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

Abstract

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.

Ethanol induces embryonic malformations by competing for retinaldehyde dehydrogenase activity during vertebrate gastrulation

“…These results clearly show that the EtOHdependent reduction in RA signaling can be rescued by Raldh2 overexpression by providing a high enough enzyme concentration for both EtOH detoxification and RA biosynthesis. Overexpression of Aldh1 has been shown to promote premature activation of RA signaling in the embryo by oxidizing the available RAL (Ang and Duester, 1999b). This observation suggested that ALDH1 could fulfill the enzymatic function of RALDH2 in the early embryo.…”

Distinct Retinoid Metabolic Functions for Alcohol Dehydrogenase Genes Adh1 and Adh4 in Protection against Vitamin A Toxicity or Deficiency Revealed in Double Null Mutant Mice

“…Studies on the second step of RA synthesis, oxidation of retinal to RA, have yielded consistent results indicating that three forms of cytosolic aldehyde dehydrogenase referred to as retinaldehyde dehydrogenase (RALDH) participate in RA synthesis (1). In vitro activities are reported for RALDH1 (4), RALDH2 (5-7), and RALDH3 (8), plus in vivo activities are reported for RALDH1 and RALDH2 (9,10). Gene disruption studies demonstrate that RALDH2 is essential for catalyzing the second step of RA synthesis during development as Raldh2 Ϫ/Ϫ embryos die at mid-gestation and almost totally lack RA (11,12).…”

Methods for detecting and identifying retinoids in tissue

“…Instead, the RA molecule is inactivated in several locations by different enzymes. These enzyme types have been demonstrated to play an essential role in retinoid signaling and show discrete domains of expression during embryonic development (Ang and Duester, 1999;Haselbeck et al, 1999;Schneider et al, 2001). …”