Hypothalamic neurosecretion and metamorphosis in Xenopus laevis. I. The effect of propylthiouracil

Control of pituitary thyroid-stimulating hormone synthesis and secretion by thyroid hormones during Xenopus metamorphosis

We used ex vivo and in vivo experiments with Xenopus laevis tadpoles to examine the hypothesis that the set-point for negative feedback on pituitary thyroid-stimulating hormone (TSH) synthesis and secretion by thyroid hormones (THs) increases as metamorphosis progresses to allow for the previously documented concomitant increase in serum TH concentrations and pituitary TSH mRNA expression during this transformative process. First, pituitaries from climactic tadpoles were cultured for up to 96 h to characterize the ability of pituitary explants to synthesize and secrete TSHβ in the absence of hypothalamic and circulating hormones. Next, pituitary explants from tadpoles NF stages 54-66 were exposed to physiologically-relevant concentrations of THs to determine whether stage-specific differences exist in pituitary sensitivity to negative feedback by THs. Finally, in vivo exposures of tadpoles to THs were conducted to confirm the results of the ex vivo experiments. When pituitaries from climactic tadpoles were removed from the influence of endogenous hormones, TSHβ mRNA expression increased late or not at all whereas the rate of TSHβ secreted into media increased dramatically, suggesting that TSH secretion, but not TSH mRNA expression, is under the negative regulation of an endogenous signal during the climactic stages of metamorphosis. Pituitaries from pre- and prometamorphic tadpoles were more sensitive to TH-induced inhibition of TSHβ mRNA expression and secretion than pituitaries from climactic tadpoles. The observed decrease in sensitivity of pituitary TSHβ mRNA expression to negative feedback by THs from premetamorphosis to metamorphic climax was confirmed by in vivo experiments in which tadpoles were reared in water containing THs. Based on the results of this study, a model is proposed to explain the seemingly paradoxical, concurrent rise in serum TH concentrations and pituitary TSH mRNA expression during metamorphosis in larval anurans.

Developmental regulation of gene expression in the thyroid gland of Xenopus laevis tadpoles

Thyroid hormones (TH) are the primary morphogen regulating amphibian metamorphosis. However, knowledge about molecular mechanisms regulating thyroid gland activity in anuran tadpoles is very scarce. In this study, we characterized gene expression profiles in thyroids of Xenopus laevis tadpoles during spontaneous metamorphosis. Using real-time PCR, elevated expression of slc5a5, tpo, tshr, and sar1a mRNAs was detected at late prometamorphic and climax stages. For dio2 and dio3 but not dio1, developmental regulation of thyroidal expression was evident from a strong up-regulation at late stages. Conversely, expression of the DNA replication markers mcm2 and pcna declined at climax stages. The presence of functional feedback mechanisms at premetamorphic stages was examined in two experiments. Stage 52 tadpoles were exposed for 72 h to 1.0 microg/l thyroxine (T4). This treatment caused reduced mRNA expression of slc5a5, tpo, and dio2, whereas no significant changes were detectable for tshr expression in thyroids and tshb expression in the pituitary. In another experiment, stage 46 tadpoles were treated with 20 mg/l sodium perchlorate (PER) for 5 and 10 days. Within this period of time, control tadpoles developed to stages 50 and 52, respectively. PER treatment resulted in up-regulation of slc5a5, tpo, and tshr mRNAs at both time points and increased dio2 mRNA expression at day 10. Effects of PER on thyroid histology were only apparent on day 10. Together, our analyses of thyroidal gene expression demonstrate a marked developmental regulation for functional markers of thyroid activity, two deiodinases as well as for DNA replication markers. Expression patterns detected in PER- and T4-treated tadpoles indicate that functional feedback signaling controlling thyroid activity is already active during premetamorphosis.

Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis

Thyroid axis disruption is an important consideration when evaluating risks associated with chemicals. Bioassay methods that include thyroid-related endpoints have been developed in a variety of species, including amphibians, whose metamorphic development is thyroid hormone (TH)-dependent. Inhibition of TH synthesis in these species leads to developmental delay, and assays designed to capture these effects take several weeks to complete. In an effort to develop a shorter term approach, the early responses of various endpoints were evaluated in Xenopus laevis throughout 8d of exposure to three TH synthesis inhibitors: methimazole (100mg/L), 6-propylthiouracil (6-PTU) (20mg/L), and perchlorate (4 mg/L). Endpoints included thyroid gland histology and cell numbers, circulating TH concentrations, and thyroidal TH and associated iodo-compounds. Thyroidal 3,5-diodo-L-tyrosine (DIT) and thyroxine (T4) were significantly reduced from day 2 onward by all three chemicals, while 3-monoiodo-L-tyrosine (MIT) was significantly reduced by methimazole and perchlorate, but not by 6-PTU. These reductions were the earliest indicators of TH synthesis inhibition. Histological effects were apparent on day 4 and became more exaggerated through day 8. However, reductions in circulating T4 and increases in thyroid gland cell numbers were not apparent until day 6. Reductions of thyroidal MIT, DIT, and T4 and circulating T4 are indicative of inhibitory effects of the chemicals on TH synthesis. Changes in thyroid histology and cell number represent compensatory effects modulated by circulating TSH. These observations establish a basis for the development of short term amphibian-based methods to evaluate thyroid axis effects using a suite of diagnostic endpoints.

Effects of tetrabromobisphenol A on larval development and thyroid hormone-regulated biomarkers of the amphibian Xenopus laevis

Effects of tetrabromobisphenol A (TBBA) on thyroid hormone (TH)-dependent morphological development and on TH-regulated biomarkers were investigated using the amphibian Xenopus laevis. Tadpoles were exposed in the Xenopus metamorphosis assay to different TBBA concentrations ranging from 2.5 to 500 microg/L for 21 days. Only the highest TBBA concentration inhibited larval development while the biomarker for negative feedback mechanism, TH receptor beta-mRNA, was not affected. Short-term exposures to TBBA, triiodothyronine (T3), and a combination of TBBA and T3 were performed to examine expression of TH-upregulated biomarkers. TBBA alone increased slightly the expression of TH receptor beta- and basic region leucin zipper transcription factor b/Zip-mRNA but inhibited their T3-induced elevation in a dose-dependent manner. Despite the fact that TBBA seems to cause only mild effects during long-term treatment on larval development, short-term exposure revealed indirect evidence that TBBA can function as a TH antagonist.

The molecular basis of thyroid hormone-dependent central nervous system remodeling during amphibian metamorphosis

Tadpole metamorphosis involves a coordinated series of changes in virtually every tissue of the body. This developmental process is induced by the single morphogen, thyroid hormone (TH). The amphibian central nervous system (CNS) is a primary target for TH, and it undergoes dramatic morphological and cytoarchitectural changes in response to the hormone. TH acts by regulating gene expression and its actions in metamorphosis are thought to result from its ability to induce tissue-specific genetic programs. Receptors for TH are ligand-dependent transcription factors whose mRNA expression is upregulated by TH during metamorphosis (receptor autoinduction). Studies on the tadpole CNS have identified four general classes of early TH response genes. These genes code for: (1) transcription factors, that are likely to be required for the expression of downstream genes (i.e. secondary response genes), (2) cellular enzymes, which carry out hormone conversions, energy transformations and may possibly mediate extranuclear effects of TH on neural cells, (3) cytoskeletal elements required for axonal development, and (4) secreted signaling molecules that control the production of TH. Recent studies suggest a critical, evolutionarily conserved role for the TH-induced transcription factor genes in controling neural cell proliferation and differentiation.

Forebrain development in control and hypothyroid larvae of Triturus cristatus carnifex

Some cytological and developmental aspects of the forebrain gray matter were studied in the control and the hypothyroid larvae of Triturus cristatus carnifex. The mitotic cell percentages in the periventricular germinal layer, the fusiform shape of the recently divided neuronal cells and the transitory gray matter cleavage, are characteristic of the immature status of the central nervous system (CNS) in this species. The hypothyroidism obtained during two months of thiourea treatment slows down the development in all the forebrain areas of the experimental larvae. Nevertheless, the nucleus olfactorius anterior and the corpus striatum, which have a longer mitotic activity with respect to the other neuronal structures, are more deeply affected by thyroid atrophy. Thyroid activity and forebrain maturation are discussed on the basis of the literature and our data with regard to their influence on behavior and evolution.