Differential expression of sea urchin Otx isoform (hpOtxE and HpOtxL) mRNAs during early development

Two distinct types of orthodenticle-related proteins (early type: HpOtxE, late type: HpOtxL) of the sea urchin, Hemicentrotus pulcherrimus, have been implicated as enhancer element binding factors of the aboral ectoderm-specific arylsulfatase (HpArs) gene. In order to understand the role of these isoforms during sea urchin development, we have isolated and characterized HpOtx gene. Here we describe the spatial expression patterns of HpOtxE and HpOtxL mRNAs and effects of overexpression of these mRNAs on embryogenesis. Whole-mount in situ hybridization using each isoform-specific probe reveals the complex and dynamic change of expression patterns among three germ layers. HpOtxE mRNA is maternally stored and exists apparently in a nonlocalized manner by the blastula stage. After hatching, HpOtxE transcripts are expressed predominantly in presumptive endoderm cells and gradually decrease during gastrulation. Signals for HpOtxL mRNA are intense at the vegetal half after hatching and subsequently, its expression is restricted to the micromere-derived cells. After primary mesenchyme cell (PMC) ingression, HpOtxL transcripts are localized at the vegetal plate and thereafter, concentrated primarily in ectoderm. Eggs injected with HpOtxE or HpOtxL mRNA develop into similar radialized structures without PMC ingression and gut invagination, whose oral-aboral axes are disrupted. Overexpression of HpOtxE induces accumulation of HpOtxL mRNA at the significantly earlier stages, though HpOtxL overexpression inhibits the accumulation of HpOtxE transcripts. Expression patterns of HpOtxE and HpOtxL in all three germ layers and dramatic morphological changes observed in the mRNA-injected embryos suggest that each HpOtx isoform has an important role in sea urchin embryogenesis.