posted on 2013-05-17, 00:00authored byThao K.
N. Nguyen, Vy M. Tran, Venkataswamy Sorna, Inger Eriksson, Akinori Kojima, Mamoru Koketsu, Duraikkannu Loganathan, Lena Kjellén, Richard I. Dorsky, Chi-Bin Chien, Balagurunathan Kuberan
Proteoglycans (PGs) modulate numerous
signaling pathways during
development through binding of their glycosaminoglycan (GAG) side
chains to various signaling molecules, including fibroblast growth
factors (FGFs). A majority of PGs possess two or more GAG side chains,
suggesting that GAG multivalency is imperative for biological functions in vivo. However, only a few studies have examined the biological
significance of GAG multivalency. In this report, we utilized a library
of bis- and tris-xylosides that produce two and three GAG chains on
the same scaffold, respectively, thus mimicking PGs, to examine the
importance of GAG valency and chain type in regulating FGF/FGFR interactions in vivo in zebrafish. A number of bis- and tris-xylosides,
but not mono-xylosides, caused an elongation phenotype upon their
injection into embryos. In situ hybridization showed
that elongated embryos have elevated expression of the FGF target
gene mkp3 but unchanged expression of reporters for
other pathways, indicating that FGF/FGFR signaling was specifically
hyperactivated. In support of this observation, elongation can be
reversed by the tyrosine kinase inhibitor SU5402, mRNA for the FGFR
antagonist sprouty4, or FGF8 morpholino. Endogenous
GAGs seem to be unaffected after xyloside treatment, suggesting that
this is a gain-of-function phenotype. Furthermore, expression of a
multivalent but not a monovalent GAG containing syndecan-1 proteoglycan
recapitulates the elongation phenotype observed with the bivalent
xylosides. On the basis of these in vivo findings,
we propose a new model for GAG/FGF/FGFR interactions in which dimerized
GAG chains can activate FGF-mediated signal transduction pathways.