How drugs interact with transporters: SGLT1 as a model

Functional characterization of SGLT1 using SSM-based electrophysiology: Kinetics of sugar binding and translocation

Beside the ongoing efforts to determine structural information, detailed functional studies on transporters are essential to entirely understand the underlying transport mechanisms. We recently found that solid supported membrane-based electrophysiology (SSME) enables the measurement of both sugar binding and transport in the Na⁺/sugar cotransporter SGLT1 (Bazzone et al, 2022a). Here, we continued with a detailed kinetic characterization of SGLT1 using SSME, determining K_M and K_D ^app for different sugars, k_obs values for sugar-induced conformational transitions and the effects of Na⁺, Li⁺, H⁺ and Cl^- on sugar binding and transport. We found that the sugar-induced pre-steady-state (PSS) charge translocation varies with the bound ion (Na⁺, Li⁺, H⁺ or Cl^-), but not with the sugar species, indicating that the conformational state upon sugar binding depends on the ion. Rate constants for the sugar-induced conformational transitions upon binding to the Na⁺-bound carrier range from 208 s^-1 for D-glucose to 95 s^-1 for 3-OMG. In the absence of Na⁺, rate constants are decreased, but all sugars bind to the empty carrier. From the steady-state transport current, we found a sequence for sugar specificity (V_max/K_M): D-glucose > MDG > D-galactose > 3-OMG > D-xylose. While K_M differs 160-fold across tested substrates and plays a major role in substrate specificity, V_max only varies by a factor of 1.9. Interestingly, D-glucose has the lowest V_max across all tested substrates, indicating a rate limiting step in the sugar translocation pathway following the fast sugar-induced electrogenic conformational transition. SGLT1 specificity for D-glucose is achieved by optimizing two ratios: the sugar affinity of the empty carrier for D-glucose is similarly low as for all tested sugars (K_D,K ^app = 210 mM). Affinity for D-glucose increases 14-fold (K_D,Na ^app = 15 mM) in the presence of sodium as a result of cooperativity. Apparent affinity for D-glucose during transport increases 8-fold (K_M = 1.9 mM) compared to K_D,Na ^app due to optimized kinetics. In contrast, K_M and K_D ^app values for 3-OMG and D-xylose are of similar magnitude. Based on our findings we propose an 11-state kinetic model, introducing a random binding order and intermediate states corresponding to the electrogenic transitions detected via SSME upon substrate binding.

Does 2-FDG PET Accurately Reflect Quantitative In Vivo Glucose Utilization?

2-Deoxy-2-¹⁸F-fluoro-d-glucose (2-FDG) with PET is undeniably useful in the clinic, being able, among other uses, to monitor change over time using the 2-FDG SUV metric. This report suggests some potentially serious caveats for this and related roles for 2-FDG PET. Most critical is the assumption that there is an exact proportionality between glucose metabolism and 2-FDG metabolism, called the lumped constant, or LC. This report describes that LC is not constant for a specific tissue and may be variable before and after disease treatment. The purpose of this work is not to deny the clinical value of 2-FDG PET; it is a reminder that when one extends the use of an appropriately qualified imaging method, new observations may arise and further validation would be necessary. The current understanding of glucose-based energetics in vivo is based on the quantification of glucose metabolic rates with 2-FDG PET, a method that permits the noninvasive assessment of various human disorders. However, 2-FDG is a good substrate only for facilitated-glucose transporters (GLUTs), not for sodium-dependent glucose cotransporters (SGLTs), which have recently been shown to be distributed in multiple human tissues. Thus, the GLUT-mediated in vivo glucose utilization measured by 2-FDG PET would be masked to the potentially substantial role of functional SGLTs in glucose transport and use. Therefore, under these circumstances, the 2-FDG LC used to quantify in vivo glucose utilization should not be expected to remain constant. 2-FDG LC variations have been especially significant in tumors, particularly at different stages of cancer development, affecting the accuracy of quantitative glucose measures and potentially limiting the prognostic value of 2-FDG, as well as its accuracy in monitoring treatments. SGLT-mediated glucose transport can be estimated using α-methyl-4-deoxy-4-¹⁸F-fluoro-d-glucopyranoside (Me-4FDG). Using both 2-FDG and Me-4FDG should provide a more complete picture of glucose utilization via both GLUT and SGLT transporters in health and disease states. Given the widespread use of 2-FDG PET to infer glucose metabolism, it is relevant to appreciate the potential limitations of 2-FDG as a surrogate for glucose metabolic rate and the potential reasons for variability in LC. Even when the readout for the 2-FDG PET study is only an SUV parameter, variability in LC is important, particularly if it changes over the course of disease progression (e.g., an evolving tumor).

Mechanistic interpretation of conventional Michaelis-Menten parameters in a transporter system

The aim was to elucidate how steps in drug translocation by a solute carrier transporter impact Michaelis-Menten parameters Km, Ki, and Vmax. The first objective was to derive a model for carrier-mediated substrate translocation and perform sensitivity analysis with regard to the impact of individual microrate constants on Km, Ki, and Vmax. The second objective was to compare underpinning microrate constants between compounds translocated by the same transporter. Equations for Km, Ki, and Vmax were derived from a six-state model involving unidirectional transporter flipping and reconfiguration. This unidirectional model is applicable to co-transporter type solute carriers, like the apical sodium-dependent bile acid transporter (ASBT) and the proton-coupled peptide cotransporter (PEPT1). Sensitivity analysis identified the microrate constants that impacted Km, Ki, and Vmax. Compound comparison using the six-state model employed regression to identify microrate constant values that can explain observed Km and Vmax values. Results yielded some expected findings, as well as some unanticipated effects of microrate constants on Km, Ki, and Vmax. Km and Ki were found to be equal for inhibitors that are also substrates. Additionally, microrate constant values for certain steps in transporter functioning influenced Km and Vmax to be low or high.

SGLT2 inhibitors act from the extracellular surface of the cell membrane

SGLT2 inhibitors are a new class of drugs that have been recently developed to treat type II diabetes. They lower glucose levels by inhibiting the renal Na⁺/glucose cotransporter SGLT2, thereby increasing the amount of glucose excreted in the urine. Pharmacodynamics studies have raised questions about how these inhibitors reach SGLT2 in the brush border membrane of the S1 and S2 segments of the renal proximal tubule: are these drugs filtered by the glomerulus and act extracellularly, or do they enter the cell and act intracellularly? To address this question we expressed hSGLT2 in HEK‐293T cells and determined the affinity of a specific hSGLT2 inhibitor, TA‐3404 (also known as JNJ‐30980924), from the extra‐ and intracellular side of the plasma membrane. Inhibition of SGLT2 activity (Na⁺/glucose currents) by TA‐3404 was determined using the whole‐cell patch clamp that allows controlling the composition of both the extracellular and intracellular solutions. We compared the results to those obtained using the nonselective SGLT inhibitor phlorizin, and to the effect of TA‐3404 on hSGLT1. Our results showed that TA‐3404 is a potent extracellular inhibitor of glucose inward SGLT2 transport (IC₅₀ 2 nmol/L) but it was ineffective from the intracellular compartment at both low (5 mmol/L) and high (150 mmol/L) intracellular NaCl concentrations. We conclude that TA‐3404 only inhibits SGLT2 from the extracellular side of the plasma membrane, suggesting that it is filtered from the blood through the glomerulus and acts from within the tubule lumen.

An emerging class of new drugs to treat diabetes is the SGLT2 inhibitors which control blood glucose levels by excreting glucose into the urine. Questions have arisen about how these inhibitors reach SGLT2 in the brush border membrane of the S1 and S2 segments of the renal proximal tubule. Using patch‐clamp technique, we characterized the effect of a canagliflozin derivative (TA‐3404) on glucose‐induced currents and we showed that the inhibitor binds to SGLT2 from the extracellular side of the plasma membrane. This provides indirect evidence that SGLT2‐specific drugs act from the tubular lumen and not from the blood through the tubular epithelium.

The transport mechanism of the human sodium/myo-inositol transporter 2 (SMIT2/SGLT6), a member of the LeuT structural family

The sodium/myo-inositol transporter 2 (SMIT2) is a member of the SLC5A gene family, which is believed to share the five-transmembrane segment inverted repeat of the LeuT structural family. The two-electrode voltage-clamp (TEVC) technique was used to measure the steady-state and the pre-steady-state currents mediated by human SMIT2 after expression in Xenopus laevis oocytes. Phlorizin is first shown to be a poor inhibitor of pre-steady-state currents for depolarizing voltage pulse. From an up to threefold difference between the apparent ON and OFF transferred charges during a voltage pulse, we also show that a fraction of the transient current recorded for very negative potentials is not a true pre-steady-state current coming from the cotransporter conformational changes. We suggest that this transient current comes from a time-dependent leak current that can reach large amplitudes when external Na(+) concentration is reduced. A kinetic model was generated through a simulated annealing algorithm. This algorithm was used to identify the optimal connectivity among 19 different kinetic models and obtain the numerical values of the associated parameters. The proposed 5-state model includes cooperative binding of Na(+) ions, strong apparent asymmetry of the energy barriers, a rate-limiting step that is likely associated with the translocation of the empty transporter, and a turnover rate of 21 s(-1). The proposed model is a proof of concept for a novel approach to kinetic modeling of electrogenic transporters and allows insight into the transport mechanism of members of the LeuT structural family at the millisecond timescale.

Effect of insulin-induced hypoglycaemia on the central nervous system: evidence from experimental studies

Insulin-induced hypoglycaemia (IIH) is a major acute complication in type 1 as well as in type 2 diabetes, particularly during intensive insulin therapy. The brain plays a central role in the counter-regulatory response by eliciting parasympathetic and sympathetic hormone responses to restore normoglycaemia. Brain glucose concentrations, being approximately 15-20% of the blood glucose concentration in humans, are rigorously maintained during hypoglycaemia through adaptions such as increased cerebral glucose transport, decreased cerebral glucose utilisation and, possibly, by using central nervous system glycogen as a glucose reserve. However, during sustained hypoglycaemia, the brain cannot maintain a sufficient glucose influx and, as the cerebral hypoglycaemia becomes severe, electroencephalogram changes, oxidative stress and regional neuronal death ensues. With particular focus on evidence from experimental studies on nondiabetic IIH, this review outlines the central mechanisms behind the counter-regulatory response to IIH, as well as cerebral adaption to avoid sequelae of cerebral neuroglycopaenia, including seizures and coma.

Bridging the gap between structure and kinetics of human SGLT1

The Na(+)-glucose cotransporter hSGLT1 is a member of a class of membrane proteins that harness Na(+) electrochemical gradients to drive uphill solute transport. Although hSGLT1 belongs to one gene family (SLC5), recent structural studies of bacterial Na(+) cotransporters have shown that Na(+) transporters in different gene families have the same structural fold. We have constructed homology models of hSGLT1 in two conformations, the inward-facing occluded (based on vSGLT) and the outward open conformations (based on Mhp1), mutated in turn each of the conserved gates and ligand binding residues, expressed the SGLT1 mutants in Xenopus oocytes, and determined the functional consequences using biophysical and biochemical assays. The results establish that mutating the ligand binding residues produces profound changes in the ligand affinity (the half-saturation concentration, K(0.5)); e.g., mutating sugar binding residues increases the glucose K(0.5) by up to three orders of magnitude. Mutation of the external gate residues increases the Na(+) to sugar transport stoichiometry, demonstrating that these residues are critical for efficient cotransport. The changes in phlorizin inhibition constant (K(i)) are proportional to the changes in sugar K(0.5), except in the case of F101C, where phlorizin K(i) increases by orders of magnitude without a change in glucose K(0.5). We conclude that glucose and phlorizin occupy the same binding site and that F101 is involved in binding to the phloretin group of the inhibitor. Substituted-cysteine accessibility methods show that the cysteine residues at the position of the gates and sugar binding site are largely accessible only to external hydrophilic methanethiosulfonate reagents in the presence of external Na(+), demonstrating that the external sugar (and phlorizin) binding vestibule is opened by the presence of external Na(+) and closes after the binding of sugar and phlorizin. Overall, the present results provide a bridge between kinetics and structural studies of cotransporters.

Structural selectivity of human SGLT inhibitors

Human Na(+)-D-glucose cotransporter (hSGLT) inhibitors constitute the newest class of diabetes drugs, blocking up to 50% of renal glucose reabsorption in vivo. These drugs have potential for widespread use in the diabetes epidemic, but how they work at a molecular level is poorly understood. Here, we use electrophysiological methods to assess how they block Na(+)-D-glucose cotransporter SGLT1 and SGLT2 expressed in human embryonic kidney 293T (HEK-293T) cells and compared them to the classic SGLT inhibitor phlorizin. Dapagliflozin [(1S)-1,5,5-anhydro-1-C-{4-chloro-3-[(4-ethoxyphenyl)methyl]phenyl}-D-glucitol], two structural analogs, and the aglycones of phlorizin and dapagliflozin were investigated in detail. Dapagliflozin and fluoro-dapagliflozin [(1S)-1,5-anhydro-1-C-{4-chloro-3-[(4-ethoxyphenyl)methyl]phenyl}-4-F-4-deoxy-D-glucitol] blocked glucose transport and glucose-coupled currents with ≈100-fold specificity for hSGLT2 (K(i) = 6 nM) over hSGLT1 (K(i) = 400 nM). As galactose is a poor substrate for SGLT2, it was surprising that galacto-dapagliflozin [(1S)-1,5-anhydro-1-C-{4-chloro-3-[(4-ethoxyphenyl)methyl]phenyl}-D-galactitol] was a selective inhibitor of hSGLT2, but was less potent than dapagliflozin for both transporters (hSGLT2 K(i) = 25 nM, hSGLT1 K(i) = 25,000 nM). Phlorizin and galacto-dapagliflozin rapidly dissociated from SGLT2 [half-time off rate (t(1/2,Off)) ≈ 20-30 s], while dapagliflozin and fluoro-dapagliflozin dissociated from hSGLT2 at a rate 10-fold slower (t(1/2,Off) ≥ 180 s). Phlorizin was unable to exchange with dapagliflozin bound to hSGLT2. In contrast, dapagliflozin, fluoro-dapagliflozin, and galacto-dapagliflozin dissociated quickly from hSGLT1 (t(1/2,Off) = 1-2 s), and phlorizin readily exchanged with dapagliflozin bound to hSGLT1. The aglycones of phlorizin and dapagliflozin were poor inhibitors of both hSGLT2 and hSGLT1 with K(i) values > 100 μM. These results show that inhibitor binding to SGLTs is composed of two synergistic forces: sugar binding to the glucose site, which is not rigid, and so different sugars will change the orientation of the aglycone in the access vestibule; and the binding of the aglycone affects the binding affinity of the entire inhibitor. Therefore, the pharmacophore must include variations in both the structure of the sugar and the aglycone.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Glucose transport by human renal Na+/D-glucose cotransporters SGLT1 and SGLT2

The human Na(+)/D-glucose cotransporter 2 (hSGLT2) is believed to be responsible for the bulk of glucose reabsorption in the kidney proximal convoluted tubule. Since blocking reabsorption increases urinary glucose excretion, hSGLT2 has become a novel drug target for Type 2 diabetes treatment. Glucose transport by hSGLT2 was studied at 37°C in human embryonic kidney 293T cells using whole cell patch-clamp electrophysiology. We compared hSGLT2 with hSGLT1, the transporter in the straight proximal tubule (S3 segment). hSGLT2 transports with surprisingly similar glucose affinity and lower concentrative power than hSGLT1: Na(+)/D-glucose cotransport by hSGLT2 was electrogenic with apparent glucose and Na(+) affinities of 5 and 25 mM, and a Na(+):glucose coupling ratio of 1; hSGLT1 affinities were 2 and 70 mM and coupling ratio of 2. Both proteins showed voltage-dependent steady-state transport; however, unlike hSGLT1, hSGLT2 did not exhibit detectable pre-steady-state currents in response to rapid jumps in membrane voltage. D-Galactose was transported by both proteins, but with very low affinity by hSGLT2 (≥100 vs. 6 mM). β-D-Glucopyranosides were either substrates or blockers. Phlorizin exhibited higher affinity with hSGLT2 (K(i) 11 vs. 140 nM) and a lower Off-rate (0.03 vs. 0.2 s⁻¹) compared with hSGLT1. These studies indicate that, in the early proximal tubule, hSGLT2 works at 50% capacity and becomes saturated only when glucose is ≥35 mM. Furthermore, results on hSGLT1 suggest it may play a significant role in the reabsorption of filtered glucose in the late proximal tubule. Our electrophysiological study provides groundwork for a molecular understanding of how hSGLT inhibitors affect renal glucose reabsorption.

Serine side chain-linked peptidomimetic conjugates of cyclic HPMPC and HPMPA: synthesis and interaction with hPEPT1

Cidofovir (HPMPC), a broad spectrum antiviral agent, cannot be administered orally due to ionization of its phosphonic acid group at physiological pH. One prodrug approach involves conversion to the cyclic form (cHPMPC, 1) and esterification by the side chain hydroxyl group of a peptidomimetic serine. Transport studies in a rat model have shown enhanced levels of total cidofovir species in the plasma after oral dosing with L-Val-L-Ser-OMe cHPMPC, 2a. To explore the possibility that 2a and its three L/D stereoisomers 2b-d undergo active transport mediated by the peptide-specific intestinal transporter PEPT1, we performed radiotracer uptake and electrophysiology experiments applying the two-electrode voltage clamp technique in Xenopus laevis oocytes overexpressing human PEPT1 (hPEPT1, SLC15A1). 2a-d did not induce inward currents, indicating that they are not transported, but the stereoisomers with an L-configuration at the N-terminal valine (2a and 2b) potently inhibited transport of the hPEPT1 substrate glycylsarcosine (Gly-Sar). A "reversed" dipeptide conjugate, L-Ser-L-Ala-OiPr cHPMPC (4), also did not exhibit detectable transport, but completely abolished the Gly-Sar signal, suggesting that affinity of the transporter for these prodrugs is not impaired by a proximate linkage to the drug in the N-terminal amino acid of the dipeptide. Single amino acid conjugates of cHPMPC (3a and 3b) or cHPMPA (5, 6a and 6b) were not transported and only weakly inhibited Gly-Sar transport. The known hPEPT1 prodrug substrate valacyclovir (7) and its L-Val-L-Val dipeptide analogue (8) were used to verify coupled transport by the oocyte model. The results indicate that the previously observed enhanced oral bioavailability of 2a relative to the parent drug is unlikely to be due to active transport by hPEPT1. Syntheses of the novel compounds 2b-d and 3-6 are described, including a convenient solid-phase method to prepare 5, 6a and 6b.

Functional expression of SGLTs in rat brain

This work provides evidence of previously unrecognized uptake of glucose via sodium-coupled glucose transporters (SGLTs) in specific regions of the brain. The current understanding of functional glucose utilization in brain is largely based on studies using positron emission tomography (PET) with the glucose tracer 2-deoxy-2-[F-18]fluoro-D-glucose (2-FDG). However, 2-FDG is only a good substrate for facilitated-glucose transporters (GLUTs), not for SGLTs. Thus, glucose accumulation measured by 2-FDG omits the role of SGLTs. We designed and synthesized two high-affinity tracers: one, α-methyl-4-[F-18]fluoro-4-deoxy-D-glucopyranoside (Me-4FDG), is a highly specific SGLT substrate and not transported by GLUTs; the other one, 4-[F-18]fluoro-4-deoxy-D-glucose (4-FDG), is transported by both SGLTs and GLUTs and will pass through the blood brain barrier (BBB). In vitro Me-4FDG autoradiography was used to map the distribution of uptake by functional SGLTs in brain slices with a comparable result from in vitro 4-FDG autoradiography. Immunohistochemical assays showed that uptake was consistent with the distribution of SGLT protein. Ex vivo 4-FDG autoradiography showed that SGLTs in these areas are functionally active in the normal in vivo brain. The results establish that SGLTs are a normal part of the physiology of specific areas of the brain, including hippocampus, amygdala, hypothalamus, and cerebral cortices. 4-FDG PET imaging also established that this BBB-permeable SGLT tracer now offers a functional imaging approach in humans to assess regulation of SGLT activity in health and disease.

Structural perspectives on secondary active transporters

Secondary active transporters catalyze the concentrative transport of substrates across lipid membranes by harnessing the energy of electrochemical ion gradients. These transporters bind their ligands on one side of the membrane, and undergo a global conformational change to release them on the other side of the membrane. Over the last few years, crystal structures have captured several bacterial secondary transporters in different states along their transport cycle, providing insight into possible molecular mechanisms. In this review, we summarize recent findings focusing on the emerging structural and mechanistic similarities between evolutionary diverse transporters. We also discuss the structural basis of substrate binding, ion coupling and inhibition viewed from the perspective of these similarities.

SGLT1, a novel cardiac glucose transporter, mediates increased glucose uptake in PRKAG2 cardiomyopathy

Human mutations in the gene PRKAG2 encoding the gamma2 subunit of AMP-activated protein kinase (AMPK) cause a glycogen storage cardiomyopathy. Transgenic mice (TG(T400N)) with the human T400N mutation exhibit inappropriate activation of AMPK and consequent glycogen storage in the heart. Although increased glucose uptake and activation of glycogen synthesis have been documented in PRKAG2 cardiomyopathy, the mechanism of increased glucose uptake has been uncertain. Wildtype (WT), TG(T400N), and TG(alpha2DN) (carrying a dominant negative, kinase dead alpha2 catalytic subunit of AMPK) mice were studied at ages 2-8 weeks. Cardiac mRNA expression of sodium-dependent glucose transporter 1 (SGLT1), but not facilitated-diffusion glucose transporter 1 (GLUT1) or GLUT4, was increased approximately 5- to 7-fold in TG(T400N) mice relative to WT. SGLT1 protein was similarly increased at the cardiac myocyte sarcolemma in TG(T400N) mice. Phlorizin, a specific SGLT1 inhibitor, attenuated cardiac glucose uptake in TG(T400N) mice by approximately 40%, but not in WT mice. Chronic phlorizin treatment reduced cardiac glycogen content by approximately 25% in TG(T400N) mice. AICAR, an AMPK activator, increased cardiac SGLT1 mRNA expression approximately 3-fold in WT mice. Relative to TG(T400N) mice, double transgenic (TG(T400N)/TG(alpha2DN)) mice had decreased ( approximately 50%) cardiac glucose uptake and decreased (approximately 70%) cardiac SGLT1 expression. TG(T400N) hearts had increased binding activity of the transcription factors HNF-1 and Sp1 to the promoter of the gene encoding SGLT1. Our data suggest that upregulation of cardiac SGLT1 is responsible for increased cardiac glucose uptake in the TG(T400N) mouse. Increased AMPK activity leads to upregulation of SGLT1, which in turn mediates increased cardiac glucose uptake.

Structure and function of Na(+)-symporters with inverted repeats

Symporters are membrane proteins that couple energy stored in electrochemical potential gradients to drive the cotransport of molecules and ions into cells. Traditionally, proteins are classified into gene families based on sequence homology and functional properties, for example the sodium glucose (SLC5 or Sodium Solute Symporter Family, SSS or SSF) and GABA (SLC6 or Neurotransmitter Sodium Symporter Family, NSS or SNF) symporter families [1-4]. Recently, it has been established that four Na(+)-symporter proteins with unrelated sequences have a common structural core containing an inverted repeat of 5 transmembrane (TM) helices [5(**)-8(**)]. Analysis of these four structures reveals that they reside in different conformations along the transport cycle providing atomic insight into the mechanism of sodium solute cotransport.