BUCHHOLZ, D.R.; TOMITA, A.; FU, L.; SHI, Y.-B.; LGRD/NICHD/NIH: A dominant positive thyroid hormone receptor induces gene regulation and metamorphic transformation in the absence of thyroid hormone in vivo in transgenic frogs

Frog metamorphosis is completely dependent on thyroid hormone. The effects of thyroid hormone are believed to be mediated by thyroid hormone receptors, although direct in vivo proof is still lacking. Here we developed a dominant positive mutant thyroid hormone receptor (dpTR) that activates genes in the absence of thyroid hormone. Induction of dpTR expression through a heat shock-inducible promoter in transgenic tadpoles led to metamorphic transformation in the gills and hind limbs. In the intestine, apoptosis and cell proliferation was induced by heat shock in transgenic animals. The morphological changes are mirrored by changes in gene expression in the tail and intestine. To determine the underlying molecular mechanism, we carried out immunoprecipitation (IP) and chromatin IP. We showed that the dpTR binds the target promoter, leading to an increase in histone acetylation, a marker of activated chromatin. Experiments performed in the presence of thiourea, which blocks endogenous thyroid hormone synthesis, show that the thyroid hormone receptor is sufficient for regulation of T3-response genes and metamorphosis. This mutant TR may have therapeutic value in being able to manipulate gene expression in vivo.