Detection of deiminated proteins in rat skin: probing with a monospecific antibody after modification of citrulline residues

Citrullination of CXCL10 and CXCL11 by peptidylarginine deiminase: a naturally occurring posttranslational modification of chemokines and new dimension of immunoregulation

Interactions between chemokines and enzymes are vital in immunoregulation. Structural protein citrullination by peptidylarginine deiminase (PAD) has been associated with autoimmunity. In this report, we identified a novel naturally occurring posttranslational modification of chemokines, that is, the deimination of arginine at position 5 into citrulline of CXC chemokine ligand 10 (CXCL10) by rabbit PAD and human PAD2. Citrullination reduced (≥ 10-fold) the chemoattracting and signaling capacity of CXCL10 for CXC chemokine receptor 3 (CXCR3) transfectants; however, it did not affect CXCR3 binding. On T lymphocytes, though, citrullinated CXCL10 remained active but was again weaker than authentic CXCL10. PAD was also able to convert CXCL11, causing an impairment of CXCR3 signaling and T-cell activation, though less pronounced than for CXCL10. Similarly, receptor binding properties of CXCL11 were not altered by citrullination. However, deimination decreased heparin binding properties of both CXCL10 and CXCL11. Overall, chemokines are the first immune modulators reported of being functionally modified by citrullination. These data provide new structure-function dimensions for chemokines in leukocyte mobilization, disclosing an anti-inflammatory role for PAD. Additionally because citrullination has severe consequences for chemokine biology, this invites to reassess the involvement and impact of PAD and citrullinated peptides in inflammation, autoimmunity, and hematologic disorders.

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

Biological functions of proteins are influenced by posttranslational modifications such as on/off switching by phosphorylation and modulation by glycosylation. Proteolytic processing regulates cytokine and chemokine activities. In this study, we report that natural posttranslational citrullination or deimination alters the biological activities of the neutrophil chemoattractant and angiogenic cytokine CXCL8/interleukin-8 (IL-8). Citrullination of arginine in position 5 was discovered on 14% of natural leukocyte-derived CXCL8(1-77), generating CXCL8(1-77)Cit(5). Peptidylarginine deiminase (PAD) is known to citrullinate structural proteins, and it may initiate autoimmune diseases. PAD efficiently and site-specifically citrullinated CXCL5, CXCL8, CCL17, CCL26, but not IL-1beta. In comparison with CXCL8(1-77), CXCL8(1-77)Cit(5) had reduced affinity for glycosaminoglycans and induced less CXCR2-dependent calcium signaling and extracellular signal-regulated kinase 1/2 phosphorylation. In contrast to CXCL8(1-77), CXCL8(1-77)Cit(5) was resistant to thrombin- or plasmin-dependent potentiation into CXCL8(6-77). Upon intraperitoneal injection, CXCL8(6-77) was a more potent inducer of neutrophil extravasation compared with CXCL8(1-77). Despite its retained chemotactic activity in vitro, CXCL8(1-77)Cit(5) was unable to attract neutrophils to the peritoneum. Finally, in the rabbit cornea angiogenesis assay, the equally potent CXCL8(1-77) and CXCL8(1-77)Cit(5) were less efficient angiogenic molecules than CXCL8(6-77). This study shows that PAD citrullinates the chemokine CXCL8, and thus may dampen neutrophil extravasation during acute or chronic inflammation.

Circulating immune complexes contain citrullinated fibrinogen in rheumatoid arthritis

Introduction

There is increasing evidence that autoantibodies and immune complexes (ICs) contribute to synovitis in rheumatoid arthritis (RA), yet the autoantigens incorporated in ICs in RA remain incompletely characterised.

Methods

We used the C1q protein to capture ICs from plasma derived from human RA and control patients. Antibodies specific for immunoglobulin were used to detect ICs, and fibrinogen antibodies were used to detect fibrinogen-containing ICs. RA and control plasma were separated by liquid chromatography, and fractions then characterised by ELISA, immunoblotting and mass spectrometry. Immunohistochemical staining was performed on rheumatoid synovial tissue.

Results

C1q-immunoassays demonstrated increased levels of IgG (p = 0.01) and IgM (p = 0.0002) ICs in plasma derived from RA patients possessing anti-cyclic citrullinated peptide (CCP+) autoantibodies as compared with healthy controls. About one-half of the anti-CCP+ RA possessed circulating ICs containing fibrinogen (p = 0.0004). Fractionation of whole RA plasma revealed citrullinated fibrinogen in the high molecular weight fractions that contained ICs. Positive correlations were observed between fibrinogen-containing ICs and anti-citrullinated fibrinogen autoantibodies, anti-CCP antibody, rheumatoid factor and certain clinical characteristics. Immunohistochemical staining demonstrated co-localisation of fibrinogen, immunoglobulin and complement component C3 in RA pannus tissue. Mass spectrometry analysis of immune complexes immunoprecipitated from RA pannus tissue lysates demonstrated the presence of citrullinated fibrinogen.

Conclusion

Circulating ICs containing citrullinated fibrinogen are present in one-half of anti-CCP+ RA patients, and these ICs co-localise with C3 in the rheumatoid synovium suggesting that they contribute to synovitis in a subset of RA patients.

Arginine deiminase has multiple regulatory roles in the biology of Giardia lamblia

The protozoan parasite Giardia lamblia uses arginine deiminase (ADI) to produce energy from free L-arginine under anaerobic conditions. In this work, we demonstrate that, in addition to its known role as a metabolic enzyme, it also functions as a peptidylarginine deiminase, converting protein-bound arginine into citrulline. G. lamblia ADI specifically binds to and citrullinates the arginine in the conserved CRGKA tail of variant-specific surface proteins (VSPs), affecting both antigenic switching and antibody-mediated cell death. During encystation, ADI translocates from the cytoplasm to the nuclei and appears to play a regulatory role in the expression of encystation-specific genes. ADI is also sumoylated, which might modulate its activity. Our findings reveal a dual role played by ADI and define novel regulatory pathways used by Giardia for survival.

Citrullination by peptidylarginine deiminase in rheumatoid arthritis

Rheumatoid arthritis (RA) is a complex, multifactorial disease with genetic and immunological aspects. Because RA is an autoimmune condition, dysregulation of the immune system is implied. Many linkage and association studies have also indicated that multiple genetic factors are associated with RA. Although the contribution of each genetic factor is small, the combination of these factors affects RA development. Previous studies have suggested that genetic changes affect the internal immunological environment, which results in autoimmune diseases. More recent genetic studies indicate that the HLA-DRB gene is the predominant cause of RA and that other non-HLA genes are also involved. We reported that peptidylarginine deiminase (gene name abbreviated to PADI, protein name abbreviated to PAD) type 4 is the one of the non-HLA genetic factors involved in RA via citrullination. Antibodies against citrullinated proteins/peptides are highly specific to RA, but the physiological roles of PADI gene, PAD proteins as their products and citrullinated proteins/peptides are obscure. However, levels of anticitrullinated protein antibodies are apparently also increased and were involved in the pathogenesis of autoimmune arthritis in mice with collagen-induced arthritis (CIA). These data suggested that citrullinated protein and anticitrullinated protein antibodies play important roles in the development of RA. This review summarizes the relationship between RA and citrullination, as well as the role of PADI4 genetics.

Mutation and citrullination modifies vimentin to a novel autoantigen for rheumatoid arthritis

OBJECTIVE

Modification of antigens represents a trigger for the generation of autoantibodies. In the pathogenesis of rheumatoid arthritis (RA), citrullination of proteins has been shown to be a critical process, and the determination of antibodies against citrullinated antigens has been a diagnostic milestone. We undertook this study to determine whether antibodies to mutated and citrullinated vimentin (MCV) could serve as a diagnostic and prognostic marker for RA.

METHODS

We identified novel isoforms of human MCV in the synovial fluid of RA patients. The significance of these disease-related modifications was investigated by the analysis of autoantibody reactivities. In a group of 1,151 RA patients, the diagnostic significance and the prognostic value of an anti-MCV enzyme-linked immunosorbent assay (ELISA) were compared with that of an anti-cyclic citrullinated peptide (anti-CCP) ELISA.

RESULTS

In RA, sensitivities of 82% and 72% were calculated for the anti-MCV and anti-CCP assays, respectively. The specificity of both assays was comparable (98% and 96%, respectively). In followup analyses of 16 RA patients with moderate disease activity (mean Disease Activity Score in 28 joints [DAS28] of 2.72) and 26 RA patients with active disease (mean DAS28 of 5.07), disease stratification of RA was possible using the anti-MCV assay (P = 0.0084). A significant correlation of anti-MCV antibodies with the DAS28 was documented (r = 0.5334, P = 0.0003), in 42 RA patients (n = 427 antibody determinations at different time points).

CONCLUSION

Antigenic properties of vimentin were determined by mutation and citrullination. Anti-MCV antibodies are a novel diagnostic marker for RA. Furthermore, they may allow monitoring and-if confirmed in even larger series of patients-stratification of disease.

NF-Y and Sp1/Sp3 are involved in the transcriptional regulation of the peptidylarginine deiminase type III gene (PADI3) in human keratinocytes

Human peptidylarginine deiminase type III gene (PADI3) encodes a crucial post-translational modification enzyme that converts protein-bound arginine residues into citrulline residues. Its expression is restricted to a few cell types, including keratinocytes in the granular layer of the epidermis and in the inner root sheath of hair follicles. In these cells, the enzyme is involved in terminal processing of intermediate filament-binding proteins such as filaggrin and trichohyalin. To study the molecular mechanisms that control the expression of PADI3 in human keratinocytes at the transcriptional level, we characterized its promoter region using human keratinocytes transfected with variously deleted fragments of the 5'-upstream region of PADI3 coupled to the luciferase gene. We found that as few as 129 bp upstream from the transcription initiation site were sufficient to direct transcription of the reporter gene. Electrophoretic mobility-shift and chromatin immunoprecipitation assays revealed that NF-Y (nuclear factor Y) and Sp1/Sp3 (specificity protein 1/3) bind to this region in vitro and in vivo. Moreover, mutation of the Sp1- or NF-Y-binding motif markedly reduced PADI3 promoter activity. Furthermore, Sp1 or NF-YA (NF-Y subunit) small interfering RNAs effectively diminished PADI3 expression in keratinocytes cultured in both low- and high-calcium medium. These data indicate that PADI3 expression is driven by Sp1/Sp3 and NF-Y binding to the promoter region.

Specific modification of peptide-bound citrulline residues

Immune reactions to citrulline-containing proteins appear to be central in the immunopathogenesis of rheumatoid arthritis. Citrulline residues are introduced into proteins by deimination of arginine residues, likely by an enzymatic process. There is a need to characterize which proteins in the inflamed joints of rheumatoid patients contain citrulline in situ. The characterization of deiminated proteins will be greatly facilitated by specific modification of peptide-bound citrulline residues that will enable specific enrichment and detection of citrulline-containing peptides. This study presents the details of such a modification method. The chemistry behind the reaction of the ureido group of citrulline with 2,3-butanedione in the presence of antipyrine is unraveled. Parameters for optimization of the reaction with respect to specificity and completeness, including the testing of different acids, reactant concentrations, and reaction time, are presented. This modification reaction is specific for citrulline residues. The modified product shows a characteristic mass shift of +238Da, as demonstrated by mass spectrometry. The product absorbs UV-Vis radiation at 464nm, and it is demonstrated that this can be used to selectively monitor citrulline-containing peptides during the separation of protein digests. Finally, the structure of the product of modified citrulline is solved by nuclear magnetic resonance spectroscopy using N-butylurea as a model substance. The results presented should facilitate the development of tags that can be used for the enrichment and subsequent detection of citrulline-containing protein fragments by mass spectrometry.

A new model for an etiology of rheumatoid arthritis: smoking may trigger HLA-DR (shared epitope)-restricted immune reactions to autoantigens modified by citrullination

OBJECTIVE

To investigate whether smoking and HLA-DR shared epitope (SE) genes may interact in triggering immune reactions to citrulline-modified proteins.

METHODS

In a case-control study involving patients with recent-onset rheumatoid arthritis (RA), we studied interactions between a major environmental risk factor (smoking), major susceptibility genes included in the SE of HLA-DR, and the presence of the most specific autoimmunity known for RA (i.e., antibodies to proteins modified by citrullination). Immunostaining for citrullinated proteins in cells from bronchoalveolar lavage fluid was used to investigate whether smoking is associated with citrullination in the lungs.

RESULTS

Previous smoking was dose-dependently associated with occurrence of anticitrulline antibodies in RA patients. The presence of SE genes was a risk factor only for anticitrulline-positive RA, and not for anticitrulline-negative RA. A major gene-environment interaction between smoking and HLA-DR SE genes was evident for anticitrulline-positive RA, but not for anticitrulline-negative RA, and the combination of smoking history and the presence of double copies of HLA-DR SE genes increased the risk for RA 21-fold compared with the risk among nonsmokers carrying no SE genes. Positive immunostaining for citrullinated proteins was recorded in bronchoalveolar lavage cells from smokers but not in those from nonsmokers.

CONCLUSION

We identified an environmental factor, smoking, that in the context of HLA-DR SE genes may trigger RA-specific immune reactions to citrullinated proteins. These data thus suggest an etiology involving a specific genotype, an environmental provocation, and the induction of specific autoimmunity, all restricted to a distinct subset of RA.

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.

Peptidylarginine deiminase isoforms are differentially expressed in the anagen hair follicles and other human skin appendages

Peptidylarginine deiminases (PAD) catalyze the conversion of arginine residues to citrullines. Five isoforms are known that present distinct tissue locations. In the epidermis, like in the skin, only PAD1, 2, and 3 are expressed. Their pattern of expression in skin appendages is not known. Here, confocal microscopy analysis using highly specific antibodies demonstrated that PAD1 and 3 are expressed in human anagen hair follicles, PAD1 and 2, in arrector pili muscles and sweat glands, whereas no PAD were detected in sebaceous glands. PAD1 was detected in the cuticle and the Huxley layer of the inner root sheath (IRS), and in the companion layer. PAD3 was localized in the medulla, and in the three layers of the IRS. Using anti-modified citrulline antibodies, we also showed that deiminated proteins appeared in the lower part of the IRS, first in the Henle layer, then in the cuticle, and finally in the Huxley layer. Our data demonstrate that PAD3 is the enzyme that deiminates trichohyalin in the medulla and the Henle layer, indicate that PAD1 and 3 are involved in the hair follicle program of differentiation, and suggest a role for PAD1 and 2 in the physiology of sweat glands and arrector pili muscles.

Citrullination of central nervous system proteins during the development of experimental autoimmune encephalomyelitis

Immunization of mammals with central nervous system (CNS)-derived proteins or peptides induces experimental autoimmune encephalomyelitis (EAE), a disease resembling the human autoimmune disease multiple sclerosis (MS). Both diseases are accompanied by destruction of a part of the of the myelin sheaths, which surround neurites in the CNS. Previous studies in MS have described alterations in the citrullination of myelin basic protein, one of the main protein constituents of the myelin sheath. Here, we show that, also during the development of EAE in mice, hypercitrullination occurs in the areas of the spinal cord that show the highest degree of inflammation and that myelin basic protein and glial fibrillary acidic protein are among the hypercitrullinated proteins. We conclude that hypercitrullination of myelin proteins in the CNS is a common phenomenon in demyelinating disease. Hypercitrullination may cause conformational changes in proteins, so the affected proteins may be involved in the pathogenesis of CNS autoimmune disease by acting as autoreactive T-cell epitopes. This is the first report in which hypercitrullination of CNS proteins in EAE is described and in which proteins other than myelin basic protein are reported to be citrullinated during autoimmune-mediated CNS inflammation.

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.

Fibrin deimination in synovial tissue is not specific for rheumatoid arthritis but commonly occurs during synovitides

Autoantibodies to deiminated (citrullinated) proteins are the most specific serological markers of rheumatoid arthritis (RA). Deimination is critical in generating the peptidic epitopes they recognize. In the synovial tissue (ST), deiminated forms of the alpha- and beta-chains of fibrin are their major autoantigenic targets (anti-human fibrin(ogen) autoantibodies (AhFibA)). We investigated whether the presence of deiminated fibrin in the ST was specific for RA, because this could explain why AhFibA are RA specific. In 13 patients with RA and 19 patients with various other rheumatological disorders, knee ST biopsies were collected in macroscopically inflamed areas identified under arthroscopy. Synovitis was histopathologically confirmed in all of the biopsies. By immunoblotting, using antisera to fibrin, Abs to citrullyl residues, and AhFibA purified from RA sera, deiminated fibrin was evidenced in ST extracts from all of the patients. Moreover, variations in the degree of fibrin deimination were observed that were not related to the disease. Immunohistochemical analysis, using Abs to citrullyl residues and an antiserum to fibrin on adjacent serial sections of ST, confirmed the results because deiminated proteins colocalized with fibrin in RA as well as in control patients. Therefore, fibrin deimination in the ST is a general phenomenon associated to any synovitis, which does not necessarily induce a B autoimmune response with production of AhFibA.

Peptidylarginine deiminase type 4, anticitrullinated peptide antibodies, and rheumatoid arthritis

Anticitrullinated peptide antibodies seem to be one of the most clinically reliable serologic markers for rheumatoid arthritis (RA). A genetic approach revealed that one of the citrullinating enzymes has a RA-susceptible variant. Peptidyl citrullination alters the chemical character of peptides and, subsequently, their antigenicity as well. This change in antigenicity of self-peptides seems to invoke citrulline-related autoimmunity. Although the precise physiologic role of citrullination is still unknown, accumulating data indicate that citrullination has a definite role in biologic phenomena, along with other posttranslational protein modifications, such as methylation and phosphorylation. In RA synovial tissue, two of five PADI isotypes are known to be expressed, and their expression is regulated at multiple steps: transcription, translation, intracellular localization, and activation/inactivation of PADI proteins. Further investigations on citrulline and PADIs from various aspects will provide a more profound understanding of RA-related autoimmunity.

Deimination of Histone H2A and H4 at Arginine 3 in HL-60 Granulocytes

Interplay of various covalent modifications of histone tails has an essential role in regulation of chromatin function. Peptidylarginine deiminase (PADI) 4 deiminates protein arginine to citrulline in a Ca(2+)-dependent manner and is present in the nucleus of granulocyte-differentiated HL-60 cells. When these cells are treated with the calcium ionophore A23187, core histone deimination occurs. To determine the deimination sites of histones, histone species were purified by reverse-phase high-performance liquid chromatography (RP-HPLC) from the cells. Immunoblotting using antimodified citrulline antibody indicated that histones H2A, H3, and H4 but not H2B were deiminated. H2A and H4 were digested with Staphylococcus aureus V8 protease, and the digests were separated by RP-HPLC. Immuno dot-blotting and mass spectrometry showed that the deiminated residues were present in H2A (1-56) and H4 (1-52) regions but not in other regions. The H2A peptide (1-56) was digested with alpha-chymotrypsin, and the deiminated peptide was separated from the corresponding nondeiminated peptide by RP-HPLC. The deiminated residue was found to be limited to residues 1-23. Similarly, digestion of the H4 peptide (1-52) with endoproteinase Asp-N and separation of the deiminated peptide from the nondeiminated peptide indicated that the deiminated residue was limited to residues 1-23. Mass spectrometry of lysylendopeptidase digests of the H2A (1-23) and H4 (1-23) peptides showed that deimination occurred at arginine 3 of the N-terminal sequence Ac-SGRGK common to H2A and H4. These results suggest that PADI4 deiminates only a restricted site of target proteins in cells. Deimination of histones is discussed in relation to chromatin structure and function.

Citrullinated proteins have increased immunogenicity and arthritogenicity and their presence in arthritic joints correlates with disease severity

Autoantibodies directed against citrulline-containing proteins have an impressive specificity of nearly 100% in patients with rheumatoid arthritis and have been suggested to be involved in the disease pathogenesis. The targeted epitopes are generated by a post-translational modification catalysed by the calcium-dependent enzyme peptidyl arginine deiminase (PAD), which converts positively charged arginine to polar but uncharged citrulline. The aim of this study was to explore the effects of citrullination on the immunogenicity of autoantigens as well as on potential arthritogenicity. Thus, immune responses to citrullinated rat serum albumin (Cit-RSA) and to unmodified rat serum albumin (RSA) were examined as well as arthritis development induced by immunisation with citrullinated rat collagen type II (Cit-CII) or unmodified CII. In addition, to correlate the presence of citrullinated proteins and the enzyme PAD4 with different stages of arthritis, synovial tissues obtained at different time points from rats with collagen-induced arthritis were examined immunohistochemically. Our results demonstrate that citrullination of the endogenous antigen RSA broke immunological tolerance, as was evident by the generation of antibodies directed against the modified protein and cross-reacting with the native protein. Furthermore we could demonstrate that Cit-CII induced arthritis with higher incidence and earlier onset than did the native counterpart. Finally, this study reveals that clinical signs of arthritis precede the presence of citrullinated proteins and the enzyme PAD4. As disease progressed into a more severe and chronic state, products of citrullination appeared specifically in the joints. Citrullinated proteins were detected mainly in extracellular deposits but could also be found in infiltrating cells and on the cartilage surface. PAD4 was detected in the cytoplasm of infiltrating mononuclear cells, from day 21 after immunisation and onwards. In conclusion, our data reveal the potency of citrullination to break tolerance against the self antigen RSA and to increase the arthritogenic properties of the cartilage antigen CII. We also show that citrullinated proteins and the enzyme PAD4 are not detectable in healthy joints, and that the appearance and amounts in arthritic joints of experimental animals are correlated with the severity of inflammation.

Peptidylarginine Deiminase Isoforms 1–3 Are Expressed in the Epidermis and Involved in the Deimination of K1 and Filaggrin

Post-translational conversion of arginine to citrulline residues is catalyzed by peptidylarginine deiminases (PAD). Although the existence of five isoforms of PAD has been reported in rodents and humans, their tissue distribution, substrate specificity, and physiological function have yet to be explored. In the epidermis, deimination of filaggrin and keratins is involved in maintaining hydration of the stratum corneum (SC), and hence the cutaneous barrier function. Here, RT-PCR, western blotting, and confocal microscopy analyses with anti-peptide antibodies highly specific for each of the PAD1-4 demonstrated that only PAD1-3 are expressed in mouse and human epidermis. PAD1 was detected in all layers, including the SC, and PAD2 in all the living layers, whereas PAD3 expression was shown to be restricted to the granular layer and lower SC. Moreover, PAD1 and 3 were observed to co-localize with (pro)filaggrin, and PAD2 to be located at the keratinocyte periphery in the stratum granulosum. We also detected PAD1 in extracts of superficial SC, where K1 is deiminated. Moreover, we showed that PAD1 and 3 are able to modify filaggrin in vitro. These data strongly suggest that each enzyme exerts a specific role in the course of epidermis differentiation.

Prediction of a coding sequence for a novel type II keratin from N-terminal sequences of mouse epidermal proteins site-specifically deiminated in embryonic development

BACKGROUND

Epidermal keratinization involves various post-translational modifications including the deimination of arginine residues. Major deiminated proteins are derived from keratin K1. Two preferred deimination sites were identified in the V subdomain of mouse K1. An antibody against one of the deiminated peptide sequences (ACP) recognized deiminated mouse and human K1, and stained the cornified layers of human and infant mouse epidermis. ACP also stained the outermost layer of mouse embryonic epidermis. Western blotting revealed minor proteins showing strong ACP-positive signals in the mouse embryonic epidermal extract in which deiminated K1 derivatives were hardly detected.

OBJECTIVE

To characterize ACP-positive proteins expressed in mouse embryonic epidermis.

METHODS

ACP-positive proteins were isolated by preparative gel electrophoresis for N-terminal sequencing followed by blast searches for matching sequences in the protein and nucleotide database.

RESULTS

We obtained N-terminal sequences of two ACP-positive proteins. A cDNA clone in the est_mouse database has an open reading frame for 202 amino acid residues containing both sequenced peptides. The deduced sequence shows typical features of the N-terminal portion of type II keratins. A virtually identical sequence to this reading frame is present in a genomic contig of chromosome 15 on which keratin type II genes are clustered. Sequential searches for overlapping cDNA clones in the est_mouse database along with similar searches in the genomic contig formulated a hypothetical cDNA sequence encoding a putative protein of 572 amino acid residues tentatively called K1-emb.

CONCLUSION

We predicted a sequence of novel type II keratin site-specifically deiminated in embryonic mouse epidermis.

Navigation within host tissues: Schistosoma mansoni and Trichobilharzia ocellata schistosomula respond to chemical gradients

After penetration of human or duck host's skin schistosomula of Schistosoma mansoni and Trichobilharzia ocellata migrate parallel to the surface in the epidermis, then they enter the dermis and venules prior to further migration. This study focuses on potential behavioural mechanisms and host cues which may enable this navigation within host tissues. We stimulated cercariae to penetrate into agar substrates and to transform to schistosomula, and analysed their orientation behaviour within chemical concentration gradients. Both species were chemotactically attracted by low molecular weight fractions of their host's serum (human, duck) and D-glucose and L-arginine were identified as attractive components in serum. They responded to gradients, which established after addition of very low concentrations of D-glucose (1 microM in T. ocellata and 2 microM in S. mansoni) and L-arginine (0.025 microM in T. ocellata and 1.0 microM in S. mansoni). The response to D-glucose was specific as other saccharides had no stimulatory activity. L-Arginine stimulated chemotactic orientation both when free and bound in peptides. However, the two species responded differently to the position of L-arginine within the peptide (terminal or subterminal), and only S. mansoni, not T. ocellata, responded to peptides occurring in serum and endothelial cells: fibronectin (1 microM), bradykinin (25 pM) and its fragment 1-5 (2.5 microM). Both species adjusted their body axis with the ventral side towards the higher concentrations of D-glucose and of L-arginine. We argue that the chemotactic orientation and the alignment of the body axis enable the parasites (i) to orientate towards deeper skin layers and avoid accidental perforation of the covering skin surface layers, (ii) to determine their position during their surface-parallel migration within the epidermis, (iii) to locate blood vessels.

Clinical and pathophysiological significance of the autoimmune response to citrullinated proteins in rheumatoid arthritis

Rheumatoid arthritis (RA) is the most frequent human autoimmune disease, affecting about 1% of the adult population worldwide. A better knowledge of the autoimmune mechanisms involved is essential. We identified the epithelial targets of various autoantibodies specifically associated to RA, as variants of (pro)filaggrin. We also showed that these targets correspond to deiminated ("citrullinated") proteins, of which arginyl residues have been posttranslationally transformed into citrullyl residues by a peptidylarginine deiminase (PAD). Moreover, we and others established that citrullyl residues are indispensable elements of the epitopes recognized by these autoantibodies but only in the context of specific aminoacid sequences. We also demonstrated that these autoantibodies to citrullinated proteins (ACPA) are secreted by plasma cells of the synovial tissue and that their major targets correspond to citrullinated forms of the alpha- and beta-chains of fibrin, abundant in the tissue. These results have allowed the development of new efficient immunochemical methods for the detection of ACPA. Some of them are already commercially available. These new methods have permitted the high diagnostic value of ACPA which are present very early in the course of the disease, and also their prognostic value, to be confirmed. ACPA detection should therefore prove to be also a very valuable tool to guide the choice of therapeutic strategies, from the earliest stages of the disease. The synthesis of ACPA in the rheumatoid synovial tissue and the existence therein of a specific antigenic target constitute a strong argument for the involvement of this specific immunological conflict in the pathophysiology of RA. Indeed, it could lead to activation of effector mechanisms with pro-inflammatory effects, thus to formation in the tissue of new fibrin deposits, secondarily citrullinated. We therefore, propose a new pathophysiological model accounting for the self-maintenance and chronicity of rheumatoid inflammation. Numerous questions about the pathophysiological significance of the autoimmune response to deiminated proteins in RA remain to be answered to confirm this model.

The presence of citrullinated proteins is not specific for rheumatoid synovial tissue

OBJECTIVE

Antibodies directed toward citrullinated proteins (e.g., anti-cyclic citrullinated peptide antibodies) are highly specific for rheumatoid arthritis (RA) and are produced locally at the site of inflammation. Although the presence of citrullinated proteins in rheumatoid synovium has been described in the literature, it is uncertain whether their presence is specific for RA. The present study was undertaken to investigate this.

METHODS

The local production of the anti-citrullinated protein antibodies was investigated by comparing the concentration of the antibodies (corrected for the total amount of IgG present) in paired samples of serum and synovial fluid from RA patients. The presence of citrullinated proteins in the synovial tissue was investigated by immunohistochemical analysis of synovial tissue from RA patients and from patients with other arthropathies, using a variety of specific antibodies to citrullinated proteins.

RESULTS

In RA patients, anti-citrullinated protein antibodies constituted a 1.4-fold higher proportion of IgG in synovial fluid compared with serum, which is indicative of a local production of the antibodies. Immunohistochemical staining of citrullinated proteins was observed in the lining layer, the sublining layer, and in extravascular fibrin deposits in inflamed synovial tissue from RA as well as non-RA patients.

CONCLUSION

The presence of citrullinated proteins in the inflamed synovium is not specific for RA, but rather, it may be an inflammation-associated phenomenon. The high specificity of the anti-citrullinated protein antibodies is, therefore, most likely the result of an abnormal humoral response to these proteins.

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.

PAD, a growing family of citrullinating enzymes: genes, features and involvement in disease

Peptidylarginine deiminase (PAD, EC 3.5.3.15) enzymes catalyze the conversion of protein-bound arginine to citrulline. This post-translational modification may have a big impact on the structure and function of the target protein. In this review, we will discuss the effects of citrullination and its involvement in several human diseases, including rheumatoid arthritis and multiple sclerosis. So far, four isotypes of PAD have been described in mammals. We describe the existence of PAD in non-mammalian vertebrates and the existence of a fifth mammalian PAD. In addition, tissue-specific expression, genomic organization and evolutionary conservation of the different PAD isotypes will be discussed in detail. This article contains supplementary material which may be viewed at the BioEssays website at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/2003/25/v25.1106.html.

Rheumatoid arthritis specific anti-Sa antibodies target citrullinated vimentin

Antibodies directed to the Sa antigen are highly specific for rheumatoid arthritis (RA) and can be detected in approximately 40% of RA sera. The antigen, a doublet of protein bands of about 50 kDa, is present in placenta and in RA synovial tissue. Although it has been stated that the Sa antigen is citrullinated vimentin, experimental proof for this claim has never been published. In this study, we investigated the precise nature of the antigen. Peptide sequences that were obtained from highly purified Sa antigen were unique to vimentin. Recombinant vimentin, however, was not recognized by anti-Sa reference sera. In vivo, vimentin is subjected to various post-translational modifications, including citrullination. Since antibodies to citrullinated proteins are known to be highly specific for RA, we investigated whether Sa is citrullinated and found that Sa indeed is citrullinated vimentin. Anti-Sa antibodies thus belong to the family of anticitrullinated protein/peptide antibodies. The presence of the Sa antigen in RA synovial tissue, and the recent observation that vimentin is citrullinated in dying human macrophages, make citrullinated vimentin an interesting candidate autoantigen in RA and may provide new insights into the potential role of citrullinated synovial antigens and the antibodies directed to them in the pathophysiology of RA.

Peptidylarginine deiminase type 4: identification of a rheumatoid arthritis-susceptible gene

Recent studies using linkage disequilibrium and SNPs uncovered a rheumatoid arthritis (RA)-susceptible haplotype in the gene encoding peptidylarginine deiminase (PADI) type 4. This gene is one of four known PADI genes that encode enzymes to change arginine into citrulline in proteins. Post-translational modifications of proteins, including peptidyl citrullination, are related to autoimmunity, and peptidyl citrulline is a known target of one of the most RA-specific autoantibodies. Further research on PADI4, its citrullination of native peptides, subsequent breakdown of tolerance, and the role of these peptides in the development of RA, is expected to bring a better understanding of autoimmunity and arthritis, and advancements in the medical care of 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.

Changing pattern of deiminated proteins in developing human epidermis

Peptidylarginine deiminases are widely distributed, calcium-ion-dependent enzymes that convert arginine residues of proteins into citrulline residues. This reaction, deimination, is thought to be an important event during the final stage of epidermal differentiation, possibly associated with integration and disintegration of keratin filaments. To elucidate the possible roles of protein deimination during human epidermal development we investigated localization of deiminated proteins using anti-citrulline peptide antibody, which preferentially recognizes citrulline residues in the V subdomains of keratin 1, and anti-chemically modified citrulline antibody, which enables detection of citrulline residues independent of amino acid sequences. Anti-chemically modified citrulline antibody, but not anti-citrulline peptide antibody stained the periderm in two-layered epidermis of 49 d and 57 d estimated gestational age. In the stratified epidermis of 88 d, 96 d, and 108 d estimated gestational age fetal skin, anti-citrulline peptide antibody and anti-chemically modified citrulline antibody staining was seen in the periderm and intermediate cell layers. After periderm cells regressed and keratinization began in the interfollicular epidermis, anti-citrulline peptide antibody and anti-chemically modified citrulline antibody were restricted to the cornified cell layers of the interfollicular epidermis, similar to the distribution patterns of that in adult epidermis. Postembedding immunoelectron microscopy showed anti-citrulline peptide antibody immunogold labeling over the cytoplasmic intermediate filament network in the periderm and the intermediate cell layers. These results demonstrate an orderly formation of deiminated proteins in different layers of embryonic epidermis and suggest important roles for peptidylarginine deiminases in human epidermal morphogenesis.

cDNA cloning, gene organization and expression analysis of human peptidylarginine deiminase type I

Peptidylarginine deiminases (PADs) catalyse a post-translational modification of proteins through the conversion of arginine residues into citrullines. The existence of four isoforms of PAD (types I, II, III and IV) encoded by four different genes, which are distinct in their substrate specificities and tissue-specific expression, was reported in rodents. In the present study, starting from epidermis polyadenylated RNA, we cloned by reverse transcriptase-PCR a full-length cDNA encoding human PAD type I. The cDNA was 2711 bp in length and encoded a 663-amino-acid sequence. The predicted protein shares 75% identity with the rat PAD type I sequence, but displays only 50-57% identity with the three other known human isoforms. We have described the organization of the human PAD type I gene on chromosome 1p36. A recombinant PAD type I was produced in Escherichia coli and shown to be enzymically active. Human PAD type I mRNAs were detected by reverse transcriptase-PCR not only in the epidermis, but also in various organs, including prostate, testis, placenta, spleen and thymus. In human epidermis extracts analysed by Western blotting, PAD type I was detected as a 70 kDa polypeptide, in agreement with its predicted molecular mass. As shown by immunohistochemistry, the enzyme was expressed in all the living layers of human epidermis, with the labelling being increased in the granular layer. This is the first description of the human PAD type I gene and the first demonstration of its expression in epidermis.

Nuclear localization of peptidylarginine deiminase V and histone deimination in granulocytes

Peptidylarginine deiminase (PAD) deiminates arginine residues in proteins to citrulline residues Ca(2+) dependently. There are four types of PADs, I, II, III, and V, in humans. We studied the subcellular distribution of PAD V in HL-60 granulocytes and peripheral blood granulocytes. Expression of green fluorescent protein-tagged PADs in HeLa cells revealed that PAD V is localized in the nucleus, whereas PAD I, II, and III are localized in the cytoplasm. PAD V deletion mutants indicated that the sequence residues 45-74 have a nuclear localization signal (NLS). A sequence feature of this NLS is a three-lysine residue cluster preceded by a proline residue and is not found in the three other PADs. Substitution of the lysine cluster by an alanine cluster abrogated the nuclear import activity. These results suggested that the NLS is a classical monopartite NLS. HL-60 granulocytes, neutrophils, and eosinophils stained with antibody specific for PAD V exhibited distinct positive signals in the nucleus. Subcellular fractionation of HL-60 granulocytes also showed the nuclear localization of the enzyme. When neutrophils were stimulated with calcium ionophore, protein deimination occurred in the nucleus. The major deiminated proteins were identified as histones H2A, H3, and H4. The implication of PAD V in histone modifications is discussed.

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.

Autoantigenic posttranslational modifications of proteins: does it apply to rheumatoid arthritis?

There are many posttranslational modifications of proteins of which all are homeostatically important either to carry out a particular structural or functional role or to allow efficient recycling of the amino acid constituents. An important feature of the modified proteins is the acquisition of autoantigenicity. That notion should have been recognized for years with the modifications of immunoglobulin G that constitute new targets for rheumatoid factors. Citrullination or the deimination of arginine residues in proteins creates epitopes that are targeted by rheumatoid autoantibodies with a diagnostic sensitivity and specificity of 40% to 70% and 92% to 99%, respectively. The how, when, and why of the responsible break in tolerance are largely speculative but apoptosis, multiple genetic and environmental influences are likely required. Identifying citrullinated proteins as autoantigens has resulted in new diagnostic and prognostic autoantibody markers for RA and studying the citrullination process and its nature and role in cell biology has provided new insights into its pathogenesis.

Identification of citrullinated rheumatoid arthritis-specific epitopes in natural filaggrin relevant for antifilaggrin autoantibody detection by line immunoassay

OBJECTIVE

To identify immunodominant epitopes in natural filaggrin that are reactive with antifilaggrin autoantibodies (AFA) in the sera of patients with rheumatoid arthritis (RA) and to explore their use in a diagnostic assay format.

METHODS

Based on the results of epitope mapping of human natural filaggrin as well as molecular modeling and computational chemistry, synthetic peptides together with recombinant citrullinated filaggrin were evaluated by a line immunoassay (LIA) for AFA detection. Diagnostic performance was assessed using 336 RA and 253 disease control sera and was compared with that of reference methods.

RESULTS

Several immunoreactive epitopes were identified in natural filaggrin, all of which contained at least 1 citrulline residue. Three antigenic substrates, including 2 synthetic peptides and recombinant citrullinated filaggrin showing maximal reactivity on LIA, were finally selected. Using the 3-antigen LIA3, overall sensitivity, specificity, and positive predictive value for RA were 65.2%, 98.0%, and 89.1%, respectively, compared with 61.9%, 98.8%, and 92.8% using the 2-antigen LIA2 (without recombinant protein). Thirty-seven percent of the rheumatoid factor (RF)-negative RA samples (30 of 81) were AFA-positive by LIA2, and 52 of 54 RF-positive control samples had no AFA detected on LIA2. Higher specificity and sensitivity were obtained by LIA2 versus anti-RA33 immunoblot, whereas good agreement was observed with antikeratin antibody testing. LIA performed significantly better than AFA immunoblotting using natural filaggrin, at a specificity level of 99% (P = 0.0047).

CONCLUSION

Citrullinated residues are present in immunoreactive epitopes of natural human filaggrin. AFA can be readily detected by citrullinated peptides in an LIA-based test, resulting in high specificity and positive predictive value for RA. The LIA could serve as a user-friendly alternative to existing immunofluorescence tests and AFA immunoblot techniques. Given its complementarity to RF, this test can be a valuable tool in the differential diagnosis of arthritis.

Sequential reorganization of cornified cell keratin filaments involving filaggrin-mediated compaction and keratin 1 deimination

The final step of keratinocyte differentiation, transition from the granular cells to the cornified cells, involves various post-translational modifications that include deimination of arginine residues. Major deiminated epidermal proteins are derived from K1. Two preferred deimination sites were identified in mouse K1, one in the V1 and the other in the V2 subdomains. An antibody against the deiminated peptide sequence in the V2 subdomain recognized not only deiminated mouse K1 but also deiminated human K1. In this study we analyzed distribution of deiminated K1 in normal human skin and in bullous congenital ichthyosiform erythroderma at light and electron microscopic levels. In normal skin the first few (1-3) cornified cell layers were positive for filaggrin and negative for the antibody against deiminated mouse K1 peptide, whereas the more superficial cells were negative for filaggrin and strongly positive for the antibody against deiminated mouse K1 peptide, indicating slightly delayed onset of K1 deimination at the initial stage of cornification. The clumped keratin in bullous congenital ichthyosiform erythroderma that was not properly compacted with filaggrin was poorly positive to the antibody against deiminated mouse K1 peptide. In addition, K1 derivatives in bullous congenital ichthyosiform erythroderma reacted poorly with the antibody against deiminated mouse K1 peptide compared with the normal control in immunoblot analyses. Our results suggest sequential reorganization of cornified cell keratin filaments involving filaggrin-mediated compaction and K1 deimination. Abnormal keratin aggregation in bullous congenital ichthyosiform erythroderma is likely to disturb the normal deimination of K1.

Deimination of arginine residues in nucleophosmin/B23 and histones in HL-60 granulocytes

Peptidylarginine deiminases (PADs) convert arginine residues in proteins into citrulline residues Ca(2+)-dependently. PAD V was recently found in granulocyte-differentiated HL-60 cells. To find a target of PAD V, we incubated HL-60 granulocytes with the calcium ionophore A23187 and studied deiminated proteins by immunocytochemistry and immunoblotting using a monospecific antibody to modified citrulline residues. Immunocytochemical signals were found in the nucleus upon incubation with A23187. Immunoblotting indicated that 40-, 18-, 17-, and 14-kDa proteins were preferentially deiminated. The 40-kDa protein, which was focused to pI 5.0 on two-dimensional gel electrophoresis, was identified as nucleophosmin/B23 by mass spectrometry. The 18-, 17-, and 14-kDa proteins extracted with 0.4 N H(2)SO(4) comigrated with histones H3, H2A, and H4, respectively, on two-dimensional gel electrophoresis specialized for histones. The citrulline content of histones amounted to about 10% of the histone molecules. We discuss the implications of deimination of these proteins for their nuclear functions.

Specific presence of intracellular citrullinated proteins in rheumatoid arthritis synovium: relevance to antifilaggrin autoantibodies

OBJECTIVE

To investigate the presence of citrullinated proteins in the synovial membrane of patients with rheumatoid arthritis (RA) and controls, and to analyze a possible relationship with antifilaggrin autoantibody (AFA) reactivity.

METHODS

Synovial biopsy samples were obtained from 88 consecutive patients undergoing needle arthroscopy for knee synovitis associated with RA (n = 36), spondylarthropathy (n = 35), osteoarthritis (n = 9), or other diagnoses (n = 8). Tissue sections were stained with 2 different anticitrulline polyclonal antibodies and an antifilaggrin monoclonal antibody (mAb). The phenotype of citrulline-positive cells and the colocalization with affinity-purified AFA were investigated by double immunofluorescence on frozen sections.

RESULTS

Studies with the first antibody showed that citrulline is expressed intracellularly in the lining and sublining layers of RA synovial tissue. Staining with the second antibody, monospecific for proteins containing modified citrulline, and with anti-inducible nitric oxide synthetase confirmed the presence of citrullinated proteins rather than free citrulline in the synovium. Citrulline-positive cells were detected in 50% of the RA patients (18 of 36) but in none of the controls (0 of 52). The anticitrulline reactivity colocalized with affinity-purified AFA reactivity, although stainings with the antifilaggrin mAb indicated the absence of filaggrin in the synovium.

CONCLUSION

Intracellular citrullinated proteins, which are not recognized by an antifilaggrin mAb, are expressed in RA but not in control synovium. The high specificity of this finding and the colocalization with AFA reactivity boost the interest in citrullinated proteins as possible triggers of autoimmune responses in RA. Moreover, this is the first description of a specific histologic marker for RA synovium.

The major synovial targets of the rheumatoid arthritis-specific antifilaggrin autoantibodies are deiminated forms of the alpha- and beta-chains of fibrin

IgG antifilaggrin autoantibodies (AFA) are the most specific serological markers of rheumatoid arthritis. In epithelial tissues, they recognize citrulline-bearing epitopes present on various molecular forms of (pro)filaggrin. Histological analysis of rheumatoid synovial membranes with an Ab to citrulline showed labeling of interstitial amorphous deposits and mononuclear cells of various types. Immunochemical analysis of exhaustive sequential extracts of the same tissues showed that they contain several deiminated (citrulline containing) proteins. Among them, two proteins, p64--78 and p55--61, present in urea-DTT and guanidine extracts, were shown by immunoblotting to be specifically targeted by AFA. By amino-terminal sequencing the proteins were identified as deiminated forms of the alpha- and beta-chains of fibrin, respectively. Their identity was confirmed using several Abs specific for the A alpha- and/or to the B beta-chain of fibrin(ogen). Moreover, AFA-positive rheumatoid arthritis (RA) sera and purified AFA were highly reactive to the A alpha- and B beta-chains of human fibrinogen only after deimination of the molecules by a peptidylarginine deiminase. Autoantibodies affinity purified from a pool of RA sera onto deiminated fibrinogen were reactive toward all of the epithelial and synovial targets of AFA. This confirmed that the autoantibodies to the deiminated A alpha-and B beta-chains of fibrinogen, the autoantibodies to the synovial proteins p64--78 and p55--61, and, lastly, AFA, constitute largely overlapping autoantibody populations. These results show that deiminated forms of fibrin deposited in the rheumatoid synovial membranes are the major target of AFA. They suggest that autoimmunization against deiminated fibrin is a critical step in RA pathogenesis.

Human peptidylarginine deiminase type III: molecular cloning and nucleotide sequence of the cDNA, properties of the recombinant enzyme, and immunohistochemical localization in human skin

Peptidylarginine deiminase catalyzes the post-translational modification of proteins through the conversion of arginine to citrulline in the presence of calcium ions. In rodents, peptidylarginine deiminase has been classified into four isoforms, types I, II, III, and IV, which are distinct in their molecular weights, substrate specificities, and tissue localization. Of these isoforms, only type III was detected in epidermis and hair follicles. Although the role of this enzyme in these tissues is not yet clear, indirect data have shown that several structural proteins such as filaggrin, trichohyalin, and keratin are substrates for peptidylarginine deiminase. In this study, we cloned the full-length cDNA of human peptidylarginine deiminase type III (3142 bp) from cultured human keratinocytes by reverse transcription-polymerase chain reaction and by rapid amplification of cDNA ends methods. This cDNA contained a 1995 bp open reading frame encoding 664 amino acids (Mr = 74 770). To explore the physicochemical and enzymatic properties of human peptidylarginine deiminase type III, we constructed a plasmid for producing a recombinant human peptidylarginine deiminase type III in bacteria. The enzymatic characteristics of the recombinant enzyme were very similar to those of the rodent peptidylarginine deiminase type III. The recombinant enzyme showed the catalytic activities toward structural proteins of epidermis and hair follicle, filaggrin and trichohyalin, in which the deiminations maxima of about 60% and 13% arginine residues were observed in filaggrin and trichohyalin, respectively. An immunohistochemical study of human scalp skin with a monospecific anti-peptidyl-arginine deiminase type III antibody revealed that the type III enzyme was localized to the inner root sheath and outer root sheath of hair follicles. Peptidylarginine deiminase type III in the inner root sheath was notable between supramatrix and keratogenous zone and was scarcely detected in cornified hair zone. The enzyme was also expressed in the cuticle layer of hair. On the other hand, expression of the enzyme in the epidermis was very low. These data imply that human peptidylarginine deiminase type III is the predominant isoform in hair follicles and may function as a modulator of hair structural proteins, including trichohyalin during hair and hair follicle formation.

Fyn tyrosine kinase participates in the compact myelin sheath formation in the central nervous system

The cellular mechanisms for spiral wrapping and compaction of myelin sheaths by oligodendrocytes are not known yet. In this study, we examined the role of fyn tyrosine kinase, which could be responsible for molecular events during the stage of myelination in the CNS. Western blot and immunohistochemical analyses revealed that fyn-deficient mice have significantly lower levels of myelin basic protein (MBP), which is required for intracellular membrane adhesion parts so-called major dense line (MDL) and thought to be essential for the stability of myelin sheath. Electron microscopy verified that the myelin ultrastructure could be used to distinguish fyn-deficient mice from wild-type mice, showing a thin and redundant myelin sheath in the corpus callosum. Further, the electron-dense 'major' line in myelin from the purified myelin fractions remained condensed, and myelin compaction was split opened in fyn-deficient mice. To determine whether there was a change in the microheterogeneity of MBP due to a post-translational event we first investigated peptidylarginine deiminase (PAD), which is an enzyme that converts arginine residues in peptides to citrulline residues. PAD immunoreactivity was observed both in the myelin from fyn-deficient and wild-type mice. By Western blot analysis we found an increase of the citrullined form of MBP. In addition, MBP from fyn-deficient mice did weakly induce vesicle aggregation properties of MBP-mediated adhesion. We concluded that although oligodendrocytes from fyn-deficient mice are able to wrap around the axon, they are unable to form compact myelin due to decreased MBP level and the presence of increased citrullinated MBP.

Decreased deiminated keratin K1 in psoriatic hyperproliferative epidermis

Citrulline-containing proteins, mainly originating from keratin K1 and formed by enzymatic deimination of arginine residues, have been identified in the cornified layers of human epidermis. We analyzed the localization and nature of the deiminated proteins in psoriatic epidermis. Immunostaining based on chemical modification of citrulline residues showed that the normal and psoriatic uninvolved epidermis contained deiminated proteins diffusely in the cornified cell layer, whereas the involved epidermis had no detectable or markedly reduced levels of deiminated proteins. Immunolabeling with polyclonal antibodies against a synthetic citrulline-containing peptide corresponding to a deiminated sequence of mouse K1 also suggested markedly decreased deiminated K1 in psoriatic involved lesions. Keratin analyses indicated that deiminated K1 present in normal and psoriatic uninvolved epidermis was not detected in the psoriatic involved epidermis. Double staining with a monoclonal antibody, 34betaB4, and the polyclonal antibodies demonstrated that epidermis with low suprabasal keratin expression was negative for deiminated K1. In contrast, intralesional acrosyringia showing decreased suprabasal keratin immunoreactivity like that of the surrounding psoriatic epidermis showed strong deiminated K1 staining. This suggests that abnormal keratin deimination is restricted to the psoriatic hyperproliferative epidermis, without affecting sweat ductal epithelia.

Identification of citrulline residues in the V subdomains of keratin K1 derived from the cornified layer of newborn mouse epidermis

Citrulline residues are detected in keratins and filaggrin in the cornified layers of mammalian epidermis. Such citrulline residues are formed by the enzymatic deimination of arginine residues by peptidylarginine deiminase (EC 3.5.3.15). Major deiminated keratins are thought to be partially degraded/disulfide-cross-linked keratin K1 based on the immunoblotting profiles. In order to obtain more definitive evidence of the deimination of keratin K1 and also to investigate its functional significance, we attempted to identify its preferred acting sites of peptidylarginine deiminase. A partially degraded keratin K1 fraction obtained from the cornified layer of newborn mouse epidermis was subjected to limited proteolytic cleavages, and the resulting deiminated peptides were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or reverse-phase high-performance liquid chromatography for N-terminal sequencing and/or amino acid analysis. At least two sites were identified, one in the V1 and the other in the V2 subdomains of keratin K1. An undecapeptide sequence covering the latter shows about 70% homology with an undecapeptide sequence in the V2 subdomain of human K1, a presumptive site of deimination. We speculated that the deimination of arginine residues in these subdomains might modulate their interactions with epidermal proteins other than keratins and filaggrin during the terminal stage of epidermal differentiation.

Molecular characterization of peptidylarginine deiminase in HL-60 cells induced by retinoic acid and 1alpha,25-dihydroxyvitamin D(3)

Three types of peptidylarginine deiminase (PAD), which converts a protein arginine residue to a citrulline residue, are widely distributed in animal tissues. Little is known about PAD of hemopoietic cells. We found that PAD activity in human myeloid leukemia HL-60 cells was induced with the granulocyte-inducing agents retinoic acid and dimethyl sulfoxide and with the monocyte-inducing agent 1alpha,25-dihydroxyvitamin D(3). We cloned and characterized a PAD cDNA from retinoic acid-induced cells. The cDNA was 2,238 base pairs long and encoded a 663-amino acid polypeptide. The HL-60 PAD had 50-55% amino acid sequence identities with the three known enzymes and 73% identity with the recently cloned keratinocyte PAD. The recombinant enzyme differs in kinetic properties from the known enzymes. Immunoblotting and Northern blotting with an antiserum against the enzyme and the cDNA, respectively, showed that a protein of approximately 67 kDa increased concomitantly with increase of mRNA of approximately 2.6 kilobases during granulocyte differentiation. During monocyte differentiation the same mRNA and protein increased as in granulocyte differentiation. Neither the enzyme activity nor the protein was found in macrophage-induced cells. These results suggested that expression of the PAD gene is tightly linked to myeloid differentiation.