The retinoid X receptor ligand, 9-cis-retinoic acid, is a potential regulator of early Xenopus development.

Abstract

Endogenous retinoids are potential regulators of vertebrate embryogenesis that have been implicated in early anterior-posterior patterning and limb-bud development. We have characterized the temporal and spatial distribution of 9-cis-retinoic acid in the Xenopus embryo and compared it to two other retinoids, afl-tmns-retinoic acid and alI-frns-retinoyl-3glucuronide. 9-cis-Retinoic acid is first detected after the midblastula transition and by the end of gastruation is localz primarily within the anterior and posterior dorsal regions ofthe embryo. Since 9-cis-retinoic acid is a 6-fold more potent dysmorphogen than trans-retinoic acid, we suggest that it is involved in the early specification of the Xenopus anterior-posterior aids.The organization of the vertebrate body plan is dependent upon communication between groups of cells in the early embryo. It has been proposed that all-trans-retinoic acid (trans-RA), a metabolite of retinol (vitamin A) and a member of a class of compounds known as retinoids, functions as one of the molecular signals in early vertebrate development (1-9). In support of this proposal, experiments have shown that trans-RA affects cellular differentiation (10) and that embryonic exposure to trans-RA causes severe malformations in a number ofvertebrate species (11-13). Furthermore, retinoid deficiency during embryonic development also produces malformations (14,15). The cellular effects of RA isomers appear to be mediated by two distinct classes of receptors within the steroid-thyroid nuclear receptor superfamily, the RA receptors (RARs) and the retinoid X receptors (RXRs), which presumably function by altering the transcription of specific target genes (16). In addition, cellular RA and retinol binding proteins (CRABPs and CRBPs) appear to modulate the localization, availability, and metabolism of these retinoids within the organism (17).Despite the ability of exogenous all-trans-RA to cause major alterations to the anterior-posterior (A-P) axis of mice (2, 3) and frogs (1, 4-6), evidence that endogenous retinoids are important modulators of early development has been difficult to obtain. Indirect evidence that retinoids are present in restricted regions of the embryo has been obtained from in vivo studies with transgenic mice (18-21) and in vitro studies with rat (22) and chicken (23) tissue explants. Chicken embryos also exhibit regional differences in the capacity to convert retinol to trans-RA (24), with the maximum activity found in Hensen's node. Since these studies have not directly identified the specific retinoids in the early embryo or measured their spatial distribution during early embryonic stages, we have addressed these questions using the amphibian embryo. The results from these studies indicate that a number of retinoids were present in the early amphibian embryo and that the levels of three of these retinoids vary both spatially and temporally in early development. Notably, 9-cis-RA was first detected after the midblastula transition and at the end of gastrulation ...

Endogenous distribution of retinoids during normal development and teratogenesis in the mouse embryo

“…In addition, there are other differences between lower vertebrates and higher vertebrates. In Xenopus (Durston et al, 1989;Kraft et al, 1994) and zebrafish (Costarides et al, unpublished results) retinal is the major retinoid present in embryos, with t-retinol playing a minor role, whereas in chicks and mouse, retinal is undetectable (Thaller and Eichele, 1987; and these results) and retinol is the major retinoid. Thus in several respects retinoid metabolism seems to be quite distinct in different classes of vertebrate.…”

“…combining the methods of Thaller and Eichele (1987) and Kraft et al (1994), separation was achieved. Fractionation of day 12 embryo extracts showed that 9-cis-RA was undetectable a t this stage of development (data not shown).…”

“…In addition, application of tRA to the anterior margin of the wing bud resulted in mirror-image duplication of the digits (Tickle et al, 1982), leading to the suggestion that tRA may be a signalling molecule involved in pattern formation. tRA is now known to be an endogenous component of mouse embryos during organogenesis (Satre and Kochhar, 19891, the amphibian spinal cord (Hunter et al, 1991), Xenopus neurulae (Durston et al, 1989;Kraft et al, 1994), the regenerating amphibian limb (Scadding and Maden, 1994), and zebrafish embryos (Costarides et al, unpublished data).…”

“…ddRA and 9-cis-RA cause digit duplication in the chick embryo (Thaller and Eichele, 1990;Thaller et al, 1993) and both 9-cis-RA and 4-0x0-RA are present in Xenopus embryos (Pijnappel et al, 1993;Kraft et al, 1994). It is not known whether any of these retinoids are present in mammalian embryos.…”

Specific teratogenic effects of different retinoic acid isomers and analogs in the developing anterior central nervous system of zebrafish

“…Nevertheless, it is known that exogenous 9 4 s RA can cause duplications in the chick wing (Thaller et al, 1993). It seems also to be involved in the dysmorphogenesis of embryonic structures in the rat (Creech Kraft and Juchau, 1993) and in Xenopus (Dawid et al, 1993;Creech Kraft et al, 1994). Most recently, Creech Kraft et al, (1994) examined the spatial and temporal distributions of endogenous 9 4 s RA in Xenopus embryos.…”