Expression of sodium-iodide symporter mRNA in the thyroid gland of Xenopus laevis tadpoles: developmental expression, effects of antithyroidal compounds, and regulation by TSH

Abstract

The uptake of iodide represents the first step in thyroid hormone synthesis by thyroid follicular cells and is mediated by the sodium-iodide symporter (NIS). In mammals, expression of NIS is stimulated by TSH and transcription of the NIS gene involves regulation by the thyroid-specific transcription factors Pax8 and Nkx2$1. In this study, we examined the mRNA expression of NIS, Pax8 and Nkx2$1 in the thyroid gland of Xenopus laevis tadpoles by semiquantitative reverse transcriptase (RT)-PCR. During spontaneous metamorphosis, NIS mRNA expression was low in premetamorphic tadpoles, increased throughout prometamorphosis, and peaked at climax stage 60. Analysis of TSH bsubunit (TSHb) mRNA in the pituitary of the same tadpoles revealed a close temporal relationship in the expression of the two genes during metamorphosis, suggesting a regulatory role of TSH in the developmental expression of NIS. Treatment of tadpoles with goitrogenic compounds (sodium perchlorate and ethylenethiourea) increased TSHb mRNA expression (approximately twofold) and caused thyroid gland hyperplasia, confirming that feedback along the pituitary-thyroid axis was operative. Analysis of gene expression in the thyroid gland revealed that goitrogen treatment was correlated with increased expression of NIS mRNA (w20-fold). In the thyroid gland organ culture experiments, bovine TSH (bTSH; 1 mU/ml) strongly induced NIS mRNA expression. This effect was mimicked by co-culture of thyroid glands with pituitaries from stage 58 tadpoles and by agents that increase intracellular cAMP (forskolin, dibutyryl-cAMP). In addition, it could be shown that thyroid glands of X. laevis tadpoles express Pax8 and Nkx2$1 mRNA in a developmentally regulated manner and that ex vivo treatment of thyroid glands with bTSH, forskolin, and cAMP analogs increased the expression of Pax8 and Nkx2$1 mRNA. This is the first report on developmental profiles and hormonal regulation of thyroid gland gene expression in amphibian tadpoles and, together, results reveal a critical role of TSH in the regulation of NIS mRNA expression in the thyroid gland of X. laevis tadpoles.

Perchlorate and ethylenethiourea induce different histological and molecular alterations in a non-mammalian vertebrate model of thyroid goitrogenesis

“…Results from these analyses confirmed previous observations of different histological phenotypes associated with each of the two treatment regimes (Opitz et al, 2006b). Moreover, for the first time, real-time PCR analyses provide information about expression profiles for a larger set of candidate genes in thyroid tissue of a non-mammalian vertebrate.…”

“…Differences in the histological appearance of thyroid glands after longer-term treatment with PER and ETU have previously been described for X. laevis tadpoles (Opitz et al, 2006b;Tietge et al, 2005). The present study therefore aimed at comparing the initial histological changes during thyroid goitrogenesis induced by PER and ETU (Fig.…”

“…As a consequence, the thyroid phenotype obtained in experimental goiter studies reflects not only the over-stimulation by TSH but also thyroidal responses to altered states of iodide turnover. This hypothesis might help to explain the variable histological phenotypes of thyroid glands that were previously observed in X. laevis tadpoles treated with different anti-thyroidal substances (Opitz et al, 2006a;Opitz et al, 2006b;Tietge et al, 2005).…”

“…Further, a conserved role of TSH for thyroid growth stimulation is suggested by numerous reports of thyroid hyperplasia following treatment of non-mammalian species with anti-thyroidal compounds (Bradford et al, 2005;Handa and Chiasson, 1980;Opitz et al, 2006a). However, although thyroidal responses to TSH have been well documented by biochemical and histological techniques, molecular analyses of thyroid tissue in non-mammalian species are still sparse (Elsalini et al, 2003;Grommen et al, 2006;Opitz et al, 2006b).…”

“…We recently demonstrated the utility of metamorphic development in X. laevis tadpoles to serve as a novel non-mammalian model for investigations on TSH-induced alterations in thyroid gene expression (Opitz et al, 2006b). However, very few genes have so far been analysed for their expression profiles in the amphibian thyroid.…”