Background
Citrullination refers to the post-translational conversion of protein arginine residues into citrulline residues, a process catalysed by peptidylarginine deiminases (PADs) 1–4 and PAD6 [
1]. Citrullination plays an important pathogenic role in anti-citrullinated protein antibody (ACPA)-positive rheumatoid arthritis (RA) [
2,
3] and, possibly, in a number of other autoimmune diseases, inflammatory diseases or neurodegenerative conditions, including multiple sclerosis [
4], Alzheimer’s disease [
5], psoriasis [
6], Sjögren’s syndrome [
7], type 1 diabetes [
8] and chronic obstructive pulmonary disease [
9].
Most studies on PAD activity and functional studies of citrullinated proteins have been based on in vitro citrullination using the reducing agent dithiothreitol (DTT) and exogenously added calcium. The calcium dependency of PADs is well described: upon binding of calcium, Cys645 in PAD4 (Cys647 in PAD2) translocates into a position in the catalytic site, where nucleophilic attack on guanidinium groups of target arginines takes place [
10,
11]. The PAD requirement for calcium is met in the extracellular space [
12] and, under certain circumstances, also intracellularly [
13]. The dependency of PADs on reducing agents is less well understood. Reduction of the active site thiol Cys645/647 is likely to precede attack on the guanidinum carbon of arginine. Thus, the redox balance might be an additional regulator for PAD’s catalytic activity. While DTT is a non-physiological synthetic molecule, reduced glutathione (GSH) is a physiological reducing agent that may facilitate PAD activity in vivo. GSH is a linear tripeptide of
l-glutamine,
l-cysteine and glycine, and contains a sulfhydryl (SH) group on the cysteinyl portion, accounting for its strong electron-donating character. GSH is the most abundant intracellular small-molecule thiol, and is essential for maintaining the thiol status of various molecules. GSH has many biological roles, including protection against reactive oxygen and nitrogen species (ROS/NOS), which are reduced by two GSH molecules forming oxidized glutathione (GSSG) in the process [
14]. GSH has been demonstrated in cytosol and in organelles of virtually all cells of the body at the low millimolar range [
15], whereas extracellular levels are two to three orders of magnitude lower [
16,
17].
Owing to involvement in many cellular functions, it is not surprising that dysregulation of GSH has been associated with various diseases [
14]. The glutathione reductase, which converts GSSG to GSH, has been found upregulated in synovial fluid (SF) from RA patients [
18], and blood from RA patients contains higher levels of GSH, as well as higher GSH:GSSG ratios, than blood from healthy controls [
19].
We hypothesize that, in addition to calcium, reducing agents are required for PADs to become enzymatically active, and that GSH is the major physiological reducing agent in this respect.
Using an assay for citrullination of matrix-bound fibrinogen, we here analyse the impact of GSH on recombinant human PADs (rhPADs), on PADs contained in SF and on PADs released from stimulated human leucocytes.
Methods
Cells, serum and SF
Blood samples were obtained from healthy donors attending the Blood Bank at Copenhagen University Hospital, Rigshospitalet, Denmark. All donors were anonymous to the investigators.
One hour after collection, serum was isolated from venous blood drawn into 10 ml dry Vacutainer tubes (BD Bioscience, Brøndby, Denmark) by centrifugation at 400 × g for 10 min at 20 °C. Pooled serum from blood group AB-positive donors, henceforward referred to as “AB serum”, was purchased from Sigma-Aldrich (St. Louis, MO, USA).
Cells were isolated from venous blood drawn into 10 ml lithium-heparin tubes (BD Bioscience). Blood leucocytes were isolated after lysis of erythrocytes in heparinized blood by incubation with ammonium chloride (In Vitro As, Fredensborg, Denmark) for 7 min. Mononuclear cells (MNCs) were isolated by gradient centrifugation of heparinized blood using LymphoPrep™ (Axis-Shield, Oslo, Norway). Before use, both cell preparations were washed twice in RPMI 1640, 25 mM Hepes containing 0.42 mM calcium nitrate, l-glutamine and gentamicin (In Vitro As).
SF samples from nine ACPA-positive RA patients, fulfilling the American College of Rheumatology 1987 diagnostic criteria [
20], were obtained from Dr Ladislav Senolt, Charles University in Prague, Czech Republic. The study was approved by the Ethics Committee of the Institute of Rheumatology and written informed consents were obtained from all patients prior to initiation of the study. Samples were centrifuged at 1900 ×
g for 10 min to remove cells and were stored at –80 °C prior to analysis.
Reagents
rhPAD2 and rhPAD4 were produced, purified and defined by means of mass concentration, as described previously [
21]. GSH was purchased from Sigma-Aldrich. The glutathione reductase inhibitor (GRI) 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylthiocarbonylamino)phenylthiocarbamoylsulfanyl]propionic acid hydrate (2-AAPA) was purchased from Sigma-Aldrich. Monoclonal mouse anti-citrullinated fibrinogen (clone 20B2; catalogue number MQ13.102) was purchased from ModiQuest (Oss, Netherlands).
Cell-free assay for PAD activity
Maxisorp plates (Nunc, Roskilde, Denmark) were coated overnight at 4 °C with 100 μl/well of 1.0 μg/ml fibrinogen (Calbiochem, Darmstadt, Germany) in coating buffer (30 mM Na2CO3, 70 mM NaHCO3, pH 9.6). Wells were washed three times and blocked in Tris-buffered saline (TBS) buffer containing 0.05 % Tween-20, pH 7.4, for 20 min at room temperature (RT). Next, the wells were incubated (100 μl/well for 180 min at RT) with: rhPAD2 and/or rhPAD4 (300 ng/ml in 100 mM Tris–HCl, pH 7.5); SF (undiluted 50 μl; diluted 1:2 in 100 mM Tris–HCl, pH 7.5); serum (diluted 1:2 in 100 mM Tris–HCl, pH 7.5); or cell culture supernatants (diluted 1:1 in 100 mM Tris–HCl, 10 mM CaCl2, pH 7.5). The reactions took place in the presence of various combinations of rhPAD2/4, DTT (1 mM), EDTA (25 mM) or GSH and CaCl2 at various concentrations, as specified in the figure legends. After three washes in washing buffer (PBS, 0.05 % Tween-20, pH 7.4), murine anti-citrullinated fibrinogen antibody (0.5 μg/ml) was incubated for 90 min at RT. After three further washes, wells were incubated with 100 μl horseradish peroxidase-conjugated polyclonal rabbit-anti mouse immunoglobulin antibodies (P0260; Dako, Glostrup, Denmark) diluted 1:1000 in washing buffer. Finally, the plates were washed three times in washing buffer and incubated with 0.4 mg/ml o-phenylene-diamine (Kem-En-Tec, Taastrup, Denmark) in developing buffer (35 mM citric acid, 65 mM Na2PO4, pH 5.0). After 10 min, the colour reaction was stopped with 1.0 M H2SO4, and the optical density (OD) was measured at 490–650 nm using the SPECTROstar nano Microplate Reader (BMG Labtech, Ortenberg, Germany). Data were processed using MARS software (BMG Labtech).
Isolated leucocytes or MNCs were added to microtitre wells coated with fibrinogen, washed and blocked as already described, and incubated for 180 min under agitation at RT. Purified cells were diluted 1:1 with RPMI 1640 to a final concentration of 5 % AB serum and, when relevant, 15 nM phorbol 12-myristate 13-acetate (PMA) and 50 μM 2-AAPA (which were incubated for 20 min with cells prior to stimulation). Cells were removed by washing four times in PBS and 0.05 % Tween-20, and plates were developed as already described.
PAD2 measurement
PAD2 was measured using an in-house ELISA, as described previously [
22]. SF was diluted 1:10 as described in [
23] and cell supernatants were diluted 1:1 with PBS containing 0.05 % Tween-20, pH 7.4.
Discussion
The physiological agent(s) responsible for reducing PADs to their physiologically active state have not been identified. In general, PAD activity studies in vitro have used DTT or a related compound not present in nature as a reducing agent. Thus, enzymatic activity of PADs contained in SF [
12,
23], in cell lysates [
25] or released by cells [
26,
27] has generally not been examined under physiological conditions, because DTT has been included in the experiments. We hypothesized that GSH is a natural reducing agent required for PADs to be enzymatically active.
Indeed, enzymatic activity of rhPAD2 and rhPAD4 was observed at GSH concentrations corresponding to those found in cytosol (i.e. around 4.5 mM) [
15]. To our knowledge, intracellular GSH levels exceeding 15 mM have not been observed in vivo, and therefore the diminished PAD activity observed at high concentrations of exogenously added GSH in this study may not be physiologically relevant.
We did not observe any PAD activity in pooled SF from RA patients without addition of GSH or DTT. Spengler et al. [
27] recently reported weak PAD activity in pure, freshly obtained SF from untreated RA patients, which was higher than in SF from OA patients, albeit 100-fold lower than the activity in the presence of DTT-containing citrullination buffer. The differences between their observations and ours may rely on their use of a different assay for protein citrullination and, possibly, usage of freshly isolated SFs. The low or absent PAD activity in SF observed here and by others [
27] suggests that an essential factor was missing for PADs to function optimally. Our finding that addition of DTT or GSH to SF strongly enhanced PAD activity indicates that a reducing agent(s) is this essential factor.
Circulating proteins produced outside joints (e.g. fibrinogen produced in the liver) are present in a citrullinated form in SF from RA patients [
28,
29], suggesting that extracellular citrullination occurs within the joints, where the calcium concentration is sufficiently high for PADs to be active. Our finding that PMA-stimulated leucocytes cultured in microtitre wells were capable of citrullinating fibrinogen coated in wells suggests that the leucocytes either released PAD in reduced form or co-released substance(s) capable of reducing PAD.
Evidence for GSH being critical for reduction of PAD came from the finding that citrullination was abrogated by addition of the highly specific GRI 2-AAPA [
24] to cells before incubation in the wells. Extracellular leucocyte PAD most probably originates from granulocytes, because PMA-stimulated MNCs devoid of granulocytes were not capable of citrullinating fibrinogen. The presence of PAD2 and of PAD activity (after addition of GSH) in supernatants from the cultures exposed to 2-AAPA suggested that 2-AAPA did not inhibit PAD release. However, we cannot rule out that 2-AAPA has effects on PAD activity other than that caused by enhancement of the GSSG:GSH ratio. The enzymatic activity of the supernatants was low compared with the activity observed in the presence of cells, suggesting that citrullination of fibrinogen took place in the close vicinity of the cells, where high local concentrations of reduced PADs could be obtained. It is likely that PADs are rapidly oxidized, and thereby inactivated, upon release from granulocytes.
With respect to citrullination of intracellular proteins, the PAD requirement for GSH is clearly met intracellularly [
15]. Indeed, intracellularly located citrullinated proteins have been observed in the synovium of RA patients, indicating that calcium concentrations high enough for PADs to be active can be reached intracellularly under certain circumstances [
13]. Accordingly, PAD enzymes are known to modulate gene expression [
30‐
32]. It is noteworthy that GSH can also be recruited to the nucleus (e.g. during the early phase of cell proliferation [
33]) and is also involved in gene expression [
34,
35].
A limitation of this study is that we cannot distinguish between PAD2 and PAD4 activity. Granulocytes are known to release both isoforms upon PMA stimulation [
26,
27]. We have shown previously that ~150-fold more rhPAD4 than rhPAD2 is required to citrullinate the epitope recognized by the detecting antibody in the assay used in this study [
12], and thus it can be speculated that PAD2 is primarily responsible for the citrullination observed. We cannot exclude an influence of other physiological reducing agents on PAD activation in vivo. Indeed, thioredoxin has been reported to be 5-fold increased in SF of RA patients compared with OA patients [
36]. However, the complete abrogation of cell-mediated citrullination by blockade of glutathione reductase suggests that GSH is predominantly responsible for reduction of PAD under normal physiological conditions.
Conclusion
This study shows for the first time that the physiological reducing agent GSH, at concentrations similar to those present in cytosol, is capable of reducing PADs to a degree that suffices for citrullination to occur. We found that the thiol status of PADs is as important as binding of calcium for PAD’s catalytic activity.
Acknowledgements
The authors wish to thank Dr Ladislav Senolt, Charles University Hospital of Prague, for providing the SF samples that made this study possible. This study was supported by the Novo Nordisk Foundation and Aase and Ejnar Danielsen Foundation.
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Competing interests
DD and CHN are inventors for two patents concerning therapeutic use of monoclonal antibodies against PADs. MEB, MAS and GJMP have no competing interests related to the work described in this article.
Authors’ contributions
DD designed the study, carried out experiments and drafted the manuscript. MEB assisted with study design, experiments and helped to revise the manuscript critically. MAS assisted with experiments and helped to revise the manuscript critically. GJMP expressed, purified and tested recombinant proteins used for the experiments and helped to revise the manuscript critically. CHN designed the study and drafted the manuscript. All authors read and approved the final manuscript.