Meeting Abstract
Iodine, an essential component of thyroid hormone, can only be obtained through the diet. The sodium iodide symporter (NIS) transports iodide across mammalian intestinal and thyroid epithelia to deliver iodide for thyroid hormone production. To determine whether a homolog of NIS performs a similar function in teleost fish, we confirmed expression of a homolog of mammalian NIS in both sub-pharyngeal thyroid follicles and intestine in multiple teleost species, indicating a conserved mechanism for intestinal-thyroid iodine transport across vertebrates. We then examined expression in these locations during development using in situ hybridization (ISH) staining of zebrafish (Danio rerio) embryos. This revealed expression of nis as early as 2 days post fertilization (dpf) along the dorsal surface of the yolk sac, suggesting a function to import iodine from yolk, potentially placed there by maternal deposition. To evaluate this possibility, RT-PCR and further in situ staining of ovarian tissue in gravid female zebrafish confirmed nis mRNA presence in the ooplasm and granulosa layer of early stage follicles. This suggests NIS can function to transport iodine into the yolk, and that maternally-deposited NIS mRNA may be available for early embryogenesis. Additionally, ISH in embryos revealed nis expression in the central nervous system throughout days 2-5 dpf, with adult whole brain ISH localizing expression in the hypothalamus, cerebellum, and optic tectum. RT-PCR on whole brain tissue from 5 species of adult fish representing 3 taxonomic orders likewise revealed robust expression. These unexpected, non-canonical locations suggest novel, as yet undescribed reproductive and neural functions of NIS in teleost species.
