Early expression of thyroid hormone receptor beta and retinoid X receptor gamma in the Xenopus embryo

The role of thyroid hormone in Xenopus metamorphosis is particularly well understood as it plays an essential role in that process. However, recent evidence suggests that thyroid hormone may play an earlier role in amphibian embryogenesis. We demonstrate that Xenopus thyroid hormone receptor beta (XTR beta) is expressed shortly after neural fold closure, and that its expression is localized to the developing retina. Retinoid X receptor gamma (RXR gamma), a potential dimerization partner for XTR beta, was also found to be expressed in the retina at early stages, and at later stages RXR gamma was also expressed in the liver diverticulum. Addition of either thyroid hormone or 9-cis retinoic acid, the ligands for XTR beta and RXR gamma, respectively, did not alter the expression of their receptors. However, the addition of thyroid hormone and 9-cis retinoic acid did alter rhodopsin mRNA expression. Addition of thyroid hormone generates a small expansion of the rhodopsin expression domain. When 9-cis retinoic acid or a combination of thyroid hormone and 9-cis retinoic acid was administered, there was a decrease in the expression domain of rhodopsin in the developing retina. These results provide evidence for an early role for XTR beta and RXR gamma in the developing Xenopus retina.     

One of the duplicated matrix metalloproteinase-9 genes is expressed in regressing tail during anuran metamorphosis

The drastic morphological changes of the tadpole are induced during the climax of anuran metamorphosis, when the concentration of endogenous thyroid hormone is maximal. The tadpole tail, which is twice as long as the body, shortens rapidly and disappears completely in several days. We isolated a cDNA clone, designated as Xl MMP-9TH, similar to the previously reported Xenopus laevis MMP-9 gene, and showed that their Xenopus tropicalis counterparts are located tandemly about 9 kb apart from each other in the genome. The Xenopus MMP-9TH gene was expressed in the regressing tail and gills and the remodeling intestine and central nervous system, and induced in thyroid hormone-treated tail-derived myoblastic cultured cells, while MMP-9 mRNA was detected in embryos. Three thyroid hormone response elements in the distal promoter and the first intron were involved in the upregulation of the Xl MMP-9TH gene by thyroid hormone in transient expression assays, and their relative positions are conserved between X. laevis and X. tropicalis promoters. These data strongly suggest that the MMP-9 gene was duplicated, and differentiated into two genes, one of which was specialized in a common ancestor of X. laevis and X. tropicalis to be expressed in degenerating and remodeling organs as a response to thyroid hormone during metamorphosis.     

Distinct expression profiles of transcriptional coactivators for thyroid hormone receptors during Xenopus laevis metamorphosis

The biological effects of thyroid hormone (T3) are mediated by the thyroid hormone receptor (TR). Amphibian metamorphosis is one of the most dramatic processes that are dependent on T3. T3 regulates a series of orchestrated developmental changes, which ultimately result in the conversion of an aquatic herbivorous tadpole to a terrestrial carnivorous frog. T3 is presumed to bind to TRs, which in turn recruit coactivators, leading to gene activation. The best-studied coactivators belong to the p160 or SRC family. Members of this family include SRC1/ NCoA-1, SRC2/TIF2/GRIP1, and SRC3/pCIP/ACTR/AIB-l/RAC-3/TRAM-1. These SRCs interact directly with liganded TR and function as adapter molecules to recruit other coactivators such as p300/CBP. Here, we studied the expression patterns of these coactivators during various stages of development. Amongst the coactivators cloned in Xenopus laevis, SRC3 was found to be dramatically upregulated during natural and T3-induced metamorphosis, and SRC2 and p300 are express     

Distinct expression profiles of transcriptional coactivators for thyroid hormone receptors during Xenopus laevis metamorphosis

The biological effects of thyroid hormone (T3) are mediated by the thyroid hormone receptor (TR). Amphibian metamorphosis is one of the most dramatic processes that are dependent on T3. T3 regulates a series of orchestrated developmental changes, which ultimately result in the conversion of an aquatic herbivorous tadpole to a terrestrial carnivorous frog. T3 is presumed to bind to TRs, which in turn recruit coactivators, leading to gene activation. The best-studied coactivators belong to the p160 or SRC family. Members of this family include SRC1/NCoA-1, SRC2/TIF2/GRIP1, and SRC3/pCIP/ACTR/AIB-1/RAC-3/TRAM-1. These SRCs interact directly with liganded TR and function as adapter molecules to recruit other coactivators such as p300/CBP. Here, we studied the expression patterns of these coactivators during various stages of development. Amongst the coactivators cloned in Xenopus laevis, SRC3 was found to be dramatically upregulated during natural and T3-induced metamorphosis, and SRC2 and p300 are expressed throughout postembryonic development with little change in their expression levels. These results support the view that these coactivators participate in gene regulation by TR during metamorphosis.     

Tissue sensitivity to thyroid hormone in athyroid Xenopus laevis larvae

Tadpoles that spontaneously arrest development and remain as larvae occur occasionally in Xenopus laevis populations. These non-metamorphosing tadpoles continue to grow, and they develop into grossly deformed giant individuals which come as close as any anurans to being truly neotenic. Giant X. laevis tadpoles that fail to metamorphose lack thyroid glands. In this study, the hypothesis that the tissues of these tadpoles nevertheless remain thyroid hormone sensitive was tested, by exposing isolated tadpole tail tips to exogenous thyroid hormone in tissue culture. The tail tips from giant tadpoles significantly shrank in response to the thyroid hormone treatment, showing that their tissue was still capable of metamorphosis. However, the amount of shrinkage was less than that observed in tail tissue from normal tadpoles. It was hypothesized that complete induction of metamorphosis may not be possible in the giant tadpoles due to a disproportionate growth and development of tissues and organs.     

Gene expression profiling to examine the thyroid hormone-disrupting activity of hydroxylated polychlorinated biphenyls in metamorphosing amphibian tadpole

Hydroxylated polychlorinated biphenyls are the metabolites produced from parent compounds by the drug-metabolizing enzyme cytochrome P450. These compounds are suspected to disrupt postembryonic neural development in the brains of mammals including humans. We studied the effects of these compounds on thyroid hormone function in the brain by using metamorphosing tadpoles of the African clawed toad (Xenopus laevis) as a model for mammalian postembryonic development. The metamorphosis assay revealed that these compounds inhibit thyroid hormone-induced metamorphosis. Genome-wide gene expression analysis in the brain following short-term exposure demonstrated that delayed metamorphosis could partially be caused by disruption of thyroid hormone-induced gene expression. Furthermore, we associated the terms of functional ontology with the genes, whose expression was disrupted by these compounds. We suggest that the use of a genome-wide analysis coupled with bioinformatics might provide an overview of the molecular mechanism underlying thyroid-disrupting activities in vivo.