Identification of gene expression indicators for thyroid axis disruption in a Xenopus laevis metamorphosis screening assay: Part 1. Effects on the brain

Thyroid hormones (TH), thyroxine (T4) and 3,5,3′-triiodothyronine (T3), play crucial roles in regulation of growth, development and metabolism in vertebrates and their actions are targets for endocrine disruptive agents. Perturbations in TH action can contribute to the development of disease states and the US Environmental Protection Agency is developing a high throughput screen using TH-dependent amphibian metamorphosis as an assay platform. Currently this methodology relies on external morphological endpoints and changes in central thyroid axis parameters. However, exposure-related changes in gene expression in TH-sensitive tissue types that occur over shorter time frames have the potential to augment this screen. This study aims to characterize and identify molecular markers in the tadpole brain. Using a combination of cDNA array analysis and real time quantitative polymerase chain reaction (QPCR), we examine the brain of tadpoles following 96 h of continuous exposure to T3, T4, methimazole, propylthiouracil, or perchlorate. This tissue was more sensitive to T4 rather than T3, even when differences in biological activity were taken into account. This implies that a simple conversion of T4 to T3 cannot fully account for T4 effects on the brain and suggests distinctive mechanisms of action for the two THs. While the brain shows gene expression alterations for methimazole and propylthiouracil, the environmental contaminant, perchlorate, had the greatest effect on the levels of mRNAs encoding proteins important in neural development and function. Our data identify gene expression profiles that can serve as exposure indicators of these chemicals.