MT1-MMP is a membrane-bound matrix metalloproteinase (MT-MMP) capable of mediating pericellular proteolysis of extracellular matrix components. MT1-MMP is therefore thought to be an important molecular tool for cellular remodeling of the surrounding matrix. To establish the biological role of this membrane proteinase we generated MT1-MMP-deficient mice by gene targeting. MT1-MMP deficiency causes craniofacial dysmorphism, arthritis, osteopenia, dwarfism, and fibrosis of soft tissues due to ablation of a collagenolytic activity that is essential for modeling of skeletal and extraskeletal connective tissues. Our findings demonstrate the pivotal function of MT1-MMP in connective tissue metabolism, and illustrate that modeling of the soft connective tissue matrix by resident cells is essential for the development and maintenance of the hard tissues of the skeleton.

As cancer cells traverse collagen-rich extracellular matrix (ECM) barriers and intravasate, they adopt a fibroblast-like phenotype and engage undefined proteolytic cascades that mediate invasive activity. Herein, we find that fibroblasts and cancer cells express an indistinguishable pericellular collagenolytic activity that allows them to traverse the ECM. Using fibroblasts isolated from gene-targeted mice, a matrix metalloproteinase (MMP)–dependent activity is identified that drives invasion independently of plasminogen, the gelatinase A/TIMP-2 axis, gelatinase B, collagenase-3, collagenase-2, or stromelysin-1. In contrast, deleting or suppressing expression of the membrane-tethered MMP, MT1-MMP, in fibroblasts or tumor cells results in a loss of collagenolytic and invasive activity in vitro or in vivo. Thus, MT1-MMP serves as the major cell-associated proteinase necessary to confer normal or neoplastic cells with invasive activity.

for highly malignant gliomas (World Health Organization grade III and IV) there is no successful treatment; patients have an average survival time of approximately 1 y after diagnosis. Glioma cells are highly invasive and infiltrate normal brain tissue, and as a result, surgical resection is always incomplete. Degradation of ECM by membrane-bound and secreted metalloproteases facilitates glioma invasion. In particular, the membrane-bound metalloproteases are pivotal for tumor invasion as they very efficiently digest extracellular matrix proteins and also activate secreted metalloproteases (1) like matrix metalloproteinase-2 (MMP-2, also known as gelatinase A), which is one of the major proteases involved in glioma invasion in mouse models (2) and probably also in humans (3). Hence, membrane-inserted metalloproteases like membrane type 1 matrix metalloproteinase (MT1-MMP) can enable gliomas to invade the brain parenchyma as single cells (4).Microglia are the intrinsic immune cells of the brain; they control the innate and the adaptive immune response in the CNS and are activated by inflammatory or other pathological stimuli (5). Activation of microglial toll-like receptors (TLRs) triggers the innate immune response and can initiate host-defense and tissue repair mechanisms, but also CNS inflammation, neurodegeneration, and trauma (5, 6). As microglial cells are attracted toward glioma in large numbers-glioma tissue consists of as much as 30% microglial cells-and because microglia density in gliomas positively correlates with malignancy, invasiveness, and grading of the tumors (7-9), we investigated if microglia may actively contribute to glioma expansion. Here, we show that soluble factors released from glioma stimulate microglial TLRs, resulting in microglial MT1-MMP expression via the TLR downstream signaling molecules MyD88 and p38 MAPK. In turn, MT1-MMP expression and activity in these immune cells promotes glioma cell invasion and tumor expansion. ResultsGlioma Associated Microglia Over-Express MT1-MMP. We analyzed the expression pattern of the matrix protease MT1-MMP in mouse and human gliomas and found the enzyme to be expressed predominantly in microglial cells closely associated with the tumors. Whereas tumor-free human brain samples showed virtually no MT1-MMP expression, we detected intense MT1-MMP labeling, especially in higher-grade gliomas. Importantly, in human samples, immunolabeling for the microglial marker Iba1 and for MT1-MMP largely overlapped [supporting information (SI) Fig. S1 A-D and Table S1]. Likewise, after injection of a human glioma cell line (U373 cells) into immunodeficient mice, we detected that microglia represent the predominant cell type contributing intratumoral MT1-MMP expression (see Fig. S1E).In our in vivo mouse model, the glioma cells were identified by stable expression of EGFP and microglial cells by immunolabeling for Iba1. In sections obtained from mice 2 weeks after intracerebral injection with isogenic glioma cells (GL261 cells), we found an increased density of mic...

During angiogenesis, endothelial cells initiate a tissue-invasive program within an interstitial matrix comprised largely of type I collagen. Extracellular matrix–degradative enzymes, including the matrix metalloproteinases (MMPs) MMP-2 and MMP-9, are thought to play key roles in angiogenesis by binding to docking sites on the cell surface after activation by plasmin- and/or membrane-type (MT) 1-MMP–dependent processes. To identify proteinases critical to neovessel formation, an ex vivo model of angiogenesis has been established wherein tissue explants from gene-targeted mice are embedded within a three-dimensional, type I collagen matrix. Unexpectedly, neither MMP-2, MMP-9, their cognate cell-surface receptors (i.e., β3 integrin and CD44), nor plasminogen are essential for collagenolytic activity, endothelial cell invasion, or neovessel formation. Instead, the membrane-anchored MMP, MT1-MMP, confers endothelial cells with the ability to express invasive and tubulogenic activity in a collagen-rich milieu, in vitro or in vivo, where it plays an indispensable role in driving neovessel formation.