Ca(2+)-dependent enhancement of [3H]noradrenaline uptake in PC12 cells through calmodulin-dependent kinases

MAP Kinase Phosphatase 3 (MKP3) Preserves Norepinephrine Transporter Activity by Modulating ERK1/2 Kinase-Mediated Gene Expression

The norepinephrine transporter (NET) mediates the clearance of norepinephrine (NE) from the extracellular space and is a target of therapeutic antidepressants and psychostimulants. Previously we identified a MAP kinase phosphatase 3 (MKP3), as an important modulator of protein kinase C (PKC) mediated internalization of the related dopamine transporter (DAT). Here we show that MKP3 decreases PKC-mediated down regulation of NET expressed in PC12 cells. We demonstrate that this process involves a PKC-stimulated decrease of NET surface expression that is dependent on dynamin. Surprisingly, MAP kinase inhibitors have no effect on the PKC-mediated regulation of NET activity, suggesting that, like PKC-mediated regulation of the DAT, the acute activation of MAP kinases is not likely to be involved. To elucidate potential mechanisms we used a substrate trap-based assay to identify extracellular-signal-regulated kinase (ERK)1/2 as the predominant substrate of MKP3. Furthermore we also established that brief chemical stabilization of a modified destabilized MKP3 does not alter PKC-mediated down regulation of NET. Finally, the expression of a dominant negative version of H-Ras, an upstream activator of ERK1/2, abolishes phorbol 12-myristate 13-acetate (PMA)-mediated down regulation of NET in a manner similar to MKP3. Taken together we propose that chronic MKP3 expression regulates surface NET through the sustained inhibition of ERK1/2 MAP kinase signaling that alters gene expression in PC12 cells. This is supported by gene expression data from naïve and MKP3-expressing PC12 cells that reveal robust decreases in gene expression of several genes in the MKP3-tranfected cells. Interestingly, caveolin-1, a protein with a critical role in membrane protein trafficking is down regulated by MKP3 expression. We further show that selective silencing of the caveolin-1 gene in naïve PC12 cells attenuates PKC-mediated downregulation of NET activity, consistent with a potential role for caveolins in regulating NET surface expression. In summary, these results suggest that chronic MKP3 expression alters the expression of genes in PC12 cells that are involved in the regulation of NET surface expression.

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.

131I-MIBG targeting of neuroblastoma cells is acutely enhanced by KCl stimulation through the calcium/calmodulin-dependent kinase pathway

The efficacy of (131)I-metaiodobenzylguanidine (MIBG) therapy relies on norepinephrine transporter (NET) function. The ionic make-up of the extracellular fluid critically controls neuronal cell activity and can also affect substrate transport. In this study, we explored the effect of treatment with elevated KCl concentration on MIBG uptake in SK-N-SH neuroblastoma cells. KCl stimulation caused a rapid increase of (131)I-MIBG uptake in a manner that was calcium-dependent and accompanied by activation of calcium/calmodulin-dependent protein kinase (CaMK)II. The effect was completely abolished by KN93, an inhibitor of CaMKI, II, and IV. STO609, a selective inhibitor of CaMK kinase required for activation of CaMKI and IV, but not CaMKII, only modestly attenuated the response. The KCl effect was also completely abrogated by ML7, a selective inhibitor of myosin light chain kinase (MLCK). This restricted form of CaMK activates myosin, which is required for vesicle trafficking. Saturation kinetic analysis revealed KCl stimulation to increase maximal transport velocity without affecting substrate affinity. In conclusion, KCl stimulation rapidly upregulates NET function through the CaMK pathway via activation of CaMKII and MLCK. These findings allow a better understanding of how NET function is acutely modulated by the ionic environment, which in turn may ultimately help improve the efficacy of (131)I-MIBG therapy.

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.

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.

Insulin reveals Akt signaling as a novel regulator of norepinephrine transporter trafficking and norepinephrine homeostasis

Noradrenergic signaling in the CNS plays an essential role in circuits involving attention, mood, memory, and stress as well as providing pivotal support for autonomic function in the peripheral nervous system. The high-affinity norepinephrine (NE) transporter (NET) is the primary mechanism by which noradrenergic synaptic transmission is terminated. Data indicate that NET function is regulated by insulin, a hormone critical for the regulation of metabolism. Given the high comorbidity of metabolic disorders such as diabetes and obesity with mental disorders such as depression and schizophrenia, we sought to determine how insulin signaling regulates NET function and thus noradrenergic homeostasis. Here, we show that acute insulin treatment, through the downstream kinase protein kinase B (Akt), significantly decreases NET surface expression in mouse hippocampal slices and superior cervical ganglion neuron boutons (sites of synaptic NE release). In vivo manipulation of insulin/Akt signaling, with streptozotocin, a drug that induces a type 1-like diabetic state in mice, also results in aberrant NET function and NE homeostasis. Notably, we also demonstrate that Akt inhibition or stimulation, independent of insulin, is capable of altering NET surface availability. These data suggest that aberrant states of Akt signaling such as in diabetes and obesity have the potential to alter NET function and noradrenergic tone in the brain. Furthermore, they provide one potential molecular mechanism by which Akt, a candidate gene for mood disorders such as schizophrenia and depression, can impact brain monoamine homeostasis.

Rab11 supports amphetamine-stimulated norepinephrine transporter trafficking

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

Reserpine-induced reduction in norepinephrine transporter function requires catecholamine storage vesicles

Treatment of rats with reserpine, an inhibitor of the vesicular monoamine transporter (VMAT), depletes norepinephrine (NE) and regulates NE transporter (NET) expression. The present study examined the molecular mechanisms involved in regulation of the NET by reserpine using cultured cells. Exposure of rat PC12 cells to reserpine for a period as short as 5min decreased [(3)H]NE uptake capacity, an effect characterized by a robust decrease in the V(max) of the transport of [(3)H]NE. As expected, reserpine did not displace the binding of [(3)H]nisoxetine from the NET in membrane homogenates. The potency of reserpine for reducing [(3)H]NE uptake was dramatically lower in SK-N-SH cells that have reduced storage capacity for catecholamines. Reserpine had no effect on [(3)H]NE uptake in HEK-293 cells transfected with the rat NET (293-hNET), cells that lack catecholamine storage vesicles. NET regulation by reserpine was independent of trafficking of the NET from the cell surface. Pre-exposure of cells to inhibitors of several intracellular signaling cascades known to regulate the NET, including Ca(2+)/Ca(2+)-calmodulin dependent kinase and protein kinases A, C and G, did not affect the ability of reserpine to reduce [(3)H]NE uptake. Treatment of PC12 cells with the catecholamine depleting agent, alpha-methyl-p-tyrosine, increased [(3)H]NE uptake and eliminated the inhibitory effects of reserpine on [(3)H]NE uptake. Reserpine non-competitively inhibits NET activity through a Ca(2+)-independent process that requires catecholamine storage vesicles, revealing a novel pharmacological method to modify NET function. Further characterization of the molecular nature of reserpine's action could lead to the development of alternative therapeutic strategies for treating disorders known to be benefitted by treatment with traditional competitive NET inhibitors.

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.

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.

The effect of streptozotocin-induced diabetes on the rat seminal vesicle: A possible pathophysiological basis for disorders of ejaculation

In the streptozotocin (STZ)-diabetic rat major increases in noradrenaline concentration and content of the seminal vesicles were evident as early as 7 weeks following induction of hyperglycemia and returned toward normal after 34 weeks of hyperglycemia. There were significant reductions in the concentration and content of dopamine at 19-42 weeks of diabetes, and small occasionally significant reductions in the content of serotonin and adrenaline, particularly around 19-26 weeks after STZ treatment. The uptake of tritiated noradrenaline in the diabetics was increased at 12 weeks compared to the controls, and decreased to control levels with increasing age. Release of tritiated noradrenline was increased in response to electrical field stimulation and high potassium solutions, and raising calcium concentration caused increased release at rest and during electrical stimulation. Immunohistochemical demonstration of tyrosine hydroxylase was increased during the period when the noradrenaline concentration and content were elevated. It is concluded that there are significant changes in the sympathetic innervation of the seminal vesicle during the course of STZ diabetes, and that alterations in the reuptake, release, and synthesis of the neurotransmitter noradrenaline may contribute to changes in the concentration of the amine in the tissue. It is possible that the changes observed are related to the remodeling and regrowth of sympathetic nerve endings damaged in the early stages of hyperglycemia. These changes may also contribute to disorders of ejaculation in diabetes.

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

The norepinephrine (NE) transporter (NET) terminates noradrenergic signaling by clearing released NE at synapses. The activity of NET can be rapidly regulated by depolarization and receptor activation via Ca2+ and kinase/phosphatase-linked pathways. The SNARE protein syntaxin 1A (SYN1A) interacts with NET and influences transporter surface trafficking and catalytic activity. In this study, we establish a link between changes in intracellular Ca2+ and SYN1A/NET interactions. SYN1A influenced NE transport only in the presence of Ca2+ in brain cortical synaptosomes. Although NET/SYN1A associations were sensitive to manipulations of Ca2+ in CHO cells, in vitro binding experiments using purified NET and SYN1A fusion proteins demonstrated a lack of direct Ca2+ sensitivity. Disruption of NET/SYN1A interaction abolished inhibition of NE transport by phorbol ester (PMA) to activate protein kinase C (PKC), but had no effect on transport inhibition by the Ca2+ calmodulin kinase (CaMK) inhibitor KN93. Furthermore, PMA enhanced Ca2+-dependent modulation of NE transport in synaptosomes. Our data reveal roles for SYN1A in the Ca2+-dependent regulation of NET, likely reliant on regulation by PKC signaling, but independent of CaMK.

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

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

Amphetamine Induces a Calcium/Calmodulin-Dependent Protein Kinase II-Dependent Reduction in Norepinephrine Transporter Surface Expression Linked to Changes in Syntaxin 1A/Transporter Complexes

Norepinephrine (NE) transporters (NETs) are high-affinity transport proteins that mediate the synaptic clearance of NE after vesicular release. NETs represent a major therapeutic target for antidepressants and are targets of multiple psychostimulants including amphetamine (AMPH) and cocaine. Recently, we demonstrated that syntaxin 1A (SYN1A) regulates NET surface expression and, through binding to the transporter's NH(2) terminus, regulates transporter catalytic function. AMPH induces NE efflux and may also regulate transporter trafficking. We monitored NET distribution and function in catecholaminergic cell lines (CAD) stably transfected with either full-length human NET (CAD-hNET) or with an hNET N-terminal deletion (CAD-hNETDelta(28-47) cells). In hNET-CAD cells, AMPH causes a slow and small reduction of surface hNET with a modest increase in hNET/SYN1A associations at the plasma membrane. In contrast, in CAD-hNETDelta(28-47) cells, AMPH induces a rapid and substantial reduction in surface hNETDelta(28-47) accompanied by a large increase in plasma membrane hNETDelta(28-47)/SYN1A complexes. We also found that AMPH in CAD-hNETDelta(28-47) cells induces a robust increase in cytosolic Ca2+ and concomitant activation of calcium/calmodulin-dependent protein kinase II (CaMKII). Inhibition of either the increase in intracellular Ca2+ or CaMKII activity blocks AMPH-stimulated hNETDelta(28-47) trafficking and the formation of hNETDelta(28-47)/SYN1A complexes. Here, we demonstrate that AMPH stimulation of CAMKII stabilizes an hNET/SYN1A complex. This hNET/SYN1A complex rapidly redistributes, upon AMPH treatment, when mechanisms supported by the transporter's NH2 terminus are eliminated.

KCl stimulation increases norepinephrine transporter function in PC12 cells

The norepinephrine transporter (NET) plays a pivotal role in terminating noradrenergic signaling and conserving norepinephrine (NE) through the process of re-uptake. Recent evidence suggests a close association between NE release and regulation of NET function. The present study evaluated the relationship between release and uptake, and the cellular mechanisms that govern these processes. KCl stimulation of PC12 cells robustly increased [3H]NE uptake via the NET and simultaneously increased [3H]NE release. KCl-stimulated increases in uptake and release were dependent on Ca2+. Treatment of cells with phorbol-12-myristate-13-acetate (PMA) or okadaic acid decreased [3H]NE uptake but did not block KCl-stimulated increases in [3H]NE uptake. In contrast, PMA increased [3H]NE release and augmented KCl-stimulated release, while okadaic acid had no effects on release. Inhibition of Ca2+-activated signaling cascades with KN93 (a Ca2+ calmodulin-dependent kinase inhibitor), or ML7 and ML9 (myosin light chain kinase inhibitors), reduced [3H]NE uptake and blocked KCl-stimulated increases in uptake. In contrast, KN93, ML7 and ML9 had no effect on KCl-stimulated [3H]NE release. KCl-stimulated increases in [3H]NE uptake were independent of transporter trafficking to the plasma membrane. While increases in both NE release and uptake mediated by KCl stimulation require Ca2+, different intracellular mechanisms mediate these two events.

The norepinephrine transporter and its regulation

For many years, the norepinephrine transporter (NET) was considered a 'static' protein that contributed to the termination of the action of norepinephrine in the synapse of noradrenergic neurons. The concept that the NET is dynamically regulated, adjusting noradrenergic transmission by changing its function and/or expression, was considered initially in the mid 1980s. Since that time, a plethora of studies demonstrate that the NET is regulated by several intracellular and extracellular signaling molecules, and that phosphorylation of the NET is a major pathway regulating its cell surface expression and thereby its function. The NET is a target of action of a number of drugs that are used long-term therapeutically or abused chronically. This has driven numerous investigations of how the NET and its function are regulated by long-term exposure to drugs. While repeated exposure to many drugs has been shown to affect NET function and expression, the intracellular mechanisms for these effects remains elusive.

Comparative studies of PC12 and mouse pheochromocytoma-derived rodent cell lines as models for the study of neuroendocrine systems

We have compared PC12 cell lines derived from different laboratories and the newly developed mouse pheochromocytoma (MPC) cell line. Morphologically, there were distinct differences in size, shape, adherence, and clumping behavior, which varied in response to different culture media, growth substrates, and nerve growth factor. Quantitative messenger ribonucleic acid (mRNA) analysis showed significant variability in the expression of the catecholaminergic biosynthetic enzymes tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase (PNMT), the noradrenaline transporter (NAT), and neuron-specific enolase (NSE) between all lines examined. Of most significance were the increased levels of PNMT mRNA in the MPC cells, which were to 15-fold greater than in the PC12 cell lines grown under the same conditions in Dulbecco modified Eagle medium (P < or = 0.05). Growth of MPC cells in Roswell Park Memorial Institute media induced a further significant increase in PNMT gene expression (P < or = 0.05). Immunohistochemistry for TH, PNMT, and NAT was generally consistent with mRNA analysis, with the MPC cells demonstrating strong immunoreactivity for PNMT. The MPC cells showed the highest levels of desipramine-sensitive [(3)H] noradrenaline uptake activity (threefold > than PC12 American Type Culture Center line, P < or = 0.05), despite relatively low levels of NAT mRNA. These results indicate that PC12 cell lines should be carefully chosen for optimal utility in the study of chromaffin cell or sympathetic neuron biology and that cell features will be influenced by type of media and substrate chosen. Furthermore, they confirm that the new MPC cell line is likely a useful model for the study of adrenergic mechanisms or studies involving NAT.

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.

Xenon prevents cellular damage in differentiated PC-12 cells exposed to hypoxia

Background

The neuroprotective effect of xenon has been demonstrated for glutamatergic neurons. In the present study it is investigated if dopaminergic neurons, i.e. nerve-growth-factor differentiated PC-12 cells, are protected as well against hypoxia-induced cell damage in the presence of xenon.

Results

Pheochromocytoma cells differentiated by addition of nerve growth factor were placed in a N₂-saturated atmosphere, a treatment that induced release of dopamine, reaching a maximum after 30 min. By determining extracellular lactate dehydrogenase concentration as marker for concomitant cellular damage, a substantial increase of enzymatic activity was found for N₂-treated cells. Replacement of N₂ by xenon in such a hypoxic atmosphere resulted in complete protection against cellular damage and prevention of hypoxia-induced dopamine release. Intracellular buffering of Ca²⁺ using the Ca-chelator 1, 2-bis(2-Aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester (BAPTA) reduced the neuroprotective effect of xenon indicating the essential participation of intracellular Ca²⁺-ions in the process of xenon-induced neuroprotection.

Conclusions

The results presented demonstrate the outstanding property of xenon to protect neuron-like cells in a hypoxic situation.

The effect of phosphorylation on amphetamine-mediated outward transport

Amphetamine elicits its locomotor-activating and drug-reinforcing effects by releasing the catecholamines dopamine and norepinephrine into the synapse. Amphetamine is a substrate of the plasmalemmal transporters for both dopamine and norepinephrine. As such, it binds to the transporters in conjunction with Na+ and Cl-, facilitating a conformational change leading the transporter to face inward. The subsequent binding of intracellular catecholamine results in an outward transport and release of the catecholamine into the synapse. Both inward and outward transport through the catecholamine transporters are regulated by protein kinases, particularly protein kinase C, but the effect of the enzyme on the two processes appears to be asymmetric. The purpose of this review is to discuss the evidence showing that protein kinase C activation facilitates outward transport through the catecholamine plasmalemmal transporters which may mediate amphetamine action in intact tissue.

Hydrogen peroxide induces loss of dopamine transporter activity: a calcium-dependent oxidative mechanism

H2O2 dose dependently inhibited dopamine uptake in PC12 cells and in striatal synaptosomes. Treatment with H2O2 resulted in a reversible reduction in Vmax, with no effect on its Km value. This suppressive effect of H2O2 could be relieved by reducing agents (dithiothreitol and cysteine). Furthermore, an oxidizer (dithiodipyridine) also markedly suppressed the dopamine transporter (DAT). Oxidative stress therefore might contribute to the action of H2O2. H2O2 appeared to modify DAT at both extracellular and intracellular sites because cumene-H2O2 (a radical generator mostly restricted to plasma membranes) at high concentrations also slightly suppressed DAT activity and the intracellular overexpression of catalase ameliorated the inhibitory effect of H2O2. Internalization was unlikely to be involved because concanavalin A, which blocked endocytosis, did not prevent the H2O2-evoked inhibition of DAT activity. Interestingly, H2O2 treatment evoked a Ca2+ influx in PC12 cells. Moreover, removal of external calcium by EGTA or reduction in the intracellular calcium level using BAPTA-AM reversed the inhibitory effect of H2O2. Conversely, depletion of intracellular calcium stores using thapsigargin did not affect the reduction in DAT activity by H2O2. Collectively, our results indicate that the DAT, one of the most important proteins controlling the dopaminergic system, is also a redox sensor. In addition, H2O2 might suppress the DAT by a Ca2+-dependent oxidative pathway.

Chronic and acute regulation of Na+/Cl- -dependent neurotransmitter transporters: drugs, substrates, presynaptic receptors, and signaling systems

Na+/Cl- -dependent neurotransmitter transporters, which constitute a gene superfamily, are crucial for limiting neurotransmitter activity. Thus, it is critical to understand their regulation. This review focuses primarily on the norepinephrine transporter, the dopamine transporter, the serotonin transporter, and the gamma-aminobutyric acid transporter GAT1. Chronic administration of drugs that alter neurotransmitter release or inhibit transporter activity can produce persistent compensatory changes in brain transporter number and activity. However, regulation has not been universally observed. Transient alterations in norepinephrine transporter, dopamine transporter, serotonin transporter, and GAT1 function and/or number occur in response to more acute manipulations, including membrane potential changes, substrate exposure, ethanol exposure, and presynaptic receptor activation/inhibition. In many cases, acute regulation has been shown to result from a rapid redistribution of the transporter between the cell surface and intracellular sites. Second messenger systems involved in this rapid regulation include protein kinases and phosphatases, of which protein kinase C has been the best characterized. These signaling systems share the common characteristic of altering maximal transport velocity and/or cell surface expression, consistent with regulation of transporter trafficking. Although less well characterized, arachidonic acid, reactive oxygen species, and nitric oxide also alter transporter function. In addition to post-translational modifications, cytoskeleton interactions and transporter oligomerization regulate transporter activity and trafficking. Furthermore, promoter regions involved in transporter transcriptional regulation have begun to be identified. Together, these findings suggest that Na+/Cl- -dependent neurotransmitter transporters are regulated both long-term and in a more dynamic manner, thereby providing several distinct mechanisms for altering synaptic neurotransmitter concentrations and neurotransmission.

Differential alteration of catecholamine release during chemical hypoxia is correlated with cell toxicity and is blocked by protein kinase C inhibitors in PC12 cells

Release of neurotransmitters, including dopamine and glutamate, has been implicated in hypoxia/ischemia-induced alterations in neuronal function and in subsequent tissue damage. Although extensive studies have been done on the mechanism underlying the changes in glutamate release, few have examined the mechanism that is responsible for the changes in catecholamines. Rat pheochromocytoma-12 (PC12) cells synthesize, store, and release catecholamines including DA and NE. Therefore, we used HPLC and ED to evaluate extracellular DA and NE concentrations in a medium during chemical hypoxia in PC12 cells. Chemical hypoxia produced by KCN induced differential release of DA and NE. Under normal glucose conditions, KCN induced release of NE, but not DA. Under glucose-free conditions, KCN-induced release of DA was elevated transiently, whereas the release of NE increased progressively. Under parallel conditions, KCN biphasically elevated the level of cytosolic free calcium ([CA(2+)](i)) in glucose-free DMEM, peaking at 95 +/- 18 nM at 1,107 +/- 151 s, followed by a new plateau level at 249 +/- 24 nM sustained from 4,243 +/- 466 to 5,263 +/- 440 s. Cell toxicity, as measured by LDH release, was increased significantly by KCN in glucose-free DMEM but was diminished in the presence of glucose, and was correlated with DA release by chemical hypoxia. The protein kinase C (PKC) inhibitor GO6976 or staurosporine inhibited KCN-induced LDH release as well as the release of NE and DA. Taken together, selective activation of DA but not NE was correlated with the LDH release by chemical hypoxia, and was diminished with GO6976 or staurosporine. These results suggest that selective activation of PKC isoforms is involved in the chemical hypoxia-induced DA release, which may lead to neuronal cell toxicity.

An investigation of noradrenaline uptake and release by the CATH.a cell line

The cell bodies of ascending noradrenergic neurons in the brain are located predominantly in the locus coeruleus. An in vitro model of locus coeruleus neurons could prove to be a useful tool in the investigation of noradrenergic neural networks and their associated pathophysiologies. The CATH.a cell line demonstrates some of the properties expected of locus coeruleus neurons, and the present study investigated the neurotransmitter uptake and release properties of the CATH.a cells. It was surprising that the CATH.a cells failed to accumulate [3H]noradrenaline ([3H]NA), suggesting the lack of a functional NA transporter. RT-PCR supported this finding by demonstrating the absence of NA transporter mRNA. Treatment of CATH.a cells with various differentiating agents failed to increase the [3H]NA uptake. Endogenous NA release was studied using HPLC detection, which revealed a lack of depolarisation-induced increases in endogenous NA release. A human NA transporter-transfected CATH.a cell line was generated (termed RUNT), and a study of the [3H]NA uptake revealed that the RUNT cells displayed significant uptake that could be blocked by cocaine (10 microM). Furthermore, the uptake capacity could be dramatically increased by differentiation of the cells with dibutyryl cyclic AMP (1 mM) for 24 h. Using dibutyryl cyclic AMP-differentiated RUNT cells, high K+ concentrations (50 mM) significantly increased [3H]NA release above basal levels.

Membrane trafficking regulates the activity of the human dopamine transporter

The trafficking of synaptic proteins is unquestionably a major determinant of the properties of synaptic transmission. Here, we present a detailed analysis of the downregulation and intracellular trafficking of the cocaine- and amphetamine-sensitive dopamine transporter (DAT), a presynaptic plasma membrane protein responsible for the regulation of extracellular DA concentrations. Using PC12 cells stably transfected with human DAT cDNA, we observe that phorbol ester activation of protein kinase C (PKC) results in decreased transporter capacity and a parallel decrease in the amount of DAT on the cell surface that is attributable to intracellular transporter sequestration. After internalization, DAT diverges to the recycling, as opposed to the degradative, arm of the endocytic pathway. This study demonstrates, for the first time, DAT endocytosis, establishes the pathways through which DAT traffics both at steady state and in response to PKC activation, and suggests that DAT recycling is likely to occur.