Identification of endogenous retinoids, enzymes, binding proteins, and receptors during early postimplantation development in mouse: Important role of retinal dehydrogenase type 2 in synthesis of all-trans-retinoic acid

Specific combinations of nuclear retinoid receptors acting as ligand-induci ble transcription factors mediate the essential role of retinoids in embryo nic development. Whereas some data exist on the expression of these recepto rs during early postimplantation development in mouse, little is known abou t the enzymes controlling the production of active ligands for the retinoid receptors. Furthermore, at early stages of mouse development virtually no data are available on the presence of endogenous retinoids. In the present study we have used a recently developed high-performance liquid chromatogra phic (HPLC) technique to identify endogenous retinoids in mouse embryos dow n to the egg cylinder stage. All-trans-retinoic acid, a ligand for the reti noic acid receptors, was detected in embryos dissected as early as 7.5 dpc (i.e., a combination of midstreak until late allantoic bud stage embryos). At these stages, we detected mRNA coding for all the retinoid receptors, re tinoid binding proteins, and two enzymes able to convert retinol to retinal (retinol dehydrogenase 5 (RDH5) and alcohol dehydrogenase 4 (ADH4)). We al so detected retinal dehydrogenase type 2 (RALDH2), an enzyme capable of oxi dising the final step in the all-trans-retinoic acid synthesis. In egg cyli nder stage mouse embryos no all-trans-retinoic acid was detected. However, at this stage its precursor all-trans-retinal was present. In accordance wi th these HPLC observations, RDH5 and ADH4 were expressed, but no transcript s coding for enzymes that oxidise retinal to retinoic acid. Therefore, our results suggest that RALDH2 is a key regulator in initiating retinoic acid synthesis sometime between the mid-primitive streak stage and the late alla ntoic bud stage in mouse embryos. (C) 2000 Academic Press.