Repression of the vertebrate organizer by Wnt8 is mediated by Vent and Vox

Abstract

Dorsoventral (DV) patterning of vertebrate embryos requires the concerted action of the Bone MorphogeneticProtein (BMP) and Wnt signaling pathways. In contrast to our understanding of the role of BMP in establishing ventral fates, our understanding of the role of Wnts in ventralizing embryos is less complete. Wnt8 is required for ventral patterning in both Xenopus and zebrafish; however, its mechanism of action remains unclear. We have used the zebrafish to address the requirement for Wnt8 in restricting the size of the dorsal organizer. Epistasis experiments suggest that Wnt8 achieves this restriction by regulating the early expression of the transcriptional repressors Vent and Vox. Our data show that vent and vox are direct transcriptional targets of Wnt8/β-catenin. Additionally, we show that Wnt8 and Bmp2b co-regulate vent and vox in a dynamic fashion. Thus, whereas both Wnt8 and zygotic BMP are ventralizing agents that regulate common target genes, their temporally different modes of action are necessary to pattern the embryo harmoniously along its DV axis.

A direct role for Wnt8 in ventrolateral mesoderm patterning

“…Additionally, while over-expression of BMP at moderate levels was found to induce wnt8 expression, high levels of BMP repress wnt8, suggesting the induction of wnt8 in response to BMP is under complex regulation (Marom et al, 1999). In zebrafish, it has been reported that Wnt8 over-expression can induce ectopic bmp2b (Agathon et al, 2003), but BMP is not necessary for normal wnt8 expression (Mullins, 1999;Ramel and Lekven, 2004;Szeto and Kimelman, 2004), suggesting an alternate transcriptional relationship in zebrafish. However, analysis of swr mutants suggests that high BMP activity can negatively regulate wnt8 at the end of gastrulation (Hammerschmidt and Mullins, 2002), similar to the relationship observed in amphibians.…”

“…Wnt and BMP activities can activate ventral genes independently of each other, but they also share some common targets (Hoppler and Moon, 1998;Marom et al, 1999;Ramel and Lekven, 2004;Szeto and Kimelman, 2004). While studies in zebrafish have highlighted a dynamic regulation of target genes by both pathways (Ramel and Lekven, 2004), it is still unclear what their relationship is: work in several model systems has shown that their relationship is complex and may involve cross-regulatory interactions at multiple levels (Hoppler and Moon, 1998;Marom et al, 1999;Fuentealba et al, 2007). Understanding the individual roles of Wnt and BMP signaling is critical for deciphering the regulation of vertebrate D/V patterning.…”

“…Studies in frogs and fish have shown that Wnt8 signaling is responsible for repressing the organizer at the onset of gastrulation (Hoppler et al, 1996;Lekven et al, 2001). In zebrafish, Wnt8 signaling limits the size of the organizer through transcriptional regulation of vent, vox, and ved (Ramel and Lekven, 2004;Ramel et al, 2005), which encode transcriptional repressors of dorsal genes (Kawahara et al, 2000a,b;Imai et al, 2001;Shimizu et al, 2002). In the absence of Wnt8, zebrafish embryos display greatly expanded dorsal fates and the concomitant loss of ventrolaterally derived cell fates (Lekven et al, 2001;Ramel and Lekven, 2004).…”

“…In zebrafish, Wnt8 signaling limits the size of the organizer through transcriptional regulation of vent, vox, and ved (Ramel and Lekven, 2004;Ramel et al, 2005), which encode transcriptional repressors of dorsal genes (Kawahara et al, 2000a,b;Imai et al, 2001;Shimizu et al, 2002). In the absence of Wnt8, zebrafish embryos display greatly expanded dorsal fates and the concomitant loss of ventrolaterally derived cell fates (Lekven et al, 2001;Ramel and Lekven, 2004). In the ectoderm, this is manifested in the expansion of neurectoderm at the expense of nonneural ectoderm.…”

“…In the mesoderm, high BMP levels during gastrulation direct progenitor cells to contribute to the tail (Agathon et al, 2003;Szeto and Kimelman, 2006;Tucker et al, 2008;Harvey et al, 2010), possibly by preventing their premature differentiation, and also influence cell convergence (Myers et al, 2002;von der Hardt et al, 2007). In zebrafish, Bmp2b co-regulates the Wnt8 target genes vent, vox, and ved during early gastrulation (Melby et al, 2000;Imai et al, 2001;Ramel et al, 2005), although the fact that BMP loss-of-function does not result in an expanded organizer (Mullins et al, 1996;Ramel and Lekven, 2004) or defective anteroposterior patterning in the gastrula (Barth et al, 1999) illustrates differences between their regulatory roles. Nonetheless, mutations in bmp2b (swr) and bmp7 (snailhouse) exhibit dramatic reductions in ventral and posterior tissues, producing a dorsalized phenotype that appears at least morphologically similar to that seen in wnt8 mutants (Mullins et al, 1996;Kishimoto et al, 1997;Schmid et al, 2000;Ramel et al, 2005).…”