Inhibition of PAD2 Improves Survival in a Mouse Model of Lethal LPS-Induced Endotoxic Shock

Analysis of the diagnostic and prognostic value of serum PAD2 in patients with sepsis in the intensive care unit

BACKGROUND

Sepsis is a common disease in the intensive care unit (ICU). In recent years, the incidence rate and mortality rate remain high. Early diagnosis of sepsis is crucial for treatment and can effectively reduce mortality. So far, the ability of serum peptidylarginine deaminase 2 (PAD2) in the diagnosis and prognosis of sepsis patients is still unclear. We conducted this study to reveal the clinical value of PAD2 as a biomarker for sepsis patients.

METHODS

A prospective study method was used to select 207 patients in the ICU of Renmin Hospital of Wuhan University from May 2022 to May 2023. They were divided into the sepsis group (n = 135) and control group (n = 72), and data were collected within 24 h of hospitalization. Sepsis patients were divided into a survival group (n = 80) and a non-survival group (n = 55) based on their 28-day survival status. Using statistical methods to evaluate the diagnostic and prognostic value of PAD2 in sepsis.

RESULTS

The serum PAD2 concentrations in the sepsis group were significantly higher than in the control group (median 16.70 vs 35.32 ng/ml, P < 0.001). Multivariate logistic regression analysis showed that the Quick Sequential Organ Failure Assessment (qSOFA), C-reactive protein (CRP), procalcitonin (PCT), and PAD2 were independent risk factors for sepsis. The Receiver operating characteristic (ROC) curve showed that the combined diagnostic value of qSOFA, CRP, PCT, and PAD2 was the highest. The serum PAD2 concentrations in the non-survival group of patients with sepsis were significantly higher than those in the survival group (median 29.26 vs 50.08 ng/ml, P < 0.05). The COX regression analysis showed that PAD2, sequential organ failure score Assessment (SOFA) score, and Acute Physiology and Chronic Health Evaluation (APACHE II) score were independent factors affecting the prognosis of sepsis patients. The ROC analysis showed that the combined prognostic value of PAD2, SOFA, and APACHE II scores was significantly higher than any single indicator. The Kaplan-Meier survival analysis showed that patients with PAD2 ≤ 48.62 ng/ml had a better prognosis.

CONCLUSION

The significant increase in serum PAD2 concentrations in patients is an independent risk factor affecting the occurrence of sepsis and 28-day mortality. The combination of PAD2 and other indicators can further improve the diagnostic and prognostic value for ICU sepsis patients.

A Narrative Review: The Role of NETs in Acute Respiratory Distress Syndrome/Acute Lung Injury

Nowadays, acute respiratory distress syndrome (ARDS) still has a high mortality rate, and the alleviation and treatment of ARDS remains a major research focus. There are various causes of ARDS, among which pneumonia and non-pulmonary sepsis are the most common. Trauma and blood transfusion can also cause ARDS. In ARDS, the aggregation and infiltration of neutrophils in the lungs have a great influence on the development of the disease. Neutrophils regulate inflammatory responses through various pathways, and the release of neutrophils through neutrophil extracellular traps (NETs) is considered to be one of the most important mechanisms. NETs are mainly composed of DNA, histones, and granuloproteins, all of which can mediate downstream signaling pathways that can activate inflammatory responses, generate immune clots, and cause damage to surrounding tissues. At the same time, the components of NETs can also promote the formation and release of NETs, thus forming a vicious cycle that continuously aggravates the progression of the disease. NETs are also associated with cytokine storms and immune balance. Since DNA is the main component of NETs, DNase I is considered a viable drug for removing NETs. Other therapeutic methods to inhibit the formation of NETs are also worthy of further exploration. This review discusses the formation and mechanism of NETs in ARDS. Understanding the association between NETs and ARDS may help to develop new perspectives on the treatment of ARDS.

Neutrophil extracellular traps and their implications in airway inflammatory diseases

Neutrophil extracellular traps (NETs) are essential for immune defense and have been increasingly recognized for their role in infection and inflammation. In the context of airway inflammatory diseases, there is growing evidence suggesting the involvement and significance of NETs. This review aims to provide an overview of the formation mechanisms and components of NETs and their impact on various airway inflammatory diseases, including acute lung injury/ARDS, asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis. By understanding the role of NETs in airway inflammation, we can gain valuable insights into the underlying pathogenesis of these diseases and identify potential targets for future therapeutic strategies that either target NETs formation or modulate their harmful effects. Further research is warranted to elucidate the complex interactions between NETs and airway inflammation and to develop targeted therapies that can effectively mitigate their detrimental effects while preserving their beneficial functions in host defense.

Neutrophil extracellular traps regulating tumorimmunity in hepatocellular carcinoma

As a component of the innate immune system, there is emerging evidence to suggest that neutrophils may play a critical role in the initiation and progression of hepatocellular carcinoma (HCC). Neutrophil extracellular traps (NETs) are web-like chromatin structures that protrude from the membranes during neutrophil activation. Recent research has shown that NETs, which are at the forefront of the renewed interest in neutrophil studies, are increasingly intertwined with HCC. By exploring the mechanisms of NETs in HCC, we aim to improve our understanding of the role of NETs and gain deeper insights into neutrophil biology. Therefore, this article provides a summary of key findings and discusses the emerging field of NETs in HCC.

Peptidylarginine deiminase 4 and ADAMTS13 activity in Staphylococcus aureus bacteraemia

Staphylococcus aureus infection is associated with increased levels of neutrophil extracellular traps (NETs) and von Willebrand factor (VWF), and with reduced activity of ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13). Peptidylarginine deiminase 4 (PAD4) contributes to NET formation and inactivates ADAMTS13 in vitro. The role of PADs in the dynamics of NETs, VWF and ADAMTS13 has not yet been studied. We thus aimed to assess the longitudinal evolution of NETs, PADs, VWF and ADAMTS13 activity in S. aureus infection. Plasma samples from S. aureus bacteraemia patients were longitudinally collected and analysed for NETs, PAD4/PAD2, VWF and ADAMTS13 activity. Correlation analyses with clinical data were performed. Recombinant PAD4 and S. aureus were assessed in vitro for their potential to modulate ADAMTS13 activity. Sixty-seven patients were included. Plasma levels of NETs, VWF, PAD4 and PAD2 were increased and ADAMTS13 activity was decreased. Levels of PADs were negatively correlated with ADAMTS13 activity. NETs were positively correlated with PADs, and negatively with ADAMTS13 activity. In vitro, recombinant PAD4 but not S. aureus reduced ADAMTS13 activity in plasma. Levels of PAD4 and PAD2 correlate with reduced ADAMTS13 activity, with neutrophils as the likely source of PAD activity in S. aureus bacteraemia. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Small molecule activates citrullination through targeting PAD2

Citrullination is a post-translational modification catalysed by peptidyl arginine deiminase (PAD) enzymes, and dysregulation of protein citrullination is involved in various pathological disorders. During the past decade, a panel of citrullination inhibitors has been developed, while small molecules activating citrullination have rarely been reported so far. In this study, we screened citrullination activator using an antibody against citrullinated histone H3 (cit-H3), and a natural compound demethoxycurcumin (DMC) significantly activated citrullination. The requirement of PAD2 for DMC-activated citrullination was confirmed by a loss of function assay. Notably, DMC directly engaged with PAD2, and showed binding selectivity among PAD family enzymes. Point mutation assay indicated that residue E352 is essential for DMC targeting PAD2. Consistently, DMC induced typical phenotypes of cells with dysregulation of PAD2 activity, including citrullination-associated cell apoptosis and DNA damage. Overall, our study not only presents a strategy for rationally screening citrullination activators, but also provides a chemical approach for activating protein citrullination. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Protein citrullination: inhibition, identification and insertion

Protein citrullination is a post-translational modification (PTM) that is catalysed by the protein arginine deiminase (PAD) family of enzymes. This PTM involves the transformation of an arginine residue into citrulline. Protein citrullination is associated with several physiological processes, including the epigenetic regulation of gene expression, neutrophil extracellular trap formation and DNA damage-induced apoptosis. Aberrant protein citrullination is relevant to several autoimmune and neurodegenerative diseases and certain forms of cancer. PAD inhibitors have shown remarkable efficacy in a range of diseases including rheumatoid arthritis (RA), lupus, atherosclerosis and ulcerative colitis. In RA, anti-citrullinated protein antibodies can be detected prior to disease onset and are thus a valuable diagnostic tool for RA. Notably, citrullinated proteins may serve more generally as biomarkers of specific disease states; however, the identification of citrullinated protein residues remains challenging owing to the small 1 Da mass change that occurs upon citrullination. Herein, we highlight the progress made so far in the development of pan-PAD and isozyme selective inhibitors as well as the identification of citrullinated proteins and the site-specific incorporation of citrulline into proteins. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Myelodysplastic neoplasm-associated U2AF1 mutations induce host defense defects by compromising neutrophil chemotaxis

Myelodysplastic neoplasm (MDS) is a hematopoietic stem cell disorder that may evolve into acute myeloid leukemia. Fatal infection is among the most common cause of death in MDS patients, likely due to myeloid cell cytopenia and dysfunction in these patients. Mutations in genes that encode components of the spliceosome represent the most common class of somatically acquired mutations in MDS patients. To determine the molecular underpinnings of the host defense defects in MDS patients, we investigated the MDS-associated spliceosome mutation U2AF1-S34F using a transgenic mouse model that expresses this mutant gene. We found that U2AF1-S34F causes a profound host defense defect in these mice, likely by inducing a significant neutrophil chemotaxis defect. Studies in human neutrophils suggest that this effect of U2AF1-S34F likely extends to MDS patients as well. RNA-seq analysis suggests that the expression of multiple genes that mediate cell migration are affected by this spliceosome mutation and therefore are likely drivers of this neutrophil dysfunction.

Development of LB244, an Irreversible STING Antagonist

The cGMP-AMP Synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway plays a critical role in sensing dsDNA localized to the cytosol, resulting in the activation of a robust inflammatory response. While cGAS-STING signaling is essential for antiviral immunity, aberrant STING activation is observed in amyotrophic lateral sclerosis (ALS), lupus, and autoinflammatory diseases such as Aicardi-Goutières syndrome (AGS) and STING associated vasculopathy with onset in infancy (SAVI). Significant efforts have therefore focused on the development of STING inhibitors. In a concurrent submission, we reported that BB-Cl-amidine inhibits STING-dependent signaling in the nanomolar range, both in vitro and in vivo. Considering this discovery, we sought to generate analogs with higher potency and proteome-wide selectivity. Herein, we report the development of LB244, which displays nanomolar potency and inhibits STING signaling with markedly enhanced proteome-wide selectivity. Moreover, LB244 mirrored the efficacy of BB-Cl-amidine in vivo. In summary, our data identify novel chemical entities that inhibit STING signaling and provide a scaffold for the development of therapeutics for treating STING-dependent inflammatory diseases.

Role and Therapeutic Targeting Strategies of Neutrophil Extracellular Traps in Inflammation

Neutrophil extracellular traps (NETs) are large DNA reticular structures secreted by neutrophils and decorated with histones and antimicrobial proteins. As a key mechanism for neutrophils to resist microbial invasion, NETs play an important role in the killing of microorganisms (bacteria, fungi, and viruses). Although NETs are mostly known for mediating microbial killing, increasing evidence suggests that excessive NETs induced by stimulation of physical and chemical components, microorganisms, and pathological factors can exacerbate inflammation and organ damage. This review summarizes the induction and role of NETs in inflammation and focuses on the strategies of inhibiting NETosis and the mechanisms involved in pathogen evasion of NETs. Furthermore, herbal medicine inhibitors and nanodelivery strategies improve the efficiency of inhibition of excessive levels of NETs.

PAD2: A potential target for tumor therapy

Peptide arginine deiminase 2(PAD2) catalyzes the conversion of arginine residues on target proteins to citrulline residues in the presence of calcium ions. This particular posttranslational modification is called citrullination. PAD2 can regulate the transcriptional activity of genes through histone citrullination and nonhistone citrullination. In this review, we summarize the evidence from recent decades and systematically illustrate the role of PAD2-mediated citrullination in tumor pathology and the regulation of tumor-associated immune cells such as neutrophils, monocytes, macrophages and T cells. Several PAD2-specific inhibitors are also presented to discuss the feasibility of anti-PAD2 therapy to treat tumors and the urgent problems to be solved. Finally, we review some recent developments in the development of PAD2 inhibitors.

ROLE OF PEPTIDYLARGININE DEIMINASE AND NEUTROPHIL EXTRACELLULAR TRAPS IN INJURIES: FUTURE NOVEL DIAGNOSTICS AND THERAPEUTIC TARGETS

ABSTRACT

Injuries lead to an early systemic inflammatory state with innate immune system activation. Neutrophil extracellular traps (NETs) are a complex of chromatin and proteins released from the activated neutrophils. Although initially described as a response to bacterial infections, NETs have also been identified in the sterile postinjury inflammatory state. Peptidylarginine deiminases (PADs) are a group of isoenzymes that catalyze the conversion of arginine to citrulline, termed citrullination or deimination. PAD2 and PAD4 have been demonstrated to play a role in NET formation through citrullinated histone 3. PAD2 and PAD4 have a variety of substrates with variable organ distribution. Preclinical and clinical studies have evaluated the role of PADs and NETs in major trauma, hemorrhage, burns, and traumatic brain injury. Neutrophil extracellular trap formation and PAD activation have been shown to contribute to the postinjury inflammatory state leading to a detrimental effect on organ systems. This review describes our current understanding of the role of PAD and NET formation following injury and burn. This is a new field of study, and the emerging data appear promising for the future development of targeted biomarkers and therapies in trauma.

Neutrophil extracellular traps in urinary tract infection

Background

Urinary tract infections (UTI) are common types of bacterial infection in children. UTI treatment is aimed to prevent complications including hypertension, proteinuria, and progression to chronic kidney disease. Activated neutrophils release chromatin-based structures associated with antimicrobial proteins called neutrophil extracellular traps (NETs). We aimed to describe the role of NET-associated markers in children with UTI as well as the role of NETs formation in a mouse model of UTI.

Materials and methods

Markers of NETs including extracellular DNA (ecDNA), myeloperoxidase (MPO) and cathelicidin were analyzed in children with febrile UTI caused by E. coli (n = 98, aged 0.3-1.3 years) and in healthy controls (n = 50, 0.5-5.2 years). Moreover, an acute experimental model of UTI was performed on PAD4 knock-out mice with diminished NETs formation (n = 18), and on wild-type mice (n = 15).

Results

Children with UTI had significantly higher urinary NETs markers including total ecDNA, nuclear DNA and mitochondrial DNA, altogether with MPO and cathelicidin. The concentrations of MPO and cathelicidin positively correlated with ecDNA (r = 0.53, p ≤ 0.001; r = 0.56, p ≤ 0.001, respectively) and the number of leukocytes in the urine (r = 0.29, p ≤ 0.05; r = 0.27, p ≤ 0.05, respectively). Moreover, urinary MPO was positively associated with cathelicidin (r = 0.61, p ≤ 0.001). In the experimental model, bacterial load in the bladder (20-fold) and kidneys (300-fold) was significantly higher in PAD4 knock-out mice than in wild-type mice.

Conclusion

Higher urinary NETs makers-ecDNA, MPO and cathelicidin and their correlation with leukocyturia in children with UTI confirmed our hypothesis about the association between NETs and UTI in children. Higher bacterial load in mice with diminished NETs formation suggests that NETs are not only a simple consequence of UTI, but might play a direct role in the prevention of pyelonephritis and other UTI complications.

Role of the PADI family in inflammatory autoimmune diseases and cancers: A systematic review

The peptidyl arginine deiminase (PADI) family is a calcium ion-dependent group of isozymes with sequence similarity that catalyze the citrullination of proteins. Histones can serve as the target substrate of PADI family isozymes, and therefore, the PADI family is involved in NETosis and the secretion of inflammatory cytokines. Thus, the PADI family is associated with the development of inflammatory autoimmune diseases and cancer, reproductive development, and other related diseases. In this review, we systematically discuss the role of the PADI family in the pathogenesis of various diseases based on studies from the past decade to provide a reference for future research.

Regulating Neutrophil PAD4/NOX-Dependent Cerebrovasular Thromboinflammation

Background: Neutrophil extracellular trap (NET) production has been implicated in the pathogenesis of thromboinflammatory conditions such as Sickle Cell Disease (SCD), contributing to heightened risk for ischemic stroke. NETs are catalyzed by the enzyme Peptidyl Arginine Deiminase 4 (PAD4) and neutrophil derived reactive oxygen species (ROS), especially NADPH oxidase (NOX) which interacts with PAD4 and is therefore critical for neutrophil function. However, the role that NOX-dependent ROS and NETs play in the accelerated cerebral microvascular thrombosis associated with thromboinflammatory conditions, such as SCD, has not been fully elucidated and is the aim of this study. Methods: The in-vitro effects of targeting PAD4 and NOX were examined using physiologically relevant NET assays with neutrophils isolated from healthy volunteers (control) and SCD patients. In addition, in-vivo intravascular effects of targeting PAD4 and NOX in the cerebral microcirculation of C57BL/6 and sickle transgenic mice (STM) were assessed using a photoactivation thrombosis model (light/dye) coupled with real-time fluorescence intravital microscopy. Results: We found that targeting PAD4 and NOX in human neutrophils significantly inhibited ionomycin dependent H3cit⁺ neutrophils. Targeting PAD4 and NOX in-vivo resulted in prolonged blood flow cessation in cerebrovascular arterioles as well as venules. Moreover, we were able to replicate the effects of PAD4 and NOX targeting in a clinical model of accelerated thromboinflammation by increasing blood flow cessation times in cerebral microvessels in STM. These findings concurred with the clinical setting i.e. neutrophils isolated from SCD patients, which possessed an attenuation of H3cit⁺ neutrophil production on targeting PAD4 and NOX. Conclusions: Taken together, our compelling data suggests that PAD4 and NOX play a significant role in neutrophil driven thromboinflammation. Targeting PAD4 and NOX limits pathological H3cit⁺ neutrophils, which may further explain attenuation of cerebral thrombosis. Overall, this study presents a viable pre-clinical model of prevention and management of thromboinflammatory complications such as ischemic stroke.

PADs and NETs in digestive system: From physiology to pathology

Peptidylarginine deiminases (PADs) are the only enzyme class known to deiminate arginine residues into citrulline in proteins, a process known as citrullination. This is an important post-translational modification that functions in several physiological and pathological processes. Neutrophil extracellular traps (NETs) are generated by NETosis, a novel cell death in neutrophils and a double-edged sword in inflammation. Excessive activation of PADs and NETs is critically implicated in their transformation from a physiological to a pathological state. Herein, we review the physiological and pathological functions of PADs and NETs, in particular, the involvement of PAD2 and PAD4 in the digestive system, from inflammatory to oncological diseases, along with related therapeutic prospects.

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.

Human Amniotic Membrane Mesenchymal Stem Cell-Synthesized PGE2 Exerts an Immunomodulatory Effect on Neutrophil Extracellular Trap in a PAD-4-Dependent Pathway through EP2 and EP4

Human amniotic membrane mesenchymal stem cells (hAM-MSC) secrete a myriad of components with immunosuppressive activities. In the present research, we aimed to describe the effect of prostaglandin E₂ (PGE₂) secreted by hAM-MSCs on neutrophil extracellular trap (NET) release and to characterize the role of its receptors (EP2/EP4) in PAD-4 and NFκB activity in neutrophils. Human peripheral blood neutrophils were ionomycin-stimulated in the presence of hAM-MSC conditioned medium (CM) treated or not with the selective PGE₂ inhibitor MF-63, PGE₂, EP2/EP4 agonists, and the selective PAD-4 inhibitor GSK-484. NET release, PAD-4, and NFκB activation were analyzed. Ionomycin induced NET release, which was inhibited in the presence of hAM-MSC-CM, while CM from hAM-MSCs treated with MF-63 prevented NET release inhibition. PGE₂ and EP2/EP4 agonists, and GSK-484 inhibited NET release. EP2/EP4 agonists and GSK-484 inhibited H3-citrullination but did not affect PAD-4 protein expression. Finally, PGE₂ and EP2/EP4 agonists and GSK-484 increased NFκB phosphorylation. Taken together, these results suggest that hAM-MSC exert their immunomodulatory activities through PGE_2, inhibiting NET release in a PAD-4-dependent pathway. This research proposes a new mechanism by which hAM-MSC exert their activities when modulating the innate immune response and inhibiting NET release.

Extracellular DNA Traps: Origin, Function and Implications for Anti-Cancer Therapies

Extracellular DNA may serve as marker in liquid biopsies to determine individual diagnosis and prognosis in cancer patients. Cell death or active release from various cell types, including immune cells can result in the release of DNA into the extracellular milieu. Neutrophils are important components of the innate immune system, controlling pathogens through phagocytosis and/or the release of neutrophil extracellular traps (NETs). NETs also promote tumor progression and metastasis, by modulating angiogenesis, anti-tumor immunity, blood clotting and inflammation and providing a supportive niche for metastasizing cancer cells. Besides neutrophils, other immune cells such as eosinophils, dendritic cells, monocytes/macrophages, mast cells, basophils and lymphocytes can also form extracellular traps (ETs) during cancer progression, indicating possible multiple origins of extracellular DNA in cancer. In this review, we summarize the pathomechanisms of ET formation generated by different cell types, and analyze these processes in the context of cancer. We also critically discuss potential ET-inhibiting agents, which may open new therapeutic strategies for cancer prevention and treatment.

Neutrophil extracellular traps are associated with enhanced procoagulant activity in liver cirrhosis patients with portal vein thrombosis

OBJECTIVE

Patients with liver cirrhosis (LC) commonly exhibit hypercoagulability and tend to develop thrombosis. Neutrophil extracellular traps (NETs) are associated with a variety of thrombotic conditions, but their possible value in portal vein thrombosis (PVT) is not known. We assessed whether NETs promote thrombosis and contribute to the procoagulant state in patients with LC.

METHODS

The circulating levels of NETs markers (myeloperoxidase, neutrophil elastase, citrullinated histone H3) were measured in 72 patients (median age, 55 years; 48 [66.7%] men) with LC from September 2020 to February 2021. Then they were divided into two groups: patients with or without PVT. NETs procoagulant activity was assessed based on thrombin-antithrombin complex (TAT complex) and Factor X. The levels of plasma markers were determined by ELISA.

RESULTS

There were 28 patients with PVT and 44 patients without PVT. The levels of NETs markers and hypercoagulability markers in the plasma of cirrhosis patients with PVT were significantly higher than those of cirrhosis patients without PVT (p < 0.05). Additionally, the levels of the NETs markers correlated with TAT complex and Factor X (Spearman correlation rho >0.73, p < 0.0001).

CONCLUSIONS

Neutrophil extracellular traps seem to enhance procoagulant activity in LC patients with PVT; thus, they may be a practical predictor of PVT as well as a rapid and easy-to-use diagnostic and treatment guide for PVT in patients with cirrhosis.

Dystrophin deficiency affects human astrocyte properties and response to damage

In addition to progressive muscular degeneration due to dystrophin mutations, 1/3 of Duchenne muscular dystrophy (DMD) patients present cognitive deficits. However, there is currently an incomplete understanding about the function of the multiple dystrophin isoforms in human brains. Here, we tested the hypothesis that dystrophin deficiency affects glial function in DMD and could therefore contribute to neural impairment. We investigated human dystrophin isoform expression with development and differentiation and response to damage in human astrocytes from control and induced pluripotent stem cells from DMD patients. In control cells, short dystrophin isoforms were up-regulated with development and their expression levels changed differently upon neuronal and astrocytic differentiation, as well as in 2-dimensional versus 3-dimensional astrocyte cultures. All DMD-astrocytes tested displayed altered morphology, proliferative activity and AQP4 expression. Furthermore, they did not show any morphological change in response to inflammatory stimuli and their number was significantly lower as compared to stimulated healthy astrocytes. Finally, DMD-astrocytes appeared to be more sensitive than controls to oxidative damage as shown by their increased cell death. Behavioral and metabolic defects in DMD-astrocytes were consistent with gene pathway dysregulation shared by lines with different mutations as demonstrated by bulk RNA-seq analysis. Together, our DMD model provides evidence for altered astrocyte function in DMD suggesting that defective astrocyte responses may contribute to neural impairment and might provide additional potential therapeutic targets.

Circulating CitH3 Is a Reliable Diagnostic and Prognostic Biomarker of Septic Patients in Acute Pancreatitis

Purpose

Acute pancreatitis (AP) is an inflammatory disease. AP starts with sterile inflammation and is often complicated with critical local or systemic infection or sepsis in severe cases. Septic AP activates peptidyl arginine deiminase (PAD) and citrullinates histone H3 (CitH3), leading to neutrophil extracellular trap (NET) formation. Investigating the role of NETs and underlying mechanisms in septic AP may facilitate developing diagnostic and therapeutic approaches. In this study, we sought to identify the expression of CitH3 in septic AP patients and to analyze the correlation of CitH3 concentration with NET components as well as clinical outcomes.

Methods

Seventy AP patients with or without sepsis (40 septic cases, 30 nonseptic cases) and 30 healthy volunteers were recruited in this study. Concentration of NET components (CitH3 and double-strain DNA) and key enzymes (PAD2/4) were measured. Clinical and laboratory characteristics of patients were recorded and analyzed.

Results

Levels of CitH3 were elevated significantly in septic AP patients compared with those in nonseptic AP and healthy volunteers. The area under the curve (AUC, 95% confidence interval) for diagnosing septic AP was 0.93 (0.86–1.003), and the cutoff was 43.05 pg/ml. Among septic AP cases (n = 40), the concentration of CitH3 was significantly increased in those who did not survive or were admitted to the intensive care unit, when compared with that in those who survived or did not require intensive care unit. Association analysis revealed that CitH3 concentration was positively correlated with PAD2, PAD4, dsDNA concentration, and Sequential Organ Failure Assessment scores.

Conclusion

CitH3 concentration increased in septic AP patients and was closely correlated with disease severity and clinical outcomes. CitH3 may potentially be a diagnostic and prognostic biomarker of septic AP.

Interaction Between Macrophage Extracellular Traps and Colon Cancer Cells Promotes Colon Cancer Invasion and Correlates With Unfavorable Prognosis

Background

Macrophage extracellular traps (METs) and tumor-infiltrating macrophages contribute to the progression of several diseases. But the role of METs and tumor-infiltrating macrophages in colon cancer (CC) has not been illuminated. In this study, we aimed to clarify the prognostic value of METs for CC patients and to explore the interaction between CC cells and METs in vitro and in vivo.

Methods

A training cohort consisting of 116 patients and a validation cohort of 94 patients were enrolled in this study. Immunofluorescence (IF) staining was conducted to determine METs formation in CC patients. Cox regression was used to perform prognostic analysis and screen out the best prognostic model. A nomogram was established to predict 5-year overall survival (OS). The correlation between METs with clinicopathological features and inflammatory markers was analyzed. The formation of METs in vitro was detected by SYTOX® green and IF staining, and the effect of METs on CC cells was detected by transwell assays. PAD2-IN-1, a selective inhibitor of peptidylarginine deiminase 2 (PAD2), was introduced to destroy the crosstalk between CC cells and METs in vitro and in vivo.

Results

METs levels were higher in CC tissues and were an independent prognostic factor for CC patients. The prognostic model consisting of age, tumors local invasion, lymph node metastasis and METs were confirmed to be consistent and accurate for predicting the 5-year OS of CC patients. Besides, METs were correlated with distant metastasis and inflammation. Through in vitro experiments, we confirmed that there was a positive feedback loop between CC cells and METs, in that METs promoted the invasion of CC cells and CC cells enhanced the production of METs, in turn. This interaction could be blocked by PAD2-IN-1 inhibitors. More importantly, animal experiments revealed that PAD2-IN-1 inhibited METs formation and CC liver metastasis in vivo.

Conclusions

METs were the potential biomarker of CC patient prognosis. PAD2-IN-1 inhibited the crosstalk between CC cells and METs in vitro and in vivo, which should be emphasized in CC therapy.

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.

Role of Neutrophils on the Ocular Surface

The ocular surface is a gateway that contacts the outside and receives stimulation from the outside. The corneal innate immune system is composed of many types of cells, including epithelial cells, fibroblasts, natural killer cells, macrophages, neutrophils, dendritic cells, mast cells, basophils, eosinophils, mucin, and lysozyme. Neutrophil infiltration and degranulation occur on the ocular surface. Degranulation, neutrophil extracellular traps formation, called NETosis, and autophagy in neutrophils are involved in the pathogenesis of ocular surface diseases. It is necessary to understand the role of neutrophils on the ocular surface. Furthermore, there is a need for research on therapeutic agents targeting neutrophils and neutrophil extracellular trap formation for ocular surface diseases.

Histone citrullination: a new target for tumors

As the main protein components of chromatin, histones play central roles in gene regulation as spools of winding DNA. Histones are subject to various modifications, including phosphorylation, acetylation, glycosylation, methylation, ubiquitination and citrullination, which affect gene transcription. Histone citrullination, a posttranscriptional modification catalyzed by peptidyl arginine deiminase (PAD) enzymes, is involved in human carcinogenesis. In this study, we highlighted the functions of histone citrullination in physiological regulation and tumors. Additionally, because histone citrullination involves forming neutrophil extracellular traps (NETs), the relationship between NETs and tumors was illustrated. Finally, the clinical application of histone citrullination and PAD inhibitors was discussed.

Epitope-Based Chicken-Derived Novel Anti-PAD2 Monoclonal Antibodies Inhibit Citrullination

The aberrant upregulation of protein arginine deiminase 2- (PAD2-) catalyzed citrullination is reported in various autoimmune diseases (rheumatoid arthritis and multiple sclerosis) and several cancers. Currently, there are no anti-PAD2 monoclonal antibodies (mAbs) that can inhibit the citrullination reaction. Here, an epitope ³⁴¹YLNRGDRWIQDEIEFGY³⁵⁷ was examined as an antigenic site of PAD2. Chickens were immunized with this epitope, and the generated mAbs were screened for its reactivity against the full-length PAD2. Enzyme-linked immunosorbent assay revealed that six mAbs, which were screened from the phage display library, crossreacted with mouse PAD2. Kinetic analysis revealed that mAbs are bound to PAD2 in the nanomolar range, which indicated a strong binding. Results of the in vitro citrullination inhibition assay revealed that the half-maximal effective concentration values of mAbs for the inhibition of histone or benzoyl-L-arginine ethyl ester citrullination were in the range of 6–75 nM which supports strong inhibition capabilities. Alanine scanning of epitope revealed that the peptide fragment ³⁴⁴RGDRWIQDEIEF³⁵⁵ was responsible for generating strong antibody responses that inhibit the PAD2-catalyzed citrullination reaction. These antibodies can aid in understanding the extracellular PAD2 function and treating diseases associated with aberrant citrullination.

Chemical biology of protein citrullination by the protein A arginine deiminases

Citrullination is a post-translational modification (PTM) that converts peptidyl-arginine into peptidyl-citrulline; citrullination is catalyzed by the protein arginine deiminases (PADs). This PTM is associated with several physiological processes, including the epigenetic regulation of gene expression, neutrophil extracellular trap formation, and DNA-damage induced apoptosis. Notably, aberrant protein citrullination is relevant to several autoimmune and neurodegenerative diseases and certain forms of cancer. As such, the PADs are promising therapeutic targets. In this review, we discuss recent advances in the development of PAD inhibitors and activity-based probes, the development and use of citrulline-specific probes in chemoproteomic applications, and methods to site-specifically incorporate citrulline into proteins.

Site-specific incorporation of citrulline into proteins in mammalian cells

Citrullination is a post-translational modification (PTM) of arginine that is crucial for several physiological processes, including gene regulation and neutrophil extracellular trap formation. Despite recent advances, studies of protein citrullination remain challenging due to the difficulty of accessing proteins homogeneously citrullinated at a specific site. Herein, we report a technology that enables the site-specific incorporation of citrulline (Cit) into proteins in mammalian cells. This approach exploits an engineered E. coli-derived leucyl tRNA synthetase-tRNA pair that incorporates a photocaged-citrulline (SM60) into proteins in response to a nonsense codon. Subsequently, SM60 is readily converted to Cit with light in vitro and in living cells. To demonstrate the utility of the method, we biochemically characterize the effect of incorporating Cit at two known autocitrullination sites in Protein Arginine Deiminase 4 (PAD4, R372 and R374) and show that the R372Cit and R374Cit mutants are 181- and 9-fold less active than the wild-type enzyme. This technology possesses the potential to decipher the biology of citrullination.

Citrullination of arginine is crucial for several physiological processes. Here the authors report the site-specific incorporation of citrulline into proteins in mammalian cells using an engineered tRNA synthetase/tRNA pair and a photocaged-citrulline.

Serum citrullinated histone H3 concentrations differentiate patients with septic verses non-septic shock and correlate with disease severity

Purpose

Microbial infection stimulates neutrophil/macrophage/monocyte extracellular trap formation, which leads to the release of citrullinated histone H3 (CitH3) catalyzed by peptidylarginine deiminase (PAD) 2 and 4. Understanding these molecular mechanisms in the pathogenesis of septic shock will be an important next step for developing novel diagnostic and treatment modalities. We sought to determine the expression of CitH3 in patients with septic shock, and to correlate CitH3 levels with PAD2/PAD4 and clinically relevant outcomes.

Methods

Levels of CitH3 were measured in serum samples of 160 critically ill patients with septic and non-septic shock, and healthy volunteers. Analyses of clinical and laboratory characteristics of patients were conducted.

Results

Levels of circulating CitH3 at enrollment were significantly increased in septic shock patients (n = 102) compared to patients hospitalized with non-infectious shock (NIC) (n = 32, p < 0.0001). The area under the curve (95% CI) for distinguishing septic shock from NIC using CitH3 was 0.76 (0.65–0.86). CitH3 was positively correlated with PAD2 and PAD4 concentrations and Sequential Organ Failure Assessment Scores [total score (r = 0.36, p < 0.0001)]. The serum levels of CitH3 at 24 h (p < 0.01) and 48 h (p < 0.05) were significantly higher in the septic patients that did not survive.

Conclusion

CitH3 is increased in patients with septic shock. Its serum concentrations correlate with disease severity and prognosis, which may yield vital insights into the pathophysiology of sepsis.

Electronic supplementary material

The online version of this article (10.1007/s15010-020-01528-y) contains supplementary material, which is available to authorized users.

Peptidylarginine Deiminases 2 Mediates Caspase-1-Associated Lethality in Pseudomonas aeruginosa Pneumonia-Induced Sepsis

BACKGROUND

Pseudomonas aeruginosa (PA) is a pathogenic bacterium that causes severe pneumonia in critically ill and immunocompromised patients. Peptidylarginine deiminase (PAD) 2, PAD4, and caspase-1 are important enzymes in mediating host response to infection. The goal of this study was to determine the interplay between PAD2, PAD4, and caspase-1 in PA pneumonia-induced sepsis.

METHODS

Pneumonia was produced in wild-type, Pad2-/-, and Pad4-/- mice by intranasal inoculation of PA (2.5 × 106 colony-forming units per mouse), and survival (n = 15/group) was monitored for 10 days. Bone marrow-derived macrophages (BMDMs) were isolated for in vitro studies. Samples were collected at specific timepoints for Western blot, bacterial load determination, and flow cytometry analysis.

RESULTS

Caspase-1-dependent inflammation was diminished in PA-inoculated Pad2-/- mice, contributing to reduced macrophage death and enhanced bacterial clearance. In addition, Pad2-/- mice exhibited improved survival and attenuated acute lung injury after PA infection. In contrast, Pad4-/- mice did not display diminished caspase-1 activation, altered bacterial loads, or improved survival.

CONCLUSIONS

Peptidylarginine deiminase 2 plays an essential role in the pathogenesis of pulmonary sepsis by mediating caspase-1 activation. This goes against previous findings of PAD4 in sepsis. Our study suggests that PAD2 is a potential therapeutic target of PA pneumonia-induced sepsis.