Differential expression of sea urchin Otx isoform (HpOtx(E) and HpOtx(L)) mRNAS during early development

Abstract

Two distinct types of orthodenticle-related proteins (early type: HpOtx(E), late type: HpOtx(L)) 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 HpOtx(E) and HpOtx(L) 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. HpOtx(E) mRNA is maternally stored and exists apparently in a nonlocalized manner by the blastula stage. After hatching, HpOtx(E) transcripts are expressed predominantly in presumptive endoderm cells and gradually decrease during gastrulation. Signals for HpOtx(L) 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, HpOtx(L) transcripts are localized at the vegetal plate and thereafter, concentrated primarily in ectoderm. Eggs injected with HpOtx(E) or HpOtx(L) mRNA develop into similar radialized structures without PMC ingression and gut invagination, whose oral-aboral axes are disrupted. Overexpression of HpOtx(E) induces accumulation of HpOtx(L) mRNA at the significantly earlier stages, though HpOtx(L) overexpression inhibits the accumulation of HpOtx(E) transcripts. Expression patterns of HpOtx(E) and HpOtx(L) 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.