High-Yield Functional Expression of Human Sodium/d-Glucose Cotransporter1 inPichia pastorisand Characterization of Ligand-Induced Conformational Changes as Studied by Tryptophan Fluorescence†

Sugar binding of sodium-glucose cotransporters analyzed by voltage-clamp fluorometry

Sugar absorption is crucial for life and relies on glucose transporters, including sodium-glucose cotransporters (SGLTs). Although the structure of SGLTs has been resolved, the substrate selectivity of SGLTs across diverse isoforms has not been determined owing to the complex substrate-recognition processes and limited analysis methods. Therefore, this study used voltage-clamp fluorometry (VCF) to explore the substrate-binding affinities of human SGLT1 in Xenopus oocytes. VCF analysis revealed high-affinity binding of D-glucose and D-galactose, which are known transported substrates. D-fructose, which is not a transported substrate, also bound to SGLT1, suggesting potential recognition despite the lack of transport activity. VCF analysis using the T287N mutant of the substrate-binding pocket, which has reduced D-glucose transport capacity, showed that its D-galactose-binding affinity exceeded its D-glucose-binding affinity. This suggests that the change in the VCF signal was due to substrate binding to the binding pocket. Both D-fructose and L-sorbose showed similar binding affinities, indicating that SGLT1 preferentially binds to pyranose-form sugars, including D-fructopyranose. Electrophysiological analysis confirmed that D-fructose binding did not affect the SGLT1 transport function. The significance of the VCF assay lies in its ability to measure sugar-protein interactions in living cells, thereby bridging the gap between structural analyses and functional characterizations of sugar transporters. Our findings also provide insights into SGLT substrate selectivity and the potential for developing medicines with reduced side effects by targeting non-glucose sugars with low bioreactivity.

Robust and flexible platform for directed evolution of yeast genetic switches

A wide repertoire of genetic switches has accelerated prokaryotic synthetic biology, while eukaryotic synthetic biology has lagged in the model organism Saccharomyces cerevisiae. Eukaryotic genetic switches are larger and more complex than prokaryotic ones, complicating the rational design and evolution of them. Here, we present a robust workflow for the creation and evolution of yeast genetic switches. The selector system was designed so that both ON- and OFF-state selection of genetic switches is completed solely by liquid handling, and it enabled parallel screen/selection of different motifs with different selection conditions. Because selection threshold of both ON- and OFF-state selection can be flexibly tuned, the desired selection conditions can be rapidly pinned down for individual directed evolution experiments without a prior knowledge either on the library population. The system's utility was demonstrated using 20 independent directed evolution experiments, yielding genetic switches with elevated inducer sensitivities, inverted switching behaviours, sensory functions, and improved signal-to-noise ratio (>100-fold induction). The resulting yeast genetic switches were readily integrated, in a plug-and-play manner, into an AND-gated carotenoid biosynthesis pathway.

Structural biology workflow for the expression and characterization of functional human sodium glucose transporter type 1 in Pichia pastoris

Heterologous expression of human membrane proteins is a challenge in structural biology towards drug discovery. Here we report a complete expression and purification process of a functional human sodium/D-glucose co-transporter 1 (hSGLT1) in Pichia pastoris as representative example of a useful strategy for any human membrane protein. hSGLT1 gene was cloned in two different plasmids to develop parallel strategies: one which includes green fluorescent protein fusion for screening optimal conditions, and another for large scale protein production for structural biology and biophysics studies. Our strategy yields at least 1 mg of monodisperse purified recombinant hSGLT1 per liter of culture, which can be characterized by circular dichroism and infrared spectroscopy as an alpha-helical fold protein. This purified hSGLT1 transports co-substrates (Na⁺ and glucose) and it is inhibited by phlorizin in electrophysiological experiments performed in planar lipid membranes.

Validation of sodium/glucose cotransporter proteins in human brain as a potential marker for temporal narrowing of the trauma formation

In many forensic cases, the existence of a traumatic brain injury (TBI) is an essential factor, and the determination of the survival time is nearly as important as the determination of whether or not a trauma exists. Since it is known that glucose uptake increases in injured brain cells in order to perpetuate the neuronal integrity, this study focuses on the pathomechanism of brain glucose supply via sodium/glucose cotransporters 1 and 2 (SGLT1, SGLT2) following traumatization. Human cerebrum tissue of male and female individuals who died following TBI was taken from the contusional and contralateral regions, as well as from individuals deceased due to cardiac arrest (control group). Total SGLT1 and SGLT2 protein expression was analyzed by immunoblotting comparing injured and non-injured tissue. The immunoreactivity in contusional cerebral cortex region began to increase 3 to 7 h following traumatization. We found that both SGLT1 and SGLT2 protein expression increased significantly 37 h post-injury compared to the control group. SGLT1 rose significantly at 52 h post-injury and peaked significantly at 72 h, while SGLT2 rose significantly at 52 and 72 h after injury. By compiling these data, we predict a standard operator via SGLT expression as a comparative expression assertion to determine post-injury survival time for unknown cases. Our result suggests that SGLT1 and SGLT2 protein expression may be useful in forensic practice as an effective target to analyze the existence of a TBI and to determine the time of the traumatization.

The Sodium Glucose Cotransporter SGLT1 Is an Extremely Efficient Facilitator of Passive Water Transport

The small intestine is void of aquaporins adept at facilitating vectorial water transport, and yet it reabsorbs ∼8 liters of fluid daily. Implications of the sodium glucose cotransporter SGLT1 in either pumping water or passively channeling water contrast with its reported water transporting capacity, which lags behind that of aquaporin-1 by 3 orders of magnitude. Here we overexpressed SGLT1 in MDCK cell monolayers and reconstituted the purified transporter into proteoliposomes. We observed the rate of osmotic proteoliposome deflation by light scattering. Fluorescence correlation spectroscopy served to assess (i) SGLT1 abundance in both vesicles and plasma membranes and (ii) flow-mediated dilution of an aqueous dye adjacent to the cell monolayer. Calculation of the unitary water channel permeability, pf, yielded similar values for cell and proteoliposome experiments. Neither the absence of glucose or Na(+), nor the lack of membrane voltage in vesicles, nor the directionality of water flow grossly altered pf Such weak dependence on protein conformation indicates that a water-impermeable occluded state (glucose and Na(+) in their binding pockets) lasts for only a minor fraction of the transport cycle or, alternatively, that occlusion of the substrate does not render the transporter water-impermeable as was suggested by computational studies of the bacterial homologue vSGLT. Although the similarity between the pf values of SGLT1 and aquaporin-1 makes a transcellular pathway plausible, it renders water pumping physiologically negligible because the passive flux would be orders of magnitude larger.

The role of transporter ectodomains in drug recognition and binding: phlorizin and the sodium–glucose cotransporter

This article reviews the role of segments of SLCs located outside the plasma membrane bilayer (ectodomains) using the inhibition of SGLTs (SLC5 family) by the aromatic glucoside phlorizin as a model system.

The complex interplay between ligand binding and conformational structure of the folate binding protein (folate receptor): Biological perspectives

This review analyzes how interplay between folate binding and changes in folate binding protein (FBP) conformation/self-association affects the biological function of FBP. Concentration-dependent, reversible self-association of hydrophobic apo-FBP at pI=7.4 is associated with decreased affinity for folate, probably due to shielding of binding sites between interacting hydrophobic patches. Titration with folate removes apo-monomers, favoring dissociation of self-associated apo-FBP into apo-monomers. Folate anchors to FBP through a network of hydrogen bonds and hydrophobic interactions, and the binding induces a conformational change with formation of hydrophilic and stable holo-FBP. Holo-FBP exhibits a ligand-mediated concentration-dependent self-association into multimers of great thermal and chemical stability due to strong intermolecular forces. Both ligand and FBP are thus protected against biological/physicochemical decomposition. In biological fluids with low FBP concentrations, e.g., saliva, semen and plasma, hydrophobic apo-monomers and hydrophilic holo-monomers associate into stable asymmetrical complexes with aberrant binding kinetics unless detergents, e.g., cholesterol or phospholipids are present.

Functional expression, purification and reconstitution of the recombinant phosphate transporter Pho89 of Saccharomyces cerevisiae

The Saccharomyces cerevisiae high-affinity phosphate transporter Pho89 is a member of the inorganic phosphate (Pi) transporter (PiT) family, and shares significant homology with the type III Na(+)/Pi symporters, hPit1 and hPit2. Currently, detailed biochemical and biophysical analyses of Pho89 to better understand its transport mechanisms are limited, owing to the lack of purified Pho89 in an active form. In the present study, we expressed functional Pho89 in the cell membrane of Pichia pastoris, solubilized it in Triton X-100 and foscholine-12, and purified it by immobilized nickel affinity chromatography combined with size exclusion chromatography. The protein eluted as an oligomer on the gel filtration column, and SDS/PAGE followed by western blotting analysis revealed that the protein appeared as bands of approximately 63, 140 and 520 kDa, corresponding to the monomeric, dimeric and oligomeric masses of the protein, respectively. Proteoliposomes containing purified and reconstituted Pho89 showed Na(+)-dependent Pi transport activity driven by an artificially imposed electrochemical Na(+) gradient. This implies that Pho89 operates as a symporter. Moreover, its activity is sensitive to the Na(+) ionophore monensin. To our knowledge, this study represents the first report on the functional reconstitution of a Pi-coupled PiT family member.

Ligand binding induces a sharp decrease in hydrophobicity of folate binding protein assessed by 1-anilinonaphthalene-8-sulphonate which suppresses self-association of the hydrophobic apo-protein

High affinity folate binding protein (FBP) regulates as a soluble protein and as a cellular receptor intracellular trafficking of folic acid, a vitamin of great importance to cell growth and division. We addressed two issues of potential importance to the biological function of FBP, a possible decrease of the surface hydrophobicity associated with the ligand-induced conformation change of FBP, and protein-inter-protein interactions involved in self-association of hydrophobic apo-FBP. The extrinsic fluorescent apolar dye 1-anilinonaphthalene-8-sulphonate (ANS) exhibited enhanced fluorescence intensity and a blueshift of emission maximum from 510-520 nm to 460-470 nm upon addition of apo-FBP indicating binding to a strongly hydrophobic environment. Neither enhancement of fluorescence nor blueshift of ANS emission maximum occurred when folate-ligated holo-FBP replaced apo-FBP. The drastic decrease in surface hydrophobicity of holo-FBP could have bearings on the biological function of FBP since changes in surface hydrophobicity have critical effects on the biological function of receptors and transport proteins. ANS interacts with exposed hydrophobic surfaces on proteins and may thereby block and prevent aggregation of proteins (chaperone-like effect). Hence, hydrophobic interactions seemed to participate in the concentration-dependent self-association of apo-FBP which was suppressed by high ANS concentrations in light scatter measurements.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

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.

Increasing gene dosage greatly enhances recombinant expression of aquaporins in Pichia pastoris

Background

When performing functional and structural studies, large quantities of pure protein are desired. Most membrane proteins are however not abundantly expressed in their native tissues, which in general rules out purification from natural sources. Heterologous expression, especially of eukaryotic membrane proteins, has also proven to be challenging. The development of expression systems in insect cells and yeasts has resulted in an increase in successful overexpression of eukaryotic proteins. High yields of membrane protein from such hosts are however not guaranteed and several, to a large extent unexplored, factors may influence recombinant expression levels. In this report we have used four isoforms of aquaporins to systematically investigate parameters that may affect protein yield when overexpressing membrane proteins in the yeast Pichia pastoris.

Results

By comparing clones carrying a single gene copy, we show a remarkable variation in recombinant protein expression between isoforms and that the poor expression observed for one of the isoforms could only in part be explained by reduced transcript levels. Furthermore, we show that heterologous expression levels of all four aquaporin isoforms strongly respond to an increase in recombinant gene dosage, independent of the amount of protein expressed from a single gene copy. We also demonstrate that the increased expression does not appear to compromise the protein folding and the membrane localisation.

Conclusions

We report a convenient and robust method based on qPCR to determine recombinant gene dosage. The method is generic for all constructs based on the pPICZ vectors and offers an inexpensive, quick and reliable means of characterising recombinant P. pastoris clones. By using this method we show that: (1) heterologous expression of all aquaporins investigated respond strongly to an increase in recombinant gene dosage (2) expression from a single recombinant gene copy varies in an isoform dependent manner (3) the poor expression observed for AtSIP1;1 is mainly caused by posttranscriptional limitations. The protein folding and membrane localisation seems to be unaffected by increased expression levels. Thus a screen for elevated gene dosage can routinely be performed for identification of P. pastoris clones with high expression levels of aquaporins and other classes of membrane proteins.

SGLT inhibitors as new therapeutic tools in the treatment of diabetes

Recently, the idea has been developed to lower blood glucose blood glucose levels in diabetes by inhibiting sugar reabsorption sugar reabsorption in the kidney kidney . The main target is thereby the early proximal tubule proximal tubule where secondary active transport secondary active transport of the sugar is mediated by the sodium-D: -glucose D-glucose cotransporter SGLT2 SGLT2 . A model substance for the inhibitors inhibitors is the O-glucoside O-glucoside phlorizin phlorizin which inhibits transport transport competitively. Its binding to the transporter involves at least two different domains: an aglucone binding aglucone binding site at the transporter surface, involving extramembranous loops extramembraneous loops , and the sugar binding sugar binding /translocation site buried in a hydrophilic pocket of the transporter. The properties of these binding sites differ between SGLT2 and SGLT1 SGLT1 , which mediates sugar absorption sugar absorption in the intestine intestine . Various O-, C-, N- and S-glucosides have been synthesized with high affinity affinity and high specificity specificity for SGLT2 SGLT2 . Some of these glucosides are in clinical trials clinical trials and have been proven to successfully increase urinary glucose excretion urinary glucose excretion and to decrease blood sugar blood sugar levels without the danger of hypoglycaemia hypoglycaemia during fasting fasting in type 2 diabetes type 2 diabetes .

Role of beta-adrenoceptors in memory consolidation: beta3-adrenoceptors act on glucose uptake and beta2-adrenoceptors on glycogenolysis

Noradrenaline, acting via beta(2)- and beta(3)-adrenoceptors (AR), enhances memory formation in single trial-discriminated avoidance learning in day-old chicks by mechanisms involving changes in metabolism of glucose and/or glycogen. Earlier studies of memory consolidation in chicks implicated beta(3)- rather than beta(2)-ARs in enhancement of memory consolidation by glucose, but did not elucidate whether stimulation of glucose uptake or of glycolysis was responsible. This study examines the role of glucose transport in memory formation using central injection of the nonselective facilitative glucose transporter (GLUT) inhibitor cytochalasin B, the endothelial/astrocytic GLUT-1 inhibitor phloretin and the Na(+)/energy-dependent endothelial glucose transporter (SGLT) inhibitor phlorizin. Cytochalasin B inhibited memory when injected into the mesopallium (avian cortex) either close to or between 25 and 45 min after training, whereas phloretin and phlorizin only inhibited memory at 30 min. This suggested that astrocytic/endothelial (GLUT-1) transport is critical at the time of consolidation, whereas a different transporter, probably the neuronal glucose transporter (GLUT-3), is important at the time of training. Inhibition of glucose transport by cytochalasin B, phloretin, or phlorizin also interfered with beta(3)-AR-mediated memory enhancement 20 min posttraining, whereas inhibition of glycogenolysis interfered with beta(2)-AR agonist enhancement of memory. We conclude that in astrocytes (1) activities of both GLUT-1 and SGLT are essential for memory consolidation 30 min posttraining; (2) neuronal GLUT-3 is essential at the time of training; and (3) beta(2)- and beta(3)-ARs consolidate memory by different mechanisms; beta(3)-ARs stimulate central glucose transport, whereas beta(2)-ARs stimulate central glycogenolysis.

Antidepressant interactions with the NMDA NR1-1b subunit

The targets for tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), and selective norepinephrine reuptake inhibitors (SNRIs) are known to be the serotonin and norepinephrine transport (reuptake) proteins which are embedded in presynaptic nerve terminals and function to bring these neurotransmitters from the synaptic cleft back into the presynaptic neuron. Using a combination of intrinsic and extrinsic fluorescence quenching, Stern-Volmer analysis, and protease protection assays, we have shown that therapeutics from each of these three classes of antidepressants bind to the extracellular S1S2 domain of the NR1-1b subunit of the NMDA receptor. These results are in agreement with recent work from our lab demonstrating the interaction of antidepressants with the S1S2 domain of the GluR2 subunit of the AMPA receptor, another member of the ionotropic glutamate receptor subfamily, as well as work from other labs, and continue the discussion of the involvement of ionotropic glutamate receptors in depression.

Functional expression and characterisation of a gut facilitative glucose transporter, NlHT1, from the phloem-feeding insect Nilaparvata lugens (rice brown planthopper)

Phloem-sap feeding Hemipteran insects have access to a sucrose-rich diet but are dependent on sucrose hydrolysis and hexose transport for carbon nutrition. A cDNA library from Nilaparvata lugens (rice brown planthopper) was screened for clones encoding potential transmembrane transporters. A selected cDNA, NlHT1, encodes a 53kDa polypeptide with sequence similarity to facilitative hexose transporters of eukaryotes and prokaryotes, including GLUT1, the human erythrocyte hexose transporter. NlHT1 was expressed as a recombinant protein in the methylotropic yeast Pichia pastoris, and was identified in a membrane fraction isolated from transformed yeast cells. Transport experiments using membrane vesicles containing NlHT1 showed that the protein is a saturable, sodium independent transporter, with a relatively low affinity for glucose (K(m) 3.0mM), which can be inhibited by cytochalasin B. Competition experiments with fructose demonstrate NlHT1 is glucose specific. In situ localisation studies revealed that NlHT1 mRNA is expressed in N. lugens gut tissue, mainly in midgut regions, and that expression is absent in hindgut and Malpighian tubules. NlHT1 is therefore likely to play an important role in glucose transport from the gut, and in carbon nutrition in vivo. This is the first report of a facilitative glucose transporter from a phloem-feeding insect pest.

Exceptional overproduction of a functional human membrane protein

Eukaryotic--especially human--membrane protein overproduction remains a major challenge in biochemistry. Heterologously overproduced and purified proteins provide a starting point for further biochemical, biophysical and structural studies, and the lack of sufficient quantities of functional membrane proteins is frequently a bottleneck hindering this. Here, we report exceptionally high production levels of a correctly folded and crystallisable recombinant human integral membrane protein in its active form; human aquaporin 1 (hAQP1) has been heterologously produced in the membranes of the methylotrophic yeast Pichia pastoris. After solubilisation and a two step purification procedure, at least 90 mg hAQP1 per liter of culture is obtained. Water channel activity of this purified hAQP1 was verified by reconstitution into proteoliposomes and performing stopped-flow vesicle shrinkage measurements. Mass spectrometry confirmed the identity of hAQP1 in crude membrane preparations, and also from purified protein reconstituted into proteoliposomes. Furthermore, crystallisation screens yielded diffraction quality crystals of untagged recombinant hAQP1. This study illustrates the power of the yeast P. pastoris as a host to produce exceptionally high yields of a functionally active, human integral membrane protein for subsequent functional and structural characterization.

A proteomic study of sodium/d-glucose cotransporter 1 (SGLT1): Topology of loop 13 and coverage of other functionally important domains

In order to obtain further information about the structure and function of human sodium/D-glucose cotransporter 1 (hSGLT1), the recombinant protein was subjected, either after reconstitution into liposomes or in its free form, to proteolysis followed by nanoscale microcapillary liquid chromatography electrospray ionization tandem mass spectrometry (LC-MS/MS). The peptides released from SGLT1 proteoliposomes by trypsin bead digestion represented the early N-terminal, loop 7, and loop 9, supporting topology models that place these domains on the extracellular side of the protein. Trypsin bead digestion generated, however, also a number of peptides derived from loop 13 whose topology with regard to the membrane is hitherto a point of debate. Sequence coverage was provided from amino acids 559 to 644, suggesting that loop 13 is almost completely accessible at the extravesicular face of the proteoliposomes. These results support the notion that major parts of loop 13, essential for the interaction with transport inhibitors in vivo, are located extracellularly in intact cells. In-gel trypsin, chymotrypsin, and in particular trypsin/chymotrypsin digestion of recombinant SGLT1 in combination with LC-MS/MS provide extensive sequence coverage of the protein, including domains involved in sugar and inhibitor binding and potential phosphorylation sites. These studies demonstrate that proteomic analysis combined with mass spectrometry is a useful tool to characterize regions of SGLT1 that are important for its function and regulation.

Three surface subdomains form the vestibule of the Na+/glucose cotransporter SGLT1

A combination of biophysical and biochemical approaches was employed to probe the topology, arrangement, and function of the large surface subdomains of SGLT1 in living cells. Using atomic force microscopy on the single molecule level, Chinese hamster ovary cells overexpressing SGLT1 were probed with atomic force microscopy tips carrying antibodies against epitopes of different subdomains. Specific single molecule recognition events were observed with antibodies against loop 6-7, loop 8-9, and loop 13-14, demonstrating the extracellular orientation of these subdomains. The addition of D-glucose in Na+-containing medium decreased the binding probability of the loop 8-9 antibody, suggesting a transport-related conformational change in the region between amino acids 339 and 356. Transport studies with mutants C345A, C351A, C355A, or C361S supported a role for these amino acids in determining the affinity of SGLT1 for D-glucose. MTSET, [2-(trimethylammonium)ethyl] methanethiosulfonate and dithiothreitol inhibition patterns on alpha-methyl-glucoside uptake by COS-7 cells expressing C255A, C560A, or C608A suggested the presence of a disulfide bridge between Cys255 and Cys608. This assumption was corroborated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry showing mass differences in peptides derived from transporters biotinylated in the absence and presence of dithiothreitol. These results indicate that loop 6-7 and loop 13-14 are connected by a disulfide bridge. This bridge brings also loop 8-9 into close vicinity with the former subdomains to create a vestibule for sugar binding.

Ligand-mediated conformational changes and positioning of tryptophans in reconstituted human sodium/d-glucose cotransporter1 (hSGLT1) probed by tryptophan fluorescence

Recombinant purified human sodium/D-glucose cotransporter1 (hSGLT1) was reconstituted in a functional form into phospholipid vesicles and its conformational states in the absence and presence of ligands and inhibitors were probed by intrinsic tryptophan fluorescence. In the presence of sodium, sugars increase intrinsic fluorescence (maximum 17%) in a saturable manner in the following order alpha-MDG >D-Glu approximately D-Gal >> D-Man >D-All, with no effect of L-Glu. Apparent affinities ranging from 0.65 to 10.4 mM were observed. In addition, D-Glu increased the accessibility of the Trps to hydrophilic collisional quenchers. On the contrary, the transport inhibitor phlorizin decreased Trps fluorescence in a sodium-dependent manner by 50% with a red shift of 4-6 nm and decreased quencher accessibility, these effects were saturable with a high affinity of 5 microM. Furthermore, the positioning of the tryptophans in the reconstituted transporter was investigated. hSGLT1 Trps fluorescence was reduced by N-bromosuccinimide treatment maximally 25% in membranes and 65% in solution. The fluorescence was also significantly but differently quenched by the lipid-soluble spin labeled probes 5-Doxyl-phosphatidylcholine (40%) and 12-Doxyl-phosphatidylcholine (26%). Depth-calculation using the parallax method suggested a location of Trps at an average depth of 10 angstrom from the center of the bilayer. These studies demonstrate the existence of different conformational states of the membrane-embedded transporter in its glucose-free form, as sodium-glucose-carrier complex and as sodium-phlorizin-carrier complex. They further indicate that most of the Trp residues in hSGLT1 are located in hydrophobic regions of the protein or in contact with the lipid bilayer of the membrane. There, they are located close to the membrane-water interface contributing to the vectorial nature of the transporter.

Tricyclic antidepressants, but not the selective serotonin reuptake inhibitor fluoxetine, bind to the S1S2 domain of AMPA receptors

The hypothesis that depression is caused solely by a decrease in synaptic availability of monoaminergic neurotransmitters has been questioned over the past two decades. Based on accumulating data, it seems more plausible that cross-talk exists between neurotransmitters in the CNS, including the glutamatergic system. Glutamate, the major fast excitatory neurotransmitter in the CNS, is the natural agonist for the ionotropic glutamate receptors, a family of ligand-gated ion channels including NMDA (N-methyl-D-aspartate), AMPA (amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid), and kainate receptors. In this work, we show that five tricyclic antidepressants bind to the S1S2 domain of the GluR2 subunit of the AMPA receptor. A combination of fluorescence quenching, Stern-Volmer analyses, and protease protection assays differentiate the binding of each antidepressant. These analyses provide no evidence for the binding of the selective serotonin reuptake inhibitor, fluoxetine, to this domain. The data presented provides further support for a role of the glutamatergic system in antidepressant activity.