dissertationHeparan sulfate (HS) chains play roles in numerous biological processes as they bind to various signaling molecules including fibroblast growth factors (FGFs). This graduate research aims at investigating the structural requirements and the biogenesis pathways of FGF8-binding HS motifs. In the first part, the importance of HS multivalency in regulating FGF8 and FGF receptor (FGFR) interactions in vivo was examined. A library of mono-, bis- or trisxylosides was injected into zebrafish embryos to stimulate the production of monomeric, dimeric or trimeric glycosaminoglycan (GAG) chains that are connected covalently, and thereby mimic naturally occurring proteoglycans. Upon their injection, bis- and trisxylosides caused an elongation phenotype whereas mono-xylosides did not. In situ hybridization and other experiments showed that FGF8/FGFR signaling was specifically hyperactivated in elongated embryos. Based on our findings, we propose a molecular model in which two covalently linked GAG chains interact with two FGF8 molecules and their cognate FGFRs and induce FGFR dimerization that leads to the elongation phenotype. This proposed molecular model was reaffirmed by the results of experiments testing syndecan-1 constructs containing zero, one, two or three HS side chains, in which multivalency again demonstrated its essential role in activating FGF8 signaling
