Meeting Abstract
The Dishevelled (Dsh) protein in the Wnt signaling pathway is essential for endomesoderm specification in sea urchin embryos. Dsh is asymmetrically enriched in a vegetal cortical domain (VCD) of the unfertilized egg, and several lines of evidence indicate that activation of Dsh in the canonical Wnt (cWnt) pathway during endomesoderm specification requires the VCD. First, while Dsh is required for activation of cWnt signaling in vegetal cells, overexpression of Dsh is not sufficient for ectopic endomesoderm formation in mesomeres. Second, a differentially modified form of Dsh accumulates in the VCD and this domain is selectively inherited by the vegetal blastomeres that nuclearize beta-catenin in early embryos. Moreover, embryological extirpation of the VCD from eggs blocks activation of cWnt signaling in embryos developing from VCD minus eggs. All of these observations indicate Dsh must work with some molecules at the VCD to regulate cWnt signaling activity and endomesoderm specification. To identify such candidate molecules, we carried out two separate molecular screens. To identify RNAs asymmetrically enriched in the egg cortex in general, we did a RNA-seq screen using eggs, egg cortices and different blastomeres from 16-cell stage embryos. To identify candidate proteins that regulate Dsh activity directly, we performed Dsh co-immunoprecipitation using lysates from eggs and isolated cortices. These screens have identified several intriguing candidates. Overexpression of one particular candidate produced severely vegetalized embryos indicating that this protein may play a critical role in the local activation of Dsh in the VCD during endomesoderm specification. We will discuss our ongoing studies to characterize the respective roles of these candidates in regulating cWnt pathway and endomesoderm specification in sea urchin embryogenesis.
