Discovery of Novel Potential Reversible Peptidyl Arginine Deiminase Inhibitor

NETs: Important players in cancer progression and therapeutic resistance

Neutrophil extracellular traps (NETs) are web-like structures composed of cytoplasmic contents, DNA chromatin and various granular proteins released by neutrophils in response to viruses, bacteria, immune complexes and cytokines. Studies have shown that NETs can promote the occurrence, development and metastasis of tumors. In this paper, the mechanism underlying the formation and degradation of NETs and the malignant biological behaviors of NETs, such as the promotion of tumor cell proliferation, epithelial mesenchymal transition, extracellular matrix remodeling, angiogenesis, immune evasion and tumor-related thrombosis, are described in detail. NETs are being increasingly studied as therapeutic targets for tumors. We have summarized strategies for targeting NETs or interfering with NET-cancer cell interactions and explored the potential application value of NETs as biomarkers in cancer diagnosis and treatment, as well as the relationship between NETs and therapeutic resistance.

Antibody discovery identifies regulatory mechanisms of protein arginine deiminase 4

Unlocking the potential of protein arginine deiminase 4 (PAD4) as a drug target for rheumatoid arthritis requires a deeper understanding of its regulation. In this study, we use unbiased antibody selections to identify functional antibodies capable of either activating or inhibiting PAD4 activity. Through cryogenic-electron microscopy, we characterized the structures of these antibodies in complex with PAD4 and revealed insights into their mechanisms of action. Rather than steric occlusion of the substrate-binding catalytic pocket, the antibodies modulate PAD4 activity through interactions with allosteric binding sites adjacent to the catalytic pocket. These binding events lead to either alteration of the active site conformation or the enzyme oligomeric state, resulting in modulation of PAD4 activity. Our study uses antibody engineering to reveal new mechanisms for enzyme regulation and highlights the potential of using PAD4 agonist and antagonist antibodies for studying PAD4-dependency in disease models and future therapeutic development.

Catalytic Antibodies May Contribute to Demyelination in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Here we report preliminary data demonstrating that some patients with myalgic encephalomyelitis/chronic fatiguesyndrome (ME/CFS) may have catalytic autoantibodies that cause the breakdown of myelin basic protein (MBP). We propose that these MBP-degradative antibodies are important to the pathophysiology of ME/CFS, particularly in the occurrence of white matter disease/demyelination. This is supported by magnetic resonance imagining studies that show these findings in patients with ME/CFS and could explain symptoms of nerve pain and muscle weakness. In this work, we performed a series of experiments on patient plasma samples where we isolated and characterized substrate-specific antibodies that digest MBP. We also tested glatiramer acetate (copaxone), an FDA approved immunomodulator to treat multiple sclerosis, and found that it inhibits ME/CFS antibody digestion of MBP. Furthermore, we found that aprotinin, which is a specific serine protease inhibitor, specifically prevents breakdown of MBP while the other classes of protease inhibitors had no effect. This coincides with the published literature describing catalytic antibodies as having serine protease-like activity. Postpandemic research has also provided several reports of demyelination in COVID-19. Because COVID-19 has been described as a trigger for ME/CFS, demyelination could play a bigger role in patient symptoms for those recently diagnosed with ME/CFS. Therefore, by studying proteolytic antibodies in ME/CFS, their target substrates, and inhibitors, a new mechanism of action could lead to better treatment and a possible cure for the disease.

Multiple Sclerosis: New Insights into Molecular Pathogenesis and Novel Platforms for Disease Treatment

BACKGROUND

Multiple sclerosis (MS), a chronic inflammatory disorder, affects the central nervous system via myelin degradation. The cause of MS is not fully known, but during recent years, our knowledge has deepened significantly regarding the different aspects of MS, including etiology, molecular pathophysiology, diagnosis and therapeutic options. Myelin basic protein (MBP) is the main myelin protein that accounts for maintaining the stability of the myelin sheath. Recent evidence has revealed that MBP citrullination or deamination, which is catalyzed by Ca2+ dependent peptidyl arginine deiminase (PAD) enzyme leads to the reduction of positive charge, and subsequently proteolytic cleavage of MBP. The overexpression of PAD2 in the brains of MS patients plays an essential role in new epitope formation and progression of the autoimmune disorder. Some drugs have recently entered phase III clinical trials with promising efficacy and will probably obtain approval in the near future. As different therapeutic platforms develop, finding an optimal treatment for each individual patient will be more challenging.

AIMS

This review provides a comprehensive insight into MS with a focus on its pathogenesis and recent advances in diagnostic methods and its present and upcoming treatment modalities.

CONCLUSION

MS therapy alters quickly as research findings and therapeutic options surrounding MS expand. McDonald's guidelines have created different criteria for MS diagnosis. In recent years, ever-growing interest in the development of PAD inhibitors has led to the generation of many reversible and irreversible PAD inhibitors against the disease with satisfactory therapeutic outcomes.

Potential peptidyl arginine deiminase type 4 inhibitors from Morinda citrifolia: a structure-based drug design approach

The World Health Organization estimates that more than 23 million individuals worldwide suffer from rheumatoid arthritis (RA), a chronic systemic autoimmune disease and experts predict that the number of RA patients may double by 2030. A substantial portion of RA patients do not respond effectively to the treatment that are already available therefore there is an urgent need of innovative new drugs. Over the past several years, Peptidyl Arginine Deiminase Type 4 (PAD4) receptors have become potential therapeutic targets for the treatment of RA. The main objective of the present study is to identify potential PAD4 inhibitors from edible fruits Morinda citrifolia. Structure based virtual screening (VS) of 60 compounds from M. citrifolia were performed to identify PAD4 inhibitors. The virtual screening of compounds resulted ten hits having XP-Glide score greater than the co-ligand (XPGS: - 8.341 kcal/mol). Three hits NF_15, NF_34, and NF_35 exhibited admirable MM-GBSA dG binding energy - 52.577, - 46.777, and - 60.711 kcal/mol, respectively. These three compounds were chosen for 100 ns molecular dynamics (MD) simulations in order to evaluate the stability and interactions. The protein-ligand complex with the highest level of stability was revealed to be NF_35. Therefore, M. citrifolia fruits may be beneficial in the treatment and prevention of rheumatoid arthritis since it contains potential hits.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-023-00147-3.

Selective inhibition of peptidyl-arginine deiminase (PAD): can it control multiple inflammatory disorders as a promising therapeutic strategy?

Peptidyl arginine deiminases (PADs) are a family of post-translational modification enzymes that irreversibly citrullinate (deiminate) arginine residues of protein and convert them to a non-classical amino acid citrulline in the presence of calcium ions. It has five isotypes, such as PAD1, PAD2, PAD3, PAD4, and PAD6, found in mammalian species. It has been suggested that increased PAD expression in various tissues contributes to the development of multiple inflammatory diseases, including rheumatoid arthritis (RA), cancer, diabetes, and neurological disorders. Elevation of PAD enzyme expression depends on several factors like rising intracellular Ca²⁺ levels, oxidative stress, and proinflammatory cytokines. PAD inhibitors originating from natural or synthetic sources can be used as a novel therapeutic approach concerning inflammatory disorders. Here, we review the pathological role of PAD in several inflammatory disorders, factors that trigger PAD expression, epigenetic role and finally, decipher the therapeutic approach of PAD inhibitors in multiple inflammatory disorders.

Peptidylarginine deiminase enzymes and citrullinated proteins in female reproductive physiology and associated diseases†

Citrullination, the post-translational modification of arginine residues, is catalyzed by the four catalytically active peptidylarginine deiminase (PAD or PADI) isozymes and alters charge to affect target protein structure and function. PADs were initially characterized in rodent uteri and, since then, have been described in other female tissues including ovaries, breast, and the lactotrope and gonadotrope cells of the anterior pituitary gland. In these tissues and cells, estrogen robustly stimulates PAD expression resulting in changes in levels over the course of the female reproductive cycle. The best-characterized targets for PADs are arginine residues in histone tails, which, when citrullinated, alter chromatin structure and gene expression. Methodological advances have allowed for the identification of tissue-specific citrullinomes, which reveal that PADs citrullinate a wide range of enzymes and structural proteins to alter cell function. In contrast to their important physiological roles, PADs and citrullinated proteins are also involved in several female-specific diseases including autoimmune disorders and reproductive cancers. Herein, we review current knowledge regarding PAD expression and function and highlight the role of protein citrullination in both normal female reproductive tissues and associated diseases.

PAD4 and Its Inhibitors in Cancer Progression and Prognosis

The systemic spread of malignancies and the risk of cancer-associated thrombosis are major clinical challenges in cancer therapy worldwide. As an important post-translational modification enzyme, peptidyl arginine deiminase 4 (PAD4) could mediate the citrullination of protein in different components (including nucleus and cytoplasm, etc.) of a variety of cells (tumor cells, neutrophils, macrophages, etc.), thus participating in gene regulation, neutrophil extracellular trap (NET) and macrophage extracellular trap (MET). Thereby, PAD4 plays an important role in enhancing the growth of primary tumors and facilitating the distant metastasis of cancer cells. In addition, it is related to the formation of cancer-associated thrombosis. Therefore, the development of PAD4-specific inhibitors may be a promising strategy for treating cancer, and it may improve patient prognosis. In this review, we describe PAD4 involvement in gene regulation, protein citrullination, and NET formation. We also discuss its potential role in cancer and cancer-associated thrombosis, and we summarize the development and application of PAD4 inhibitors.

Molecular Mechanisms of Rhabdomyolysis-Induced Kidney Injury: From Bench to Bedside

Rhabdomyolysis-induced acute kidney injury (RIAKI) occurs following damage to the muscular sarcolemma sheath, resulting in the leakage of myoglobin and other metabolites that cause kidney damage. Currently, the sole recommended clinical treatment for RIAKI is aggressive fluid resuscitation, but other potential therapies, including pretreatments for those at risk for developing RIAKI, are under investigation. This review outlines the mechanisms and clinical significance of RIAKI, investigational treatments and their specific targets, and the status of ongoing research trials.

The Role of Neutrophils as a Driver in Hepatic Ischemia-Reperfusion Injury and Cancer Growth

The innate immune system plays an essential role in the response to sterile inflammation and its association with liver ischemia and reperfusion injury (IRI). Liver IRI often manifests during times of surgical stress such as cancer surgery or liver transplantation. Following the initiation of liver IRI, stressed hepatocytes release damage-associated molecular patterns (DAMPs) which promote the infiltration of innate immune cells which then initiate an inflammatory cascade and cytokine storm. Upon reperfusion, neutrophils are among the first cells that infiltrate the liver. Within the liver, neutrophils play an important role in fueling tissue damage and tumor progression by promoting the metastatic cascade through the formation of Neutrophil Extracellular Traps (NETs). NETs are composed of web-like DNA structures containing proteins that are released in response to inflammatory stimuli in the environment. Additionally, NETs can aid in mediating liver IRI, promoting tumor progression, and most recently, in mediating early graft rejection in liver transplantation. In this review we aim to summarize the current knowledge of innate immune cells, with a focus on neutrophils, and their role in mediating IRI in mouse and human diseases, including cancer and transplantation. Moreover, we will investigate the interaction of Neutrophils with varying subtypes of other cells. Furthermore, we will discuss the role and different treatment modalities in targeting Neutrophils and NETs to prevent IRI.

Recent progress in the mechanistic understanding of NET formation in neutrophils

Neutrophils are the most abundant circulating white blood cells and one of the major cell types of the innate immune system. Neutrophil extracellular traps (NETs) are a result of the extracellular release of nuclear chromatin from the ruptured nuclear envelope and plasma membrane. The externalized chromatin is an ancient defense weapon for animals to entrap and kill microorganisms in the extracellular milieu, thus protecting animals ranging from lower invertebrates to higher vertebrates. Although the externalized chromatin has the advantage of acting as anti-infective to protect against infections, extracellular chromatin might be problematic in higher vertebrate animals as they have an adaptive immune system that can trigger further immune or autoimmune responses. NETs and their associated nuclear and/or cytoplasmic components may induce sterile inflammation, immune, and autoimmune responses, leading to various human diseases. Though important in human pathophysiology, the cellular and molecular mechanisms of NET formation (also called NETosis) are not well understood. Given that nuclear chromatin forms the backbone of NETs, the nucleus is the root of the nuclear DNA extracellular traps. Thus, nuclear chromatin decondensation, along with the rupture of nuclear envelope and plasma membrane, is required for nuclear chromatin extracellular release and NET formation. So far, most of the literature focuses on certain signaling pathways, which are involved in NET formation but without explanation of cellular events and morphological changes described above. Here, we have summarized emerging evidence and discuss new mechanistic understanding, with our perspectives, in NET formation in neutrophils.

Inhibition of PAD4-mediated NET formation by cl-amidine prevents diabetes development in nonobese diabetic mice

Many evidences indicated that neutrophil extracellular traps (NETs) play pathogenic roles in type 1 diabetes (T1D). Peptidylarginine deiminases 4 (PAD4) has been proved to be indispensable for generation of NETs. In the current study, we investigated whether oral administration of cl-amidine, an effective inhibitor of PAD4, protects non-obese diabetic (NOD) mice from T1D development. Female NOD mice were orally administrated with cl-amidine (5 μg/g body weight) from the age of 8 weeks up to 16 weeks. It showed that cl-amidine inhibit NET formation in vitro and in vivo. The onset of T1D was delayed nearly 8 weeks and the incidence of disease was significantly decreased in cl-amidine treated mice compared with the control group. Moreover, cl-amidine decreased the serum levels of anti-citrullinated peptide antibody (ACPA) and anti-neutrophil cytoplasmic antibodies (ANCA) in NOD mice. Also, it decreased generation of T1D autoantibodies such as glutamic acid decarboxylase antibody (GADA), tyrosine phosphatase-related islet antigen-2 antibody (IA2A) and zinc transporter 8 antibody (ZnT8A), which were strongly correlated with the reduced serum PAD4 and MPO-DNA levels. Furthermore, cl-amidine administration inhibited pancreatic inflammation and increased frequency of regulatory T cells in pancreatic lymph nodes (PLNs). In addition, cl-amidine improved gut barrier dysfunction and decreased the serum level of lipopolysaccharide (LPS), which was positively correlated with the NETs markers (PAD4 and MPO-DNA) and T1D autoantibody IA2A. In conclusion, our data showed that orally delivery of cl-amidine effectively prevent T1D development and suggested inhibition of PAD4-dependent NET formation as a potential way of clinical treatment in T1D.

Citrullination in the pathology of inflammatory and autoimmune disorders: recent advances and future perspectives

Numerous post-translational modifications (PTMs) govern the collective metabolism of a cell through altering the structure and functions of proteins. The action of the most prevalent PTMs, encompassing phosphorylation, methylation, acylations, ubiquitination and glycosylation is well documented. A less explored protein PTM, conversion of peptidylarginine to citrulline, is the subject of this review. The process of citrullination is catalysed by peptidylarginine deiminases (PADs), a family of conserved enzymes expressed in a variety of human tissues. Accumulating evidence suggest that citrullination plays a significant role in regulating cellular metabolism and gene expression by affecting a multitude of pathways and modulating the chromatin status. Here, we will discuss the biochemical nature of arginine citrullination, the enzymatic machinery behind it and also provide information on the pathological consequences of citrullination in the development of inflammatory diseases (rheumatoid arthritis, multiple sclerosis, psoriasis, systemic lupus erythematosus, periodontitis and COVID-19), cancer and thromboembolism. Finally, developments on inhibitors against protein citrullination and recent clinical trials providing a promising therapeutic approach to inflammatory disease by targeting citrullination are discussed.

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.

PAD Inhibitors as a Potential Treatment for SARS-CoV-2 Immunothrombosis

Since the discovery of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December 2019, the virus's dynamicity has resulted in the evolution of various variants, including the delta variant and the more novel mu variant. With a multitude of mutant strains posing as challenges to vaccine efficacy, it is critical that researchers embrace the development of pharmacotherapeutics specific to SARS-CoV-2 pathophysiology. Neutrophil extracellular traps and their constituents, including citrullinated histones, display a linear connection with thrombotic manifestations in COVID-19 patients. Peptidylarginine deiminases (PADs) are a group of enzymes involved in the modification of histone arginine residues by citrullination, allowing for the formation of NETs. PAD inhibitors, specifically PAD-4 inhibitors, offer extensive pharmacotherapeutic potential across a broad range of inflammatory diseases such as COVID-19, through mediating NETs formation. Although numerous PAD-4 inhibitors exist, current literature has not explored the depth of utilizing these inhibitors clinically to treat thrombotic complications in COVID-19 patients. This review article offers the clinical significance of PAD-4 inhibitors in reducing thrombotic complications across various inflammatory disorders like COVID-19 and suggests that these inhibitors may be valuable in treating the origin of SARS-CoV-2 immunothrombosis.

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.

RGD Peptide and PAD4 Inhibitor-Loaded Gold Nanorods for Chemo-Photothermal Combined Therapy to Inhibit Tumor Growth, Prevent Lung Metastasis and Improve Biosafety

Purpose

A targeted drug delivery system that combines protein-arginine deiminase type-4 (PAD4) inhibitors YW3-56 (356) with PTT of NPs is constructed to both decrease the accumulation of gold in metabolic organs and reduce the dose of chemotherapeutic agents.

Patients and Methods

In vitro cytotoxicity test and in vivo S180 tumor-bearing mice model were used to compare antitumor activity of 356-modified gold nanospheres and nanorods. The A549 tumor-bearing mice model was also exploited in antitumor assessment. In addition, ICP-MS, blood cell analyzer and blood biochemistry analyzer are applied for assessing the biosafety of NPs.

Results

Both 356-modified gold nanospheres and nanorods showed antitumor activity. However, 356-loaded gold nanorods are found to have better tumor inhibitory activity than 356-loaded gold nanospheres in the presence of laser and without laser irradiation. Thus, 356-loaded gold nanorods are selected to be applied for chemo-photothermal combined therapy on in vivo. We find that combination therapy could inhibit tumor growth and reduce lung tumor metastasis and inflammatory infiltration compared with individual therapy. It triggers apoptosis in tumor tissue observed by TUNEL assay and TEM pictures.

Conclusion

Thus, an RGD targeting and PAD4 inhibitor-loaded system are established based on chemo-photothermal combined therapy. It could inhibit tumor growth, prevent lung metastasis and improve biosafety.

Lipoproteins from Staphylococcus aureus Drive Neutrophil Extracellular Trap Formation in a TLR2/1- and PAD-Dependent Manner

Neutrophils, polymorphonuclear leukocytes (PMN), play a critical role in the innate immune response to Staphylococcus aureus, a pathogen that continues to be associated with significant morbidity and mortality. Neutrophil extracellular trap (NET) formation is involved in ensnaring and killing of S. aureus, but this host-pathogen interaction also leads to host tissue damage. Importantly, NET components including neutrophil proteases are under consideration as therapeutic targets in a variety of disease processes. Although S. aureus lipoproteins are recognized to activate cells via TLRs, specific mechanisms of interaction with neutrophils are poorly delineated. We hypothesized that a lipoprotein-containing cell membrane preparation from methicillin-resistant S. aureus (MRSA-CMP) would elicit PMN activation, including NET formation. We investigated MRSA-CMP-elicited NET formation, regulated elastase release, and IL-8 production in human neutrophils. We studied PMN from healthy donors with or without a common single-nucleotide polymorphism in TLR1, previously demonstrated to impact TLR2/1 signaling, and used cell membrane preparation from both wild-type methicillin-resistant S. aureus and a mutant lacking palmitoylated lipoproteins (lgt). MRSA-CMP elicited NET formation, elastase release, and IL-8 production in a lipoprotein-dependent manner. TLR2/1 signaling was involved in NET formation and IL-8 production, but not elastase release, suggesting that MRSA-CMP-elicited elastase release is not mediated by triacylated lipoproteins. MRSA-CMP also primed neutrophils for enhanced NET formation in response to a subsequent stimulus. MRSA-CMP-elicited NET formation did not require Nox2-derived reactive oxygen species and was partially dependent on the activity of peptidyl arginine deiminase (PAD). In conclusion, lipoproteins from S. aureus mediate NET formation via TLR2/1 with clear implications for patients with sepsis.

Targeting citrullination in autoimmunity: insights learned from preclinical mouse models

INTRODUCTION

Aberrant citrullination and excessive peptidylarginine deiminase (PAD) activity are detected in numerous challenging autoimmune diseases such as rheumatoid arthritis, inflammatory bowel diseases, systemic lupus erythematosus, multiple sclerosis, and type 1 diabetes. Because excessive PAD activity is a common denominator in these diseases, PADs are interesting potential therapeutic targets for future therapies.

AREAS COVERED

This review summarizes the advances made in the design of PAD inhibitors, their utilization and therapeutic potential in preclinical mouse models of autoimmunity. Relevant literature encompasses studies from 1994 to 2021 that are available on PubMed.gov.

EXPERT OPINION

Pan-PAD inhibition is a promising therapeutic strategy for autoimmune diseases. Drugs achieving pan-PAD inhibition were capable of ameliorating, reversing, and preventing clinical symptoms in preclinical mouse models. However, the implications for PADs in key biological processes potentially present a high risk for clinical complications and could hamper the translation of PAD inhibitors to the clinic. We envisage that PAD isozyme-specific inhibitors will improve the understanding the role of PAD isozymes in disease pathology, reduce the risk of side-effects and enhance prospects for future clinical translation.

The Role of Neutrophil Extracellular Traps in Periodontitis

Periodontitis is a chronic, destructive disease of periodontal tissues caused by multifaceted, dynamic interactions. Periodontal bacteria and host immunity jointly contribute to the pathological processes of the disease. The dysbiotic microbial communities elicit an excessive immune response, mainly by polymorphonuclear neutrophils (PMNs). As one of the main mechanisms of PMN immune response in the oral cavity, neutrophil extracellular traps (NETs) play a crucial role in the initiation and progression of late-onset periodontitis. NETs are generated and released by neutrophils stimulated by various irritants, such as pathogens, host-derived mediators, and drugs. Chromatin and proteins are the main components of NETs. Depending on the characteristics of the processes, three main pathways of NET formation have been described. NETs can trap and kill pathogens by increased expression of antibacterial components and identifying and trapping bacteria to restrict their spread. Moreover, NETs can promote and reduce inflammation, inflicting injuries on the tissues during the pro-inflammation process. During their long-term encounter with NETs, periodontal bacteria have developed various mechanisms, including breaking down DNA of NETs, degrading antibacterial proteins, and impacting NET levels in the pocket environment to resist the antibacterial function of NETs. In addition, periodontal pathogens can secrete pro-inflammatory factors to perpetuate the inflammatory environment and a friendly growth environment, which are responsible for the progressive tissue damage. By learning the strategies of pathogens, regulating the periodontal concentration of NETs becomes possible. Some practical ways to treat late-onset periodontitis are reducing the concentration of NETs, administering anti-inflammatory therapy, and prescribing broad-spectrum and specific antibacterial agents. This review mainly focuses on the mechanism of NETs, pathogenesis of periodontitis, and potential therapeutic approaches based on interactions between NETs and periodontal pathogens.

Innate Immune Cells and Hypertension: Neutrophils and Neutrophil Extracellular Traps (NETs)

Uncontrolled immune system activation amplifies end-organ injury in hypertension. Nonetheless, the exact mechanisms initiating this exacerbated inflammatory response, thereby contributing to further increases in blood pressure (BP), are still being revealed. While participation of lymphoid-derived immune cells has been well described in the hypertension literature, the mechanisms by which myeloid-derived innate immune cells contribute to T cell activation, and subsequent BP elevation, remains an active area of investigation. In this article, we critically analyze the literature to understand how monocytes, macrophages, dendritic cells, and polymorphonuclear leukocytes, including mast cells, eosinophils, basophils, and neutrophils, contribute to hypertension and hypertension-associated end-organ injury. The most abundant leukocytes, neutrophils, are indisputably increased in hypertension. However, it is unknown how (and why) they switch from critical first responders of the innate immune system, and homeostatic regulators of BP, to tissue-damaging, pro-hypertensive mediators. We propose that myeloperoxidase-derived pro-oxidants, neutrophil elastase, neutrophil extracellular traps (NETs), and interactions with other innate and adaptive immune cells are novel mechanisms that could contribute to the inflammatory cascade in hypertension. We further posit that the gut microbiota serves as a set point for neutropoiesis and their function. Finally, given that hypertension appears to be a key risk factor for morbidity and mortality in COVID-19 patients, we put forth evidence that neutrophils and NETs cause cardiovascular injury post-coronavirus infection, and thus may be proposed as an intriguing therapeutic target for high-risk individuals. © 2021 American Physiological Society. Compr Physiol 11:1575-1589, 2021.

Peptidylarginine Deiminase Inhibition Prevents Diabetes Development in NOD Mice

Protein citrullination plays a role in several autoimmune diseases. Its involvement in murine and human type 1 diabetes has recently been recognized through the discovery of antibodies and T-cell reactivity against citrullinated peptides. In the current study, we demonstrate that systemic inhibition of peptidylarginine deiminases (PADs), the enzymes mediating citrullination, through BB-Cl-amidine treatment, prevents diabetes development in NOD mice. This prevention was associated with reduced levels of citrullination in the pancreas, decreased circulating autoantibody titers against citrullinated glucose-regulated protein 78, and reduced spontaneous neutrophil extracellular trap formation of bone marrow-derived neutrophils. Moreover, BB-Cl-amidine treatment induced a shift from Th1 to Th2 cytokines in the serum and an increase in the frequency of regulatory T cells in the blood and spleen. In the pancreas, BB-Cl-amidine treatment preserved insulin production and was associated with a less destructive immune infiltrate characterized by reduced frequencies of effector memory CD4⁺ T cells and a modest reduction in the frequency of interferon-γ-producing CD4⁺ and CD8⁺ T cells. Our results point to a role of citrullination in the pathogenesis of autoimmune diabetes, with PAD inhibition leading to disease prevention through modulation of immune pathways. These findings provide insight in the potential of PAD inhibition for treating autoimmune diseases like type 1 diabetes.

The Role of Citrullination in Inflammatory Bowel Disease: A Neglected Player in Triggering Inflammation and Fibrosis?

Citrullination is a posttranslational modification of proteins mediated by a specific family of enzymes called peptidylarginine deiminases (PAD). Dysregulation of these enzymes is involved in the etiology of various diseases, from cancer to autoimmune disorders. In inflammatory bowel disease (IBD), data for a role of citrullination in the disease process are starting to accumulate at different experimental levels including gene expression analyses, RNA, and protein quantifications. Most data have been generated in ulcerative colitis, but data in Crohn disease are lacking so far. In addition, the citrullination of histones is the fundamental process promoting inflammation through the formation of neutrophil extracellular traps (NETs). Interestingly, NETs have also been shown to activate fibroblasts into myofibroblasts in fibrotic interstitial lung disease. Therefore, citrullination merits more thorough study in the bowel to determine its role in driving disease complications such as fibrosis. In this review we describe the process of citrullination and the different players in this pathway, the role of citrullination in autoimmunity with a special focus on IBD, the emerging role for citrullination and NETs in triggering fibrosis, and, finally, how this process could be therapeutically targeted.

Periodontal Pathogens' strategies disarm neutrophils to promote dysregulated inflammation

Periodontitis is an irreversible, chronic inflammatory disease where inflammophilic pathogenic microbial communities accumulate in the gingival crevice. Neutrophils are a major component of the innate host response against bacterial challenge, and under homeostatic conditions, their microbicidal functions typically protect the host against periodontitis. However, a number of periodontal pathogens developed survival strategies to evade neutrophil microbicidal functions while promoting inflammation, which provides a source of nutrients for bacterial growth. Research on periodontal pathogens has largely focused on a few established species: Tannerella forsythia, Treponema denticola, Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans, and Porphyromonas gingivalis. However, advances in culture-independent techniques have facilitated the identification of new bacterial species in periodontal lesions, such as the two Gram-positive anaerobes, Filifactor alocis and Peptoanaerobacter stomatis, whose characterization of pathogenic potential has not been fully described. Additionally, there is not a full understanding of the pathogenic mechanisms used against neutrophils by organisms that are abundant in periodontal lesions. This presents a substantial barrier to the development of new approaches to prevent or ameliorate the disease. In this review, we first summarize the neutrophil functions affected by the established periodontal pathogens listed above, denoting unknown areas that still merit a closer look. Then, we review the literature on neutrophil functions and the emerging periodontal pathogens, F. alocis and P. stomatis, comparing the effects of the emerging microbes to that of established pathogens, and speculate on the contribution of these putative pathogens to the progression of periodontal disease.

Citrullination of extracellular histone H3.1 reduces antibacterial activity and exacerbates its proteolytic degradation

BACKGROUND

Cystic fibrosis (CF), involves excessive airway accumulation of neutrophils, often in parallel with severe infection caused by Pseudomonas aeruginosa. Free histones are known to possess bactericidal properties, but the degree of antibacterial activity exerted on specific lung-based pathogens is largely unknown. Neutrophils have a high content of peptidyl deiminase 4 (PADI4), which citrullinate cationic peptidyl-arginines. In histone H3.1, several positions in the NH₂-terminal tail are subject to citrullination.

METHODS

Full-length and segmented histone subunit H3.1 was investigated for bactericidal activity towards P. aeruginosa (strain PAO1). PADI4-induced citrullination of histone H3.1 was assessed for antibacterial activity towards P. aeruginosa. Next, the effect of neutrophil elastase (NE)-mediated proteolysis of histone H3.1 was investigated. Finally, PADI4, H3.1, and citrullinated H3.1 were examined in healthy control and CF patient lung tissues.

RESULTS

Full-length histone H3.1 and sections of the histone H3.1 tail, displayed bactericidal activity towards P. aeruginosa. These antibacterial effects were reduced following citrullination by PADI4 or proteolysis by NE. Interestingly, citrullination of histone H3.1 exacerbated NE-mediated degradation. In CF lung tissue, citrullinated histone H3.1 and PADI4 immunoreactivity was abundant. Degraded histone H3.1 was detected in the sputum of CF patients but was absent in the sputum of healthy controls.

CONCLUSIONS

Citrullination impairs the antibacterial activity of histone H3.1 and exacerbates its proteolytic degradation by NE. Citrullination is likely to play an important role during resolution of acute inflammation. However, in chronic inflammation akin to CF, citrullination may dampen host defense and promote pathogen survival, as exemplified by P. aeruginosa.

Neutrophil extracellular traps-associated protein peptidyl arginine deaminase 4 immunohistochemical expression in ulcerative colitis and its association with the prognostic predictors

Neutrophil extracellular traps (NETs) are incriminated in several immune and inflammatory diseases including ulcerative colitis (UC). Analysis of colonic tissues for NETs-related markers in UC carries prognostic and therapeutic implications. This work aims to evaluate the immunohistochemical (IHC) expression of NETs-associated-protein arginine deaminase 4 (PAD4) in colonic biopsies from UC patients in comparison to normal colon (NC). Association between PAD4 expression level and histopathologic grade, patient's therapeutic response and other clinicopathological prognostic predictors in UC are determined. This cohort study included biopsies from 42 UC patients and 11 NC controls. Clinicopathological data including patient's age at diagnosis, gender, presenting symptoms, anatomical disease extent, extra-intestinal manifestations, type and response to therapy and surgical interventions were recorded and tabulated. Histopathological grading of disease activity and associated epithelial changes were assessed. PAD4 immunostaining was conducted using Horseradish Peroxidase technique and scored semiquantitatively considering intensity and percentage of nuclear staining of lamina propria inflammatory cells. Appropriate statistical tests were applied. Anti-PAD4 was localized mainly in the nuclei of lamina propria infiltrating neutrophils. It was expressed more significantly in UC (95.2 %) compared to NC (p 0.001). Increased PAD4 expression level was significantly associated with increasing histopathologic grade, anatomical disease extent, lacking response to therapy and subjection to radical surgery (p:0.001, = 0.038, 0.046, 0.046 respectively). Age, gender, presenting symptoms, extra-intestinal manifestations and epithelial changes showed insignificant associations. This study characterizes a subset of UC patients with high histopathological grade of activity, pancolonic involvement, strong/moderate PAD4 expression levels and who are unresponsive to routine medical therapeutic regimens rendering them candidates for radical surgery. In conjunction with histopathological grading, IHC evaluation of PAD4 in UC is recommended to guide patient's selection for targeted therapy using the novel-discovered selective PAD4 inhibitors.

Anti-Citrullinated Protein Antibodies in Patients with Rheumatoid Arthritis: Biological Effects and Mechanisms of Immunopathogenesis

Individuals with high anti-citrullinated protein antibody (ACPA) titers have an increased risk of developing rheumatoid arthritis (RA). Although our knowledge of the generation and production of ACPAs has continuously advanced during the past decade, our understanding on the pathogenic mechanisms of how ACPAs interact with immune cells to trigger articular inflammation is relatively limited. Citrullination disorders drive the generation and maintenance of ACPAs, with profound clinical significance in patients with RA. The loss of tolerance to citrullinated proteins, however, is essential for ACPAs to exert their pathogenicity. N-linked glycosylation, cross-reactivity and the structural interactions of ACPAs with their citrullinated antigens further direct their biological functions. Although questions remain in the pathogenicity of ACPAs acting as agonists for a receptor-mediated response, immune complex (IC) formation, complement system activation, crystallizable fragment gamma receptor (FcγR) activation, cross-reactivity to joint cartilage and neutrophil extracellular trap (NET)-related mechanisms have all been suggested recently. This paper presents a critical review of the characteristics and possible biological effects and mechanisms of the immunopathogenesis of ACPAs in patients with RA.

Thrombus NET content is associated with clinical outcome in stroke and myocardial infarction

OBJECTIVE

To investigate whether immune cell composition and content of neutrophil extracellular traps (NETs) in relation to clinical outcome are different between acute ischemic stroke (AIS) and acute myocardial infarction (AMI), we performed histologic analysis and correlated results with clinical and procedural parameters.

METHODS

We retrieved thrombi from patients with AIS (n = 71) and AMI (n = 72) during endovascular arterial recanalization and analyzed their immune cell composition and NET content by immunohistology. We then associated thrombus composition with procedural parameters and outcome in AIS and with cardiac function in patients with AMI. Furthermore, we compared AIS thrombi with AMI thrombi and differentiated Trial of Org 10172 in Acute Stroke Treatment classifications to address potential differences in thrombus pathogenesis.

RESULTS

Amounts of leukocytes (p = 0.133) and neutrophils (p = 0.56) were similar between AIS and AMI thrombi. Monocytes (p = 0.0052), eosinophils (p < 0.0001), B cells (p < 0.0001), and T cells (p < 0.0001) were more abundant in stroke compared with AMI thrombi. NETs were present in 100% of patients with AIS and 20.8% of patients with AMI. Their abundance in thrombi was associated with poor outcome scores in patients with AIS and with reduced ejection fraction in patients with AMI.

CONCLUSION

In our detailed histologic analysis of arterial thrombi, thrombus composition and especially abundance of leukocyte subsets differed between patients with AIS and AMI. The presence and amount of NETs were associated with patients' outcome after AIS and AMI, supporting a critical impact of NETs on thrombus stability in both conditions.

Novel biomarkers for the evaluation of aging-induced proteinopathies

Proteinopathies are characterized by aging related accumulation of misfolded protein aggregates. Irreversible covalent modifications of aging proteins may significantly affect the native three dimentional conformation of proteins, alter their function and lead to accumulation of misfolded protein as dysfunctional aggregates. Protein misfolding and accumulation of aberrant proteins are known to be associated with aging-induced proteinopathies such as amyloid ß and tau proteins in Alzheimer's disease, α-synuclein in Parkinson's disease and islet amyloid polypeptides in Type 2 diabetes mellitus. Protein oxidation processes such as S-nitrosylation, dityrosine formation and some of the newly elucidated processes such as carbamylation and citrullination recently drew the attention of researchers in the field of Gerontology. Studying over these processes and illuminating their relations between proteinopathies may help to diagnose early and even to treat age related disorders. Therefore, we have chosen to concentrate on aging-induced proteinopathic nature of these novel protein modifications in this review.

Increased Circulating Levels of Neutrophil Extracellular Traps During Cardiopulmonary Bypass

Background

The intensity of inflammatory response triggered by cardiopulmonary bypass (CPB) during cardiac surgery has been associated with adverse outcomes. Neutrophils might contribute to organ injury through the liberation of DNA histone-based structures named “neutrophil extracellular traps” (NETs). Our objective was to assess circulating NETs levels before and after cardiac surgery in low-risk and high-risk patients.

Methods

This prospective cohort study included 2 groups of patients undergoing elective cardiac surgery with the use of CPB. The first group consisted of low-risk patients (European System for Cardiac Operative Risk Evaluation II ≤ 1%), and the second group included high-risk patients (European System for Cardiac Operative Risk Evaluation II ≥ 5%). Blood samples were drawn pre-CPB and 3 hours post-CPB separation. Measurements of circulating NETs, interleukin-6, C-reactive protein, myeloperoxidase, citrullinated histone 3, and pentraxin-related protein 3 levels were performed at each time point.

Results

Twenty-four patients, 12 high-risk and 12 low-risk patients, were included. Circulating NETs measurements changed from a median of 0.054 before CPB to 0.084 at 3 hours post-CPB separation, with a median increase of 0.037 (P < 0.001) per patient. No difference was noted between the high-risk and low-risk groups. A linear relationship was found between the circulating NETs measurements 3 hours post-CPB and CPB duration (ß = 0.047; confidence interval, 0.012-0.081; P = 0.01 R² = 0.27). A correlation was found between the change in NETs with citrullinated histone 3 and myeloperoxidase levels, but not between NETs and other inflammatory biomarkers.

Conclusions

Circulating NETs measurements increases during cardiac surgery with postsurgical levels proportional to CPB duration. The clinical significance of NETs production during cardiac surgery should be further investigated.

An Overview of the Intrinsic Role of Citrullination in Autoimmune Disorders

A protein undergoes many types of posttranslation modification. Citrullination is one of these modifications, where an arginine amino acid is converted to a citrulline amino acid. This process depends on catalytic enzymes such as peptidylarginine deiminase enzymes (PADs). This modification leads to a charge shift, which affects the protein structure, protein-protein interactions, and hydrogen bond formation, and it may cause protein denaturation. The irreversible citrullination reaction is not limited to a specific protein, cell, or tissue. It can target a wide range of proteins in the cell membrane, cytoplasm, nucleus, and mitochondria. Citrullination is a normal reaction during cell death. Apoptosis is normally accompanied with a clearance process via scavenger cells. A defect in the clearance system either in terms of efficiency or capacity may occur due to massive cell death, which may result in the accumulation and leakage of PAD enzymes and the citrullinated peptide from the necrotized cell which could be recognized by the immune system, where the immunological tolerance will be avoided and the autoimmune disorders will be subsequently triggered. The induction of autoimmune responses, autoantibody production, and cytokines involved in the major autoimmune diseases will be discussed.

Peptidylarginine deiminase 4 (PAD4) activity in early rheumatoid arthritis

Objectives: Peptidylarginine deiminases (PADs) are a family of calcium-dependent enzymes catalysing the conversion of arginine residues to citrulline, which may constitute a risk factor in rheumatoid arthritis (RA) pathogenesis. We investigated PAD activation by serum (PAD_Act) in early RA, and the associations between PAD activation and disease characteristics, treatment response, and progression of radiographic damage.Method: Sera from disease-modifying anti-rheumatic drug (DMARD)-naïve early RA patients (n = 225), classified according to the 2010 American College of Rheumatology/European League Against Rheumatism criteria, and healthy controls (n = 63) were analysed for PAD4 activating capacity at 0, 3, 12, and 24 months using a high-performance liquid chromatography fluorometric method. Associations for PAD_Act were evaluated by Mann-Whitney U and chi-squared tests. Changes in PAD_Act levels were compared using the Wilcoxon signed-rank test.Results: PAD_Act positivity occurred in 42% (n = 95) of the patients and was more prevalent in anti-citrullinated protein antibody (ACPA)-positive vs ACPA-negative patients (47% vs 20%, p = 0.002), but not in rheumatoid factor (RF)-positive vs RF-negative patients (44% vs 38%, p = 0.49). PAD_Act-positive were younger than PAD_Act-negative patients [mean ± sd 48.7 ± 13.5 vs 53.2 ± 13.7 years, p = 0.011]. Median [25th, 75th percentile] PAD_Act levels were higher in patients than in controls (8768 [7491, 11 393] vs 7046 [6347, 7906], p < 0.0001) and decreased after initiation of DMARD treatment, but were not associated with treatment response or progression of radiographic damage after 2 years of follow-up.Conclusion: Serum capacity to activate PAD4 was associated with ACPA and RF positivity and earlier disease onset in early RA patients, and decreased after initiation of DMARD treatment, indicating that anti-PAD treatment could potentially be beneficial in RA.

Neutrophil Extracellular Traps in the Establishment and Progression of Renal Diseases

Uncontrolled inflammatory and immune responses are often involved in the development of acute and chronic forms of renal injury. Neutrophils are innate immune cells recruited early to sites of inflammation, where they produce pro-inflammatory cytokines and release mesh-like structures comprised of DNA and granular proteins known as neutrophil extracellular traps (NETs). NETs are potentially toxic, contribute to glomerular injury, activate autoimmune processes, induce vascular damage, and promote kidney fibrosis. Evidence from multiple studies suggests that an imbalance between production and clearance of NETs is detrimental for renal health. Hence strategies aimed at modulating NET-associated processes could have a therapeutic impact on a myriad of inflammatory diseases that target the kidney. Here, we summarize the role of NETs in the pathogenesis of renal diseases and their mechanisms of tissue damage.