Background Cyclin-dependent kinases (CDKs) are recognized as a regulator of cell cycle progression, and CDK activation is regulated by CDK inhibitors. Whereas, many study reported the interaction between inflammation and osteoarthritis. Recently, Marves reported that p21 deficiency mice was susceptible to LPS-induced inflammation. However, how p21 works responsible for persistent inflammation of joints that occurs with osteoarthritis is unknown. The aim of this study is to investigate the p21 function about progressions of osteoarthritis in vivo.
Objectives P21 knockout mice (B6.129S6 (Cg)-Cdkn1atm1Led/J) and C57Bl6/J (Wild type) mice were obtained from The Jackson Laboratory (Bar Harbor, Maine). Respectively 84 male mice at 10 week-old were used in this experiment.
Methods Destabilization of the medial meniscus (DMM) was induced in the right knee and the sham group in the left knee. We sacrificed mice at 7 time points, non-operative group (control), postoperative 1day, 3day, 1week, 2weeks, 4weeks, and 8weeks. Each group contained 6 male mice. We collected blood samples and evaluate IL-1β expression by IL-1β ELISA Kit (ab100704: abcam). Post operated 1week and 8weeks, we sacrificed mice and scanned knee joint using μCT and watched changing of bone structure. Coronal histological sections were stained with H-E, Safranin O, and expression of F4/80, PCNA, IL-1β was analyzed by immunohistochemistry.
Results ELISA: Serum levels of IL-1β were significantly elevated in p21(-/-) mice at each time points except control group (Fig. 1). P21(-/-) mice showed long lasting elevation of IL-1β compared with WT after DMM surgery. After postoperative 1w, IL-1β begin to increase slowly until postoperative 8weeks.
Histrogical evaluation: We evaluated synovitis with H-E stain section. At postoperative 1 week, both p21(-/-) and WT showed synovial membrane outgrowth. At 8week, synovitis settled and volume of synovial membrane decreased in WT. However, synovial inflammation was still occurred in p21(-/-) mice (Fig. 2). F4/80 and PCNA immunohistochemistry showed surface linier of synovial membranes were strongly stained in p21(-/-) mice compared with WT (Fig. 3, 4). Safranin O staining in joint cartilage tissues were evaluated by OARSI histopathology classification. P21(-/-) mice post DMM surgery at 8 weeks model showed grade 6 in medial tibial plateau, grade 5 in medial femoral condyle. WT mice showed grade 0.5 in medial tibial plateau, grade 3 in medial femoral condyle (Fig. 5). These suggested that p21 deficiency was susceptible to osteoarthritis.
μCT analysis: At control group there is no difference between p21(-/-) and WT mice about their appearance (Fig. 6a). P21(-/-) showed subchondral bone sclerotic change and generated multiple mineralized tissue compared with WT at 8 weeks post-surgery (Fig. 6b).
Conclusions We demonstrated that the p21 deficiency was susceptible to osteoarthritis change in DMM models with systemic IL-1β expression, synovial macrophage infiltration (F4/80), and synovial cell proliferation (PCNA). Therefore, we conclude that p21 deficiency accelerates osteoarthritis with inflammation.
Mavers M, Cuda CM, et al. Cyclin-dependent kinase inhibitor p21, via its C-terminal domain, is essential for resolution of murine inflammatory arthritis. Arthritis and rheumatism. 2012;64(1):141-52.
Disclosure of Interest None declared