FGF-Stimulated Outgrowth and Proliferation of Limb Mesoderm Is Dependent on Syndecan-3

Abstract

The outgrowth of the mesoderm of the developing limb bud in response to the apical ectodermal ridge (AER) is mediated at least in part by members of the FGF family. Recent studies have indicated that FGFs need to interact with heparan sulfate proteoglycans in order to bind to and activate their specific cell surface receptors. Syndecan-3 is an integral membrane heparan sulfate proteoglycan that is highly expressed by the distal mesodermal cells of the chick limb bud that are undergoing proliferation and outgrowth in response to the AER. Here we report that maintenance of high-level syndecan-3 expression by the subridge mesoderm of the chick limb bud is directly or indirectly dependent on the AER, since its expression is severely impaired in the distal mesoderm of the limb buds of limbless and wingless mutant embryos which lack functional AERs capable of directing the outgrowth of limb mesoderm. We have also found that exogenous FGF-2 maintains a domain of high-level syndecan-3 expression in the outgrowing mesodermal cells of explants of the posterior mesoderm of normal limb buds cultured in the absence of the AER and in the outgrowing subapical mesoderm of explants of limbless mutant limb buds which lack a functional AER. These results suggest that the domain of high-level syndecan-3 expression in the subridge mesoderm of normal limb buds is maintained by FGFs produced by the AER. Finally, we report that polyclonal antibodies against a syndecan-3 fusion protein inhibit the ability of FGF-2 to promote the proliferation and outgrowth of the posterior subridge mesoderm of limb buds cultured in the absence of the AER. These results suggest that syndecan-3 plays an essential role in limb outgrowth by mediating the interaction of FGFs produced by the AER with the underlying mesoderm of the limb bud.

Distinctive Expression Patterns of Heparan Sulfate O-Sulfotransferases and Regional Differences in Heparan Sulfate Structure in Chick Limb Buds

“…For example, chick syndecan-3 expressed in the distal mesenchymal cells underlying the AER plays an important role during limb outgrowth by regulating the interaction of FGFs with the underlying mesoderm (20). On the other hand, our results indicate that 6-O-sulfated structures of HS are preferentially found in the proximal regions of the limb buds.…”

“…In addition, HSPGs may bind growth factors in the extracellular matrix and thereby alter their stability and modulate their effective concentrations (17)(18)(19). Syndecan-3 (integral membrane HSPG) plays such a role in chick limb outgrowth by controlling the FGF signaling between the AER and mesoderm (20). Glypican-3 (glycosylphosphatidylinositol-anchored cell surface HSPG) controls cellular responses to bone morphogenetic protein-4 (BMP-4) in limb patterning (21).…”

The roles of FGFs in the early development of vertebrate limbs

“…One function is to stimulate the proliferation of cells in the progress zone and thus produce the new cells required for limb outgrowth. Studies employing markers for cell proliferation have directly demonstrated that FGFs are mitogens for limb bud mesenchyme (Aono and Ide 1988;Dealy et al 1997). Moreover, ectopic expression of Fgf2 on the anterior side of the wing bud apparently causes excess proliferation of mesenchymal cells, resulting in abnormal thickening and splitting of the skeletal primordia (Riley et al 1993).…”

Syndecan-3 Is a Selective Regulator of Chondrocyte Proliferation

“…Antibody Interference Assays in Cultured Chondrocytes-In previous studies by others, antibodies raised against syndecan-3 were used to interfere with its function and determine its role in developmental processes (30). Thus, we used a similar approach to test the role of syndecan-3 in chondrocyte proliferation.…”