Mechanical injury of cartilage explants causes specific time-dependent changes in chondrocyte gene expression

Objective. Joint injury in young adults leads to an increased risk of developing osteoarthritis (OA) later in life. This study was undertaken to determine if injurious mechanical compression of cartilage explants results in changes at the level of gene transcription that may lead to subsequent degradation of the cartilage. Methods. Cartilage was explanted from the femoropatellar groove of newborn calves. Levels of messenger RNA encoding matrix molecules, proteases, their natural inhibitors, transcription factors, and cytokines were assessed in free swelling control cultures as compared with cartilage cultures at 1, 2, 4, 6, 12, and 24 hours after application of a single injurious compression. Results. Gene-expression levels measured in noninjured, free swelling cartilage varied over 5 orders of magnitude. Matrix molecules were the most highly expressed of the genes tested, while cytokines, matrix metalloproteinases (MMPs), aggrecanases (ADAMTS5), and transcription factors showed lower expression levels. Matrix molecules showed little change in expression after injurious compression, whereas MMP-3 increased similar to 250-fold, ADAMTS-5 increased similar to 40-fold, and tissue inhibitor of metalloproteinases I increased similar to 12-fold above the levels in free swelling cultures. Genes typically used as internal controls, GAPDH and beta-actin, increased expression levels -4-fold after injury, making them unsuitable for use as normalization genes in this study. The expression levels of tumor necrosis factor a and interleukin-1 beta, cytokines known to be involved in the progression of OA, did not change in the chondrocytes after injury. Conclusion. Changes in the level of gene expression after mechanical injury are gene specific and time dependent. The quantity of specific proteins may be altered as a result of these changes in gene expression, which may eventually lead to degradation at the tissue level and cause a compromise in cartilage structure and function.