Oligomerization and trafficking of the human dopamine transporter. Mutational analysis identifies critical domains important for the functional expression of the transporter

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.

Early-life perfluorooctanoic acid exposure disrupts the function of dopamine transporter protein with glycosylation changes implicating the links between decreased dopamine levels and disruptive behaviors in larval zebrafish

Exposure to perfluorooctanoic acid (PFOA) during early embryonic development is associated with the increased risk of developmental neurotoxicity and neurobehavioral disorders in children. In our previous study, we demonstrated that exposure to PFOA affected locomotor activity and disrupted dopamine-related gene expression in zebrafish larvae. Consequently, we continue to study the dopaminergic system with a focus on dopamine levels and dopamine's effect on behaviors in relation to PFOA exposure. In the present study, we found a decrease in dopamine levels in larval zebrafish. We studied the dopamine transporter (DAT) protein, which is responsible for regulating dopamine levels through the reuptake of dopamine in neuronal cells. We demonstrated that exposure to PFOA disrupted the glycosylation process of DAT, inhibited its uptake function, and induced endoplasmic reticulum (ER) stress in dopaminergic cells. Besides, we conducted a light-dark preference test on larval zebrafish and observed anxiety/depressive-like behavioral changes following exposure to PFOA. Dopamine is one of the most prominent neurotransmitters that significantly influences human behavior, with low dopamine levels being associated with impairments such as anxiety and depression. The anxiety-like response in zebrafish larvae exposure to PFOA implies the link with the reduced dopamine levels. Taken together, we can deduce that glycosylation changes in DAT lead to dysfunction of DAT to regulate dopamine levels, which in turn alters behavior in larval zebrafish. Therefore, alternation in dopamine levels may play a pivotal role in the development of anxiety/depressive-like behavioral changes induced by PFOA.

Breaking the rules of SLC6 transporters: Export of the human creatine transporter-1 from the endoplasmic reticulum is supported by its N-terminus

Mutations in the human creatine transporter 1 (CRT1/SLC6A8) cause the creatine transporter deficiency syndrome, which is characterized by intellectual disability, epilepsy, autism, and developmental delay. The vast majority of mutations cause protein misfolding and hence reduce cell surface expression. Hence, it is important to understand the molecular machinery supporting folding and export of CRT1 from the endoplasmic reticulum (ER). All other SLC6 members thus far investigated rely on a C-terminal motif for binding the COPII-component SEC24 to drive their ER export; their N-termini are dispensable. Here, we show that, in contrast, in CRT1 the C-terminal ER-export motif is cryptic and it is the N-terminus, which supports ER export. This conclusion is based on the following observations: (i) siRNA-induced depletion of individual SEC24 isoforms revealed that CRT1 relied on SEC24C for ER export. However, mutations of the C-terminal canonical ER-export motif of CRT1 did not impair its cell surface delivery. (ii) Nevertheless, the C-terminal motif of CRT1 was operational in a chimeric protein comprising the serotonin transporter (SERT/SLC6A4) and the C-terminus of CRT1. (iii) Tagging of the N-terminus-but not the C-terminus-with yellow fluorescent protein (YFP) resulted in ER retention. (iv) Serial truncations of the N-terminus showed that removal of ≥51 residues of CRT1 impaired surface delivery, because the truncated CRT1 were confined to the ER. (v) Mutation of P51 to alanine also reduced cell surface delivery of CRT1 and relieved its dependence on SEC24C. Thus, the ER-export motif in the N-terminus of CRT1 overrides the canonical C-terminal motif.

Bacterial over-production of the functionally active human SLC38A2 (SNAT2) exploiting the mistic tag: a cheap and fast tool for testing ligands

BACKGROUND

SLC38A2 is a ubiquitously expressed Na+-dependent transporter specific for small and medium neutral amino acids. It is involved in human pathologies, such as type II diabetes and cancer. Despite its relevance in human physio-pathology, structure/function relationship studies and identification of ligands with regulatory roles are still in infancy.

METHODS AND RESULTS

The cDNA coding for SLC38A2 was cloned in the pET-28-Mistic vector, and the BL21 codon plus RIL strain was transformed with the recombinant construct. 0.5% glucose and oxygen availability were crucial for protein expression. The over-expressed hSNAT2-Mistic chimera was cleaved on column and purified by nickel-chelating affinity chromatography, with a yield of about 60 mg/Liter cell culture. The purified hSNAT2 was reconstituted in proteoliposomes in an active form with a right-side-out orientation with respect to the native membrane.

CONCLUSIONS

The addition of a Mistic tag at the N-terminus of the SNAT2 protein was crucial for its over-expression and purification. The purified protein was functionally active, representing a powerful tool for performing structure/function studies and testing ligands as inhibitors and/or activators.

Dessert or Poison? The Roles of Glycosylation in Alzheimer's, Parkinson's, Huntington's Disease, and Amyotrophic Lateral Sclerosis

Ministry of Education and Key Laboratory of Neurons and glial cells of the central nervous system (CNS) are modified by glycosylation and rely on glycosylation to achieve normal neural function. Neurodegenerative disease is a common disease of the elderly, affecting their healthy life span and quality of life, and no effective treatment is currently available. Recent research implies that various glycosylation traits are altered during neurodegenerative diseases, suggesting a potential implication of glycosylation in disease pathology. Herein, we summarized the current knowledge about glycosylation associated with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic lateral sclerosis (ALS) pathogenesis, focusing on their promising functional avenues. Moreover, we collected research aimed at highlighting the need for such studies to provide a wealth of disease-related glycosylation information that will help us better understand the pathophysiological mechanisms and hopefully specific glycosylation information to provide further diagnostic and therapeutic directions for neurodegenerative diseases.

Structures and membrane interactions of native serotonin transporter in complexes with psychostimulants

The serotonin transporter (SERT) is a member of the SLC6 neurotransmitter transporter family that mediates serotonin reuptake at presynaptic nerve terminals. SERT is the target of both therapeutic antidepressant drugs and psychostimulant substances such as cocaine and methamphetamines, which are small molecules that perturb normal serotonergic transmission by interfering with serotonin transport. Despite decades of studies, important functional aspects of SERT such as the oligomerization state of native SERT and its interactions with potential proteins remain unresolved. Here, we develop methods to isolate SERT from porcine brain (pSERT) using a mild, nonionic detergent, utilize fluorescence-detection size-exclusion chromatography to investigate its oligomerization state and interactions with other proteins, and employ single-particle cryo-electron microscopy to elucidate the structures of pSERT in complexes with methamphetamine or cocaine, providing structural insights into psychostimulant recognition and accompanying pSERT conformations. Methamphetamine and cocaine both bind to the central site, stabilizing the transporter in an outward open conformation. We also identify densities attributable to multiple cholesterol or cholesteryl hemisuccinate (CHS) molecules, as well as to a detergent molecule bound to the pSERT allosteric site. Under our conditions of isolation, we find that pSERT is best described as a monomeric entity, isolated without interacting proteins, and is ensconced by multiple cholesterol or CHS molecules.

Dopamine Transporter Deficiency Syndrome (DTDS): Expanding the Clinical Phenotype and Precision Medicine Approaches

Infantile parkinsonism-dystonia due to dopamine transporter deficiency syndrome (DTDS) is an ultrarare childhood movement disorder caused by biallelic loss-of-function mutations in the SLC6A3 gene. Advances in genomic analysis have revealed an evolving spectrum of SLC6A3-related neurological and neuropsychiatric disorders. Since the initial clinical and genetic characterisation of DTDS in 2009, there have been thirty-one published cases with a variety of protein-truncating variants (nonsense variants, splice-site changes, and deletions) and missense changes. Amino acid substitutions result in mutant proteins with impaired dopamine transporter function due to reduced transporter activity, impaired dopamine binding, reduced cell-surface expression, and aberrant posttranslational protein modification with impaired glycosylation. In this review, we provide an overview of the expanding clinical phenotype of DTDS and the precision therapies in development, including pharmacochaperones and gene therapy.

Molecular mechanism of hyperactivation conferred by a truncation of TRPA1

A drastic TRPA1 mutant (R919*) identified in CRAMPT syndrome patients has not been mechanistically characterized. Here, we show that the R919* mutant confers hyperactivity when co-expressed with wild type (WT) TRPA1. Using functional and biochemical assays, we reveal that the R919* mutant co-assembles with WT TRPA1 subunits into heteromeric channels in heterologous cells that are functional at the plasma membrane. The R919* mutant hyperactivates channels by enhancing agonist sensitivity and calcium permeability, which could account for the observed neuronal hypersensitivity-hyperexcitability symptoms. We postulate that R919* TRPA1 subunits contribute to heteromeric channel sensitization by altering pore architecture and lowering energetic barriers to channel activation contributed by the missing regions. Our results expand the physiological impact of nonsense mutations, reveal a genetically tractable mechanism for selective channel sensitization, uncover insights into the process of TRPA1 gating, and provide an impetus for genetic analysis of patients with CRAMPT or other stochastic pain syndromes.

Evaluation and Validation of Commercially Available Dopamine Transporter Antibodies

With a wide variety of dopamine transporter (DAT) antibodies available commercially, it is important to validate which antibodies provide sufficient immunodetection for reproducibility purpose and for accurate analysis of DAT levels and/or location. Commercially available DAT antibodies that are commonly used were tested in western blotting (WB) on wild-type (WT) and DAT-knock-out (DAT-KO) brain tissue and with immunohistology (IH) techniques against coronal slices of unilaterally lesioned 6-OHDA rats, in addition to wild-type and DAT-knock-out mice. DAT-KO mice and unilateral 6-OHDA lesions in rats were used as a negative control for DAT antibody specificity. Antibodies were tested at various concentrations and rated based on signal detection varying from no signal to optimal signal detection. Commonly used antibodies, including AB2231 and PT-22 524-1-AP, did not provide specific DAT signals in WB and IH. Although certain antibodies provided a good DAT signal, such as SC-32258, D6944, and MA5-24796, they also presented nonspecific bands in WB. Many DAT antibodies did not detect the DAT as advertised, and this characterization of DAT antibodies may provide a guide for immunodetection of DAT for molecular studies.

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.

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.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Dopamine transporter function fluctuates across sleep/wake state: potential impact for addiction

The dopamine transporter (DAT) has been implicated in a variety of arousal-related processes including the regulation of motor activity, learning, motivated behavior, psychostimulant abuse, and, more recently, sleep/wake state. We previously demonstrated that DAT uptake regulates fluctuations in extracellular dopamine (DA) in the striatum across the light/dark cycle with DA levels at their highest during the dark phase and lowest during the light phase. Despite this evidence, whether fluctuations in DA uptake across the light/dark cycle are associated with changes in sleep/wake has not been tested. To address this, we employed a combination of sleep/wake recordings, fast scan cyclic voltammetry, and western blotting to examine whether sleep/wake state and/or light/dark phase impact DA terminal neurotransmission in male rats. Further, we assessed whether variations in plasma membrane DAT levels and/or phosphorylation of the threonine 53 site on the DAT accounts for fluctuations in DA neurotransmission. Given the extensive evidence indicating that psychostimulants increase DA through interactions with the DAT, we also examined to what degree the effects of cocaine at inhibiting the DAT vary across sleep/wake state. Results demonstrated a significant association between individual sleep/wake states and DA terminal neurotransmission, with higher DA uptake rate, increased phosphorylation of the DAT, and enhanced cocaine potency observed after periods of sleep. These findings suggest that sleep/wake state influences DA neurotransmission in a manner that is likely to impact a host of DA-dependent processes including a variety of neuropsychiatric disorders.

Direct coupling of oligomerization and oligomerization-driven endocytosis of the dopamine transporter to its conformational mechanics and activity

Dopamine transporter (DAT) mediates the reuptake of synaptically released dopamine, and thus controls the duration and intensity of dopamine neurotransmission. Mammalian DAT has been observed to form oligomers, although the mechanisms of oligomerization and its role in DAT activity and trafficking remain largely unknown. We discovered a series of small molecule compounds that stabilize trimers and induce high-order oligomers of DAT and concomitantly promote its clathrin-independent endocytosis. Using a combination of chemical cross-linking, fluorescence resonance energy transfer microscopy, antibody-uptake endocytosis assay, live-cell lattice light sheet microscopy, ligand binding and substrate transport kinetics analyses, and molecular modeling and simulations, we investigated molecular basis of DAT oligomerization and endocytosis induced by these compounds. Our study showed that small molecule–induced DAT oligomerization and endocytosis are favored by the inward-facing DAT conformation and involve interactions of four hydrophobic residues at the interface between transmembrane (TM) helices TM4 and TM9. Surprisingly, a corresponding quadruple DAT mutant displays altered dopamine transport kinetics and increased cocaine-analog binding. The latter is shown to originate from an increased preference for outward-facing conformation and inward-to-outward transition. Taken together, our results demonstrate a direct coupling between conformational dynamics of DAT, functional activity of the transporter, and its oligomerization leading to endocytosis. The high specificity of such coupling for DAT makes the TM4-9 hub a new target for pharmacological modulation of DAT activity and subcellular localization.

Effect of palmitoylation on the dimer formation of the human dopamine transporter

The human dopamine transporter (hDAT) is one in three members of the monoamine transporter family (MAT). hDAT is essential for regulating the dopamine concentration in the synaptic cleft through dopamine reuptake into the presynaptic neuron; thereby controlling hDAT dopamine signaling. Dysfunction of the transporter is linked to several psychiatric disorders. hDAT and the other MATs have been shown to form oligomers in the plasma membrane, but only limited data exists on which dimeric and higher order oligomeric states are accessible and energetically favorable. In this work, we present several probable dimer conformations using computational coarse-grained self-assembly simulations and assess the relative stability of the different dimer conformations using umbrella sampling replica exchange molecular dynamics. Overall, the dimer conformations primarily involve TM9 and/or TM11 and/or TM12 at the interface. Furthermore, we show that a palmitoyl group (palm) attached to hDAT on TM12 modifies the free energy of separation for interfaces involving TM12, suggesting that S-palmitoylation may change the relative abundance of dimers involving TM12 in a biological context. Finally, a comparison of the identified interfaces of hDAT and palmitoylated hDAT to the human serotonin transporter interfaces and the leucine transporter interface, suggests similar dimer conformations across these protein family.

Leucine heptad motifs within transmembrane domains affect function and oligomerization of human organic anion transporting polypeptide 1B1

Organic anion transporting polypeptides (OATPs) are transmembrane proteins responsible for the uptake of a wide range of endogenous compounds and clinically important drugs. The liver-specific OATP1B1 serves crucial roles in the removal of many orally administered drugs. The proper function of the transporter hence is essential for the pharmacokinetics of various therapeutic agents. Membrane proteins tend to form oligomers that are important for their stability, targeting and/or interactions with the substrates. Previous study in our laboratory revealed that OATP1B1 may form homo-oligomers and that a GXXXG motif localized at transmembrane domain 8 (TM8) may affect its oligomerization. In the current study, three short-form leucine heptad repeats within the transmembrane domains of OATP1B1 were investigated. It was found that the disruption of leucine heptad repeats within TM3 dramatically reduced the uptake function and protein-protein association of OATP1B1; while within TM8, only L378 is essential for the function of OATP1B1 and alanine replacement of L378 exhibited no effect on the oligomerization. The fragmental expression of TM3 interfered with the association of OATP1B1 homo-oligomers as well as its association with OATP1B3, which is also selectively expressed at human hepatocytes, suggesting that the region may be shared by both transporters for their protein-protein interactions.

Optogenetically-induced multimerization of the dopamine transporter increases uptake and trafficking to the plasma membrane

The dopamine transporter (DAT) is essential for the reuptake of the released neurotransmitter dopamine (DA) in the brain. Psychostimulants, methamphetamine and cocaine, have been reported to induce the formation of DAT multimeric complexes in vivo and in vitro. The interpretation of DAT multimer function has been primarily in the context of compounds that induce structural and functional modifications of the DAT, complicating the understanding of the significance of DAT multimers. To examine multimerization in the absence of DAT ligands as well as in their presence, we developed a novel, optogenetic fusion chimera of cryptochrome 2 and DAT with an mCherry fluorescent reporter (Cry2-DAT). Using blue light to induce Cry2-DAT multimeric protein complex formation, we were able to simultaneously test the functional contributions of DAT multimerization in the absence or presence of substrates or inhibitors with high spatiotemporal precision. We found that blue light-stimulated Cry2-DAT multimers significantly increased IDT307 uptake and MFZ 9-18 binding in the absence of ligands as well as after methamphetamine and nomifensine treatment. Blue light-induced Cry2-DAT multimerization increased colocalization with recycling endosomal marker Rab11 and had decreased presence in Rab5-positive early endosomes and Rab7-positive late endosomes. Our data suggest that the increased uptake and binding results from induced and rapid trafficking of DAT multimers to the plasma membrane. Our data suggest that DAT multimers may function to help maintain DA homeostasis.

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.

The Amino Terminus of LeuT Changes Conformation in an Environment Sensitive Manner

Neurotransmitter:sodium symporters are highly expressed in the human brain and catalyze the uptake of substrate through the plasma membrane by using the electrochemical gradient of sodium as the energy source. The bacterial homolog LeuT, a small amino acid transporter isolated from the bacteria Aquifex aeolicus, is the founding member of the family and has been crystallized in three conformations. The N-terminus is structurally well defined and strongly interacts with the transporter core in the outward-facing conformations. However, it could not be resolved in the inward-facing conformation, which indicates enhanced mobility. Here we investigate conformations and dynamics of the N-terminus, by combining molecular dynamics simulations with experimental verification using distance measurements and accessibility studies. We found strongly increased dynamics of the N-terminus, but also that helix TM1A is subject to enhanced mobility. TM1A moves towards the transporter core in the membrane environment, reaching a conformation that is closer to the structure of LeuT with wild type sequence, indicating that the mutation introduced to create the inward-facing structure might have altered the position of helix TM1A. The mobile N-terminus avoids entering the open vestibule of the inward-facing state, as accessibility studies do not show any reduction of quenching by iodide of a fluorophore attached to the N-terminus.

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

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

Intrastriatal alpha-synuclein fibrils in monkeys: spreading, imaging and neuropathological changes

Several studies have demonstrated that intrastriatal injections of fibrillar α-synuclein in rodent brain induced a Parkinson's disease-like propagation of Lewy body pathology with significant nigrostriatal neurodegeneration. This study evaluated the pathological features when exogenous α-synuclein preformed fibrils were injected into the putamen of non-human primates. Eight cynomolgus monkeys received unilateral intraputamen injections of α-synuclein preformed fibrils and four monkeys received sham surgery. Monkeys were assessed with 123I-PE2I single-photon emission computerized tomography scans targeting the dopamine transprter at baseline, 3, 6, 9, 12, and 15 months. Imaging revealed a robust increase in dopamine transporter binding, an effect confirmed by port-mortem immunohistochemical analyses, suggesting that upregulation of dopamine transporter occurs as part of an early pathological process. Histochemistry and immunohistochemistry revealed that α-synuclein preformed fibrils injections into the putamen induced intraneuronal inclusions positive for phosphorylated α-synuclein in ipsilateral substantia nigra and adjacent to the injection site. α-Synuclein inclusions were thioflavin-S-positive suggesting that the inclusions induced by α-synuclein preformed fibrils exhibited pathological properties similar to amyloid-like Lewy body pathology in Parkinson's disease brains. The α-synuclein preformed fibrils resulted in Lewy pathology in the ipsilateral substantia nigra with significant reduction (-29.30%) of dopaminergic neurons as compared with controls. Nigral neurons with α-synuclein inclusions exhibited a phenotypic downregulation of the dopamine markers tyrosine hydroxylase and Nurr1. Taken together, our findings demonstrate that α-synuclein preformed fibrils induce a synucleinopathy in non-human primates with authentic Lewy pathology and nigrostriatal changes indicative of early Parkinson's disease.

Trimerization of dopamine transporter triggered by AIM-100 binding: Molecular mechanism and effect of mutations

Recent work demonstrated the propensity of dopamine transporters (DATs) to form trimers or higher oligomers, enhanced upon binding a furopyrimidine, AIM-100. AIM-100 binding promotes DAT endocytosis and thereby moderates dopaminergic transmission. Despite the neurobiological significance of these events, the molecular mechanisms that underlie the stabilization of DAT trimer and the key interactions that modulate the trimerization of DAT, and not serotonin transporter SERT, remain unclear. In the present study, we determined three structural models, termed trimer-W238, -C306 and -Y303, for possible trimerization of DATs . To this aim, we used structural data resolved for DAT and its structural homologs that share the LeuT fold, advanced computational modeling and simulations, site-directed mutagenesis experiments and live-cell imaging assays. The models are in accord with the versatility of LeuT fold to stabilize dimeric or higher order constructs. Selected residues show a high propensity to occupy interfacial regions. Among them, D231-W238 in the extracellular loop EL2, including the intersubunit salt-bridge forming pair D231/D232-R237 (not present in SERT) (in trimer-W238), the loop EL3 (trimers-C306 and -Y303), and W497 on the intracellularly exposed IL5 loop (trimer-C306) and its spatial neighbors (e.g. K525) near the C-terminus are computationally predicted and experimentally confirmed to play important roles in enabling the correct folding and/or oligomerization of DATs in the presence of AIM-100. The study suggests the possibility of controlling the effective transport of dopamine by altering the oligomerization state of DAT upon small molecule binding, as a possible intervention strategy to modulate dopaminergic signaling. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Palmitoylation by Multiple DHHC Enzymes Enhances Dopamine Transporter Function and Stability

The dopamine transporter (DAT) is a plasma membrane protein that mediates the reuptake of extracellular dopamine (DA) and controls the spatiotemporal dynamics of dopaminergic neurotransmission. The transporter is subject to fine control that tailors clearance of transmitter to physiological demands, and dysregulation of reuptake induced by psychostimulant drugs, transporter polymorphisms, and signaling defects may impact transmitter tone in disease states. We previously demonstrated that DAT undergoes complex regulation by palmitoylation, with acute inhibition of the modification leading to rapid reduction of transport activity and sustained inhibition of the modification leading to transporter degradation and reduced expression. Here, to examine mechanisms and outcomes related to increased modification, we coexpressed DAT with palmitoyl acyltransferases (PATs), also known as DHHC enzymes, which catalyze palmitate addition to proteins. Of 12 PATs tested, DAT palmitoylation was stimulated by DHHC2, DHHC3, DHHC8, DHHC15, and DHHC17, with others having no effect. Increased modification was localized to previously identified palmitoylation site Cys580 and resulted in upregulation of transport kinetics and elevated transporter expression mediated by reduced degradation. These findings confirm palmitoylation as a regulator of multiple DAT properties crucial for appropriate DA homeostasis and identify several potential PAT pathways linked to these effects. Defects in palmitoylation processes thus represent possible mechanisms of transport imbalances in DA disorders.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

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

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

The SUMO-Conjugase Ubc9 Prevents the Degradation of the Dopamine Transporter, Enhancing Its Cell Surface Level and Dopamine Uptake

The dopamine transporter (DAT) is a plasma membrane protein responsible for the uptake of released dopamine back to the presynaptic terminal and ending dopamine neurotransmission. The DAT is the molecular target for cocaine and amphetamine as well as a number of pathological conditions including autism spectrum disorders, attention-deficit hyperactivity disorder (ADHD), dopamine transporter deficiency syndrome (DTDS), and Parkinson’s disease. The DAT uptake capacity is dependent on its level in the plasma membrane. In vitro studies show that DAT functional expression is regulated by a balance of endocytosis, recycling, and lysosomal degradation. However, recent reports suggest that DAT regulation by endocytosis in neurons is less significant than previously reported. Therefore, additional mechanisms appear to determine DAT steady-state level and functional expression in the neuronal plasma membrane. Here, we hypothesize that the ubiquitin-like protein small ubiquitin-like modifier 1 (SUMO1) increases the DAT steady-state level in the plasma membrane. In confocal microscopy, fluorescent resonance energy transfer (FRET), and Western blot analyses, we demonstrate that DAT is associated with SUMO1 in the rat dopaminergic N27 and DAT overexpressing Human Embryonic Kidney cells (HEK)-293 cells. The overexpression of SUMO1 and the Ubc9 SUMO-conjugase induces DAT SUMOylation, reduces DAT ubiquitination and degradation, enhancing DAT steady-state level. In addition, the Ubc9 knock-down by interference RNA (RNAi) increases DAT degradation and reduces DAT steady-state level. Remarkably, the Ubc9-mediated SUMOylation increases the expression of DAT in the plasma membrane and dopamine uptake capacity. Our results strongly suggest that SUMOylation is a novel mechanism that plays a central role in regulating DAT proteostasis, dopamine uptake, and dopamine signaling in neurons. For that reason, the SUMO pathway including SUMO1, SUMO2, Ubc9, and DAT SUMOylation, can be critical therapeutic targets in regulating DAT stability and dopamine clearance in health and pathological states.

Dopamine transporter forms stable dimers in the live cell plasma membrane in a phosphatidylinositol 4,5-bisphosphate-independent manner

The human dopamine transporter (hDAT) regulates the level of the neurotransmitter dopamine (DA) in the synaptic cleft and recycles DA for storage in the presynaptic vesicular pool. Many neurotransmitter transporters exist as oligomers, but the physiological role of oligomerization remains unclear; for example, it has been speculated to be a prerequisite for amphetamine-induced release and protein trafficking. Previous studies point to an oligomeric quaternary structure of hDAT; however, the exact stoichiometry and the fraction of co-existing oligomeric states are not known. Here, we used single-molecule brightness analysis to quantify the degree of oligomerization of heterologously expressed hDAT fused to monomeric GFP (mGFP–hDAT) in Chinese hamster ovary (CHO) cells. We observed that monomers and dimers of mGFP–hDAT co-exist and that higher-order molecular complexes of mGFP–hDAT are absent at the plasma membrane. The mGFP–hDAT dimers were stable over several minutes, and the fraction of dimers was independent of the mGFP–hDAT surface density. Furthermore, neither oxidation nor depletion of cholesterol had any effect on the fraction of dimers. Unlike for the human serotonin transporter (hSERT), in which direct binding of phosphatidylinositol 4,5-bisphosphate (PIP₂) stabilized the oligomers, the stability of mGFP–hDAT dimers was PIP₂ independent.

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.

Mode-Selective Raman Imaging of Dopamine-Human Dopamine Transporter Interaction in Live Cells

Dopamine (DA) is the catecholamine neurotransmitter which interacts with dopamine receptors (DARs) to generate dopaminergic signals in the nervous system. Dopamine transporter (DAT) interacts with DA to maintain DA's homeostasis in synaptic and perisynaptic space. DAT and DARs have great importance in the central nervous system (CNS) because they are associated with the targeted binding of drugs. Interactions of DA, its analogue with DARs, or DAT have been studied extensively to understand the mechanism of the dopaminergic signaling process and several neurodegenerative diseases, including schizophrenia, Parkinson's diseases, addiction, attention deficit hyperactivity disorder, and bipolar disorder. However, there is still a lack of a risk-free, label-free, and minimally invasive imaging approach to probe the interaction between DA and DAT or DARs. Here, we probed the DA, human dopamine transporter (hDAT), and DA-hDAT interactions in live cells using combined approach of two-photon excited (2PE) fluorescence imaging and mode-selective Raman measurement. We utilized the signature Raman peak at 1287 cm^-1 to probe the location of DA and 807 and 1076 cm^-1 to probe the DA-hDAT interaction in live cells. We found that the combined approach of mode-selective Raman imaging, 2PE fluorescence imaging, and computational methods is successful to probe and confirm the DA-hDAT interactions in living cells. The probing of the interactions of DARs or DAT with DA or other targeting drugs is crucial for the diagnosis and cure of several neurodegenerative diseases. Also, this analytical approach could be extended to probe other types of protein-ligand interactions.

Regulation of monoamine transporters and receptors by lipid microdomains: implications for depression

Lipid microdomains ("rafts") are dynamic, nanoscale regions of the plasma membrane enriched in cholesterol and glycosphingolipids, that possess distinctive physicochemical properties including higher order than the surrounding membrane. Lipid microdomain integrity is thought to affect neurotransmitter signaling by regulating membrane-bound protein signaling. Among the proteins potentially affected are monoaminergic receptors and transporters. As dysfunction of monoaminergic neurotransmission is implicated in major depressive disorder and other neuropsychiatric conditions, interactions with lipid microdomains may be of clinical importance. This systematic review evaluates what is known about the molecular relationships of monoamine transporter and receptor regulation to lipid microdomains. The PubMed/MeSH database was searched for original studies published in English through August 2017 concerning relationships between lipid microdomains and serotonin, dopamine and norepinephrine transporters and receptors. Fifty-seven publications were identified and assessed. Strong evidence implicates lipid microdomains in the regulation of serotonin and norepinephrine transporters; serotonin 1A, 2A, 3A, and 7A receptors; and dopamine D1 and β2 adrenergic receptors. Results were conflicting or more complex regarding lipid microdomain associations with the dopamine transporter, D2, D3, and D5 receptors; and negative with respect to β1 adrenergic receptors. Indirect evidence suggests that antidepressants, lipid-lowering drugs, and polyunsaturated fatty acids may exert effects on depression and suicide by altering the lipid milieu, thereby affecting monoaminergic transporter and receptor signaling. The lipid composition of membrane subdomains is involved in localization and trafficking of specific monoaminergic receptors and transporters. Elucidating precise mechanisms whereby lipid microdomains modulate monoamine neurotransmission in clinical contexts can have critical implications for pharmacotherapeutic targeting.

Thermostabilization and purification of the human dopamine transporter (hDAT) in an inhibitor and allosteric ligand bound conformation

The human dopamine transporter (hDAT) plays a major role in dopamine homeostasis and regulation of neurotransmission by clearing dopamine from the extracellular space using secondary active transport. Dopamine is an essential monoamine chemical messenger that regulates reward seeking behavior, motor control, hormonal release, and emotional response in humans. Psychostimulants such as cocaine primarily target the central binding site of hDAT and lock the transporter in an outward-facing conformation, thereby inhibiting dopamine reuptake. The inhibition of dopamine reuptake leads to accumulation of dopamine in the synapse causing heightened signaling. In addition, hDAT is implicated in various neurological disorders and disease-associated neurodegeneration. Despite its significance, the structural studies of hDAT have proven difficult. Instability of hDAT in detergent micelles has been a limiting factor in its successful biochemical, biophysical, and structural characterization. To overcome this hurdle, we identified ligands that stabilize hDAT in detergent micelles. We then screened ~200 single residue mutants of hDAT using a high-throughput scintillation proximity assay and identified a thermostable variant (I248Y). Here we report a robust strategy to overexpress and successfully purify a thermostable variant of hDAT in an inhibitor and allosteric ligand bound conformation.

Linking Glycation and Glycosylation With Inflammation and Mitochondrial Dysfunction in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder, affecting about 6.3 million people worldwide. PD is characterized by the progressive degeneration of dopaminergic neurons in the Substantia nigra pars compacta, resulting into severe motor symptoms. The cellular mechanisms underlying dopaminergic cell death in PD are still not fully understood, but mitochondrial dysfunction, oxidative stress and inflammation are strongly implicated in the pathogenesis of both familial and sporadic PD cases. Aberrant post-translational modifications, namely glycation and glycosylation, together with age-dependent insufficient endogenous scavengers and quality control systems, lead to cellular overload of dysfunctional proteins. Such injuries accumulate with time and may lead to mitochondrial dysfunction and exacerbated inflammatory responses, culminating in neuronal cell death. Here, we will discuss how PD-linked protein mutations, aging, impaired quality control mechanisms and sugar metabolism lead to up-regulated abnormal post-translational modifications in proteins. Abnormal glycation and glycosylation seem to be more common than previously thought in PD and may underlie mitochondria-induced oxidative stress and inflammation in a feed-forward mechanism. Moreover, the stress-induced post-translational modifications that directly affect parkin and/or its substrates, deeply impairing its ability to regulate mitochondrial dynamics or to suppress inflammation will also be discussed. Together, these represent still unexplored deleterious mechanisms implicated in neurodegeneration in PD, which may be used for a more in-depth knowledge of the pathogenic mechanisms, or as biomarkers of the disease.

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

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

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

Ascorbic acid increases SVCT2 localization at the plasma membrane by accelerating its trafficking from early secretory compartments and through the endocytic-recycling pathway

Ascorbic acid (Asc) is an antioxidant molecule essential for physiological functions. The concentration of extracellular Asc increases during synaptic transmission and renal reabsorption. These phenomena induce an increase of the Sodium-dependent-Vitamin-C-transporter 2 (SVCT2) at plasma membrane (PM) localization, as we previously demonstrated in neuronal and non-neuronal cells. Hence, the aim of this study was to evaluate intracellular SVCT2 trafficking kinetics in response to Asc. We observed two peaks of SVCT2 localization and function at the PM (at 5-10 min, "acute response", and 30-60 min, "post-acute response") when cells were incubated with Asc. We defined that the post-acute response was dependent on SVCT2 located in early secretory compartments, and its trafficking was abolished with Tunicamycin and Brefeldin A treatment. Moreover, using the RUSH system to retain and synchronize cargo secretion through the secretory pathway we demonstrated that the post-acute response increases SVCT2 trafficking kinetics from the ER to the PM suggesting the retention of SVCT2 at the early secretory pathway when Asc is absent. However, these observations do not explain the increased SVCT2 levels at the PM during the "acute" response, suggesting the involvement of a faster mechanism in close proximity with the PM. To investigate the possible role of endosomal compartments, we tested the effect of endocytosis inhibition. Expression of dominant-negative (DN) versions of the GTPase-dynamin II and clathrin-accessory protein AP180 showed a significant increase in SVCT2 levels at the PM. Moreover, expression of Rab11-DN, a GTPase implicated in cargo protein recycling from endosomes to the PM showed a similar outcome, strongly indicating that Asc impacts SVCT2 trafficking during the acute response. Therefore, our results revealed two mechanisms by which Asc modulates SVCT2 levels at the PM, one at the early secretory pathway and another at the endocytic compartments. We propose that these two mechanisms have key protective implications in the homeostasis of metabolically active and specialized tissues.

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

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

Structural and Functional Characterization of the Interaction of Snapin with the Dopamine Transporter: Differential Modulation of Psychostimulant Actions

The importance of dopamine (DA) neurotransmission is emphasized by its direct implication in several neurological and psychiatric disorders. The DA transporter (DAT), target of psychostimulant drugs, is the key protein that regulates spatial and temporal activity of DA in the synaptic cleft via the rapid reuptake of DA into the presynaptic terminal. There is strong evidence suggesting that DAT-interacting proteins may have a role in its function and regulation. Performing a two-hybrid screening, we identified snapin, a SNARE-associated protein implicated in synaptic transmission, as a new binding partner of the carboxyl terminal of DAT. Our data show that snapin is a direct partner and regulator of DAT. First, we determined the domains required for this interaction in both proteins and characterized the DAT-snapin interface by generating a 3D model. Using different approaches, we demonstrated that (i) snapin is expressed in vivo in dopaminergic neurons along with DAT; (ii) both proteins colocalize in cultured cells and brain and, (iii) DAT and snapin are present in the same protein complex. Moreover, by functional studies we showed that snapin produces a significant decrease in DAT uptake activity. Finally, snapin downregulation in mice produces an increase in DAT levels and transport activity, hence increasing DA concentration and locomotor response to amphetamine. In conclusion, snapin/DAT interaction represents a direct link between exocytotic and reuptake mechanisms and is a potential target for DA transmission modulation.

Dimer Interface of the Human Serotonin Transporter and Effect of the Membrane Composition

The oligomeric state of membrane proteins has recently emerged in many cases as having an effect on their function. However, the intrinsic dynamics of their spatial organization in cells and model systems makes it challenging to characterize. Here we use molecular dynamics (MD) simulations at multiple resolutions to determine the dimer conformation of the human serotonin transporter (hSERT). From self-assembly simulations we predict dimer candidates and subsequently quantify their relative strength. We use umbrella sampling (US) replica exchange MD simulations for which we present extensive analysis of their efficiency and improved sampling compared to regular US MD simulations. The data shows that the most stable hSERT dimer interface is symmetrical and involves transmembrane helix 12 (TM12), similar to the crystal structure of the bacterial homologue LeuT, but with a slightly different orientation. We also describe the supramolecular organization of hSERT from a 250 μs self-assembly simulation. Finally, the effects of the presence of phosphatidylinositol bisphosphate or cholesterol in the membrane model has been quantified for the TM12-TM12 predicted interface. Collectively, the presented data bring new insight to the area of protein and lipid interplay in biological membranes.

Functional properties of dopamine transporter oligomers after copper linking

Although it is universally accepted that dopamine transporters (DATs) exist in monomers, dimers and tetramers (i.e. dimers of dimers), it is not known whether the oligomeric organization of DAT is a prerequisite for its ability to take up dopamine (DA), or whether each DAT protomer, the subunit of quaternary structure, functions independently in terms of DA translocation. In this study, copper phenanthroline (CuP) was used to selectively target surface DAT: increasing concentrations of CuP gradually cross-linked natural DAT dimers in LLC-PK₁ cells stably expressing hDAT and thereby reduced DA uptake functionality until all surface DATs were inactivated. DATs that were not cross-linked by CuP showed normal DA uptake with DA K_m at ~ 0.5 μM and DA efflux with basal and amphetamine-induced DA efflux as much as control values. The cocaine analog 2β-carbomethoxy-3β-[4-fluorophenyl]-tropane (CFT) was capable to bind to copper-cross-linked DATs, albeit with an affinity more than fivefold decreased (K_d of CFT = 109 nM after cross-linking vs 19 nM before). A kinetic analysis is offered describing the changing amounts of dimers and monomers with increasing [CuP], allowing the estimation of dimer functional activity compared with a DAT monomer. Consonant with previous conclusions for serotonin transporter and NET that only one protomer of an oligomer is active at the time, the present data indicated a functional activity of the DAT dimer of 0.74 relative to a monomer.

Human bile acid transporter ASBT (SLC10A2) forms functional non-covalent homodimers and higher order oligomers

The human apical sodium-dependent bile acid transporter, hASBT/SLC10A2, plays a central role in cholesterol homeostasis via the efficient reabsorption of bile acids from the distal ileum. hASBT has been shown to self-associate in higher order complexes, but while the functional role of endogenous cysteines has been reported, their implication in the oligomerization of hASBT remains unresolved. Here, we determined the self-association architecture of hASBT by site-directed mutagenesis combined with biochemical, immunological and functional approaches. We generated a cysteine-less form of hASBT by creating point mutations at all 13 endogenous cysteines in a stepwise manner. Although Cysless hASBT had significantly reduced function correlated with lowered surface expression, it featured an extra glycosylation site that facilitated its differentiation from wt-hASBT on immunoblots. Decreased protein expression was associated with instability and subsequent proteasome-dependent degradation of Cysless hASBT protein. Chemical cross-linking of wild-type and Cysless species revealed that hASBT exists as an active dimer and/or higher order oligomer with apparently no requirement for endogenous cysteine residues. This was further corroborated by co-immunoprecipitation of differentially tagged (HA-, Flag-) wild-type and Cysless hASBT. Finally, Cysless hASBT exhibited a dominant-negative effect when co-expressed with wild-type hASBT which validated heterodimerization/oligomerization at the functional level. Combined, our data conclusively demonstrate the functional existence of hASBT dimers and higher order oligomers irrespective of cysteine-mediated covalent bonds, thereby providing greater understanding of its topological assembly at the membrane surface.

Amphetamine Reverses Escalated Cocaine Intake via Restoration of Dopamine Transporter Conformation

Cocaine abuse disrupts dopamine system function, and reduces cocaine inhibition of the dopamine transporter (DAT), which results in tolerance. Although tolerance is a hallmark of cocaine addiction and a DSM-V criterion for substance abuse disorders, the molecular adaptations producing tolerance are unknown, and testing the impact of DAT changes on drug taking behaviors has proven difficult. In regard to treatment, amphetamine has shown efficacy in reducing cocaine intake; however, the mechanisms underlying these effects have not been explored. The goals of this study were twofold; we sought to (1) identify the molecular mechanisms by which cocaine exposure produces tolerance and (2) determine whether amphetamine-induced reductions in cocaine intake are connected to these mechanisms. Using cocaine self-administration and fast-scan cyclic voltammetry in male rats, we show that low-dose, continuous amphetamine treatment, during self-administration or abstinence, completely reversed cocaine tolerance. Amphetamine treatment also reversed escalated cocaine intake and decreased motivation to obtain cocaine as measured in a behavioral economics task, thereby linking tolerance to multiple facets of cocaine use. Finally, using fluorescence resonance energy transfer imaging, we found that cocaine tolerance is associated with the formation of DAT-DAT complexes, and that amphetamine disperses these complexes. In addition to extending our basic understanding of DATs and their role in cocaine reinforcement, we serendipitously identified a novel therapeutic target: DAT oligomer complexes. We show that dispersion of oligomers is concomitant with reduced cocaine intake, and propose that pharmacotherapeutics aimed at these complexes may have potential for cocaine addiction treatment.SIGNIFICANCE STATEMENT Tolerance to cocaine's subjective effects is a cardinal symptom of cocaine addiction and a DSM-V criterion for substance abuse disorders. However, elucidating the molecular adaptions that produce tolerance and determining its behavioral impact have proven difficult. Using cocaine self-administration in rats, we link tolerance to cocaine effects at the dopamine transporter (DAT) with aberrant cocaine-taking behaviors. Further, tolerance was associated with multi-DAT complexes, which formed after cocaine exposure. Treatment with amphetamine deconstructed DAT complexes, reversed tolerance, and decreased cocaine seeking. These data describe the behavioral consequence of cocaine tolerance, provide a putative mechanism for its development, and suggest that compounds that disperse DAT complexes may be efficacious treatments for cocaine addiction.

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.

Chromogranins: from discovery to current times

The discovery in 1953 of the chromaffin granules as co-storage of catecholamines and ATP was soon followed by identification of a range of uniquely acidic proteins making up the isotonic vesicular storage complex within elements of the diffuse sympathoadrenal system. In the mid-1960s, the enzymatically inactive, major core protein, chromogranin A was shown to be exocytotically discharged from the stimulated adrenal gland in parallel with the co-stored catecholamines and ATP. A prohormone concept was introduced when one of the main storage proteins collectively named granins was identified as the insulin release inhibitory polypeptide pancreastatin. A wide range of granin-derived biologically active peptides have subsequently been identified. Both chromogranin A and chromogranin B give rise to antimicrobial peptides of relevance for combat of pathogens. While two of the chromogranin A-derived peptides, vasostatin-I and pancreastatin, are involved in modulation of calcium and glucose homeostasis, respectively, vasostatin-I and catestatin are important modulators of endothelial permeability, angiogenesis, myocardial contractility, and innate immunity. A physiological role is now evident for the full-length chromogranin A and vasostatin-I as circulating stabilizers of endothelial integrity and in protection against myocardial injury. The high circulating levels of chromogranin A and its fragments in patients suffering from various inflammatory diseases have emerged as challenges for future research and clinical applications.

Transporter oligomerization: form and function

Transporters are integral membrane proteins with central roles in the efficient movement of molecules across biological membranes. Many transporters exist as oligomers in the membrane. Depending on the individual transport protein, oligomerization can have roles in membrane trafficking, function, regulation and turnover. For example, our recent studies on UapA, a nucleobase ascorbate transporter, from Aspergillus nidulans, have revealed both that dimerization of this protein is essential for correct trafficking to the membrane and the structural basis of how one UapA protomer can affect the function of the closely associated adjacent protomer. Here, we review the roles of oligomerization in many particularly well-studied transporters and transporter families.

Identifying novel members of the Wntless interactome through genetic and candidate gene approaches

Wnt signaling is an important pathway that regulates several aspects of embryogenesis, stem cell maintenance, and neural connectivity. We have recently determined that opioids decrease Wnt secretion, presumably by inhibiting the recycling of the Wnt trafficking protein Wntless (Wls). This effect appears to be mediated by protein-protein interaction between Wls and the mu-opioid receptor (MOR), the primary cellular target of opioid drugs. The goal of this study was to identify novel protein interactors of Wls that are expressed in the brain and may also play a role in reward or addiction. Using genetic and candidate gene approaches, we show that among a variety of protein, Wls interacts with the dopamine transporter (target of cocaine), cannabinoid receptors (target of THC), Adenosine A2A receptor (target of caffeine), and SGIP1 (endocytic regulator of cannabinoid receptors). Our study shows that aside from opioid receptors, Wntless interacts with additional proteins involved in reward and/or addiction. Future studies will determine whether Wntless and WNT signaling play a more universal role in these processes.

Targeting of dopamine transporter to filopodia requires an outward-facing conformation of the transporter

Dopamine transporter (DAT) has been shown to accumulate in filopodia in neurons and non-neuronal cells. To examine the mechanisms of DAT filopodial targeting, we used quantitative live-cell fluorescence microscopy, and compared the effects of the DAT inhibitor cocaine and its fluorescent analog JHC1-64 on the plasma membrane distribution of wild-type DAT and two non-functional DAT mutants, R60A and W63A, that do not accumulate in filopodia. W63A did not bind JHC1-64, whereas R60A did, although less efficiently compared to the wild-type DAT. Molecular dynamics simulations predicted that R60A preferentially assumes an outward-facing (OF) conformation through compensatory intracellular salt bridge formation, which in turn favors binding of cocaine. Imaging analysis showed that JHC1-64-bound R60A mutant predominantly localized in filopodia, whereas free R60A molecules were evenly distributed within the plasma membrane. Cocaine binding significantly increased the density of R60A, but not that of W63A, in filopodia. Further, zinc binding, known to stabilize the OF state, also increased R60A concentration in filopodia. Finally, amphetamine, that is thought to disrupt DAT OF conformation, reduced the concentration of wild-type DAT in filopodia. Altogether, these data indicate that OF conformation is required for the efficient targeting of DAT to, and accumulation in, filopodia.

Allosteric modulation of human dopamine transporter activity under conditions promoting its dimerization

The human dopamine (DA) transporter (hDAT) is a key regulator of neurotransmission and a target for antidepressants and addictive drugs. Despite the recent resolution of dDAT structures from Drosophila melanogaster, complete understanding of its mechanism of function and even information on its biological assembly is lacking. The resolved dDAT structures are monomeric, but growing evidence suggests that hDAT might function as a multimer, and its oligomerization may be relevant to addictive drug effects. Here, using structure-based computations, we examined the possible mechanisms of hDAT dimerization and its dynamics in a lipid bilayer. Using a combination of site-directed mutagenesis, DA-uptake, and cross-linking experiments that exploited the capacity of Cys-306 to form intermonomeric disulfide bridges in the presence of an oxidizing agent, we tested the effects of mutations at transmembrane segment (TM) 6 and 12 helices in HEK293 cells. The most probable structural model for hDAT dimer suggested by computations and experiments differed from the dimeric structure resolved for the bacterial homolog, LeuT, presumably because of a kink at TM12 preventing favorable monomer packing. Instead, TM2, TM6, and TM11 line the dimer interface. Molecular dynamics simulations of the dimeric hDAT indicated that the two subunits tend to undergo cooperative structural changes, both on local (extracellular gate opening/closure) and global (transition between outward-facing and inward-facing states) scales. These observations suggest that hDAT transport properties may be allosterically modulated under conditions promoting dimerization. Our study provides critical insights into approaches for examining the oligomerization of neurotransmitter transporters and sheds light on their drug modulation.

Phospho-specific antibodies targeting the amino terminus of the human dopamine transporter

The dopamine transporter (DAT), which mediates the inactivation of released dopamine through its reuptake, is the primary molecular target for the actions of psychostimulants. An increasing number of studies support an essential role for phosphorylation of serines (Ser) in the distal amino (N) terminus of DAT in regulating its function. Still, the molecular details of the regulation of phosphorylation and its impact on function are not fully understood. To address this, we have developed and characterized two distinct phospho-antibodies that recognize human DAT when it is phosphorylated at Ser7 or Ser12. Our data show that treatment of cells with phorbol 12-myristate 13-acetate (PMA), amphetamine (AMPH) or okadaic acid (OA) leads to an increase in the phosphorylation of DAT at both residues and that these responses are dependent on the activity of protein kinase C. We also show that AMPH-induced and OA-induced phosphorylation of DAT are dependent on Ca²⁺/calmodulin-dependent protein kinase α. Our data further suggest that the lipid raft localization of DAT is necessary for efficient N-terminal phosphorylation and for the associated behavioral effects of AMPH, demonstrating the potential of these novel antibodies as powerful tools to study DAT regulation and function in vivo.

The sigma-1 receptor modulates dopamine transporter conformation and cocaine binding and may thereby potentiate cocaine self-administration in rats

The dopamine transporter (DAT) regulates dopamine (DA) neurotransmission by recapturing DA into the presynaptic terminals and is a principal target of the psychostimulant cocaine. The sigma-1 receptor (σ₁R) is a molecular chaperone, and its ligands have been shown to modulate DA neuronal signaling, although their effects on DAT activity are unclear. Here, we report that the prototypical σ₁R agonist (+)-pentazocine potentiated the dose response of cocaine self-administration in rats, consistent with the effects of the σR agonists PRE-084 and DTG (1,3-di-o-tolylguanidine) reported previously. These behavioral effects appeared to be correlated with functional changes of DAT. Preincubation with (+)-pentazocine or PRE-084 increased the B_max values of [³H]WIN35428 binding to DAT in rat striatal synaptosomes and transfected cells. A specific interaction between σ₁R and DAT was detected by co-immunoprecipitation and bioluminescence resonance energy transfer assays. Mutational analyses indicated that the transmembrane domain of σ₁R likely mediated this interaction. Furthermore, cysteine accessibility assays showed that σ₁R agonist preincubation potentiated cocaine-induced changes in DAT conformation, which were blocked by the specific σ₁R antagonist CM304. Moreover, σ₁R ligands had distinct effects on σ₁R multimerization. CM304 increased the proportion of multimeric σ₁Rs, whereas (+)-pentazocine increased monomeric σ₁Rs. Together these results support the hypothesis that σ₁R agonists promote dissociation of σ₁R multimers into monomers, which then interact with DAT to stabilize an outward-facing DAT conformation and enhance cocaine binding. We propose that this novel molecular mechanism underlies the behavioral potentiation of cocaine self-administration by σ₁R agonists in animal models.