Wnt5b signaling in zebrafish development and disease

Wnts are secreted glycoproteins that activate signaling pathways important in development and disease. The canonical Wnt pathway activates nuclear localization of β-catenin, a multi-functional protein involved in cell adhesion and transcription. Other Wnt pathways, which appear to be independent of nuclear β-catenin, are collectively categorized as non-canonical pathways. Non-canonical Wnt pathways are involved in a variety of biological processes such as establishing planar cell polarity (PCP), stimulating intra-cellular calcium release and antagonizing canonical Wnt signaling. Wnt proteins share remarkable structural similarity, yet can trigger diverse pathways that lead to distinct or even opposing signaling outputs. To address this central question in biology, we focused on two fundamental aspects of Wnt signaling: the differential expression of Wnt ligands and the action of Wnt receptors. We specifically studied how zebrafish embryos regulate the expression of Wnt5, a particular Wnt protein capable of inducing all known Wnt pathways, and whether/how alternative Wnt receptors Frizzled (Fz) and Related to Tyrosine Kinase (Ryk) convey Wnt5 signals to distinguishable cellular molecular responses. Non-canonical Wnts are largely believed to act as permissive cues for embryonic development via Fz receptors in vertebrate cell movement. In this thesis, I provide evidence that Wnt5b acts as an instructive cue during gastrulation, and spatial-temporal control of Wnt5b expression is essential for its function in embryogenesis. I integrated in silico and in vivo methods to identify cis-regulatory elements that modulate embryonic expression pattern of zebrafish wnt5b. I further showed that epigenetic changes of these elements may be contributing to increased tumor incidence in adult zebrafish. In addition to Fz, Wnt ligands can signal through an alternative receptor Ryk. However, Wnt-Ryk signaling during embryogenesis is less well characterized. We found that Ryk deficiency impairs Wnt5b-induced calcium activity and directional cell movement. Upon Wnt5b stimulation, Fz2, but not Ryk, recruits Disheveled to the cell membrane, suggesting that Fz2 and Ryk mediate separate pathways. We further demonstrated that Ryk-expressing cells migrate away from the Wnt5b source by promoting polarized protrusive activity and conclude that full-length Ryk transduces directional Wnt5b signals in development.