soluble plasma fibronectin (formerly called "cold-insoluble globulin", or CIg) is a major protein component of blood plasma (300 μg/ml) and is produced in the liver by hepatocytes.
insoluble cellular fibronectin is a major component of the extracellular matrix. It is secreted by various cells, primarily fibroblasts, as a soluble protein dimer and is then assembled into an insoluble matrix in a complex cell-mediated process.
Fibronectin exists as a protein dimer, consisting of two nearly identical polypeptide chains linked by a pair of C-terminaldisulfide bonds.[9] Each fibronectin subunit has a molecular weight of ~230–~275 kDa[10] and contains three types of modules: type I, II, and III. All three modules are composed of two anti-parallel β-sheets resulting in a Beta-sandwich; 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.[11]
Three regions of variable splicing occur along the length of the fibronectin protomer. One or both of the "extra" type III modules (EIIIA and EIIIB) may be present in cellular fibronectin, 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 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 protein-binding domains along the length of a fibronectin monomer. There are four fibronectin-binding domains, allowing fibronectin to associate with other fibronectin molecules.[9] 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 α5β1 and αVβ3 integrins on the cell surface. The "synergy site" is in III9 and has a role in modulating fibronectin's association with α5β1integrins.[12] Fibronectin also contains domains for fibrin-binding (I1–5, I10–12), collagen-binding (I6–9), fibulin-1-binding (III13–14), heparin-binding and syndecan-binding (III12–14).[9]
Fibronectin plays a crucial role in wound healing.[13][14] Along with fibrin, plasma fibronectin is deposited at the site of injury, forming a blood clot that stops bleeding and protects the underlying tissue. As repair of the injured tissue continues, fibroblasts and macrophages begin to remodel the area, degrading the proteins that form the provisional blood clot matrix and replacing them with a matrix that more resembles the normal, surrounding tissue. Fibroblasts secrete proteases, including 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 wound healing. Fragmenting fibronectin further exposes its V-region, which contains the site for α4β1integrin binding. These fragments of fibronectin are believed to enhance the binding of α4β1 integrin-expressing cells, allowing them to adhere to and forcefully contract the surrounding matrix.
Cellular fibronectin is assembled into an insolublefibrillarmatrix in a complex cell-mediated process.[18] Fibronectin matrix assembly begins when soluble, compact fibronectin dimers are secreted from cells, often fibroblasts. These soluble dimers bind to α5β1integrin receptors on the cell surface and aid in clustering the integrins. The local concentration of integrin-bound fibronectin increases, allowing bound fibronectin molecules to more readily interact with one another. Short fibronectin fibrils then begin to form between adjacent cells. As matrix assembly proceeds, the soluble fibrils are converted into larger insoluble fibrils that comprise the extracellular matrix.
Fibronectin's shift from soluble to insoluble 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 integrin receptors. This 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 fibrils to branch and stabilize into an insoluble fibronectin matrix.
A transmembrane protein, CD93, has been shown to be essential for fibronectin matrix assembly (fibrillogenesis) in human dermal blood endothelial cells.[19] As a consequence, knockdown of CD93 in these cells resulted in the disruption of the fibronectin fibrillogenesis. Moreover, the CD93 knockout mice retinas displayed disrupted fibronectin matrix at the retinal sprouting front.[19]
FN1-FGFR1 fusion is frequent in phosphaturic mesenchymal tumours.[24][25]
Role in wound healing
Fibronectin has profound effects on wound healing, including the formation of proper substratum for migration and growth of cells during the development and organization of granulation tissue, as well as remodeling and resynthesis of the connective tissue matrix.[26] The biological significance of fibronectin in vivo was studied during the mechanism of wound healing.[26] Plasma fibronectin levels are decreased in acute inflammation or following surgical trauma and in patients with disseminated intravascular coagulation.[27]
Fibronectin is located in the extracellular matrix of embryonic and adult tissues (not in the basement membranes of the adult tissues), but may be more widely distributed in inflammatory lesions. During blood clotting, the fibronectin remains associated with the clot, covalently cross-linked to fibrin with the help of Factor XIII (fibrin-stabilizing factor).[28][29]Fibroblasts play a major role in wound healing by adhering to fibrin. Fibroblast adhesion to fibrin requires fibronectin, and was strongest when the fibronectin was cross-linked to the fibrin. Patients with Factor XIII deficiencies display impairment in wound healing as fibroblasts don't grow well in fibrin lacking Factor XIII. Fibronectin promotes particle phagocytosis by both macrophages and fibroblasts. Collagen deposition at the wound site by fibroblasts takes place with the help of fibronectin. Fibronectin was also observed to be closely associated with the newly deposited 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 (reticulin). An in vitro study with native collagen demonstrated that fibronectin binds to type III collagen rather than other types.[30]
In vivo vs in vitro
Plasma fibronectin, which is synthesized by hepatocytes,[31] and fibronectin synthesized by culturedfibroblasts are similar but not identical; immunological, structural, and functional differences have been reported.[32] It is likely that these differences result from differential processing of a single nascent mRNA. Nevertheless, plasma fibronectin can be insolubilized into the tissue extracellular matrixin vitro and in vivo. Both plasma and cellular fibronectins in the matrix form high molecular weight, disulfide-bondedmultimers. The mechanism of formation of these multimers is not presently 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 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.[32]
Interactions
Besides integrin, fibronectin binds to many other host and non-host molecules. For example, it has been shown to interact with proteins such fibrin, 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, heparan sulfate.
^Wierzbicka-Patynowski I, Schwarzbauer JE (August 2003). "The ins and outs of fibronectin matrix assembly". Journal of Cell Science. 116 (Pt 16): 3269–76. doi:10.1242/jcs.00670. PMID12857786. S2CID16975447.
^Lee JC, Jeng YM, Su SY, Wu CT, Tsai KS, Lee CH, Lin CY, Carter JM, Huang JW, Chen SH, Shih SR, Mariño-Enríquez A, Chen CC, Folpe AL, Chang YL, Liang CW (March 2015). "Identification of a novel FN1-FGFR1 genetic fusion as a frequent event in phosphaturic mesenchymal tumour". The Journal of Pathology. 235 (4): 539–45. doi:10.1002/path.4465. PMID25319834. S2CID9887919.
^Bruhn HD, Heimburger N (1976). "Factor-VIII-related antigen and cold-insoluble globulin in leukemias and carcinomas". Haemostasis. 5 (3): 189–192. doi:10.1159/000214134. PMID1002003.
^Lapiere JC, Chen JD, Iwasaki T, Hu L, Uitto J, Woodley DT (November 1994). "Type VII collagen specifically binds fibronectin via a unique subdomain within the collagenous triple helix". The Journal of Investigative Dermatology. 103 (5): 637–41. doi:10.1111/1523-1747.ep12398270. PMID7963647.
^Zhou Y, Li L, Liu Q, Xing G, Kuai X, Sun J, Yin X, Wang J, Zhang L, He F (May 2008). "E3 ubiquitin ligase SIAH1 mediates ubiquitination and degradation of TRB3". Cellular Signalling. 20 (5): 942–8. doi:10.1016/j.cellsig.2008.01.010. PMID18276110.
Further reading
ffrench-Constant C (December 1995). "Alternative splicing of fibronectin--many different proteins but few different functions". Experimental Cell Research. 221 (2): 261–71. doi:10.1006/excr.1995.1374. PMID7493623.
Przybysz M, Katnik-Prastowska I (2002). "[Multifunction of fibronectin]" [Multifunction of fibronectin]. Postȩpy Higieny I Medycyny Doświadczalnej (in Polish). 55 (5): 699–713. PMID11795204.
Rameshwar P, Oh HS, Yook C, Gascon P, Chang VT (2003). "Substance p-fibronectin-cytokine interactions in myeloproliferative disorders with bone marrow fibrosis". Acta Haematologica. 109 (1): 1–10. doi:10.1159/000067268. PMID12486316. S2CID25830801.
Schmidt DR, Kao WJ (January 2007). "The interrelated role of fibronectin and interleukin-1 in biomaterial-modulated macrophage function". Biomaterials. 28 (3): 371–82. doi:10.1016/j.biomaterials.2006.08.041. PMID16978691.
