Mechanism for cocaine blocking the transport of dopamine: insights from molecular modeling and dynamics simulations

In vitro and in vivo stability of a highly efficient long-acting cocaine hydrolase

It is recognized as a promising therapeutic strategy for cocaine use disorder to develop an efficient enzyme which can rapidly convert cocaine to physiologically inactive metabolites. We have designed and discovered a series of highly efficient cocaine hydrolases, including CocH5-Fc(M6) which is the currently known as the most efficient cocaine hydrolase with both the highest catalytic activity against (-)-cocaine and the longest biological half-life in rats. In the present study, we characterized the time courses of protein appearance, pH, structural integrity, and catalytic activity against cocaine in vitro and in vivo of a CocH5-Fc(M6) bulk drug substance produced in a bioreactor for its in vitro and in vivo stability after long-time storage under various temperatures (- 80, - 20, 4, 25, or 37 °C). Specifically, all the tested properties of the CocH5-Fc(M6) protein did not significantly change after the protein was stored at any of four temperatures including - 80, - 20, 4, and 25 °C for ~ 18 months. In comparison, at 37 °C, the protein was less stable, with a half-life of ~ 82 days for cocaine hydrolysis activity. Additionally, the in vivo studies further confirmed the linear elimination PK profile of CocH5-Fc(M6) with an elimination half-life of ~ 9 days. All the in vitro and in vivo data on the efficacy and stability of CocH5-Fc(M6) have consistently demonstrated that CocH5-Fc(M6) has the desired in vitro and in vivo stability as a promising therapeutic candidate for treatment of cocaine use disorder.

Targeting dopamine transporter to ameliorate cognitive deficits in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the pathologic deposition of amyloid and neurofibrillary tangles in the brain, leading to neuronal damage and defective synapses. These changes manifest as abnormalities in cognition and behavior. The functional deficits are also attributed to abnormalities in multiple neurotransmitter systems contributing to neuronal dysfunction. One such important system is the dopaminergic system. It plays a crucial role in modulating movement, cognition, and behavior while connecting various brain areas and influencing other neurotransmitter systems, making it relevant in neurodegenerative disorders like AD and Parkinson's disease (PD). Considering its significance, the dopaminergic system has emerged as a promising target for alleviating movement and cognitive deficits in PD and AD, respectively. Extensive research has been conducted on dopaminergic neurons, receptors, and dopamine levels as critical factors in cognition and memory in AD. However, the exact nature of movement abnormalities and other features of extrapyramidal symptoms are not fully understood yet in AD. Recently, a previously overlooked element of the dopaminergic system, the dopamine transporter, has shown significant promise as a more effective target for enhancing cognition while addressing dopaminergic system dysfunction in AD.

Epigenetic and Long-Term Effects of Nicotine on Biology, Behavior, and Health

Tobacco and nicotine use are associated with disease susceptibility and progression. Health challenges associated with nicotine and smoking include developmental delays, addiction, mental health and behavioral changes, lung disease, cardiovascular disease, endocrine disorders, diabetes, immune system changes, and cancer. Increasing evidence suggests that nicotine-associated epigenetic changes may mediate or moderate the development and progression of a myriad of negative health outcomes. In addition, nicotine exposure may confer increased lifelong susceptibility to disease and mental health challenges through alteration of epigenetic signaling. This review examines the relationship between nicotine exposure (and smoking), epigenetic changes, and maladaptive outcomes that include developmental disorders, addiction, mental health challenges, pulmonary disease, cardiovascular disease, endocrine disorders, diabetes, immune system changes, and cancer. Overall, findings support the contention that nicotine (or smoking) associated altered epigenetic signaling is a contributing factor to disease and health challenges.

The relationship between sleep and appetitive conditioning: A systematic review and meta-analysis

This systematic review and meta-analysis (PROSPERO registration animal/human studies: CRD42021234793/CRD42021234790) examined the relationship between sleep and appetitive conditioning. Inclusion criteria included: a) appetitive conditioning paradigm; b) measure of conditioning; c) sleep measurement and/or sleep loss; d) human and/etor non-human animal samples; and e) written in English. Searches of seven databases returned 3777 publications. The final sample consisted of 42 studies using primarily animal samples and involving food- and drug-related conditioning tasks. We found sleep loss disrupted appetitive conditioning of food rewards (p < 0.001) but potentiated appetitive conditioning of drug rewards (p < 0.001). Furthermore, sleep loss negatively impacted extinction learning irrespective of the reward type. Post-learning sleep was associated with increases in REM sleep (p = 0.02). Findings suggest sleep loss potentiates the impact of psychoactive substances in a manner likely to produce an increased risk of problematic substance use. In obese/overweight populations, sleep loss may be associated with deficits in the conditioning and extinction of reward-related behaviours. Further research should assess the relationship between sleep and appetitive conditioning in humans.

Prescription psychostimulants for cocaine use disorder: A review from molecular basis to clinical approach

Cocaine use is a public health concern in many countries worldwide, particularly in the Americas and Oceania. Overdose deaths involving stimulants, such as cocaine, have been increasing markedly in North America, especially with concurrent opioid involvement. To date, no pharmacological treatment is available to treat stimulant (including cocaine) use disorders. Prescription psychostimulants (PPs) could be useful to treat cocaine use disorder (CUD) as they share the pharmacological effects with cocaine, as evidenced by a recent meta-analysis that assessed 38 randomized clinical trials (RCTs). PPs were found to promote sustained abstinence and reduce drug use in patients with CUD. The aim of this paper is to provide a narrative review of the clinical pharmacology of PPs and comment on the current stage of evidence supporting PPs to treat CUD. We also propose a model of care that integrates PPs with evidence-based psychosocial interventions (such as cognitive-behavioural therapy [CBT] and contingency management [CM]), a harm reduction approach and case management with social support.

Fast Prediction of Binding Affinities of SARS-CoV-2 Spike Protein and Its Mutants with Antibodies through Intermolecular Interaction Modeling-Based Machine Learning

Since the introduction of the novel SARS-CoV-2 virus (COVID-19) in late 2019, various new variants have appeared with mutations that confer resistance to the vaccines and monoclonal antibodies that were developed in response to the wild-type virus. As we continue through the pandemic, an accurate and efficient methodology is needed to help predict the effects certain mutations will have on both our currently produced therapeutics and those that are in development. Using published cryo-electron microscopy and X-ray crystallography structures of the spike receptor binding domain region with currently known antibodies, in the present study, we created and cross-validated an intermolecular interaction modeling-based multi-layer perceptron machine learning approach that can accurately predict the mutation-caused shifts in the binding affinity between the spike protein (wild-type or mutant) and various antibodies. This validated artificial intelligence (AI) model was used to predict the binding affinity (Kd) of reported SARS-CoV-2 antibodies with various variants of concern, including the most recently identified "Deltamicron" (or "Deltacron") variant. This AI model may be employed in the future to predict the Kd of developed novel antibody therapeutics to overcome the challenging antibody resistance issue and develop structural bases for the effects of both current and new mutants of the spike protein. In addition, the similar AI strategy and approach based on modeling of the intermolecular interactions may be useful in development of machine learning models predicting binding affinities for other protein-protein binding systems, including other antibodies binding with their antigens.

Development of a Highly Efficient Long-Acting Cocaine Hydrolase Entity to Accelerate Cocaine Metabolism

It is particularly challenging to develop a truly effective pharmacotherapy for cocaine use disorder (CUD) treatment. Accelerating cocaine metabolism via hydrolysis at cocaine benzoyl ester using an efficient cocaine hydrolase (CocH) is known as a promising pharmacotherapeutic approach to CUD treatment. Preclinical and clinical studies on our first CocH (CocH1), in its human serum albumin-fused form known as TV-1380, have demonstrated the promise of a general concept of CocH-based pharmacotherapy for CUD treatment. However, the biological half-life of TV-1380 (t_1/2 = 8 h in rats, associated with t_1/2 = 43-77 h in humans) is not long enough for practical treatment of cocaine dependence, which requires enzyme injection for no more than once weekly. Through protein fusion of a human butyrylcholinesterase mutant (denoted as CocH5) with a mutant (denoted as Fc(M6)) of Fc from human IgG1, we have designed, prepared, and tested a new fusion protein (denoted as CocH5-Fc(M6)) for its pharmacokinetic profile and in vivo catalytic activity against (-)-cocaine. CocH5-Fc(M6) represents the currently most efficient long-acting cocaine hydrolase with both the highest catalytic activity against (-)-cocaine and the longest elimination half-life (t_1/2 = 229 ± 5 h) in rats. As a result, even at a single modest dose of 3 mg/kg, CocH5-Fc(M6) can significantly and effectively accelerate the metabolism of cocaine in rats for at least 60 days. In addition, ∼70 nM CocH5-Fc(M6) in plasma was able to completely block the toxicity and physiological effects induced by intraperitoneal injection of a lethal dose of cocaine (60 mg/kg).

An Increase in Peripheral Temperature following Cocaine Administration Is Mediated through Activation of Dopamine D2 Receptor in Rats

Drug addiction has become a worldwide problem, affecting millions of people across the globe. While the majority of mechanistic studies on drug addiction have been focused on the central nervous system, including the mesolimbic dopamine system, the peripheral actions of drugs of abuse remain largely unknown. Our preliminary study found that the systemic injection of cocaine increased peripheral skin temperature. This led us to our present study, which investigated the mechanisms underlying the increase in peripheral temperature following cocaine injection. Male Sprague Dawley rats were anesthetized with pentobarbital sodium, and peripheral skin temperature measurements were taken using a thermocouple needle microprobe and an infrared thermal camera. Cocaine injection caused an acute rise in peripheral body temperature, but not core body temperature, about 10 min after injection, and the temperature increases were occluded by systemic injection of dopamine D2 receptor antagonist L741,626, but not D1 receptor antagonist SCH23390. In addition, systemic administration of bromocriptine, a dopamine D2 receptor agonist, significantly increased peripheral temperature. Infrared thermal imaging showed that the thermal increases following cocaine injection were predominantly in the distal areas of the forelimbs and hindlimbs, relative to core body temperature. Treatment with cocaine or bromocriptine decreased the size of skin blood vessels without affecting the expression of dopamine D2 receptors. These results suggest that increased peripheral temperature in skin following cocaine injection is associated with the activation of the dopamine D2 receptor.

The Use of Drosophila to Understand Psychostimulant Responses

The addictive properties of psychostimulants such as cocaine, amphetamine, methamphetamine, and methylphenidate are based on their ability to increase dopaminergic neurotransmission in the reward system. While cocaine and methamphetamine are predominately used recreationally, amphetamine and methylphenidate also work as effective therapeutics to treat symptoms of disorders including attention deficit and hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). Although both the addictive properties of psychostimulant drugs and their therapeutic efficacy are influenced by genetic variation, very few genes that regulate these processes in humans have been identified. This is largely due to population heterogeneity which entails a requirement for large samples. Drosophila melanogaster exhibits similar psychostimulant responses to humans, a high degree of gene conservation, and allow performance of behavioral assays in a large population. Additionally, amphetamine and methylphenidate reduce impairments in fly models of ADHD-like behavior. Therefore, Drosophila represents an ideal translational model organism to tackle the genetic components underlying the effects of psychostimulants. Here, we break down the many assays that reliably quantify the effects of cocaine, amphetamine, methamphetamine, and methylphenidate in Drosophila. We also discuss how Drosophila is an efficient and cost-effective model organism for identifying novel candidate genes and molecular mechanisms involved in the behavioral responses to psychostimulant drugs.

Cocaine hydrolase blocks cocaine-induced dopamine transporter trafficking to the plasma membrane

Cocaine blocks dopamine uptake via dopamine transporter (DAT) on plasma membrane of neuron cells and, as a result, produces the high and induces DAT trafficking to plasma membrane which contributes to the drug seeking or craving. In this study, we first examined the dose dependence of cocaine-induced DAT trafficking and hyperactivity in rats, demonstrating that cocaine at an intraperitoneal dose of 10 mg/kg or higher led to redistribution of most DAT to the plasma membrane while inducing significant hyperactivity in rats. However, administration of 5-mg/kg cocaine (ip) did not significantly induce DAT trafficking or hyperactivity in rats. So the threshold (intraperitoneal) dose of cocaine that can significantly induce DAT trafficking or hyperactivity should be between 5 and 10 mg/kg. These data suggest that when a cocaine dose is high enough to induce significant hyperactivity, it can also significantly induce DAT trafficking to the plasma membrane. Further, the threshold brain cocaine concentration required to induce significant hyperactivity and DAT trafficking was estimated to be ~2.0 ± 0.8 μg/g. Particularly, for treatment of cocaine abuse, previous studies demonstrated that an exogenous cocaine-metabolizing enzyme, for example, CocH3-Fc(M3), can effectively block cocaine-induced hyperactivity. However, it was unknown whether an enzyme could also effectively block cocaine-induced DAT trafficking to the plasma membrane. This study demonstrates, for the first time, that the enzyme is also capable of effectively blocking cocaine from reaching the brain even with a lethal dose of 60-mg/kg cocaine (ip) and, thus, powerfully preventing cocaine-induced physiological effects such as the hyperactivity and DAT trafficking.

Molecular mechanisms of action of stimulant novel psychoactive substances that target the high-affinity transporter for dopamine

Drug misuse is a significant social and public health problem worldwide. Misused substances exert their neurobehavioural effects through changing neural signalling within the brain, many of them leading to substance dependence and addiction in the longer term. Among drugs with addictive liability, there are illicit classical stimulants such as cocaine and amphetamine, and their more recently available counterparts known as novel psychoactive substances (NPS). Stimulants normally increase dopamine availability in the brain, including the pathway implicated in reward-related behaviour. This pattern is observed in both animal and human brain. The main biological target of stimulants, both classical and NPS, is the dopamine transporter (DAT) implicated in the dopamine-enhancing effects of these drugs. This article aims at reviewing research on the molecular mechanisms underpinning the interactions between stimulant NPS, such as benzofurans, cathinones or piperidine derivatives and DAT, to achieve a greater understanding of the core phenomena that decide about the addictive potential of stimulant NPS. As the methodology is essential in the process of experimental research in this area, we review the applications of in vitro, in vivo and in silico approaches. The latter, including molecular dynamics, attracts the focus of the present review as the method of choice in molecular and atomistic investigations of the mechanisms of addiction of stimulant NPS. Research of this kind is of interest to not only scientists but also health professionals as updated knowledge of NPS, their modes of action and health risks, is needed to tackle the challenges posed by NPS misuse.

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.

Anhydroecgonine methyl ester, a cocaine pyrolysis product, contributes to cocaine-induced rat primary hippocampal neuronal death in a synergistic and time-dependent manner

Crack cocaine users are simultaneously exposed to volatilized cocaine and to its main pyrolysis product, anhydroecgonine methyl ester (AEME). Although the neurotoxic effects of cocaine have been extensively studied, little is known about AEME or its combination. We investigated cell death processes using rat primary hippocampal cells exposed to cocaine (2 mM), AEME (1 mM) and their combination (C + A), after 1, 3, 6 and 12 h. Cocaine increased LC3 I after 6 h and LC3 II after 12 h, but reduced the percentage of cells with acid vesicles, suggesting failure in the autophagic flux, which activated the extrinsic apoptotic pathway after 12 h. AEME neurotoxicity did not involve the autophagic process; rather, it activated caspase-9 after 6 h and caspase-8 after 12 h leading to a high percentage of cells in early apoptosis. C + A progressively reduced the percentage of undamaged cells, starting after 3 h; it activated both apoptotic pathways after 6 h, and was more neurotoxic than cocaine and AEME alone. Also, C + A increased the phosphorylation of p62 after 12 h, but there was little difference in LC3 I or II, and a small percentage of cells with acid vesicles at all time points investigated. In summary, the present study provides new evidence for the neurotoxic mechanism and timing response of each substance alone and in combination, indicating that AEME is more than just a biological marker for crack cocaine consumption, as it may intensify and hasten cocaine neurotoxicity.

Psychoactive Drugs-From Chemical Structure to Oxidative Stress Related to Dopaminergic Neurotransmission. A Review

Nowadays, more and more young people want to experience illegal, psychoactive substances, without knowing the risks of exposure. Besides affecting social life, psychoactive substances also have an important effect on consumer health. We summarized and analyzed the published literature data with reference to the mechanism of free radical generation and the link between chemical structure and oxidative stress related to dopaminergic neurotransmission. This review presents data on the physicochemical properties, on the ability to cross the blood brain barrier, the chemical structure activity relationship (SAR), and possible mechanisms by which neuronal injuries occur due to oxidative stress as a result of drug abuse such as "bath salts", amphetamines, or cocaine. The mechanisms of action of ingested compounds or their metabolites involve intermediate steps in which free radicals are generated. The brain is strongly affected by the consumption of such substances, facilitating the induction of neurodegenerative diseases. It can be concluded that neurotoxicity is associated with drug abuse. Dependence and oxidative stress are linked to inhibition of neurogenesis and the onset of neuronal death. Understanding the pathological mechanisms following oxidative attack can be a starting point in the development of new therapeutic targets.

Clinical potential of a rationally engineered enzyme for treatment of cocaine dependence: Long-lasting blocking of the psychostimulant, discriminative stimulus, and reinforcing effects of cocaine

It is a grand challenge to develop a truly effective treatment of substance use disorder (SUD), particularly for cocaine and other drugs without an FDA-approved treatment available, because a truly effective therapy must effectively block the drug's physiological and reinforcing effects during the entire period of treatment in order to achieve the long-time abstinence required by the FDA. Whether a biologic, such as monoclonal antibody, vaccine, or therapeutic enzyme, can be truly effective for SUD treatment or not has been the subject of extensive debate. The main debate question is whether a biologic, particularly an exogenous enzyme, can effectively block the drug's reinforcing effect. In this report, we demonstrate that a modest dose of a recently redesigned long-acting cocaine hydrolase, CocH3-Fc(M6), can be used to effectively block the psychostimulant, discriminative stimulus, and reinforcing effects of cocaine for a sufficiently long period of time. For example, a dose of 3 mg/kg CocH3-Fc(M6) completely blocked the discriminative stimulus and reinforcing effects for 24/25 days and continued to significantly attenuate/decrease the cocaine effects for at least 29 days in rats. All the animal data consistently suggest that the long-acting cocaine hydrolase is a truly promising candidate of enzyme therapy for treatment of cocaine use disorder.

Recent Advances and Challenges of the Drugs Acting on Monoamine Transporters

Background:

The human Monoamine Transporters (hMATs), primarily including hSERT, hNET and hDAT, are important targets for the treatment of depression and other behavioral disorders with more than the availability of 30 approved drugs.

Objective:

This paper is to review the recent progress in the binding mode and inhibitory mechanism of hMATs inhibitors with the central or allosteric binding sites, for the benefit of future hMATs inhibitor design and discovery. The Structure-Activity Relationship (SAR) and the selectivity for hit/lead compounds to hMATs that are evaluated by in vitro and in vivo experiments will be highlighted.

Methods:

PubMed and Web of Science databases were searched for protein-ligand interaction, novel inhibitors design and synthesis studies related to hMATs.

Results:

Literature data indicate that since the first crystal structure determinations of the homologous bacterial Leucine Transporter (LeuT) complexed with clomipramine, a sizable database of over 100 experimental structures or computational models has been accumulated that now defines a substantial degree of structural variability hMATs-ligands recognition. In the meanwhile, a number of novel hMATs inhibitors have been discovered by medicinal chemistry with significant help from computational models.

Conclusion:

The reported new compounds act on hMATs as well as the structures of the transporters complexed with diverse ligands by either experiment or computational modeling have shed light on the poly-pharmacology, multimodal and allosteric regulation of the drugs to transporters. All of the studies will greatly promote the Structure-Based Drug Design (SBDD) of structurally novel scaffolds with high activity and selectivity for hMATs.

Structure-Based Design and Discovery of a Long-Acting Cocaine Hydrolase Mutant with Improved Binding Affinity to Neonatal Fc Receptor for Treatment of Cocaine Abuse

Despite decades of efforts to develop a pharmacotherapy for cocaine abuse treatment, there is still no FDA-approved treatment of diseases associated with this commonly abused drug. Our previously designed highly efficient cocaine hydrolases (CocHs) and the corresponding Fc-fusion proteins (e.g., CocH3-Fc) are recognized as potentially promising therapeutic enzyme candidates for cocaine abuse treatment, but all with limited biological half-lives. In order to prolong the biological half-life and, thus, decrease the required frequency of the enzyme administration for cocaine abuse treatment, we have modeled the Fc-fusion CocH binding with neonatal Fc receptor (FcRn) in the present study. This approach led to the design and testing of CocH3-Fc(M6), a CocH3-Fc mutant with nearly 100-fold increased binding affinity: from K_d = ~ 4 μM to K_d = 43 nM. As a result, CocH3-Fc(M6) indeed revealed a markedly prolonged biological half-life (t_1/2 = 206 ± 7 h or ~ 9 days) in rats, longer than other known Fc-fusion protein drugs such as abatacept and alefacept (for other therapeutic purposes) in the same species (rats). It has been demonstrated that a single dose of 3 mg/kg CocH3-Fc(M6) effectively blocked 20 mg/kg cocaine-induced hyperactivity on day 18 after CocH3-Fc(M6) administration. This is the first attempt to rationally design long-acting Fc-fusion enzyme mutant based on combined computational modeling and experimental measurement of the Fc-fusion CocH binding with FcRn. The similar structure-based design strategy may be used to prolong the biological half-lives of other Fc-fusion protein drugs.

Highly sensitive detection for cocaine using an aptamer–cocaine–aptamer method

In this study, a thiol-modified aptamer was immobilized on a gold nanoparticle-modified molybdenum disulfide composite material (MoS₂@AuNPs), which could remove the interference of physical adsorption and reduce false positive signals.

The impact of substance abuse on HIV-mediated neuropathogenesis in the current ART era

Approximately 37 million people worldwide are infected with human immunodeficiency virus (HIV). One highly significant complication of HIV infection is the development of HIV-associated neurocognitive disorders (HAND) in 15-55% of people living with HIV (PLWH), that persists even in the antiretroviral therapy (ART) era. The entry of HIV into the central nervous system (CNS) occurs within 4-8 days after peripheral infection. This establishes viral reservoirs that may persist even in the presence of ART. Once in the CNS, HIV infects resident macrophages, microglia, and at low levels, astrocytes. In response to chronic infection and cell activation within the CNS, viral proteins, inflammatory mediators, and host and viral neurotoxic factors produced over extended periods of time result in neuronal injury and loss, cognitive deficits and HAND. Substance abuse is a common comorbidity in PLWH and has been shown to increase neuroinflammation and cognitive disorders. Additionally, it has been associated with poor ART adherence, and increased viral load in the cerebrospinal fluid (CSF), that may also contribute to increased neuroinflammation and neuronal injury. Studies have examined mechanisms that contribute to neuroinflammation and neuronal damage in PLWH, and how substances of abuse exacerbate these effects. This review will focus on how substances of abuse, with an emphasis on methamphetamine (meth), cocaine, and opioids, impact blood brain barrier (BBB) integrity and transmigration of HIV-infected and uninfected monocytes across the BBB, as well as their effects on monocytes/macrophages, microglia, and astrocytes within the CNS. We will also address how these substances of abuse may contribute to HIV-mediated neuropathogenesis in the context of suppressive ART. Additionally, we will review the effects of extracellular dopamine, a neurotransmitter that is increased in the CNS by substances of abuse, on HIV neuropathogenesis and how this may contribute to neuroinflammation, neuronal insult, and HAND in PLWH with active substance use. Lastly, we will discuss some potential therapies to limit CNS inflammation and damage in HIV-infected substance abusers.

Characterization of a Dopamine Transporter and Its Splice Variant Reveals Novel Features of Dopaminergic Regulation in the Honey Bee

Dopamine is an important neuromodulator involved in reward-processing, movement control, motivational responses, and other aspects of behavior in most animals. In honey bees (Apis mellifera), the dopaminergic system has been implicated in an elaborate pheromonal communication network between individuals and in the differentiation of females into reproductive (queen) and sterile (worker) castes. Here we have identified and characterized a honey bee dopamine transporter (AmDAT) and a splice variant lacking exon 3 (AmDATΔex3). Both transcripts are present in the adult brain and antennae as well as at lower levels within larvae and ovaries. When expressed separately in the Xenopus oocyte system, AmDAT localizes to the oocyte surface whereas the splice variant is retained at an internal membrane. Oocytes expressing AmDAT exhibit a 12-fold increase in the uptake of [3H]dopamine relative to non-injected oocytes, whereas the AmDATΔex3-expressing oocytes show no change in [3H]dopamine transport. Electrophysiological measurements of AmDAT activity revealed it to be a high-affinity, low-capacity transporter of dopamine. The transporter also recognizes noradrenaline as a major substrate and tyramine as a minor substrate, but does not transport octopamine, L-Dopa, or serotonin. Dopamine transport via AmDAT is inhibited by cocaine in a reversible manner, but is unaffected by octopamine. Co-expression of AmDAT and AmDATΔex3 in oocytes results in a substantial reduction in AmDAT-mediated transport, which was also detected as a significant decrease in the level of AmDAT protein. This down-regulatory effect is not attributable to competition with AmDATΔex3 for ER ribosomes, nor to a general inhibition of the oocyte's translational machinery. In vivo, the expression of both transcripts shows a high level of inter-individual variability. Gene-focused, ultra-deep amplicon sequencing detected methylation of the amdat locus at ten 5'-C-phosphate-G-3' dinucleotides (CpGs), but only in 5-10% of all reads in whole brains or antennae. These observations, together with the localization of the amdat transcript to a few clusters of dopaminergic neurons, imply that amdat methylation is positively linked to its transcription. Our findings suggest that multiple cellular mechanisms, including gene splicing and epigenomic communication systems, may be adopted to increase the potential of a conserved gene to contribute to lineage-specific behavioral outcomes.

Autism-linked dopamine transporter mutation alters striatal dopamine neurotransmission and dopamine-dependent behaviors

The precise regulation of synaptic dopamine (DA) content by the dopamine transporter (DAT) ensures the phasic nature of the DA signal, which underlies the ability of DA to encode reward prediction error, thereby driving motivation, attention, and behavioral learning. Disruptions to the DA system are implicated in a number of neuropsychiatric disorders, including attention deficit hyperactivity disorder (ADHD) and, more recently, Autism Spectrum Disorder (ASD). An ASD-associated de novo mutation in the SLC6A3 gene resulting in a threonine to methionine substitution at site 356 (DAT T356M) was recently identified and has been shown to drive persistent reverse transport of DA (i.e. anomalous DA efflux) in transfected cells and to drive hyperlocomotion in Drosophila melanogaster. A corresponding mutation in the leucine transporter, a DAT-homologous transporter, promotes an outward-facing transporter conformation upon substrate binding, a conformation possibly underlying anomalous dopamine efflux. Here we investigated in vivo the impact of this ASD-associated mutation on DA signaling and ASD-associated behaviors. We found that mice homozygous for this mutation display impaired striatal DA neurotransmission and altered DA-dependent behaviors that correspond with some of the behavioral phenotypes observed in ASD.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Molecular Docking Studies of Methamphetamine and Amphetamine- Related Derivatives as an Inhibitor against Dopamine Receptor

BACKGROUND

The catecholamines such as dopamine, norepinephrine, and epinephrine are neurotransmitters that regulate different physiological functions of the central nervous system. Some evidence suggests that the degeneration of dopamine neurons in the substantia nigra contributes to Parkinson's Disease (PD), which is a neurodegenerative disorder and it is responsible for the major symptoms of PD. It is suggested that replenishment of striatal dopamine through the oral administration of the dopamine precursor, levodopa, can compensate for the lack of endogenously produced dopamine. Some studies have shown competitive inhibition of dopamine receptor such as methamphetamine, and other amphetamine-related derivatives, which block dopamine receptor activity to uptake dopamine.

METHODS

In this study, 3D structures of amphetamine, methamphetamine, cocaine, methylphenidate, cathinone, MDMA, and mephedrone were obtained from the PubChem database, which has reported some evidence about their inhibitory effect with dopamine receptor. Then, these structures were provided for molecular docking analysis by Autodock Vina software. Eventually, the binding energies between docked dopamine receptor and them were calculated and their interactions were prognosticated.

RESULTS

Our results indicated that all chemicals can interact with dopamine receptor molecule in the active site of dopamine and the minimum binding energies belong to Cocaine and Methylphenidate with -7.9 Kcal/mol and -7.2 Kcal/mol, respectively.

CONCLUSION

It might be concluded that amphetamine, methamphetamine, cocaine, methylphenidate, cathinone, MDMA, and mephedrone could act as potential inhibitors of DA receptor for dopamine uptake, which could cause degenerative disorders.

Development of Fc-Fused Cocaine Hydrolase for Cocaine Addiction Treatment: Catalytic and Pharmacokinetic Properties

Cocaine abuse is a worldwide public health and social problem without a US Food and Drug Administration (FDA)-approved medication. Accelerating cocaine metabolism that produces biologically inactive metabolites by administration of an efficient cocaine hydrolase (CocH) has been recognized as a promising strategy for cocaine abuse treatment. However, the therapeutic effects of CocH are limited by its short biological half-life (e.g., 8 h or shorter in rats). In this study, we designed and prepared a set of Fc-fusion proteins constructed by fusing Fc(M3) with CocH3 at the N-terminus of CocH3. A linker between the two protein domains was optimized to improve both the biological half-life and catalytic activity against cocaine. It has been concluded that Fc(M3)-G₆S-CocH3 not only has fully retained the catalytic efficiency of CocH3 against cocaine but also has the longest biological half-life (e.g., ∼ 136 h in rats) among all of the long-acting CocHs identified so far. A single dose (0.2 mg/kg, IV) of Fc(M3)-G₆S-CocH3 was able to significantly attenuate 15 mg/kg cocaine-induced hyperactivity for at least 11 days (268 h) after the Fc(M3)-G₆S-CocH3 administration.

HIV-Associated Neurocognitive Disorder

In the last two decades, several advancement studies have increased the care of HIV-infected individuals. Specifically, the development for preparation of combination antiretroviral therapy has resulted in a dramatic decline in the rate of deaths from AIDS. The term “HIV-associated neurocognitive disorder” (HAND) has been used to distinguish the spectrum of neurocognitive dysfunction associated with HIV infection. HIV can pass to the CNS during the early stages of infection and last in the CNS. CNS inflammation and infection lead to the development of HAND. The brain can serve as a sanctuary for ongoing HIV replication, even when the systemic viral suppression has been achieved. HAND can remain in patients treated with combination antiretroviral therapy, and its effect on survival, quality of life, and everyday functioning make it a significant unresolved problem. This chapter discusses details of the computational modeling studies on mechanisms and structures of human dopamine transporter (hDAT) and its interaction with HIV-1 trans activator of transcription (Tat).

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.

Cocaine Potency at the Dopamine Transporter Tracks Discrete Motivational States During Cocaine Self-Administration

Although the dopamine transporter (DAT) is the primary site of action for cocaine, and the dopamine system is known to mediate the reinforcing effects of cocaine, the dopaminergic variations underlying individual differences in cocaine self-administration behaviors are not fully understood. Recent advances in the application of economic principles to operant tasks in rodents have allowed for the within-subject, within-session determination of both consummatory and appetitive responding for reinforcers. Here we combined a behavioral economics approach with cocaine self-administration and ex vivo voltammetric recording of dopamine signaling in the core of the nucleus accumbens of rats to determine the relationship between dopamine signaling and discrete aspects of cocaine taking and seeking. We found neither dopamine release or uptake tracked individual differences in cocaine consumption or the reinforcing efficacy of cocaine. Cocaine potency at the DAT was correlated with reinforcing efficacy, but was not related to cocaine consumption. Further, we introduce a novel analysis that determines perseverative responding within the same procedure, and find that cocaine potency at the DAT also tracks differences in perseverative responding. Together, we demonstrate that cocaine effects at the DAT determine the reinforcing efficacy of cocaine, and perseverative responding for sub-threshold doses of cocaine that do not maintain responding when presented in isolation. Surprisingly, we find that variations in cocaine potency do not account for differences in cocaine consumption, suggesting that satiation for cocaine is determined by other targets or mechanisms. Finally, we outline a novel approach for relating drug-target interactions and potency to discrete motivational states during a single self-administration session.

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.

The molecular basis of talin2's high affinity toward β1-integrin

Talin interacts with β-integrin tails and actin to control integrin activation, thus regulating focal adhesion dynamics and cell migration. There are two talin genes, Tln1 and Tln2, which encode talin1 and talin2, and it is generally believed that talin2 functions redundantly with talin1. However, we show here that talin2 has a higher affinity to β1-integrin tails than talin1. Mutation of talin2 S339 to leucine, which can cause Fifth Finger Camptodactyly, a human genetic disease, completely disrupted its binding to β–integrin tails. Also, substitution of talin1 C336 with Ser enhanced the affinity of talin1, whereas substitution of talin2 S339 with Cys diminished that of talin2. Further computational modeling analysis shows that talin2 S339 formed a hydrogen bond with E353, which is critical for inducing key hydrogen bonds between talin2 N326 and β1-integrin R760, and between talin2 K327 and β1-integrin D759. Mutation at any of these residues significantly diminished the interaction of talin2 with β1- integrin tails. These hydrogen bonds were not observed in talin1/β1-integrin, but did exist in talin1^C336S/β1-integrin complex. These results suggest that talin2 S339 forms a hydrogen bond with E353 to mediate its high affinity to β1-integrin.

Guidelines for Homology Modeling of Dopamine, Norepinephrine, and Serotonin Transporters

The human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT) are carriers of neurotransmitters and targets for many drugs. Pioneering works in the past three years to elucidate experimental models of the Drosophila dDAT and human hSERT structures will rapidly impact the field of neuroscience. Here, we evaluated automated homology-based human models of these transporters, employing systematic physics-based, knowledge-based, and empirical-based check. Modeling guidelines were conveyed with attention to the central binding site (S1), secondary binding site (S2), and the extracellular loops EL2 and EL4. Application of new experimental models (dDAT and hSERT) will improve the accuracy of homology models, previously utilizing prokaryotic leucine transporter (LeuT) structure, and provide better predictions of ligand interactions, which is required for understanding of cellular mechanisms and for development of novel therapeutics.

Computational modeling of human dopamine transporter structures, mechanism and its interaction with HIV-1 transactivator of transcription

This is a brief review of computational modeling studies on the detailed structures and mechanism of human dopamine transporter (hDAT), as well as its interaction with HIV-1 transactivator of transcription (Tat). Extensive molecular modeling, docking and dynamics simulations have resulted in reasonable structural models of hDAT in three typical conformational states, its dopamine uptake mechanism and its interaction with Tat. The obtained hDAT models in different conformational states and their complexes with dopamine and Tat have provided novel structural and mechanistic insights concerning how hDAT uptakes dopamine and how Tat affects the dopamine uptake by hDAT. The computational insights, that are consistent with available experimental data, should be valuable for future rational design of novel therapeutic strategies for treatment of HIV-associated neurocognitive disorders.

Exploring the Inhibitory Mechanism of Approved Selective Norepinephrine Reuptake Inhibitors and Reboxetine Enantiomers by Molecular Dynamics Study

Selective norepinephrine reuptake inhibitors (sNRIs) provide an effective class of approved antipsychotics, whose inhibitory mechanism could facilitate the discovery of privileged scaffolds with enhanced drug efficacy. However, the crystal structure of human norepinephrine transporter (hNET) has not been determined yet and the inhibitory mechanism of sNRIs remains elusive. In this work, multiple computational methods were integrated to explore the inhibitory mechanism of approved sNRIs (atomoxetine, maprotiline, reboxetine and viloxazine), and 3 lines of evidences were provided to verify the calculation results. Consequently, a binding mode defined by interactions between three chemical moieties in sNRIs and eleven residues in hNET was identified as shared by approved sNRIs. In the meantime, binding modes of reboxetine's enantiomers with hNET were compared. 6 key residues favoring the binding of (S, S)-reboxetine over that of (R, R)-reboxetine were discovered. This is the first study reporting that those 11 residues are the common determinants for the binding of approved sNRIs. The identified binding mode shed light on the inhibitory mechanism of approved sNRIs, which could help identify novel scaffolds with improved drug efficacy.

Reusable split-aptamer-based biosensor for rapid detection of cocaine in serum by using an all-fiber evanescent wave optical biosensing platform

A rapid, facile, and sensitive assay of cocaine in biological fluids is important to prevent illegal abuse of drugs. A two-step structure-switching aptasensor has been developed for cocaine detection based on evanescent wave optical biosensing platform. In the proposed biosensing platform, two tailored aptamer probes were used to construct the molecular structure switching. In the existence of cocaine, two fragments of cocaine aptamer formed a three-way junction quickly, and the fluorophore group of one fragment was effectively quenched by the quencher group of the other one. The tail of the three-way junction hybridized with the cDNA sequences immobilized on the optical fiber biosensor. Fluorescence was excited by evanescent wave, and the fluorescence signal was proportional to cocaine concentration. Cocaine was detected in 450 s (300 s for incubation and 150 s for detection and regeneration) with a limit of detection (LOD) of 165.2 nM. The proposed aptasensor was evaluated in human serum samples, and it exhibited good recovery, precision, and accuracy without complicated sample pretreatments.

HIV-1 transgenic rats display an increase in [(3)H]dopamine uptake in the prefrontal cortex and striatum

HIV viral proteins within the central nervous system are associated with the development of neurocognitive impairments in HIV-infected individuals. Dopamine transporter (DAT)-mediated dopamine transport is critical for normal dopamine homeostasis. Abnormal dopaminergic transmission has been implicated as a risk determinant of HIV-induced neurocognitive impairments. Our published work has demonstrated that transactivator of transcription (Tat)-induced inhibition of DAT is mediated by allosteric binding site(s) on DAT, not the interaction with the dopamine uptake site. The present study investigated whether impaired DAT function induced by Tat exposure in vitro can be documented in HIV-1 transgenic (HIV-1Tg) rats. We assessed kinetic analyses of [(3)H]dopamine uptake into prefrontal and striatal synaptosomes of HIV-1Tg and Fisher 344 rats. Compared with Fisher 344 rats, the capacity of dopamine transport in the prefrontal cortex (PFC) and striatum of HIV-1Tg rats was increased by 34 and 32 %, respectively. Assessment of surface biotinylation indicated that DAT expression in the plasma membrane was reduced in PFC and enhanced in striatum, respectively, of HIV-1Tg rats. While the maximal binding sites (B max) of [(3)H]WIN 35,428 was decreased in striatum of HIV-1Tg rats, an increase in DAT turnover proportion was found, relative to Fisher 344 rats. Together, these findings suggest that neuroadaptive changes in DAT function are evidenced in the HIV-1Tg rats, perhaps compensating for viral-protein-induced abnormal dopaminergic transmission. Thus, our study provides novel insights into understanding mechanism underlying neurocognitive impairment evident in neuroAIDS.

Monoamine transporters: insights from molecular dynamics simulations

The human monoamine transporters (MATs) facilitate the reuptake of the neurotransmitters serotonin, dopamine, and norepinephrine from the synaptic cleft. Imbalance in monoaminergic neurotransmission is linked to various diseases including major depression, attention deficit hyperactivity disorder, schizophrenia, and Parkinson's disease. Inhibition of the MATs is thus an important strategy for treatment of such diseases. The MATs are sodium-coupled transport proteins belonging to the neurotransmitter/Na(+) symporter (NSS) family, and the publication of the first high-resolution structure of a NSS family member, the bacterial leucine transporter LeuT, in 2005, proved to be a major stepping stone for understanding this family of transporters. Structural data allows for the use of computational methods to study the MATs, which in turn has led to a number of important discoveries. The process of substrate translocation across the membrane is an intrinsically dynamic process. Molecular dynamics simulations, which can provide atomistic details of molecular motion on ns to ms timescales, are therefore well-suited for studying transport processes. In this review, we outline how molecular dynamics simulations have provided insight into the large scale motions associated with transport of the neurotransmitters, as well as the presence of external and internal gates, the coupling between ion and substrate transport, and differences in the conformational changes induced by substrates and inhibitors.

Insights to ligand binding to the monoamine transporters-from homology modeling to LeuBAT and dDAT

Understanding of drug binding to the human biogenic amine transporters (BATs) is essential to explain the mechanism of action of these pharmaceuticals but more importantly to be able to develop new and improved compounds to be used in the treatment of depression or drug addiction. Until recently no high resolution structure was available of the BATs and homology modeling was a necessity. Various studies have revealed experimentally validated binding modes of numerous ligands to the BATs using homology modeling. Here we examine and discuss the similarities between the binding models of substrates, antidepressants, psychostimulants, and mazindol in homology models of the human BATs and the recently published crystal structures of the Drosophila dopamine transporter and the engineered protein, LeuBAT. The comparison reveals that careful computational modeling combined with experimental data can be utilized to predict binding of molecules to proteins that agree very well with crystal structures.

Molecular mechanism of HIV-1 Tat interacting with human dopamine transporter

Nearly 70% of HIV-1-infected individuals suffer from HIV-associated neurocognitive disorders (HAND). HIV-1 transactivator of transcription (Tat) protein is known to synergize with abused drugs and exacerbate the progression of central nervous system (CNS) pathology. Cumulative evidence suggest that the HIV-1 Tat protein exerts the neurotoxicity through interaction with human dopamine transporter (hDAT) in the CNS. Through computational modeling and molecular dynamics (MD) simulations, we develop a three-dimensional (3D) structural model for HIV-1 Tat binding with hDAT. The model provides novel mechanistic insights concerning how HIV-1 Tat interacts with hDAT and inhibits dopamine uptake by hDAT. In particular, according to the computational modeling, Tat binds most favorably with the outward-open state of hDAT. Residues Y88, K92, and Y470 of hDAT are predicted to be key residues involved in the interaction between hDAT and Tat. The roles of these hDAT residues in the interaction with Tat are validated by experimental tests through site-directed mutagensis and dopamine uptake assays. The agreement between the computational and experimental data suggests that the computationally predicted hDAT-Tat binding mode and mechanistic insights are reasonable and provide a new starting point to design further pharmacological studies on the molecular mechanism of HIV-1-associated neurocognitive disorders.

A label-free aptamer-fluorophore assembly for rapid and specific detection of cocaine in biofluids

We report a rapid and specific aptamer-based method for one-step cocaine detection with minimal reagent requirements. The feasibility of aptamer-based detection has been demonstrated with sensors that operate via target-induced conformational change mechanisms, but these have generally exhibited limited target sensitivity. We have discovered that the cocaine-binding aptamer MNS-4.1 can also bind the fluorescent molecule 2-amino-5,6,7-trimethyl-1,8-naphthyridine (ATMND) and thereby quench its fluorescence. We subsequently introduced sequence changes into MNS-4.1 to engineer a new cocaine-binding aptamer (38-GC) that exhibits higher affinity to both ligands, with reduced background signal and increased signal gain. Using this aptamer, we have developed a new sensor platform that relies on the cocaine-mediated displacement of ATMND from 38-GC as a result of competitive binding. We demonstrate that our sensor can detect cocaine within seconds at concentrations as low as 200 nM, which is 50-fold lower than existing assays based on target-induced conformational change. More importantly, our assay achieves successful cocaine detection in body fluids, with a limit of detection of 10.4, 18.4, and 36 μM in undiluted saliva, urine, and serum samples, respectively.

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.

Discovery of novel-scaffold monoamine transporter ligands via in silico screening with the S1 pocket of the serotonin transporter

Discovery of new inhibitors of the plasmalemmal monoamine transporters (MATs) continues to provide pharmacotherapeutic options for depression, addiction, attention deficit disorders, psychosis, narcolepsy, and Parkinson’s disease. The windfall of high-resolution MAT structural information afforded by X-ray crystallography has enabled the construction of credible computational models. Elucidation of lead compounds, creation of compound structure–activity series, and pharmacologic testing are staggering expenses that could be reduced by using a MAT computational model for virtual screening (VS) of structural libraries containing millions of compounds. Here, VS of the PubChem small molecule structural database using the S1 (primary substrate) ligand pocket of a serotonin transporter homology model yielded 19 prominent “hit” compounds. In vitro pharmacology of these VS hits revealed four structurally unique MAT substrate uptake inhibitors with high nanomolar affinity at one or more of the three MATs. In vivo characterization of three of these hits revealed significant activity in a mouse model of acute depression at doses that did not elicit untoward locomotor effects. This constitutes the first report of MAT inhibitor discovery using exclusively the primary substrate pocket as a VS tool. Novel-scaffold MAT inhibitors offer hope of new medications that lack the many classic adverse effects of existing antidepressant drugs.

Changes in dopamine transporter binding in nucleus accumbens following chronic self-administration cocaine: heroin combinations

Concurrent use of cocaine and heroin (speedball) has been shown to exert synergistic effects on dopamine neurotransmission in the nucleus accumbens (NAc), as observed by significant increases in extracellular dopamine levels and compensatory elevations in the maximal reuptake rate of dopamine. The present studies were undertaken to determine whether chronic self-administration of cocaine, heroin or a combination of cocaine:heroin led to compensatory changes in the abundance and/or affinity of high- and low-affinity DAT binding sites. Saturation binding of the cocaine analog [(125) I] 3β-(4-iodophenyl)tropan-2β-carboxylic acid methyl ester ([(125) I]RTI-55) in rat NAc membranes resulted in binding curves that were best fit to two-site binding models, allowing calculation of dissociation constant (Kd ) and binding density (Bmax ) values corresponding to high- and low-affinity DAT binding sites. Scatchard analysis of the saturation binding curves clearly demonstrate the presence of high- and low- affinity binding sites in the NAc, with low-affinity sites comprising 85 to 94% of the binding sites. DAT binding analyses revealed that self-administration of cocaine and a cocaine:heroin combination increased the affinity of the low-affinity site for the cocaine congener RTI-55 compared to saline. These results indicate that the alterations observed following chronic speedball self-administration are likely due to the cocaine component alone; thus further studies are necessary to elaborate upon the synergistic effect of cocaine:heroin combinations on the dopamine system in the NAc.

Conformational changes in dopamine transporter intracellular regions upon cocaine binding and dopamine translocation

The dopamine transporter (DAT), a member of the neurotransmitter:sodium symporter family, mediates the reuptake of dopamine at the synaptic cleft. DAT is the primary target for psychostimulants such as cocaine and amphetamine. We previously demonstrated that cocaine binding and dopamine transport alter the accessibility of Cys342 in the third intracellular loop (IL3). To study the conformational changes associated with the functional mechanism of the transporter, we made cysteine substitution mutants, one at a time, from Phe332 to Ser351 in IL3 of the background DAT construct, X7C, in which 7 endogenous cysteines were mutated. The accessibility of the 20 engineered cysteines to polar charged sulfhydryl reagents was studied in the absence and presence of cocaine or dopamine. Of the 11 positions that reacted with methanethiosulfonate ethyl ammonium, as evidenced by inhibition of ligand binding, 5 were protected against this inhibition by cocaine and dopamine (S333C, S334C, N336C, M342C and T349C), indicating that reagent accessibility is affected by conformational changes associated with inhibitor and substrate binding. In some of the cysteine mutants, transport activity is disrupted, but can be rescued by the presence of zinc, most likely because the distribution between inward- and outward-facing conformations is restored by zinc binding. The experimental data were interpreted in the context of molecular models of DAT in both the inward- and outward-facing conformations. Differences in the solvent accessible surface area for individual IL3 residues calculated for these states correlate well with the experimental accessibility data, and suggest that protection by ligand binding results from the stabilization of the outward-facing configuration. Changes in the residue interaction networks observed from the molecular dynamics simulations also revealed the critical roles of several positions during the conformational transitions. We conclude that the IL3 region of DAT undergoes significant conformational changes in transitions necessary for both cocaine binding and substrate transport.

Antagonist-induced conformational changes in dopamine transporter extracellular loop two involve residues in a potential salt bridge

Ligand-induced changes in the conformation of extracellular loop (EL) 2 in the rat (r) dopamine transporter (DAT) were examined using limited proteolysis with endoproteinase Asp-N and detection of cleavage products by epitope-specific immunoblotting. The principle N-terminal fragment produced by Asp-N was a 19kDa peptide likely derived by proteolysis of EL2 residue D174, which is present just past the extracellular end of TM3. Production of this fragment was significantly decreased by binding of cocaine and other uptake blockers, but was not affected by substrates or Zn(2+), indicating the presence of a conformational change at D174 that may be related to the mechanism of transport inhibition. DA transport activity and cocaine analog binding were decreased by Asp-N treatment, suggesting a requirement for EL2 integrity in these DAT functions. In a previous study we demonstrated that ligand-induced protease resistance also occurred at R218 on the C-terminal side of rDAT EL2. Here using substituted cysteine accessibility analysis of human (h) DAT we confirm cocaine-induced alterations in reactivity of the homologous R219 and identify conformational sensitivity of V221. Focused molecular modeling of D174 and R218 based on currently available Aquifex aeolicus leucine transporter crystal structures places these residues within 2.9Å of one another, suggesting their proximity as a structural basis for their similar conformational sensitivities and indicating their potential to form a salt bridge. These findings extend our understanding of DAT EL2 and its role in transport and binding functions.

Mutation of tyrosine 470 of human dopamine transporter is critical for HIV-1 Tat-induced inhibition of dopamine transport and transporter conformational transitions

HIV-1 Tat protein plays a crucial role in perturbations of the dopamine (DA) system. Our previous studies have demonstrated that Tat decreases DA uptake, and allosterically modulates DA transporter (DAT) function. In the present study, we have found that Tat interacts directly with DAT, leading to inhibition of DAT function. Through computational modeling and simulations, a potential recognition binding site of human DAT (hDAT) for Tat was predicted. Mutation of tyrosine470 (Y470H) attenuated Tat-induced inhibition of DA transport, implicating the functional relevance of this residue for Tat binding to hDAT. Y470H reduced the maximal velocity of [³H]DA uptake without changes in the K(m) and IC₅₀ values for DA inhibition of DA uptake but increased DA uptake potency for cocaine and GBR12909, suggesting that this residue does not overlap with the binding sites in hDAT for substrate but is critical for these inhibitors. Furthermore, Y470H also led to transporter conformational transitions by affecting zinc modulation of DA uptake and WIN35,428 binding as well as enhancing basal DA efflux. Collectively, these findings demonstrate Tyr470 as a functional recognition residue in hDAT for Tat-induced inhibition of DA transport and transporter conformational transitions. The consequence of mutation at this residue is to block the functional binding of Tat to hDAT without affecting physiological DA transport.

Adolescence methylphenidate treatment in a rodent model of attention deficit/hyperactivity disorder: dopamine transporter function and cellular distribution in adulthood

Attention deficit/hyperactivity disorder (ADHD) is attributed to dysfunction of the prefrontal cortex. Methylphenidate, an inhibitor of dopamine and norepinephrine transporters (DAT and NET, respectively), is a standard treatment for ADHD. The Spontaneously Hypertensive Rat (SHR) is a well-established animal model of ADHD. Our previous results showed that methylphenidate treatment in adolescent SHR enhanced cocaine self-administration during adulthood, and alterations in DAT function in prefrontal cortex play a role in this response. Importantly, prefrontal cortex subregions, orbitofrontal cortex (OFC) and medial prefrontal cortex (mPFC), have been shown to have distinct roles in ADHD and cocaine self-administration. In the current study, SHR, Wistar-Kyoto (WKY) and Wistar (WIS) rats received a therapeutically relevant dose of methylphenidate (1.5mg/kg, p.o.) or vehicle during adolescence and then OFC and mPFC DAT function and cellular expression were assessed during adulthood. In both OFC and mPFC, no strain differences in Vmax or Km for dopamine uptake into synaptosomes were found between vehicle-treated SHR, WKY and WIS. Methylphenidate increased DAT Vmax in SHR mPFC and decreased DAT Vmax in WKY OFC. Also, methylphenidate decreased DAT Km in WIS OFC. Further, methylphenidate did not alter DAT cellular localization, indicating that methylphenidate treatment during adolescence regulated DAT function in SHR mPFC in a trafficking-independent manner. Thus, the increase in mPFC DAT function was an SHR-specific long term consequence of methylphenidate treatment during adolescence, which may be responsible for the treatment-induced alterations in behavior including the observed increases in cocaine self-administration.

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.

Modulation by cocaine of dopamine receptors through miRNA-133b in zebrafish embryos

The use of cocaine during pregnancy can affect the mother and indirectly might alter the development of the embryo/foetus. Accordingly, in the present work our aim was to study in vivo (in zebrafish embryos) the effects of cocaine on the expression of dopamine receptors and on miR-133b. These embryos were exposed to cocaine hydrochloride (HCl) at 5 hours post-fertilization (hpf) and were then collected at 8, 16, 24, 48 and 72 hpf to study the expression of dopamine receptors, drd1, drd2a, drd2b and drd3, by quantitative real time PCR (qPCR) and in situ hybridization (ISH, only at 24 hpf). Our results indicate that cocaine alters the expression of the genes studied, depending on the stage of the developing embryo and the type of dopamine receptor. We found that cocaine reduced the expression of miR-133b at 24 and 48 hpf in the central nervous system (CNS) and at the periphery by qPCR and also that the spatial distribution of miR-133b was mainly seen in somites, a finding that suggests the involvement of miR-133b in the development of the skeletal muscle. In contrast, at the level of the CNS miR-133b had a weak and moderate expression at 24 and 48 hpf. We also analysed the interaction of miR-133b with the Pitx3 and Pitx3 target genes drd2a and drd2b, tyrosine hydroxylase (th) and dopamine transporter (dat) by microinjection of the Pitx3-3'UTR sequence. Microinjection of Pitx3-3'UTR affected the expression of pitx3, drd2a, drd2b, th and dat. In conclusion, in the present work we describe a possible mechanism to account for cocaine activity by controlling miR-133b transcription in zebrafish. Via miR-133b cocaine would modulate the expression of pitx3 and subsequently of dopamine receptors, dat and th. These results indicate that miRNAs can play an important role during embryogenesis and in drug addiction.