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Cathepsin g Degrades Both Glycosylated and Unglycosylated Regions of Lubricin, a Synovial Mucin

Journal article
Authors Shan Huang
Kristina A Thomsson
Chunsheng Jin
Sally Alweddi
A. Struglics
Ola Rolfson
Lena Björkman
S. Kalamajski
T. A. Schmidt
G. D. Jay
R. Krawetz
Niclas G. Karlsson
T. Eisler
Published in Scientific Reports
Volume 10
Issue 1
ISSN 2045-2322
Publication year 2020
Published at Institute of Medicine, Department of Rheumatology and Inflammation Research
Institute of Biomedicine, Department of Medical Biochemistry and Cell Biology
Institute of Clinical Sciences, Department of Orthopaedics
Language en
Subject categories Cell and Molecular Biology


Lubricin (PRG4) is a mucin type protein that plays an important role in maintaining normal joint function by providing lubrication and chondroprotection. Improper lubricin modification and degradation has been observed in idiopathic osteoarthritis (OA), while the detailed mechanism still remains unknown. We hypothesized that the protease cathepsin G (CG) may participate in degrading lubricin in synovial fluid (SF). The presence of endogenous CG in SF was confirmed in 16 patients with knee OA. Recombinant human lubricin (rhPRG4) and native lubricin purified from the SF of patients were incubated with exogenous CG and lubricin degradation was monitored using western blot, staining by Coomassie or Periodic Acid-Schiff base in gels, and with proteomics. Full length lubricin (∼300 kDa), was efficiently digested with CG generating a 25-kDa protein fragment, originating from the densely glycosylated mucin domain (∼250 kDa). The 25-kDa fragment was present in the SF from OA patients, and the amount was increased after incubation with CG. A CG digest of rhPRG4 revealed 135 peptides and 72 glycopeptides, and confirmed that the protease could cleave in all domains of lubricin, including the mucin domain. Our results suggest that synovial CG may take part in the degradation of lubricin, which could affect the pathological decrease of the lubrication in degenerative joint disease. © 2020, The Author(s).

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