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]
Fibronectin genetic variation as a protective factor against Alzheimer's disease
A specific genetic variation in Fibronectin gene was shown to reduce the risk of developing Alzheimer's disease in a multicenter, multiethnic genetic epidemiology and functional genomics study. This effect is believed to be through enhancing the brain's ability to clear the toxic waste and protein accummulation through blood-brain-barrier. [33]
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.
^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.