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 fo r endoderm and aboral ectoderm formation during sea urchin embryogenesis. S pOtx target genes were repressed by fusing the SpOtx homeodomain to an acti ve repression domain of Drosophila Engrailed. The Engrailed-SpOtx fusion pr otein reduced the expression of endoderm- and aboral ectoderm-specific gene s and inhibited the formation of endoderm and aboral ectoderm cell types. C oexpressing activated beta-catenin with Engrailed-SpOtx did not overcome th e inhibition of endoderm and aboral ectoderm formation, suggesting that SpO tx functioned either downstream of or parallel to nuclear beta-catenin. Emb ryos expressing C-cadherin, which blocks nuclear translocation of beta-cate nin, have defects in endoderm and aboral ectoderm formation. Coexpressing S pOtx with C-cadherin restored aboral ectoderm-specific gene expression and aboral ectoderm morphology, but with C-cadherin present, SpOtx was not suff icient for endoderm formation. Our results show that SpOtx plays a key role in the activation of aboral ectoderm- and endoderm-specific gene expressio n and, in addition, suggest that SpOtx mediates some of beta-catenin's func tions in endoderm and aboral ectoderm formation. (C) 1999 Academic Press.