Extracellular matrix

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The extracellular matrix (ECM) is a dynamic structure that contains a complex mixture of molecules, including elastin, collagens, laminins, fibronectin, proteoglycans and other insoluble molecules. The ECM not only provides a structural framework to support cells and divide tissues (Klein et al. 2004a;Mott & Werb 2004), but also acts as a reservoir for biologically active molecules, such as cytokines, chemokines, growth factors and apoptotic ligands.

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The specific structure and composition of the ECM meets the demands of specialized tissues (Klein et al. 2004a). Upon interaction with cells through receptor-mediating signaling induced by growth factors, cytokines and other cell adhesion receptors, the ECM controls cell behavior and creates an influential cellular environment (Brew, Dinakarpandian, & Nagase 2000;Massova et al. 1998). These ECM-cell interactions are crucial for the normal development and function of the organism. To maintain homeostasis, the remodeling of ECM is highly regulated. It is modulated through proteolytic systems that control the hydrolytic degradation of a variety of ECM molecules. By regulating the composition of the ECM structure, these proteolytic systems play a significant role in the control of signals sent by the ECM components. This affects cell migration, proliferation, differentiation and death. Uncontrolled proteolysis is an important pathogenic mechanism observed in a variety of diseases (Massova et al. 1998). The function of these proteolytic systems is very complex, as more than 500 genes encoding proteases or protease-like proteins exist in the human genome (Puente et al. 2003).

The majority of endopeptidases are divided into four different groups: 1) serine proteases, 2) cysteine proteases, 3) aspartic proteases and 4) metalloproteinases. The metalloproteinases are further divided into five subgroups that include the Metzincin superfamily and the gluzincin, inuzincin, carboxypeptidase and DD carboxypeptidase subgroups. The Metzincin superfamily includes five multigene families, the serralysins, the pappalysins, the astacins, the adamalysins (or ADAMs) and the matrix metalloproteinases (MMPs) (Sternlicht & Werb 2001). The MMPs are also known as matrixins and are the main enzymes responsible for the ECM degradation resulting in ECM turnover. The first MMP was described 45 years ago in experiments designed to explain how a metamorphosing tadpole lost its collagen-rich tail, leading to normal maturation to the frog (Gross & Lapiere 1962). This enzyme was shown to be important for the normal degradation of connective tissue in the tadpole tail. Following that discovery, MMPs were found to be present in all living organisms, from the simplest bacteria to the most complex systems in mammals. The physiological role of MMPs has been proven by experimental work using transgenes encoding MMPs, Tissue Inhibitors of Metalloproteinases (TIMPs), mutagenesis for specific MMPs or TIMPs genes (Vu & Werb 2000). These molecules take part in homeostatic mechanisms, such as tissue restoration, remodeling and repair, which are important in normal biological processes, such as fetal tissue development, organ morphogenesis,

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