Citrullination under physiological and pathological conditions

Comparative evaluation of semaphorin-4D, peptidylarginine deiminase-2, and matrix metalloproteinase-8 levels of gingival crevicular fluid in periodontally healthy and Stage III periodontitis smoker and non-smoker patients before and after non-surgical periodontal therapy

BACKGROUND

This study was designed to assess the influence of non-surgical periodontal therapy (NSPT) on gingival crevicular fluid (GCF) levels of semaphorin-4D (SEMA-4D), peptidylarginine deiminase-2 (PAD-2), and matrix metalloproteinase-8 (MMP-8) levels in periodontally healthy, Stage III periodontitis non-smoker and smoker patients.

METHODS

Sixty patients were equally divided into three groups, Group I: Periodontally healthy, Group II: Non-smokers with Stage III periodontitis, and Group III: Smokers with Stage III periodontitis. The patients underwent NSPT with clinical and biochemical parameters examined at baseline and 3 months post therapy. GCF was collected for levels of SEMA-4D, PAD-2, and MMP-8 through enzyme-linked immunosorbent assay (ELISA).

RESULTS

Greater values of PPD (8.06 ± 0.19 mm), CAL (8.94 ± 0.19 mm), PI (2.58 ± 0.19) while lower PBI (1.39 ± 0.19%) and GI (1.72 ± 0.19) scores were seen in Group III as compared to Group II, which reduced significantly from baseline to 3 months in both the groups after NSPT. Minimum values of SEMA-4D, PAD-2, and MMP-8 levels in GCF were seen for Group I, which increased incrementally to Group II and III. Also, among Group II and III the SEMA-4D, PAD-2, and MMP-8 levels in GCF reduced from baseline to 3 months indicating a favorable response within the tissues.

CONCLUSION

Greater levels in GCF of Levels of SEMA-4D, PAD-2, and MMP-8 in Group II and III, which reduced significantly post NSPT, implied that these biomarkers play a pivotal role in the inflammatory process and can be utilized for early diagnosis.

Comprehensive characterization of protein modifications using mass spectrometry and dry blood spots

PURPOSE

The main objective of this study is to characterize and analyze modified peptides in DBS samples. This includes deciphering their specific PTMs and understanding their potential impact on the population or disease cohort under study.

EXPERIMENTAL DESIGN

Using mass spectrometry-based proteomic approaches, we performed a comprehensive analysis of DBS samples. Our focus was on the identification and quantification of modified peptides. We also took advantage of recent advances in DBS mass spectrometry to ensure accurate detection and quantification.

RESULTS

A comprehensive analysis identified 972 modified peptides in DBS samples. Of these, a subset of 211 peptides was consistently present in all samples, highlighting their potential biological importance and relevance. This indicates a diverse spectrum of PTMs in the proteome of DBS samples.

CONCLUSIONS AND CLINICAL RELEVANCE

Integration of mass spectrometry and proteomics has revealed a broad spectrum of modified peptides in DBS samples and highlighted their importance in biological processes and disease progression. Accurate detection of these PTMs may be critical for risk stratification and disease management. This study improves the understanding of molecular mechanisms underlying biological processes and disease development, providing important insights for clinical applications.

Peptidyl Arginine Deiminases in Chronic Diseases: A Focus on Rheumatoid Arthritis and Interstitial Lung Disease

Protein citrullination is accomplished by a broad enzyme family named Peptidyl Arginine Deiminases (PADs), which makes this post-translational modification in many proteins that perform physiological and pathologic mechanisms in the body. Due to these modifications, citrullination has become a significant topic in the study of pathological processes. It has been related to some chronic and autoimmune diseases, including rheumatoid arthritis (RA), interstitial lung diseases (ILD), multiple sclerosis (MS), and certain types of cancer, among others. Antibody production against different targets, including filaggrin, vimentin, and collagen, results in an immune response if they are citrullinated, which triggers a continuous inflammatory process characteristic of autoimmune and certain chronic diseases. PAD coding genes (PADI1 to PADI4 and PADI6) harbor variations that can be important in these enzymes' folding, activity, function, and half-life. However, few studies have considered these genetic factors in the context of chronic diseases. Exploring PAD pathways and their role in autoimmune and chronic diseases is a major topic in developing new pharmacological targets and valuable biomarkers to improve diagnosis and prevention. The present review addresses and highlights genetic, molecular, biochemical, and physiopathological factors where PAD enzymes perform a major role in autoimmune and chronic diseases.

Impact of GSK199 and GSK106 binding on protein arginine deiminase IV stability and flexibility: A computational approach

Protein arginine deiminase IV (PAD4) is a potential target for diseases including rheumatoid arthritis and cancers. Currently, GSK199 is a potent, selective yet reversible PAD4 inhibitor. Its derivative, GSK106, on the other hand, was reported as an inactive compound when tested against PAD4 assay. Although they had similar skeleton, their impact towards PAD4 structural and flexibility is unknown. In order to fill the research gap, the impact of GSK199 and GSK106 binding towards PAD4 stability and flexibility is investigated via a combination of computational methods. Molecular docking indicates that GSK199 and GSK106 are capable to bind at PAD4 pocket by using its back door with -10.6 kcal/mol and -9.6 kcal/mol, respectively. The simulations of both complexes were stable throughout 100 ns. The structure of PAD4 exhibited a tighter packing in the presence of GSK106 compared to GSK199. The RMSF analysis demonstrates significant changes between the PAD4-GSK199 and PAD4-GSK106 simulations in the regions containing residues 136, 160, 220, 438, and 606. The Molecular Mechanics Poisson-Boltzmann Surface Area (MMPBSA) analysis shows a marked difference in binding free energies, with -11.339 kcal/mol for the PAD4-GSK199 complex and 1.063 kcal/mol for the PAD4-GSK106 complex. The hydrogen bond analysis revealed that the GSK199 and GSK106 binding to PAD4 are assisted by six hydrogen bonds and three hydrogen bonds, respectively. The visualisation of the MD simulations revealed that GSK199 remained in the PAD4 pocket, whereas GSK106 shifted away from the catalytic site. Meanwhile, molecular dockings of benzoyl arginine amide (BAEE) substrate have shown that BAEE is able to bind to PAD4 catalytic site when GSK106 was present but not when GSK199 occupied the site. Overall, combination of computational approaches successfully described the behaviour of binding pocket of PAD4 structure in the presence of the active and inactive compounds.

Citrullination Post-Translational Modification: State of the Art of Brain Tumor Investigations and Future Perspectives

The present review aims to describe the state of the art of research studies investigating the citrullination post-translational modification in adult and pediatric brain tumors. After an introduction to the deimination reaction and its occurrence in proteins and polypeptide chains, the role of the citrullination post-translational modification in physiological as well as pathological states, including cancer, is summarized, and the recent literature and review papers on the topic are examined. A separate section deals with the specific focus of investigation of the citrullination post-translational modification in relation to brain tumors, examining the state of the art of the literature that mainly concerns adult and pediatric glioblastoma and posterior fossa pediatric tumors. We examined the literature on this emerging field of research, and we apologize in advance for any possible omission. Although only a few studies inspecting citrullination in brain tumors are currently available, the results interestingly highlighted different profiles of the citrullinome associated with different histotypes. The data outlined the importance of this post-translational modification in modulating cancer invasion and chemoresistance, influencing key factors involved in apoptosis, cancer cell communication through extracellular vesicle release, autophagy, and gene expression processes, which suggests the prospect of taking citrullination as a target of cancer treatment or as a source of potential diagnostic and prognostic biomarkers for potential clinical applications in the future.

Clinical characteristics of rheumatoid arthritis patients complicated with pulmonary nontuberculous mycobacterial disease: A cross-sectional case series study

OBJECTIVES

Pulmonary nontuberculous mycobacterial disease (pNTM) is a common pulmonary complication of rheumatoid arthritis (RA), but their association has rarely been researched. We aimed to reveal the clinical characteristics of RA with pNTM.

METHODS

Among all the RA patients who visited Tenri hospital from April 2017 to March 2018, we enrolled those fulfilling the 2007 ATS/IDSA diagnostic criteria of pNTM, and sex- and age- matched control group at a ratio of 1:5. Demographic characteristics were compared between the two groups.

RESULTS

Among 865 RA patients, 35 (4.0%) patients were complicated with pNTM. RA patients with pNTM had significantly lower BMI and higher rheumatoid factor (RF) and anti-citrullinated protein antibody (ACPA) positivity. Bronchiectasis was the most frequent lesion, followed by clusters of small nodules, patchy consolidation and cavity. Multivariable logistic regression analysis revealed bronchiectasis as a strong independent associated factor of pNTM. Treatment for pNTM was needed in 14 of the 35 (40%) RA patients with pNTM and sputum negative conversion was accomplished in 11 of the 14 cases (78.6%).

CONCLUSIONS

RA patients with lower BMI, RF/ACPA positivity, and bronchiectasis were associated with pNTM. Treatment for pNTM may attain sputum negative conversion and radiological improvement in patients with RA.

Anti-citrullinated protein antibodies as biomarkers in rheumatoid arthritis

INTRODUCTION

The serological biomarker anti-citrullinated protein antibodies (ACPAs) may have several functions but is especially important for the diagnosis of rheumatoid arthritis (RA) along with clinical symptoms.

AREAS COVERED

This review provides an overview of ACPAs, which are useful in RA diagnostics and may improve our understanding of disease etiology. PubMed was searched with combinations of words related to antibodies recognizing epitopes containing the post-translationally modified amino acid citrulline in combination with rheumatoid arthritis; cyclic citrullinated peptide, CCP, anti-CCP, anti-citrullinated protein antibodies, ACPA, citrullination, peptide/protein arginine deiminase, PAD, filaggrin, vimentin, keratin, collagen, perinuclear factor, EBNA1, EBNA2, and others. From this search, we made a qualitative extract of publications relevant to the discovery, characterization, and clinical use of these antibodies in relation to RA. We highlight significant findings and identify areas for improvement.

EXPERT OPINION

ACPAs have high diagnostic sensitivity and specificity for RA and recognize citrullinated epitopes from several proteins. The best-performing single epitope originates from Epstein-Barr Virus nuclear antigen 2 and contains a central Cit-Gly motif, which is recognized by ACPAS when located in a flexible peptide structure. In addition, ACPAs may also have prognostic value, especially in relation to early treatment, although ACPAs' main function is to aid in the diagnosis of RA.

Progression of pre-rheumatoid arthritis to clinical disease of joints: Potential role of mesenchymal stem cells

Rheumatoid arthritis (RA) related autoimmunity is developed at mucosal sites due to the interplay between genetic risk factors and environmental triggers. The pre-RA phase that leads to anti-citrullinated protein antibodies, rheumatoid factor, and other autoantibodies spread in the systemic circulation may not affect articular tissue for years until a mysterious second hit triggers the localization of RA-related autoimmunity in joints. Several players in the joint microenvironment mediate the synovial innate and adaptive immunological processes, eventually leading to clinical synovitis. There still exists a gap in the early phase of RA pathogenesis, i.e., the progression of diseases from the systemic circulation to joints. The lack of better understanding of these events results in the inability to answer questions about why only after a certain point of time the disease appears in joints and why in some cases, it simply remains latent and doesn't affect joints at all. In the current review, we focused on the immunomodulatory and regenerative role of mesenchymal stem cells and associated exosomes in RA pathology. We also highlighted the age-related dysregulations in activities of mesenchymal stem cells and how that might trigger homing of systemic autoimmunity to joints.

Hypercitrullination and anti-citrullinated protein antibodies in chronic apical periodontitis, a laboratory investigation. Does autoimmunity contribute to the pathogenesis?

AIM

The presence of Gram-negative anaerobic bacteria, in particular, Porphyromonas gingivalis (P. gingivalis) in periapical granulomas predicts the generation of citrullinated proteins in the lesion. Citrullination of proteins may lead to the formation of anti-citrullinated autoantibodies (ACPA-s) initiating the formation of an autoimmune loop which may contribute to the perpetuation of inflammatory reactions and tissue damage in chronic apical periodontitis. The objective of this study was to demonstrate the formation of citrullinated proteins in chronic apical periodontitis and whether they can act as autoantigens.

METHODOLOGY

Twenty-five periapical granulomas (n = 25) were investigated in the study. Healthy periodontal tissue samples served as normal control tissue (n = 6). The peptidyl-citrulline level was determined with the dot blot method. ACPA levels were analysed using anti-citrullinated cyclic peptide (anti-CCP) EDIA kit. Differences between periapical granuloma and control samples were assessed using Mann-Whitney U tests. p Values <.05 were considered as statistically significant.

RESULTS

Protein concentrations, peptidyl-citrulline levels and anti-CCP ratios were compared between periapical granuloma and healthy control groups. Multiple linear regression analysis revealed significant (p = .042) hypercitrullination in periapical granuloma samples. Moreover, there was a significant difference in the ACPA ratios between periapical granuloma (2.03 ± 0.30) and healthy control (0.63 ± 0.17) groups (p = .01). Seventeen of 25 periapical granuloma samples (17/25; 68%), whereas one out of six control samples (1/6; 17%) were shown to be positive for the presence of ACPA.

CONCLUSIONS

This is the first study detecting the presence of citrullinated peptides and APCA in periapical granuloma, suggesting the contribution of autoimmune reactions in the pathogenesis and perpetuation of chronic apical periodontitis.

Differential Protein Citrullination in Human ER- and ER+ Tumor and Adjacent Healthy Breast Tissue

Post-translational modification of arginine to citrulline is catalyzed by members of the peptidylarginine deiminase (PAD) family. Dysregulation of this catalysis is a significant driver of the pathogenesis of numerous inflammatory diseases, including cancer. However, dysregulation of PAD activity has not been examined in breast cancer with respect to hormone receptor status. In this study, we measured PAD enzyme levels using Western blotting and investigated protein citrullination using a mass spectrometry-based proteomics approach in primary estrogen receptor negative (ER-) or positive (ER+) breast tumor and matched adjacent normal tissue. Our findings reveal 72 and 41 citrullinated proteins in ER- tumor and adjacent healthy tissue, respectively, where 20 of these proteins are common between the two groups. We detected 64 and 49 citrullinated proteins in ER+ tumor and adjacent healthy tissue, respectively, where 32 proteins are common. Interestingly, upon comparison of ER- and ER+ tumor tissue, only 32 citrullinated proteins are shared between the two and the rest are unique to the tumor's receptor status. Using the STRING database for protein-protein interaction network analysis, these proteins are involved in protein-folding events (i.e., heat shock proteins) in ER- samples and blood-clotting events (i.e., fibulin) in ER+ samples. Constituents of the extracellular matrix structure (i.e., collagen and fibrinogen) were found in both. Herein, we establish evidence that supports the role of this unique post-translational modification in breast cancer biology. Finally, to aid drug discovery against citrullination, we developed a liquid chromatography-ultraviolet method to measure PAD enzymatic activity and optimized glucagon-like peptide II to quantitatively measure the ability of PADs to citrullinate its substrate.

A subset of antibodies targeting citrullinated proteins confers protection from rheumatoid arthritis

Although elevated levels of anti-citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis (RA), the in vivo functions of these antibodies remain unclear. Here, we have expressed monoclonal ACPAs derived from patients with RA, and analyzed their functions in mice, as well as their specificities. None of the ACPAs showed arthritogenicity nor induced pain-associated behavior in mice. However, one of the antibodies, clone E4, protected mice from antibody-induced arthritis. E4 showed a binding pattern restricted to skin, macrophages and dendritic cells in lymphoid tissue, and cartilage derived from mouse and human arthritic joints. Proteomic analysis confirmed that E4 strongly binds to macrophages and certain RA synovial fluid proteins such as α-enolase. The protective effect of E4 was epitope-specific and dependent on the interaction between E4-citrullinated α-enolase immune complexes with FCGR2B on macrophages, resulting in increased IL-10 secretion and reduced osteoclastogenesis. These findings suggest that a subset of ACPAs have therapeutic potential in RA.

Histone Citrullination Mediates a Protective Role in Endothelium and Modulates Inflammation

NETosis is a key host immune process against a pathogenic infection during innate immune activation, consisting of a neutrophil "explosion" and, consequently, NET formation, containing mainly DNA, histones, and other nuclear proteins. During sepsis, an exacerbated immune host response to an infection occurs, activating the innate immunity and NETosis events, which requires histone H3 citrullination. Our group compared the circulating histone levels with those citrullinated H3 levels in plasma samples of septic patients. In addition, we demonstrated that citrullinated histones were less cytotoxic for endothelial cells than histones without this post-translational modification. Citrullinated histones did not affect cell viability and did not activate oxidative stress. Nevertheless, citrullinated histones induced an inflammatory response, as well as regulatory endothelial mechanisms. Furthermore, septic patients showed elevated levels of circulating citrullinated histone H3, indicating that the histone citrullination is produced during the first stages of sepsis, probably due to the NETosis process.

High Presence of NETotic Cells and Neutrophil Extracellular Traps in Vaginal Discharges of Women with Vaginitis: An Exploratory Study

Infectious vaginitis is a microbiological syndrome of great importance in public health that affects millions of women worldwide. However, no studies have explored the phenomenon of the production of the neutrophil extracellular traps (NETs) that are released into the female reproductive tract in these pathologies. This study aimed to determine the presence of NETosis in vaginal discharges of women with bacterial vaginosis, candidiasis, and trichomoniasis by characterizing NETs. Extracellular DNA with neutrophil elastase and citrullinated histones was identified to confirm the NET components (n = 10). The concentration, phenotypes of NETs, and number of NETotic cells were determined. The results showed an increase in NETotic cells in women with Candida albicans (CA) and Trichomonas vaginalis (TV) and an increase in NETs in TV-induced vaginitis. Samples of CA- and TV-infected women showed different NET phenotypes (diffNETs, sprNETs, and aggNETs); diffNETs were found in high concentrations in samples with CA and were increased in three types of NETs in TV infections. Samples with intermediate microbiota and bacterial vaginosis showed increased NETotic cells while the intermediate microbiota presented a higher concentration of NETs. Therefore, alterations in the microbiota and the presence of fungal and parasitic infections are important stimuli for the activation and induction of NETosis, and their cytotoxic effects could enhance tissue damage.

Anti-cyclic citrullinated peptide antibody-positive rheumatoid arthritis caused by bacterial organizing pneumonia in a patient with Sjogren’s syndrome

A 58-year-old woman with a history of Sjogren’s syndrome was admitted to our hospital with cough, decreased right lung breath sounds and arthralgia in both thumbs. Chest computed tomography showed consolidation with air bronchogram in the right lung. Levels of anti-cyclic citrullinated peptide antibody and rheumatoid factor levels were significantly elevated. She was diagnosed with rheumatoid arthritis induced by bacterial organizing pneumonia. Treatment with salazosulfapyridine was added for rheumatoid arthritis and arthralgia gradually improved. This case highlights that respiratory infections could lead to anti-cyclic citrullinated peptide antibody-positive rheumatoid arthritis in patients with Sjogren’s syndrome.

Anti-citrullinated protein antibody specificities and pulmonary fibrosis in relation to genetic loci in early rheumatoid arthritis

OBJECTIVES

Pulmonary manifestations in RA are common comorbidities, but the underlying mechanisms are largely unknown. The added value of a multiplex of ACPA and genetic risk markers was evaluated for the development of pulmonary fibrosis (PF) in an inception cohort.

METHODS

A total of 1184 patients with early RA were consecutively included and followed prospectively from the index date until death or 31 December 2016. The presence of 21 ACPA fine specificities was analysed using a custom-made microarray chip (Thermo Fisher Scientific, Uppsala, Sweden). Three SNPs, previously found related to PF were evaluated, rs2609255 (FAM13A), rs111521887 (TOLLIP) and rs35705950 (MUC5B). ACPA and genetic data were available for 841 RA patients, of whom 50 developed radiologically defined PF.

RESULTS

In unadjusted analyses, 11 ACPA specificities were associated with PF development. In multiple variable analyses, six ACPA specificities were associated with increased risk of PF: vimentin (Vim)60-75, fibrinogen (Fib)β62-78 (72), Fibα621-635, Bla26, collagen (C)II359-369 and F4-CIT-R (P < 0.01 to P < 0.05). The number of ACPA specificities was also related to PF development (P < 0.05 crude and adjusted models). In multiple variable models respectively adjusted for each of the SNPs, the number of ACPA specificities (P < 0.05 in all models), anti-Vim60-75 (P < 0.05, in all models), anti-Fibβ62-78 (72) (P < 0.001 to P < 0.05), anti-CII359-369 (P < 0.05 in all models) and anti-F4-CIT-R AQ4 (P < 0.01 to P < 0.05), anti-Fibα621-635 (P < 0.05 in one) and anti-Bla26 (P < 0.05 in two) were significantly associated with PF development.

CONCLUSION

The development of PF in an inception cohort of RA patients was associated with both presence of certain ACPA and the number of ACPA specificities and risk genes.

Novel Approaches in the Immunotherapy of Multiple Sclerosis: Cyclization of Myelin Epitope Peptides and Conjugation with Mannan

Multiple Sclerosis (MS) is a serious autoimmune disease. The patient in an advanced state of the disease has restrained mobility and remains handicapped. It is therefore understandable that there is a great need for novel drugs and vaccines for the treatment of MS. Herein we summarise two major approaches applied for the treatment of the disease using peptide molecules alone or conjugated with mannan. The first approach focuses on selective myelin epitope peptide or peptide mimetic therapy alone or conjugated with mannan, and the second on immune-therapy by preventing or controlling disease through the release of appropriate cytokines. In both approaches the use of cyclic peptides offers the advantage of increased stability from proteolytic enzymes. In these approaches, the synthesis of myelin epitope peptides conjugated to mannan is of particular interest as this was found to protect mice against experimental autoimmune encephalomyelitis, an animal model of MS, in prophylactic and therapeutic protocols. Protection was peptide-specific and associated with reduced antigen-specific T cell proliferation. The aim of the studies of these peptide epitope analogs is to understand their molecular basis of interactions with human autoimmune T-cell receptor and a MS-associated human leucocyte antigen (HLA)-DR2b. This knowledge will lead the rational design to new beneficial non-peptide mimetic analogs for the treatment of MS. Some issues of the use of nanotechnology will also be addressed as a future trend to tackle the disease. We highlight novel immunomodulation and vaccine-based research against MS based on myelin epitope peptides and strategies developed in our laboratories.

Preclinical models of arthritis for studying immunotherapy and immune tolerance

Increasingly earlier identification of individuals at high risk of rheumatoid arthritis (RA) (eg, with autoantibodies and mild symptoms) improves the feasibility of preventing or curing disease. The use of antigen-specific immunotherapies to reinstate immunological self-tolerance represent a highly attractive strategy due to their potential to induce disease resolution, in contrast to existing approaches that require long-term treatment of underlying symptoms.

Preclinical animal models have been used to understand disease mechanisms and to evaluate novel immunotherapeutic approaches. However, models are required to understand critical processes supporting disease development such as the breach of self-tolerance that triggers autoimmunity and the progression from asymptomatic autoimmunity to joint pain and bone loss. These models would also be useful in evaluating the response to treatment in the pre-RA period.

This review proposes that focusing on immune processes contributing to initial disease induction rather than end-stage pathological consequences is essential to allow development and evaluation of novel immunotherapies for early intervention. We will describe and critique existing models in arthritis and the broader field of autoimmunity that may fulfil these criteria. We will also identify key gaps in our ability to study these processes in animal models, to highlight where further research should be targeted.

Salivary citrullinated proteins in rheumatoid arthritis and associated periodontal disease

Periodontal disease (PD) can be an important precipitating factor in the production of citrullinated proteins. Its importance is emphasized, but it is not the only way to produce citrullinated proteins. The aim of the current study was to determine the periodontal conditions and the salivary citrullinated protein content in patients with rheumatoid arthritis (RA) compared to healthy controls. We also wished to correlate citrullinated protein levels in the saliva and serum biomarkers with the periodontal status and temporomandibular joint (TMJ) involvement of patients with RA. Twenty-three patients with RA and 17 healthy controls participated the study. Saliva samples were taken: citrulline content of saliva was measured. Blood test results for patients with RA were collected. TMJ disorders were described. Cariological and periodontal indices were registered. Periodontal conditions and periodontal staging were also registered. Comparison of measured values between groups was performed. Intragroup correlation of patients’ values was counted. The prevalence of TMJ complaints was significantly higher in the RA group (8/23) versus controls (1/17). The patients with RA had worse periodontal condition because more patients with RA had gingivitis with a significantly higher bleeding on probing (BOP) (RA: 22.4 ± 25.0%; controls: 6.36 ± 11.6%; p = 0.018). Gingival index (GI) was also significantly higher in the patients than in controls (RA: 0.68 ± 0.58; controls: 0.19 ± 0.38; p = 0.010). The citrullinated protein (relative) content of saliva did not differ significantly (p = 0.147) between patients with RA (1102.2 ± 530.8) and healthy controls (1873.1 ± 1594.9). In RA, the salivary anti-CCP levels positively correlated with PD staging (R = 0.464, p = 0.039) . Control subjects more commonly had healthy gingiva than RA patients. Moreover, in the control group more individuals had intact and reduced height periodontium than periodontitis compared to the RA group. There was no significant difference in the levels of salivary citrulline between patients with RA and controls, despite the significant differences in their periodontal status. Thus, salivary citrulline levels are not associated with RA disease severity.

Advances in the diagnosis of autoimmune diseases based on citrullinated peptides/proteins

Introduction: Autoimmune diseases are still one of the hard obstacles associated with humanity. There are many exogenous and endogenous etiological factors behind autoimmune diseases, which may be combined or dispersed to stimulate the autoimmune responses. Protein citrullination represents one of these factors. Harnessing specific citrullinated proteins/peptides could early predict and/or diagnose some of the autoimmune diseases. Many generations of diagnostic tools based on citrullinated peptides with comparable specificity/sensitivity are available worldwide.Areas covered: In this review, we discuss the deimination reaction behind the citrullination of most known autoantigens targeted, different generations of diagnostic tools based on citrullinated probes with specificity/sensitivity of each as well as newly developed assays. Furthermore, the most advanced molecular analytical tools to detect the citrullinated residues in the biological fluid and their performance are also evaluated, providing new avenues to early detect autoimmune diseases with high accuracy.Expert opinion: With the current specificity/sensitivity tools available for autoimmune disease detection, emphasis must be placed on developing more advance and effective, early, rapid, and simple diagnostic devices for autoimmune disease monitoring (similar to a portable device for sugar test at home). The molecular analytical devices with dual and/or multiplexe functions should be more simplified and invested in clinical laboratories.

Interplay of Microbiota and Citrullination in the Immunopathogenesis of Rheumatoid Arthritis

Microbiota is a balanced ecosystem that has important functions to the host health including development, defense, digestion, and absorption of dietary fibers and minerals, vitamin synthesizes, protection, and training the host immune system. On the other hand, its dysbiosis is linked to many human diseases such as rheumatoid arthritis (RA). The RA is an inflammatory autoimmune disorder caused by genetic and environmental factors; microbiota may be considered as a risk environmental factor for it. Citrullination is a post-translation modification (PMT) that converts the amino acid arginine to amino acid citrulline in certain proteins. These citrullinated proteins are recognized as a foreign antigen by the immune system resulting in the upregulation of inflammatory action such as in RA. The current work highlights the effect of both gut and oral microbiota dysbiosis on the development of RA, as well as discusses how the alteration in microbiota composition leads to the overgrowth of some bacterial species that entangled in RA pathogenicity. The evidence suggested that some oral and gut microbial species such as Porphyromonas gingivalis and Prevotella copri, respectively, contribute to RA pathogenesis. During dysbiosis, these bacteria can mediate the citrullination of either human or bacteria proteins to trigger an immune response that leads to the generation of autoantibodies.

Evaluation of galectin-3, peptidylarginine deiminase-4 and tumor necrosis factor-α levels in gingival crevicular fluid for periodontal health, gingivitis and stage III grade C periodontitis: A pilot study

BACKGROUND

Comparing the gingival crevicular fluid (GCF) levels of galectin-3, peptidylarginine deiminase 4 (PAD4) and tumor necrosis factor-alpha (TNF-α) in individuals with stage III grade C periodontitis and gingivitis and with healthy periodontium was the purpose of this clinical research.

METHODS

Sixty systemically healthy and non-smoker individuals consisting of stage III grade C periodontitis (group S3P/n = 20), gingivitis (group G/n = 20), and periodontally healthy (group HP/n = 20) were recruited for this research. Clinical parameters such as probing depth, clinical attachment level, gingival index, plaque index, and bleeding on probing were recorded in periodontal charts. Enzyme-linked immunosorbent assay method was used in evaluating the GCF levels of galectin-3, PAD4, and TNF-α for study groups.

RESULTS

The GCF galectin-3 total amount was highest in group S3P compared with group G and group HP (P <0.05). Its total amount was also higher in group G compared with group HP (P <0.05). The GCF PAD4 total amount was higher in group S3P compared with group HP (P <0.05) but was similar with group G (P >0.05). Its total amounts were also similar in group G and group HP (P >0.05). The GCF TNF-α total amounts were similar in group S3P and group G (P >0.05) but significantly greater than the group HP (P ˂0.05). The GCF galectin-3, PAD4, and TNF-α concentrations were lower in the group S3P and group G compared with the group HP (P <0.05). There were significant positive correlations between GCF galectin-3 total amount and all clinical parameters (P ˂0.01) and also between GCF galectin-3 and TNF-α total amounts (P ˂0.01). There was no correlation between PAD4 and clinical parameters, or between PAD4 and TNF-α (P >0.05).

CONCLUSIONS

Galectin-3 and PAD4 may be involved in the periodontal disease pathogenesis considering the elevated levels of these molecules in periodontal disease. These biomarkers may be used in the diagnosis of periodontal diseases.

Rethink About the Role of Rheumatoid Factor and Anti-citrullinated Protein Antibody in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease characterized by joint inflammation and extra-articular manifestations. Many questions in the pathogenesis, clinical manifestation, and disease spectrum are answered after the discovery of the first autoantibody namely rheumatoid factor (RF). The finding of the second autoantibody named anti-citrullinated protein antibody (ACPA), which unearths the importance of protein citrullination process. It further provides the insight how immune cells and complement interact to perpetuate the inflammatory response. These two autoantibodies pave the way for our better understanding of RA. This review article focuses on the history, pathophysiology, and clinical association of these two autoantibodies in RA.

Bioorthogonal protein labelling enables the study of antigen processing of citrullinated and carbamylated auto-antigens

Proteolysis is fundamental to many biological processes. In the immune system, it underpins the activation of the adaptive immune response: degradation of antigenic material into short peptides and presentation thereof on major histocompatibility complexes, leads to activation of T-cells. This initiates the adaptive immune response against many pathogens. Studying proteolysis is difficult, as the oft-used polypeptide reporters are susceptible to proteolytic sequestration themselves. Here we present a new approach that allows the imaging of antigen proteolysis throughout the processing pathway in an unbiased manner. By incorporating bioorthogonal functionalities into the protein in place of methionines, antigens can be followed during degradation, whilst leaving reactive sidechains open to templated and non-templated post-translational modifications, such as citrullination and carbamylation. Using this approach, we followed and imaged the post-uptake fate of the commonly used antigen ovalbumin, as well as the post-translationally citrullinated and/or carbamylated auto-antigen vinculin in rheumatoid arthritis, revealing differences in antigen processing and presentation.

Insights of rheumatoid arthritis biomarkers

Rheumatoid arthritis is a chronic, autoimmune connective tissue disease. In addition to joint involvement, extra-articular changes and organ complications also occur in the course of the disease. Untreated disease leads to disability and premature death. Therefore, it is important to recognise and begin treatment early. Based on the presence of rheumatoid factor and antibodies against citrullinated peptides, we can distinguish two forms of the disease: seropositive and seronegative. Research continues to elucidate the mechanisms of the onset of the disease, as well as to uncover factors that induce and influence the activity of the disease. The presence of markers that initially appear and affect the course of the disease can potentially aid in patient treatment. In this article, we have collected biomarkers of rheumatoid arthritis that are well understood as well as those that have been recently described.

Regulation of polyamine homeostasis through an antizyme citrullination pathway

This study reveals an uncovered mechanism for the regulation of polyamine homeostasis through protein arginyl citrullination of antizyme (AZ), a natural inhibitor of ornithine decarboxylase (ODC). ODC is critical for the cellular production of polyamines. AZ binds to ODC dimers and promotes the degradation of ODC via the 26S proteasome. This study demonstrates the protein citrullination of AZ catalyzed by peptidylarginine deiminase type 4 (PAD4) both in vitro and in cells. Upon PAD4 activation, the AZ protein was citrullinated and accumulated, leading to higher levels of ODC proteins in the cell. In the PAD4-overexpressing and activating cells, the levels of ODC enzyme activity and the product putrescine increased with the level of citrullinated AZ proteins and PAD4 activity. Suppressing cellular PAD4 activity reduces the cellular levels of ODC and downregulates cellular polyamines. Furthermore, citrullination of AZ in the C-terminus attenuates AZ function in the inhibition, binding, and degradation of ODC. This paper provides evidence to illustrate that PAD4-mediated AZ citrullination upregulates cellular ODC and polyamines by retarding ODC degradation, thus interfering with the homeostasis of cellular polyamines, which may be an important pathway regulating AZ functions that is relevant to cancer biology.

Elucidating Citrullination by Mass Spectrometry and Its Role in Disease Pathogenesis

This review focuses on discussing key mechanisms in disease pathogenesis mediated by the protein post-translational modification citrullination. These processes are discussed in depth in the context of complex diseases such as rheumatoid arthritis, cancer, central nervous system disorders, and cardiovascular disease. Additionally, a critical evaluation of challenges in laboratory detection of citrullination sites is also outlined. In this context, the role of mass spectrometry is discussed with a focus on contemporary techniques that offer promising options to detect the exact site of protein citrullination. Novel methods described in the paper have the potential to detect and quantify the occurrence of post-translational modification sites for diagnosis and therapeutic purposes with a high degree of specificity and sensitivity. Furthermore, they offer a much faster performance than traditional techniques making them ideal for large-scale experimentation.

Peptidyl arginine deiminase 4 and its potential role in Alzheimer's disease

Alzheimer's disease (AD) is the main cause of dementia, and its pathogenesis is still not clear. Peptidyl arginine deiminases 4(PAD4) as one of the important members of PAD family, is the only protein with nuclear transfer function, it can regulate the expression of many proteins through citrullinating histone. PAD4 can also interact with many transcription factors, involved in regulating gene expression. PAD4 expression is closely related to the inflammatory factors secreted, cell autophagy, tumorigenesis and other neurodegenerative diseases. More importantly, PAD4 and its citrullinated protein were found in cortical and hippocampal neurons of AD patients. To study the expression and regulatory pathway of PAD4 in vivo and in vitro experiments on AD may be of helpful to elucidate the pathogenesis of AD. Meanwhile, detection of anti-citrullinated antibody will have potential value as novel biomarkers of AD.

Evaluation of peptidylarginine deiminase 4 and PADI4 polymorphisms in sepsis-induced acute kidney injury

BACKGROUND

The aim of this study is to evaluate the peptidylarginine deiminase 4 (PAD 4) concentration and PADI4 polymorphisms as predictors of acute kidney injury (AKI) development, the need for renal replacement therapy (RRT), and mortality in patients with septic shock.

METHODS

We included all individuals aged ≥ 18 years, with a diagnosis of septic shock at ICU admission. Blood samples were taken within the first 24 hours of the patient's admission to determine serum PAD4 concentration and its PADI4 polymorphism (rs11203367) and (rs874881). Patients were monitored during their ICU stay and the development of SAKI was evaluated. Among the patients in whom SAKI developed, mortality and the need for RRT were also evaluated.

RESULTS

There were 99 patients, 51.5% of whom developed SAKI and of these, 21.5% needed RRT and 80% died in the ICU. There was no difference between PAD4 concentration (p = 0.116) and its polymorphisms rs11203367 (p = 0.910) and rs874881 (p = 0.769) in patients in whom SAKI did or did not develop. However, PAD4 had a positive correlation with plasma urea concentration (r = 0.269 and p = 0.007) and creatinine (r = 0.284 and p = 0.004). The PAD4 concentration and PADI4 polymorphisms were also not associated with RRT and with mortality in patients with SAKI.

CONCLUSION

PAD4 concentration and its polymorphisms were not associated with SAKI development, the need for RRT, or mortality in patients with septic shock. However, PAD4 concentrations were associated with creatinine and urea levels in these patients.

Deimination, Intermediate Filaments and Associated Proteins

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

[Protein arginine deiminase of oral microbiome plays a causal role in the polyarthritis rheumatoid initiating]

In the last decade, the association between the periodontitis and rheumatoid arthritis (RA) has been established, suggesting that oral microbiome plays a causal role by initiating this chronic autoimmune inflammatory disease of articulation. Both pathogenesis are similar in term of chronic inflammation, tissue breakdown and bone resorption. Molecular aspects have also revealed that citrullination, a post-translational modification catalyzed by peptidyl-arginine deiminases (PADs), is involved in both diseases. For RA, citrullinated proteins production leads to the synthesis the of anti-citrullinated protein antibodies triggering the loss of immune tolerance. In humans, five PADs have been identified. Recently, studies have found that only Porphyromonas species possess PAD. Thus, a major periodontal pathogen, Porphyromonas gingivalis, is able to generate citrullinated epitopes, and could consequently induce anti-citrullinated protein antibodies. In this review, citrullination process, periodontitis and RA are described to put them in relation with molecular, clinical and epidemiological studies establishing the association between periodontitis and RA.

Epigenetic biomarkers for human sepsis and septic shock: insights from immunosuppression

Sepsis is a life-threatening condition that occurs when the body responds to an infection damaging its own tissues. Sepsis survivors sometimes suffer from immunosuppression increasing the risk of death. To our best knowledge, there is no 'gold standard' for defining immunosuppression except for a composite clinical end point. As the immune system is exposed to epigenetic changes during and after sepsis, research that focuses on identifying new biomarkers to detect septic patients with immunoparalysis could offer new epigenetic-based strategies to predict short- and long-term pathological events related to this life-threatening state. This review describes the most relevant epigenetic mechanisms underlying alterations in the innate and adaptive immune responses described in sepsis and septic shock, and their consequences for immunosuppression states, providing several candidates to become epigenetic biomarkers that could improve sepsis management and help predict immunosuppression in postseptic patients.

Peptidylarginine Deiminase IV Regulates Breast Cancer Stem Cells via a Novel Tumor Cell-Autonomous Suppressor Role

Peptidylarginine deiminases (PADI) catalyze posttranslational modification of many target proteins and have been suggested to play a role in carcinogenesis. Citrullination of histones by PADI4 was recently implicated in regulating embryonic stem and hematopoietic progenitor cells. Here, we investigated a possible role for PADI4 in regulating breast cancer stem cells. PADI4 activity limited the number of cancer stem cells (CSC) in multiple breast cancer models in vitro and in vivo. Mechanistically, PADI4 inhibition resulted in a widespread redistribution of histone H3, with increased accumulation around transcriptional start sites. Interestingly, epigenetic effects of PADI4 on the bulk tumor cell population did not explain the CSC phenotype. However, in sorted tumor cell populations, PADI4 downregulated expression of master transcription factors of stemness, NANOG and OCT4, specifically in the cancer stem cell compartment, by reducing the transcriptionally activating H3R17me2a histone mark at those loci; this effect was not seen in the non-stem cells. A gene signature reflecting tumor cell-autonomous PADI4 inhibition was associated with poor outcome in human breast cancer datasets, consistent with a tumor-suppressive role for PADI4 in estrogen receptor-positive tumors. These results contrast with known tumor-promoting effects of PADI4 on the tumor stroma and suggest that the balance between opposing tumor cell-autonomous and stromal effects may determine net outcome. Our findings reveal a novel role for PADI4 as a tumor suppressor in regulating breast cancer stem cells and provide insight into context-specific effects of PADI4 in epigenetic modulation. SIGNIFICANCE: These findings demonstrate a novel activity of the citrullinating enzyme PADI4 in suppressing breast cancer stem cells through epigenetic repression of stemness master transcription factors NANOG and OCT4.

Measuring ACPA in the general population or primary care: is it useful?

Rheumatoid arthritis (RA) is associated with a significant disease burden and high costs for society. Because the disease has identifiable preclinical stages, screening and prevention have become a possibility in RA. Anticitrullinated peptide antibodies (ACPAs) are arguably the most likely candidate biomarker to screen for RA. This paper reviews the evidence for the use of ACPAs as a screening test in the broader general population, to identify individuals at high risk of subsequent onset of RA. We will review the diagnostic properties of the test and its positive and negative predictive value in different settings. We will discuss how ACPA testing could effectively be integrated in a broader screening strategy for RA.

Interaction between food antigens and the immune system: Association with autoimmune disorders

It has been shown that environmental factors such as infections, chemicals, and diet play a major role in autoimmune diseases; however, relatively little attention has been given to food components as the most prevalent modifiers of these afflictions. This review summarizes the current body of knowledge related to different mechanisms and associations between food proteins/peptides and autoimmune disorders. The primary factor controlling food-related immune reactions is the oral tolerance mechanism. The failure of oral tolerance triggers immune reactivity against dietary antigens, which may initiate or exacerbate autoimmune disease when the food antigen shares homology with human tissue antigens. Because the conformational fit between food antigens and a host's self-determinants has been determined for only a few food proteins, we examined evidence related to the reaction of affinity-purified disease-specific antibody with different food antigens. We also studied the reaction of monoclonal or polyclonal tissue-specific antibodies with various food antigens and the reaction of food-specific antibodies with human tissue antigens. Examining the assembled information, we postulated that chemical modification of food proteins by different toxicants in food may result in immune reaction against modified food proteins that cross-react with tissue antigens, resulting in autoimmune reactivity. Because we are what our microbiome eats, food can change the gut commensals, and toxins can breach the gut barrier, penetrating into different organs where they can initiate autoimmune response. Conversely, there are also foods and supplements that help maintain oral tolerance and microbiome homeostasis. Understanding the potential link between specific food consumption and autoimmunity in humans may lay the foundation for further research about the proper diet in the prevention of autoimmune diseases.

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.

Gums and joints: is there a connection? Part two: the biological link

Rheumatoid arthritis (RA) and periodontitis (PD) are inflammatory diseases characterised by an exacerbated immune-inflammatory reaction that leads to the destruction of bone and other connective tissues that share numerous similarities. Although a significant and independent association between these two conditions has been described, the pathophysiological processes that may explain this relationship remain unknown and multiple theories have been proposed. This review presents the most important theories currently proposed to explain the biological link between RA and PD.

Citrulline Effect Is a Characteristic Feature of Deiminated Peptides in Tandem Mass Spectrometry

Tandem mass spectrometry of peptides is of utmost importance in proteomics. Collision-induced dissociation usually generates y type fragment ion series from tryptic peptides, carrying information on their primary structure. Amino acid side chains or differences in their basicity could alter fragmentation processes considerably. The well-known proline effect is a cleavage preference at the N-terminus of proline residues in peptides, usually yielding a very abundant y ion while suppressing others. Previously, we reported a similar phenomenon occurring at the C-terminus of citrulline residues and coined the term Cit effect. To confirm the presence of Cit effect in large proteomic datasets, we analyzed 293 peptides containing Cit residues based on the human proteome database mining work of Lee et al. (2018). The occurrence of Cit effect was found to be 44%. Comparing bond scissions at the amide linkage between Cit-Zzz (citrulline followed by a specified residue) to Aaa1-Aaa2 (Aaa can be any residue except Cit), 5 Cit-Zzz cleavages were significantly (CL = 95.0%) more frequent in > 85% of the cases in terms of relative sequential base beak occurrence. We used Pro effect to compare with Cit effect and obtained very similar results. On the other hand, our study showed that Cit effect is slightly inferior in the overall incidence to Pro effect (50% vs. 33%, CL = 95%) among deiminated peptides when Pro residues were also present in the sequence. Our results suggest that Cit effect is a characteristic feature and a possible biasing factor of deiminated peptides which can confirm the position of citrullination sites.

How Autoantibodies Regulate Osteoclast Induced Bone Loss in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease, characterized by autoimmunity that triggers joint inflammation and tissue destruction. Traditional concepts of RA pathogenesis have strongly been focused on inflammation. However, more recent evidence suggests that autoimmunity per se modulates the disease and in particular bone destruction during the course of RA. RA-associated bone loss is caused by increased osteoclast differentiation and activity leading to rapid bone resorption. Autoimmunity in RA is based on autoantibodies such as rheumatoid factor (RF) and autoantibodies against citrullinated proteins (ACPA). These autoantibodies exert effector functions on immune cells and on bone resorbing osteoclasts, thereby facilitating bone loss. This review summarizes potential pathways involved in increased destruction of bone tissue in RA, particularly focusing on the direct and indirect actions of autoantibodies on osteoclast generation and function.

Post-Translational Modifications of Proteins: Novel Insights in the Autoimmune Response in Rheumatoid Arthritis

Post-translational modifications (PTM) are chemical changes mostly catalyzed by enzymes that recognize specific target sequences in specific proteins. These modifications play a key role in regulating the folding of proteins, their targeting to specific subcellular compartments, their interaction with ligands or other proteins, and eventually their immunogenic properties. Citrullination is the best characterized PTM in the field of rheumatology, with antibodies anticyclic citrullinated peptides being the gold standard for the diagnosis of rheumatoid arthritis (RA). In recent years, growing evidence supports not only that a wide range of proteins are subject to citrullination and can trigger an autoimmune response in RA, but also that several other PTMs such as carbamylation and acetylation occur in patients with this disease. This induces a wide spectrum of autoantibodies, as biomarkers, with different sensitivity and specificity for diagnosis, which may be linked to peculiar clinical manifestations and/or response to treatment. The purpose of this review article is to critically summarize the available literature on antibodies against post-translationally modified proteins, in particular antibodies against citrullinated proteins (ACPA) and antibodies against modified proteins (AMPA), and outline their diagnostic and prognostic role to be implemented in clinical practice for RA patients.

Discovery of Novel Potential Reversible Peptidyl Arginine Deiminase Inhibitor

Citrullination, a posttranslational modification, is catalyzed by peptidylarginine deiminases (PADs), a unique family of enzymes that converts peptidyl-arginine to peptidyl-citrulline. Overexpression and/or increased PAD activity is observed in rheumatoid arthritis (RA), Alzheimer’s disease, multiple sclerosis, and cancer. Moreover, bacterial PADs, such as Porphyromonas gingivalis PAD (PPAD), may have a role in the pathogenesis of RA, indicating PADs as promising therapeutic targets. Herein, six novel compounds were examined as potential inhibitors of human PAD4 and PPAD, and compared to an irreversible PAD inhibitor, Cl-amidine. Four of the tested compounds (compounds 2, 3, 4, and 6) exhibited a micromolar-range inhibition potency against PAD4 and no effect against PPAD in the in vitro assays. Compound 4 was able to inhibit the PAD4-induced citrullination of H3 histone with higher efficiency than Cl-amidine. In conclusion, compound 4 was highly effective and presents a promising direction in the search for novel RA treatment strategies.

Peptidylarginine Deiminase Inhibitor Cl-Amidine Attenuates Cornification and Interferes with the Regulation of Autophagy in Reconstructed Human Epidermis

Deimination, a post-translational modification catalyzed by a family of enzymes called peptidylarginine deiminases (PADs), is the conversion of arginine into citrulline residues in a protein. Deimination has been associated with numerous physiological and pathological processes. Our aim was to study its implication in the homeostasis of human epidermis, where three PADs are expressed, namely PAD1, 2, and 3. Three-dimensional reconstructed human epidermis (RHEs) were treated for 2 days with increased concentrations (0-800 μM) of Cl-amidine, a specific PAD inhibitor. Cl-amidine treatments inhibited deimination in a dose-dependent manner and were not cytotoxic for keratinocytes. At 800 μM , Cl-amidine was shown to reduce deimination by half, alter keratinocyte differentiation, decrease the number of corneocyte layers, significantly increase the number of transitional cells, induce clustering of mitochondria and of heterogeneous vesicles in the cytoplasm of granular keratinocytes, and upregulate the expression of autophagy proteins, including LC3-II, sestrin-2, and p62/SQSTM1. LC3 and PADs were further shown to partially co-localize in the upper epidermis. These results demonstrated that Cl-amidine treatments slow down cornification and alter autophagy in the granular layer. They suggest that PAD1 and/or PAD3 play a role in the constitutive epidermal autophagy process that appears as an important step in cornification.

Thioredoxin Modulates Protein Arginine Deiminase 4 (PAD4)-Catalyzed Citrullination

Protein citrullination is a post-translational modification catalyzed by the protein arginine deiminases (PADs). This modification plays a crucial role in the pathophysiology of numerous autoimmune disorders including RA. Recently, there has been a growing interest in investigating physiological regulators of PAD activity to understand the primary cause of the associated disorders. Apart from calcium, it is well-documented that a reducing environment activates the PADs. Although the concentration of thioredoxin (hTRX), an oxidoreductase that maintains the cellular reducing environment, is elevated in RA patients, its contribution toward RA progression or PAD activity has not been explored. Herein, we demonstrate that hTRX activates PAD4. Kinetic characterization of PAD4 using hTRX as the reducing agent yielded parameters that are comparable to those obtained with a routinely used non-physiological reducing agent, e.g., DTT, suggesting the importance of hTRX in PAD regulation under physiological conditions. Furthermore, we show that various hTRX mutants, including redox inactive hTRX variants, are capable of activating PAD4. This indicates a mechanism that does not require oxidoreductase activity. Indeed, we observed non-covalent interactions between PAD4 and hTRX variants, and propose that these redox-independent interactions are sufficient for hTRX-mediated PAD4 activation.

Arginine Citrullination at the C-Terminal Domain Controls RNA Polymerase II Transcription

The post-translational modification of key residues at the C-terminal domain of RNA polymerase II (RNAP2-CTD) coordinates transcription, splicing, and RNA processing by modulating its capacity to act as a landing platform for a variety of protein complexes. Here, we identify a new modification at the CTD, the deimination of arginine and its conversion to citrulline by peptidyl arginine deiminase 2 (PADI2), an enzyme that has been associated with several diseases, including cancer. We show that, among PADI family members, only PADI2 citrullinates R1810 (Cit1810) at repeat 31 of the CTD. Depletion of PADI2 or loss of R1810 results in accumulation of RNAP2 at transcription start sites, reduced gene expression, and inhibition of cell proliferation. Cit1810 is needed for interaction with the P-TEFb (positive transcription elongation factor b) kinase complex and for its recruitment to chromatin. In this way, CTD-Cit1810 favors RNAP2 pause release and efficient transcription in breast cancer cells.

Potential Impact of Oral Inflammations on Cardiac Functions and Atrial Fibrillation

Inflammation may be a risk factor for atrial fibrillation (AF). Oral infections frequently lead to chronic inflammation, such as gingivitis, periodontitis, and endodontic lesions. In this narrative review, we consider five basic pathogenic mechanisms that involve oral infections and inflammations in the pathogenesis of AF: (1) low level bacteremia by which oral bacteria enter the blood stream at inflamed sites of the oral cavity and invade the heart; (2) Systemic inflammation induced by inflammatory mediators, which are released from the sites of oral inflammation into the blood stream, affecting cardiac remodeling; (3) autoimmunity against molecular structures expressed in the heart caused by the host immune response to specific components of oral pathogens; (4) potentially arrhythmic effects mediated by activation of the autonomous nervous system triggered by oral inflammations; and (5) arrhythmic effects resulting from specific bacterial toxins that are produced by oral pathogenic bacteria. A number of studies support the involvement of all five mechanisms, suggesting a potentially complex contribution of oral inflammations to the pathogenesis of AF.