Dimerized glycosaminoglycan chains increase FGF signaling during zebrafish development.

Area of Science:

• Developmental Biology
• Molecular Biology
• Biochemistry

Background:

• Proteoglycans (PGs) are key regulators of signaling pathways during development.
• Their glycosaminoglycan (GAG) side chains bind signaling molecules like fibroblast growth factors (FGFs).
• The biological significance of GAG multivalency, where PGs have multiple GAG chains, is understudied.

Purpose of the Study:

• To investigate the role of GAG valency and chain type in regulating FGF/FGFR interactions.
• To examine the in vivo biological significance of GAG multivalency using zebrafish embryos.

Main Methods:

• Utilized a library of bis- and tris-xylosides to mimic PGs with multiple GAG chains.
• Injected xylosides into zebrafish embryos and assessed developmental phenotypes.
• Employed in situ hybridization to analyze gene expression of FGF target genes and pathway reporters.
• Used inhibitors (SU5402, sprouty4 mRNA, FGF8 morpholino) to confirm FGF/FGFR pathway involvement.

Main Results:

• Injection of bis- and tris-xylosides, but not mono-xylosides, induced an embryonic elongation phenotype.
• Elongated embryos showed specifically hyperactivated FGF/FGFR signaling, indicated by elevated mkp3 expression.
• The elongation phenotype was reversible with FGF/FGFR pathway inhibitors, confirming pathway specificity.
• Expression of multivalent syndecan-1, but not monovalent GAGs, recapitulated the elongation phenotype.

Conclusions:

• GAG multivalency is critical for regulating FGF/FGFR interactions and developmental signaling.
• A model is proposed where dimerized GAG chains activate FGF-mediated signal transduction.
• This gain-of-function phenotype highlights the importance of GAG chain arrangement in vivo.