27.6
Les Fibronectines Connectent les Cellules à la MEC
La fibronectine est une glycoprotéine adhésive présente dans la matrice extracellulaire des tissus embryonnaires et adultes. Ces molécules aident prin…
Les fibronectines sont des glycoprotéines adhésives présentes dans la matrice extracellulaire. Leur rôle principal est de connecter les cellules à la matrice.
Les fibronectines se composent de deux grandes sous-unités qui sont reliées à leur C-terminal par un couple de liaisons disulfure, formant un dimère. Les deux sous-unités contiennent divers domaines répétitifs qui peuvent se lier aux fibrilles de collagène, à l’héparine des protéoglycanes et aux intégrines - les récepteurs d’adhésion cellulaire. De cette façon, les fibronectines relient les cellules aux composants de la matrice.
La liaison avec l’intégrine est grandement influencée par la présence de résidus d’arginine, de glycine et d’aspartate, ou répétitions RGD, comme on le voit dans les domaines de type III - l’un des principaux domaines répétitifs dans les fibronectines.
Une fois liées à la fibronectine, les intégrines peuvent se lier aux filaments d'actine, formant un pont entre les composants extracellulaires et le cytosquelette de la cellule. Cette liaison transmet la tension, étirant les fibronectines et exposant leurs sites de liaison cryptiques auxquels plusieurs dimères de fibronectine peuvent ensuite se lier, formant des fibrilles de fibronectine.
Ces fibrilles de fibronectine aident à la migration et à la prolifération des cellules épidermiques, facilitant divers processus biologiques comme la cicatrisation des plaies.
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Q1: What is the basic structure of a fibronectin molecule?
Fibronectin consists of two identical polypeptide subunits joined at their C-terminal by disulfide bonds, forming a dimer. Both subunits contain repeating domains that bind to various extracellular components including collagen fibrils, heparin, proteoglycans, and integrins. This dimeric structure enables fibronectin to bridge cells with the extracellular matrix.
Q2: How do fibronectins connect cells to the extracellular matrix?
Fibronectins bind to integrins, cell-adhesion receptors on the cell surface, through RGD repeats in their type III domains. Once bound to fibronectin, integrins connect to actin filaments in the cytoskeleton. This creates a bridge linking extracellular matrix components with the cell's internal structure, enabling cell attachment and communication with the matrix.
Q3: What role do RGD repeats play in fibronectin binding?
RGD repeats, composed of arginine, glycine, and aspartate residues, are found in fibronectin's type III domains and greatly influence integrin binding. These sequences are critical recognition sites that allow integrins to bind fibronectin with high specificity. RGD-mediated binding initiates the cascade that connects cells to matrix components and facilitates cellular responses.
Q4: How do fibronectin fibrils form in the extracellular matrix?
When integrins bind fibronectin, tension is transmitted through the molecule, stretching it and exposing cryptic binding sites. Multiple fibronectin dimers then bind to these exposed sites, forming fibronectin fibrils. These fibrils support epidermal cell migration and proliferation, facilitating biological processes like wound healing and tissue remodeling.
Q5: What are the two types of fibronectin found in vertebrates?
Vertebrates have soluble plasma fibronectin, produced by hepatocytes and found in blood plasma, and insoluble cellular fibronectin, located in the extracellular matrix. Plasma fibronectin plays a role in blood clotting by depositing at injury sites alongside fibrin. Fibroblasts produce the insoluble cellular fibronectin that forms the extracellular matrix structure.
Q6: What happens to plasma fibronectin after it is deposited at an injury site?
Plasma fibronectin is deposited along with fibrin at injury sites to help stop bleeding. After clotting occurs, proteases secreted by fibroblasts digest the plasma fibronectin. Meanwhile, fibroblasts produce insoluble cellular fibronectin that forms the extracellular matrix, replacing the temporary clot with permanent tissue structure.
Q7: How do fibronectins interact with collagen and proteoglycans in the matrix?
Fibronectin's repeating domains bind directly to collagen fibrils and heparin of proteoglycans, anchoring these major extracellular matrix components. By simultaneously binding integrins on cells and collagen and proteoglycans in the matrix, fibronectins act as molecular bridges. This multi-binding capability makes fibronectins essential adhesive glycoproteins that organize and stabilize the extracellular matrix structure.