Peptidyl arginine deiminase type 2 (PAD‐2) and PAD‐4 but not PAD‐1, PAD‐3, and PAD‐6 are expressed in rheumatoid arthritis synovium in close association with tissue inflammation

Abstract

Objective. Autoantibodies to citrullinated proteins (ACPAs) are specific for rheumatoid arthritis (RA) and probably are involved in its pathophysiology. Citrullyl residues, posttranslationally generated by peptidyl arginine deiminase (PAD), are indispensable components of ACPA-targeted epitopes. The aim of this study was to identify which PAD isotypes are expressed in the synovial tissue (ST) of patients with RA and are involved in the citrullination of fibrin, the major synovial target of ACPAs.Methods. Expression of all PAD isotypes, including the recently described PAD type 6 (PAD-6), was explored by reverse transcription-polymerase chain reaction and immunoblotting, first in blood-derived mononuclear leukocytes from healthy donors, then in ST samples from 16 patients with RA and 11 control patients (4 with other arthritides and 7 with osteoarthritis [OA]). In ST samples from patients with RA, PADs were localized by immunohistochemistry.Results. In lymphocytic and monocytic cells and, similarly, in ST samples from patients with RA, the PAD-2, PAD-4, and PAD-6 genes were found to be transcribed, but only PAD-2 and PAD-4 enzymes were detected. PAD-2 was also expressed in ST from control patients, including those with OA, while PAD-4 was preferentially expressed in ST from patients with other arthritides. In RA, the expression levels of PAD-2 and PAD-4 were correlated with the intensity of inflammation (cell infiltration, hypervascularization, and synovial lining hyperplasia), and both enzymes were demonstrable within or in the vicinity of citrullinated fibrin deposits.Conclusion. PAD-2 and PAD-4 are the only PAD isotypes expressed in the ST of patients with RA and those with other arthritides. Inflammatory cells are a major source, but PAD-4 also comes from hyperplastic synoviocytes. Both isotypes are probably involved in the citrullination of fibrin.

Potential protein targets of the peptidylarginine deiminase 2 and peptidylarginine deiminase 4 enzymes in rheumatoid synovial tissue and its possible meaning

“…For example, PAD2 citrullinates β/γ-actin, whereas PAD4 citrullinates the H3 histone. Additionally, functional PAD2 and PAD4 isoforms are predominantly active in rheumatoid synovial tissue (29)(30)(31). Furthermore, the in vitro assays demonstrated that fibrinogen, α-enolase, and other PAD-modified proteins become autoantigens, and patients with RA produce autoantibodies against citrullinated peptides.…”

Peptidylarginine deiminase and protein citrullination in prion diseases

“…40 In this disease, upregulated expression of PAD2 and PAD4 as well as increased citrullinated proteins are key aspects of the abnormalities seen in the synovial tissue. [41][42][43] Over the past 10 years, many research groups have demonstrated that antibodies against citrullinated proteins (ACPAs) are present in ~70% of RA patients and that the presence of ACPAs is highly specific for RA. In addition, ACPA-positive patients were shown to have a more severe disease course than RA patients without ACPAs.…”

Citrullinated fibrinogen shows defects in FPA and FPB release and fibrin polymerization catalyzed by thrombin

“…As the expression of PAD2 and PAD4 has been demonstrated in the synovial tissue of RA patients [19], the fibrinogen in the extravasated plasma that passes into the synovium might be deiminated at residues Aα16Arg and Bβ14Arg, resulting in citrullinated fibrinogen. In addition, elevated levels of thrombin in the RA synovial fluid were demonstrated by measurement of thrombin-antithrombin III complexes [20]; therefore, some fibrinogen (non-citrullinated fibrinogen at residues Aα16Arg and Bβ14Arg) might be converted into fibrin by thrombin and deposited on synovial tissue, before finally being deiminated by PAD2 and PAD4.…”