Retinoic acid synthesis in the prevertebrate amphioxus involves retinol oxidation

All- trans-retinoic acid (RA) contributes to the establishment of the anterior-posterior (AP) axis in chordates. In vertebrates, all- trans-retinol is oxidized to RA by two oxidative steps. However, the controversy about the enzymes responsible for retinol oxidation (ADH vs RDH) and the fact that some candidates are absent in cephalochordates questioned retinol oxidation in this lineage. Retinoid quantitation has revealed that Branchiostoma floridae adults contain both retinol and retinoic acid as well as retinal, the intermediate in the metabolic pathway. Furthermore, our data show that the developmental effects of retinol treatment are comparable to those reported for RA. SEM analysis revealed mouth and gill slit aberrations due to a posteriorization effect, also visualized by changes in the beta-galactosidase pattern. Overall, these findings support the idea that amphioxus metabolizes endogenous retinol to retinoic acid and suggest a common oxidative pathway for RA in the chordate phylum.