REGULATION OF NA TRANSPORT FUNCTION BY CFTR IN THYROID EPITHELIAL CELLS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Thyroid hormonogenesis requires iodide uptake and release into the thyroid follicular lumen. The apical exit of iodide is thought to be mediated by Pendrin, an anion exchanger encoded by the SLC26A4 gene. However, several lines of evidence suggest that alternative pathways for apical iodide efflux exist in the thyroid gland. By researching the existing literature, as well as by mining microarray databases, we identified transporters that potentially mediate iodide efflux. These include the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and ClC-5, a Cl-/H+ exchanger, both of which previously have been identified in thyroid tissue. Polarized primary cultures were grown from wild type and CFTR -/- pig thyroids to facilitate evaluations of anion transport using short-circuit current (Isc) measurements as well as 125I fluxes. Isc studies indicate the presence of cAMP-activated anion (chloride and iodide) secretion that is abolished in CFTR-/- pig thyroid cultures. Wild type thyroid expresses SLC26A7, another candidate anion transport protein, based on measurements of both the mRNA and protein levels, using qRT-PCR and immunoblotting, respectively. Interestingly, SLC26A7 protein expression is abolished in CFTR -/- thyroid cultures, although mRNA expression did not differ significantly. The biophysical and pharmacological profile of SLC26A7 currents will be further examined in expression systems and resultant information exploited to probe not only its polarized expression in thyroid cultures, but also its functional impact on iodide secretion. In addition, future studies using polarized primary cultures will test the effects of ClC-5 and/or SLC26A7 shRNA knockdown, to evaluate their relative contributions to vectorial iodide transport.