| dc.description.abstract | Iodine, a non-metallic trace element, is one of the heaviest elements essential for normal biological function. Vertebrate animals must obtain iodine from the environment to manufacture thyroid hormones and sufficient iodine supply is thus critical for normal endocrine function. In mammals, iodine uptake from the environment is achieved by the sodium iodide symporter (NIS), but little is known outside a few mammalian orders about how animals obtain iodine. Because the basic biochemical pathways for thyroid hormone synthesis and receptor activation appear conserved in vertebrates, the mechanisms through which non-mammalian species accumulate iodine for thyroid hormone manufacture have also been assumed to be similar. However, few studies have identified the NIS gene or characterized the regulation of its activity in fish. The perciform teleost fish, red drum (Sciaenops ocellatus) is easily obtained from hatcheries and has been used extensively for nutritional and thyroid hormone research, therefore providing an informative species in which to study the mechanisms of iodide uptake and utilization in a nonmammalian species.
I developed a fast, non-lethal methodology using Positron Emission Tomography and Computed Tomography PET/CT imaging of 124I injected fish to produce three dimensional images identifying areas of active radioiodide uptake in the subpharyngeal region of red drum, thus allowing me to establish a methodology for consistently obtaining thyroid tissue. I then confirmed that an NIS homolog is expressed in red drum thyroid, stomach and intestine, but found little evidence for its expression in gills.
Utilizing quantitative RT-PCR, I demonstrated that thyroid stimulating hormone stimulated nis expression in red drum subpharyngeal thyroid tissue but not any other nis expressing tissue, similar to mammalian thyroid NIS. nis expression in red drum subpharyngeal, stomach, intestine, and gill tissues was relatively unchanged during feeding, salinity transfer, and environmental iodide exposure, suggesting that these fish have physiological mechanisms of dietary iodine uptake, thyroid hormone synthesis, and iodide recycling similar to those described in mammals. Minimal nis expression in gills across a variety of osmotic environments and nutritional conditions suggests that this transporter does not serve as a mechanism for direct environmental iodide uptake via gills. | en |
