Retinoic acid receptors and cellular retinoid binding proteins. I. A systematic study of their differential pattern of transcription during mouse organogenesis.

We report here the gene expression patterns, as revealed by in situ hybridisation, of the retinoic acid receptors alpha, beta and gamma (RAR-alpha, -beta and -gamma), and the cellular binding proteins for retinol and retinoic acid (CRBP, CRABP) in non-neural tissues of mouse embryos during the period of organogenesis. At all stages, RAR-alpha transcripts were almost ubiquitous, whereas the distribution of transcripts of the other four genes was distinctive in all systems. At early stages in the formation of an organ, the expression patterns were different in the epithelium, the adjacent mesenchyme, and in mesenchyme more distant from the epithelium, suggesting a role for RA and RA receptors in epithelial-mesenchymal tissue interactions. In the developing face, limb bud and genital tubercle, where large expanses of mesenchyme are present, differential patterns of expression were established before the onset of overt tissue differentiation, suggesting some significance for pattern formation in these regions. The distribution of RAR-beta transcripts in tracheobronchial, intestinal and genital tract epithelial is consistent with the possibility that RAR-beta plays a role in mediating retinoid effects on the differentiated stage of these epithelia. Possible developmental roles of RARs in relation to the expression patterns of other genes are discussed. CRBP expression domains showed a high degree of overlap with RAR-beta and RAR-gamma, and a mutual exclusivity with CRABP expression domains. Correlation of these expression patterns with the morphogenetic effects of vitamin A deficiency and retinoid excess lead us to propose that the function of CRBP is to store and release retinol where high levels of RA are required for specific morphogenetic processes, while CRABP serves to sequester RA in regions where normal developmental functions require RA levels to be low. Where both binding protein genes are expressed in a non-overlapping pattern within a large area of mesenchyme, a gradient of free RA may be created between them by release of retinol-derived RA from CRBP-expressing cells, with binding to CRABP enhancing the steepness of the decline in concentration distant to the source.