Requirement of SpOtx in cell fate decisions in the sea urchin embryo and possible role as a mediator of beta-catenin signaling.

We show here that the homeodomain transcription factor SpOtx is required for endoderm and aboral ectoderm formation during sea urchin embryogenesis. SpOtx target genes were repressed by fusing the SpOtx homeodomain to an active repression domain of Drosophila Engrailed. The Engrailed-SpOtx fusion protein reduced the expression of endoderm- and aboral ectoderm-specific genes and inhibited the formation of endoderm and aboral ectoderm cell types. Coexpressing activated beta-catenin with Engrailed-SpOtx did not overcome the inhibition of endoderm and aboral ectoderm formation, suggesting that SpOtx functioned either downstream of or parallel to nuclear beta-catenin. Embryos expressing C-cadherin, which blocks nuclear translocation of beta-catenin, have defects in endoderm and aboral ectoderm formation. Coexpressing SpOtx with C-cadherin restored aboral ectoderm-specific gene expression and aboral ectoderm morphology, but with C-cadherin present, SpOtx was not sufficient for endoderm formation. Our results show that SpOtx plays a key role in the activation of aboral ectoderm- and endoderm-specific gene expression and, in addition, suggest that SpOtx mediates some of beta-catenin's functions in endoderm and aboral ectoderm formation.