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

Detection of Microorganisms Using Artificial Siderophore-FeIII Complex-Modified Substrates

Four FeIII complexes of typical artificial siderophore ligands containing catecholate and/or hydroxamate groups of tricatecholate, biscatecholate-monohydroxamate, monocatecholate-bishydroxamate, and trihydroxamate type artificial siderophores (K3[FeIIILC3], K2[FeIIILC2H1], K[FeIIILC1H2], and [FeIIILH3]) were modified on Au substrate surfaces. Their abilities to adsorb microorganisms were investigated using scanning electron microscopy, quartz crystal microbalance, and AC impedance methods. The artificial siderophore-iron complexes modified on Au substrates (FeLC3/Au, FeLC2H1/Au, FeLC1H2/Au, and FeLH3/Au) showed the selective immobilization behavior for various microorganisms, depending on the structural features of the artificial siderophores (the number of catecholate and hydroxamate arms). Their specificities corresponded well with the structural characteristics of natural siderophores released by microorganisms and used for FeIII ion uptake. These findings suggest that they were generated via specific interactions between the artificial siderophore-FeIII complexes and the receptors on microorganism surfaces. Our observations revealed that the FeL/Au systems may be potentially used as effective microbe-capturing probes that can enable rapid and simple detection and identification of various microorganisms.

[Microorganism Immobilization Device Using Artificial Siderophores]

Inspired by the mechanism by which microorganisms utilize siderophores to ingest iron, four different FeIII complexes of typical artificial siderophore ligands containing catecholate and/or hydroxamate groups, K3[FeIII-LC3], K2[FeIII-LC2H1], K[FeIII-LC1H2], and [FeIII-LH3], were prepared. They were modified on an Au substrate surface (Fe-L/Au) and applied as microorganism immobilization devices for fast, sensitive, selective detection of microorganisms, where H6LC3, H5LC2H1, H4LC1H2, and H3LH3 denote the tri-catecholate, biscatecholate-monohydroxamate, monocatecholate-bishydroxamate, and tri-hydroxamate type of artificial siderophores, respectively. Their adsorption properties for the several microorganisms were investigated using scanning electron microscopy (SEM), quartz crystal microbalance (QCM), and electric impedance spectroscopy (EIS) methods. The artificial siderophore-iron complexes modified on the Au substrates Fe-LC3/Au, Fe-LC2H1/Au, Fe-LC1H2/Au, and Fe-LH3/Au showed specific microorganism immobilization behavior with selectivity based on the structure of the artificial siderophores. Their specificities corresponded well with the structural characteristics of natural siderophores that microorganisms release from the cell and/or use to take up an iron. These findings suggest that release and uptake are achieved through specific interactions between the artificial siderophore-FeIII complexes and receptors on the cell surfaces of microorganisms. This study revealed that Fe-L/Au systems have specific potential to serve as effective immobilization probes of microorganisms for rapid, selective detection and identification of a variety of microorganisms.

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.

Neutrophils discriminate live from dead bacteria by integrating signals initiated by Fprs and TLRs

The mechanisms whereby neutrophils respond differentially to live and dead organisms are unknown. We show here that neutrophils produce 5- to 30-fold higher levels of the Cxcl2 chemokine in response to live bacteria, compared with killed bacteria or isolated bacterial components, despite producing similar levels of Cxcl1 or pro-inflammatory cytokines. Secretion of high levels of Cxcl2, which potently activates neutrophils by an autocrine mechanism, requires three signals. The first two signals are provided by two different sets of signal peptides released by live bacteria, which selectively activate formylated peptide receptor 1 (Fpr1) and Fpr2, respectively. Signal 3 originates from Toll-like receptor activation by microbial components present in both live and killed bacteria. Mechanistically, these signaling pathways converge at the level of the p38 MAP kinase, leading to activation of the AP-1 transcription factor and to Cxcl2 induction. Collectively, our data demonstrate that the simultaneous presence of agonists for Fpr1, Fpr2, and Toll-like receptors represents a unique signature associated with viable bacteria, which is sensed by neutrophils and induces Cxcl2-dependent autocrine cell activation.

Autonomic Nervous System Neuroanatomical Alterations Could Provoke and Maintain Gastrointestinal Dysbiosis in Autism Spectrum Disorder (ASD): A Novel Microbiome-Host Interaction Mechanistic Hypothesis

Dysbiosis secondary to environmental factors, including dietary patterns, antibiotics use, pollution exposure, and other lifestyle factors, has been associated to many non-infective chronic inflammatory diseases. Autism spectrum disorder (ASD) is related to maternal inflammation, although there is no conclusive evidence that affected individuals suffer from systemic low-grade inflammation as in many psychological and psychiatric diseases. However, neuro-inflammation and neuro-immune abnormalities are observed within ASD-affected individuals. Rebalancing human gut microbiota to treat disease has been widely investigated with inconclusive and contradictory findings. These observations strongly suggest that the forms of dysbiosis encountered in ASD-affected individuals could also originate from autonomic nervous system (ANS) functioning abnormalities, a common neuro-anatomical alteration underlying ASD. According to this hypothesis, overactivation of the sympathetic branch of the ANS, due to the fact of an ASD-specific parasympathetic activity deficit, induces deregulation of the gut-brain axis, attenuating intestinal immune and osmotic homeostasis. This sets-up a dysbiotic state, that gives rise to immune and osmotic dysregulation, maintaining dysbiosis in a vicious cycle. Here, we explore the mechanisms whereby ANS imbalances could lead to alterations in intestinal microbiome-host interactions that may contribute to the severity of ASD by maintaining the brain-gut axis pathways in a dysregulated state.

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.

Biological Roles of Lipocalins in Chemical Communication, Reproduction, and Regulation of Microbiota

Major evolutionary transitions were always accompanied by genetic remodelling of phenotypic traits. For example, the vertebrate transition from water to land was accompanied by rapid evolution of olfactory receptors and by the expansion of genes encoding lipocalins, which - due to their transporting functions - represent an important interface between the external and internal organic world of an individual and also within an individual. Similarly, some lipocalin genes were lost along other genes when this transition went in the opposite direction leading, for example, to cetaceans. In terrestrial vertebrates, lipocalins are involved in the transport of lipophilic substances, chemical signalling, odour reception, antimicrobial defence and background odour clearance during ventilation. Many ancestral lipocalins have clear physiological functions across the vertebrate taxa while many other have - due to pleiotropic effects of their genes - multiple or complementary functions within the body homeostasis and development. The aim of this review is to deconstruct the physiological functions of lipocalins in light of current OMICs techniques. We concentrated on major findings in the house mouse in comparison to other model taxa (e.g., voles, humans, and birds) in which all or most coding genes within their genomes were repeatedly sequenced and their annotations are sufficiently informative.

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.

Symbioses of Cyanobacteria in Marine Environments: Ecological Insights and Biotechnological Perspectives

Cyanobacteria are a diversified phylum of nitrogen-fixing, photo-oxygenic bacteria able to colonize a wide array of environments. In addition to their fundamental role as diazotrophs, they produce a plethora of bioactive molecules, often as secondary metabolites, exhibiting various biological and ecological functions to be further investigated. Among all the identified species, cyanobacteria are capable to embrace symbiotic relationships in marine environments with organisms such as protozoans, macroalgae, seagrasses, and sponges, up to ascidians and other invertebrates. These symbioses have been demonstrated to dramatically change the cyanobacteria physiology, inducing the production of usually unexpressed bioactive molecules. Indeed, metabolic changes in cyanobacteria engaged in a symbiotic relationship are triggered by an exchange of infochemicals and activate silenced pathways. Drug discovery studies demonstrated that those molecules have interesting biotechnological perspectives. In this review, we explore the cyanobacterial symbioses in marine environments, considering them not only as diazotrophs but taking into consideration exchanges of infochemicals as well and emphasizing both the chemical ecology of relationship and the candidate biotechnological value for pharmaceutical and nutraceutical applications.

The Role of Formyl Peptide Receptors in Permanent and Low-Grade Inflammation: Helicobacter pylori Infection as a Model

Formyl peptide receptors (FPRs) are cell surface pattern recognition receptors (PRRs), belonging to the chemoattractant G protein-coupled receptors (GPCRs) family. They play a key role in the innate immune system, regulating both the initiation and the resolution of the inflammatory response. FPRs were originally identified as receptors with high binding affinity for bacteria or mitochondria N-formylated peptides. However, they can also bind a variety of structurally different ligands. Among FPRs, formyl peptide receptor-like 1 (FPRL1) is the most versatile, recognizing N-formyl peptides, non-formylated peptides, and synthetic molecules. In addition, according to the ligand nature, FPRL1 can mediate either pro- or anti-inflammatory responses. Hp(2-20), a Helicobacter pylori-derived, non-formylated peptide, is a potent FPRL1 agonist, participating in Helicobacter pylori-induced gastric inflammation, thus contributing to the related site or not-site specific diseases. The aim of this review is to provide insights into the role of FPRs in H. pylori-associated chronic inflammation, which suggests this receptor as potential target to mitigate both microbial and sterile inflammatory diseases.

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.

Receptors Mediating Host-Microbiota Communication in the Metaorganism: The Invertebrate Perspective

Multicellular organisms live in close association with a plethora of microorganism, which have a profound effect on multiple host functions. As such, the microbiota and its host form an intimate functional entity, termed the metaorganism or holobiont. But how does the metaorganism communicate? Which receptors recognize microbial signals, mediate the effect of the microbiota on host physiology or regulate microbiota composition and homeostasis? In this review we provide an overview on the function of different receptor classes in animal host-microbiota communication. We put a special focus on invertebrate hosts, including both traditional invertebrate models such as Drosophila melanogaster and Caenorhabditis elegans and “non-model” invertebrates in microbiota research. Finally, we highlight the potential of invertebrate systems in studying mechanism of host-microbiota interactions.

Exploring Biased Agonism at FPR1 as a Means to Encode Danger Sensing

Ligand-based selectivity in signal transduction (biased signaling) is an emerging field of G protein-coupled receptor (GPCR) research and might allow the development of drugs with targeted activation profiles. Human formyl peptide receptor 1 (FPR1) is a GPCR that detects potentially hazardous states characterized by the appearance of N-formylated peptides that originate from either bacteria or mitochondria during tissue destruction; however, the receptor also responds to several non-formylated agonists from various sources. We hypothesized that an additional layer of FPR signaling is encoded by biased agonism, thus allowing the discrimination of the source of threat. We resorted to the comparative analysis of FPR1 agonist-evoked responses across three prototypical GPCR signaling pathways, i.e., the inhibition of cAMP formation, receptor internalization, and ERK activation, and analyzed cellular responses elicited by several bacteria- and mitochondria-derived ligands. We also included the anti-inflammatory annexinA1 peptide Ac2-26 and two synthetic ligands, the W-peptide and the small molecule FPRA14. Compared to the endogenous agonists, the bacterial agonists displayed significantly higher potencies and efficacies. Selective pathway activation was not observed, as both groups were similarly biased towards the inhibition of cAMP formation. The general agonist bias in FPR1 signaling suggests a source-independent pathway selectivity for transmission of pro-inflammatory danger signaling.

Bacterial MgrB peptide activates chemoreceptor Fpr3 in mouse accessory olfactory system and drives avoidance behaviour

Innate immune chemoreceptors of the formyl peptide receptor (Fpr) family are expressed by vomeronasal sensory neurons (VSNs) in the accessory olfactory system. Their biological function and coding mechanisms remain unknown. We show that mouse Fpr3 (Fpr-rs1) recognizes the core peptide motif f-MKKFRW that is predominantly present in the signal sequence of the bacterial protein MgrB, a highly conserved regulator of virulence and antibiotic resistance in Enterobacteriaceae. MgrB peptide can be produced and secreted by bacteria, and is selectively recognized by a subset of VSNs. Exposure to the peptide also stimulates VSNs in freely behaving mice and drives innate avoidance. Our data shows that Fpr3 is required for neuronal detection and avoidance of peptides derived from a conserved master virulence regulator of enteric bacteria.

Nigericin Promotes NLRP3-Independent Bacterial Killing in Macrophages

Altered microbiota has been associated with a number of diseases, including inflammatory bowel diseases, diabetes, and cancer. This dysregulation is thought to relate the host inflammatory response to enteric pathogens. Macrophages play a key role in host response to microbes and are involved in bacterial killing and clearance. This process is partially mediated through the potassium efflux-dependent, cytosolic, PYCARD-containing inflammasome protein complex. Surprisingly, we discovered an alternative mechanism for bacterial killing, independent of the NLRP3 inflammasome/PYCARD. Using the NLRP3 inflammasome-deficient Raw 264.7 and PYCARD-deficient J77 macrophages, which both lack PYCARD, we found that the potassium efflux activator nigericin enhances bacterial killing. Macrophage response to nigericin was examined by RT gene profiling and subsequent qPCR, which demonstrated altered expression of a series of genes involved in the IL-18 bacterial killing pathway. Based on our results we propose a model of bacterial killing, unrelated to NLRP3 inflammasome activation in macrophage cells. Improving understanding of the molecular pathways driving bacterial clearance within macrophage cells will aid in the development of novel immune-targeted therapeutics in a number of diseases.

Chemotactic Ligands that Activate G-Protein-Coupled Formylpeptide Receptors

Leukocyte infiltration is a hallmark of inflammatory responses. This process depends on the bacterial and host tissue-derived chemotactic factors interacting with G-protein-coupled seven-transmembrane receptors (GPCRs) expressed on the cell surface. Formylpeptide receptors (FPRs in human and Fprs in mice) belong to the family of chemoattractant GPCRs that are critical mediators of myeloid cell trafficking in microbial infection, inflammation, immune responses and cancer progression. Both murine Fprs and human FPRs participate in many patho-physiological processes due to their expression on a variety of cell types in addition to myeloid cells. FPR contribution to numerous pathologies is in part due to its capacity to interact with a plethora of structurally diverse chemotactic ligands. One of the murine Fpr members, Fpr2, and its endogenous agonist peptide, Cathelicidin-related antimicrobial peptide (CRAMP), control normal mouse colon epithelial growth, repair and protection against inflammation-associated tumorigenesis. Recent developments in FPR (Fpr) and ligand studies have greatly expanded the scope of these receptors and ligands in host homeostasis and disease conditions, therefore helping to establish these molecules as potential targets for therapeutic intervention.

G-protein-coupled formyl peptide receptors play a dual role in neutrophil chemotaxis and bacterial phagocytosis

A dogma of innate immunity is that neutrophils use G-protein–coupled receptors (GPCRs) for chemoattractant to chase bacteria through chemotaxis and then use phagocytic receptors coupled with tyrosine kinases to destroy opsonized bacteria via phagocytosis. Our current work showed that G-protein–coupled formyl peptide receptors (FPRs) directly mediate neutrophil phagocytosis. Mouse neutrophils lacking formyl peptide receptors (Fpr1/2^–/–) are defective in the phagocytosis of Escherichia coli and the chemoattractant N-formyl-Met-Leu-Phe (fMLP)-coated beads. fMLP immobilized onto the surface of a bead interacts with FPRs, which trigger a Ca²⁺ response and induce actin polymerization to form a phagocytic cup for engulfment of the bead. This chemoattractant GPCR/Gi signaling works independently of phagocytic receptor/tyrosine kinase signaling to promote phagocytosis. Thus, in addition to phagocytic receptor-mediated phagocytosis, neutrophils also utilize the chemoattractant GPCR/Gi signaling to mediate phagocytosis to fight against invading bacteria.

Transcriptomic and Proteomic Profiling Revealed High Proportions of Odorant Binding and Antimicrobial Defense Proteins in Olfactory Tissues of the House Mouse

Mammalian olfaction depends on chemosensory neurons of the main olfactory epithelia (MOE), and/or of the accessory olfactory epithelia in the vomeronasal organ (VNO). Thus, we have generated the VNO and MOE transcriptomes and the nasal cavity proteome of the house mouse, Mus musculus musculus. Both transcriptomes had low levels of sexual dimorphisms, while the soluble proteome of the nasal cavity revealed high levels of sexual dimorphism similar to that previously reported in tears and saliva. Due to low levels of sexual dimorphism in the olfactory receptors in MOE and VNO, the sex-specific sensing seems less likely to be dependent on receptor repertoires. However, olfaction may also depend on a continuous removal of background compounds from the sites of detection. Odorant binding proteins (OBPs) are thought to be involved in this process and in our study Obp transcripts were most expressed along other lipocalins (e.g., Lcn13, Lcn14) and antimicrobial proteins. At the level of proteome, OBPs were highly abundant with only few being sexually dimorphic. We have, however, detected the major urinary proteins MUP4 and MUP5 in males and females and the male-biased central/group-B MUPs that were thought to be abundant mainly in the urine. The exocrine gland-secreted peptides ESP1 and ESP22 were male-biased but not male-specific in the nose. For the first time, we demonstrate that the expression of nasal lipocalins correlates with antimicrobial proteins thus suggesting that their individual variation may be linked to evolvable mechanisms that regulate natural microbiota and pathogens that regularly enter the body along the ‘eyes-nose-oral cavity’ axis.

Impact of Intestinal Peptides on the Enteric Nervous System: Novel Approaches to Control Glucose Metabolism and Food Intake

The gut is one of the most important sources of bioactive peptides in the body. In addition to their direct actions in the brain and/or peripheral tissues, the intestinal peptides can also have an impact on enteric nervous neurons. By modifying the endogenousproduction of these peptides, one may expect modify the "local" physiology such as glucose absorption, but also could have a "global" action via the gut-brain axis. Due to the various origins of gut peptides (i.e., nutrients, intestinal wall, gut microbiota) and the heterogeneity of enteric neurons population, the potential physiological parameters control by the interaction between the two partners are multiple. In this review, we will exclusively focus on the role of enteric nervous system as a potential target of gut peptides to control glucose metabolism and food intake. Potential therapeutic strategies based on per os administration of gut peptides to treat type 2 diabetes will be described.

On the tear proteome of the house mouse (Mus musculus musculus) in relation to chemical signalling

Mammalian tears are produced by lacrimal glands to protect eyes and may function in chemical communication and immunity. Recent studies on the house mouse chemical signalling revealed that major urinary proteins (MUPs) are not individually unique in Mus musculus musculus. This fact stimulated us to look for other sexually dimorphic proteins that may—in combination with MUPs—contribute to a pool of chemical signals in tears. MUPs and other lipocalins including odorant binding proteins (OBPs) have the capacity to selectively transport volatile organic compounds (VOCs) in their eight-stranded beta barrel, thus we have generated the tear proteome of the house mouse to detect a wider pool of proteins that may be involved in chemical signalling. We have detected significant male-biased (7.8%) and female-biased (7%) proteins in tears. Those proteins that showed the most elevated sexual dimorphisms were highly expressed and belong to MUP, OBP, ESP (i.e., exocrine gland-secreted peptides), and SCGB/ABP (i.e., secretoglobin) families. Thus, tears may have the potential to elicit sex-specific signals in combination by different proteins. Some tear lipocalins are not sexually dimorphic—with MUP20/darcin and OBP6 being good examples—and because all proteins may flow with tears through nasolacrimal ducts to nasal and oral cavities we suggest that their roles are wider than originally thought. Also, we have also detected several sexually dimorphic bactericidal proteins, thus further supporting an idea that males and females may have adopted alternative strategies in controlling microbiota thus yielding different VOC profiles.

On-resin N-formylation of peptides: a head-to-head comparison of reagents in solid-phase synthesis of ligands for formyl peptide receptors

4-Nitrophenyl formate was found to be the most convenient reagent in solid-phase formylation of peptides with a high formylation degree within 20 min to 3 h depending on reaction temperature and length of peptide.