Article Text
Abstract
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 Ca2+ is available. Such high Ca2+ concentrations are normally not present in living cells. In macrophages, which are abundant in the inflamed RA synovium, vimentin is specifically citrullinated after Ca2+ 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.
- rheumatoid arthritis
- peptidylarginine deiminase
- citrullination
- macrophages
- vimentin
- ACR, American College of Rheumatology
- anti-MC, anti-modified citrulline antibodies
- CCP, cyclic citrullinated peptide
- HUGO, human genome organisation
- NP-40, Nonidet P40
- PAD, peptidylarginine deiminase
- PB, peripheral blood
- PBMCs, peripheral blood mononuclear cells
- PMCA, plasma membrane Ca2+ pump
- RA, rheumatoid arthritis
- RT-PCR, reverse transcriptase-polymerase chain reaction
- SDS-PAGE, sodium dodecyl sulphate-polyacrylamide gel electrophoresis
- SF, synovial fluid
- SFMCs, synovial fluid mononuclear cells
- UTR, untranslated terminal region
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- ACR, American College of Rheumatology
- anti-MC, anti-modified citrulline antibodies
- CCP, cyclic citrullinated peptide
- HUGO, human genome organisation
- NP-40, Nonidet P40
- PAD, peptidylarginine deiminase
- PB, peripheral blood
- PBMCs, peripheral blood mononuclear cells
- PMCA, plasma membrane Ca2+ pump
- RA, rheumatoid arthritis
- RT-PCR, reverse transcriptase-polymerase chain reaction
- SDS-PAGE, sodium dodecyl sulphate-polyacrylamide gel electrophoresis
- SF, synovial fluid
- SFMCs, synovial fluid mononuclear cells
- UTR, untranslated terminal region