Abstract
Epiboly, the spreading and the thinning of the blastoderm to cover the yolk cell and close the blastopore in fish embryos, is central to the process of gastrulation. Despite its fundamental importance, little is known about the molecular mechanisms that control this coordinated cell movement. By a combination of knockdown studies and rescue experiments in zebrafish (Danio rerio), we show that epiboly relies on the molecular networking of syntenin with syndecan heparan sulphate proteoglycans, which act as co-receptors for adhesion molecules and growth factors. Furthermore, we show that the interaction of syntenin with phosphatidylinositol 4,5-bisphosphate (PIP2) and with the small GTPase ADP-ribosylation factor 6 (Arf6), which regulate the endocytic recycling of syndecan, is necessary for epiboly progression. Analysis of the earliest cellular defects suggests a role for syntenin in the autonomous vegetal expansion of the yolk syncytial layer and the rearrangement of the actin cytoskeleton in extra-embryonic tissues, but not in embryonic cell fate determination. This study identifies the importance of the syntenin-syndecan-PIP2-Arf6 complex for the progression of fish epiboly and establishes its key role in directional cell movements during early development.