Effect of a high systemic background level of vitamin A on the teratogenicity of all-trans-retinoic acid given either acutely or subacutely

Sonic Hedgehog Signaling Is Required for Cyp26 Expression during Embryonic Development

Deciphering how signaling pathways interact during development is necessary for understanding the etiopathogenesis of congenital malformations and disease. In several embryonic structures, components of the Hedgehog and retinoic acid pathways, two potent players in development and disease are expressed and operate in the same or adjacent tissues and cells. Yet whether and, if so, how these pathways interact during organogenesis is, to a large extent, unclear. Using genetic and experimental approaches in the mouse, we show that during development of ontogenetically different organs, including the tail, genital tubercle, and secondary palate, Sonic hedgehog (SHH) loss-of-function causes anomalies phenocopying those induced by enhanced retinoic acid signaling and that SHH is required to prevent supraphysiological activation of retinoic signaling through maintenance and reinforcement of expression of the Cyp26 genes. Furthermore, in other tissues and organs, disruptions of the Hedgehog or the retinoic acid pathways during development generate similar phenotypes. These findings reveal that rigidly calibrated Hedgehog and retinoic acid activities are required for normal organogenesis and tissue patterning.

Prenatal effects of retinoic acid on lumbar spinal cord development and liver antioxidants in rats

During embryonic and early postnatal development, retinoic acid (RA) regulates genes that control neuronal differentiation and neurite outgrowth from the neural tube. The effects of high levels of RA on the CNS can be detected via nitric oxide (NO), which plays a crucial role in neural transmission. The aim of the study was to investigate the prenatal influence of high levels of RA on postnatal development of nitrergic structures in lumbar spinal cord and antioxidant status. RA was administered orally at a dose of 10mg/kg body weight to pregnant female Wistar rats during days 8-10 of gestation. Neuronal nitric oxide synthase (nNOS) of lumbar spinal cord sections was processed for visualization via nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry on postnatal day one, day twenty-one and in adults. The results suggest that prenatal administration of high levels of RA is not associated with postnatal morphological changes in nNOS-positive neurons in the rat lumbar spinal cord. An estimation of the activity of enzymes related to the storage of retinoid in the liver showed possible side effects. Suppression and deepening of superoxide dismutase activity persisted into adulthood, and a concurrent downregulation of glutathione reductase was noted. A decrease in reduced glutathione persisted until adulthood when other compensatory mechanisms were probably active to maintain an appropriate level.

Induction of beta-retinoic acid receptor mRNA by teratogenic doses of retinoids in murine fetuses

Retinoic acid is required for normal growth and development, however excessive doses are teratogenic. Recently several nuclear retinoic acid receptors (RAR) have been identified and postulated to mediate the response of retinoic acid at the gene level. We wished to determine if alpha-RAR mRNA or beta-RAR mRNA levels are modulated by teratogenic doses of retinoic acid in vivo. We have found that beta-RAR mRNA levels in 9-day-gestation mouse conceptuses are increased as early as 3 h after administration of a completely teratogenic dose of retinoic acid (100 mg/kg body weight; b.w.) and reach a maximum of approximately sixfold after 6 h of treatment. Maternal liver and maternal kidney demonstrated a similar pattern of increase in beta-RAR mRNA, however this was only approximately threefold. Retinoic acid dose-response experiments demonstrated a reduced increase of beta-RAR mRNA levels with 10 mg/kg b.w. (minimally teratogenic dose), and no increase with a more-physiological dose of 1 mg/kg b.w. in the conceptuses. beta-RAR mRNA levels were elevated in 18-day-gestation fetuses to a similar extent to that observed in the 9-day-gestation conceptuses. Therefore, the twofold difference in the extent to which beta-RAR mRNA levels increase does not occur because the fetuses are at a developmental stage that is sensitive to the teratogenic effects of retinoic acid. Finally, treatment with another teratogenic retinoid, etretinate, and a nonteratogenic retinoid, retinoyl beta-glucuronide, both resulted in increase in the level of beta-RAR mRNA in the conceptuses and the maternal tissues. Therefore, an increase in beta-RAR mRNA levels caused by treatment with retinoids does not necessarily commit a fetus to undergo an abnormal pattern of development characteristic of teratogenic retinoids.