Proteomic analysis of human norepinephrine transporter complexes reveals associations with protein phosphatase 2A anchoring subunit and 14-3-3 proteins

Venlafaxine antagonizes the noradrenaline-promoted colon cancer progression by inhibiting the norepinephrine transporter

Epidemiological studies have demonstrated that the use of antidepressants is associated with a decreased risk of colorectal cancer (CRC); however, the mechanisms behind this association are yet unknown. Adrenergic system contributes to the stress-related tumor progression, with norepinephrine (NE) mainly secreted from adrenergic nerve fibers. Norepinephrine serotonin reuptake inhibitors are successfully used antidepressants. This study demonstrates that a widely used antidepressant venlafaxine (VEN) antagonizes NE-promoted colon cancer in vivo and in vitro. Bioinformatic analysis suggested that NE transporter (NET, SLC6A2), a target of VEN, was closely associated with the prognosis of clinical patients with CRC. In addition, the knockdown of NET antagonized the effect of NE. The NET-protein phosphatase 2 scaffold subunit alpha/phosphorylated Akt/vascular endothelial growth factor pathway partially mediates the antagonizing effect of VEN on NE's actions in colon cancer cells. These were also confirmed by in vivo experiments. Our findings revealed for the first time that, in addition to its primary function as a transporter, NET also promotes NE-enhanced colon cancer cell proliferation, tumor angiogenesis, and tumor growth. This provides direct experimental and mechanistic evidence for the use of antidepressant VEN in the treatment of CRC and a therapeutic potential for repurposing existing drugs as an anti-cancer approach to improve the prognosis of patients with CRC.

Dephosphorylation of human dopamine transporter at threonine 48 by protein phosphatase PP1/2A up-regulates transport velocity

Several protein kinases, including protein kinase C, Ca²⁺/calmodulin-dependent protein kinase II, and extracellular signal-regulated kinase, play key roles in the regulation of dopamine transporter (DAT) functions. These functions include surface expression, internalization, and forward and reverse transport, with phosphorylation sites for these kinases being linked to distinct regions of the DAT N terminus. Protein phosphatases (PPs) also regulate DAT activity, but the specific residues associated with their activities have not yet been elucidated. In this study, using co-immunoprecipitation followed by MS and immunoblotting analyses, we demonstrate the association of DAT with PP1 and PP2A in the mouse brain and heterologous cell systems. By applying MS in conjunction with a metabolic labeling method, we defined a PP1/2A-sensitive phosphorylation site at Thr-48 in human DAT, a residue that has not been previously reported to be involved in DAT phosphorylation. Site-directed mutagenesis of Thr-48 to Ala (T48A) to prevent phosphorylation enhanced dopamine transport kinetics, supporting a role for this residue in regulating DAT activity. Moreover, T48A-DAT displayed increased palmitoylation, suggesting that phosphorylation/dephosphorylation at this site has an additional regulatory role and reinforcing a previously reported reciprocal relationship between C-terminal palmitoylation and N-terminal phosphorylation.

Serotonin Transporter Associated Protein Complexes Are Enriched in Synaptic Vesicle Proteins and Proteins Involved in Energy Metabolism and Ion Homeostasis

The serotonin transporter (SERT) mediates Na⁺-dependent high-affinity serotonin uptake and plays a key role in regulating extracellular serotonin concentration in the brain and periphery. To gain novel insight into SERT regulation, we conducted a comprehensive proteomics screen to identify components of SERT-associated protein complexes in the brain by employing three independent approaches. In vivo SERT complexes were purified from rat brain using an immobilized high-affinity SERT ligand, amino-methyl citalopram. This approach was combined with GST pulldown and yeast two-hybrid screens using N- and C-terminal cytoplasmic transporter domains as bait. Potential SERT associated proteins detected by at least two of the interaction methods were subjected to gene ontology analysis resulting in the identification of functional protein clusters that are enriched in SERT complexes. Prominent clusters include synaptic vesicle proteins, as well as proteins involved in energy metabolism and ion homeostasis. Using subcellular fractionation and electron microscopy we provide further evidence that SERT is indeed associated with synaptic vesicle fractions, and colocalizes with small vesicular structures in axons and axon terminals. We also show that SERT is found in close proximity to mitochondrial membranes in both, hippocampal and neocortical regions. We propose a model of the SERT interactome, in which SERT is distributed between different subcellular compartments through dynamic interactions with site-specific protein complexes. Finally, our protein interaction data suggest novel hypotheses for the regulation of SERT activity and trafficking, which ultimately impact on serotonergic neurotransmission and serotonin dependent brain functions.

Ca2+ dependent surface trafficking of norepinephrine transporters depends on threonine 30 and Ca2+ calmodulin kinases

The antidepressant-sensitive norepinephrine (NE) transporter (NET) inactivates NE released during central and peripheral neuronal activity by transport into presynaptic cells. Altered NE clearance due to dysfunction of NET has been associated with the development of mental illness and cardiovascular diseases. NET activity in vivo is influenced by stress, neuronal activity, hormones and drugs. We investigated the mechanisms of Ca²⁺ regulation of NET and found that Ca²⁺ influenced both V_max and K_m for NE transport into cortical synaptosomes. Changes in extracellular Ca²⁺ triggered rapid and bidirectional surface trafficking of NET expressed in cultured cells. Deletion of residues 28-47 in the NET NH₂-terminus abolished the Ca²⁺ effect on surface trafficking. Mutagenesis studies identified Thr30 in this region as the essential residue for both Ca²⁺- dependent phosphorylation and trafficking of NET. Depolarization of excitable cells increased surface NET in a Thr30 dependent manner. A proteomic analysis, RNA interference, and pharmacological inhibition supported roles of CaMKI and CaMKII in Ca²⁺-modulated NE transport and NET trafficking. Depolarization of primary noradrenergic neurons in culture with elevated K⁺ increased NET surface expression in a process that required external Ca²⁺ and depended on CaMK activity. Hippocampal NE clearance in vivo was also stimulated by depolarization, and inhibitors of CaMK signaling prevented this stimulation. In summary, Ca²⁺ signaling influenced surface trafficking of NET through a CaMK-dependent mechanism requiring Thr30.

Guidelines for Homology Modeling of Dopamine, Norepinephrine, and Serotonin Transporters

The human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT) are carriers of neurotransmitters and targets for many drugs. Pioneering works in the past three years to elucidate experimental models of the Drosophila dDAT and human hSERT structures will rapidly impact the field of neuroscience. Here, we evaluated automated homology-based human models of these transporters, employing systematic physics-based, knowledge-based, and empirical-based check. Modeling guidelines were conveyed with attention to the central binding site (S1), secondary binding site (S2), and the extracellular loops EL2 and EL4. Application of new experimental models (dDAT and hSERT) will improve the accuracy of homology models, previously utilizing prokaryotic leucine transporter (LeuT) structure, and provide better predictions of ligand interactions, which is required for understanding of cellular mechanisms and for development of novel therapeutics.

Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters

Modulation of neurotransmission by the monoamines dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is critical for normal nervous system function. Precise temporal and spatial control of this signaling in mediated in large part by the actions of monoamine transporters (DAT, NET, and SERT, respectively). These transporters act to recapture their respective neurotransmitters after release, and disruption of clearance and reuptake has significant effects on physiology and behavior and has been linked to a number of neuropsychiatric disorders. To ensure adequate and dynamic control of these transporters, multiple modes of control have evolved to regulate their activity and trafficking. Central to many of these modes of control are the actions of protein kinases, whose actions can be direct or indirectly mediated by kinase-modulated protein interactions. Here, we summarize the current state of our understanding of how protein kinases regulate monoamine transporters through changes in activity, trafficking, phosphorylation state, and interacting partners. We highlight genetic, biochemical, and pharmacological evidence for kinase-linked control of DAT, NET, and SERT and, where applicable, provide evidence for endogenous activators of these pathways. We hope our discussion can lead to a more nuanced and integrated understanding of how neurotransmitter transporters are controlled and may contribute to disorders that feature perturbed monoamine signaling, with an ultimate goal of developing better therapeutic strategies.

PP2A as a master regulator of the cell cycle

Protein phosphatase 2A (PP2A) plays a critical multi-faceted role in the regulation of the cell cycle. It is known to dephosphorylate over 300 substrates involved in the cell cycle, regulating almost all major pathways and cell cycle checkpoints. PP2A is involved in such diverse processes by the formation of structurally distinct families of holoenzymes, which are regulated spatially and temporally by specific regulators. Here, we review the involvement of PP2A in the regulation of three cell signaling pathways: wnt, mTOR and MAP kinase, as well as the G1→S transition, DNA synthesis and mitotic initiation. These processes are all crucial for proper cell survival and proliferation and are often deregulated in cancer and other diseases.

Proteomic approaches and identification of novel therapeutic targets for alcoholism

Recent studies have shown that gene regulation is far more complex than previously believed and does not completely explain changes at the protein level. Therefore, the direct study of the proteome, considerably different in both complexity and dynamicity to the genome/transcriptome, has provided unique insights to an increasing number of researchers. During the past decade, extraordinary advances in proteomic techniques have changed the way we can analyze the composition, regulation, and function of protein complexes and pathways underlying altered neurobiological conditions. When combined with complementary approaches, these advances provide the contextual information for decoding large data sets into meaningful biologically adaptive processes. Neuroproteomics offers potential breakthroughs in the field of alcohol research by leading to a deeper understanding of how alcohol globally affects protein structure, function, interactions, and networks. The wealth of information gained from these advances can help pinpoint relevant biomarkers for early diagnosis and improved prognosis of alcoholism and identify future pharmacological targets for the treatment of this addiction.

Regulated norepinephrine transporter interaction with the neurokinin-1 receptor establishes transporter subcellular localization

Neurokinin-1 receptor (NK1R) mediates down-regulation of human norepinephrine (NE) transporter (hNET) via protein kinase C (PKC). However, native NET regulation by NK1R and the mechanism by which NK1R targets NET among other potential effectors are unknown. Effect of NK1R activation on native NET regulation and NET/NK1R interaction were studied using rat brain synaptosomes expressing native NET and NK1R as well as human placental trophoblast (HTR) cells coexpressing WT-hNET or NK1R/PKC-resistant hNET-T258A,S259A double mutant (NET-DM) and hNK1R. The selective NK1R agonist, GR73632, and Substance-P (SP) inhibited NE transport and reduced plasma membrane expression of NET and NK1R. Pretreatment with the NK1R antagonist, EMEND (aprepitant) prevented these NK1R-mediated effects. Immunoprecipitation experiments showed that NET forms stable complexes with NK1R. In HTR cells, combined biotinylation and immunoprecipitation studies revealed plasma membrane localization of NET·NK1R complexes. Receptor activation resulted in the internalization of NET·NK1R complexes. Lipid raft and immunoprecipitation analyses revealed the presence of NET·NK1R complexes exclusively in non-raft membrane fractions under basal/unstimulated conditions. However, NK1R activation led to translocation of NET·NK1R complexes to raft-rich membrane fractions. Importantly, PKCα was found in association with raft-localized NET following SP treatment. Similar to WT-NET, PKC-resistant NET-DM was found in association with NK1R exclusively in non-raft fractions. However, SP treatment failed to translocate NET-DM·NK1R complexes from non-raft fractions to raft fractions. Collectively, these results suggest that NK1R forms physical complexes with NET and that the receptor-mediated Thr(258) + Ser(259) motif-dependent translocation of NET·NK1R complexes into raft-rich microdomains facilitates NET/NK1R interaction with PKCα to coordinate spatially restricted NET regulation.

Spinocerebellar ataxia type 12

SCA12 is a late-onset, autosomal dominant, slowly progressive disorder. Action tremor is the usual presenting sign. Subsequent development of ataxia and hyperreflexia suggests spinocerebellar ataxia. In the index SCA12 kindred, which resides in North America and is of German ancestry, parkinsonism, anxiety, depression, and cognitive dysfunction are not uncommon. SCA12 is linked to a CAG repeat expansion mutation in exon 7 of PPP2R2B, a gene that encodes Bβ, a regulatory subunit of protein phosphatase 2A (PP2A). CAG repeats number 7-28 in normal individuals and 55-78 in SCA12 patients. The mechanism by which this mutation leads to SCA12 has not been determined. The CAG expansion in PPP2R2B has promoter function in vitro. CAG length correlates with increased Bβ expression. There is no evidence that this CAG expansion results in polyglutamine production. In addition to the North. American SCA12 kindred, multiple SCA12 families have been found in Northern India that are not related to the index SCA12 kindred. SCA12 has been reported, rarely, in Singapore and China. Action tremor, anxiety, and depression in SCA12 have responded to usual treatments for these disorders. SCA12 may be considered in patients who present with action tremor and later develop signs of cerebellar and cortical dysfunction.

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.

Cocaine up-regulation of the norepinephrine transporter requires threonine 30 phosphorylation by p38 mitogen-activated protein kinase

The norepinephrine (NE) transporter (NET) regulates NE signaling by rapidly clearing synaptic NE. Cocaine binds NET and modulates NE transport. These actions contribute to rewarding effects and abuse liability of cocaine. Activation of mitogen-activated protein kinase (MAPK) cascades is implicated in cocaine-induced neuroadaptations. However, the role of MAPK and the mechanisms involved in cocaine modulation of NET are not clear. Acute intra-peritoneal injections of cocaine (20 mg/kg body weight) to rats resulted in increased NE uptake by prefrontal cortex (PFC) synaptosomes with a parallel increase in the surface expression of endogenous NET. Cocaine also enhanced the immunoreactivity of phospho-p38 MAPK in the PFC synaptosomes without affecting the total p38 MAPK. In vitro cocaine (30-50 μM) treatment of rat PFC synaptosomes increased native NET function, surface expression, and phosphorylation in a manner sensitive to p38 MAPK inhibition by PD169316. We next examined cocaine-elicited effects on wild-type human NET (hNET) expressed heterologously in human placental trophoblast cells to gain more insights into the mechanisms involved. Cocaine treatment of hNET expressing human placental trophoblast cells up-regulated the function, surface expression, and phosphorylation of hNET in a PD169316-sensitive manner. In addition, cocaine inhibited constitutive endocytosis of hNET. Mutational analysis of serine and threonine residues revealed that substitution of threonine 30, located at the amino terminus of hNET with alanine (T30A-hNET), abolished cocaine-induced up-regulation of NET function, surface expression, and phosphorylation. Furthermore, cocaine did not alter T30A-hNET endocytosis. These studies identify a novel molecular mechanism that cocaine-activated p38 MAPK-mediated phosphorylation of NET-T30 dictates surface NET availability, and hence, NE transport.

Molecular targets of alcohol action: Translational research for pharmacotherapy development and screening

Alcohol abuse and dependence are multifaceted disorders with neurobiological, psychological, and environmental components. Research on other complex neuropsychiatric diseases suggests that genetically influenced intermediate characteristics affect the risk for heavy alcohol consumption and its consequences. Diverse therapeutic interventions can be developed through identification of reliable biomarkers for this disorder and new pharmacological targets for its treatment. Advances in the fields of genomics and proteomics offer a number of possible targets for the development of new therapeutic approaches. This brain-focused review highlights studies identifying neurobiological systems associated with these targets and possible pharmacotherapies, summarizing evidence from clinically relevant animal and human studies, as well as sketching improvements and challenges facing the fields of proteomics and genomics. Concluding thoughts on using results from these profiling technologies for medication development are also presented.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

Rab11 supports amphetamine-stimulated norepinephrine transporter trafficking

The norepinephrine transporter (NET) is a presynaptic plasma membrane protein that mediates reuptake of synaptically released norepinephrine. NET is also a major target for medications used for the treatment of depression, attention deficit/hyperactivity disorder, narcolepsy, and obesity. NET is regulated by numerous mechanisms, including catalytic activation and membrane trafficking. Amphetamine (AMPH), a psychostimulant and NET substrate, has also been shown to induce NET trafficking. However, neither the molecular basis nor the nature of the relevant membrane compartments of AMPH-modulated NET trafficking has been defined. Indeed, direct visualization of drug-modulated NET trafficking in neurons has yet to be demonstrated. In this study, we used a recently developed NET antibody and the presence of large presynaptic boutons in sympathetic neurons to examine basal and AMPH-modulated NET trafficking. Specifically, we establish a role for Rab11 in AMPH-induced NET trafficking. First, we found that, in cortical slices, AMPH induces a reduction in surface NET. Next, we observed AMPH-induced accumulation and colocalization of NET with Rab11a and Rab4 in presynaptic boutons of cultured neurons. Using tagged proteins, we demonstrated that NET and a truncated Rab11 effector (FIP2DeltaC2) do not redistribute in synchrony, whereas NET and wild-type Rab11a do. Analysis of various Rab11a/b mutants further demonstrates that Rab11 regulates NET trafficking. Expression of the truncated Rab11a effector (FIP2DeltaC2) attenuates endogenous Rab11 function and prevented AMPH-induced NET internalization as does GDP-locked Rab4 S22N. Our data demonstrate that AMPH leads to an increase of NET in endosomes of single boutons and varicosities in a Rab11-dependent manner.

Involvement of threonine 258 and serine 259 motif in amphetamine-induced norepinephrine transporter endocytosis

D-amphetamine (AMPH) down-regulates the norepinephrine transporter (NET), although the exact trafficking pathways altered and motifs involved are not known. Therefore, we examined the cellular and molecular mechanisms involved in AMPH-induced NET regulation in human placental trophoblast cells expressing the wild-type (WT)-hNET and the hNET double mutant (DM)-bearing protein kinase C (PKC)-resistant T258A + S259A motif. NET function and surface expression were significantly reduced in cells expressing WT-hNET but not in cells expressing hNET-DM following AMPH treatment. AMPH inhibited plasma membrane recycling of both WT-hNET and hNET-DM. In contrast, AMPH stimulated endocytosis of WT-hNET, and did not affect hNET-DM endocytosis. Although PKC or calcium/calmodulin- dependent kinase-II (CaMKII) inhibition or depletion of calcium failed to block AMPH-mediated down-regulation of WT-hNET, NET-specific blocker desipramine completely prevented AMPH-induced down-regulation. Furthermore, AMPH treatment had no effect on phospho-CaMKII immunoreactivity. The inhibitory potency of AMPH was highest on hNET-DM, intermediary on T258A and S259A single mutants and lowest on WT-hNET. Single mutants exhibited partial resistance to AMPH-mediated down-regulation. AMPH accumulation was similar in cells expressing WT-hNET or hNET-DM. The results demonstrate that reduced plasma membrane insertion and enhanced endocytosis account for AMPH-mediated NET down-regulation, and provide the first evidence that T258/S259 motif is involved only in AMPH-induced NET endocytosis that is desipramine-sensitive, but PKC and CaMKII independent.

Subcellular localization of the antidepressant-sensitive norepinephrine transporter

BACKGROUND

Reuptake of synaptic norepinephrine (NE) via the antidepressant-sensitive NE transporter (NET) supports efficient noradrenergic signaling and presynaptic NE homeostasis. Limited, and somewhat contradictory, information currently describes the axonal transport and localization of NET in neurons.

RESULTS

We elucidate NET localization in brain and superior cervical ganglion (SCG) neurons, aided by a new NET monoclonal antibody, subcellular immunoisolation techniques and quantitative immunofluorescence approaches. We present evidence that axonal NET extensively colocalizes with syntaxin 1A, and to a limited degree with SCAMP2 and synaptophysin. Intracellular NET in SCG axons and boutons also quantitatively segregates from the vesicular monoamine transporter 2 (VMAT2), findings corroborated by organelle isolation studies. At the surface of SCG boutons, NET resides in both lipid raft and non-lipid raft subdomains and colocalizes with syntaxin 1A.

CONCLUSION

Our findings support the hypothesis that SCG NET is segregated prior to transport from the cell body from proteins comprising large dense core vesicles. Once localized to presynaptic boutons, NET does not recycle via VMAT2-positive, small dense core vesicles. Finally, once NET reaches presynaptic plasma membranes, the transporter localizes to syntaxin 1A-rich plasma membrane domains, with a portion found in cholera toxin-demarcated lipid rafts. Our findings indicate that activity-dependent insertion of NET into the SCG plasma membrane derives from vesicles distinct from those that deliver NE. Moreover, NET is localized in presynaptic membranes in a manner that can take advantage of regulatory processes targeting lipid raft subdomains.

Two allelic isoforms of the serotonin transporter from Schistosoma mansoni display electrogenic transport and high selectivity for serotonin

The human blood fluke Schistosoma mansoni is the primary cause of schistosomiasis, a debilitating disease that affects 200 million individuals in over 70 countries. The biogenic amine serotonin is essential for the survival of the parasite and serotonergic proteins are potential novel drug targets for treating schistosomiasis. Here we characterize two novel serotonin transporter gene transcripts, SmSERT-A and SmSERT-B, from S.mansoni. Southern blot analysis shows that the two mRNAs are the products of different alleles of a single SmSERT gene locus. The two SmSERT forms differ in three amino acid positions near the N-terminus of the protein. Both SmSERTs are expressed in the adult form and in the sporocyst form (infected snails) of the parasite, but are absent from all other stages of the parasite's complex life cycle. Heterologous expression of the two cDNAs in mammalian cells resulted in saturable, sodium-dependent serotonin transport activity with an apparent affinity for serotonin comparable to that of the human serotonin transporter. Although the two SmSERTs are pharmacologically indistinguishable from each other, efflux experiments reveal notably higher substrate selectivity for serotonin compared with their mammalian counterparts. Several well-established substrates for human SERT including (+/-)MDMA, S-(+)amphetamine, RU 24969, and m-CPP are not transported by SmSERTs, underscoring the higher selectivity of the schistosomal isoforms. Voltage-clamp recordings of SmSERT substrate-elicited currents confirm the substrate selectivity observed in efflux experiments and suggest that it may be possible to exploit the electrogenic nature of SmSERT to screen for compounds that target the parasite in vivo.

Association of changes in norepinephrine and serotonin transporter expression with the long-term behavioral effects of antidepressant drugs

Previous work has shown that repeated desipramine treatment causes downregulation of the norepinephrine transporter (NET) and persistent antidepressant-like effects on behavior, ie effects observed 2 days after discontinuation of drug treatment when acute effects are minimized. The present study examined whether this mechanism generalizes to other antidepressants and also is evident for the serotonin transporter (SERT). Treatment of rats for 14 days with 20 mg/kg per day protriptyline or 7.5 mg/kg per day sertraline reduced NET and SERT expression, respectively, in cerebral cortex and hippocampus; these treatments also induced a persistent antidepressant-like effect on forced-swim behavior. Increased serotonergic neurotransmission likely mediated the behavioral effect of sertraline, as it was blocked by inhibition of serotonin synthesis with p-chlorophenylalanine; a parallel effect was observed previously for desipramine and noradrenergic neurotransmission. Treatment with 20 mg/kg per day reboxetine for 42, but not 14, days reduced NET expression; antidepressant-like effects on behavior were observed for both treatment durations. Treatment for 14 days with 70 mg/kg per day venlafaxine, which inhibits both the NET and SERT, or 10 mg/kg per day phenelzine, a monoamine oxidase inhibitor, produced antidepressant-like effects on behavior without altering NET or SERT expression. For all drugs tested, reductions of NET and SERT protein were not accompanied by reduced NET or SERT mRNA in locus coeruleus or dorsal raphe nucleus, respectively. Overall, the present results suggest an important, though not universal, role for NET and SERT regulation in the long-term behavioral effects of antidepressants. Understanding the mechanisms underlying transporter regulation in vivo may suggest novel targets for the development of antidepressant drugs.

Norepinephrine transport-mediated gene expression in noradrenergic neurogenesis

Background

We have identified a differential gene expression profile in neural crest stem cells that is due to deletion of the norepinephrine transporter (NET) gene. NET is the target of psychotropic substances, such as tricyclic antidepressants and the drug of abuse, cocaine. NET mutations have been implicated in depression, anxiety, orthostatic intolerance and attention deficit hyperactivity disorder (ADHD). NET function in adult noradrenergic neurons of the peripheral and central nervous systems is to internalize norepinephrine from the synaptic cleft. By contrast, during embryogenesis norepinephrine (NE) transport promotes differentiation of neural crest stem cells and locus ceruleus progenitors into noradrenergic neurons, whereas NET inhibitors block noradrenergic differentiation. While the structure of NET und the regulation of NET function are well described, little is known about downstream target genes of norepinephrine (NE) transport.

Results

We have prepared gene expression profiles of in vitro differentiating wild type and norepinephrine transporter-deficient (NETKO) mouse neural crest cells using long serial analysis of gene expression (LongSAGE). Comparison analyses have identified a number of important differentially expressed genes, including genes relevant to neural crest formation, noradrenergic neuron differentiation and the phenotype of NETKO mice. Examples of differentially expressed genes that affect noradrenergic cell differentiation include genes in the bone morphogenetic protein (BMP) signaling pathway, the Phox2b binding partner Tlx2, the ubiquitin ligase Praja2, and the inhibitor of Notch signaling, Numbl. Differentially expressed genes that are likely to contribute to the NETKO phenotype include dopamine-β-hydroxylase (Dbh), tyrosine hydroxylase (Th), the peptide transmitter 'cocaine and amphetamine regulated transcript' (Cart), and the serotonin receptor subunit Htr3a. Real-time PCR confirmed differential expression of key genes not only in neural crest cells, but also in the adult superior cervical ganglion and locus ceruleus. In addition to known genes we have identified novel differentially expressed genes and thus provide a valuable database for future studies.

Conclusion

Loss of NET function during embryonic development in the mouse deregulates signaling pathways that are critically involved in neural crest formation and noradrenergic cell differentiation. The data further suggest deregulation of signaling pathways in the development and/or function of the NET-deficient peripheral, central and enteric nervous systems.

Norepinephrine transporter regulation mediates the long-term behavioral effects of the antidepressant desipramine

The relationship between the ability of repeated desipramine treatment to cause downregulation of the norepinephrine transporter (NET) and produce antidepressant-like effects on behavior was determined. Treatment of rats with 15 mg/kg per day desipramine reduced NET expression, measured by (3)H-nisoxetine binding and SDS-PAGE/immunoblotting, in cerebral cortex and hippocampus and reduced the time of immobility in the forced-swim test. The antidepressant-like effect on forced-swim behavior was evident 2 days following discontinuation of desipramine treatment when plasma and brain levels of desipramine and its major metabolite desmethyldesipramine were not detectable. Reduced NET expression resulted in reduced norepinephrine uptake, measured in vitro, and increased noradrenergic neurotransmission, measured in vivo using microdialysis. Overall, the dose-response and time-of-recovery relationships for altered NET expression matched those for production of antidepressant-like effects on behavior. The importance of increased noradrenergic neurotransmission in the persistent antidepressant-like effect on behavior was confirmed by demonstrating that it was blocked by inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine. The present results suggest an important role for NET regulation in the long-term behavioral effects of desipramine and are consistent with clinical data suggesting that enhanced noradrenergic neurotransmission is necessary, but not sufficient, for its antidepressant actions. Understanding the mechanisms underlying NET regulation in vivo may suggest novel targets for therapeutic intervention in the treatment of depression.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

Desvenlafaxine Succinate Identifies Novel Antagonist Binding Determinants in the Human Norepinephrine Transporter

Desvenlafaxine succinate (DVS) is a recently introduced antagonist of the human norepinephrine and serotonin transporters (hNET and hSERT, respectively), currently in clinical development for use in the treatment of major depressive disorder and vasomotor symptoms associated with menopause. Initial evaluation of the pharmacological properties of DVS (J Pharmacol Exp Ther 318:657-665, 2006) revealed significantly reduced potency for the hNET expressed in membranes compared with whole cells when competing for [(3)H]nisoxetine (NIS) binding. Using hNET in transfected human embryonic kidney-293 cells, this difference in potency for DVS at sites labeled by [(3)H]NIS was found to distinguish DVS, the DVS analog rac-(1-[1-(3-chloro-phenyl)-2-(4-methylpiperazin-1-yl)-ethyl]cyclohexanol (WY-46824), methylphenidate, and the cocaine analog 3beta-(4-iodophenyl)tropane-2beta-carboxylic acid methyl ester (RTI-55) from other hNET antagonists, such as NIS, mazindol, tricyclic antidepressants, and cocaine. These differences seem not to arise from preparation-specific perturbations of ligand intrinsic affinity or antagonist-specific surface trafficking but rather from protein conformational alterations that perturb the relationships between distinct hNET binding sites. In an initial search for molecular features that differentially define antagonist binding determinants, we document that Val148 in hNET transmembrane domain 3 selectively disrupts NIS binding but not that of DVS.

Defining the dopamine transporter proteome by convergent biochemical and in silico analyses

Monoamine transporters play a key role in neuronal signaling by mediating reuptake of neurotransmitters from the synapse. The function of the dopamine transporter (DAT), an important member of this family of transporters, is regulated by multiple signaling mechanisms, which result in altered cell surface trafficking of DAT. Protein-protein interactions are likely critical for this mode of transporter regulation. In this study, we identified proteins associated with DAT by immunoprecipitation (IP) followed by mass spectrometry. We identified 20 proteins with diverse cellular functions that can be classified as trafficking proteins, cytoskeletal proteins, ion channels and extracellular matrix-associated proteins. DAT was found to associate with the voltage-gated potassium channel Kv2.1 and synapsin Ib, a protein involved in regulating neurotransmitter release. An in silico analysis provided evidence for common transcriptional regulation of the DAT proteome genes. In summary, this study identified a network of proteins that are primary candidates for functional regulation of the DAT, an important player in mechanisms of mental disorders and drug addiction.

Attenuation of the norepinephrine transporter activity and trafficking via interactions with α-synuclein

Alpha-synuclein (alpha-Syn) has been studied in the context of Parkinson's disease, but its normative role remains elusive. We have shown that alpha-Syn regulates the homeostasis of dopaminergic and serotonergic synapses, through trafficking of the dopamine and serotonin transporter, respectively. In the present study we sought to determine if alpha-Syn could also modulate noradrenergic signaling, by studying its interactions with the norepinephrine transporter (NET). We co-transfected Ltk- cells with increasing amounts of alpha-Syn DNA and a constant amount of NET DNA, and observed a progressive decrease (68%) in [3H]-NE uptake in cells co-transfected with a ratio of 3:1 alpha-Syn:NET DNA. The Kd of transport did not change, but increasing alpha-Syn caused a decrease in the Vmax of the transporter, from 2.27+/-0.14 to 0.89+/-0.15 pmol/min/10(5) cells, with NET expression alone or 4:1 ratio of alpha-Syn:NET transfection, respectively. Decreases in surface biotinylation and [3H]-nisoxetine binding kinetics in intact cells revealed that NET cell surface expression was attenuated in correlation to the amount of alpha-Syn co-transfected into cells. The interaction between NET and alpha-Syn occurred via the NAC domain of alpha-Syn, the region directly responsible for self-aggregation. These findings are the first to show that alpha-Syn has a central role in the homeostasis of noradrenergic neurons. Together with our previous studies on dopamine and serotonin transporters, we propose that a primary physiological role of alpha-Syn may be to regulate the homeostasis of monoamines in synapses, through modulatory interactions of the protein with monoaminergic transporters.

Oligomerization of neurotransmitter transporters: a ticket from the endoplasmic reticulum to the plasma membrane

Cellular localization of neurotransmitter transporters is important for the precise control of synaptic transmission. By removing the neurotransmitters from the synaptic cleft, these transporters terminate signalling and affect duration and intensity of neurotransmission. Thus, a lot of work has been invested in the determination of the cellular compartment to which neurotransmitter transporters localize. In particular, the polarized distribution has received substantial attention. However, trafficking of transporters in the early secretory pathway has been largely ignored. Oligomer formation is a prerequisite for newly formed transporters to pass the stringent quality control mechanisms of the endoplasmic reticulum (ER), and this quaternary structure is also the preferred state which transporters reside in at the plasma membrane. Only properly assembled transporters are able to recruit the coatomer coat proteins that are needed for ER-to-Golgi trafficking. In this review, we will start with a brief description on transporter oligomerization that underlies ER-to-Golgi trafficking, followed by an introduction to ER-to-Golgi trafficking of neurotransmitter transporters. Finally, we will discuss the importance of oligomer formation for the pharmacological action of the illicitly used amphetamines and its derivatives.

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.

The dopamine transporter proteome

Dopamine (DA) uptake through the neuronal plasma membrane DA transporter (DAT) is essential for the maintenance of normal DA homeostasis in the brain. The DAT-mediated re-uptake system limits not only the intensity but also the duration of DA actions at presynaptic and postsynaptic receptors. This protein is the primary target for cocaine and amphetamine, both highly addictive and major substances of abuse worldwide. DAT is also the molecular target for therapeutic agents used in the treatment of mental disorders, such as attention deficit hyperactivity disorder and depression. Given the role played by the DAT in regulation of DA neurotransmission and its contribution to the abuse potential of psychostimulants, it becomes not only important but also necessary to understand the functional regulation of this protein. To investigate the cellular and molecular mechanisms associated with DAT function and regulation, our laboratory and others have embarked on a systematic search for DAT protein-protein interactions. Recently, a growing number of proteins have been shown to interact with DAT. These novel interactions might be important in the assembly, targeting, trafficking and/or regulation of transporter function. In this review, I summarize the main findings obtained from the characterization of DAT-interacting proteins and discuss the functional implications of these novel interactions. Based on these new data, I propose to use the term DAT proteome to explain how interacting proteins regulate DAT function. These novel interactions might help define new mechanisms associated with the function of the transporter.

Getting the Message Across: A Recent Transporter Structure Shows the Way

Efforts to define the mechanisms governing neurotransmitter uptake and drug action have moved into high gear with the publication of a high-resolution structure of a leucine transporter from Aquifex aeolicus, a bacterial member of the SLC6 transporter family. Solved with the substrate leucine bound, the new structure corroborates extensive biochemical and mutagenesis studies performed with related mammalian neurotransmitter transporters and provides exciting suggestions as to how coupling arises between ions and substrates to permit efficient neurotransmitter clearance.

14-3-3 proteins in membrane protein transport

14-3-3 proteins affect the cell surface expression of several unrelated cargo membrane proteins, e.g., MHC II invariant chain, the two-pore potassium channels KCNK3 and KCNK9, and a number of different reporter proteins exposing Arg-based endoplasmic reticulum localization signals in mammalian and yeast cells. These multimeric membrane proteins have a common feature in that they all expose coatomer protein complex I (COPI)- and 14-3-3-binding motifs. 14-3-3 binding depends on phosphorylation of the membrane protein in some and on multimerization of the membrane protein in other cases. Evidence from mutant proteins that are unable to interact with either COPI or 14-3-3 and from yeast cells with an altered 14-3-3 content suggests that 14-3-3 proteins affect forward transport in the secretory pathway. Mechanistically, this could be explained by clamping, masking, or scaffolding. In the clamping mechanism, 14-3-3 binding alters the conformation of the signal-exposing tail of the membrane protein, whereas masking or scaffolding would abolish or allow the interaction of the membrane protein with other proteins or complexes. Interaction partners identified as putative 14-3-3 binding partners in affinity purification approaches constitute a pool of candidate proteins for downstream effectors, such as coat components, coat recruitment GTPases, Rab GTPases, GTPase-activating proteins (GAPs), guanine-nucleotide exchange factors (GEFs) and motor proteins.

Effect of prolonged exposure to milnacipran on norepinephrine transporter in cultured bovine adrenal medullary cells

The antidepressants milnacipran and paroxetine are used clinically worldwide. In the present study, we report here the effects of treatment with milnacipran and paroxetine on the functional activity, binding sites, and mRNA of the norepinephrine (NE) transporter (NET) in cultured bovine adrenal medullary cells. In acute treatment with antidepressants for 20 min, both milnacipran and paroxetine competitively inhibited NET function in cultured adrenal medullary cells. Prolonged treatment of adrenal medullary cells with milnacipran produced time (48-96h)- and concentration (35-355 nM)-dependent increases in [3H]NE uptake and [3H]DMI binding without any increase in NET mRNA. At a high concentration (800 nM, 72 h), paroxetine suppressed [3H]NE uptake. To examine whether milnacipran-induced [3H]NE uptake is mediated by newly synthesized mRNAs or proteins, we used actinomycin D, an inhibitor of DNA-dependent RNA polymerase, and cycloheximide, an inhibitor of ribosomal protein synthesis. Cycloheximide (1 micorM, 72 h) abolished the effect of milnacipran on [3H]NE uptake, while the stimulatory effect of milnacipran was observed in actinomycin D-treated cells. The present findings suggest that prolonged exposure to milnacipran up-regulates the NET function, probably through a post-transcriptional process of NET or other proteins.