Interaction of Fibronectin with Proteoglycans

Certain heparin-binding sequences in fibronrectin seem to cooperate with its integrin a5b1-binding domain (modules III9-10) to enhance the spreading of cells. Although fibroblasts will adhere to surfaces coated with cell-binding fragments (e.g., 110k CBF) containing modules III9-10, they fail to spread and form focal contacts the way they do on whole Fn unless the Hep-2 fragment is also present [ref]. A similar effect occurs in melanoma cells through a weaker heparin-binding site in the third type III domain [ref]. In the former case it is thought that Hep-2 exerts its effects by binding to one or more cell-surface heparan sulfate proteoglycans (HSPGs) to induce additional signals through activation of PKC [ref]. The notion that a heparin-like moeity is involved is strengthened by the observation that mutation of the same basic residues in Fn module III-13 that diminish heparin binding also diminish the ability of that domain to induce focal contacts [ref]. Treatment of cells with enzymes that remove GAG moeities from HSPGs has a similar effect. The target of Hep-2 binding has not been proven but a suggested candidate is syndecan-4, a widespread HSPG that is enriched in focal adhesions and whose over-expression in and of itself will induce the formation of focal adhesions [ref] even in cells that are incapable of attaching GAG chains [ref]. HSPGs are prone to aggregate, perhaps even more so the core proteins lacking GAG moeities. Antibodies to syndecan-4 can also trigger spreading of cells grown on Fn-CBF, presumably through clustering. However, studies with fibroblasts isolated from Syndecan-4 deficient mice are able to form focal contacts when cultured on Fn suggesting that additional actors are lurking off stage.

A direct interaction between syndecan-4 and fibronectin has yet to be thoroughly characterized. As mentioned above, binding of Fn to heparan sulfates is generally much weaker than to heparin. However, Woods et al [ref] using an affinity electrophoresis method for binding of recombinant 40kDa Hep-2 to a preparation of GAGs that was derived directly from purified Syndecan-4, obtained a Kd of 68 nM, even stronger than that observed with heparin. This is an important finding that needs to be confirmed by another method. It implies that there is something special about the GAG moieties on syndecan-4 and it would be of great interest to determine the relevant structures.

The Hep-2 domain also functions in concert with the a4b1-binding region (CS1) to support the motility of certain cell types including melanoma cells [ref] and neural crest cells [ref]. At least in the former case this does not appear to involve the GAG-binding properties of Hep-2 but rather a specific peptide sequence IDAPS in module III-14 that, like LDVPS in CS1, is recognized by a4b1 .

Some proteoglycans are reported to interact with fibronectin through their core proteins [ref1; ref2].