The role of conserved tryptophan and acidic residues in the human dopamine transporter as characterized by site-directed mutagenesis

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Cryo-EM structure of the human NKCC1 transporter reveals mechanisms of ion coupling and specificity

The sodium-potassium-chloride transporter NKCC1 of the SLC12 family performs Na⁺ -dependent Cl^- - and K⁺ -ion uptake across plasma membranes. NKCC1 is important for regulating cell volume, hearing, blood pressure, and regulation of hyperpolarizing GABAergic and glycinergic signaling in the central nervous system. Here, we present a 2.6 Å resolution cryo-electron microscopy structure of human NKCC1 in the substrate-loaded (Na⁺ , K⁺ , and 2 Cl^- ) and occluded, inward-facing state that has also been observed for the SLC6-type transporters MhsT and LeuT. Cl^- binding at the Cl1 site together with the nearby K⁺ ion provides a crucial bridge between the LeuT-fold scaffold and bundle domains. Cl^- -ion binding at the Cl2 site seems to undertake a structural role similar to conserved glutamate of SLC6 transporters and may allow for Cl^- -sensitive regulation of transport. Supported by functional studies in mammalian cells and computational simulations, we describe a putative Na⁺ release pathway along transmembrane helix 5 coupled to the Cl2 site. The results provide insight into the structure-function relationship of NKCC1 with broader implications for other SLC12 family members.

The substrate import mechanism of the human serotonin transporter

The serotonin transporter (SERT) initiates the reuptake of extracellular serotonin in the synapse to terminate neurotransmission. The cryogenic electron microscopy structures of SERT bound to ibogaine and the physiological substrate serotonin resolved in different states have provided a glimpse of the functional conformations at atomistic resolution. However, the conformational dynamics and structural transitions to intermediate states are not fully understood. Furthermore, the molecular basis of how serotonin is recognized and transported remains unclear. In this study, we performed unbiased microsecond-long simulations of the human SERT to investigate the structural dynamics to various intermediate states and elucidated the complete substrate import pathway. Using Markov state models, we characterized a sequential order of conformational-driven ion-coupled substrate binding and transport events and calculated the free energy barriers of conformation transitions associated with the import mechanism. We find that the transition from the occluded to inward-facing state is the rate-limiting step for substrate import and that the substrate decreases the free energy barriers to achieve the inward-facing state. Our study provides insights on the molecular basis of dynamics-driven ion-substrate recognition and transport of SERT that can serve as a model for other closely related neurotransmitter transporters.

A non-helical region in transmembrane helix 6 of hydrophobic amino acid transporter MhsT mediates substrate recognition

MhsT of Bacillus halodurans is a transporter of hydrophobic amino acids and a homologue of the eukaryotic SLC6 family of Na+ -dependent symporters for amino acids, neurotransmitters, osmolytes, or creatine. The broad range of transported amino acids by MhsT prompted the investigation of the substrate recognition mechanism. Here, we report six new substrate-bound structures of MhsT, which, in conjunction with functional studies, reveal how the flexibility of a Gly-Met-Gly (GMG) motif in the unwound region of transmembrane segment 6 (TM6) is central for the recognition of substrates of different size by tailoring the binding site shape and volume. MhsT mutants, harboring substitutions within the unwound GMG loop and substrate binding pocket that mimick the binding sites of eukaryotic SLC6A18/B0AT3 and SLC6A19/B0AT1 transporters of neutral amino acids, exhibited impaired transport of aromatic amino acids that require a large binding site volume. Conservation of a general (G/A/C)ΦG motif among eukaryotic members of SLC6 family suggests a role for this loop in a common mechanism for substrate recognition and translocation by SLC6 transporters of broad substrate specificity.

Mutational effects of human dopamine transporter at tyrosine88, lysine92, and histidine547 on basal and HIV-1 Tat-inhibited dopamine transport

Dysregulation of dopaminergic system induced by HIV-1 Tat protein-mediated direct inhibition of the dopamine transporter (DAT) has been implicated as a mediating factor of HIV-1 associated neurocognitive disorders. We have reported that single point mutations on human DAT (hDAT) at tyrosine88 (Y88F), lysine92 (K92M), and histidine547 (H547A) differentially regulate basal dopamine uptake but diminish Tat-induced inhibition of dopamine uptake by changing dopamine transport process. This study evaluated the effects of double (Y88F/H547A) and triple (Y88F/K92M/H547A) mutations on basal dopamine uptake, Tat-induced inhibition of DAT function, and dynamic transport process. Compared to wild-type hDAT, the Vmax values of [3H]Dopamine uptake were increased by 96% in Y88F/H547A but decreased by 97% in Y88F/K92M/H547A. [3H]WIN35,428 binding sites were not altered in Y88F/H547A but decreased in Y88F/K92M/H547A. Y88F/H547A mutant attenuated Tat-induced inhibition of dopamine uptake observed in wild-type hDAT. Y88F/H547A displayed an attenuation of zinc-augmented [3H]WIN35,428 binding, increased basal dopamine efflux, and reduced amphetamine-induced dopamine efflux, indicating this mutant alters transporter conformational transitions. These findings further demonstrate that both tyrosine88 and histidine547 on hDAT play a key role in stabilizing basal dopamine transport and Tat-DAT integration. This study provides mechanistic insights into developing small molecules to block multiple sites in DAT for Tat binding.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.

Dopamine transporter oligomerization involves the scaffold domain, but spares the bundle domain

The human dopamine transporter (hDAT) is located on presynaptic neurons, where it plays an essential role in limiting dopaminergic signaling by temporarily curtailing high neurotransmitter concentration through rapid re-uptake. Transport by hDAT is energized by transmembrane ionic gradients. Dysfunction of this transporter leads to disease states, such as Parkinson’s disease, bipolar disorder or depression. It has been shown that hDAT and other members of the monoamine transporter family exist in oligomeric forms at the plasma membrane. Several residues are known to be involved in oligomerization, but interaction interfaces, oligomer orientation and the quarternary arrangement in the plasma membrane remain poorly understood. Here we examine oligomeric forms of hDAT using a direct approach, by following dimerization of two randomly-oriented hDAT transporters in 512 independent simulations, each being 2 μs in length. We employed the DAFT (docking assay for transmembrane components) approach, which is an unbiased molecular dynamics simulation method to identify oligomers, their conformations and populations. The overall ensemble of a total of >1 ms simulation time revealed a limited number of symmetric and asymmetric dimers. The identified dimer interfaces include all residues known to be involved in dimerization. Importantly, we find that the surface of the bundle domain is largely excluded from engaging in dimeric interfaces. Such an interaction would typically lead to inhibition by stabilization of one conformation, while substrate transport relies on a large scale rotation between the inward-facing and the outward-facing state.

The human dopamine transporter efficiently removes the neurotransmitter dopamine from the synaptic cleft. Alteration of dopamine transporter function is associated with several neurological diseases, including mood disorders or attention-deficit hyperactivity disorder, but is also a major player in addiction and drug abuse. Functional studies have revealed that not only is transporter oligomerization involved in surface expression and endocytosis, but, more importantly, in reverse transport (efflux) of dopamine that is triggered by amphetamine-like drugs of abuse. Structural knowledge of transporter oligomerization is largely missing. We performed a large scale comprehensive computational study on transporter oligomerization to reveal dimer geometries and the residues involved in the interfaces. The dimer conformations we find in our dataset are fully consistent with all available experimental data in the literature, but also show novel interfaces. We further verified all dimer geometries by free energy calculations. Our results identified an unpredicted—but for the mechanism of substrate transport essential—property: the bundle domain, which moves during the transport cycle, is excluded from contributing to dimer interfaces, thereby allowing for unrestrained movements necessary to translocate substrates through the membrane.

Small molecule induced oligomerization, clustering and clathrin-independent endocytosis of the dopamine transporter

Clathrin-independent endocytosis (CIE) mediates internalization of many transmembrane proteins but the mechanisms of cargo recruitment during CIE are poorly understood. We found that the cell-permeable furopyrimidine AIM-100 promotes dramatic oligomerization, clustering and CIE of human and mouse dopamine transporters (DAT), but not of their close homologues, norepinephrine and serotonin transporters. All effects of AIM-100 on DAT and the occupancy of substrate binding sites in the transporter were mutually exclusive, suggesting that AIM-100 may act by binding to DAT. Surprisingly, AIM-100-induced DAT endocytosis was independent of dynamin, cholesterol-rich microdomains and actin cytoskeleton, implying that a novel endocytic mechanism is involved. AIM-100 stimulated trafficking of internalized DAT was also unusual: DAT accumulated in early endosomes without significant recycling or degradation. We propose that AIM-100 augments DAT oligomerization through an allosteric mechanism associated with the DAT conformational state, and that oligomerization-triggered clustering leads to a coat-independent endocytosis and subsequent endosomal retention of DAT.

Latch and trigger role for R445 in DAT transport explains molecular basis of DTDS

A recent study reports on five different mutations as sources of dopamine transporter (DAT) deficiency syndrome (DTDS). One of these mutations, R445C, is believed to be located on the intracellular side of DAT distal to the primary (S1) or secondary (S2) sites to which substrate binding is understood to occur. Thus, the molecular mechanism by which the R445C mutation results in DAT transport deficiency has eluded explanation. However, the recently reported X-ray structures of the endogenous amine transporters for dDAT and hSERT revealed the presence of a putative salt bridge between R445 and E428 suggesting a possible mechanism. To evaluate whether the R445C effect is a result of a salt bridge interaction, the mutants R445E, E428R, and the double mutant E428R/R445E were generated. The single mutants R445E and E428R displayed loss of binding and transport properties of the substrate [³H]DA and inhibitor [³H]CFT at the cell surface while the double mutant E428R/R445E, although nonfunctional, restored [³H]DA and [³H]CFT binding affinity to that of WT. Structure based analyses of these results led to a model wherein R445 plays a dual role in normal DAT function. R445 acts as a component of a latch in its formation of a salt bridge with E428 which holds the primary substrate binding site (S1) in place and helps enforce the inward closed protein state. When this salt bridge is broken, R445 acts as a trigger which disrupts a local polar network and leads to the release of the N-terminus from its position inducing the inward closed state to one allowing the inward open state. In this manner, both the loss of binding and transport properties of the R445C variant are explained.

A Markov State-based Quantitative Kinetic Model of Sodium Release from the Dopamine Transporter

The dopamine transporter (DAT) belongs to the neurotransmitter:sodium symporter (NSS) family of membrane proteins that are responsible for reuptake of neurotransmitters from the synaptic cleft to terminate a neuronal signal and enable subsequent neurotransmitter release from the presynaptic neuron. The release of one sodium ion from the crystallographically determined sodium binding site Na2 had been identified as an initial step in the transport cycle which prepares the transporter for substrate translocation by stabilizing an inward-open conformation. We have constructed Markov State Models (MSMs) from extensive molecular dynamics simulations of human DAT (hDAT) to explore the mechanism of this sodium release. Our results quantify the release process triggered by hydration of the Na2 site that occurs concomitantly with a conformational transition from an outward-facing to an inward-facing state of the transporter. The kinetics of the release process are computed from the MSM, and transition path theory is used to identify the most probable sodium release pathways. An intermediate state is discovered on the sodium release pathway, and the results reveal the importance of various modes of interaction of the N-terminus of hDAT in controlling the pathways of release.

Dopamine Transporter Amino and Carboxyl Termini Synergistically Contribute to Substrate and Inhibitor Affinities

Extracellular dopamine and serotonin concentrations are determined by the presynaptic dopamine (DAT) and serotonin (SERT) transporters, respectively. Numerous studies have investigated the DAT and SERT structural elements contributing to inhibitor and substrate binding. To date, crystallographic studies have focused on conserved transmembrane domains, where multiple substrate binding and translocation features are conserved. However, it is unknown what, if any, role the highly divergent intracellular N and C termini contribute to these processes. Here, we used chimeric proteins to test whether DAT and SERT N and C termini contribute to transporter substrate and inhibitor affinities. Replacing the DAT N terminus with that of SERT had no effect on DA transport V_max but significantly decreased DAT substrate affinities for DA and amphetamine. Similar losses in uptake inhibition were observed for small DAT inhibitors, whereas substituting the DAT C terminus with that of SERT affected neither substrate nor inhibitor affinities. In contrast, the N-terminal substitution was completely tolerated by the larger DAT inhibitors, which exhibited no loss in apparent affinity. Remarkably, all affinity losses were rescued in DAT chimeras encoding both SERT N and C termini. The sensitivity to amino-terminal substitution was specific for DAT, because replacing the SERT N and/or C termini affected neither substrate nor inhibitor affinities. Taken together, these findings provide compelling experimental evidence that DAT N and C termini synergistically contribute to substrate and inhibitor affinities.

Impact of disruption of secondary binding site S2 on dopamine transporter function

The structures of the leucine transporter, drosophila dopamine transporter, and human serotonin transporter show a secondary binding site (designated S2 ) for drugs and substrate in the extracellular vestibule toward the membrane exterior in relation to the primary substrate recognition site (S1 ). The present experiments are aimed at disrupting S2 by mutating Asp476 and Ile159 to Ala. Both mutants displayed a profound decrease in [(3) H]DA uptake compared with wild-type associated with a reduced turnover rate kcat . This was not caused by a conformational bias as the mutants responded to Zn(2+) (10 μM) similarly as WT. The dopamine transporters with either the D476A or I159A mutation both displayed a higher Ki for dopamine for the inhibition of [3H](-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane binding than did the WT transporter, in accordance with an allosteric interaction between the S1 and S2 sites. The results provide evidence in favor of a general applicability of the two-site allosteric model of the Javitch/Weinstein group from LeuT to dopamine transporter and possibly other monoamine transporters. X-ray structures of transporters closely related to the dopamine (DA) transporter show a secondary binding site S2 in the extracellular vestibule proximal to the primary binding site S1 which is closely linked to one of the Na(+) binding sites. This work examines the relationship between S2 and S1 sites. We found that S2 site impairment severely reduced DA transport and allosterically reduced S1 site affinity for the cocaine analog [(3) H]CFT. Our results are the first to lend direct support for the application of the two-site allosteric model, advanced for bacterial LeuT, to the human DA transporter. The model states that, after binding of the first DA molecule (DA1 ) to the primary S1 site (along with Na(+) ), binding of a second DA (DA2 ) to the S2 site triggers, through an allosteric interaction, the release of DA1 and Na(+) into the cytoplasm.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

Dopamine transporter oligomerization: impact of combining protomers with differential cocaine analog binding affinities

Previous studies point to quaternary assembly of dopamine transporters (DATs) in oligomers. However, it is not clear whether the protomers function independently in the oligomer. Is each protomer an entirely separate unit that takes up dopamine and is inhibited by drugs known to block DAT function? In this work, human embryonic kidney 293 cells were co-transfected with DAT constructs possessing differential binding affinities for the phenyltropane cocaine analog, [³H]WIN35,428. It was assessed whether the binding properties in co-expressing cells capable of forming hetero-oligomers differ from those in preparations obtained from mixed singly transfected cells where such oligomers cannot occur. A method is described that replaces laborious 'mixing' experiments with an in silico method predicting binding parameters from those observed for the singly expressed constructs. Among five pairs of constructs tested, statistically significant interactions were found between protomers of wild-type (WT) and D313N, WT and D345N, and WT and D436N. Compared with predicted Kd values of [³H]WIN35,428 binding to the non-interacting pairs, the observed affinity of the former pair was increased 1.7 fold while the latter two were reduced 2.2 and 4.1 fold, respectively. This is the first report of an influence of protomer composition on the properties of a DAT inhibitor, indicating cooperativity within the oligomer. The dopamine transporter (DAT) can exist as an oligomer but it is unknown whether the protomers function independently. The present results indicate that protomers that are superpotent or deficient in cocaine analog binding can confer enhanced or reduced potency to the oligomer, respectively. In this respect, positive or negative cooperativity is revealed in the DAT oligomer.

Computational and biochemical docking of the irreversible cocaine analog RTI 82 directly demonstrates ligand positioning in the dopamine transporter central substrate-binding site

The dopamine transporter (DAT) functions as a key regulator of dopaminergic neurotransmission via re-uptake of synaptic dopamine (DA). Cocaine binding to DAT blocks this activity and elevates extracellular DA, leading to psychomotor stimulation and addiction, but the mechanisms by which cocaine interacts with DAT and inhibits transport remain incompletely understood. Here, we addressed these questions using computational and biochemical methodologies to localize the binding and adduction sites of the photoactivatable irreversible cocaine analog 3β-(p-chlorophenyl)tropane-2β-carboxylic acid, 4'-azido-3'-iodophenylethyl ester ([(125)I]RTI 82). Comparative modeling and small molecule docking indicated that the tropane pharmacophore of RTI 82 was positioned in the central DA active site with an orientation that juxtaposed the aryliodoazide group for cross-linking to rat DAT Phe-319. This prediction was verified by focused methionine substitution of residues flanking this site followed by cyanogen bromide mapping of the [(125)I]RTI 82-labeled mutants and by the substituted cysteine accessibility method protection analyses. These findings provide positive functional evidence linking tropane pharmacophore interaction with the core substrate-binding site and support a competitive mechanism for transport inhibition. This synergistic application of computational and biochemical methodologies overcomes many uncertainties inherent in other approaches and furnishes a schematic framework for elucidating the ligand-protein interactions of other classes of DA transport inhibitors.

Dopamine reuptake transporter (DAT) "inverse agonism"--a novel hypothesis to explain the enigmatic pharmacology of cocaine

The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable subjective effects such as drug "highs" or euphoria in drug-experienced human volunteers. Moreover they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to conclude that the highly unusual stimulant profile of cocaine and related compounds, eg methylphenidate, is not mediated by monoamine reuptake inhibition alone. We describe the experimental findings which suggest cocaine serves as a negative allosteric modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of transport. This results in a firing-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an inverse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a new class of monoaminergic drugs; DAT "inverse agonists". This article is part of the Special Issue entitled 'CNS Stimulants'.

Defining the blanks--pharmacochaperoning of SLC6 transporters and ABC transporters

SLC6 family members and ABC transporters represent two extremes: SLC6 transporters are confined to the membrane proper and only expose small segments to the hydrophilic milieu. In ABC transporters the hydrophobic core is connected to a large intracellular (eponymous) ATP binding domain that is comprised of two discontiguous repeats. Accordingly, their folding problem is fundamentally different. This can be gauged from mutations that impair the folding of the encoded protein and give rise to clinically relevant disease phenotypes: in SLC6 transporters, these cluster at the protein–lipid interface on the membrane exposed surface. Mutations in ABC-transporters map to the interface between nucleotide binding domains and the coupling helices, which provide the connection to the hydrophobic core. Folding of these mutated ABC-transporters can be corrected with ligands/substrates that bind to the hydrophobic core. This highlights a pivotal role of the coupling helices in the folding trajectory. In contrast, insights into pharmacochaperoning of SLC6 transporters are limited to monoamine transporters – in particular the serotonin transporter (SERT) – because of their rich pharmacology. Only ligands that stabilize the inward facing conformation act as effective pharmacochaperones. This indicates that the folding trajectory of SERT proceeds via the inward facing conformation. Mutations that impair folding of SLC6 family members can be transmitted as dominant or recessive alleles. The dominant phenotype of the mutation can be rationalized, because SLC6 transporters are exported in oligomeric form from the endoplasmic reticulum (ER). Recessive transmission requires shielding of the unaffected gene product from the mutated transporter in the ER. This can be accounted for by a chaperone-COPII (coatomer protein II) exchange model, where proteinaceous ER-resident chaperones engage various intermediates prior to formation of the oligomeric state and subsequent export from the ER. It is likely that the action of pharmacochaperones is contingent on and modulated by these chaperones.

Nonclassical pharmacology of the dopamine transporter: atypical inhibitors, allosteric modulators, and partial substrates

The dopamine transporter (DAT) is a sodium-coupled symporter protein responsible for modulating the concentration of extraneuronal dopamine in the brain. The DAT is a principle target of various psychostimulant, nootropic, and antidepressant drugs, as well as certain drugs used recreationally, including the notoriously addictive stimulant cocaine. DAT ligands have traditionally been divided into two categories: cocaine-like inhibitors and amphetamine-like substrates. Whereas inhibitors block monoamine uptake by the DAT but are not translocated across the membrane, substrates are actively translocated and trigger DAT-mediated release of dopamine by reversal of the translocation cycle. Because both inhibitors and substrates increase extraneuronal dopamine levels, it is often assumed that all DAT ligands possess an addictive liability equivalent to that of cocaine. However, certain recently developed ligands, such as atypical benztropine-like DAT inhibitors with reduced or even a complete lack of cocaine-like rewarding effects, suggest that addictiveness is not a constant property of DAT-affecting compounds. These atypical ligands do not conform to the classic preconception that all DAT inhibitors (or substrates) are functionally and mechanistically alike. Instead, they suggest the possibility that the DAT exhibits some of the ligand-specific pleiotropic functional qualities inherent to G-protein-coupled receptors. That is, ligands with different chemical structures induce specific conformational changes in the transporter protein that can be differentially transduced by the cell, ultimately eliciting unique behavioral and psychological effects. The present overview discusses compounds with conformation-specific activity, useful not only as tools for studying the mechanics of dopamine transport, but also as leads for medication development in addictive disorders.

Mutational analysis of the high-affinity zinc binding site validates a refined human dopamine transporter homology model

The high-resolution crystal structure of the leucine transporter (LeuT) is frequently used as a template for homology models of the dopamine transporter (DAT). Although similar in structure, DAT differs considerably from LeuT in a number of ways: (i) when compared to LeuT, DAT has very long intracellular amino and carboxyl termini; (ii) LeuT and DAT share a rather low overall sequence identity (22%) and (iii) the extracellular loop 2 (EL2) of DAT is substantially longer than that of LeuT. Extracellular zinc binds to DAT and restricts the transporter‚s movement through the conformational cycle, thereby resulting in a decrease in substrate uptake. Residue H293 in EL2 praticipates in zinc binding and must be modelled correctly to allow for a full understanding of its effects. We exploited the high-affinity zinc binding site endogenously present in DAT to create a model of the complete transmemberane domain of DAT. The zinc binding site provided a DAT-specific molecular ruler for calibration of the model. Our DAT model places EL2 at the transporter lipid interface in the vicinity of the zinc binding site. Based on the model, D206 was predicted to represent a fourth co-ordinating residue, in addition to the three previously described zinc binding residues H193, H375 and E396. This prediction was confirmed by mutagenesis: substitution of D206 by lysine and cysteine affected the inhibitory potency of zinc and the maximum inhibition exerted by zinc, respectively. Conversely, the structural changes observed in the model allowed for rationalizing the zinc-dependent regulation of DAT: upon binding, zinc stabilizes the outward-facing state, because its first coordination shell can only be completed in this conformation. Thus, the model provides a validated solution to the long extracellular loop and may be useful to address other aspects of the transport cycle.

The dopamine transporter (DAT) regulates dopaminergic neurotransmission in the brain and is implicated in numerous human disease states. DAT is unique among the monoamine neurotransmitter transporter family because its substrate transport is inhibited by extracellular zinc. DAT homology models rely upon the crystal structure of LeuT solved in 2005. LeuT and DAT share a relatively low overall sequence identity of 22%. In addition, the length of the second extracellular loop of DAT exceeds that of LeuT by 21 residues. The zinc binding site cannot be directly modeled from the LeuT template alone because of these differences. Current available homology models of DAT focused on substrate or inhibitor binding rather than on the second extracellular loop. We exploited the specificity of the zinc binding site to build and calibrate a DAT homology model of the complete transmembrane domain. Our model predicted that the zinc binding site in DAT consists of four zinc co-ordinating residues rather than three that had been previously identified. We verified this hypothesis by site-directed mutagenesis and uptake inhibition studies.

Importance of cholesterol in dopamine transporter function

The conformation and function of the dopamine transporter (DAT) can be affected by manipulating membrane cholesterol, yet there is no agreement as to the impact of cholesterol on the activity of lipid-raft localized DATs compared with non-raft DATs. Given the paucity of information regarding the impact of cholesterol on substrate efflux by the DAT, this study explores its influence on the kinetics of DAT-mediated DA efflux induced by dextroamphetamine, as measured by rotating disk electrode voltammetry (RDEV). Treatment with methyl-β-cyclodextrin (mβCD), which effectively depletes total membrane cholesterol--uniformly affecting cholesterol-DAT interactions in both raft and non-raft membrane domains--reduced both DA uptake and efflux rate. In contrast, disruption of raft-localized DAT by cholesterol chelation with nystatin had no effect, arguing against a vital role for raft-localized DAT in substrate uptake or efflux. Supranormal repletion of cholesterol-depleted cells with the analog desmosterol, a non-raft promoting sterol, was as effective as cholesterol itself in restoring transport rates. Further studies with Zn(2+) and the conformationally biased W84L DAT mutant supported the idea that cholesterol is important for maintaining the outward-facing DAT with normal rates of conformational interconversions. Collectively, these results point to a role for direct cholesterol-DAT interactions in regulating DAT function.

The atypical stimulant and nootropic modafinil interacts with the dopamine transporter in a different manner than classical cocaine-like inhibitors

Modafinil is a mild psychostimulant with pro-cognitive and antidepressant effects. Unlike many conventional stimulants, modafinil has little appreciable potential for abuse, making it a promising therapeutic agent for cocaine addiction. The chief molecular target of modafinil is the dopamine transporter (DAT); however, the mechanistic details underlying modafinil's unique effects remain unknown. Recent studies suggest that the conformational effects of a given DAT ligand influence the magnitude of the ligand's reinforcing properties. For example, the atypical DAT inhibitors benztropine and GBR12909 do not share cocaine's notorious addictive liability, despite having greater binding affinity. Here, we show that the binding mechanism of modafinil is different than cocaine and similar to other atypical inhibitors. We previously established two mutations (W84L and D313N) that increase the likelihood that the DAT will adopt an outward-facing conformational state—these mutations increase the affinity of cocaine-like inhibitors considerably, but have little or opposite effect on atypical inhibitor binding. Thus, a compound's WT/mutant affinity ratio can indicate whether the compound preferentially interacts with a more outward- or inward-facing conformational state. Modafinil displayed affinity ratios similar to those of benztropine, GBR12909 and bupropion (which lack cocaine-like effects in humans), but far different than those of cocaine, β-CFT or methylphenidate. Whereas treatment with zinc (known to stabilize an outward-facing transporter state) increased the affinity of cocaine and methylphenidate two-fold, it had little or no effect on the binding of modafinil, benztropine, bupropion or GBR12909. Additionally, computational modeling of inhibitor binding indicated that while β-CFT and methylphenidate stabilize an “open-to-out” conformation, binding of either modafinil or bupropion gives rise to a more closed conformation. Our findings highlight a mechanistic difference between modafinil and cocaine-like stimulants and further demonstrate that the conformational effects of a given DAT inhibitor influence its phenomenological effects.

Site-directed mutations near transmembrane domain 1 alter conformation and function of norepinephrine and dopamine transporters

The human dopamine and norepinephrine transporters (hDAT and hNET, respectively) control neurotransmitter levels within the synaptic cleft and are the site of action for amphetamine (AMPH) and cocaine. We investigated the role of a threonine residue within the highly conserved and putative phosphorylation sequence RETW, located just before transmembrane domain 1, in regulating hNET and hDAT function. The Thr residue was mutated to either alanine or aspartate. Similar to the inward facing T62D-hDAT, T58D-hNET demonstrated reduced [(3)H]DA uptake but enhanced basal DA efflux compared with hNET with no further effect of AMPH. The mutations had profound effects on substrate function and binding. The potency of substrates to inhibit [(3)H]DA uptake and compete with radioligand binding was increased in T→A and/or T→D mutants. Substrates, but not inhibitors, demonstrated temperature-sensitive effects of binding. Neither the functional nor the binding potency for hNET blockers was altered from wild type in hNET mutants. There was, however, a significant reduction in potency for cocaine and benztropine to inhibit [(3)H]DA uptake in T62D-hDAT compared with hDAT. The potency of these drugs to inhibit [(3)H](-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane-1,5-napthalenedisulfonate (WIN35,428) binding was not increased, demonstrating a discordance between functional and binding site effects. Taken together, these results concur with the notion that the T→D mutation in RETW alters the preferred conformation of both hNET and hDAT to favor one that is more inward facing. Although substrate activity and binding are primarily altered in this conformation, the function of inhibitors with distinct structural characteristics may also be affected.

Interrelation of dopamine transporter oligomerization and surface presence as studied with mutant transporter proteins and amphetamine

Our previous work suggested a role for oligomerization in regulating dopamine transporter (DAT) internalization, with d-amphetamine dissociating DAT oligomers and monomers being endocytosed. This model was put to detailed testing in the present work with the use of DAT constructs differentially tagged with Myc or Flag, reversal of tags in co-immunoprecipitation and cross-linking assays, and application of antibodies against different tags in biotinylation experiments. Upon pairing wild-type (WT) DAT with W84L mutant, effects of d-amphetamine on oligomerization (decrease) but not surface DAT are observed. Internalization of W84L monomers appears to be slow as inferred from the inability of d-amphetamine to reduce surface Myc upon co-expressing Flag-WT with Myc-W84L but not Myc-WT with Flag-W84L, and from the sluggish Myc-W84L endocytosis rate (both with or without d-amphetamine). Results obtained for D313N, D345N, or D436N mutants can all be accommodated by a model in which D-amphetamine is unable to dissociate mutant protomers from oligomers (tetramers or higher-order assemblies) that contain them; this interpretation is confirmed in experiments with both tag reversal in co-expression and antibody reversal in western blotting. Upon co-transfecting Myc- and Flag-tagged constructs, resulting tetramers can be calculated to be composed of different species (MycMycMycMyc, MycMycMycFlag, MycMycFlagFlag, MycFlagFlagFlag, and FlagFlagFlagFlag), but it is shown that outcomes predicted by models based on MycMycFlagFlag oligomers are not changed in a major way by the occurrence of the additional species.

Bivalent phenethylamines as novel dopamine transporter inhibitors: evidence for multiple substrate-binding sites in a single transporter

Bivalent ligands--compounds incorporating two receptor-interacting moieties linked by a flexible chain--often exhibit profoundly enhanced binding affinity compared with their monovalent components, implying concurrent binding to multiple sites on the target protein. It is generally assumed that neurotransmitter sodium symporter (NSS) proteins, such as the dopamine transporter (DAT), contain a single domain responsible for recognition of substrate molecules. In this report, we show that molecules possessing two substrate-like phenylalkylamine moieties linked by a progressively longer aliphatic spacer act as progressively more potent DAT inhibitors (rather than substrates). One compound bearing two dopamine (DA)-like pharmacophoric 'heads' separated by an 8-carbon linker achieved an 82-fold gain in inhibition of [(3)H] 2beta-carbomethoxy-3beta-(4-fluorophenyl)-tropane (CFT) binding compared with DA itself; bivalent compounds with a 6-carbon linker and heterologous combinations of DA-, amphetamine- and beta-phenethylamine-like heads all resulted in considerable and comparable gains in DAT affinity. A series of short-chain bivalent-like compounds with a single N-linkage was also identified, the most potent of which displayed a 74-fold gain in binding affinity. Computational modelling of the DAT protein and docking of the two most potent bivalent (-like) ligands suggested simultaneous occupancy of two discrete substrate-binding domains. Assays with the DAT mutants W84L and D313N--previously employed by our laboratory to probe conformation-specific binding of different structural classes of DAT inhibitors--indicated a bias of the bivalent ligands for inward-facing transporters. Our results strongly indicate the existence of multiple DAT substrate-interaction sites, implying that it is possible to design novel types of DAT inhibitors based upon the 'multivalent ligand' strategy.

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.

Interaction of catechol and non-catechol substrates with externally or internally facing dopamine transporters

Our previous work suggested that collapsing the Na(+) gradient and membrane potential converts the dopamine (DA) transporter (DAT) to an inward-facing conformation with a different substrate binding profile. Here, DAT expressing human embryonic kidney 293 cells were permeabilized with digitonin, disrupting ion/voltage gradients and allowing passage of DAT substrates. The potency of p-tyramine and other non-catechols (d-amphetamine, beta-phenethylamine, MPP(+)) in inhibiting cocaine analog binding to DAT in digitonin-treated cells was markedly weakened to a level similar to that observed in cell-free membranes. In contrast, the potency of DA and another catechol, norepinephrine, was not significantly changed by the same treatment, whereas epinephrine showed only a modest reduction. These findings suggest that catechol substrates interact symmetrically with both sides of DAT and non-catechol substrates, favoring binding to outward-facing transporter. In the cocaine analog binding assay, the mutant W84L displayed enhanced intrinsic binding affinity for substrates in interacting with both outward- and inward-facing states; D313N showed wild-type-like symmetric binding; but D267L and E428Q showed an apparent improvement in the permeation pathway from the external face towards the substrate site. Thus, the structure of both substrate and transporter play a role in the sidedness and mode of interaction between them.

Role of the dopamine transporter in the action of psychostimulants, nicotine, and other drugs of abuse

A number of studies over the last two decades have demonstrated the critical importance of dopamine (DA) in the behavioral pharmacology and addictive properties of abused drugs. The DA transporter (DAT) is a major target for drugs of abuse in the category of psychostimulants, and for methylphenidate (MPH), a drug used for treating attention deficit hyperactivity disorder (ADHD), which can also be a psychostimulant drug of abuse. Other drugs of abuse such as nicotine, ethanol, heroin and morphine interact with the DAT in more indirect ways. Despite the different ways in which drugs of abuse can affect DAT function, one evolving theme in all cases is regulation of the DAT at the level of surface expression. DAT function is dynamically regulated by multiple intracellular and extracellular signaling pathways and several protein-protein interactions. In addition, DAT expression is regulated through the removal (internalization) and recycling of the protein from the cell surface. Furthermore, recent studies have demonstrated that individual differences in response to novel environments and psychostimulants can be predicted based on individual basal functional DAT expression. Although current knowledge of multiple factors regulating DAT activity has greatly expanded, many aspects of this regulation remain to be elucidated; these data will enable efforts to identify drugs that might be used therapeutically for drug dependence therapeutics.

Interaction of cocaine-, benztropine-, and GBR12909-like compounds with wild-type and mutant human dopamine transporters: molecular features that differentially determine antagonist-binding properties

The widely abused psychostimulant cocaine is thought to elicit its reinforcing effects primarily via inhibition of the neuronal dopamine transporter (DAT). However, not all DAT inhibitors share cocaine's behavioral profile, despite similar or greater affinity for the DAT. This may be due to differential molecular interactions with the DAT. Our previous work using transporter mutants with altered conformational equilibrium (W84L and D313N) indicated that benztropine and GBR12909 interact with the DAT in a different manner than cocaine. Here, we expand upon these previous findings, studying a number of structurally different DAT inhibitors for their ability to inhibit [(3)H]CFT binding to wild-type, W84L and D313N transporters. We systematically tested structural intermediates between cocaine and benztropine, structural hybrids of benztropine and GBR12909 and a number of other structurally heterologous inhibitors. Derivatives of the stimulant desoxypipradrol (2-benzhydrylpiperidine) exhibited a cocaine-like binding profile with respect to mutation, whereas compounds possessing the diphenylmethoxy moiety of benztropine and GBR12909 were dissimilar to cocaine-like compounds. In tests with specific isomers of cocaine and tropane analogues, compounds with 3alpha stereochemistry tended to exhibit benztropine-like binding, whereas those with 3beta stereochemistry were more cocaine-like. Our results point to the importance of specific molecular features--most notably the presence of a diphenylmethoxy moiety--in determining a compound's binding profile. This study furthers the concept of using DAT mutants to differentiate cocaine-like inhibitors from atypical inhibitors in vitro. Further studies of the molecular features that define inhibitor-transporter interaction could lead to the development of DAT inhibitors with differential clinical utility.

Labeling of dopamine transporter transmembrane domain 1 with the tropane ligand N-[4-(4-azido-3-[125I]iodophenyl)butyl]-2beta-carbomethoxy-3beta-(4-chlorophenyl)tropane implicates proximity of cocaine and substrate active sites

The novel photoaffinity ligand N-[4-(4-azido-3-(125)I-iodophenyl)-butyl]-2-beta-carbomethoxy-3beta-(4-chlorophenyl) tropane ([(125)I]MFZ 2-24) was used to investigate the site for cocaine binding on the dopamine transporter (DAT). [(125)I]MFZ 2-24 irreversibly labeled both rat striatal and expressed human DAT with high affinity and appropriate pharmacological specificity. Tryptic proteolysis of [(125)I]MFZ 2-24 labeled DAT followed by epitope-specific immunoprecipitation demonstrated that the ligand becomes adducted almost exclusively to transmembrane domains (TMs) 1-2. Further localization of [(125)I]MFZ 2-24 incorporation achieved by proteolyzing labeled wild-type and methionine mutant DATs with cyanogen bromide identified the sequence between residues 68 and 80 in TM1 as the ligand adduction site. This is in marked contrast to the previously identified attachment of the photoaffinity label [(125)I]RTI 82 in TM6. Because [(125)I]MFZ 2-24 and [(125)I]RTI 82 possess identical tropane pharmacophores and differ only in the placement of the reactive azido moieties, their distinct incorporation profiles identify the regions of the protein adjacent to different aspects of the cocaine molecule. These findings thus strongly support the direct interaction of cocaine on DAT with TM1 and TM6, both of which have been implicated by mutagenesis and homology to a bacterial leucine transporter as active sites for substrates. These results directly establish the proximity of TMs 1 and 6 in DAT and suggest that the mechanism of transport inhibition by cocaine involves close interactions with multiple regions of the substrate permeation pathway.

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.

Investigation of the functional roles of the MELAL and GQXXRXG motifs of the human noradrenaline transporter using cysteine mutants

The study examines the roles of the highly conserved MELAL and GQXXRXG motifs, located in the second transmembrane domain and the first intracellular loop of the human noradrenaline transporter (hNET). We have previously shown that this region does not directly participate in the NET substrate translocation pathway [Sucic, S., and Bryan-Lluka, L.J., 2005. Roles of transmembrane domain 2 and the first intracellular loop in human noradrenaline transporter function: pharmacological and SCAM analysis. J. Neurochem. 94, 1620-1630.], while the current report focuses on the importance of this region in determining other functional properties of the hNET. Mutation to cysteine of the wild-type residues was carried out by site-directed mutagenesis of hNET cDNA. The wild-type and mutant hNETs were expressed in transiently transfected COS-7 cells and the effects of these mutations were pharmacologically examined. The results indicate that the GQXXRXG motif is important for the binding of cocaine, but not antidepressants. The hN120C mutant caused an 11-fold increase in the binding affinity of cocaine, compared to the wild-type hNET, while hQ118C, hY119C, hR121C and hE122C showed smaller increases. Interestingly, the apparent affinities of cocaine for some of these mutants were either decreased or unchanged, contrasting with the effects observed from the binding studies. The hE113C mutant in the MELAL motif caused very marked (over 400-fold) reductions in the binding affinities of substrates, but had no effects on the binding affinities of cocaine or antidepressants. Overall, the MELAL and GQXXRXG motifs are important determinants of NET cell surface expression and substrate and inhibitor binding. The results further suggest that the binding sites for substrates, cocaine and antidepressants on the NET are distinct but overlapping.

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.

Substrates and inhibitors display different sensitivity to expression level of the dopamine transporter in heterologously expressing cells

The use of heterologous expression systems for studying dopamine (DA) transporter (DAT) function has provided important information corroborating and complementing in situ obtained knowledge. Preliminary experiments with human embryonic kidney cells (HEK293) heterologously expressing varying amounts of DAT suggested fluctuations in the potency of cocaine in inhibiting DA uptake and led to the present systematic assessment of the impact of the density of DAT on its function. Transiently expressing intact HEK293 cells, transfected with increasing amounts of DAT cDNA, displayed increasing levels of surface DAT, binding of the cocaine analog [(3)H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane ([(3)H]CFT), and uptake of [(3)H]DA, [(3)H]N-methyl-4-phenylpyridinium ([(3)H]MPP(+)), [(3)H]norepinephrine, and [(3)H]serotonin. However, the amount of DAT cDNA and the DAT expression level required to produce 50% of maximal activity was threefold higher for CFT binding than for DA uptake. Increased DAT expression was accompanied by weakened potency in inhibiting [(3)H]DA uptake for cocaine, CFT, benztropine, and its analog JHW025, GBR 12909 and mazindol; their potency in inhibiting [(3)H]CFT binding was unaffected. Inhibition of uptake by the substrates DA, m-tyramine, d-amphetamine, or MPP(+) was also unaffected. Increasing DAT in stably expressing HEK293 cells by stimulation of gene expression with sodium butyrate also decreased the uptake inhibitory potency of a number of the above blockers without affecting the interaction between substrates and DAT. The present results prompt discussion of models explaining how factors regulating DAT expression at the plasma membrane can regulate DAT function and pharmacology.

The norepinephrine transporter in physiology and disease

The norepinephrine transporter (NET) terminates noradrenergic signalling by rapid re-uptake of neuronally released norepinephrine (NE) into presynaptic terminals. NET exerts a fine regulated control over NE-mediated behavioural and physiological effects including mood, depression, feeding behaviour, cognition, regulation of blood pressure and heart rate. NET is a target of several drugs which are therapeutically used in the treatment or diagnosis of disorders among which depression, attention-deficit hyperactivity disorder and feeding disturbances are the most common. Individual genetic variations in the gene encoding the human NET (hNET), located at chromosome 16q12.2, may contribute to the pathogenesis of those diseases. An increasing number of studies concerning the identification of single nucleotide polymorphisms in the hNET gene and their potential association with disease as well as the functional investigation of naturally occurring or induced amino acid variations in hNET have contributed to a better understanding of NET function, regulation and genetic contribution to disorders. This review will reflect the current knowledge in the field of NET from its initial discovery until now.

Conserved serine residues in serotonin transporter contribute to high-affinity cocaine binding

Serotonin transporter (SERT) is one of the key protein targets of cocaine. Despite intensive studies, it is not clear where cocaine binds to its targets and what residues are involved in cocaine binding. We have cloned the serotonin transporter from silkworm (Bombyx mori, bmSERT). When expressed in cultured cells, bmSERT is over 20-fold less sensitive to cocaine than Drosophila melanogaster SERT (dmSERT). We performed species-scanning mutagenesis using bmSERT and dmSERT. There are two adjacent threonine residues in transmembrane domain 12 of bmSERT where the corresponding residues are two serines in dmSERT and in all known mammalian monoamine transporters. Replacing the serine residues with threonines in dmSERT reduces cocaine sensitivity; while switching the two threonine residues in bmSERT to serines increased cocaine sensitivity. Mutations at the corresponding residues in dopamine transporter also changed cocaine affinity. Our results suggest that the conserved serine residues in SERT contribute to high-affinity cocaine binding.

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

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

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.

A pincer-like configuration of TM2 in the human dopamine transporter is responsible for indirect effects on cocaine binding

The second transmembrane segment (TM2) of DAT and other neurotransmitter transporters has been proposed to play a role in oligomerization as well as in cocaine binding. In an attempt to determine whether TM2 contributes to the binding site and/or transport pathway of DAT, we mutated to cysteine, one at a time, 25 residues in TM2 - from Phe98 to Gln122 - in an appropriate DAT background construct. Four of the mutants, F98C, G110C, P112C, and E117C, did not express at the cell surface, and G121C was inactive, despite its presence on the cell surface. Of the 21 mutants that expressed, none of the substituted cysteines reacted with MTSEA biotin-CAP, and none of the 20 functional mutants was sensitive to MTSEA or MTSET. Thus, TM2 does not appear to be water-accessible, based both on the lack of functional effects of charged MTS derivatives, and on the biochemical determination of lack of reaction with a biotinylated MTS derivative. This leads to the conclusion that TM2 does not contribute directly to the substrate-binding site or the transport pathway, and suggests that the observed effect of mutations in this region on cocaine binding is indirect. Three mutants, M106C, V107C and I108C, were crosslinked by treatment with HgCl(2). This crosslinking was inhibited by the presence of the cocaine analogue MFZ 2-12, likely due to a conformational rearrangement in TM2 upon inhibitor binding. However, the lack of crosslinking of cysteines substituted for Leu99, Leu113 and Leu120 - three of the residues that along with Met106 form a leucine heptad repeat in TM2 - makes it unlikely that this leucine repeat plays a role in symmetrical TM2 dimerization. Importantly, a high-resolution structure of LeuT, a sodium-dependent leucine transporter that is sufficiently homologous to DAT to suggest a high degree of structural similarity, became available while this manuscript was under review. We have taken advantage of this structure to explore further and interpret our experimental results in a rigorous structural context.

A triple mutation in the second transmembrane domain of mouse dopamine transporter markedly decreases sensitivity to cocaine and methylphenidate

Previously, we reported that Phe105 in transmembrane domain 2 of the mouse dopamine transporter (DAT) is crucial for high-affinity cocaine binding. In the current study, we investigated whether other residues surrounding Phe105 also affect the potency of cocaine inhibition. After three rounds of sequential random mutagenesis at these residues, we found a triple mutant (L104V, F105C and A109V) of mouse DAT that retained over 50% uptake activity and was 69-fold less sensitive to cocaine inhibition when compared with the wild-type mouse DAT. The triple mutation also resulted in a 47-fold decrease in sensitivity to methylphenidate inhibition, suggesting that the binding sites for cocaine and methylphenidate may overlap. In contrast, the inhibition of dopamine uptake by amphetamine or methamphetamine was not significantly changed by the mutations, suggesting that the binding sites for the amphetamines differ from those for cocaine and methylphenidate. Such functional but cocaine-insensitive DAT mutants can be used to generate a knock-in mouse line to study the role of DAT in cocaine addiction.

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.

A comprehensive atlas of the topography of functional groups of the dopamine transporter

The neuronal dopamine transporter (DAT) is a transmembrane transporter that clears DA from the synaptic cleft. Knowledge of DAT functional group topography is a prerequisite for understanding the molecular basis of transporter function, the actions of psychostimulant drugs, and mechanisms of dopaminergic neurodegeneration. Information concerning the molecular interactions of drugs of abuse (such as cocaine, amphetamine, and methamphetamine) with the DAT at the functional group level may also aid in the development of compounds useful as therapeutic agents for the treatment of drug abuse. This review will provide a cumulative and comprehensive focus on the amino acid functional group topography of the rat and human DATs, as revealed by protein chemical modification and the techniques of site-directed mutagenesis. The results from these studies, represented mostly by site-directed mutagenesis, can be classified into several main categories: modifications without substantial affects on substrate transport, DAT membrane expression, or cocaine analog binding; those modifications which alter both substrate transport and cocaine analog binding; and those that affect DAT membrane expression. Finally, some modifications can selectively affect either substrate transport or cocaine analog binding. Taken together, these literature results show that domains for substrates and cocaine analogs are formed by interactions with multiple and sometimes distinct DAT functional groups.

Interactions of cations and anions with the binding of uptake blockers to the dopamine transporter

Uptake blockers and substrates are likely to recognise a common binding domain on the dopamine neuronal transporter (DAT). Among cations that form ionic gradients at the level of the cellular plasma membrane, Na+ is the only one that can stimulate their binding. The binding stimulation appears over Na+ concentrations ranging from 0 to 10-60 mM; at higher Na+ concentrations, binding reaches a plateau or decreases, according to the uptake blocker that is studied. The majority of the other cations, including K+, Ca2+, Mg2+ and Tris+, inhibit the binding of uptake blockers. Several metals impair binding to the DAT and/or the dopamine transport, but, under specific conditions, some of them, and chiefly Zn2+, stimulate binding. The complex relationships between cations, uptake blockers and the DAT suggest that cations recognise at least three different sites: the first one, site 1, is for cation-induced binding inhibition; the second one, site 2, is for Na+-induced binding stimulation; and the third one, site 3, is for Zn2+-induced binding stimulation. Modelling of the interactions between Na+, K+ and radioligands allows a better understanding of the effects of cations at sites 1 and 2, and of uptake blockers at site 1. Some anions also facilitate the binding of uptake blockers to the DAT, as far as they are associated with Na+. The dependence of the binding of dopamine on ions could be involved in its preferential inward transport and used by uptake blockers for their own binding to the DAT.

Predicting functional sites in proteins: site-specific evolutionary models and their application to neurotransmitter transporters

Currently there exist several computational methods for predicting the functional sites in a set of homologous proteins based on their sequences. Due to difficulties in defining the functional site in a protein, it is not trivial to compare the performance of these methods, evaluate their limitations and quantify improvements by new approaches. Here, we use extensive mutation data from two proteins, Lac repressor and subtilisin, to perform such an analysis. Along with the evaluation of existing approaches, we describe a site class model of evolution as a tool to predict functional sites in proteins. The results indicate that this model, which simulates the evolution process at the amino acid level using site-specific substitution matrices, provides the most accurate information on functional sites in a given protein family. Secondly, we present an application of this model to neurotransmitter transporters, a superfamily of proteins of which we have limited experimental knowledge. Based on this application we present testable hypotheses regarding the mechanism of action of these proteins.

Interaction between dopamine and its transporter: role of intracellular sodium ions and membrane potential

The present study addresses the effect of intracellular Na(+) and membrane potential on the binding of dopamine (DA) to the dopamine transporter (DAT). Perforation of plasma membranes of DAT-expressing cells with gramicidin diminished DA uptake and decreased the potency (increases K(i)) of DA in inhibiting the binding of cocaine analog [(3)H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT). It also compromised the ability of external Na(+) to reduce DA K(i). No substantial effect on DA K(i) was observed upon gramicidin treatment in Na(+)-free buffer, membrane depolarization with high [K(+)](o), or elevation of [Na(+)](i) with monensin under non-depolarizing conditions. Elevation of DA K(i) was greater at more positive potentials when [Na(+)](i) was raised to a similar level, or at higher [Na(+)](i) when the membrane was depolarized to a similar level. In cells expressing D313N DAT, DA K(i) was significantly higher but less sensitive to gramicidin than that in wild-type (WT) cells. In contrast, DA K(i) in cell-free membranes was insensitive to Na(+), gramicidin, and D313N mutation. The data suggest that (i) intracellular Na(+) plays a role in affecting the external access to DA binding sites at DAT on depolarized plasma membranes of cells, and (ii) access to DA binding sites in cell-free membranes may occur from the intracellular side of the membrane. Unlike DA binding, CFT binding to both cells and membranes was sensitive to Na(+) and D313N mutation but insensitive to gramicidin, consistent with exclusively external access to sites that are different from but conformationally linked to those for DA.

The Role of N-Glycosylation in Function and Surface Trafficking of the Human Dopamine Transporter

The present study addressed the role of N-linked glycosylation of the human dopamine transporter (DAT) in its function with the help of mutants, in which canonical N-glycosylation sites have been removed (N181Q, N181Q,N188Q, and N181Q,N188Q,N205Q), expressed in human embryonic kidney-293 cells. Removal of canonical sites produced lower molecular weight species as did enzymatic deglycosylation or blockade of glycosylation, and all three canonical sites were found to carry sugars. Prevention of N-glycosylation reduced both surface and intracellular DAT. Although partially or non-glycosylated DAT was somewhat less represented at the surface, no evidence was found for preferential exclusion of such material from the plasma membrane, indicating that glycosylation is not essential for DAT expression. Non-glycosylated DAT was less stable at the surface as revealed by apparently enhanced endocytosis, consonant with weaker DAT immunofluorescence at the cell surface and stronger presence in cytosol in confocal analysis of the double and triple mutant. Non-glycosylated DAT did not transport dopamine as efficiently as wild-type DAT as judged from the sharp reduction in uptake V(max), and prevention of N-glycosylation enhanced the potency of cocaine-like drugs in inhibiting dopamine uptake into intact cells without changing their affinity for DAT when measured in membrane preparations prepared from these cells. Thus, non-glycosylated DAT at the cell surface displays appreciably reduced catalytic activity and altered inhibitor sensitivity compared with wild type.

Mutation of Trp84 and Asp313 of the dopamine transporter reveals similar mode of binding interaction for GBR12909 and benztropine as opposed to cocaine

The different psychomotor-stimulant effects of cocaine, GBR12909, and benztropine may partially stem from their different molecular actions on the dopamine transporter (DAT). To explore this possibility, we examined binding of these inhibitors to mutated DATs with altered Na(+) dependence of DAT activities and with enhanced binding of a cocaine analog, [(3)H]2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane (CFT). In [(3)H]CFT competition assays with intact cells, the mutation-induced change in the ability of Na(+) to enhance the apparent affinity of CFT, cocaine, GBR12909, and benztropine was inhibitor-independent. Thus, for the four inhibitors, the curve of [Na(+)] versus apparent ligand affinity was steeper at W84L compared with wild type, shallower at D313N, and flat at W84LD313N. At each mutant, the apparent affinity of CFT and cocaine was enhanced regardless of whether Na(+) was present. However, the apparent affinity of GBR12909 and benztropine for W84L was reduced in the absence of Na(+) but near normal in the presence of 130 mm Na(+), and that for D313N and W84LD313N was barely changed. At the single mutants, the alterations in Na(+) dependence and apparent affinity of the four inhibitors were comparable between [(3)H]CFT competition assays and [(3)H]dopamine uptake inhibition assays. These results demonstrate that DAT inhibitors producing different behavioral profiles can respond in an opposite way when residues of the DAT protein are mutated. For GBR12909 and benztropine, their cocaine-like changes in Na(+) dependence suggest that they prefer a DAT state similar to that for cocaine. However, their cocaine-unlike changes in apparent affinity argue that they, likely via their diphenylmethoxy moiety, share DAT binding epitopes that are different from those for cocaine.

Human dopamine transporter gene variation: effects of protein coding variants V55A and V382A on expression and uptake activities

The human dopamine transporter (DAT, SLC6A3) is an important 15 exon gene for dopamine neurotransmission and dopamine recycling. Common exon 15 variable number tandem repeat variants can be associated with attention deficit/hyperactivity disorder. Rarer single nucleotide polymorphisms produce missense variants including V55A and V382A. We now report studies of the functional influences of these DAT protein-coding variants. In COS cell transient-expression assays, V382A displays about half of the dopamine uptake velocity Vmax and cocaine analog binding Bmax values of wildtype DAT. V382A lowers dopamine's potency in inhibiting cocaine analog binding by six-fold. Cells expressing V382A or mixtures of V382A and wildtype DAT both display reduced plasma membrane and increased perinuclear expression, consistent with dominant effects of V328A on expression. V55A expresses normally but reveals a 1.7-fold-lower Km for dopamine uptake. Individuals with these human DAT protein variants could display altered dopamine systems.

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.

Aspartate 345 of the Dopamine Transporter Is Critical for Conformational Changes in Substrate Translocation and Cocaine Binding

The present study elucidated the role of aspartate 345, a residue conserved in the third intracellular loop of all Na+/Cl(-)-dependent neurotransmitter transporters, in conformational changes of the dopamine (DA) transporter. Asparagine substitution (D345N) resulted in near normal transporter expression on the cell surface but caused extremely low Vmax and Km values for DA uptake, converted the inhibitory effect of Zn2+ on DA uptake to a stimulatory one, and eliminated reverse transport. The cocaine-like inhibitor 2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane or the selective DA transporter inhibitor GBR12935 bound to D345N with a normal affinity and still inhibited DA uptake potently. However, the mutation reduced the binding capacity of the surface transporter for these two inhibitors by 90% or more. Moreover, the binding activity of D345N can be significantly improved by Zn2+ but not by Na+. These results are consistent with a defect in reorientation of the substrate-binding site to the extracellular side, leading to a loss of the outward-facing conformational state where external DA binds to initiate uptake and the inhibitors bind to initiate uptake inhibition. Alanine or glutamate substitution produced a similar phenotype, suggesting that both the negative charge and the residue volume at position 345 are vital. Furthermore, in intact cells, cocaine potentiated the reaction of the membrane-impermeant sulfhydryl reagent methanethiosulfonate ethyltrimethylammonium with the extracellularly located endogenous cysteines of D345N but not those of wild type, and this potentiation was blocked upon K+ substitution for Na+. Thus, cocaine binding to D345N likely induces a different and Na(+)-dependent conformational change, which may contribute to its Na(+)-dependent uptake inhibitory activity.

Identification of Intracellular Residues in the Dopamine Transporter Critical for Regulation of Transporter Conformation and Cocaine Binding

Recently we showed evidence that mutation of Tyr-335 to Ala (Y335A) in the human dopamine transporter (hDAT) alters the conformational equilibrium of the transport cycle. Here, by substituting, one at a time, 16 different bulky or charged intracellular residues, we identify three residues, Lys-264, Asp-345, and Asp-436, the mutation of which to alanine produces a phenotype similar to that of Y335A. Like Y335A, the mutants (K264A, D345A, and D436A) were characterized by low uptake capacity that was potentiated by Zn(2+). Moreover, the mutants displayed lower affinity for cocaine and other inhibitors, suggesting a role for these residues in maintaining the structural integrity of the inhibitor binding crevice. The conformational state of K264A, Y335A, and D345A was investigated by assessing the accessibility to MTSET ([2-(trimethylammonium)ethyl]-methanethiosulfonate) of a cysteine engineered into position 159 (I159C) in transmembrane segment 3 of the MTSET-insensitive "E2C" background (C90A/C306A). Unlike its effect at the corresponding position in the homologous norepinephrine transporter (NET I155C), MTSET did not inhibit uptake mediated by E2C I159C. Furthermore, no inhibition was observed upon treatment with MTSET in the presence of dopamine, cocaine, or Zn(2+). Without Zn(2+), E2C I159C/K264A, E2C I159C/Y335A, and E2C I159C/D345A were also not inactivated by MTSET. In the presence of Zn(2+) (10 microm), however, MTSET (0.5 mm) caused up to approximately 60% inactivation. As in NET I155C, this inactivation was protected by dopamine and enhanced by cocaine. These data are consistent with a Zn(2+)-dependent partial reversal of a constitutively altered conformational equilibrium in the mutant transporters. They also suggest that the conformational equilibrium produced by the mutations resembles that of the NET more than that of the DAT. Moreover, the data provide evidence that the cocaine-bound state of both DAT mutants and of the NET is structurally distinct from the cocaine-bound state of the DAT.