Wnt5b–Ryk pathway provides directional signals to regulate gastrulation movement

Noncanonical Wnts are largely believed to act as permissive cues for vertebrate cell movement via Frizzled (Fz). In addition to Fz, Wnt ligands are known to regulate neurite outgrowth through an alternative receptor related to tyrosine kinase (Ryk). However, Wnt–Ryk signaling during embryogenesis is less well characterized. In this study, we report a role for Wnt5b as an instructive cue to regulate gastrulation movements through Ryk. In zebrafish, Ryk deficiency impairs Wnt5b-induced Ca2+ activity and directional cell movement. Wnt5b–Ryk signaling promotes polarized cell protrusions. Upon Wnt5b stimulation, Fz2 but not Ryk recruits Dishevelled to the cell membrane, suggesting that Fz2 and Ryk mediate separate pathways. Using co-culture assays to generate directional Wnt5b cues, we demonstrate that Ryk-expressing cells migrate away from the Wnt5b source. We conclude that full-length Ryk conveys Wnt5b signals in a directional manner during gastrulation.

Vegetal rotation, a new gastrulation movement involved in the internalization of the mesoderm and endoderm in Xenopus

A main achievement of gastrulation is the movement of the endoderm and mesoderm from the surface of the embryo to the interior. Despite its fundamental importance, this internalization process is not well understood in amphibians. We show that in Xenopus, an active distortion of the vegetal cell mass, vegetal rotation, leads to a dramatic expansion of the blastocoel floor and a concomitant turning around of the marginal zone which constitutes the first and major step of mesoderm involution. This vigorous inward surging of the vegetal region into the blastocoel can be analyzed in explanted slices of the gastrula, and is apparently driven by cell rearrangement. Thus, the prospective endoderm, previously thought to be moved passively, provides the main driving force for the internalization of the mesendoderm during the first half of gastrulation. For further involution, and for normal positioning of the involuted mesoderm and its rapid advance toward the animal pole, fibronectin-independent interaction with the blastocoel roof is required.