An extracellular loop region of the serotonin transporter may be involved in the translocation mechanism

In silico analysis of a SLC6A4 G100V mutation in lung cancers

SLC6A4 is a serotonin re-uptake transporter which has been a target for anti-depressant therapies but recently some mutations have been described in cancer cells. Here, we characterize mutations in SLC6A4 that appear in cancer cells. We employed several validated computational and artificial intelligence algorithms to characterize the mutations. We identified a previously uncharacterized G100V mutation in lung cancers. In sillico structural analysis reveals that this mutation may affect SLC6A4 ligand binding and subsequently its function. We also identified several other mutations that may affect the structure of the protein. This preliminary analysis highlights the role of SLC6A4 in human cancers.

Extracellular loops matter - subcellular location and function of the lysine transporter Lyp1 from Saccharomyces cerevisiae

Yeast amino acid transporters of the APC superfamily are responsible for the proton motive force-driven uptake of amino acids into the cell, which for most secondary transporters is a reversible process. The l-lysine proton symporter Lyp1 of Saccharomyces cerevisiae is special in that the Michaelis constant from out-to-in transport ( K m out → in ) is much lower than K m in → out , which allows accumulation of l-lysine to submolar concentration. It has been proposed that high intracellular lysine is part of the antioxidant mechanism of the cell. The molecular basis for the unique kinetic properties of Lyp1 is unknown. We compared the sequence of Lyp1 with APC para- and orthologues and find structural features that set Lyp1 apart, including differences in extracellular loop regions. We screened the extracellular loops by alanine mutagenesis and determined Lyp1 localization and activity and find positions that affect either the localization or activity of Lyp1. Half of the affected mutants are located in the extension of extracellular loop 3 or in a predicted α-helix in extracellular loop 4. Our data indicate that extracellular loops not only connect the transmembrane helices but also serve functionally important roles.

Allosteric Modulation of Neurotransmitter Transporters as a Therapeutic Strategy

Neurotransmitter transporters (NTTs) are involved in the fine-tuning of brain neurotransmitter homeostasis. As such, they are implicated in a plethora of complex behaviors, including reward, movement, and cognition. During recent decades, compounds that modulate NTT functions have been developed. Some of them are in clinical use for the management of different neuropsychiatric conditions. The majority of these compounds have been found to selectively interact with the orthosteric site of NTTs. Recently, diverse allosteric sites have been described in a number of NTTs, modulating their function. A more complex NTT pharmacology may be useful in the development of novel therapeutics. Here, we summarize current knowledge on such modulatory allosteric sites, with specific focus on their pharmacological and therapeutic potential.

Post-translational modifications of serotonin transporter

The serotonin transporter (SERT) is an oligomeric glycoprotein with two sialic acid residues on each of two complex oligosaccharide molecules. Studies using in vivo and in vitro model systems demonstrated that diverse post-translational modifications, including phosphorylation, glycosylation, serotonylation, and disulfide bond formation, all favorably influences SERT conformation and allows the transporter to function most efficiently. This review discusses the post-translational modifications and their importance on the structure, maturation, and serotonin (5-HT) uptake ability of SERT. Finally, we discuss how these modifications are altered in diabetes mellitus and subsequently impairs the 5-HT uptake ability of SERT.

Role of N-glycosylation in renal betaine transport

The osmolyte and folding chaperone betaine is transported by the renal Na(+)-coupled GABA (γ-aminobutyric acid) symporter BGT-1 (betaine/GABA transporter 1), a member of the SLC6 (solute carrier 6) family. Under hypertonic conditions, the transcription, translation and plasma membrane (PM) insertion of BGT-1 in kidney cells are significantly increased, resulting in elevated betaine and GABA transport. Re-establishing isotonicity involves PM depletion of BGT-1. The molecular mechanism of the regulated PM insertion of BGT-1 during changes in osmotic stress is unknown. In the present study, we reveal a link between regulated PM insertion and N-glycosylation. Based on homology modelling, we identified two sites (Asn(171) and Asn(183)) in the extracellular loop 2 (EL2) of BGT-1, which were investigated with respect to trafficking, insertion and transport by immunogold-labelling, electron microscopy (EM), mutagenesis and two-electrode voltage clamp measurements in Xenopus laevis oocytes and uptake of radiolabelled substrate into MDCK (Madin-Darby canine kidney) and HEK293 (human embryonic kidney) cells. Trafficking and PM insertion of BGT-1 was clearly promoted by N-glycosylation in both oocytes and MDCK cells. Moreover, association with N-glycans at Asn(171) and Asn(183) contributed equally to protein activity and substrate affinity. Substitution of Asn(171) and Asn(183) by aspartate individually caused no loss of BGT-1 activity, whereas the double mutant was inactive, suggesting that N-glycosylation of at least one of the sites is required for function. Substitution by alanine or valine at either site caused a dramatic loss in transport activity. Furthermore, in MDCK cells PM insertion of N183D was no longer regulated by osmotic stress, highlighting the impact of N-glycosylation in regulation of this SLC6 transporter.

Importance of the Extracellular Loop 4 in the Human Serotonin Transporter for Inhibitor Binding and Substrate Translocation

The serotonin transporter (SERT) terminates serotonergic neurotransmission by performing reuptake of released serotonin, and SERT is the primary target for antidepressants. SERT mediates the reuptake of serotonin through an alternating access mechanism, implying that a central substrate site is connected to both sides of the membrane by permeation pathways, of which only one is accessible at a time. The coordinated conformational changes in SERT associated with substrate translocation are not fully understood. Here, we have identified a Leu to Glu mutation at position 406 (L406E) in the extracellular loop 4 (EL4) of human SERT, which induced a remarkable gain-of-potency (up to >40-fold) for a range of SERT inhibitors. The effects were highly specific for L406E relative to six other mutations in the same position, including the closely related L406D mutation, showing that the effects induced by L406E are not simply charge-related effects. Leu(406) is located >10 Å from the central inhibitor binding site indicating that the mutation affects inhibitor binding in an indirect manner. We found that L406E decreased accessibility to a residue in the cytoplasmic pathway. The shift in equilibrium to favor a more outward-facing conformation of SERT can explain the reduced turnover rate and increased association rate of inhibitor binding we found for L406E. Together, our findings show that EL4 allosterically can modulate inhibitor binding within the central binding site, and substantiates that EL4 has an important role in controlling the conformational equilibrium of human SERT.

Extracellular loop 3 of the noradrenaline transporter contributes to substrate and inhibitor selectivity

The human noradrenaline transporter (NET) and 5-hydroxytryptamine (5-HT) transporter (SERT) are inhibited by antidepressants and psychoactive drugs such as cocaine. Both substrates and inhibitors bind in the transmembrane core of the protein, but molecular divergence at the binding site is not sufficient to account for the NET-selective and SERT-selective inhibition of the antidepressants, desipramine and citalopram, respectively. We considered that the poorly conserved third extracellular loop may contribute to these differences. We substituted single amino acid residues of the third extracellular loop in NET for equivalents from SERT, transiently transfected COS-7 cells with WT NET, 13 mutant NETs and WT SERT, and measured [(3)H]noradrenaline uptake, [(3)H]nisoxetine binding and [(3)H]5-HT uptake. Mutants F299W, Y300Q, R301K and K303L, at the C-terminal end of EL3, all showed significantly decreased [(3)H]nisoxetine binding, indicative of a reduced cell surface expression. Most mutants differed little, if at all, from WT NET regarding [(3)H]noradrenaline uptake; however, the I297P mutant showed no significant uptake activity despite intact cell surface expression, and the A293F mutant showed a significantly slower transporter turnover than WT NET in addition to [(3)H]5-HT uptake that was significantly greater than that of WT NET. The A293F mutation also decreased desipramine potency and increased the inhibition of [(3)H]noradrenaline uptake by citalopram compared to WT NET. These results suggest that the third extracellular loop allosterically regulates the ability of the transmembrane domains to transport substrates and bind inhibitors and thus contributes to the selectivity of substrates and antidepressants for NET and SERT.

A second extracellular site is required for norepinephrine transport by the human norepinephrine transporter

The human norepinephrine transporter (NET) is implicated in many neurological disorders and is a target of tricyclic antidepressants and nisoxetine (NX). We used molecular docking simulations to guide the identification of residues likely to affect substrate transport and ligand interactions at NET. Mutations to alanine identified a hydrophobic pocket in the extracellular cavity of NET, comprising residues Thr80, Phe317, and Tyr317, which was critical for efficient norepinephrine (NE) transport. This secondary NE substrate site (NESS-2) overlapped the NX binding site, comprising Tyr84, Phe317, and Tyr317, and was positioned ∼11 Å extracellular to the primary site for NE (NESS-1). Thr80 in NESS-2 appeared to be critical in positioning NE for efficient translocation to NESS-1. Three residues identified as being involved in gating the reverse transport of NE (Arg81, Gln314, and Asp473) did not affect NE affinity for NESS-1. Mutating residues adjacent to NESS-2 abolished NET expression (D75A and L76A) or appeared to affect NET folding (S419A), suggesting important roles in stabilizing NET structure, whereas W308A and F388A at the top of NESS-2 abolished both NE transport and NX binding. Our findings are consistent with a multistep model of substrate transport by NET, for which a second, shallow extracellular NE substrate site (NESS-2) is required for efficient NE transport by NET.

The role of ERp44 in maturation of serotonin transporter protein

In heterologous and endogenous expression systems, we studied the role of ERp44 and its complex partner endoplasmic reticulum (ER) oxidase 1-α (Ero1-Lα) in mechanisms regulating disulfide bond formation for serotonin transporter (SERT), an oligomeric glycoprotein. ERp44 is an ER lumenal chaperone protein that favors the maturation of disulfide-linked oligomeric proteins. ERp44 plays a critical role in the release of proteins from the ER via binding to Ero1-Lα. Mutation in the thioredoxin-like domain hampers the association of ERp44C29S with SERT, which has three Cys residues (Cys-200, Cys-209, and Cys-109) on the second external loop. We further explored the role of the protein chaperones through shRNA knockdown experiments for ERp44 and Ero1-Lα. Those efforts resulted in increased SERT localization to the plasma membrane but decreased serotonin (5-HT) uptake rates, indicating the importance of the ERp44 retention mechanism in the proper maturation of SERT proteins. These data were strongly supported with the data received from the N-biotinylaminoethyl methanethiosulfonate (MTSEA-biotin) labeling of SERT on ERp44 shRNA cells. MTSEA-biotin only interacts with the free Cys residues from the external phase of the plasma membrane. Interestingly, it appears that Cys-200 and Cys-209 of SERT in ERp44-silenced cells are accessible to labeling by MTSEA-biotin. However, in the control cells, these Cys residues are occupied and produced less labeling with MTSEA-biotin. Furthermore, ERp44 preferentially associated with SERT mutants (C200S, C209S, and C109A) when compared with wild type. These interactions with the chaperone may reflect the inability of Cys-200 and Cys-209 SERT mutants to form a disulfide bond and self-association as evidenced by immunoprecipitation assays. Based on these collective findings, we hypothesize that ERp44 together with Ero1-Lα plays an important role in disulfide formation of SERT, which may be a prerequisite step for the assembly of SERT molecules in oligomeric form.

Structures of LeuT in bicelles define conformation and substrate binding in a membrane-like context

Neurotransmitter sodium symporters (NSSs) catalyze the uptake of neurotransmitters into cells, terminating neurotransmission at chemical synapses. Consistent with the role of NSSs in the central nervous system, they are implicated in multiple diseases and disorders. LeuT, from Aquifex aeolicus, is a prokaryotic ortholog of the NSS family and has contributed to our understanding of the structure, mechanism and pharmacology of NSSs. At present, however, the functional state of LeuT in crystals grown in the presence of n-octyl-β-D-glucopyranoside (β-OG) and the number of substrate binding sites are controversial issues. Here we present crystal structures of LeuT grown in DMPC-CHAPSO bicelles and demonstrate that the conformations of LeuT-substrate complexes in lipid bicelles and in β-OG detergent micelles are nearly identical. Furthermore, using crystals grown in bicelles and the substrate leucine or the substrate analog selenomethionine, we find only a single substrate molecule in the primary binding site.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

Emerging structure-function relationships defining monoamine NSS transporter substrate and ligand affinity

Monoamine transporters are a group of transmembrane neurotransmitter sodium symporter (NSS) transporters that play a crucial role in regulating biogenic monoamine concentrations at peripheral and central synapses. Given the key role played by serotonin, dopamine and noradrenaline in addictive and disease states, structure-function studies have been conducted to help guide the development of improved central nervous system therapeutics. Extensive pharmacological, immunological and biochemical studies, in conjunction with three-dimensional homology modeling, have been performed to structurally and functionally characterise the monoamine transporter substrate permeation pathway, substrate selectivity, and binding sites for ions, substrates and inhibitors at the molecular level. However, only recently has it been possible to start to construct an accurate molecular interaction network for the monoamine transporters and their corresponding substrates and inhibitors. Crystal structures of Aquifex aeolicus leucine transporter (LeuT(Aa)), a homologous protein to monoamine transporters that has been experimentally demonstrated to share similar structural folds with monoamine transporters, have been determined in complex with amino acids and inhibitors. The molecular interactions of leucine and tricyclic antidepressants (TCA) has supported many of the predictions based on the mutational studies. Models constructed from LeuT(Aa) are now allowing a rational approach to further clarify the molecular determinants of NSS transporter-ligand complexes, and potentially the ability to better manipulate drug specificity and affinity. In this review, we compare the structure-function relationships of other SLC6 NSS family transporters with monoamine transporters, and discuss possible mechanisms involved in substrate binding and transport, and modes of inhibition by TCAs.

Alteration of sugar-induced conformational changes of the melibiose permease by mutating Arg141 in loop 4-5

The melibiose permease (MelB) from Escherichia coli couples the uptake of melibiose to that of Na+, Li+, or H+. In this work, we applied attenuated total reflection Fourier transform infrared (ATR-FTIR) difference spectroscopy to obtain information about the structural changes involved in substrate interaction with the R141C mutant and with the wild-type MelB reacted with N-ethylmaleimide (NEM). These modified permeases have the ability to bind the substrates but fail to transport them. It is shown that the sugar-induced ATR-FTIR difference spectra of the R141C mutant are different from those corresponding to the Cys-less permease from which it is derived. There are alterations of peaks assigned to turns and beta-structures located most likely in loop 4-5. In addition, and quite notably, a peak at 1659 cm(-1), assigned to changes at the level of one alpha-helix subpopulation, disappears in the melibiose-induced difference spectrum in the presence of Na+, suggesting a reduction of the conformational change capacity of the mutated MelB. These helices may involve structural components that couple the cation- and sugar-binding sites. On the other hand, MelB-NEM difference spectra are proportionally less disrupted than the R141C ones. Hence, the transport cycle of these two permeases, modified at two different loops, is most likely impaired at a different stage. It is proposed that the R141C mutant leads to the generation of a partially defective ternary complex that is unable to catalyze the subsequent conformational change necessary for substrate translocation.

Assessing the ability of sequence-based methods to provide functional insight within membrane integral proteins: a case study analyzing the neurotransmitter/Na+ symporter family

Background

Efforts to predict functional sites from globular proteins is increasingly common; however, the most successful of these methods generally require structural insight. Unfortunately, despite several recent technological advances, structural coverage of membrane integral proteins continues to be sparse. ConSequently, sequence-based methods represent an important alternative to illuminate functional roles. In this report, we critically examine the ability of several computational methods to provide functional insight within two specific areas. First, can phylogenomic methods accurately describe the functional diversity across a membrane integral protein family? And second, can sequence-based strategies accurately predict key functional sites? Due to the presence of a recently solved structure and a vast amount of experimental mutagenesis data, the neurotransmitter/Na⁺ symporter (NSS) family is an ideal model system to assess the quality of our predictions.

Results

The raw NSS sequence dataset contains 181 sequences, which have been aligned by various methods. The resultant phylogenetic trees always contain six major subfamilies are consistent with the functional diversity across the family. Moreover, in well-represented subfamilies, phylogenetic clustering recapitulates several nuanced functional distinctions. Functional sites are predicted using six different methods (phylogenetic motifs, two methods that identify subfamily-specific positions, and three different conservation scores). A canonical set of 34 functional sites identified by Yamashita et al. within the recently solved LeuT_Aa structure is used to assess the quality of the predictions, most of which are predicted by the bioinformatic methods. Remarkably, the importance of these sites is largely confirmed by experimental mutagenesis. Furthermore, the collective set of functional site predictions qualitatively clusters along the proposed transport pathway, further demonstrating their utility. Interestingly, the various prediction schemes provide results that are predominantly orthogonal to each other. However, when the methods do provide overlapping results, specificity is shown to increase dramatically (e.g., sites predicted by any three methods have both accuracy and coverage greater than 50%).

Conclusion

The results presented herein clearly establish the viability of sequence-based bioinformatic strategies to provide functional insight within the NSS family. As such, we expect similar bioinformatic investigations will streamline functional investigations within membrane integral families in the absence of structure.

Ibogaine, a noncompetitive inhibitor of serotonin transport, acts by stabilizing the cytoplasm-facing state of the transporter

Ibogaine, a hallucinogenic alkaloid with purported anti-addiction properties, inhibited serotonin transporter (SERT) noncompetitively by decreasing V(max) with little change in the K(m) for serotonin (5-HT). Ibogaine also inhibited binding to SERT of the cocaine analog 2beta-2-carbomethoxy-3-(4-[(125)I]iodophenyl)tropane. However, inhibition of binding was competitive, increasing the apparent K(D) without much change in B(max). Ibogaine increased the reactivity of cysteine residues positioned in the proposed cytoplasmic permeation pathway of SERT but not at nearby positions out of that pathway. In contrast, cysteines placed at positions in the extracellular permeation pathway reacted at slower rates in the presence of ibogaine. These results are consistent with the proposal that ibogaine binds to and stabilizes the state of SERT from which 5-HT dissociates to the cytoplasm, in contrast with cocaine, which stabilizes the state that binds extracellular 5-HT.

Characterization of a serotonin transporter in the parasitic flatworm, Schistosoma mansoni: cloning, expression and functional analysis

The biogenic amine serotonin (5-hydroxytryptamine: 5HT) is a widely distributed neuroactive substance of vertebrates and invertebrates. Among parasitic flatworms, in particularly the bloodfluke, Schistosoma mansoni, 5HT is an important modulator of neuromuscular function and metabolism. Previous work has shown that schistosomes take up 5HT from host blood via a carrier mediated mechanism. This transport is thought to contribute to the control of schistosome motility in the bloodstream and is essential for survival of the parasite. Here we provide the first molecular evidence for the existence of a 5HT transporter in S. mansoni. A cDNA showing high homology with plasma membrane serotonin transporters (SERT) from other species was cloned and characterized by heterologous expression in cultured HEK293 cells. Functional studies showed that the recombinant schistosome transporter (SmSERT) mediates specific and saturable [(3)H]-5HT transport with a K(t)=1.30+/-0.05 microM. The heterologously expressed protein was inhibited by classic SERT blockers (clomipramine, fluoxetine, citalopram) and the same drugs also inhibited [(3)H]-5HT uptake by intact schistosomula in culture, suggesting that SmSERT may be responsible for this transport. Conventional (end-point) and real-time quantitative RT-PCR analyses determined that SmSERT is expressed both in the free-living stage (cercaria) and parasitic forms of S. mansoni but the expression level is significantly higher in the parasites. These results suggest that SmSERT is upregulated following cercarial transformation, possibly to mediate the recruitment of exogenous 5HT from the host.

The Conserved Glutamate (Glu136) in Transmembrane Domain 2 of the Serotonin Transporter Is Required for the Conformational Switch in the Transport Cycle

The alternate access model provides the theoretical framework for understanding how transporters translocate hydrophilic substrates across the lipid bilayer. The model postulates at least two conformations of a transporter, an outward and an inward facing conformation, which seal the translocation pathway to the interior and exterior of the cell, respectively. It is not clear how the conformational switch is triggered in neurotransmitter/sodium symporters, but Na+ is likely to play an essential role. Here, we focused on Glu136 of the serotonin transporter (SERT); this residue is conserved in transmembrane domain 2 of neurotransmitter/sodium symporters and related proteins. Three substitutions were introduced, resulting in SERT-E136D, SERT-E136Q, and SERT-E136A, which were all correctly inserted into the plasma membrane. SERT-E136Q and SERT-E136A failed to support substrate influx into cells, whereas SERT-E136D did so at a reduced rate. Binding experiments with the inhibitor 2beta-[3H]carbomethoxy-3beta-(4-iodophenyl)tropane (beta-[3H]CIT) supported the conjecture that the mutant transporters preferentially adopted the inward facing conformation: beta-[3H]CIT interacted with SERT in a manner consistent with binding to the outward facing state. Accordingly, the Na+-induced acceleration of beta-[3H]CIT association was most pronounced in wild-type SERT, followed by SERT-E136D > SERT-E136Q > SERT-E136A. Similarly, SERT-E136Q supported substrate efflux in a manner indistinguishable from wild-type SERT, whereas SERT-E136A was inactive. Thus, in the absence of Glu136, the conformational equilibrium of SERT is shifted progressively (SERT-E136D > SERT-E136Q > SERT-E136A) to the inward facing conformation.

Structure/function relationships in serotonin transporter: new insights from the structure of a bacterial transporter

Serotonin transporter (SERT) serves the important function of taking up serotonin (5-HT) released during serotonergic neurotransmission. It is the target for important therapeutic drugs and psychostimulants. SERT catalyzes the influx of 5-HT together with Na+ and Cl- in a 1:1:1 stoichiometry. In the same catalytic cycle, there is coupled efflux of one K+ ion. SERT is one member of a large family of amino acid and amine transporters that is believed to utilize similar mechanisms of transport. A bacterial member of this family was recently crystallized, revealing the structural basis of these transporters. In light of the new structure, previous results with SERT have been re-interpreted, providing new insight into the substrate binding site, the permeation pathway, and the conformational changes that occur during the transport cycle.

Serotonin transporter mutations associated with obsessive-compulsive disorder and phosphorylation alter binding affinity for inhibitors

Mutants of serotonin transporter that are altered in their regulation by cGMP were tested for the ability of cocaine and the antidepressant drugs imipramine, sertraline, citalopram and fluoxetine to inhibit serotonin transport. Mutation at Ile-425 to valine, found in some patients with obsessive-compulsive disorder, altered the response of SERT to cGMP (Kilic, F., Murphy, D.L., Rudnick, G., 2003. A human serotonin transporter mutation causes constitutive activation of transport activity. Mol. Pharmacol. 64, 440-446). This mutation selectively decreased the potency of sertraline for inhibiting serotonin transport. The potencies of imipramine, citalopram, fluoxetine and cocaine for inhibiting transport were not affected by this mutation. In binding measurements with the cocaine analog 2beta-carbomethoxy-3beta-(4-[(125)I]-iodophenyl)-tropane (beta-CIT), sertraline potency was reduced by the I425V mutation but citalopram potency was unchanged. Mutation at the site of cGMP-dependent phosphorylation, Thr-276, decreased the potency of each of the drugs tested. This effect was also observed in studies with beta-CIT where both citalopram and sertraline were less potent at displacing this high-affinity ligand. These results support an influence of Thr-276 on the conformation of inhibitor binding sites of serotonin transporter, and also suggest that the sertraline binding site contains unique determinants that are not shared with the other tested inhibitors.

Crystal structure of a bacterial homologue of Na+/Cl--dependent neurotransmitter transporters

Na+/Cl--dependent transporters terminate synaptic transmission by using electrochemical gradients to drive the uptake of neurotransmitters, including the biogenic amines, from the synapse to the cytoplasm of neurons and glia. These transporters are the targets of therapeutic and illicit compounds, and their dysfunction has been implicated in multiple diseases of the nervous system. Here we present the crystal structure of a bacterial homologue of these transporters from Aquifex aeolicus, in complex with its substrate, leucine, and two sodium ions. The protein core consists of the first ten of twelve transmembrane segments, with segments 1-5 related to 6-10 by a pseudo-two-fold axis in the membrane plane. Leucine and the sodium ions are bound within the protein core, halfway across the membrane bilayer, in an occluded site devoid of water. The leucine and ion binding sites are defined by partially unwound transmembrane helices, with main-chain atoms and helix dipoles having key roles in substrate and ion binding. The structure reveals the architecture of this important class of transporter, illuminates the determinants of substrate binding and ion selectivity, and defines the external and internal gates.

Cysteine-scanning mutagenesis of serotonin transporter intracellular loop 2 suggests an alpha-helical conformation

Like other proteins involved in neurotransmitter transport, serotonin transporter (SERT) activity is regulated by multiple intracellular signal transduction pathways. The second intracellular loop (IL2) of SERT contains consensus sequences for cGMP-dependent protein kinase and protein kinase C. A 24-residue region of SERT including IL2, from Ile-270 through Ser-293, was analyzed by cysteine-scanning mutagenesis and chemical modification. 2-(Aminoethyl)methanethiosulfonate hydrobromide (MTSEA) failed to inhibit serotonin transport or binding of the cocaine analog 2beta-carbomethoxy-3beta-(4-[125I]iodophenyl)tropane (beta-CIT) in intact cells expressing these mutants, but it inactivated beta-CIT binding in membrane preparations. From the pattern of sensitivity, IL2 appears to extend from Trp-271 through Ile-290, a significantly longer region than that initially predicted by hydropathy analysis. Six mutants reacted with MTSEA much faster than the others, and the pattern of the more reactive mutations suggested that IL2 is in an alpha-helical conformation. Some of the mutants had significantly elevated transport rates, suggesting a possible mechanism for the regulation of SERT activity.

Conformationally sensitive residues in extracellular loop 5 of the Na+/dicarboxylate co-transporter

The Na+/dicarboxylate co-transporter, NaDC-1, from the kidney and small intestine, transports three sodium ions together with one divalent anion substrate, such as succinate2-. A previous study (Pajor, A. M. (2001) J. Biol. Chem. 276, 29961-29968), identified four amino acids, Ser-478, Ala-480, Ala-481, and Thr-482, near the extracellular end of transmembrane helix (TM) 9 that are likely to form part of the permeation pathway of the transporter. All four cysteine-substituted mutants were sensitive to inhibition by the membrane-impermeant reagent [2-(trimethylammonium)ethyl]-methanethiosulfonate (MTSET) and protected by substrate. In the present study, we continued the cysteine scan through extracellular loop 5 and TM10, from Thr-483 to Val-528. Most cysteine substitutions were well tolerated, although cysteine mutations of some residues, particularly within the TM, produced proteins that were not expressed on the plasma membrane. Six residues in the extracellular loop (Thr-483, Thr-484, Leu-485, Leu-487, Ile-489, and Met-493) were sensitive to chemical labeling by MTSET, depending on the conformational state of the protein. Transport inhibition by MTSET could be prevented by substrate regardless of temperature, suggesting that the likely mechanism of substrate protection is steric hindrance rather than large-scale conformational changes associated with translocation. We conclude that extracellular loop 5 in NaDC-1 appears to have a functional role, and it is likely to be located in or near the substrate translocation pore in the protein. Conformational changes in the protein affect the accessibility of the residues in extracellular loop 5 and provide further evidence of large-scale changes in the structure of NaDC-1 during the transport cycle.

Cysteine residues in the organic anion transporter mOAT1

Mouse organic anion transporter 1 (mOAT1) belongs to a family of organic anion transporters, which play critical roles in the body disposition of clinically important drugs, including anti-HIV therapeutics, anti-tumour drugs, antibiotics, anti-hypertensives and anti-inflammatories. mOAT1-mediated transport of organic anion PAH ( p -aminohippurate) in HeLa cells was inhibited by the cysteine-modifying reagent PCMBS (p-chloromercuribenzenesulphonate). Therefore the role of cysteine residues in the function of mOAT1 was examined by site-directed mutagenesis. All 13 cysteine residues in mOAT1 were replaced by alanine, singly or in combination. Single replacement of these residues had no significant effect on mOAT1-mediated PAH transport, indicating that no individual cysteine residue is necessary for function. Multiple replacements at a C-terminal region (C335/379/427/434A; Cys(335/379/427/434)-->Ala) resulted in a substantial decrease in transport activity. A simultaneous replacement of all 13 cysteine residues (C-less) led to a complete loss of transport function. The decreased or lack of transport activity of the mutants C335/379/427/434A and C-less was due to the impaired trafficking of the mutant transporters to the cell surface. These results suggest that although cysteine residues are not required for function in mOAT1, their presence appears to be important for the targeting of the transporter to the plasma membrane. We also showed that, although all cysteine mutants of mOAT1 were sensitive to the inhibition by PCMBS, C49A was less sensitive than the wild-type mOAT1, suggesting that the modification of Cys49 may play a role in the inhibition of mOAT1 by PCMBS.

Role of Glycosylation in the Organic Anion Transporter OAT1

Organic anion transporters (OAT) play essential roles in the body disposition of clinically important anionic drugs, including antiviral drugs, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. We reported previously (Kuze, K., Graves, P., Leahy, A., Wilson, P., Stuhlmann, H., and You, G. (1999) J. Biol. Chem. 274, 1519-1524) that tunicamycin, an inhibitor of asparagine-linked glycosylation, significantly inhibited organic anion transport in COS-7 cells expressing a mouse organic anion transporter (mOAT1), suggesting an important role of glycosylation in mOAT1 function. In the present study, we investigated the effect of disrupting putative glycosylation sites in mOAT1 as well as its human counterpart, hOAT1, by mutating asparagine to glutamine and assessing mutant transporters in HeLa cells. We showed that the putative glycosylation site Asp-39 in mOAT1 was not glycosylated but the corresponding site (Asp-39) in hOAT1 was glycosylated. Disrupting Asp-39 resulted in a complete loss of transport activity in both mOAT1 and hOAT1 without affecting their cell surface expression, suggesting that the loss of function is not because of deglycosylation of Asp-39 per se but rather is likely because of the change of this important amino acid critically involved in the substrate binding. Single replacement of asparagines at other sites had no effect on transport activity indicating that glycosylation at individual sites is not essential for OAT function. In contrast, a simultaneous replacement of all asparagines in both mOAT1 and hOAT1 impaired the trafficking of the transporters to the plasma membrane. In summary, we provided the evidence that 1) Asp-39 is crucially involved in substrate recognition of OAT1, 2) glycosylation at individual sites is not required for OAT1 function, and 3) glycosylation plays an important role in the targeting of OAT1 onto the plasma membrane. This study is the first molecular identification and characterization of glycosylation of OAT1 and may provide important insights into the structure-function relationships of the organic anion transporter family.

Uptake inhibitors but not substrates induce protease resistance in extracellular loop two of the dopamine transporter

Changes in protease sensitivity of extracellular loop two (EL2) of the dopamine transporter (DAT) during inhibitor and substrate binding were examined using trypsin proteolysis and epitope-specific immunoblotting. In control rat striatal membranes, proteolysis of DAT in a restricted region of EL2 was produced by 0.001 to 10 microg/ml trypsin. However, in the presence of the dopamine uptake blockers [2-(diphenylmethoxyl) ethyl]-4-(3phenylpropyl) piperazine (GBR 12909), mazindol, 2beta-carbomethoxy-3beta-(4-flourophenyl)tropane (beta-CFT), nomifensine, benztropine, or (-)-cocaine, 100- to 1000-fold higher concentrations of trypsin were required to produce comparable levels of proteolysis. Protease resistance induced by ligands was correlated with their affinity for DAT binding, was not observed with Zn2+, (+)-cocaine, or inhibitors of norepinephrine or serotonin transporters, and was not caused by altered catalytic activity of trypsin. Together, these results support the hypothesis that the interaction of uptake inhibitors with DAT induces a protease-resistant conformation in EL2. In contrast, binding of substrates did not induce protease resistance in EL2, suggesting that substrates and inhibitors interact with DAT differently during binding. To assess the effects of EL2 proteolysis on DAT function, the binding and transport properties of trypsin-digested DAT were assayed with [3H]CFT and [3H]dopamine. Digestion decreased the Bmax for binding and the Vmax for uptake in amounts that were proportional to the extent of proteolysis, indicating that the structural integrity of EL2 is required for maintenance of both DAT binding and transport functions. Together this data provides novel information about inhibitor and substrate interactions at EL2, possibly relating the protease resistant DAT conformation to a mechanism of transport inhibition.

Role of proline residues in the expression and function of the human noradrenaline transporter

The aim was to investigate the roles of proline residues in extracellular loop 2 (P172, P183, P188 and P209) and transmembrane domains 2, 5, 11 and 12 (P108, P270, P526, P551, P552 and P570) in determining noradrenaline transporter (NET) expression and function. Mutants of human NET with these residues mutated to alanine were pharmacologically characterized. Mutation of P108, P270 and P526 disrupted cell surface expression, from [3H]nisoxetine binding and confocal microscopy data. Mutations of P526, P551 and P570 reduced transporter turnover (Vmax of [3H]noradrenaline uptake/Bmax of [3H]nisoxetine binding) by 1.5-1.7-fold compared with wild-type NET, so these residues might be involved in conformational changes associated with substrate translocation. Conversely, mutations of P172, P183, P188 and P209 increased Vmax/Bmax by 2-3-fold compared with wild-type, indicating that the presence of these proline residues limits turnover of the NET. The mutations had few effects on apparent affinities of substrates or affinities of inhibitors, except decreases in inhibitor affinities after mutations of the P270 and P570 residues, and increases after mutation of the P526 residue. Hence, proline residues in extracellular loop 2 and in transmembrane domains have a range of roles in determining expression and function of the NET.

Modulation of Na,K-ATPase by the gamma subunit: studies with transfected cells and transmembrane mimetic peptides

The enzymatic activity of the Na,K-ATPase, or sodium pump, is modulated by members of the so-called FXYD family of transmembrane proteins. The best characterized member, FXYD2, also referred to as the gamma subunit, has been shown to decrease the apparent Na+ affinity and increase the apparent ATP affinity of the pump. The effect on ATP affinity had been ascribed to the cytoplasmic C-terminal end of the protein, whereas recent observations suggest that the transmembrane (TM) segment of gamma mediates the Na+ affinity effect. Here we use a novel approach involving synthetic transmembrane mimetic peptides to demonstrate unequivocally that the TM domain of gamma effects the shift in apparent Na+ affinity. Specifically, we show that incubation of these peptides with membranes containing alphabeta pumps modulates Na+ affinity in a manner similar to transfected full-length gamma subunit. Using mutated gamma peptides and transfected proteins, we also show that a specific glycine residue, Gly-41, which is associated with a form of familial renal hypomagnesemia when mutated to Arg, is important for this kinetic effect, whereas Gly-35, located on an alternate face of the transmembrane helix, is not. The peptide approach allows for the analysis of mutants that fail to be expressed in a transfected system.

Probing conformational changes in neurotransmitter transporters: a structural context

The Na+/Cl-dependent neurotransmitter transporters, a family of proteins responsible for the reuptake of neurotransmitters and other small molecules from the synaptic cleft, have been the focus of intensive research in recent years. The biogenic amine transporters, a subset of this larger family, are especially intriguing as they are the targets for many psychoactive compounds, including cocaine and amphetamines, as well as many antidepressants. In the absence of a high-resolution structure for any transporter in this family, research into the structure-function relationships of these transporters has relied on analysis of the effects of site-directed mutagenesis as well as of chemical modification of reactive residues. The aim of this review is to establish a structural context for the experimental study of these transporters through various computational approaches and to highlight what is known about the conformational changes associated with function in these transporters. We also present a novel numbering scheme to assist in the comparison of aligned positions between sequences of the neurotransmitter transporter family, a comparison that will be of increasing importance as additional experimental data is amassed.

Creatine transporter activity and content in the rat heart supplemented by and depleted of creatine

The intracellular creatine concentration is an important bioenergetic parameter in cardiac muscle. Although creatine uptake is known to be via a NaCl-dependent creatine transporter (CrT), its localization and regulation are poorly understood. We investigated CrT kinetics in isolated perfused hearts and, by using cardiomyocytes, measured CrT content at the plasma membrane or in total lysates. Rats were fed control diet or diet supplemented with creatine or the creatine analog beta-guanidinopropionic acid (beta-GPA). Creatine transport in control hearts followed saturation kinetics with a K(m) of 70 +/- 13 mM and a V(max) of 3.7 +/- 0.07 nmol x min(-1) x g wet wt(-1). Creatine supplementation significantly decreased the V(max) of the CrT (2.7 +/- 0.17 nmol x min(-1) x g wet wt(-1)). This was matched by an approximately 35% decrease in the plasma membrane CrT; the total CrT pool was unchanged. Rats fed beta-GPA exhibited a >80% decrease in tissue creatine and increase in beta-GPA(total). The V(max) of the CrT was increased (6.0 +/- 0.25 nmol x min(-1) x g wet wt(-1)) and the K(m) decreased (39.8 +/- 3.0 mM). The plasma membrane CrT increased about fivefold, whereas the total CrT pool remained unchanged. We conclude that, in heart, creatine transport is determined by the content of a plasma membrane isoform of the CrT but not by the total cellular CrT pool.

The role of the conserved GXXXRXG motif in the expression and function of the human norepinephrine transporter

Highly conserved motifs in the monoamine transporters, e.g. the human norepinephrine transporter (hNET) GXXXRXG motif which was the focus of the present study, are likely to be important structural features in determining function. This motif was investigated by mutating the glycines to glutamate (causing loss of function) and alanine, and the arginine to glycine. The effects of hG117A, hR121G and hG123A mutations on function were examined in COS-7 cells and compared to hNET. Substrate K(m) values were decreased for hG117A and hG123A, and their K(i) values for inhibition of [3H]nisoxetine binding were decreased 3-4-fold and 4-6-fold, respectively. Transporter turnover was reduced to 65% of hNET for hG117A and hR121G and to 28% for hG123A, suggesting that substrate translocation is impaired. K(i) values of nisoxetine and desipramine for inhibition of [3H]norepinephrine uptake were increased by 5-fold for hG117A, with no change for cocaine. The K(i) value of cocaine was increased by 3-fold for hG123A, with no change for nisoxetine and desipramine. However, there were no effects of the mutations on the K(d) of [3H]nisoxetine binding or K(i) values of desipramine or cocaine for inhibition of [3H]nisoxetine binding. Hence, glycine residues of the GXXXRXG motif are important determinants of NET expression and function, while the arginine residue does not have a major role. This study also showed that antidepressants and psychostimulants have different NET binding sites and provided the first evidence that different sites on the NET are involved in the binding of inhibitors and their competitive inhibition of substrate uptake.

A cocaine insensitive chimeric insect serotonin transporter reveals domains critical for cocaine interaction

Serotonin transporters are key target sites for clinical drugs and psychostimulants, such as fluoxetine and cocaine. Molecular cloning of a serotonin transporter from the central nervous system of the insect Manduca sexta enabled us to define domains that affect antagonist action, particularly cocaine. This insect serotonin transporter transiently expressed in CV-1 monkey kidney cells exhibits saturable, high affinity Na+ and Cl- dependent serotonin uptake, with estimated Km and Vmax values of 436 +/- 19 nm and 3.8 +/- 0.6 x 10-18 mol.cell.min-1, respectively. The Manduca high affinity Na+/Cl- dependent transporter shares 53% and 74% amino acid identity with the human and fruit fly serotonin transporters, respectively. However, in contrast to serotonin transporters from these two latter species, the Manduca transporter is inhibited poorly by fluoxetine (IC50 = 1.23 micro m) and cocaine (IC50 = 12.89 micro m). To delineate domains and residues that could play a role in cocaine interaction, the human serotonin transporter was mutated to incorporate unique amino acid substitutions, detected in the Manduca homologue. We identified a domain in extracellular loop 2 (amino acids 148-152), which, when inserted into the human transporter, results in decreased cocaine sensitivity of the latter (IC50 = 1.54 micro m). We also constructed a number of chimeras between the human and Manduca serotonin transporters (hSERT and MasSERT, respectively). The chimera, hSERT1-146/MasSERT106-587, which involved N-terminal swaps including transmembrane domains (TMDs) 1 and 2, was remarkably insensitive to cocaine (IC50 = 180 micro m) compared to the human (IC50 = 0.431 micro m) and Manduca serotonin transporters. The chimera MasSERT1-67/hSERT109-630, which involved only the TMD1 swap, showed greater sensitivity to cocaine (IC50 = 0.225 micro m) than the human transporter. Both chimeras showed twofold higher serotonin transport affinity compared to human and Manduca serotonin transporters. Our results show TMD1 and TMD2 affect the apparent substrate transport and antagonist sensitivity by possibly providing unique conformations to the transporter. The availability of these chimeras facilitates elucidation of specific amino acids involved in interactions with cocaine.

New creatine transporter assay and identification of distinct creatine transporter isoforms in muscle

Despite the pivotal role of creatine (Cr) and phosphocreatine (PCr) in muscle metabolism, relatively little is known about sarcolemmal creatine transport, creatine transporter (CRT) isoforms, and subcellular localization of the CRT proteins. To be able to quantify creatine transport across the sarcolemma, we have developed a new in vitro assay using rat sarcolemmal giant vesicles. The rat giant sarcolemmal vesicle assay reveals the presence of a specific high-affinity and saturable transport system for Cr in the sarcolemma (Michaelis-Menten constant 52.4 +/- 9.4 microM and maximal velocity value 17.3 +/- 3.1 pmol x min(-1) x mg vesicle protein(-1)), which cotransports Cr into skeletal muscle together with Na(+) and Cl(-) ions. The regulation of Cr transport in giant vesicles by substrates, analogs, and inhibitors, as well as by phorbol 12-myristate 13-acetate and insulin, was studied. Two antibodies raised against COOH- and NH(2)-terminal synthetic peptides of CRT sequences both recognize two major polypeptides on Western blots with apparent molecular masses of 70 and 55 kDa, respectively. The highest CRT expression occurs in heart, brain, and kidney, and although creatine kinase is absent in liver cells, CRT is also found in this tissue. Surprisingly, immunofluorescence staining of cultured adult rat heart cardiomyocytes with specific anti-CRT antibodies, as well as cell fractionation and cell surface biotinylation studies, revealed that only a minor CRT species with an intermediate molecular mass of approximately 58 kDa is present in the sarcolemma, whereas the previously identified major CRT-related protein species of 70 and 55 kDa are specifically located in mitochondria. Our studies indicate that mitochondria may represent a major compartment of CRT localization, thus providing a new aspect to the current debate about the existence and whereabouts of intracellular Cr and PCr compartments that have been inferred from [(14)C]PCr/Cr measurements in vivo as well as from recent in vivo NMR studies.

Distinct regulatory effects of the Na,K-ATPase gamma subunit

The two variants of the gamma subunit of the rat renal sodium pump, gamma(a) and gamma(b), have similar effects on the Na,K-ATPase. Both increase the affinity for ATP due to a shift in the enzyme's E(1) <--> E(2) conformational equilibrium toward E(1). In addition, both increase K(+) antagonism of cytoplasmic Na(+) activation. To gain insight into the structural basis for these distinct effects, extramembranous N-terminal and C-terminal mutants of gamma were expressed in rat alpha1-transfected HeLa cells. At the N terminus, the variant-distinct region was deleted (gammaNDelta7) or replaced by alanine residues (gammaN7A). At the C terminus, four (gamma(a)CDelta4) or ten (gamma(a)CDelta10) residues were deleted. None of these mutations abrogates the K(+)/Na(+) antagonism as evidenced in a similar increase in K'(Na) seen at high (100 mm) K(+) concentration. In contrast, the C-terminal as well as N-terminal deletions (gammaNDelta7, gamma(a)CDelta4, and gamma(a)CDelta10) abolished the decrease in K'(ATP) seen with wild-type gamma(a) or gamma(b). It is concluded that different regions of the gamma chain mediate the distinct functional effects of gamma, and the effects can be long-range. In the transmembrane region, the impact of G41R replacement was analyzed since this mutation is associated with autosomal dominant renal Mg(2+)-wasting in man (Meij, I. C., Koenderink, J. B., van Bokhoven, H., Assink, K. F. H., Groenestege, W. T., de Pont, J. J. H. H. M., Bindels, R. J. M., Monnens, L. A. H., Van den Heuvel, L. P. W. J., and Knoers, N. V. A. M. (2000) Nat. Genet. 26, 265-266). The results show that Gly-41 --> Arg prevents trafficking of gamma but not alphabeta pumps to the cell surface and abrogates functional effects of gamma on alphabeta pumps. These findings underscore a potentially important role of gamma in affecting solute transport, in this instance Mg(2+) reabsorption, consequent to its primary effect on the sodium pump.

Tyrosine residue 271 of the norepinephrine transporter is an important determinant of its pharmacology

The aim was to examine the functional importance in the norepinephrine transporter (NET) of (i) the phenylalanine residue at position 531 in transmembrane domain (TMD) 11 by mutating it to tyrosine in the rat (rF531Y) and human (hF531Y) NETs and (ii) the highly conserved tyrosine residues at positions 249 in TMD 4 of human NET (hNET) (mutated to alanine: hY249A) and 271 in TMD 5, by mutating to alanine (hY271A), phenylalanine (hY271F) and histidine (hY271H). The effects of the mutations on NET function were examined by expressing the mutant and wildtype NETs in COS-7 cells and measuring the K(m) and V(max) for uptake of the substrates, [3H]norepinephrine, [3H]MPP(+) and [3H]dopamine, the K(D) and B(max) for [3H]nisoxetine binding and the K(i) of the inhibitors, nisoxetine, desipramine and cocaine, for inhibition of [3H]norepinephrine uptake. The K(m) values of the substrates were lower for the mutants at amino acid 271 than hNET and unaffected for the other mutants, and each mutant had a significantly lower V(max) than NET for substrate uptake. The mutations at position 271 caused an increase in the K(i) or K(D) values of nisoxetine, desipramine and cocaine, but there were no effects for the other mutations. Hence, the 271 tyrosine residue in TMD 5 is an important determinant of NET function, with the mutants showing an increase in the apparent affinities of substrates and a decrease in the apparent affinities of inhibitors, but the 249 tyrosine and 531 phenylalanine residues do not have a major role in determining NET function.

Species-scanning mutagenesis of the serotonin transporter reveals residues essential in selective, high-affinity recognition of antidepressants

The serotonin transporter (SERT) is a high-affinity sodium/chloride-dependent neurotransmitter transporter responsible for reuptake of serotonin from the extracellular space. SERT is a selective target of several clinically important antidepressants. In a cross-species analysis comparing human and bovine SERTs, the kinetic parameters for serotonin uptake were found to be similar, however, the pharmacological profiles of the two transporters differ. Following transient expression in COS-1 cells, IC(50) values were determined for several antidepressants and psychostimulants. The potencies of the antidepressants citalopram, fluoxetine, paroxetine and imipramine were several-fold higher at hSERT compared with bSERT. No species selectivity was observed for the antidepressants fluvoxamine, and sertraline or for the psychostimulants cocaine, the cocaine analogue beta-carbomethoxy-3beta-(4-iodophenyl)tropane, or for 3,4-methylenedioxymethamphetamine (MDMA). Analysis of six hSERT/bSERT chimeras and subsequent species-scanning mutagenesis of each isoform revealed methionine-180, tyrosine-495, and phenylalanine-513 to be responsible for the increase in citalopram and paroxetine potencies at hSERT and methionine-180 and phenylalanine-513 to confer species selectivity at hSERT for fluoxetine and imipramine. Results were obtained by doing the forward, bovine to human, mutations and confirmed by doing the reverse mutations. Citalopram analogues were used to define the roles of methionine-180, tyrosine-495, and phenylalanine-513 and to reveal molecular interactions with individual functional groups of citalopram. We suggest that methionine-180 interacts with the heterocyclic nucleus of citalopram or stabilizes the binding pocket and phenylalanine-513 to be a steric blocker of antidepressant recognition.

Down-regulation of the rat serotonin transporter upon exposure to a selective serotonin reuptake inhibitor

The serotonin transporter (SERT) terminates serotonergic neurotransmission by rapid reuptake of 5-hydroxytryptamine (5-HT) into the nerve terminal or axonal varicosities. SERT represents the target of various antidepressants which inhibit 5-HT transport and are widely used for the pharmacotherapy of depression. Here, we have analyzed the function of SERT stably expressed in HEK 293 cells upon exposure to citalopram, a selective serotonin reuptake inhibitor (SSRI), with respect to 5-HT transport activity and protein expression as estimated by ligand binding experiments. Our results show that long-term exposure to an SSRI causes a down-regulation of transport activity as revealed by a reduction of the maximal transport rate, without affecting substrate affinity, accompanied by a decrease in ligand binding sites.

The transport activity of the Na+-Ca2+ exchanger NCX1 expressed in HEK 293 cells is sensitive to covalent modification of intracellular cysteine residues by sulfhydryl reagents

Membrane permeable N-ethylmaleimide (NEM) and (2-aminoethyl)methanethiosulfonatehydrobromide (MTSEA) inhibited the rat brain Na(+)-Ca(2+) exchanger RBE-2 (NCX1.5) expressed in HEK 293 cells in a dose dependent manner. 50% inhibition was obtained at 1 mm MTSEA and 1.65 mm NEM. External application of membrane impermeable [2-(trimethylammonium) ethyl]methanethiosulfonatebromide (MTSET) and sodium(2-sulfonatoethyl)methanethiosulfonate (MTSES) did not inhibit the transport activity in whole cells. Following reconstitution, however, of RBE-2 transfected cell proteins into proteoliposomes, external application of MTSET and MTSES led to a decrease in transport activity to 42.7 (S.D. = 9.1) and 51% (S.D. = 10.14), respectively. Similar results were obtained also when the rat heart isoform RHE-1 (NCX1.1) or the rat brain isoform RBE-1 (NCX1.4) was expressed. NEM and MTSEA inhibited Na(+) gradient-dependent Ca(2+) uptake also in HEK 293 cells expressing RBE-2/C14A/C20S/ C122S/C780S (numbering corresponds to RBE-2), a mutant in which all putative extracellular cysteines were exchanged. To study the accessibility of different cysteines to covalent modification, surface biotinylation of cells expressing the wild type exchanger and its mutants was carried out using 3-(N-maleimidylpropionyl)biocytin. Surface biotinylation revealed immunoreactive protein derived from the wild type Na(+)-Ca(2+) exchanger only if the transfected cells were exposed to the reducing agent Tris(2-carboxyethyl)phosphine. No reduction was needed when the single cysteine mutants of RBE-2, C14A, C20S, and C780S, were expressed. Treatment of the cells expressing these mutants with MTSET before biotinylation, led to a decrease in the amount of exchanger protein that was revealed. No immunoreactive protein was detected when the quadruple mutant RBE-2, C14A/C20S/C122S/C780S, was biotinylated, suggesting that no additional cysteines are accessible directly from the extracellular face of the membrane. Permeabilizing the cells expressing RBE-2/C14A/C20S/ C122S/C780S with streptolysin O resulted in biotinylation of the exchanger protein. Its amount decreased if exposure to NEM preceded streptolysin O treatment. Our results suggest that Na(+)-Ca(2+) exchange activity is inhibited by covalent modification with sulfhydryl reagents of cysteine residues that are accessible from the cytoplasmic face of the membrane.

Determination of residues in the norepinephrine transporter that are critical for tricyclic antidepressant affinity

The norepinephrine (NET) and dopamine (DAT) transporters are highly homologous proteins, displaying many pharmacological similarities. Both transport dopamine with higher affinity than norepinephrine and are targets for the psychostimulants cocaine and amphetamine. However, they strikingly contrast in their affinities for tricyclic antidepressants (TCA). Previous studies, based on chimeric proteins between DAT and NET suggest that domains ranging from putative transmembrane domain (TMD) 5 to 8 are involved in the high affinity binding of TCA to NET. We substituted 24 amino acids within this region in the human NET with their counterparts in the human DAT, resulting in 22 different mutants. Mutations of residues located in extra- or intracytoplasmic loops have no effect on binding affinity of neither TCA nor cocaine. Three point mutations in TMD6 (F316C), -7 (V356S), and -8 (G400L) induced a loss of TCA binding affinity of 8-, 5-, and 4-fold, respectively, without affecting the affinity of cocaine. The triple mutation F316C/V356S/G400L produced a 40-fold shift in desipramine affinity. These three residues are strongly conserved in all TCA-sensitive transporters cloned in mammalian and nonmammalian species. A strong shift in TCA affinity (IC(50)) was also observed for double mutants F316C/D336T (35-fold) and S399P/G400L (80-fold for nortriptyline and 1000-fold for desipramine). Reverse mutations P401S/L402G in hDAT did not elicit any gain in TCA affinities, whereas C318F and S358V resulted in a 3- and 10-fold increase in affinity, respectively. Our results clearly indicate that two residues located in TMD6 and -7 of hNET may play an important role in TCA interaction and that a critical region in TMD8 is likely to be involved in the tertiary structure allowing the high affinity binding of TCA.

Functional role of critical stripe residues in transmembrane span 7 of the serotonin transporter. Effects of Na+, Li+, and methanethiosulfonate reagents

Mutations at critical residue positions in transmembrane span 7 (TM7) of the serotonin transporter affect the Na(+) dependence of transport. It was possible that these residues, which form a stripe along one side of the predicted alpha-helix, formed part of a water-filled pore for Na(+). We tested whether cysteine substitutions in TM7 were accessible to hydrophilic, membrane-impermeant methanethiosulfonate (MTS) reagents. Although all five cysteine-containing mutants tested were sensitive to these reagents, noncysteine control mutants at the same positions were in most cases equally sensitive. In all cases, MTS sensitivity could be traced to changes in accessibility of a native cysteine residue in extracellular loop 1, Cys-109. Moreover, none of the TM7 cysteines reacted with the biotinylating reagent MTSEA-biotin when tested in the C109A background. It is thus unlikely that the critical stripe forms part of a water-filled pore. Instead, studies of the ion dependence of the reaction between Cys-109 and MTS reagents lead to the conclusion that TM7 is involved in propagating conformational changes caused by ion binding, perhaps as part of the translocation mechanism. The critical stripe residues on TM7 probably represent a close contact region between TM7 and one or more other TMs in the transporter's three-dimensional structure.

Serotonin transporter inhibitors: synthesis and binding potency of 2'-methyl- and 3'-methyl-6-nitroquipazine

Racemic 2'-methyl- and 3'-methyl-6-nitroquipazine ligands were selected as targets, synthesized and evaluated at the serotonin transporter employing an in vitro competitive inhibition assay with [3H]paroxetine and rat cortical membrane. The 2'-methyl-6-nitroquipazine was found to be 50 times more potent than the 3'-methyl-substituted counterpart and of comparable potency to the known high affinity agent 5-iodo-6-nitroquipazine.

Cyclosporin A inhibits creatine uptake by altering surface expression of the creatine transporter

The immunosuppressive drug cyclosporin A (CsA) inhibited the hCRT-1 cDNA-induced creatine uptake in Xenopus oocytes and the endogenous creatine uptake in cultured C(2)C(12) muscle cells in a dose- and time-dependent manner. FK506, another potent immunosuppressant, was unable to mimic the effect of CsA suggesting that the inhibitory effect of CsA was specific. To delineate the mechanism underlying, we investigated the effect of CsA on the K(m) and V(max) of creatine transport and also on the cell surface distribution of the creatine transporter. Although CsA treatment did not affect the K(m) (20-24 microm) for creatine, it significantly decreased the V(max) of creatine uptake in both oocytes and muscle cells. CsA treatment reduced the cell surface expression level of the creatine transporter in the muscle cells by approximately 60% without significantly altering its total expression level, and the reduction in the cell surface expression paralleled the decrease in creatine uptake. Taken together, our results suggest that CsA inhibited creatine uptake by altering the surface abundance of the creatine transporter. We propose that CsA impairs the targeting of the creatine transporter by inhibiting the function of an associated cyclophilin, resulting in an apparent loss in surface expression of the creatine transporter. Our results also suggest that prolonged exposure to CsA may result in chronically creatine-depleted muscle, which may be a cause for the development of CsA-associated clinical myopathies in organ transplant patients.

Serotonin, dopamine and norepinephrine transporters in the central nervous system and their inhibitors

An overview is presented on progress made in the research on neuronal transporters of serotonin, dopamine and norepinephrine in the central nervous system. Tools developed by molecular biology, such as expression of cloned transporters, their mutants and chimera in non-neuronal cells offered the opportunity to study the putative domains for binding of substrates and uptake inhibitors and discover factors in the regulation of the transporter function. The study of the distribution of monoamine transporters in human brain became possible by the development of selective radiolabelled transport inhibitors. The relationships between the chemical structure of the uptake inhibitors and the affinity for the monoamine transporters is reported, and the (potential) therapeutic applications of the compounds are discussed.

The accessibility of a novel reentrant loop of the glutamate transporter GLT-1 is restricted by its substrate

The excitatory neurotransmitter glutamate is removed from the synaptic cleft by several related sodium- and potassium-coupled transporters. They thereby restrict the neurotoxicity of this transmitter. Based on the accessibility of single cysteines to the large sulfhydryl reagent 3-N-maleimidyl(propionyl)biocytin, we have proposed a topological model for the astroglial glutamate transporter GLT-1 (Grunewald, M., Bendahan, A. and Kanner, B. I. (1998) Neuron 21, 623-632). Because of several unexpected observations, we have investigated the topological disposition of 19 cysteine residues engineered into a loop proposed to be intracellular. We have probed the accessibility of these cysteines to small and large sulfhydryl reagents. The impermeant hydrophilic sulfhydryl reagent [(2-trimethylammonium)ethyl] methanethiosulfonate inhibits transport activity only at two of these positions, weakly at G365C and potently at A364C. Glutamate and its nontransportable analogue dihydrokainate markedly protect A364C transporters against this impermeant reagent. Using a biotinylated maleimide, we found that, among the 14 mutants tested with it, only A364C is accessible to it from the extracellular side. This, together with our previous observations, indicates that the loop-including amino acid residues 354, 359, 373, and 379-is largely intracellular, but a short region of it forms a reentrant pore-loop-like structure, the accessibility of which is dependent on the conformation of the transporter.