Development of a Xenopus laevis Metamorphosis Model for Detection of Thyroid System-Disrupting Chemicals
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Beschreibung
Thyroid hormones (TH) are critically important for the regulation of diverse biological processes associated with vertebrate development. Increasing evidence for disruption of normal thyroid system function by environmental chemicals have led to concerns that exposure to substances altering TH economy or peripheral TH signalling may result in adverse health effects in humans and wildlife, particularly if exposure occurs during early development. In this study, a conceptual and methodological framework of a novel amphibian testing strategy for detection of thyroid system disruption was developed, using tadpoles of the South African clawed frog Xenopus laevis as test organisms. Given the multitude of possible modes of action involved in thyroid system disruption, this study aimed at the development of a longer-term Xenopus Metamorphosis Assay (XEMA) focussing on the detection of chemical disruption of TH economy and a complementary short-term Gene Expression in Xenopus (GenEX) assay for sensitive identification of alterations in peripheral TH action. For the XEMA development, several test protocol variants were evaluated towards their utility to detect both acceleration of development by thyroxine (T4) as well as developmental retardation by the anti-thyroidal compounds propylthiouracil (PTU) and ethylenethiourea (ETU). Comparative analyses of agonist and antagonist effect patterns revealed that a 21-d XEMA protocol using initial stage 51 tadpoles provides for the highest sensitivity for detection of agonists (e.g., T4) and very robust responses to anti-thyroidal compounds (e.g., PTU). Using the sensitive 21-d assay protocol, further studies aimed at an evaluation of histological and molecular endpoints towards their utility to enhance the diagnostic identification of thyroid system disruption in X. laevis tadpoles. To this end, temporal and concentration-dependent response profiles could be characterized for a set of histological parameters (e.g., colloid depletion, follicle distension, follicular cell hypertrophy and hyperplasia). Concerning molecular markers of thyroid disruption, determination of mRNA expression of thyroid-stimulating hormone (TSH) in the pituitary and sodium-iodide symporter (NIS) in thyroid tissue were found to enhance the diagnostic value of the XEMA testing approach. Complementary to the XEMA, a protocol for a short-term GenEX assay was evaluated towards detection of agonists and antagonists of TH signalling in X. laevis tadpoles. The GenEX assay exploits analysis of TH-dependent gene expression as a means to detect altered TH action in tadpole tissue following short-term treatment of premetamorphic tadpoles with test compounds. A 48-h GenEX test protocol was established and successfully validated for detection of agonistic, partial agonistic and antagonistic activities using synthetic TH agonists (GC 1) and antagonists (NH 3). Together, the results of this study demonstrate that the complementary use of the two testing tools XEMA and GenEX provides a comprehensive means to identify thyroid system-disrupting substances acting via different modes of action. Moreover, given the conservation of thyroid function across vertebrate classes, the amphibian testing strategy is proposed as a viable surrogate model for the assessment of thyroid system disruption in vertebrates.
Autor*in
Robert Opitz
Themen in »Development of a Xenopus laevis Metamorphosis Model for Detection of Thyroid System-Disrupting Chemicals«
amphibian development endocrine disruption metamorphosis thyroid hormone