Waif1/5T4 Inhibits Wnt/β-Catenin Signaling and Activates Noncanonical Wnt Pathways by Modifying LRP6 Subcellular Localization

SummaryWnt proteins can activate distinct signaling pathways, but little is known about the mechanisms regulating pathway selection. Here we show that the metastasis-associated transmembrane protein Wnt-activated inhibitory factor 1 (Waif1/5T4) interferes with Wnt/β-catenin signaling and concomitantly activates noncanonical Wnt pathways. Waif1 inhibits β-catenin signaling in zebrafish and Xenopus embryos as well as in mammalian cells, and zebrafish waif1a acts as a direct feedback inhibitor of wnt8-mediated mesoderm and neuroectoderm patterning during zebrafish gastrulation. Waif1a binds to the Wnt coreceptor LRP6 and inhibits Wnt-induced LRP6 internalization into endocytic vesicles, a process that is required for pathway activation. Thus, Waif1a modifies Wnt/β-catenin signaling by regulating LRP6 subcellular localization. In addition, Waif1a enhances β-catenin-independent Wnt signaling in zebrafish embryos and Xenopus explants by promoting a noncanonical function of Dickkopf1. These results suggest that Waif1 modulates pathway selection in Wnt-receiving cells

Germ cells in the crustacean Parhyale hawaiensis depend on Vasa protein for their maintenance but not for their formation

Germ cells are a population of cells that do not differentiate to form somatic tissue but form the egg and sperm that ensure the reproduction of the organism. To understand how germ cells form, holds a key for identifying what sets them apart from all other cells of the organism. There are large differences between embryos regarding where and when germ cells form but the expression of Vasa protein is a common trait of germ cells. We studied the role of vasa during germ cell formation in the crustacean Parhyale hawaiensis. In a striking difference to the posterior specification of the group of germ cells in the arthropod model Drosophila, all germ cells in Parhyale originate from a single germ line progenitor cell of the 8-cell stage. We found vasa RNA ubiquitously distributed from 1-cell to 16-cell stage in Parhyale and localized to the germ cells from 32-cell stage onwards. Localization of vasa RNA to the germ cells is controlled by its 3'UTR and this could be mimicked by fluorescently labeled 3'UTR RNA. Vasa protein was first detectable at the 100-cell stage. MO-mediated inhibition of vasa translation caused germ cells to die after gastrulation. This means that in Parhyale Vasa protein is not required for the initial generation of the clone of germ cells but is required for their subsequent proliferation and maintenance. It also means that the role of vasa changed substantially during an evolutionary switch in the crustaceans by Parhyale from the specification of a group of germ cells to that of a single germ line progenitor. This is the first functional study of vasa in an arthropod beyond Drosophila