The Na+/I- symporter (NIS) is the plasma membrane glycoprotein that mediates active I- uptake in the thyroid gland, lactating breast, and other tissues. NIS-mediated uptake is the basis for the use of radioiodide for the diagnosis and treatment of thyroid tumors. However, the majority of thyroid cancers exhibit lower I- transport because NIS is intracellularly retained. To exert its function in vectorial I- accumulation, NIS is selectively expressed at the basolateral membrane in all tissues where NIS function has been investigated. Interestingly, we show that the absorption of dietary I- occurs in the small intestine, where NIS is functionally expressed in the brush border, i.e., apically, in rodent and human villi. For these reasons, we sought to study NIS polarized plasma membrane targeting.
We found that progressive truncations of the intracellular carboxy-terminus (Ct) reduce NIS plasma membrane localization and consequently the Vmax for I- transport, whereas the Km is unaffected. Using an epithelial polarized cell model, we identified the segment between positions 609 and 611 as the molecular determinant of NIS basolateral targeting. Altogether, these findings pose new intriguing biological questions on how different epithelia selectively interpret the polarized sorting signals in the NIS Ct. In addition to providing a model for investigating the physiology of I- transport, a transepithelial transport system has the distinctive advantage of allowing substrate depletion in the compartment where the transporter is targeted, in contrast to transport experiments in nonpolarized cells, where the extracellular concentration of the substrate is not significantly altered by the activity of the transporter. Thus, transepithelial I- transport inhibition experiments enabled us to uncover that perchlorate (ClO4-), a competitive inhibitor of NIS and an environmental pollutant, is actively transported by NIS. Further characterization revealed the unprecedented feature that NIS transports different substrates with different stoichiometries: 2:1 electrogenic for Na+/I- transport and 1:1 electroneutral for ClO4-, suggesting that other transporters may exhibit the same property. In addition, we showed that substitutions at position 93 change NIS transport stoichiometry and substrate specificity. In in vivo experiments, we have shown that NIS also actively concentrates ClO4- in the lactating mammary gland, thus reducing the availability of I- in the milk, which is the only source of I- for the newborn, and directly inhibiting I- uptake in the newborn’s thyroid. Together with the finding that NIS is functionally expressed in the intestine, these results indicate that ClO4- exposure from water contamination may result in a greater environmental health risk than previously acknowledged.
In summary, the investigation of a cell biology question, i.e., NIS polarized plasma membrane localization, has provided us with the tools to discover that NIS actively transports ClO4- with a different stoichiometry than I-, thus paving the way to studying two very different areas, the mechanism of anion selectivity and coupling and the public health significance of ClO4-
