13-P038 Spatio-temporal expression of heparan sulfate glycosaminoglycan chains modulates distribution of Fgf signaling during mammalian embryogenesis

Heparan sulfate (HS) proteoglycans comprising HS glycosaminoglycan chains (HS-chains) and core proteins function in the membrane and extracellular matrix (ECM) as co-receptors for growth factors; i.e., they modulate activity of signaling factors via controlling their extracellular stabilization and movement. However, precise mechanisms by which HS-chains regulate complex mammalian morphogenetic processes remain to be elucidated. This study demonstrated the pivotal roles played by HS-chains in spatio-temporal distribution of Fgf signaling in the mouse embryo. We identified the transgene insertion allele of Ext2 which catalyzes elongation of HS-chains. Ext2deficient embryos displayed early embryonic defects due to lack of HS-chains. Marker expression analyses indicated that HSchains are essential for response to Fgf signaling. Localization of FGF4, FGFR2, HS-chains and syndecan-1 during extraembryonic ectoderm development revealed that HS-chains control the FGF ligand distribution in target cells. Accordingly, HSchains expression is distinctively specific to those areas in which Fgf signaling is potentially active. Additional fine mosaic analyses with single cell resolution involving chimeras demonstrated that cell-autonomous or immediately adjoining expression of membrane-associated HS-chains is crucial for early embryonic development, supporting that HS-chains function in the distribution but not movement of FGF primarily. Given that 22 FGF ligands, 4 FGF receptors and 12 membrane-associated core proteins are redundantly expressed in the mouse embryo, we hypothesized that spatiallyand temporally-specific expression of Ext2-dependent HS-chains contribute to FGF distribution during mammalian morphogenesis, e.g., formation of morphogen gradients.