Abstract
The requirement for vitamin A in reproduction was first recognized in the early 1900’s, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.
Highlights
It has been nearly 100 years since the essential micronutrient, vitamin A, was first described.In 1913 McCollum and Davis reported that the addition of an ether extract from egg yolk or butter, but not lard or olive oil, could reinstate growth in rats maintained for several months on a purified ration of casein, carbohydrates and salt mixtures [1]
This suggests that some factor other than retinoic acid (RA) from the mesonephros may be affected by CYP26, and control entry into meiosis
These key experiments showed that compound retinoic acid receptor (RAR) mutants die either in utero or shortly after birth, and recapitulate most of the defects described as part of the vitamin A deficiency syndrome [61,88,89,90,91,92,93,94,95,103]
Summary
It has been nearly 100 years since the essential micronutrient, vitamin A, was first described. The metabolic scheme in which vitamin A (retinol) generates the vitamin A aldehyde (retinaldehyde) to support synthesis of the visual pigments, and its further irreversible oxidation to the vitamin A acid (all-trans retinoic acid, RA) that supports growth and tissue maintenance was first reported in the landmark paper by Dowling and Wald and this metabolic scheme stands essentially unchanged today (Figure 1A) [11]. Vitamin A and metabolites are lipophilic compounds that are generally found in association with serum and cellular binding proteins [24]. The RAR/RXR complex is bound to a specific sequence of DNA called the retinoic acid response element (RARE). The. DNA to which the RAR/RXR heterodimer binds is called a retinoic acid response element (RARE). RAREs serve as enhancer elements and when occupied by the RA/RAR/RXR complex, facilitate chromatin opening and changes in RA target gene transcriptional activity [34,35]. Note in this review retinoid is a term that refers to compounds structurally-related to retinol, and in this review, is used to refer to vitamin A and its metabolites
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