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 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 receptor isoforms and to investigate other factors that modify the activity of the receptors in different tissues. Progress: 1. Functions of TRb isoforms in differentiation and homeostasis. To determine the biological functions for TRb1, we have studied a mouse model in which TRb1 has been specifically deleted while leaving the TRb2 isoform intact. This model incorporates a marker gene cassette in the TRb1-specific exon that allows detailed analysis of the cell-specific expression of TRb1. This study has revealed novel functions for TRb1 in the cochlea and in the maintenance of hearing during aging. Age-related hearing loss is widespread in human populations but the causes are incompletely understood. These findings indicate a previously unknown function for a thyroid hormone receptor in the maintenace of cochlear function and the preservation of hearing. 2. These studies also revealed that specific expression of TRb1 in a previously unrecognized cortical cell population in the adrenal gland. TRb1 mediates hypertrophic responses to thyroid hormone, suggesting direct functions for thyroid hormone in control of the adrenal gland. Moreover, this 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. 3. We continue to investigate transcriptional mechanisms that direct the 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 different tissues to thyroid hormone. 4. Target genes for TRb isoforms in development and homeostasis. To study this critical question regarding the mechanisms by which thyroid hormone determines cellular differentiation or function, we have continued our investigation of candidate target genes for TRb isoforms. These studies involve the use of a variety of molecular and genomic approaches to identify changes in gene expression patterns and candidate binding sites for these receptor isoforms in different tissues.
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