A Dominant-Negative Thyroid Hormone Receptor Blocks Amphibian Metamorphosis by Retaining Corepressors at Target Genes

Abstract

The total dependence of amphibian metamorphosis on thyroid hormone (T₃) provides a unique vertebrate model for studying the molecular mechanism of T₃ receptor (TR) function in vivo. In vitro transcription and developmental expression studies have led to a dual function model for TR in amphibian development, i.e., TRs act as transcriptional repressors in premetamorphic tadpoles and as activators during metamorphosis. We examined molecular mechanisms of TR action in T3-induced metamorphosis by using dominant-negative receptors (dnTR) ubiquitously expressed in transgenic Xenopus laevis. We showed that T₃-induced activation of T₃ target genes and morphological changes are blocked in dnTR transgenic animals. By using chromatin immunoprecipitation, we show that dnTR bound to target promoters, which led to retention of corepressors and continued histone deacetylation in the presence of T₃. These results thus provide direct in vivo evidence for the first time for a molecular mechanism of altering gene expression by a dnTR. The correlation between dnTR-mediated gene repression and inhibition of metamorphosis also supports a key aspect of the dual function model for TR in development: during T₃-induced metamorphosis, TR functions as an activator via release of corepressors and promotion of histone acetylation and gene activation.

ACKNOWLEDGMENTS

We thank A. Shreiber and D. D. Brown for the gift of the dnTR plasmid.

D.B. and S.-C.V.H. contributed equally to this study.