Ion-coupled neurotransmitter transport: thermodynamic vs. kinetic determinations of stoichiometry

Thermodynamic first law efficiency of membrane proteins

Proteins are nature's biomolecular machines. Proteins, such as transporters, pumps and motors, have complex function/operating-machinery/mechanisms, comparable to the macro-scaled machines that we encounter in our daily life. These proteins, as it is for their macro-scaled counterparts, convert (part of) other/various forms of energy into work. In this study, we are performing the first law analysis on a set of proteins, including the dopamine transporter, glycine transporters I and II, glutamate transporter, sodium-potassium pump and Ca²⁺ ATPase. Each of these proteins operates on a thermodynamic/mechanic cycle to perform their function. In each of these cycles, they receive energy from a source, convert part of this energy into work and reject the remaining part of the energy to the environment. Conservation of energy principle was applied to the thermodynamic/mechanic cycle of each protein, and thermodynamic first law efficiency was evaluated for each cycle, which shows how much of the energy input per cycle was converted into useful work. Interestingly, calculations based on experimental data indicate that proteins can operate under a range of efficiencies, which vary based on the extracellular and intracellular ion and substrate concentrations. The lowest observed first law efficiency was 50%, which is a very high value if compared to the efficiency of the macro-scaled heat engines we encounter in our daily lives.Communicated by Ramaswamy H. Sarma.

Modification of a Putative Third Sodium Site in the Glycine Transporter GlyT2 Influences the Chloride Dependence of Substrate Transport

Neurotransmitter removal from glycine-mediated synapses relies on two sodium-driven high-affinity plasma membrane GlyTs that control neurotransmitter availability. Mostly glial GlyT1 is the main regulator of glycine synaptic levels, whereas neuronal GlyT2 promotes the recycling of synaptic glycine and supplies neurotransmitter for presynaptic vesicle refilling. The GlyTs differ in sodium:glycine symport stoichiometry, showing GlyT1 a 2:1 and GlyT2 a 3:1 sodium:glycine coupling. Sodium binds to the GlyTs at two conserved Na⁺ sites: Na1 and Na2. The location of GlyT2 Na3 site remains unknown, although Glu650 has been involved in the coordination. Here, we have used comparative MD simulations of a GlyT2 model constructed by homology to the crystalized DAT from Drosophila melanogaster by placing the Na3 ion at two different locations. By combination of in silico and experimental data obtained by biochemical and electrophysiological analysis of GlyTs mutants, we provide evidences suggesting the GlyT2 third sodium ion is held by Glu-250 and Glu-650, within a region with robust allosteric properties involved in cation-specific sensitivity. Substitution of Glu650 in GlyT2 by the corresponding methionine in GlyT1 reduced the charge-to-flux ratio to the level of GlyT1 without producing transport uncoupling. Chloride dependence of glycine transport was almost abolished in this GlyT2 mutant but simultaneous substitution of Glu250 and Glu650 by neutral amino acids rescued chloride sensitivity, suggesting that protonation/deprotonation of Glu250 substitutes chloride function. The differential behavior of equivalent GlyT1 mutations sustains a GlyT2-specific allosteric coupling between the putative Na3 site and the chloride site.

Structural basis for recognition of diverse antidepressants by the human serotonin transporter

Selective serotonin reuptake inhibitors are clinically prescribed antidepressants that act by increasing the local concentrations of neurotransmitters at synapses and in extracellular spaces via blockade of the serotonin transporter. Here we report X-ray structures of engineered thermostable variants of the human serotonin transporter bound to the antidepressants sertraline, fluvoxamine, and paroxetine. The drugs prevent serotonin binding by occupying the central substrate-binding site and stabilizing the transporter in an outward-open conformation. These structures explain how residues within the central site orchestrate binding of chemically diverse inhibitors and mediate transporter drug selectivity.

Effect of Dimerization on the Dynamics of Neurotransmitter:Sodium Symporters

Dimerization is a common feature among the members of the neurotransmitter:sodium symporter (NSS) family of membrane proteins. Yet, the effect of dimerization on the mechanism of action of NSS members is not fully understood. In this study, we examined the collective dynamics of two members of the family, leucine transporter (LeuT) and dopamine transporter (DAT), to assess the significance of dimerization in modulating the functional motions of the monomers. We used to this aim the anisotropic network model (ANM), an efficient and robust method for modeling the intrinsic motions of proteins and their complexes. Transporters belonging to the NSS family are known to alternate between outward-facing (OF) and inward-facing (IF) states, which enables the uptake and release of their substrate (neurotransmitter) respectively, as the substrate is transported from the exterior to the interior of the cell. In both LeuT and DAT, dimerization is found to alter the collective motions intrinsically accessible to the individual monomers in favor of the functional transitions (OF ↔ IF), suggesting that dimerization may play a role in facilitating transport.

Revised Ion/Substrate Coupling Stoichiometry of GABA Transporters

The purpose of this review is to highlight recent evidence in support of a 3 Na⁺: 1 Cl^-: 1 GABA coupling stoichiometry for plasma membrane GABA transporters (SLC6A1 , SLC6A11 , SLC6A12 , SLC6A13 ) and how the revised stoichiometry impacts our understanding of the contribution of GABA transporters to GABA homeostasis in synaptic and extrasynaptic regions in the brain under physiological and pathophysiological states. Recently, our laboratory probed the GABA transporter stoichiometry by analyzing the results of six independent measurements, which included the shifts in the thermodynamic transporter reversal potential caused by changes in the extracellular Na⁺, Cl^-, and GABA concentrations, as well as the ratio of charge flux to substrate flux for Na⁺, Cl^-, and GABA under voltage-clamp conditions. The shifts in the transporter reversal potential for a tenfold change in the external concentration of Na⁺, Cl^-, and GABA were 84 ± 4, 30 ± 1, and 29 ± 1 mV, respectively. Charge flux to substrate flux ratios were 0.7 ± 0.1 charges/Na⁺, 2.0 ± 0.2 charges/Cl^-, and 2.1 ± 0.1 charges/GABA. We then compared these experimental results with the predictions of 150 different transporter stoichiometry models, which included 1-5 Na⁺, 0-5 Cl^-, and 1-5 GABA per transport cycle. Only the 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry model correctly predicts the results of all six experimental measurements. Using the revised 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry, we propose that the GABA transporters mediate GABA uptake under most physiological conditions. Transporter-mediated GABA release likely takes place under pathophysiological or extreme physiological conditions.

Electrophysiological Actions of Synthetic Cathinones on Monoamine Transporters

Products containing psychoactive synthetic cathinones, such as mephedrone and 3,4-methylenedioxypyrovalerone (MDPV) are prevalent in our society. Synthetic cathinones are structurally similar to methamphetamine, and numerous synthetics have biological activity at dopamine, serotonin, and norepinephrine transporters. Importantly, monoamine transporters co-transport sodium ions along with their substrate, and movement of substrates and ions through the transporter can generate measurable ionic currents. Here we review how electrophysiological information has enabled us to determine how synthetic cathinones affect transporter-mediated currents in cells that express these transporters. Specifically, drugs that act as transporter substrates induce inward depolarizing currents when cells are held near their resting membrane potential, whereas drugs that act as transporter blockers induce apparent outward currents by blocking an inherent inward leak current. We have employed the two-electrode voltage-clamp technique in Xenopus laevis oocytes overexpressing monoamine transporters to determine whether synthetic cathinones found in the so-called bath salts products behave as blockers or substrates. We also examined the structure-activity relationships for synthetic cathinone analogs related to the widely abused compound MDPV, a common constituent in "bath salts" possessing potent actions at the dopamine transporter.

Energy landscape of LeuT from molecular simulations

The bacterial sodium-coupled leucine transporter (LeuT) has been broadly used as a structural model for understanding the structure-dynamics-function of mammalian neurotransmitter transporters as well as other solute carriers that share the same fold (LeuT fold), as the first member of the family crystallographically resolved in multiple states: outward-facing open, outward-facing occluded, and inward-facing open. Yet, a complete picture of the energy landscape of (sub)states visited along the LeuT transport cycle has been elusive. In an attempt to visualize the conformational spectrum of LeuT, we performed extensive simulations of LeuT dimer dynamics in the presence of substrate (Ala or Leu) and co-transported Na(+) ions, in explicit membrane and water. We used both conventional molecular dynamics (MD) simulations (with Anton supercomputing machine) and a recently introduced method, collective MD, that takes advantage of collective modes of motions predicted by the anisotropic network model. Free energy landscapes constructed based on ∼40 μs trajectories reveal multiple substates occluded to the extracellular (EC) and/or intracellular (IC) media, varying in the levels of exposure of LeuT to EC or IC vestibules. The IC-facing transmembrane (TM) helical segment TM1a shows an opening, albeit to a smaller extent and in a slightly different direction than that observed in the inward-facing open crystal structure. The study provides insights into the spectrum of conformational substates and paths accessible to LeuT and highlights the differences between Ala- and Leu-bound substates.

A current view of serotonin transporters

Serotonin transporters (SERTs) are largely recognized for one aspect of their function—to transport serotonin back into the presynaptic terminal after its release. Another aspect of their function, however, may be to generate currents large enough to have physiological consequences. The standard model for electrogenic transport is the alternating access model, in which serotonin is transported with a fixed ratio of co-transported ions resulting in net charge per cycle. The alternating access model, however, cannot account for all the observed currents through SERT or other monoamine transporters.  Furthermore, SERT agonists like ecstasy or antagonists like fluoxetine generate or suppress currents that the standard model cannot support.  Here we survey evidence for a channel mode of transport in which transmitters and ions move through a pore. Available structures for dopamine and serotonin transporters, however, provide no evidence for a pore conformation, raising questions of whether the proposed channel mode actually exists or whether the structural data are perhaps missing a transient open state.

Chloride requirement for monoamine transporters

This review focuses on the Cl(-) requirement for dopamine, serotonin, and norepinephrine (DA, 5-HT, and NE) transport and induced current via the transporters for these transmitters, DAT, SERT, and NET. Indirect evidence exists for the passage of Cl(-) ions through monoamine transporters; however, direct evidence is sparse. An unanswered question is why in some preparations, notably native neurons, it appears that Cl(-) ions carry the current through DAT, whereas in heterologous expression systems Na(+) ions carry the current often referred to as the uncoupled current. It is suggested that different functional states in monoamine transporters represent conformational states that carry dominantly Cl(-) or Na(+). Structures of monoamine transporters contribute enormously to structure-function relationships; however, thus far no structural features support the functionally relevant ionic currents that are known to exist in monoamine transporters.

X-ray structures of Drosophila dopamine transporter in complex with nisoxetine and reboxetine

Most antidepressants elicit their therapeutic benefits through selective blockade of Na(+)/Cl(-)-coupled neurotransmitter transporters. Here we report X-ray structures of the Drosophila melanogaster dopamine transporter in complexes with the polycyclic antidepressants nisoxetine or reboxetine. The inhibitors stabilize the transporter in an outward-open conformation by occupying the substrate-binding site. These structures explain how interactions between the binding pocket and substituents on the aromatic rings of antidepressants modulate drug-transporter selectivity.

An N-terminal threonine mutation produces an efflux-favorable, sodium-primed conformation of the human dopamine transporter

The dopamine transporter (DAT) reversibly transports dopamine (DA) through a series of conformational transitions. Alanine (T62A) or aspartate (T62D) mutagenesis of Thr62 revealed T62D-human (h)DAT partitions in a predominately efflux-preferring conformation. Compared with wild-type (WT), T62D-hDAT exhibits reduced [(3)H]DA uptake and enhanced baseline DA efflux, whereas T62A-hDAT and WT-hDAT function in an influx-preferring conformation. We now interrogate the basis of the mutants' altered function with respect to membrane conductance and Na(+) sensitivity. The hDAT constructs were expressed in Xenopus oocytes to investigate if heightened membrane potential would explain the efflux characteristics of T62D-hDAT. In the absence of substrate, all constructs displayed identical resting membrane potentials. Substrate-induced inward currents were present in oocytes expressing WT- and T62A-hDAT but not T62D-hDAT, suggesting equal bidirectional ion flow through T62D-hDAT. Utilization of the fluorescent DAT substrate ASP(+) [4-(4-(dimethylamino)styryl)-N-methylpyridinium] revealed that T62D-hDAT accumulates substrate in human embryonic kidney (HEK)-293 cells when the substrate is not subject to efflux. Extracellular sodium (Na(+) e) replacement was used to evaluate sodium gradient requirements for DAT transport functions. The EC50 for Na(+) e stimulation of [(3)H]DA uptake was identical in all constructs expressed in HEK-293 cells. As expected, decreasing [Na(+)]e stimulated [(3)H]DA efflux in WT- and T62A-hDAT cells. Conversely, the elevated [(3)H]DA efflux in T62D-hDAT cells was independent of Na(+) e and commensurate with [(3)H]DA efflux attained in WT-hDAT cells, either by removal of Na(+) e or by application of amphetamine. We conclude that T62D-hDAT represents an efflux-willing, Na(+)-primed orientation-possibly representing an experimental model of the conformational impact of amphetamine exposure to hDAT.

X-ray structure of dopamine transporter elucidates antidepressant mechanism

Antidepressants targeting Na(+)/Cl(-)-coupled neurotransmitter uptake define a key therapeutic strategy to treat clinical depression and neuropathic pain. However, identifying the molecular interactions that underlie the pharmacological activity of these transport inhibitors, and thus the mechanism by which the inhibitors lead to increased synaptic neurotransmitter levels, has proven elusive. Here we present the crystal structure of the Drosophila melanogaster dopamine transporter at 3.0 Å resolution bound to the tricyclic antidepressant nortriptyline. The transporter is locked in an outward-open conformation with nortriptyline wedged between transmembrane helices 1, 3, 6 and 8, blocking the transporter from binding substrate and from isomerizing to an inward-facing conformation. Although the overall structure of the dopamine transporter is similar to that of its prokaryotic relative LeuT, there are multiple distinctions, including a kink in transmembrane helix 12 halfway across the membrane bilayer, a latch-like carboxy-terminal helix that caps the cytoplasmic gate, and a cholesterol molecule wedged within a groove formed by transmembrane helices 1a, 5 and 7. Taken together, the dopamine transporter structure reveals the molecular basis for antidepressant action on sodium-coupled neurotransmitter symporters and elucidates critical elements of eukaryotic transporter structure and modulation by lipids, thus expanding our understanding of the mechanism and regulation of neurotransmitter uptake at chemical synapses.

How LeuT shapes our understanding of the mechanisms of sodium-coupled neurotransmitter transporters

Neurotransmitter transporters are ion-coupled symporters that drive the uptake of neurotransmitters from neural synapses. In the past decade, the structure of a bacterial amino acid transporter, leucine transporter (LeuT), has given valuable insights into the understanding of architecture and mechanism of mammalian neurotransmitter transporters. Different conformations of LeuT, including a substrate-free state, inward-open state, and competitive and non-competitive inhibitor-bound states, have revealed a mechanistic framework for the transport and transport inhibition of neurotransmitters. The current review integrates our understanding of the mechanistic and pharmacological properties of eukaryotic neurotransmitter transporters obtained through structural snapshots of LeuT.

S(+)amphetamine induces a persistent leak in the human dopamine transporter: molecular stent hypothesis

BACKGROUND AND PURPOSE

Wherever they are located, dopamine transporters (DATs) clear dopamine (DA) from the extracellular milieu to help regulate dopaminergic signalling. Exposure to amphetamine (AMPH) increases extracellular DA in the synaptic cleft, which has been ascribed to DAT reverse transport. Increased extracellular DA prolongs postsynaptic activity and reinforces abuse and hedonic behaviour.

EXPERIMENTAL APPROACH

Xenopus laevis oocytes expressing human (h) DAT were voltage-clamped and exposed to DA, R(-)AMPH, or S(+)AMPH.

KEY RESULTS

At -60mV, near neuronal resting potentials, S(+)AMPH induced a depolarizing current through hDAT, which after removing the drug, persisted for more than 30 min. This persistent leak in the absence of S(+)AMPH was in contrast to the currents induced by R(-)AMPH and DA, which returned to baseline immediately after their removal. Our data suggest that S(+)AMPH and Na(+) carry the initial S(+)AMPH-induced current, whereas Na+ and Cl(-) carry the persistent leak current. We propose that the persistent current results from the internal action of S(+)AMPH on hDAT because the temporal effect was consistent with S(+)AMPH influx, and intracellular S(+)AMPH activated the effect. The persistent current was dependent on Na(+) and was blocked by cocaine. Intracellular injection of S(+)AMPH also activated a DA-induced persistent leak current.

CONCLUSIONS AND IMPLICATIONS

We report a hitherto unknown action of S(+)AMPH on hDAT that potentially affects AMPH-induced DA release. We propose that internal S(+)AMPH acts as a molecular stent that holds the transporter open even after external S(+)AMPH is removed. Amphetamine-induced persistent leak currents are likely to influence dopaminergic signalling, DA release mechanisms, and amphetamine abuse.

4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP+) is a fluorescent substrate for the human serotonin transporter

Monoamine transporters terminate synaptic neurotransmission and are molecular targets for antidepressants and psychostimulants. Fluorescent reporters can monitor real-time transport and are amenable for high-throughput screening. However, until now, their use has mostly been successful to study the catecholamine transporters but not the serotonin (5HT) transporter. Here, we use fluorescence microscopy, electrophysiology, pharmacology, and molecular modeling to compare fluorescent analogs of 1-methyl-4-phenylpyridinium (MPP(+)) as reporters for the human serotonin transporter (hSERT) in single cells. The fluorescent substrate 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) exhibits superior fluorescence uptake in hSERT-expressing HEK293 cells than other MPP(+) analogs tested. APP(+) uptake is Na(+)- and Cl(-)-dependent, displaced by 5HT, and inhibited by fluoxetine, suggesting APP(+) specifically monitors hSERT activity. ASP(+), which was previously used to study catecholamine transporters, is 10 times less potent than APP(+) at inhibiting 5HT uptake and has minimal hSERT-mediated uptake. Furthermore, in hSERT-expressing oocytes voltage-clamped to -60 mV, APP(+) induced fluoxetine-sensitive hSERT-mediated inward currents, indicating APP(+) is a substrate, whereas ASP(+) induced hSERT-mediated outward currents and counteracted 5HT-induced hSERT currents, indicating ASP(+) possesses activity as an inhibitor. Extra-precise ligand receptor docking of APP(+) and ASP(+) in an hSERT homology model showed both ASP(+) and APP(+) docked favorably within the active region; accordingly, comparable concentrations are required to elicit their opposite electrophysiological responses. We conclude APP(+) is better suited than ASP(+) to study hSERT transport fluorometrically.

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.

The N terminus of monoamine transporters is a lever required for the action of amphetamines

The serotonin transporter (SERT) terminates neurotransmission by removing serotonin from the synaptic cleft. In addition, it is the site of action of antidepressants (which block the transporter) and of amphetamines (which induce substrate efflux). We explored the functional importance of the N terminus in mediating the action of amphetamines by focusing initially on the highly conserved threonine residue at position 81, a candidate site for phosphorylation by protein kinase C. Molecular dynamics simulations of the wild type SERT, compared with its mutations SERT^T81A and SERT^T81D, suggested structural changes in the inner vestibule indicative of an opening of the inner vestibule. Predictions from this model (e.g. the preferential accumulation of SERT^T81A in the inward conformation, its reduced turnover number, and a larger distance between its N and C termini) were verified. Most importantly, SERT^T81A (and the homologous mutations in noradrenaline and dopamine) failed to support amphetamine-induced efflux, and this was not remedied by aspartate at this position. Amphetamine-induced currents through SERT^T81A were comparable with those through the wild type transporter. Both abundant Na⁺ entry and accumulation of SERT^T81A in the inward facing conformation ought to favor amphetamine-induced efflux. Thus, we surmised that the N terminus must play a direct role in driving the transporter into a state that supports amphetamine-induced efflux. This hypothesis was verified by truncating the first 64 amino acids and by tethering the N terminus to an additional transmembrane helix. Either modification abolished amphetamine-induced efflux. We therefore conclude that the N terminus of monoamine transporters acts as a lever that sustains reverse transport.

The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

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.

Mechanisms of neurotransmitter release by amphetamines: a review

Amphetamine and substituted amphetamines, including methamphetamine, methylphenidate (Ritalin), methylenedioxymethamphetamine (ecstasy), and the herbs khat and ephedra, encompass the only widely administered class of drugs that predominantly release neurotransmitter, in this case principally catecholamines, by a non-exocytic mechanism. These drugs play important medicinal and social roles in many cultures, exert profound effects on mental function and behavior, and can produce neurodegeneration and addiction. Numerous questions remain regarding the unusual molecular mechanisms by which these compounds induce catecholamine release. We review current issues on the two apparent primary mechanisms--the redistribution of catecholamines from synaptic vesicles to the cytosol, and induction of reverse transport of transmitter through plasma membrane uptake carriers--and on additional drug effects that affect extracellular catecholamine levels, including uptake inhibition, effects on exocytosis, neurotransmitter synthesis, and metabolism.

Substrate binding stoichiometry and kinetics of the norepinephrine transporter

The human norepinephrine (NE) transporter (hNET) attenuates neuronal signaling by rapid NE clearance from the synaptic cleft, and NET is a target for cocaine and amphetamines as well as therapeutics for depression, obsessive-compulsive disorder, and post-traumatic stress disorder. In spite of its central importance in the nervous system, little is known about how NET substrates, such as NE, 1-methyl-4-tetrahydropyridinium (MPP+), or amphetamine, interact with NET at the molecular level. Nor do we understand the mechanisms behind the transport rate. Previously we introduced a fluorescent substrate similar to MPP+, which allowed separate and simultaneous binding and transport measurement (Schwartz, J. W., Blakely, R. D., and DeFelice, L. J. (2003) J. Biol. Chem. 278, 9768-9777). Here we use this substrate, 4-(4-(dimethylamino)styrl)-N-methyl-pyridinium (ASP+), in combination with green fluorescent protein-tagged hNETs to measure substrate-transporter stoichiometry and substrate binding kinetics. Calibrated confocal microscopy and fluorescence correlation spectroscopy reveal that hNETs, which are homomultimers, bind one substrate molecule per transporter subunit. Substrate residence at the transporter, obtained from rapid on-off kinetics revealed in fluorescence correlation spectroscopy, is 526 micros. Substrate residence obtained by infinite dilution is 1000 times slower. This novel examination of substrate-transporter kinetics indicates that a single ASP+ molecule binds and unbinds thousands of times before being transported or ultimately dissociated from hNET. Calibrated fluorescent images combined with mass spectroscopy give a transport rate of 0.06 ASP+/hNET-protein/s, thus 36,000 on-off binding events (and 36 actual departures) occur for one transport event. Therefore binding has a low probability of resulting in transport. We interpret these data to mean that inefficient binding could contribute to slow transport rates.

EmrE, a Multidrug Transporter from Escherichia coli, Transports Monovalent and Divalent Substrates with the Same Stoichiometry

Multidrug transporters recognize and transport substrates with apparently little common structural features. At times these substrates are neutral, negatively, or positively charged, and only limited information is available as to how these proteins deal with the energetic consequences of transport of substrates with different charges. Multidrug transporters and drug-specific efflux systems are responsible for clinically significant resistance to chemotherapeutic agents in pathogenic bacteria, fungi, parasites, and human cancer cells. Understanding how these efflux systems handle different substrates may also have practical implications in the development of strategies to overcome the resistance mechanisms mediated by these proteins. Here, we compare transport of monovalent and divalent substrates by EmrE, a multidrug transporter from Escherichia coli, in intact cells and in proteoliposomes reconstituted with the purified protein. The results demonstrated that whereas the transport of monovalent substrates involves charge movement (i.e. electrogenic), the transport of divalent substrate does not (i.e. electroneutral). Together with previous results, these findings suggest that an EmrE dimer exchanges two protons per substrate molecule during each transport cycle. In intact cells, under conditions where the only driving force is the electrical potential, EmrE confers resistance to monovalent substrates but not to divalent ones. In the presence of proton gradients, resistance to both types of substrates is detected. The finding that under some conditions EmrE does not remove certain types of drugs points out the importance of an in-depth understanding of mechanisms of action of multidrug transporters to devise strategies for coping with the problem of multidrug resistance.

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.

Ionic currents in the human serotonin transporter reveal inconsistencies in the alternating access hypothesis

We have investigated the conduction states of human serotonin transporter (hSERT) using the voltage clamp, cut-open frog oocyte method under different internal and external ionic conditions. Our data indicate discrepancies in the alternating access model of cotransport, which cannot consistently explain substrate transport and electrophysiological data. We are able simultaneously to isolate distinct external and internal binding sites for substrate, which exert different effects upon currents conducted by hSERT, in contradiction to the alternating access model. External binding sites of coupled Na ions are likewise simultaneously accessible from the internal and external face. Although Na and Cl are putatively cotransported, they have opposite effects on the internal face of the transporter. Finally, the internal K ion does not compete with internal 5-hydroxytryptamine for empty transporters. These data can be explained more readily in the language of ion channels, rather than carrier models distinguished by alternating access mechanisms: in a channel model of coupled transport, the currents represent different states of the same permeation path through hSERT and coupling occurs in a common pore.

Distinct recognition of substrates by the human and Drosophila serotonin transporters

The human and Drosophila serotonin transporters (hSERT and dSERT, respectively) were used to explore differences in substrate properties. hSERT and dSERT showed similar Km values for 5-hydroxytryptamine (5-HT; serotonin) transport (1.2 and 0.9 micro M, respectively), suggesting similar recognition of 5-HT by the two species variants. Although dSERT cell surface expression was approximately 8-fold lower than that of hSERT, dSERT does appear to have a 2-fold faster turnover number for inward transport of 5-HT. Interestingly, another substrate, N-methyl-4-phenylpyridinium (MPP+), was transported only by hSERT. However, MPP+ inhibited 5-HT uptake in both species variants with similar potencies. Two cross-species chimeras, H1-118D119-627 and H1-281D282-476H477-638, were also unable to transport MPP+, implicating the role of transmembrane domains V to IX in the substrate permeation pathway. Based on exchange experiments, certain substituted-amphetamines also appear to be poor substrates at dSERT. Two-electrode voltage-clamp studies in oocytes confirmed that the amphetamines do not possess substrate-like properties for dSERT. Our data suggest distinct molecular recognition among SERT substrate classes that influence translocation mechanisms.

Cotransporters as molecular water pumps

Molecular water pumps are membrane proteins of the cotransport type in which a flux of water is coupled to substrate fluxes by a mechanism within the protein. Free energy can be exchanged between the fluxes. Accordingly, the flux of water may be relatively independent of the external water chemical potential and can even proceed uphill. In short, water is being cotransported. The evidence for water cotransport is reviewed with particular emphasis on electrogenic cotransporters expressed in Xenopus oocytes under voltage clamped conditions. Phenomena such as uphill water transport, tight coupling between water transport and clamp current, cotransport of small hydrophilic molecules, and shifts in reversal potentials with osmolarity are discussed with examples from the Na+/glutamate and Na+/glucose cotransporters. Unstirred layers and electrode artifacts as alternative explanations for such cotransport can be ruled out for both experimental and theoretical reasons. Indeed, substrate fluxes mediated by channels or ionophores generate much smaller water fluxes than those observed with cotransporters. Theoretical models, using reasonable values for the intracellular diffusion coefficient, indicate the presence of only small unstirred layers in the membranes studied.

Water transport by the human Na+-coupled glutamate cotransporter expressed in Xenopus oocytes

The water transport properties of the human Na+-coupled glutamate cotransporter (EAAT1) were investigated. The protein was expressed in Xenopus laevis oocytes and electrogenic glutamate transport was recorded by two-electrode voltage clamp, while the concurrent water transport was monitored as oocyte volume changes. Water transport by EAAT1 was bimodal. Water was cotransported along with glutamate and Na+ by a mechanism within the protein. The transporter also sustained passive water transport in response to osmotic challenges. The two modes could be separated and could proceed in parallel. The cotransport modality was characterized in solutions of low Cl- concentration. Addition of glutamate promptly initiated an influx of 436 +/- 55 water molecules per unit charge, irrespective of the clamp potential. The cotransport of water occurred in the presence of adverse osmotic gradients. In accordance with the Gibbs equation, energy was transferred within the protein primarily from the downhill fluxes of Na+ to the uphill fluxes of water. Experiments using the cation-selective ionophore gramicidin showed no unstirred layer effects. Na+ currents in the ionophore did not lead to any significant initial water movements. In the absence of glutamate, EAAT1 contributed a passive water permeability (Lp) of (11.3 +/- 2.0) x 10(-6) cm s(-1) (osmol l(-1))(-1). In the presence of glutamate, Lp was about 50 % higher for both high and low Cl- concentrations. The physiological role of EAAT1 as a molecular water pump is discussed in relation to cellular volume homeostasis in the nervous system.

K(+)-induced reversal of astrocyte glutamate uptake is limited by compensatory changes in intracellular Na+

Glutamate uptake is coupled to counter-transport of K+, and high external K+ concentrations can induce reversal of glutamate uptake in whole-cell patch-clamp and isolated membrane preparations. However, high external K+ causes little or no reversal of glutamate uptake in intact astrocytes, suggesting a regulatory mechanism not evident in membrane preparations. One mechanism by which intact cells could limit the effects of altered extracellular ion concentrations on glutamate transport is by compensatory changes in intracellular Na+ concentrations. This possibility was examined using astrocyte cultures treated in two ways to reduce the driving force for glutamate uptake: incubation in high K+ (with reciprocal reduction in Na+), and incubation with metabolic inhibitors to induce ATP depletion. ATP depletion produced a rise in intracellular Na+, a collapse of the membrane sodium gradient and a massive reversal of glutamate uptake. By contrast, incubation in high K+/low Na+ medium did not significantly alter the sodium gradient and did not induce glutamate uptake reversal. The sodium gradient was shown to be maintained under these conditions by compensatory reductions in intracellular Na+ that approximately matched the reductions in extracellular Na+. These findings suggest a mechanism by which astrocytes may limit reversal of glutamate uptake under high K+/low Na+ conditions, and further suggest a general mechanism by which Na(+)-dependent transport processes could be shielded from fluctuating extracellular ion concentrations.

Phosphorylation and sequestration of serotonin transporters differentially modulated by psychostimulants

Many psychotropic drugs interfere with the reuptake of dopamine, norepinephrine, and serotonin. Transport capacity is regulated by kinase-linked pathways, particularly those involving protein kinase C (PKC), resulting in transporter phosphorylation and sequestration. Phosphorylation and sequestration of the serotonin transporter (SERT) were substantially impacted by ligand occupancy. Ligands that can permeate the transporter, such as serotonin or the amphetamines, prevented PKC-dependent SERT phosphorylation. Nontransported SERT antagonists such as cocaine and antidepressants were permissive for SERT phosphorylation but blocked serotonin effects. PKC-dependent SERT sequestration was also blocked by serotonin. These findings reveal activity-dependent modulation of neurotransmitter reuptake and identify previously unknown consequences of amphetamine, cocaine, and antidepressant action.

Transmembrane domain I contributes to the permeation pathway for serotonin and ions in the serotonin transporter

Mutation of a conserved Asp (D98) in the rat serotonin (5HT) transporter (rSERT) to Glu (D98E) led to decreased 5HT transport capacity, diminished coupling to extracellular Na+ and Cl-, and a selective loss of antagonist potencies (cocaine, imipramine, and citalopram but not paroxetine or mazindol) with no change in 5HT Km value. D98E, which extends the acidic side chain by one carbon, affected the rank-order potency of substrate analogs for inhibition of 5HT transport, selectively increasing the potency of two analogs with shorter alkylamine side chains, gramine, and dihydroxybenzylamine. D98E also increased the efficacy of gramine relative to 5HT for inducing substrate-activated currents in Xenopus laevis oocytes, but these currents were noticeably dependent on extracellular medium acidification. I-V profiles for substrate-independent and -dependent currents indicated that the mutation selectively impacts ion permeation coupled to 5HT occupancy. The ability of the D98E mutant to modulate selective aspects of substrate recognition, to perturb ion dependence as well as modify substrate-induced currents, suggests that transmembrane domain I plays a critical role in defining the permeation pathway of biogenic amine transporters.