Chimeric dopamine-norepinephrine transporters delineate structural domains influencing selectivity for catecholamines and 1-methyl-4-phenylpyridinium

Effects of SRI-32743, a Novel Quinazoline Structure-Based Compound, on HIV-1 Tat and Cocaine Interaction with Norepinephrine Transporter

Prolonged exposure to HIV-1 transactivator of transcription (Tat) protein dysregulates monoamine transmission, a physiological change implicated as a key factor in promoting neurocognitive disorders among people living with HIV. We have demonstrated that in vivo expression of Tat in Tat transgenic mice decreases dopamine uptake through both dopamine transporter (DAT) and norepinephrine transporter (NET) in the prefrontal cortex. Further, our novel allosteric inhibitor of monoamine transporters, SRI-32743, has been shown to attenuate Tat-inhibited dopamine transport through DAT and alleviates Tat-potentiated cognitive impairments. The current study reports the pharmacological profiles of SRI-32743 in basal and Tat-induced inhibition of human NET (hNET) function. SRI-32743 exhibited less affinity for hNET binding than desipramine, a classical NET inhibitor, but displayed similar potency for inhibiting hDAT and hNET activity. SRI-32743 concentration-dependently increased hNET affinity for [3H]DA uptake but preserved the Vmax of dopamine transport. SRI-32743 slowed the cocaine-mediated dissociation of [3H]Nisoxetine binding and reduced both [3H]DA and [3H]MPP+ efflux but did not affect d-amphetamine-mediated [3H]DA release through hNET. Finally, we determined that SRI-32743 attenuated a recombinant Tat1-86-induced decrease in [3H]DA uptake via hNET. Our findings demonstrated that SRI-32743 allosterically disrupts the recombinant Tat1-86-hNET interaction, suggesting a potential treatment for HIV-infected individuals with concurrent cocaine abuse.

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.

Mutations of tyrosine 467 in the human norepinephrine transporter attenuate HIV-1 Tat-induced inhibition of dopamine transport while retaining physiological function

Dysregulation of dopaminergic transmission induced by the HIV-1 transactivator of transcription (Tat) has been implicated as a central factor in the development of HIV-1 associated neurocognitive disorders (HAND). We have demonstrated that the tyrosine470 residue of the human dopamine transporter (hDAT) plays a critical role in Tat-hDAT interaction. Based on the computational modeling predictions, the present study sought to examine the mutational effects of the tyrosine467 residue of the human norepinephrine transporter (hNET), a corresponding residue of the hDAT tyrosine470, on Tat-induced inhibition of reuptake of dopamine through the hNET. Mutations of the hNET tyrosine467 to a histidine (Y467H) or a phenylalanine (Y467F) displayed similar kinetic properties of reuptake of [3H]dopamine and [3H]norepinephrine in PC12 cells expressing wild-type hNET and its mutants. Compared to wild-type hNET, neither of Y467H or Y467F altered Bmax and Kd values of [3H]WIN35,428 binding, whereas Y467H but not Y467F decreased the Bmax of [3H]nisoxetine binding without changes in Kd. Y467H also increased the affinity of nisoxetine for inhibiting [3H]dopamine uptake relative to wild-type hNET. Recombinant Tat1-86 (140 nM) induced a significant reduction of [3H]dopamine uptake in wild-type hNET, which was attenuated in both Y467H and Y467F. Compared to wild-type hNET, neither Y467H or Y467F altered [3H]dopamine efflux in CHO cells expressing WT hNET and mutants, whereas Y467F but not Y467H decreased [3H]MPP+ efflux. These results demonstrate tyrosine467 as a functional recognition residue in the hNET for Tat-induced inhibition of dopamine transport and provide a novel insight into the molecular basis for developing selective compounds that target Tat-NET interactions in the context of HAND.

The "www" of Xenopus laevis Oocytes: The Why, When, What of Xenopus laevis Oocytes in Membrane Transporters Research

After 50 years, the heterologous expression of proteins in Xenopus laevis oocytes is still essential in many research fields. New approaches and revised protocols, but also classical methods, such as the two-electrode voltage clamp, are applied in studying membrane transporters. New and old methods for investigating the activity and the expression of Solute Carriers (SLC) are reviewed, and the kinds of experiment that are still useful to perform with this kind of cell are reported. Xenopus laevis oocytes at the full-grown stage have a highly efficient biosynthetic apparatus that correctly targets functional proteins at the defined compartment. This small protein factory can produce, fold, and localize almost any kind of wild-type or recombinant protein; some tricks are required to obtain high expression and to verify the functionality. The methodologies examined here are mainly related to research in the field of membrane transporters. This work is certainly not exhaustive; it has been carried out to be helpful to researchers who want to quickly find suggestions and detailed indications when investigating the functionality and expression of the different members of the solute carrier families.

Molecular characterization of the serotonergic transporter from the cestode Echinococcus granulosus: pharmacology and potential role in the nervous system

Echinococcus granulosus, the etiological agent of human cystic echinococcosis (formerly known as hydatid disease), represents a serious worldwide public health problem with limited treatment options. The essential role played by the neuromuscular system in parasite survival and the relevance of serotonin (5-HT) in parasite movement and development make the serotonergic system an attractive source of drug targets. In this study, we cloned and sequenced a cDNA coding for the serotonin transporter from E. granulosus (EgSERT). Bioinformatic analyses suggest that EgSERT has twelve transmembrane domains with highly conserved ligand and ionic binding sites but a less conserved allosteric site compared with the human orthologue (HsSERT). Modeling studies also suggest a good degree of conservation of the overall structure compared with HsSERT. Functional and pharmacological studies performed on the cloned EgSERT confirm that this protein is indeed a serotonin transporter. EgSERT is specific for 5-HT and does not transport other neurotransmitters. Typical monoamine transport inhibitors also displayed inhibitory activities towards EgSERT, but with lower affinity than for the human SERT (HsSERT), suggesting a high divergence of the cestode transporter compared with HsSERT. In situ hybridization studies performed in the larval protoscolex stage suggest that EgSERT is located in discrete regions that are compatible with the major ganglia of the serotonergic nervous system. The pharmacological properties, the amino acidic substitutions at important functional regions compared with the HsSERT, and the putative role of EgSERT in the nervous system suggest that it could be an important target for pharmacological intervention.

Structure Modeling of the Norepinephrine Transporter

The norepinephrine transporter (NET) is one of the monoamine transporters. Its X-ray crystal structure has not been obtained yet. Inhibitors of human NET (hNET) play a major role in the treatment of many central and peripheral nervous system diseases. In this study, we focused on the spatial structure of a NET constructed by homology modeling on Drosophila melanogaster dopamine transporter templates. We further examined molecular construction of primary binding pocket (S1) together with secondary binding site (S2) and extracellular loop 4 (EL4). The next stage involved docking of transporter inhibitors: Reboxetine, duloxetine, desipramine, and other commonly used drugs. The procedure revealed the molecular orientation of residues and disclosed ones that are the most important for ligand binding: Phenylalanine F72, aspartic acid D75, tyrosine Y152, and phenylalanine F317. Aspartic acid D75 plays a key role in recognition of the basic amino group present in monoamine transporter inhibitors and substrates. The study also presents a comparison of hNET models with other related proteins, which could provide new insights into their interaction with therapeutics and aid future development of novel bioactive compounds.

Monoamine Transporters: Basic Biology with Clinical Implications

Biogenic amine transporters mediate two important steps in the reuptake and recycling of monoamines released by neurons in the central nervous system. First, high-affinity transporters found in the plasma membrane of neurons and glial cells mediate the removal of neurotransmitter from the extracellular space, thus terminating the action of the monoamines serotonin, norepinephrine, and dopamine. Within the cell, vesicular transporters repackage monoamines into vesicles for additional cycles of release. Two gene families are involved in the transport of the biogenic amines—the Na+/Cl--dependent plasma membrane carriers and the H+-dependent vesicular amine carriers. These transporters are known to regulate neurotransmitter con centrations in monoaminergic pathways and are the primary targets for a wide variety of clinically important antidepressants, antihypertensives, stimulants, and stimulant drugs of abuse. The Neuroscientist 1:259-267, 1995

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

The norepinephrine transporter (NET) mediates reuptake of synaptically released norepinephrine in central and peripheral noradrenergic neurons. The molecular processes governing availability of NET in the plasma membrane are poorly understood. Here we use the fluorescent cocaine analogue JHC 1-64, as well as several other approaches, to investigate the trafficking itinerary of NET in live noradrenergic neurons. Confocal imaging revealed extensive constitutive internalization of JHC 1-64-labeled NET in the neuronal somata, proximal extensions and presynaptic boutons. Phorbol 12-myristate 13-acetate increased intracellular accumulation of JHC 1-64-labeled NET and caused a parallel reduction in uptake capacity. Internalized NET strongly colocalized with the "long loop" recycling marker Rab11, whereas less overlap was seen with the "short loop" recycling marker Rab4 and the late endosomal marker Rab7. Moreover, mitigating Rab11 function by overexpression of dominant negative Rab11 impaired NET function. Sorting of NET to the Rab11 recycling compartment was further supported by confocal imaging and reversible biotinylation experiments in transfected differentiated CATH.a cells. In contrast to NET, the dopamine transporter displayed markedly less constitutive internalization and limited sorting to the Rab11 recycling compartment in the differentiated CATH.a cells. Exchange of domains between the two homologous transporters revealed that this difference was determined by non-conserved structural elements in the intracellular N terminus. We conclude that NET displays a distinct trafficking itinerary characterized by continuous shuffling between the plasma membrane and the Rab11 recycling compartment and that the functional integrity of the Rab11 compartment is critical for maintaining proper presynaptic NET function.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

Binding site residues control inhibitor selectivity in the human norepinephrine transporter but not in the human dopamine transporter

The transporters for norepinephrine and dopamine (NET and DAT, respectively) constitute the molecular targets for recreational drugs and therapeutics used in the treatment of psychiatric disorders. Despite a strikingly similar amino acid sequence and predicted topology between these transporters, some inhibitors display a high degree of selectivity between NET and DAT. Here, a systematic mutational analysis of non-conserved residues within the extracellular entry pathway and the high affinity binding site in NET and DAT was performed to examine their role for selective inhibitor recognition. Changing the six diverging residues in the central binding site of NET to the complementary residues in DAT transferred a DAT-like pharmacology to NET, showing that non-conserved binding site residues in NET are critical determinants for inhibitor selectivity. In contrast, changing the equivalent residues in the central site of DAT to the corresponding residues in NET had modest effects on the same inhibitors, suggesting that non-conserved binding site residues in DAT play a minor role for selective inhibitor recognition. Our data points towards distinct structural determinants governing inhibitor selectivity in NET and DAT, and provide important new insight into the molecular basis for NET/DAT selectivity of therapeutic and recreational drugs.

APP+, a fluorescent analogue of the neurotoxin MPP+, is a marker of catecholamine neurons in brain tissue, but not a fluorescent false neurotransmitter

We have previously introduced fluorescent false neurotransmitters (FFNs) as optical reporters that enable visualization of individual dopaminergic presynaptic terminals and their activity in the brain. In this context, we examined the fluorescent pyridinium dye 4-(4-dimethylamino)phenyl-1-methylpyridinium (APP+), a fluorescent analogue of the dopaminergic neurotoxin MPP+, in acute mouse brain tissue. APP+ is a substrate for the dopamine transporter (DAT), norepinephrine transporter (NET), and serotonin transporter (SERT), and as such represented a candidate for the development of new FFN probes. Here we report that APP+ labels cell bodies of catecholaminergic neurons in the midbrain in a DAT- and NET-dependent manner, as well as fine dopaminergic axonal processes in the dorsal striatum. APP+ destaining from presynaptic terminals in the dorsal striatum was also examined under the conditions inducing depolarization and exocytotic neurotransmitter release. Application of KCl led to a small but significant degree of destaining (approximately 15% compared to control), which stands in contrast to a nearly complete destaining of the new generation FFN agent, FFN102. Electrical stimulation of brain slices at 10 Hz afforded no significant change in the APP+ signal. These results indicate that the majority of the APP+ signal in axonal processes originates from labeled organelles including mitochondria, whereas only a minor component of the APP+ signal represents the releasable synaptic vesicular pool. These results also show that APP+ may serve as a useful probe for identifying catecholaminergic innervations in the brain, although it is a poor candidate for the development of FFNs.

Secalonic acid A protects dopaminergic neurons from 1-methyl-4-phenylpyridinium (MPP⁺)-induced cell death via the mitochondrial apoptotic pathway

Secalonic acid A (SAA) is a natural compound found in marine fungi. We have reported that SAA can attenuate the cytotoxicity of colchicine in rat cortical neurons. Whether SAA can also inhibit the neurotoxicity of 1-methyl-4-phenylpyridinium (MPP(+)) in dopaminergic neurons has not been investigated. Here, we show that pretreatment with 1 μM SAA significantly rescued tyrosine hydroxylase (TH)-positive neurons from MPP(+)-induced neurotoxicity in primary dopaminergic neuron culture. Moreover, SAA at doses of 0.15 mg/kg and 0.75 mg/kg increased the number of dopaminergic neurons and upregulated striatal dopamine in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson's disease mice experiments. We also show that SAA significantly attenuated cytotoxicity induced by 2.5 mM MPP(+) in SH-SY5Y cells. These results indicate that the activation of JNK, p38 mitogen activated protein kinase (MAPK) and caspase-3 during apoptosis triggered by MPP(+) could be suppressed by SAA; on the other hand, an MPP(+)-induced increase in the expression of Bax in SH-SY5Y cells was blocked by SAA. These results indicate that inhibition of the phosphorylation of JNK and p38 MAPK, down-regulation of Bax expression, and suppression of caspase-3 activation are involved in the protective effects of SAA against MPP(+) toxicity in SH-SY5Y cells. SAA may rescue dopaminergic neurons from MPP(+)-induced cell death through the mitochondrial apoptotic pathway.

Transport of biogenic amine neurotransmitters at the mouse blood-retina and blood-brain barriers by uptake1 and uptake2

Uptake1 and uptake2 transporters are involved in the extracellular clearance of biogenic amine neurotransmitters at synaptic clefts. We looked for them at the blood-brain barrier (BBB) and blood-retina barriers (BRB), where they could be involved in regulating the neurotransmitter concentration and modulate/terminate receptor-mediated effects within the neurovascular unit (NVU). Uptake2 (Oct1-3/Slc22a1-3, Pmat/Slc29a4) and Mate1/Slc47a1 transporters are also involved in the transport of xenobiotics. We used in situ carotid perfusion of prototypic substrates like [(3)H]-1-methyl-4-phenylpyridinium ([(3)H]-MPP(+)), [(3)H]-histamine, [(3)H]-serotonin, and [(3)H]-dopamine, changes in ionic composition and genetic deletion of Oct1-3 carriers to detect uptake1 and uptake2 at the BBB and BRB. We showed that uptake1 and uptake2 are involved in the transport of [(3)H]-dopamine and [(3)H]-MPP(+) at the blood luminal BRB, but not at the BBB. These functional studies, together with quantitative RT-PCR and confocal imaging, suggest that the mouse BBB lacks uptake1 (Net/Slc6a2, Dat/Slc6a3, Sert/Slc6a4), uptake2, and Mate1 on both the luminal and abluminal sides. However, we found evidence for functional Net and Oct1 transporters at the luminal BRB. These heterogeneous transport properties of the brain and retina NVUs suggest that the BBB helps protect the brain against biogenic amine neurotransmitters in the plasma while the BRB has more of a metabolic/endocrine role.

Novel C-1 substituted cocaine analogs unlike cocaine or benztropine

Despite a wealth of information on cocaine-like compounds, there is no information on cocaine analogs with substitutions at C-1. Here, we report on (R)-(-)-cocaine analogs with various C-1 substituents: methyl (2), ethyl (3), n-propyl (4), n-pentyl (5), and phenyl (6). Analog 2 was equipotent to cocaine as an inhibitor of the dopamine transporter (DAT), whereas 3 and 6 were 3- and 10-fold more potent, respectively. None of the analogs, however, stimulated mouse locomotor activity, in contrast to cocaine. Pharmacokinetic assays showed compound 2 occupied mouse brain rapidly, as cocaine itself; moreover, 2 and 6 were behaviorally active in mice in the forced-swim test model of depression and the conditioned place preference test. Analog 2 was a weaker inhibitor of voltage-dependent Na+ channels than cocaine, although 6 was more potent than cocaine, highlighting the need to assay future C-1 analogs for this activity. Receptorome screening indicated few significant binding targets other than the monoamine transporters. Benztropine-like "atypical" DAT inhibitors are known to display reduced cocaine-like locomotor stimulation, presumably by their propensity to interact with an inward-facing transporter conformation. However, 2 and 6, like cocaine, but unlike benztropine, exhibited preferential interaction with an outward-facing conformation upon docking in our DAT homology model. In summary, C-1 cocaine analogs are not cocaine-like in that they are not stimulatory in vivo. However, they are not benztropine-like in binding mechanism and seem to interact with the DAT similarly to cocaine. The present data warrant further consideration of these novel cocaine analogs for antidepressant or cocaine substitution potential.

A second extracellular site is required for norepinephrine transport by the human norepinephrine transporter

The human norepinephrine transporter (NET) is implicated in many neurological disorders and is a target of tricyclic antidepressants and nisoxetine (NX). We used molecular docking simulations to guide the identification of residues likely to affect substrate transport and ligand interactions at NET. Mutations to alanine identified a hydrophobic pocket in the extracellular cavity of NET, comprising residues Thr80, Phe317, and Tyr317, which was critical for efficient norepinephrine (NE) transport. This secondary NE substrate site (NESS-2) overlapped the NX binding site, comprising Tyr84, Phe317, and Tyr317, and was positioned ∼11 Å extracellular to the primary site for NE (NESS-1). Thr80 in NESS-2 appeared to be critical in positioning NE for efficient translocation to NESS-1. Three residues identified as being involved in gating the reverse transport of NE (Arg81, Gln314, and Asp473) did not affect NE affinity for NESS-1. Mutating residues adjacent to NESS-2 abolished NET expression (D75A and L76A) or appeared to affect NET folding (S419A), suggesting important roles in stabilizing NET structure, whereas W308A and F388A at the top of NESS-2 abolished both NE transport and NX binding. Our findings are consistent with a multistep model of substrate transport by NET, for which a second, shallow extracellular NE substrate site (NESS-2) is required for efficient NE transport by NET.

Structural exploration of (3S,6S)-6-benzhydryl-N-benzyltetrahydro-2H-pyran-3-amine analogues: identification of potent triple monoamine reuptake inhibitors as potential antidepressants

To further explore the basic structural motifs (3S,6S)-6-benzhydryl-N-benzyltetrahydro-2H-pyran-3-amine and (2S,4R,5R)-2-benzhydryl-5-(benzylamino)tetrahydro-2H-pyran-4-ol, developed by our research group, for monoamine transport inhibition, we designed and synthesized various structurally altered analogues. The new compounds were tested for their affinities for the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET) in rat brain by measuring their capacity to inhibit the uptake of [(3)H]DA, [(3)H]5-HT, and [(3)H]NE, respectively. Our results point to novel compounds with a TUI, DNRI, SNRI, or SSRI profile. Among the TUIs, compound 2 g exhibited a balanced potency for all three monoamine transporters (K(i): 60, 79, and 70.3 nM for DAT, SERT, and NET, respectively). In the rat forced swim test, compound 2 g produced a significant decrease in immobility in drug-treated rats relative to vehicle, indicating a potential antidepressant property.

4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP+) is a fluorescent substrate for the human serotonin transporter

Monoamine transporters terminate synaptic neurotransmission and are molecular targets for antidepressants and psychostimulants. Fluorescent reporters can monitor real-time transport and are amenable for high-throughput screening. However, until now, their use has mostly been successful to study the catecholamine transporters but not the serotonin (5HT) transporter. Here, we use fluorescence microscopy, electrophysiology, pharmacology, and molecular modeling to compare fluorescent analogs of 1-methyl-4-phenylpyridinium (MPP(+)) as reporters for the human serotonin transporter (hSERT) in single cells. The fluorescent substrate 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) exhibits superior fluorescence uptake in hSERT-expressing HEK293 cells than other MPP(+) analogs tested. APP(+) uptake is Na(+)- and Cl(-)-dependent, displaced by 5HT, and inhibited by fluoxetine, suggesting APP(+) specifically monitors hSERT activity. ASP(+), which was previously used to study catecholamine transporters, is 10 times less potent than APP(+) at inhibiting 5HT uptake and has minimal hSERT-mediated uptake. Furthermore, in hSERT-expressing oocytes voltage-clamped to -60 mV, APP(+) induced fluoxetine-sensitive hSERT-mediated inward currents, indicating APP(+) is a substrate, whereas ASP(+) induced hSERT-mediated outward currents and counteracted 5HT-induced hSERT currents, indicating ASP(+) possesses activity as an inhibitor. Extra-precise ligand receptor docking of APP(+) and ASP(+) in an hSERT homology model showed both ASP(+) and APP(+) docked favorably within the active region; accordingly, comparable concentrations are required to elicit their opposite electrophysiological responses. We conclude APP(+) is better suited than ASP(+) to study hSERT transport fluorometrically.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

Proteins interacting with monoamine transporters: current state and future challenges

Plasma membrane and vesicular transporters for the biogenic amines, dopamine, norepinephrine, and serotonin, represent a group of proteins that play a crucial role in the regulation of neurotransmission. Clinically, mono amine transporters are the primary targets for the actions of many therapeutic agents used to treat mood disorders, as well as the site of action for highly addictive psychostimulants such as cocaine, amphetamine, methamphetamine, and 3,4-methylenedioxymethamphetamine. Over the past decade, the use of approaches such as yeast two-hybrid and proteomics has identified a multitude of transporter interacting proteins, suggesting that the function and regulation of these transporters are more complex than previously anticipated. With the increasing number of interacting proteins, the rules dictating transporter synthesis, assembly, targeting, trafficking, and function are beginning to be deciphered. Although many of these protein interactions have yet to be fully characterized, current knowledge is beginning to shed light on novel transporter mechanisms involved in monoamine homeostasis, the molecular actions of psychostimulants, and potential disease mechanisms. While future studies resolving the spatial and temporal resolution of these, and yet unknown, interactions will be needed, the realization that monoamine transporters do not work alone opens the path to a plethora of possible pharmacological interventions.

Molecular determinants for selective recognition of antidepressants in the human serotonin and norepinephrine transporters

Inhibitors of the serotonin transporter (SERT) and norepinephrine transporter (NET) are widely used in the treatment of major depressive disorder. Although SERT/NET selectivity is a key determinant for the therapeutic properties of these drugs, the molecular determinants defining SERT/NET selectivity are poorly understood. In this study, the structural basis for selectivity of the SERT selective inhibitor citalopram and the structurally closely related NET selective inhibitor talopram is delineated. A systematic structure-activity relationship study allowed identification of the substituents that control activity and selectivity toward SERT and NET and revealed a common pattern showing that SERT and NET have opposite preference for the stereochemical configuration of these inhibitors. Mutational analysis of nonconserved SERT/NET residues within the central substrate binding site was performed to determine the molecular basis for inhibitor selectivity. Changing only five residues in NET to the complementary residues in SERT transferred a SERT-like affinity profile for R- and S-citalopram into NET, showing that the selectivity of these compounds is determined by amino acid differences in the central binding site of the transporters. In contrast, the activity of R- and S-talopram was largely unaffected by any mutations within the central substrate binding site of SERT and NET and in the outer vestibule of NET, suggesting that citalopram and talopram bind to distinct sites on SERT and NET. Together, these findings provide important insight into the molecular basis for SERT/NET selectivity of antidepressants, which can be used to guide rational development of unique transporter inhibitors with fine-tuned transporter selectivity.

Similar potency of catechin and its enantiomers in alleviating 1-methyl-4-phenylpyridinium ion cytotoxicity in SH-SY5Y cells

OBJECTIVES

Previously, the flavonoid (±)-catechin was shown to exert potent neuroprotective action in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's disease model. The purpose of this study was to investigate whether the different enantiomers of catechin ((+)-catechin, (-)-catechin and (±)-catechin, a 50:50 mixture of (+)-catechin and (-)-catechin) could protect SH-SY5Y cells against 1-methyl-4-phenylpyridinium ion (MPP(+) ) toxicity by decreasing the generation of oxygen free radicals. The inhibitive effect of (±)-catechin on JNK/c-Jun activation was investigated.

METHODS

The effects of (+)-catechin, (-)-catechin or (±)-catechin in protecting against MPP(+) toxicity were evaluated and compared in SH-SY5Y cells by testing the release of lactate dehydrogenase. The generation of reactive oxygen species (ROS) was measured by immunochemistry and the phosphorylation level of JNK/c-Jun was determined by Western blotting.

KEY FINDINGS

In SH-SY5Y cells, (+)-catechin, (-)-catechin or (±)-catechin reduced apoptosis induced by MPP(+) and decreased ROS generation caused by MPP(+) . Different enantiomers of catechin showed protective effects at similar potency. Moreover (±)-catechin decreased JNK/c-Jun phosphorylation which was increased by MPP(+).

CONCLUSIONS

  Catechin and its two enantiomers could protect SH-SY5Y cells against MPP(+) cytotoxicity at a similar potency. Antioxidative stress and inhibition of the JNK/c-Jun signalling pathway might have been involved in the neuroprotective mechanisms of catechin against MPP(+) cytotoxicity in SH-SY5Y cells.

Functional characterization of rat plasma membrane monoamine transporter in the blood-brain and blood-cerebrospinal fluid barriers

This study investigated the expression and functional roles of rat plasma membrane monoamine transporter (rPMAT) in the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier by using in vitro brain barrier model cells (TR-BBB13 and TR-CSFB3 cells) and multiple in vivo experimental techniques. Quantitative reverse transcription-polymerase chain reaction analysis showed relatively high expression of rPMAT mRNA in TR-BBB13 and TR-CSFB3 cells. 1-Methyl-4-phenylpyridinium (MPP(+) ) was transported into rPMAT-expressing cells in a sodium-independent manner. [(3) H]MPP(+) was taken up concentration dependently by TR-BBB13 and TR-CSFB3 cells with K(m) values similar to that of rPMAT-expressing cells. [(3) H]MPP(+) transports into these cells were markedly inhibited by serotonin, dopamine, and cationic drugs. rPMAT small interfering RNA (siRNA) significantly suppressed the [(3) H]MPP(+) uptake by TR-BBB13 cells. Intracerebrally injected [(3) H]MPP(+) was eliminated from the brain parenchymal region, whereas brain [(3) H]MPP(+) uptake did not increase with time during in situ brain perfusion, suggesting that the brain-to-blood transport across the BBB predominates over the blood-to-brain transport. Brain microdialysis studies revealed that the elimination across the BBB was significantly decreased by coperfusion of unlabelled MPP(+) , serotonin, or dopamine. [(3) H]MPP(+) was also eliminated from the CSF. These findings suggest that PMAT in brain barriers functions as the brain-to-blood transporter to regulate brain concentrations of organic cations including monoamines and cationic neurotoxins.

Site-directed mutagenesis in the study of membrane transporters

One of the major goals in membrane transporter research is to understand how transporter proteins work at the molecular level. Ideally, this research would be carried out with a detailed knowledge of the three-dimensional structure of the protein. However, in the absence of atomic resolution structures for many membrane transporters other molecular tools need to be employed. In vitro site-directed mutagenesis is one method that has the capacity to provide both structural information and identification of the role of individual residues and/or regions of a protein that are involved in function.

Recent advances in the understanding of the interaction of antidepressant drugs with serotonin and norepinephrine transporters

The biogenic monoamine transporters are integral membrane proteins that perform active transport of extracellular dopamine, serotonin and norepinephrine into cells. These transporters are targets for therapeutic agents such as antidepressants, as well as addictive substances such as cocaine and amphetamine. Seminal advances in the understanding of the structure and function of this transporter family have recently been accomplished by structural studies of a bacterial transporter, as well as medicinal chemistry and pharmacological studies of mammalian transporters. This feature article focuses on antidepressant drugs that act on the serotonin and/or the norepinephrine transporters. Specifically, we focus on structure-activity relationships of these drugs with emphasis on relationships between their molecular properties and the current knowledge of transporter structure.

Activation of NF-kappaB p65/c-Rel dimer is associated with neuroprotection elicited by mGlu5 receptor agonists against MPP(+) toxicity in SK-N-SH cells

Nuclear factor-kappaB (NF-kappaB) is a transcriptional regulator of neuron survival eliciting diverse effects according to the specific composition of its active dimer. While p50/p65 mediates neurodegenerative events, c-Rel-containing dimers promote cell survival. Stimulation of metabotropic glutamate receptors type 5 (mGlu5) reduces neuron vulnerability to amyloid-beta through activation of anti-apoptotic, c-Rel-dependent transcription of Bcl-X(L) pathway. We here evaluated the protective activity of mGlu5 agonists in dopaminergic SK-N-SH cells exposed to 1-methyl-4-phenylpyridinium (MPP(+)), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causing parkinsonism in experimental animals. MPP(+) produced a concentration-dependent cell loss. Activation of mGlu5 receptors by CHPG (1 mM) and 3HPG (50 microM) abolished the toxic effect produced by 3 microM MPP(+). The neuroprotection was associated with activation of NF-kappaB p65/c-Rel dimer and reduction of p50/p65. These effects were prevented by the mGlu5 receptor antagonist MPEP (5 microM). It is suggested that mGlu5 receptor agonists through activation of a c-Rel-dependent anti-apoptotic pathway can rescue dopaminergic cell from mitochondrial toxicity.

Dopamine modulates mitochondrial function in viable SH-SY5Y cells possibly via its interaction with complex I: relevance to dopamine pathology in schizophrenia

Deleterious effects of dopamine (DA) involving mitochondrial dysfunction have an important role in DA-associated neuronal disorders, including schizophrenia and Parkinson's disease. DA detrimental effects have been attributed to its ability to be auto-oxidized to toxic reactive oxygen species. Since, unlike Parkinson's disease, schizophrenia does not involve neurodegenerative processes, we suggest a novel mechanism by which DA impairs mitochondrial function without affecting cell viability. DA significantly dissipated mitochondrial membrane potential (delta psi m) in SH-SY5Y cells. Bypassing complex I prevented the DA-induced depolarization. Moreover, DA inhibited complex I but not complex II activity in disrupted mitochondria, suggesting complex I participation in DA-induced mitochondrial dysfunction. We further demonstrated that intact mitochondria can accumulate DA in a saturated manner, with an apparent Km=122.1+/-28.6 nM and Vmax=1.41+/-0.15 pmol/mg protein/min, thereby enabling the interaction between DA and complex I. DA accumulation was an energy and Na+-dependent process. The pharmacological profile of mitochondrial DA uptake differed from that of other characterized DA transporters. Finally, relevance to schizophrenia is demonstrated by an abnormal interaction between DA and complex I in schizophrenic patients. These results suggest a non-lethal interaction between DA and mitochondria possibly via complex I, which can better explain DA-related pathological processes observed in non-degenerative disorders, such as schizophrenia.

Cytotoxicity of β-carbolines in dopamine transporter expressing cells: Structure–activity relationships

Some beta-carbolines (BC) are natural constituents in the human brain deriving from tryptophan, tryptamine, and serotonin. In vitro and animal experiments suggest that BC-cations may cause neurodegeneration with a higher vulnerability of dopaminergic than of other neurons. Despite the possible implication of the BC-cations in the pathogenesis of Parkinson's disease (PD), the underlying mechanisms are poorly understood. The present study further explores the structural requirements for the cytotoxic effects of BCs and searches for additional compounds involved in the pathogenesis of PD. Previous studies were now extended to serotonin-derived BCs, tetrahydro-BCs, a BC-dimer, and a BC-enantiomer to reveal possible stereoselectivity. Neutral, rather lipophilic BCs may pass the plasma membrane and the outer and inner mitochondrial membranes by diffusion whereas the cationic, more polar compounds, can be transported by the dopamine transporter (DAT). In the present study, 4 out of 17 BC-cations caused DAT-independent toxicity. This number is unexpected in view of previous findings that all BC-cations are transported by DAT. 3-Carboxylated and 6-methoxylated BCs were poor substrates. The size alone does not seem to be a limiting factor. A dimeric BC-cation was readily transported by the DAT despite its much larger structure compared to dopamine. Furthermore, (R)-enantiomers were preferentially transported. The neutral BCs were approximately one order of magnitude less toxic than the cationic BCs. There are considerable differences of the transport efficiency between the BCs. Potent cytotoxic tetrahydro-BCs were detected. Because precursor tetrahydro-BCs are present in the brain, the search for the occurrence of these compounds in human brain is warranted.

Molecular cloning and functional expression of the murine noradrenaline transporter

The cDNA of the murine noradrenaline transporter (mNAT) was cloned from the RNA of the placenta of a C57BL/6 mouse. The cloned mNAT differs from a previously published sequence in two amino acids within the C-terminal region. A cDNA obtained from an inbred mouse strain showed a further amino acid exchange (Ile(505)Val) within the fifth intracellular loop. The pharmacological properties of both, the wild-type mNAT and the variant (mNAT-I(505)V), were studied in human embryonic kidney HEK293 cells transfected with the corresponding cDNA. The kinetic constants for transport (K (m), V (max)) of [(3)H]noradrenaline ([(3)H]-NA) and binding (K (D), B (max)) of the selective NAT inhibitor [(3)H]nisoxetine were not different between the two isoforms; the mean kinetic constants amounted to about 4 microM and 120pmol/mg protein for K (m) and V (max) and 6nM and 18pmol/mg protein for K (D) and B (max), respectively. [(3)H]-NA transport by both isoforms showed the typical properties of an NAT because it was dependent on sodium and chloride and inhibited with almost identical K (i) values by various NAT substrates and inhibitors. The only significant pharmacological difference identified between the two mNAT isoforms was an about threefold higher affinity for cocaine of the very rare mNAT-I(505)V variant.

Thyronamines inhibit plasma membrane and vesicular monoamine transport

Thyroid hormone has long been known to have important transcriptional regulatory activities. Recently, however, the presence of endogenous derivatives of thyroid hormone, thyronamines, has been reported in various mammalian tissues. These derivatives have potent in vitro activity with a class of orphan G-protein-coupled receptors, the trace amine-associated receptors, and profound in vivo effects when administered to mice. We report here a novel neuromodulatory role for thyronamines. In synaptosomal preparations and heterologous expression systems, thyronamines act as specific dopamine and norepinephrine reuptake inhibitors. Thyronamines also inhibit the transport of monoamines into synaptic vesicles. These observations expand the nontranscriptional role of thyroid hormone derivatives and may help to explain the pharmacological effects of thyronamines in vivo.

Identification of essential loops and residues of glucosyltransferase V (GtrV) of Shigella flexneri

Lipopolysaccharide (LPS), particularly the O-antigen component, is one of many virulence determinants necessary for Shigella flexneri pathogenesis. O-antigen modification is mediated by glucosyltransferase (gtr) genes encoded by temperate serotype-converting bacteriophages. The gtrV and gtrX genes encode the GtrV and GtrX glucosyltransferases, respectively. These are integral membrane proteins, which catalyze the transfer of a glucosyl residue via an alpha1,3 linkage to rhamnose II and rhamnose I of the O-antigen unit. This mediates conversion of S. flexneri serotype Y to serotype 5a and X, respectively. Essential regions in the topology of GtrV protein were identified by in vivo recombination and a PCR-mediated approach. A series of GtrX-GtrV and GtrV-GtrX chimeric proteins were constructed based on the fact that GtrV and GtrX share sequence similarity. Analysis of their respective serotype conversion abilities led to the identification of two important periplasmic loops: loops No 2 and No 10 located in the N- and C-termini, respectively. Within these two loops, three conserved motifs were identified; two in loop No 2 and one in loop No 10. These conserved motifs contain acidic residues which were shown to be critical for GtrV function.

The norepinephrine transporter in physiology and disease

The norepinephrine transporter (NET) terminates noradrenergic signalling by rapid re-uptake of neuronally released norepinephrine (NE) into presynaptic terminals. NET exerts a fine regulated control over NE-mediated behavioural and physiological effects including mood, depression, feeding behaviour, cognition, regulation of blood pressure and heart rate. NET is a target of several drugs which are therapeutically used in the treatment or diagnosis of disorders among which depression, attention-deficit hyperactivity disorder and feeding disturbances are the most common. Individual genetic variations in the gene encoding the human NET (hNET), located at chromosome 16q12.2, may contribute to the pathogenesis of those diseases. An increasing number of studies concerning the identification of single nucleotide polymorphisms in the hNET gene and their potential association with disease as well as the functional investigation of naturally occurring or induced amino acid variations in hNET have contributed to a better understanding of NET function, regulation and genetic contribution to disorders. This review will reflect the current knowledge in the field of NET from its initial discovery until now.

Cerebellar neurotransmission in attention-deficit/hyperactivity disorder: Does dopamine neurotransmission occur in the cerebellar vermis?

Children and adolescents with attention-deficit/hyperactivity disorder (ADHD) have smaller cerebellar volumes, particularly in the posterior-inferior cerebellar vermis (lobules VIII-X). Functional activation of the human cerebellar vermis following stimulant administration has also been repeatedly demonstrated. There is no well-characterized dopaminergic pathway that projects to the posterior-inferior cerebellar vermis, although the dopamine transporter (DAT) and tyrosine hydroxylase (TH) have been localized in the posterior-inferior vermis in the non-human primate by immunohistochemistry. We hypothesized that DA neurotransmission may occur in localized "hot spots" in the cerebellar vermis, and if so, that differences in such neurotransmission might be relevant to the pathophysiology of ADHD. To investigate this hypothesis, cerebellar tissue was obtained from rats and non-human primates. Catecholamines were extracted and analyzed using HPLC with coulometric detection. A regional gradient of norepinephrine (NE) and DA was found throughout the cerebellum with NE levels always roughly 10-40-fold higher than DA in both rats and monkeys. In addition, in vivo microdialysis studies were performed in the rat posterior-inferior cerebellar vermis in anesthetized animals. Significant NE overflow was observed over baseline following reverse microdialysis induced release by potassium or d-amphetamine. DA overflow was not observed over baseline for potassium stimulation, but was significant for d-amphetamine stimulation. These studies refute the hypothesis that DA neurotransmission normally occurs in the rat cerebellar vermis, but highlight that vermal DA is released by d-amphetamine. The presence of DAT may therefore allow for enhanced regulation of NE and not regulation of released DA.

Dopamine transporter-mediated cytotoxicity of 6-hydroxydopamine in vitro depends on expression of mutant alpha-synucleins related to Parkinson's disease

6-Hydroxydopamine (6-OHDA) is widely used to produce animal models of Parkinson's disease (PD) by selectively destroying the nigro-striatal dopaminergic systems, but selective toxicity of 6-OHDA towards dopaminergic cells in vitro remains controversial. Mutant (A30P and A53T) alpha-synuclein isoforms cause increased vulnerability of cells towards various toxic insults and enhance dopamine transporter (DAT)-mediated toxicity of the selective dopaminergic neurotoxin and mitochondrial complex I inhibitor MPP(+) in vitro. Here we extend our recent studies on DAT-mediated toxicity to elucidate the mechanisms involved in selective dopaminergic toxicity of 6-OHDA. We studied the cytotoxicity as well as the toxic mechanisms of 6-OHDA in human embryonic kidney HEK-293 cells ectopically co-expressing mutant alpha-synucleins and the human DAT protein. 6-OHDA showed half-maximal toxic concentration (TC(50)) of 88 microM in HEK-hDAT cells without alpha-synuclein expression after 24 h, whereas the TC(50) values significantly decreased to 58 and 39 microM by expression of A30P and A53T alpha-synuclein, respectively. alpha-Synuclein expression did not affect 6-OHDA toxicity in HEK-293 cells not expressing the DAT. Analysis of intracellular parameters of cellular energy metabolism revealed that the co-expression of mutant alpha-synucleins in HEK-hDAT cells accelerates the reduction of intracellular net ATP levels and ATP/ADP ratios induced by 6-OHDA. Uptake function of the DAT was not altered by expression of alpha-synuclein isoforms. Our data suggest a mechanism of 6-OHDA-induced dopaminergic toxicity involving an interaction of mutant alpha-synucleins with the DAT molecule and subsequent acceleration of cellular energy depletion that might be relevant for the pathogenesis of PD.

ADHD and the dopamine transporter: are there reasons to pay attention?

The catecholamine dopamine (DA) plays an important role as a neurotransmitter in the brain in circuits linked to motor function, reward, and cognition. The presynaptic DA transporter (DAT) inactivates DA following release and provides a route for non-exocytotic DA release (efflux) triggered by amphetamines. The synaptic role of DATs first established through antagonist studies and more recently validated through mouse gene-knockout experiments, raises questions as to whether altered DAT structure or regulation support clinical disorders linked to compromised DA signaling, including drug abuse, schizophrenia, and attention deficit hyperactivity disorder (ADHD). As ADHD appears to have highly heritable components and the most commonly prescribed therapeutics for ADHD target DAT, studies ranging from brain imaging to genomic and genetic analyses have begun to probe the DAT gene and its protein for possible contributions to the disorder and/or its treatment. In this review, after a brief overview of ADHD prevalence and diagnostic criteria, we examine the rationale and experimental findings surrounding a role for human DAT in ADHD. Based on the available evidence from our lab and labs of workers in the field, we suggest that although a common variant within the human DAT (hDAT) gene (SLC6A3) is unlikely to play a major role in the ADHD, contributions of hDAT to risk maybe most evident in phenotypic subgroups. The in vitro and in vivo validation of functional variants, pursued for contributions to endophenotypes in a within family approach, may help elucidate DAT and DA contributions to ADHD and its treatment.

Dysregulation of dopamine signaling in the dorsal striatum inhibits feeding

Dopamine signaling is an important component of many goal-directed behaviors, such as feeding. Acute disruption of dopamine signaling using pharmacological agents tends to inhibit normal feeding behaviors in rodents. Likewise, genetically engineered dopamine-deficient (DD) mice are unable to initiate sufficient feeding and will starve by approximately 3 weeks of age if untreated. Adequate feeding by DD mice can be achieved by daily administration of L-3,4-dihydroxyphenylalanine (L-dopa), a precursor of dopamine, which can be taken up by dopaminergic neurons, converted to dopamine, and released in a regulated manner. In contrast, adequate feeding cannot be restored with apomorphine (APO), a mixed agonist that activates D1 and D2 receptors. Viral restoration of dopamine production in neurons that project to the dorsal striatum also restores feeding in DD mice. Administration of amphetamine (AMPH) or nomifensine (NOM), drugs which increase synaptic dopamine concentration, inhibits food intake in virally rescued DD mice (vrDD) as in control animals. These results indicate that the dysregulation of dopamine signaling in the dorsal striatum is sufficient to induce hypophagia and suggest that regulated release of dopamine in that brain region is essential for normal feeding and, probably, many other goal-directed behaviors.

Differences in interactions with the dopamine transporter as revealed by diminishment of Na+ gradient and membrane potential: Dopamine versus other substrates

In heterologous cells expressing the dopamine transporter (DAT), simultaneous elevation of intracellular Na(+) and depolarization of the membrane with gramicidin reduced the potency of various DAT substrates, including dopamine, d-amphetamine, beta-phenethylamine, p-tyramine, and MPP(+), in inhibiting binding of the cocaine analog [(3)H]CFT, with the greatest reduction observed for d-amphetamine. In rat striatal synaptosomes, gramicidin exerted similar effects; in addition, the potency of d-amphetamine was reduced by the Na(+)-channel activator veratridine. The latter effect was counteracted by the Na(+)-channel blocker tetrodotoxin. In broken membranes, where, as the situation with gramicidin, both sides of the non-polarized membrane were exposed to 130 mM Na(+), gramicidin was ineffective. Dopamine had a potency for membrane preparations that was not significantly different from that for control cells or synaptosomes, while other substrates had potencies for membrane preparations that were reduced to a level similar to those observed in gramicidin-treated cells or synaptosomes. These results suggest that diminishing Na(+) gradient and membrane potential may convert DAT to a conformational state that dopamine could easily bind to when gaining free access to its intracellular portion. In contrast, non-dopamine substrates may not be able to readily interact with this state from either side of the membrane.

Quantitative 1H magnetic resonance spectroscopic imaging determines therapeutic immunization efficacy in an animal model of Parkinson's disease

Nigrostriatal degeneration, the pathological hallmark of Parkinson's disease (PD), is mirrored by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. MPTP-treated animals show the common behavioral, motor, and pathological features of human disease. We demonstrated previously that adoptive transfer of Copaxone (Cop-1) immune cells protected the nigrostriatal dopaminergic pathway in MPTP-intoxicated mice. Herein, we evaluated this protection by quantitative proton magnetic resonance spectroscopic imaging (1H MRSI). 1H MRSI performed in MPTP-treated mice demonstrated that N-acetyl aspartate (NAA) was significantly diminished in the substantia nigra pars compacta (SNpc) and striatum, regions most affected in human disease. When the same regions were coregistered with immunohistochemical stains for tyrosine hydroxylase, numbers of neuronal bodies and termini were similarly diminished. MPTP-intoxicated animals that received Cop-1 immune cells showed NAA levels, in the SNpc and striatum, nearly equivalent to PBS-treated animals. Moreover, adoptive transfer of immune cells from ovalbumin-immunized to MPTP-treated mice failed to alter NAA levels or protect dopaminergic neurons and their projections. These results demonstrate that 1H MRSI can evaluate dopaminergic degeneration and its protection by Cop-1 immunization strategies. Most importantly, the results provide a monitoring system to assess therapeutic outcomes for PD.

Effects of zotepine and olanzapine on noradrenaline transporter in cultured bovine adrenal medullary cells

BACKGROUND

Previously, it was demonstrated that the inhibitory effects of atypical antipsychotic drugs such as clozapine and risperidone on noradrenaline transporter (NAT) might in part be associated with their clinical profile. The present study examined the effects of zotepine on NAT in the cells and compared them with those of olanzapine.

MATERIALS AND METHODS

Adrenal medullary cells were isolated by a method of collagenase digestion of slices of fresh bovine adrenal medulla and the cells were plated at a density of 4 x 10(6) cells. Cells were incubated with [3H]noradrenaline (NA) in the presence or absence of zotepine or olanzapine. The amount of radioactivity taken into the cells was counted by a liquid scintillation counter. Plasma membranes of bovine adrenal medulla were prepared, and the binding of [3H]desipramine (DMI) was determined by incubating the membrane suspension in binding buffer together with zotepine or olanzapine. Specific binding of [3H] DMI was defined as that binding which was inhibited by nisoxetine.

RESULTS

Both zotepine (10-1000 ng/ml) and olanzapine (10-1000 ng/ml) decreased [3H]NA uptake in a concentration-dependent manner. The IC50 values of zotepine and olanzapine on [3H]NA uptake were 10 +/- 4 and 14 +/- 8 ng/ml, respectively. Eadie-Hofstee analysis of [3H]NA uptake showed that treatment with zotepine and olanzapine decreased the V(max) of uptake without changing the K(m). Both zotepine (10-1000 ng/ml) and olanzapine (30-1000 ng/ml) inhibited [3H]DMI binding in a concentration-dependent manner. The IC50 values of zotepine and olanzapine on [3H]DMI binding were 50 +/- 18, and 120 +/- 38 ng/ml, respectively. Scatchard plot analysis of [3H]DMI binding showed that zotepine and olanzapine decreased the B(max) of binding without altering the K(d).

CONCLUSIONS

The inhibitory effects of zotepine and olanzapine might be responsible in part for their clinical profile.

Proline mutations induce negative-dosage effects on uptake velocity of the dopamine transporter

Ala and Gly substitutions for Pro 101 (P101) located in transmembrane domain 2 of the dopamine transporter (DAT) abolished transport activity but did not disrupt plasma membrane expression. Due to the high conservation of P101 in all neurotransmitter transporters and the capability of Pro to add flexibility to helices, we hypothesized that P101 contributes to the dynamic feature of substrate translocation. To test this hypothesis, here we analysed transport activity for DAT mutants where this Pro was mutated into different amino acids, including Ser, Val, Leu and Phe. The transmembrane domain 2 helix of P101F, unlike the other mutants, was computationally predicted to have a Van der Waals energy threefold higher than the wild-type helix. P101F mutant expression was consistently disrupted in COS cells. Among all the other mutants that express normally, P101V, with a side-chain size close to that of Pro, restores the transport activity of P101A by sevenfold. Most importantly, P101V, P101L and P101S display negative-dosage effects on dopamine (DA) transport, i.e. the velocity-concentration curve for DA uptake does not show a plateau with increasing [DA] but rather peaks and then goes down. These data support the view that P101 of DAT plays an essential role in DA translocation.

Single nucleotide polymorphisms in the human norepinephrine transporter gene affect expression, trafficking, antidepressant interaction, and protein kinase C regulation

The role of norepinephrine (NE) in attention, memory, affect, stress, heart rate, and blood pressure implicates NE in psychiatric and cardiovascular disease. The norepinephrine transporter (NET) mediates reuptake of released catecholamines, thus playing a role in the limitation of signaling strength in the central and peripheral nervous systems. Nonsynonymous single nucleotide polymorphisms (SNPs) in the human NET (hNET) gene that influence transporter function can contribute to disease, such as the nonfunctional transporter, A457P, identified in orthostatic intolerance. Here, we examine additional amino acid variants that have been identified but not characterized in populations that include cardiovascular phenotypes. Variant hNETs were expressed in COS-7 cells and were assayed for protein expression and trafficking using cell-surface biotinylation and Western blot analysis, transport of radiolabeled substrate, antagonist interaction, and regulation through protein kinase C (PKC)-linked pathways by the phorbol ester beta-phorbol-12-myristate-13-acetate. We observed functional perturbations in 6 of the 10 mutants studied. Several variants were defective in trafficking and transport, with the most dramatic effect observed for A369P, which was completely devoid of the fully glycosylated form of transporter protein, was retained intracellularly, and lacked any transport activity. Furthermore, A369P and another trafficking variant, N292T, impeded surface expression of hNET when coexpressed. F528C demonstrated increased transport and, remarkably, exhibited both insensitivity to down-regulation by PKC and a decrease in potency for the tricyclic antidepressant desipramine. These findings reveal functional deficits that are likely to compromise NE signaling in SNP carriers in the population and identify key regions of NET contributing to transporter biosynthesis, activity, and regulation.

A genetic screen in Caenorhabditis elegans for dopamine neuron insensitivity to 6-hydroxydopamine identifies dopamine transporter mutants impacting transporter biosynthesis and trafficking

The presynaptic dopamine (DA) transporter (DAT) is a major determinant of synaptic DA inactivation, an important target for psychostimulants including cocaine and amphetamine, and a mediator of DA neuron vulnerability to the neurotoxins 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium ion. To exploit genetic approaches for the study of DATs and neural degeneration, we exploited the visibility of green fluorescent protein (GFP)-tagged DA neurons in transgenic nematodes to implement a forward genetic screen for suppressors of 6-OHDA sensitivity. In our initial effort, we identified three novel dat-1 alleles conferring 6-OHDA resistance. Two of the dat-1 alleles derive from point mutations in conserved glycine residues (G55, G90) in contiguous DAT-1 transmembrane domains (TM1 and TM2, respectively), whereas the third allele results in altered translation of the transporter's COOH terminus. Our studies reveal biosynthetic, trafficking and functional defects in the DAT-1 mutants, exhibited both in vitro and in vivo. These studies validate a forward genetic approach to the isolation of DA neuron-specific toxin suppressors and point to critical contributions of the mutated residues, as well as elements of the DAT-1 COOH terminus, to functional expression of catecholamine transporters in neurons.

sgk1, a member of an RNA cluster associated with cell death in a model of Parkinson's disease

In an effort to gain deeper insight into the molecular processes underlying neurodegeneration in Parkinson's disease, we performed gene expression profiling at several early time points after MPTP-injection into old (1-year) mice. We used a PCR-based gene expression profiling method, digital expression pattern display (DEPD), a method of very high sensitivity and reproducibility, which displays almost all transcripts of a tissue. To identify cell death-associated genes, we defined clusters of differentially expressed transcripts with expression behaviour that correlated with the temporal profile of cell death progression and characterized one of these cell death clusters further. We selected one of the strongest regulated genes, the serum and glucocorticoid-regulated kinase 1 (sgk1), and validated its differential expression by Northern blot analysis, semiquantitative PCR and in situ hybridization. Up-regulation of sgk1 (i) coincides with the onset of dopaminergic cell death in both the 8-week acute and 1-year subacute MPTP models, (ii) spans the entire brain, (iii) is attenuated by the l-deprenyl-mediated inhibition of the MPTP conversion to its active metabolite MPP+ and (iv) is not induced by dehydration. This study demonstrated that the combination of the DEPD technology, clustering analysis and a detailed histopathology is a useful tool for elucidating molecular pathways in neurodegenerative diseases.