Mechanisms of cell fate specification in sea urchin embryos: Signaling pathways that pattern fates along the embryonic axes

Cell-cell signaling is important for cell fate specification along the animal-vegetal axis in sea urchins. One important signaling source is the vegetal-most blastomeres, the molecular nature of which has remained unidentified. Recently, components of the WNT signaling pathway, GSK-3beta and beta-catenin, have been implicated in this process. We tested TCF/LEF transcription factors, the nuclear effector of the WNT pathway, in animal-vegetal patterning. We show that expression of a dominant negative TCF results in animalization of the embryo, while an activated TCF leads to vegetalization of the embryo. The activity of endogenous sea urchin TCF is potentiated by LiCl, a known GSK-3beta inhibitor, consistent with an in vivo interaction between endogenous beta-catenin and TCF. We also provide evidence indicating that all beta-catenin's activity in specification along the animal-vegetal axis is mediated by TCF. Using an inducible form of TCF, we show that TCF transcriptional activity affects animal-vegetal axis patterning before the 60-cell stage.; In sea urchin embryos, the oral-aboral axis is the embryonic ventral-dorsal axis, which is not fixed until after fertilization. Molecular nature of the oral-aboral axis patterning is poorly understood, although classical experiments have shown that Nickel ion specifically promotes the oral ectoderm fate. Here we show that overexpression of activin, a member of the TGFbeta superfamily, can generate excess oral ectoderm at the expense of the aboral ectoderm, without affecting the animal-vegetal axis patterning. This result suggests the involvement of TGFbeta pathway in the oral-aboral axis specification.; Both these signaling pathways mainly influence ectodermal and endodermal territories. We were also interested in determining key elements of mesodermal specification in sea urchins. Twist protein is required for mesoderm specification in Drosophila, C. elegans and vertebrates. Because of sea urchin's unique phylogenetic position, we tested molecular and functional conservation of sea urchin twist gene. While we find that molecularly twist gene is well conserved in sea urchin, we are unable to link twist to early sea urchin mesodermal specification, suggesting a possible divergence of its role during evolution.