Fibronectin

Fibronectin is a high-is_associated_with::molecular weight (~440kDa) is_associated_with::glycoprotein of the is_associated_with::extracellular matrix that binds to membrane-spanning is_associated_with::receptor proteins called is_associated_with::integrins. Similar to integrins, fibronectin binds extracellular matrix components such as is_associated_with::collagen, is_associated_with::fibrin, and is_associated_with::heparan sulfate is_associated_with::proteoglycans (e.g. is_associated_with::syndecans).

Fibronectin exists as a is_associated_with::protein dimer, consisting of two nearly identical is_associated_with::monomers linked by a pair of is_associated_with::disulfide bonds. The fibronectin protein is produced from a single gene, but is_associated_with::alternative splicing of its is_associated_with::pre-mRNA leads to the creation of several is_associated_with::isoforms.

Two types of fibronectin are present in is_associated_with::vertebrates:
 * soluble plasma fibronectin (formerly called "cold-insoluble globulin", or CIg) is a major protein component of is_associated_with::blood plasma (300 μg/ml) and is produced in the is_associated_with::liver by is_associated_with::hepatocytes.
 * insoluble cellular fibronectin is a major component of the extracellular matrix. It is secreted by various cells, primarily is_associated_with::fibroblasts, as a soluble is_associated_with::protein dimer and is then assembled into an insoluble matrix in a complex cell-mediated process.

Fibronectin plays a major role in is_associated_with::cell adhesion, growth, migration, and differentiation, and it is important for processes such as is_associated_with::wound healing and is_associated_with::embryonic development. Altered fibronectin expression, degradation, and organization has been associated with a number of is_associated_with::pathologies, including cancer and is_associated_with::fibrosis.

Structure
Fibronectin exists as a protein dimer, consisting of two nearly identical is_associated_with::polypeptide chains linked by a pair of is_associated_with::C-terminal is_associated_with::disulfide bonds. Each fibronectin is_associated_with::monomer has a molecular weight of 230–250 is_associated_with::kDa and contains three types of modules: type I, II, and III. All three modules are composed of two anti-parallel is_associated_with::β-sheets; however, type I and type II are stabilized by intra-chain disulfide bonds, while type III modules do not contain any disulfide bonds. The absence of disulfide bonds in type III modules allows them to partially unfold under applied force.

Three regions of variable splicing occur along the length of the fibronectin is_associated_with::protomer. One or both of the "extra" type III modules (EIIIA and EIIIB) may be present in cellulafibronectin, but they are never present in plasma fibronectin. A "variable" V-region exists between III14–15 (the 14th and 15th type III module). The V-region structure is different from the type I, II, and III modules, and its presence and length may vary. The V-region contains the binding site for is_associated_with::α4β1 integrins. It is present in most cellular fibronectin, but only one of the two subunits in a plasma fibronectin dimer contains a V-region sequence.

The modules are arranged into several functional and is_associated_with::protein-binding domains along the length of a fibronectin is_associated_with::monomer. There are four fibronectin-binding domains, allowing fibronectin to associate with other fibronectin molecules. One of these fibronectin-binding domains, I1–5, is referred to as the "assembly domain", and it is required for the initiation of fibronectin matrix assembly. Modules III9–10 correspond to the "cell-binding domain" of fibronectin. The RGD sequence (Arg–Gly–Asp) is located in III10 and is the site of cell attachment via is_associated_with::α5β1 and is_associated_with::αVβ3 integrins on the cell surface. The "synergy site" is in III9 and has a role in modulating fibronectin's association with is_associated_with::α5β1 is_associated_with::integrins. Fibronectin also contains domains for is_associated_with::fibrin-binding (I1–5, I10–12), is_associated_with::collagen-binding (I6–9), is_associated_with::fibulin-1-binding (III13–14), is_associated_with::heparin-binding and is_associated_with::syndecan-binding (III12–14).

Function
Fibronectin has numerous functions that ensure the normal functioning of is_associated_with::vertebrate organisms. It is involved in is_associated_with::cell adhesion, growth, migration, and differentiation. Cellular fibronectin is assembled into the is_associated_with::extracellular matrix, an insoluble network that separates and supports the organs and tissues of an organism.

Fibronectin plays a crucial role in is_associated_with::wound healing. Along with is_associated_with::fibrin, plasma fibronectin is deposited at the site of injury, forming a is_associated_with::blood clot that stops bleeding and protects the underlying tissue. As repair of the injured tissue continues, is_associated_with::fibroblasts and is_associated_with::macrophages begin to remodel the area, degrading the proteins that form the provisional is_associated_with::blood clot matrix and replacing them with a matrix that more resembles the normal, surrounding tissue. Fibroblasts secrete is_associated_with::proteases, including is_associated_with::matrix metalloproteinases, that digest the plasma fibronectin, and then the fibroblasts secrete cellular fibronectin and assemble it into an insoluble matrix. Fragmentation of fibronectin by proteases has been suggested to promote wound contraction, a critical step in is_associated_with::wound healing. Fragmenting fibronectin further exposes its V-region, which contains the site for is_associated_with::α4β1 is_associated_with::integrin-binding. These fragments of fibronectin are believed to enhance is_associated_with::α4β1 is_associated_with::integrins-expressing cell binding, allowing them to adhere to and forcefully contract the surrounding matrix.

Fibronectin is necessary for is_associated_with::embryogenesis, and inactivating the is_associated_with::gene for fibronectin results in early embryonic lethality. Fibronectin is important for guiding cell attachment and migration during is_associated_with::embryonic development. In is_associated_with::mammalian development, the absence of fibronectin leads to defects in is_associated_with::mesodermal, is_associated_with::neural tube, and is_associated_with::vascular development. Similarly, the absence of a normal fibronectin matrix in developing is_associated_with::amphibians causes defects in is_associated_with::mesodermal patterning and inhibits is_associated_with::gastrulation.

Fibronectin is also found in normal human saliva, which helps prevent colonization of the is_associated_with::oral cavity and is_associated_with::pharynx by potentially is_associated_with::pathogenic bacteria.

Matrix assembly
Cellular fibronectin is assembled into an is_associated_with::insoluble is_associated_with::fibrillar matrix in a complex cell-mediated process. Fibronectin matrix assembly begins when soluble, compact fibronectin dimers are is_associated_with::secreted from cells, often is_associated_with::fibroblasts. These soluble dimers bind to is_associated_with::α5β1 is_associated_with::integrin receptors on the cell surface and aid in clustering the is_associated_with::integrins. The local is_associated_with::concentration of is_associated_with::integrin-bound fibronectin increases, allowing bound fibronectin molecules to more readily interact with one another. Short fibronectin is_associated_with::fibrils then begin to form between adjacent cells. As matrix assembly proceeds, the soluble fibrils are converted into larger insoluble fibrils that comprise the is_associated_with::extracellular matrix.

Fibronectin’s shift from is_associated_with::soluble to insoluble is_associated_with::fibrils proceeds when cryptic fibronectin-binding sites are exposed along the length of a bound fibronectin molecule. Cells are believed to stretch fibronectin by pulling on their fibronectin-bound is_associated_with::integrin receptors. This is_associated_with::force partially unfolds the fibronectin ligand, unmasking cryptic fibronectin-binding sites and allowing nearby fibronectin molecules to associate. This fibronectin-fibronectin interaction enables the soluble, cell-associated is_associated_with::fibrils to branch and stabilize into an insoluble fibronectin matrix.

Role in cancer
Several of the morphological changes observed in is_associated_with::tumors and tumor-derived is_associated_with::cell lines have been attributed to decreased fibronectin expression, increased fibronectin degradation, and/or decreased expression of fibronectin-binding receptors, such as is_associated_with::α5β1 is_associated_with::integrins.

Fibronectin has been implicated in is_associated_with::carcinoma development. In is_associated_with::lung carcinoma, fibronectin expression is increased, especially in is_associated_with::non-small cell lung carcinoma. The adhesion of lung carcinoma cells to fibronectin enhances tumorigenicity and confers resistance to is_associated_with::apoptosis-inducing is_associated_with::chemotherapeutic agents. Fibronectin has been shown to stimulate the is_associated_with::gonadal steroids that interact with is_associated_with::vertebrate is_associated_with::androgen receptors, which are capable of controlling the expression of is_associated_with::cyclin D and related is_associated_with::genes involved in is_associated_with::cell cycle control. These observations suggest that fibronectin may promote lung tumor growth/survival and resistance to therapy, and it could represent a novel target for the development of new is_associated_with::anticancer drugs.

Fibronectin 1 acts as a potential is_associated_with::biomarker for is_associated_with::radioresistance.

Role in wound healing
Fibronectin has profound effects on is_associated_with::wound healing, including the formation of proper substratum for migration and growth of cells during the development and organization of is_associated_with::granulation tissue, as well as remodeling and resynthesis of the connective tissue matrix. The biological significance of fibronectin in vivo was studied during the mechanism of wound healing. Plasma fibronectin levels are decreased in acute inflammation or following surgical trauma and in patients with is_associated_with::disseminated intravascular coagulation.

Fibronectin is located in the ECM of embryonic and adult tissues (not in the is_associated_with::basement membranes of the adult tissues), but may be more widely distributed in inflammatory lesions. During clotting of the plasma, the fibronectin remains associated with the clot, covalently cross-linked to the is_associated_with::fibrin with the help of is_associated_with::factor XIII (fibrin-stabilizing factor). is_associated_with::Fibroblasts play a major role in wound healing by adhering to fibrin. The adhesion of fibroblast to the fibrin requires the presence of fibronectin and it was maximum when the fibronectin is crosss-linked to the fibrin. Patients with factor XIII deficiencies display impairment in wound healing and also that the fibroblasts don’t grow in the fibrin which is deficient in this factor. Fibronectin promotes phagocytosis of the particles, not only by the macrophages, but also by the fibroblasts. The deposition of collagen in the wounded site by the fibroblasts takes place with the help of fibronectin. Fibronectin was also observed to be closely associated with the newly deposited is_associated_with::collagen fibrils. Based on the size and histological staining characteristics of the fibrils, it is likely that at least in part they are composed of type III collagen (is_associated_with::reticulin). The in vitro study with the native collagen has demonstrated that the fibronectin binds to type III collagen rather than to any other types.

In vivo vs in vitro
Plasma fibronectin, which is synthesized by is_associated_with::hepatocytes, and fibronectin synthesized by cultured is_associated_with::fibroblasts are similar but not identical; immunological, structural, and functional differences have been reported. It is likely that these differences result from differential processing of a single nascent mRNA. Nevertheless, plasma fibronectin can be insolubilized into the tissue is_associated_with::extracellular matrix in vitro and in vivo. Both plasma and cellular fibronectins in the matrix form high molecular weight, disulfide-bonded multimers. The mechanism of formation of these multimers is not known. Plasma fibronectin has been shown to contain two free sulfhydryls per subunit (X), and cellular fibronectin has been shown to contain at least one. These sulfhydryls probably are buried within the tertiary structure, because sulfhydryls are exposed when the fibronectin is denatured. Such denaturation results in the oxidation of free sulfhydryls and formation of disulfide-bonded fibronectin multimers. This has led to speculation that the free sulfhydryls may be involved in formation of disulfide-bonded fibronectin multimers in the extracellular matrix. Consistent with this, sulfhydryl modification of fibronectin with is_associated_with::N-ethylmaleimide prevents binding to cell layers. Tryptic cleavage patterns of multimeric fibronectin do not reveal the disulfide-bonded fragments that would be expected if multimerization involved one or both of the free sulfhydryls. The free sulfhydryls of fibronectin are not required for the binding of fibronectin to the cell layer or for its subsequent incorporation into the extracellular matrix. Disulfide-bonded multimerization of fibronectin in the cell layer occurs by disulfide bond exchange in the disulfide-rich amino-terminal one-third of the molecule.

Interactions
Besides integrin, fibronectin binds to many other host and non-host molecules. For example, it has been shown to interact with proteins such is_associated_with::fibrin, is_associated_with::tenascin, TNF-α, BMP-1, rotavirus NSP-4, and many fibronectin-binding proteins from bacteria (like FBP-A; FBP-B on the N-terminal domain), as well as the glycosaminoglycan, is_associated_with::heparan sulfate.

Fibronectin has been shown to interact with:
 * is_associated_with::CD44
 * COL7A1,
 * LPA,
 * is_associated_with::IGFBP3,
 * TNC, and
 * is_associated_with::TRIB3.