HEPARAN SULFATES MEDIATE THE BINDING OF BASIC FIBROBLAST GROWTH-FACTOR TO A SPECIFIC RECEPTOR ON NEURAL PRECURSOR CELLS

Heparan sulfate proteoglycans are thought to be obligatory for receptor binding and subsequent mitogenic activity of basic fibroblast growth factor (FGF-2). In a previous study (Nurcombe V., Ford, M. D., Wildschut, J., Bartlett, P. F. (1993) Science 260, 103-106) we have shown that primary cultures of mouse neuroepithelial cells and a cell line derived from them, 2.3D, secrete a heparan sulfate proteoglycan with a high affinity for EGF-2. In this study, a combination of affinity chromatography and gel chromatography was used to further isolate heparan sulfate side chains with high affinity for FGF-2. These active chains had an average molecular weight of 18,000-20,000. In order to determine whether heparan sulfate chains with specificity for FGF-2 also displayed selectivity for the different FGF receptors, peptides designed to the heparin-binding region of the receptors were used in competitive inhibition studies. The structure of the predicted heparin-binding domain of the FGF receptor 1 was modeled on the basis of its presumed secondary and tertiary structure homology with immunoglobulin loops. These results suggested that many of the basic residues within the second immunoglobulin loop of the FGF receptor 1 form a basic domain in the molecule and therefore form part of a heparin-binding site. Peptides homologous to this region of FGF receptor 1 were shown to inhibit mitogenesis in 2.3D cells, while those to FGF receptor types 2, 3, and 4 did not. A reverse transcriptase-polymerase chain reaction assay designed to detect expression of the four FGF receptors types demonstrated that FGF receptors 1 and 3 were present on the 2.3D cell line but that receptors 2 and 4 were not. These findings indicate that unique heparan sulfate domains interact with specific cell-surface receptors to direct cellular responses.