Differential Internalization Rates and Postendocytic Sorting of the Norepinephrine and Dopamine Transporters Are Controlled by Structural Elements in the N Termini

Activity-dependent constraints on catecholamine signaling

Catecholamine signaling is thought to modulate cognition in an inverted-U relationship, but the mechanisms are unclear. We measured norepinephrine and dopamine release, postsynaptic calcium responses, and interactions between tonic and phasic firing modes under various stimuli and conditions. High tonic activity in vivo depleted catecholamine stores, desensitized postsynaptic responses, and decreased phasic transmission. Together, these findings provide a more complete understanding of the inverted-U relationship, offering insights into psychiatric disorders and neurodegenerative diseases with impaired catecholamine signaling.

Illuminating the monoamine transporters: Fluorescently labelled ligands to study dopamine, serotonin and norepinephrine transporters

Fluorescence microscopy has revolutionized the visualization of physiological processes in live-cell systems. With the recent innovations in super resolution microscopy, these events can be examined with high precision and accuracy. The development of fluorescently labelled small molecules has provided a significant advance in understanding the physiological relevance of targeted proteins that can now be visualized at the cellular level. One set of physiologically important target proteins are the monoamine transporters (MATs) that play an instrumental role in maintaining monoamine signalling homeostasis. Understanding the mechanisms underlying their regulation and dysregulation is fundamental to treating several neuropsychiatric conditions such as attention deficit hyperactivity disorder (ADHD), anxiety, depression and substance use disorders. Herein, we describe the rationale behind the small molecule design of fluorescently labelled ligands (FLL) either as MAT substrates or inhibitors as well as their applications to advance our understanding of this class of transporters in health and disease.

Activity-dependent constraints on catecholamine signaling

Catecholamine signaling is thought to modulate cognition in an inverted-U relationship, but the mechanisms are unclear. We measured norepinephrine and dopamine release, postsynaptic calcium responses, and interactions between tonic and phasic firing modes under various stimuli and conditions. High tonic activity in vivo depleted catecholamine stores, desensitized postsynaptic responses, and decreased phasic transmission. Together this provides a clearer understanding of the inverted-U relationship, offering insights into psychiatric disorders and neurodegenerative diseases with impaired catecholamine signaling.

A density-based enrichment measure for assessing colocalization in single-molecule localization microscopy data

Dual-color single-molecule localization microscopy (SMLM) provides unprecedented possibilities for detailed studies of colocalization of different molecular species in a cell. However, the informational richness of the data is not fully exploited by current analysis tools that often reduce colocalization to a single value. Here, we describe a tool specifically designed for determination of co-localization in both 2D and 3D from SMLM data. The approach uses a function that describes the relative enrichment of one molecular species on the density distribution of a reference species. The function reframes the question of colocalization by providing a density-context relevant to multiple biological questions. Moreover, the function visualize enrichment (i.e. colocalization) directly in the images for easy interpretation. We demonstrate the approach’s functionality on both simulated data and cultured neurons, and compare it to current alternative measures. The method is available in a Python function for easy and parameter-free implementation.

Full information gained from single-molecule localisation microscopy (SMLM) isn't exploited by current analysis tools. Here the authors report relative enrichment which uses a density-based colocalisation measure for both 2D and 3D SMLM data; they apply it to both simulated data and cultured neurons.

Targeted Mass Spectrometry-Based Approach for the Determination of Intrinsic Internalization Kinetics of Cell-Surface Membrane Protein Targets

Successful development of targeted therapeutics aimed at the elimination of diseased cells relies on the target properties and the therapeutics that target them. Currently, target properties have been evaluated through antibody-dependent semiquantitative approaches such as flow cytometry, Western blotting, or microscopy. Since antibodies can alter target properties following binding, antibody-dependent approaches provide at best skewed measurements for target intrinsic properties. To circumvent, here we attempted to develop an antibody-free targeted mass spectrometry-based (ATM) strategy to measure the surface densities and the intrinsic rates (K_int) of CD38 internalization in multiple myeloma cell lines. Using cell-surface biotinylation in conjunction with differential mass tagging to separate inward CD38 molecules from the outbound and nascent ones, the ATM approach revealed diversities in measured CD38 K_int values of 0.239 min^-1 S.E. ± 0.076, 0.109 min^-1 S.E. ± 0.032, and 0.058 min^-1 S.E. ± 0.001 for LP1, NCIH929, and MOLP8 cell lines, respectively. Together with CD38 surface densities, intrinsic K_int values aligned well with the tumor penetration model and supported the outcomes for tumor regression in mouse xenografts upon drug treatment. Additionally, the ATM approach can evaluate molecules with fast K_int as we determined for CTLA4 protein. We believe that the ATM approach has the potential to evaluate diverse cell-surface targets as part of the pharmacological assessment in drug discovery.

Illuminating the norepinephrine transporter: fluorescent probes based on nisoxetine and talopram

The utilization of fluorescent ligands to study the monoamine transporters (MATs) has increased our knowledge of their function and distribution in live cell systems. In this study, we extend SAR for nisoxetine and talopram as parent compounds, to identify high affinity rhodamine-labeled fluorescent probes for the norepinephrine transporter (NET). Nisoxetine-based fluorescent probe 6 demonstrated high binding affinity (K _i = 43 nM) for NET and an overall selectivity compared to the other transporters for dopamine (DAT; K _i = 1540 nM) and serotonin (SERT; K _i = 785 nM) in competitive radioligand binding assays. Using confocal microscopy, compound 6 was shown to stain both NET and SERT, but not DAT, at low nanomolar concentrations, in transporter-expressing cells.

Dynamic control of the dopamine transporter in neurotransmission and homeostasis

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

Novel Fluorescent Ligands Enable Single-Molecule Localization Microscopy of the Dopamine Transporter

The dopamine transporter (DAT) is critical for spatiotemporal control of dopaminergic neurotransmission and is the target for therapeutic agents, including ADHD medications, and abused substances, such as cocaine. Here, we develop new fluorescently labeled ligands that bind DAT with high affinity and enable single-molecule detection of the transporter. The cocaine analogue MFZ2-12 (1) was conjugated to novel rhodamine-based Janelia Fluorophores (JF₅₄₉ and JF₆₄₆). High affinity binding of the resulting ligands to DAT was demonstrated by potent inhibition of [³H]dopamine uptake in DAT transfected CAD cells and by competition radioligand binding experiments on rat striatal membranes. Visualization of binding was substantiated by confocal or TIRF microscopy revealing selective binding of the analogues to DAT transfected CAD cells. Single particle tracking experiments were performed with JF₅₄₉-conjugated DG3-80 (3) and JF₆₄₆-conjugated DG4-91 (4) on DAT transfected CAD cells enabling quantification and categorization of the dynamic behavior of DAT into four distinct motion classes (immobile, confined, Brownian, and directed). Finally, we show that the ligands can be used in direct stochastic optical reconstruction microscopy (dSTORM) experiments permitting further analyses of DAT distribution on the nanoscale. In summary, these novel fluorescent ligands are promising new tools for studying DAT localization and regulation with single-molecule resolution.

Quantum dots reveal heterogeneous membrane diffusivity and dynamic surface density polarization of dopamine transporter

The presynaptic dopamine transporter mediates rapid reuptake of synaptic dopamine. Although cell surface DAT trafficking recently emerged as an important component of DAT regulation, it has not been systematically investigated. Here, we apply our single quantum dot (Qdot) tracking approach to monitor DAT plasma membrane dynamics in several heterologous expression cell hosts with nanometer localization accuracy. We demonstrate that Qdot-tagged DAT proteins exhibited highly heterogeneous membrane diffusivity dependent on the local membrane topography. We also show that Qdot-tagged DATs were localized away from the flat membrane regions and were dynamically retained in the membrane protrusions and cell edges for the duration of imaging. Single quantum dot tracking of wildtype DAT and its conformation-defective coding variants (R60A and W63A) revealed a significantly accelerated rate of dysfunctional DAT membrane diffusion. We believe our results warrant an in-depth investigation as to whether compromised membrane dynamics is a common feature of brain disorder-derived DAT mutants.

Tales of tails in transporters

Cell nutrition, detoxification, signalling, homeostasis and response to drugs, processes related to cell growth, differentiation and survival are all mediated by plasma membrane (PM) proteins called transporters. Despite their distinct fine structures, mechanism of function, energetic requirements, kinetics and substrate specificities, all transporters are characterized by a main hydrophobic body embedded in the PM as a series of tightly packed, often intertwined, α-helices that traverse the lipid bilayer in a zigzag mode, connected with intracellular or extracellular loops and hydrophilic N- and C-termini. Whereas longstanding genetic, biochemical and biophysical evidence suggests that specific transmembrane segments, and also their connecting loops, are responsible for substrate recognition and transport dynamics, emerging evidence also reveals the functional importance of transporter N- and C-termini, in respect to transport catalysis, substrate specificity, subcellular expression, stability and signalling. This review highlights selected prototypic examples of transporters in which their termini play important roles in their functioning.

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.

The Dopamine Transporter Recycles via a Retromer-Dependent Postendocytic Mechanism: Tracking Studies Using a Novel Fluorophore-Coupling Approach

Presynaptic reuptake, mediated by the dopamine (DA) transporter (DAT), terminates DAergic neurotransmission and constrains extracellular DA levels. Addictive and therapeutic psychostimulants inhibit DA reuptake and multiple DAT coding variants have been reported in patients with neuropsychiatric disorders. These findings underscore that DAT is critical for DA neurotransmission and homeostasis. DAT surface availability is regulated acutely by endocytic trafficking, and considerable effort has been directed toward understanding mechanisms that govern DAT's plasma membrane expression and postendocytic fate. Multiple studies have demonstrated DAT endocytic recycling and enhanced surface delivery in response to various stimuli. Paradoxically, imaging studies have not detected DAT targeting to classic recycling endosomes, suggesting that internalized DAT targets to either degradation or an undefined recycling compartment. Here, we leveraged PRIME (PRobe Incorporation Mediated by Enzyme) labeling to couple surface DAT directly to fluorophore, and tracked DAT's postendocytic itinerary in immortalized mesencephalic cells. Following internalization, DAT robustly targeted to retromer-positive endosomes, and DAT/retromer colocalization was observed in male mouse dopaminergic somatodendritic and terminal regions. Short hairpin RNA-mediated Vps35 knockdown revealed that DAT endocytic recycling requires intact retromer. DAT also targeted rab7-positive endosomes with slow, linear kinetics that were unaffected by either accelerating DAT internalization or binding a high-affinity cocaine analog. However, cocaine increased DAT exit from retromer-positive endosomes significantly. Finally, we found that the DAT carboxy-terminal PDZ-binding motif was required for DAT recycling and exit from retromer. These results define the DAT recycling mechanism and provide a unifying explanation for previous, seemingly disparate, DAT endocytic trafficking findings.SIGNIFICANCE STATEMENT The neuronal dopamine (DA) transporter (DAT) recaptures released DA and modulates DAergic neurotransmission, and a number of DAT coding variants have been reported in several DA-related disorders, including infantile parkinsonism, attention-deficit/hyperactivity disorder and autism spectrum disorder. DAT is also competitively inhibited by psychostimulants with high abuse potential. Therefore, mechanisms that acutely affect DAT availability will likely exert significant impact on both normal and pathological DAergic homeostasis. Here, we explore the cellular mechanisms that acutely control DAT surface expression. Our results reveal the intracellular mechanisms that mediate DAT endocytic recycling following constitutive and regulated internalization. In addition to shedding light on this critical process, these findings resolve conflict among multiple, seemingly disparate, previous reports on DAT's postendocytic fate.

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

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

Dopamine Transporter Amino and Carboxyl Termini Synergistically Contribute to Substrate and Inhibitor Affinities

Extracellular dopamine and serotonin concentrations are determined by the presynaptic dopamine (DAT) and serotonin (SERT) transporters, respectively. Numerous studies have investigated the DAT and SERT structural elements contributing to inhibitor and substrate binding. To date, crystallographic studies have focused on conserved transmembrane domains, where multiple substrate binding and translocation features are conserved. However, it is unknown what, if any, role the highly divergent intracellular N and C termini contribute to these processes. Here, we used chimeric proteins to test whether DAT and SERT N and C termini contribute to transporter substrate and inhibitor affinities. Replacing the DAT N terminus with that of SERT had no effect on DA transport V_max but significantly decreased DAT substrate affinities for DA and amphetamine. Similar losses in uptake inhibition were observed for small DAT inhibitors, whereas substituting the DAT C terminus with that of SERT affected neither substrate nor inhibitor affinities. In contrast, the N-terminal substitution was completely tolerated by the larger DAT inhibitors, which exhibited no loss in apparent affinity. Remarkably, all affinity losses were rescued in DAT chimeras encoding both SERT N and C termini. The sensitivity to amino-terminal substitution was specific for DAT, because replacing the SERT N and/or C termini affected neither substrate nor inhibitor affinities. Taken together, these findings provide compelling experimental evidence that DAT N and C termini synergistically contribute to substrate and inhibitor affinities.