Developmental roles of the retinoic acid receptors

Retinoic acid, one of the principle active metabolites of vitamin A (retinol), is believed to be essential for numerous developmental and physiological processes. Vitamin A deprivation (VAD) during development leads to numerous congenital defects. Previous studies of retinoic acid receptor (RAR) deficient mice failed to reveal any of these VAD-induced defects. This finding suggested that either the RARs are functionally redundant or that they are not critically required during development. In order to address these possibilities, we derived a number of RAR compound mutants. Unlike RAR single mutants, these compound null mutants died either in utero or shortly following birth. Histological analysis revealed essentially all of the defects characteristic of fetal VAD. A number of additional malformations, not described in previous VAD studies, were also observed. These included defects of the ocular and salivary glands and their ducts, the skeletal elements of the fore-and hindlimbs, and the cervical region of the axial skeleton. In addition, with the exception of derivatives forming within the first pharyngeal arch, most of the elements derived from mesectoderm emanating from cranial and hindbrain levels were affected. A number of these mutants also exhibited supernumerary cranial skeletal elements characteristics of the reptilian skull. A summary of the defects found in these RAR double mutants is presented.     

Nuclear all-trans retinoic acid receptors

The present study was undertaken to investigate the effects of selenite (Se^IV) and selenate (Se^VI) on the all-trans retinoic acid (RA)-nuclear retinoic acid receptor (RAR) complex formation in rat liver. We also present the data on the in vitro effects of Se^IV on the RARα and the type I iodothyronine 5′-deiodinase gene expression in the GH₄C₁ rat pituitary tumor cells. Se^IV at 1.0 μmol/L was found to reduce (p<0.05) the RA specific binding to RAR in rat liver. Dithiothreitol (DTT), a protective agent for sulfhydryl groups, was found to be slightly effective in protecting the RAR binding properties when affected by Se^IV. Se^VI at 0.1 μmol/L reduced (p<0.05) the RA specific binding to RAR in liver, as well. Seleno-l-methionine (Se-^II) when compared tol-methionine did not exert any inhibitory effect on the formation of the RA-RAR complex. Se^IV (up to 2.5 μmol/L) has no inhibitory effect on GH₄C₁ cell proliferation as well as the prolactin secretion. Se^IV at 1.0 μmol/L significantly decreases the rate of mRNA synthesis and/or degradation of the α form of the RAR and causes the enhancement of the type I iodothyronine 5′-deiodinase gene expression in GH₄C₁ cells. The results based on in vitro experiments suggest that inorganic selenium may affect the RA specific binding to their cognate receptor molecules, and it may reduce expression of the gene encoding the RARα, with the cell vitality and the cell growth remaining unchanged.     

Retinoic acid and retinoic acid receptors in craniofacial development

Interest in retinoids and craniofacial development originated independently from nutritional and teratological studies; however, the site of action of retinoids in normal development remains contentious. Recent transgenic strategies have shown that retinoic acid and nuclear retinoid receptors are required for the morphogenetic specification of cranial neural crest cells and their mesenchymal derivatives during craniofacial development. Interestingly, while some aspects of the RA teratogenicity have been shown to be receptor-mediated, there is as yet no clear evidence that this is the case for the embryonic head and face. Hox genes are one important set of targets for RA in the developing neural primordium and cranial neural crest, but it remains unclear as to how retinoid-mediated regulation of such targets is realized as the morphogenetic specification of cell fate.     

Retinoid receptors involved in the effects of retinoic acid on rat testis development

We have previously shown that retinoic acid (RA) is able to act on the development of Leydig, Sertoli, and germ cells in the testis in culture (Livera et al., Biol Reprod 2000; 62:1303-1314). To identify which receptors mediate these effects, we have now added selective agonists and antagonists of retinoic acid receptors (RARs) or retinoid X receptors (RXRs) in the same organotypic culture system. The RAR alpha agonist mimicked most of the effects of RA on the cultured fetal or neonatal testis, whereas the RAR beta, gamma, and pan RXR agonists did not. The RAR alpha agonist decreased the testosterone production, the number of gonocytes, and the cAMP response to FSH of fetal testis explanted at 14.5 days postconception (dpc). The RAR alpha agonist disorganized the cords of the 14.5-dpc cultured testis and increased the cord diameter in cultured 3-days-postpartum (dpp) testis in the same way as RA. All these RA effects could be reversed by an RAR alpha antagonist and were unchanged by an RAR beta/gamma antagonist. The RAR beta agonist, however, increased Sertoli cell proliferation in the 3-dpp testis in the same way as RA, and this effect was blocked by an RAR beta antagonist. The RAR gamma and the pan RXR agonists had no selective effect. These results suggest that all the effects of RA on development of the fetal and neonatal testis are mediated via RAR alpha, except for its effect on Sertoli cell proliferation, which involves RAR beta.