Membrane-type matrix metalloproteinase I (MT1-MMP)-deficient mice were found to have severe defects in skeletal development and angiogenesis. The craniofacial, axial, and appendicular skeletons were severely affected, leading to a short and domed skull, marked deceleration of postnatal growth, and death by 3 wk of age. Shortening of bones is a consequence of decreased chondrocyte proliferation in the proliferative zone of the growth plates. Defective vascular invasion of cartilage leads to enlargement of hypertrophic zones of growth plates and delayed formation of secondary ossification centers in long bones. In an in vivo corneal angiogenesis assay, null mice did not have angiogenic response to implanted FGF-2, suggesting that the defect in angiogenesis is not restricted to cartilage alone. In tissues from null mice, activation of latent matrix metalloproteinase 2 was deficient, suggesting that MT1-MMP is essential for its activation in vivo.
Matrix metalloproteinases (MMPs) are a family of Zndependent enzymes that are essential for extracellular matrix (ECM) turnover in normal and pathological conditions (1, 2). The MMPs are produced as latent proenzymes, and can be inhibited by specific tissue inhibitors of metalloproteinases (TIMPs). The enzymes can be divided into two structurally distinct subgroups, i.e., membrane-type (MT-MMP) and secreted MMPs. The secreted MMPs include interstitial collagenases (MMP-1, -8, and -13), which degrade fibrillar collagens, gelatinases (type IV collagenases, MMP-2 and -9) with high activity against gelatin and type IV collagen, and stromelysins (MMP-3, -10, and -11), which degrade a variety of collagenous and noncollagenous ECM proteins. Although the secreted MMPs have different expression patterns, there seems to be considerable redundancy and overlap between them with respect to function. Thus, mice deficient for MMP-2 (3), MMP-3 (4), MMP-7 (5), MMP-9 (6), MMP-10 (7), or MMP-12 (8) are all viable. Of these, only the MMP-9-deficient mice have been reported to show developmental abnormalities, which involve the growth plate and endochondral ossification (6).To date, five genetically distinct MT-MMPs (Mmp14-17, Mmp 21) have been identified (9-14). These enzymes (except MMP-17, which is glycosylphosphatidylinositol anchored) are singlepass type I membrane proteins with an extracellular N terminus containing the catalytic domain and a short C-terminal cytoplasmic domain. The prototypic MT-MMP, MT1-MMP (also termed MMP-14), was first identified as a cellular receptor and activator for pro-MMP-2 (9), but both MT1-MMP and MT2-MMP (MMP-15) have also been shown to have activity against a variety of ECM proteins, including gelatin, fibronectin, vitronectin, fibrillar collagens, and aggrecan (15, 16). MT1-MMP is widely expressed in cultured cells and tissues during development (17), but its strictly regulated spatial and temporal expression indicates more specific roles for this enzyme (17, 18). A crucial role for MT1-MMP for bone growth was recently demonstrated in MT1-MMP-deficient mi...
