Thyroid hormone influences a variety of processes including development, metabolism and growth. Thyroid hormone action is mediated by nuclear receptors which interact with regulatory regions of genes stimulated or inhibited by thyroid hormone. There are 2 thyroid hormone receptor (TR) genes, alpha and beta, with a number of isoforms products from each gene. The relative abundance of these receptors varies in development and in different tissues in the body. A disorder in humans, Resistance to Thyroid Hormone (RTH), is associated with defects in these nuclear receptors. A variety of phenotypes are seen in RTH-affected individuals, ranging from growth delay and mental retardation to attention deficit disorder. The proposal studies will define how these processes are mediated by thyroid hormone and identify those actions specific for TR isoforms. These studies should also indicate how TR defects produce the abnormalities seen in RTH and provide a model to assess the response to treatment with thyroid hormone and its analogs. The phenotype of RTH can result from complete absence of the TR beta gene or by point mutations in the TR beta carboxy terminus, which act as dominant negative inhibitors of thyroid hormone action. Homologous recombination techniques have been used to target and inactivate the TR beta gene in mice, creating a TR beta deletion model of RTH. In addition to studying these mice, Dr. Brent will create mice with a point mutation in the TR beta gene that will produce a dominant negative receptor typical of the other genotype producing RTH. He will analyze both lines of mice, focusing on the role of TR in the regulation of nervous system development and function, cardiac function, metabolism and the dynamics of fat storage. Additionally, a TR beta isoform exclusively expressed in the brain and retina, TR beta 2 will be selectively inactivated and the resultant mice analyzed. Abnormalities in brain development and thyroid hormone regulation of pituitary hormones will be studied. Each targeted embryonic stem cell line will be used to create whole animals and will also be studied in an in vitro model in which both alleles are inactivated. Embryonic stem cells, under defined conditions, can be differentiated into a variety of cell types including erythrocytes, cardiac myocytes and neurons. This in vitro system will complement the in vivo studies and provide a model to study tissue specific gene regulation under more defined conditions.