Citrullination of CXCL8 by peptidylarginine deiminase alters receptor usage, prevents proteolysis, and dampens tissue inflammation

Neutrophil-derived PAD4 induces citrullination of CKMT1 exacerbates mucosal inflammation in inflammatory bowel disease

Peptidyl arginine deiminase 4 (PAD4) plays a pivotal role in infection and inflammatory diseases by facilitating the formation of neutrophil extracellular traps (NETs). However, the substrates of PAD4 and its exact role in inflammatory bowel disease (IBD) remain unclear. In this study, we employed single-cell RNA sequencing (scRNA-seq) and substrate citrullination mapping to decipher the role of PAD4 in intestinal inflammation associated with IBD. Our results demonstrated that PAD4 deficiency alleviated colonic inflammation and restored intestinal barrier function in a dextran sulfate sodium (DSS)-induced colitis mouse model. scRNA-seq analysis revealed significant alterations in intestinal cell populations, with reduced neutrophil numbers and changes in epithelial subsets upon PAD4 deletion. Gene expression analysis highlighted pathways related to inflammation and epithelial cell function. Furthermore, we found that neutrophil-derived extracellular vesicles (EVs) carrying PAD4 were secreted into intestinal epithelial cells (IECs). Within IECs, PAD4 citrullinates mitochondrial creatine kinase 1 (CKMT1) at the R242 site, leading to reduced CKMT1 protein stability via the autophagy pathway. This action compromises mitochondrial homeostasis, impairs intestinal barrier integrity, and induces IECs apoptosis. IEC-specific depletion of CKMT1 exacerbated intestinal inflammation and apoptosis in mice with colitis. Clinical analysis of IBD patients revealed elevated levels of PAD4, increased CKMT1 citrullination, and decreased CKMT1 expression. In summary, our findings highlight the crucial role of PAD4 in IBD, where it modulates IECs plasticity via CKMT1 citrullination, suggesting that PAD4 may be a potential therapeutic target for IBD.

Peptidylarginine Deiminase Type 2 Predicts Tumor Progression and Poor Prognosis in Patients with Curatively Resected Biliary Tract Cancer

(1) Background: PADI2 is a post-translational modification (PTM) enzyme that catalyzes citrullination, which then triggers autoimmune disease and cancer. This study aimed to evaluate the prognostic value of peptidylarginine deiminase 2 (PADI2) protein expression in biliary tract cancer (BTC) patients. (2) Methods: Using immunohistochemistry, the PADI2 protein expression in BTC tissues was analyzed. The correlations between PADI2 protein expression and clinicopathologic characteristics were analyzed using Chi-square tests. The Kaplan-Meier procedure was used for comparing survival distributions. We used Cox proportional hazards regression for univariate and multivariate analyses. From 2014 to 2020, 30 resected BTC patients were enrolled in this study. (3) Results: Patients with high PADI2 protein expression were associated with shorter progress-free survival (PFS; p = 0.041), disease-specific survival (DSS; p = 0.025), and overall survival (OS; p = 0.017) than patients with low PADI2 protein expression. (4) Conclusions: The results indicated that PADI2 protein expression was an independent poor prognostic factor for BTC patients regarding PFS, DSS, and OS.

The chemokines CXCL8 and CXCL12: molecular and functional properties, role in disease and efforts towards pharmacological intervention

Chemokines are an indispensable component of our immune system through the regulation of directional migration and activation of leukocytes. CXCL8 is the most potent human neutrophil-attracting chemokine and plays crucial roles in the response to infection and tissue injury. CXCL8 activity inherently depends on interaction with the human CXC chemokine receptors CXCR1 and CXCR2, the atypical chemokine receptor ACKR1, and glycosaminoglycans. Furthermore, (hetero)dimerization and tight regulation of transcription and translation, as well as post-translational modifications further fine-tune the spatial and temporal activity of CXCL8 in the context of inflammatory diseases and cancer. The CXCL8 interaction with receptors and glycosaminoglycans is therefore a promising target for therapy, as illustrated by multiple ongoing clinical trials. CXCL8-mediated neutrophil mobilization to blood is directly opposed by CXCL12, which retains leukocytes in bone marrow. CXCL12 is primarily a homeostatic chemokine that induces migration and activation of hematopoietic progenitor cells, endothelial cells, and several leukocytes through interaction with CXCR4, ACKR1, and ACKR3. Thereby, it is an essential player in the regulation of embryogenesis, hematopoiesis, and angiogenesis. However, CXCL12 can also exert inflammatory functions, as illustrated by its pivotal role in a growing list of pathologies and its synergy with CXCL8 and other chemokines to induce leukocyte chemotaxis. Here, we review the plethora of information on the CXCL8 structure, interaction with receptors and glycosaminoglycans, different levels of activity regulation, role in homeostasis and disease, and therapeutic prospects. Finally, we discuss recent research on CXCL12 biochemistry and biology and its role in pathology and pharmacology.

Platelets derived citrullinated proteins and microparticles are potential autoantibodies ACPA targets in RA patients

Citrullinated neoepitopes have emerged as key triggers of autoantibodies anti-citrullinated protein antibodies (ACPA) synthesis in rheumatoid arthritis (RA) patients. Apart from their critical role in homeostasis and thrombosis, platelets have a significant contribution to inflammation as well. Although anuclear in nature, platelets have an intricate post-translational modification machinery. Till now, citrullination in platelets and its contribution to trigger autoantibodies ACPA production in RA is an unexplored research direction. Herein, we investigated the expression of peptidylarginine deiminase (PAD) enzymes and citrullinated proteins/peptides in the human platelets and platelet derived microparticles (PDP). Both PAD4 mRNA and protein, but not the other PAD isoforms, are detectable in the human platelets. With a strict filtering criterion,108 citrullination sites present on 76 proteins were identified in the human platelets, and 55 citrullinated modifications present on 37 different proteins were detected in the PDPs. Among them, some are well-known citrullinated autoantigens associated with RA. Citrullinated forms of thrombospondin-1, β-actin, and platelet factor-4 (also known as CXCL4) are highly immunogenic and bound by autoantibodies ACPA. Furthermore, ACPA from RA sera and synovial fluids recognized citrullinated proteins from platelets and significantly activated them as evidenced by P-selectin upregulation and sCD40 L secretion. These results clearly demonstrate the presence of citrullinated autoantigens in platelets and PDPs, thus could serve as potential targets of ACPA in RA.

Thirty-five years since the discovery of chemotactic cytokines, interleukin-8 and MCAF: A historical overview

Inflammation is a host defense response to various invading stimuli, but an excessive and persistent inflammatory response can cause tissue injury, which can lead to irreversible organ damage and dysfunction. Excessive inflammatory responses are believed to link to most human diseases. A specific type of leukocyte infiltration into invaded tissues is required for inflammation. Historically, the underlying molecular mechanisms of this process during inflammation were an enigma, compromising research in the fields of inflammation, immunology, and pathology. However, the pioneering discovery of chemotactic cytokines (chemokines), monocyte-derived neutrophil chemotactic factor (MDNCF; interleukin [IL]-8, CXCL8) and monocyte chemotactic and activating factor (MCAF; monocyte chemotactic factor 1 [MCP-1], CCL2) in the late 1980s finally enabled us to address this issue. In this review, we provide a historical overview of chemokine research over the last 35 years.

Microbial pathways to subvert host immunity generate citrullinated neoantigens targeted in rheumatoid arthritis

The specific association between antibodies to citrullinated proteins and rheumatoid arthritis (RA) has centered interest on understanding why citrullinated proteins become immunogenic in this disease, which is believed to inform the origins of autoimmunity in RA. Since citrullination is a physiologic post-translational modification (PTM), one theory is that conditions promoting abnormal citrullination are initiators of self-reactive immune responses to citrullinated proteins in RA. Foremost candidates that dysregulate the normal balance of citrullination are microbial agents, which can exploit citrullination as an effector mechanism to subvert host antimicrobial activities and maximize their progeny. Here, we will use the host-pathogen interface as a unifying model to link microbe-induced citrullination and the loss of immunological tolerance to citrullinated antigens in RA.

Stepwise Collision Energy-Resolved Tandem Mass Spectrometric Experiments for the Improved Identification of Citrullinated Peptides

The use of tandem mass spectrometry (MS/MS) is a fundamental prerequisite of reliable protein identification and quantification in mass-spectrometry-based proteomics. In bottom-up and middle-down proteomics, proteins are identified by the characteristic fragments of their constituting peptides. Post-translational modifications (PTMs) often further complicate proteome analyses. Citrullination is an increasingly studied PTM converting arginines to citrullines (Cit, X) and is implicated in several autoimmune and neurological diseases as well as different types of cancer. Confirmation of citrullination is known to be very challenging since it results in the same molecular mass change as Asn/Gln deamidation. In this study, we explore which MS/MS characteristics can be used for the reliable identification of citrullination. We synthesized several peptides incorporating Cit residues that model enzymatic cleavages of different proteins with verified or putative citrullination. Collision-induced dissociation was used to investigate the energy dependence of Byonic and Mascot scores and confirmed sequence coverage (CSC) along with the neutral loss of HNCO characteristic to citrulline side chains. We found that although the recommended values (19-45 V) for ramped collision energy settings cover the optimal Mascot, Byonic, or %CSC scores effectively, the diagnostic HNCO loss from precursors and fragments may reach their maximum intensities at lower and higher collision energies, respectively. Therefore, we suggest broadening the ramp range to ∼5-60 V to obtain more favorable identification rates for citrullinated peptides. We also found that Byonic was more successful in correctly identifying citrullinated peptides with deamidated residues than Mascot.

The virtues and vices of protein citrullination

The post-translational modification of proteins expands the regulatory scope of the proteome far beyond what is achievable through genome regulation. The field of protein citrullination has seen significant progress in the last two decades. The small family of peptidylarginine deiminase (PADI or PAD) enzymes, which catalyse citrullination, have been implicated in virtually all facets of molecular and cell biology, from gene transcription and epigenetics to cell signalling and metabolism. We have learned about their association with a remarkable array of disease states and we are beginning to understand how they mediate normal physiological functions. However, while the biochemistry of PADI activation has been worked out in exquisite detail in vitro, we still lack a clear mechanistic understanding of the processes that regulate PADIs within cells, under physiological and pathophysiological conditions. This review summarizes and discusses the current knowledge, highlights some of the unanswered questions of immediate importance and gives a perspective on the outlook of the citrullination field.

CXCL8 in Tumor Biology and Its Implications for Clinical Translation

The chemokine CXCL8 has been found to play an important role in tumor progression in recent years. CXCL8 activates multiple intracellular signaling pathways by binding to its receptors (CXCR1/2), and plays dual pro-tumorigenic roles in the tumor microenvironment (TME) including directly promoting tumor survival and affecting components of TME to indirectly facilitate tumor progression, which include facilitating tumor cell proliferation and epithelial-to-mesenchymal transition (EMT), pro-angiogenesis, and inhibit anti-tumor immunity. More recently, clinical trials indicate that CXCL8 can act as an independently predictive biomarker in patients receiving immune checkpoint inhibitions (ICIs) therapy. Preclinical studies also suggest that combined CXCL8 blockade and ICIs therapy can enhance the anti-tumor efficacy, and several clinical trials are being conducted to evaluate this therapy modality.

Interleukin-8: An evolving chemokine

Early in the 1980s several laboratories mistakenly reported that partially purified interleukin-1 (IL-1) was chemotactic for neutrophils. However, further investigations by us, revealed that our purified IL-1 did not have neutrophil chemotactic activity and this activity in the LPS-stimulated human monocyte conditioned media could clearly be separated from IL-1 activity on HPLC gel filtration. This motivated Teizo Yoshimura and Kouji Matsushima to purify the monocyte-derived neutrophil chemotactic factor (MDNCF), present in LPS conditioned media and molecularly clone the cDNA for MDNCF. They found that MDNCF protein (later renamed IL-8, and finally termed CXCL8) is first translated as a precursor form consisting of 99 amino acid residues and the signal peptide is then removed, leading to the secretion and processing of biologically active IL-8 of 72 amino acid form (residues 28-99). There are four cysteine residues forming two disulfide linkage and 14 basic amino acid residues which result in a very basic property for the binding of IL-8 to heparan sulfate-proteoglycan. The IL-8 gene consists of 4 exons and 3 introns. IL-8 is produced by various types of cells in inflammation. The 5'-flanking region of IL-8 gene contains several nuclear factor binding sites, and NF-κB in combination with AP-1 or C/EBP synergistically activates IL-8 gene in response to IL-1 and TNFα. Two receptors exist for IL-8, CXCR1 and CXCR2 in humans, which belong to γ subfamily of GTP binding protein (G-protein) coupled rhodopsin-like 7 transmembrane domain receptors. Rodents express CXCR2 and do not produce IL-8, but produce numerous homologues instead. Once IL-8 binds to the receptor, β and γ subunits of G-protein are released from Gα (Gαi2 in neutrophils) and activate PI3Kγ, PLCβ2/β3, PLA2 and PLD. Gαi2 inhibits adenyl cyclase to decrease cAMP levels. Small GTPases Ras/Rac/Rho/cdc42/Rap1, PKC and AKT (PKB) exist down-stream of β and γ subunits and regulate cell adhesion, actin polymerization, membrane protrusion, and eventually cell migration. PLCβ activation generates IP3 and induces Ca⁺⁺ mobilization, DAG generation to activate protein kinase C to lead granule exocytosis and respiratory burst. MDNCF was renamed interleukin 8 (IL-8) at the International Symposium on Novel Neutrophil Chemotactic Activating Polypeptides, London, UK in 1989. The discovery of IL-8 prompted us to also purify and molecularly clone the cDNA of MCAF/MCP-1 responsible for monocyte chemotaxis, and other groups to identify a large family of chemotactic cytokines capable of attracting other types of leukocytes. In 1992, most of the investigators contributing to the discovery of this new family of chemotactic cytokines gathered in Baden, Austria and agreed to name this family "chemokines" and subsequently established the CXCL/CCL and CXCR/CCR nomenclature. The discovery of chemokines resulted in solving the long-time enigma concerning the mechanism of cell type specific leukocyte infiltration into inflamed tissues and provided a molecular basis for immune and hematopoietic cell migration and interactions under physiological as well as pathological conditions. To our surprise based on its recently identified multifunctional activities, IL-8 has evolved from a neutrophil chemoattractant to a promising therapeutic target for a wide range of inflammatory and neoplastic diseases. IL-8 was initially characterized as a chemoattractant of neutrophils engaged in acute inflammation and then discovered to also be chemotactic for endothelial cells with a major role in angiogenesis. These two activities of IL-8 foster its stimulatory effect on tumor growth. This is abetted by recent additional discoveries showing that IL-8 has stimulatory effects on stem cells and can therefore directly promote the growth of receptor expressing cancer stem cells. IL-8 by interacting with bone marrow stem/progenitor cells has also the capacity to mobilize and release hematopoietic cells into the peripheral circulation. This includes the mobilization of neutrophilic myeloid-derived suppressor cells (N-MDSC) to infiltrate into tumors and thus further promotes the immune escape of tumors. Finally, the capacity of IL-8 to induce trans-differentiation of epithelial cancer cells into mesenchymal phenotype (EMT) increases the malignancy of tumors by promoting their metastatic spread and resistance to chemotherapeutics and cytotoxic immune cells. These observations have stimulated considerable current efforts to develop receptor antagonists for IL-8 and humanized anti-IL-8 antibody for the therapy of cancer, particularly in combination with immune checkpoint inhibitors, such as anti-PD-1/PD-L1 antibodies.

A Hairy Cituation - PADIs in Regeneration and Alopecia

In this Review article, we focus on delineating the expression and function of Peptidyl Arginine Delminases (PADIs) in the hair follicle stem cell lineage and in inflammatory alopecia. We outline our current understanding of cellular processes influenced by protein citrullination, the PADI mediated posttranslational enzymatic conversion of arginine to citrulline, by exploring citrullinomes from normal and inflamed tissues. Drawing from other stem cell lineages, we detail the potential function of PADIs and specific citrullinated protein residues in hair follicle stem cell activation, lineage specification and differentiation. We highlight PADI3 as a mediator of hair shaft differentiation and display why mutations in PADI3 are linked to human alopecia. Furthermore, we propose mechanisms of PADI4 dependent fine-tuning of the hair follicle lineage progression. Finally, we discuss citrullination in the context of inflammatory alopecia. We present how infiltrating neutrophils establish a citrullination-driven self-perpetuating proinflammatory circuitry resulting in T-cell recruitment and activation contributing to hair follicle degeneration. In summary, we aim to provide a comprehensive perspective on how citrullination modulates hair follicle regeneration and contributes to inflammatory alopecia.

Role of protein deimination in cardiovascular diseases: potential new avenues for diagnostic and prognostic biomarkers

INTRODUCTION

Arginine deimination (citrullination) is a post-translational modification catalyzed by a family of peptidyl arginine deiminase (PAD) enzymes. Cell-based functional studies and animal models have manifested the key role of PADs in various cardiovascular diseases (CVDs).

AREA COVERED

This review summarizes the latest developments in the role of PADs in CVD pathogenesis. It focuses on the PAD functions and diverse citrullinated proteins in cardiovascular conditions like deep vein thrombosis, ischemia/reperfusion, and atherosclerosis. Identification of PAD isoforms and citrullinated targets are essential for directing diagnosis and clinical intervention. Finally, anti-citrullinated protein antibodies (ACPAs) are addressed as an independent risk factor for cardiovascular events. A search of PubMed biomedical literature from the past ten years was performed with a combination of the following keywords: PAD/PADI, deimination/citrullination, autoimmune, fibrosis, NET, neutrophil, macrophage, inflammation, inflammasome, cardiovascular, heart disease, myocardial infarction, ischemia, atherosclerosis, thrombosis, and aging. Additional papers from retrieved articles were also considered.

EXPERT OPINION

PADs are unique family of enzymes that converts peptidyl-arginine to -citrulline in protein permanently. Overexpression or increased activity of PAD has been observed in various CVDs with acute and chronic inflammation as the background. Importantly, far beyond being simply involved in forming neutrophil extracellular traps (NETs), accumulating evidence indicated PAD activation as a trigger for numerous processes, such as transcriptional regulation, endothelial dysfunction, and thrombus formation. In summary, the findings so far have testified the important role of deimination in cardiovascular biology, while more basic and translational studies are essential to further exploration.

The Chemokine Systems at the Crossroads of Inflammation and Energy Metabolism in the Development of Obesity

Obesity is characterized as a complex and multifactorial excess accretion of adipose tissue accompanied with alterations in the immune and metabolic responses. Although the chemokine systems have been documented to be involved in the control of tissue inflammation and metabolism, the dual role of chemokines and chemokine receptors in the pathogenesis of the inflammatory milieu and dysregulated energy metabolism in obesity remains elusive. The objective of this review is to present an update on the link between chemokines and obesity-related inflammation and metabolism dysregulation under the light of recent knowledge, which may present important therapeutic targets that could control obesity-associated immune and metabolic disorders and chronic complications in the near future. In addition, the cellular and molecular mechanisms of chemokines and chemokine receptors including the potential effect of post-translational modification of chemokines in the regulation of inflammation and energy metabolism will be discussed in this review.

Peptidylarginine Deiminase 2 in Host Immunity: Current Insights and Perspectives

Peptidylarginine deiminases (PADs) are a group of enzymes that catalyze post-translational modifications of proteins by converting arginine residues into citrullines. Among the five members of the PAD family, PAD2 and PAD4 are the most frequently studied because of their abundant expression in immune cells. An increasing number of studies have identified PAD2 as an essential factor in the pathogenesis of many diseases. The successes of preclinical research targeting PAD2 highlights the therapeutic potential of PAD2 inhibition, particularly in sepsis and autoimmune diseases. However, the underlying mechanisms by which PAD2 mediates host immunity remain largely unknown. In this review, we will discuss the role of PAD2 in different types of cell death signaling pathways and the related immune disorders contrasted with functions of PAD4, providing novel therapeutic strategies for PAD2-associated pathology.

A highly efficient method for the production and purification of recombinant human CXCL8

Chemokines play diverse and fundamental roles in the immune system and human disease, which has prompted their structural and functional characterisation. Production of recombinant chemokines that are folded and bioactive is vital to their study but is limited by the stringent requirements of a native N-terminus for receptor activation and correct disulphide bonding required to stabilise the chemokine fold. Even when expressed as fusion proteins, overexpression of chemokines in E. coli tends to result in the formation of inclusion bodies, generating the additional steps of solubilisation and refolding. Here we present a novel method for producing soluble chemokines in relatively large amounts via a simple two-step purification procedure with no requirements for refolding. CXCL8 produced by this method has the correct chemokine fold as determined by NMR spectroscopy and in chemotaxis assays was indistinguishable from commercially available chemokines. We believe that this protocol significantly streamlines the generation of recombinant chemokines.

Impact of Posttranslational Modification in Pathogenesis of Rheumatoid Arthritis: Focusing on Citrullination, Carbamylation, and Acetylation

Rheumatoid arthritis (RA) is caused by prolonged periodic interactions between genetic, environmental, and immunologic factors. Posttranslational modifications (PTMs) such as citrullination, carbamylation, and acetylation are correlated with the pathogenesis of RA. PTM and cell death mechanisms such as apoptosis, autophagy, NETosis, leukotoxic hypercitrullination (LTH), and necrosis are related to each other and induce autoantigenicity. Certain microbial infections, such as those caused by Porphyromonasgingivalis, Aggregatibacter actinomycetemcomitans, and Prevotella copri, can induce autoantigens in RA. Anti-modified protein antibodies (AMPA) containing anti-citrullinated protein/peptide antibodies (ACPAs), anti-carbamylated protein (anti-CarP) antibodies, and anti-acetylated protein antibodies (AAPAs) play a role in pathogenesis as well as in prediction, diagnosis, and prognosis. Interestingly, smoking is correlated with both PTMs and AMPAs in the development of RA. However, there is lack of evidence that smoking induces the generation of AMPAs.

Peptidylarginine deiminases 4 as a promising target in drug discovery

Peptidylarginine deaminase 4 (PAD4) is a crucial post-translational modifying enzyme catalyzing the conversion of arginine into citrulline residues, and mediating the formation of neutrophil extracellular traps (NETs). PAD4 plays a vital role in the occurrence and development of cardiovascular diseases, autoimmune diseases, and various tumors. Therefore, PAD4 is considered as a promising drug target for disease diagnosis and treatment. More and more efforts are devoted to developing highly efficient and selective PAD4 inhibitors via high-throughput screening, structure-based drug design and structure-activity relationship study. This article outlined the physiological and pathological functions of PAD4, and corresponding representative small molecule inhibitors reported in recent years.

PAD2-mediated citrullination of Fibulin-5 promotes elastogenesis

The formation of elastic fibers is active only in the perinatal period. How elastogenesis is developmentally regulated is not fully understood. Citrullination is a unique form of post-translational modification catalyzed by peptidylarginine deiminases (PADs), including PAD1-4. Its physiological role is largely unknown. By using an unbiased proteomic approach of lung tissues, we discovered that FBLN5 and LTBP4, two key elastogenic proteins, were temporally modified in mouse and human lungs. We further demonstrated that PAD2 citrullinated FBLN5 preferentially in young lungs compared to adult lungs. Genetic ablation of PAD2 resulted in attenuated elastogenesis in vitro and age-dependent emphysema in vivo. Mechanistically, citrullination protected FBLN5 from proteolysis and subsequent inactivation of its elastogenic activity. Furthermore, citrullinated but not native FBLN5 partially rescued in vitro elastogenesis in the absence of PAD activity. Our data uncover a novel function of citrullination, namely promoting elastogenesis, and provide additional insights to how elastogenesis is regulated.

Citrullination and PAD Enzyme Biology in Type 1 Diabetes - Regulators of Inflammation, Autoimmunity, and Pathology

The generation of post-translational modifications (PTMs) in human proteins is a physiological process leading to structural and immunologic variety in proteins, with potentially altered biological functions. PTMs often arise through normal responses to cellular stress, including general oxidative changes in the tissue microenvironment and intracellular stress to the endoplasmic reticulum or immune-mediated inflammatory stresses. Many studies have now illustrated the presence of 'neoepitopes' consisting of PTM self-proteins that induce robust autoimmune responses. These pathways of inflammatory neoepitope generation are commonly observed in many autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis, and type 1 diabetes (T1D), among others. This review will focus on one specific PTM to self-proteins known as citrullination. Citrullination is mediated by calcium-dependent peptidylarginine deiminase (PAD) enzymes, which catalyze deimination, the conversion of arginine into the non-classical amino acid citrulline. PADs and citrullinated peptides have been associated with different autoimmune diseases, notably with a prominent role in the diagnosis and pathology of rheumatoid arthritis. More recently, an important role for PADs and citrullinated self-proteins has emerged in T1D. In this review we will provide a comprehensive overview on the pathogenic role for PADs and citrullination in inflammation and autoimmunity, with specific focus on evidence for their role in T1D. The general role of PADs in epigenetic and transcriptional processes, as well as their crucial role in histone citrullination, neutrophil biology and neutrophil extracellular trap (NET) formation will be discussed. The latter is important in view of increasing evidence for a role of neutrophils and NETosis in the pathogenesis of T1D. Further, we will discuss the underlying processes leading to citrullination, the genetic susceptibility factors for increased recognition of citrullinated epitopes by T1D HLA-susceptibility types and provide an overview of reported autoreactive responses against citrullinated epitopes, both of T cells and autoantibodies in T1D patients. Finally, we will discuss recent observations obtained in NOD mice, pointing to prevention of diabetes development through PAD inhibition, and the potential role of PAD inhibitors as novel therapeutic strategy in autoimmunity and in T1D in particular.

From ELISA to Immunosorbent Tandem Mass Spectrometry Proteoform Analysis: The Example of CXCL8/Interleukin-8

With ELISAs one detects the ensemble of immunoreactive molecules in biological samples. For biomolecules undergoing proteolysis for activation, potentiation or inhibition, other techniques are necessary to study biology. Here we develop methodology that combines immunosorbent sample preparation and nano-scale liquid chromatography—tandem mass spectrometry (nano-LC-MS/MS) for proteoform analysis (ISTAMPA) and apply this to the aglycosyl chemokine CXCL8. CXCL8, the most powerful human chemokine with neutrophil chemotactic and –activating properties, occurs in different NH₂-terminal proteoforms due to its susceptibility to site-specific proteolytic modification. Specific proteoforms display up to 30-fold enhanced activity. The immunosorbent ion trap top-down mass spectrometry-based approach for proteoform analysis allows for simultaneous detection and quantification of full-length CXCL8(1-77), elongated CXCL8(-2-77) and all naturally occurring truncated CXCL8 forms in biological samples. For the first time we demonstrate site-specific proteolytic activation of CXCL8 in synovial fluids from patients with chronic joint inflammation and address the importance of sample collection and processing.

Neutrophil and Eosinophil DNA Extracellular Trap Formation: Lessons From Pathogenic Fungi

Fungal infections represent a worldwide health problem. Fungal pathogens are responsible for a variety of conditions, including superficial diseases, allergic pathologies and potentially lethal invasive infections. Neutrophils and eosinophils have been implicated as effector cells in several pathologies. Neutrophils are major effector cells involved in the control of fungal infections and exhibit a plethora of antifungal mechanisms, such as phagocytosis, reactive oxygen species production, degranulation, extracellular vesicle formation, and DNA extracellular trap (ET) release. Eosinophils are polymorphonuclear cells classically implicated as effector cells in the pathogenesis of allergic diseases and helminthic infections, although their roles as immunomodulatory players in both innate and adaptive immunity are currently recognized. Eosinophils are also endowed with antifungal activities and are abundantly found in allergic conditions associated with fungal colonization and sensitization. Neutrophils and eosinophils have been demonstrated to release their nuclear and mitochondrial DNA in response to many pathogens and pro-inflammatory stimuli. ETs have been implicated in the killing and control of many pathogens, as well as in promoting inflammation and tissue damage. The formation of ETs by neutrophils and eosinophils has been described in response to pathogenic fungi. Here, we provide an overview of the mechanisms involved in the release of neutrophil and eosinophil ETs in response to fungal pathogens. General implications for understanding the formation of ETs and the roles of ETs in fungal infections are discussed.

Citrullination regulates wound responses and tissue regeneration in zebrafish

Calcium is an important early signal in wound healing, yet how these early signals promote regeneration remains unclear. Peptidylarginine deiminases (PADs), a family of calcium-dependent enzymes, catalyze citrullination, a post-translational modification that alters protein function and has been implicated in autoimmune diseases. We generated a mutation in the single zebrafish ancestral pad gene, padi2, that results in a loss of detectable calcium-dependent citrullination. The mutants exhibit impaired resolution of inflammation and regeneration after caudal fin transection. We identified a new subpopulation of cells displaying citrullinated histones within the notochord bead following tissue injury. Citrullination of histones in this region was absent, and wound-induced proliferation was perturbed in Padi2-deficient larvae. Taken together, our results show that Padi2 is required for the citrullination of histones within a group of cells in the notochord bead and for promoting wound-induced proliferation required for efficient regeneration. These findings identify Padi2 as a potential intermediary between early calcium signaling and subsequent tissue regeneration.

A Mixture of Tocopherol Acetate and L-Menthol Synergistically Promotes Hair Growth in C57BL/6 Mice

Oral finasteride and topical minoxidil are single components approved by the US FDA for treating hair loss. Some other compounds originating from natural products are also traditionally used for promoting hair growth. In this study, observations of treated keratinocyte cells were used to demonstrate that tocopherol acetate, L-menthol, and stevioside exert an effect on cell regeneration. Furthermore, these were topically applied to the shaved skin of C57BL/6 mice to observe their effects on hair growth. A mixture of tocopherol acetate, L-menthol, and stevioside showed the highest potential for promoting hair growth in vivo. In in vivo experiments, the mixture of tocopherol acetate, L-menthol, and stevioside was more effective than tocopherol acetate or L-menthol alone in promoting hair growth. The transcriptome analysis of skin from the dorsal side of a mouse treated with tocopherol acetate or L-menthol versus vehicle revealed key changes in keratin, keratin-associated protein, forkhead box, sonic hedgehog, fibroblast growth factor 10, desmoglein 4, deoxyribonuclease 1-like 2, and cadherin 3, known to play roles in promoting hair growth.

Deimination, Intermediate Filaments and Associated Proteins

Deimination (or citrullination) is a post-translational modification catalyzed by a calcium-dependent enzyme family of five peptidylarginine deiminases (PADs). Deimination is involved in physiological processes (cell differentiation, embryogenesis, innate and adaptive immunity, etc.) and in autoimmune diseases (rheumatoid arthritis, multiple sclerosis and lupus), cancers and neurodegenerative diseases. Intermediate filaments (IF) and associated proteins (IFAP) are major substrates of PADs. Here, we focus on the effects of deimination on the polymerization and solubility properties of IF proteins and on the proteolysis and cross-linking of IFAP, to finally expose some features of interest and some limitations of citrullinomes.

Citrullination as a novel posttranslational modification of matrix metalloproteinases

Matrix metalloproteinases (MMPs) are enzymes with critical roles in biology and pathology. Glycosylation, nitrosylation and proteolysis are known posttranslational modifications (PTMs) regulating intrinsically the activities of MMPs. We discovered MMP citrullination by peptidyl arginine deiminases (PADs) as a new PTM. Upon hypercitrullination, MMP-9 acquired a higher affinity for gelatin than control MMP-9. Furthermore, hypercitrullinated proMMP-9 was more efficiently activated by MMP-3 compared to control MMP-9. JNJ0966, a specific therapeutic inhibitor of MMP-9 activation, inhibited the activation of hypercitrullinated proMMP-9 by MMP-3 significantly less in comparison with control proMMP-9. The presence of citrullinated/homocitrullinated MMP-9 was detected in vivo in neutrophil-rich sputum samples of cystic fibrosis patients. In addition to citrullination of MMP-9, we report efficient citrullination of MMP-1 and lower citrullination levels of MMP-3 and MMP-13 by PAD2 in vitro. In conclusion, citrullination of MMPs is a new PTM worthy of additional biochemical and biological studies.

Differential Expression of TOM34, AL1A1, PADI2 and KLRBA in NNK Induced Lung Cancer in Wistar Rats and their Implications

BACKGROUND

Lung cancer is the most common cancer with a high mortality rate. The diagnosis only at advanced stages and lack of effective treatment are the main factors responsible for high mortality. Tobacco smoke is the major responsible factor for inflammation and tumor development in lungs.

OBJECTIVE

The present study was carried out to identify differentially expressed proteins and elucidate their role in carcinogenesis.

METHODS

The lung cancer was developed in Wistar rats by using NNK as carcinogen and cancer development was confirmed by histopathological examination. The 2D SDS PAGE was used to analyse total proteins and find out differentially expressed proteins in NNK treated lung tissue vis-a-vis control tissue. The findings of proteomic analysis were further validated by quantification of corresponding transcripts using Real Time PCR. Finally, Cytoscape was used to find out protein-protein interaction.

RESULTS

The histopathological examinations showed neoplasia at 9th month after NNK treatment. The proteomic analysis revealed several differentially expressed proteins, four of which were selected for further studies. (TOM34, AL1A1, PADI2 and KLRBA) that were up regulated in NNK treated lung tissue. The real time analysis showed over expression of the genes coding for the selected proteins. Thus, the proteomic and transcriptomic data corroborate each other. Further, these proteins showed interaction with the members of NF-κB family and STAT3.

CONCLUSION

We conclude that these proteins play a substantial role in the induction of lung cancer through NF-κB and STAT3 pathway. Therefore, these may have the potential to be used as therapeutic targets and for early detection of lung cancer.

Truncation of CXCL8 to CXCL8(9-77) enhances actin polymerization and in vivo migration of neutrophils

CXCL8 is the principal human neutrophil-attracting chemokine and a major mediator of inflammation. The chemokine exerts its neutrophil-chemotactic and neutrophil-activating activities via interaction with glycosaminoglycans (GAGs) and activation of the G protein-coupled receptors (GPCRs) CXCR1 and CXCR2. Natural CXCL8 displays an exceptional degree of amino (NH₂ )-terminal heterogeneity. Most CXCL8 forms result from proteolytic processing of authentic CXCL8(1-77). Here, we compared the potencies to activate and recruit neutrophils of the 3 most abundant natural CXCL8 forms: full-length 77 amino acid CXCL8 and the 2 major natural truncated forms lacking 5 or 8 NH₂ -terminal amino acids. NH₂ -terminal truncation hardly affected the capacity of CXCL8 to induce shedding of CD62L or to up-regulate the expression of the adhesion molecules CD11a, CD11b, or CD15 on human neutrophils. In addition, the potency of CXCL8 to induce neutrophil degranulation and its effect on phagocytosis remained unaltered upon removal of 5 or 8 NH₂ -terminal residues. However, NH₂ -terminal truncation strongly potentiated CXCL8-induced actin polymerization. CXCL8(6-77) and CXCL8(9-77) showed a comparable capacity to induce Ca²⁺ signaling in human neutrophils and to direct in vitro neutrophil migration. Strikingly, the ability of CXCL8(9-77) to recruit neutrophils into the peritoneal cavity of mice was significantly enhanced compared to CXCL8(6-77). These results suggest that NH₂ -terminal truncation influences specific biological activities of CXCL8 and indicate that CXCL8(9-77) may be the most potent neutrophil-attracting CXCL8 form in vivo.

Citrullinome of Porphyromonas gingivalis Outer Membrane Vesicles: Confident Identification of Citrullinated Peptides

Porphyromonas gingivalis is a key pathogen in chronic periodontitis and has recently been mechanistically linked to the development of rheumatoid arthritis via the activity of peptidyl arginine deiminase generating citrullinated epitopes in the periodontium. In this project the outer membrane vesicles (OMV) from P. gingivalis W83 wild-type (WT), a W83 knock-out mutant of peptidyl arginine deiminase (ΔPPAD), and a mutant strain expressing PPAD with the active site cysteine mutated to alanine (C351A), have been analyzed using a two-dimensional HFBA-based separation system combined with LC-MS. For optimal and positive identification and validation of citrullinated peptides and proteins, high resolution mass spectrometers and strict MS search criteria were utilized. This may have compromised the total number of identified citrullinations but increased the confidence of the validation. A new two-dimensional separation system proved to increase the strength of validation, and along with the use of an in-house build program, Citrullia, we establish a fast and easy semi-automatic (manual) validation of citrullinated peptides. For the WT OMV we identified 78 citrullinated proteins having a total of 161 citrullination sites. Notably, in keeping with the mechanism of OMV formation, the majority (51 out of 78) of citrullinated proteins were predicted to be exported via the inner membrane and to reside in the periplasm or being translocated to the bacterial surface. Citrullinated surface proteins may contribute to the pathogenesis of rheumatoid arthritis. For the C351A-OMV a single citrullination site was found and no citrullinations were identified for the ΔPPAD-OMV, thus validating the unbiased character of our method of citrullinated peptide identification.

Citrullination of fibrinogen by peptidylarginine deiminase 2 impairs fibrin clot structure

BACKGROUND

Citrullination is the post-translational conversion of arginine into citrulline in proteins. The reaction is catalyzed by peptidylarginine deiminase (PAD), of which five isoforms exist. Fibrinogen is a substrate for PAD2 and PAD4, and citrullinated fibrinogen (cFBG) has been detected in patients with inflammatory diseases. In purified systems, cFBG is known to inhibit the release of fibrinopeptide A (FPA) and B (FPB) and impairs fibrin polymerization. However, the effect of cFBG on fibrin structure and fibrinolysis in a plasma environment remains unclear. We hypothesized that citrullination of fibrinogen impairs fibrin properties.

METHODS

Fibrinogen was citrullinated by recombinant PAD2 and PAD4. The impact of cFBG on fibrin structure was investigated by turbidity measurements in fibrinogen-deficient plasma spiked with cFBG or native fibrinogen.

RESULTS

Citrullination of fibrinogen by PAD2 dose-dependently reduced the rate of fibrin polymerization, as well as the overall hemostasis potential of fibrin, the maximum velocity of fibrin formation, the fibrin mass/length ratio, and the lysis of fibrin clots.

CONCLUSION

Citrullination of fibrinogen by PAD2 affects not only fibrin polymerization but also fibrin fiber properties, indicating that the fibrin network formed in the presence of cFBG may influence hemostasis. Our results suggest that citrullination of fibrinogen alters the composition of fibrin fibers which may lead to a looser fibrin network that is more susceptible to fibrinolysis and thereby affecting the hemostatic balance.

Citrullinated Inhibitor of DNA Binding 1 Is a Novel Autoantigen in Rheumatoid Arthritis

Objective

To explore the intrinsic role of inhibitor of DNA binding 1 (ID‐1) in rheumatoid arthritis (RA) fibroblast‐like synoviocytes (FLS) and to investigate whether ID‐1 is citrullinated and autoantigenic in RA.

Methods

RA patient serum ID‐1 levels were measured before and after infliximab treatment. RA FLS were transfected with a clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR‐associated protein 9 construct targeting ID‐1 to examine the effects of ID‐1 deletion. RA synovial fluid (SF) and homogenized synovial tissue (ST) were immunoprecipitated for ID‐1 and measured for citrullinated residues using an enzyme‐linked immunosorbent assay and Western blotting. Liquid chromatography tandem mass spectrometry (LC‐MS/MS) was performed on in vitro–citrullinated recombinant human ID‐1 (cit–ID‐1) to localize the sites of citrullination. Normal and RA sera and SF were analyzed by immunodot blotting for anti–citrullinated protein antibodies (ACPAs) to cit–ID‐1.

Results

RA patient serum ID‐1 levels positively correlated with several disease parameters and were reduced after infliximab treatment. RA FLS displayed reduced growth and a robust increase in interleukin‐6 (IL‐6) and IL‐8 production upon deletion of ID‐1. ID‐1 immunodepletion significantly reduced the levels of citrullinated residues in RA SF, and citrullinated ID‐1 was detected in homogenized RA ST (n = 5 samples; P < 0.05). Immunodot blot analyses revealed ACPAs to cit–ID‐1 but not to native ID‐1, in RA peripheral blood (PB) sera (n = 30 samples; P < 0.001) and SF (n = 18 samples; P < 0.05) but not in normal PB sera. Following analyses of LC‐MS/MS results for citrullination sites and corresponding reactivity in immunodot assays, we determined the critical arginines in ID‐1 for autoantigenicity: R33, R52, and R121.

Conclusion

Novel roles of ID‐1 in RA include regulation of FLS proliferation and cytokine secretion as well as autoantigenicity following citrullination.

Release of active peptidylarginine deiminase into the circulation during acute inflammation induced by coronary artery bypass surgery

Purpose: Peptidylarginine deiminase (PAD) catalyzes citrullination, a post-translational modification that can alter structure, function and antigenicity of proteins. Citrullination in the lungs due to smoking is believed to initiate an anti-citrulline immune response in rheumatoid arthritis. Citrullination in other inflamed organs has also been demonstrated, but it is not known whether smoking or inflammatory processes in general result in release of relevant amounts of PAD into the circulation with potential to cause citrullination of proteins at various anatomical sites. Coronary artery bypass grafting (CABG) using cardiopulmonary bypass (CPB) induces an acute systemic inflammation response. In the present study, we investigate whether smoking or acute systemic inflammation causes release of PAD into the circulation.

Patients and methods: This study included 36 patients with coronary heart disease (16 smokers and 20 non-smokers) undergoing CABG surgery with CPB. Circulating levels of PAD2 and PAD4, PAD activity, the neutrophil activation markers MPO, MMP-9 and lipocalin-2, the cytokines IL-6 and IL-10, and the chemokine CXCL8 were measured 2 hrs preoperatively and 2 hrs postoperatively.

Results: At baseline, serum PAD2 and PAD4 concentration did not differ between smokers and non-smokers. However, serum from non-smokers contained higher PAD activity than serum from smokers. Circulating PAD2 levels and PAD activity increased markedly in both groups after surgery, as did all neutrophil activation markers, cytokines and chemokine. PAD2 levels correlated with neutrophil activation markers, but not with cytokine and chemokine levels.

Conclusion: Blood levels of PAD2 did not differ significantly between smokers and non-smokers, but smokers had decreased PAD activity in the circulation. PAD2 levels and PAD activity increased in blood during inflammation induced by CABG with CPB. This suggests that acute inflammation, ischemia or reperfusion, or a combination of these, leads to systemic spreading of enzymatically active PAD, which may affect protein function and induce generation of citrullinated self-antigens.

An ancient mechanism of arginine-specific substrate cleavage: What's 'up' with NSP4?

The recently discovered neutrophil serine protease 4 (NSP4) is the fourth member of the NSP family, which includes the well-studied neutrophil elastase, proteinase 3 and cathepsin G. Like the other three NSP members, NSP4 is synthesized by myeloid precursors in the bone marrow and, after cleavage of the two-amino acid activation peptide, is stored as an active protease in azurophil granules of neutrophils. Based on its primary amino acid sequence, NSP4 is predicted to have a shallow S1 specificity pocket with elastase-like substrate specificity. However, NSP4 was found to preferentially cleave after an arginine residue. Structural studies resolved this paradox by revealing an unprecedented mechanism of P1-arginine recognition. In contrast to the canonical mechanism in which the P1-arginine residue points down into a deep S1 pocket, the arginine side chain adopts a surface-exposed 'up' conformation in the NSP4 active site. This conformation is stabilized by the Phe190 residue, which serves as a hydrophobic platform for the aliphatic portion of the arginine side chain, and a network of hydrogen bonds between the arginine guanidium group and the NSP4 residues Ser192 and Ser216. This unique configuration allows NSP4 to cleave even after naturally modified arginine residues, such as citrulline and methylarginine. This non-canonical mechanism, characterized by the hallmark 'triad' Phe190-Ser192-Ser216, is largely preserved throughout evolution starting with bony fish, which appeared about 400 million years ago. Although the substrates and physiological role of NSP4 remain to be determined, its remarkable evolutionary conservation, restricted tissue expression and homology to other neutrophil serine proteases anticipate a function in immune-related processes.

Progression on Citrullination of Proteins in Gastrointestinal Cancers

The citrullination modification (Cit) of proteins has received increasing attention in recent years. This kind of protein modification was first discovered in autoimmune diseases such as rheumatoid arthritis. The citrullination modification process is catalyzed by the peptidyl arginine deiminases (PADIs) family. A well-known citrullination of histone involves the key mechanism of neutrophil extracellular traps (NETs) of inflammation in the peripheral blood. Further studies revealed that citrullination modification of proteins also involves in carcinogenesis in human being. Citrullinated proteins disturbed the stability of proteins and caused DNA damages. There is increasing evidence that citrullinated proteins can be used as potential targets for cancer diagnosis or treatment. This review introduces the concept of citrullination modification of proteins, substrate proteins, examining methods and biological significances.

Differential Effects of Posttranslational Modifications of CXCL8/Interleukin-8 on CXCR1 and CXCR2 Internalization and Signaling Properties

CXCL8 or interleukin (IL)-8 directs neutrophil migration and activation through interaction with CXCR1 and CXCR2 that belong to the family of G protein-coupled receptors (GPCRs). Naturally occurring posttranslational modifications of the NH₂-terminal region of CXCL8 affect its biological activities, but the underlying molecular mechanisms are only partially understood. Here, we studied the implications of site-specific citrullination and truncation for the signaling potency of CXCL8. Native CXCL8(1-77), citrullinated [Cit5]CXCL8(1-77) and the major natural isoform CXCL8(6-77) were chemically synthesized and tested in internalization assays using human neutrophils. Citrullinated and truncated isoforms showed a moderately enhanced capacity to induce internalization of CXCR1 and CXCR2. Moreover, CXCL8-mediated activation of Gαi-dependent signaling through CXCR1 and CXCR2 was increased upon modification to [Cit5]CXCL8(1-77) or CXCL8(6-77). All CXCL8 variants promoted recruitment of β-arrestins 1 and 2 to CXCR1 and CXCR2. Compared to CXCL8(1-77), CXCL8(6-77) showed an enhanced potency to recruit β-arrestin 2 to both receptors, while for [Cit5]CXCL8(1-77) only the capacity to induce β-arrestin 2 recruitment to CXCR2 was increased. Both modifications had no biasing effect, i.e., did not alter the preference of CXCL8 to activate either Gαi-protein or β-arrestin-dependent signaling through its receptors. Our results support the concept that specific chemokine activities are fine-tuned by posttranslational modifications.

Evaluation of four clinical laboratory parameters for the diagnosis of myalgic encephalomyelitis

Background

Myalgic encephalomyelitis (ME) is a complex and debilitating disease that often initially presents with flu-like symptoms, accompanied by incapacitating fatigue. Currently, there are no objective biomarkers or laboratory tests that can be used to unequivocally diagnosis ME; therefore, a diagnosis is made when a patient meets series of a costly and subjective inclusion and exclusion criteria. The purpose of the present study was to evaluate the utility of four clinical parameters in diagnosing ME.

Methods

In the present study, we utilized logistic regression and classification and regression tree analysis to conduct a retrospective investigation of four clinical laboratory in 140 ME cases and 140 healthy controls.

Results

Correlations between the covariates ranged between [− 0.26, 0.61]. The best model included the serum levels of the soluble form of CD14 (sCD14), serum levels of prostaglandin E2 (PGE₂), and serum levels of interleukin 8, with coefficients 0.002, 0.249, and 0.005, respectively, and p-values of 3 × 10⁻⁷, 1 × 10⁻⁵, and 3 × 10⁻³, respectively.

Conclusions

Our findings show that these parameters may help physicians in their diagnosis of ME and may additionally shed light on the pathophysiology of this disease.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1696-z) contains supplementary material, which is available to authorized users.

Burning controversies in NETs and autoimmunity: The mysteries of cell death and autoimmune disease

The causes and mechanisms of autoimmune disease pose continuing challenges to the scientific community. Recent clues implicate a peculiar feature of neutrophils, their ability to release nuclear chromatin in the form of neutrophil extracellular traps (NETs), in the induction or progression of autoimmune disease. Efforts to define the beneficial versus detrimental effects of NET release have, as yet, only partially revealed mechanisms that guide this process. Evidence suggests that the process of NET release is highly regulated, but the details of regulation remain controversial and obscure. Without a better understanding of the factors that initiate and control NET formation, the judicious modification of neutrophil behaviour for medically useful purposes appears remote. We highlight gaps and inconsistencies in published work, which make NETs and their role in health and disease a puzzle that deserves more focused attention.

Pathological roles of the homeostatic chemokine CXCL12

CXCL12 is a CXC chemokine that traditionally has been classified as a homeostatic chemokine. It contributes to physiological processes such as embryogenesis, hematopoiesis and angiogenesis. In contrast to these homeostatic functions, increased expression of CXCL12 in general, or of a specific CXCL12 splicing variant has been demonstrated in various pathologies. In addition to this increased or differential transcription of CXCL12, also upregulation of its receptors CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3) contributes to the onset or progression of diseases. Moreover, posttranslational modification of CXCL12 during disease progression, through interaction with locally produced molecules or enzymes, also affects CXCL12 activity, adding further complexity. As CXCL12, CXCR4 and ACKR3 are broadly expressed, the number of pathologies wherein CXCL12 is involved is growing. In this review, the role of the CXCL12/CXCR4/ACKR3 axis will be discussed for the most prevalent pathologies. Administration of CXCL12-neutralizing antibodies or small-molecule antagonists of CXCR4 or ACKR3 delays disease onset or prevents disease progression in cancer, viral infections, inflammatory bowel diseases, rheumatoid arthritis and osteoarthritis, asthma and acute lung injury, amyotrophic lateral sclerosis and WHIM syndrome. On the other hand, CXCL12 has protective properties in Alzheimer's disease and multiple sclerosis, has a beneficial role in wound healing and has crucial homeostatic properties in general.

Peroxynitrite Exposure of CXCL12 Impairs Monocyte, Lymphocyte and Endothelial Cell Chemotaxis, Lymphocyte Extravasation in vivo and Anti-HIV-1 Activity

CXCL12 is a chemotactic cytokine that attracts many different cell types for homeostasis and during inflammation. Under stress conditions, macrophages and granulocytes produce factors such as peroxynitrite as a consequence of their oxidative response. After short incubations of CXCL12 with peroxynitrite, the gradual nitration of Tyr7, Tyr61, or both Tyr7 and Tyr61 was demonstrated with the use of mass spectrometry, whereas longer incubations caused CXCL12 degradation. Native CXCL12 and the nitrated forms, [3-NT⁶¹]CXCL12 and [3-NT^7/61]CXCL12, were chemically synthesized to evaluate the effects of Tyr nitration on the biological activity of CXCL12. All CXCL12 forms had a similar binding affinity for heparin, the G protein-coupled chemokine receptor CXCR4 and the atypical chemokine receptor ACKR3. However, nitration significantly enhanced the affinity of CXCL12 for chondroitin sulfate. Internalization of CXCR4 and β-arrestin 2 recruitment to CXCR4 was significantly reduced for [3-NT^7/61]CXCL12 compared to CXCL12, whereas β-arrestin 2 recruitment to ACKR3 was similar for all CXCL12 variants. [3-NT^7/61]CXCL12 was weaker in calcium signaling assays and in in vitro chemotaxis assays with monocytes, lymphocytes and endothelial cells. Surprisingly, nitration of Tyr61, but not Tyr7, partially protected CXCL12 against cleavage by the specific serine protease CD26. In vivo, the effects were more pronounced compared to native CXCL12. Nitration of any Tyr residue drastically lowered lymphocyte extravasation to joints compared to native CXCL12. Finally, the anti-HIV-1 activity of [3-NT⁷]CXCL12 and [3-NT^7/61]CXCL12 was reduced, whereas CXCL12 and [3-NT⁶¹]CXCL12 were equally potent. In conclusion, nitration of CXCL12 occurs readily upon contact with peroxynitrite and specifically nitration of Tyr7 fully reduces its in vitro and in vivo biological activities.

A guide to chemokines and their receptors

The chemokines (or chemotactic cytokines) are a large family of small, secreted proteins that signal through cell surface G protein-coupled heptahelical chemokine receptors. They are best known for their ability to stimulate the migration of cells, most notably white blood cells (leukocytes). Consequently, chemokines play a central role in the development and homeostasis of the immune system, and are involved in all protective or destructive immune and inflammatory responses. Classically viewed as inducers of directed chemotactic migration, it is now clear that chemokines can stimulate a variety of other types of directed and undirected migratory behavior, such as haptotaxis, chemokinesis, and haptokinesis, in addition to inducing cell arrest or adhesion. However, chemokine receptors on leukocytes can do more than just direct migration, and these molecules can also be expressed on, and regulate the biology of, many nonleukocytic cell types. Chemokines are profoundly affected by post-translational modification, by interaction with the extracellular matrix (ECM), and by binding to heptahelical 'atypical' chemokine receptors that regulate chemokine localization and abundance. This guide gives a broad overview of the chemokine and chemokine receptor families; summarizes the complex physical interactions that occur in the chemokine network; and, using specific examples, discusses general principles of chemokine function, focusing particularly on their ability to direct leukocyte migration.

Neutrophils: Beneficial and Harmful Cells in Septic Arthritis

Septic arthritis is an inflammatory joint disease that is induced by pathogens such as Staphylococcus aureus. Infection of the joint triggers an acute inflammatory response directed by inflammatory mediators including microbial danger signals and cytokines and is accompanied by an influx of leukocytes. The recruitment of these inflammatory cells depends on gradients of chemoattractants including formylated peptides from the infectious agent or dying cells, host-derived leukotrienes, complement proteins and chemokines. Neutrophils are of major importance and play a dual role in the pathogenesis of septic arthritis. On the one hand, these leukocytes are indispensable in the first-line defense to kill invading pathogens in the early stage of disease. However, on the other hand, neutrophils act as mediators of tissue destruction. Since the elimination of inflammatory neutrophils from the site of inflammation is a prerequisite for resolution of the acute inflammatory response, the prolonged stay of these leukocytes at the inflammatory site can lead to irreversible damage to the infected joint, which is known as an important complication in septic arthritis patients. Thus, timely reduction of the recruitment of inflammatory neutrophils to infected joints may be an efficient therapy to reduce tissue damage in septic arthritis.

Rheumatoid arthritis and citrullination

PURPOSE OF REVIEW

Dysregulated citrullination is a key element that drives the production and maintenance of antibodies to citrullinated proteins, a hallmark in rheumatoid arthritis (RA). This article reviews recent literature on the origin of citrullinated antigens in RA.

RECENT FINDINGS

The study of synovial fluid from patients with RA has provided important insights into the identity of citrullinated proteins that accumulate in the RA joint (the RA citrullinome) and mechanisms that control their generation.

SUMMARY

Citrullinating enzymes (peptidylarginine deiminases, PADs) are tightly controlled to limit their hyperactivation. Calcium and redox conditions are important regulators of PAD activity. Studies suggest that citrullination is dysregulated both intra- and extracellularly in RA. In neutrophils, host (i.e., perforin and the membrane attack complex) and bacterial (i.e., toxins) pore-forming proteins induce prominent calcium influx, cytolysis, and hyperactivation of PADs. These factors likely drive hypercitrullination in the RA joint and at extraarticular sites of disease initiation, respectively. As oxidizing conditions present in the extracellular environment are known to inactivate PADs, extracellular citrullination in RA probably requires the constant release of active enzymes from dying cells and may be accelerated by autoantibodies that activate PADs.

The unique structural and functional features of CXCL12

The CXC chemokine CXCL12 is an important factor in physiological and pathological processes, including embryogenesis, hematopoiesis, angiogenesis and inflammation, because it activates and/or induces migration of hematopoietic progenitor and stem cells, endothelial cells and most leukocytes. Therefore, CXCL12 activity is tightly regulated at multiple levels. CXCL12 has the unique property of existing in six splice variants in humans, each having a specific tissue distribution and in vivo activity. Controlled splice variant transcription and mRNA stability determine the CXCL12 expression profile. CXCL12 fulfills its functions in homeostatic and pathological conditions by interacting with its receptors CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3) and by binding to glycosaminoglycans (GAGs) in tissues and on the endothelium to allow a proper presentation to passing leukocytes. Homodimerizaton and heterodimerization of CXCL12 and its receptors can alter their signaling activity, as exemplified by the synergy between CXCL12 and other chemokines in leukocyte migration assays. Receptor binding may also initiate CXCL12 internalization and its subsequent removal from the environment. Furthermore, CXCL12 activity is regulated by posttranslational modifications. Proteolytic removal of NH₂- or COOH-terminal amino acids, citrullination of arginine residues by peptidyl arginine deiminases or nitration of tyrosine residues reduce CXCL12 activity. This review summarizes the interactions of CXCL12 with the cellular environment and discusses the different levels of CXCL12 activity regulation.

Glycosaminoglycan Interactions with Chemokines Add Complexity to a Complex System

Chemokines have two types of interactions that function cooperatively to control cell migration. Chemokine receptors on migrating cells integrate signals initiated upon chemokine binding to promote cell movement. Interactions with glycosaminoglycans (GAGs) localize chemokines on and near cell surfaces and the extracellular matrix to provide direction to the cell movement. The matrix of interacting chemokine–receptor partners has been known for some time, precise signaling and trafficking properties of many chemokine–receptor pairs have been characterized, and recent structural information has revealed atomic level detail on chemokine–receptor recognition and activation. However, precise knowledge of the interactions of chemokines with GAGs has lagged far behind such that a single paradigm of GAG presentation on surfaces is generally applied to all chemokines. This review summarizes accumulating evidence which suggests that there is a great deal of diversity and specificity in these interactions, that GAG interactions help fine-tune the function of chemokines, and that GAGs have other roles in chemokine biology beyond localization and surface presentation. This suggests that chemokine–GAG interactions add complexity to the already complex functions of the receptors and ligands.

Regulation of Chemokine Function: The Roles of GAG-Binding and Post-Translational Nitration

The primary function of chemokines is to direct the migration of leukocytes to the site of injury during inflammation. The effects of chemokines are modulated by several means, including binding to G-protein coupled receptors (GPCRs), binding to glycosaminoglycans (GAGs), and through post-translational modifications (PTMs). GAGs, present on cell surfaces, bind chemokines released in response to injury. Chemokines bind leukocytes via their GPCRs, which directs migration and contributes to local inflammation. Studies have shown that GAGs or GAG-binding peptides can be used to interfere with chemokine binding and reduce leukocyte recruitment. Post-translational modifications of chemokines, such as nitration, which occurs due to the production of reactive species during oxidative stress, can also alter their biological activity. This review describes the regulation of chemokine function by GAG-binding ability and by post-translational nitration. These are both aspects of chemokine biology that could be targeted if the therapeutic potential of chemokines, like CXCL8, to modulate inflammation is to be realised.