Thyroid hormone mediates a remarkable range of functions in many tissues and organ systems. These functions are important both in development and adult homeostasis. The breadth of responses raises a key question concerning the mechanisms that determine the nature, time and place of a given response to thyroid hormone. How does one hormone elicit so many different responses? Thyroid hormone receptors (TR) act as ligand-regulated transcription factors and occupy a key position in the chain of events that produce the cellular response. Two receptor genes, Thrb and Thra, encode several TR isoforms that are expressed in different developmental and tissue-specific patterns. Thus, the ability to express a given receptor isoform in a particular tissue provides a means of conferring a specific biological response. This project investigates the mechanisms that direct the unique expression patterns of different TR isoforms as a mechanism that determines specific functions of thyroid hormone. This project also investigates specific developmental and homeostatic functions for the different receptor isoforms encoded by the Thra and Thrb genes. Progress: 1. The Thrb gene has a large and complex structure, spanning about 400 kb on human chromosome 3 or mouse chromosome 14. Our studies have mapped functional control regions of the Thrb gene in mouse models. We are pursuing studies to investigate the mechanisms mediated by these regions that direct tissue-specific expression of the TRb receptor isoforms including in the pituitary, cochlea and retina. The unusual multi-functional nature of these control regions provides a model system of biological importance in which to investigate how transcriptional mechanisms regulate chromatin structure and gene activity in different tissues. To investigate the potential significance of genomic control regions in determining the thyroid hormone response in human tissues, the control regions of the mouse Thrb gene have been compared with the corresponding regions of the human THRB gene (collaboration with Dr F. Celi, NIDDK). 2. Functions of TRa1 and TRb isoforms in tissue differentiation and homeostasis. We continue to investigate tissue- and cell-specific functions or the different TR isoforms expressed by the Thra and Thrb genes in mamalian model systems. To determine the specific biological functions for the TRb1 isoform, we are studying a mouse model in which TRb1 has been specifically deleted while leaving the TRb2 isoform intact. This model also incoporates a marker gene cassette in the TRb1-specific exon that allows detailed analysis of the cell-specific expression of this isoform in different tissues. 3. Target genes for TRb isoforms in tissue development and homeostasis. To study this critical question regarding the mechanisms by which thyroid hormone produces changes in cellular differentiation or function, we are continuing in our investigation of candidate target genes for TRb isoforms. These studies involve the use of a variety of molecular approaches to identify changes in gene expression patterns and candidate binding sites for these receptor isoforms in the genome in different tissues.

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