Nonstandard abbreviations used: anterior cruciate ligament (ACL); bone morphogenetic protein (BMP); constitutively active activin receptor-like kinase 3 (ALK3 CA ); constitutively active MKK6 (MKK6 CA ); hemagglutinin (HA); matrix metalloproteinase (MMP); medial meniscus (MM); osteoarthritis (OA); receptorregulated Smad (R-Smad); synovial fibroblast (SF); TGF-β-activating kinase 1 (TAK1).
Conflict of interest:The authors have declared that no conflict of interest exists.
IntroductionInjury to the articular cartilage occurs under various pathological conditions such as trauma, inflammation, and aging (1), and cartilage injury is followed by osteoarthritic changes of the affected joints. Osteoarthritis is the most common degenerative joint disorder, affecting nearly half of the elderly population. Osteoarthritis is characterized by degradation of articular cartilage and overgrowth of cartilage and bone, known as osteophytes, at the periphery of the articular surface, which results in pain and loss of joint function (1, 2). Microscopically, loss of proteoglycan and fibrillation of the articular surface are observed at the early stage of arthritis. At later stages, clefts are formed, and at the end stage, erosive changes in the articular cartilage appear. The high prevalence of this disease results in high costs for treating patients, and therefore the development of good therapeutics for osteoarthritis is a matter of great urgency. Because of the limited capacity of spontaneous healing, the regeneration of intact articular cartilage is one of the most challenging issues in the orthopedic field (3, 4). Transplantation of autologous chondrocytes or mesenchymal progenitor cells and autogenous osteochondral transplantation (mosaicplasty) have been successfully utilized for the repair of focal osteochondral defects (3, 5-11). However, the application of these technologies is limited to small defects due to the difficulty of obtaining a sufficient amount of cells or tissues. Synovium is a thin tissue lining the nonarticular surfaces of diarthrodial joints (12). Synovial tissues contain various types of cells, including type A cells, macrophage lineage cells, and type B cells, which are specialized synovial fibroblasts (SFs). It is now widely recognized that synovial tissues are involved primarily in the pathogenesis of arthritic joint disorders such as rheumatoid arthritis by producing the matrix-degenerating enzymes cystein proteases and matrix metalloproteinases (MMPs) and the proinflammatory cytokines interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) (12). We previously reported that SFs express a high level of receptor activator of NF-κB ligand, the osteoclast differentiation factor belonging to the TNF-α superfamily (13). In contrast to such catabolic actions, there is accumulating evidence that synovial cells have anabolic effects, leading to bone and cartilage production. Hunziker and Rosenburg reported that synovial cells can migrate into partial-thickness articular cartilage defects, where they prolifera...