Authentic matrix vesicles contain active metalloproteases (MMP): A role for matrix vesicle-associated MMP-13 in activation of transforming growth factor-β

Matrix vesicles (MV) play a key role in the initiation of cartilage mineralization. Although many components in these microstructures have been identified, the specific function of each component is still poorly understood. In this study, we show that metalloproteases (MMP), MMP-2, -9, and -13 are associated with MV isolated from growth plate cartilage. In addition, we provide evidence that MV contain transforming growth factor-β (TGF-β) and that MV-associated MMP-13 is capable of activating latent TGF-β. To determine whether MMPs are associated directly with MV, vesicles isolated from growth plate cartilage were sequentially treated with hyaluronidase, NaCl, and bacterial collagenase to remove matrix proteins and other components attached to their outer surface. Finally, the vesicles were incubated with detergent to rupture the MV membrane and expose components that are inside the vesicles. Each treated MV fraction was subjected to substrate zymography, immunoblotting, and substrate activity assay. Whereas active MMP-13 was lost after combined treatment with hyaluronidase and NaCl, MMP-2 and -9 activities were still retained in the pellet fraction even after detergent treatment, suggesting that the gelatinases, MMP-2 and -9, are integral components of MV. In addition, MV contain TGF-β in the small latent complex, and MMP-13 associated with the MV surface was responsible for activation of TGF-β. Since the amount of TGF-β activated by hypertrophic chondrocytes increased with mineral appearance in serum-free chondrocyte cultures, a role for active MV-associated MMPs is suggested in activation of TGF-β seen during late chondrocyte hypertrophy and mineralization of growth plate cartilage.