Biosynthesis, N-glycosylation, and surface trafficking of biogenic amine transporter proteins

Signaling mechanisms involved in the acute effects of estradiol on 5-HT clearance

Estradiol was found previously to have an antidepressant-like effect and to block the ability of selective serotonin reuptake inhibitors (SSRIs) to have an antidepressant-like effect. The antidepressant-like effect of estradiol was due to estrogen receptor β (ERβ) and/or GPR30 activation, whereas estradiol's blockade of the effect of an SSRI was mediated by ERα. This study focuses on investigating signaling pathways as well as interacting receptors associated with these two effects of estradiol. In vivo chronoamperometry was used to measure serotonin transporter (SERT) function. The effect of local application of estradiol or selective agonists for ERα (PPT) or ERβ (DPN) into the CA3 region of the hippocampus of ovariectomized (OVX) rats on 5-hydroxytryptamine (5-HT) clearance as well as on the ability of fluvoxamine to slow 5-HT clearance was examined after selective blockade of signaling pathways or that of interacting receptors. Estradiol- or DPN-induced slowing of 5-HT clearance mediated by ERβ was blocked after inhibition of MAPK/ERK1/2 but not of PI3K/Akt signaling pathways. This effect also involved interactions with TrkB, and IGF-1 receptors. Estradiol's or PPT's inhibition of the fluvoxamine-induced slowing of 5-HT clearance mediated by ERα, was blocked after inhibition of either MAPK/ERK1/2 or PI3K/Akt signaling pathways. This effect involved interactions with the IGF-1 receptor and with the metabotropic glutamate receptor 1, but not with TrkB. This study illustrates some of the signaling pathways required for the effects of estradiol on SERT function, and particularly shows that ER subtypes elicit different as well as common signaling pathways for their actions.

Enhanced assymetrical noradrenergic transmission in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

The ablation of olfactory bulb induces critical changes in dopamine, and monoamine oxidase activity in the brain stem. Growing evidence supports the participation of this telencephalic region in the regulation blood pressure and cardiovascular activity but little is known about its contribution to hypertension. We have previously reported that in the olfactory bulb of normotensive rats endothelins enhance noradrenergic activity by increasing tyrosine hydroxylase activity and norepinephrine release. In the present study we sought to establish the status of noradrenergic activity in the olfactory bulb of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Different steps in norepinephrine transmission including tyrosine hydroxylase activity, neuronal norepinephrine release and uptake were assessed in the left and right olfactory bulb of DOCA-salt hypertensive rats. Increased tyrosine hydroxylase activity, and decreased neuronal norepinephrine uptake were observed in the olfactory bulb of DOCA-salt hypertensive rats. Furthermore the expression of tyrosine hydroxylase and its phosphorylated forms were also augmented. Intriguingly, asymmetrical responses between the right and left olfactory bulb of normotensive and hypertensive rats were observed. Neuronal norepinephrine release was increased in the right but not in the left olfactory bulb of DOCA-salt hypertensive rats, whereas non asymmetrical differences were observed in normotensive animals. Present findings indicate that the olfactory bulb of hypertensive rats show an asymmetrical increase in norepinephrine activity. The observed changes in noradrenergic transmission may likely contribute to the onset and/or progression of hypertension in this animal model.

A dialogue between the immune system and brain, spoken in the language of serotonin

Neuropsychiatric disorders have long been linked to both immune system activation and alterations in serotonin (5-HT) signaling. In the CNS, the contributions of 5-HT modulate a broad range of targets, most notably, hypothalamic, limbic and cortical circuits linked to the control of mood and mood disorders. In the periphery, many are aware of the production and actions of 5-HT in the gut but are unaware that the molecule and its receptors are also present in the immune system where evidence suggests they contribute to the both innate and adaptive responses. In addition, there is clear evidence that the immune system communicates to the brain via both humoral and neuronal mechanisms, and that CNS 5-HT neurons are a direct or indirect target for these actions. Following a brief primer on the immune system, we describe our current understanding of the synthesis, release, and actions of 5-HT in modulating immune function, including the expression of 5-HT biosynthetic enzymes, receptors, and transporters that are typically studied with respect to the roles in the CNS. We then orient our presentation to recent findings that pro-inflammatory cytokines can modulate CNS 5-HT signaling, leading to a conceptualization that among the many roles of 5-HT in the body is an integrated physiological and behavioral response to inflammatory events and pathogens. From this perspective, altered 5-HT/immune conversations are likely to contribute to risk for neurobehavioral disorders historically linked to compromised 5-HT function or ameliorated by 5-HT targeted medications, including depression and anxiety disorders, obsessive-compulsive disorder (OCD), and autism. Our review raises the question as to whether genetic variation impacting 5-HT signaling genes may contribute to maladaptive behavior as much through perturbed immune system modulation as through altered brain mechanisms. Conversely, targeting the immune system for therapeutic development may provide an important opportunity to treat mental illness.

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.

Impact of ovarian hormones on the modulation of the serotonin transporter by fluvoxamine

Most preclinical studies examining the mechanism(s) of action of antidepressants are carried out using male animals. Blockade of serotonin transporter (SERT) function by selective serotonin reuptake inhibitors (SSRIs) is the initial event that triggers a not completely understood process that results in clinical improvement in depression. To investigate whether there are differences in the ability of SSRIs to inhibit the SERT between male and female rats at different phases of the estrous cycle, clearance of locally applied serotonin (5-HT) was measured by in vivo chronoamperometry. Local application of the SSRI, fluvoxamine, directly into the CA3 area of hippocampus increased significantly 5-HT clearance time parameters in male rats and female rats in estrus or diestrus, but not in proestrus. The contribution of ovarian steroids to this result was investigated in ovariectomized (OVX) rats treated with estradiol benzoate (EB) and/or progesterone (P). In OVX-control rats, fluvoxamine increased clearance time parameters, whereas EB and/or P treatment blocked this effect, consistent with what was seen in female rats in proestrus. This effect was gender-specific, since treatment of castrated rats with EB/P had no effect on the ability of fluvoxamine to slow 5-HT clearance. The time course of hormonal effects showed that 1-60 min after local application of 17-beta-estradiol (E(2)) into the CA3 region of OVX rats, fluvoxamine had no effect on clearance time of 5-HT. E(2)-BSA mimicked E(2)'s effects at 10 min but not at 60 min. Pretreatment with estrogen receptor antagonists blocked the effects of E(2). The finding that acutely both estradiol and progesterone can inhibit the ability of an SSRI to slow the clearance of 5-HT, may have important implications for the use of SSRIs in women.

The comparative distributions of the monoamine transporters in the rodent, monkey, and human amygdala

The monoamines in the amygdala modulate multiple aspects of emotional processing in the mammalian brain, and organic or pharmacological dysregulation of these systems can result in affective pathologies. Knowledge of the normal distribution of these neurotransmitters, therefore, is central to our understanding of both the normal processes regulated by the amygdala and the pathological conditions associated with monoaminergic dysregulation. The monoaminergic transporters have proven to be accurate and reliable markers of the distributions of their substrates. The purpose of this review was twofold: First, to briefly recount the functional relevance of dopamine, serotonin, and norepinephrine transmission in the amygdala, and second, to describe and compare the distributions of the monoamine transporters in the rodent, monkey, and human brain. The transporters were found to be heterogeneously distributed in the amygdala. The dopamine transporter (DAT) is consistently found to be extremely sparsely distributed, however the various accounts of its subregional topography are inconsistent, making any cross-species comparisons difficult. The serotonin transporter (SERT) had the greatest overall degree of labeling of the three markers, and was characterized by substantial inter-species variability in its relative distribution. The norepinephrine transporter (NET) was shown to possess an intermediate level of labeling, and like the SERT, its distribution is not consistent across the three species. The results of these comparisons indicate that caution should be exercised when using animal models to investigate the complex processes modulated by the monoamines in the amygdala, as their relative contributions to these functions may differ across species.

AQP1 gene expression is upregulated by arginine vasopressin and cyclic AMP agonists in trophoblast cells

Aquaporins (AQPs) are water channels that regulate water flow in many tissues. As AQP1 is a candidate to regulate placental fluid exchange, we sought to investigate the effect of arginine vasopressin (AVP) and cAMP agonists on AQP1 gene expression in first trimester-derived extravillous cytotrophoblasts (HTR-8/Svneo) and two highly proliferative carcinoma trophoblast-like cell lines but with a number of functional features of the syncytiotrophoblast namely; JAR and JEG-3 cells. Our data demonstrated that AVP (0.1 nM) significantly increased the expression of AQP1 mRNA at 10 h in HTR-8/SVneo and JEG-3 cells (P<0.05). Both SP-cAMP, a membrane-permeable and phosphodiesterase resistant cAMP, and forskolin, an adenylate cyclase stimulator significantly increased AQP1 mRNA expression in all cell lines after 2 h in a dose-dependent manner (P<0.05) with a parallel increase in protein expression. In the time course study, 5 microM of either SP-cAMP or forskolin significantly stimulated AQP1 mRNA expression after 2 h in HTR-8/SVneo cells and after 10 h in JAR and JEG-3 cells. AQP1 protein expression was highest after 20 h in both HTR-8/SVneo and JEG-3 cells (P<0.05). AVP-stimulated cAMP elevation was blocked in the presence of 9-(tetrahydro-2'-furyl) adenine (SQ22536) (100 microM), a cell-permeable adenylate cyclase inhibitor (P<0.05). These results indicate that in trophoblasts-like cells AQP1 gene expression is upregulated by both AVP and cAMP agonists. Furthermore, our data demonstrate that a cAMP-dependent pathway is responsible for the AVP effect on AQP1. Thus, modulation of AQP1 expression by maternal hormones may regulate invasion and fetal-placental-amnion water homeostasis during gestation.

Current progress and future challenges in the biochemical diagnosis and treatment of pheochromocytomas and paragangliomas

Findings from five independent studies - with close to 350 patients with pheochromocytoma and more than 2,500 in whom the tumor was excluded - indicate that measurements of plasma free metanephrines provide an overall diagnostic sensitivity of 98% and specificity of 92%. The recommendation that initial testing for the tumor should always include measurements of either plasma or urinary fractionated metanephrines results from recognition of the high diagnostic sensitivity of measurements of plasma metanephrines. The few patients with pheochromocytoma in whom the test may not yield a positive result include those with very small tumors or microscopic disease and others with tumors that do not produce norepinephrine and epinephrine. Such patients are typically normotensive and do not exhibit symptoms of catecholamine excess. Additional measurements of methoxytyramine can be useful for detecting those tumors that produce only dopamine. Suboptimal diagnostic specificity and difficulties in distinguishing true- from false-positive elevations of plasma metanephrines remain challenges for diagnosis. Improvements in analytical technology (e.g., liquid chromatography with tandem mass spectrometry) and new strategies for follow-up testing provide possible solutions to these problems. The single most important remaining clinical care challenge is the development of effective cures for patients with malignant disease. Current treatments, none of which are truly satisfactory, include chemotherapy and radiopharmaceutical therapy with (131)I-labelled M-iodobenzylguanidine or radioactive somatostatin analogues. Improvements in treatment may in the future come from several fronts, but proof of efficacy ideally will require well-coordinated multicenter prospective trials in larger numbers of patients than in previous studies.

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.

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.

Nicotine increases dopamine transporter function in rat striatum through a trafficking-independent mechanism

In previous in vivo voltammetry studies, acute nicotine administration increased striatal dopamine clearance. The current study aimed to determine whether nicotine also increases [(3)H]dopamine uptake across the time course of the previous voltammetry studies and whether dopamine transporter trafficking to the cell surface mediates the nicotine-induced augmentation of dopamine clearance in striatum. Rats were administered nicotine (0.32 mg/kg, s.c.); striatal synaptosomes were obtained 5, 10, 40 or 60 min later. Nicotine increased (25%) the V(max) of [(3)H]dopamine uptake at 10 and 40 min. To determine whether the increase in V(max) was due to an increase in dopamine transporter density, [(3)H]GBR 12935 (1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride) binding was performed using rat striatal membranes; no differences were found between nicotine and saline-control groups at 5, 10 or 40 min post-injection, indicating that nicotine did not increase striatal dopamine transporter density; however, [(3)H]GBR 12935 binding assays determine both cell surface and intracellular dopamine transporter. Changes in cellular dopamine transporter localization in striatum were determined using biotinylation and subfractionation approaches; no differences between nicotine and saline-control groups were observed at 10 and 40 min post-injection. These results suggest that the nicotine-induced increase in dopamine uptake and clearance in striatum may occur via a trafficking-independent mechanism.

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.

Water channels and zymogen granules in salivary glands

Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly water-permeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 - AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca(2+)](i) and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca(2+) mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca(2+) signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca(2+)](i). In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

Cell-type-specific limitation on in vivo serotonin storage following ectopic expression of the Drosophila serotonin transporter, dSERT

The synaptic machinery for neurotransmitter storage is cell-type specific. Although most elements of biosynthesis and transport have been identified, it remains unclear whether additional factors may be required to maintain this specificity. The Drosophila serotonin transporter (dSERT) is normally expressed exclusively in serotonin (5-HT) neurons in the CNS. Here we examine the effects of ectopic transcriptional expression of dSERT in the Drosophila larval CNS. We find a surprising limitation on 5-HT storage following ectopic expression of dSERT and green fluorescence protein-tagged dSERT (GFP-dSERT). When dSERT transcription is driven ectopically in the CNS, 5-HT is detectable only in 5-HT, dopamine (DA), and a very limited number of additional neurons. Addition of exogenous 5-HT does not dramatically broaden neuronal storage sites, so this limitation is only partly due to restricted intercellular diffusion of 5-HT. Furthermore, this limitation is not due to gross mislocalization of dSERT, because cells lacking or containing 5-HT show similar levels and subcellular distribution of GFP-dSERT protein, nor is it due to lack of the vesicular transporter, dVMAT. These data suggest that a small number of neurons selectively express factor(s) required for 5-HT storage, and potentially for function of dSERT.

The serotonin transporter (SLC6A4) is present in B-cell clones of diverse malignant origin: probing a potential anti-tumor target for psychotropics

Following our previous description of the serotonin transporter (SERT) acting as a conduit to 5-hydroxytryptamine (5-HT)-mediated apoptosis, specifically in Burkitt's lymphoma, we now detail its expression among a broad spectrum of B cell malignancy, while exploring additional SERT substrates for potential therapeutic activity. SERT was readily detected in derived B cell lines with origins as diverse as B cell precursor acute lymphoblastic leukemia, mantle cell lymphoma, diffuse large B cell lymphoma, and multiple myeloma. Concentration and timecourse kinetics for the antiproliferative and proapoptotic activities of the amphetamine derivatives fenfluramine (an appetite suppressant) and 3,4-methylenedioxymethamphetamine (MDMA; "Ecstasy") revealed them as being similar to the endogenous indoleamine. A tricyclic antidepressant, clomipramine, instead mirrored the behavior of the selective serotonin reuptake inhibitor fluoxetine, both being effective in the low micromolar range. A majority of neoplastic clones were sensitive to one or more of the serotonergic compounds. Dysregulated bcl-2 expression, either by t(14;18)(q32;q21) translocation or its introduction as a constitutively active transgene, provided protection from proapoptotic but not antiproliferative outcomes. These data indicate a potential for SERT as a novel anti-tumor target for amphetamine analogs, while evidence is presented that the seemingly more promising antidepressants are likely impacting malignant B cells independently of the transporter itself.

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.

Serotonin and cocaine-sensitive inactivation of human serotonin transporters by methanethiosulfonates targeted to transmembrane domain I

To explore aqueous accessibility and functional contributions of transmembrane domain (TM) 1 in human serotonin transporter (hSERT) proteins, we utilized the largely methanethiosulfonate (MTS) insensitive hSERT C109A mutant and mutated individual residues of hSERT TM1 to Cys followed by tests of MTS inactivation of 5-hydroxytryptamine (5-HT) transport. Residues in TM1 cytoplasmic to Gly-94 were largely unaffected by Cys substitution, whereas the mutation of residues extracellular to Ile-93 variably diminished transport activity. TM1 Cys substitutions displayed differential sensitivity to MTS reagents, with residues more cytoplasmic to Asp-98 being largely insensitive to MTS inactivation. Aminoethylmethanethiosulfonate (MTSEA), [2-(trimethylammonium) ethyl]methanethiosulfonate bromide (MTSET), and sodium (2-sulfonatoethyl)-methanethiosulfonate (MTSES) similarly and profoundly inactivated 5-HT transport by SERT mutants D98C, G100C, W103C, and Y107C. MTSEA uniquely inactivated transport activity of S91C, G94C, Y95C but increased activity at I108C. MTSEA and MTSET, but not MTSES, inactivated transport function at N101C. Notably, 5-HT provided partial to complete protection from MTSET inactivation for D98C, G100C, N101C, and Y107C. Equivalent blockade of MTSET inactivation at N101C was observed with 5-HT at both room temperature and at 4 degrees C, inconsistent with major conformational changes leading to protection. Notably, cocaine also protected MTSET inactivation of G100C and N101C, although MTS incubations with N101C that eliminate 5-HT transport do not preclude cocaine analog binding nor its inhibition by 5-HT. 5-HT modestly enhanced the inactivation by MTSET at I93C and Y95C, whereas cocaine significantly enhanced MTSET sensitivity at Y107C and I108C. In summary, our studies reveal physical differences in TM1 accessibility to externally applied MTS reagents and reveal sites supporting substrate and antagonist modulation of MTS inactivation. Moreover, we identify a limit to accessibility for membrane-impermeant MTS reagents that may reflect aspects of an occluded permeation pathway.

Dissociation of high-affinity cocaine analog binding and dopamine uptake inhibition at the dopamine transporter

Cocaine initiates its euphoric effects by binding to the dopamine transporter (DAT), blocking uptake of synaptic dopamine. It has been hypothesized that the DAT transmembrane aspartic acid residue D79 forms an ionic interaction with charged nitrogen atoms in both dopamine and cocaine. We examined the consequences of novel and previously studied mutations of the D79 residue on DAT uptake of [3H]dopamine, DAT binding of the cocaine analog [3H]WIN 35,428, and drug inhibition of each process, all under identical conditions. The rat D79E DAT mutation decreased dopamine uptake Vmax by 7-fold and decreased dopamine turnover by 4-fold. Wild-type DAT displayed near-perfect agreement in the uptake and binding inhibition potencies for substrates, but cocaine and other nonsubstrate inhibitor drugs were approximately 3-fold less potent in uptake than in binding assays. Apparent affinities for substrates were unaffected by the D79E mutation unless the catechol moiety was modified. Strikingly, potencies for nonsubstrate inhibitors in uptake and binding assays matched for D79E DAT, because of a 3-fold lowering of binding affinities relative to WT DAT. The present findings reveal a complex role for D79 in determining substrate specificity and high-affinity binding of DAT inhibitors. We propose that at least two discrete inhibitor-binding DAT conformations or populations exist and that the DAT conformation/population responsible for inhibitor high-affinity binding is less responsible for dopamine uptake. The findings may be extensible to other psychostimulants and antidepressants that display discrepancies between binding affinity and monoamine uptake inhibition potency and may be relevant to development of a long-sought "cocaine antagonist".

Initial conditions of serotonin transporter kinetics and genotype: influence on SSRI treatment trial outcome

BACKGROUND

Fifty-one patients with major depression were classified for 5-HTT promoter region polymorphism and platelet 5-HTT kinetics before treatment with fluoxetine, and then examined for treatment outcome.

METHODS

Dose was stratified from 1.25 mg to 40 mg per day to allow for the possibility that one genotype could express a lower-dose fluoxetine response. A repeated-measures analysis of variance of 24-item Hamilton depression change through baseline, 1-week placebo lead-in, and 6, 12, and 18 weeks treatment was done to test a genotype effect on outcome.

RESULTS

Genotype had a significant effect on outcome (F = 4.7, p <.02), with the initial affinity constant (K(m)) (F = 11.9, p =.001), and dose (F = 6.0, p <.02) being significant covariates on outcome as well. The gene effect, however, was complex in that the 5-HTT promoter region insertion showed two effects: both a placebo response effect (F = 4, p <.025), and a drug dose response effect (r =.40, p <.01). The long allele group was more responsive to placebo, as well as more responsive to drug dose than was the short allele group.

CONCLUSIONS

This is the first study to examine the antidepressant dose-response relationship to 5-HTT kinetics and genetics. The findings indicate that both the initial affinity and genotype of 5-HTT may contribute in unique ways to the variation in the outcome of depression treatment trials.

5-Hydroxytryptamine drives apoptosis in biopsylike Burkitt lymphoma cells: reversal by selective serotonin reuptake inhibitors

Serotonin (5-HT), a well-known neurotransmitter of the central nervous system, has been implicated in diverse aspects of immune regulation. Here we show that 5-HT can efficiently drive programmed cell death in established Burkitt lymphoma (BL) lines that remain faithful to the original biopsy phenotype (group 1). Group 1 BL cells cultured in the presence of 5-HT exhibited marked suppression of DNA synthesis that was accompanied by extensive apoptosis-serotonin-driven apoptosis was complete within 24 hours, was preceded by early caspase activation, and was accompanied by a decline in mitochondrial membrane potential. BL cells that had drifted to a lymphoblastic group 3 phenotype were relatively resistant to these actions of serotonin, and the forced ectopic expression of either bcl-2 or bcl-x(L) provided substantial protection from 5-HT-induced apoptosis. 5-HT receptor antagonists (SDZ205-557, granisetron, methysergide) failed to inhibit serotonin-induced apoptosis, whereas the selective serotonin reuptake inhibitors (SSRI)-fluoxetine (Prozac), paroxetine (Paxil), and citalopram (Celexa)-substantially blocked the monoamine actions. Western blot analysis showed that BL cells expressed protein for the 5-HT transporter, and transport assays confirmed active uptake of serotonin by the cells. Unlike what was suggested for neuronal cells, there was no evidence that intracellular oxidative metabolites were responsible for the 5-HT-induced programmed death of BL cells. These data indicate that serotonin drives apoptosis in biopsylike BL cells after its entry through an active transport mechanism, and they suggest a novel therapeutic modality for Burkitt lymphoma.

Mutations in the Caenorhabditis elegans serotonin reuptake transporter MOD-5 reveal serotonin-dependent and -independent activities of fluoxetine

We isolated two mutants defective in the uptake of exogenous serotonin (5-HT) into the neurosecretory motor neurons of Caenorhabditis elegans. These mutants were hypersensitive to exogenous 5-HT and hyper-responsive in the experience-dependent enhanced slowing response to food modulated by 5-HT. The two allelic mutations defined the gene mod-5 (modulation of locomotion defective), which encodes the only serotonin reuptake transporter (SERT) in C. elegans. The selective serotonin reuptake inhibitor fluoxetine (Prozac) potentiated the enhanced slowing response, and this potentiation required mod-5 function, establishing a 5-HT- and SERT-dependent behavioral effect of fluoxetine in C. elegans. By contrast, other responses of C. elegans to fluoxetine were independent of MOD-5 SERT and 5-HT. Further analysis of the MOD-5-independent behavioral effects of fluoxetine could lead to the identification of novel targets of fluoxetine and could facilitate the development of more specific human pharmaceuticals.

Cocaine and antidepressant-sensitive biogenic amine transporters exist in regulated complexes with protein phosphatase 2A

Presynaptic transporter proteins regulate the clearance of extracellular biogenic amines after release and are important targets for multiple psychoactive agents, including amphetamines, cocaine, and antidepressant drugs. Recent studies reveal that dopamine (DA), norepinephrine (NE), and serotonin (5-HT) transporters (DAT, NET, and SERT, respectively) are rapidly regulated by direct or receptor-mediated activation of cellular kinases, particularly protein kinase C (PKC). With SERTs, PKC activation results in activity-dependent transporter phosphorylation and sequestration. Protein phosphatase 1/2A (PP1/PP2A) inhibitors, such as okadaic acid (OA) and calyculin A, also promote SERT phosphorylation and functional downregulation. How kinase, phosphatase, and transporter activities are linked mechanistically is unclear. In the present study, we found that okadaic acid-sensitive phosphatase activity is enriched in SERT immunoprecipitates from human SERT stably transfected cells. Moreover, blots of these immunoprecipitates reveal the presence of PP2A catalytic subunit (PP2Ac), findings replicated using brain preparations. Whole-cell treatments with okadaic acid or calyculin A diminished SERT/PP2Ac associations. Phorbol esters, which trigger SERT phosphorylation, also diminish SERT/PP2Ac associations, effects that can be blocked by PKC antagonists as well as the SERT substrate 5-HT. Similar transporter/PP2Ac complexes were also observed in coimmunoprecipitation studies with NETs and DATs. Our findings provide evidence for the existence of regulated heteromeric assemblies involving biogenic amine transporters and PP2A and suggest that the dynamic stability of these complexes may govern transporter phosphorylation and sequestration.

Cloning of dopamine, norepinephrine and serotonin transporters from monkey brain: relevance to cocaine sensitivity

We used RT-PCR to clone monoamine transporters from Macaca mulatta, Macaca fasicularis and Saimiri sciureus (dopamine transporter; DAT) and Macaca mulatta (norepinephrine transporter; NET and serotonin transporter; SERT). Monkey DAT, NET and SERT proteins were >98% homologous to human and, when expressed in HEK-293 cells, displayed drug affinities and uptake kinetics that were highly correlated with monkey brain or human monoamine transporters. In contrast to reports of other species, we discovered double (leucine for phenylalanine 143 and arginine for glutamine 509; Variant I) and single (proline for leucine 355; Variant II) amino acid variants of DAT. Variant I displayed dopamine transport kinetics and binding affinities for various DAT blockers (including cocaine) versus [3H] CFT (WIN 35, 428) that were identical to wild-type DAT (n=7 drugs; r(2)=0.991). However, we detected a six-fold difference in the affinity of cocaine versus [3H] cocaine between Variant I (IC(50): 488+/-102 nM, SEM, n=3) and wild-type DAT (IC(50): 79+/-8.2 nM, n=3, P<0.05). Variant II was localized intracellularly in HEK-293 cells, as detected by confocal microscopy, and had very low levels of binding and dopamine transport. Also discovered was a novel exon 5 splice variant of NET that displayed very low levels of transport and did not bind cocaine. With NetPhos analysis, we detected a number of highly conserved putative phosphorylation sites on extracellular as well as intracellular loops of the DAT, NET, and SERT, which may be functional for internalized transporters. The homology and functional similarity of human and monkey monoamine transporters further support the value of primates in investigating the role of monoamine transporters in substance abuse mechanisms, neuropsychiatric disorders and development of diagnostic and therapeutic agents.

Arg-425 of the citrate transporter CitP is responsible for high affinity binding of di- and tricarboxylates

The citrate transporter of Leuconostoc mesenteroides (CitP) catalyzes exchange of divalent anionic citrate from the medium for monovalent anionic lactate, which is an end product of citrate degradation. The exchange generates a membrane potential and thus metabolic energy for the cell. The mechanism by which CitP transports both a divalent and a monovalent substrate was the subject of this investigation. Previous studies indicated that CitP is specific for substrates containing a 2-hydroxycarboxylate motif, HO-CR(2)-COO(-). CitP has a high affinity for substrates that have a "second" carboxylate at one of the R groups, such as divalent citrate and (S)-malate (Bandell, M., and Lolkema, J. S. (1999) Biochemistry 38, 10352-10360). Monovalent anionic substrates that lack this second carboxylate were found to bind with a low affinity. In the present study we have constructed site-directed mutants, changing Arg-425 into a lysine or a cysteine residue. By using two substrates, i.e. (S)-malate and 2-hydroxyisobutyrate, the substrate specificity of the mutants was analyzed. In both mutants the affinity for divalent (S)-malate was strongly decreased, whereas the affinity for monovalent 2-hydroxyisobutyrate was not. The largest effect was seen when the arginine was changed into the neutral cysteine, which reduced the affinity for (S)-malate over 50-fold. Chemical modification of the R425C mutant with the sulfhydryl reagent 2-aminoethyl methanethiosulfonate, which restores the positive charge at position 425, dramatically reactivated the mutant transporter. The R425C and R425K mutants revealed a substrate protectable inhibition by other sulfhydryl reagents and the lysine reagent 2,4,6-trinitrobenzene sulfonate, respectively. It is concluded that Arg-425 complexes the charged carboxylate present in divalent substrates but that is absent in monovalent substrates, and thus plays an important role in the generation of the membrane potential.

Aquaporin water channel in salivary glands

Water secretion from salivary glands, which are innervated by parasympathetic and sympathetic nerves, occurs in response to the stimulation by neurotransmitters. In general, parasympathetic or sympathetic stimulation produces a high flow of saliva as a result of the activation of M3 muscarinic or alpha1-adrenergic receptors, respectively. The secretory mechanisms of fluid secretion were osmotically regulated in response to a transepithelial ion gradient generated by ion transport systems that were located in the apical or basolateral membranes of the acinar cells. Recently, the identification of water-specific channels, or aquaporins (AQPs), in many mammalian tissue and cell types has provided insight into the molecular basis of water movement across biological membranes. It has been reported that several AQPs are expressed in salivary glands and especially AQP5 plays an important role in fluid secretion. This review will focus on the role of AQP5 in the movement of water across the apical plasma membrane in relation to the physiology and pathophysiology of salivary glands.

Phosphorylation and sequestration of serotonin transporters differentially modulated by psychostimulants

Many psychotropic drugs interfere with the reuptake of dopamine, norepinephrine, and serotonin. Transport capacity is regulated by kinase-linked pathways, particularly those involving protein kinase C (PKC), resulting in transporter phosphorylation and sequestration. Phosphorylation and sequestration of the serotonin transporter (SERT) were substantially impacted by ligand occupancy. Ligands that can permeate the transporter, such as serotonin or the amphetamines, prevented PKC-dependent SERT phosphorylation. Nontransported SERT antagonists such as cocaine and antidepressants were permissive for SERT phosphorylation but blocked serotonin effects. PKC-dependent SERT sequestration was also blocked by serotonin. These findings reveal activity-dependent modulation of neurotransmitter reuptake and identify previously unknown consequences of amphetamine, cocaine, and antidepressant action.

Transmembrane domain I contributes to the permeation pathway for serotonin and ions in the serotonin transporter

Mutation of a conserved Asp (D98) in the rat serotonin (5HT) transporter (rSERT) to Glu (D98E) led to decreased 5HT transport capacity, diminished coupling to extracellular Na+ and Cl-, and a selective loss of antagonist potencies (cocaine, imipramine, and citalopram but not paroxetine or mazindol) with no change in 5HT Km value. D98E, which extends the acidic side chain by one carbon, affected the rank-order potency of substrate analogs for inhibition of 5HT transport, selectively increasing the potency of two analogs with shorter alkylamine side chains, gramine, and dihydroxybenzylamine. D98E also increased the efficacy of gramine relative to 5HT for inducing substrate-activated currents in Xenopus laevis oocytes, but these currents were noticeably dependent on extracellular medium acidification. I-V profiles for substrate-independent and -dependent currents indicated that the mutation selectively impacts ion permeation coupled to 5HT occupancy. The ability of the D98E mutant to modulate selective aspects of substrate recognition, to perturb ion dependence as well as modify substrate-induced currents, suggests that transmembrane domain I plays a critical role in defining the permeation pathway of biogenic amine transporters.

Regulated phosphorylation and trafficking of antidepressant-sensitive serotonin transporter proteins

Presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) mediate antidepressant-sensitive clearance of 5-HT following release. Although we have been aware for decades that SERT-mediated 5-HT clearance can be modulated by exogenous agents including serotonin-selective reuptake inhibitors, amphetamines, and cocaine, we have had little reason to speculate that SERT activity was actively controlled through endogenous pathways. Recent studies indicate that SERTs are likely to be trafficked to specific plasma membrane subdomains to achieve localized clearance of 5-HT, and that the number of SERTs resident in the plasma membrane is controlled through kinase- and phosphatase-linked pathways. In particular, roles for protein kinase C and phosphatase 2A become apparent through studies with enzyme activators and inhibitors in SERT-transfected cells, where SERT proteins are rapidly phosphorylated in parallel with transporter redistribution and loss of functional uptake capacity. Based on our findings, and the studies of others in native tissues and transfected cells, we propose a model whereby SERTs are organized in a macromolecular complex in the plasma membrane that may serve to locate reuptake activity near release sites. Although many elements of this model remain hypothetical, our findings suggest a much more dynamic picture of transporter-mediated 5-HT reuptake than typically described and suggest opportunities both for the development of new SERT regulatory agents and for the identification of regulatory pathways that may be compromised in mental illness.