Calcium-dependent interactions of the human norepinephrine transporter with syntaxin 1A

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.

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.

Targeting drug sensitivity predictors: New potential strategies to improve pharmacotherapy of human brain disorders

One of the main challenges in medicine is the lack of efficient drug therapies for common human disorders. For example, although depressed patients receive powerful antidepressants, many often remain resistant to psychopharmacotherapy. The growing recognition of complex interplay between the drug targets and the predictors of drug sensitivity requires an improved understanding of these two key aspects of drug action and their potentially shared molecular networks. Here, we apply the concept of endophenotypes and their interplay to drug action and sensitivity. Based on these analyses, we postulate that novel drugs may be developed by targeting specific molecular pathways that integrate drug targets with drug sensitivity predictors.

Dysregulation of Norepinephrine Release in the Absence of Functional Synaptotagmin 7

Synaptotagmin 7 (Syt7) is expressed in cardiac sympathetic nerve terminals where norepinephrine (NE) is released in both Ca(2+)-dependent exocytosis and Ca(2+)-independent norepinephrine transporter (NET)-mediated overflow. The role of Syt7 in the regulation of NE release from cardiac sympathetic nerve terminals is tested by employing a Syt7 knock-in mouse line that expresses a non-functional mutant form of Syt7. In cardiac sympathetic nerve terminals prepared from these Syt7 knock-in mice, the Ca(2+)-dependent component of NE release was diminished. However, these terminals displayed upregulated function of NET (∼130% of controls) and a significant increase in Ca(2+)-independent NE overflow (∼140% of controls), which is greater than the Ca(2+)-dependent component of NE exocytosis occurring in wild-type controls. Consistent with a significant increase in NE overflow, the Syt7 knock-in mice showed significantly higher blood pressures compared to those of littermate wild-type and heterozygous mice. Our results indicate that the lack of functional Syt7 dysregulates NE release from cardiac sympathetic nerve terminals.

Functional mechanisms of neurotransmitter transporters regulated by lipid-protein interactions of their terminal loops

The physiological functions of neurotransmitter:sodium symporters (NSS) in reuptake of neurotransmitters from the synapse into the presynaptic nerve have been shown to be complemented by their involvement, together with non-plasma membrane neurotransmitter transporters, in the reverse transport of substrate (efflux) in response to psychostimulants. Recent experimental evidence implicates highly anionic phosphatidylinositol 4,5-biphosphate (PIP(2)) lipids in such functions of the serotonin (SERT) and dopamine (DAT) transporters. Thus, for both SERT and DAT, neurotransmitter efflux has been shown to be strongly regulated by the presence of PIP(2) lipids in the plasma membrane, and the electrostatic interaction of the N-terminal region of DAT with the negatively charged PIP(2) lipids. We examine the experimentally established phenotypes in a structural context obtained from computational modeling based on recent crystallographic data. The results are shown to set the stage for a mechanistic understanding of physiological actions of neurotransmitter transporters in the NSS family of membrane proteins. This article is part of a Special Issue entitled: Lipid-protein interactions.

Synaptosome-related (SNARE) genes and their interactions contribute to the susceptibility and working memory of attention-deficit/hyperactivity disorder in males

BACKGROUNDS

N-ethylmaleimide-sensitive attachment protein receptor (SNARE) complex involved in neurotransmission via exocytosis was implicated in attention-deficit/hyperactivity disorder (ADHD). The present study investigated the influence of SNARE related genes and their interaction on ADHD susceptibility and their cognitive functions.

METHODS

We genotyped eight single nucleotide polymorphisms (SNP) of Syntaxin 1A (STX1A), vesicle-associated membrane protein 2 (VAMP2) and synaptosomal-associated protein 25 kDa (SNAP25) and conducted case-control studies in 1404 male ADHD and 617 male controls. Quantitative analyses were performed for genotypes and performance on the Rey-Osterrieth complex figure test (RCFT), digit span test and Stroop test in 383 ADHD males. In addition, we explored gene-gene interactions by generalized multifactor dimensionality reduction (GMDR) followed with logistic regression and analyses of covariance for verifying.

RESULTS

Genotypic distribution of rs875342 of STX1A was significantly different between ADHD and controls. The SNPs, rs363039 of SNAP25 and rs1150 of VAMP2, were significantly associated with RCFT scores, while rs875342 of STX1A with digit span. We found genetic interaction models between these three genes and ADHD susceptibility as well as working memory function evaluated by RCFT.

CONCLUSION

SNARE complex genes and their interactions may play a significant role in susceptibility and working memory of ADHD.

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.

Ganglion-specific impairment of the norepinephrine transporter in the hypertensive rat

Hypertension is associated with enhanced cardiac sympathetic transmission, although the exact mechanisms underlying this are still unknown. We hypothesized that defective function of the norepinephrine uptake transporter (NET) may contribute to the sympathetic phenotype of the spontaneously hypertensive rat, and that this may occur before the development of hypertension itself. The dynamic kinetics of NET were monitored temporally using a novel fluorescent assay of the transporter in cultured postganglionic sympathetic neurons from the cardiac stellate ganglion, the superior cervical ganglion, the celiac ganglia/superior mesenteric ganglia, and the renal sympathetic chain. All NET activity was blocked by desipramine. NET rate was significantly impaired in cardiac stellate sympathetic neurons from the prehypertensive spontaneously hypertensive rat compared with age-matched normotensive Wistar-Kyoto rats. A similar response was seen in hypertensive spontaneously hypertensive rats stellate sympathetic neurons. However, no reduction in transporter rate was observed at either age in the other major noncardiac sympathetic ganglia. Depolarization of cardiac stellate neurons by electrical field stimulation further potentiated the difference in transporter rate observed between the hypertensive and normotensive rats at both developmental ages. In conclusion, dysregulation of the norepinephrine transporter in the hypertensive rat is ganglion-specific, where NET impairment in the stellate neurons may contribute to the increased cardiac norepinephrine spillover seen in hypertension.

Stimulation of norepinephrine transporter function by fasudil, a Rho kinase inhibitor, in cultured bovine adrenal medullary cells

Norepinephrine transporter (NET) regulates noradrenergic synaptic transmission by controlling extracellular levels of norepinephrine (NE). The small GTPase, RhoA, and its downstream effector Rho kinase (ROCK) are involved in the regulation of actin cytoskeleton and focal adhesion/stress fiber formation, which may play an important role in various functions of the sympathetic nervous system. We report here the effect of fasudil, a ROCK inhibitor, on the functions of NET in cultured bovine adrenal medullary cells as a model of sympathetic neurons. Treatment of bovine adrenal medullary cells with fasudil caused an increase in [(3)H]NE uptake in time (8-120 h) and concentration (10-100 μM)-dependent manner. Another ROCK inhibitor, Y-27632 (10-100 μM, 1 day), also increased [(3)H]NE uptake by the cells. Kinetics analysis of the effect of fasudil on NE transport showed a significant increase in the V (max) of NE transport with little change in K (m). When both extracellular and intracellular Ca(2+) were removed by the deprivation of extracellular Ca(2+) and BAPTA-AM, a cell-permeable Ca(2+) chelator, [(3)H]NE uptake induced by fasudil was completely abolished. Nocodazole, an inhibitor of microtubule polymerization, but not cytochalasin D, an inhibitor of actin polymerization, suppressed the stimulatory effect of fasudil on [(3)H]NE uptake. The present findings suggest that the ROCK inhibitor fasudil up-regulates NET function in a Ca(2+)-dependent and/or nocodazole-sensitive pathway in adrenal medullary cells.

Proteins interacting with monoamine transporters: current state and future challenges

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

A conserved asparagine residue in transmembrane segment 1 (TM1) of serotonin transporter dictates chloride-coupled neurotransmitter transport

Na(+)- and Cl(-)-dependent uptake of neurotransmitters via transporters of the SLC6 family, including the human serotonin transporter (SLC6A4), is critical for efficient synaptic transmission. Although residues in the human serotonin transporter involved in direct Cl(-) coordination of human serotonin transport have been identified, the role of Cl(-) in the transport mechanism remains unclear. Through a combination of mutagenesis, chemical modification, substrate and charge flux measurements, and molecular modeling studies, we reveal an unexpected role for the highly conserved transmembrane segment 1 residue Asn-101 in coupling Cl(-) binding to concentrative neurotransmitter uptake.

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.

The neuronal glycine transporter GLYT2 associates with membrane rafts: functional modulation by lipid environment

The neuronal glycine transporter GLYT2 is a plasma membrane protein that removes the neurotransmitter glycine from the synaptic cleft, thereby aiding the pre-synaptic terminal reloading and the termination of the glycinergic signal. Missense mutations in the gene encoding GLYT2 (SLC6A5) cause hyperekplexia in humans. The activity of GLYT2 seems to be highly regulated. In this report, we demonstrate that GLYT2 is associated with membrane rafts in the plasma membrane of brainstem terminals and neurons. The transporter is localized to Triton X-100-insoluble light synaptosomal membranes together with flotillin-1, a marker protein for membrane rafts, in a methyl-beta-cyclodextrin (MbetaCD)-sensitive manner. In brainstem primary neurons, the GLYT2 punctuate pattern visualized by confocal microscopy was modified by cholesterol depletion with MbetaCD, unlike other non-raft neuronal markers. GLYT2-associated gold particles were observed by electron microscopy on purified rafts from brainstem synaptosomes. Furthermore, either in brainstem terminals and cultured neurons, the pharmacological reduction of the levels of raft components, cholesterol and sphingomyelin, impairs both the association of GLYT2 with membrane rafts and its transport activity. Thus, GLYT2 may require membrane raft location for optimal function, and therefore the lipid environment may constitute a new mechanism to modulate GLYT2.

Upregulation of norepinephrine transporter function by prolonged exposure to nicotine in cultured bovine adrenal medullary cells

Nicotine acts on nicotinic acetylcholine receptors in the adrenal medulla and brain, thereby stimulating the release of monoamines such as norepinephrine (NE). In the present study, we examined the effects of prolonged exposure to nicotine on NE transporter (NET) activity in cultured bovine adrenal medullary cells. Treatment of adrenal medullary cells with nicotine increased [(3)H]NE uptake in both a time- (1-5 days) and concentration-dependent (0.1-10 muM) manner. Kinetic analysis showed that nicotine induced an increase in the V (max) of [(3)H]NE uptake with little change in K (m). This increase in NET activity was blocked by cycloheximide, an inhibitor of ribosomal protein synthesis, but not by actinomycin D, a DNA-dependent RNA polymerase inhibitor. [(3)H]NE uptake induced by nicotine was strongly inhibited by hexamethonium and mecamylamine but not by alpha-bungarotoxin, and was abolished by elimination of Ca(2+) from the culture medium. KN-93, an inhibitor of Ca(2+)/calmodulin-dependent protein kinase II, attenuated not only nicotine-induced [(3)H]NE uptake but also (45)Ca(2+) influx in the cells. The present findings suggest that long-term exposure to nicotine increases NET activity through a Ca(2+)-dependent post-transcriptional process in the adrenal medulla.

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.

Syntaxin 1A regulates dopamine transporter activity, phosphorylation and surface expression

We investigated the functional relationship between the soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) protein syntaxin 1A (syn 1A) and the dopamine transporter (DAT) by treating rat striatal tissue with Botulinum Neurotoxin C (BoNT/C) and co-transfecting syn 1A with DAT in non-neuronal cells, followed by analysis of DAT activity, phosphorylation, and regulation. Treatment of striatal slices with BoNT/C resulted in elevated dopamine (DA) transport Vmax and reduced DAT phosphorylation, while heterologous co-expression of syn 1A led to reduction in DAT surface expression and transport Vmax. Syn 1A was present in DAT immunoprecipitation complexes, supporting a direct or indirect interaction between the proteins. Phorbol ester regulation of DA transport activity was retained in BoNT/C-treated synaptosomes and syn 1A transfected cells, demonstrating that protein kinase C (PKC) and syn 1A effects occur through independent processes. These findings reveal a novel mechanism for regulation of DAT activity and phosphorylation, and suggest the potential for syn 1A to impact DA neurotransmission through effects on reuptake.

Dynamic monitoring of NET activity in mature murine sympathetic terminals using a fluorescent substrate

BACKGROUND AND PURPOSE

To validate a fluorescence approach for monitoring norepinephrine transporter (NET) transport rate in mature sympathetic terminals, and to determine how prejunctional muscarinic receptors affect NET rate.

EXPERIMENTAL APPROACH

Confocal imaging of a fluorescent NET substrate [neurotransmitter transporter uptake assay (NTUA)] as it accumulates in the mature sympathetic nerve terminals of the mouse isolated vas deferens. Fluorescence recovery after photobleaching (FRAP), enhanced green fluorescence protein (EGFP)-transgenic mice and contraction studies were also used.

KEY RESULTS

NTUA fluorescence accumulated linearly in nerve terminals, an effect that was prevented with NET inhibition with desipramine (1 microM). Such accumulation was reversed by amphetamine (10 microM), which is known to reverse the direction of transport of NET substrates. NTUA labelling was not present in cholinergic terminals (identified using EGFP fluorescence expressed in transgenic mice under a choline acetyltransferase promoter). FRAP experiments, altered nerve terminal distribution with reserpine pretreatment and co-imaging in terminals filled with a cytoplasmic marker (Alexa 594 dextran) indicated that the NTUA labelling was largely confined to vesicles within varicosities; vesicular exchange between varicosities was rare. The rate of NTUA accumulation was slower in the presence of the muscarinic agonist carbachol (10 microM) demonstrating muscarinic inhibition of NET rate.

CONCLUSIONS AND IMPLICATIONS

A straightforward protocol now exists to monitor NET transport rate at the level of the single nerve terminal varicosity, providing a useful tool to understand the physiology of NET regulation, the action of NET inhibitors on mature sympathetic terminals, dynamic vesicular tracking and to identify sympathetic terminals from mixed terminal populations in living organs.

Slow intracellular accumulation of GABA(A) receptor delta subunit is modulated by brain-derived neurotrophic factor

GABA(A) receptors composed of the gamma2 and delta subunits have distinct properties, functions and subcellular localization, and pathological conditions differentially modulate their surface expression. Recent studies demonstrate that acute seizure activity accelerated trafficking of the gamma2 and beta2/3 subunits but not that of the delta subunit. The trafficking of the gamma2 and beta2/3 subunits is relatively well understood but that of the delta subunit has not been studied. We compared intracellular accumulation of the delta and gamma2 subunits in cultured hippocampal neurons using an antibody feeding technique. Intracellular accumulation of the delta subunit peaked between 3 and 6 h, whereas, maximum internalization of the gamma2 subunit took 30 min. In the organotypic hippocampal slice cultures internalization of the delta subunit studied using a biotinylation assay revealed highest accumulation between 3 and 5 h and that of the gamma2 subunit between 15 and 45 min. The surface half-life of the delta subunit was 171 min in cultured hippocampal neurons and 102 min in the organotypic hippocampal slice cultures. In the subsequent studies, internalization of the delta subunit was found to be dependent on network activity but independent of ligand-binding. Brain-derived neurotrophic factor (BDNF) reduced buildup of the delta subunit in the cytoplasmic compartments and increased its surface expression, and this BDNF effect was independent of network activity. BDNF effect was mediated by activation of TrkB receptors, PLCgamma and PKC. Increase in the basal PKC activity augmented cell surface stability of the delta subunit. These results suggest that rate of intracellular accumulation of the delta subunit was distinct and modulated by BDNF.

Rab 11 regulates constitutive dopamine transporter trafficking and function in N2A neuroblastoma cells

The dopamine transporter (DAT) is a crucial regulator of dopaminergic neurotransmission which undergoes constitutive and substrate-mediated trafficking to and from the membrane. Although, considerable research has been done to elucidate the regulation of substrate-stimulated DAT trafficking, less is known about which trafficking proteins are involved in constitutive DAT trafficking. Rab proteins are GTPases known to regulate the trafficking of proteins to and from specific endocytic compartments. Rabs 8 and 11, in particular, are involved in trafficking proteins from intracellular compartments to the plasma membrane. In this study, we sought to determine whether Rabs 8 and 11 would modulate DAT activity and trafficking in N2A neuroblastoma cells. We used Rab mutations known to confer constitutively active or dominant negative activity of these proteins to investigate the role of Rab activity in constitutive DAT trafficking and function. We found that constitutively active Rab 11 upregulates DAT function and surface expression while neither the constitutively active nor the dominant negative mutant of Rab 8 had any effect on DA uptake. Furthermore, immunofluorescence experiments revealed that dominant negative Rab 11 overexpression results in decreased surface DAT indicating a necessary function of Rab 11 in DAT trafficking to the plasma membrane. These data show for the first time a functional role of Rab proteins in the constitutive recycling of DAT to the plasma membrane.

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.

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.

Regulated interactions of the norepineprhine transporter by the actin and microtubule cytoskeletons

One role of the actin cytoskeleton is to maintain the structural morphology and activity of the pre-synaptic terminal. We sought to determine if the actin cytoskeleton plays a role in regulating interactions between the norepinephrine transporter (NET) and alpha-Synuclein (alpha-Syn), two proteins expressed in the pre-synaptic terminal. In cells transfected with either 0.5 microg/mL or 3 microg/mL of alpha-Syn and 1 microg/mL of NET DNA, treatment with cytochalasin D, an actin depolymerizing agent, caused a dose-dependent decrease and increase, respectively, in [3H]-NE uptake. Protein interactions between NET, beta-actin, and alpha-Syn were modified, along with levels of surface transporters. Treatment of primary brainstem neurons and frontal cortex synaptosomes with cytochalasin D caused a 115% and 28% increase, respectively, in NET activity. Depolymerization of both actin and microtubules did not alter NET activity in cells with 0.5 microg/mL alpha-Syn, but caused an increase in [3H]-NE uptake in cells transfected with 3 microg/mL of alpha-Syn and primary neurons. This is the first direct demonstration of NET activity being regulated via actin and modulated by interactions with alpha-Syn.

Regulation of the norepinephrine transporter by alpha-synuclein-mediated interactions with microtubules

alpha-Synuclein (alpha-Syn) regulates catecholaminergic neurotransmission. We demonstrate that alpha-Syn regulates the activity and surface expression of the norepinephrine transporter (NET), depending on its expression levels. In cells co-transfected with NET and low amounts of alpha-Syn, NET activity and cell surface expression were increased and protein interactions with alpha-Syn decreased, compared with cells transfected with NET alone. Converse effects were observed at higher levels of alpha-Syn expression. Treatment with nocodazole and other microtubule (MT) destabilizers abolished the expression-dependent bimodal regulation of NET by alpha-Syn. At low alpha-Syn levels, nocodazole had no effect on NET surface expression or protein interactions, while inducing increases in these measures at higher levels. Cells that were transfected with NET alone displayed no sensitivity to nocodazole, indicating that alpha-Syn expression was necessary for the MT-dependent changes in NET activity. MT destabilizers also caused a significant increase in [(3)H]-NE uptake in brainstem primary neurons and synaptosomes from the frontal cortex, but not striatal synaptosomes. These findings suggest that the surface localization and activity of NET is modulated by alpha-Syn in a manner that is both dependent on interactions with the MT cytoskeleton and varies across brain regions.