The cell. 2.Extracellular matrix
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Glycoproteins are key molecules for the cohesion of tissues.
Fibronectin binds extracellular molecules such as collagen, proteoglycans, fibrin, as well as cell surface receptors.
Tenascin binds proteoglycans as well as integrins and other cell membrane receptors.
Laminin is a main component of the basal lamina.
Metalloproteinases and serine proteases allow remodelling and recycling of the extracellular matrix.
Cells are attached to the extracellular matrix, which at the same time is a network of molecules linked to each other. Most of the molecular adhesions in the extracellular matrix are based in protein-protein interactions, but protein-carbohydrates adhesions also helps to strengthen the molecular network. There are three ways of adhesions that maintain the structural integrity of tissues: cell-cell, cell-extracellular matrix, and molecules of the extracellular matrix between each other. Some glycoproteins located at the extracellular space have a role in adhesion of the extracellular matrix network.
Fibronectins are glycoproteins composed of two polypeptides joined by disulfide bridges, and contain several molecular domains that recognize and bind to glycosaminoglycans, proteoglycans, fibrin, heparin and some transmembrane proteins such as integrins. Thus, connections are established between different extracellular matrix molecules, but also between extracellular matrix and cells. Fibronectins are found as insoluble fibers in solid tissues, or like soluble molecules in body fluids such as blood. Besides maintaining extracellular matrix cohesion, fibronectins have many other functions. Por example, during embryonic development, cells move from one side to another through adhesion paths made of fibronectins.
Drawing of a fibronectin molecule. It is composed of two amino acid chains joined together by disulfide bridges close to the carboxyl terminal. Molecular domains that interact with and join to other molecules are indicated. (Modified from Pankov and Yamada, 2002).
Tenascin forms a family of large size proteins that can bind between each other, and are found in several types of tissues, including embryonary tissues, wounds, and tumors. They are able to bind to cell membrane proteins such as integrins, immunoglobulin receptors, proteoglycans, and annexin II. Tenascin also interacts with other proteins of the extracellular matrix such as fibronectin and certain proteoglycans.
Laminin is a main component of the basal lamina. It is made up of three polypeptides, alpha, beta and gamma, joined together by disulfide bridges. There are 5 types of alpha chains, 3 of beta chains, and 3 of gamma chains, which may be combined to form different types of laminin, although not all combinations are possible, and only 12 laminin subtypes have been found. Laminin is synthesized by epithelium, muscle cells, neurons, and bone marrow cells. Most of these cells release laminin to basal lamina, which is a sheet of extracellular matriz that separates these cells from connective tissue. Besides the structural function in the molecular network of the basal lamina, laminin also influences cell behavior and differentiation through interactions with integrins. That is why mutation of laminin usually leads to pathological processes.
Other adhesion glycoproteins found in the extracellular matrix are fibrin, which binds membrane receptors of platelets during blood coagulation, osteopontin, which is found in bone and kidney, and the so called binding protein, which binds to proteoglycans in cartilage. There are many others.
Extracellular matrix turnover: metalloproteinases
Extracellular matrix of animals is in constant recycling by a process of degradation and synthesis of molecules, which is under the cell control. Degradation is carried out by metalloproteinase enzymes. They are associated to the external layer of plasma membrane as free molecules, or they may be inserted in the plasma membrane. In both cases the catalytic domain is extracellular. Metalloproteinases are synthesized as inactive forms, known as prometalloproteinases, and are activated by proteolytic cleavage, which is carried out by enzymes located in the plasma membrane. In mammals, there are more than 20 types of metalloproteinases, which degradate different types of extracellular matrix molecules. A type of metalloproteinase can degradate several types of molecules, but is mostly active on one type. Thus, there are collagenases, gelatinases, and some others, depending on their main substrate.
Besides maintaining homeostasis, metalloproteinases are key players in the extracellular matrix remodeling after certain signals, such as hormones, during pathological processes like inflammation, during the repairing of wounded tissues, in tumor metastasis, and during embryonic development. Another role of metalloproteinases is to release molecules that are retained by the extracellular matrix, which become free signals for neighbor cells. Metalloproteins are produced by fibroblasts, but also by epithelium, chondrocytes, osteoblasts, leukocytes, as well as by cancer cells.
Bosman FT, Stamenkovic I.. 2003. Functional structure and composition of the extracellular matrix. Journal of pathology. 200:423-428.
Hynes RO. 1999. Cell adhesion: old and new questions. Trends in cell biology. 9:M33-M37.
Luo BH, et al. 2007. Structural basis of integrin regulation and signaling. Annual review of immunology. 25:619-647.
Pankov R, Yamada KM. 2002. Fibronectin at a glance. Journal of cell science. 115:3861-3863.
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Updated: 2017-09-06. 17:00
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo