-12 and WKYMV altered the intracellular lipid profile of LPS-stimulated macrophages, revealing an increased concentration of several glycerophospholipids, suggesting regulation of inflammatory pathways triggered by LPS. Overall, our data indicate the therapeutic potential of Ac_9-12 and WKYMV via Fpr1 or Fpr2-activation in the inflammatory response and macrophage activation.

Stimulation of the Migration and Expansion of Adult Mouse Neural Stem Cells by the FPR2-Specific Peptide WKYMVm

Neural stem cells (NSCs) are multipotent cells capable of self-renewal and differentiation into different nervous system cells. Mouse NSCs (mNSCs) are useful tools for studying neurogenesis and the therapeutic applications of neurodegenerative diseases in mammals. Formyl peptide receptor 2 (FPR2), expressed in the central nervous system and brain, is involved in the migration and differentiation of murine embryonic-derived NSCs. In this study, we explored the effect of FPR2 activation in adult mNSCs using the synthetic peptide Trp-Lys-Tyr-Met-Val-D-Met-NH2 (WKYMVm), an agonist of FPR2. After isolation of NSCs from the subventricular zone of the adult mouse brain, they were cultured in two culture systems—neurospheres or adherent monolayers—to demonstrate the expression of NSC markers and phenotypes. Under different conditions, mNSCs differentiated into neurons and glial cells such as astrocytes, microglia, and oligodendrocytes. Treatment with WKYMVm stimulated the chemotactic migration of mNSCs. Moreover, WKYMVm-treated mNSCs were found to promote proliferation; this result was confirmed by the expansion of mNSCs in Matrigel and the increase in the number of Ki67-positive cells. Incubation of mNSCs with WKYMVm in a supplement-free medium enhanced the survival rate of the mNSCs. Together, these results suggest that WKYMVm-induced activation of FPR2 stimulates cellular responses in adult NSCs.

Emerging contributions of formyl peptide receptors to neurodegenerative diseases

Inflammation is a central element of many neurodegenerative diseases. Formyl peptide receptors (FPRs) can trigger several receptor-dependent signal transduction pathways that play a key role in neuroinflammation and neurodegeneration. They are chemotactic receptors that help to regulate pro- and anti-inflammatory responses in most mammals. FPRs are primarily expressed in the immune and nervous systems where they interact with a complex pattern of pathogen-derived and host-endogenous molecules. Mounting evidence points towards a contribution of FPRs - via neuropathological ligands such as Amyloid beta, and neuroprotective ligands such as Humanin, Lipoxin A4, and Annexin A1 - to multiple pathological aspects of neurodegenerative diseases. In this review, we aim to summarize the interplay of FPRs with neuropathological and neuroprotective ligands. Next, we depict their capability to trigger a number of ligand-dependent cell signaling pathways and their potential to interact with additional intracellular cofactors. Moreover, we highlight first studies, demonstrating that a pharmacological inhibition of FPRs helps to ameliorate neuroinflammation, which may pave the way towards novel therapeutic strategies.

A novel computer vision-based assessment of neutrophil chemotaxis in patients with severe infection

Objectives

To evaluate the value of chemotactic function of neutrophils in patients with severe infections.

Methods

A computer vision‐based cellular chemotaxis analysis platform was established for the dynamic assessment of neutrophil chemotaxis. Fifty‐three patients in the intensive care unit were eligible for the study. In parallel, 142 healthy volunteers were recruited to detect and establish the normal values for chemotactic function. Four chemotactic function indicators were determined–chemotaxis distance (CD), chemotaxis cell ratio (CCR), chemotaxis index (CI) and maximum speed of chemotaxis (V_max). The chemotaxis function scores (CFS) were calculated for further correlation analysis with clinical data.

Results

The normal ranges of indicators were established as CD ≥ 1755.85 µm, CCR ≥ 3.34%, CI ≥ 39.63, V_max ≥ 14.63 µm min⁻¹ and CFS ≥ 15. We found that the chemotactic function of neutrophils in patients suffering from infections was significantly impaired. The mean values of CD, CCR, CI, V_max and CFS were 1452.8 µm (P < 0.0001), 3.1% (P < 0.0001), 34.5 (P < 0.0001), 12.2 µm min⁻¹ (P < 0.0001) and 9 (P < 0.0001), respectively. CD and CFS were significantly negatively correlated with the APACHE II score (rCD = −0.55, rCFS = −0.39), SOFA score (rCD = −0.68, rCFS = −0.56), procalcitonin concentration (rCD = −0.60, rCFS = −0.5) and the expression of P2RX1 (rCD = −0.76, rCFS = −0.56), respectively.

Conclusions

CD, CCR, CI and V_max can well reflect the neutrophil chemotactic function in patients with severe infections. CFS systematically indicated neutrophil function and has promising clinical application prospects.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Formyl peptide receptor 2, as an important target for ligands triggering the inflammatory response regulation: a link to brain pathology

Formyl peptide receptors (FPRs) belong to the family of seven-transmembrane G protein-coupled receptors. Among them, FPR2 is a low affinity receptor for N-formyl peptides and is considered the most promiscuous member of FPRs. FPR2 is able to recognize a broad variety of endogenous or exogenous ligands, ranging from lipid to proteins and peptides, including non-formylated peptides. Due to this property FPR2 has the ability to modulate both pro- and anti-inflammatory response, depending on the nature of the bound agonist and on the different recognition sites of the receptor. Thus, FPR2 takes part not only in the proinflammatory response but also in the resolution of inflammation (RoI) processes. Recent data have indicated that the malfunction of RoI may be the background for some central nervous system (CNS) disorders. Therefore, much interest is focused on endogenous molecules called specialized pro-resolving mediators (SPMs), as well as on new synthetic FPR2 agonists, which kick-start the resolution of inflammation (RoI) and modulate its course. Here, we shed some light on the general characteristics of the FPR family in humans and in the experimental animals. Moreover, we present a guide to understanding the "double faced" action of FPR2 activation in the context of immune-related diseases of the CNS.

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.

From immune to olfactory expression: neofunctionalization of formyl peptide receptors

Variations in gene expression patterns represent a powerful source of evolutionary innovation. In a rodent living about 70 million years ago, a genomic accident led an immune formyl peptide receptor (FPR) gene to hijack a vomeronasal receptor regulatory sequence. This gene shuffling event forced an immune pathogen sensor to transition into an olfactory chemoreceptor, which thus moved from sensing the internal world to probing the outside world. We here discuss the evolution of the FPR gene family, the events that led to their neofunctionalization in the vomeronasal organ and the functions of immune and vomeronasal FPRs.

The formyl peptide receptor agonist Ac2-26 alleviates neuroinflammation in a mouse model of pneumococcal meningitis

Background

Bacterial meningitis is still a cause of severe neurological disability. The brain is protected from penetrating pathogens by 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. FPRs show a broad spectrum of ligands, including pro- and anti-inflammatory ones. Here, we investigated the effects of the annexin A1 mimetic peptide Ac2-26 in a mouse model of pneumococcal meningitis.

Methods

Wildtype (WT) and Fpr1- and Fpr2-deficient mice were intrathecally infected with Streptococcus pneumoniae D39 (type 2). Subsequently, the different mice groups were treated by intraperitoneal injections of Ac2-26 (1 mg/kg body weight) 2, 8, and 24 h post-infection. The extent of inflammation was analyzed in various brain regions by means of immunohistochemistry and real-time reverse transcription polymerase chain reaction (RT-PCR) 30 h post-infection.

Results

Ac2-26-treated WT mice showed less severe neutrophil infiltration, paralleled by a reduced induction of pro-inflammatory glial cell responses in the hippocampal formation and cortex. While meningitis was ameliorated in Ac2-26-treated Fpr1-deficient mice, this protective effect was not observed in Fpr2-deficient mice. Irrespective of Ac2-26 treatment, inflammation was more severe in Fpr2-deficient compared to Fpr1-deficient mice.

Conclusions

In summary, this study demonstrates anti-inflammatory properties of Ac2-26 in a model of bacterial meningitis, which are mediated via FPR2, but not FPR1. Ac2-26 and other FPR2 modulators might be promising targets for the development of novel therapies for Streptococcus pneumoniae-induced meningitis.

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.

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.

BML-111 suppresses TGF-β1-induced lung fibroblast activation in vitro and decreases experimental pulmonary fibrosis in vivo

Pulmonary fibrosis is an aggressive end‑stage disease. Transforming growth factor‑β1 (TGF‑β1) mediates lung fibroblast activation and is essential for the progress of pulmonary fibrosis. BML‑111, a lipoxinA4 (LXA4) receptor (ALX) agonist, has been reported to possess anti‑fibrotic properties. The present study aimed to elucidate whether BML‑111 inhibits TGF‑β1‑induced mouse embryo lung fibroblast (NIH3T3 cell line) activation in vitro and bleomycin (BLM)‑induced pulmonary fibrosis in vivo. In vitro experiments demonstrated that BML‑111 treatment inhibits TGF‑β1‑induced NIH3T3 cell viability and the expression of smooth muscle α actin (α‑SMA), fibronectin and total collagen. Furthermore, this suppressive effect was associated with mothers against decapentaplegic homolog (Smad)2/3, extracellular signal‑regulated kinase (ERK) and Akt phosphorylation interference. In vivo experiments revealed that BML‑111 treatment markedly improved survival rate and ameliorated the destruction of lung tissue structure. It also reduced interleukin‑1β (IL‑1β), tumor necrosis factor‑α (TNF‑α) and TGF‑β1 expression in the BLM intratracheal mouse model. In addition, the expression ofα‑SMA and extracellular matrix (ECM) deposition (total collagen, hydroxyproline and fibronectin) were also suppressed following BML‑111 treatment. However, BOC‑2, an antagonist of ALX, partially weakened the effects of BML‑111. In conclusion, these results indicated that BML‑111 inhibits TGF‑β1‑induced fibroblasts activation and alleviates BLM‑induced pulmonary fibrosis. Therefore, BML‑111 may be used as a potential therapeutic agent for pulmonary fibrosis treatment.

Ongoing Lung Inflammation and Disease Progression in Mice after Smoking Cessation: Beneficial Effects of Formyl-Peptide Receptor Blockade

The most important risk factor for chronic obstructive pulmonary disease (COPD) is cigarette smoking. Until now, smoking cessation (SC) is the only treatment effective in slowing down the progression of the disease. However, in many cases SC may only relieve the airflow obstruction and inflammatory response. Consequently, a persistent lung inflammation in ex-smokers is associated with progressive deterioration of respiratory functions. This is an increasingly important clinical problem whose mechanistic basis remains poorly understood. Available therapies do not adequately suppress inflammation and are not able to stop the vicious cycle that is at the basis of persistent inflammation. In addition, in mice after SC an ongoing inflammation and progressive lung deterioration is observed. After 4 months of smoke exposure mice show mild emphysematous changes. Lung inflammation is still present after SC, and emphysema progresses during the next 6-month period of observation. Destruction of alveolar walls is associated with airways remodeling (goblet cell metaplasia and peribronchiolar fibrosis). Modulation of formyl-peptide receptor signaling with antagonists mitigates inflammation and prevents deterioration of lung structures. This study suggests an important role for N-formylated peptides in the progression and exacerbation of COPD. Modulating formyl-peptide receptor signal should be explored as a potential new therapy for COPD.

Hematopoietic cell-derived RELMα regulates hookworm immunity through effects on macrophages

Resistin-like molecule α (RELMα) is a highly secreted protein in type 2 (Th2) cytokine-induced inflammation including helminth infection and allergy. In infection with Nippostrongylus brasiliensis (Nb), RELMα dampens Th2 inflammatory responses. RELMα is expressed by immune cells, and by epithelial cells (EC); however, the functional impact of immune versus EC-derived RELMα is unknown. We generated bone marrow (BM) chimeras that were RELMα deficient (RELMα^{-/ -} ) in BM or non BM cells and infected them with Nb. Non BM RELMα^-/- chimeras had comparable inflammatory responses and parasite burdens to RELMα^+/+ mice. In contrast, both RELMα^-/- and BM RELMα^-/- mice exhibited increased Nb-induced lung and intestinal inflammation, correlated with elevated Th2 cytokines and Nb killing. CD11c⁺ lung macrophages were the dominant BM-derived source of RELMα and can mediate Nb killing. Therefore, we employed a macrophage-worm co-culture system to investigate whether RELMα regulates macrophage-mediated Nb killing. Compared to RELMα^{+ /+} macrophages, RELMα^-/- macrophages exhibited increased binding to Nb and functionally impaired Nb development. Supplementation with recombinant RELMα partially reversed this phenotype. Gene expression analysis revealed that RELMα decreased cell adhesion and Fc receptor signaling pathways, which are associated with macrophage-mediated helminth killing. Collectively, these studies demonstrate that BM-derived RELMα is necessary and sufficient to dampen Nb immune responses, and identify that one mechanism of action of RELMα is through inhibiting macrophage recruitment and interaction with Nb. Our findings suggest that RELMα acts as an immune brake that provides mutually beneficial effects for the host and parasite by limiting tissue damage and delaying parasite expulsion.

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.

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.

Formyl-peptide receptor 2 governs leukocyte influx in local Staphylococcus aureus infections

Leukocytes express formyl-peptide receptors (FPRs), which sense microbe-associated molecular pattern (MAMP) molecules, leading to leukocyte chemotaxis and activation. We recently demonstrated that phenol-soluble modulin (PSM) peptides from highly pathogenic Staphylococcus aureus are efficient ligands for the human FPR2. How PSM detection by FPR2 impacts on the course of S. aureus infections has remained unknown. We characterized the specificity of mouse FPR2 (mFpr2) using a receptor-transfected cell line, homeobox b8 (Hoxb8), and primary neutrophils isolated from wild-type (WT) or mFpr2^-/- mice. The influx of leukocytes into the peritoneum of WT and mFpr2^-/- mice was analyzed. We demonstrate that mFpr2 is specifically activated by PSMs in mice, and they represent the first secreted pathogen-derived ligands for the mFpr2. Intraperitoneal infection with S. aureus led to lower numbers of immigrated leukocytes in mFpr2^-/- compared with WT mice at 3 h after infection, and this difference was not observed when mice were infected with an S. aureus PSM mutant. Our data support the hypothesis that the mFpr2 is the functional homolog of the human FPR2 and that a mouse infection model represents a suitable model for analyzing the role of PSMs during infection. PSM recognition by mFpr2 shapes leukocyte influx in local infections, the typical infections caused by S. aureus-Weiss, E., Hanzelmann, D., Fehlhaber, B., Klos, A., von Loewenich, F. D., Liese, J., Peschel, A., Kretschmer, D. Formyl-peptide receptor 2 governs leukocyte influx in local Staphylococcus aureus infections.

Formyl Peptide Receptor 1-Mediated Glial Cell Activation in a Mouse Model of Cuprizone-Induced Demyelination

Multiple sclerosis (MS) is a chronic degenerative disease of the central nervous system that is characterized by myelin abnormalities, oligodendrocyte pathology, and concomitant glia activation. Unclear are the factors triggering gliosis and demyelination. New findings suggest an important role of the innate immune response in the initiation and progression of active demyelinating lesions. The innate immune response is induced by pathogen-associated or danger-associated molecular patterns, which are identified by pattern recognition receptors (PRRs), including the G-protein coupled with formyl peptide receptors (FPRs). Glial cells, the immune cells of the central nervous system, also express the PRRs. In this study, we used the cuprizone mice model to investigate the expression of the FPR1 in the course of cuprizone-induced demyelination In addition, we used FPR1-deficient mice to analyze glial cell activation through immunohistochemistry and real-time RT-PCR in cuprizone model. Our results revealed a significantly increased expression of FPR1 in the cortex of cuprizone-treated mice. FPR1-deficient mice showed a slight but significant decrease of demyelination in the corpus callosum compared to the wild-type mice. Furthermore, FPR1 deficiency resulted in reduced glial cell activation and mRNA expression of microglia/macrophages markers, as well as pro- and anti-inflammatory cytokines in the cortex, compared to wild-type mice after cuprizone-induced demyelination. Combined together, these results suggest that the FPR1 is an important part of the innate immune response in the course of cuprizone-induced demyelination.

Formyl peptide receptors promotes neural differentiation in mouse neural stem cells by ROS generation and regulation of PI3K-AKT signaling

This study aimed to determine whether formyl peptide receptors (FPRs) regulated the differentiation of neural stem cells (NSCs). FPRs promote the migration of NSCs both in vitro and in vivo. However, the role of FPRs during differentiation of NSCs is unknown. Analysis by Western blot showed significantly increased expression of FPR1 and FPR2 during differentiation of NSCs. The activation of FPRs promotes NSCs to differentiate into neurons with more primary neurites and branch points and longer neurites per cell. Meanwhile, this activation also inhibits the differentiation of NSC into astrocytes. This bidirectional effect can be inhibited by the FPRs-specific inhibitor. Moreover, it was found that the activation of FPRs increased the generation of reactive oxygen species (ROS) and phosphorylation of AKT in the NSCs, while N-acetylcysteine and LY294002 inhibited the FPRs-stimulated increase in ROS generation and AKT phosphorylation, and blocked the FPRs-stimulated neural differentiation into neurons. Therefore, FPRs-stimulated neural differentiation was mediated via ROS and PI3K-AKT signaling pathways. Collectively, the present findings provided a novel insight into the functional role of FPRs in neurogenesis, with important implications for its potential use as a candidate for treating brain or spinal cord injury.

The sensing of bacteria: emerging principles for the detection of signal sequences by formyl peptide receptors

The ability to detect specific chemical signatures released by bacteria and other microorganisms is a fundamental feature of immune defense against pathogens. There is increasing evidence that chemodetection of such microorganism-associated molecular patterns (MAMPs) occurs at many places in the body including specific sets of chemosensory neurons in the mammalian nose. Formyl peptide receptors (FPRs) are a unique family of G protein-coupled receptors (GPCRs) that can detect the presence of bacteria and function as chemotactic receptors. Here, we highlight the recent discovery of a vast family of natural FPR agonists, the bacterial signal peptides (or signal sequences), thus providing new insight into the molecular mechanisms of bacterial sensing by human and mouse FPRs. Signal peptides in bacteria are formylated, N-terminal protein signatures required for directing the transfer of proteins through the plasma membrane. After their cleavage and release, signal peptides are available for FPR detection and thus provide a previously unrecognized MAMP. With over 170 000 predicted sequences, bacterial signal peptides represent one of the largest families of GPCR ligands and one of the most complex classes of natural activators of the innate immune system. By recognizing a conserved three-dimensional peptide motif, FPRs employ an unusual detection mechanism that combines structural promiscuity with high specificity and sensitivity, thus solving the problem of detecting thousands of distinct sequences yet maintaining selectivity. How signal peptides are released by bacteria and sensed by GPCRs and how these processes shape the responses of other cells and whole organisms represents an important topic for future research.

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.

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.

Formylpeptide Receptors Promote the Migration and Differentiation of Rat Neural Stem Cells

Neural stem cells (NSCs) bear characteristics for proliferation, migration and differentiation into three main neural cell type(s): neurons, astrocytes and/or oligodendrocytes. Formylpeptide receptors (Fprs), belonging to the family of G protein-coupled chemoattractant receptors, have been detected on neurons in the central nervous system (CNS). Here, we report that Fpr1 and Fpr2 are expressed on NSCs as detected with immunohistochemistry, RT-PCR and WB assays. In addition, Fpr1 and Fpr2 promoted NSC migration through F-actin polymerization and skewed NSC differentiation to neurons. Our study demonstrates a unique role of Fpr1 and Fpr2 in NSCs and opens a novel window for cell replacement therapies for brain and spinal cord injury.

Strain-specific Loss of Formyl Peptide Receptor 3 in the Murine Vomeronasal and Immune Systems

Formyl peptide receptor 3 (Fpr3, also known as Fpr-rs1) is a G protein-coupled receptor expressed in subsets of sensory neurons of the mouse vomeronasal organ, an olfactory substructure essential for social recognition. Fpr3 has been implicated in the sensing of infection-associated olfactory cues, but its expression pattern and function are incompletely understood. To facilitate visualization of Fpr3-expressing cells, we generated and validated two new anti-Fpr3 antibodies enabling us to analyze acute Fpr3 protein expression. Fpr3 is not only expressed in murine vomeronasal sensory neurons but also in bone marrow cells, the primary source for immune cell renewal, and in mature neutrophils. Consistent with the notion that Fpr3 functions as a pathogen sensor, Fpr3 expression in the immune system is up-regulated after stimulation with a bacterial endotoxin (lipopolysaccharide). These results strongly support a dual role for Fpr3 in both vomeronasal sensory neurons and immune cells. We also identify a large panel of mouse strains with severely altered expression and function of Fpr3, thus establishing the existence of natural Fpr3 knock-out strains. We attribute distinct Fpr3 expression in these strains to the presence or absence of a 12-nucleotide in-frame deletion (Fpr3Δ424-435). In vitro calcium imaging and immunofluorescence analyses demonstrate that the lack of four amino acids leads to an unstable, truncated, and non-functional receptor protein. The genome of at least 19 strains encodes a non-functional Fpr3 variant, whereas at least 13 other strains express an intact receptor. These results provide a foundation for understanding the in vivo function of Fpr3.

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.

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.

FPR2/ALXR agonists and the resolution of inflammation

The resolution of inflammation (RoI), once believed to be a passive process, has lately been shown to be an active and delicately orchestrated process. During the resolution phase of acute inflammation, novel mediators, including lipoxins and resolvins, which are members of the specialized pro-resolving mediators of inflammation, are produced. FPR2/ALXR, a receptor modulated by some of these lipids as well as by peptides (e.g., annexin A1), has been shown to be one of the receptors involved in the RoI. The aim of this perspective is to present the concept of the RoI from a medicinal chemistry point of view and to highlight the effort of the research community to discover and develop anti-inflammatory/pro-resolution small molecules to orchestrate inflammation by activation of FPR2/ALXR.

Deficiency of formyl peptide receptor 1 and 2 is associated with increased inflammation and enhanced liver injury after LPS-stimulation

Introduction

Formyl peptide-receptor 1 and 2 (FPR1 and FPR2) in mice were identified as receptors with contrary affinity for the PAMP fMLF. Formyl-methionyl-leucyl-phenylalanine is either part of the bacterial membrane and is secreted by the mitochondria of eukaryotic ceslls during apoptosis. Furthermore FPR1 and 2 are described as highly relevant factors for the chemotaxis of immune cells. Their role during the acute liver injury has not been investigated yet.

Materials and Methods

Constitutive knockout mice for FPR1 (mFPR1^-/-), FPR2 (mFPR2^-/-) and wild type (WT) mice were challenged with LPS i.p. for 3 h and 6 h. Liver and serum were sampled for further analysis.

Results

Liver transaminases were elevated in all mice 3 h and 6 h post LPS stimulation. Gene expression analysis displayed a reduced expression of the pro-inflammatory cytokines IL-6 and CXCL1 after 3 h in the mFPR1^-/- compared to wild type and mFPR2^-/- mice. After 6 h, IL-6, TNF-α and CXCL1 were significantly higher in mice lacking mFPR1 or 2. Consistent to these findings the numbers of CD11b⁺ and Ly6G⁺ immune cells were altered in the livers. The analysis of TLR2 and TLR4 revealed time and genotype specific changes in theirs gene expression. Additionally, the liver in mFPR1- and mFPR2-deficient mice seem to be more susceptible to apoptosis by showing a significant higher number of TUNEL⁺-cells in the liver than WT-mice and displayed less Ki67-positive nuclei in the liver.

Conclusion

The results suggest a prominent role of FPRs in the regulation of the hepatic inflammatory response after LPS induced liver injury. Deletion of mFPR1 or mFPR2 leads to deregulation of the inflammatory response compared to WT mice, associated with more severe liver injury represented by higher levels of transaminases, apoptotic cells and a reduced regenerative capacity.

Heterologously expressed formyl peptide receptor 2 (FPR2/ALX) does not respond to lipoxin A₄

Lipoxin A₄ (LXA₄) has been described as an anti-inflammatory mediator, which exerts its effects through the formyl peptide receptor FPR2, also known as ALX. However, there has been a controversy whether or not cells expressing FPR2/ALX, such as neutrophils, respond to LXA₄. We, therefore, systematically examined the ability of the human and murine forms of the receptor to respond to LXA₄. We show that both receptor orthologues responded to the FPR2/ALX peptide agonist WKYMVM when expressed heterologously. In contrast, LXA₄ from different sources neither increased [Ca²⁺](i) and extracellular-signal-regulated kinase (ERK) phosphorylation, nor did it induce a decrease in cAMP levels or a translocation of β-arrestin. Also, several LXA₄ analogs were found to be unable to signal through FPR2/ALX. We conclude that FPR2/ALX is not activated by LXA₄ and that the molecular mechanism by which LXA₄ functions still needs to be identified.

Functional characterization of three mouse formyl peptide receptors

The evolutionary relationship and functional correlation between human formyl peptide receptors (FPRs) and their mouse counterparts remain incompletely understood. We examined three members of the mouse formyl peptide receptor subfamily (mFprs) and found that they differ in agonist preference and cellular distributions. When stably expressed in transfected rat basophilic leukemia (RBL-2H3) cells, mFpr1 was readily activated by N-formylated peptides derived from Listeria monocytogenes (fMIVTLF), Staphylococcus aureus (fMIFL), and mitochondria (fMMYALF). In contrast, the Escherichia coli-derived fMLF was 1000-fold less potent. The aforementioned peptides were much less efficacious at mFpr2, which responded better to the synthetic hexapeptide WKYMVm, the synthetic agonists Quin-C1 (a substituted quinazolinone), and compound 43 (a nitrosylated pyrazolone derivative). Saturation binding assays showed that mFpr1 and mFpr2 were expressed at similar levels on the cell surface, although their affinity for N-formyl-Met-Leu-Phe-Ile-Ile-Lys-fluorescein isothiocyanate varied by more than 1000-fold [dissociation constant (K(d)) values of 2.8 nM for mFpr1 and 4.8 μM for mFpr2]). Contrary to these receptors, mFpr-rs1 responded poorly to all the previously mentioned peptides that were tested. Fluorescent microscopy revealed an intracellular distribution pattern of mFpr-rs1. On the basis of these results, we conclude that mFpr1 is an ortholog of human FPR1 with certain pharmacologic properties of human FPR2/ALX, whereas mFpr2 has much lower affinity for formyl peptides. The intracellular distribution of mFpr-rs1 suggests an evolutionary correlation with human FPR3.

Formyl peptide receptors from immune and vomeronasal system exhibit distinct agonist properties

The formyl peptide receptor (Fpr) family is well known for its contribution to immune defense against pathogens in human and rodent leukocytes. Recently, several structurally related members of these receptors were discovered in sensory neurons of the mouse vomeronasal organ (VNO), key detectors of pheromones and related semiochemicals. Although the biological role of vomeronasal Fprs is not yet clear, the known contribution of other Fprs to host immune defense suggested that they could contribute to vomeronasal pathogen sensing. Precise knowledge about the agonist properties of mouse Fprs is required to determine their function. We expressed all seven mouse and three human Fprs using an in vitro system and tested their activation with 32 selected compounds by conducting high throughput calcium measurements. We found an intriguing functional conservation between human and mouse immune Fprs that is most likely a consequence of closely similar biological constraints. By contrast, our data suggest a neofunctionalization of the vomeronasal Fprs. We show that the vomeronasal receptor mFpr-rs1 can be activated robustly by W-peptide and structural derivatives but not by other typical ligands of immune Fprs. mFpr-rs1 exhibits a stereo-selective preference for peptides containing d-amino acids. The same peptide motifs are contained in pathogenic microorganisms. Thus, the ligand profile of mFpr-rs1 is consistent with a role in vomeronasal pathogen sensing.

Transcriptional variations mediated by an alternative promoter of the FPR3 gene

Formyl peptide receptor 3 (FPR3) is a potential player in innate immunity and appears with FPR2 as a FPR cluster during primate evolution. Comparative genome analyses indicate that a segmental duplication (SD) event upstream of the FPR3 gene after the divergence of New and Old World monkeys led to the emergence of an alternative promoter. In this study we combined computational and experimental approaches to identify a FPR3 gene that is controlled by an alternative promoter derived during a SD event. Its transcriptional activity was detected by quantitative reverse transcription polymerase chain reaction. Human alternative transcripts (FPR3-1 and FPR3-2) showed tissue-specific patterns with strong expressions in lung or uterus, while the FPR3-1 transcript of rhesus macaque is broadly expressed in various tissues. Overall, transcriptional variations of FPR3 occur by an alternative promoter during primate evolution.

Characterization of Fpr-rs8, an atypical member of the mouse formyl peptide receptor gene family

The formyl peptide receptor gene family encodes G protein-coupled receptors for phagocyte chemoattractants, including bacteria- and mitochondria-derived N-formylpeptides. The human family has 3 functional genes, whereas the mouse family has 7 functional genes and 2 possible pseudogenes (ΨFpr-rs2 and ΨFpr-rs3). Here we characterize ΨFpr-rs2, a duplication of Fpr-rs2. Compared to Fpr-rs2, the ΨFpr-rs2 ORF is 186 nucleotides shorter but 98% identical. Due to a deletion and frame shift, the sequences lack homology from amino acid 219-289. Both transcripts were detected constitutively in multiple immune organs; however, ΨFpr-rs2 was consistently less abundant than Fpr-rs2. LPS induced expression of ΨFpr-rs2, but not Fpr-rs2, in spleen and bone marrow. Both transcripts were detected constitutively in thioglycollate-elicited peritoneal neutrophils, whereas only Fpr-rs2 was detected in thioglycollate-elicited peritoneal macrophages. Both transcripts were induced in LPS-stimulated macrophages. ΨFpr-rs2-GFP fusion protein appeared in cytoplasm but not plasma membrane of transfected HEK 293 cells, whereas Fpr-rs2-GFP labeled only plasma membrane. Survival of ΨFpr-rs2(-/-) mice was 33% shorter than that of wild-type and heterozygous littermates (p < 0.05), but no signature pathology was identified. Since ΨFpr-rs2 is expressed in phagocytes and regulated by bacterial products, and may affect longevity, we propose renaming it Fpr-rs8, an atypical member of the formyl peptide receptor gene family.

Reduced fear memory and anxiety-like behavior in mice lacking formylpeptide receptor 1

N-formylpeptide receptor 1 (FPR1) is a G protein-coupled receptor that mediates pro-inflammatory chemotactic responses by phagocytic leukocytes to N-formylpeptides produced by bacteria or mitochondria. Mice lacking Fpr1 (Fpr1 (-/-) mice) have increased susceptibility to challenge with certain bacteria. FPR1 is also a receptor for annexin-1, which mediates the anti-inflammatory effects of glucocorticoids as well as negative feedback by glucocorticoids of the hypothalamic-pituitary-adrenocortical axis. However, homeostatic functions of FPR1 in the neuroendocrine system have not previously been defined. Here we show that in systematic behavioral testing Fpr1 (-/-) mice exhibited increased exploratory activity, reduced anxiety-like behavior, and impaired fear memory, but normal spatial memory and learning capacity. Consistent with this, the homeostatic serum level of corticosterone in Fpr1 (-/-) mice was significantly lower compared with wild-type mice. The data implicate Fpr1 in modulation of anxiety-like behavior and fear memory by regulating glucocorticoid production.

Resolvin D1 decreases adipose tissue macrophage accumulation and improves insulin sensitivity in obese-diabetic mice

Type 2 diabetes and obesity have emerged as global public health crises. Adipose tissue expansion in obesity promotes accumulation of classically activated macrophages that perpetuate chronic inflammation and sustain insulin resistance. Acute inflammation normally resolves in an actively orchestrated series of molecular and cellular events that ensures return to homeostasis after an inflammatory insult, a process regulated in part by endogenous lipid mediators such as the resolvins. In this study, we sought to determine whether stimulating resolution with resolvin D1 (RvD1) improves insulin sensitivity by resolving chronic inflammation associated with obesity. In male leptin receptor-deficient (db/db) mice, treatment with RvD1 (2 μg/kg) improved glucose tolerance, decreased fasting blood glucose, and increased insulin-stimulated Akt phosphorylation in adipose tissue relative to vehicle-treated mice. Treatment with RvD1 increased adiponectin production, while expression of IL-6 in adipose tissue was decreased. The formation of crown-like structures rich in inflammatory F4/80(+)CD11c(+) macrophages was reduced by >50% in adipose tissue by RvD1 and was associated with an increased percentage of F4/80(+) cells expressing macrophage galactose-type C-type lectin 1 (MGL-1), a marker of alternatively activated macrophages. These results suggest that stimulating resolution with the endogenous proresolving mediator RvD1 could provide a novel therapeutic strategy for treating obesity-induced diabetes.

Molecular and evolutionary analyses of formyl peptide receptors suggest the absence of VNO-specific FPRs in primates

Formyl peptide receptors (FPRs) were observed to expand in rodents and were recently suggested as candidate vomeronasal chemosensory receptors. Since vomeronasal chemosensory receptors usually underwent positive selection and evolved concordantly with the vomeronasal organ (VNO) morphology, we surveyed FPRs in primates in which VNO morphology is greatly diverse and thus it would provide us a clearer view of VNO-FPRs evolution. By screening available primate genome sequences, we obtained the FPR repertoires in representative primate species. As a result, we did not find FPR family size expansion in primates. Further analyses showed no evolutionary force variance between primates with or without VNO structure, which indicated that there was no functional divergence among primates FPRs. Our results suggest that primates lack the VNO-specific FPRs and the FPR expansion is not a common phenomenon in mammals outside rodent lineage, regardless of VNO complexity.

Inhibition of neutrophil migration in mice by mouse formyl peptide receptors 1 and 2 dual agonist: indication of cross-desensitization in vivo

It has been reported that the stimulation of neutrophils with N-formyl-Met-Leu-Phe (fMLF), an agonist for formyl peptide receptor (Fpr) 1, renders cells unresponsive to other chemoattractants in vitro. This is known as cross-desensitization, but its functional relevance in neutrophil migration in vivo has not been investigated. Here, we show that precedent stimulation of mouse neutrophils with compound 43, a non-peptidyl agonist for mouse Fpr1 and Fpr2, rendered the cells unresponsive to a second stimulation with C5a, leukotriene B₄, or keratinocyte-derived cytokine (KC) in calcium mobilization and chemotaxis assays in vitro. The expression of chemokine (C-X-C motif) receptor 2 (CXCR2) on the surface of neutrophils was concomitantly diminished by stimulating the cells with the compound. Moreover, oral administration of the compound to mice before they were exposed to lipopolysaccharide (LPS) aerosol resulted in a dose-dependent reduction in the neutrophil count in bronchoalveolar lavage fluid. The expression of CXCR2 on blood neutrophils was also reduced in the compound-administered mice. The recipient mice that underwent adoptive transfer of fluorescence-labelled neutrophils that had been incubated with the compound showed a substantial decrease in neutrophil counts in bronchoalveolar lavage fluid after they were exposed to LPS, when compared with the control mice to which vehicle-treated neutrophils had been transferred. These results are consistent with the idea that the agonist for Fpr1 and Fpr2 induced cross-desensitization in neutrophils and attenuated neutrophil migration into the airways. Our results also revealed the unpredicted effect of an Fpr1 and Fpr2 dual agonist, which may act as a functional antagonist for multiple chemoattractant receptors in vivo.