Fibronectin at a glance

Abstract

Fibronectin (FN) mediates a wide variety of cellular interactions with the extracellular matrix (ECM) and plays important roles in cell adhesion,migration, growth and differentiation(Mosher, 1989;Carsons, 1989;Hynes, 1990;Yamada and Clark, 1996). FN is widely expressed by multiple cell types and is critically important in vertebrate development, as demonstrated by the early embryonic lethality of mice with targeted inactivation of the FN gene(George et al., 1993). Although FN has been studied for more than two decades, this remarkably complex molecule is still the subject of exciting discoveries, such as finding new integrin- and heparin-binding sites(Mostafavi-Pour et al., 2001;Liao et al., 2002) or even a new form of the molecule (Zhao et al.,2001) that mediates a particular viral infection(Liu and Collodi, 2002).FIG1FN usually exists as a dimer composed of two nearly identical ∼250 kDa subunits linked covalently near their C-termini by a pair of disulfide bonds(see poster). Each monomer consists of three types of repeating units (termed FN repeats): type I (purple rectangles), type II (green octagons) and type III(red ovals). FN contains 12 type I repeats, two type II repeats and 15-17 type III repeats, which together account for approximately 90% of the FN sequence. Type I repeats are about 40 amino-acid residues in length and contain two disulfide bonds; type II repeats comprise a stretch of approximately 60 amino acids and two intrachain disulfide bonds; and type III repeats are about 90 residues long without any disulfide bonds. All three types of FN repeat are also found in other molecules, suggesting that FN evolved through exon shuffling (Patel et al.,1987).Although FN molecules are the product of a single gene, the resulting protein can exist in multiple forms that arise from alternative splicing of a single pre-mRNA that can generate as many as 20 variants in human FN (for reviews, see ffrench-Constant,1995; Kosmehl et al.,1996) (left panel, Plasma and Cellular fibronectin). A major type of splicing occurs within the central set of type III repeats (left panel, FN III7 to FN III15). Exon usage or skipping leads to inclusion or exclusion of either of two type III repeats — EDB (also termed EIIIB or EDII and located between FN repeats III7 and III8) and EDA (also called EIIIA or EDI and located between FN repeats III11 and III12). This `yes or no' type of splicing of FN ED domains is found in many vertebrates, including Xenopus, chickens, rats and humans.A third region of alternative splicing is localized to a non-homologous stretch called the V (variable in length) or IIICS (type III connecting segment) region. The structural variations in this region are more complex and species dependent (left panel, lower four gray boxes). In most species studied to date, except chicken, this region can be either partially or completely included or excluded; for example, in human FN, there can be five different V region variants.1 A fourth type of splicing is found in cartilage, where the predominant form of FN [termed (V+C)-] lacks the entire V region along with the FN III15 and FN I10 repeats. Interestingly, this FN isoform exists not only as a homodimer but also in an unusual monomeric configuration(left panel, Single-chain fibronectins). Recently, another single-chain FN(termed FN2) has been described in zebrafish, together with a FN1 form that is very similar to FNs identified in other vertebrates. The truncated FN2 form results from a fifth type of splicing in zebrafish (left panel, Single-chain fibronectins).1In chicken, the whole 120 amino acid residues of the V region can be included or a 44 amino acid segment from the 5′ end can be excluded(creating V76), but the whole V region is never missing. A similar mechanism leads to exclusion of a 25 amino acid fragment in rat, generating V95 that can be detected together with V0 and V120 forms. Splicing of the V region is even more complicated in human where segments from both 5′ (25aa) and 3′ (31aa) ends can be omitted independently (creating V95 and V89 correspondingly) or together (V64) producing five different V regions.FN is an abundant soluble constituent of plasma (300 μg/ml) and other body fluids and also part of the insoluble extracellular matrix. On the basis of its solubility, FN can be subdivided into two forms — soluble plasma FN (pFN) and less-soluble cellular (cFN) FN. Plasma FN is synthesized predominantly in the liver by hepatocytes and shows a relatively simple splicing pattern. The alternatively spliced EDA and EDB domains are almost always absent from plasma FN, although both V0 and V+ are present (left panel,Plasma fibronectin). Cellular FN consists of a much larger and more heterogeneous group of FN isoforms that result from cell-type-specific and species-specific splicing patterns (left panel, Cellular fibronectin). Thus,alternative splicing of precursor mRNA from the single FN gene has the capacity to produce a large number of variant