Profiling cathepsin activity in osteoarthritis (OA) articular cartilage. Cryosections of intact cartilage (IC)/degenerated cartilage (DC), 5-μm thick, stained with GB123 and 4′: 6-diamidino-2-phenylindole (DAPI) then analyzed for cathepsin activity using imageJ intensity software. (A) Representative phase images overlaid with GB123 fluorescence, sections of IC and DC at 60- and 10-times magnification (n = 5). Red and blue fluorescence are Cy5 (GB123) and DAPI, respectively. (B) Cathepsin activity as a function of cartilage distance from the articular surface (that is, superficial zone) and up to 1.5 mm deep (left panel): right panel, overall quantification of cathepsin activity surrounding chondrocyte clusters (that is, cartilage extracellular matrix (ECM)). Plots of band intensity (image-J software) show SD surrounding an average data point. Values were generated by subtracting the background from images of cathepsin-stained cell clusters. Statistical analysis was by non-parametric Mann-Whitney analysis, assuming P <0.05 to be statistically significant.