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Retinoic acid receptors and nuclear orphan receptors in the development of Xenopus laevis.
Dreyer C
,
Ellinger-Ziegelbauer H
.
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Nuclear hormone receptors are ligand-activated transcription factors that regulate the expression of target genes by binding to hormone responsive elements (HRE) in their 5' upstream region. Retinoids which are known for their teratogenicity and which have a potential role in the specification of the anteroposterior axis of vertebrate embryos regulate transcription via a hormone-like mechanism by activating nuclear retinoic acid receptors, designated RAR and RXR. Of the several isoforms of RAR found in embryos of Xenopus laevis, xRAR gamma 2 appears to be the most abundant. During the early retinoic acid-sensitive period of development, the total amount of xRAR gamma 2 transcript and protein is increased and a highly specific pattern of expression emerges. During neurulation, the receptor is predominantly found in the dorsal posterior region, in the head endomesoderm, and in the rostral hindbrain. The dependence of this pattern on mesoderm induction and on neural induction is discussed. Contrasting with the elaborate pattern of xRAR gamma 2, the FTZ-F1-related nuclear orphan receptors (xFF1rA/B) are ubiquitous nuclear proteins in Xenopus embryos, as are the peroxisome proliferator-activated receptors xPPAR alpha and beta. PPARs are activated by polyunsaturated fatty acids and regulate the synthesis of enzymes involved in lipid metabolism. Later in development, the isoforms xPPAR alpha, beta, and gamma attain different tissue specificities.
Fig. 1. Treatment of embryos with retinoic acid. Phenotypes observed in tadpoles after treatment of embryos at the blastula stage 9 with a 30 min pulse of 10-6M all-trans-RA. The untreated control is shown at the bottom
Fig. 2. Localization of xRAR-gamma mRNA in embryos at the neurula stage. Detection of xRAR-gamma mRNA on a sagittal section from a stage 16 neurula embryo by in situ hybridization as described (Ellinger-Ziegelbauer and Dteyeer, 1991). Anterior is to the right
Fig. 3. Retinoic acid receptor mRNA and protein expression during
embryogenesis of X. laev;s. (al Developmental Northern blocs of
polyA~ RNA from gastrula (stage 10; lane 1), midneurula (stage 16; lane
2), early railbud (stage 22; lane 3), and late tailbud (stage 29; lane 4)
embryos probed with antisense RNA specific of xRARy2 (upper panel),
and heterologous mouse RAR{3 (lower pane/). Note rhar the autoradiograph
shown in lower panel was exposed 10 times longer than that
shown in the upper panel. The double band in the lower panel corresponds
to ribosomal RNA (data not shown). Ibl Developmental Wesrern
blot using a polyclonal antiserum raised against mosr of the D/EiF
domain of xRARy2 IEllinger-Z,egelbauer and Drever. 1993!. Stages
according to Nieuwkoop and Faber (1967) are indicated at the bottom of
each lane.
Fig. 4. Immunodetection of nuclear receptors in embryos of X. laev;s. (a-d) Whole-mount immunofluorescent staining of stage 20 late neurula
embryos with anti-xRARYfDEFJ. Sections of 4 tJm were cut after embedding in Technovit fE/linger-Ziegelbauer and Dreyer. 1993). (a) Posterior region;
(b) region of the hindbrain of sagittal section; (c) part of transverse section showing the cement gland; (d) anterior part of sagittal section showing
the forebrain (F)and the cement gland (CG).Anterior is to the right in (a,b,d!. Ie-g) Staining of frozen sections with anti-xPPARj3(e,f), or preimmune
serum (g). (e,g) ovarian section showing previtellogenic oocytes; (f) dorsoposterior part of stage 20 neurula. (h) Dorsal part of a transverse section
from a stage 23 early tai/bud embryo sectioned after whole-mount staining with anti-xFF1rAlDEFdJ
