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METHODS IN CELL PHYSIOLOGY
1Laboratory of Medical Genetics, Department of Internal Medicine, Cardiology and Hepatology, University of Bologna, Bologna; and 2Laboratory of Molecular Genetics, Istituto Giannina Gaslini, Genova, Italy
Submitted 26 May 2006 ; accepted in final form 15 September 2006
The sodium iodide symporter (NIS) mediates iodide (I) transport in the thyroid gland and other tissues and is of increasing importance as a therapeutic target and nuclear imaging reporter. NIS activity in vitro is currently measured with radiotracers and electrophysiological techniques. We report on the development of a novel live cell imaging assay of NIS activity using the I-sensitive and genetically encodable yellow fluorescent protein (YFP) variant YFP-H148Q/I152L. In FRTL-5 thyrocytes stably expressing YFP-H148Q/I152L, I induced a rapid and reversible decrease in cellular fluorescence characterized by 1) high affinity for extracellular I (35 µM), 2) inhibition by the NIS inhibitor perchlorate, 3) extracellular Na+ dependence, and 4) TSH dependence, suggesting that fluorescence changes are due to I influx via NIS. Individual cells within a population of FRTL-5 cells exhibited a 3.5-fold variation in the rate of NIS-mediated I influx, illustrating the utility of YFP-H148Q/I152L to detect cell-to-cell difference in NIS activity. I also caused a perchlorate-sensitive decrease in YFP-H148Q/I152L fluorescence in COS-7 cells expressing NIS but not in cells lacking NIS. These results demonstrate that YFP-H148Q/I152L is a sensitive biosensor of NIS-mediated I uptake in thyroid cells and in nonthyroidal cells following gene transfer and suggest that fluorescence detection of cellular I may be a useful tool by which to study the pathophysiology and pharmacology of NIS.
thyroid; fluorescence microscopy; FRTL-5 cells
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