Cartilage extracellular matrix (ECM) degradation and angiogenesis are critical for skeletal development in general and for endochondral ossification in particular. Matrix metalloproteinases (MMPs) play an important role in these processes. MMP-14 (or MT1-MMP, membrane type I-MMP), an integral membrane MMP, is prominently expressed during skeletal development. In vitro, MMP-14 cleaves ECM molecules such as collagen I and aggrecan. It is the cell surface activator of progelatinase A and collagenase-3, the most active collagen II protease. These observations predicted a significant role for MMP-14 in cartilage ECM proteolysis and skeletal development. We have confirmed this by generation of transgenic null mice lacking MMP-14, via homologous recombination in embryonic stem cells. The abnormalities in MMP-14 deficient mice include defective cartilage resorption, decreased chondrocyte proliferation, delayed formation of secondary ossification centers, and defective angiogenesis. The consequences of these abnormalities are severe dwarfism and early death. The precise molecular pathways leading up to this phenotype are unknown and will be investigated in this proposal. We will address the effects of the transgene on parameters of chondrocyte performance and bone growth, and on expression of genes for stage specific chondrocyte markers and molecules related to angiogenesis. Experiments are described to determine whether MMP-14 directly resorbs cartilage extracellular matrix or whether it acts through activation of other metalloproteases. Since secondary ossification center formation is delayed, we will study angiogenesis using in vitro and in vivo assays in these mice. In view of preliminary studies which suggest interference with the growth hormone-insulin-like growth factor (IGF) axis in these mice, we will investigate a role for MMP-14 in mediating the bioavailability of free IGF via proteolysis of IGF binding proteins.
