Molecular Factors in PAD2 (PADI2) and PAD4 (PADI4) Are Associated with Interstitial Lung Disease Susceptibility in Rheumatoid Arthritis Patients

Around 50% of rheumatoid arthritis (RA) patients show some extra-articular manifestation, with the lung a usually affected organ; in addition, the presence of anti-citrullinated protein antibodies (ACPA) is a common feature, which is caused by protein citrullination modifications, catalyzed by the peptidyl arginine deiminases (PAD) enzymes. We aimed to identify single nucleotide variants (SNV) in PADI2 and PADI4 genes (PAD2 and PAD4 proteins, respectively) associated with susceptibility to interstitial lung disease (ILD) in RA patients and the PAD2 and PAD4 levels. Material and methods: 867 subjects were included: 118 RA-ILD patients, 133 RA patients, and 616 clinically healthy subjects (CHS). Allelic discrimination was performed in eight SNVs using qPCR, four in PADI2 and four in PADI4. The ELISA technique determined PAD2 and PAD4 levels in serum and bronchoalveolar lavage (BAL) samples, and the population structure was evaluated using 14 informative ancestry markers. Results: The rs1005753-GG (OR = 4.9) in PADI2 and rs11203366-AA (OR = 3.08), rs11203367-GG (OR = 2.4) in PADI4 are associated with genetic susceptibility to RA-ILD as well as the ACTC haplotype (OR = 2.64). In addition, the PAD4 protein is increased in RA-ILD individuals harboring the minor allele homozygous genotype in PADI4 SNVs. Moreover, rs1748033 in PADI4, rs2057094, and rs2076615 in PADI2 are associated with RA susceptibility. In conclusion, in RA patients, single nucleotide variants in PADI4 and PADI2 are associated with ILD susceptibility. The rs1748033 in PADI4 and two different SNVs in PADI2 are associated with RA development but not ILD. PAD4 serum levels are increased in RA-ILD patients.

Role of hypochlorous acid (HOCl) and other inflammatory mediators in the induction of macrophage extracellular trap formation

The infiltration of activated leukocytes, including macrophages, at sites of inflammation and the formation and presence of hypochlorous acid (HOCl) are interlinked hallmarks of many debilitating disease processes, including atherosclerosis, arthritis, neurological and renal disease, diabetes and obesity. The production of extracellular traps by activated leukocytes in response to a range of inflammatory stimuli is increasingly recognised as an important process within a range of disease settings. We show that exposure of human monocyte-derived macrophages to pathophysiological levels of HOCl results in the dose-dependent extrusion of DNA and histones into the cellular supernatant, consistent with extracellular trap formation. Concurrent with, but independent of these findings, macrophage exposure to HOCl also resulted in an immediate and sustained cytosolic accumulation of Ca²⁺, culminating in the increased production of cytokines and chemokines. Polarisation of the macrophages prior to HOCl exposure revealed a greater propensity for inflammatory M1 macrophages to produce extracellular traps, whereas alternatively-activated M2 macrophages were less susceptible to HOCl insult. M1 macrophages also produced extracellular traps on exposure to phorbol myristate acetate (PMA), interleukin-8 (IL-8) and tumour necrosis factor α (TNFα). Taken together, these data indicate a potential role for macrophages in mediating extracellular trap formation, which may be relevant in pathological conditions characterised by chronic inflammation or excessive HOCl formation.

Relative efficiencies of peptidylarginine deiminase 2 and 4 in generating target sites for anti-citrullinated protein antibodies in fibrinogen, alpha-enolase and histone H3

OBJECTIVE

Peptidylarginine deiminase 2 (PAD2) and PAD4 are expressed in the synovium of rheumatoid arthritis (RA) patients and catalyze citrullination of arginine residues in proteins targeted by anti-citrullinated protein antibodies (ACPAs). Little is known about the relative importance of PAD2 and PAD4 in generating citrullinated self-antigens. Here we investigate the ability of PAD2 and PAD4 to generate citrullinated targets for ACPAs in four human proteins.

METHODS

Synovial fluid (SF) and plasma were collected from 42 RA patients. Human fibrinogen, human alpha-enolase (ENO1), human histone H3, and human serum albumin (HSA) were citrullinated in vitro by PAD2 or PAD4. The total degree of citrullination was determined using the anti-modified citrulline approach. Antibody binding to native and citrullinated proteins was measured by ELISA.

RESULTS

ACPAs within pooled SF from multiple RA patients reacted equally well with, and cross-reacted with, PAD2- and PAD4-citrullinated fibrinogen. ACPAs from most individual patient SF and plasma samples bound equally well to PAD2- and PAD4-citrullinated fibrinogen or ENO1. When histone H3 was used as target, PAD4 was generally superior in generating epitopes recognized by ACPAs. No binding to citrullinated HSA was observed.

CONCLUSION

In most patients, PAD2 and PAD4 are equally efficient in generating citrullinated target sites for ACPAs in fibrinogen and ENO1. The binding of autoantibodies to histone H3 was generally higher after citrullination with PAD4 than with PAD2. Citrullinated HSA is not a target for ACPAs.

Accumulation of citrullinated glial fibrillary acidic protein in a mouse model of bile duct ligation-induced hepatic fibrosis

Hepatic stellate cells (HSCs) play pivotal roles in hepatic fibrosis as they synthesize glial fibrillary acidic protein (GFAP), which is increased in activated HSCs. GFAP-expressing HSCs and myofibroblasts accumulate in and around hepatic fibrosis lesions. Peptidylarginine deiminase 2 (PAD2) is responsible for the citrullination of GFAP (cit-GFAP). However, the involvement of PAD2 and cit-GFAP in hepatic fibrosis remains unclear. To determine the expression of PAD2 and cit-GFAP in hepatic fibrosis, C57BL/6 mice underwent bile duct ligation (BDL) or a sham operation. In BDL livers, the expression of PAD2 and its enzyme activity were significantly increased compared with controls. In addition, PAD2-postitive cells were rarely observed in only the portal vein and the small bile duct in sham-operated livers, whereas an increased number of PAD2-positive cells were detected in the bile duct and Glisson’s sheath in BDL livers. Interestingly, PAD2 was colocalized with α-SMA-positive cells and CK19-positive cells in BDL livers, indicating upregulated PAD2 in activated HSCs and portal fibroblasts of the livers of BDL mice. We also found that citrullinated proteins were highly accumulated in the livers of BDL mice compared with controls. Moreover, the expression level of GFAP and the amount of cit-GFAP were higher in BDL livers than in control livers. In correlation with PAD2 localization, cit-GFAP was observed in α-SMA-positive and CK19-positive cells in the livers of BDL mice. These results suggest that the increased expression and activation of PAD2 along with increased citrullinated proteins, specifically cit-GFAP, may play important roles in the pathogenesis of hepatic fibrosis.

Modifying Enzymes Are Elicited by ER Stress, Generating Epitopes That Are Selectively Recognized by CD4+ T Cells in Patients With Type 1 Diabetes

In spite of tolerance mechanisms, some individuals develop T-cell-mediated autoimmunity. Posttranslational modifications that increase the affinity of epitope presentation and/or recognition represent one means through which self-tolerance mechanisms can be circumvented. We investigated T-cell recognition of peptides that correspond to modified β-cell antigens in subjects with type 1 diabetes. Modified peptides elicited enhanced proliferation by autoreactive T-cell clones. Endoplasmic reticulum (ER) stress in insulinoma cells increased cytosolic calcium and the activity of tissue transglutaminase 2 (tTG2). Furthermore, stressed human islets and insulinomas elicited effector responses from T cells specific for modified peptides, suggesting that ER stress-derived tTG2 activity generated deamidated neoepitopes that autoreactive T cells recognized. Patients with type 1 diabetes had large numbers of T cells specific for these epitopes in their peripheral blood. T cells with these specificities were also isolated from the pancreatic draining lymph nodes of cadaveric donors with established diabetes. Together, these results suggest that self-antigens are enzymatically modified in β-cells during ER stress, giving rise to modified epitopes that could serve to initiate autoimmunity or to further broaden the antigenic repertoire, activating potentially pathogenic CD4⁺ T cells that may not be effectively eliminated by negative selection.

The expression of mRNA for peptidylarginine deiminase type 2 and type 4 in bone marrow CD34+ cells in rheumatoid arthritis

OBJECTIVES

Antibodies directed to citrullinated proteins are highly specific for rheumatoid arthritis (RA). Citrullination is catalyzed by peptidylarginine deiminase (PAD) enzymes. The current study examined the mRNA expression of PADI2 and PADI4 in bone marrow (BM) CD34+ cells from RA patients.

METHODS

CD34+ cells were purified from BM samples obtained from 48 RA patients and from 30 osteoarthritis (OA) patients during joint operations via aspiration from the iliac crest. The expression of mRNAs for PADI2, PADI4 and Sp1 was examined by quantitative reverse transcription PCR.

RESULTS

The expression of mRNA for PADI2 was significantly higher in RA BM CD34+ cells than OA BM CD34+ cells. The expression of mRNAs for PADI4 and Sp1 in RA BM CD34+ cells appeared to be increased compared to OA BM CD34+ cells, although it did not reach the statistical significance. The levels of mRNAs for PADI2, PADI4 and Sp1 were not correlated with serum C-reactive protein or with the administration of methotrexate or oral steroids. Finally, the level of PADI2 mRNA as well as that of PADI4 mRNA was significantly correlated with the level of Sp1 mRNA in RA BM CD34+ cells.

CONCLUSIONS

These results indicate that the mRNA expression of PADI2, PADI4 and Sp1 is upregulated in RA BM CD34+ cells independently of the systemic inflammation or treatment regimen. Moreover, the data suggest that the enhanced mRNA expression of PADI2 and PADI4 in BM CD34+ cells might be a result of the enhanced expression of Sp1 gene in RA BM CD34+ cells.

Activation of Peptidylarginine Deiminase in the Salivary Glands of Balb/c Mice Drives the Citrullination of Ro and La Ribonucleoproteins

The goal of the present study was to determine whether peptidylarginine deiminase PAD2 and PAD4 enzymes are present in Balb/c mouse salivary glands and whether they are able to citrullinate Ro and La ribonucleoproteins. Salivary glands from Balb/c mice were cultured in DMEM and supplemented with one of the following stimulants: ATP, LPS, TNF, IFNγ, or IL-6. A control group without stimulant was also evaluated. PAD2, PAD4, citrullinated peptides, Ro60, and La were detected by immunohistochemistry and double immunofluorescence. PAD2 and PAD4 mRNAs and protein expression were detected by qPCR and Western blot analysis. PAD activity was assessed using an antigen capture enzyme-linked immunosorbent assay. LPS, ATP, and TNF triggered PAD2 and PAD4 expression; in contrast, no expression was detected in the control group (p < 0.001). PAD transcription slightly increased in response to stimulation. Additionally, PAD2/4 activity modified the arginine residues of a reporter protein (fibrinogen) in vitro. PADs citrullinated Ro60 and La ribonucleoproteins in vivo. Molecular stimulants induced apoptosis in ductal cells and the externalization of Ro60 and La ribonucleoproteins onto apoptotic membranes. PAD enzymes citrullinate Ro and La ribonucleoproteins, and this experimental approach may facilitate our understanding of the role of posttranslational modifications in the pathophysiology of Sjögren's syndrome.

Identification of Padi2 as a novel angiogenesis-regulating gene by genome association studies in mice

Recent findings indicate that growth factor-driven angiogenesis is markedly influenced by genetic variation. This variation in angiogenic responsiveness may alter the susceptibility to a number of angiogenesis-dependent diseases. Here, we utilized the genetic diversity available in common inbred mouse strains to identify the loci and candidate genes responsible for differences in angiogenic response. The corneal micropocket neovascularization assay was performed on 42 different inbred mouse strains using basic fibroblast growth factor (bFGF) pellets. We performed a genome-wide association study utilizing efficient mixed-model association (EMMA) mapping using the induced vessel area from all strains. Our analysis yielded five loci with genome-wide significance on chromosomes 4, 8, 11, 15 and 16. We further refined the mapping on chromosome 4 within a haplotype block containing multiple candidate genes. These genes were evaluated by expression analysis in corneas of various inbred strains and in vitro functional assays in human microvascular endothelial cells (HMVECs). Of these, we found the expression of peptidyl arginine deiminase type II (Padi2), known to be involved in metabolic pathways, to have a strong correlation with a haplotype shared by multiple high angiogenic strains. In addition, inhibition of Padi2 demonstrated a dosage-dependent effect in HMVECs. To investigate its role in vivo, we knocked down Padi2 in transgenic kdrl:zsGreen zebrafish embryos using morpholinos. These embryos had disrupted vessel formation compared to control siblings. The impaired vascular pattern was partially rescued by human PADI2 mRNA, providing evidence for the specificity of the morphant phenotype. Taken together, our study is the first to indicate the potential role of Padi2 as an angiogenesis-regulating gene. The characterization of Padi2 and other genes in associated pathways may provide new understanding of angiogenesis regulation and novel targets for diagnosis and treatment of a wide variety of angiogenesis-dependent diseases.

Smoking is associated with increased levels of extracellular peptidylarginine deiminase 2 (PAD2) in the lungs

OBJECTIVES

Smoking is a well-established risk factor in rheumatoid arthritis (RA), and citrullination of self-antigens plays a pathogenic role in the majority of patients. Increased numbers of peptidylarginine deiminase 2 (PAD2)-containing macrophages have been demonstrated in bronchoalveolar lavage (BAL) fluid from smokers, but intracellularly located PAD cannot be responsible for citrullination of extracellular self-antigens. We aimed to establish a link between smoking and extracellular PAD2 in the lungs.

METHODS

BAL fluid samples were obtained from 13 smokers and 11 nonsmoking controls. Total protein content and C-reactive protein (CRP) concentration were determined after separating cells from the samples. PAD2 content in cell-free BAL fluids was measured by means of a PAD2-specific sandwich ELISA.

RESULTS

Significantly increased levels of soluble PAD2 were detected in cell-free BAL fluids from smokers as compared to non-smokers (p=0.018). The PAD2 content correlated with the overall CRP levels (p=0.009) and cell count (p=0.016).

CONCLUSIONS

This first demonstration of increased levels of extracellular PAD2 in the lungs of smokers supports the hypothesis that smoking promotes extracellular citrullination of proteins. This may represent a pathological event upstream for the production of anti-citrullinated protein antibodies (ACPAs) among RA patients carrying HLA-molecules capable of binding citrullinated self-peptides.

Targeted H3R26 deimination specifically facilitates estrogen receptor binding by modifying nucleosome structure

Transcription factor binding to DNA in vivo causes the recruitment of chromatin modifiers that can cause changes in chromatin structure, including the modification of histone tails. We previously showed that estrogen receptor (ER) target gene activation is facilitated by peptidylarginine deiminase 2 (PAD2)-catalyzed histone H3R26 deimination (H3R26Cit). Here we report that the genomic distributions of ER and H3R26Cit in breast cancer cells are strikingly coincident, linearly correlated, and observed as early as 2 minutes following estradiol treatment. The H3R26Cit profile is unlike that of previously described histone modifications and is characterized by sharp, narrow peaks. Paired-end MNase ChIP-seq indicates that the charge-neutral H3R26Cit modification facilitates ER binding to DNA by altering the fine structure of the nucleosome. Clinically, we find that PAD2 and H3R26Cit levels correlate with ER expression in breast tumors and that high PAD2 expression is associated with increased survival in ER+ breast cancer patients. These findings provide insight into how transcription factors gain access to nucleosomal DNA and implicate PAD2 as a novel therapeutic target for ER+ breast cancer.

Transcription factors bind to DNA to activate and repress gene transcription. Many transcription factors, particularly nuclear receptors, associate with their cognate DNA element in a highly dynamic manner in vivo. Highly acetylated histone tails and DNase sensitive chromatin are amenable to the initial binding of transcription factors. Upon binding to DNA, transcription factor binding recruits remodelers and coactivators that can cause a concomitant increase in accessibility and acetylation. Herein, we show that estrogen receptor recruitment of a histone deiminase causes the positively charged H3R26 residue to be neutralized. This modification changes the fine structure of the nucleosome particle and facilitates estrogen receptor binding. Lastly, we find that high deiminase expression is associated with increased survival in estrogen receptor-positive breast cancer patients.

Vimentin is involved in peptidylarginine deiminase 2-induced apoptosis of activated Jurkat cells

Peptidylarginine deiminase type 2 (PADI2) deiminates (or citrullinates) arginine residues in protein to citrulline residues in a Ca2+-dependent manner, and is found in lymphocytes and macrophages. Vimentin is an intermediate filament protein and a well-known substrate of PADI2. Citrullinated vimentin is found in ionomycin-induced macrophage apoptosis. Citrullinated vimentin is the target of anti-Sa antibodies, which are specific to rheumatoid arthritis, and play a critical role in the pathogenesis of the disease. To investigate the role of PADI2 in apoptosis, we generated a Jurkat cell line that overexpressed the PADI2 transgene from a tetracycline-inducible promoter, and used a combination of 12-O-tetradecanoylphorbol-13-acetate and ionomycin to activate Jurkat cells. We found that PADI2 overexpression reduced the cell viability of activated Jurkat cells in1a dose- and time-dependent manner. The PADI2-overexpressed and -activated Jurkat cells presented typical manifestations of apoptosis, and exhibited greater levels of citrullinated proteins, including citrullinated vimentin. Vimentin overexpression rescued a portion of the cells from apoptosis. In conclusion, PADI2 overexpression induces apoptosis in activated Jurkat cells. Vimentin is involved in PADI2-induced apoptosis. Moreover, PADI2-overexpressed Jurkat cells secreted greater levels of vimentin after activation, and expressed more vimentin on their cell surfaces when undergoing apoptosis. Through artificially highlighting PADI2 and vimentin, we demonstrated that PADI2 and vimentin participate in the apoptotic mechanisms of activated T lymphocytes. The secretion and surface expression of vimentin are possible ways of autoantigen presentation to the immune system.

PADI2 is significantly associated with rheumatoid arthritis

Citrullination, a posttranslational modification of peptidyl arginine to citrulline, plays an essential role in rheumatoid arthritis (RA). Citrullination is catalyzed by a group of peptidylarginine deiminases (PADs) including PADI 1, 2, 3, 4 and 6. Many studies have indicated that the gene encoding PADI4 is a factor in susceptibility to RA. Some studies have detected PADI2 expression in RA synovial tissues, suggesting that PADI2 also plays an important role in the disease. This study evaluated the possible association between the PADI2-encoding gene and RA. Seventeen tag SNPs across the PAD locus were genotyped using a custom-designed Illumina 96-SNP VeraCode microarray. Peripheral blood samples were collected from patients with RA (n = 267), ankylosing spondylitis (AS, n = 51) and healthy controls (n = 160). The results of genotyping were verified using Sequenom MassARRAY in an independent cohort of 307 patients with RA, 324 patients with AS and 509 healthy controls. A western blot analysis was performed using synovial tissue from patients with RA (n = 7), osteoarthritis (OA, n = 7) and AS (n = 5) to determine the levels of expression of PADI2. A microarray analysis revealed a significant association between three selected PADI2 SNPs (rs2235926, rs2057094, rs2076616) and the presence of RA. The increased susceptibility to RA associated with rs2235926 (OR = 1.706733, 95% CI = [1.576366–1.866587], p = 0.000839) and rs2057094 (OR = 1.360432, 95% CI = [1.065483–1.869482], p = 0.003291) was further confirmed by the Sequenom MassARRAY. No tag SNPs in the PADI2 locus showed a significant association with AS. Increased expression of PADI2 was detected in RA synovial tissues compared with samples from patients with OA and AS. PADI2 is significantly associated with RA and may be involved in the pathogenesis of the disease.

Potential role for PAD2 in gene regulation in breast cancer cells

The peptidylarginine deiminase (PAD) family of enzymes post-translationally convert positively charged arginine residues in substrate proteins to the neutral, non-standard residue citrulline. PAD family members 1, 2, 3, and 6 have previously been localized to the cell cytoplasm and, thus, their potential to regulate gene activity has not been described. We recently demonstrated that PAD2 is expressed in the canine mammary gland epithelium and that levels of histone citrullination in this tissue correlate with PAD2 expression. Given these observations, we decided to test whether PAD2 might localize to the nuclear compartment of the human mammary epithelium and regulate gene activity in these cells. Here we show, for the first time, that PAD2 is specifically expressed in human mammary gland epithelial cells and that a portion of PAD2 associates with chromatin in MCF-7 breast cancer cells. We investigated a potential nuclear function for PAD2 by microarray, qPCR, and chromatin immunoprecipitation analysis. Results show that the expression of a unique subset of genes is disregulated following depletion of PAD2 from MCF-7 cells. Further, ChIP analysis of two of the most highly up- and down-regulated genes (PTN and MAGEA12, respectively) found that PAD2 binds directly to these gene promoters and that the likely mechanism by which PAD2 regulates expression of these genes is via citrullination of arginine residues 2-8-17 on histone H3 tails. Thus, our findings define a novel role for PAD2 in gene expression in human mammary epithelial cells.

Modulation of Peptidyl Arginine Deiminase 2 and Implication for Neurodegeneration

PURPOSE

To demonstrate that elevated pressure increases the peptidyl arginine deiminase 2 (PAD2) expression in cultured astrocytes in vitro that can be modulated by pharmacological agents modulating intracellular calcium.

METHODS

Isolated rat brain astrocytes were subjected to pressure treatment. Western and immunohistochemical analyses detected PAD2 protein expression. Calcium measurements were achieved employing fluorescence-based microscopic imaging and quantification system. Experiments were repeated with human optic nerve head-derived astrocytes.

RESULTS

PAD2 has recently been shown to be associated with glaucomatous optic nerve. Astrocytes subjected to pressure (25-100 mmHg) show elevated level of PAD2, increased intracellular calcium, and concomitant citrullination but not significant cell death. PAD2 expression in response to elevated pressure may play a role in glaucomatous neurodegeneration. Pressure-treated astrocytes were also subjected to thapsigargin (50-250 nM) treatment, but it is unclear whether this had any further effect in increasing PAD2 expression. Conversely, treatment with calcium chelating agent BAPTA-AM (50-250 nM) results in decreased intracellular calcium concentration and PAD2.

CONCLUSIONS

These results suggest calcium modulation could be exploited as therapeutic strategy to modulate pressure-induced PAD2 expression and citrullination.

Age-related reduction in retinal deimination levels in the F344BN rat

Increased deimination and peptidyl arginine deiminase type 2 (PAD2) expression has been observed in age-related neurodegenerative diseases without discrimination between their aging and disease component. Here, we describe reduced levels of deimination commensurate with reduced protein, mRNA and activity of peptidylarginine deiminase type 2 in the retina, optic nerve and plasma of aged rats when compared to young rats. The decrease was significant in the ganglion cell layer, inner plexiform layer and inner nuclear layer. Because our observations suggest reduced deimination is a consequence of aging, we conclude that increased deimination must be a consequence of disease. Our findings are important to understand late-onset and progressive diseases such as glaucoma, pseudoexfoliation syndrome, age-related macular degeneration and Oguchi's disease.

Peptidyl argininedeiminase 2 CpG island in multiple sclerosis white matter is hypomethylated

In previous studies, we documented increased citrullinated myelin basic protein (MBP) was present in MBP isolated from multiple sclerosis (MS) normal appearing white matter (NAWM). This increase was due to the myelin enzyme peptidyl argininedeiminase 2 (PAD2). In this study, we show that methylation of cytosine of the PAD2 promoter in DNA from MS NAWM was decreased to one-third of the level of that in DNA from normal white matter. The PAD2 promoter in DNA from thymus obtained from the same MS patients and white matter DNA from Alzheimer's, Huntington's, and Parkinson's was not hypomethylated. DNA demethylase activity in supernatants prepared from NAWM of MS patients was 2-fold higher than the DNA demethylase from normal, Alzheimer's, Huntington's and Parkinson's disease white matter. The amount of PAD2 enzyme and citrullinated MBP was increased in MS NAWM. The decreased methylation of cytosines in the PAD2 promoter may explain the increased synthesis of PAD2 protein that is responsible for the increased amount of citrullinated MBP, which in turn results in loss of myelin stability in MS brain.

Microglial expression of peptidylarginine deiminase 2 in the prenatal rat brain

Peptidylarginine deiminases (PADs) are Ca^2t+-dependant post-translational modification enzymes that catalyze the citrullination of protein arginyl residues. PAD type 2 (PAD2) is thought to be involved in some processes of neurodegeneration and myelination in the central nervous system. In this study, we found PAD2-positive cells in rat cerebra in 19-to 21-day old embryos, i.e. at a developmental stage well before myelination begins. Most of the cells were microglial marker-positive cells found mainly in the prospective medulla, and others were microglial marker-negative cells found mainly in the prospective dentate gyrus of the hippocampus. The former seemed to be in an activated state as judged by morphological criteria. The specificity of the enzyme activity, immunoblotting and reverse transcriptase-polymerase chain reaction analyses revealed that these cells expressed PAD2 and not PAD1, PAD3 or PAD4. Our data is indicative of microglial expression of PAD2 in the prenatal developing cerebrum.

Methylation of arginine residues interferes with citrullination by peptidylarginine deiminases in vitro

Peptidylarginine deiminase (PAD) enzymes catalyze the conversion of arginine residues in proteins to citrulline residues. Citrulline is a non-standard amino acid that is not incorporated in proteins during translation, but can be generated post-translationally by the PAD enzymes. Although the existence of citrulline residues in proteins has been known for a long time, only a few proteins have been reported to contain this amino acid under normal conditions. These include the nuclear histones, which also contain a wide variety of other post-translational modifications, as for instance methylation of arginine residues. It has been suggested that citrullination and methylation of arginine residues are competing processes and that PAD enzymes might "reverse" the methylation of arginine residues by converting monomethylated arginine into citrulline. However, conflicting data have been reported on the capacity of PADs to citrullinate monomethylated peptidylarginine. Using synthetic peptides that contain either arginine or methylated arginine residues, we show that the human PAD2, PAD3 and PAD4 enzymes and PAD enzyme present in several mouse tissues in vitro can only convert non-methylated peptidylarginine into peptidylcitrulline and that hPAD6 does not show any deiminating activity at all. A comparison of bovine histones either treated or untreated with PAD by amino acid analysis also supported the interference of deimination by arginine methylation. Taken together, these data indicate that it is unlikely that methyl groups at the guanidino position of peptidylarginine can be removed by peptidylarginine deiminases, which has important implications for the recently reported role of these enzymes in gene regulation.

Experimental autoimmune encephalomyelitis induction in peptidylarginine deiminase 2 knockout mice

During the development of multiple sclerosis the destruction of the myelin sheath surrounding the neurites is accompanied by citrullination of several central nervous system (CNS) proteins, including myelin basic protein and glial fibrillary acidic protein. In experimental autoimmune encephalomyelitis (EAE), a disease induced in animals by immunization with proteins or peptides from the CNS, the animals develop symptoms similar to multiple sclerosis (MS). The increased levels of citrullinated CNS proteins associated with MS are also observed during the development of EAE. To study the role of CNS protein citrullination in EAE development, we induced EAE with a peptide derived from myelin oligodendrocyte glycoprotein (MOG(35-55)) in mice lacking the peptidylarginine deiminase 2 (PAD2) protein, because this enzyme was the most likely candidate to be involved in catalyzing CNS protein citrullination in the diseased state. Even though the PAD2 knockout mice displayed a dramatic reduction in the amount of citrullination present in the CNS, indicating that PAD2 is indeed responsible for the majority of detectable citrullination observed in EAE, the development of EAE was not impaired by genetic deletion of PAD2, suggesting that PAD2 catalyzed citrullination is not essential to the development of EAE.

Citrullination preferentially proceeds in glomerular Bowman's capsule and increases in obstructive nephropathy

BACKGROUND

Peptidylarginine deiminases (PADs) are a group of posttranslational modification enzymes that citrullinate (deiminate) protein arginine residues, yielding citrulline residues. Citrullination of arginine residues abolishes their positive charge, markedly altering their structure. We undertook this study to investigate the actions of PADs in the kidney.

METHODS

In male rats, we ligated the unilateral ureter, then analyzed the obstructed and contralateral kidneys 1 week later. Controls were rats simultaneously given sham operations. In another experiment, we ligated unilateral ureters of eight rats, four of which received a ureter-bladder anastomosis 1 week later. These rats were subjected to histologic examinations 5 weeks after unilateral ureteral obstruction (UUO).

RESULTS

Reverse transcription-polymerase chain reaction (RT-PCR) revealed that, of PADs (type I, II, III, and IV), only PAD type II was expressed in kidneys. Western blot study showed that PAD type II expression and citrullinated protein content increased greatly in kidneys that underwent unilateral ureteral ligation compared to that in contralateral or sham-operated kidneys. Immunohistochemical analyses revealed that PAD type II was preferentially expressed by parietal epithelial cells and that only in Bowman's capsule were proteins citrullinated. Additionally, these PAD type II and citrullinated proteins in obstructed nephropathy were significantly attenuated by the release of the obstruction. Proteome analysis revealed that one of citrullinated proteins in the kidney should be actin.

CONCLUSION

This result indicates that PAD type II and citrullinated proteins are suitable markers of Bowman's capsule. Not only are these markers preferentially expressed in Bowman's capsules but their expression is also increased in damaged kidneys by UUO, features that promise the further clarification of kidney diseases.

Regulation of the expression of peptidylarginine deiminase type II gene (PADI2) in human keratinocytes involves Sp1 and Sp3 transcription factors

Peptidylarginine deiminases (PAD) convert protein-bound arginine residues into citrulline residues in a Ca(2+) ion-dependent manner. Among the five isoforms (PAD1, 2, 3, 4, and 6) existing in rodents and humans, PAD2 is the most widely expressed in both species, tissues, and organs. In order to study the mechanisms regulating the expression of the human PAD2 gene, PADI2, we characterized its promoter region using transfected human keratinocytes. A series of reporter gene constructions derived from the 2 kb region upstream of the transcription initiation site defined a minimal promoter sequence from nucleotides -132 to -41. This PADI2 region is GC-rich and lacks canonical TATA and CAAT boxes. Investigation of cis-acting elements in the region, further deletion analyses and electrophoretic mobility shift assays using specific antibodies revealed four Sp1-binding sites and identified Sp1 and Sp3 as binding factors important for the promoter activity. These results suggest that Sp1/Sp3 cooperation may provide a mechanism to control the transcription of PADI2.

Abnormal accumulation of citrullinated proteins catalyzed by peptidylarginine deiminase in hippocampal extracts from patients with Alzheimer's disease

Citrullinated proteins are the products of a posttranslational process in which arginine residues undergo modification into citrulline residues when catalyzed by peptidylarginine deiminases (PADs) in a calcium ion-dependent manner. In our previous report, PAD2 expressed mainly in the rat cerebrum became activated early in the neurodegenerative process. To elucidate the involvement of protein citrullination in human neuronal degeneration, we examined whether citrullinated proteins are produced during Alzheimer's disease (AD). By Western blot analysis with antimodified citrulline antibody, citrullinated proteins of varied molecular weights were detected in hippocampal tissues from patients with AD but not normal humans. Two of the citrullinated proteins were identified as vimentin and glial fibrillary acidic protein (GFAP) by using two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry. Interestingly, PAD2 was detected in hippocampal extracts from AD and normal brains, but the amount of PAD2 in the AD tissue was markedly greater. Histochemical analysis revealed citrullinated proteins throughout the hippocampus, especially in the dentate gyrus and stratum radiatum of CA1 and CA2 areas. However, no citrullinated proteins were detected in the normal hippocampus. PAD2 immunoreactivity was also ubiquitous throughout both the AD and the normal hippocampal areas. PAD2 enrichment coincided well with citrullinated protein positivity. Double immunofluorescence staining revealed that citrullinated protein- and PAD2-positive cells also coincided with GFAP-positive cells, but not all GFAP-positive cells were positive for PAD2. As with GFAP, which is an astrocyte-specific marker protein, PAD2 is distributed mainly in astrocytes. These collective results, the abnormal accumulation of citrullinated proteins and abnormal activation of PAD2 in hippocampi of patients with AD, strongly suggest that PAD has an important role in the onset and progression of AD and that citrullinated proteins may become a useful marker for human neurodegenerative diseases.

Comparison of enzymatic properties between hPADI2 and hPADI4

In the sera of rheumatoid arthritis (RA) patients, autoantibodies directed to citrullinated proteins are found with high specificity for RA. Peptidylarginine deiminases (PADIs) are enzymes responsible for protein citrullination. Among many isoforms of PADIs, only PADI4 has been identified as an RA-susceptibility gene. To understand the mechanisms of the initiation and progression of RA, we compared the properties of two PADIs, human PADI2 and human PADI4, which are present in the synovial tissues of RA patients. We confirmed their precise distribution in the RA synovium and compared the stability, Ca2+ dependency, optimal pH range, and substrate specificity. Small but significant differences were found in the above-mentioned properties between hPADI2 and hPADI4. Using LC/MS/MS analysis, we identified the sequences in human fibrinogen indicating that hPADI2 and hPADI4 citrullinate in different manners. Our results indicate that hPADI2 and hPADI4 have different roles under physiological and pathological conditions. Further studies are needed for the better understanding of the role of hPADIs in the initiation and progression of RA.

Expression and activity of citrullinating peptidylarginine deiminase enzymes in monocytes and macrophages

BACKGROUND

Antibodies directed to proteins containing the non-standard amino acid citrulline, are extremely specific for rheumatoid arthritis (RA). Peptidylcitrulline can be generated by post-translational conversion of arginine residues. This process, citrullination, is catalysed by a group of calcium dependent peptidylarginine deiminase (PAD) enzymes.

OBJECTIVE

To investigate the expression and activity of four isotypes of PAD in peripheral blood and synovial fluid cells of patients with RA.

RESULTS

The data presented here show that citrullination of proteins by PAD enzymes is a process regulated at three levels: transcription-in peripheral blood PAD2 and PAD4 mRNAs are expressed predominantly in monocytes; PAD4 mRNA is not detectable in macrophages, translation-translation of PAD2 mRNA is subject to differentiation stage-specific regulation by its 3' UTR, and activation-the PAD proteins are only activated when sufficient Ca(2+) is available. Such high Ca(2+) concentrations are normally not present in living cells. In macrophages, which are abundant in the inflamed RA synovium, vimentin is specifically citrullinated after Ca(2+) influx.

CONCLUSION

PAD2 and PAD4 are the most likely candidate PAD isotypes for the citrullination of synovial proteins in RA. Our results indicate that citrullinated vimentin is a candidate autoantigen in RA.

Citrullination of synovial proteins in murine models of rheumatoid arthritis

OBJECTIVE

Antibodies directed to citrulline-containing proteins are highly specific for rheumatoid arthritis (RA) and can be detected in up to 80% of patients with RA. Citrulline is a nonstandard amino acid that can be incorporated into proteins only by posttranslational modification of arginine by peptidylarginine deiminase (PAD) enzymes. The objective of this study was to investigate the presence of anticitrulline antibodies, PAD enzymes, and citrullinated antigens in mouse models of both acute and chronic destructive arthritis: streptococcal cell wall (SCW)-induced arthritis and collagen-induced arthritis (CIA), respectively.

METHODS

Synovial tissue biopsy specimens were obtained from naive mice, mice with CIA, and mice with SCW-induced arthritis. The expression of messenger RNA (mRNA) for PAD enzymes was analyzed by reverse transcriptase-polymerase chain reaction; the presence of PAD proteins and their products (citrullinated proteins) was analyzed by Western blotting and by immunolocalization. The presence of anticitrullinated protein antibodies was investigated by an anti-cyclic citrullinated peptide (anti-CCP) enzyme-linked immunosorbent assay (ELISA) and an ELISA using in vitro citrullinated fibrinogen.

RESULTS

In both mouse models, PAD type 2 (PAD2) mRNA was present in the synovium but was not translated into PAD2 protein. In contrast, PAD4 mRNA, although absent from healthy synovium, was readily transcribed and translated by polymorphonuclear neutrophils infiltrating the synovial tissue during inflammation. As a consequence, several synovial proteins were subjected to citrullination. One of these proteins was identified as fibrin, which has been reported to be citrullinated also in synovium of patients with RA. Although generation of citrullinated antigens during synovial inflammation in the mice was eminent, no anti-CCP antibodies could be detected.

CONCLUSION

Citrullination of synovial antigens is an active process during joint inflammation in both mice and humans, but the induction of autoantibodies directed to these proteins is a more specific phenomenon, detectable only in human RA patients.

Human peptidylarginine deiminase type II: molecular cloning, gene organization, and expression in human skin

Peptidylarginine deiminases (PADs) are posttranslational modification enzymes that convert protein arginine to citrulline residues in a calcium ion-dependent manner. Rodents have four isoforms of PAD (types I, II, III, and IV), each of which is distinct in substrate and tissue specificity. In fact, the only tissue in which all four PAD mRNAs have been detected is the epidermis. In this study, we found PAD activity in HSC-1 human cutaneous squamous carcinoma cells in vitro, and this activity increased during cultivation. Using a homology-based strategy, we cloned a full-length cDNA encoding human PAD type II. The cDNA was 2348 bp long and encoded a 665-amino-acid sequence with a predicted molecular mass of 75 kDa. The predicted protein shared 93% identity with the rat and mouse PAD type II sequence. Alignment of the amino acid sequences from both species revealed notable conservation in the C-terminal region, suggesting the presence of a functional region such as an enzyme catalytic site and/or a calcium-binding domain. Gene organization analysis established that human PAD type II on chromosome 1p35.2-p35.21 spanned more than 50 kb and contained 16 exons and 15 introns. A recombinant PAD protein subsequently produced in Escherichia coli proved to be enzymatically active, with substrate specificities similar to those of the rat PAD type II. In an immunohistochemical study of human skin, the type II enzyme was expressed by all the living epidermal layers, suggesting that PAD type II is functionally important during terminal differentiation of epidermal keratinocytes.

Increased and type II-specific expression of peptidylarginine deiminase in activated microglia but not hyperplastic astrocytes following kainic acid-evoked neurodegeneration in the rat brain

Peptidylarginine deiminases (PADs) are a group of posttranslational modification enzymes that convert protein arginine residues to citrulline residues. In the rat cerebrum, the type II PAD is thought to be expressed mainly in glial cells, especially astrocytes, and to become activated early in the neurodegenerative process. To determine whether hyperplastic glial cells express PAD type II, we examined the rat brain after kainic acid-evoked neurodegeneration. Western blot and reverse transcriptase-polymerase chain reaction analyses revealed increased and type II-specific expression of PAD in the brain at 4-7 days after kainate administration. Immunocytochemically, no PAD type II immunoreactivity was observed in the glial fibrillary acidic protein-positive astrocytes, but such immunoreactivity was present coincident with a microglial marker recognized by Bandeiraea simplicifolia isolectin B4 in the damaged regions. These results clearly indicate that PAD type II is specifically and abundantly expressed in activated microglial cells and suppressed in hyperplastic astrocytes following neurodegeneration.