Phosphorylation and regulation of psychostimulant-sensitive neurotransmitter transporters

Palmitoylation Regulates Human Serotonin Transporter Activity, Trafficking, and Expression and Is Modulated by Escitalopram

In the central nervous system, serotonergic signaling modulates sleep, mood, and cognitive control. During serotonergic transmission, the synaptic concentration of serotonin is tightly controlled in a spatial and temporal manner by the serotonin transporter (SERT). Dysregulation of this process is implicated in the pathogenesis of major-depressive, obsessive-compulsive, and autism-spectrum disorders, which makes SERT a primary target for prescription therapeutics, most notably selective serotonin reuptake inhibitors (SSRIs). S-Palmitoylation, the reversible addition of a 16-carbon fatty acid to proteins, is an increasingly recognized dynamic post-translational modification responsible for modulating protein kinetics, trafficking, and localization patterns in response to physiologic/cellular stimuli. In this study, we reveal that human SERTs are a target for palmitoylation, and using the irreversible palmitoyl acyltransferase inhibitor 2-bromopalmitate (2BP), we have identified several associated functions. Using a lower dose of 2BP in shorter time frames, inhibition of palmitoylation was associated with reductions in SERT Vmax, without changes in Km or surface expression. With higher doses of 2BP for longer time intervals, inhibition of palmitoylation was consistent with the loss of cell surface and total SERT protein, suggesting palmitoylation is an important mechanism in regulating SERT trafficking and maintenance of SERT protein through biogenic or anti-degradative processes. Additionally, we have identified that treatment with the SSRI escitalopram decreases SERT palmitoylation analogous to 2BP, reducing SERT surface expression and transport capacity. Ultimately, these results reveal that palmitoylation is a major regulatory mechanism for SERT kinetics and trafficking and may be the mechanism responsible for escitalopram-induced internalization and ultimately decreased cellular SERT protein levels.

Palmitoylation regulates human serotonin transporter activity, trafficking, and expression and is modulated by escitalopram

In the central nervous system, serotonergic signaling modulates sleep, mood, and cognitive control. During neuronal transmission, the synaptic concentration of serotonin is tightly controlled in a spatial and temporal manner by the serotonin transporter (SERT). Dysregulation of serotonergic signaling is implicated in the pathogenesis of major-depressive, obsessive-compulsive, and autism-spectrum disorders, which makes SERT a primary target for prescription therapeutics, most notably selective-serotonin reuptake inhibitors (SSRIs). S-palmitoylation is an increasingly recognized dynamic post-translational modification, regulating protein kinetics, trafficking, and localization patterns upon physiologic/cellular stimuli. In this study, we reveal that human SERTs are a target for palmitoylation, and using the irreversible palmitoyl acyl-transferase inhibitor, 2-bromopalmitate (2BP) we have identified several associated functions. Using a lower dose of 2BP in shorter time frames, inhibition of palmitoylation was associated with reductions in SERT V max , without changes in K m or surface expression. With higher doses of 2BP for longer time intervals, inhibition of palmitoylation was consistent with the loss of cell surface and total SERT protein, suggesting palmitoylation is an important mechanism in regulating SERT trafficking and maintenance of SERT protein through biogenic or anti-degradative processes. Additionally, we have identified that treatment with the SSRI escitalopram decreases SERT palmitoylation analogous to 2BP, reducing SERT surface expression and transport capacity. Ultimately, these results reveal palmitoylation is a major regulatory mechanism for SERT kinetics and trafficking and may be the mechanism responsible for escitalopram-induced internalization and loss of total SERT protein.

Structure and Gating Dynamics of Na+/Cl- Coupled Neurotransmitter Transporters

Neurotransmitters released at the neural synapse through vesicle exocytosis are spatiotemporally controlled by the action of neurotransmitter transporters. Integral membrane proteins of the solute carrier 6 (SLC6) family are involved in the sodium and chloride coupled uptake of biogenic amine neurotransmitters including dopamine, serotonin, noradrenaline and inhibitory neurotransmitters including glycine and γ-amino butyric acid. This ion-coupled symport works through a well-orchestrated gating of substrate through alternating-access, which is mediated through movements of helices that resemble a rocking-bundle. A large array of commercially prescribed drugs and psychostimulants selectively target neurotransmitter transporters thereby modulating their levels in the synaptic space. Drug-induced changes in the synaptic neurotransmitter levels can be used to treat depression or neuropathic pain whereas in some instances prolonged usage can lead to habituation. Earlier structural studies of bacterial neurotransmitter transporter homolog LeuT and recent structure elucidation of the Drosophila dopamine transporter (dDAT) and human serotonin transporter (hSERT) have yielded a wealth of information in understanding the transport and inhibition mechanism of neurotransmitter transporters. Computational studies based on the structures of dDAT and hSERT have shed light on the dynamics of varied components of these molecular gates in affecting the uphill transport of neurotransmitters. This review seeks to address structural dynamics of neurotransmitter transporters at the extracellular and intracellular gates and the effect of inhibitors on the ligand-binding pocket. We also delve into the effect of additional factors including lipids and cytosolic domains that influence the translocation of neurotransmitters across the membrane.

Multifaceted Regulations of the Serotonin Transporter: Impact on Antidepressant Response

Serotonin transporter, SERT (SLC64A for solute carrier family 6, member A4), is a twelve transmembrane domain (TMDs) protein that assumes the uptake of serotonin (5-HT) through dissipation of the Na⁺ gradient established by the electrogenic pump Na/K ATPase. Abnormalities in 5-HT level and signaling have been associated with various disorders of the central nervous system (CNS) such as depression, obsessive-compulsive disorder, anxiety disorders, and autism spectrum disorder. Since the 50s, SERT has raised a lot of interest as being the target of a class of antidepressants, the Serotonin Selective Reuptake Inhibitors (SSRIs), used in clinics to combat depressive states. Because of the refractoriness of two-third of patients to SSRI treatment, a better understanding of the mechanisms regulating SERT functions is of priority. Here, we review how genetic and epigenetic regulations, post-translational modifications of SERT, and specific interactions between SERT and a set of diverse partners influence SERT expression, trafficking to and away from the plasma membrane and activity, in connection with the neuronal adaptive cell response to SSRI antidepressants.

An acute, epitope-specific modification in the dopamine transporter associated with methamphetamine-induced neurotoxicity

Preclinical studies demonstrate that repeated, high-dose methamphetamine administrations rapidly decrease plasmalemmal dopamine uptake, which may contribute to aberrant dopamine accumulation, reactive species generation, and long-term dopaminergic deficits. The present study extends these findings by demonstrating a heretofore unreported, epitope-specific modification in the dopamine transporter caused by a methamphetamine regimen that induces these deficits. Specifically, repeated, high-dose methamphetamine injections (4 × 10 mg/kg/injection, 2-h intervals) rapidly decreased immunohistochemical detection of striatal dopamine transporter as assessed 1 h after the final methamphetamine exposure. In contrast, neither a single high dose (1 × 10 mg/kg) nor repeated injections of a lower dose (4 × 2 mg/kg/injection) induced this change. The high-dose regimen-induced alteration was only detected using antibodies directed against the N-terminus. Immunohistochemical staining using antibodies directed against the C-terminus did not reveal any changes. The high-dose regimen also did not alter dopamine transporter expression as assessed using [(125) I]RTI-55 autoradiography. These data suggest that the repeated, high-dose methamphetamine regimen alters the N-terminus of the dopamine transporter. Further, these data may be predictive of persistent dopamine deficits caused by the stimulant. Future studies of the signaling cascades involved should provide novel insight into potential mechanisms underlying the physiological and pathophysiological regulation of the dopamine transporter.

Dopamine-sensitive signaling mediators modulate psychostimulant-induced ultrasonic vocalization behavior in rats

The mesolimbic dopamine system plays a major role in psychostimulant-induced ultrasonic vocalization (USV) behavior in rodents. Within this system, psychostimulants elevate synaptic concentrations of dopamine thereby leading to exaggerated activation of postsynaptic dopamine receptors within the D1-like and D2-like subfamilies. Dopamine receptor stimulation activate several transmembrane signaling systems and cognate intracellular mediators; downstream activation of transcription factors then conveys the information from receptor activation to appropriate modulation of cellular and physiologic functions. We previously showed that cocaine-induced USV behavior was associated with enhanced expression of the neurotrophin BDNF. Like cocaine, amphetamine also increases synaptic dopamine levels, albeit primarily through facilitating dopamine release. Therefore, in the present study we investigated whether amphetamine and cocaine similarly activate dopamine-linked signaling cascades to regulate intracellular mediators leading to induction of USV behavior. The results show that amphetamine increased the emission of 50 kHz USVs and this effect was blocked by SCH23390, a D1 receptor antagonist. Similar to cocaine, amphetamine increased BDNF protein expression in discrete brain regions, while pretreatment with K252a, a trkB neurotrophin receptor inhibitor, significantly reduced amphetamine-induced USV behavior. Inhibition of cyclic-AMP/PKA signaling with H89 or inhibition of PLC signaling with U73122 significantly blocked both the acute and subchronic amphetamine-induced USV behavior. In contrast, pharmacologic inhibition of either pathway enhanced cocaine-induced USV behavior. Although cocaine and amphetamine similarly modulate neurotrophin expression and USV, the molecular mechanisms by which these psychostimulants differentially activate dopamine receptor subtypes or other monoaminergic systems may be responsible for the distinct aspects of behavioral responses.

N-terminal tagging of the dopamine transporter impairs protein expression and trafficking in vivo

The dopamine transporter (DAT) is the primary protein responsible for the uptake of dopamine from the extracellular space back into presynaptic neurons. As such, it plays an important role in the cessation of dopaminergic neurotransmission and in the maintenance of extracellular dopamine homeostasis. Here, we report the development of a new BAC transgenic mouse line that expresses DAT with an N-terminal HA-epitope (HAD-Tg). In this line, two copies of the HA-DAT BAC are incorporated into the genome, increasing DAT mRNA levels by 47%. Despite the increase in mRNA levels, HAD-Tg mice show no significant increase in the level of DAT protein in the striatum, indicating a defect in protein trafficking or stability. By crossing HAD-Tg mice with DAT knockout mice (DAT-KO), we engineered mice that exclusively express HA-tagged DAT in the absence of endogenous DAT (DAT-KO/HAD-Tg). We show that DAT-KO/HAD-Tg mice express only 8.5% of WT DAT levels in the striatum. Importantly, the HA-tagged DAT that is present in DAT-KO/HAD-Tg mice is functional, as it is able to partially rescue the DAT-KO hyperactive phenotype. Finally, we provide evidence that the HA-tagged DAT is retained in the cell body based on a reduction in the striatum:midbrain protein ratio. These results demonstrate that the presence of the N-terminal tag leads to impaired DAT protein expression in vivo due in part to improper trafficking of the tagged transporter, and highlight the importance of the N-terminus in the transport of DAT to striatal terminals.

An N-terminal threonine mutation produces an efflux-favorable, sodium-primed conformation of the human dopamine transporter

The dopamine transporter (DAT) reversibly transports dopamine (DA) through a series of conformational transitions. Alanine (T62A) or aspartate (T62D) mutagenesis of Thr62 revealed T62D-human (h)DAT partitions in a predominately efflux-preferring conformation. Compared with wild-type (WT), T62D-hDAT exhibits reduced [(3)H]DA uptake and enhanced baseline DA efflux, whereas T62A-hDAT and WT-hDAT function in an influx-preferring conformation. We now interrogate the basis of the mutants' altered function with respect to membrane conductance and Na(+) sensitivity. The hDAT constructs were expressed in Xenopus oocytes to investigate if heightened membrane potential would explain the efflux characteristics of T62D-hDAT. In the absence of substrate, all constructs displayed identical resting membrane potentials. Substrate-induced inward currents were present in oocytes expressing WT- and T62A-hDAT but not T62D-hDAT, suggesting equal bidirectional ion flow through T62D-hDAT. Utilization of the fluorescent DAT substrate ASP(+) [4-(4-(dimethylamino)styryl)-N-methylpyridinium] revealed that T62D-hDAT accumulates substrate in human embryonic kidney (HEK)-293 cells when the substrate is not subject to efflux. Extracellular sodium (Na(+) e) replacement was used to evaluate sodium gradient requirements for DAT transport functions. The EC50 for Na(+) e stimulation of [(3)H]DA uptake was identical in all constructs expressed in HEK-293 cells. As expected, decreasing [Na(+)]e stimulated [(3)H]DA efflux in WT- and T62A-hDAT cells. Conversely, the elevated [(3)H]DA efflux in T62D-hDAT cells was independent of Na(+) e and commensurate with [(3)H]DA efflux attained in WT-hDAT cells, either by removal of Na(+) e or by application of amphetamine. We conclude that T62D-hDAT represents an efflux-willing, Na(+)-primed orientation-possibly representing an experimental model of the conformational impact of amphetamine exposure to hDAT.

Characterization of intracellular regions in the human serotonin transporter for phosphorylation sites

In the central nervous system, synaptic levels of the monoamine neurotransmitter serotonin are mainly controlled by the serotonin transporter (SERT), and drugs used in the treatment of various psychiatric diseases have SERT as primary target. SERT is a phosphoprotein that undergoes phosphorylation/dephosphorylation during transporter regulation by multiple pathways. In particular, activation and/or inhibition of kinases including PKC, PKG, p38MAPK, and CaMKII modulate SERT function and trafficking. The molecular mechanisms by which kinase activity is linked to SERT regulation are poorly understood, including the identity of specific phosphorylated residues. To elucidate SERT phosphorylation sites, we have generated peptides corresponding to the entire intracellular region of human SERT and performed in vitro phosphorylation assays with a panel of kinases suggested to be involved in SERT regulation or for which canonical phosphorylation sites are predicted. Peptide analysis by liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to identify and quantify site-specific phosphorylation. Five residues located in the N- and C-termini and in intracellular loop 1 and 2 were identified as phosphorylation sites; Ser149, Ser277, and Thr603 for PKC, Ser13 for CaMKII, and Thr616 for p38MAPK. Possible regulatory roles of these potential phosphoacceptors for SERT function and surface expression were investigated using phospho-mimicking and phosphodeficient mutations, coexpression of constitutively active kinases and pharmacological kinase induction in a heterologous expression system. Our results suggest that Ser277 is involved in an initial phase of PKC-mediated down-regulation of SERT. The five identified sites can guide future studies of direct links between SERT phosphorylation and regulatory processes.

Interleukin-1 beta enhances endocytosis of glial glutamate transporters in the spinal dorsal horn through activating protein kinase C

Excessive activation of glutamate receptors in spinal dorsal horn neurons is a key mechanism leading to abnormal neuronal activation in pathological pain conditions. Previous studies have shown that activation of glutamate receptors in the spinal dorsal horn is enhanced by impaired glial glutamate transporter functions and proinflammatory cytokines including interleukin-1 beta (IL-1β). In this study, we for the first time revealed that spinal glial glutamate transporter activities in the neuropathic animals are attenuated by endogenous IL-1β. Specifically, we demonstrated that nerve injury results in an increased expression of IL-1β and activation of PKC in the spinal dorsal horn as well as suppression of glial glutamate uptake activities. We provided evidence that the nerve-injury induced suppression of glial glutamate uptake is at least in part ascribed to endogenous IL-1β and activation of PKC in the spinal dorsal horn. IL-1β reduces glial glutamate transporter activities through enhancing the endocytosis of both GLT-1 and GLAST glial glutamate transporters. The IL-1β induced trafficking of glial glutamate transporters is through the calcium/PKC signaling pathway, and the dynamin-dependent endocytosis, which is dependent on the integrity of actin filaments. The signaling pathway regulating glial glutamate transporters revealed in this study provides novel targets to attenuate aberrant activation of glutamate receptors in the spinal dorsal horn, which could ultimately help the development of analgesics.

Long-term exposure of RN46A cells expressing serotonin transporter (SERT) to a cAMP analog up-regulates SERT activity and is accompanied by neural differentiation of the cells

To examine the functional regulation of serotonin transporter (SERT) by cAMP, we examined whether SERT uptake activity was affected by dibutyryl cAMP (dbcAMP), a cAMP analog, in SERT-transfected RN46A cells derived from embryonic rat raphe neurons. Long-term exposure (> 4 h) of dbcAMP (1 mM) to SERT-expressing RN46A cells significantly up-regulated SERT activity. In addition, a selective PKA activator, but not a selective EPAC activator, increased the serotonin uptake activity of SERT, suggesting that this regulation was mainly mediated via PKA. Time-dependent up-regulation of SERT activity by dbcAMP was accompanied by neural differentiation of RN46A cells. Further investigation of dbcAMP-induced up-regulation of SERT revealed that dbcAMP elevated SERT protein levels without affecting SERT mRNA transcription. The chase assay for residual SERT protein revealed that dbcAMP slowed its degradation rate. Immunohistochemical analysis revealed that plasma membrane-localized SERT was more abundant in dbcAMP-treated cells than in non-treated cells, suggesting that dbcAMP up-regulated SERT by decreasing its degradation and increasing its plasma membrane expression. These results raise the possibility that the elevation of intracellular cAMP up-regulated SERT function through a mechanism linked to the differentiation of RN46A cells and show the importance of SERT function during the developmental process of the serotonergic nervous system.

Phosphorylation of dopamine transporter serine 7 modulates cocaine analog binding

As an approach to elucidating dopamine transporter (DAT) phosphorylation characteristics, we examined in vitro phosphorylation of a recombinant rat DAT N-terminal peptide (NDAT) using purified protein kinases. We found that NDAT becomes phosphorylated at single distinct sites by protein kinase A (Ser-7) and calcium-calmodulin-dependent protein kinase II (Ser-13) and at multiple sites (Ser-4, Ser-7, and Ser-13) by protein kinase C (PKC), implicating these residues as potential sites of DAT phosphorylation by these kinases. Mapping of rat striatal DAT phosphopeptides by two-dimensional thin layer chromatography revealed basal and PKC-stimulated phosphorylation of the same peptide fragments and comigration of PKC-stimulated phosphopeptide fragments with NDAT Ser-7 phosphopeptide markers. We further confirmed by site-directed mutagenesis and mass spectrometry that Ser-7 is a site for PKC-stimulated phosphorylation in heterologously expressed rat and human DATs. Mutation of Ser-7 and nearby residues strongly reduced the affinity of rat DAT for the cocaine analog (-)-2β-carbomethoxy-3β-(4-fluorophenyl) tropane (CFT), whereas in rat striatal tissue, conditions that promote DAT phosphorylation caused increased CFT affinity. Ser-7 mutation also affected zinc modulation of CFT binding, with Ala and Asp substitutions inducing opposing effects. These results identify Ser-7 as a major site for basal and PKC-stimulated phosphorylation of native and expressed DAT and suggest that Ser-7 phosphorylation modulates transporter conformational equilibria, shifting the transporter between high and low affinity cocaine binding states.

Dopamine transporter phosphorylation site threonine 53 regulates substrate reuptake and amphetamine-stimulated efflux

In the central nervous system, levels of extraneuronal dopamine are controlled primarily by the action of the dopamine transporter (DAT). Multiple signaling pathways regulate transport activity, substrate efflux, and other DAT functions through currently unknown mechanisms. DAT is phosphorylated by protein kinase C within a serine cluster at the distal end of the cytoplasmic N terminus, whereas recent work in model cells revealed proline-directed phosphorylation of rat DAT at membrane-proximal residue Thr(53). In this report, we use mass spectrometry and a newly developed phospho-specific antibody to positively identify DAT phosphorylation at Thr(53) in rodent striatal tissue and heterologous expression systems. Basal phosphorylation of Thr(53) occurred with a stoichiometry of ~50% and was strongly increased by phorbol esters and protein phosphatase inhibitors, demonstrating modulation of the site by signaling pathways that impact DAT activity. Mutations of Thr(53) to prevent phosphorylation led to reduced dopamine transport V(max) and total apparent loss of amphetamine-stimulated substrate efflux, supporting a major role for this residue in the transport kinetic mechanism.

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.

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.

The ugly side of amphetamines: short- and long-term toxicity of 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy'), methamphetamine and D-amphetamine

Amphetamine ('Speed'), methamphetamine ('Ice') and its congener 3,4-methylenedioxymethamphetamine (MDMA; 'Ecstasy') are illicit drugs abused worldwide for their euphoric and stimulant effects. Despite compelling evidence for chronic MDMA neurotoxicity in animal models, the physiological consequences of such toxicity in humans remain unclear. In addition, distinct differences in the metabolism and pharmacokinetics of MDMA between species and different strains of animals prevent the rationalisation of realistic human dose paradigms in animal studies. Here, we attempt to review amphetamine toxicity and in particular MDMA toxicity in the pathogenesis of exemplary human pathologies, independently of confounding environmental factors such as poly-drug use and drug purity.

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.

Regulation of monoamine transporters: Role of transporter phosphorylation

Presynaptic biogenic amine transporters mediate reuptake of released amines from the synapse, thus regulating serotonin, dopamine and norepinephrine neurotransmission. Medications utilized in the treatment of depression, attention deficit-hyperactivity disorder and other psychiatric disorders possess high affinity for amine transporters. In addition, amine transporters are targets for psychostimulants. Altered expression of biogenic amine transporters has long been implicated in several psychiatric and degenerative disorders. Therefore, appropriate regulation and maintenance of biogenic amine transporter activity is critical for the maintenance of normal amine homoeostasis. Accumulating evidence suggests that cellular protein kinases and phosphatases regulate amine transporter expression, activity, trafficking and degradation. Amine transporters are phosphoproteins that undergo dynamic control under the influence of various kinase and phosphatase activities. This review presents a brief overview of the role of amine transporter phosphorylation in the regulation of amine transport in the normal and diseased brain. Understanding the molecular mechanisms by which phosphorylation events affect amine transporter activity is essential for understanding the contribution of transporter phosphorylation to the regulation of monoamine neurotransmission and for identifying potential new targets for the treatment of various brain diseases.

Regulation of the dopamine transporter: aspects relevant to psychostimulant drugs of abuse

Dopaminergic signaling in the brain is primarily modulated by dopamine transporters (DATs), which actively translocate extraneuronal dopamine back into dopaminergic neurons. Transporter proteins are highly dynamic, continuously trafficking between plasmalemmal and endosomal membranes. Changes in DAT membrane trafficking kinetics can rapidly regulate dopaminergic tone by altering the number of transporters present at the cell surface. Various psychostimulant DAT ligands-acting either as amphetamine-like substrates or cocaine-like nontranslocated inhibitors-affect transporter trafficking, triggering rapid insertion or removal of plasmalemmal DATs. In this review, we focus on the effects of psychostimulants of addiction (particularly D-methamphetamine and cocaine) on DAT regulation and membrane trafficking, with an emphasis on how these drugs may influence intracellular signaling cascades and transporter-associated scaffolding proteins to affect DAT regulation. In addition, we consider involvement of presynaptic receptors for dopamine and other ligands in DAT regulation. Finally, we discuss possible implications of transporter regulation to the putative toxicity of several substituted amphetamine derivatives commonly used as recreational drugs, as well as to the design of therapeutics for cocaine addiction.

Altered dopamine transporter function and phosphorylation following chronic cocaine self-administration and extinction in rats

Cocaine binds with the dopamine transporter (DAT), an effect that has been extensively implicated in its reinforcing effects. However, persisting adaptations in DAT regulation after cocaine self-administration have not been extensively investigated. Here, we determined the changes in molecular mechanisms of DAT regulation in the caudate-putamen (CPu) and nucleus accumbens (NAcc) of rats with a history of cocaine self-administration, followed by 3weeks of withdrawal under extinction conditions (i.e., no cocaine available). DA uptake was significantly higher in the CPu of cocaine-experienced animals as compared to saline-yoked controls. DAT V(max) was elevated in the CPu without changes in apparent affinity for DA. In spite of elevated CPu DAT activity, total and surface DAT density and DAT-PP2Ac (protein phosphatase 2A catalytic subunit) interaction remained unaltered, although p-Ser- DAT phosphorylation was elevated. In contrast to the CPu, there were no differences between cocaine and saline rats in the levels of DA uptake, DAT V(max) and K(m) values, total and surface DAT, p-Ser-DAT phosphorylation, or DAT-PP2Ac interactions in the NAcc. These results show that chronic cocaine self-administration leads to lasting, regionally specific alterations in striatal DA uptake and DAT-Ser phosphorylation. Such changes may be related to habitual patterns of cocaine-seeking observed during relapse.

Enteropathogenic Escherichia coli infection inhibits intestinal serotonin transporter function and expression

BACKGROUND & AIMS

Serotonin transporter (SERT) plays a critical role in regulating serotonin (5-hydroxytryptamine [5-HT]) availability in the gut. Elevated 5-HT levels are associated with diarrheal conditions such as irritable bowel syndrome and enteric infections. Whether alteration in SERT activity contributes to the pathophysiology of diarrhea induced by the food-borne pathogen enteropathogenic Escherichia coli (EPEC) is not known. The present studies examined the effects of EPEC infection on SERT activity and expression in intestinal epithelial cells and elucidated the underlying mechanisms.

METHODS

Caco-2 cells as a model of human intestinal epithelia and EPEC-infected C57BL/6J mouse model of infection were utilized. SERT activity was measured as Na(+) and Cl(-) dependent (3)[H] 5-HT uptake. SERT expression was measured by real-time quantitative reverse-transcription polymerase chain reaction, Western blotting, and immunofluorescence studies.

RESULTS

Infection of Caco-2 cells with EPEC for 30-120 minutes decreased apical SERT activity (P < .001) in a type 3 secretion system dependent manner and via involvement of protein tyrosine phosphatases. EPEC infection decreased V(max) of the transporter; whereas cell surface biotinylation studies revealed no alteration in the cellular or plasma membrane content of SERT in Caco-2 cells. EPEC infection of mice (24 hours) reduced SERT immunostaining with a corresponding decrease in SERT messenger RNA levels, 5-HT uptake, and mucosal 5-HT content in the small intestine.

CONCLUSIONS

Our results demonstrate inhibition of SERT by EPEC and define the mechanisms underlying these effects. These data may aid in the development of a novel pharmacotherapy to modulate the serotonergic system in treatment of infectious diarrheal diseases.

Nongenomic mechanisms of physiological estrogen-mediated dopamine efflux

Background

Neurological diseases and neuropsychiatric disorders that vary depending on female life stages suggest that sex hormones may influence the function of neurotransmitter regulatory machinery such as the dopamine transporter (DAT).

Results

In this study we tested the rapid nongenomic effects of several physiological estrogens [estradiol (E₂), estrone (E₁), and estriol (E₃)] on dopamine efflux via the DAT in a non-transfected, NGF-differentiated, rat pheochromocytoma (PC12) cell model that expresses membrane estrogen receptors (ERs) α, β, and GPR30. We examined kinase, ionic, and physical interaction mechanisms involved in estrogenic regulation of the DAT function. E₂-mediated dopamine efflux is DAT-specific and not dependent on extracellular Ca²⁺-mediated exocytotic release from vesicular monoamine transporter vesicles (VMATs). Using kinase inhibitors we also showed that E₂-mediated dopamine efflux is dependent on protein kinase C and MEK activation, but not on PI3K or protein kinase A. In plasma membrane there are ligand-independent associations of ERα and ERβ (but not GPR30) with DAT. Conditions which cause efflux (a 9 min 10^-9 M E₂ treatment) cause trafficking of ERα (stimulatory) to the plasma membrane and trafficking of ERβ (inhibitory) away from the plasma membrane. In contrast, E₁ and E₃ can inhibit efflux with a nonmonotonic dose pattern, and cause DAT to leave the plasma membrane.

Conclusion

Such mechanisms explain how gender biases in some DAT-dependent diseases can occur.

Role of the dopamine transporter in the action of psychostimulants, nicotine, and other drugs of abuse

A number of studies over the last two decades have demonstrated the critical importance of dopamine (DA) in the behavioral pharmacology and addictive properties of abused drugs. The DA transporter (DAT) is a major target for drugs of abuse in the category of psychostimulants, and for methylphenidate (MPH), a drug used for treating attention deficit hyperactivity disorder (ADHD), which can also be a psychostimulant drug of abuse. Other drugs of abuse such as nicotine, ethanol, heroin and morphine interact with the DAT in more indirect ways. Despite the different ways in which drugs of abuse can affect DAT function, one evolving theme in all cases is regulation of the DAT at the level of surface expression. DAT function is dynamically regulated by multiple intracellular and extracellular signaling pathways and several protein-protein interactions. In addition, DAT expression is regulated through the removal (internalization) and recycling of the protein from the cell surface. Furthermore, recent studies have demonstrated that individual differences in response to novel environments and psychostimulants can be predicted based on individual basal functional DAT expression. Although current knowledge of multiple factors regulating DAT activity has greatly expanded, many aspects of this regulation remain to be elucidated; these data will enable efforts to identify drugs that might be used therapeutically for drug dependence therapeutics.

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.

A juxtamembrane mutation in the N terminus of the dopamine transporter induces preference for an inward-facing conformation

The human dopamine transporter (hDAT) regulates synaptic dopamine (DA) levels and is the site of action of abused and therapeutic drugs. Here we study the effect of a threonine residue (Thr62 in hDAT) that is highly conserved within a canonical phosphorylation site (RETW) in the juxtamembrane N-terminal region of monoamine transporters. In stably transfected human embryonic kidney 293T cells, expression of T62D-hDAT was reduced compared with hDAT or T62A-hDAT. T62D-hDAT displayed dramatically reduced [(3)H]dopamine up-take but exhibited a higher basal dopamine efflux compared with hDAT or T62A-hDAT, as determined by measurements of [(3)H]dopamine efflux and amperometry. The high constitutive efflux in T62D-hDAT precluded the measurement of amphetamine-stimulated [(3)H]dopamine efflux, but when dopamine was added internally into voltage-clamped T62D-hDAT cells, amphetamine-induced efflux comparable with hDAT was detected by amperometry. In accordance with findings that Zn(2+) can rescue reduced DA uptake in mutant transporters that are predominantly inward-facing, micromolar concentrations of Zn(2+) markedly potentiated [(3)H]dopamine uptake in T62D-hDAT and permitted the measurement of amphetamine-stimulated dopamine efflux. These results suggest that T62D-hDAT prefers an inward-facing conformation in the transition between inward- and outward-facing conformations. For T62A-hDAT, however, the measured 50% reduction in both [(3)H]dopamine uptake and [(3)H]dopamine efflux was consistent with a slowed transition between inward- and outward-facing conformations. The mechanism underlying the important functional role of Thr62 in hDAT activity suggested by these findings is examined in a structural context using dynamic simulations of a three-dimensional molecular model of DAT.

Amphetamine and methamphetamine differentially affect dopamine transporters in vitro and in vivo

The psychostimulants d-amphetamine (AMPH) and methamphetamine (METH) release excess dopamine (DA) into the synaptic clefts of dopaminergic neurons. Abnormal DA release is thought to occur by reverse transport through the DA transporter (DAT), and it is believed to underlie the severe behavioral effects of these drugs. Here we compare structurally similar AMPH and METH on DAT function in a heterologous expression system and in an animal model. In the in vitro expression system, DAT-mediated whole-cell currents were greater for METH stimulation than for AMPH. At the same voltage and concentration, METH released five times more DA than AMPH and did so at physiological membrane potentials. At maximally effective concentrations, METH released twice as much [Ca(2+)](i) from internal stores compared with AMPH. [Ca(2+)](i) responses to both drugs were independent of membrane voltage but inhibited by DAT antagonists. Intact phosphorylation sites in the N-terminal domain of DAT were required for the AMPH- and METH-induced increase in [Ca(2+)](i) and for the enhanced effects of METH on [Ca(2+)](i) elevation. Calmodulin-dependent protein kinase II and protein kinase C inhibitors alone or in combination also blocked AMPH- or METH-induced Ca(2+) responses. Finally, in the rat nucleus accumbens, in vivo voltammetry showed that systemic application of METH inhibited DAT-mediated DA clearance more efficiently than AMPH, resulting in excess external DA. Together these data demonstrate that METH has a stronger effect on DAT-mediated cell physiology than AMPH, which may contribute to the euphoric and addictive properties of METH compared with AMPH.

Haloperidol regulates the phosphorylation level of the MEK-ERK-p90RSK signal pathway via protein phosphatase 2A in the rat frontal cortex

Haloperidol, a classical antipsychotic drug, affects the extracellular signal-regulated kinase (ERK) pathway in the brain. However, findings are inconsistent and the mechanism by which haloperidol regulates ERK is poorly understood. Therefore, we examined the ERK pathway and the related protein phosphatase 2A (PP2A) in detail after haloperidol administration. Haloperidol (0.5 and 1 mg/kg) induced biphasic changes in the phosphorylation level of mitogen-activated protein kinase kinase (MEK), ERK, and p90 ribosomal S6 kinase (p90RSK) without changing Raf-1 phosphorylation. Fifteen minutes after haloperidol administration, MEK-ERK-p90RSK phosphorylation increased, whilst PP2A activity decreased. At 60 min, the reverse was observed and the binding of PP2A to MEK and ERK increased. Higher dosages of haloperidol (2 and 4 mg/kg), affected neither MEK-ERK-p90RSK phosphorylation nor PP2A activity. Accordingly, PP2A regulates acute dose- and time-dependent changes in MEK-ERK-p90RSK phosphorylation after haloperidol treatment. These findings suggest the involvement of a dephosphorylating mechanism in the acute action of haloperidol.

Phorbol ester induced trafficking-independent regulation and enhanced phosphorylation of the dopamine transporter associated with membrane rafts and cholesterol

We examined the mechanisms involved in protein kinase C (PKC)-dependent down-regulation of dopamine transporter (DAT) activity and cell surface expression by treating heterologously expressing cells with the clathrin-mediated endocytosis inhibitor concanavalin A (Con A) or the cholesterol depleter/membrane raft disrupter methyl-beta-cyclodextrin (MbetaC) prior to treatment with the PKC activator phorbol 12-myristate, 13-acetate (PMA). Con A blocked PMA-induced surface reductions of DAT but only partially inhibited down-regulation, while MbetaC partially blocked down-regulation but did not inhibit loss of cell surface DAT, demonstrating that PKC-induced DAT down-regulation occurs by a combination of trafficking and non-trafficking processes. Using density-gradient centrifugation, we found that DATs are distributed approximately equally between Triton-insoluble, cholesterol-rich membrane rafts and Triton-soluble non-raft membranes. DATs in both populations are present at the cell surface and are active for dopamine and cocaine binding. PMA-induced loss of cell surface DAT occurred only from non-raft populations, demonstrating that non-raft DATs are regulated by trafficking events and indicating the likelihood that the cholesterol-dependent non-trafficking regulatory mechanism occurs in rafts. PMA did not affect the DAT raft-non-raft distribution but stimulated the phosphorylation of DAT to a substantially greater level in rafts than non-rafts. These findings reveal a previously unknown role for cholesterol in DAT function and demonstrate the presence of distinct subcellular DAT populations that possess multiple regulatory differences that may impact dopaminergic neurotransmission.

Dysregulation of dopamine transporter trafficking and function after abstinence from cocaine self-administration in rats: evidence for differential regulation in caudate putamen and nucleus accumbens

The profound alterations produced by cocaine on dopamine (DA) neurotransmission raise the possibility that dopamine transporter (DAT)-expressing neurons may modify DA transport in response to repeated cocaine exposure to maintain the appropriate efficiency of DA clearance. In this study, we determined the changes in molecular mechanisms of DAT regulation in rats with a history of repeated cocaine self-administration followed by 3 weeks of abstinence. Using ex vivo caudate putamen (CPu) and nucleus accumbens (NAcc) synaptosomal preparations, we found that DA uptake was significantly higher in the CPu and NAcc of cocaine-experienced animals compared with yoked saline animals. Surface distribution, p-Ser phosphorylation, and protein phosphatase 2A catalytic subunit (PP2Ac) interaction of DAT were all altered in the CPu. Maximal velocity (V(max)) values were elevated both in the CPu and NAcc of cocaine-experienced rats compared with saline controls. Although there was no change in the apparent affinity for DA in the CPu, increased DA affinity was evident in the NAcc. Consistent with elevated DAT activity in cocaine-experienced animals, a higher level of surface DAT, DAT-PP2Ac association, and decreased serine phosphorylation of DAT were observed in the CPu, but not in the NAcc. These results, for the first time, suggest that chronic cocaine self-administration followed by abstinence leads to persisting alterations in normal DAT trafficking and catalytic regulatory cascades in the CPu and NAcc in a brain region-specific manner.

Functional comparison of full-length and N-terminal-truncated octopamine transporters from Lepidoptera

We have cloned two new lepidopteran octopamine transporters (OATs), members of the solute-linked carrier family 6 (SLC6) of nutrient transporters, from the CNS of the European corn borer Ostrinia nubilalis and the cabbage white Pieris rapae. Comparison of these sequences with the previously cloned OAT from the cabbage looper Trichoplusia ni showed that the T. ni OAT sequence previously reported was truncated by 74 amino acids at the N-terminus. The cytoplasmic N-termini deduced here are considerably longer than the N-termini of other monoamine transporters in the SLC6 family and contain many more high-probability serine- and threonine-phosphorylation sites. Monoamine uptake and competitive inhibition studies on baculovirus-infected Sf9 cells expressing these three cloned OATs indicate that they are able to transport tyramine, octopamine and dopamine with high affinity (K(m) and K(i) range, 0.4 microM-2.7 microM) and capacity ((3)H-dopamine uptake by TrnOAT, 2.5 pmol/well/min). We aimed to examine the role of the N-terminus of OAT by comparing the properties of the full-length T. ni OAT with those of the previously reported N-truncated version. Results for the new full-length T. ni OAT showed no difference in the protein's affinity for octopamine or dopamine, although at low levels of viral infection it did show slightly higher transport activity ((3)H-dopamine uptake by truncated TrnOAT, 1.5 pmol/well/min). Treatment of Sf9 cells expressing full-length or truncated TrnOAT with a variety of protein kinase activators and inhibitors, however, did not change transporter activity. Neither an intact N-terminus, nor apparently a particular phosphorylation state of this extended N-terminus, is required for OAT to transport monoamines.

The functional impact of SLC6 transporter genetic variation

Solute carrier 6 (SLC6) is a gene family of ion-coupled plasma membrane cotransporters, including transporters of neurotransmitters, amino acids, and osmolytes that mediate the movement of their substrates into cells to facilitate or regulate synaptic transmission, neurotransmitter recycling, metabolic function, and fluid homeostasis. Polymorphisms in transporter genes may influence expression and activity of transporters and contribute to behavior, traits, and disease. Determining the relationship between the monoamine transporters and complex psychiatric disorders has been a particular challenge that is being met by evolving approaches. Elucidating the functional consequences of and interactions among polymorphic sites is advancing our understanding of this relationship. Examining the influence of environmental influences, especially early-life events, has helped bridge the gap between genotype and phenotype. Refining phenotypes, through assessment of endophenotypes, specific behavioral tasks, medication response, and brain network properties has also improved detection of the impact of genetic variation on complex behavior and disease.

Phosphorylation of threonine residue 276 is required for acute regulation of serotonin transporter by cyclic GMP

Cellular protein kinases, phosphatases, and other serotonin transporter (SERT) interacting proteins participate in several signaling mechanisms regulating SERT activity. The molecular mechanisms of protein kinase G (PKG)-mediated SERT regulation and the site of transporter phosphorylation were investigated. Treatment of rat midbrain synaptosomes with 8-bromo-cGMP increased SERT activity, and the increase was selectively blocked by PKG inhibitors. The V(max) value for serotonin (5-HT) transport increased following cGMP treatment. However, surface biotinylation studies showed no change in SERT surface abundance following PKG activation. (32)P metabolic labeling experiments showed increased SERT phosphorylation in the presence of cGMP that was abolished by selectively inhibiting PKG. Phosphoamino acid analysis revealed that cGMP-stimulated native SERT phosphorylation occurred only on threonine residues. When added to CHO-1 cells expressing SERT, 8-bromo-cGMP stimulated 5-HT transport and SERT phosphorylation. Mutation of SERT threonine 276 to alanine completely abolished cGMP-mediated stimulation of 5-HT transport and SERT phosphorylation. Although the T276A mutation had no significant effect on 5-HT transport or SERT protein expression, mutation to aspartate (T276D) increased the level of 5-HT uptake to that of cGMP-stimulated 5-HT uptake in wild-type SERT-expressing cells and was no longer sensitive to cGMP. These findings provide the first identification of a phosphorylation site in SERT and demonstrate that phosphorylation of Thr-276 is required for cGMP-mediated SERT regulation. They also constitute the first evidence that in the central nervous system PKG activation stimulates endogenous SERT activity by a trafficking-independent mechanism.

Rhesus monkey trace amine-associated receptor 1 signaling: enhancement by monoamine transporters and attenuation by the D2 autoreceptor in vitro

Trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor that directly responds to endogenous monoamines as well as amphetamine-related psychostimulants, including methamphetamine. In the present study, we demonstrate TAAR1 mRNA and protein expression in rhesus monkey brain regions associated with monoaminergic systems, variable cellular distribution of TAAR1 in rhesus monkey brain, and TAAR1 coexpression with the dopamine transporter (DAT) in a subset of dopamine neurons in both rhesus monkey and mouse substantia nigra. On this basis, we evaluated rhesus monkey TAAR1 activation by different compounds and its functional relation with monoamine transporters and the dopamine D2 receptor (D2) short isoform (D2s) autoreceptor in vitro using a cAMP response element-luciferase assay. TAAR1 activation by monoamines and amphetamine-related compounds was greatly enhanced by coexpression of dopamine, norepinephrine, or serotonin transporters, and the activation enhancement was blocked by monoamine transporter inhibitors. This enhancement did not occur in control experiments in which the dopamine D1 receptor (D1) was substituted for TAAR1. Furthermore, activation of TAAR1 by dopamine was completely inhibited by D2s when coexpressed with TAAR1, and this inhibition was blocked by the D2 antagonist raclopride. Last, dopamine activation of TAAR1 could induce c-FOS-luciferase expression but only in the presence of DAT, whereas dopamine activation of D1 resulted in equivalent c-FOS expression in the presence or absence of DAT. Together, these data reveal a broad agonist spectrum for TAAR1, a functional relation of TAAR1 with monoamine transporters and D2s, and a mechanism by which D2 receptor drugs can influence brain monoaminergic function and have efficacy through affecting TAAR1 signaling.

Serotonin transport and serotonin transporter‐mediated antidepressant recognition are controlled by 5‐HT2Breceptor signaling in serotonergic neuronal cells

The plasma membrane 5-HT transporter (SERT) is the major protagonist in regulating extracellular 5-HT concentration and constitutes the target of drugs used to treat a host of metabolic and psychiatric disorders. The exact mechanisms sustaining SERT function still remain elusive. The present work exploits the properties of the 1C11 neuroectodermal progenitor, which acquires, upon 4 days of differentiation, a functional SERT within an integrated serotonergic phenotype to investigate regulatory mechanisms involved in SERT onset and functions. We show that poly(A) addition precedes SERT mRNA translation on day 2 of the serotonergic program. The newly translated transporter molecules immediately bind cocaine. Day 4 must be awaited to monitor antidepressant recognition and 5-HT uptake. Because external 5-HT reduces both 5-HT transport and SERT antidepressant binding, we identify 5-HT(2B) receptors as key players in controlling the overall 5-HT transport system. In the absence of external 5-HT, 5-HT(2B) receptor coupling to NO production ensures SERT phosphorylation to basal level and maximal 5-HT uptake. In the presence of 5-HT, the 5-HT(2B) receptor-PKC coupling promotes additional phosphorylations of both SERT and Na(+),K(+)-ATPase alpha-subunit, impairing the electrochemical gradient necessary to 5-HT uptake. SERT hyperphosphorylation also affects antidepressant recognition. Finally, such 5-HT(2B) receptor-mediated control of SERT activity operates in primary neurons from raphe nuclei. Altogether, our data shed new light on the 5-HT-driven post-translational modifications involved in the control of SERT activity.

Regulation of the dopamine transporter by phosphorylation

The dopamine transporter (DAT) is a neuronal phosphoprotein and target for psychoactive drugs that plays a critical role in terminating dopaminergic transmission by reuptake of dopamine from the synaptic space. Control of DAT activity and plasma membrane expression are therefore central to drug actions and the spatial and temporal regulation of synaptic dopamine levels. DATs rapidly traffic between the plasma membrane and endosomal compartments in both constitutive and protein kinase C-dependent manners. Kinase activators, phosphatase inhibitors, and transported substrates modulate DAT phosphorylation and activity, but the underlying mechanisms and role of phosphorylation in these processes are poorly understood. Complex adaptive changes in DAT function potentially related to these processes are also induced by psychostimulant and therapeutic transport blockers such as cocaine and methylphenidate. This chapter provides an overview of the current state of knowledge regarding DAT phosphorylation and its relationship to transporter activity and trafficking. A better understanding of how dopaminergic neurons regulate DAT function and the role of phosphorylation may lead to the identification of novel therapeutic targets for the treatment and prevention of dopaminergic disorders.

An Initial Assessment of the Systemic Pharmacokinetics of Botulinum Toxin

Botulinum toxin is an extraordinarily potent molecule that has an unusually long duration of action. Despite this, there is little information available on natural mechanisms for metabolism or elimination and virtually no information on pharmacologically induced mechanisms for metabolism and elimination. Therefore, a number of experiments were performed on laboratory animals that addressed two major issues: 1) the effect of blood on the structure, function, and biologic half-life of the toxin, and 2) the effect of neutralizing antibodies on half-life and elimination of circulating toxin. In the first series of studies, the metabolic transformation of toxin was assessed by incubating it in blood for varying lengths of time. At each time point, aliquots were examined to determine the amount of toxin, the structure of toxin, the catalytic activity of toxin, and the neuromuscular blocking activity of toxin. This work demonstrated that blood did not alter any characteristic of the toxin molecule. Experiments were also done in which toxin was administered to mice and rats at doses that produced clinical poisoning. The results demonstrated that the elimination half-life for native (nonmetabolized) toxin in blood and serum was 230 to 260 min. During the second series of studies, the rate of elimination of circulating toxin was studied in the presence of antibodies directed against the carboxyl-terminal half of the toxin molecule. This work demonstrated that neutralizing antibodies 1) enhanced clearance of toxin from the circulation and 2) enhanced tissue accumulation of toxin, particularly in liver and spleen.

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.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Effects of continuous administration of paroxetine on ligand binding site and expression of serotonin transporter protein in mouse brain

Selective serotonin reuptake inhibitors (SSRIs), such as paroxetine, are utilized in the treatment of depression and anxiety disorders. Although SSRIs potently interfere with the activity of brain serotonin transporter (SERT) after acute treatment, clinical improvement of psychiatric diseases is observed only after the repeated administration for several weeks (2-6 weeks). The present study was undertaken to investigate the effects of continuous administration of paroxetine on specific [3H]paroxetine binding sites and expression of SERT protein in mouse brain. Mice continuously and subcutaneously received paroxetine at doses of 2.67 or 13.3 mumol/kg/day for 21 days by using osmotic minipumps, and the steady-state plasma drug levels were within the range of reported concentrations in the clinical therapy. Continuous administration of paroxetine at theses doses produced significant (25-46%) reduction of [3H]paroxetine binding in each brain region (cerebral cortex, striatum, hippocampus, thalamus, midbrain) of mice. In Western blot analysis, expression levels of SERT protein in the thalamus and midbrain of mice were significantly (51% and 61%, respectively) decreased on day 21 after the implantation of minipumps at the higher dose. In conclusion, this study has firstly shown that continuous administration of paroxetine induces significant reduction of not only ligand binding sites of SERT but the protein expression level in mouse brain. Such down-regulation of SERT may partly underlie the therapeutic effect of long-term treatment with SSRIs in human.

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.

Psychoactive substrates stimulate dopamine transporter phosphorylation and down-regulation by cocaine-sensitive and protein kinase C-dependent mechanisms

Dopamine transporters (DATs) undergo intracellular sequestration and functional down-regulation upon exposure to psychostimulant substrates. To investigate the potential mechanism underlying these responses, we examined the acute in vitro and in vivo effects of amphetamine and methamphetamine (METH) on phosphorylation and down-regulation of rat DAT using wild type and N-terminal truncation mutants. Phosphorylation of DAT assessed by (32)PO(4) metabolic labeling was increased up to 2-fold by in vitro treatment of rDAT LLC-PK(1) cells with amphetamine or METH and was similarly increased in rat striatal tissue by in vitro application or in vivo injection of METH. The dopamine transport blocker (-)-cocaine did not affect DAT phosphorylation but prevented the phosphorylation increase induced by METH. Phosphorylation of DAT induced by METH was also prevented by the protein kinase C blocker bisindoylmaleimide I and was absent in an N-terminally truncated protein that lacks the first 21 residues including 6 serines that also represent the site of phorbol ester induced phosphorylation. Down-regulation of transport induced by METH was also cocaine- and protein kinase C-dependent but was retained in the N-terminal truncation mutant. These results demonstrate that transport or binding of METH stimulates DAT phosphorylation and down-regulation by a mechanism that requires protein kinase C but that METH-induced down-regulation can occur independently of direct transporter phosphorylation. The finding that DAT phosphorylation is stimulated by amphetamines reveals a previously unknown effect of these drugs that is not produced by cocaine and may be related to reinforcement.

Differential effects of dopamine and psychoactive drugs on dopamine transporter phosphorylation and regulation

The dopamine transporter (DAT) is a phosphoprotein whose activity and phosphorylation state are acutely regulated by both protein kinase C (PKC) and substrate transport. DAT is a major site of action for psychostimulant and therapeutic drugs that either block transport or are transported substrates, but the effects of such drugs on DAT phosphorylation and regulation are not well understood. To examine these issues we subjected rDAT LLC-PK(1) cells to acute in vitro pretreatments with the endogenous, psychostimulant, and therapeutic compounds dopamine (DA), (-)-cocaine, 2 beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (beta-CFT), GBR 12909, mazindol, and methylphenidate (MPH), in the presence or absence of the PKC activator phorbol 12 myristate 13 acetate (PMA), followed by analysis of DAT metabolic phosphorylation and transport activity. Basal phosphorylation of DAT was not affected by any of the uptake blockers tested, and PMA-stimulated phosphorylation was not affected by cocaine, beta-CFT, mazindol or MPH, but was strongly suppressed by GBR 12909. Pretreatment of cells with cocaine or MPH had no effect on subsequent DA transport activity or the extent of PMA-induced transport down-regulation, whereas GBR 12909 inhibited PMA-induced DAT internalization. These findings indicate that these DAT phosphorylation and down-regulation properties are unaffected by some classes of uptake blocking drugs, but that differential regulatory effects may be exerted by GBR compounds. Pretreatment of cells with DA had no obvious effect on basal or PMA-stimulated DAT phosphorylation but led to cocaine-blockable transport down-regulation. DA-induced down-regulation was blocked by the PKC inhibitor bisindoylmaleimide I and was not additive with down-regulation induced by PMA, consistent with PKC serving as a common step and point of integration for these DA and PMA induced processes. The results of this study provide information on the potential for endogenous and psychoactive compounds to modulate or be modulated by DAT phosphorylation-mediated regulatory mechanisms that may contribute to drug behavioral or therapeutic properties.

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.

Visualization of Binding and Transcytosis of Botulinum Toxin by Human Intestinal Epithelial Cells

Botulinum toxin is an unusually potent oral poison, which means that the toxin must have an efficient mechanism for escaping the lumen of the gut to reach the general circulation. Previous work involving iodination of toxin and analysis of its movement demonstrated a specific process of transepithelial transport. In the present study, botulinum toxin labeled with Alexa Fluor 488 was used to visualize the discrete steps of binding, internalization, transcytosis, and release. The data revealed that binding sites for the toxin were distributed across the apical surface of epithelial cells, and there was no evidence of significant clustering. The amount of toxin bound to receptors at saturation was too large to be accommodated in a single wave of endocytosis. Toxin that entered epithelial cells did not remain in the vicinity of the endocytosing membrane, which is in striking contrast to events in neuronal cells. Instead, the toxin began to spread across the length of cells, eventually being released on the basolateral surface. Migration of toxin through epithelial cells required redistribution to the cell periphery. This migration pattern could be attributed to the large and centrally located nucleus, which physically displaced transport vesicles. Transcytosed toxin began to reach the contralateral surface within ca. 5 min, and transcytosis was essentially complete within 20 to 30 min.

Deficits in dopamine clearance and locomotion in hypoinsulinemic rats unmask novel modulation of dopamine transporters by amphetamine

Insulin affects brain reward pathways and there is converging evidence that this occurs through insulin regulation of the dopamine (DA) transporter (DAT). In rats made hypoinsulinemic by fasting, synaptosomal DA uptake is reduced. Interestingly, [3H]DA uptake is increased in hypoinsulinemic rats with a history of amphetamine self-administration. The possibility that amphetamine and insulin act in concert to regulate DAT activity prompted this study. Here we show that [3H]DA uptake, measured in vitro and clearance of exogenously applied DA in vivo, is significantly reduced in rats made hypoinsulinemic by a single injection of streptozotocin. Strikingly, amphetamine (1.78 mg/kg, given every other day for 8 days) restored DA clearance in streptozotocin-treated rats but was without effect on DA clearance in saline-treated rats. Basal locomotor activity of streptozotocin-treated rats was lower compared to control rats; however, in streptozotocin-treated rats, hyperlocomotion induced by amphetamine increased over successive amphetamine injections. In saline-treated rats the locomotor stimulant effect of amphetamine remained stable across the four amphetamine injections. These results provide exciting new evidence that actions of amphetamine on DA neurotransmission are insulin-dependent and further suggest that exposure to amphetamine may cause long-lasting changes in DAT function.

Effect of mirtazapine treatment on serotonin transporter in blood peripheral lymphocytes of major depression patients

Lymphocytes from human peripheral blood exhibit a series of markers of neurotransmitters, such as specific receptors and transporters. A reduction of serotonin transporters and an increase of them has been reported after treatment with fluoxetine in depressed patients. The aim of this study was to determine if the administration of an antidepressant with a different mechanism of action, such as mirtazapine, could produce a similar effect. Twenty eight patients (age 41.40+/-2.45) were diagnosed following the criteria for major depression by the Structured Clinical Interview for Disorders of Axis I of the American Psychiatric Association. Severity was measured by Hamilton Scale and by Beck Inventory for Depression, scores of 30.88+/-7.48 and 30.24+/-10.88, respectively, prior to treatment. Samples from control subjects were obtained alternating with patients before and after the administration of the antidepressant: twenty eight and twenty four, respectively (age 38.80+/-2.95). Mirtazapine was given in a dose of 30 mg/day for 6 weeks. Blood lymphocytes were isolated by density gradient from patients and controls before and after treatment. There was a partial response according to clinical evaluation and scores of the Scale and the Inventory. Serotonin transporters were labeled with [3H] paroxetine. Number of sites (B(max)) were 10.86+/-2.60 and 12.58+/-2.71 fmol/10(6) cells for both groups of controls. The depressed patients had a significant reduction of serotonin transporters in their lymphocytes before treatment and an increase after it, with B(max) values of 6.52+/-0.49 and 15.61+/-0.49 fmol/10(6) cells, respectively. There were no significant differences in the affinity for the ligand. Concentrations of serotonin or noradrenaline in lymphocytes were not modified before the treatment, although there was a significant decrease after taking 30 mg/day of the antidepressant for 6 weeks. Mirtazapine, not being a serotonin reuptake inhibitor, did increase the number of transporters in lymphocytes of major depression patients, indicating a complex mechanism, not only directly related to the transporter, but involved in the therapeutic response.

Evidence for biphasic effects of protein kinase C on serotonin transporter function, endocytosis, and phosphorylation

The serotonin transporter (SERT) regulates 5-hydroxytryptamine (serotonin) (5-HT) neurotransmission and is a high-affinity target for antidepressants and psychostimulants. In the present study, we investigated the mechanisms that contribute to a previously unidentified biphasic regulation of endogenous SERTs expressed in the platelets. Treatment of rat platelets with beta-phorbol 12-myristate 13-acetate (PMA) for 5 min or less resulted in a rapid inhibition of SERT involving changes in intrinsic activity of the transporter (increased K(m) and decreased V(max)). beta-PMA treatment for 30 min or more produced a sustained inhibition of SERT with a decrease only in the V(max). Whereas inhibition of SERT activity was detected from 1 to 45 min after phorbol ester addition, the decrease in surface SERT required at least 30 min of phorbol ester incubation. Increased endocytosis of SERT accounted for the decrease in surface SERT at the later point. Protein kinase C (PKC)-mediated phosphorylation of SERT occurs on the plasma membrane during the initial phase of rapid transporter inhibition, and later, the phosphorylated SERT enters the intracellular pool. beta-PMA-induced phosphorylation of SERT occurs initially on serine residues(s) and then on threonine residue(s). The initial serine phosphorylation corresponded to the first phase of rapid inhibition mediated by changes in intrinsic activity and/or silencing of SERT. The later phosphorylation on threonine residue(s) corresponded to the later phase of sustained inhibition mediated by an enhanced endocytosis of SERT. Together, these data reveal that in platelets, SERT function is regulated by PKC in a biphasic manner involving both trafficking-dependent and independent mechanisms and that these two events occur at distinct phases of transporter phosphorylation.

A role for p38 mitogen-activated protein kinase in the regulation of the serotonin transporter: evidence for distinct cellular mechanisms involved in transporter surface expression

The serotonin transporter (SERT) is regulated by various signaling mechanisms that may operate to maintain appropriate levels of synaptic serotonin (5-HT). We demonstrate that one of the mitogen-activated protein kinases (MAPKs), p38 MAPK, regulates SERT. Treatment of rat midbrain synaptosomes with p38 MAPK-specific inhibitors, PD169316 [4-(4-fluorophenyl)-2-(4-nitrophenyl)-5-(4-pyridyl)-1H-imidazole] or SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole], reduced 5-HT uptake. An additive SERT inhibition by PD169316 and beta-phorbol 12-myristate 13-acetate (beta-PMA) indicated the involvement of a protein kinase C (PKC)-independent MAPK pathway. Kinetic studies indicated a significant decrease in the transport capacity (V(max)) after PD169316 treatment of synaptosomes. Biotinylation studies showed reduced SERT proteins in the plasma membrane of synaptosomes after p38 MAPK inhibition and PKC activation. Phosphorylation studies using synaptosomes revealed decreased SERT phosphorylation by PD169316 but increased phosphorylation by beta-PMA. d-Amphetamine enhanced SERT basal phosphorylation and PD169316 blocked this effect. SERT interaction with protein phosphatase 2A catalytic subunit and syntaxin 1A decreased after PD169316 or beta-PMA treatment of synaptosomes. In synaptosomes, PKC activation but not p38 MAPK inhibition resulted in SERT redistribution from cholesterolrich lipid raft fractions to nonlipid raft fractions. The presence of phospho-p38 MAPK in synaptosomes and human embryonic kidney 293 (HEK-293) cells suggested the presence of constitutively active p38 MAPK in these preparations. Cotransfection of HEK-293 cells with SERT and a constitutively active form of MAP kinase kinase 3b(E) [MKK3b(E)] increased 5-HT transport, and RNA interference targeted to p38 MAPK inhibited 5-HT uptake, confirming the involvement of active p38 MAPK in SERT expression. Although PD169316 inhibited SERT insertion to the plasma membrane, beta-PMA increased SERT internalization in HEK-293 cells. Together, these results indicate a distinct role of p38 MAPK in SERT regulation.