A model system of the dynamic loading occurring in synovial joints: the biological effect of plowing on pristine cartilage.

Mechanical stress is listed as a main risk factor for cartilage degradation. The aim of this study was to investigate the biological response of cartilage to dynamic loading such as plowing. Cartilage strips harvested from bovine nasal septum were submitted to plowing using a cylindrical indenter, applying a constant normal force in the vertical axis and moving at constant speed in the horizontal axis. After plowing, cell viability, gene expression and glycosaminoglycan (GAG) release were measured with conventional assays. The cell-viability assay and qRT-PCR showed that plowing induces cell death and matrix metalloproteinase 3 (MMP-3) upregulation. The addition of actinomycin D, before or after plowing, confirmed that plowing was responsible for the observed MMP-3 upregulation. Even if the transcriptions of the tissue inhibitor of metalloproteinase (TIMP-1), aggrecan (Agg), collagen type I (Coll1), collagen type II (Coll2) and fibronectin (Fn) were not significantly affected by plowing, actinomycin D treatment revealed that plowing induces a strong increase in TIMP-1 and Coll1 messenger RNA content and influences the gene regulation of Agg, Coll2 and Fn. Furthermore, plowed cartilage explants exhibited enhanced GAG release. Application of hydroxamate MMP inhibitor after loading showed that plowing induces GAG release via the activation of catabolic enzymes. Plowing causes cell death of the chondrocytes closer to the surface as well as matrix damage, observed as GAG loss. Moreover, in healthy chondrocytes, plowing promotes the production and activation of catabolic enzymes like MMP-3.