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

Mitochondrial damage-associated molecular patterns: A new insight into metabolic inflammation in type 2 diabetes mellitus

The pathogenesis of diabetes is accompanied by increased levels of inflammatory factors, also known as "metabolic inflammation", which runs through the whole process of the occurrence and development of the disease. Mitochondria, as the key site of glucose and lipid metabolism, is often accompanied by mitochondrial function damage in type 2 diabetes mellitus (T2DM). Damaged mitochondria release pro-inflammatory factors through damage-related molecular patterns that activate inflammation pathways and reactions to oxidative stress, further aggravate metabolic disorders, and form a vicious circle. Currently, the pathogenesis of diabetes is still unclear, and clinical treatment focuses primarily on symptomatic intervention of the internal environment of disorders of glucose and lipid metabolism with limited clinical efficacy. The proinflammatory effect of mitochondrial damage-associated molecular pattern (mtDAMP) in T2DM provides a new research direction for exploring the pathogenesis and intervention targets of T2DM. Therefore, this review covers the most recent findings on the molecular mechanism and related signalling cascades of inflammation caused by mtDAMP in T2DM and discusses its pathogenic role of it in the pathological process of T2DM to search potential intervention targets.

WD40 Repeat Protein 26 Negatively Regulates Formyl Peptide Receptor-1 Mediated Wound Healing in Intestinal Epithelial Cells

N-formyl peptide receptors (FPRs) serve as phagocyte pattern-recognition receptors that play a crucial role in the regulation of host defense against infection. Epithelial cells also express FPRs, and their activation during inflammation or injury results in enhanced epithelial migration and proliferation and improved mucosal wound repair. However, signaling mechanisms that govern epithelial FPR1 activity are not well understood. This study identified a novel FPR1-interacting protein, WD40 repeat protein (WDR)-26, which negatively regulates FPR1-mediated wound healing in intestinal epithelial cells. We show that WDR26-mediated inhibition of wound repair is mediated through the inhibition of Rac family small GTPase 1 and cell division cycle 42 activation, as well as downstream intracellular reactive oxygen species production. Furthermore, on FPR1 activation with N-formyl-methionyl-leucyl phenylalanine, WDR26 dissociates from FPR1, resulting in the activation of downstream cell division cycle 42/Rac family small GTPase 1 signaling, increased epithelial cell migration, and mucosal wound repair. These findings elucidate a novel regulatory function of WDR26 in FPR1-mediated wound healing in intestinal epithelial cells.

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

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

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

The formyl peptide fMLF primes platelet activation and augments thrombus formation

Essentials The role of formyl peptide receptor 1 (FPR1) and its ligand, fMLF, in the regulation of platelet function, hemostasis, and thrombosis is largely unknown. Fpr1-deficient mice and selective inhibitors for FPR1 were used to investigate the function of fMLF and FPR1 in platelets. N-formyl-methionyl-leucyl-phenylalanine primes platelet activation and augments thrombus formation, mainly through FPR1 in platelets. Formyl peptide receptor 1 plays a pivotal role in the regulation of platelet function.

BACKGROUND

Formyl peptide receptors (FPRs) play pivotal roles in the regulation of innate immunity and host defense. The FPRs include three family members: FPR1, FPR2/ALX, and FPR3. The activation of FPR1 by its high-affinity ligand, N-formyl-methionyl-leucyl-phenylalanine (fMLF) (a bacterial chemoattractant peptide), triggers intracellular signaling in immune cells such as neutrophils and exacerbates inflammatory responses to accelerate the clearance of microbial infection. Notably, fMLF has been demonstrated to induce intracellular calcium mobilization and chemotaxis in platelets that are known to play significant roles in the regulation of innate immunity and inflammatory responses. Despite a plethora of research focused on the roles of FPR1 and its ligands such as fMLF on the modulation of immune responses, their impact on the regulation of hemostasis and thrombosis remains unexplored.

OBJECTIVE

To determine the effects of fMLF on the modulation of platelet reactivity, hemostasis, and thrombus formation.

METHODS

Selective inhibitors for FPR1 and Fpr1-deficient mice were used to determine the effects of fMLF and FPR1 on platelets using various platelet functional assays.

RESULTS

N-formyl-methionyl-leucyl-phenylalanine primes platelet activation through inducing distinctive functions and enhances thrombus formation under arterial flow conditions. Moreover, FPR1 regulates normal platelet function as its deficiency in mouse or blockade in human platelets using a pharmacological inhibitor resulted in diminished agonist-induced platelet activation.

CONCLUSION

Since FPR1 plays critical roles in numerous disease conditions, its influence on the modulation of platelet activation and thrombus formation may provide insights into the mechanisms that control platelet-mediated complications under diverse pathological settings.

Formyl Met-Leu-Phe-Stimulated FPR1 Phosphorylation in Plate-Adherent Human Neutrophils: Enhanced Proteolysis but Lack of Inhibition by Platelet-Activating Factor

N-formyl-Met-Leu-Phe (fMLF) is a model PAMP/DAMP driving human PMN to sites of injury/infection utilizing the GPCR, FPR1. We examined a microtiter plate format for measurement of FPR1 phosphorylation in adherent PMN at high densities and found that a new phosphosensitive FPR1 fragment, 25K-FPR1, accumulates in SDS-PAGE extracts. 25K-FPR1 is fully inhibited by diisopropylfluorophosphate PMN pretreatment but is not physiologic, as its formation failed to be significantly perturbed by ATP depletion, time and temperature of adherence, or adherence mechanism. 25K-FPR1 was minimized by extracting fMLF-exposed PMN in lithium dodecylsulfate at 4°C prior to reduction/alkylation. After exposure of adherent PMN to a 5 log range of PAF before or after fMLF, unlike in suspension PMN, no inhibition of fMLF-induced FPR1 phosphorylation was observed. However, PAF induced the release of 40% of PMN lactate dehydrogenase, implying significant cell lysis. We infer that PAF-induced inhibition of fMLF-dependent FPR1 phosphorylation observed in suspension PMN does not occur in the unlysed adherent PMN. We speculate that although the conditions of the assay may induce PAF-stimulated necrosis, the cell densities on the plates may approach levels observed in inflamed tissues and provide for an explanation of PAF's divergent effects on FPR1 phosphorylation as well as PMN function.

Proteomic analysis of human epileptic neocortex predicts vascular and glial changes in epileptic regions

Epilepsy is a common neurological disorder, which is not well understood at the molecular level. Exactly why some brain regions produce epileptic discharges and others do not is not known. Patients who fail to respond to antiseizure medication (refractory epilepsy) can benefit from surgical removal of brain regions to reduce seizure frequency. The tissue removed in these surgeries offers an invaluable resource to uncover the molecular and cellular basis of human epilepsy. Here, we report a proteomic study to determine whether there are common proteomic patterns in human brain regions that produce epileptic discharges. We analyzed human brain samples, as part of the Systems Biology of Epilepsy Project (SBEP). These brain pieces are in vivo electrophysiologically characterized human brain samples withdrawn from the neocortex of six patients with refractory epilepsy. This study is unique in that for each of these six patients the comparison of protein expression was made within the same patient: a more epileptic region was compared to a less epileptic brain region. The amount of epileptic activity was defined for each patient as the frequency of their interictal spikes (electric activity between seizures that is a parameter strongly linked to epilepsy). Proteins were resolved from three subcellular fractions, using a 2D differential gel electrophoresis (2D-DIGE), revealing 31 identified protein spots that changed significantly. Interestingly, glial fibrillary acidic protein (GFAP) was found to be consistently down regulated in high spiking brain tissue and showed a strong negative correlation with spike frequency. We also developed a two-step analysis method to select for protein species that changed frequently among the patients and identified these proteins. A total of 397 protein spots of interest (SOI) were clustered by protein expression patterns across all samples. These clusters were used as markers and this analysis predicted proteomic changes due to both histological differences and molecular pathways, revealed by examination of gene ontology clusters. Our experimental design and proteomic data analysis predicts novel glial changes, increased angiogenesis, and changes in cytoskeleton and neuronal projections between high and low interictal spiking regions. Quantitative histological staining of these same tissues for both the vascular and glial changes confirmed these findings, which provide new insights into the structural and functional basis of neocortical epilepsy.

Regulation of N-Formyl Peptide Receptor Signaling and Trafficking by Arrestin-Src Kinase Interaction

Arrestins were originally described as proteins recruited to ligand-activated, phosphorylated G protein-coupled receptors (GPCRs) to attenuate G protein-mediated signaling. It was later revealed that arrestins also mediate GPCR internalization and recruit a number of signaling proteins including, but not limited to, Src family kinases, ERK1/2, and JNK3. GPCR-arrestin binding and trafficking control the spatial and temporal activity of these multi-protein complexes. In previous reports, we concluded that N-formyl peptide receptor (FPR)-mediated apoptosis, which occurs upon receptor stimulation in the absence of arrestins, is associated with FPR accumulation in perinuclear recycling endosomes. Under these conditions, inhibition of Src kinase and ERK1/2 prevented FPR-mediated apoptosis. To better understand the role of Src kinase in this process, in the current study we employed a previously described arrestin-2 (arr2) mutant deficient in Src kinase binding (arr2-P91G/P121E). Unlike wild type arrestin, arr2-P91G/P121E did not inhibit FPR-mediated apoptosis, suggesting that Src binding to arrestin-2 prevents apoptotic signaling. However, in cells expressing this mutant, FPR-mediated apoptosis was still blocked by inhibition of Src kinase activity, suggesting that activation of Src independent of arrestin-2 binding is involved in FPR-mediated apoptosis. Finally, while Src kinase inhibition prevented FPR-mediated-apoptosis in the presence of arr2-P91G/P121E, it did not prevent FPR-arr2-P91G/P121E accumulation in the perinuclear recycling endosome. On the contrary, inhibition of Src kinase activity mediated the accumulation of activated FPR-wild type arrestin-2 in recycling endosomes without initiating FPR-mediated apoptosis. Based on these observations, we conclude that Src kinase has two independent roles following FPR activation that regulate both FPR-arrestin-2 signaling and trafficking.

Phosphoproteomic analysis of the mouse brain mu-opioid (MOP) receptor

Many in vitro data have shown that the efficacy of several opioid drugs is correlated with differential mu-opioid (MOP) receptor phosphorylation. Label-free semiquantitative on-line nanoflow liquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) analyses were performed to compare the endogenous MOP receptor phosphorylation patterns of mice administered with morphine, etonitazene and fentanyl. The analysis identified S363, T370 and S375 as phosphorylated residues in the carboxy-terminus. Only T370 and S375 were regulated by agonists, with a higher propensity to promote double phosphorylation for high efficacy agonists. Our study provides confirmation that differential agonist-driven multi-site phosphorylation of MOP receptor occurs in vivo and validate the use of MS to study endogenous GPCR phosphorylation.

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.

Anti-inflammatory effects of secondary metabolites of marine Pseudomonas sp. in human neutrophils are through inhibiting P38 MAPK, JNK, and calcium pathways

Activated neutrophils play a significant role in the pathogenesis of many inflammatory diseases. The metabolites of marine microorganisms are increasingly employed as sources for developing new drugs; however, very few marine drugs have been studied in human neutrophils. Herein, we showed that secondary metabolites of marine Pseudomonas sp. (N11) significantly inhibited superoxide anion generation and elastase release in formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-activated human neutrophils, with IC50 values of 0.67±0.38 µg/ml and 0.84±0.12 µg/ml, respectively. In cell-free systems, neither superoxide anion-scavenging effect nor inhibition of elastase activity was associated with the suppressive effects of N11. N11 inhibited the phosphorylation of p38 MAP kinase and JNK, but not Erk and Akt, in FMLP-induced human neutrophils. Also, N11 dose-dependently attenuated the transient elevation of intracellular calcium concentration in activated neutrophils. In contrast, N11 failed to alter phorbol myristate acetate-induced superoxide anion generation, and the inhibitory effects of N11 were not reversed by protein kinase A inhibitor. In conclusion, the anti-inflammatory effects of N11 on superoxide anion generation and elastase release in activated human neutrophils are through inhibiting p38 MAP kinase, JNK, and calcium pathways. Our results suggest that N11 has the potential to be developed to treat neutrophil-mediated inflammatory diseases.

Cardioprotective potential of annexin-A1 mimetics in myocardial infarction

Myocardial infarction (MI) and its resultant heart failure remains a major cause of death in the world. The current treatments for patients with MI are revascularization with thrombolytic agents or interventional procedures. These treatments have focused on restoring blood flow to the ischemic tissue to prevent tissue necrosis and preserve organ function. The restoration of blood flow after a period of ischemia, however, may elicit further myocardial damage, called reperfusion injury. Pharmacological interventions, such as antioxidant and Ca(2+) channel blockers, have shown premises in experimental settings; however, clinical studies have shown limited success. Thus, there is a need for the development of novel therapies to treat reperfusion injury. The therapeutic potential of glucocorticoid-regulated anti-inflammatory mediator annexin-A1 (ANX-A1) has recently been recognized in a range of systemic inflammatory disorders. ANX-A1 binds to and activates the family of formyl peptide receptors (G protein-coupled receptor family) to inhibit neutrophil activation, migration and infiltration. Until recently, studies on the cardioprotective actions of ANX-A1 and its peptide mimetics (Ac2-26, CGEN-855A) have largely focused on its anti-inflammatory effects as a mechanism of preserving myocardial viability following I-R injury. Our laboratory provided the first evidence of the direct protective action of ANX-A1 on myocardium, independent of inflammatory cells in vitro. We now review the potential for ANX-A1 based therapeutics to be seen as a "triple shield" therapy against myocardial I-R injury, limiting neutrophil infiltration and preserving both cardiomyocyte viability and contractile function. This novel therapy may thus represent a valuable clinical approach to improve outcome after MI.

Human neutrophil formyl peptide receptor phosphorylation and the mucosal inflammatory response

Bacterial/mitochondrial fMLF analogs bind FPR1, driving accumulation/activation of PMN at sites of infection/injury, while promoting wound healing in epithelia. We quantified levels of UFPR1 and TFPR1 in isolated PMN by use of phosphosensitive NFPRb and phosphorylation-independent NFPRa antibodies. UFPR1 and total TFPR were assessed inflamed mucosa, observed in human IBD. In isolated PMN after fMLF stimulation, UFPR1 declined 70% ((fMLF)EC50 = 11 ± 1 nM; t1/2 = 15 s) and was stable for up to 4 h, whereas TFPR1 changed only slightly. Antagonists (tBoc-FLFLF, CsH) and metabolic inhibitor NaF prevented the fMLF-dependent UFPR1 decrease. Annexin A1 fragment Ac2-26 also induced decreases in UFPR1 ((Ac2-26)EC50 ∼ 3 µM). Proinflammatory agents (TNF-α, LPS), phosphatase inhibitor (okadaic acid), and G-protein activator (MST) modestly increased (fMLF)EC50, 2- to 4-fold, whereas PTX, Ca(2+) chelators (EGTA/BAPTA), H2O2, GM-CSF, ENA-78, IL-1RA, and LXA4 had no effect. Aggregation-inducing PAF, however, strongly inhibited fMLF-stimulated UFPR1 decreases. fMLF-driven PMN also demonstrated decreased UFPR1 after traversing monolayers of cultured intestinal epithelial cells, as did PMN in intestinal mucosal samples, demonstrating active inflammation from UC patients. Total TFPR remained high in PMN within inflamed crypts, migrating through crypt epithelium, and in the lamina propria-adjoining crypts, but UFPR1 was only observed at some peripheral sites on crypt aggregates. Loss of UFPR1 in PMN results from C-terminal S/T phosphorylation. Our results suggest G protein-insensitive, fMLF-dependent FPR1 phosphorylation in isolated suspension PMN, which may manifest in fMLF-driven transmigration and potentially, in actively inflamed tissues, except at minor discrete surface locations of PMN-containing crypt aggregates.

Phosphosite mapping of HIP-55 protein in mammalian cells

In the present study, hematopoietic progenitor kinase 1 (HPK1)-interacting protein of 55 kDa (HIP-55) protein was over-expressed in HEK293 cells, which was genetically attached with 6x His tag. The protein was purified by nickel-charged resin and was then subjected to tryptic digestion. The phosphorylated peptides within the HIP-55 protein were enriched by TiO₂ affinity chromatography, followed by mass spectrometry analysis. Fourteen phosphorylation sites along the primary structure of HIP-55 protein were identified, most of which had not been previously reported. Our results indicate that bio-mass spectrometry coupled with manual interpretation can be used to successfully identify the phosphorylation modification in HIP-55 protein in HEK293 cells.