The role of conserved tryptophan and acidic residues in the human dopamine transporter as characterized by site-directed mutagenesis

Role of proline residues in the expression and function of the human noradrenaline transporter

The aim was to investigate the roles of proline residues in extracellular loop 2 (P172, P183, P188 and P209) and transmembrane domains 2, 5, 11 and 12 (P108, P270, P526, P551, P552 and P570) in determining noradrenaline transporter (NET) expression and function. Mutants of human NET with these residues mutated to alanine were pharmacologically characterized. Mutation of P108, P270 and P526 disrupted cell surface expression, from [3H]nisoxetine binding and confocal microscopy data. Mutations of P526, P551 and P570 reduced transporter turnover (Vmax of [3H]noradrenaline uptake/Bmax of [3H]nisoxetine binding) by 1.5-1.7-fold compared with wild-type NET, so these residues might be involved in conformational changes associated with substrate translocation. Conversely, mutations of P172, P183, P188 and P209 increased Vmax/Bmax by 2-3-fold compared with wild-type, indicating that the presence of these proline residues limits turnover of the NET. The mutations had few effects on apparent affinities of substrates or affinities of inhibitors, except decreases in inhibitor affinities after mutations of the P270 and P570 residues, and increases after mutation of the P526 residue. Hence, proline residues in extracellular loop 2 and in transmembrane domains have a range of roles in determining expression and function of the NET.

Glycosyl modification facilitates homo- and hetero-oligomerization of the serotonin transporter. A specific role for sialic acid residues

The serotonin transporter (SERT) is an oligomeric glycoprotein with two sialic acid residues on each of two complex oligosaccharide molecules. In this study, we investigated the contribution of N-glycosyl modification to the structure and function of SERT in two model systems: wild-type SERT expressed in sialic acid-defective Lec4 Chinese hamster ovary (CHO) cells and a mutant form (after site-directed mutagenesis of Asn-208 and Asn-217 to Gln) of SERT, QQ, expressed in parental CHO cells. In both systems, SERT monomers required modification with both complex oligosaccharide residues to associate with each other and to function in homo-oligomeric forms. However, defects in sialylated N-glycans did not alter surface expression of the SERT protein. Furthermore, in heterologous (CHO and Lec4 cells) and endogenous (placental choriocarcinoma JAR cells) expression systems, we tested whether glycosyl modification also manipulates the hetero-oligomeric interactions of SERT, specifically with myosin IIA. SERT is phosphorylated by cGMP-dependent protein kinase G through interactions with anchoring proteins, and myosin is a protein kinase G-anchoring protein. A physical interaction between myosin and SERT was apparent; however, defects in sialylated N-glycans impaired association of SERT with myosin as well as the stimulation of the serotonin uptake function in the cGMP-dependent pathway. We propose that sialylated N-glycans provide a favorable conformation to SERT that allows the transporter to function most efficiently via its protein-protein interactions.

Substrate-induced trafficking of the dopamine transporter in heterologously expressing cells and in rat striatal synaptosomal preparations

Dopamine transporter (DAT) trafficking was assessed by functional measurements of dopamine uptake and by biotinylation of surface proteins followed by gel electrophoresis and Western blotting. In human embryonic kidney (HEK)-293 cells expressing human DAT (HEK-hDAT), pretreatment with dopamine (0.1-100 microM) followed by washout caused reductions in subsequent dopamine uptake (reflected in Vmax) with effective dopamine concentrations in the 10 to 100 microM range and pretreatment times of 10 to 60 min. Reductions assessed after 60-min pretreatment with 100 microM dopamine corresponded with decreases measured in surface DAT by the noncleavable biotin method, which were caused, at least in part, by enhanced endocytosis as monitored with cleavable biotin. Pretreatment of rat striatal synaptosomes with dopamine (10 and 100 microM) also caused reductions in DAT uptake activity (Vmax), and again the underlying mechanism seemed to be a diminished presence of DAT at the surface of synaptosomes as measured by the noncleavable biotin method. The copresence of cocaine during pretreatment with dopamine prevented the down-regulation of surface DAT. The present results show that DAT surface residency can be regulated by substrate acting on it, not only in cells heterologously expressing DAT but also in situ in rat brain tissue.

The top 20 dopamine transporter mutants: structure–function relationships and cocaine actions

Our laboratory and others elucidated the primary amino acid sequences of the dopamine transporter (DAT) by cloning its cDNA and genomic sequences more than 12 years ago. Motivations for this work included the ideas that cocaine's interactions with DAT accounted for its rewarding properties and that selective inhibitors of DAT/cocaine interactions might thus provide good anticocaine medications. Such ideas supported interest in the detailed structure-function relationships of cocaine/DAT interactions, and in the construction and characterization of extensive series of site-directed DAT mutants. We can now select the most interesting 20 cocaine-analog selective mutations of the more than 100 single- and multiple amino acid substitution mutations that we have characterized. These mutants selectively reduce the affinities of the mutant DATs for cocaine analogs, but (absolutely or relatively) spare their affinities for dopamine. Several themes relevant to cocaine/DAT interactions emerge from these mutants. First, such mutations are found in a number of different DAT domains. Secondly, many but not all of these mutations lie in groups, near each other and near the same faces of presumably helical DAT transmembrane domains. Third, most are also conserved in the serotonin transporter (SERT), a transporter that is now strongly implicated in cocaine reward based on data from knockout mice. We discuss the results from these "top 20" mutants in light of the strengths and limitations of current DAT models and data from other studies. Taken together, these studies appear to indicate direct or indirect participation of several specific portions of DAT in selective recognition of cocaine analogs. These studies provide a strong basis for redirected studies aimed at producing dopamine- and serotonin-sparing cocaine antagonists that would represent combined DAT/SERT disinhibitors.

Na+ and the substrate permeation pathway in dopamine transporters

Advances have been made in characterizing the relationship between Na+ and the substrate permeation pathway in the dopamine transporter. This review covers the role of Na+ in co-transport with dopamine as well as in the recognition of dopamine. Apparent recognition depends on the preparation studied: it differs between intact cells heterologously expressing the dopamine transporter and membranes prepared from these cells. In our search for amino acid residues in the transporter involved in Na+ action, W84 and D313 were found to play a special role in cation interaction, with evidence for regulation of both Na+ and H+ sensitivity. Mutation of D313 to N appeared to decrease the affinity for the dopamine transporter in intact cells, not by altering recognition per se. A model is proposed in which access of dopamine, not recognition itself, is regulated by D313 and Na+. Thus, the role of external Na+ in intact cell preparations is to turn dopamine transporters to the externally facing form, allowing access of dopamine to its binding site.

Na+ stimulates binding of dopamine to the dopamine transporter in cells but not in cell-free preparations

Although Na+ is crucial for the function of the dopamine (DA) transporter (DAT), its role in the substrate binding step has been questioned. To address this issue, we investigated the effect of Na+ on DA binding by measuring the potency of DA in inhibiting the binding of the cocaine analogue [3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT) in intact cells expressing DAT in their plasma membranes and in membranes isolated from these cells. In cells, Na+ substantially enhanced the potency of DA in inhibiting CFT binding. This effect of Na+ was independent of buffer compositions and substitutes (sucrose vs. NMDG), more pronounced at 4 degrees C than 25 degrees C, and correlated with its stimulatory effect on DA uptake Km. Removing extracellular Na+ had little effect on intracellular concentrations of Na+ and K+, or on membrane potential. These data suggest that extracellular Na+ most likely acts at the transporter level to enhance the binding of external DA during the transport cycle. In contrast, in cell-free membrane preparations the Na+ stimulation was abolished without impairment of the potency of DA in inhibiting CFT binding, regardless of whether sucrose was used to maintain the buffer osmolarity. The difference in Na+ dependence for DA to inhibit CFT binding between plasma membranes of intact cells and isolated membranes raises the possibility that intracellular ion environment, alone or in combination with other cellular factors, plays a critical role in determining DA-DAT interaction and the integration of Na+ modulation in this interaction.

Cocaine affinity decreased by mutations of aromatic residue phenylalanine 105 in the transmembrane domain 2 of dopamine transporter

Dopamine transporter (DAT) is a major target of cocaine, one of the most abused drugs. Major efforts have been focused on defining residues in DAT involved in cocaine binding. We have isolated the Drosophila melanogaster DAT (dDAT) cDNA, which is 10-fold less sensitive to cocaine than the mammalian DATs. Replacing transmembrane domain 2 (TM2) of mouse DAT (mDAT) with dDAT sequence reduced cocaine sensitivity. The reciprocal construct exhibited increased cocaine sensitivity. Switching residue 105 in TM2, a phenylalanine conserved in all mammalian DATs, to methionine, the corresponding residue in dDAT, resulted in a functional transporter with cocaine sensitivity 4-fold lower. Replacing F105 with alanine, leucine, isoleucine, serine, threonine, asparagine, or glutamine resulted in transporters with low transport activity. In contrast, changing F105 to the other aromatic residues tyrosine or tryptophan retained more than 75% transport activity and high cocaine sensitivity. Most significantly, the reciprocal construct, switching the methionine in dDAT at the corresponding residue to phenylalanine, increased cocaine sensitivity 3-fold. Finally, the mDAT mutant with a cysteine at this position had normal transport activity but exhibited cocaine sensitivity that was 15-fold lower. These results suggest that F105 in mDAT contributes to high-affinity cocaine binding. The functional cocaine-insensitive mutants provide tools for the study of the mechanism of cocaine addiction.

The role of the conserved GXXXRXG motif in the expression and function of the human norepinephrine transporter

Highly conserved motifs in the monoamine transporters, e.g. the human norepinephrine transporter (hNET) GXXXRXG motif which was the focus of the present study, are likely to be important structural features in determining function. This motif was investigated by mutating the glycines to glutamate (causing loss of function) and alanine, and the arginine to glycine. The effects of hG117A, hR121G and hG123A mutations on function were examined in COS-7 cells and compared to hNET. Substrate K(m) values were decreased for hG117A and hG123A, and their K(i) values for inhibition of [3H]nisoxetine binding were decreased 3-4-fold and 4-6-fold, respectively. Transporter turnover was reduced to 65% of hNET for hG117A and hR121G and to 28% for hG123A, suggesting that substrate translocation is impaired. K(i) values of nisoxetine and desipramine for inhibition of [3H]norepinephrine uptake were increased by 5-fold for hG117A, with no change for cocaine. The K(i) value of cocaine was increased by 3-fold for hG123A, with no change for nisoxetine and desipramine. However, there were no effects of the mutations on the K(d) of [3H]nisoxetine binding or K(i) values of desipramine or cocaine for inhibition of [3H]nisoxetine binding. Hence, glycine residues of the GXXXRXG motif are important determinants of NET expression and function, while the arginine residue does not have a major role. This study also showed that antidepressants and psychostimulants have different NET binding sites and provided the first evidence that different sites on the NET are involved in the binding of inhibitors and their competitive inhibition of substrate uptake.

Cationic interactions at the human dopamine transporter reveal binding conformations for dopamine distinguishable from those for the cocaine analog 2 alpha-carbomethoxy-3 alpha-(4-fluorophenyl)tropane

In membrane preparations, CFT, a phenyltropane cocaine analog, and dopamine (DA) interact with the recombinant human dopamine transporter (hDAT) in Na+ -free medium. Na+ markedly increased the transporter's affinity for CFT, but had little or no effect on DA potency for inhibiting CFT binding. Raising [Na+ ] from 20 to 155 mm reduced Li+ -induced increase in DA K (i), but not CFT K (d). The presence of 155 mm Na+ enhanced the tolerance to low pH of CFT Kd but not DA Ki. Leucine substitution for tryptophan 84 (W84L) in transmembrane domain (TM) 1 or asparagine substitution for aspartate 313 (D313N) in TM 6 did not or only modestly enhance the affinity of Na+ -independent CFT binding, and retained the near normal ability of DA, Li+, K+, or H+ to inhibit this binding. However, the mutations significantly enhanced the Na+ stimulation of CFT binding as well as the Na+ antagonism against Li+ and H+ inhibition of CFT binding. In contrast, the mutations neither changed the Na+ -insensitive feature of DA Ki nor enhanced the Na+ protection of DA Ki against Li+ 's inhibitory effect, though they caused Na+ protection of DA Ki against H+ 's inhibitory action. These results are consistent with the existence of binding conformations for DA that are distinguishable from those for CFT, and with a differential association of cation interactions with DA and CFT binding. The mutations likely alter Na+ -bound state(s) of hDAT, preferentially strengthening the positive allosteric coupling between Na+ and CFT binding, and reducing the impact of Li+ or H+ on the CFT binding.

Dopamine transporter mutants with cocaine resistance and normal dopamine uptake provide targets for cocaine antagonism

Cocaine's blockade of dopamine reuptake by brain dopamine transporters (DAT) is a central feature of current understanding of cocaine reward and addiction. Empirical screening of small-molecule chemical libraries has thus far failed to provide successful cocaine blockers that allow dopamine reuptake in the presence of cocaine and provide cocaine "antagonism". We have approached this problem by assessing expression, dopamine uptake, and cocaine analog affinities of 56 DAT mutants in residues located in or near transmembrane domains likely to play significant roles in cocaine recognition and dopamine uptake. A phenylalanine-to-alanine mutant in putative DAT transmembrane domain 3, F154A, retains normal dopamine uptake, lowers cocaine affinity 10-fold, and reduces cocaine stereospecificity. Such mutants provide windows into DAT structures that could serve as targets for selective cocaine blockers and document how combined strategies of mutagenesis and small molecule screening may improve our abilities to identify and design compounds with such selective properties.

Generation of an activating Zn(2+) switch in the dopamine transporter: mutation of an intracellular tyrosine constitutively alters the conformational equilibrium of the transport cycle

Binding of Zn(2+) to the endogenous Zn(2+) binding site in the human dopamine transporter leads to potent inhibition of [(3)H]dopamine uptake. Here we show that mutation of an intracellular tyrosine to alanine (Y335A) converts this inhibitory Zn(2+) switch into an activating Zn(2+) switch, allowing Zn(2+)-dependent activation of the transporter. The tyrosine is part of a conserved YXX Phi trafficking motif (X is any residue and Phi is a residue with a bulky hydrophobic group), but Y335A did not show alterations in surface targeting or protein kinase C-mediated internalization. Despite wild-type levels of surface expression, Y335A displayed a dramatic decrease in [(3)H]dopamine uptake velocity (V(max)) to less than 1% of the wild type. In addition, Y335A showed up to 150-fold decreases in the apparent affinity for cocaine, mazindol, and related inhibitors whereas the apparent affinity for several substrates was increased. However, the presence of Zn(2+) in micromolar concentrations increased the V(max) up to 24-fold and partially restored the apparent affinities. The capability of Zn(2+) to restore transport is consistent with a reversible, constitutive shift in the distribution of conformational states in the transport cycle upon mutation of Tyr-335. We propose that this shift is caused by disruption of intramolecular interactions important for stabilizing the transporter in a conformation in which extracellular substrate can bind and initiate transport, and accordingly that Tyr-335 is critical for regulating isomerization between discrete states in the transport cycle.