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 different tissue responses raises a key question concerning the mechanisms that determine the nature, time and place of a given response to thyroid hormone. How does this 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 elicit 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 and tissue-specific functions of different TRb receptor isoforms encoded by the Thrb gene.
The aim i s to reveal the developmental and homeostatic functions for these TR isoforms and to investigate other factors that modify the activity of the receptors in different tissues. The project addresses: 1. Functions of TRb isoforms in differentiation and homeostasis. To determine the biological functions for TRb isoforms, we have studied mouse models in which TRb1 or TRb2 have been specifically deleted. Our studies demonstrated novel, cell-specific expression patterns of TRb1 in the cochlea and in the adrenal cortex. In the auditory system, we determined that TRb1 promotes the maintenance of hearing during aging. The findings suggest that thyroid hormone may be a factor that contributes to age-related hearing loss, which is widespread in human populations. In the adrenal gland, TRb1 is expressed in a previously unrecognized cortical cell population and mediates hypertrophic responses to thyroid hormone, suggesting direct functions for this hormone over adrenal gland functions. This cortical cell population is sexually-dimorphic, being transient in males but persistent in females, indicating that thyroid hormone may influence gender-specific differences in adrenal gland function. 2. We continue to investigate the mechanisms that direct differential expression of the TRb1 and TRb2 isoforms of the Thrb gene. The gene has a complex structure, spanning about 400 kb on human chromosome 3 or mouse chromosome 14. The unusual multi-functional nature of the enhancer regions that control expression of the TRb1 and TRb2-specific promoters of the gene in different tissues provides a model system in which to investigate how the regulation of chromatin structure and gene activity determine the sensitivity of tissues to thyroid hormone. 3. We have determined that deiodinase enzymes in several different target tissues (cochlea, retina) provide a critical level of control over the activity of TR isoforms. Recent evidence indicates that type 3 deiodinase, an inactivating enzyme, is critical in testis development. The findings also indicate that the testis are subject to control by TRa1 and that type 3 deiodinase coordinates the level of active ligand available for this receptor isoform. The findings support the proposal that specific receptor isoforms and deiodinases function closely together in a given tissue to determine the nature and time of the response. 4. Target genes for TRb isoforms in natural tissues. To study this critical question regarding the mechanisms by which thyroid hormone determines cellular differentiation and function, we have continued our investigation of candidate target genes for TRb isoforms. These studies involve a variety of molecular and genomic approaches to identify changes in gene expression patterns and genomic DNA binding sites for these receptor isoforms in different tissues.
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