Dietary I− Absorption

Expression and Regulation of the Na+/I− Symporter in the Intestine

Juan Pablo Nicola*; Nancy Carrasco†,1; Ana María Masini-Repiso*,1    * Departamento de Bioquímica Clínica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina
† Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut, USA
1 Corresponding authors: email address: nancy.carrasco@yale.edu, amasini@fcq.unc.edu.ar

1 The Importance of Iodide in Human Health

Iodide (I−) uptake in the thyroid gland is the first step in the biosynthesis of thyroid hormones—triiodothyronine (T3) and thyroxine (T4) (Portulano, Paroder-Belenitsky, & Carrasco, 2014). Thyroid hormones are the only iodine-containing hormones in vertebrates and are required for the development and maturation of the central nervous system, skeletal muscle, and lungs in the fetus and the newborn. They are also primary regulators of intermediate metabolism and effect pleiotropic modulation in virtually all organs and tissues throughout life (Yen, 2001).

Iodine is an extremely scarce element in the environment and is supplied to the body exclusively through the diet. Insufficient dietary I− intake may cause mild to severe hypothyroidism and subsequently goiter, stunted growth, retarded psychomotor development, and even cretinism (impairment of physical growth and irreversible mental retardation due to severe thyroid hormone deficiency during childhood) (Zimmermann, 2009). I− deficiency-associated diseases are the most common preventable cause of mental retardation in the world and were slated for global eradication by iodination of table salt by the year 1990 by the World Health Organization. Although significant progress has been made, there were still an estimated 1.88 billion people suffering from insufficient I− intake in 2011 (Andersson, Karumbunathan, & Zimmermann, 2012).

As iodine is an irreplaceable component of thyroid hormones, normal thyroid physiology relies on adequate dietary I− intake, gastrointestinal I− absorption, and proper I− accumulation in thyrocytes. Therefore, the evolution of a highly efficient system to avidly accumulate I− appears to be a physiological adaptation to compensate for the environmental scarcity of iodine.

2 The Na+/I− Symporter

The thyroid gland has developed a remarkably efficient system to ensure an adequate supply of I− for thyroid hormone biosynthesis. Under physiological conditions, the thyroid concentrates I− approximately 40-fold with respect to the plasma concentration (Wolff & Maurey, 1961). Moreover, the ability of the thyroid to concentrate I− has provided the molecular basis for the use of radioiodide in the diagnosis, treatment, and follow-up of thyroid pathology (Bonnema & Hegedus, 2012; Reiners, Hanscheid, Luster, Lassmann, & Verburg, 2011). A major breakthrough in the field—as important as the introduction of radioactive I− isotopes into the study of thyroid physiology near the middle of the twentieth century (Hertz, Roberts, Means, & Evans, 1940)—was the identification of the complementary DNA (cDNA) encoding the Na+/I− symporter (NIS), the protein that mediates I− transport in the thyroid (Dai, Levy, & Carrasco, 1996). The identification of NIS started a new era of intensive I− research.

2.1 Molecular identification of NIS

The journey toward the identification of NIS began with the isolation of poly(A+) RNA from FRTL-5 cells, a line of highly differentiated rat thyroid-derived cells which, microinjected into Xenopus laevis oocytes, produced Na+-dependent I− transport (Vilijn & Carrasco, 1989). Thereafter, the cDNA encoding NIS was isolated by expression cloning in X. laevis oocytes using cDNA libraries generated from FRTL-5 cells (Dai et al., 1996). The full nucleotide sequence revealed an open reading frame of 1,854 nucleotides encoding a protein of 618 amino acids. Shortly thereafter, the screening of a human thyroid cDNA library with rat NIS probes enabled the identification of human NIS (Smanik et al., 1996), which exhibits 84% identity and 93% similarity to rat NIS. The human NIS gene was mapped to chromosome 19p13.11 and comprises 15 exons with an open reading frame of 1,929 nucleotides, giving rise to a protein of 643 amino acids (Smanik, Ryu, Theil, Mazzaferri, & Jhiang, 1997).

NIS is an intrinsic plasma membrane glycoprotein. The current, experimentally tested NIS secondary structure model shows a hydrophobic protein with 13 transmembrane segments (TMSs), an extracellular amino terminus and an intracellular carboxy terminus (Levy et al., 1997, 1998; Fig. 1A). Moreover, NIS is a highly N-glycosylated protein, although N-glycosylation is not essential for I− transport or NIS trafficking to the plasma membrane (Levy et al., 1998).

NIS-driven active transport of I− into the thyroid is electrogenic and relies on the driving force of the Na+ gradient generated by the Na+/K+ ATPase and the electrical potential across the plasma membrane. By coupling the inward transport of Na+ down its electrochemical gradient to the translocation of I− against its electrochemical gradient across the plasma membrane, NIS avidly concentrates I− into the cells (Dai et al., 1996; Eskandari et al., 1997).

Like all membrane transporters, NIS belongs to the solute-carrier gene (SLC) superfamily. In particular, NIS is a member of solute-carrier family 5A (SLC5A) and has been designated SLC5A5 according to the Human Genome Organization (HUGO) Gene Nomenclature Committee. To date, the only crystal structure of a member of SLC5A is that of the Vibrio parahaemolyticus Na+/galactose transporter (vSGLT), a bacterial homologue of the human SGLT1 (SLC5A1) (Faham et al., 2008). Despite the lack of sequence homology, as predicted by De la Vieja, Reed, Ginter, and Carrasco (2007), the structure of vSGLT revealed the same fold—an inverted topology repeat and unwound helices in regions critical for substrate binding—and a Na+ coordination similar to that observed in the high-resolution (1.65 Å) crystal structure of the leucine transporter (LeuT) from Aquifex aeolicus (LeuT) (Yamashita, Singh, Kawate, Jin, & Gouaux, 2005). Remarkably, NIS shares significant identity (27%) and homology (58%) with vSGLT—almost as much as SGLT1 does (31% identity, 62%...