Na+ coordination at the Na2 site of the Na+/I- symporter (original) (raw)
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Journal of Biological Chemistry, 2007
The Na ؉ /I ؊ symporter (NIS) is a key plasma membrane glycoprotein that mediates Na ؉-dependent active I ؊ transport in the thyroid, lactating breast, and other tissues. The OH group of the side chain at position 354 in transmembrane segment (TMS) IX of NIS has been demonstrated to be essential for NIS function, as revealed by the study of the congenital I ؊ transport defect-causing T354P NIS mutation. TMS IX has the most -OH group-containing amino acids (Ser and Thr) of any TMS in NIS. We have thoroughly characterized the functional significance of all Ser and Thr in TMS IX in NIS, as well as of other residues in TMS IX that are highly conserved in other transporters of the SLC5A protein family. Here we show that five -OH group-containing residues (Thr-351, Ser-353, Thr-354, Ser-356, and Thr-357) and Asn-360, all of which putatively face the same side of the helix in TMS IX, plus Asp-369, located in the membrane/cytosol interface, play key roles in NIS function and seem to be involved in Na ؉ binding/translocation.
Advances in Na+/I− symporter (NIS) research in the thyroid and beyond
Molecular and Cellular Endocrinology, 2003
The Na + /I − symporter (NIS) is a plasma membrane glycoprotein that mediates active iodide uptake in the thyroid-the essential first step in thyroid hormone biosynthesis-and in other tissues, such as salivary and lactating mammary glands. Thyroidal radioiodide uptake has been used for over 60 years in the diagnosis and effective treatment of thyroid cancer and other diseases. However, the NIS cDNA was only isolated in 1996 by expression cloning in Xenopus laevis oocytes, marking the beginning of the molecular characterization of NIS and the study of its regulation, both in the thyroid and other tissues. One of the most exciting current areas of NIS research-radioiodide treatment of extrathyroidal cancers-was launched by the discovery of functional expression of endogenous NIS in breast cancer and by the ectopic transfer of the NIS gene into otherwise non NIS-expressing cancers. This review summarizes the main findings in NIS research, emphasizing the most recent developments.
Mechanism of anion selectivity and stoichiometry of the Na+/I- symporter (NIS)
Proceedings of the National Academy of Sciences, 2011
I − uptake in the thyroid, the first step in thyroid hormone biosynthesis, is mediated by the Na þ ∕I − symporter (NIS) with an electrogenic 2Na þ : 1I − stoichiometry. We have obtained mechanistic information on NIS by characterizing the congenital I − transport defect-causing NIS mutant G93R. This mutant is targeted to the plasma membrane but is inactive. Substitutions at position 93 show that the longer the side chain of the neutral residue at this position, the higher the K m for the anion substrates. Unlike WT NIS, which mediates symport of Na þ and the environmental pollutant perchlorate electroneutrally, G93T/N/Q/E/D NIS, strikingly, do it electrogenically with a 2∶1 stoichiometry. Furthermore, G93E/Q NIS discriminate between anion substrates, a discovery with potential clinical relevance. A 3D homology model of NIS based on the structure of the bacterial Na þ ∕galactose transporter identifies G93 as a critical player in the mechanism of the transporter: the changes from an outwardly to an inwardly open conformation during the transport cycle use G93 as a pivot.
Thyroid Na+/I− symporter Mechanism, stoichiometry, and specificity
Journal of Biological …, 1997
The rat thyroid Na+/I- symporter (NIS) was expressed in Xenopus laevis oocytes and characterized using electrophysiological, tracer uptake, and electron microscopic methods. NIS activity was found to be electrogenic and Na+-dependent (Na+ > Li+ > H+). The apparent affinity constants for Na+ and I- were 28 +/- 3 mM and 33 +/- 9 microM, respectively. Stoichiometry of Na+/anion cotransport was 2:1. NIS was capable of transporting a wide variety of anions (I-, ClO3-, SCN-, SeCN-, NO3-, Br-, BF4-, IO4-, BrO3-, but perchlorate (ClO4-) was not transported. In the absence of anion substrate, NIS exhibited a Na+-dependent leak current (approximately 35% of maximum substrate-induced current) with an apparent Na+ affinity of 74 +/- 14 mM and a Hill coefficient (n) of 1. In response to step voltage changes, NIS exhibited current transients that relaxed with a time constant of 8-14 ms. Presteady-state charge movements (integral of the current transients) versus voltage relations obey a Bol...
The Na + /I − Symporter (NIS): Mechanism and Medical Impact
Endocrine Reviews, 2014
The Na ϩ /I Ϫ symporter (NIS) is the plasma membrane glycoprotein that mediates active I Ϫ transport in the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine. In the thyroid, NISmediated I Ϫ uptake plays a key role as the first step in the biosynthesis of the thyroid hormones, of which iodine is an essential constituent. These hormones are crucial for the development of the central nervous system and the lungs in the fetus and the newborn and for intermediary metabolism at all ages. Since the cloning of NIS in 1996, NIS research has become a major field of inquiry, with considerable impact on many basic and translational areas. In this article, we review the most recent findings on NIS, I Ϫ homeostasis, and related topics and place them in historical context. Among many other issues, we discuss the current outlook on iodide deficiency disorders, the present stage of understanding of the structure/function properties of NIS, information gleaned from the characterization of I Ϫ transport deficiency-causing NIS mutations, insights derived from the newly reported crystal structures of prokaryotic transporters and 3-dimensional homology modeling, and the novel discovery that NIS transports different substrates with different stoichiometries. A review of NIS regulatory mechanisms is provided, including a newly discovered one involving a K ϩ channel that is required for NIS function in the thyroid. We also cover current and potential clinical applications of NIS, such as its central role in the treatment of thyroid cancer, its promising use as a reporter gene in imaging and diagnostic procedures, and the latest studies on NIS gene transfer aimed at extending radioiodide treatment to extrathyroidal cancers, including those involving specially engineered NIS molecules.
The Thyroid Na+/I- Symporter: Molecular Characterization and Genomic Regulation
Molecular Imaging and Radionuclide Therapy, 2016
Iodide (I-) is an essential constituent of the thyroid hormones triiodothyronine (T 3) and thyroxine (T 4), and the iodide concentrating mechanism of the thyroid gland is essential for the synthesis of these hormones. In addition, differential uptake of iodine isotopes (radioiodine) is a key modality for the diagnosis and therapy of thyroid cancer. The sodium dependent iodide transport activity of the thyroid gland is mainly attributed to the functional expression of the Na + /I-Symporter (NIS) localized at the basolateral membrane of thyrocytes. In this paper, we review and summarize current data on molecular characterization, on structure and function of NIS protein, as well as on the transcriptional regulation of NIS encoding gene in the thyroid gland. We also propose that a better and more precise understanding of NIS gene regulation at the molecular level in both healthy and malignant thyroid cells may lead to the identification of small molecule candidates. These could then be translated into clinical practice for better induction and more effective modulation of radioiodine uptake in dedifferentiated thyroid cancer cells and in their distant metastatic lesions.
Physiological sodium concentrations enhance the iodide affinity of the Na+/I− symporter
Nature Communications, 2014
The Na + /I − symporter (NIS) mediates active I − transport-the first step in thyroid hormonogenesis-with a 2Na + :1I − stoichiometry. NIS-mediated 131 I − treatment of thyroid cancer post-thyroidectomy is the most effective targeted internal radiation cancer treatment available. Here, to uncover mechanistic information on NIS, we use statistical thermodynamics to obtain K d s and estimate the relative populations of the different NIS species during Na + /anion binding and transport. We show that, although the affinity of NIS for I − is low (K d =224μM), it increases when Na + is bound (K d =22.4μM). However, this K d is still much higher than the submicromolar physiological I − concentration. To overcome this, NIS takes advantage of the extracellular Na + concentration and the pronounced increase in its own affinity for I − and for the second Na + elicited by binding of the first. Thus, at physiological Na + concentrations, ~79% of NIS molecules are occupied by two Na + ions and ready to bind and transport I − .
Physiological reviews, 2000
The Na(+)/I(-) symporter (NIS) is an intrinsic membrane protein that mediates the active transport of iodide into the thyroid and other tissues, such as salivary glands, gastric mucosa, and lactating mammary gland. NIS plays key roles in thyroid pathophysiology as the route by which iodide reaches the gland for thyroid hormone biosynthesis and as a means for diagnostic scintigraphic imaging and for radioiodide therapy in hyperthyroidism and thyroid cancer. The molecular characterization of NIS started with the 1996 isolation of a cDNA encoding rat NIS and has since continued at a rapid pace. Anti-NIS antibodies have been prepared and used to study NIS topology and its secondary structure. The biogenesis and posttranslational modifications of NIS have been examined, a thorough electrophysiological analysis of NIS has been conducted, the cDNA encoding human NIS (hNIS) has been isolated, the genomic organization of hNIS has been elucidated, the regulation of NIS by thyrotropin and I(-)...
Endocrinology, 2008
I− is actively transported into thyrocytes via the Na+/I− symporter (NIS), a key glycoprotein located on the basolateral plasma membrane. The cDNA encoding rat NIS was identified in our laboratory, where an extensive structure/function characterization of NIS is being conducted. Several NIS mutants have been identified as causes of congenital I− transport defect (ITD), including V59E NIS. ITD is characterized by low thyroid I− uptake, low saliva/plasma I− ratio, hypothyroidism, and goiter and may cause mental retardation if untreated. Studies of other ITD-causing NIS mutants have revealed valuable information regarding NIS structure/function. V59E NIS was reported to exhibit as much as 30% of the activity of wild-type NIS. However, this observation was at variance with the patients’ phenotype of total lack of activity. We have thoroughly characterized V59E NIS and studied several amino acid substitutions at position 59. We demonstrated that, in contrast to the previous report, V59E ...
The Journal of endocrinology, 2008
The active transport of iodide from the bloodstream into thyroid follicular cells is mediated by the Na+/I- symporter (NIS). We studied mouse NIS (mNIS) and found that it catalyzes iodide transport into transfected cells more efficiently than human NIS (hNIS). To further characterize this difference, we compared (125)I uptake in the transiently transfected human embryonic kidney (HEK) 293 cells. We found that the V(max) for mNIS was four times higher than that for hNIS, and that the iodide transport constant (K(m)) was 2.5-fold lower for hNIS than mNIS. We also performed immunocytolocalization studies and observed that the subcellular distribution of the two orthologs differed. While the mouse protein was predominantly found at the plasma membrane, its human ortholog was intracellular in approximately 40% of the expressing cells. Using cell surface protein-labeling assays, we found that the plasma membrane localization frequency of the mouse protein was only 2.5-fold higher than tha...