How dopamine transporter interacts with dopamine: insights from molecular modeling and simulation

By performing homology modeling, molecular docking, and molecular dynamics simulations, we have developed three-dimensional (3D) structural models of both dopamine transporter and dopamine transporter-dopamine complex in the environment of lipid bilayer and solvent water. According to the simulated structure of dopamine transporter-dopamine complex, dopamine was orientated in a hydrophobic pocket at the midpoint of the membrane. The modeled 3D structures provide some detailed structural and mechanistic insights concerning how dopamine transporter (DAT) interacts with dopamine at atomic level, extending our mechanistic understanding of the dopamine reuptake with the help of Na(+) ions. The general features of the modeled 3D structures are consistent with available experimental data. Based on the modeled structures, our calculated binding free energy (DeltaG(bind) = -6.4 kcal/mol) for dopamine binding with DAT is also reasonably close to the experimentally derived DeltaG(bind) value of -7.4 kcal/mol. Finally, a possible dopamine-entry pathway, which involves formation and breaking of the salt bridge between side chains of Arg(85) and Asp(476), is proposed based on the results obtained from the modeling and molecular dynamics simulation. The new structural and mechanistic insights obtained from this computational study are expected to stimulate future, further biochemical and pharmacological studies on the detailed structures and mechanisms of DAT and other homologous transporters.     

Substrates and inhibitors display different sensitivity to expression level of the dopamine transporter in heterologously expressing cells

The use of heterologous expression systems for studying dopamine (DA) transporter (DAT) function has provided important information corroborating and complementing in situ obtained knowledge. Preliminary experiments with human embryonic kidney cells (HEK293) heterologously expressing varying amounts of DAT suggested fluctuations in the potency of cocaine in inhibiting DA uptake and led to the present systematic assessment of the impact of the density of DAT on its function. Transiently expressing intact HEK293 cells, transfected with increasing amounts of DAT cDNA, displayed increasing levels of surface DAT, binding of the cocaine analog [(3)H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane ([(3)H]CFT), and uptake of [(3)H]DA, [(3)H]N-methyl-4-phenylpyridinium ([(3)H]MPP(+)), [(3)H]norepinephrine, and [(3)H]serotonin. However, the amount of DAT cDNA and the DAT expression level required to produce 50% of maximal activity was threefold higher for CFT binding than for DA uptake. Increased DAT expression was accompanied by weakened potency in inhibiting [(3)H]DA uptake for cocaine, CFT, benztropine, and its analog JHW025, GBR 12909 and mazindol; their potency in inhibiting [(3)H]CFT binding was unaffected. Inhibition of uptake by the substrates DA, m-tyramine, d-amphetamine, or MPP(+) was also unaffected. Increasing DAT in stably expressing HEK293 cells by stimulation of gene expression with sodium butyrate also decreased the uptake inhibitory potency of a number of the above blockers without affecting the interaction between substrates and DAT. The present results prompt discussion of models explaining how factors regulating DAT expression at the plasma membrane can regulate DAT function and pharmacology.     

Calmodulin kinase II interacts with the dopamine transporter C terminus to regulate amphetamine-induced reverse transport

Efflux of dopamine through the dopamine transporter (DAT) is critical for the psychostimulatory properties of amphetamines, but the underlying mechanism is unclear. Here we show that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays a key role in this efflux. CaMKIIalpha bound to the distal C terminus of DAT and colocalized with DAT in dopaminergic neurons. CaMKIIalpha stimulated dopamine efflux via DAT in response to amphetamine in heterologous cells and in dopaminergic neurons. CaMKIIalpha phosphorylated serines in the distal N terminus of DAT in vitro, and mutation of these serines eliminated the stimulatory effects of CaMKIIalpha. A mutation of the DAT C terminus impairing CaMKIIalpha binding also impaired amphetamine-induced dopamine efflux. An in vivo role for CaMKII was supported by chronoamperometry measurements showing reduced amphetamine-induced dopamine efflux in response to the CaMKII inhibitor KN93. Our data suggest that CaMKIIalpha binding to the DAT C terminus facilitates phosphorylation of the DAT N terminus and mediates amphetamine-induced dopamine efflux.     

The dopamine transporter constitutively internalizes and recycles in a protein kinase C-regulated manner in stably transfected PC12 cell lines

The dopamine transporter (DAT) removes dopamine from the extracellular milieu and is potently inhibited by number of psychoactive drugs, including cocaine, amphetamines, and methylphenidate (Ritalin). Multiple lines of evidence demonstrate that protein kinase C (PKC) down-regulates dopamine transport, primarily by redistributing DAT from the plasma membrane to endosomal compartments, although the mechanisms facilitating transporter sequestration are not defined. Here, we demonstrate that DAT constitutively internalizes and recycles in rat pheochromocytoma (PC12) cells. Temperature blockades demonstrated basal internalization and reliance on recycling to maintain DAT cell surface levels. In contrast, recycling blockade with bafilomycin A1 significantly decreased transferrin receptor (TfR) surface expression but had no effect on DAT surface levels, suggesting that DAT and TfR traffic via distinct endosomal mechanisms. Kinetic analyses reveal robust constitutive DAT cycling to and from the plasma membrane, independent of transporter expression levels. In contrast, phorbol ester-mediated PKC activation accelerated DAT endocytosis and attenuated transporter recycling in a manner sensitive to DAT expression levels. These data demonstrate constitutive DAT trafficking and that PKC-mediated DAT sequestration is achieved by a combination of accelerated internalization and reduced recycling. Additionally, the differential sensitivity to expression level exhibited by constitutive and regulated DAT trafficking suggests that these two processes are mediated by independent cellular mechanisms.     

The uptake inhibitors cocaine and benztropine differentially alter the conformation of the human dopamine transporter

The binding affinity of the cocaine analog [(3)H]2 beta-carbomethoxy-3beta-(4-fluorophenyl) tropane (WIN) for the dopamine transporter (DAT) is increased by the reaction of Cys-90, at the extracellular end of the first transmembrane segment, with methanethiosulfonate (MTS) reagents. Cocaine enhances the reaction of Cys-90 with the sulfhydryl reagents, thereby augmenting the increase in binding. In contrast, cocaine decreases the reaction of Cys-135 and Cys-342, endogenous cysteines in cytoplasmic loops, with MTS reagents. Because this reaction inhibits [(3)H]WIN binding, cocaine protects against the loss of binding caused by reaction of these cysteines. In the present work, we compare the abilities of DAT inhibitors and substrates to affect the reaction of Cys-90, Cys-135, and Cys-342 with MTS ethyltrimethylammonium (MTSET). The results indicate that the different abilities of compounds to protect against the MTSET-induced inhibition of binding are attributable to differences in their abilities to attenuate the inhibitory effects of modification of Cys-135 and Cys-342 as well as to enhance the reaction with Cys-90 and the resulting potentiation of binding. The inhibitor benztropine was unique in its inability to protect Cys-135. Moreover, whereas cocaine, WIN, mazindol, and dopamine enhanced the reaction of Cys-90 with MTSET, benztropine had no effect on this reaction. These two features combine to give benztropine its weak potency in protecting ligand binding to wild-type DAT from MTSET. These results indicate that different inhibitors of DAT, such as cocaine and benztropine, produce different conformational changes in the transporter. There are differences in the psychomotor stimulant-like effects of these compounds, and it is possible that the different behavioral effects of these DAT inhibitors stem from their different molecular actions on DAT.     

The UT-A1 urea transporter interacts with snapin, a SNARE-associated protein

The UT-A1 urea transporter mediates rapid transepithelial urea transport across the inner medullary collecting duct and plays a major role in the urinary concentrating mechanism. To transport urea, UT-A1 must be present in the plasma membrane. The purpose of this study was to screen for UT-A1-interacting proteins and to study the interactions of one of the identified potential binding partners with UT-A1. Using a yeast two-hybrid screen of a human kidney cDNA library with the UT-A1 intracellular loop (residues 409-594) as bait, we identified snapin, a ubiquitously expressed SNARE-associated protein, as a novel UT-A1 binding partner. Deletion analysis indicated that the C-terminal coiled-coil domain (H2) of snapin is required for UT-A1 interaction. Snapin binds to the intracellular loop of UT-A1 but not to the N- or C-terminal fragments. Glutathione S-transferase pulldown experiments and co-immunoprecipitation studies verified that snapin interacts with native UT-A1, SNAP23, and syntaxin-4 (t-SNARE partners), indicating that UT-A1 participates with the SNARE machinery in rat kidney inner medulla. Confocal microscopic analysis of immunofluorescent UT-A1 and snapin showed co-localization in both the cytoplasm and in the plasma membrane. When we co-injected UT-A1 with snapin cRNA in Xenopus oocytes, urea influx was significantly increased. In the absence of snapin, the influx was decreased when UT-A1 was combined with t-SNARE components syntaxin-4 and SNAP23. We conclude that UT-A1 may be linked to the SNARE machinery via snapin and that this interaction may be functionally and physiologically important for urea transport.     

The tandem PDZ protein Syntenin interacts with the aminoacyl tRNA synthetase complex in a lysyl-tRNA synthetase-dependent manner

Syntenin-1 is a tandem PDZ protein that binds a diverse array of signaling molecules that are often associated with cell adhesion and intracellular trafficking. With the use of a MS-based functional proteomics approach, we identified several members of the aminoacyl-tRNA synthetase macromolecular (ARS) complex in a syntenin-1 pull down assay. Interaction of these proteins with syntenin-1 was confirmed by co-immunoprecipitation from cultured cells. We demonstrate a direct interaction of syntenin-1 with lysyl-tRNA synthetase (KRS), which contains a PDZ binding motif at its C-terminus. This motif is important for the interaction of the entire complex with syntenin-1. A point mutation in the PDZ2 domain of syntenin-1 abrogates interaction with KRS. As a result, other components of the ARS complex no longer co-immunoprecipitate with syntenin-1. We further show that syntenin-1 regulates KRS activity. These findings suggest that syntenin-1 is an adaptor modulating the activity of KRS.     

Pin1 interacts with c-Myb in a phosphorylation-dependent manner and regulates its transactivation activity

Activity and stability of the proto-oncogene c-Myb are regulated by post-translational modifications, though the molecular mechanisms underlying such control are only partially understood. Here we describe the functional interaction of c-Myb with Pin1, an isomerase that binds to phosphorylated Ser/Thr-Pro motifs. We found that co-expression of c-Myb and Pin1 led to a net increase of c-Myb transactivation activity, both on reporter constructs as well as on an endogenous target gene. DNA-binding studies revealed that Pin1 did not increase the association of c-Myb with its response element in DNA. The increase of c-Myb transactivation activity was strictly dependent on the presence of an active catalytic center in Pin1. We provide evidence that c-Myb and Pin1 physically interacted, both upon ectopic expression of the proteins in HEK-293 cells as well as in the more physiological setting of HL60 cells, where c-Myb and Pin1 are resident proteins. By point mutating each individual Ser/Thr-Pro motif in c-Myb as well as by using deletion mutants we show that S528 in the EVES-motif was the docking site for Pin1. Mass spectrometry confirmed that S528 is phosphorylated in vivo. Finally, functional studies showed that mutation of S528 to alanine almost abolished the increase of transactivation activity by Pin1. This study reveals a new paradigm by which phosphorylation controls c-Myb function.     

1-Methyl-4-phenylpyridinium-induced down-regulation of dopamine transporter function correlates with a reduction in dopamine transporter cell surface expression

The mechanisms whereby 1-methyl-4-phenylpyridinium (MPP(+)) mediates cell death and Parkinsonism are still unclear. We have shown that dopamine transporter (DAT) is required for MPP(+)-mediated cytotoxicity in HEK-293 cells stably transfected with human DAT. Furthermore, MPP(+) produced a concentration- and time-dependent reduction in the uptake of [3H]dopamine. We observed a significant decrease in [3H]WIN 35428 binding in the intact cells with MPP(+). The saturation analysis of the [3H]WIN 35428 binding obtained from total membrane fractions revealed a decrease in the transporter density (B(max)) with an increase in the dissociation equilibrium constant (K(d)) after MPP(+) treatment. Furthermore, biotinylation assays confirmed that MPP(+) reduced both plasma membrane and intracellular DAT immunoreactivity. Taken together, these findings suggest that the reduction in cell surface DAT protein expression in response to MPP(+) may be a contributory factor in the down-regulation of DAT function while enhanced lysosomal degradation of DAT may signal events leading to cellular toxicity.     

Effect of nootropic drugs on the dopamine release and dopamine uptake in structures of the rat striatum

Nootropics increase the overflow of dopamine from rat striatum slices in a concentration dependent manner, but without relation to their clinical effectiveness. The influence of a nootropic drugs and of amphetamine on the stimulus induced dopamine release points to a relationship between nootropic and nooanaleptic activity, on the one hand, and transmitter release, on the other. Dopamine re-uptake is not altered by nootropics like piracetam.     

NHERF2/SIP-1 interacts with mouse SRY via a different mechanism than human SRY

In mammals, male sex determination is controlled by the SRY protein, which drives differentiation of the bipotential embryonic gonads into testes by activating the Sertoli cell differentiation program. The morphological effects of SRY are well documented; however, its molecular mechanism of action remains unknown. Moreover, SRY proteins display high sequence variability among mammalian species, which makes protein motifs difficult to delineate. We previously isolated SIP-1/NHERF2 as a human SRY-interacting protein. SIP-1/NHERF2, a PDZ protein, interacts with the C-terminal extremity of the human SRY protein. Here we showed that the interaction of SIP-1/NHERF2 and SRY via the SIP-1/NHERF2 PDZ1 domain is conserved in mice. However, the interaction occurs via a domain that is internal to the mouse SRY protein and involves a different recognition mechanism than human SRY. Furthermore, we show that mouse and human SRY induce nuclear accumulation of the SIP-1/NHERF2 protein in cultured cells. Finally, a transgenic mouse line expressing green fluorescent protein under the control of the mouse Sry promoter allowed us to show that SRY and SIP-1/NHERF2 are co-expressed in the nucleus of pre-Sertoli cells during testis determination. Taken together, our results suggested that the function of SIP-1/NHERF2 as an SRY cofactor during testis determination is conserved between human and mouse.     

Physarum myosin light chain interacts with actin in a Ca2+-dependent manner

Actin-modulating activity was analysed with the 16,131-dalton calcium-binding light chain (CaLc, Kobayashi et al. (1988) J. Biol. Chem. 263, 305-313) of Physarum myosin, which is under an inhibitory Ca-control (Kohama and Kendrick-Jones (1986) J. Biochem. 99, 1433-1446). When skeletal muscle actin was polymerized in the presence of CaLc and Ca2+, increases in both viscosity and birefringence were reduced under high shear conditions. However, CaLc did not inhibit actin polymerization under no or low shearing forces, which was demonstrated by a variety of methods including fluorescence intensity measurements using pyrenyl actin. We propose that actin polymerized in the presence of CaLc and Ca2+ is easily fragmented under high shearing forces to produce the changes in viscosity and birefringence.     

Reaction of oxidized dopamine with endogenous cysteine residues in the human dopamine transporter

There is evidence to suggest that dopamine (DA) oxidizes to form dopamine ortho-quinone (DAQ), which binds covalently to nucleophilic sulfhydryl groups on protein cysteinyl residues. This reaction has been shown to inhibit dopamine uptake, as well as other biological processes. We have identified specific cysteine residues in the human dopamine transporter (hDAT) that are modified by this electron-deficient substrate analog. DAQ reactivity was inferred from its effects on the binding of [(3)H]2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane (beta-CFT) to hDAT cysteine mutant constructs. One construct, X5C, had four cysteines mutated to alanine and one to phenylalanine (Cys(90)A, Cys(135)A, C306A, C319F and Cys(342)A). In membrane preparations 1 mM DAQ did not affect [(3)H]beta-CFT binding to X5C hDAT, in contrast to its effect in wild-type hDAT in which it reduced the B:(max) value by more than half. Wild-type cysteines were substituted back into X5C, one at a time, and the ability of DAQ to inhibit [(3)H]beta-CFT binding was assessed. Reactivity of DAQ with Cys(90) increased the affinity of [(3)H]beta-CFT for the transporter, whereas reactivity with Cys(135) decreased the affinity of [(3)H]beta-CFT. DAQ did not change the K:(D) for [(3)H]beta-CFT binding to wild-type. The reactivity of DAQ at Cys(342) decreased B:(max) to the same degree as wild-type. The latter result suggests that Cys(342) is the wild-type residue most responsible for DAQ-induced inhibition of [(3)H]beta-CFT binding.