Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2

Abstract

The concentration of glucose in plasma is held within narrow limits (4-10 mmol/l), primarily to ensure fuel supply to the brain. Kidneys play a role in glucose homeostasis in the body by ensuring that glucose is not lost in the urine. Three membrane proteins are responsible for glucose reabsorption from the glomerular filtrate in the proximal tubule: sodium-glucose cotransporters SGLT1 and SGLT2, in the apical membrane, and GLUT2, a uniporter in the basolateral membrane. 'Knockout' of these transporters in mice and men results in the excretion of filtered glucose in the urine. In humans, intravenous injection of the plant glucoside phlorizin also results in excretion of the full filtered glucose load. This outcome and the finding that, in an animal model, phlorizin reversed the symptoms of diabetes, has stimulated the development and successful introduction of SGLT2 inhibitors, gliflozins, in the treatment of type 2 diabetes mellitus. Here we summarise the current state of our knowledge about the physiology of renal glucose handling and provide background to the development of SGLT2 inhibitors for type 2 diabetes treatment.

Keywords: GLUTs; Gliflozins; Glucose; Inhibitors; Kidney; Phlorizin; Proximal tubule; Review; SGLTs; Type 2 diabetes mellitus.

Fig. 1

A drawing illustrating a cross-sectional view of the human kidney, showing the location of one of the 1 million nephrons in the kidney. The renal vein and renal artery are shown in black and red, respectively. The average daily filtered glucose load, urinary glucose excretion and glucose reabsorption in healthy adults are also shown. Phlorizin (Pz) increases glucose excretion to the filtered load and eliminates glucose reabsorption. This figure is available as part of a downloadable slideset

Fig. 2

Glucose reabsorption by the proximal tubule. (a) Schematic representation of a single nephron, the functional unit of the kidney. (b) Glucose concentration (mmol/l) measured in micropuncture studies as fluid flows from the glomerulus along the tubule [51]. SGLT2 and GLUT2 are responsible for glucose reabsorption in the S1 and S2 segments, and SGLT1 and GLUT2 are responsible for glucose reabsorption in the S3 segment . Adapted from [6], distributed under the terms of the CC BY 4.0 Attribution License (http://creativecommons.org/licenses/by/4.0/). This figure is available as part of a downloadable slideset

Fig. 3

Reabsorption of glucose in the proximal tubule. (a) Epithelial cells of the S1 and S2 segments of the proximal tubule express SGLT2 on the apical membrane and GLUT2 on the basolateral membrane. (b) Epithelial cells of the S3 segment express SGLT1 on the apical membrane and GLUT2 on the basolateral membrane. In both S1/S2 and S3 segments, glucose reabsorption occurs, first via glucose transport across the apical membrane by SGLTs and then by passive glucose exit towards the plasma via GLUT2. The sodium gradient across the apical membrane is maintained by the basolateral Na⁺/K⁺ pump. At an extracellular NaCl concentration of 150 mmol/l, a membrane potential of −50 mV and at 37°C, the human SGLT2 has a K_m for glucose of 5 mmol/l, a K_i for phlorizin of 11 nmol/l and a sodium:glucose coupling ratio of 1:1. Under the same conditions, human SGLT1 has a glucose K_m of 2 mmol/l, a phlorizin K_i of 140 nmol/l, and a sodium:glucose coupling ratio of 2:1. Adapted from [6], distributed under the terms of the CC BY 4.0 Attribution License (http://creativecommons.org/licenses/by/4.0/). This figure is available as part of a downloadable slideset

Fig. 4

Urinary excretion of glucose PET tracers, 2-FDG and Me-4FDG in wild-type, Glut2^−/−, Sglt1^−/− and Sglt2^−/− mice. The total amount of 2-FDG and Me-4FDG in the urinary bladder of representative mice as a function of time after intravenous injection of radiotracer (11 MBq) is shown. The data were fitted to a three-compartmental model for glomerular filtration, reabsorption and urinary excretion, showing that the excretion of 2-FDG in wild-type mice, and Me-4FDG in the Glut2^−/− mice was equivalent to the filtered glucose load. The excretion of Me-4FDG was greater in Sglt1^−/− than Sglt2^−/− mice. The entire filtered load of Me-4FDG was reabsorbed in wild-type mice. Adapted from [27] distributed under the terms of the CC BY-NC 4.0 Attribution License (https://creativecommons.org/licenses/by-nc/4.0/). This figure is available as part of a downloadable slideset

Fig. 5

(a) Time course of Me-4FDG excretion into the urinary bladder of a rat. Me-4FDG (300 MBq) was injected intravenously into rats and excretion into the urinary bladder measured using microPET. Its excretion into the urinary bladder (MBq) is plotted as a function of time before and after injection of 1 mg/kg dapagliflozin (Dapa; an SGLT2 inhibitor) at 20 min. Injection of dapagliflozin causes the rapid excretion of Me-4FDG into the urinary bladder. Similar results were obtained with intravenous phlorizin (A. S. Yu and C. Ghezzi, unpublished results). (b) SGLT2 distribution in a rat, analysed using F-Dapa microPET. Animals were injected with F-Dapa (300 MBq) and the distribution of the tracer was imaged at 60 min using microPET. F-Dapa binding to organs is shown in control conditions (white bars) or after competition with cold dapagliflozin (black bars). Three-dimensional regions of interest (ROIs) were drawn over each organ and the data are presented as a percentage of the initial dose per tissue weight (%ID/g), with the exception of the bladder, for which values are presented as percentage ID per total bladder volume. Data are presented as the apparent density of SGLT2 in each organ, as means + SEM. ***p ≤ 0.001 vs control. The data show that functional SGLT2 is only expressed in the kidney. (c, d) Location of SGLT2 in the mouse kidney, visualised using F-Dapa and ex vivo microautoradiography and H&E staining. A mouse was injected with 148 MBq F-Dapa and after 15 min the kidney was removed and processed. (c) Aligned autoradiogram and H&E-stained image of the whole mouse kidney. Scale bar, 1 mm. (d) F-Dapa binding to the tubules surrounding a glomerulus. These images show that dapagliflozin is filtered by glomeruli in the outer renal cortex and then binds to SGLT2 in the early proximal tubule. Scale bar, 100 μm. Parts (b–d) are adapted with permission of American Society of Nephrology from [32]; permission conveyed through Copyright Clearance Center, Inc. This figure is available as part of a downloadable slideset

Fig. 6

A mechanical model for sodium-coupled sugar transport. Sodium (green circle) binds first to the extracellular side (‘OUT’; state 1) to open the outer gate (state 2), permitting the sugar (glucose; yellow hexagon) to bind and be trapped in the bound site (state 3). The binding of both substrates induces a conformational change to an ‘inward facing’ conformation, resulting in the opening of the inner gate (state 4) and the release of Na⁺ and sugar into the cell interior. After the release of both substrates, the inner gate closes to form the inward facing ligand-free conformation (state 5). The cycle is completed by the change in conformation to the outward facing ligand-free (state 1). Phlorizin binds at the second point in the process (state 2). Adapted from [6], distributed under the terms of the CC BY 4.0 Attribution License (http://creativecommons.org/licenses/by/4.0/). This figure is available as part of a downloadable slideset

Fig. 7

Homology model of the human SGLT2 based on the inward facing, occluded conformation of vSGLT (as described in [19]). Helices are represented as tubes. For clarity, helices −1 and 11–14 have been removed and helices 1, 2 and 10 are depicted as transparent. TM3 is coloured orange. Highlighted are the residues forming the glucose-binding site and the inner and outer gates EL8a and EL8b are helices in the external loop linking TM7 and TM8. This figure is available as part of a downloadable slideset

Fig. 8

The external vestibule of human SGLT1 in the outward facing, sodium-bound conformation (Fig. 6, state 2). The vestibule was mapped using fluorescent reagents covalently bound to cysteine residues in the sugar-binding site, e.g. tetramethylrhodamine (TAMRA) bound to Y290C. The location of transmembrane helices (TM) of the structural model of SGLT1 are shown (some helices have been removed for clarity), along with the boundary of the 600 Å³ vestibule (blue area) bounded by the outer ends of TM1, TM2, TM3, TM6, TM9 and TM10. Reproduced from [47], distributed under the terms of the Creative Commons Attribution-NonCommerical-NoDerivatives International License 4.0 (CC BY-NC-ND; https://creativecommons.org/licenses/by-nc-nd/4.0/). This figure is available as part of a downloadable slideset