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

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.

Comparative analysis of the mouse and human peptidylarginine deiminase gene clusters reveals highly conserved non-coding segments and a new human gene, PADI6

Peptidylarginine deiminases (PADs) convert arginine residues in proteins into citrullines. They are suspected to be involved in multiple sclerosis and rheumatoid arthritis pathophysiology, and they play a role in epidermis homeostasis and possibly in regulation of gene expression through histone modification. In humans, four isoforms encoded by the genes PADI1-4 are known so far. We here report the characterization and comparative analysis of the human (355 kb) and mouse (240 kb) PAD gene clusters on chromosomes 1p35-36 and 4E1, respectively. We characterized an as yet unknown human PADI6 gene, and cloned the corresponding cDNA encoding a 694-amino-acid protein. RT-PCR analysis showed a rather restricted pattern of tissue-specific expression, mainly in ovary, testis and peripheral blood leukocytes. Nucleotide substitution rates suggest that PADI genes are under purifying selection. Comparative analysis of the human and mouse sequences identified 251 conserved non-coding segments predominantly clustered within the promoter regions, the large (>10 kb) first intron of each of the genes PADI1-3, and an 8 kb PADI1-2 intergenic region. The presence of numerous transcription factor binding sites suggests the segments are putative regulatory elements. This study is the first description of the human PADI6 gene and encoded protein, and the first step towards a better understanding of the coordinated regulation of PADI gene expression.

Structural basis for Ca2+-induced activation of human PAD4

Peptidylarginine deiminase 4 (PAD4) is a Ca(2+)-dependent enzyme that catalyzes the conversion of protein arginine residues to citrulline. Its gene is a susceptibility locus for rheumatoid arthritis. Here we present the crystal structure of Ca(2+)-free wild-type PAD4, which shows that the polypeptide chain adopts an elongated fold in which the N-terminal domain forms two immunoglobulin-like subdomains, and the C-terminal domain forms an alpha/beta propeller structure. Five Ca(2+)-binding sites, none of which adopt an EF-hand motif, were identified in the structure of a Ca(2+)-bound inactive mutant with and without bound substrate. These structural data indicate that Ca(2+) binding induces conformational changes that generate the active site cleft. Our findings identify a novel mechanism for enzyme activation by Ca(2+) ions, and are important for understanding the mechanism of protein citrullination and for developing PAD-inhibiting drugs for the treatment of rheumatoid arthritis.

Identification of 45 novel SNPs in the 83-kb region containing peptidylarginine deiminase types 1 and 3 loci on chromosomal band 1p36.13

Peptidylarginine deiminases (PADIs) are post-translational modification enzymes that catalyze the conversion of protein-bound arginine residues into citrulline residues in the presence of calcium ions. Among PADIs, PADI4 was identified as a rheumatoid arthritis-susceptibility gene (Suzuki et al. in Nat Genet 34:395, 2003). We identified a total of 87 single nucleotide polymorphisms (SNPs) in PADI1 and PADI3 gene loci. Following a comparison of our data with SNPs in the dbSNP database in the National Center for Biotechnology Information, 45 SNPs are considered to be novel: 33 were identified in the PADI1 gene locus and 12 in the PADI3 gene locus. We also identified two insertion-deletion polymorphisms in introns of the PADI1. The high-resolution map that we constructed in this study will serve as a useful resource for analyzing gene scans of complex diseases mapped to this local segment on chromosomal band 1p36.13.

Peptidylarginine deiminase, the arginine to citrulline converting enzyme, is frequently recognized by sera of patients with rheumatoid arthritis, systemic lupus erythematosus and primary Sjögren syndrome

OBJECTIVE

Antibodies to citrulline-containing epitopes of filaggrin are highly specific for rheumatoid arthritis (RA). We studied whether the enzyme peptidylarginine deiminase (PAD), responsible for the post-translational modification of peptide-bound arginine residues to citrulline, constitutes an antigen for patients with RA.

METHODS

IgG antibodies to PAD were measured by enzyme-linked immunosorbent assay (ELISA) in sera from patients with RA, systemic lupus erythematosus (SLE), primary Sjögren syndrome (pSS), multiple sclerosis (MS) and healthy controls.

RESULTS

Compared to healthy controls, raised levels of IgG antibodies to PAD were found in 50 of 57 recent-onset RA patients (88%) and in 40 (70%) of the same 57 patients 3 years later (p<0.0001 for both comparisons). Eleven of 51 (22%) patients with RA of long duration, 19/43 (44%) patients with SLE and 16/19 (84%) patients with pSS, but none of 20 patients with MS, had elevated anti-PAD levels.

CONCLUSION

The arginine-citrulline converting enzyme PAD was recognized as a new antigen against which patients with inflammatory rheumatic diseases frequently show IgG class antibodies.

Arginine Deiminase Uses an Active-Site Cysteine in Nucleophilic Catalysis ofl-Arginine Hydrolysis

Arginine deiminase (EC 3.5.3.6) catalyzes the hydrolysis of l-arginine to citrulline and ammonium ion, which is the first step of the microbial l-arginine degradation pathway. The deiminase conserves the active-site Cys-His-Asp motif found in several related enzymes that catalyze group-transfer reactions from the guanidinium center of arginine-containing substrates. For each of these enzymes, nucleophilic catalysis by the conserved Cys has been postulated but never tested. In this communication we report the results from rapid quench studies of single-turnover reactions carried out with recombinant Pseudomonas aeruginosa arginine deiminase and limiting [14C-1]l-arginine. The citrulline-formation and arginine-decay curves measured at 25 degrees C were fitted to yield apparent rate constants k = 3.6 +/- 0.1 s-1 and k = 4.2 +/- 0.1 s-1, respectively. The time course for the formation (k =13 s-1) and decay (k = 6.5 s-1) of 14C-labeled enzyme defined a kinetically competent intermediate. Under the same reaction conditions, the Cys406Ser mutant failed to form the 14C-labeled enzyme intermediate. These results, along with the recently reported enzyme X-ray structure (Galkin, A.; Kulakova, L.; Sarikaya, E.; Lim, K.; Howard, A.; Herzberg, O. J. Biol. Chem. 2004, 279, 14001-14008, evidence a reaction pathway in which l-arginine deimination proceeds via a covalent enzyme intermediate formed by ammonia displacement from the arginine guanidinum carbon by the active-site Cys406.

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.