Isoforms of retinoic acid receptor beta expressed in the chicken embryo

Classical pathways of gene regulation by retinoids

Retinoic acid receptors were discovered during early studies of the actions and mechanisms of essential vitamins. Vitamin A is metabolized in the body to retinoic acid (RA) which is a key compound in the control of many developmental processes in chordates. These functions are mediated by a subfamily of nuclear receptors, divided into two classes, the retinoic acid receptors (RAR) and the retinoid X receptors (RXR). Each class is encoded by three closely related genes that are located on different chromosomes. The three proteins in each class are designated α, β and γ, respectively. A wealth of structural studies have shown that they all share the same architecture including a DNA-binding domain connected by a flexible linker to the ligand and co-activator binding domain. Retinoic acid incorporation into the ligand-binding domain leads to a conformational change enabling the formation of RAR homodimers or RAR/RXR heterodimers that in turn bind specifically to target DNA sequences. The consensus sequences located on the promotors of regulated genes are known as retinoic acid response elements (RARE). The activated RAR/RXR homodimers recruit co-activators with histone acetylase activity leading to an opening of the chromatin structure and enabling downstream transcription of regulated genes. These canonical pathways describe the control mechanism for the majority of developmental processes mediated by retinoic acid and its derivatives.

Mechanism of asymmetric ovarian development in chick embryos

In most animals, the gonads develop symmetrically, but most birds develop only a left ovary. A possible role for estrogen in this asymmetric ovarian development has been proposed in the chick, but the mechanism underlying this process is largely unknown. Here, we identify the molecular mechanism responsible for this ovarian asymmetry. Asymmetric PITX2 expression in the left presumptive gonad leads to the asymmetric expression of the retinoic-acid (RA)-synthesizing enzyme, RALDH2, in the right presumptive gonad. Subsequently, RA suppresses expression of the nuclear receptors Ad4BP/SF-1 and estrogen receptor alpha in the right ovarian primordium. Ad4BP/SF-1 expressed in the left ovarian primordium asymmetrically upregulates cyclin D1 to stimulate cell proliferation. These data suggest that early asymmetric expression of PITX2 leads to asymmetric ovarian development through up- or downregulation of RALDH2, Ad4BP/SF-1, estrogen receptor alpha and cyclin D1.

Cloning of complementary deoxyribonucleic acids encoding quail (Coturnix coturnix japonica) retinoic acid receptor ss isoforms and changes in their gene expression during gonadotropic growth

Retinoids have important effects on the development of the reproductive system, where they act via their specific nuclear receptors: retinoic acid receptors (RARalpha, ss, gamma) and retinoid X receptors (RXRalpha, ss, gamma). The research reported here was conducted in an effort to clone quail RARbeta+ cDNA (qRARbeta) and to evaluate the expression of qRARbeta+ mRNAs in different tissues and during the development of gonadotropic organs. Two complete cDNAs of qRARbeta1 and qRARbeta2 were isolated by a combination of reverse transcription-polymerase chain reaction and 5'- and 3'-rapid amplification of cDNA ends techniques. An RNase protection assay revealed the widespread expression of qRARbeta1 and beta2 with large tissue-specific variations. The qRARbeta1 isoform was predominant in the testis, whereas qRARbeta2 was dominant in the other tissues examined with the exception of the brain, where both isoforms were almost equally expressed. In the developing testes, the qRARbeta1 mRNA level was high between 30 and 40 days of age, the period during which the testes grew rapidly. The level declined thereafter to its initial level. In contrast, qRARbeta2 mRNA did not exhibit obvious changes. In the developing oviducts, both qRARbeta1 and beta2 mRNAs reached their peak levels by 30 days of age, just before the rapid development of the oviduct occurred, and then decreased to almost undetectable levels when the oviduct developed to the laying stage (over 2.88 g in weight). Similar expression patterns of qRARbeta1 and beta2 were also observed in the developing follicles from the prehierarchical (<2-mm diameter) to the largest preovulatory follicle. In contrast, neither qRARbeta1 nor beta2 mRNA exhibited developmental changes in the brain. These results suggest that RARbeta+ may play an important role in the development of the reproductive systems of birds.

Cell-type-specific regulation of the retinoic acid receptor mediated by the orphan nuclear receptor TLX

Malformations in the eye can be caused by either an excess or deficiency of retinoids. An early target gene of the retinoid metabolite, retinoic acid (RA), is that encoding one of its own receptors, the retinoic acid receptor beta (RARbeta). To better understand the mechanisms underlying this autologous regulation, we characterized the chick RARbeta2 promoter. The region surrounding the transcription start site of the avian RARbeta2 promoter is over 90% conserved with the corresponding region in mammals and confers strong RA-dependent transactivation in primary cultured embryonic retina cells. This response is selective for RAR but not retinoid X receptor-specific agonists, demonstrating a principal role for RAR(s) in retina cells. Retina cells exhibit a far higher sensitivity to RA than do fibroblasts or osteoblasts, a property we found likely due to expression of the orphan nuclear receptor TLX. Ectopic expression of TLX in fibroblasts resulted in increased sensitivity to RA induction, an effect that is conserved between chick and mammals. We have identified a cis element, the silencing element relieved by TLX (SET), within the RARbeta2 promoter region which confers TLX- and RA-dependent transactivation. These results indicate an important role for TLX in autologous regulation of the RARbeta gene in the eye.

All-trans retinoic acid stimulates and maintains neurite outgrowth in nerve growth factor-supported developing chick embryonic sympathetic neurons

In explanted embryonic chick sympathetic neurons, all-trans retinoic acid (RA) as well as nerve growth factor (NGF) were found to be required for neuronal survival and neurite outgrowth at early stages of development (day 6.5-7) in agreement with previous work (Rodriguez-Tébar and Rohrer [1991] Development 112:813-820). The dependence of neurons on all-trans RA for survival diminished at later stages of development. However, all-trans RA was found to be needed at all stages of development in order to maximize neurite outgrowth. Further, removal of all-trans RA from the cultures led to a rapid degeneration of the formed neurites, demonstrating the essentiality of all-trans RA for both the development of neurites, and for the maintenance of existing neurites in cultured embryonic sympathetic neurons. The mechanism whereby all-trans RA exerts its effects on embryonic sympathetic neurons may involve activation of the nuclear retinoic acid and retinoid-X receptor (RAR and RXR) families. The results of Northern blot analyses and/or reverse transcriptase-polymerase chain reaction (RT-PCR) studies show that embryonic sympathetic ganglia express RAR beta, RAR gamma and RXR gamma mRNAs. RXR gamma mRNA is expressed at highest levels in immature neurons that are not yet responsive to NGF (day 6.5-7) and message levels decline with increasing developmental age. In contrast, RAR beta transcript levels are barely detectable at day 6.5-7, and increase approximately 4-fold in ganglia from embryos at day 8.5-9 and decline thereafter. RT-PCR studies show that RAR gamma mRNA is expressed both early (day 6.5-7) and late (day 15) in ganglionic development. Transcripts for the NGF receptors, p75NGFR and p140trk were also examined. The appearance of a single 2.7 kb p140trk transcript coincides with the time when RAR beta mRNA is maximally expressed, raising the possibility that NGF receptors may be targets of retinoid action. Evidence is also presented that all-trans RA may enhance neurite outgrowth by mechanisms other than simply inducing NGF-responsiveness of neurons.

Differential expression of retinoic acid receptor-beta isoforms during chick limb ontogeny

Retinoids influence both morphogenetic events and differentiation during development of the vertebrate limb. These effects are mediated through nuclear retinoid receptors, which modulate target gene expression. We report here the cloning and characterization of three promoter- and splicing-variants of the retinoic acid receptor-beta (RAR-beta) from chick. These receptor isoforms are independently expressed during limb development. RAR beta 2 but not RAR beta 1 transcripts are enriched three-fold in the posterior limb bud, reflecting the increased RA concentrations in this region. RAR beta 1 transcripts are initially present throughout the limb bud mesenchyme and ectoderm, then become restricted within perichondrial regions and loose connective tissue of the limb. RAR beta 1 expression closely overlaps that of NCAM (neural cell adhesion molecule) and tenascin in non-neuronal tissues. RAR beta 2 transcripts are present within a subset of those limb tissues which express RAR beta 1. In the early limb bud RAR beta 2 transcripts are detected in proximal limb mesenchyme and in the initial mesenchymal condensate. In older limbs RAR beta 2 mRNAs are abundant in cells lateral to the digit cartilage. Neither RAR beta 1 nor RAR beta 2 transcripts are associated specifically with regions of limb cell death. The differential expression and regulation of RAR beta isoforms suggests these variants may have different roles in limb development.

Retinoic acid receptor isoform beta 2 is an early marker for alimentary tract and central nervous system positional specification in the chicken

In this study I describe the distribution of one variant of retinoic acid receptor-beta (RAR-beta), the RAR-beta 2 isoform, during the stages before organogenesis of the chick embryo. Unlike the situation in older embryos, at these stages its distribution does not differ qualitatively from that of all RAR-beta transcripts. During the presomite headfold stage, RAR-beta 2 transcripts are simultaneously upregulated in two different locations. These locations define positional identities within the anlage of the chick alimentary tract and within the central nervous system (CNS). As development proceeds the transcript expression maintains its spatial restriction within those two regions. At presomite stages RAR-beta 2 transcripts are enriched within the proamnion, which contains the presumptive foregut and precardiac cells; somewhat later it is present within the foregut endoderm at the site where foregut and the lateral amniocardiac vesicles fuse to form the coelom and cardiac tube. As the foregut continues its caudal extension, RAR-beta 2 expression defines an anteroposterior boundary at the level of the pharynx within the alimentary tract. The second expression site of RAR-beta 2 mRNA first appears within the posterior neural plate at the level where Hensen's node commences its caudal regression. This boundary lies at the border between the future rhombomeres 5 and 6 within the hindbrain. Expression of RAR-beta 2 transcripts is also spatially restricted within some migrating cranial neural crest cells. The expression of RAR-beta 2 in cranial neural crest cells is consistent with what is known about the mechanisms by which cranial neural crest cell fate is determined. These data support the hypothesis that retinoids may contribute to positional specification of anteroposterior body axis, and perhaps also to the formation and identity of the developing alimentary tract and heart tube.

The role of retinoids in normal and abnormal embryonic craniofacial morphogenesis

The objective of this article is to evaluate the role of retinoids in the developing head and face. This article covers two lines of evidence that strongly support a role for retinoids in craniofacial development. First, the specific effects of exogenous retinoids on the head and face are covered and mechanisms for the specificity discussed. Second, the function of endogenous retinoids in facial development is discussed in relation to the distribution of retinoid-binding substances in the face. Finally, the interaction of retinoids with other genes known to be expressed in the face as well as other factors required for facial growth is discussed.

Analysis of high dysmorphogenic activity of Ro 13-6307, a tetramethylated tetralin analog of retinoic acid

Certain synthetic retinoids differ widely from retinoic acid (RA) in teratogenic potency, being much more or much less effective than RA. It is assumed that the potency of a retinoid may depend on the nature of its interaction with cellular binding components (nuclear retinoic acid receptors or cytoplasmic binding proteins) and, as in the case of retinoids that are mammalian teratogens, on factors that determine its accessibility to the embryo. To investigate some of the factors that contribute to potency, we used a new synthetic retinoid Ro 13-6307 that differs in structure from RA in having an aromatic ring inserted in its side chain along with gem dimethyl modification of the natural cyclohexenyl ring. Pregnant ICR mice were given a single oral dose (0, 1, or 10 mg/kg) on day 11 of gestation, and the resultant teratogenic outcome was monitored on day 17. Direct effects on cell differentiation were obtained by exposing high density cultures of limb bud mesenchymal cells to a range of concentrations (0.3 ng/ml-3 micrograms/ml) of Ro 13-6307 and scoring for chondrogenic suppression. Concentrations reaching the embryo after maternal administration of Ro 13-6307 were measured by HPLC to quantify the analog for a period of 4 h after administration of the oral dose. We found that this retinoid was 40-fold as active as RA in both inducing teratogenesis and suppressing chondrogenesis, yet its concentration in the affected embryo was only a fraction of that achieved after an equivalent dose of RA was employed in a similar protocol.(ABSTRACT TRUNCATED AT 250 WORDS)