Expressions of Raldh3 and Raldh4 during zebrafish early development.

Retinoic acid (RA) plays crucial roles in vertebrate embryogenesis. Four retinal dehydrogenases (Raldhs) that are responsible for RA synthesis have been characterized in mammals. However, only Raldh2 ortholog is identified in zebrafish. Here, we report the identification of raldh3 and raldh4 genes in zebrafish. The predicted proteins encoded by zebrafish raldh3 and raldh4 exhibit 70.0% and 73.5% amino acid identities with mouse Raldh3 and Raldh4, respectively. RT-PCR analyses reveal that both raldh3 and raldh4 mRNAs are present in early development. However, whole mount in situ hybridization shows that raldh3 mRNA is first expressed in the developing eye region of zebrafish embryos at 10-somite stage. At 24 hpf (hours post fertilization), raldh3 mRNA is expressed in the ventral retina of eye. At 36 hpf, the mRNA is also expressed in otic vesicle in addition to ventral retina, and it maintains its expression pattern till 2 dpf (days post fertilization). At 3 dpf, raldh3 mRNA becomes very weak in ventral retina but is present in otic vesicle at a high level. At 5 dpf and 7 dpf, raldh3 is no longer expressed in eyes but is expressed in otic vesicles at a very low level. raldh4 mRNA is initially detected in developing liver and intestine regions at 2 dpf embryos. Its expression level becomes very high in these two regions of embryos from 3 dpf to 5 dpf. Analysis on the sections of 5 dpf embryos reveals that raldh4 is expressed in the epithelium of intestine. At 7 dpf, raldh4 mRNA is only weakly expressed in the epithelium of intestinal bulb.