The genomic structure and the expression profile of the Xenopus laevis transthyretin gene.

Transthyretin (TTR) is a major thyroid hormone-binding protein in the amphibian tadpole whose plasma mRNA and protein levels are altered during metamorphosis. While the temporal and spatial expression patterns and genomic structure of the TTR gene are well studied in higher vertebrates, detailed expression pattern in the extrahepatic tissues, the transcriptional regulation, and the genomic structure have not yet been identified in amphibians. In this study, we attempted to elucidate these mechanisms. Here, we determined the genomic structure of the Xenopus laevis TTR gene including 5'-flanking regions, and examined TTR expression patterns in several tissues. The TTR gene of X. laevis is composed of 4 exons and 3 introns, and the nucleotide sequence of intron 1 is not similar to that previously reported. This suggests that the TTR gene of X. laevis was duplicated and the gene cloned in this study was the other copy of previously reported gene. We also found that TTR was primarily transcribed in the liver of both tadpoles and adults. In the adult liver, TTR transcripts were more abundant in males than females. In higher vertebrates, the expression of TTR is controlled by several transcription factors including forkhead box A2 (FoxA2). However, in the X. laevis liver, FoxA2 expression patterns were not similar to TTR. We also found that exogenous FoxA2 increased the X. laevis TTR promoter-driven luciferase activity. These results suggest that, in amphibian, the expression of TTR is regulated partially by FoxA2, and that another system may exist to control TTR expression.

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.