Sequence variation in the human dopamine transporter gene in children with attention deficit hyperactivity disorder

Aberrant extracellular dopamine clearance in the prefrontal cortex exhibits ADHD-like behavior in NCX3 heterozygous mice

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that involves dopaminergic dysfunction in the prefrontal cortex (PFC), manifesting hyperactivity, inattention, and cognitive deficits. However, the ADHD-associated candidate genes underlying dopaminergic neurotransmission alterations remain poorly defined. Here, we identified the abundant localization of sodium-calcium exchanger 3 (NCX3) levels in the dopaminergic neurons of the ventral tegmental area, a major source of dopaminergic innervation to the PFC. We confirmed that NCX3 knockdown in N27 cells caused aberrant dopamine influx through the strong interaction between calcium/calmodulin-dependent protein kinase II alpha and dopamine transporter. In addition, we assessed behavioral changes and underlying molecular properties in NCX3 heterozygous (NCX3+/-) mice. NCX3+/- mice exhibited hyperactivity, cognitive deficits, and social dysfunction which were alleviated by treating with methylphenidate. Furthermore, NCX3+/- mice displayed a persistent elevation of basal dopamine levels and decreased extracellular levels of dopamine triggered by social stimuli in the PFC of NCX3+/- mice. In agreement with the rise in extracellular dopamine levels in the PFC, NCX3+/- mice showed activation of dopamine D1 receptor signaling pathways in the PFC compared to wild-type mice. Thus, deficiency of NCX3 leads to impaired dopaminergic neurotransmission in the PFC, which likely accounts for the ADHD-like behavior in NCX3+/- mice.

Dopamine transporter endocytic trafficking: Neuronal mechanisms and potential impact on DA-dependent behaviors

The dopamine (DA) transporter (DAT) is a major determinant of DAergic neurotransmission, and is a primary target for addictive and therapeutic psychostimulants. Evidence accumulated over decades in cell lines and in vitro preparations revealed that DAT function is acutely regulated by membrane trafficking. Many of these findings have recently been validated in vivo and in situ, and several behavioral and physiological findings raise the possibility that regulated DAT trafficking may impact DA signaling and DA-dependent behaviors. Here we review key DAT trafficking findings across multiple systems, and discuss the cellular mechanisms that mediate DAT trafficking, as well as the endogenous receptors and signaling pathways that drive regulated DAT trafficking. We additionally discuss recent findings that DAT trafficking dysfunction correlates to perturbations in DA signaling and DA-dependent behaviors.

Forward genetic screen of the C. elegans million mutation library reveals essential, cell-autonomous contributions of BBSome proteins to dopamine signaling

The nematode Caenorhabditis elegans is well known for its ability to support forward genetic screens to identify molecules involved in neuronal viability and signaling. The proteins involved in C. elegans dopamine (DA) regulation are highly conserved across evolution, with prior work demonstrating that the model can serve as an efficient platform to identify novel genes involved in disease-associated processes. To identify novel players in DA signaling, we took advantage of a recently developed library of pre-sequenced mutant nematodes arising from the million mutation project (MMP) to identify strains that display the DA-dependent swimming-induced-paralysis phenotype (Swip). Our screen identified novel mutations in the dopamine transporter encoding gene dat-1, whose loss was previously used to identify the Swip phenotype, as well as multiple genes with previously unknown connections to DA signaling. Here, we present our isolation and characterization of one of these genes, bbs-1, previously linked to the function of primary cilia in worms and higher organisms, including humans, and where loss-of-function mutations result in a human disorder known as Bardet-Biedl syndrome. Our studies of C. elegans BBS-1 protein, as well as other proteins that are known to be assembled into a higher order complex (the BBSome) reveal that functional or structural disruption of this complex leads to exaggerated C. elegans DA signaling to produce Swip via a cell-autonomous mechanism. We provide evidence that not only does the proper function of cilia in C. elegans DA neurons support normal swimming behavior, but also that bbs-1 maintains normal levels of DAT-1 trafficking or function via a RHO-1 and SWIP-13/MAPK-15 dependent pathway where mutants may contribute to Swip independent of altered ciliary function. Together, these studies demonstrate novel contributors to DA neuron function in the worm and demonstrate the utility and efficiency of forward genetic screens using the MMP library.

Kappa Opioid Receptor Antagonism Restores Phosphorylation, Trafficking and Behavior induced by a Disease Associated Dopamine Transporter Variant

Aberrant dopamine (DA) signaling is implicated in schizophrenia, bipolar disorder (BPD), autism spectrum disorder (ASD), substance use disorder, and attention-deficit/hyperactivity disorder (ADHD). Treatment of these disorders remains inadequate, as exemplified by the therapeutic use of d-amphetamine and methylphenidate for the treatment of ADHD, agents with high abuse liability. In search for an improved and non-addictive therapeutic approach for the treatment of DA-linked disorders, we utilized a preclinical mouse model expressing the human DA transporter (DAT) coding variant DAT Val559, previously identified in individuals with ADHD, ASD, or BPD. DAT Val559, like several other disease-associated variants of DAT, exhibits anomalous DA efflux (ADE) that can be blocked by d-amphetamine and methylphenidate. Kappa opioid receptors (KORs) are expressed by DA neurons and modulate DA release and clearance, suggesting that targeting KORs might also provide an alternative approach to normalizing DA-signaling disrupted by perturbed DAT function. Here we demonstrate that KOR stimulation leads to enhanced surface trafficking and phosphorylation of Thr53 in wildtype DAT, effects achieved constitutively by the Val559 mutant. Moreover, these effects can be rescued by KOR antagonism of DAT Val559 in ex vivo preparations. Importantly, KOR antagonism also corrected in vivo DA release as well as sex-dependent behavioral abnormalities observed in DAT Val559 mice. Given their low abuse liability, our studies with a construct valid model of human DA associated disorders reinforce considerations of KOR antagonism as a pharmacological strategy to treat DA associated brain disorders.

Mouse models for inherited monoamine neurotransmitter disorders

Several mouse models have been developed to study human defects of primary and secondary inherited monoamine neurotransmitter disorders (iMND). As the field continues to expand, current defects in corresponding mouse models include enzymes and a molecular co-chaperone involved in monoamine synthesis and metabolism (PAH, TH, PITX3, AADC, DBH, MAOA, DNAJC6), tetrahydrobiopterin (BH4) cofactor synthesis and recycling (adGTPCH1/DRD, arGTPCH1, PTPS, SR, DHPR), and vitamin B6 cofactor deficiency (ALDH7A1), as well as defective monoamine neurotransmitter packaging (VMAT1, VMAT2) and reuptake (DAT). No mouse models are available for human DNAJC12 co-chaperone and PNPO-B6 deficiencies, disorders associated with recessive variants that result in decreased stability and function of the aromatic amino acid hydroxylases and decreased neurotransmitter synthesis, respectively. More than one mutant mouse is available for some of these defects, which is invaluable as different variant-specific (knock-in) models may provide more insights into underlying mechanisms of disorders, while complete gene inactivation (knock-out) models often have limitations in terms of recapitulating complex human diseases. While these mouse models have common phenotypic traits also observed in patients, reflecting the defective homeostasis of the monoamine neurotransmitter pathways, they also present with disease-specific manifestations with toxic accumulation or deficiency of specific metabolites related to the specific gene affected. This review provides an overview of the currently available models and may give directions toward selecting existing models or generating new ones to investigate novel pathogenic mechanisms and precision therapies.

Efficacy and safety of established and off-label ADHD drug therapies for cognitive impairment or attention-deficit hyperactivity disorder symptoms in bipolar disorder: A systematic review by the ISBD Targeting Cognition Task Force

BACKGROUND

Abnormalities in dopamine and norepinephrine signaling are implicated in cognitive impairments in bipolar disorder (BD) and attention-deficit hyperactivity disorder (ADHD). This systematic review by the ISBD Targeting Cognition Task Force therefore aimed to investigate the possible benefits on cognition and/or ADHD symptoms and safety of established and off-label ADHD therapies in BD.

METHODS

We included studies of ADHD medications in BD patients, which involved cognitive and/or safety measures. We followed the procedures of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) 2020 statement. Searches were conducted on PubMed, Embase and PsycINFO from inception until June 2023. Two authors reviewed the studies independently using the Revised Cochrane Collaboration's Risk of Bias tool for Randomized trials.

RESULTS

Seventeen studies were identified (N = 2136), investigating armodafinil (k = 4, N = 1581), methylphenidate (k = 4, N = 84), bupropion (k = 4, n = 249), clonidine (k = 1, n = 70), lisdexamphetamine (k = 1, n = 25), mixed amphetamine salts (k = 1, n = 30), or modafinil (k = 2, n = 97). Three studies investigated cognition, four ADHD symptoms, and 10 the safety. Three studies found treatment-related ADHD symptom reduction: two involved methylphenidate and one amphetamine salts. One study found a trend towards pro-cognitive effects of modafinil on some cognitive domains. No increased risk of (hypo)mania was observed. Five studies had low risk of bias, eleven a moderate risk, and one a serious risk of bias.

CONCLUSIONS

Methylphenidate or mixed amphetamine salts may improve ADHD symptoms in BD. However, there is limited evidence regarding the effectiveness on cognition. The medications produced no increased mania risk when used alongside mood stabilizers. Further robust studies are needed to assess cognition in BD patients receiving psychostimulant treatment alongside mood stabilizers.

Leaky lessons learned: Efflux prone dopamine transporter variant reveals sex and circuit specific contributions of D2 receptor signalling to neuropsychiatric disease

Aberrant dopamine (DA) signalling has been implicated in various neuropsychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), schizophrenia, bipolar disorder (BPD) and addiction. The availability of extracellular DA is sculpted by the exocytotic release of vesicular DA and subsequent transporter-mediated clearance, rendering the presynaptic DA transporter (DAT) a crucial regulator of DA neurotransmission. D2-type DA autoreceptors (D2ARs) regulate multiple aspects of DA homeostasis, including (i) DA synthesis, (ii) vesicular release, (iii) DA neuron firing and (iv) the surface expression of DAT and DAT-mediated DA clearance. The DAT Val559 variant, identified in boys with ADHD or ASD, as well as in a girl with BPD, supports anomalous DA efflux (ADE), which we have shown drives tonic activation of D2ARs. Through ex vivo and in vivo studies of the DAT Val559 variant using transgenic knock-in mice, we have uncovered a circuit and sex-specific capacity of D2ARs to regulate DAT, which consequently disrupts DA signalling and behaviour differently in males and females. Our studies reveal the ability of the construct-valid DAT Val559 model to elucidate endogenous mechanisms that support DA signalling, findings that may be of translational and/or therapeutic importance.

Gene therapy for neurotransmitter-related disorders

Inborn errors of neurotransmitter (NT) metabolism are a group of rare, heterogenous diseases with predominant neurological features, such as movement disorders, autonomic dysfunction, and developmental delay. Clinical overlap with other disorders has led to delayed diagnosis and treatment, and some conditions are refractory to oral pharmacotherapies. Gene therapies have been developed and translated to clinics for paediatric inborn errors of metabolism, with 38 interventional clinical trials ongoing to date. Furthermore, efforts in restoring dopamine synthesis and neurotransmission through viral gene therapy have been developed for Parkinson's disease. Along with the recent European Medicines Agency (EMA) and Medicines and Healthcare Products Regulatory Agency (MHRA) approval of an AAV2 gene supplementation therapy for AADC deficiency, promising efficacy and safety profiles can be achieved in this group of diseases. In this review, we present preclinical and clinical advances to address NT-related diseases, and summarise potential challenges that require careful considerations for NT gene therapy studies.

Characterization of Behavioral Phenotypes in Heterozygous DAT Rat Based on Pedigree

Dopamine is an essential neurotransmitter whose key roles include movement control, pleasure and reward, attentional and cognitive skills, and regulation of the sleep/wake cycle. Reuptake is carried out by the dopamine transporter (DAT; DAT1 SLC6A3 gene). In order to study the effects of hyper-dopaminergia syndrome, the gene was silenced in rats. DAT-KO rats show stereotypical behavior, hyperactivity, a deficit in working memory, and an altered circadian cycle. In addition to KO rats, heterozygous (DAT-HET) rats show relative hypofunction of DAT; exact phenotypic effects are still unknown and may depend on whether the sire or the dam was KO. Our goal was to elucidate the potential importance of the parental origin of the healthy or silenced allele and its impact across generations, along with the potential variations in maternal care. We thus generated specular lines to study the effects of (grand) parental roles in inheriting the wild or mutated allele. MAT-HETs are the progeny of a KO sire and a WT dam; by breeding MAT-HET males and KO females, we obtained subjects with a DAT -/- epigenotype, named QULL, to reflect additional epigenetic DAT modulation when embryos develop within a hyper-dopaminergic KO uterus. We aimed to verify if any behavioral anomaly was introduced by a QULL (instead of KO) rat acting as a direct father or indirect maternal grandfather (or both). We thus followed epigenotypes obtained after three generations and observed actual effects on impaired maternal care of the offspring (based on pedigree). In particular, offspring of MAT-HET-dam × QULL-sire breeding showed a compulsive and obsessive phenotype. Although the experimental groups were all heterozygous, the impact of having a sire of epigenotype QULL (who developed in the uterus of a KO grand-dam) has emerged clearly. Along the generations, the effects of the DAT epigenotype on the obsessive/compulsive phenotype do vary as a function of the uterine impact on either allele in one's genealogical line.

Thrsp Gene and the ADHD Predominantly Inattentive Presentation

There are three presentations of attention-deficit/hyperactivity disorder (ADHD): the predominantly inattention (ADHD-PI), predominantly hyperactive-impulsive (ADHD-HI), and combined (ADHD-C) presentations of ADHD. These may represent distinct childhood-onset neurobehavioral disorders with separate etiologies. ADHD diagnoses are behaviorally based, so investigations into potential etiologies should be founded on behavior. Animal models of ADHD demonstrate face, predictive, and construct validity when they accurately reproduce elements of the symptoms, etiology, biochemistry, and disorder treatment. Spontaneously hypertensive rats (SHR/NCrl) fulfill many validation criteria and compare well with clinical cases of ADHD-C. Compounding the difficulty of selecting an ideal model to study specific presentations of ADHD is a simple fact that our knowledge regarding ADHD neurobiology is insufficient. Accordingly, the current review has explored a potential animal model for a specific presentation, ADHD-PI, with acceptable face, predictive, and construct validity. The Thrsp gene could be a biomarker for ADHD-PI presentation, and THRSP OE mice could represent an animal model for studying this distinct ADHD presentation.

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

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

Male DAT Val559 Mice Exhibit Compulsive Behavior under Devalued Reward Conditions Accompanied by Cellular and Pharmacological Changes

Identified across multiple psychiatric disorders, the dopamine (DA) transporter (DAT) Ala559Val substitution triggers non-vesicular, anomalous DA efflux (ADE), perturbing DA neurotransmission and behavior. We have shown that DAT Val559 mice display a waiting impulsivity and changes in cognitive performance associated with enhanced reward motivation. Here, utilizing a within-subject, lever-pressing paradigm designed to bias the formation of goal-directed or habitual behavior, we demonstrate that DAT Val559 mice modulate their nose poke behavior appropriately to match context, but demonstrate a perseverative checking behavior. Although DAT Val559 mice display no issues with the cognitive flexibility required to acquire and re-learn a visual pairwise discrimination task, devaluation of reward evoked habitual reward seeking in DAT Val559 mutants in operant tasks regardless of reinforcement schedule. The direct DA agonist apomorphine also elicits locomotor stereotypies in DAT Val559, but not WT mice. Our observation that dendritic spine density is increased in the dorsal medial striatum (DMS) of DAT Val559 mice speaks to an imbalance in striatal circuitry that might underlie the propensity of DAT Val559 mutants to exhibit compulsive behaviors when reward is devalued. Thus, DAT Val559 mice represent a model for dissection of how altered DA signaling perturbs circuits that normally balance habitual and goal-directed behaviors.

Behaviorally penetrant, anomalous dopamine efflux exposes sex and circuit dependent regulation of dopamine transporters

Virtually all neuropsychiatric disorders display sex differences in prevalence, age of onset, and/or clinical symptomology. Although altered dopamine (DA) signaling is a feature of many of these disorders, sex-dependent mechanisms uniquely responsive to DA that drive sex-dependent behaviors remain unelucidated. Previously, we established that anomalous DA efflux (ADE) is a prominent feature of the DA transporter (DAT) variant Val559, a coding substitution identified in two male-biased disorders: attention-deficit/hyperactivity disorder and autism spectrum disorder. In vivo, Val559 ADE induces activation of nigrostriatal D2-type DA autoreceptors (D2ARs) that magnifies inappropriate, nonvesicular DA release by elevating phosphorylation and surface trafficking of ADE-prone DAT proteins. Here we demonstrate that DAT Val559 mice exhibit sex-dependent alterations in psychostimulant responses, social behavior, and cognitive performance. In a search for underlying mechanisms, we discovered that the ability of ADE to elicit D2AR regulation of DAT is both sex and circuit-dependent, with dorsal striatum D2AR/DAT coupling evident only in males, whereas D2AR/DAT coupling in the ventral striatum is exclusive to females. Moreover, systemic administration of the D2R antagonist sulpiride, which precludes ADE-driven DAT trafficking, can normalize DAT Val559 behavioral changes unique to each sex and without effects on the opposite sex or wildtype mice. Our studies support the sex- and circuit dependent capacity of D2ARs to regulate DAT as a critical determinant of the sex-biased effects of perturbed DA signaling in neurobehavioral disorders. Moreover, our work provides a cogent example of how a shared biological insult drives alternative physiological and behavioral trajectories as opposed to resilience.

Translational relevance of forward genetic screens in animal models for the study of psychiatric disease

Psychiatric disorders represent a significant burden in our societies. Despite the convincing evidence pointing at gene and gene-environment interaction contributions, the role of genetics in the etiology of psychiatric disease is still poorly understood. Forward genetic screens in animal models have helped elucidate causal links. Here we discuss the application of mutagenesis-based forward genetic approaches in common animal model species: two invertebrates, nematodes (Caenorhabditis elegans) and fruit flies (Drosophila sp.); and two vertebrates, zebrafish (Danio rerio) and mice (Mus musculus), in relation to psychiatric disease. We also discuss the use of large scale genomic studies in human populations. Despite the advances using data from human populations, animal models coupled with next-generation sequencing strategies are still needed. Although with its own limitations, zebrafish possess characteristics that make them especially well-suited to forward genetic studies exploring the etiology of psychiatric disorders.

Allosteric Modulator KM822 Attenuates Behavioral Actions of Amphetamine in Caenorhabditis elegans through Interactions with the Dopamine Transporter DAT-1

Aberrant dopamine (DA) signaling is associated with several psychiatric disorders, such as autism, bipolar disorder, addiction, and Parkinson's disease, and several medications that target the DA transporter (DAT) can induce or treat these disorders. In addition, psychostimulants, such as cocaine and D-amphetamine (AMPH), rely on the competitive interactions with the transporter's substrate binding site to produce their rewarding effects. Agents that exhibit noncompetitive, allosteric modulation of DAT remain an important topic of investigation due to their potential therapeutic applications. We previously identified a novel allosteric modulator of human DAT, KM822, that can decrease the affinity of cocaine for DAT and attenuate cocaine-elicited behaviors; however, whether DAT is the sole mediator of KM822 actions in vivo is unproven given the large number of potential off-target sites. Here, we provide in silico and in vitro evidence that the allosteric site engaged by KM822 is conserved between human DAT and Caenorhabditis elegans DAT-1. KM822 binds to a similar pocket in DAT-1 as previously identified in human DAT. In functional dopamine uptake assays, KM822 affects the interaction between AMPH and DAT-1 by reducing the affinity of AMPH for DAT-1. Finally, through a combination of genetic and pharmacological in vivo approaches we provide evidence that KM822 diminishes the behavioral actions of AMPH on swimming-induced paralysis through a direct allosteric modulation of DAT-1. More broadly, our findings demonstrate allosteric modulation of DAT as a behavior modifying strategy and suggests that Caenorhabditis elegans can be operationalized to identify and investigate the interactions of DAT allosteric modulators. SIGNIFICANCE STATEMENT: We previously demonstrated that the dopamine transporter (DAT) allosteric modulator KM822 decreases cocaine affinity for human DAT. Here, using in silico and in vivo genetic approaches, we extend this finding to interactions with amphetamine, demonstrating evolutionary conservation of the DAT allosteric site. In Caenorhabditis elegans, we report that KM822 suppresses amphetamine behavioral effects via specific interactions with DAT-1. Our findings reveal Caenorhabditis elegans as a new tool to study allosteric modulation of DAT and its behavioral consequences.

Identification of Chromosomal Regions Linked to Autism-Spectrum Disorders: A Meta-Analysis of Genome-Wide Linkage Scans

Background: Autism spectrum disorder (ASD) is a clinically and genetically heterogeneous group of pervasive neurodevelopmental disorders with a strong hereditary component. Although, genome-wide linkage scans (GWLS) and association studies (GWAS) have previously identified hundreds of ASD risk gene loci, the results remain inconclusive. Method: We performed a heterogeneity-based genome search meta-analysis (HEGESMA) of 15 genome scans of autism and ASD. Results: For strictly defined autism, data were analyzed across six separate genome scans. Region 7q22-q34 reached statistical significance in both weighted and unweighted analyses, with evidence of significantly low between-scan heterogeneity. For ASDs (data from 12 separate scans), chromosomal regions 5p15.33-5p15.1 and 15q22.32-15q26.1 reached significance in both weighted and unweighted analyses but did not reach significance for either low or high heterogeneity. Region 1q23.2-1q31.1 was significant in unweighted analyses with low between-scan heterogeneity. Finally, region 8p21.1-8q13.2 reached significant linkage peak in all our meta-analyses. When we combined all available genome scans (15), the same results were produced. Conclusions: This meta-analysis suggests that these regions should be further investigated for autism susceptibility genes, with the caveat that autism spectrum disorders have different linkage signals across genome scans, possibly because of the high genetic heterogeneity of the disease.

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.

Altered Development of Mesencephalic Dopaminergic Neurons in SIDS: New Insights into Understanding Sudden Infant Death Pathogenesis

Sudden infant death syndrome (SIDS) is defined as the unexpected sudden death of an infant under 1 year of age that remains unexplained after a thorough case investigation. The SIDS pathogenesis is still unknown; however, abnormalities in brain centers that control breathing and arousal from sleep, including dramatic changes in neurotransmitter levels, have been supposed in these deaths. This is the first study focusing on mesencephalic dopaminergic neurons, so far extensively studied only in animals and human neurological diseases, in SIDS. Dopaminergic structures in midbrain sections of a large series of sudden infant deaths (36 SIDS and 26 controls) were identified using polyclonal rabbit antibodies against tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, and the dopamine transporter, a membrane protein specifically expressed in dopaminergic cells. Dopamine-immunolabeled neurons were observed concentrated in two specific structures: the pars compacta of the substantia nigra and in the subnucleus medialis of the periaqueductal gray matter. Anatomical and functional degenerations of dopaminergic neurons in these regions were observed in most SIDS cases but never in controls. These results indicate that dopamine depletion, which is already known to be linked especially to Parkinson's disease, is strongly involved even in SIDS pathogenesis.

Dominant-negative actions of a dopamine transporter variant identified in patients with parkinsonism and neuropsychiatric disease

Dysfunctional dopaminergic neurotransmission is central to movement disorders and mental diseases. The dopamine transporter (DAT) regulates extracellular dopamine levels, but the genetic and mechanistic link between DAT function and dopamine-related pathologies is not clear. Particularly, the pathophysiological significance of monoallelic missense mutations in DAT is unknown. Here, we use clinical information, neuroimaging, and large-scale exome-sequencing data to uncover the occurrence and phenotypic spectrum of a DAT coding variant, DAT-K619N, which localizes to the critical C-terminal PSD-95/Discs-large/ZO-1 homology–binding motif of human DAT (hDAT). We identified the rare but recurrent hDAT-K619N variant in exome-sequenced samples of patients with neuropsychiatric diseases and a patient with early-onset neurodegenerative parkinsonism and comorbid neuropsychiatric disease. In cell cultures, hDAT-K619N displayed reduced uptake capacity, decreased surface expression, and accelerated turnover. Unilateral expression in mouse nigrostriatal neurons revealed differential effects of hDAT-K619N and hDAT-WT on dopamine-directed behaviors, and hDAT-K619N expression in Drosophila led to impairments in dopamine transmission with accompanying hyperlocomotion and age-dependent disturbances of the negative geotactic response. Moreover, cellular studies and viral expression of hDAT-K619N in mice demonstrated a dominant-negative effect of the hDAT-K619N mutant. Summarized, our results suggest that hDAT-K619N can effectuate dopamine dysfunction of pathological relevance in a dominant-negative manner.

Physiology and Pharmacology of Neurotransmitter Transporters

Regulation of the ability of a neurotransmitter [our focus: serotonin, norepinephrine, dopamine, acetylcholine, glycine, and gamma-aminobutyric acid (GABA)] to reach its receptor targets is regulated in part by controlling the access the neurotransmitter has to receptors. Transporters, located at both the cellular plasma membrane and in subcellular vesicles, carry a myriad of responsibilities that include enabling neurotransmitter release and controlling uptake of neurotransmitter back into a cell or vesicle. Driven largely by electrochemical gradients, these transporters move neurotransmitters. The regulation of the transporters themselves through changes in expression and/or posttranslational modification allows for fine-tuning of this system. Transporters have been best recognized as targets for psychoactive stimulants and remain a mainstay target of primarily central nervous system (CNS) acting drugs for treatment of debilitating diseases such as depression and anxiety. Studies reveal, however, that transporters are found and functional in tissues outside the CNS (gastrointestinal and cardiovascular tissues, for example). The importance of neurotransmitter transporters is underscored with discoveries that dysfunction of transporters can cause life-changing disease. This article provides a high-level review of major classes of both plasma membrane transporters and vesicular transporters. © 2021 American Physiological Society. Compr Physiol 11:2279-2295, 2021.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Dynamic control of the dopamine transporter in neurotransmission and homeostasis

The dopamine transporter (DAT) transports extracellular dopamine into the intracellular space contributing to the regulation of dopamine neurotransmission. A reduction of DAT density is implicated in Parkinson's disease (PD) by neuroimaging; dopamine turnover is dopamine turnover is elevated in early symptomatic PD and in presymptomatic individuals with monogenic mutations causal for parkinsonism. As an integral plasma membrane protein, DAT surface expression is dynamically regulated through endocytic trafficking, enabling flexible control of dopamine signaling in time and space, which in turn critically modulates movement, motivation and learning behavior. Yet the cellular machinery and functional implications of DAT trafficking remain enigmatic. In this review we summarize mechanisms governing DAT trafficking under normal physiological conditions and discuss how PD-linked mutations may disturb DAT homeostasis. We highlight the complexity of DAT trafficking and reveal DAT dysregulation as a common theme in genetic models of parkinsonism.

General Overview of Organic Cation Transporters in Brain

Inhibitors of Na⁺/Cl^- dependent high affinity transporters for norepinephrine (NE), serotonin (5-HT), and/or dopamine (DA) represent frequently used drugs for treatment of psychological disorders such as depression, anxiety, obsessive-compulsive disorder, attention deficit hyperactivity disorder, and addiction. These transporters remove NE, 5-HT, and/or DA after neuronal excitation from the interstitial space close to the synapses. Thereby they terminate transmission and modulate neuronal behavioral circuits. Therapeutic failure and undesired central nervous system side effects of these drugs have been partially assigned to neurotransmitter removal by low affinity transport. Cloning and functional characterization of the polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), OCT3 (SLC22A3) and the plasma membrane monoamine transporter PMAT (SLC29A4) revealed that every single transporter mediates low affinity uptake of NE, 5-HT, and DA. Whereas the organic transporters are all located in the blood brain barrier, OCT2, OCT3, and PMAT are expressed in neurons or in neurons and astrocytes within brain areas that are involved in behavioral regulation. Areas of expression include the dorsal raphe, medullary motoric nuclei, hypothalamic nuclei, and/or the nucleus accumbens. Current knowledge of the transport of monoamine neurotransmitters by the organic cation transporters, their interactions with psychotropic drugs, and their locations in the brain is reported in detail. In addition, animal experiments including behavior tests in wildtype and knockout animals are reported in which the impact of OCT2, OCT3, and/or PMAT on regulation of salt intake, depression, mood control, locomotion, and/or stress effect on addiction is suggested.

Functional and Biochemical Consequences of Disease Variants in Neurotransmitter Transporters: A Special Emphasis on Folding and Trafficking Deficits

Neurotransmitters, such as γ-aminobutyric acid, glutamate, acetyl choline, glycine and the monoamines, facilitate the crosstalk within the central nervous system. The designated neurotransmitter transporters (NTTs) both release and take up neurotransmitters to and from the synaptic cleft. NTT dysfunction can lead to severe pathophysiological consequences, e.g. epilepsy, intellectual disability, or Parkinson’s disease. Genetic point mutations in NTTs have recently been associated with the onset of various neurological disorders. Some of these mutations trigger folding defects in the NTT proteins. Correct folding is a prerequisite for the export of NTTs from the endoplasmic reticulum (ER) and the subsequent trafficking to their pertinent site of action, typically at the plasma membrane. Recent studies have uncovered some of the key features in the molecular machinery responsible for transporter protein folding, e.g., the role of heat shock proteins in fine-tuning the ER quality control mechanisms in cells. The therapeutic significance of understanding these events is apparent from the rising number of reports, which directly link different pathological conditions to NTT misfolding. For instance, folding-deficient variants of the human transporters for dopamine or GABA lead to infantile parkinsonism/dystonia and epilepsy, respectively. From a therapeutic point of view, some folding-deficient NTTs are amenable to functional rescue by small molecules, known as chemical and pharmacological chaperones.

Neurobehavioral changes arising from early life dopamine signaling perturbations

Dopamine (DA) signaling is critical to the modulation of multiple brain functions including locomotion, reinforcement, attention and cognition. The literature provides strong evidence that altered DA availability and actions can impact normal neurodevelopment, with both early and enduring consequences on anatomy, physiology and behavior. An appreciation for the developmental contributions of DA signaling to brain development is needed to guide efforts to preclude and remedy neurobehavioral disorders, such as attention-deficit/hyperactivity disorder, addiction, bipolar disorder, schizophrenia and autism spectrum disorder, each of which exhibits links to DA via genetic, cellular and/or pharmacological findings. In this review, we highlight research pursued in preclinical models that use genetic and pharmacological approaches to manipulate DA signaling at sensitive developmental stages, leading to changes at molecular, circuit and/or behavioral levels. We discuss how these alterations can be aligned with traits displayed by neuropsychiatric diseases. Lastly, we review human studies that evaluate contributions of developmental perturbations of DA systems to increased risk for neuropsychiatric disorders.

In Situ Regulated Dopamine Transporter Trafficking: There's No Place Like Home

Dopamine (DA) is critical for motivation, reward, movement initiation, and learning. Mechanisms that control DA signaling have a profound impact on these important behaviors, and additionally play a role in DA-related neuropathologies. The presynaptic SLC6 DA transporter (DAT) limits extracellular DA levels by clearing released DA, and is potently inhibited by addictive and therapeutic psychostimulants. Decades of evidence support that the DAT is subject to acute regulation by a number of signaling pathways, and that endocytic trafficking strongly regulates DAT availability and function. DAT trafficking studies have been performed in a variety of model systems, including both in vitro and ex vivo preparations. In this review, we focus on the breadth of DAT trafficking studies, with specific attention to, and comparison of, how context may influence DAT's response to different stimuli. In particular, this overview highlights that stimulated DAT trafficking not only differs between in vitro and ex vivo environments, but also is influenced by both sex and anatomical subregions.

New Insights on the Effects of Methylphenidate in Attention Deficit Hyperactivity Disorder

This narrative review describes an overview of the multiple effects of methylphenidate (MPH) in attention-deficit/hyperactivity disorder (ADHD) and its potential neurobiological targets. It addressed the following aspects: 1) MPH effects on attention and executive functions in ADHD; 2) the relation between MPH efficacy and dopamine transporter gene (DAT) polymorphism; and 3) the role of MPH as an epigenetic modulator in ADHD. Literature analysis showed that MPH, the most commonly used psychostimulant in the therapy of ADHD, acts on multiple components of the disorder. Marked improvements in attentional and executive dysfunction have been observed in children with ADHD during treatment with MPH, as well as reductions in neurological soft signs. MPH efficacy may be influenced by polymorphisms in the DAT, and better responses to treatment were associated with the 10/10 genotype. Innovative lines of research have suggested that ADHD etiopathogenesis and its neuropsychological phenotypes also depend on the expression levels of human endogenous retrovirus (HERV). In particular, several studies have revealed that ADHD is associated with HERV-H over-expression and that MPH administration results in decreased expression levels of this retroviral family and a reduction in the main symptoms of the disorder. In conclusion, there is a confirmed role for MPH as an elective drug in the therapy of ADHD alone or in association with behavioral therapy. Its effectiveness can vary based on DAT polymorphisms and can act as a modulator of HERV-H gene expression, pointing to targets for a precision medicine approach.

Abnormal Behavior of Zebrafish Mutant in Dopamine Transporter Is Rescued by Clozapine

Dopamine transporter (SLC6A3) deficiency causes infantile Parkinson disease, for which there is no effective therapy. We have explored the effects of genetically deleting SLC6A3 in zebrafish. Unlike the wild-type, slc6a3−/− fish hover near the tank bottom, with a repetitive digging-like behavior. slc6a3−/− fish manifest pruning and cellular loss of particular tyrosine hydroxylase-immunoreactive neurons in the midbrain. Clozapine, an effective therapeutic for treatment-resistant schizophrenia, rescues the abnormal behavior of slc6a3−/− fish. Clozapine also reverses the abnormalities in the A8 region of the mutant midbrain. By RNA sequencing analysis, clozapine increases the expression of erythropoietin pathway genes. Transgenic over-expression of erythropoietin in neurons of slc6a3−/− fish partially rescues the mutant behavior, suggesting a potential mechanistic basis for clozapine's efficacy.

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DAT mutation in zebrafish causes digging behavior and loss of specific midbrain neurons

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Clozapine restores normal behavior and neuronal morphology of mutant fish

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Clozapine increases expression of erythropoietin pathway genes

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Transgenic expression of erythropoietin partially rescues the mutant behavior

The SERT Met172 Mouse: An Engineered Model To Elucidate the Contributions of Serotonin Signaling to Cocaine Action

Cocaine abuse and addiction remain highly prevalent and, unfortunately, poorly treated. It is well-known that essential aspects of cocaine's addictive actions involve the drug's ability to block the presynaptic dopamine (DA) transporter (DAT), thereby elevating extracellular levels of DA in brain circuits that subserve reward, reinforcement, and habit. Less well appreciated are the multiple DA-independent actions of cocaine, activities that we and others believe contribute key pieces to the puzzle of cocaine addiction, treatment, and relapse. In particular, a significant body of work points to altered serotonin (5-HT) signaling as one such component, not surprising given that, relative to DAT, cocaine acts as potently to block the 5-HT transporter (SERT) as to block DAT, and thereby elevates extracellular 5-HT levels throughout the brain when reward-eliciting DA elevations occur. To elucidate the contribution of SERT antagonism to the actions of cocaine, we engineered a mouse model that significantly reduces cocaine potency at SERT without disrupting the expression or function of SERT in vivo. In this short Perspective, we review the rationale for development of the SERT Met172 model, the studies that document the pharmacological impact of the Ile172Met substitution in vitro and in vivo, and our findings with the model that demonstrate serotonergic contributions to the genetic, physiological, and behavioral actions of cocaine.

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.

Dopamine-dependent, swimming-induced paralysis arises as a consequence of loss of function mutations in the RUNX transcription factor RNT-1

Dopamine (DA) is a neurotransmitter with actions across phylogeny that modulate core behaviors such as motor activity, reward, attention, and cognition. Perturbed DA signaling in humans is associated with multiple disorders, including addiction, ADHD, schizophrenia, and Parkinson's disease. The presynaptic DA transporter exerts powerful control on DA signaling by efficient clearance of the neurotransmitter following release. As in vertebrates, Caenorhabditis elegans DAT (DAT-1) constrains DA signaling and loss of function mutations in the dat-1 gene result in slowed crawling on solid media and swimming-induced paralysis (Swip) in water. Previously, we identified a mutant line, vt34, that exhibits robust DA-dependent Swip. vt34 exhibits biochemical and behavioral phenotypes consistent with reduced DAT-1 function though vt34; dat-1 double mutants exhibit an enhanced Swip phenotype, suggesting contributions of the vt34-associated mutation to additional mechanisms that lead to excess DA signaling. SNP mapping and whole genome sequencing of vt34 identified the site of the molecular lesion in the gene B0412.2 that encodes the Runx transcription factor ortholog RNT-1. Unlike dat-1 animals, but similar to other loss of function rnt-1 mutants, vt34 exhibits altered male tail morphology and reduced body size. Deletion mutations in both rnt-1 and the bro-1 gene, which encodes a RNT-1 binding partner also exhibit Swip. Both vt34 and rnt-1 mutations exhibit reduced levels of dat-1 mRNA as well as the tyrosine hydroxylase ortholog cat-2. Although reporter studies indicate that rnt-1 is expressed in DA neurons, its re-expression in DA neurons of vt34 animals fails to fully rescue Swip. Moreover, as shown for vt34, rnt-1 mutation exhibits additivity with dat-1 in generating Swip, as do rnt-1 and bro-1 mutations, and vt34 exhibits altered capacity for acetylcholine signaling at the neuromuscular junction. Together, these findings identify a novel role for rnt-1 in limiting DA neurotransmission and suggest that loss of RNT-1 may disrupt function of both DA neurons and body wall muscle to drive Swip.

Serotonin transporter inhibition and 5-HT2C receptor activation drive loss of cocaine-induced locomotor activation in DAT Val559 mice

Dopamine (DA) signaling dysfunction is believed to contribute to multiple neuropsychiatric disorders including attention-deficit/hyperactivity disorder (ADHD). The rare DA transporter (DAT) coding substitution Ala559Val found in subjects with ADHD, bipolar disorder and autism, promotes anomalous DA efflux in vitro and, in DAT Val559 mice, leads to increased reactivity to imminent handling, waiting impulsivity, and enhanced motivation for reward. Here, we report that, in contrast to amphetamine and methylphenidate, which induce significant locomotor activation, cocaine administration to these mice elicits no locomotor effects, despite retention of conditioned place preference (CPP). Additionally, cocaine fails to elevate extracellular DA. Given that amphetamine and methylphenidate, unlike cocaine, lack high-affinity interactions with the serotonin (5-HT) transporter (SERT), we hypothesized that the lack of cocaine-induced hyperlocomotion in DAT Val559 mice arises from SERT blockade and augmented 5-HT signaling relative to cocaine actions on wildtype animals. Consistent with this idea, the SERT blocker fluoxetine abolished methylphenidate-induced locomotor activity in DAT Val559 mice, mimicking the effects seen with cocaine. Additionally, a cocaine analog (RTI-113) with greater selectivity for DAT over SERT retains locomotor activation in DAT Val559 mice. Furthermore, genetic elimination of high-affinity cocaine interactions at SERT in DAT Val559 mice, or specific inhibition of 5-HT_2C receptors in these animals, restored cocaine-induced locomotion, but did not restore cocaine-induced elevations of extracellular DA. Our findings reveal a significant serotonergic plasticity arising in the DAT Val559 model that involves enhanced 5-HT_2C signaling, acting independently of striatal DA release, capable of suppressing the activity of cocaine-sensitive motor circuits.

Single Quantum Dot Imaging Reveals PKCβ-Dependent Alterations in Membrane Diffusion and Clustering of an Attention-Deficit Hyperactivity Disorder/Autism/Bipolar Disorder-Associated Dopamine Transporter Variant

The dopamine transporter (DAT) is a transmembrane protein that terminates dopamine signaling in the brain by driving rapid dopamine reuptake into presynaptic nerve terminals. Several lines of evidence indicate that DAT dysfunction is linked to neuropsychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD), bipolar disorder (BPD), and autism spectrum disorder (ASD). Indeed, individuals with these disorders have been found to express the rare, functional DAT coding variant Val559, which confers anomalous dopamine efflux (ADE) in vitro and in vivo. To elucidate the impact of the DAT Val559 variant on membrane diffusion dynamics, we implemented our antagonist-conjugated quantum dot (QD) labeling approach to monitor the lateral mobility of single particle-labeled transporters in transfected HEK-293 and SK-N-MC cells. Our results demonstrate significantly higher diffusion coefficients of DAT Val559 compared to those of DAT Ala559, effects likely determined by elevated N-terminal transporter phosphorylation. We also provide pharmacological evidence that PKCβ-mediated signaling supports enhanced DAT Val559 membrane diffusion rates. Additionally, our results are complimented with diffusion rates of phosphomimicked and phosphorylation-occluded DAT variants. Furthermore, we show DAT Val559 has a lower propensity for membrane clustering, which may be caused by a mutation-derived shift out of membrane microdomains leading to faster lateral membrane diffusion rates. These findings further demonstrate a functional impact of DAT Val559 and suggest that changes in transporter localization and lateral mobility may sustain ADE and contribute to alterations in dopamine signaling underlying multiple neuropsychiatric disorders.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Region-Specific Regulation of Presynaptic Dopamine Homeostasis by D2 Autoreceptors Shapes the In Vivo Impact of the Neuropsychiatric Disease-Associated DAT Variant Val559

Disruptions of dopamine (DA) signaling contribute to a broad spectrum of neuropsychiatric disorders, including attention-deficit hyperactivity disorder (ADHD), addiction, bipolar disorder, and schizophrenia. Despite evidence that risk for these disorders derives from heritable variation in DA-linked genes, a better understanding is needed of the molecular and circuit context through which gene variation drives distinct disease traits. Previously, we identified the DA transporter (DAT) variant Val559 in subjects with ADHD and established that the mutation supports anomalous DAT-mediated DA efflux (ADE). Here, we demonstrate that region-specific contributions of D₂ autoreceptors (D2AR) to presynaptic DA homeostasis dictate the consequences of Val559 expression in adolescent male mice. We show that activation of D2ARs in the WT dorsal striatum (DS), but not ventral striatum (VS), increases DAT phosphorylation and surface trafficking. In contrast, the activity of tyrosine hydroxylase (TH) is D2AR-dependent in both regions. In the DS but not VS of Val559 mice, tonic activation of D2ARs drives a positive feedback loop that promotes surface expression of efflux-prone DATs, raising extracellular DA levels and overwhelming DAT-mediated DA clearance capacity. Whereas D2ARs that regulate DAT are tonically activated in the Val559 DS, D2ARs that regulate TH become desensitized, allowing maintenance of cytosolic DA needed to sustain ADE. Together with prior findings, our results argue for distinct D2AR pools that regulate DA synthesis versus DA release and inactivation and offer a clear example of how the penetrance of gene variation can be limited to a subset of expression sites based on differences in intersecting regulatory networks.SIGNIFICANCE STATEMENT Altered dopamine (DA) signaling has been linked to multiple neuropsychiatric disorders. In an effort to understand and model disease-associated DAergic disturbances, we previously screened the DA transporter (DAT) in subjects with attention-deficit hyperactivity disorder (ADHD) and identified multiple, functionally impactful, coding variants. One of these variants, Val559, supports anomalous DA efflux (ADE) and in transgenic mice leads to changes in locomotor patterns, psychostimulant sensitivity, and impulsivity. Here, we show that the penetrance of Val559 ADE is dictated by region-specific differences in how presynaptic D₂-type autoreceptors (D2ARs) constrain DA signaling, biasing phenotypic effects to dorsal striatal projections. The Val559 model illustrates how the impact of genetic variation underlying neuropsychiatric disorders can be shaped by the differential engagement of synaptic regulatory mechanisms.

Neuropsychiatric disease-associated genetic variants of the dopamine transporter display heterogeneous molecular phenotypes

Genetic factors are known to significantly contribute to the etiology of psychiatric diseases such as attention deficit hyperactivity disorder (ADHD) and autism spectrum and bipolar disorders, but the underlying molecular processes remain largely elusive. The dopamine transporter (DAT) has received continuous attention as a potential risk factor for psychiatric disease, as it is critical for dopamine homeostasis and serves as principal target for ADHD medications. Constrain metrics for the DAT-encoding gene, solute carrier family 6 member 3 (SLC6A3), indicate that missense mutations are under strong negative selection, pointing to pathophysiological outcomes when DAT function is compromised. Here, we systematically characterized six rare genetic variants of DAT (I312F, T356M, D421N, A559V, E602G, and R615C) identified in patients with neuropsychiatric disorders. We evaluated dopamine uptake and ligand interactions, along with ion coordination and electrophysiological properties, to elucidate functional phenotypes, and applied Zn²⁺ exposure and a substituted cysteine-accessibility approach to identify shared structural changes. Three variants (I312F, T356M, and D421N) exhibited impaired dopamine uptake associated with changes in ligand binding, ion coordination, and distinct conformational disturbances. Remarkably, we found that all three variants displayed gain-of-function electrophysiological phenotypes. I312F mediated an increased uncoupled anion conductance previously suggested to modulate neuronal excitability. T356M and D421N both mediated a cocaine-sensitive leakage of cations, which for T356M was potentiated by Zn²⁺, concurrent with partial functional rescue. Collectively, our findings support that gain of disruptive functions due to missense mutations in SLC6A3 may be key to understanding how dopaminergic dyshomeostasis arises in heterozygous carriers.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.

Is dopamine transporter-mediated dopaminergic signaling in the retina a noninvasive biomarker for attention-deficit/ hyperactivity disorder? A study in a novel dopamine transporter variant Val559 transgenic mouse model

Background

Dopamine (DA) is a critical neuromodulator in the retina. Disruption of retinal DA synthesis and signaling significantly attenuates light-adapted, electroretinogram (ERG) responses, as well as contrast sensitivity and acuity. As these measures can be detected noninvasively, they may provide opportunities to detect disease processes linked to perturbed DA signaling. Recently, we identified a rare, functional DA transporter (DAT, SLC6A3) coding substitution, Ala559Val, in subjects with attention-deficit/hyperactivity disorder (ADHD), demonstrating that DAT Val559 imparts anomalous DA efflux (ADE) with attendant physiological, pharmacological, and behavioral phenotypes. To understand the broader impact of ADE on ADHD, noninvasive measures sensitive to DAT reversal are needed.

Methods

Here, we explored this question through ERG-based analysis of retinal light responses, as well as HPLC measurements of retinal DA in DAT Val559 mice.

Results

Male mice homozygous (HOM) for the DAT Val559 variant demonstrated increased, light-adapted ERG b-wave amplitudes compared to wild type (WT) and heterozygous (HET) mice, whereas dark-adapted responses were indistinguishable across genotypes. The elevated amplitude of the photopic light responses in HOM mice could be mimicked in WT mice by applying D₁ and D₄ DA receptor agonists and suppressed in HOM mice by introducing D₄ antagonist, supporting elevated retinal DA signaling arising from ADE. Following the challenge with amphetamine, WT exhibited an increase in light-adapted response amplitudes, while HOM did not. Total retinal DA content was similar across genotypes. Interestingly, female DAT Val559 HOM animals revealed no significant difference in photopic ERG responses when compared with WT and HET littermates.

Conclusions

These data reveal that noninvasive, in vivo evaluation of retinal responses to light can reveal physiological signatures of ADE, suggesting a possible approach to the segregation of neurobehavioral disorders based on the DAT-dependent control of DA signaling.

Phosphorylation of the Amino Terminus of the Dopamine Transporter: Regulatory Mechanisms and Implications for Amphetamine Action

Amphetamines (AMPHs) are potent psychostimulants that are widely used and abused, with profound medical and societal impact. Their actions at dopaminergic neurons are thought to mediate their therapeutic efficacy as well as their liability for abuse and dependence. AMPHs target the dopamine transporter (DAT), the plasmalemmal membrane protein that mediates the inactivation of released dopamine (DA) through its reuptake. AMPHs act as substrates for DAT and are known to cause mobilization of dopamine (DA) to the cell exterior via DAT-mediated reverse transport (efflux). It has become increasingly evident that the mechanisms that regulate AMPH-induced DA efflux are distinct from those that regulate DA uptake. Central to these mechanisms is the phosphorylation of the DAT amino (N)-terminus, which has been repeatedly demonstrated to facilitate DAT-mediated DA efflux, without impacting other aspects of DAT physiology. This review aims to summarize the current status of knowledge regarding DAT N-terminal phosphorylation and its regulation by protein modulators and the membrane microenvironment. A better understanding of these mechanisms may lead to the identification of novel therapeutic approaches that interfere selectively with the pharmacological effects of AMPHs without altering the physiological function of DAT.

Functional coding variation in the presynaptic dopamine transporter associated with neuropsychiatric disorders drives enhanced motivation and context-dependent impulsivity in mice

Recent genetic analyses have provided evidence that clinical commonalities associated with different psychiatric diagnoses often have shared mechanistic underpinnings. The development of animal models expressing functional genetic variation attributed to multiple disorders offers a salient opportunity to capture molecular, circuit and behavioral alterations underlying this hypothesis. In keeping with studies suggesting dopaminergic contributions to attention-deficit hyperactivity disorder (ADHD), bipolar disorder (BPD) and autism spectrum disorder (ASD), subjects with these diagnoses have been found to express a rare, functional coding substitution in the dopamine (DA) transporter (DAT), Ala559Val. We developed DAT Val559 knock-in mice as a construct valid model of dopaminergic alterations that drive multiple clinical phenotypes, and here evaluate the impact of lifelong expression of the variant on impulsivity and motivation utilizing the 5- choice serial reaction time task (5-CSRTT) and Go/NoGo as well as tests of time estimation (peak interval analysis), reward salience (sucrose preference), and motivation (progressive ratio test). Our findings indicate that the DAT Val559 variant induces impulsivity behaviors that are dependent upon the reward context, with increased impulsive action observed when mice are required to delay responding for a reward, whereas mice are able to withhold responding if there is a probability of reward for a correct rejection. Utilizing peak interval and progressive ratio tests, we provide evidence that impulsivity is likely driven by an enhanced motivational phenotype that also may drive faster task acquisition in operant tasks. These data provide critical validation that DAT, and more generally, DA signaling perturbations can drive impulsivity that can manifest in specific contexts and not others, and may rely on motivational alterations, which may also drive increased maladaptive reward seeking.

The Atypical MAP Kinase SWIP-13/ERK8 Regulates Dopamine Transporters through a Rho-Dependent Mechanism

The neurotransmitter dopamine (DA) regulates multiple behaviors across phylogeny, with disrupted DA signaling in humans associated with addiction, attention-deficit/ hyperactivity disorder, schizophrenia, and Parkinson's disease. The DA transporter (DAT) imposes spatial and temporal limits on DA action, and provides for presynaptic DA recycling to replenish neurotransmitter pools. Molecular mechanisms that regulate DAT expression, trafficking, and function, particularly in vivo, remain poorly understood, though recent studies have implicated rho-linked pathways in psychostimulant action. To identify genes that dictate the ability of DAT to sustain normal levels of DA clearance, we pursued a forward genetic screen in Caenorhabditis elegans based on the phenotype swimming-induced paralysis (Swip), a paralytic behavior observed in hermaphrodite worms with loss-of-function dat-1 mutations. Here, we report the identity of swip-13, which encodes a highly conserved ortholog of the human atypical MAP kinase ERK8. We present evidence that SWIP-13 acts presynaptically to insure adequate levels of surface DAT expression and DA clearance. Moreover, we provide in vitro and in vivo evidence supporting a conserved pathway involving SWIP-13/ERK8 activation of Rho GTPases that dictates DAT surface expression and function.SIGNIFICANCE STATEMENT Signaling by the neurotransmitter dopamine (DA) is tightly regulated by the DA transporter (DAT), insuring efficient DA clearance after release. Molecular networks that regulate DAT are poorly understood, particularly in vivo Using a forward genetic screen in the nematode Caenorhabditis elegans, we implicate the atypical mitogen activated protein kinase, SWIP-13, in DAT regulation. Moreover, we provide in vitro and in vivo evidence that SWIP-13, as well as its human counterpart ERK8, regulate DAT surface availability via the activation of Rho proteins. Our findings implicate a novel pathway that regulates DA synaptic availability and that may contribute to risk for disorders linked to perturbed DA signaling. Targeting this pathway may be of value in the development of therapeutics in such disorders.

Palmitoylation mechanisms in dopamine transporter regulation

The neurotransmitter dopamine (DA) plays a key role in several biological processes including reward, mood, motor activity and attention. Synaptic DA homeostasis is controlled by the dopamine transporter (DAT) which transports extracellular DA into the presynaptic neuron after release and regulates its availability to receptors. Many neurological disorders such as schizophrenia, bipolar disorder, Parkinson disease and attention-deficit hyperactivity disorder are associated with imbalances in DA homeostasis that may be related to DAT dysfunction. DAT is also a target of psychostimulant and therapeutic drugs that inhibit DA reuptake and lead to elevated dopaminergic neurotransmission. We have recently demonstrated the acute and chronic modulation of DA reuptake activity and DAT stability through S-palmitoylation, the linkage of a 16-carbon palmitate group to cysteine via a thioester bond. This review summarizes the properties and regulation of DAT palmitoylation and describes how it serves to affect various transporter functions. Better understanding of the role of palmitoylation in regulation of DAT function may lead to identification of therapeutic targets for modulation of DA homeostasis in the treatment of dopaminergic disorders.

Sequence determinants of the Caenhorhabditis elegans dopamine transporter dictating in vivo axonal export and synaptic localization

The monoamine neurotransmitter dopamine (DA) acts across phylogeny to modulate both simple and complex behaviors. The presynaptic DA transporter (DAT) is a major determinant of DA signaling capacity in ensuring efficient extracellular DA clearance. In humans, DAT is also a major target for prescribed and abused psychostimulants. Multiple structural determinants of DAT function and regulation have been defined, though largely these findings have arisen from heterologous expression or ex vivo cell culture studies. Loss of function mutations in the gene encoding the Caenhorhabditis elegans DAT (dat-1) produces rapid immobility when animals are placed in water, a phenotype termed swimming-induced paralysis (Swip). The ability of a DA neuron-expressed, GFP-tagged DAT-1 fusion protein (GFP::DAT-1) to localize to synapses and rescue Swip in these animals provides a facile approach to define sequences supporting DAT somatic export and function in vivo. In prior studies, we found that truncation of the last 25 amino acids of the DAT-1 C-terminus (Δ25) precludes Swip rescue, supported by a deficit in GFP::DAT-1 synaptic localization. Here, we further defined the elements within Δ25 required for DAT-1 export and function in vivo. We identified two conserved motifs (⁵⁸⁴KW⁵⁸⁵ and ⁵⁹¹PYRKR⁵⁹⁵) where mutation results in a failure of GFP::DAT-1 to be efficiently exported to synapses and restore DAT-1 function. The ⁵⁸⁴KW⁵⁸⁵ motif conforms to a sequence proposed to support SEC24 binding, ER export from the endoplasmic reticulum (ER), and surface expression of mammalian DAT proteins, whereas the ⁵⁹¹PYRKR⁵⁹⁵ sequence conforms to a 3R motif identified as a SEC24 binding site in vertebrate G-protein coupled receptors. Consistent with a potential role of SEC24 orthologs in DAT-1 export, we demonstrated DA neuron-specific expression of a sec-24.2 transcriptional reporter. Mutations of the orthologous C-terminal sequences in human DAT (hDAT) significantly reduced transporter surface expression and DA uptake, despite normal hDAT protein expression. Although, hDAT mutants retained SEC24 interactions, as defined in co-immunoprecipitation studies. However, these mutations disrupted the ability of SEC24D to enhance hDAT surface expression. Our studies document an essential role of conserved DAT C-terminal sequences in transporter somatic export and synaptic localization in vivo, that add further support for important roles for SEC24 family members in efficient transporter trafficking.

Research Domain Criteria versus DSM V: How does this debate affect attempts to model corticostriatal dysfunction in animals?

For decades, the nosology of mental illness has been based largely upon the descriptions in the Diagnostic and Statistical Manual of the American Psychiatric Association (DSM). A recent challenge to the DSM approach to psychiatric nosology from the National Institute on Mental Health (USA) defines Research Domain Criteria (RDoC) as an alternative. For RDoC, psychiatric illnesses are not defined as discrete categories, but instead as specific behavioral dysfunctions irrespective of DSM diagnostic categories. This approach was driven by two primary weaknesses noted in the DSM: (1) the same symptoms occur in very different disease states; and (2) DSM criteria lack grounding in the underlying biological causes of mental illness. RDoC intends to ground psychiatric nosology in those underlying mechanisms. This review addresses the suitability of RDoC vs. DSM from the view of modeling mental illness in animals. A consideration of all types of psychiatric dysfunction is beyond the scope of this review, which will focus on models of conditions associated with frontostriatal dysfunction.

Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters

Modulation of neurotransmission by the monoamines dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is critical for normal nervous system function. Precise temporal and spatial control of this signaling in mediated in large part by the actions of monoamine transporters (DAT, NET, and SERT, respectively). These transporters act to recapture their respective neurotransmitters after release, and disruption of clearance and reuptake has significant effects on physiology and behavior and has been linked to a number of neuropsychiatric disorders. To ensure adequate and dynamic control of these transporters, multiple modes of control have evolved to regulate their activity and trafficking. Central to many of these modes of control are the actions of protein kinases, whose actions can be direct or indirectly mediated by kinase-modulated protein interactions. Here, we summarize the current state of our understanding of how protein kinases regulate monoamine transporters through changes in activity, trafficking, phosphorylation state, and interacting partners. We highlight genetic, biochemical, and pharmacological evidence for kinase-linked control of DAT, NET, and SERT and, where applicable, provide evidence for endogenous activators of these pathways. We hope our discussion can lead to a more nuanced and integrated understanding of how neurotransmitter transporters are controlled and may contribute to disorders that feature perturbed monoamine signaling, with an ultimate goal of developing better therapeutic strategies.

Regulation of the Dopamine and Vesicular Monoamine Transporters: Pharmacological Targets and Implications for Disease

Dopamine (DA) plays a well recognized role in a variety of physiologic functions such as movement, cognition, mood, and reward. Consequently, many human disorders are due, in part, to dysfunctional dopaminergic systems, including Parkinson's disease, attention deficit hyperactivity disorder, and substance abuse. Drugs that modify the DA system are clinically effective in treating symptoms of these diseases or are involved in their manifestation, implicating DA in their etiology. DA signaling and distribution are primarily modulated by the DA transporter (DAT) and by vesicular monoamine transporter (VMAT)-2, which transport DA into presynaptic terminals and synaptic vesicles, respectively. These transporters are regulated by complex processes such as phosphorylation, protein-protein interactions, and changes in intracellular localization. This review provides an overview of 1) the current understanding of DAT and VMAT2 neurobiology, including discussion of studies ranging from those conducted in vitro to those involving human subjects; 2) the role of these transporters in disease and how these transporters are affected by disease; and 3) and how selected drugs alter the function and expression of these transporters. Understanding the regulatory processes and the pathologic consequences of DAT and VMAT2 dysfunction underlies the evolution of therapeutic development for the treatment of DA-related disorders.

The rare DAT coding variant Val559 perturbs DA neuron function, changes behavior, and alters in vivo responses to psychostimulants

Despite the critical role of the presynaptic dopamine (DA) transporter (DAT, SLC6A3) in DA clearance and psychostimulant responses, evidence that DAT dysfunction supports risk for mental illness is indirect. Recently, we identified a rare, nonsynonymous Slc6a3 variant that produces the DAT substitution Ala559Val in two male siblings who share a diagnosis of attention-deficit hyperactivity disorder (ADHD), with other studies identifying the variant in subjects with bipolar disorder (BPD) and autism spectrum disorder (ASD). Previously, using transfected cell studies, we observed that although DAT Val559 displays normal total and surface DAT protein levels, and normal DA recognition and uptake, the variant transporter exhibits anomalous DA efflux (ADE) and lacks capacity for amphetamine (AMPH)-stimulated DA release. To pursue the significance of these findings in vivo, we engineered DAT Val559 knock-in mice, and here we demonstrate in this model the presence of elevated extracellular DA levels, altered somatodendritic and presynaptic D2 DA receptor (D2R) function, a blunted ability of DA terminals to support depolarization and AMPH-evoked DA release, and disruptions in basal and psychostimulant-evoked locomotor behavior. Together, our studies demonstrate an in vivo functional impact of the DAT Val559 variant, providing support for the ability of DAT dysfunction to impact risk for mental illness.

SLC6A3 coding variant Ala559Val found in two autism probands alters dopamine transporter function and trafficking

Emerging evidence associates dysfunction in the dopamine (DA) transporter (DAT) with the pathophysiology of autism spectrum disorder (ASD). The human DAT (hDAT; SLC6A3) rare variant with an Ala to Val substitution at amino acid 559 (hDAT A559V) was previously reported in individuals with bipolar disorder or attention-deficit hyperactivity disorder (ADHD). We have demonstrated that this variant is hyper-phosphorylated at the amino (N)-terminal serine (Ser) residues and promotes an anomalous DA efflux phenotype. Here, we report the novel identification of hDAT A559V in two unrelated ASD subjects and provide the first mechanistic description of its impaired trafficking phenotype. DAT surface expression is dynamically regulated by DAT substrates including the psychostimulant amphetamine (AMPH), which causes hDAT trafficking away from the plasma membrane. The integrity of DAT trafficking directly impacts DA transport capacity and therefore dopaminergic neurotransmission. Here, we show that hDAT A559V is resistant to AMPH-induced cell surface redistribution. This unique trafficking phenotype is conferred by altered protein kinase C β (PKCβ) activity. Cells expressing hDAT A559V exhibit constitutively elevated PKCβ activity, inhibition of which restores the AMPH-induced hDAT A559V membrane redistribution. Mechanistically, we link the inability of hDAT A559V to traffic in response to AMPH to the phosphorylation of the five most distal DAT N-terminal Ser. Mutation of these N-terminal Ser to Ala restores AMPH-induced trafficking. Furthermore, hDAT A559V has a diminished ability to transport AMPH, and therefore lacks AMPH-induced DA efflux. Pharmacological inhibition of PKCβ or Ser to Ala substitution in the hDAT A559V background restores AMPH-induced DA efflux while promoting intracellular AMPH accumulation. Although hDAT A559V is a rare variant, it has been found in multiple probands with neuropsychiatric disorders associated with imbalances in DA neurotransmission, including ADHD, bipolar disorder, and now ASD. These findings provide valuable insight into a new cellular phenotype (altered hDAT trafficking) supporting dysregulated DA function in these disorders. They also provide a novel potential target (PKCβ) for therapeutic interventions in individuals with ASD.

Genetic targeting of the amphetamine and methylphenidate-sensitive dopamine transporter: on the path to an animal model of attention-deficit hyperactivity disorder

Alterations in dopamine (DA) signaling underlie the most widely held theories of molecular and circuit level perturbations that lead to risk for attention-deficit hyperactivity disorder (ADHD). The DA transporter (DAT), a presynaptic reuptake protein whose activity provides critical support for DA signaling by limiting DA action at pre- and postsynaptic receptors, has been consistently associated with ADHD through pharmacological, behavioral, brain imaging and genetic studies. Currently, the animal models of ADHD exhibit significant limitations, stemming in large part from their lack of construct validity. To remedy this situation, we have pursued the creation of a mouse model derived from a functional nonsynonymous variant in the DAT gene (SLC6A3) of ADHD probands. We trace our path from the identification of these variants to in vitro biochemical and physiological studies to the production of the DAT Val559 mouse model. We discuss our initial findings with these animals and their promise in the context of existing rodent models of ADHD.