A network of phosphatidylinositol (4,5)-bisphosphate (PIP(2)) binding sites on the dopamine transporter regulates amphetamine behavior in Drosophila Melanogaster

Assessment of voluntary drug and alcohol intake in Drosophila melanogaster using a modified one-tube capillary feeding assay

BACKGROUND

The capillary feeding assay measures intakes of solutions containing ethanol and other drugs of abuse in the fruit fly Drosophila melanogaster. Existing single-tube and two-choice variations of the assay conflate drug intake with nutrient intake and utilize conditions that decrease lifespan of flies, suggesting these conditions might serve as a physiological stressor.

NEW METHOD

The current experiments used a novel variation of a single-tube capillary feeding assay with flies maintained on standard, semi-soft food throughout the experiment, and offered a drug in a moderately preferred sucrose solution. Preference or aversion for a drug-containing solution was assessed relative to a control sucrose solution.

RESULTS

We found concentration-dependent preferences for solutions containing ethanol or fencamfamine, aversions to solutions containing amphetamine, caffeine, muscimol, nicotine, pentobarbital, or picrotoxin, and a nominal aversion to solutions containing methamphetamine. Some aversions were found only in male flies, although low overall intake by female flies likely contributed to this apparent sex-specific effect.

COMPARISON WITH EXISTING METHOD(S)

The continuous availability of semi-soft food in our assay to decouple the need for nutrients from intake of the drug-containing solution. In addition, our assay uses only a single capillary tube per vial of flies, making it less resource intensive than two-choice capillary feeding assays.

CONCLUSIONS

The availability of standard food in our modified, one-tube capillary feeding assay should prove to be useful modification of the capillary feeding assay for studies of drug intake.

Fusobacterium nucleatum determines the expression of amphetamine-induced behavioral responses through an epigenetic phenomenon

Amphetamines (AMPHs) are psychostimulants commonly used for the treatment of neuropsychiatric disorders. They are also misused (AMPH use disorder; AUD), with devastating outcomes. Recent studies have implicated dysbiosis in the pathogenesis of AUD. However, the mechanistic roles of microbes in AUD are unknown. Fusobacterium nucleatum (Fn) is a bacterium that increases in abundance in both rats and humans upon AMPH exposure. Fn releases short-chain fatty acids (SCFAs), bacterial byproducts thought to play a fundamental role in the gut-brain axis as well as the pathogenesis of AUD. We demonstrate that in gnotobiotic Drosophila melanogaster, colonization with Fn or dietary supplementation of the SCFA butyrate, a potent inhibitor of histone deacetylases (HDACs), enhances the psychomotor and rewarding properties of AMPH as well as its ability to promote male sexual motivation. Furthermore, solely HDAC1 RNAi targeted inhibition recapitulates these enhancements, pointing to a specific process underlying this Fn phenomenon. Of note is that the expression of these AMPH behaviors is determined by the increase in extracellular dopamine (DA) levels that result from AMPH-induced reversal of DA transporter (DAT) function, termed non-vesicular DA release (NVDR). The magnitude of AMPH-induced NVDR is dictated, at least in part, by DAT expression levels. Consistent with our behavioral data, we show that Fn, butyrate, and HDAC1 inhibition enhance NVDR by elevating DAT expression. Thus, the participation of Fn in AUD stems from its ability to release butyrate and inhibit HDAC1. These data offer a microbial target and probiotic interventions for AUD treatment.

Epigenetic regulation-mediated disorders in dopamine transporter endocytosis: A novel mechanism for the pathogenesis of Parkinson's disease

Mechanisms such as DNA methylation, histone modifications, and non-coding RNA regulation may impact the endocytosis of dopamine transporter (DAT) by influencing processes like neuronal survival, thereby contributing to the initiation and progression of Parkinson's Disease (PD). Some small molecule inhibitors or natural bioactive compounds have the potential to modulate epigenetic processes, thereby reversing induced pluripotent stem cells (iPSCs) reprogramming and abnormal differentiation, offering potential therapeutic effects for PD. Although no specific DNA modification enzyme directly regulates DAT endocytosis, enzymes such as DNA methyltransferases (DNMTs) may indirectly influence DAT endocytosis by regulating the expression of genes associated with this process. DNA modifications impact DAT endocytosis by modulating key signaling pathways, including the (protein kinase C) PKC and D2 receptor (D2R) pathways. Key enzymes involved in RNA modifications that influence DAT endocytosis include m6A methyltransferases and other related enzymes. This regulation impacts the synthesis and function of proteins involved in DAT endocytosis, thereby indirectly affecting the process itself. RNA modifications regulate DAT endocytosis through various indirect pathways, as well as histone modifications. Key enzymes influence the expression of genes associated with DAT endocytosis by modulating the chromatin's accessibility and compaction state. These enzymes control the expression of proteins involved in regulating endocytosis, promoting endosome formation, and facilitating recycling processes. Through the modulation exerted by these enzymes, the speed of DAT endocytosis and recycling patterns are indirectly regulated, establishing a crucial epigenetic control point for the regulation of neurotransmitter transport. Based on this understanding, we anticipate that targeting these processes could lead to favorable therapeutic effects for early PD pathogenesis.

Caenorhabditis elegans for opioid addiction research

The problem of drug addiction has become a profound societal problem worldwide. A better understanding of the neurobiological basis of addiction and the discovery of more effective treatments are needed. Recent studies have shown that many mechanisms that underlie addiction exist in more primitive organisms, including the nematode Caenorhabditis elegans (C. elegans). C. elegans is also hypothesized to possess a functional opioid-like system, including the endogenous opioid-like peptide NLP-24 and opioid-like receptor NPR-17. Opioids, such as morphine, are thought to cause addiction-like behavior by activating dopamine nerves in C. elegans via the opioid-like system. Accumulating evidence suggests that C. elegans is an excellent animal model for identifying molecular mechanisms of addiction.

Parkinson's disease gene, Synaptojanin1, dysregulates the surface maintenance of the dopamine transporter

Missense mutations of PARK20/SYNJ1 (synaptojanin1/Synj1) were found in complex forms of familial Parkinsonism. However, the Synj1-regulated molecular and cellular changes associated with dopaminergic dysfunction remain unknown. We now report a fast depletion of evoked dopamine and impaired maintenance of the axonal dopamine transporter (DAT) in the Synj1 haploinsufficient (Synj1+/-) neurons. While Synj1 has been traditionally known to facilitate the endocytosis of synaptic vesicles, we provide in vitro and in vivo evidence demonstrating that Synj1 haploinsufficiency results in an increase of total DAT but a reduction of the surface DAT. Synj1+/- neurons exhibit maladaptive DAT trafficking, which could contribute to the altered DA release. We showed that the loss of surface DAT is associated with the impaired 5'-phosphatase activity and the hyperactive PI(4,5)P2-PKCβ pathway downstream of Synj1 deficiency. Thus, our findings provided important mechanistic insight for Synj1-regulated DAT trafficking integral to dysfunctional DA signaling, which might be relevant to early Parkinsonism.

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.

A Conserved Intramolecular Ion-Pair Plays a Critical but Divergent Role in Regulation of Dimerization and Transport Function among the Monoamine Transporters

The monoamine transporters, including the serotonin transporter (SERT), dopamine transporter (DAT), and norepinephrine transporter (NET), are the therapeutic targets for the treatment of many neuropsychiatric disorders. Despite significant progress in characterizing the structures and transport mechanisms of these transporters, the regulation of their transport functions through dimerization or oligomerization remains to be understood. In the present study, we identified a conserved intramolecular ion-pair at the third extracellular loop (EL3) connecting TM5 and TM6 that plays a critical but divergent role in the modulation of dimerization and transport functions among the monoamine transporters. The disruption of the ion-pair interactions by mutations induced a significant spontaneous cross-linking of a cysteine mutant of SERT and an increase in cell surface expression but with an impaired specific transport activity. On the other hand, similar mutations of the corresponding ion-pair residues in both DAT and NET resulted in an opposite effect on their oxidation-induced dimerization, cell surface expression, and transport function. Reversible biotinylation experiments indicated that the ion-pair mutations slowed down the internalization of SERT but stimulated the internalization of DAT. In addition, cysteine accessibility measurements for monitoring SERT conformational changes indicated that substitution of the ion-pair residues resulted in profound effects on the rate constants for cysteine modification in both the extracellular and cytoplasmatic substrate permeation pathways. Furthermore, molecular dynamics simulations showed that the ion-pair mutations increased the interfacial interactions in a SERT dimer but decreased it in a DAT dimer. Taken together, we propose that the transport function is modulated by the equilibrium between monomers and dimers on the cell surface, which is regulated by a potential compensatory mechanism but with different molecular solutions among the monoamine transporters. The present study provided new insights into the structural elements regulating the transport function of the monoamine transporters through their dimerization.

Parkinson's disease gene, Synaptojanin1, dysregulates the surface maintenance of the dopamine transporter

Missense mutations of PARK20/SYNJ1 (synaptojanin1/Synj1) have been linked to complex forms of familial parkinsonism, however, the molecular and cellular changes associated with dopaminergic dysfunction remains unknown. We now report fast depletion of evoked dopamine (DA) and altered maintenance of the axonal dopamine transporter (DAT) in the Synj1+/- neurons. While Synj1 has been traditionally known to facilitate the endocytosis of synaptic vesicles, we demonstrated that axons of cultured Synj1+/- neurons exhibit an increase of total DAT but a reduction of the surface DAT, which could be exacerbated by neuronal activity. We revealed that the loss of surface DAT is specifically associated with the impaired 5'-phosphatase activity of Synj1 and the hyperactive downstream PI(4,5)P2-PKCβ pathway. Thus, our findings provided important mechanistic insight for Synj1-regulated DAT trafficking integral to dysfunctional DA signaling in early parkinsonism.

Oligomerization of Monoamine Transporters

Transporters of the monoamine transporter (MAT) family regulate the uptake of important neurotransmitters like dopamine, serotonin, and norepinephrine. The MAT family functions using the electrochemical gradient of ions across the membrane and comprises three transporters, dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). MAT transporters have been observed to exist in monomeric states to higher-order oligomeric states. Structural features, allosteric modulation, and lipid environment regulate the oligomerization of MAT transporters. NET and SERT oligomerization are regulated by levels of PIP2 present in the membrane. The kink present in TM12 in the MAT family is crucial for dimer interface formation. Allosteric modulation in the dimer interface hinders dimer formation. Oligomerization also influences the transporters' function, trafficking, and regulation. This chapter will focus on recent studies on monoamine transporters and discuss the factors affecting their oligomerization and its impact on their function.

The Stage-Based Model of Addiction-Using Drosophila to Investigate Alcohol and Psychostimulant Responses

Addiction is a progressive and complex disease that encompasses a wide range of disorders and symptoms, including substance use disorder (SUD), for which there are few therapeutic treatments. SUD is the uncontrolled and chronic use of substances despite the negative consequences resulting from this use. The progressive nature of addiction is organized into a testable framework, the neurobiological stage-based model, that includes three behavioral stages: (1) binge/intoxication, (2) withdrawal/negative affect, and (3) preoccupation/anticipation. Human studies offer limited opportunities for mechanistic insights into these; therefore, model organisms, like Drosophila melanogaster, are necessary for understanding SUD. Drosophila is a powerful model organism that displays a variety of SUD-like behaviors consistent with human and mammalian substance use, making flies a great candidate to study mechanisms of behavior. Additionally, there are an abundance of genetic tools like the GAL4/UAS and CRISPR/Cas9 systems that can be used to gain insight into the molecular mechanisms underlying the endophenotypes of the three-stage model. This review uses the three-stage framework and discusses how easily testable endophenotypes have been examined with experiments using Drosophila, and it outlines their potential for investigating other endophenotypes.

CHARMM-GUI Membrane Builder: Past, Current, and Future Developments and Applications

Molecular dynamics simulations of membranes and membrane proteins serve as computational microscopes, revealing coordinated events at the membrane interface. As G protein-coupled receptors, ion channels, transporters, and membrane-bound enzymes are important drug targets, understanding their drug binding and action mechanisms in a realistic membrane becomes critical. Advances in materials science and physical chemistry further demand an atomistic understanding of lipid domains and interactions between materials and membranes. Despite a wide range of membrane simulation studies, generating a complex membrane assembly remains challenging. Here, we review the capability of CHARMM-GUI Membrane Builder in the context of emerging research demands, as well as the application examples from the CHARMM-GUI user community, including membrane biophysics, membrane protein drug-binding and dynamics, protein-lipid interactions, and nano-bio interface. We also provide our perspective on future Membrane Builder development.

Cardiolipin and OPA1 Team up for Methamphetamine-Induced Locomotor Activity by Promoting Neuronal Mitochondrial Fusion in the Nucleus Accumbens of Mice

Mitochondria are highly dynamic organelles with coordinated cycles of fission and fusion occurring continuously to satisfy the energy demands in the complex architecture of neurons. How mitochondria contribute to addicted drug-induced adaptable mitochondrial networks and neuroplasticity remains largely unknown. Through liquid chromatography-mass spectrometry-based lipidomics, we first analyzed the alteration of the mitochondrial lipidome of three mouse brain areas in methamphetamine (METH)-induced locomotor activity and conditioned place preference. The results showed that METH remodeled the mitochondrial lipidome of the hippocampus, nucleus accumbens (NAc), and striatum in both models. Notably, mitochondrial hallmark lipid cardiolipin (CL) was specifically increased in the NAc in METH-induced hyperlocomotor activity, which was accompanied by an elongated giant mitochondrial morphology. Moreover, METH significantly boosted mitochondrial respiration and ATP generation as well as the copy number of mitochondrial genome DNA in the NAc. By screening the expressions of mitochondrial dynamin-related proteins, we found that repeated METH significantly upregulated the expression of long-form optic atrophy type 1 (L-OPA1) and enhanced the interaction of L-OPA1 with CL, which may promote mitochondrial fusion in the NAc. On the contrary, neuronal OPA1 depletion in the NAc not only recovered the dysregulated mitochondrial morphology and synaptic vesicle distribution induced by METH but also attenuated the psychomotor effect of METH. Collectively, upregulated CL and OPA1 cooperate to mediate METH-induced adaptation of neuronal mitochondrial dynamics in the NAc, which correlates with the psychomotor effect of METH. These findings propose a potential therapeutic approach for METH addiction by inhibiting neuronal mitochondrial fusion.

Syntaxin 1 Ser14 phosphorylation is required for nonvesicular dopamine release

Amphetamine (AMPH) is a psychostimulant that is commonly abused. The stimulant properties of AMPH are associated with its ability to increase dopamine (DA) neurotransmission. This increase is promoted by nonvesicular DA release mediated by reversal of DA transporter (DAT) function. Syntaxin 1 (Stx1) is a SNARE protein that is phosphorylated at Ser14 by casein kinase II. We show that Stx1 phosphorylation is critical for AMPH-induced nonvesicular DA release and, in Drosophila melanogaster, regulates the expression of AMPH-induced preference and sexual motivation. Our molecular dynamics simulations of the DAT/Stx1 complex demonstrate that phosphorylation of these proteins is pivotal for DAT to dwell in a DA releasing state. This state is characterized by the breakdown of two key salt bridges within the DAT intracellular gate, causing the opening and hydration of the DAT intracellular vestibule, allowing DA to bind from the cytosol, a mechanism that we hypothesize underlies nonvesicular DA release.

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.

Phosphatidylinositol 4,5-bisphosphate (PIP2) facilitates norepinephrine transporter dimerization and modulates substrate efflux

The plasmalemmal norepinephrine transporter (NET) regulates cardiovascular sympathetic activity by clearing extracellular norepinephrine in the synaptic cleft. Here, we investigate the subunit stoichiometry and function of NET using single-molecule fluorescence microscopy and flux assays. In particular, we show the effect of phosphatidylinositol 4,5-bisphosphate (PIP2) on NET oligomerization and efflux. NET forms monomers (~60%) and dimers (~40%) at the plasma membrane. PIP2 depletion results in a decrease in the average oligomeric state and decreases NET-mediated substrate efflux while not affecting substrate uptake. Mutation of the putative PIP2 binding residues R121, K334, and R440 to alanines does not affect NET dimerization but results in decreased substrate efflux that is not altered upon PIP2 depletion; this indicates that PIP2 interactions with these residues affect NET-mediated efflux. A dysregulation of norepinephrine and PIP2 signaling have both been implicated in neuropsychiatric and cardiovascular diseases. This study provides evidence that PIP2 directly regulates NET organization and function.

Nanoscopic dopamine transporter distribution and conformation are inversely regulated by excitatory drive and D2 autoreceptor activity

The nanoscopic organization and regulation of individual molecular components in presynaptic varicosities of neurons releasing modulatory volume neurotransmitters like dopamine (DA) remain largely elusive. Here we show, by application of several super-resolution microscopy techniques to cultured neurons and mouse striatal slices, that the DA transporter (DAT), a key protein in varicosities of dopaminergic neurons, exists in the membrane in dynamic equilibrium between an inward-facing nanodomain-localized and outward-facing unclustered configuration. The balance between these configurations is inversely regulated by excitatory drive and DA D2 autoreceptor activation in a manner dependent on Ca2+ influx via N-type voltage-gated Ca2+ channels. The DAT nanodomains contain tens of transporters molecules and overlap with nanodomains of PIP2 (phosphatidylinositol-4,5-bisphosphate) but show little overlap with D2 autoreceptor, syntaxin-1, and clathrin nanodomains. The data reveal a mechanism for rapid alterations of nanoscopic DAT distribution and show a striking link of this to the conformational state of the transporter.

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.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

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.

The Role of the Dopamine Transporter in the Effects of Amphetamine on Sleep and Sleep Architecture in Drosophila

The dopamine transporter (DAT) mediates the inactivation of released dopamine (DA) through its reuptake, and thereby plays an important homeostatic role in dopaminergic neurotransmission. Amphetamines exert their stimulant effects by targeting DAT and inducing the reverse transport of DA, leading to a dramatic increase of extracellular DA. Animal models have proven critical to investigating the molecular and cellular mechanisms underlying transporter function and its modulation by psychostimulants such as amphetamine. Here we establish a behavioral model for amphetamine action using adult Drosophila melanogaster. We use it to characterize the effects of amphetamine on sleep and sleep architecture. Our data show that amphetamine induces hyperactivity and disrupts sleep in a DA-dependent manner. Flies that do not express a functional DAT (dDAT null mutants) have been shown to be hyperactive and to exhibit significantly reduced sleep at baseline. Our data show that, in contrast to its action in control flies, amphetamine decreases the locomotor activity of dDAT null mutants and restores their sleep by modulating distinct aspects of sleep structure. To begin to explore the circuitry involved in the actions of amphetamine on sleep, we also describe the localization of dDAT throughout the fly brain, particularly in neuropils known to regulate sleep. Together, our data establish Drosophila as a robust model for studying the regulatory mechanisms that govern DAT function and psychostimulant action.

Bile Acids Gate Dopamine Transporter Mediated Currents

Bile acids (BAs) are molecules derived from cholesterol that are involved in dietary fat absorption. New evidence supports an additional role for BAs as regulators of brain function. Sterols such as cholesterol interact with monoamine transporters, including the dopamine (DA) transporter (DAT) which plays a key role in DA neurotransmission and reward. This study explores the interactions of the BA, obeticholic acid (OCA), with DAT and characterizes the regulation of DAT activity via both electrophysiology and molecular modeling. We expressed murine DAT (mDAT) in Xenopus laevis oocytes and confirmed its functionality. Next, we showed that OCA promotes a DAT-mediated inward current that is Na+-dependent and not regulated by intracellular calcium. The current induced by OCA was transient in nature, returning to baseline in the continued presence of the BA. OCA also transiently blocked the DAT-mediated Li+-leak current, a feature that parallels DA action and indicates direct binding to the transporter in the absence of Na+. Interestingly, OCA did not alter DA affinity nor the ability of DA to promote a DAT-mediated inward current, suggesting that the interaction of OCA with the transporter is non-competitive, regarding DA. Docking simulations performed for investigating the molecular mechanism of OCA action on DAT activity revealed two potential binding sites. First, in the absence of DA, OCA binds DAT through interactions with D421, a residue normally involved in coordinating the binding of the Na+ ion to the Na2 binding site (Borre et al., J. Biol. Chem., 2014, 289, 25764-25773; Cheng and Bahar, Structure, 2015, 23, 2171-2181). Furthermore, we uncover a separate binding site for OCA on DAT, of equal potential functional impact, that is coordinated by the DAT residues R445 and D436. Binding to that site may stabilize the inward-facing (IF) open state by preventing the re-formation of the IF-gating salt bridges, R60-D436 and R445-E428, that are required for DA transport. This study suggests that BAs may represent novel pharmacological tools to regulate DAT function, and possibly, associated behaviors.

Identification by proximity labeling of novel lipidic and proteinaceous potential partners of the dopamine transporter

Dopamine (DA) transporters (DATs) are regulated by trafficking and modulatory processes that probably rely on stable and transient interactions with neighboring proteins and lipids. Using proximity-dependent biotin identification (BioID), we found novel potential partners for DAT, including several membrane proteins, such as the transmembrane chaperone 4F2hc, the proteolipid M6a and a potential membrane receptor for progesterone (PGRMC2). We also detected two cytoplasmic proteins: a component of the Cullin1-dependent ubiquitination machinery termed F-box/LRR-repeat protein 2 (FBXL2), and the enzyme inositol 5-phosphatase 2 (SHIP2). Immunoprecipitation (IP) and immunofluorescence studies confirmed either a physical association or a close spatial proximity between these proteins and DAT. M6a, SHIP2 and the Cullin1 system were shown to increase DAT activity in coexpression experiments, suggesting a functional role for their association. Deeper analysis revealed that M6a, which is enriched in neuronal protrusions (filopodia or dendritic spines), colocalized with DAT in these structures. In addition, the product of SHIP2 enzymatic activity (phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2]) was tightly associated with DAT, as shown by co-IP and by colocalization of mCherry-DAT with a specific biosensor for this phospholipid. PI(3,4)P2 strongly stimulated transport activity in electrophysiological recordings, and conversely, inhibition of SHIP2 reduced DA uptake in several experimental systems including striatal synaptosomes and the dopaminergic cell line SH-SY5Y. In summary, here we report several potential new partners for DAT and a novel regulatory lipid, which may represent new pharmacological targets for DAT, a pivotal protein in dopaminergic function of the brain.

Dopaminergic Ric GTPase activity impacts amphetamine sensitivity and sleep quality in a dopamine transporter-dependent manner in Drosophila melanogaster

Dopamine (DA) is required for movement, sleep, and reward, and DA signaling is tightly controlled by the presynaptic DA transporter (DAT). Therapeutic and addictive psychostimulants, including methylphenidate (Ritalin; MPH), cocaine, and amphetamine (AMPH), markedly elevate extracellular DA via their actions as competitive DAT inhibitors (MPH, cocaine) and substrates (AMPH). DAT silencing in mice and invertebrates results in hyperactivity, reduced sleep, and blunted psychostimulant responses, highlighting DAT's essential role in DA-dependent behaviors. DAT surface expression is not static; rather it is dynamically regulated by endocytic trafficking. PKC-stimulated DAT endocytosis requires the neuronal GTPase, Rit2, and Rit2 silencing in mouse DA neurons impacts psychostimulant sensitivity. However, it is unknown whether or not Rit2-mediated changes in psychostimulant sensitivity are DAT-dependent. Here, we leveraged Drosophila melanogaster to test whether the Drosophila Rit2 ortholog, Ric, impacts dDAT function, trafficking, and DA-dependent behaviors. Orthologous to hDAT and Rit2, dDAT and Ric directly interact, and the constitutively active Ric mutant Q117L increased dDAT surface levels and function in cell lines and ex vivo Drosophila brains. Moreover, DAergic RicQ117L expression caused sleep fragmentation in a DAT-dependent manner but had no effect on total sleep and daily locomotor activity. Importantly, we found that Rit2 is required for AMPH-stimulated DAT internalization in mouse striatum, and that DAergic RicQ117L expression significantly increased Drosophila AMPH sensitivity in a DAT-dependent manner, suggesting a conserved impact of Ric-dependent DAT trafficking on AMPH sensitivity. These studies support that the DAT/Rit2 interaction impacts both baseline behaviors and AMPH sensitivity, potentially by regulating DAT trafficking.

Psychomotor impairments and therapeutic implications revealed by a mutation associated with infantile Parkinsonism-Dystonia

Parkinson disease (PD) is a progressive, neurodegenerative disorder affecting over 6.1 million people worldwide. Although the cause of PD remains unclear, studies of highly penetrant mutations identified in early-onset familial parkinsonism have contributed to our understanding of the molecular mechanisms underlying disease pathology. Dopamine (DA) transporter (DAT) deficiency syndrome (DTDS) is a distinct type of infantile parkinsonism-dystonia that shares key clinical features with PD, including motor deficits (progressive bradykinesia, tremor, hypomimia) and altered DA neurotransmission. Here, we define structural, functional, and behavioral consequences of a Cys substitution at R445 in human DAT (hDAT R445C), identified in a patient with DTDS. We found that this R445 substitution disrupts a phylogenetically conserved intracellular (IC) network of interactions that compromise the hDAT IC gate. This is demonstrated by both Rosetta molecular modeling and fine-grained simulations using hDAT R445C, as well as EPR analysis and X-ray crystallography of the bacterial homolog leucine transporter. Notably, the disruption of this IC network of interactions supported a channel-like intermediate of hDAT and compromised hDAT function. We demonstrate that Drosophila melanogaster expressing hDAT R445C show impaired hDAT activity, which is associated with DA dysfunction in isolated brains and with abnormal behaviors monitored at high-speed time resolution. We show that hDAT R445C Drosophila exhibit motor deficits, lack of motor coordination (i.e. flight coordination) and phenotypic heterogeneity in these behaviors that is typically associated with DTDS and PD. These behaviors are linked with altered dopaminergic signaling stemming from loss of DA neurons and decreased DA availability. We rescued flight coordination with chloroquine, a lysosomal inhibitor that enhanced DAT expression in a heterologous expression system. Together, these studies shed some light on how a DTDS-linked DAT mutation underlies DA dysfunction and, possibly, clinical phenotypes shared by DTDS and PD.

SLC6 transporter oligomerization

Transporters of the solute carrier 6 (SLC6) family mediate the reuptake of neurotransmitters such as dopamine, norepinephrine, serotonin, GABA, and glycine. SLC6 family members are 12 transmembrane helix‐spanning proteins that operate using the transmembrane sodium gradient for transport. These transporters assume various quaternary arrangements ranging from monomers to complex stoichiometries with multiple subunits. Dopamine and serotonin transporter oligomerization has been implicated in trafficking of newly formed proteins from the endoplasmic reticulum to the plasma membrane with a pre‐fixed assembly. Once at the plasma membrane, oligomers are kept fixed in their quaternary assembly by interaction with phosphoinositides. While it remains unclear how oligomer formation precisely affects physiological transporter function, it has been shown that oligomerization supports the activity of release‐type psychostimulants. Most recently, single molecule microscopy experiments unveiled that the stoichiometry differs between individual members of the SLC6 family. The present overview summarizes our understanding of the influence of plasma membrane constituents on transporter oligomerization, describes the known interfaces between protomers and discusses open questions.

Direct dopamine terminal regulation by local striatal microcircuitry

Regulation of axonal dopamine release by local microcircuitry is at the hub of several biological processes that govern the timing and magnitude of signaling events in reward-related brain regions. An important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters - as well heterosynaptic mechanisms such as retrograde signaling from postsynaptic cholinergic and dynorphin systems, among others. Additionally, modulation of dopamine release via diffusible messengers, such as nitric oxide and hydrogen peroxide, allows for various metabolic factors to quickly and efficiently regulate dopamine release and subsequent signaling. Here we review how these mechanisms work in concert to influence the timing and magnitude of striatal dopamine signaling, independent of action potential activity at the level of dopaminergic cell bodies in the midbrain, thereby providing a parallel pathway by which dopamine can be modulated. Understanding the complexities of local regulation of dopamine signaling is required for building comprehensive frameworks of how activity throughout the dopamine system is integrated to drive signaling and control behavior.