HUMAN OSTEOARTHRITIC CARTILAGE MATRIX VESICLES GENERATE BOTH CALCIUM PYROPHOSPHATE DIHYDRATE AND APATITE IN-VITRO

Calcium crystals in osteoarthritic (OA) joints promote enzymatic degra dation of articular tissues. Matrix vesicles provide a nidus for calci um crystal formation in chick epiphyseal and mature porcine articular cartilage. In order to examine a potential role for matrix vesicles fr om OA cartilage in generating pathologic crystals, we sought to determ ine whether vesicles derived from human OA cartilage (OAMV) could mine ralize; and we characterized the resultant mineral species. OAMV were isolated and examined for alkaline phosphatase (AP) and nucleoside tri phosphate pyrophosphohydrolase (NTPPPH) activity. OAMV ATP-dependent a nd independent mineralization were measured in a radiometric biominera lization assay, and newly formed OAMV crystals were examined using Fou rier transform infrared spectroscopy (FTIR) and compensated polarized light microscopy. The mean specific activity of OAMV AP was approximat ely 6 times higher and NTPPPH activity 11 times lower than that of pre viously characterized, mature, porcine, articular cartilage vesicles. OAMV progressively precipitated Ca-45 over time both in the presence a nd absence of ATP. The FTIR spectra of mineral formed in ATP-dependent assays most closely resembled the standard spectrum for calcium pyrop hosphate dihydrate (CPPD). The FTIR spectra of OAMV mineral formed in the absence of ATP closely resembled apatite. These data support the h ypothesis that OAMV may form mineral phases of two key crystals found in degenerating cartilage and provide further evidence for the role of matrix vesicles in pathologic articular cartilage biomineralization.