Complementary domains of retinoic acid production and degradation in the early chick embryo

Excess retinoids as well as retinoid deprivation cause abnormal development , suggesting that retinoid homeostasis is critical for proper. morphogenesi s. RALDH-2 and CYP26, two key enzymes that carry out retinoic acid (RA) syn thesis and degradation, respectively, were cloned from the chick and show s ignificant homology with their orthologs in other vertebrates. Expression p atterns of RALDH-2 and CYP26 genes were determined in the early chick embry o by in situ hybridization. During gastrulation and neurulation RALDH-2 and CYP26 were expressed in nonoverlapping regions, with RALDH-2 transcripts l ocalized to the presumptive presomitic and lateral plate mesoderm and CYP2B mRNA to the presumptive mid- and forebrain. The two domains of expression were separated by an approximately 300-mu m-wide gap, encompassing the pres umptive hindbrain. In the limb region, a similar spatial segregation of RAL DH-2 and CYP26 expression was found at stages 14 and 15. Limb region mesode rm expressed RALDH-2, whereas the overlying limb ectoderm expressed CYP26. RA-synthesizing and -degrading enzymatic activities were measured biochemic ally in regions expressing RALDH-2 or CYP26. Regions expressing RALDH-2 gen erated RA efficiently from precursor retinal but degraded RA only inefficie ntly. Conversely, tissue expressing CYP26 efficiently degraded but did not synthesize RA. Localized regions of RA synthesis and degradation mediated b y these two enzymes may therefore provide a mechanism to regulate RA homeos tasis spatially in vertebrate embryos. (C) 1999 Academic Press.