Vitamin A deficiency and the expression of retinoic acid receptors during early cardiogenesis in quail embryo

The vitamin A deficiency-associated lethal syndrome observed in avian embryos may be linked to dysfunction of vitamin A-dependent genes. We tested this hypothesis in a quail embryo model by examining the expression of retinoic acid receptors (RARs) and cytosolic retinoic acid binding protein (CRABP) in normal and vitamin A-deficient embryos during early development. RAR alpha and RAR gamma mRNA were expressed at the same level in normal and vitamin A-deficient embryos during all stages studied. Expression of CRABP I was low in normal and vitamin A-deficient quail embryos during early development, but increased rapidly at later stages. Two transcripts of RAR beta, 3.2 and 3.5 kb, were detected in quail embryos during developmental stages 6-12. In normal emryos the level of the 3.2-kb isoform increased as embryonic development advanced. The expression of the 3.5-kb transcript was significantly decreased in vitamin A-deficient embryos, while the 3.2-kb transcript was undetectable by northern analysis. In situ hybridization of stage 7-8 normal quail embryos using a chicken RAR beta 2 riboprobe revealed that RAR beta 2 expression was predominantly associated with the cell populations in heart-forming regions, somites, neural folds, notochord and the presumptive thyroid. In stark contrast, in the vitamin A-deficient quail embryo RARP2 was not expressed in any of the above cell populations. We conclude that the expressions of RAR beta and CRABP I are developmentally regulated. Additionally, the expression of RAR beta 2 during early embryogenesis is regulated by vitamin A status. We propose that RARP2 plays an important role in the mechanism of action of retinoids in early avian development. The lack of expression of RAR beta 2 may be linked to abnormalities of the cardiovascular system.