Stimulation of premature retinoic acid synthesis in Xenopus embryos following premature expression of aldehyde dehydrogenase ALDH1.

In order for nuclear retinoic acid receptors to mediate retinoid signaling, the ligand retinoic acid must first be produced from its vitamin A precursor retinal. Biochemical studies have shown that retinal can be metabolized in vitro to retinoic acid by members of the aldehyde dehydrogenase enzyme family, including ALDH1. Here we describe the first direct evidence that ALDH1 plays a physiological role in retinoic acid synthesis by analysis of retinoid signaling in Xenopus embryos, which have plentiful stores of maternally derived retinal. The Xenopus ALDH1 gene was cloned and shown to be highly conserved with chick and mammalian homologs. Xenopus ALDH1 was not expressed at blastula and gastrula stages, but was expressed at the neurula stage. We used a retinoic acid bioassay to demonstrate that retinoic acid is normally undetectable in embryos from fertilization to the initial gastrula stage, but that a tremendous increase in retinoic acid occurs during neurulation when ALDH1 is first expressed. Overexpression of ALDH1 by injection of Xenopus embryos with mRNAs encoding the mouse, chick or Xenopus ALDH1 homologs induced high levels of retinoic acid detection during the blastula stage. Thus, premature expression of ALDH1 stimulates premature synthesis of retinoic acid. These findings reveal an important conserved role for ALDH1 in retinoic acid synthesis in vivo, and demonstrate that conversion of retinoids from the aldehyde form to the carboxylic acid form is a crucial regulatory step in retinoid signaling.