Thyrotropin (TSH) is a pituitary-secreted glycoprotein hormone that is related to the gonadotropins (follicle-stimulating hormone, FSH and luteinizing hormone, LH). TSH interacts with its thyroidal receptors to induce the synthesis and secretion of thyroid hormones (T3 and T4). TSH receptors were originally thought to be solely expressed in the thyroid, however recently, TSH receptors have been found in extra-thyroidal tissues (e.g., adipose tissue (lipolysis), lymphocytes (immunoregulation), brain and pituitary) and convincing functional significance has been ascribed to their activation. The principal investigators recently cloned the first non-mammalian TSH receptor from the striped bass, a marine bony fish. This cDNA was isolated from the gonads of the fish (as opposed to the thyroid where they are expected to be expressed). Although this was the first report of the expression of TSH receptors in the gonads of any vertebrate, these findings have since been confirmed in other species of fish. Visualization of the striped bass TSH receptor mRNA in the cytoplasm of both developing oocytes and spermatocytes suggest that TSH may play a direct role in the development of sperm and egg rather than merely being "maternal" mRNA stored for use during embryo development. This results of this research will determine if TSH plays a direct role in gametogenesis or embryogenesis, or both. Although it has been established that thyroid hormones (as regulated by TSH) regulate gonadal physiology both in mammals and fish, this study could provide the first evidence of a direct role of TSH at the level of the gonad. An essential requirement for the action of a hormone is the timely expression of its receptor in the target tissue. Even though the expression of the TSH receptor gene in fish gametes has been confirmed, a test of the hypothesis that TSH plays a direct role in gametogenesis will be conducted by correlating gamete development with seasonal changes in gene expression, production of receptor protein, and localization of the protein within the gametes. In addition, TSH receptor transcripts will be detected and quantified, and receptor protein will be localized in embryos and larvae in order to test the alternate hypothesis that the TSH-receptor transcripts are stored maternal mRNA required for embryogenesis. Proof of active TSH receptor in gametes will strongly suggest i) a direct role of TSH in gamete development, ii) a putative role of TSH receptor in the disruption of gametogenesis as a causative factor of failure of fish spawning, and iii) a basis for the re-evaluation of gonadal expression of TSH receptor in higher vertebrates. Although studies of ligand-induced activation of the TSH receptor on the processes of gamete development and maturation would be invaluable to conclusively demonstrate the direct role of TSH in gametogenesis, sufficient quantities of homologous striped bass TSH are not available at this time.
