Fibronectin is a large glycoprotein found in plasma (plasma fibronectin) and the extracellular matrix (cellular fibronectin). The former type was initially named cold-insoluble globulin (Morrison et al. 1948). In the 1970s, several laboratories independently characterized cellular fibronectin, naming it surface glycoprotein antigen (Vaheri and Ruoslahti 1974), large external transformation sensitive protein (LETS) (Hynes and Bye 1974), and cell surface protein (Yamada and Weston 1974), among others. When the common identity of these proteins was recognized, the name fibronectin (from the Latin fibra, meaning “fiber” and nectere meaning “to bind”) was adopted.
The expression of fibronectin is necessary for embryonic development: though gastrulation appears to progress normally in fibronectin knockout mice, the embryos start to exhibit significant defects shortly thereafter (George et al. 1993). Defects include the absence of a notochord and somites, as well as severe vascular abnormalities. The embryos die before the tenth day of development.
From an evolutionary perspective, fibronectin is a relative newcomer to the extracellular matrix. While collagens, laminins, and thrombospondins have been identified in the extracellular matrix of sponges and the mesoglea of cnidarians, fibronectin is only found in representatives of the phylum Chordata (Ozbek et al. 2010). Fibronectin has yet to be identified in cephalochordates, and only fibronectin-like predicted proteins are found in tunicates. However, the fibronectin found in vertebrates ranging from lampreys to birds and mammals has a domain architecture that is highly conserved (Adams et al. 2015).
Patterns of Expression
Fibronectin is highly expressed during wound healing, and the form that is expressed is the embryonic form (i.e., with the ED-A and ED-B variable domains) (ffrench-Constant et al. 1989). The highest levels of expression are within the granulation tissue and nearby dermis; migrating epidermal cells do not appear to express fibronectin, though they are migrating on a fibronectin-rich matrix. Similarly, fibronectin with the alternatively spliced FNIII domains reappears in crushed adult rat sciatic nerves, tumors, and renal disease (Schwarzbauer and DeSimone 2011).
Fibronectin is probably best known for its ability to promote cell adhesion and, under many conditions, cell motility. Cells cultured on fibronectin typically spread and form extensive lamellae and lamellipodia as well as robust stress fibers and focal adhesion complexes (Fig. 2a–c). Proteolytic fragments that retained the adhesive properties of intact fibronectin helped to narrow down the region of the molecule responsible for promoting cell adhesion, and eventually peptides were used to identify the motif RGDS, found in the tenth FNIII domain, as the cell-binding sequence (Pierschbacher and Ruoslahti 1984). The RGDS peptides were then used to elute fibronectin receptors from an affinity column (Pytela et al. 1985). Others took a different approach to find fibronectin receptors: antibodies known to label cells at points where actin filaments aligned with fibronectin fibrils in culture were used to screen expression libraries. The resulting clones were sequenced and named integrin (Tamkun et al. 1986); the integrins were eventually shown to be the same receptors that were eluted from the fibronectin affinity column using RGDS peptides. Integrins are now the most widely studied family of extracellular matrix receptors, and many of the properties of fibronectin can be explained by signaling initiated by fibronectin/integrin interactions.
Integrins are a family of dimeric extracellular receptors composed of alpha and beta subunits. Fibronectin has been shown to interact with at least 12 different integrin dimers, and the interaction sites have been mapped (Fig. 2d). The tenth FNIII domain, with its RGDS motif, is able to interact with nine different integrins. Two of these, the original fibronectin receptor α5β1 as well as αIIbβ3 (first identified in the 1970s as a platelet surface protein), bind more strongly in the presence of the synergy site PHSRN found in the adjacent ninth FNIII. Interestingly, several integrin-binding sites have been mapped to the ED-A and V domains, demonstrating the potential for alternative splicing to impact the function of fibronectin. The integrin-binding motifs in the 5th and 14th FNIII domains, as well as in the V region, are phylogenetically conserved; the RGD motif in the 10th FNIII domain is missing from lampreys and cartilaginous fish (Adams et al. 2015). Fibronectin/integrin interactions have been shown to be critical for fibronectin fibrillogenesis (Schwarzbauer and DeSimone 2011), as well as in promoting cell adhesion, migration, proliferation, differentiation, and survival (e.g., Vega and Schwarzbauer 2016).
Fibronectin can interact with fibrin, heparin, and collagen, and the regions where these molecules interact have been mapped (Fig. 2d). Syndecans are able to interact with the so-called heparin II binding region of fibronectin and modify fibronectin signaling through integrins (Leonova and Galzitskaya 2013). The same region (FNIII domains 12 through 14) is able to bind to growth factors, including most members of the PDGF and FGF families, as well as several members of the TGFβ and neurotrophin families of growth factors (Martino and Hubbell 2010). Fibronectin as a reservoir of growth factors can (1) concentrate growth factors near the cell surface and thereby increase their effects, (2) prevent the growth factors from being internalized thereby keeping them active for a longer period, and (3) sequester soluble growth factors for potential release following matrix degradation.
Bacteria have evolved a number of cell surface proteins that interact with fibronectin (Henderson et al. 2011), and bacterial binding to fibronectin probably plays roles in colonization and bacteria-host cell interactions. Staphylococcus aureus, which was first shown to interact with fibronectin in the 1970s, can use its fibronectin-binding proteins to invade endothelial cells, epithelia, osteoblasts, and keratinocytes, where they can avoid the host immune response (Henderson et al. 2011). This invasion appears to be the result of the bacterial proteins causing conformational changes in fibronectin, which in turn leads to integrin clustering and the uptake of the pathogens (Maurer et al. 2015).
Fibronectin is present in the stroma of many solid tumors, and its expression is associated with increased distant metastases and, in breast cancer patients, reduced survival rates (Insua-Rodríguez and Oskarsson 2016). The fibronectin found in tumor stroma can be expressed either by the tumor cells themselves or by surrounding fibroblasts. Growth factors secreted by the tumor cells can lead to the expression of fibronectin by fibroblasts in the stroma, creating new niches for metastases and increasing the motility of the tumor cells. The importance of fibronectin in tumor growth and metastasis is illustrated by studies of the microRNA let-7g. Decreased levels of let-7g expression in breast cancer are associated with increased metastasis and reduced survival (Qian et al. 2011). Expression profile analysis shows that the fibronectin gene FN1 is a direct downstream target of let-7g. Estrogen and epidermal growth factor decrease let-7g expression, leading to an increase in the deposition of fibronectin. This may explain, in part, the prometastatic properties of these growth factors.
As described above, fibronectin with the alternatively spliced ED-A and ED-B FNIII domains is abundant both in embryos and in healing wounds (ffrench-Constant et al. 1989). When mice are engineered to express only fibronectin without the ED-A FNIII domain, they undergo normal development but show abnormal healing of full thickness skin wounds (Muro et al. 2003). The wounds display abnormal interactions between the epidermis and the underlying granulation tissue, and over time the newly formed epidermis forms ulcers that become infiltrated with macrophages and polymorphonuclear leukocytes.
Fibronectin is a glycoprotein found in plasma and the extracellular matrix. Though encoded on a single gene, diversity of function is generated through alternative splicing. Its expression is widespread both in the embryo and in adult tissues, though alternatively spliced variants that dominate in the embryo are re-expressed in wounds and tumors. Fibronectin can promote cell adhesion, spreading and motility, and is able to interact with at least a dozen integrins. Many integrins recognize the RGD motif found in the tenth FNIII domain; others recognize alternatively spliced domains. Regions within fibronectin that bind to collagen, heparin, fibrin, and growth factors have been determined. High levels of fibronectin in tumors are associated with an increased metastasis and a poor clinical outcome. Future directions include the identification of therapeutic approaches that interfere with the role of fibronectin in promoting tumor growth and metastasis.
- Schwarzbauer JE, DeSimone DW. Fibronectins, their fibrillogenesis, and in vivo functions. Cold Spring Harb Perspect Biol. 2011;3(7). doi:10.1101/cshperspect.a005041.Google Scholar