The Role of N-Glycosylation in Function and Surface Trafficking of the Human Dopamine Transporter

The hypocretin-orexin system regulates cocaine self-administration via actions on the mesolimbic dopamine system

Recent evidence suggests that the hypocretin-orexin system participates in the regulation of reinforcement processes. The current studies examined the extent to which hypocretin neurotransmission regulates behavioral and neurochemical responses to cocaine, and behavioral responses to food reinforcement. These studies used a combination of fixed ratio, discrete trials, progressive ratio and threshold self-administration procedures to assess whether the hypocretin 1 receptor antagonist, SB-334867, reduces cocaine self-administration in rats. Progressive ratio sucrose self-administration procedures were also used to assess the extent to which SB-334867 reduces responding to a natural reinforcer in food-restricted and food-sated rats. Additionally, these studies used microdialysis and in vivo voltammetry in rats to examine whether SB-334867 attenuates the effects of cocaine on dopamine signaling within the nucleus accumbens core. Furthermore, in vitro voltammetry was used to examine whether hypocretin knockout mice display attenuated dopamine responses to cocaine. Results indicate that when SB-334867 was administered peripherally or within the ventral tegmental area, it reduced the motivation to self-administer cocaine and attenuated cocaine-induced enhancement of dopamine signaling. SB-334867 also reduced the motivation to self-administer sucrose in food-sated but not food-restricted rats. Finally, hypocretin knockout mice displayed altered baseline dopamine signaling and reduced dopamine responses to cocaine. Combined, these studies suggest that hypocretin neurotransmission participates in reinforcement processes, likely through modulation of the mesolimbic dopamine system. Additionally, the current observations suggest that the hypocretin system may provide a target for pharmacotherapies to treat cocaine addiction.

Adenylyl cyclase type 5 protein expression during cardiac development and stress

Adenylyl cyclase (AC) types 5 and 6 (AC5 and AC6) are the two major AC isoforms expressed in the mammalian heart that mediate signals from beta-adrenergic receptor stimulation. Because of the unavailability of isoform-specific antibodies, it is difficult to ascertain the expression levels of AC5 protein in the heart. Here we demonstrated the successful generation of an AC5 isoform-specific mouse monoclonal antibody and studied the expression of AC5 protein during cardiac development in different mammalian species. The specificity of the antibody was confirmed using heart and brain tissues from AC5 knockout mice and from transgenic mice overexpressing AC5. In mice, the AC5 protein was highest in the brain but was also detectable in all organs studied, including the heart, brain, lung, liver, stomach, kidney, skeletal muscle, and vascular tissues. Western blot analysis showed that AC5 was most abundant in the neonatal heart and declined to basal levels in the adult heart. AC5 protein increased in the heart with pressure-overload left ventricular hypertrophy. Thus this new AC5 antibody demonstrated that this AC isoform behaves similarly to fetal type genes, such as atrial natriuretic peptide; i.e., it declines with development and increases with pressure-overload hypertrophy.

Infantile parkinsonism-dystonia: a dopamine "transportopathy"

The dopamine transporter (DAT) retrieves the neurotransmitter dopamine from the synaptic cleft at dopaminergic synapses. Variations in solute carrier family 6A, member 3 (SLC6A3/DAT1), the human gene encoding DAT, have been implicated in attention deficit hyperactivity and bipolar disorders, and DAT is a prominent site of action for drugs such as amphetamines and cocaine. In this issue of the JCI, Kurian et al. report that an autosomal recessive infantile parkinsonism-dystonia is caused by loss-of-function mutations in DAT that impair dopamine reuptake (see the related article beginning on page 1595). Though this might be predicted to result in dopamine excess in the synaptic cleft, it likely also causes depletion of presynaptic dopamine stores and possibly downregulation of postsynaptic dopamine receptor function, resulting in impairments in dopaminergic neurotransmission consistent with the clinical presentation. This is the first report of a genetic alteration in DAT function underlying a parkinsonian disorder.

A single nucleotide polymorphism in the human serotonin transporter introduces a new site for N-linked glycosylation

The human serotonin transporter (hSERT) is responsible for reuptake of serotonin (5-HT) from the synaptic cleft and is target for antidepressant medicine. Differential hSERT activity caused by genetic polymorphisms is believed to affect the risk of developing depression and, moreover, to affect the response to antidepressant therapy. The hSERT contains in the second extracellular loop (EL2) two sites for N-linked glycosylation that are critical for functional transporter expression. Here we examine a non-synonymous single nucleotide polymorphism (SNP) in EL2 that gives rise to a potential third glycosylation site due to substitution of a lysine at position 201 with an asparagine (K201N). In agreement with introduction of an additional glycosylation site, western blot analysis showed migration of hSERT K201N corresponding to a higher molecular weight than wild type hSERT upon expression in both HEK293 cells and primary cultures of cortical neurons. An increase in molecular weight was not observed after removal of glycans with peptide N-glycosidase F (PNGase F). Quantitative analysis of western blots indicated significantly increased total transporter expression ( approximately 30%) for hSERT K201N as compared to hSERT in both cell systems. The increase in expression was accompanied by corresponding significant increases in the number of [(3)H]citalopram binding sites and in the V(max) for [(3)H]5-HT uptake. Characterization of mutants carrying all possible combinations of glycosylation sites demonstrated clear correlation between the number of glycosylation sites and the level of transporter activity, and showed that K201N could substitute for either one of the two original glycosylation sites.

Trafficking of dopamine transporters in psychostimulant actions

Brain dopamine (DA) plays a pivotal role in drug addiction. Since the plasma membrane DA transporter (DAT) is critical for terminating DA neurotransmission, it is important to understand how DATs are regulated and this regulation impacts drug addiction. The number of cell surface DATs is controlled by constitutive and regulated endocytic trafficking. Psychostimulants impact this trafficking. Amphetamines, DAT substrates, cause rapid up-regulation and slower down-regulation of DAT whereas cocaine, a DAT inhibitor, increases surface DATs. Recent reports have begun to elucidate the molecular mechanisms of these psychostimulant effects and link changes in DAT trafficking to psychostimulant-induced reward/reinforcement in animal models.

Homozygous loss-of-function mutations in the gene encoding the dopamine transporter are associated with infantile parkinsonism-dystonia

Genetic variants of the SLC6A3 gene that encodes the human dopamine transporter (DAT) have been linked to a variety of neuropsychiatric disorders, particularly attention deficit hyperactivity disorder. In addition, the homozygous Slc6a3 knockout mouse displays a hyperactivity phenotype. Here, we analyzed 2 unrelated consanguineous families with infantile parkinsonism-dystonia (IPD) syndrome and identified homozygous missense SLC6A3 mutations (p.L368Q and p.P395L) in both families. Functional studies demonstrated that both mutations were loss-of-function mutations that severely reduced levels of mature (85-kDa) DAT while having a differential effect on the apparent binding affinity of dopamine. Thus, in humans, loss-of-function SLC6A3 mutations that impair DAT-mediated dopamine transport activity are associated with an early-onset complex movement disorder. Identification of the molecular basis of IPD suggests SLC6A3 as a candidate susceptibility gene for other movement disorders associated with parkinsonism and/or dystonic features.

Methylmercury inhibits dopaminergic function in rat pup synaptosomes in an age-dependent manner

Methylmercury (MeHg) is an environmental neurotoxicant that is especially harmful during brain development. Previously, we found greater sensitivity to MeHg-induced oxidative stress and greater loss of mitochondrial membrane potential in synaptosomes from early postnatal rats than in synaptosomes from older rat pups and adults. Here, we determine whether MeHg exposure also leads to greater changes in dopamine (DA) levels and dopamine transporter (DAT) function in synaptosomes from early postnatal rats. We report that MeHg exposure leads to DAT inhibition, and increases the levels of released DA compared to control; further, the effects are much greater in synaptosomes prepared from postnatal day (PND) 7 rats than in synaptosomes from PND 14 or PND 21 animals. In addition to the effects of MeHg in young rats, we observed age-dependent differences in dopaminergic function in unexposed synaptosomes: synaptosomal DA levels increased with age, whereas medium (released) DA levels were high at PND 7 and were lower in PND 14 and PND 21 synaptosomes. DAT activity increased slightly from PND 7 to PND 14 and then increased more strongly to PND 21, suggesting that higher DA release, in addition to the lower DAT activity seen in PND 7 animals, was responsible for the age differences in levels of released DA. These results demonstrate that MeHg affects the dopaminergic system during early development; it thus may contribute to the neurobehavioral effects seen in MeHg-exposed children.

A closer look at amphetamine-induced reverse transport and trafficking of the dopamine and norepinephrine transporters

Amphetamine (AMPH) and its derivatives are regularly used in the treatment of a wide array of disorders such as attention-deficit hyperactivity disorder (ADHD), obesity, traumatic brain injury, and narcolepsy (Prog Neurobiol 75:406-433, 2005; J Am Med Assoc 105:2051-2054, 1935; J Am Acad Child Adolesc Psychiatry 41:514-521, 2002; Neuron 43:261-269, 2004; Annu Rev Pharmacol Toxicol 47:681-698, 2007; Drugs Aging 21:67-79, 2004). Despite the important medicinal role for AMPH, it is more widely known for its psychostimulant and addictive properties as a drug of abuse. The primary molecular targets of AMPH are both the vesicular monoamine transporters (VMATs) and plasma membrane monoamine-dopamine (DA), norepinephrine (NE), and serotonin (5-HT)-transporters. The rewarding and addicting properties of AMPH rely on its ability to act as a substrate for these transporters and ultimately increase extracellular levels of monoamines. AMPH achieves this elevation in extracellular levels of neurotransmitter by inducing synaptic vesicle depletion, which increases intracellular monoamine levels, and also by promoting reverse transport (efflux) through plasma membrane monoamine transporters (J Biol Chem 237:2311-2317, 1962; Med Exp Int J Exp Med 6:47-53, 1962; Neuron 19:1271-1283, 1997; J Physiol 144:314-336, 1958; J Neurosci 18:1979-1986, 1998; Science 237:1219-1223, 1987; J Neurosc 15:4102-4108, 1995). This review will focus on two important aspects of AMPH-induced regulation of the plasma membrane monoamine transporters-transporter mediated monoamine efflux and transporter trafficking.

Rapid substrate-induced down-regulation in function and surface localization of dopamine transporters: rat dorsal striatum versus nucleus accumbens

The dopamine transporter (DAT) substrates dopamine, d-amphetamine (AMPH), and methamphetamine are known to rapidly and transiently reduce DAT activity and/or surface expression in dorsal striatum and heterologous expression systems. We sought to determine if similar substrate-induced regulation of DATs occurs in rat nucleus accumbens. In dorsal striatum synaptosomes, brief (15-min) in vitro substrate pre-exposure markedly decreased maximal [(3)H]dopamine uptake velocity whereas identical substrate pre-exposure in nucleus accumbens synaptosomes produced a smaller, non-significant reduction. However, 45 min after systemic AMPH administration, maximal ex vivo [(3)H]dopamine uptake velocity was significantly reduced in both brain regions. Protein kinase C inhibition blocked AMPH's down-regulation of DAT activity. DAT synaptosomal surface expression was not modified following either the brief in vitro or in vivo AMPH pre-exposure but was reduced after a longer (1-h) in vitro pre-exposure in both brain regions. Together, our findings suggest that relatively brief substrate exposure results in greater down-regulation of DAT activity in dorsal striatum than in nucleus accumbens. Moreover, exposure to AMPH appears to regulate striatal DATs in a biphasic manner, with an initial protein kinase C-dependent decrease in DAT-mediated uptake velocity and then, with longer exposure, a reduction in DAT surface expression.

Proline-directed phosphorylation of the dopamine transporter N-terminal domain

Phosphorylation of the dopamine transporter (DAT) on N-terminal serines and unidentified threonines occurs concomitantly with protein kinase C (PKC)- and substrate-induced alterations in transporter activity, subcellular distribution, and dopamine efflux, but the residues phosphorylated and identities of protein kinases and phosphatases involved are not known. As one approach to investigating these issues, we recombinantly expressed the N-terminal tail of rat DAT (NDAT) and examined its phosphorylation and dephosphorylation properties in vitro. We found that NDAT could be phosphorylated to significant levels by PKCalpha, PKA, PKG, and CaMKII, which catalyzed serine phosphorylation, and ERK1, JNK, and p38, which catalyzed threonine phosphorylation. We identified Thr53, present in a membrane proximal proline-directed kinase motif as the NDAT site phosphorylated in vitro by ERK1, JNK and p38, and confirmed by peptide mapping and mutagenesis that Thr53 is phosphorylated in vivo. Dephosphorylation studies showed that protein phosphatase 1 catalyzed near-complete in vitro dephosphorylation of PKCalpha-phosphorylated NDAT, similar to its in vivo and in vitro effects on native DAT. These findings demonstrate the ability of multiple enzymes to directly recognize the DAT N-terminal domain and for kinases to act at multiple distinct sites. The strong correspondence between NDAT and rDAT phosphorylation characteristics suggests the potential for the enzymes that are active on NDAT in vitro to act on DAT in vivo and indicates the usefulness of NDAT for guiding future DAT phosphorylation analyses.

Chronic methylphenidate treatment enhances striatal dopamine neurotransmission after experimental traumatic brain injury

Traumatic brain injury (TBI) results in functional deficits that often are effectively treated clinically with the neurostimulant, methylphenidate (MPH). We hypothesized that daily MPH administration would reverse striatal neurotransmission deficits observed in the controlled cortical impact (CCI) model of TBI. CCI or naïve rats received daily injections of MPH (5 mg/kg) or saline for 14 days and were assessed on day 15 using fast scan cyclic voltammetry. Dopamine (DA) transporter (DAT) localization, DA-related proteins, and transcription factor (c-fos) expression were also assessed. CCI resulted in reduced electrically evoked overflow of DA and maximal velocity of DA clearance (V(max)). In contrast, CCI was associated with a decrease in the apparent K(M) of DAT. Daily dose of MPH after CCI resulted in robust increases in evoked DA overflow and V(max) as well as increased apparent K(M). Reductions in total striatal DAT expression occurred after CCI and were not further affected by MPH. In contrast, membrane-bound striatal DAT levels were increased in both CCI groups. MPH post-CCI significantly increased striatal c-fos levels compared with saline. These results support the hypothesis that daily MPH improves striatal DA neurotransmission after CCI. DAT expression and transcriptional changes affecting DA protein function may underlie the injury and MPH-induced alterations in neurotransmission observed.

Dopamine transporter endocytic determinants: carboxy terminal residues critical for basal and PKC-stimulated internalization

Dopamine (DA) reuptake terminates dopaminergic neurotransmission and is mediated by DA transporters (DATs). Acute protein kinase C (PKC) activation accelerates DAT internalization rates, thereby reducing DAT surface expression. Basal DAT endocytosis and PKC-stimulated DAT functional downregulation rely on residues within the 587-596 region, although whether PKC-induced DAT downregulation reflects transporter endocytosis mechanisms linked to those controlling basal endocytosis rates is unknown. Here, we define residues governing basal and PKC-stimulated DAT endocytosis. Alanine substituting DAT residues 587-590 1) abolished PKC stimulation of DAT endocytosis, and 2) markedly accelerated basal DAT internalization, comparable to that of wildtype DAT during PKC activation. Accelerated basal DAT internalization relied specifically on residues 588-590, which are highly conserved among SLC6 neurotransmitter transporters. Our results support a model whereby residues within the 587-590 stretch may serve as a locus for a PKC-sensitive braking mechanism that tempers basal DAT internalization rates.

Cocaine increases stimulated dopamine release more in periadolescent than adult rats

The neural mechanisms responsible for the enhanced adolescent vulnerability for initiating drug abuse are unclear. We investigated whether age differences in dopamine neurotransmission could explain cocaine's enhanced psychomotor effects in the periadolescent rat. Electrical stimulation of the medial forebrain bundle of anesthetized post-natal age 28 days (PN28) and PN65 rats elicited dopamine release in caudate nucleus and nucleus accumbens core before and after 15 mg/kg cocaine i.p. Extracellular dopamine concentrations were greater in PN65 than PN28 caudate following 20 and 60 Hz stimulations and in the PN65 nucleus accumbens following 60 Hz stimulations. Cocaine increased dopamine concentrations elicited by 20 Hz stimulations 3-fold in the adult, but almost 9-fold in periadolescent caudate. Dopamine release rate was lower in the periadolescent caudate although total dopamine clearance was similar to that of adults. The periadolescent caudate achieved adult levels of clearance by compensating for a lower V(max) with higher uptake affinity. Tighter regulation of extracellular dopamine by the higher uptake/release ratio in periadolescents led to greater increases after cocaine. In nucleus accumbens, dopamine release and V(max) were lower in periadolescents than adults, but uptake affinity and cocaine effects were similar. Immaturity of dopamine neurotransmission in dorsal striatum may underlie enhanced acute responses to psychostimulants in adolescent rats and suggests a mechanism for the greater vulnerability of adolescent humans to drug addiction.

Ethanol potentiates dopamine uptake and increases cell surface distribution of dopamine transporters expressed in SK-N-SH and HEK-293 cells

Ethanol increases dopaminergic release in the reward and reinforcement areas of the brain. The primary protein responsible for terminating dopamine (DA) neurotransmission is the plasma membrane-bound dopamine transporter (DAT). In vitro electrophysiological and biochemical studies in Xenopus laevis oocytes have previously shown ethanol potentiates DAT function and increases transporter-binding sites. The potentiating effect of ethanol on the transporter is eliminated in Xenopus oocytes by the DAT mutation glycine 130 to threonine. However, ethanol's action on DAT functional regulation has yet to be examined in mammalian cell expression systems. To further understand the molecular mechanisms of ethanol's action on DAT, we determined the direct mechanistic action of short-term (< or =2 h) ethanol exposure on transporter function and cell surface distribution in non-neuronal human embryonic kidney cells-293 (HEK-293) and neuronal SK-N-SH neuroblastoma cells expressing the transporter. Wild-type or G130T mutant DAT were overexpressed in HEK-293 and SK-N-SH cells. Ethanol potentiated DAT mediated [(3)H]DA uptake in a dose (25, 50, 100 mM), but not time dependent manner in cells expressing wild-type DAT. Ethanol-induced potentiation of uptake was significantly reduced in cells expressing the G130T mutant. Analysis of DA uptake kinetic parameters indicates 100-mM ethanol exposure increased [(3)H]DA uptake velocity (V(max)), while affinity for DA (K(m)) remained unchanged. The effect of ethanol on wild-type DAT surface expression was measured by biotinylation cell surface labeling. DAT surface expression increased 40%-50% after 1-h, 100-mM ethanol exposure. These studies show ethanol potentiates DAT functional regulation in both neuronal and non-neuronal cells, suggesting a direct mechanistic action of ethanol on transporter trafficking in mammalian systems. Our findings demonstrate ethanol's action on DAT function and regulation is consistent across multiple model systems.

Amphetamine-induced decreases in dopamine transporter surface expression are protein kinase C-independent

Amphetamine (AMPH) is a potent dopamine (DA) transporter (DAT) inhibitor that markedly increases extracellular DA levels. In addition to its actions as a DAT antagonist, acute AMPH exposure induces DAT losses from the plasma membrane, implicating transporter-specific membrane trafficking in amphetamine's actions. Despite reports that AMPH modulates DAT surface expression, the trafficking mechanisms leading to this effect are currently not defined. We recently reported that DAT residues 587-596 play an integral role in constitutive and protein kinase C (PKC)-accelerated DAT internalization. In the current study, we tested whether the structural determinants required for PKC-stimulated DAT internalization are necessary for AMPH-induced DAT sequestration. Acute amphetamine exposure increased DAT endocytic rates, but DAT carboxy terminal residues 587-590, which are required for PKC-stimulated internalization, were not required for AMPH-accelerated DAT endocytosis. AMPH decreased DAT endocytic recycling, but did not modulate transferrin receptor recycling, suggesting that AMPH does not globally diminish endocytic recycling. Finally, treatment with a PKC inhibitor demonstrated that AMPH-induced DAT losses from the plasma membrane were not dependent upon PKC activity. These results suggest that the mechanisms responsible for AMPH-mediated DAT internalization are independent from those governing PKC-sensitive DAT endocytosis.

Substrates dissociate dopamine transporter oligomers

Substrate-induced endocytic trafficking of dopamine transporter (DAT) has been observed, but little is known about the regulation of DAT oligomerization by substrate. The present study investigates the effect on substrates on DAT oligomerization and explores a potential link with the presence of DAT at the cell surface in human embryonic kidney cells transiently or stably expressing N-terminal tagged DAT constructs. Dopamine (100 microM) or amphetamine (2-10 microM) reduced Myc-DAT coimmunoprecipitated along with Flag-DAT (oligomeric DAT) in tandem with a reduction in surface DAT determined by biotinylation. Dopamine (10-1000 microM) and amphetamine (0.2-200 microM) reduced DAT oligomerization as assessed by cross-linking with copper sulfate phenanthroline or Cu2+. Inhibition of endocytosis by 10 microM phenylarsine oxide or 450 mM sucrose counteracted the effect of 10 microM DA or 2 microM amphetamine in reducing DAT cross-linking. In addition to overall similarities between the results with the two cross-linking agents and between the results with the two different endocytosis inhibitors, some differences were noted as well, likely related to the efficiency of the cross-linking process and the sulfhydryl-reactive properties of phenylarsine oxide, respectively. The present results are the first to indicate regulation of oligomerization of an solute carrier family 6 transporter, the DAT, by substrates that act at DAT. In addition, the present study opens up the possibility of an important linkage between oligomerization of DAT and endocytic or other modulatory mechanisms impacting surface DAT.

Spatial and temporal expression patterns of the epithelial cell adhesion molecule (EpCAM/EGP-2) in developing and adult kidneys

BACKGROUND

The epithelial cell adhesion molecule (EpCAM) is expressed by most epithelia and is involved in processes fundamental for morphogenesis, including cell-cell adhesion, proliferation, differentiation, and migration. Previously, a role for EpCAM in pancreatic morphogenesis was confirmed in vitro. Furthermore, changes in the EpCAM expression pattern were found in developing lung and thymus and in the regenerating liver. Therefore, EpCAM was proposed to be a morphoregulatory molecule.

METHODS

Using immunohistochemistry, the expression pattern of human and murine homologues of EpCAM was characterized in adult and embryonic kidneys from humans and human-EpCAM (hEpCAM)-transgenic mice.

RESULTS

EpCAM expression was found in the ureteric bud throughout nephrogenesis. EpCAM was not expressed in the metanephric mesenchyme. In comma- and S-shaped bodies, both metanephric mesenchyme derived structures, EpCAM expression appeared by E13.5. In adult kidneys, most epithelia expressed varying levels of EpCAM, as confirmed by double staining for human EpCAM and segment-specific nephron markers. Podocytes were EpCAM negative. At the cellular level, the EpCAM expression shifted from apical in embryonic to basolateral in adult kidneys.

CONCLUSIONS

The spatiotemporal expression pattern of EpCAM changes during nephrogenesis. In the adult kidney, the expression varies markedly along the nephron. These data provide a basis for further studies on EpCAM in developing and adult kidneys.

Characterization of a novel gene, BcMF7, that is expressed preferentially in pollen of Brassica campestris L. ssp. chinensis Makino

Pollen formation is important for plant sexual reproduction. To identify the genes that are involved in pollen formation, we performed the genome-wide transcriptional profiling in the flower buds of both male meiotic cytokinesis (mmc) mutant and its wild-type plants of Brassica campestris L. ssp. chinensis, syn. B. rapa L. ssp. chinensis. cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis showed that the mmc mutation resulted in changes in expression of a variety of genes. BcMF7, a transcript-derived fragment (TDF) accumulated in the wild-type flower buds was further characterized. The BcMF7 gene has 1161 bp in length with two introns. The full-length BcMF7 cDNA has 609 bp in length and encodes a protein of 129 amino acids. The deduced amino acid sequence of BcMF7 protein shares no similarity to any function-known protein in Swiss-Prot database, but has 8 protein kinase C phosphorylation sites, 2 caselin kinase II phosphorylation sites, 2 tyrosine kinase phosphorylation sites, 2 N-glycosylation sites and 2 N-myristolyation sites. Spatial and temporal expression patterns analysis showed that BcMF7 was expressed exclusively in pollen. The expression signal of BcMF7 was first detected at the tetrad stage of microspore development, reached a peak level at the uninucleate stage, and decreased to a slightly low level at the mature pollen stage. All these results show that BcMF7 may play a certain role in the signal transduction during pollen development.

Regulation of dopamine transporter function and cell surface expression by D3 dopamine receptors

D(3) dopamine receptors are expressed by dopamine neurons and are implicated in the modulation of presynaptic dopamine neurotransmission. The mechanisms underlying this modulation remain ill defined. The dopamine transporter, which terminates dopamine transmission via reuptake of released neurotransmitter, is regulated by receptor- and second messenger-linked signaling pathways. Whether D3 receptors regulate dopamine transporter function is unknown. We addressed this issue using a fluorescent imaging technique that permits real time quantification of dopamine transporter function in living single cells. Accumulation of the fluorescent dopamine transporter substrate trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium (ASP(+)) in human embryonic kidney cells expressing human dopamine transporter was saturable and temperature-dependent. In cells co-expressing dopamine transporter and D3 receptors, the D2/D3 agonist quinpirole produced a rapid, concentration-dependent, and pertussis toxin-sensitive increase of ASP(+) uptake. Similar agonist effects were observed in Neuro2A cells and replicated in human embryonic kidney cells using a radioligand uptake assay in which binding to and activation of D3 receptors by [(3)H]dopamine was prevented. D3 receptor stimulation activated phosphoinositide 3-kinase and MAPK. Inhibition of either kinase prevented the quinpirole-induced increase in uptake. D3 receptor activation differentially affected dopamine transporter function and subcellular distribution depending on the duration of agonist exposure. Biotinylation experiments revealed that the rapid increase of uptake was associated with increased cell surface and decreased intracellular expression and increased dopamine transporter exocytosis. In contrast, prolonged agonist exposure reduced uptake and transporter cell surface expression. These results demonstrate that D3 receptors regulate dopamine transporter function and identify a novel mechanism by which D3 receptors regulate extracellular dopamine concentrations.

Monoamine transporters and psychostimulant addiction

Psychostimulants are a broadly defined class of drugs that stimulate the central and peripheral nervous systems as their primary pharmacological effect. The abuse liability of psychostimulants is well established and represents a significant public health concern. An extensive literature documents the critical importance of monoamines (dopamine, serotonin and norepinephrine) in the behavioral pharmacology and addictive properties of psychostimulants. In particular, the dopamine transporter plays a primary role in the reinforcing and behavioral-stimulant effects of psychostimulants in animals and humans. Moreover, both serotonin and norepinephrine systems can reliably modulate the neurochemical and behavioral effects of psychostimulants. However, there is a growing body of evidence that highlights complex interactions among additional neurotransmitter systems. Cortical glutamatergic systems provide important regulation of dopamine function, and inhibitory amino acid gamma-aminobutyric acid (GABA) systems can modulate basal dopamine and glutamate release. Repeated exposure to psychostimulants can lead to robust and enduring changes in neurobiological substrates, including monoamines, and corresponding changes in sensitivity to acute drug effects on neurochemistry and behavior. Significant advances in the understanding of neurobiological mechanisms underlying psychostimulant abuse and dependence have guided pharmacological treatment strategies to improve clinical outcome. In particular, functional agonist treatments may be used effectively to stabilize monoamine neurochemistry, influence behavior and lead to long-term abstinence. However, additional clinical studies are required in order to identify safe and efficacious pharmacotherapies.

A meta-analysis of association studies between the 10-repeat allele of a VNTR polymorphism in the 3'-UTR of dopamine transporter gene and attention deficit hyperactivity disorder

The association between the 10-repeat allele of the dopamine transporter gene (DAT) and attention deficit hyperactivity disorder (ADHD) is uncertain. This study aimed to conduct a meta-analysis of the association between the 10-repeat allele of a variable number tandem repeat (VNTR) polymorphism in the 3'-untranslated region (UTR) of the DAT1 gene and ADHD. We pooled up 18 published transmission disequilibrium test (TDT) studies between the 40-base pair VNTR polymorphism in the3'-UTR of the DAT1 gene and ADHD. It included a total of 1,373 informative meioses, 7 haplotype-based haplotype relative risk (HHRR) studies, and 6 case-control-based association studies. There were statistically significant evidences for heterogeneity of the odds ratio in TDT and HHRR studies (P < 0.10), but not in case-control studies. The results of random effects model showed small but significant association between ADHD and the DAT1 gene in TDT studies (OR = 1.17, 95% CI = 1.05-1.30, chi-square = 8.11, df = 1, P = 0.004), but not in HHRR and case-control studies. The 10-repeat allele of a VNTR polymorphism in the 3'-UTR the DAT1 gene has a small but significant role in the genetic susceptibility of ADHD. These meta-analysis findings support the involvement of the dopamine system genes in ADHD liability variation. However, more work is required to further identify the functional allelic variants/mutations that are responsible for this association.

D2 receptors regulate dopamine transporter function via an extracellular signal-regulated kinases 1 and 2-dependent and phosphoinositide 3 kinase-independent mechanism

The dopamine transporter (DAT) terminates dopamine (DA) neurotransmission by reuptake of DA into presynaptic neurons. Regulation of DA uptake by D(2) dopamine receptors (D(2)R) has been reported. The high affinity of DA and other DAT substrates for the D(2)R, however, has complicated investigation of the intracellular mechanisms mediating this effect. The present studies used the fluorescent DAT substrate, 4-[4-(diethylamino)-styryl]-N-methylpyridinium iodide (ASP(+)) with live cell imaging techniques to identify the role of two D(2)R-linked signaling pathways, extracellular signal-regulated kinases 1 and 2 (ERK1/2), and phosphoinositide 3 kinase (PI3K) in mediating D(2)R regulation of DAT. Addition of the D(2)/D(3) receptor agonist quinpirole (0.1-10 muM) to human embryonic kidney cells coexpressing human DAT and D(2) receptor (short splice variant, D(2S)R) induced a rapid, concentration-dependent and pertussis toxin-sensitive increase in ASP(+) accumulation. The D(2)/D(3) agonist (S)-(+)-(4aR, 10bR)-3,4,4a, 10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride (PD128907) also increased ASP(+) accumulation. D(2S)R activation increased phosphorylation of ERK1/2 and Akt, a major target of PI3K. The mitogen-activated protein kinase kinase inhibitor 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD98059) prevented the quinpirole-evoked increase in ASP(+) accumulation, whereas inhibition of PI3K was without effect. Fluorescence flow cytometry and biotinylation studies revealed a rapid increase in DAT cell-surface expression in response to D(2)R stimulation. These experiments demonstrate that D(2S)R stimulation increases DAT cell surface expression and therefore enhances substrate clearance. Furthermore, they show that the increase in DAT function is ERK1/2-dependent but PI3K-independent. Our data also suggest the possibility of a direct physical interaction between DAT and D(2)R. Together, these results suggest a novel mechanism by which D(2S)R autoreceptors may regulate DAT in the central nervous system.

BcMF13, a new reproductive organ-specific gene from Brassica rapa. ssp. chinensis, affects pollen development

A transcript-derived fragment (GenBank accession number DN237920.1) accumulated in the wild-type flower buds of Chinese cabbage (Brassica campestris L. ssp. chinensis Makino, syn. B. rapa ssp. chinensis) was isolated and further investigated. The full length DNA and cDNA of the fragment were cloned by rapid amplification of cDNA ends. The gene, BcMF13, encodes a protein of 73 amino acids and is interrupted by an intron of 106 bp (GenBank accession number EF158459). Southern blot analysis revealed that BcMF13 could be a single-copy gene in the Chinese cabbage genome. Sequence blast analysis showed that BcMF13 was a new gene. In EST database, those sequences share 96-98% identity with BcMF13 cDNA all came from flower buds, microspores, anthers of Brassica, which proved BcMF13 homologs closely related to the development of male gametogenesis in Brassica. RT-PCR discovered that it is exclusively expressed in stage four and five flower buds of fertile line, strongly expressed in stamens. Successful suppression of BcMF13 gene expression by RNA antisense strategy greatly reduced the normal pollen grains, suggesting that BcMF13 was essential in pollen development in Brassica.

Two N-linked glycans are required to maintain the transport activity of the bile salt export pump (ABCB11) in MDCK II cells

The aim of this study was to determine the role of N-linked glycosylation in protein stability, intracellular trafficking, and bile acid transport activity of the bile salt export pump [Bsep (ATP-binding cassette B11)]. Rat Bsep was fused with yellow fluorescent protein, and the following mutants, in which Asn residues of putative glycosylation sites (Asn(109), Asn(116), Asn(122), and Asn(125)) were sequentially replaced with Gln, were constructed by site-directed mutagenesis: single N109Q, double N109Q + N116Q, triple N109Q + N116Q + N122Q, and quadruple N109Q + N116Q + N122Q + N125Q. Immunoblot and glycosidase cleavage analysis demonstrated that each site was glycosylated. Removal of glycans decreased taurocholate transport activity as determined in polarized MDCK II cells. This decrease resulted from rapid decay of the mutant Bsep protein; biochemical half-lives were 3.76, 3.65, 3.24, 1.35, and 0.52 h in wild-type, single-mutant, double-mutant, triple-mutant, and quadruple-mutant cells, respectively. Wild-type and single- and double-mutant proteins were distributed exclusively along the apical membranes, whereas triple- and quadruple-mutant proteins remained intracellular. MG-132 but not bafilomycin A(1) extended the half-life, suggesting a role for the proteasome in Bsep degradation. To determine whether a specific glycosylation site or the number of glycans was critical for protein stability, we studied the protein expression of combinations of N-glycan-deficient mutants and observed that Bsep with one glycan was considerably unstable compared with Bsep harboring two or more glycans. In conclusion, at least two N-linked glycans are required for Bsep protein stability, intracellular trafficking, and function in the apical membrane.

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.

Fluctuation of the dopamine uptake inhibition potency of cocaine, but not amphetamine, at mammalian cells expressing the dopamine transporter

Cocaine, amphetamines and other psychostimulants inhibit synaptic dopamine uptake by interfering with dopamine transporter (DAT) function. The resultant potentiation of dopaminergic neurotransmission is associated with psychostimulant addiction. Fluctuations in dopamine uptake inhibition potency (DUIP) were observed for classical DAT blockers including cocaine, mazindol, methylphenidate (Ritalintrade mark) and benztropine in CHO cells expressing wild type DAT; cocaine potency also decreased in DAT-expressing non-neuronal COS-7 cells and neuronal N2A neuroblastoma cells. In contrast, the DAT substrate (+)-amphetamine did not display this DUIP fluctuation. In parallel experiments, no fluctuation was observed for the apparent binding affinities of these 5 drugs. The DUIP decrease appeared to correlate with an increase in cell surface DAT expression level, as measured by B(max) values and confocal microscopy. The fact that the DUIP profile of amphetamine diverged from that of the classical DAT blockers is consistent with the idea of fundamental differences between the mechanisms of abused psychostimulant DAT substrates and inhibitors. Identification of the cellular factors that underlie the DAT inhibitor DUIP fluctuation phenomenon may be relevant to anti-psychostimulant drug discovery efforts.

Loss of N-linked glycosylation reduces urea transporter UT-A1 response to vasopressin

The vasopressin-regulated urea transporter (UT)-A1 is a transmembrane protein with two glycosylated forms of 97 and 117 kDa; both are derived from a single 88-kDa core protein. However, the precise molecular sites and the function for UT-A1 N-glycosylation are not known. In this study, we compared Madin-Darby canine kidney cells stably expressing wild-type (WT) UT-A1 to Madin-Darby canine kidney cell lines stably expressing mutant UT-A1 lacking one (A1m1, A1m2) or both glycosylation sites (m1m2). Site-directed mutagenesis revealed that UT-A1 has two glycosylation sites at Asn-279 and -742. Urea flux is stimulated by 10 nM vasopressin (AVP) or 10 microM forskolin (FSK) in WT cells. In contrast, m1m2 cells have a delayed and significantly reduced maximal urea flux. A 15-min treatment with AVP and FSK significantly increased UT-A1 cell surface expression in WT but not in m1m2 cells, as measured by biotinylation. We confirmed this finding using immunostaining. Membrane fractionation of the plasma membrane, Golgi, and endoplasmic reticulum revealed that AVP or FSK treatment increases UT-A1 abundance in both Golgi and plasma membrane compartments in WT but not in m1m2 cells. Pulse-chase experiments showed that UT-A1 half-life is reduced in m1m2 cells compared with WT cells. Our results suggest that mutation of the N-linked glycosylation sites reduces urea flux by reducing UT-A1 half-life and decreasing its accumulation in the apical plasma membrane. In vivo, inner medullary collecting duct cells may regulate urea uptake by altering UT-A1 glycosylation in response to AVP stimulation.

Role of dopamine transporter (DAT) in dopamine transport across the nasal mucosa

Dopamine is a catecholamine neurotransmitter necessary for motor functions. Its deficiency has been observed in several neurological disorders, but replacement of endogenous dopamine via oral or parenteral delivery is limited by poor absorption, rapid metabolism and the inability of dopamine to cross the blood-brain barrier. The intranasal administration of dopamine, however, has resulted in improved central nervous system (CNS) bioavailability compared to that obtained following intravenous delivery. Portions of the nasal mucosa are innervated by olfactory neurons expressing dopamine transporter (DAT) which is responsible for the uptake of dopamine within the central nervous system. The objective of these studies was to study the role of DAT in dopamine transport across the bovine olfactory and nasal respiratory mucosa. Western blotting studies demonstrated the expression of DAT and immunohistochemistry revealed its epithelial and submucosal localization within the nasal mucosa. Bidirectional transport studies over a 0.1-1 mM dopamine concentration range were carried out in the mucosal-submucosal and submucosal-mucosal directions to quantify DAT activity, and additional transport studies investigating the ability of GBR 12909, a DAT inhibitor, to decrease dopamine transport were conducted. Dopamine transport in the mucosal-submucosal direction was saturable and was decreased in the presence of GBR 12909. These studies demonstrate the activity of DAT in the nasal mucosa and provide evidence that DAT-mediated dopamine uptake plays a role in the absorption and distribution of dopamine following intranasal administration.

Dopaminergic marker proteins in the substantia nigra of human immunodeficiency virus type 1-infected brains

With the advent of highly active antiretroviral therapy, human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) is becoming a more chronic, manageable disease; nevertheless, the prevalence of neurological complications of AIDS is increasing. In this study, protein levels of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the substantia nigra of HIV-infected brains and -seronegative controls were determined by immunoblotting. The immunoreactivity of neuronal specific enolase (NSE) was used to assess cell loss. Although there were no changes in levels of immunoreactive DAT or NSE proteins in HIV brains, levels of immunoreactive TH were significantly reduced, relative to controls. These results suggest that decreases in TH, the rate-limiting enzyme of dopamine synthesis, may be a factor in the neurological manifestations of HIV infection.

Effects of N-linked glycosylation on the creatine transporter

The CRT (creatine transporter) is a member of the Na+- and Cl--dependent neurotransmitter transporter family and is responsible for the import of creatine into cells, and thus is important for cellular energy metabolism. We established for CRT an expression system in HEK-293 cells that allowed biochemical, immunological and functional analysis of CRT wild-type and glycosylation-deficient mutants. Analysis of HA (haemagglutinin)-tagged CRT-NN (wild-type rat CRT with an HA-tag at the C-terminus) revealed several monomeric immunoreactive species with apparent molecular masses of 58, 48 and 43 kDa. The 58 kDa species was shown to be plasma-membrane-resident by EndoHf (endoglycosidase Hf) and PNGase F (peptide N-glycosidase F) treatments and represents fully glycosylated CRT, whereas the 48 kDa and 43 kDa species were glycosylation intermediates and non-glycosylated CRT respectively. Glycosylation-deficient mutants (Asn192Asp, Asn197Asp and Asn192Asp/Asn197Asp) showed altered electrophoretic mobility, indicating that CRT is indeed N-glycosylated. In addition, a prominent CRT band in the range of 75-91 kDa was also detected. Pharmacological inhibition of N-linked glycosylation by tunicamycin in CRT-NN-expressing cells gave a similar reduction in molecular mass, corroborating the finding that Asn192 and Asn197 are major N-glycosylation sites in CRT. Although the apparent Km was not significantly affected in glycosylation-deficient mutants compared with CRT-NN, we measured reduced Vmax values for all mutants (21-28% residual activity), and 51% residual activity after enzymatic deglycosylation of surface proteins in intact CRT-NN cells by PNGase F. Moreover, immunocytochemical analysis of CRT-NN- and CRT-DD-expressing cells (where CRT-DD represents a non-glycosylated double mutant of CRT, i.e. Asn192Asp/Asn197Asp) showed a lower abundance of CRT-DD in the plasma membrane. Taken together, our results suggest that plasma-membrane CRT is glycosylated and has an apparent monomer molecular mass of 58 kDa. Furthermore, N-linked glycosylation is neither exclusively important for the function of CRT nor for surface trafficking, but affects both processes. These findings may have relevance for closely related neurotransmitter transporter family members.

Long chain base tolerance in Saccharomyces cerevisiae is induced by retrograde signals from the mitochondria

Saccharomyces cerevisiae cells lacking their mitochondrial DNA (rho0 cells) respond to this loss of genetic information by induction of a program of nuclear gene expression called the retrograde response. Expression of genes involved in multidrug resistance and sphingolipid biosynthesis is coordinately induced in rho0 cells by the zinc cluster transcription factor Pdr3p. In this report, we identify a membrane protein involved in control of intracellular levels of a sphingolipid precursor as a transcriptional target of the Pdr3p-mediated retrograde response. These sphingolipid precursors are called long chain bases (LCBs) and increased LCB levels are growth inhibitory. This membrane protein has been designated Rsb1p and has previously been shown to act as a LCB transporter protein and to be a component of the endoplasmic reticulum. These earlier studies used an amino-terminal truncated form of Rsb1p. Here we employ a full-length form of Rsb1p and find that this protein is localized to the plasma membrane and is modified by N-linked glycosylation. Two glycosylation sites are present in the Rsb1p and both are required for normal LCB resistance. Mutational analysis of the RSB1 promoter revealed that two Pdr3p binding sites are present and both of these are required for normal retrograde induction of transcription. LCB tolerance is strongly increased in rho0 cells but this increase is ablated in rho0 rsb1Delta cells. Together, these data indicate Pdr3p activation of RSB1 transcription is an important feature of the retrograde response allowing normal detoxification of an endogenous sphingolipid precursor.

Human genetics and pharmacology of neurotransmitter transporters

Biogenic amine neurotransmitters are released from nerve terminals and activate pre- and postsynaptic receptors. Released neurotransmitters are sequestered by transporters into presynaptic neurons, a major mode of their inactivation in the brain. Genetic studies of human biogenic amine transporter genes, including the dopamine transporter (hDAT; SLC6A3), the serotonin transporter (hSERT; SLC6A4), and the norepinephrine transporter (hNET; SLC6A2) have provided insight into how genomic variations in these transporter genes influence pharmacology and brain physiology. Genetic variants can influence transporter function by various mechanisms, including substrate affinities, transport velocity, transporter expression levels (density), extracellular membrane expression, trafficking and turnover, and neurotransmitter release. It is increasingly apparent that genetic variants of monoamine transporters also contribute to individual differences in behavior and neuropsychiatric disorders. This chapter summarizes current knowledge of transporters with a focus on genomic variations, expression variations, pharmacology of protein variants, and known association with human diseases.

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.

Rapid delivery of the dopamine transporter to the plasmalemmal membrane upon amphetamine stimulation

The dopamine transporter, DAT, is a primary regulator of dopamine (DA) signaling at the synapse. Persistent stimulation with the substrate amphetamine (AMPH) promotes DAT internalization. AMPH rapidly elicits DA efflux, yet its effect on DAT trafficking at short times is unknown. We examined the rapid effect of AMPH on DAT trafficking in rat striatal synaptosomes using biotinylation to label surface DAT. Within 30s of treatment with 3 microM AMPH, synaptosomal DAT surface expression increased to 163% of control and remained elevated through at least 1 min before returning to control levels at 2.5 min. The increase in surface DAT was cocaine-sensitive but was not produced by DA itself. A 1-min preincubation with AMPH did not alter [(3)H]DA uptake, but did result in a higher basal DA efflux and efflux elicited in the presence of AMPH as compared to vehicle pretreatment. Reversible biotinylation experiments demonstrated that the AMPH-stimulated rise in surface DAT is due to an increase in the delivery of DAT to the plasmalemmal membrane rather than a reduction of the endocytic process. These studies suggest that AMPH has a biphasic effect on DAT trafficking and acts rapidly to regulate DAT in the plasmalemmal membrane.

Regulation of monoamine transporters: influence of psychostimulants and therapeutic antidepressants

Synaptic neurotransmission in the central nervous system (CNS) requires the precise control of the duration and the magnitude of neurotransmitter action at specific molecular targets. At the molecular level, neurotransmitter signaling is dynamically regulated by a diverse set of macromolecules including biosynthetic enzymes, secretory proteins, ion channels, pre- and postsynaptic receptors and transporters. Monoamines, 5-hydroxytryptamine or serotonin (5-HT), norepinephrine (NE), and dopamine (DA) play an important modulatory role in the CNS and are involved in numerous physiological functions and pathological conditions. Presynaptic plasma membrane transporters for 5-HT (SERT), NE (NET), and DA (DAT), respectively, control synaptic actions of these monoamines by rapidly clearing the released amine. Monoamine transporters are the sites of action for widely used antidepressants and are high affinity molecular targets for drugs of abuse including cocaine, amphetamine, and 3,4-methylenedioxymetamphetamine (MDMA) "Ecstasy." Monoamine transporters also serve as molecular gateways for neurotoxins. Emerging evidence indicates that regulation of transporter function and surface expression can be rapidly modulated by "intrinsic" transporter activity itself, and antidepressant and psychostimulant drugs that block monoamine transport have a profound effect on transporter regulation. Therefore, disregulations in the functioning of monoamine transporters may underlie many disorders of transmitter imbalance such as depression, attention deficit hyperactivity disorder, and schizophrenia. This review integrates recent progress in understanding the molecular mechanisms of monoamine transporter regulation, in particular, posttranscriptional regulation by phosphorylation and trafficking linked to cellular protein kinases, protein phosphatases, and transporter interacting proteins. The review also discusses the possible role of psychostimulants and antidepressants in influencing monoamine transport regulation.

Expression studies of naturally occurring human dopamine transporter variants identifies a novel state of transporter inactivation associated with Val382Ala

Multiple, rare, human dopamine (DA) transporter (hDAT, SLC6A3) coding variants have been described, though to date they have been incompletely characterized. Here we present studies analyzing the function and regulation of five naturally occurring coding variants, V55A, R237Q, V382A, A559V and E602G, expressed in COS-7 and SH-SY5Y cells. All variants, except V382A, exhibited levels of surface protein expression and DA transport activity comparable to hDAT. V382A, divergent at the most highly conserved residue among reported hDAT variants, exhibited significantly diminished surface expression, likely derived from inefficient plasma membrane delivery. Moreover, a greater expression of V382A protein was required to achieve comparable levels of transport to hDAT, consistent with a loss of transport function. V382A displayed a decrease in sensitivity to phorbol ester (PMA)-induced internalization, as well as an altered substrate selectivity for DA versus norepinephrine (NE). Analysis of PMA-induced V382A internalization revealed a trafficking-independent action of PMA, consistent with the existence of a surface-localized, transport-inactive state.

Variants of the primate vesicular monoamine transporter-2

The vesicular monoamine transporter-2 (VMAT2) sequesters monoamine neurotransmitters into vesicles and prevents neurotoxicity. Human or monkey striatum generated three VMAT2 immunoreactive proteins of approximately 75 kDa, approximately 52-55 kDa, and approximately 45 kDa. The approximately 55-kDa band is considered the unglycosylated native protein. Deglycosylation of the VMAT2 from striatum or human VMAT2 expressed in HEK293 cells yielded a approximately 45-kDa, but not a 55-kDa immunoreactive band. We investigated this apparent mismatch between observed molecular size and predicted size.

Nonclassical, distinct endocytic signals dictate constitutive and PKC-regulated neurotransmitter transporter internalization

Neurotransmitter transporters are critical for synaptic neurotransmitter inactivation. Transporter inhibitors markedly increase the duration and magnitude of synaptic transmission, underscoring the importance of transporter activity in neurotransmission. Recent studies indicate that membrane trafficking dynamically governs neuronal transporter cell-surface presentation in a protein kinase C-regulated manner, suggesting that transporter trafficking profoundly affects synaptic signaling. However, the molecular architecture coupling neurotransmitter transporters to the endocytic machinery is not defined. Here, we identify nonclassical, distinct endocytic signals in the dopamine transporter (DAT) that are necessary and sufficient to drive constitutive and protein kinase C-regulated DAT internalization. The DAT internalization signal is conserved across SLC6 neurotransmitter carriers and is functional in the homologous norepinephrine transporter, suggesting that this region is likely to be the endocytic signal for all SLC6 neurotransmitter transporters. The DAT endocytic signal does not conform to classic internalization motifs, suggesting that SLC6 neurotransmitter transporters may have evolved unique endocytic mechanisms.

IRIP, a new ischemia/reperfusion-inducible protein that participates in the regulation of transporter activity

We report the identification and characterization of a new ischemia/reperfusion-inducible protein (IRIP), which belongs to the SUA5/YrdC/YciO protein family. IRIP cDNA was isolated in a differential display analysis of an ischemia/reperfusion-treated kidney RNA sample. Mouse IRIP mRNA was expressed in all tissues tested, the highest level being in the testis, secretory, and endocrine organs. Besides ischemia/reperfusion, endotoxemia also activated the expression of IRIP in the liver, lung, and spleen. The transporter regulator RS1 was identified as an IRIP-interacting protein in yeast two-hybrid screening. The interaction between IRIP and RS1 was further confirmed in coimmunoprecipitation assays. A possible role of IRIP in regulating transporter activity was subsequently investigated. IRIP overexpression inhibited endogenous 1-methyl-4-phenylpyridinium (MPP+) uptake activity in HeLa cells. The activities of exogenous organic cation transporters (OCT2 and OCT3), organic anion transporter (OAT1), and monoamine transporters were also inhibited by IRIP. Conversely, inhibition of IRIP expression by small interfering RNA or antisense RNA increased MPP+ uptake. We measured transport kinetics of OCT2-mediated uptake and demonstrated that IRIP overexpression significantly decreased V(max) but did not affect K(m). On the basis of these results, we propose that IRIP regulates the activity of a variety of transporters under normal and pathological conditions.

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.

The dopamine transporter and attention-deficit/hyperactivity disorder

The high incidence of attention-deficit/hyperactivity disorder (ADHD) and escalating use of ADHD medications present a compelling case for clarifying the pathophysiology of, and developing laboratory or radiologic tests for, ADHD. Currently, the majority of specific genes implicated in ADHD encode components of catecholamine signaling systems. Of these, the dopamine transporter (DAT) is a principal target of the most widely used antihyperactivity medications (amphetamine and methylphenidate); the DAT gene is associated with ADHD, and some studies have detected abnormal levels of the DAT in brain striatum of ADHD subjects. Medications for ADHD interfere with dopamine transport by brain-region- and drug-specific mechanisms, indirectly activating dopamine- and possibly norepinephrine-receptor subtypes that are implicated in enhancing attention and experiential salience. The most commonly used DAT-selective ADHD medications raise extracellular dopamine levels in DAT-rich brain regions. In brain regions expressing both the DAT and the norepinephrine transporter (NET), the relative contributions of dopamine and norepinephrine to ADHD pathophysiology and therapeutic response are obfuscated by the capacity of the NET to clear dopamine as well as norepinephrine. Thus, ADHD medications targeting DAT or NET might disperse dopamine widely and consign dopamine storage and release to regulation by noradrenergic, as well as dopaminergic neurons.

N-linked glycosylation facilitates processing and cell surface expression of rat luteinizing hormone receptor

The extracellular domain of the luteinizing hormone (LH) receptor has six potential N-linked glycosylation sites. Although previous studies have shown that mutation of the first three sites results in decreased ligand binding at the cell surface, the role of glycosylation in LH receptor processing is not understood. In the present study, we examined whether mutation of the first three sites has any affect on receptor synthesis, processing, and degradation of the mutant receptors. The data show that mutation of N77, N152, or N173 did not affect receptor synthesis, but did significantly reduce processing of the receptor precursor to the mature, cell surface form. Furthermore, defective processing was due to increased degradation of the precursor rather than increased turnover of cell surface receptors. Thus, lack of glycosylation decreases LH receptor processing and targets the receptors for degradation thereby leading to decreased cell surface expression. These results show that glycosylation of the LH receptor plays an important role in receptor processing and cell surface expression.

Interaction between a hydroxypiperidine analogue of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine and Aspartate 68 in the human dopamine transporter

Compound (+)-R,R-D-84 is an optically active trans-hydroxy-substituted derivative of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine (D-164). As a hydroxypiperidine analog of GBR 12935, (+)-R,R-D-84 is a candidate dopamine transporter compound for the treatment of cocaine dependence. The present work addresses the functional activity of (+)-R,R-D-84 at monoamine transporters and its potential molecular mechanism involving acidic amino acids (D and E). The selectivity for the dopamine vs. serotonin transporter of (+)-R,R-D-84 was greater than that of (-)-S,S-D-83, its enantiomer, and the selectivity of both compounds was greater than that of GBR 12909 (diphenyl-fluorinated GBR 12935). Only (+)-R,R-D-84 displayed improved selectivity vs. the norepinephrine transporter. D313N or E215Q mutation did not alter the pattern of affinities (measured by membrane binding of the cocaine analog [3H]CFT) for the dopamine transporter of (+)-R,R-D-84, (-)-S,S-D-83, D-164 (non-hydroxylated analog), or GBR 12909. In contrast, D68N mutation specifically lowered the affinity of (+)-R,R-D-84, pointing to a role for D68 in the interaction with (+)-R,R-D-84, possibly through hydrogen bonding between the hydroxyl and the carboxyl group of D68 which is lacking in N68. The present results, combined with behavioral data, implicate D68 in the dopamine transporter in cocaine antagonist activity of (+)-R,R-D-84.

A comprehensive atlas of the topography of functional groups of the dopamine transporter

The neuronal dopamine transporter (DAT) is a transmembrane transporter that clears DA from the synaptic cleft. Knowledge of DAT functional group topography is a prerequisite for understanding the molecular basis of transporter function, the actions of psychostimulant drugs, and mechanisms of dopaminergic neurodegeneration. Information concerning the molecular interactions of drugs of abuse (such as cocaine, amphetamine, and methamphetamine) with the DAT at the functional group level may also aid in the development of compounds useful as therapeutic agents for the treatment of drug abuse. This review will provide a cumulative and comprehensive focus on the amino acid functional group topography of the rat and human DATs, as revealed by protein chemical modification and the techniques of site-directed mutagenesis. The results from these studies, represented mostly by site-directed mutagenesis, can be classified into several main categories: modifications without substantial affects on substrate transport, DAT membrane expression, or cocaine analog binding; those modifications which alter both substrate transport and cocaine analog binding; and those that affect DAT membrane expression. Finally, some modifications can selectively affect either substrate transport or cocaine analog binding. Taken together, these literature results show that domains for substrates and cocaine analogs are formed by interactions with multiple and sometimes distinct DAT functional groups.

Parkin increases dopamine uptake by enhancing the cell surface expression of dopamine transporter

Mutations of parkin, a protein-ubiquitin E3 ligase, are linked to Parkinson's disease (PD). Although a variety of parkin substrates have been identified, none of these is selectively expressed in dopaminergic neurons, whose degeneration plays a critical role in PD. Here we show that parkin significantly increased dopamine uptake in the human dopaminergic neuroblastoma cell line SH-SY5Y. This effect was accompanied by increased V(max) of dopamine uptake and unchanged K(m). Consistent with this, increased binding sites for dopamine transporter (DAT) ligand were observed in SH-SY5Y cells overexpressing parkin. The results were confirmed when parkin was transfected in HEK293 cells stably expressing DAT. In these cells, parkin enhanced the ubiquitination and degradation of DAT, increased its cell surface expression, and augmented dopamine uptake. The effects of parkin were significantly abrogated by its PD-causing mutations. Because the cell surface expression of functional DAT requires its oligomerization, misfolded DAT, induced either by the protein glycosylation inhibitor tunicamycin or by its C-terminal truncation, significantly attenuated cell surface expression of native DAT and reduced dopamine uptake. Expression of parkin, but not its T240R mutant, significantly alleviated these detrimental effects of misfolded DAT. Thus, our studies suggest that parkin increases dopamine uptake by enhancing the ubiquitination and degradation of misfolded DAT, so as to prevent it from interfering with the oligomerization and cell surface expression of native DAT. This function of parkin would enhance the precision of dopaminergic transmission, increase the efficiency of dopamine utilization, and reduce dopamine toxicity on neighboring cells.