Studies of the biogenic amine transporters 15. Identification of novel allosteric dopamine transporter ligands with nanomolar potency

Unlocking the Potential of High-Quality Dopamine Transporter Pharmacological Data: Advancing Robust Machine Learning-Based QSAR Modeling

The dopamine transporter (DAT) plays a critical role in the central nervous system and has been implicated in numerous psychiatric disorders. The ligand-based approaches are instrumental to decipher the structure-activity relationship (SAR) of DAT ligands, especially the quantitative SAR (QSAR) modeling. By gathering and analyzing data from literature and databases, we systematically assemble a diverse range of ligands binding to DAT, aiming to discern the general features of DAT ligands and uncover the chemical space for potential novel DAT ligand scaffolds. The aggregation of DAT pharmacological activity data, particularly from databases like ChEMBL, provides a foundation for constructing robust QSAR models. The compilation and meticulous filtering of these data, establishing high-quality training datasets with specific divisions of pharmacological assays and data types, along with the application of QSAR modeling, prove to be a promising strategy for navigating the pertinent chemical space. Through a systematic comparison of DAT QSAR models using training datasets from various ChEMBL releases, we underscore the positive impact of enhanced data set quality and increased data set size on the predictive power of DAT QSAR models.

Discovery and Development of Monoamine Transporter Ligands

Monoamine transporters (MATs) are targets of a wide range of compounds that have been developed as therapeutic treatments for various neuropsychiatric and neurodegenerative disorders such as depression, ADHD, neuropathic pain, anxiety disorders, stimulant use disorders, epilepsy, and Parkinson's disease. The MAT family is comprised of three main members - the dopamine transporter (DAT), the norepinephrine transporter (NET), and the serotonin transporter (SERT). These transporters are through reuptake responsible for the clearance of their respective monoamine substrates from the extracellular space. The determination of X-ray crystal structures of MATs and their homologues bound with various substrates and ligands has resulted in a surge of structure-function-based studies of MATs to understand the molecular basis of transport function and the mechanism of various ligands that ultimately result in their behavioral effects. This review focusses on recent examples of ligand-based structure-activity relationship studies trying to overcome some of the challenges associated with previously developed MAT inhibitors. These studies have led to the discovery of unique and novel structurally diverse MAT ligands including allosteric modulators. These novel molecular scaffolds serve as leads for designing more effective therapeutic interventions by modulating the activities of MATs and ultimately their associated neurotransmission and behavioral effects.

SRI-32743, a novel allosteric modulator, attenuates HIV-1 Tat protein-induced inhibition of the dopamine transporter and alleviates the potentiation of cocaine reward in HIV-1 Tat transgenic mice

Cocaine abuse increases the incidence of HIV-1-associated neurocognitive disorders. We have demonstrated that HIV-1 transactivator of transcription (Tat) allosterically modulates dopamine (DA) reuptake through the human DA transporter (hDAT), potentially contributing to Tat-induced cognitive impairment and potentiation of cocaine conditioned place preference (CPP). This study determined the effects of a novel allosteric modulator of DAT, SRI-32743, on the interactions of HIV-1 Tat, DA, cocaine, and [³H]WIN35,428 with hDAT in vitro. SRI-32743 (50 nM) attenuated Tat-induced inhibition of [³H]DA uptake and decreased the cocaine-mediated dissociation of [³H]WIN35,428 binding in CHO cells expressing hDAT, suggesting a SRI-32743-mediated allosteric modulation of the Tat-DAT interaction. In further in vivo studies utilizing doxycycline-inducible Tat transgenic (iTat-tg) mice, 14 days of Tat expression significantly reduced the recognition index by 31.7% in the final phase of novel object recognition (NOR) and potentiated cocaine-CPP 2.7-fold compared to responses of vehicle-treated control iTat-tg mice. The Tat-induced NOR deficits and potentiation of cocaine-CPP were not observed in saline-treated iTat-tg or doxycycline-treated G-tg (Tat-null) mice. Systemic administration (i.p.) of SRI-32743 prior to behavioral testing ameliorated Tat-induced impairment of NOR (at a dose of 10 mg/kg) and the Tat-induced potentiation of cocaine-CPP (at doses of 1 or 10 mg/kg). These findings demonstrate that Tat and cocaine interactions with DAT may be regulated by compounds interacting at the DAT allosteric modulatory sites, suggesting a potential therapeutic intervention for HIV-infected patients with concurrent cocaine abuse.

Mutations of tyrosine 467 in the human norepinephrine transporter attenuate HIV-1 Tat-induced inhibition of dopamine transport while retaining physiological function

Dysregulation of dopaminergic transmission induced by the HIV-1 transactivator of transcription (Tat) has been implicated as a central factor in the development of HIV-1 associated neurocognitive disorders (HAND). We have demonstrated that the tyrosine470 residue of the human dopamine transporter (hDAT) plays a critical role in Tat-hDAT interaction. Based on the computational modeling predictions, the present study sought to examine the mutational effects of the tyrosine467 residue of the human norepinephrine transporter (hNET), a corresponding residue of the hDAT tyrosine470, on Tat-induced inhibition of reuptake of dopamine through the hNET. Mutations of the hNET tyrosine467 to a histidine (Y467H) or a phenylalanine (Y467F) displayed similar kinetic properties of reuptake of [3H]dopamine and [3H]norepinephrine in PC12 cells expressing wild-type hNET and its mutants. Compared to wild-type hNET, neither of Y467H or Y467F altered Bmax and Kd values of [3H]WIN35,428 binding, whereas Y467H but not Y467F decreased the Bmax of [3H]nisoxetine binding without changes in Kd. Y467H also increased the affinity of nisoxetine for inhibiting [3H]dopamine uptake relative to wild-type hNET. Recombinant Tat1-86 (140 nM) induced a significant reduction of [3H]dopamine uptake in wild-type hNET, which was attenuated in both Y467H and Y467F. Compared to wild-type hNET, neither Y467H or Y467F altered [3H]dopamine efflux in CHO cells expressing WT hNET and mutants, whereas Y467F but not Y467H decreased [3H]MPP+ efflux. These results demonstrate tyrosine467 as a functional recognition residue in the hNET for Tat-induced inhibition of dopamine transport and provide a novel insight into the molecular basis for developing selective compounds that target Tat-NET interactions in the context of HAND.

Binding Mode of Human Norepinephrine Transporter Interacting with HIV-1 Tat

The increase of HIV infection in macrophages results in HIV proteins being released, like HIV Tat which impairs the function of monoamine transporters. HIV-infected patients have displayed increased synaptic levels of dopamine (DA) due to reduced binding and function of monoamine transporters such as the norepinephrine transporter (NET) and the dopamine transporter (DAT). Development of a three-dimensional model of the HIV-1 Tat-human NET (hNET) binding complex would help reveal how HIV-1 Tat causes toxicity in the neuron by affecting DA uptake. Here we use computational techniques such as molecular modeling to study microscopic properties and molecular dynamics of the HIV-1 Tat-hNET binding. These modeling techniques allow us to analyze noncovalent interactions and observe residue-residue contacts to verify a model structure. The modeling results studied here show that HIV-1 Tat-hNET binding is highly dynamic and that HIV-1 Tat preferentially binds to hNET in its outward-open state. In particular, HIV-1 Tat forms hydrogen bond interactions with side chains of hNET residues Y84, K88, and T544. The favorable hydrogen bonding interactions of HIV-1 Tat with the hNET side chain residues Y84 and T544 have been validated by our subsequently performed DA uptake activity assays and site-directed mutagenesis, suggesting that the modeled HIV-1 Tat-hNET binding mode is reasonable. These mechanistic and structural insights gained through homology models discussed in this study are expected to encourage the pursuit of pharmacological and biochemical studies on HIV-1 Tat interacting with hNET mechanisms and detailed structures.

Allosteric Modulation of Neurotransmitter Transporters as a Therapeutic Strategy

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

Cell-Based Radiotracer Binding and Uptake Inhibition Assays: A Comparison of In Vitro Methods to Assess the Potency of Drugs That Target Monoamine Transporters

High-affinity monoamine transporters are targets for prescribed medications and stimulant drugs of abuse. Therefore, assessing monoamine transporter activity for candidate medications and newly-emerging drugs of abuse provides essential information for industry, academia, and public health. Radiotracer binding and uptake inhibition are the gold standard assays for determining drug–transporter interaction profiles. The combined results from such assays yield a unique biochemical fingerprint for each compound. Over time, different assay methods have been developed to assess transporter activity, and the comparability of data across various assay platforms remains largely unclear. Here, we compare the effects of six well-established stimulants in two different cell-based uptake inhibition assays, one method using adherent cells and the other using suspended cells. Furthermore, we compare the data from transfected cell lines derived from different laboratories and data reported from rat synaptosomes. For transporter inhibitors, IC₅₀ values obtained by the two experimental methods were comparable, but using different transfected cell lines yielded disparate results. For transporter substrates, differences between the two cell lines were less pronounced but the drugs displayed different inhibition potencies when evaluated by the two methods. Our study illustrates the inherent limitations when comparing transporter inhibition data from different laboratories and stresses the importance of including appropriate control experiments with reference compounds when investigating new drugs of interest.

Pharmacological Characterization of a Betaine/GABA Transporter 1 (BGT1) Inhibitor Displaying an Unusual Biphasic Inhibition Profile and Anti-seizure Effects

Focal epileptic seizures can in some patients be managed by inhibiting γ-aminobutyric acid (GABA) uptake via the GABA transporter 1 (GAT1) using tiagabine (Gabitril®). Synergistic anti-seizure effects achieved by inhibition of both GAT1 and the betaine/GABA transporter (BGT1) by tiagabine and EF1502, compared to tiagabine alone, suggest BGT1 as a target in epilepsy. Yet, selective BGT1 inhibitors are needed for validation of this hypothesis. In that search, a series of BGT1 inhibitors typified by (1R,2S)-2-((4,4-bis(3-methylthiophen-2-yl)but-3-en-yl)(methyl)amino)cyclohexanecarboxylic acid (SBV2-114) was developed. A thorough pharmacological characterization of SBV2-114 using a cell-based [³H]GABA uptake assay at heterologously expressed BGT1, revealed an elusive biphasic inhibition profile with two IC₅₀ values (4.7 and 556 μM). The biphasic profile was common for this structural class of compounds, including EF1502, and was confirmed in the MDCK II cell line endogenously expressing BGT1. The possibility of two binding sites for SBV2-114 at BGT1 was assessed by computational docking studies and examined by mutational studies. These investigations confirmed that the conserved residue Q299 in BGT1 is involved in, but not solely responsible for the biphasic inhibition profile of SBV2-114. Animal studies revealed anti-seizure effects of SBV2-114 in two mouse models, supporting a function of BGT1 in epilepsy. However, as SBV2-114 is apparent to be rather non-selective for BGT1, the translational relevance of this observation is unknown. Nevertheless, SBV2-114 constitutes a valuable tool compound to study the molecular mechanism of an emerging biphasic profile of BGT1-mediated GABA transport and the putative involvement of two binding sites for this class of compounds.

How to rescue misfolded SERT, DAT and NET: targeting conformational intermediates with atypical inhibitors and partial releasers

Point mutations in the coding sequence for solute carrier 6 (SLC6) family members result in clinically relevant disorders, which are often accounted for by a loss-of-function phenotype. In many instances, the mutated transporter is not delivered to the cell surface because it is retained in the endoplasmic reticulum (ER). The underlying defect is improper folding of the transporter and is the case for many of the known dopamine transporter mutants. The monoamine transporters, i.e. the transporters for norepinephrine (NET/SLC6A2), dopamine (DAT/SLC6A3) and serotonin (SERT/SLC6A4), have a rich pharmacology; hence, their folding-deficient mutants lend themselves to explore the concept of pharmacological chaperoning. Pharmacochaperones are small molecules, which bind to folding intermediates with exquisite specificity and scaffold them to a folded state, which is exported from the ER and delivered to the cell surface. Pharmacochaperoning of mutant monoamine transporters, however, is not straightforward: ionic conditions within the ER are not conducive to binding of most typical monoamine transporter ligands. A collection of compounds exists, which are classified as atypical ligands because they trap monoamine transporters in unique conformational states. The atypical binding mode of some DAT inhibitors has been linked to their anti-addictive action. Here, we propose that atypical ligands and also compounds recently classified as partial releasers can serve as pharmacochaperones.

Insights into the mechanism and pharmacology of neurotransmitter sodium symporters

Neurotransmitter sodium symporters (NSS) belong to the SLC6 family of solute carriers and play an essential role in neurotransmitter homeostasis throughout the body. In the past decade, structural studies employing bacterial orthologs of NSSs have provided insight into the mechanism of neurotransmitter transport. While the overall architecture of SLC6 transporters is conserved among species, in comparison to the bacterial homologs, the eukaryotic SLC6 family members harbor differences in amino acid sequence and molecular structure, which underpins their functional and pharmacological diversity, as well as their ligand specificity. Here, we review the structures and mechanisms of eukaryotic NSSs, focusing on the molecular basis for ligand recognition and on transport mechanism.

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 V_max 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.

Functional Selectivity and Partial Efficacy at the Monoamine Transporters: A Unified Model of Allosteric Modulation and Amphetamine-Induced Substrate Release

All clinically approved drugs targeting the plasmalemmal transporters for dopamine, norepinephrine, and serotonin act either as competitive uptake inhibitors or as amphetamine-like releasers. Monoamine transporter (MAT) ligands that allosterically affect MAT-mediated substrate uptake, release, or both were recently discovered. Their modes of action have not yet been explained in a unified framework. Here, we go beyond competitive inhibitors and classic amphetamines and introduce concepts for partial efficacy at and allosteric modulation of MATs. After we elaborate on a kinetic account for amphetamine action, we provide an explanation for partial release (i.e., the observation that some amphetamines are less efficacious than others in inducing monoamine efflux). We then elucidate mechanisms of allosteric inhibition and stimulation of MATs, which can be functionally selective for either substrate uptake or amphetamine-induced release. These concepts are integrated into a parsimonious kinetic framework, which relies exclusively on physiologic transport modes (without any deviation from an alternating access mechanism). The model posits cooperative substrate and Na⁺ binding and functional selectivity by conformational selection (i.e., preference of the allosteric modulators for the substrate-loaded or substrate-free states of the transporter). Thus, current knowledge about the kinetics of monoamine transport is sufficiently detailed to provide a quantitative description of the releasing action of amphetamines, of substrate uptake, and of selective modulation thereof by allosteric modulators.

Interactions between Cocaine and the Putative Allosteric Dopamine Transporter Ligand SRI-31142

Drugs that inhibit the dopamine (DA) transporter (DAT) include both therapeutic agents and abused drugs. Recent studies identified a novel series of putative allosteric DAT inhibitors, but the in vivo effects of these compounds are unknown. This study examined the abuse-related behavioral and neurochemical effects produced in rats by SRI-31142 [2-(7-methylimidazo[1,2-a]pyridin-6-yl)-N-(2-phenyl-2-(pyridin-4-yl)ethyl)quinazolin-4-amine], one compound from this series. In behavioral studies, intracranial self-stimulation (ICSS) was used to compare the effects produced by SRI-31142, the abused and nonselective DAT inhibitor cocaine, and the selective DAT inhibitor GBR-12935 [1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine]. In neurochemical studies, in vivo microdialysis was used to compare the effects of SRI-31142 and cocaine on levels of DA and serotonin in nucleus accumbens (NAc). The effects of SRI-31142 in combination with cocaine were also examined in both procedures. In contrast to cocaine and GBR-12935, SRI-31142 failed to produce abuse-related increases in ICSS or NAc DA; instead, SRI-31142 only decreased ICSS and NAc DA at a dose that was also sufficient to block cocaine-induced increases in ICSS and NAc DA. Pharmacokinetic studies suggested low but adequate brain penetration of SRI-31142, in vitro binding studies failed to identify likely non-DAT targets, and in vitro functional assays failed to confirm DA uptake inhibition in an assay of DAT-mediated fluorescent signals in live cells. These results indicate that SRI-31142 does not produce cocaine-like abuse-related effects in rats. SRI-31142 may have utility to block cocaine effects and may warrant further study as a candidate pharmacotherapy; however, the role of DAT in mediating these effects is unclear, and side effects may be a limiting factor.

Identification of the benztropine analog [125I]GA II 34 binding site on the human dopamine transporter

The dopamine transporter (DAT) is a neuronal membrane protein that is responsible for reuptake of dopamine (DA) from the synapse and functions as a major determinant in control of DA neurotransmission. Cocaine and many psychostimulant drugs bind to DAT and block reuptake, inducing DA overflow that forms the neurochemical basis for euphoria and addiction. Paradoxically, however, some ligands such as benztropine (BZT) bind to DAT and inhibit reuptake but do not produce these effects, and it has been hypothesized that differential mechanisms of binding may stabilize specific transporter conformations that affect downstream neurochemical or behavioral outcomes. To investigate the binding mechanisms of BZT on DAT we used the photoaffinity BZT analog [¹²⁵I]N-[n-butyl-4-(4‴-azido-3‴-iodophenyl)]-4',4″-difluoro-3α-(diphenylmethoxy)tropane ([¹²⁵I]GA II 34) to identify the site of cross-linking and predict the binding pose relative to that of previously-examined cocaine photoaffinity analogs. Biochemical findings show that adduction of [¹²⁵I]GA II 34 occurs at residues Asp79 or Leu80 in TM1, with molecular modeling supporting adduction to Leu80 and a pharmacophore pose in the central S1 site similar to that of cocaine and cocaine analogs. Substituted cysteine accessibility method protection analyses verified these findings, but identified some differences in structural stabilization relative to cocaine that may relate to BZT neurochemical outcomes.

In Vitro Assays for the Functional Characterization of the Dopamine Transporter (DAT)

Detailed in this unit are protocols for studying the in vitro uptake of dopamine (DA) as a means for defining the functional characteristics of dopamine transporters. All assays are performed using commercially available cell lines that transiently express the transporter under investigation. The three main assays provided are: a kinetic assay to calculate the affinity (K_M ) and maximal velocity (V_max ) of radiolabeled DA uptake into cells; concentration-response assays to measure the potencies (IC₅₀ /K_i values) of test compounds as transport inhibitors; and an efflux assay to assess the ability and potency (EC₅₀ ) of a ligand to elicit reverse transport of DA accumulated in the cell. Although the methods are described using DAT and its ligands, the same procedure can be employed for studying serotonin and norepinephrine transporters as well. © 2017 by John Wiley & Sons, Inc.

Overview of Monoamine Transporters

The dopamine (DAT), serotonin (SERT), and norepinephrine (NET) transporters, which are collectively referred to as monoamine transporters (MATs), play significant roles in regulating the neuronal response to these neurotransmitters. MATs terminate the action of these neurotransmitters by translocating them from the synaptic space into the presynaptic neurons. These three transmitters are responsible for controlling a number of physiological, emotional, and behavioral functions, with their transporters being the site of action of drugs employed for the treatment of a variety of conditions, including depression, anxiety, ADHD, schizophrenia, and psychostimulant abuse. Provided in this unit is information on the localization and regulation of MATs and the structural components of these proteins most responsible for the translocation process. Also included is a brief description of the evolution of ligands that interact with these transporters, as well as current theories concerning the pharmacological effects of substances that interact with these sites, including the molecular mechanisms of action of uptake inhibitors and allosteric modulators. Data relating to the presence, structure, and functions of allosteric modulators are included as well. The aim of this review is to provide background information on MATs to those who are new to this field, with a focus on the therapeutic potential of compounds that interact with these substrate transport sites. © 2017 by John Wiley & Sons, Inc.

Inhibitor mechanisms in the S1 binding site of the dopamine transporter defined by multi-site molecular tethering of photoactive cocaine analogs

Dopamine transporter (DAT) blockers like cocaine and many other abused and therapeutic drugs bind and stabilize an inactive form of the transporter inhibiting reuptake of extracellular dopamine (DA). The resulting increases in DA lead to the ability of these drugs to induce psychomotor alterations and addiction, but paradoxical findings in animal models indicate that not all DAT antagonists induce cocaine-like behavioral outcomes. How this occurs is not known, but one possibility is that uptake inhibitors may bind at multiple locations or in different poses to stabilize distinct conformational transporter states associated with differential neurochemical endpoints. Understanding the molecular mechanisms governing the pharmacological inhibition of DAT is therefore key for understanding the requisite interactions for behavioral modulation and addiction. Previously, we leveraged complementary computational docking, mutagenesis, peptide mapping, and substituted cysteine accessibility strategies to identify the specific adduction site and binding pose for the crosslinkable, photoactive cocaine analog, RTI 82, which contains a photoactive azide attached at the 2β position of the tropane pharmacophore. Here, we utilize similar methodology with a different cocaine analog N-[4-(4-azido-3-I-iodophenyl)-butyl]-2-carbomethoxy-3-(4-chlorophenyl)tropane, MFZ 2-24, where the photoactive azide is attached to the tropane nitrogen. In contrast to RTI 82, which crosslinked into residue Phe319 of transmembrane domain (TM) 6, our findings show that MFZ 2-24 adducts to Leu80 in TM1 with modeling and biochemical data indicating that MFZ 2-24, like RTI 82, occupies the central S1 binding pocket with the (+)-charged tropane ring nitrogen coordinating with the (-)-charged carboxyl side chain of Asp79. The superimposition of the tropane ring in the three-dimensional binding poses of these two distinct ligands provides strong experimental evidence for cocaine binding to DAT in the S1 site and the importance of the tropane moiety in competitive mechanisms of DA uptake inhibition. These findings set a structure-function baseline for comparison of typical and atypical DAT inhibitors and how their interactions with DAT could lead to the loss of cocaine-like behaviors.

The role of human dopamine transporter in NeuroAIDS

HIV-associated neurocognitive disorder (HAND) remains highly prevalent in HIV infected individuals and represents a special group of neuropathological disorders, which are associated with HIV-1 viral proteins, such as transactivator of transcription (Tat) protein. Cocaine abuse increases the incidence of HAND and exacerbates its severity by enhancing viral replication. Perturbation of dopaminergic transmission has been implicated as a risk factor of HAND. The presynaptic dopamine (DA) transporter (DAT) is essential for DA homeostasis and dopaminergic modulation of the brain function including cognition. Tat and cocaine synergistically elevate synaptic DA levels by acting directly on human DAT (hDAT), ultimately leading to dysregulation of DA transmission. Through integrated computational modeling and experimental validation, key residues have been identified in hDAT that play a critical role in Tat-induced inhibition of DAT and induce transporter conformational transitions. This review presents current information regarding neurological changes in DAT-mediated dopaminergic system associated with HIV infection, DAT-mediated adaptive responses to Tat as well as allosteric modulatory effects of novel compounds on hDAT. Understanding the molecular mechanisms by which Tat induces DAT-mediated dysregulation of DA system is of great clinical interest for identifying new targets for an early therapeutic intervention for HAND.

Allosteric modulatory effects of SRI-20041 and SRI-30827 on cocaine and HIV-1 Tat protein binding to human dopamine transporter

Dopamine transporter (DAT) is the target of cocaine and HIV-1 transactivator of transcription (Tat) protein. Identifying allosteric modulatory molecules with potential attenuation of cocaine and Tat binding to DAT are of great scientific and clinical interest. We demonstrated that tyrosine 470 and 88 act as functional recognition residues in human DAT (hDAT) for Tat-induced inhibition of DA transport and transporter conformational transitions. Here we investigated the allosteric modulatory effects of two allosteric ligands, SRI-20041 and SRI-30827 on cocaine binding on wild type (WT) hDAT, Y470 H and Y88 F mutants. Effect of SRI-30827 on Tat-induced inhibition of [³H]WIN35,428 binding was also determined. Compared to a competitive DAT inhibitor indatraline, both SRI-compounds displayed a similar decrease (30%) in IC₅₀ for inhibition of [³H]DA uptake by cocaine in WT hDAT. The addition of SRI-20041 or SRI-30827 following cocaine slowed the dissociation rate of [³H]WIN35,428 binding in WT hDAT relative to cocaine alone. Moreover, Y470H and Y88F hDAT potentiate the inhibitory effect of cocaine on DA uptake and attenuate the effects of SRI-compounds on cocaine-mediated dissociation rate. SRI-30827 attenuated Tat-induced inhibition of [³H]WIN35,428 binding. These observations demonstrate that tyrosine 470 and 88 are critical for allosteric modulatory effects of SRI-compounds on the interaction of cocaine with hDAT.

Structure-activity relationship studies of citalopram derivatives: examining substituents conferring selectivity for the allosteric site in the 5-HT transporter

BACKGROUND AND PURPOSE

The 5-HT transporter (SERT) is a target for antidepressant drugs. SERT possesses two binding sites: the orthosteric (S1) binding site, which is the presumed target for current SERT inhibitors, and an allosteric (S2) site for which potential therapeutic effects are unknown. The antidepressant drug citalopram displays high-affinity S1 binding and low-affinity S2 binding. To elucidate a possible therapeutic role of allosteric inhibition of SERT, a drug that specifically targets the allosteric site is required. The purpose of this study was to find a compound having higher selectivity towards the S2 site.

EXPERIMENTAL APPROACH

We performed a systematic structure-activity relationship study based on the scaffold of citalopram and the structurally closely related congener, talopram, which shows low-affinity S1 binding in SERT. The role of the four chemical substituents, which distinguish citalopram from talopram in conferring selectivity towards the S1 and S2 site, respectively, was assessed by determining the binding of 14 citalopram/talopram analogous to the S1 and S2 binding sites in SERT using membranes of COS7 cells transiently expressing SERT.

KEY RESULTS

The structure-activity relationship study revealed that dimethyl citalopram possesses the highest affinity for the allosteric site relative to the S1 site in SERT and has approximately twofold selectivity for the allosteric site relative to the S1 site in SERT.

CONCLUSIONS AND IMPLICATIONS

The compound could be a useful lead for future synthesis of drugs with high affinity and high selectivity towards the allosteric binding site.

Insights into the Modulation of Dopamine Transporter Function by Amphetamine, Orphenadrine, and Cocaine Binding

Human dopamine (DA) transporter (hDAT) regulates dopaminergic signaling in the central nervous system by maintaining the synaptic concentration of DA at physiological levels, upon reuptake of DA into presynaptic terminals. DA translocation involves the co-transport of two sodium ions and the channeling of a chloride ion, and it is achieved via alternating access between outward-facing (OF) and inward-facing states of DAT. hDAT is a target for addictive drugs, such as cocaine, amphetamine (AMPH), and therapeutic antidepressants. Our recent quantitative systems pharmacology study suggested that orphenadrine (ORPH), an anticholinergic agent and anti-Parkinson drug, might be repurposable as a DAT drug. Previous studies have shown that DAT-substrates like AMPH or -blockers like cocaine modulate the function of DAT in different ways. However, the molecular mechanisms of modulation remained elusive due to the lack of structural data on DAT. The newly resolved DAT structure from Drosophila melanogaster opens the way to a deeper understanding of the mechanism and time evolution of DAT–drug/ligand interactions. Using a combination of homology modeling, docking analysis, molecular dynamics simulations, and molecular biology experiments, we performed a comparative study of the binding properties of DA, AMPH, ORPH, and cocaine and their modulation of hDAT function. Simulations demonstrate that binding DA or AMPH drives a structural transition toward a functional form predisposed to translocate the ligand. In contrast, ORPH appears to inhibit DAT function by arresting it in the OF open conformation. The analysis shows that cocaine and ORPH competitively bind DAT, with the binding pose and affinity dependent on the conformational state of DAT. Further assays show that the effect of ORPH on DAT uptake and endocytosis is comparable to that of cocaine.