**2. The SGLT family**

## **2.1. Synonyms**

SGLT1-6, Gene Symbols: *SLC5*, sodium-glucose symporters

## **2.2. Summary**

The model of active, ATP-dependent glucose transport against a concentration gradient was proposed in 1960 by Bob Crane [5]. Intestinal reabsorption of glucose by the intestinal epithelium through transporters requires sodium symport which is ATP dependent via coupling to the sodium/potassium (Na+ /K+ ) pump. Mechanistically, the inward sodium gradient at the apical site of epithelial cells is maintained by the ATP-driven, active extrusion of sodium at the basolateral membrane (Figure 3, Figure 5B). The sodium dependent glucose transporters (SGLTs) are members of a larger gene family (>200 genes) of sodium:solute symporters (SSF) that contain a common SSF motif in the fifth transmembrane region [1]. The human SGLT protein family (SLC5A) comprises 11 isoforms that structurally are characterized by 14 transmembrane domains, where the N-and C-termini face the extracellular (luminal) side of the cell. The 11 family members share an amino acid identity of 21-70%. A broad range of substrates are transported by proteins encoded by the *SLC5* genes.

GLUT2 might not be exclusively involved in uptake of this ketohexose by the hepatocyte [38]. GLUT9 is highly ex‐ pressed in liver and due to its capability to transport uric acid its proposed function in humans might be the release of uric acid from the liver. In mice GLUT9 is required for uric acid uptake into liver for further breakdown by uricase to allantoin [39]. Whether GLUT9 also contributes to hexose transport, namely fructose, in hepatocytes is currently un‐ known. Fructose that is taken up by the liver mainly feeds into triglyceride synthesis, and via ATP depletion stimulates AMP-deaminase and thereby purine degradation leading to an increased generation of uric acid. **B-Insulin stimulat‐ ed glucose uptake into skeletal muscle and adipose tissue**. In muscle and fat, glucose uptake is stimulated by insu‐ lin. After insulin binds to its receptor, auto-phosphorylation of the receptor occurs that triggers a signalling cascade that finally leads to a translocation of GLUT4 vesicles from an intracellular pool to the plasma membrane. The acute increase of GLUT4 molecules at the cell surface leads to an increase in glucose uptake and represents the rate-limiting step in insulin-stimulated glucose uptake in adipose and muscle tissues. **C**-**Pancreatic β-cells secrete insulin in re‐ sponse to elevations in blood glucose**. GLUT2 mediates glucose uptake into β-cells. Phosphorylation of glucose by glucokinase is the rate limiting step of glycolysis which increases the ATP to ADP ratio of the cell leading to closure of the KATP channel and subsequent opening of the of Ca++channels caused presumably by changes of the plasma mem‐ brane polarization. The opening of Ca++channels raises intracellular Ca++concentrations and induces exocytosis of the

As the brain's main energy source glucose needs to be transported across the blood-brain barrier. This process is facilitated by GLUT1 (Figure 1). Insulin-stimulated clearance of blood glucose through uptake into skeletal muscle, the heart and adipose tissue, the rate-limiting step is defined by translocation of GLUT4 from an intracellular compartment to the plasma membrane; and it's this signalling cascade that represents insulin sensitivity (Figure 4B). The secretion of insulin by the pancreatic β-cells of the islets of Langerhans is dependent on GLUT2 which functions as a β-cell glucose sensor (Figure 4C). However, the involvement of sugar transporters in the regulation of processes such as brain glucose sensing or glucose transport

This chapter summarizes the principal characteristics of SGLT [1] and GLUT-[4] mediated sugar transport, primarily focusing on the human transporters, with emphasis on the current understanding of their physiology, based on inherited disorders and syndromes in humans

The model of active, ATP-dependent glucose transport against a concentration gradient was proposed in 1960 by Bob Crane [5]. Intestinal reabsorption of glucose by the intestinal epithelium through transporters requires sodium symport which is ATP dependent via

gradient at the apical site of epithelial cells is maintained by the ATP-driven, active extrusion of sodium at the basolateral membrane (Figure 3, Figure 5B). The sodium dependent glucose transporters (SGLTs) are members of a larger gene family (>200 genes) of sodium:solute symporters (SSF) that contain a common SSF motif in the fifth transmembrane region [1]. The

) pump. Mechanistically, the inward sodium

/K+

in the mammary gland are not yet well understood.

SGLT1-6, Gene Symbols: *SLC5*, sodium-glucose symporters

and phenotypes of genetically modified mice.

coupling to the sodium/potassium (Na+

**2. The SGLT family**

**2.1. Synonyms**

**2.2. Summary**

insulin granules.

8 Glucose Homeostasis

**Figure 5. Proposed models for the mechanism of facilitative and active glucose transport across cellular mem‐ branes.A-A 6-state model is proposed for SGLT mediated glucose transport.** The empty transporter is assumed to have a valence of-2 (1). Sugar transport is initiated upon binding of two sodium ions to the open form of the outside gate (2). In the next step, glucose binds to the transporter, which induces a conformational change from an outward to an inward occluded state (3, 4). Upon open of the inward gate, the glucose is released into the cytoplasm before the sodium (5). The transport cycle is completed by a conformational change to return the ligand-free inward facing (6) structure to the ligand-free outward-facing structure (1). **B-Model of GLUT mediated glucose transport.** Glucose binds to an outward-facing site of the transporter (1) which induces a conformational changes that allows movement of the hexose through the protein (2-3). After the release of the hexose from its inward-facing binding site into the cytosol the transporter undergoes a reverse conformational change (4-1).

The focus of the current chapter is on the sodium-dependent glucose transporters within the *SLC5* gene family, namely SGLT1-5, and the closely related, based on sequence homology and substrate specificity, sodium driven myoinositol transporter SMIT1 (SLC5A3) and SMIT2 (SGLT6/SLC5A11).

More distant relatives of the *SLC5A* gene family are the iodide transporters NIS (sodium-iodide symporter [SLC5A5]) and AIT (apical iodide transporter [SLC5A11]), the Na+/Cl-/choline transporter (CHT, [SLC5A7]) and the sodium-dependent multivitamin transporter (SMVT, [SLC5A6]). NIS and AIT are expressed in the thyroid gland. While NIS is responsible for iodide uptake which is required for production of T3 and T4, AIT is thought to catalyse the movement of iodide from the thyrocyte cytoplasm to the lumen of the gland. SMVT is widely expressed, while

SGLT3 for which imino sugars-containing an amine group in place of a hydroxyl group-are ligands. Based on mutational analysis of SGLTs and the crystal structure of vSGLT, which is 32% identical to human SGLT1, residues that coordinate substrate recognition have been shown to be relatively conserved. An exception to that is the human SGLT3, a glucose sensor, which can be converted to a functional transporter based on a single amino acid exchange. In addition to glucose and other monosaccharides, SGLTs also transport glycosides. Those can be either substrates such as indican and arbutin, or actual inhibitors such as the highly potent, classic competitive SGLT inhibitor phlorizin, a naturally occurring β-glucoside (see below). The ability to recognize galactose as substrate by SLC5 family members has been attributed to

Ion selectivity and stoichiometry has been well characterized for SGLTs. The transporters are

required to drive SGLT2 activity. Despite crystallographic information for vSGLT, the electron density is not sufficient to assign binding sites for small single ions such as sodium. However, using mutational analysis and superimposition of structural models from the solute symport‐ ers vSGLT, LeuT, and Mhp1, a sodium binding site for vSGLT is suggested to be close to the sugar binding residues in transmembrane domains 1 and 8. The predicted cation binding site

The mechanism of sodium-driven glucose transport has been intensively investigated for SGLT1, applying various methodologies that allow the kinetics of transport to be deter‐ mined using heterologous expression of the transporter in *Xenopus laevis* oocytes. From the kinetic measurements a 6-state equilibrium model is proposed, where conformational changes dependent on cation and sugar binding, transport and cytoplasmic release are integrated. The six kinetic states describe the "empty" transporter, the sodium bound form, and the sodium and glucose bound transporter at the external and internal plasma

SGLT1 mediated glucose transport has been characterized regarding its kinetics, conforma‐ tional changes and the significance of residues for substrate/inhibitor binding. However, many questions remain unanswered such as the precise identity of the second-sodium binding site for SGLT1, and the location of the phlorizin binding site in SGLT1 and SGLT2, which may be of relevance for SGLT2 selective inhibitors that, recently approved, represent a new treatment

In 1987, the laboratory of Ernest Wright cloned the first sodium-dependent glucose transporter from rabbit intestinal mRNA by an expressing cloning strategy using *Xenopus laevis* oocytes [1]. SGLT1 is primarily expressed in the brush border membrane of mature enterocytes in the small intestine and catalyses the absorption of the dietary sugars glucose and galactose from the gut lumen. SGLT1 is also expressed in the kidney on the luminal

(Km=4mM). While Li<sup>+</sup>

(Km=9mM) and H<sup>+</sup>

ions bind to SGLT1 and 3, and only one Na+

to glucose transport stoichiometry

Mammalian Sugar Transporters http://dx.doi.org/10.5772/58325

(Km=7µM) can

is

11

the presence of a threonine corresponding to amino acid 460 in human SGLT1.

in vSGLT appears to allow accessibility to the cytoplasmic aqueous phase.

selective for the cotransport cation Na+

membrane surfaces (Figure 5A).

**2.4. SGLT1 (***SLC5A1)*

is established for SGLT1-3, where two Na+

replace Na+, no other monovalent cation is accepted. The Na+

option for Type 2 diabetes mellitus (T2DM) (see below).

CHT is mainly found in the central nervous system. Biochemically, the CHT mediates Na+/ choline co-transport in a chloride dependent manner.
