INITIATION OF RETINOID SIGNALING IN PRIMITIVE STREAK MOUSE EMBRYOS - SPATIOTEMPORAL EXPRESSION PATTERNS OF RECEPTORS AND METABOLIC ENZYMES FOR LIGAND SYNTHESIS

The requirement of vitamin A (retinol) for successful completion of ve rtebrate embryogenesis is well established. Retinoid signaling involve s a two-step metabolic event in which retinol is first converted to re tinal, and then retinal is converted to the active ligand retinoic aci d, which modulates the transcriptional activity of a nuclear retinoic acid receptor (RAR). During mouse embryogenesis, retinoic acid is not detected at 6.5 days of embryonic development (E6.5) when gastrulation first initiates, but it is detected at E7.5 and later. This suggests that retinoid signaling during embryogenesis may be initiated during t he primitive streak stage. Here we have used whole-mount in situ hybri dization to examine E6.5-E8.5 mouse embryos for expression of RAR alph a, RAR beta, RAR gamma, and two enzymes, class IV alcohol dehydrogenas e (ADH-IV) and class I aldehyde dehydrogenase (ALDH-I), which have bee n shown to have retinol and retinal dehydrogenase activities, respecti vely. At E6.5, RAR alpha mRNA was expressed ubiquitously in embryonic and extraembryonic tissues, RAR gamma mRNA was detected throughout all embryonic tissues, but mRNAs for RAR beta, ADH-TV and ALDH-I were not detected. By E7.5, RAR alpha mRNA was still ubiquitous, RAR beta mRNA was now observed in presumptive hindbrain ectoderm and adjacent mesen chyme, RAR gamma mRNA was still observed in all embryonic tissues, and ADH-TV as well as ALDH-I mRNAs were now both expressed in primitive s treak mesoderm. In E8.5 embryos, RAR alpha mRNA was still ubiquitous, RAR beta mRNA was present in the caudal hindbrain as well as the close d neural tube and foregut, RAR gamma mRNA was widespread but most prev alent in caudal embryonic tissues, and mRNAs for both ADH-TV and ALDH- I were expressed in cranial mesenchyme, somites, and paraxial mesoderm . Thus, ADH-IV and ALDH-I, two metabolic enzymes able to convert retin ol to retinoic acid, are both initially expressed in primitive streak mesoderm at E7.5 when retinoic acid is first detectable. On the other hand, RAR alpha and RAR gamma expression is widespread and present at E6.5 prior to retinoic acid detection. These results suggest that upre gulation of ADH-IV and ALDH-I gene expression in primitive streak meso derm may lead to retinoic acid synthesis and initiation of retinoid si gnaling during mouse embryogenesis. (C) 1997 Wiley Liss, Inc.