ABSTRACT
Citrullination is the post-translational modification of arginine residues by peptidylarginine deiminases (PADIs). Fibrinogen is one substrate of PADIs under physiological conditions. Fibrinogen is an important factor for blood coagulation and inducing inflammation. The citrullinated form of fibrinogen appears in rheumatoid arthritis synovial tissue together with the production of autoantibodies that target self-peptides containing citrulline. However, whether the function of fibrinogen changes after citrullination remains unclear. We found that citrullinated fibrinogen markedly impairs the function of thrombin-catalysed fibrin polymerization and also inhibits fibrin formation. Increased citrullinated fibrinogen might thus affect the balance between coagulation and fibrinolysis and alter antigenicity under physiological conditions. These data suggest that citrullination of proteins could physiologically change functions and subsequently generate pro-inflammatory conditions and autoimmune reactions.
Subject(s)
Citrulline/chemistry , Fibrin/chemistry , Fibrinogen/physiology , Thrombin/chemistry , Arginine/chemistry , Blood Coagulation , Catalysis , Edetic Acid/chemistry , Fibrinogen/chemistry , Humans , Inflammation , Kinetics , Peptides/chemistry , Substrate Specificity , Time FactorsABSTRACT
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.
Subject(s)
Hydrolases/metabolism , Amino Acid Sequence , Arginine/chemistry , Arginine/metabolism , Arthritis, Rheumatoid/enzymology , Arthritis, Rheumatoid/pathology , Calcium/pharmacology , Catalysis , Citrulline/chemistry , Citrulline/metabolism , Fibrinogen/chemistry , Fibrinogen/metabolism , Filaggrin Proteins , Humans , Hydrogen-Ion Concentration , Immunohistochemistry , Intermediate Filament Proteins/chemistry , Intermediate Filament Proteins/metabolism , Kinetics , Molecular Sequence Data , Protein Denaturation , Protein-Arginine Deiminase Type 2 , Protein-Arginine Deiminase Type 4 , Protein-Arginine Deiminases , Substrate Specificity , Synovial Membrane/enzymology , Synovial Membrane/pathologyABSTRACT
Individuals with rheumatoid arthritis frequently have autoantibodies to citrullinated peptides, suggesting the involvement of the peptidylarginine deiminases citrullinating enzymes (encoded by PADI genes) in rheumatoid arthritis. Previous linkage studies have shown that a susceptibility locus for rheumatoid arthritis includes four PADI genes but did not establish which PADI gene confers susceptibility to rheumatoid arthritis. We used a case-control linkage disequilibrium study to show that PADI type 4 is a susceptibility locus for rheumatoid arthritis (P = 0.000008). PADI4 was expressed in hematological and rheumatoid arthritis synovial tissues. We also identified a haplotype of PADI4 associated with susceptibility to rheumatoid arthritis that affected stability of transcripts and was associated with levels of antibody to citrullinated peptide in sera from individuals with rheumatoid arthritis. Our results imply that the PADI4 haplotype associated with susceptibility to rheumatoid arthritis increases production of citrullinated peptides acting as autoantigens, resulting in heightened risk of developing the disease.
