An important model system for understanding nuclear receptor signaling during development is metamorphosis of the frog Xenopus laevis. Thyroid hormone induces remarkable functional and morphogenetic changes as the larval tadpole transitions into the adult frog. A great deal of progress has been made on the molecular mechanisms of thyroid hormone receptor function in all vertebrates. Besides characterization of the highly conserved thyroid hormone receptor genes themselves, several receptor interacting proteins have been identified that mediate their ability to repress or activate transcription in the absence or presence of ligand, respectively. These coactivators and corepressors, collectively termed coregulators, recruit complexes that modify target gene chromatin or modulate RNA polymerase II access to hormone responsive promoters. Differential expression and/or activity of coregulator complexes have been suggested to influence cell and promoter specific responses to a given hormone, but these experiments have mostly relied on transient transfection and in vitro transcription systems or gene knockout technologies that deprive the animal of these genes from fertilization onward. Our central hypothesis is that the balance of ligand-dependent coactivation and ligand-independent corepression is critical for proper temporal and spatial responses to thyroid hormone during metamorphosis. To test this hypothesis, we will develop transgenic tadpoles that allow us to determine the precise pattern of thyroid hormone receptor mediated transcription in living tadpoles. We will then correlate coactivator and corepressor gene expression with the onset of developmental competence to respond to natural and synthetic Iigands and the tissue specificity of those responses. The relative importance of corepressor and coactivator interactions with the thyroid hormone receptor in vivo will then be tested by two strategies: first, by treatment with NH-3, a compound that disrupts both coactivator and corepressor interaction with the thyroid hormone receptor; and second, by tissue specific and inducible expression of full-length and dominant negative corepressor proteins. Finally, we will analyze gene expression networks regulated by TH in proliferating regions of the brain and jaw cartilage. We are particularly interested in determining the molecular basis for the selective agonism of NH-3 on neuronal versus cartilage proliferation. This selective agonism/antagonism is a common feature of estrogen receptor modulators such as tamoxifen but has not been previously described for thyroid hormone receptors. These last studies will provide new information on gene expression cascades leading to cellular proliferation in two tissues that are dependent on TH for proper development in both amphibians and mammals, including humans. In summary, Xenopus laevis metamorphosis is a powerful model for determining the role of thyroid hormone receptors and their coregulators in tissue specific control of gene expression networks during vertebrate development.
