The role of external loop regions in serotonin transport. Loop scanning mutagenesis of the serotonin transporter external domain

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.

Two Lignan Glycosides from Albizia julibrissin Durazz. Noncompetitively Inhibit Serotonin Transporter

Albizia julibrissin Durazz. is one of the most common herbs used for depression and anxiety treatment, but its molecular basis and mechanism of action as an antidepressant or anxiolytic drug are not understood. In this study, we separated and identified two lignan glycosides that inhibit serotonin transporter (SERT) noncompetitively by decreasing Vmax with little change in Km for its fluorescence substrate. In addition, treatment with lignan glycosides did not alter total and cell surface expression levels of the transporter protein. The two compounds decreased the accessibility of a cysteine residue placed in the extracellular substrate permeation pathway by inducing a conformational shift toward an outward-closed state of SERT. These results are consistent with molecular docking for the association of the lignan glycosides to the allosteric site in SERT. The present work supports the proposal that these compounds act on SERT by a novel underlying mechanism of action different from that of conventional antidepressant drugs.

Extracellular loops of the serotonin transporter act as a selectivity filter for drug binding

The serotonin transporter (SERT) shapes serotonergic neurotransmission by retrieving its eponymous substrate from the synaptic cleft. Ligands that discriminate between SERT and its close relative, the dopamine transporter DAT, differ in their association rate constant rather than their dissociation rate. The structural basis for this phenomenon is not known. Here we examined the hypothesis that the extracellular loops 2 (EL2) and 4 (EL4) limit access to the ligand-binding site of SERT. We employed an antibody directed against EL4 (residues 388–400) and the antibody fragments 8B6 scFv (directed against EL2 and EL4) and 15B8 Fab (directed against EL2) and analyzed their effects on the transport cycle of and inhibitor binding to SERT. Electrophysiological recordings showed that the EL4 antibody and 8B6 scFv impeded the initial substrate-induced transition from the outward to the inward-facing conformation but not the forward cycling mode of SERT. In contrast, binding of radiolabeled inhibitors to SERT was enhanced by either EL4- or EL2-directed antibodies. We confirmed this observation by determining the association and dissociation rate of the DAT-selective inhibitor methylphenidate via electrophysiological recordings; occupancy of EL2 with 15B8 Fab enhanced the affinity of SERT for methylphenidate by accelerating its binding. Based on these observations, we conclude that (i) EL4 undergoes a major movement during the transition from the outward to the inward-facing state, and (ii) EL2 and EL4 limit access of inhibitors to the binding of SERT, thus acting as a selectivity filter. This insight has repercussions for drug development.

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.

Phylogenetic relationships and protein modelling revealed two distinct subfamilies of group II HKT genes between crop and model grasses

Molecular evolution of large protein families in closely related species can provide useful insights on structural functional relationships. Phylogenetic analysis of the grass-specific group II HKT genes identified two distinct subfamilies, I and II. Subfamily II was represented in all species, whereas subfamily I was identified only in the small grain cereals and possibly originated from an ancestral gene duplication post divergence from the coarse grain cereal lineage. The core protein structures were highly analogous despite there being no more than 58% amino acid identity between members of the two subfamilies. Distinctly variable regions in known functional domains, however, indicated functional divergence of the two subfamilies. The subsets of codons residing external to known functional domains predicted signatures of positive Darwinian selection potentially identifying new domains of functional divergence and providing new insights on the structural function and relationships between protein members of the two subfamilies.

Addressing the Complexity of Tourette's Syndrome through the Use of Animal Models

Tourette's syndrome (TS) is a neurodevelopmental disorder characterized by fluctuating motor and vocal tics, usually preceded by sensory premonitions, called premonitory urges. Besides tics, the vast majority—up to 90%—of TS patients suffer from psychiatric comorbidities, mainly attention deficit/hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD). The etiology of TS remains elusive. Genetics is believed to play an important role, but it is clear that other factors contribute to TS, possibly altering brain functioning and architecture during a sensitive phase of neural development. Clinical brain imaging and genetic studies have contributed to elucidate TS pathophysiology and disease mechanisms; however, TS disease etiology still is poorly understood. Findings from genetic studies led to the development of genetic animal models, but they poorly reflect the pathophysiology of TS. Addressing the role of neurotransmission, brain regions, and brain circuits in TS disease pathomechanisms is another focus area for preclinical TS model development. We are now in an interesting moment in time when numerous innovative animal models are continuously brought to the attention of the public. Due to the diverse and largely unknown etiology of TS, there is no single preclinical model featuring all different aspects of TS symptomatology. TS has been dissected into its key symptomst hat have been investigated separately, in line with the Research Domain Criteria concept. The different rationales used to develop the respective animal models are critically reviewed, to discuss the potential of the contribution of animal models to elucidate TS disease mechanisms.

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.

Novel structure--function information on biogenic amine transporters revealed by site-directed mutagenesis and alkylation

The study reported by Wenge and Bönisch in this issue provides critical structural information regarding extracellular loop 2 (EL2) of the human norepinephrine transporter (NET). A systematic search among all 10 cysteine and 13 histidine residues in NET led to His222 in EL2 as the target for N-ethylmaleimide: its alkylation interferes with [(3)H]nisoxetine binding, indicating the part of EL2 containing His 222 reaches back into the protein interior where it prevents access by nisoxetine to its binding site. Thus, EL2 in human NET does much more than conformationally assisting substrate translocation. The present study underscores the importance of site-directed mutagenesis approaches to elucidate structural features that cannot be deduced from crystals of homolog proteins. In the case of NET, the closest crystal structure is that of the homolog LeuT, but EL2 is difficult to align with 22 less loop residues in LeuT than in NET. The present results could only be achieved by the systematic mutagenesis study of all cysteines and all histidines in NET.

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.

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.

X-ray structures of LeuT in substrate-free outward-open and apo inward-open states

Neurotransmitter sodium symporters are integral membrane proteins that remove chemical transmitters from the synapse and terminate neurotransmission mediated by serotonin, dopamine, noradrenaline, glycine and GABA (γ-aminobutyric acid). Crystal structures of the bacterial homologue, LeuT, in substrate-bound outward-occluded and competitive inhibitor-bound outward-facing states have advanced our mechanistic understanding of neurotransmitter sodium symporters but have left fundamental questions unanswered. Here we report crystal structures of LeuT mutants in complexes with conformation-specific antibody fragments in the outward-open and inward-open states. In the absence of substrate but in the presence of sodium the transporter is outward-open, illustrating how the binding of substrate closes the extracellular gate through local conformational changes: hinge-bending movements of the extracellular halves of transmembrane domains 1, 2 and 6, together with translation of extracellular loop 4. The inward-open conformation, by contrast, involves large-scale conformational changes, including a reorientation of transmembrane domains 1, 2, 5, 6 and 7, a marked hinge bending of transmembrane domain 1a and occlusion of the extracellular vestibule by extracellular loop 4. These changes close the extracellular gate, open an intracellular vestibule, and largely disrupt the two sodium sites, thus providing a mechanism by which ions and substrate are released to the cytoplasm. The new structures establish a structural framework for the mechanism of neurotransmitter sodium symporters and their modulation by therapeutic and illicit substances.

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.

Transmembrane domain 6 of the human serotonin transporter contributes to an aqueously accessible binding pocket for serotonin and the psychostimulant 3,4-methylene dioxymethamphetamine

The plasma membrane serotonin (5-HT) transporter (SERT, SLC6A4) clears 5-HT after release at nerve termini and is targeted by both antidepressant medications and psychostimulants (e.g. MDMA, cocaine). Homology modeling of human SERT (hSERT), based on high resolution structures of the microbial SLC6 family member LeuT(Aa), along with biochemical studies of wild type and mutant transporters, predicts transmembrane (TM) domains 1, 3, 6, and 8 comprise the 5-HT-binding pocket. We utilized the substituted cysteine accessibility method along with surface and site-specific biotinylation to probe TM6 for aqueous accessibility and differential interactions with 5-HT and psychostimulants. Our results are consistent with TM6 being composed of an aqueous-accessible, alpha-helical extracellular domain (TM6a) that is separated by a central, unwound section from a cytoplasmically localized domain (TM6b) with limited aqueous accessibility. The substitution G338C appears to lock hSERT in an outward-facing conformation that, although accessible to aminoethylmethanethiosulfonate-biotin, 5-HT, and citalopram, is incapable of inward 5-HT transport. Transport of 5-HT by G338C can be partially restored by the TM1 mutation Y95F. With regard to methanethiosulfonate (MTS) inactivation of uptake, TM6a Cys mutants demonstrate Na(+)-dependent [2-(trimethylammonium)ethyl]-MTS sensitivity. Studies with the centrally located substitution S336C reveal features of a common binding pocket for 5-HT and 3,4-methylenedioxymethamphetamine (MDMA). Interestingly, the substitution I333C reveals an MDMA-induced conformation not observed with 5-HT. In the context of prior studies on TM1, our findings document shared and unique features of TM6 contributing to hSERT aqueous accessibility, ligand recognition, and conformational dynamics.

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.

Assessing the validity of current mouse genetic models of obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD is a major cause of disability; however, the genetic factors and pathophysiological mechanisms underlying this complex, heterogeneous disorder remain largely unknown. During the past decade, a number of putative mouse genetic models of OCD have been developed for the purpose of studying the neural mechanisms underlying this disorder and developing novel treatments. This review presents and evaluates these experimental preparations to date. Models using knockout or transgenic approaches, as well as those examining variation in genetically diverse populations, are evaluated and discussed.

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.

Homology Modeling of the Serotonin Transporter: Insights into the Primary Escitalopram-binding Site

The serotonin transporter (SERT) is one of the neurotransmitter transporters that plays a critical role in the regulation of endogenous amine concentrations and therefore is an important target for therapeutic agents affecting the central nervous system. The recently published, high resolution X-ray structure of the closely related amino acid transporter, Aquifex aeolicus leucine transporter (LeuT), provides an opportunity to develop a three-dimensional model of the structure of SERT. We present herein a homology model of SERT using LeuT as the template and containing escitalopram as a bound ligand. Our model explains selectivities known from mutational studies and varying ligand data, which are discussed and illustrated in the paper.

Localization of cocaine analog [125I]RTI 82 irreversible binding to transmembrane domain 6 of the dopamine transporter

The site of cocaine binding on the dopamine transporter (DAT) was investigated using the photoactivatable irreversible cocaine analog [125I]3beta-(p-chlorophenyl)tropane-2beta-carboxylic acid, 4'-azido-3'-iodophenylethyl ester ([125I]RTI 82). The incorporation site of this compound was mapped to transmembrane domains (TMs) 4-6 using epitope-specific immunoprecipitation of trypsin fragments and further localized using cyanogen bromide (CNBr), which hydrolyzes proteins on the C-terminal side of methionine residues. CNBr hydrolysis of [125I]RTI 82-labeled rat striatal and expressed human DATs produced fragments of approximately 5-10 kDa consistent with labeling between Met(271/272) or Met(290) in TM5 to Met(370/371) in TM7. To further define the incorporation site, substitution mutations were made that removed endogenous methionines and inserted exogenous methionines in combinations that would generate labeled CNBr fragments of distinct masses depending on the labeling site. The results obtained were consistent with the presence of TM6 but not TMs 4, 5, or 7 in the labeled fragments, with additional support for these conclusions obtained by epitope-specific immunoprecipitation and secondary digestion of CNBr fragments with endoproteinase Lys-C. The final localization of [125I]RTI 82 incorporation to rat DAT Met(290)-Lys(336) and human DAT I291M to R344M provides positive evidence for the proximity of cocaine binding to TM6. Residues in and near DAT TM6 regulate transport and transport-dependent conformational states, and TM6 forms part of the substrate permeation pathway in the homologous Aquifex aeolicus leucine transporter. Cocaine binding near TM6 may thus overlap the dopamine translocation pathway and function to inhibit TM6 structural rearrangements necessary for transport.

Comparison of the monoamine transporters from human and mouse in their sensitivities to psychostimulant drugs

Background

The plasma membrane neurotransmitter transporters terminate neurotransmissions by the reuptake of the released neurotransmitters. The transporters for the monoamines dopamine, norepinephrine, and serotonin (DAT, NET, and SERT) are targets for several popular psychostimulant drugs of abuse. The potencies of the psychostimulant on the monoamine transporters have been studied by several laboratories. However, there are significant discrepancies in the reported data with differences up to 60-fold. In addition, the drug potencies of the 3 monoamine transporters from mouse have not been compared in the same experiments or along side the human transporters. Further studies and systematic comparisons are needed.

Results

In this study, we compared the potencies of five psychostimulant drugs to inhibit human and mouse DAT, SERT and NET in the same cellular background. The K_I values of cocaine to inhibit the 3 transporters are within a narrow range of 0.2 to 0.7 μM. In comparison, methylphenidate inhibited DAT and NET at around 0.1 μM, while it inhibited SERT at around 100 μM. The order of amphetamine potencies was NET (K_I = 0.07–0.1 μM), DAT (K_I ≈ 0.6 μM), and SERT (K_I between 20 to 40 μM). The results for methamphetamine were similar to those for amphetamine. In contrast, another amphetamine derivative, MDMA (3–4 methylenedioxymethamphetamine), exhibited higher potency at SERT than at DAT. The human and mouse transporters were similar in their sensitivities to each of the tested drugs (K_I values are within 4-fold).

Conclusion

The current and previous studies support the following conclusions: 1) cocaine blocks all 3 monoamine transporters at similar concentrations; 2) methylphenidate inhibits DAT and NET well but a 1000-fold higher concentration of the drug is required to inhibit SERT; 3) Amphetamine and methamphetamine are most potent at NET, while being 5- to 9-fold less potent at DAT, and 200- to 500-fold less potent at SERT; 4) MDMA has moderately higher apparent affinity for SERT and NET than for DAT. The relative potencies of a drug to inhibit DAT, NET and SERT suggest which neurotransmitter systems are disrupted the most by each of these stimulants and thus the likely primary mechanism of drug action.

Serotonin transporter function is an early step in left-right patterning in chick and frog embryos

The neurotransmitter serotonin has been shown to regulate a number of embryonic patterning events in addition to its crucial role in the nervous system. Here, we examine the role of two serotonin transporters, the plasma membrane serotonin transporter (SERT) and the vesicular monoamine transporter (VMAT), in embryonic left-right asymmetry. Pharmacological or genetic inhibitors of either SERT or VMAT specifically randomized the laterality of the heart and viscera in Xenopus embryos. This effect takes place during cleavage stages, and is upstream of the left-sided gene XNR-1. Targeted microinjection of an SERT-dominant negative construct confirmed the necessity for SERT function in embryonic laterality and revealed that the descendants of the right ventral blastomere are the most dependent upon SERT signaling in left-right patterning. Moreover, the importance of SERT and VMAT in laterality is conserved in chick embryos, being upstream of the early left-sided gene Shh. Endogenous transcripts of SERT and VMAT are expressed from the initiation of the primitive streak in chick and are asymmetrically expressed in Hensen's node. Taken together our data characterize two new right-sided markers in chick gastrulation, identify a novel, early component of the left-right pathway in two vertebrate species, and reveal a new biological role for serotonin transport.

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.

Recognition of psychostimulants, antidepressants, and other inhibitors of synaptic neurotransmitter uptake by the plasma membrane monoamine transporters

The plasma membrane monoamine transporters terminate neurotransmission by removing dopamine, norepinephrine, or serotonin from the synaptic cleft between neurons. Specific inhibitors for these transporters, including the abused psychostimulants cocaine and amphetamine and the tricyclic and SSRI classes of antidepressants, exert their physiological effects by interfering with synaptic uptake and thus prolonging the actions of the monoamine. Pharmacological, biochemical, and immunological characterization of the many site-directed, chimeric, and deletion mutants generated for the plasma membrane monoamine transporters have revealed much about the commonalities and dissimilarities between transporter substrate, ion, and inhibitor binding sites. Mutations that alter the binding affinity or substrate uptake inhibition potency of inhibitors by at least 3-fold are the focus of this review. These findings are clarifying the picture regarding substrate uptake inhibitor/transporter protein interactions at the level of the drug pharmacophore and the amino acid residue, information necessary for rational design of novel medications for substance abuse and a variety of psychiatric disorders.

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.

Single Nucleotide Polymorphisms in the Human Norepinephrine Transporter Gene Affect Expression, Trafficking, Antidepressant Interaction, and Protein Kinase C Regulation

The role of norepinephrine (NE) in attention, memory, affect, stress, heart rate, and blood pressure implicates NE in psychiatric and cardiovascular disease. The norepinephrine transporter (NET) mediates reuptake of released catecholamines, thus playing a role in the limitation of signaling strength in the central and peripheral nervous systems. Nonsynonymous single nucleotide polymorphisms (SNPs) in the human NET (hNET) gene that influence transporter function can contribute to disease, such as the nonfunctional transporter, A457P, identified in orthostatic intolerance. Here, we examine additional amino acid variants that have been identified but not characterized in populations that include cardiovascular phenotypes. Variant hNETs were expressed in COS-7 cells and were assayed for protein expression and trafficking using cell-surface biotinylation and Western blot analysis, transport of radiolabeled substrate, antagonist interaction, and regulation through protein kinase C (PKC)-linked pathways by the phorbol ester beta-phorbol-12-myristate-13-acetate. We observed functional perturbations in 6 of the 10 mutants studied. Several variants were defective in trafficking and transport, with the most dramatic effect observed for A369P, which was completely devoid of the fully glycosylated form of transporter protein, was retained intracellularly, and lacked any transport activity. Furthermore, A369P and another trafficking variant, N292T, impeded surface expression of hNET when coexpressed. F528C demonstrated increased transport and, remarkably, exhibited both insensitivity to down-regulation by PKC and a decrease in potency for the tricyclic antidepressant desipramine. These findings reveal functional deficits that are likely to compromise NE signaling in SNP carriers in the population and identify key regions of NET contributing to transporter biosynthesis, activity, and regulation.

Interaction between a hydroxypiperidine analogue of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine and Aspartate 68 in the human dopamine transporter

Compound (+)-R,R-D-84 is an optically active trans-hydroxy-substituted derivative of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine (D-164). As a hydroxypiperidine analog of GBR 12935, (+)-R,R-D-84 is a candidate dopamine transporter compound for the treatment of cocaine dependence. The present work addresses the functional activity of (+)-R,R-D-84 at monoamine transporters and its potential molecular mechanism involving acidic amino acids (D and E). The selectivity for the dopamine vs. serotonin transporter of (+)-R,R-D-84 was greater than that of (-)-S,S-D-83, its enantiomer, and the selectivity of both compounds was greater than that of GBR 12909 (diphenyl-fluorinated GBR 12935). Only (+)-R,R-D-84 displayed improved selectivity vs. the norepinephrine transporter. D313N or E215Q mutation did not alter the pattern of affinities (measured by membrane binding of the cocaine analog [3H]CFT) for the dopamine transporter of (+)-R,R-D-84, (-)-S,S-D-83, D-164 (non-hydroxylated analog), or GBR 12909. In contrast, D68N mutation specifically lowered the affinity of (+)-R,R-D-84, pointing to a role for D68 in the interaction with (+)-R,R-D-84, possibly through hydrogen bonding between the hydroxyl and the carboxyl group of D68 which is lacking in N68. The present results, combined with behavioral data, implicate D68 in the dopamine transporter in cocaine antagonist activity of (+)-R,R-D-84.

Cysteine-scanning mutagenesis of the fifth external loop of serotonin transporter

External loop 5 (EL5) of serotonin transporter was analyzed by mutating each of the residues from Thr-480 to Ala-511, one at a time, with cysteine. Cysteine was well-tolerated at most positions, although G485C, Y495C, and E508C had low transport activities. Replacement with cysteine rendered mutants G484C-P499C sensitive to partial or complete inactivation by [2-(trimethylammonium)ethyl] methanethiosulfonate and (2-sulfonatoethyl) methanethiosulfonate. Within this sensitive region, the rates of reaction varied by over 2 orders of magnitude. Rates of inactivation were not significantly affected by removal of Na(+) or by addition of cocaine or serotonin. These results suggest that modification of EL5 interferes with the transport process but is not sensitive to substrate and ion binding.

Molecular model of the neural dopamine transporter

The dopamine transporter (DAT) regulates the action of dopamine by reuptake of the neurotransmitter into presynaptic neurons, and is the main molecular target of amphetamines and cocaine. DAT and the Na+/H+ antiporter (NhaA) are secondary transporter proteins that carry small molecules across a cell membrane against a concentration gradient, using ion gradients as energy source. A 3-dimensional projection map of the E. coli NhaA has confirmed a topology of 12 membrane spanning domains, and was previously used to construct a 3-dimensional NhaA model with 12 trans-membrane alpha-helices (TMHs). The NhaA model, and site directed mutagenesis data on DAT, were used to construct a detailed 3-dimensional DAT model using interactive molecular graphics and empiric force field calculations. The model proposes a dopamine transport mechanism involving TMHs 1, 3, 4, 5, 7 and 11. Asp79, Tyr252 and Tyr274 were the primary cocaine binding residues. Binding of cocaine or its analogue, (-)-2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT), seemed to lock the transporter in an inactive state, and thus inhibit dopamine transport. The present model may be used to design further experimental studies of the molecular structure and mechanisms of DAT and other secondary transporter proteins.

Zn2+ Inhibits Glycine Transport by Glycine Transporter Subtype 1b

In the central nervous system, glycine is a co-agonist with glutamate at the N-methyl-D-aspartate subtype of glutamate receptors and also an agonist at inhibitory, strychnine-sensitive glycine receptors. The GLYT1 subtypes of glycine transporters (GLYTs) are responsible for regulation of glycine at excitatory synapses, whereas a combination of GLYT1 and GLYT2 subtypes of glycine transporters are used at inhibitory glycinergic synapses. Zn2+ is stored in synaptic vesicles with glutamate in a number of regions of the brain and is believed to play a role in modulation of excitatory neurotransmission. In this study we have investigated the actions of Zn2+ on the glycine transporters, GLYT1b and GLYT2a, expressed in Xenopus laevis oocytes and we demonstrate that Zn2+ is a noncompetitive inhibitor of GLYT1 but has no effect on GLYT2. We have also investigated the molecular basis for these differences and the relationship between the Zn2+ and proton binding sites on GLYT1. Using site-directed mutagenesis, we identified 2 histidine residues, His-243 in the large second extracellular loop (ECL2) and His-410 in the fourth extracellular loop (ECL4), as two coordinates in the Zn2+ binding site of GLYT1b. In addition, our study suggests that the molecular determinants of proton regulation of GLYT1b are localized to the 2 histidine residues (His-410 and His-421) of ECL4. The ability of Zn2+ and protons to regulate the rate of glycine transport by interacting with residues situated in ECL4 of GLYT1b suggests that this region may influence the substrate translocation mechanism.

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.

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.

Effect of sodium lithium and proton concentrations on the electrophysiological properties of the four mouse GABA transporters expressed in Xenopus oocytes

Mouse GABA transporters belong to the family of Na(+) and Cl(-) dependent neurotransmitter transporter. GABA transport, by these family members, was shown to be electrogenic and driven by sodium ions. It was demonstrated that, as in several other transporters, sodium binding and release by GAT1, GAT3 and BGT-1, the canine homolog of GAT2, resulted in the appearance of presteady-state currents. In this work we show that each of the four GABA transporters exhibit unique presteady-state currents when expressed in Xenopus oocytes. The properties of the presteady-state currents correspond to the transporters affinities to Na(+). At 100 mM GAT1 exhibited symmetric presteady-state currents at all imposed potentials, whereas GAT2 exhibited asymmetric presteady-state currents exclusively at negative imposed potentials, GAT3 or GAT4 exhibited presteady-state currents predominantly at positive imposed potentials. GABA uptake by GAT2 and GAT4 was much more sensitive to external pH than GAT1 and GAT3. Reducing the external Na(+) concentration rendered the GABA uptake activity by GAT1 and GAT3 to be sensitive to pH. Lowering the external pH reduced the Na(+) affinity of GAT1. Substitution of the external Na(+) to Li(+) resulted in the appearance of leak currents exclusively at negative potentials in Xenopus oocyte expressing GAT1 and GAT3. Low Na(+) concentrations inhibited the leak currents of GAT1 but Na(+) had little effect on the leak currents of GAT3. Washing of occluded Na(+) in GAT1 enhanced the leak currents. Similarly addition of GABA in the presence of 80 mM Li(+), that presumably accelerated the release of the bound Na(+), also induced the leak currents. Conversely, addition of GABA to GAT3 expressing oocytes, in the presence of 80 mM Li(+), inhibited the leak currents.

A human serotonin transporter mutation causes constitutive activation of transport activity

A rarely occurring variant of human serotonin transporter (hSERT) was tested for its functional consequences in HeLa and COS-7 cells. The variant, in which Ile-425 is converted to Val, was significantly different from wild type with respect to its catalytic properties. In both cell types, rates of serotonin (5-HT) transport were higher for the I425V variant. Both an increase in Vmax and a decrease in KM caused this increase in rate. The increase in Vmax was not accounted for by increases in transporter expression or in the distribution of transporter between the cell surface and intracellular pools. The decrease in KM was accompanied by a decrease in the KD for binding of the cocaine analog 2beta-carbomethoxy-3beta-(4-[125I]iodophenyl)tropane. In both HeLa and COS-7 cells, the nitric oxide donor S-nitroso-N-acetylpenicillamine increased the activity of wild-type hSERT to that of the variant but did not change the activity of the I425V variant. This stimulation was prevented by the presence of oxyhemoglobin, which quenches nitric oxide, and by an inhibitor of guanylyl cyclase.

A mutation in the human norepinephrine transporter gene (SLC6A2) associated with orthostatic intolerance disrupts surface expression of mutant and wild-type transporters

The norepinephrine transporter (NET) mediates reuptake of norepinephrine released from neurons, and, as such, it is an important regulator of noradrenergic neurotransmission. Recently, our laboratory reported a polymorphism in the human NET (hNET) gene A457P in an individual with the autonomic disorder orthostatic intolerance (OI). The presence of the hNET-A457P allele tracked with elevated heart rates and plasma NE levels in family members. hNET-A457P lacks >98% transport activity in several heterologous expression systems. In the present work, Western blot and biotinylation analyses performed in transiently transfected COS-7 cells revealed impairment in processing of hNET-A457P to the fully glycosylated form and a decrease in surface expression to approximately 30% of hNET-wild type (hNET-wt). Because the hNET-A457P mutation is carried on a single allele in OI subjects, we examined the influence of cotransfection of hNET-wt and hNET-A457P and found that hNET-A457P exerts a dominant-negative effect on hNET-wt uptake activity. Experiments to determine oligomerization as a potential mechanism of the dominant-negative effect demonstrated that hNET-A457P coimmunoprecipitates with, and diminishes surface expression of, hNET-wt. These results reveal that hNET-A457P causes a conformational disruption that interferes with transporter biosynthetic progression and trafficking of both the mutant transporter and hNET-wt. These results elucidate a molecular mechanism for the disrupted NE homeostasis and cardiovascular function evident in OI patients with the hNET-A457P mutation.

A second site rescue mutation partially restores functional expression to the serotonin transporter mutant V382P

Transmembrane span 7 (TM7) of the serotonin transporter (SERT) was previously subjected to random mutagenesis, and the mutation V382P was found to abolish transport activity. Val-382 lies next to a threonine residue in the native sequence, creating a TP motif in this mutant. On the basis of molecular modeling studies, which have shown that the presence of a TP motif produces a very large kink in an alpha-helix, it was hypothesized that this motif could be the source of V382P's deleterious effects. We tested this hypothesis by producing second site mutations in the V382P construct that removed the TP motif: T381A-V382P and T381V-V382P. These mutants were tested for the recovery of serotonin transport and binding activities and for expression at the cell surface. The TM7 alpha-helix was modeled computationally, using Biased Monte Carlo simulations to quantify the conformational preferences of the wild type and mutant helices. The double mutation T381A-V382P, which was predicted by modeling to produce a smaller perturbing bend in TM7, was indeed found to allow partial rescue of transport activity. The double mutation T381V-V382P, on the other hand, did not rescue transport activity. Computational analysis of this mutant predicted a markedly different conformational preference from either the V382P or the T381A-V382P mutants. These studies show that changes in the structure of TM7 exert a strong influence on SERT's ability to achieve a mature, properly folded, cell surface conformation.

Differential effect of pH on sodium binding by the various GABA transporters expressed in Xenopus oocytes

Mouse GABA transporters belong to the family of Na(+)- and Cl(-)-dependent neurotransmitter transporters. The four GABA transporters exhibit unique presteady-state currents when expressed in Xenopus oocytes. The properties of the presteady-state currents correspond to their different affinities to Na(+). In the presence of 20 microM GABA and at pH 7.5, the half-maximal uptake activity was 47, 120, 25 and 35 mM Na(+) for GAT1, GAT2, GAT3 and GAT4, respectively. The appearance of presteady-state currents at positive or negative imposed potentials was in correlation with the affinity to Na(+). Changing the external pH differentially affected the GABA uptake and the presteady-state activities of the various GABA transporters. It is suggested that protons compete with Na(+) on its binding site; however, the proton binding is not productive and is unable to drive GABA 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.

Early fluorescence signals detect transitions at mammalian serotonin transporters

The mammalian serotonin transporters rSERT or hSERT were expressed in oocytes and labeled with sulforhodamine-MTS. The endogenous Cys-109 residue contributes most of the signal, and the labeled transporter shows normal function. The SERT fluorescence decreases in the presence of 5-HT and also depends on the inorganic substrates of SERT. The fluorescence also increases with membrane depolarization. During voltage-jump experiments, fluorescence relaxations show little inactivation or history dependence. The fluorescence signal has a voltage dependence similar to that of the prepriming step of the previously described voltage-dependent transient current. However, the fluorescence relaxations are the fastest voltage-dependent events yet studied at SERT; their time constants of approximately 8-30 ms are severalfold faster than the prepriming or inactivation phases of the transient currents. These fluorescence signals are interpreted within the framework of the gate-lumen-gate model. The signals may monitor initial events at the outer gate.

A putative three-dimensional arrangement of the human serotonin transporter transmembrane helices: a tool to aid experimental studies

The human serotonin transporter is the molecular target for selective serotonin reuptake inhibitor drugs which are being used for treatment of depression. A three-dimensional model of the membrane spanning parts of the transporter was constructed. The transporter was assumed to consist of 12 transmembrane alpha-helices. The model was based on published experimental data of cocaine binding to mutant transporters, amino acid sequence analysis, and interactive molecular graphics. The model suggests that a high affinity cocaine binding site is situated in a region of the model where Asp98 acts like an anchor, while a putative low affinity site is situated in another region with Glu508 as the anchoring amino acid. A series of docking experiments with various reuptake inhibitors were conducted, using interactive molecular graphics techniques combined with energy calculations and analysis of the transporter-ligand complexes. Experiments involving molecular mapping of ligand binding areas may benefit from using the current model in experimental design. From the current model, several amino acids were proposed as prime candidates for mutagenesis and subsequent ligand binding studies. Also for evaluation of results from site directed mutagenesis experiments with SERT and similar transporters we assume the model will be helpful.

A conformationally sensitive residue on the cytoplasmic surface of serotonin transporter

Serotonin transporter (SERT) contains a single reactive external cysteine residue at position 109 (Chen, J. G., Liu-Chen, S., and Rudnick, G. (1997) Biochemistry 36, 1479-1486) and seven predicted cytoplasmic cysteines. A mutant of rat SERT (X8C) in which those eight cysteine residues were replaced by other amino acids retained approximately 32% of wild type transport activity and approximately 56% of wild type binding activity. In contrast to wild-type SERT or the C109A mutant, X8C was resistant to inhibition of high affinity cocaine analog binding by the cysteine reagent 2-(aminoethyl)methanethiosulfonate hydrobromide (MTSEA) in membrane preparations from transfected cells. Each predicted cytoplasmic cysteine residue was reintroduced, one at a time, into the X8C template. Reintroduction of Cys-357, located in the third intracellular loop, restored MTSEA sensitivity similar to that of C109A. Replacement of only Cys-109 and Cys-357 was sufficient to prevent MTSEA sensitivity. Thus, Cys-357 was the sole cytoplasmic determinant of MTSEA sensitivity in SERT. Both serotonin and cocaine protected SERT from inactivation by MTSEA at Cys-357. This protection was apparently mediated through a conformational change following ligand binding. Although both ligands bind in the absence of Na(+) and at 4 degrees C, their ability to protect Cys-357 required Na(+) and was prevented at 4 degrees C. The accessibility of Cys-357 to MTSEA inactivation was increased by monovalent cations. The K(+) ion, which is believed to serve as a countertransport substrate for SERT, was the most effective ion for increasing Cys-357 reactivity.

Substrate-induced conformational changes of extracellular loop 1 in the glycine transporter GLYT2

The neurotransmitter glycine is removed from the synaptic cleft by two Na(+)-and Cl(-)-dependent transporters, the glial (GLYT1) and neuronal (GLYT2) glycine transporters. GLYT2 lacks a conserved cysteine in the first hydrophilic loop (EL1) that is reactive to [2-(trimethylammonium)ethyl] methanethiosulfonate (MTSET) in related transporters. A chimeric GLYT2 (GLYT2a-EL1) that contains GLYT1 sequences in this region, including the relevant cysteine, was sensitive to the reagent, and its sensitivity was decreased by co-substrates. We combined cysteine-specific biotinylation to detect transporter-reagent interactions with MTSET inactivation assays and temperature dependence analysis to study the mechanism by which Cl(-), Na(+), and glycine reduce methanethiosulfonate reagent inhibition. We demonstrate a Na(+) protective effect rather than an increased susceptibility to the reagent exerted by Li(+), as reported for the serotonin transporter. The different inhibition, protection, and reactivation properties between GLYT2a-EL1 and serotonin transporter suggest that EL1 is a source of structural heterogeneity involved in the specific effect of lithium on serotonin transport. The protection by Na(+) or Cl(-) on GLYT2a-EL1 was clearly dependent on temperature, suggesting that EL1 is not involved in ion binding but is subjected to ion-induced conformational changes. Na(+) and Cl(-) were required for glycine protection, indicating the necessity of prior ion interaction with the transporter for the binding of glycine. We conclude that EL1 acts as a fluctuating hinge undergoing sequential conformational changes during the transport cycle.

A lithium-induced conformational change in serotonin transporter alters cocaine binding, ion conductance, and reactivity of Cys-109

Inactivation of serotonin transporter (SERT) expressed in HeLa cells by [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) occurred much more readily when Na(+) in the reaction medium was replaced with Li(+). This did not result from a protective effect of Na(+) but rather from a Li(+)-specific increase in the reactivity of Cys-109 in the first external loop of the transporter. Li(+) alone of the alkali cations caused this increase in reactivity. Replacing Na(+) with N-methyl-d-glucamine (NMDG(+)) did not reduce the affinity of cocaine for SERT, as measured by displacement of a high affinity cocaine analog, but replacement of Na(+) with Li(+) led to a 2-fold increase in the K(D) for cocaine. The addition of either cocaine or serotonin (5-HT) protected SERT against MTSET inactivation. When SERT was expressed in Xenopus oocytes, inward currents were elicited by superfusing the cell with 5-HT (in the presence of Na(+)) or by replacing Na(+) with Li(+) but not NMDG(+). MTSET treatment of oocytes in Li(+) but not in Na(+) decreased both 5-HT and Li(+) induced currents, although 5-HT-induced currents were inhibited to a greater extent. Na(+) antagonized the effects of Li(+) on both inactivation and current. These results are consistent with Li(+) inducing a conformational change that exposes Cys-109, decreases cocaine affinity, and increases the uncoupled inward current.

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.

Structural probing of a microdomain in the dopamine transporter by engineering of artificial Zn2+ binding sites

Previously, we have identified three Zn(2+) binding residues in an endogenous Zn(2+) binding site in the human dopamine transporter (hDAT): (193)His in extracellular loop 2 (ECL 2), (375)His at the external end of transmembrane segment (TM) 7, and (396)Glu at the external end of TM 8. Here we have generated a series of artificial Zn(2+) binding sites in a domain situated around the external ends of TMs 7 and 8 by taking advantage of the well-defined structural constraints for binding of the zinc(II) ion. Initially, we found that the Zn(2+)-coordinating (193)His in ECL 2 could be substituted with a histidine inserted at the i - 4 position relative to (375)His in TM 7. In this mutant (H193K/M371H), Zn(2+) potently inhibited [(3)H]dopamine uptake with an IC(50) value of 7 microM as compared to a value of 300 microM for the control (H193K). These data are consistent with the presence of an alpha-helical configuration of TM 7. This inference was further corroborated by the observation that no increase in the apparent Zn(2+) affinity was observed following introduction of histidines at the i - 2, i - 3, and i - 5 positions. In contrast, introduction of histidines at positions i + 2, i + 3, and i + 4 all resulted in potent inhibition of [(3)H]dopamine uptake by Zn(2+) (IC(50) = 3-32 microM). These observations are inconsistent with continuation of the helix beyond position 375 and indicate an approximate boundary between the end of the helix and the succeeding loop. In summary, the data presented here provide new insight into the structure of a functionally important domain in the hDAT and illustrate how engineering of Zn(2+) binding sites can be a useful approach for probing both secondary and tertiary structure relationships in membrane proteins of unknown structure.

Structure and function of the dopamine transporter

The dopamine transporter mediates uptake of dopamine into neurons and is a major target for various pharmacologically active drugs and environmental toxins. Since its cloning, much information has been obtained regarding its structure and function. Binding domains for dopamine and various blocking drugs including cocaine are likely formed by interactions with multiple amino acid residues, some of which are separate in the primary structure but lie close together in the still unknown tertiary structure. Chimera and site-directed mutagenesis studies suggest the involvement of both overlapping and separate domains in the interaction with substrates and blockers, whereas recent findings with sulfhydryl reagents selectively targeting cysteine residues support a role for conformational changes in the binding of blockers such as cocaine. The dopamine transporter can also operate in reverse, i.e. in an efflux mode, and recent mutagenesis experiments show different structural requirements for inward and outward transport. Strong evidence for dopamine transporter domains selectively influencing binding of dopamine or cocaine analogs has not yet emerged, although the development of a cocaine antagonist at the level of the transporter remains a possibility.