Adenosine receptor, protein kinase G, and p38 mitogen-activated protein kinase-dependent up-regulation of serotonin transporters involves both transporter trafficking and activation

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

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

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

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

Effects of flurbiprofen on the functional regulation of serotonin transporter and its misfolded mutant

Flurbiprofen, a nonsteroidal anti-inflammatory drug, reportedly exhibits chemical chaperone activity. Herein, we investigated the role of flurbiprofen in regulating serotonin transporter (SERT) function via membrane trafficking. We used COS-7 cells transiently expressing wild-type (WT) SERT or a C-terminus-deleted mutant of SERT (SERTΔCT), a misfolded protein. Flurbiprofen treatment reduced the expression of immaturely glycosylated SERT and enhanced the expression of maturely glycosylated SERT. In addition, we observed increased serotonin uptake in SERT-expressing cells. These results suggest that flurbiprofen modulates SERT function by promoting membrane trafficking. In SERTΔCT-expressing cells, flurbiprofen reduced the protein expression and uptake activity of SERTΔCT. Furthermore, flurbiprofen inhibited the formation of SERTΔCT aggregates. Studies using flurbiprofen enantiomers suggested that these effects of flurbiprofen on SERT were not mediated via cyclooxygenase inhibition. The levels of GRP78/BiP, an endoplasmic reticulum (ER) stress marker, were assessed to elucidate whether flurbiprofen can ameliorate SERTΔCT-induced ER stress. Interestingly, flurbiprofen induced GRP78/BiP expression only under ER stress conditions and not under steady-state conditions. In HRD1 E3 ubiquitin ligase knockdown cells, flurbiprofen affected the ER-associated degradation system. Collectively, the findings suggest that flurbiprofen may function as an inducer of molecular chaperones, in addition to functioning as a chemical chaperone.

Antidepressant Like Effect of Ascorbic Acid in Mice: Possible Involvement of NO-sGC-cGMP Signaling

The present study was designed to determine the antidepressant like activity of ascorbic acid (AA) in mice. Further the influence of NO-sGC-cGMP signaling in the antidepressant like effect of AA in mice was determined. Male swiss albino mice were used in the present study. Mice in the control group received saline and fluoxetine (10 mg/kg, i.p.) was used as the standard antidepressant drug. AA (50, 100 and 150 mg/kg, i.p.) was administered to the mice and depression related behavior were determined using tail suspension test (TST) and forced swim test (FST). Further the whole brain nitrite and serotonin levels were also determined. It was observed that the administration of AA (100 mg/kg, i.p.) reversed the depression like behavior in mice in TST and FST. AA (100 mg/kg, i.p.) treatment decreased the level of nitrite and increased the level of serotonin in the brain of mice significantly as compared to control. Further the behavioral and neurochemical effect of AA (50 mg/kg, i.p) was studied in NO modulator [NO donor: L-Arginine (50 mg/kg, i.p); NO-sGC inhibitor: methylene blue (1 mg/kg, i.p.) and cGMP modulator: sildenafil (1 mg/kg, i.p.)] pretreated mice. It was observed that the pretreatment of NO donor and cGMP modulator counteracted the effect conferred by AA (50 mg/kg, i.p). While the pretreatment of NO-sGC inhibitor potentiated the effect conferred by AA (50 mg/kg, i.p). The present study suggested that the AA confer antidepressant like effect in mice and NO-sGC-cGMP signaling pathway influence the antidepressant like effect of AA in mice.

Rare Opportunities for Insights Into Serotonergic Contributions to Brain and Bowel Disorders: Studies of the SERT Ala56 Mouse

Altered structure, expression, and regulation of the presynaptic serotonin (5-HT) transporter (SERT) have been associated with multiple neurobehavioral disorders, including mood disorders, obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). Opportunities to investigate mechanistic links supporting these associations were spurred with the identification of multiple, rare human SERT coding variants in a study that established a male-specific linkage of ASD to a linkage marker on chromosome 17 which encompassed the location of the SERT gene (SLC6A4). We have explored the most common of these variants, SERT Ala56, in vitro and in vivo. Results support a tonic elevation of 5-HT transport activity in transfected cells and human lymphoblasts by the variant in vitro that leads to an increased 5-HT clearance rate in vivo when studied in the SERT Ala56 mouse model, along with altered sensitivity to SERT regulatory signaling pathways. Importantly, hyperserotonemia, or an elevated whole blood 5-HT, level, was found in SERT Ala56 mice, reproducing a well-replicated trait observed in a significant fraction of ASD subjects. Additionally, we found multiple biochemical, physiological, and behavioral alterations in the SERT Ala56 mice that can be analogized to those observed in ASD and its medical comorbidities. The similarity of the functional impact of the SERT Ala56 variant to the consequences of p38α MAPK activation, ascribed to the induction of a biased conformation of the transporter toward an outward-facing conformation, has resulted in successful efforts to restore normal behavioral and bowel function via pharmacological and genetic p38α MAPK targeting. Moreover, the ability of the inflammatory cytokine IL-1β to enhance SERT activity via a p38α MAPK-dependent pathway suggests that the SERT Ala56 conformation mimics that of a chronic inflammatory state, supporting findings in ASD of elevated inflammatory cytokine levels. In this report, we review studies of the SERT Ala56 variant, discussing opportunities for continued insight into how chronically altered synaptic 5-HT homeostasis can drive reversible, functional perturbations in 5-HT sensitive pathways in the brain and periphery, and how targeting the SERT regulome, particularly through activating pathways such as those involving IL-1β/p38α MAPK, may be of benefit for neurobehavioral disorders, including ASD.

Serotonin Transporter Ala276 Mouse: Novel Model to Assess the Neurochemical and Behavioral Impact of Thr276 Phosphorylation In Vivo

The serotonin (5-HT) transporter (SERT) is a key regulator of 5-HT signaling and is a major target for antidepressants and psychostimulants. Human SERT coding variants have been identified in subjects with obsessive-compulsive disorder (OCD) and autism spectrum disorder (ASD) that impact transporter phosphorylation, cell surface trafficking and/or conformational dynamics. Prior to an initial description of a novel mouse line expressing the non-phosphorylatable SERT substitution Thr276Ala, we review efforts made to elucidate the structure and conformational dynamics of SERT with a focus on research implicating phosphorylation at Thr276 as a determinant of SERT conformational dynamics. Using the high-resolution structure of human SERT in inward- and outward-open conformations, we explore the conformation dependence of SERT Thr276 exposure, with results suggesting that phosphorylation is likely restricted to an inward-open conformation, consistent with prior biochemical studies. Assessment of genotypes from SERT/Ala276 heterozygous matings revealed a deviation from Mendelian expectations, with reduced numbers of Ala276 offspring, though no genotype differences were seen in growth or physical appearance. Similarly, no genotype differences were evident in midbrain or hippocampal 5-HT levels, midbrain and hippocampal SERT mRNA or midbrain protein levels, nor in midbrain synaptosomal 5-HT uptake kinetics. Behaviorally, SERT Ala276 homozygotes appeared normal in measures of anxiety and antidepressant-sensitive stress coping behavior. However, these mice displayed sex-dependent alterations in repetitive and social interactions, consistent with circuit-dependent requirements for Thr276 phosphorylation underlying these behaviors. Our findings indicate the utility of SERT Ala276 mice in evaluation of developmental, functional and behavioral consequences of regulatory SERT phosphorylation in vivo.

Syntaxin 3 interacts with serotonin transporter and regulates its function

Herein, we investigated the functional association of the serotonin transporter (SERT) with syntaxin-3 (STX3). We first overexpressed SERT and STX3 in various cells and examined their interaction, localization, and functional association. Immunoprecipitation studies revealed that STX3 interacted with SERT when expressed in COS-7 cells. Immunocytochemical studies revealed that SERT and STX3 were colocalized in the endoplasmic reticulum (ER) and Golgi apparatus. STX3 overexpression significantly reduced the uptake activity of SERT by attenuating its plasma membrane expression, suggesting that overexpressed STX3 anchors SERT in the ER and Golgi apparatus. STX3 knockdown did not affect the uptake activity of SERT but altered its glycosylation state. To elucidate the association of STX3 with SERT under physiological conditions, rather than overexpressing cells, we investigated this interaction in polarized Caco-2 cells, which endogenously express both proteins. Immunocytochemical studies revealed that SERT and STX3 were localized in microvilli-like structures at the apical plasma membrane. STX3 knockdown marginally but significantly decreased the serotonin uptake activity of Caco-2 cells, suggesting that STX3 positively regulates SERT function in Caco-2 cells, as opposed to SERT regulation by STX3 in overexpressing cells. Collectively, STX3 may colocalize with SERT during SERT membrane trafficking and regulate SERT function in an STX3-expressing lesion-dependent manner.

Role of the E3 ubiquitin ligase HRD1 in the regulation of serotonin transporter function

To elucidate the regulation of serotonin transporter (SERT) function via its membrane trafficking, we investigated the involvement of the ubiquitin E3 ligase HRD1 (HMG-CoA reductase degradation protein), which participates in endoplasmic reticulum (ER)-associated degradation (ERAD), in the functional regulation of SERT. Cells transiently expressing wild-type SERT or a SERT C-terminal deletion mutant (SERTΔCT), a SERT protein predicted to be misfolded, were used for experiments. Studies using HRD1-overexpressing or HRD1-knockdown cells demonstrated that HRD1 is involved in SERT proteolysis. Overexpression of HRD1 promoted SERT ubiquitination, the effect of which was augmented by treatment with the proteasome inhibitor MG132. Immunoprecipitation studies revealed that HRD1 interacts with SERT in the presence of MG132. In addition, HRD1 was intracellularly colocalized with SERT, especially with aggregates of SERTΔCT in the ER. HRD1 also affected SERT uptake activity in accordance with the expression levels of the SERT protein. These results suggest that HRD1 contributes to the membrane trafficking and functional regulation of SERT through its involvement in ERAD-mediated SERT degradation.

Effects of 9,10-phenanthrenequione on antioxidant indices and metabolite profiles in Takifugu obscurus plasma

Derived from polycyclic aromatic hydrocarbons (PAHs), oxygenated-PAHs (oxy-PAHs) may pose hazards to aquatic organisms, which remain largely unknown. Takifugu obscurus is an important anadromous fish species of high economic and ecological values. In the present study, T. obscurus was acutely exposed to 44.29 µg l-1 9,10-phenanthrenequione (9,10-PQ) for 96 h. Changes of antioxidant indices and metabolite profiles in plasma were compared between 9,10-PQ treatment and the control. The results showed that 9,10-PQ treatment significantly increased malondialdehyde (MDA) content during 6 to 96 h, increased superoxide dismutase (SOD) and catalase (CAT) activities at 6 h, but decreased them at 96 h. These results indicated that 9,10-PQ induced oxidative stress to fish. Ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) analysis revealed that four metabolic pathways were influenced in response to treatment with 9,10-PQ, including glycerophospholipid metabolism, phenylalanine, tyrosine and tryptophan biosynthesis, purine metabolism and sulfur metabolism. These pathways are associated with antioxidant mechanisms, biosynthesis of neurotransmitters and innate immune functions. Thus, the as-obtained results confirmed that 9,10-PQ induced oxidative stress and raised concerns of neurotoxicity and immunotoxicity to fish. Overall, the present study posed a high environmental risk of oxy-PAHs to aquatic ecosystems.

Ex vivo Quantitative Proteomic Analysis of Serotonin Transporter Interactome: Network Impact of the SERT Ala56 Coding Variant

Altered serotonin (5-HT) signaling is associated with multiple brain disorders, including major depressive disorder (MDD), obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). The presynaptic, high-affinity 5-HT transporter (SERT) tightly regulates 5-HT clearance after release from serotonergic neurons in the brain and enteric nervous systems, among other sites. Accumulating evidence suggests that SERT is dynamically regulated in distinct activity states as a result of environmental and intracellular stimuli, with regulation perturbed by disease-associated coding variants. Our lab identified a rare, hypermorphic SERT coding substitution, Gly56Ala, in subjects with ASD, finding that the Ala56 variant stabilizes a high-affinity outward-facing conformation (SERT^∗) that leads to elevated 5-HT uptake in vitro and in vivo. Hyperactive SERT Ala56 appears to preclude further activity enhancements by p38α mitogen-activated protein kinase (MAPK) and can be normalized by pharmacological p38α MAPK inhibition, consistent with SERT Ala56 mimicking, constitutively, a high-activity conformation entered into transiently by p38α MAPK activation. We hypothesize that changes in SERT-interacting proteins (SIPs) support the shift of SERT into the SERT^∗ state which may be captured by comparing the composition of SERT Ala56 protein complexes with those of wildtype (WT) SERT, defining specific interactions through comparisons of protein complexes recovered using preparations from SERT^–/– (knockout; KO) mice. Using quantitative proteomic-based approaches, we identify a total of 459 SIPs, that demonstrate both SERT specificity and sensitivity to the Gly56Ala substitution, with a striking bias being a loss of SIP interactions with SERT Ala56 compared to WT SERT. Among this group are previously validated SIPs, such as flotillin-1 (FLOT1) and protein phosphatase 2A (PP2A), whose functions are believed to contribute to SERT microdomain localization and regulation. Interestingly, our studies nominate a number of novel SIPs implicated in ASD, including fragile X mental retardation 1 protein (FMR1) and SH3 and multiple ankyrin repeat domains protein 3 (SHANK3), of potential relevance to long-standing evidence of serotonergic contributions to ASD. Further investigation of these SIPs, and the broader networks they engage, may afford a greater understanding of ASD as well as other brain and peripheral disorders associated with perturbed 5-HT signaling.

Human Serotonin Transporter Coding Variation Establishes Conformational Bias with Functional Consequences

The antidepressant-sensitive serotonin (5-HT) transporter (SERT) dictates rapid, high-affinity clearance of the neurotransmitter in both the brain and periphery. In a study of families with multiple individuals diagnosed with autism spectrum disorder (ASD), we previously identified several, rare, missense coding variants that impart elevated 5-HT transport activity, relative to wild-type SERT, upon heterologous expression as well as in ASD subject lymphoblasts. The most common of these variants, SERT Ala56, located in the transporter's cytosolic N-terminus, has been found to confer in transgenic mice hyperserotonemia, an ASD-associated biochemical trait, an elevated brain 5-HT clearance rate, and ASD-aligned behavioral changes. Hyperfunction of SERT Ala56 has been ascribed to a change in 5-HT KM, though the physical basis of this change has yet to be elucidated. Through assessments of fluorescence resonance energy transfer (FRET) between cytosolic N- and C-termini, sensitivity to methanethiosulfonates, and capacity for N-terminal tryptic digestion, we obtain evidence for mutation-induced conformational changes that support an open-outward 5-HT binding conformation in vitro and in vivo. Aspects of these findings were also evident with another naturally occurring C-terminal SERT coding variant identified in our ASD study, Asn605. We conclude that biased conformations of surface resident transporters that can impact transporter function and regulation are an unappreciated consequence of heritable and disease-associated SERT coding variation.

Multifaceted Regulations of the Serotonin Transporter: Impact on Antidepressant Response

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

RETRACTED: Placental serotonin systems in pregnancy metabolic complications associated with maternal obesity and gestational diabetes mellitus

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). The publication was retracted by request of the authors following an investigation by Monash University performed following its Procedures for Investigating Code Breaches and in accordance with the Australian Code for the Responsible Conduct of Research. The University concluded on the balance of probability that a significant part of the text in the paper was included without knowledge, without consent and without correct attribution of the original author who, at the time, was a student at the University. The results discussed in the review article are still scientifically valid. ☆ This article is part of a Special Issue entitled: Membrane Transporters and Receptors in Pregnancy Metabolic Complications edited by Luis Sobrevia.

Juvenile Arthritis Patients Suffering from Chronic Inflammation Have Increased Activity of Both IDO and GTP-CH1 Pathways But Decreased BH4 Efficacy: Implications for Well-Being, Including Fatigue, Cognitive Impairment, Anxiety, and Depression

Juvenile idiopathic arthritis (JIA) represents joint inflammation with an unknown cause that starts before the age of 16, resulting in stiff and painful joints. In addition, JIA patients often report symptoms of sickness behavior. Recent animal studies suggest that proinflammatory cytokines produce sickness behavior by increasing the activity of indoleamine-2,3-dioxygenase (IDO) and guanosinetriphosphate⁻cyclohydrolase-1 (GTP⁻CH1). Here, it is hypothesized that inflammation in JIA patients enhances the enzymatic activity of IDO and GTP-CH1 and decreases the co-factor tetrahydrobiopterin (BH4). These compounds play a crucial role in the synthesis and metabolism of neurotransmitters. The aim of our study was to reveal whether inflammation affects both the GTP-CH1 and IDO pathway in JIA patients. Serum samples were collected from twenty-four JIA patients. In these samples, the concentrations of tryptophan (TRP), kynurenine (KYN), tyrosine (TYR), neopterin, and phenylalanine (PHE) were measured. An HPLC method with electrochemical detection was developed to quantify tryptophan, kynurenine, and tyrosine. Neopterin and phenylalanine were quantified by ELISA. The KYN/TRP ratio was measured as an index of IDO activity, while the PHE/TYR ratio was measured as an index of BH4 activity. Neopterin concentrations were used as an indirect measure of GTP-CH1 activity. JIA patients with high disease activity showed higher levels of both neopterin and kynurenine, and a higher ratio of both KYN/TRP and PHE/TYR and lower tryptophan levels than clinically inactive patients. Altogether, these data support our hypothesis that inflammation increases the enzymatic activity of both IDO and GTP-CH1 but decreases the efficacy of the co-factor BH4. In the future, animal studies are needed to investigate whether inflammation-induced changes in these enzymatic pathways and co-factor BH4 lower the levels of the brain neurotransmitters glutamate, noradrenaline, dopamine, serotonin, and melatonin, and consequently, whether they may affect fatigue, cognition, anxiety, and depression. Understanding of these complex neuroimmune interactions provides new possibilities for Pharma-Food interventions to improve the quality of life of patients suffering from chronic inflammation.

SKF-10047, a prototype Sigma-1 receptor agonist, augmented the membrane trafficking and uptake activity of the serotonin transporter and its C-terminus-deleted mutant via a Sigma-1 receptor-independent mechanism

The serotonin transporter (SERT) is functionally regulated via membrane trafficking. Our previous studies have demonstrated that the SERT C-terminal deletion mutant (SERTΔCT) showed a robust decrease in its membrane trafficking and was retained in the endoplasmic reticulum (ER), suggesting that SERTΔCT is an unfolded protein that may cause ER stress. The Sigma-1 receptor (SigR1) has been reported to attenuate ER stress via its chaperone activity. In this study, we investigated the effects of SKF-10047, a prototype SigR1 agonist, on the membrane trafficking and uptake activity of SERT and SERTΔCT expressed in COS-7 cells. Twenty-four hours of SKF-10047 treatment (>200 μM) accelerated SERT membrane trafficking and robustly upregulated SERTΔCT activity. Interestingly, these effects of SKF-10047 on SERT functions were also found in cells in which SigR1 expression was knocked down by shRNA, suggesting that SKF-10047 exerted these effects on SERT via a mechanism independent of SigR1. A cDNA array study identified several candidate genes involved in the mechanism of action of SKF-10047. Among them, Syntaxin3, a member of the SNARE complex, was significantly upregulated by 48 h of SKF-10047 treatment. These results suggest that SKF-10047 is a candidate for ER stress relief.

Single Quantum Dot Tracking Reveals Serotonin Transporter Diffusion Dynamics are Correlated with Cholesterol-Sensitive Threonine 276 Phosphorylation Status in Primary Midbrain Neurons

Serotonin transporter (SERT) terminates serotonin signaling in the brain by enabling rapid clearance of the neurotransmitter. SERT dysfunction has been associated with a variety of psychiatric disorders, including depression, anxiety, and autism. Visualizing SERT behavior at the single molecule level in endogenous systems remains a challenge. In this study, we utilize quantum dot (QD) single particle tracking (SPT) to capture SERT dynamics in primary rat midbrain neurons. Membrane microenvironment, specifically membrane cholesterol, plays a key role in SERT regulation and has been found to affect SERT conformational state. We sought to determine how reduced cholesterol content affects both lateral mobility and phosphorylation of conformationally sensitive threonine 276 (Thr276) in endogenous SERT using two different methods of cholesterol manipulation, statins and methyl-β-cyclodextrin. Both chronic and acute cholesterol depletion increased SERT lateral diffusion, radial displacement along the membrane, mobile fraction, and Thr276 phosphorylation levels. Overall, this work has provided new insights about endogenous neuronal SERT mobility and its associations with membrane cholesterol and SERT phosphorylation status.

Bacterial Lipopolysaccharide Increases Serotonin Metabolism in Both Medial Prefrontal Cortex and Nucleus Accumbens in Male Wild Type Rats, but Not in Serotonin Transporter Knockout Rats

It is well known that bacterial lipopolysaccharides (LPS) both increases proinflammatory cytokines and produces sickness behavior, including fatigue and anhedonia (i.e., the inability to experience pleasure). Previously, we have shown that intraperitoneally (i.p.) administered LPS increased extracellular monoamine metabolite levels in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), which was completely, or at least partly, prevented by pretreatment with a triple reuptake inhibitor that also blocks the serotonin (5-HT) transporter (SERT). This suggests indirectly, that LPS may enhance SERT transporter activity, and consequently, increase removal of 5-HT from the synaptic cleft, and increase metabolism of 5-HT. In the present study, we focus more specifically on the role of SERT in this increased metabolism by using rats, that differ in SERT expression. Therefore, the effects of an intraperitoneal LPS injection on extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were investigated by in vivo microdialysis in the NAc and mPFC of wild type (SERT^+/+), heterozygous (SERT^+/−) and knockout (SERT^−/−) rats. Here, we show that LPS-induced 5-HIAA formation in male rats, is significantly increased in SERT^+/+ rats in both the NAc and mPFC, whereas this increase is partly or totally abolished in SERT^+/− and SERT^−/− rats, respectively. Thus, the present study supports the hypothesis that systemic LPS in male rats increases SERT function and consequently enhances 5-HT uptake and metabolism in both the NAc and mPFC.

Pathologic role of nitrergic neurotransmission in mood disorders

Mood disorders are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although over the past 40 years the biogenic amine models have provided meaningful links with the clinical phenomena of, and the pharmacological treatments currently employed in, mood disorders, there is still a need to examine the contribution of other systems to the neurobiology and treatment of mood disorders. This article reviews the current literature describing the potential role of nitric oxide (NO) signaling in the pathophysiology and thereby the treatment of mood disorders. The hypothesis has arisen from several observations including (i) altered NO levels in patients with mood disorders; (ii) antidepressant effects of NO signaling blockers in both clinical and pre-clinical studies; (iii) interaction between conventional antidepressants/mood stabilizers and NO signaling modulators in several biochemical and behavioral studies; (iv) biochemical and physiological evidence of interaction between monoaminergic (serotonin, noradrenaline, and dopamine) system and NO signaling; (v) interaction between neurotrophic factors and NO signaling in mood regulation and neuroprotection; and finally (vi) a crucial role for NO signaling in the inflammatory processes involved in pathophysiology of mood disorders. These accumulating lines of evidence have provided a new insight into novel approaches for the treatment of mood disorders.

The Development of Screening Methods to Identify Drugs to Limit ER Stress Using Wild-type and Mutant Serotonin Transporter

The function of the serotonin transporter (SERT) is regulated by its membrane trafficking. Previously, we showed that the C-terminus-deleted mutant of SERT (SERTΔCT) exhibited an aberrant membrane trafficking and subsequent retention at the endoplasmic reticulum (ER). In addition, we found that proteasome inhibitor-induced ER stress resulted in the impairment of SERT membrane trafficking and retention of SERT at the ER, an impairment very similar to that of SERTΔCT. Based on the result that the chemical chaperone 4-phnylbutulic acid (4-PBA), which relieves ER stress, accelerated the membrane trafficking and upregulated SERT activity, we hypothesized that drugs that facilitate the membrane trafficking of SERT would have potential therapeutic effects on an ER stress-related disease. In this study, we aimed to develop simple screening methods for such drugs using SERT. We first validated the serotonin uptake assay using fluorescent substrates. This simple and reliable assay method was useful for screening for drugs that affected the wild-type SERT but not SERTΔCT. In addition, we verified an assay focusing on the formation of SERTΔCT aggregates. The drugs 4-PBA and SKF-10047 facilitated the trafficking of SERT to the membrane and reduced SERTΔCT aggregates, indicating that the drugs with such characters could be potential candidates for ER stress relief. For both assays, we clarified the usefulness of a high-content screening microscope. These results could pave the way for high-throughput screening for such drugs.

Modulation of serotonin transporter function by kappa-opioid receptor ligands

Kappa opioid receptor (KOR) agonists produce dysphoria and psychotomimesis. While KOR agonists produce pro-depressant-like effects, KOR antagonists produce anti-depressant-like effects in rodent models. The cellular mechanisms and downstream effector(s) by which KOR ligands produce these effects are not clear. KOR agonists modulate serotonin (5-HT) transmission in the brain regions implicated in mood and motivation regulation. Presynaptic serotonin transporter (SERT) activity is critical in the modulation of synaptic 5-HT and, subsequently, in mood disorders. Detailing the molecular events of KOR-linked SERT regulation is important for examining the postulated role of this protein in mood disorders. In this study, we used heterologous expression systems and native tissue preparations to determine the cellular signaling cascades linked to KOR-mediated SERT regulation. KOR agonists U69,593 and U50,488 produced a time and concentration dependent KOR antagonist-reversible decrease in SERT function. KOR-mediated functional down-regulation of SERT is sensitive to CaMKII and Akt inhibition. The U69,593-evoked decrease in SERT activity is associated with a decreased transport V_max, reduced SERT cell surface expression, and increased SERT phosphorylation. Furthermore, KOR activation enhanced SERT internalization and decreased SERT delivery to the membrane. These data demonstrate that KOR activation decreases 5-HT uptake by altering SERT trafficking mechanisms and phosphorylation status to reduce the functional availability of surface SERT.

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.

Role of brainstem serotonin in analgesia produced by low-intensity exercise on neuropathic pain after sciatic nerve injury in mice

Physical exercise is a low-cost, safe, and efficient intervention for the reduction of neuropathic chronic pain in humans. However, the underlying mechanisms for how exercise reduces neuropathic pain are not yet well understood. Central monoaminergic systems play a critical role in endogenous analgesia leading us to hypothesize that the analgesic effect of low-intensity exercise occurs through activation of monoaminergic neurotransmission in descending inhibitory systems. To test this hypothesis, we induced peripheral nerve injury (PNI) by crushing the sciatic nerve. The exercise intervention consisted of low-intensity treadmill running for 2 weeks immediately after injury. Animals with PNI showed an increase in pain-like behaviors that were reduced by treadmill running. Reduction of serotonin (5-hydroxytryptamine) synthesis using the tryptophan hydroxylase inhibitor para-chlorophenylalanine methyl ester prevented the analgesic effect of exercise. However, blockade catecholamine synthesis with the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine had no effect. In parallel, 2 weeks of exercise increased brainstem levels of the 5-HT and its metabolites (5-hydroxyindoleacetic acid), decreased expression of the serotonin transporter, and increased expression of 5-HT receptors (5HT-1B, 2A, 2C). Finally, PNI-induced increase in inflammatory cytokines, tumor necrosis factor-alpha, and interleukin-1 beta, in the brainstem, was reversed by 2 weeks of exercise. These findings provide new evidence indicating that low-intensity aerobic treadmill exercise suppresses pain-like behaviors in animals with neuropathic pain by enhancing brainstem 5-HT neurotransmission. These data provide a rationale for the analgesia produced by exercise to provide an alternative approach to the treatment of chronic neuropathic pain.

Physical and functional interactions between the serotonin transporter and the neutral amino acid transporter ASCT2

The activity of serotonergic systems depends on the reuptake of extracellular serotonin via its plasma membrane serotonin [5-HT (5-hydroxytryptamine)] transporter (SERT), a member of the Na+/Cl−-dependent solute carrier 6 family. SERT is finely regulated by multiple molecular mechanisms including its physical interaction with intracellular proteins. The majority of previously identified SERT partners that control its functional activity are soluble proteins, which bind to its intracellular domains. SERT also interacts with transmembrane proteins, but its association with other plasma membrane transporters remains to be established. Using a proteomics strategy, we show that SERT associates with ASCT2 (alanine–serine–cysteine–threonine 2), a member of the solute carrier 1 family co-expressed with SERT in serotonergic neurons and involved in the transport of small neutral amino acids across the plasma membrane. Co-expression of ASCT2 with SERT in HEK (human embryonic kidney)-293 cells affects glycosylation and cell-surface localization of SERT with a concomitant reduction in its 5-HT uptake activity. Conversely, depletion of cellular ASCT2 by RNAi enhances 5-HT uptake in both HEK-293 cells and primary cultured mesencephalon neurons. Mimicking the effect of ASCT2 down-regulation, treatment of HEK-293 cells and neurons with the ASCT2 inhibitor D-threonine also increases 5-HT uptake. Moreover, D-threonine does not enhance further the maximal velocity of 5-HT uptake in cells depleted of ASCT2. Collectively, these findings provide evidence for a complex assembly involving SERT and a member of another solute carrier family, which strongly influences the subcellular distribution of SERT and the reuptake of 5-HT.

Control of serotonin transporter phosphorylation by conformational state

Serotonin transporter (SERT) is responsible for reuptake and recycling of 5-hydroxytryptamine (5-HT; serotonin) after its exocytotic release during neurotransmission. Mutations in human SERT are associated with psychiatric disorders and autism. Some of these mutations affect the regulation of SERT activity by cGMP-dependent phosphorylation. Here we provide direct evidence that this phosphorylation occurs at Thr276, predicted to lie near the cytoplasmic end of transmembrane helix 5 (TM5). Using membranes from HeLa cells expressing SERT and intact rat basophilic leukemia cells, we show that agents such as Na(+) and cocaine that stabilize outward-open conformations of SERT decreased phosphorylation and agents that stabilize inward-open conformations (e.g., 5-HT, ibogaine) increased phosphorylation. The opposing effects of the inhibitors cocaine and ibogaine were each reversed by an excess of the other inhibitor. Inhibition of phosphorylation by Na(+) and stimulation by ibogaine occurred at concentrations that induced outward opening and inward opening, respectively, as measured by the accessibility of cysteine residues in the extracellular and cytoplasmic permeation pathways, respectively. The results are consistent with a mechanism of SERT regulation that is activated by the transport of 5-HT, which increases the level of inward-open SERT and may lead to unwinding of the TM5 helix to allow phosphorylation.

A requirement of serotonergic p38α mitogen-activated protein kinase for peripheral immune system activation of CNS serotonin uptake and serotonin-linked behaviors

Alterations in central serotonin (5-hydroxytryptamine, 5-HT) neurotransmission and peripheral immune activation have been linked to multiple neuropsychiatric disorders, including depression, schizophrenia and autism. The antidepressant-sensitive 5-HT transporter (SERT, SLC6A4), a critical determinant of synaptic 5-HT inactivation, can be regulated by pro-inflammatory cytokine signaling. Systemic innate immune system activation via intraperitoneal lipopolysaccharide (LPS) injection rapidly elevates brain SERT activity and 5-HT clearance. Moreover, the pro-inflammatory cytokine interleukin (IL)-1β rapidly stimulates SERT activity in raphe nerve terminal preparations ex vivo, effects that are attenuated by pharmacological p38 MAPK inhibition. To establish a role of serotonergic p38α MAPK signaling in LPS/IL-1β-induced SERT regulation and attendant behavioral responses, we pursued studies in mice that afford conditional elimination of p38α MAPK in 5-HT neurons (p38α(5HT-)). We found p38α(5HT-) and control (p38α(5HT+)) littermates to be indistinguishable in viability and growth and to express equivalent levels of SERT protein and synaptosomal 5-HT transport activity. Consistent with pharmacological studies, however, IL-1β fails to increase SERT activity in midbrain synaptosomes prepared from p38α(5HT-) animals. Moreover, although LPS elevated plasma corticosterone and central/peripheral pro-inflammatory cytokines in p38α(5HT-) animals, elevations in midbrain SERT activity were absent nor were changes in depressive and anxiety-like behaviors observed. Our studies support an obligate role of p38α MAPK signaling in 5-HT neurons for the translation of immune activation to SERT regulation and 5-HT-modulated behaviors.

Integrin β3 Haploinsufficiency Modulates Serotonin Transport and Antidepressant-Sensitive Behavior in Mice

Converging lines of evidence have identified genetic interactions between the serotonin transporter (SERT) gene and ITGB3, which encodes the β3 subunit that forms the αIIbβ3 and αvβ3 integrin receptor complexes. Here we examine the consequences of haploinsufficiency in the mouse integrin β3 subunit gene (Itgb3) on SERT function and selective 5-hydroxytryptamine (5-HT) reuptake inhibitor (SSRI) effectiveness in vivo. Biochemical fractionation studies and immunofluorescent staining of murine brain slices reveal that αvβ3 receptors and SERTs are enriched in presynaptic membranes from several brain regions and that αvβ3 colocalizes with a subpopulation of SERT-containing synapses in raphe nuclei. Notably, we establish that loss of a single allele of Itgb3 in murine neurons is sufficient to decrease 5-HT uptake by SERT in midbrain synaptosomes. Pharmacological assays to elucidate the αvβ3-mediated mechanism of reduced SERT function indicate that decreased integrin β3 subunit expression scales down the population size of active SERT molecules and, as a consequence, lowers the effective dose of SSRIs. These data are consistent with the existence of a subpopulation of SERTs that are tightly modulated by integrin αvβ3 and significantly contribute to global SERT function at 5-HT synapses in the midbrain. Importantly, our screen of a normal human population for single nucleotide polymorphisms in human ITGB3 identified a variant associated with reductions in integrin β3 expression levels that parallel our mouse findings. Thus, polymorphisms in human ITGB3 may contribute to the differential responsiveness of select patients to SSRIs.

Akt-mediated regulation of antidepressant-sensitive serotonin transporter function, cell-surface expression and phosphorylation

The present study is focused on the cellular basis for Akt-mediated SERT regulation. SERT has been implicated in mood disorders. SERT is a primary target for antidepressants used in the therapeutic intervention of psychiatric disorders.

Role of nitric oxide in the behavioral and neurochemical effects of IB-MECA in zebrafish

RATIONALE

The adenosine A3 receptor and the nitric oxide (NO) pathway regulate the function and localization of serotonin transporters (SERTs). These transporters regulate extracellular serotonin levels, which are correlated with defensive behavior.

OBJECTIVE

The purpose of this study was to understand the role of the A3AR on anxiety and arousal models in zebrafish, and whether this role is mediated by the nitrergic modulation of serotonin uptake.

METHODS

The effects of IB-MECA (0.01 and 0.1 mg/kg) were assessed in a series of behavioral tasks in adult zebrafish, as well as on extracellular serotonin levels in vivo and serotonin uptake in brain homogenates. Finally, the interaction between IB-MECA and drugs blocking voltage-dependent calcium channels (VDCCs), NO synthase, and SERT was analyzed.

RESULTS

At the lowest dose, IB-MECA decreased bottom dwelling and scototaxis, while at the highest dose, it also decreased shoaling, startle probability, and melanophore responses. These effects were accompanied by an increase in brain extracellular serotonin levels. IB-MECA also concentration-dependently increased serotonin uptake in vitro. The effects of IB-MECA on extracellular 5-HT, scototaxis, and geotaxis were blocked by L-NAME, while only the effects on 5-HT and scototaxis were blocked by verapamil. In vitro, the increase in 5-HT uptake was dependent on VDCCs and NO. Finally, fluoxetine blocked the effect of IB-MECA on scototaxis, but not geotaxis.

CONCLUSION

These results suggest that the effect of IB-MECA on scototaxis are mediated by a VDCC-NO-SERT pathway. While NO seems to mediate the effects of IB-MECA on geotaxis, neither VDCCs nor SERT seems to be involved in this process.

Importance of the Extracellular Loop 4 in the Human Serotonin Transporter for Inhibitor Binding and Substrate Translocation

The serotonin transporter (SERT) terminates serotonergic neurotransmission by performing reuptake of released serotonin, and SERT is the primary target for antidepressants. SERT mediates the reuptake of serotonin through an alternating access mechanism, implying that a central substrate site is connected to both sides of the membrane by permeation pathways, of which only one is accessible at a time. The coordinated conformational changes in SERT associated with substrate translocation are not fully understood. Here, we have identified a Leu to Glu mutation at position 406 (L406E) in the extracellular loop 4 (EL4) of human SERT, which induced a remarkable gain-of-potency (up to >40-fold) for a range of SERT inhibitors. The effects were highly specific for L406E relative to six other mutations in the same position, including the closely related L406D mutation, showing that the effects induced by L406E are not simply charge-related effects. Leu(406) is located >10 Å from the central inhibitor binding site indicating that the mutation affects inhibitor binding in an indirect manner. We found that L406E decreased accessibility to a residue in the cytoplasmic pathway. The shift in equilibrium to favor a more outward-facing conformation of SERT can explain the reduced turnover rate and increased association rate of inhibitor binding we found for L406E. Together, our findings show that EL4 allosterically can modulate inhibitor binding within the central binding site, and substantiates that EL4 has an important role in controlling the conformational equilibrium of human SERT.

Targeting human central nervous system protein kinases: An isoform selective p38αMAPK inhibitor that attenuates disease progression in Alzheimer's disease mouse models

The first kinase inhibitor drug approval in 2001 initiated a remarkable decade of tyrosine kinase inhibitor drugs for oncology indications, but a void exists for serine/threonine protein kinase inhibitor drugs and central nervous system indications. Stress kinases are of special interest in neurological and neuropsychiatric disorders due to their involvement in synaptic dysfunction and complex disease susceptibility. Clinical and preclinical evidence implicates the stress related kinase p38αMAPK as a potential neurotherapeutic target, but isoform selective p38αMAPK inhibitor candidates are lacking and the mixed kinase inhibitor drugs that are promising in peripheral tissue disease indications have limitations for neurologic indications. Therefore, pursuit of the neurotherapeutic hypothesis requires kinase isoform selective inhibitors with appropriate neuropharmacology features. Synaptic dysfunction disorders offer a potential for enhanced pharmacological efficacy due to stress-induced activation of p38αMAPK in both neurons and glia, the interacting cellular components of the synaptic pathophysiological axis, to be modulated. We report a novel isoform selective p38αMAPK inhibitor, MW01-18-150SRM (=MW150), that is efficacious in suppression of hippocampal-dependent associative and spatial memory deficits in two distinct synaptic dysfunction mouse models. A synthetic scheme for biocompatible product and positive outcomes from pharmacological screens are presented. The high-resolution crystallographic structure of the p38αMAPK/MW150 complex documents active site binding, reveals a potential low energy conformation of the bound inhibitor, and suggests a structural explanation for MW150’s exquisite target selectivity. As far as we are aware, MW150 is without precedent as an isoform selective p38MAPK inhibitor or as a kinase inhibitor capable of modulating in vivo stress related behavior.

Salvinorin A regulates dopamine transporter function via a kappa opioid receptor and ERK1/2-dependent mechanism

Salvinorin A (SalA), a selective κ-opioid receptor (KOR) agonist, produces dysphoria and pro-depressant like effects. These actions have been attributed to inhibition of striatal dopamine release. The dopamine transporter (DAT) regulates dopamine transmission via uptake of released neurotransmitter. KORs are apposed to DAT in dopamine nerve terminals suggesting an additional target by which SalA modulates dopamine transmission. SalA produced a concentration-dependent, nor-binaltorphimine (BNI)- and pertussis toxin-sensitive increase of ASP(+) accumulation in EM4 cells coexpressing myc-KOR and YFP-DAT, using live cell imaging and the fluorescent monoamine transporter substrate, trans 4-(4-(dimethylamino)-styryl)-N-methylpyridinium) (ASP(+)). Other KOR agonists also increased DAT activity that was abolished by BNI pretreatment. While SalA increased DAT activity, SalA treatment decreased serotonin transporter (SERT) activity and had no effect on norepinephrine transporter (NET) activity. In striatum, SalA increased the Vmax for DAT mediated DA transport and DAT surface expression. SalA up-regulation of DAT function is mediated by KOR activation and the KOR-linked extracellular signal regulated kinase-½ (ERK1/2) pathway. Co-immunoprecipitation and BRET studies revealed that DAT and KOR exist in a complex. In live cells, DAT and KOR exhibited robust FRET signals under basal conditions. SalA exposure caused a rapid and significant increase of the FRET signal. This suggests that the formation of KOR and DAT complexes is promoted in response to KOR activation. Together, these data suggest that enhanced DA transport and decreased DA release resulting in decreased dopamine signalling may contribute to the dysphoric and pro-depressant like effects of SalA and other KOR agonists.

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

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

Nucleoside transporters in the purinome

The purinome is a rich complex of proteins and cofactors that are involved in fundamental aspects of cellular homeostasis and cellular responses. The purinome is evolutionarily ancient and is made up of thousands of members. Our understanding of the mechanisms linking some parts of this complex network and the physiological relevance of the various connections is well advanced. However, our understanding of other parts of the purinome is less well developed. Our research focuses on the adenosine or nucleoside transporters (NTs), which are members of the membrane purinome. Nucleoside transporters are integral membrane proteins that are responsible for the flux of nucleosides, such as adenosine, and nucleoside analog drugs, used in a variety of anti-cancer, anti-viral and anti-parasite therapies, across cell membranes. Nucleoside transporters form the SLC28 and SLC29 families of solute carriers and the protein members of these families are widely distributed in human tissues including the central nervous system (CNS). NTs modulate purinergic signaling in the CNS primarily through their effects on modulating prevailing adenosine levels inside and outside the cell. By clearing the extracellular milieu of adenosine, NTs can terminate adenosine receptor-dependent signaling and this raises the possibility of regulatory feedback loops that tie together receptor signaling with transporter function. Despite the important role of NTs as modulators of purinergic signaling in the human body, very little is known about the nature or underlying mechanisms of regulation of either the SLC28 or SLC29 families, particularly within the context of the CNS purinome. Here we provide a brief overview of our current understanding of the regulation of members of the SLC29 family and highlight some interesting avenues for future research.

Serotonin and serotonin transport in the regulation of lactation

Serotonin (5-HT), classically known as a neurotransmitter involved in regulating sleep, appetite, memory, sexual behavior, neuroendocrine function and mood is also synthesized in epithelial cells located in many organs throughout the body, including the mammary gland. The function of epithelial 5-HT is dependent on the expression of the 5-HT receptors in a particular system. The conventional components of a classic 5-HT system are found within the mammary gland; synthetic enzymes (tryptophan hydroxylase I, aromatic amino acid decarboxylase), several 5-HT receptors and the 5-HT reuptake transporter (SERT). In the mammary gland, two actions of 5-HT through two different 5-HT receptor subtypes have been described: negative feedback on milk synthesis and secretion, and stimulation of parathyroid hormone related-protein, a calcium-mobilizing hormone. As with neuronal systems, the regulation of 5-HT activity is multifactorial, but one seminal component is reuptake of 5-HT from the extracellular space following its release. Importantly, the wide availability of selective 5-HT reuptake inhibitors (SSRI) allows the manipulation of 5-HT activity in a biological system. Here, we review the role of 5-HT in mammary gland function, review the biochemistry, genetics and physiology of SERT, and discuss how SERT is vital to the function of the mammary gland.

Serotonin-induced down-regulation of cell surface serotonin transporter

The serotonin transporter (SERT) terminates serotonergic signaling and enables refilling of synaptic vesicles by mediating reuptake of serotonin (5-HT) released into the synaptic cleft. The molecular and cellular mechanisms controlling SERT activity and surface expression are not fully understood. Here we demonstrate that the substrate 5-HT itself causes acute down-regulation of SERT cell surface expression. To assess surface SERT expression by ELISA, we used a SERT variant (TacSERT) where the N-terminus of SERT was fused to the intracellular tail of the extracellularly FLAG-tagged single-membrane spanning protein Tac. In stably transfected HEK293 cells, 5-HT caused a dose-dependent reduction in TacSERT surface signal with an EC50 value equivalent to the Km value observed for 5-HT uptake. The 5-HT-induced reduction in surface signal reached maximum within 40-60min and was blocked by the selective SERT inhibitor S-citalopram. 5-HT-induced reduction in SERT expression was further supported by surface biotinylation experiments showing 5-HT-induced reduction in wild type SERT plasma membrane levels. Moreover, preincubation with 5-HT lowered the Vmax for 5-HT uptake in cultured raphe serotonergic neurons, indicting that endogenous cell-surface resident SERT likewise is down-regulated in the presence of substrate.

Lipopolysaccharide increases degradation of central monoamines: an in vivo microdialysis study in the nucleus accumbens and medial prefrontal cortex of mice

Peripheral administration of lipopolysaccharide (LPS) in rodents induces anhedonia, i.e. the inability to experience pleasure. Recently, we reported that serotonin transporter (SERT) function is required for LPS-induced anhedonia. Less is known about the effect of LPS on the biological activity of dopamine transporters (DAT) and norepinephrine transporters (NET). Therefore, in vivo microdialysis was performed in the nucleus accumbens and medial prefrontal cortex of C57BL6/J mice exposed to saline or LPS (133 µg/kg i.p.). To investigate the possible involvement of different monoamine transporters, the triple reuptake inhibitor DOV 216,303 or saline was i.p. injected 30 min before the saline/LPS injection. The dose of LPS, shown to decrease responding for brain stimulation reward in mice, significantly increased extracellular levels of monoamine metabolites (5-HIAA, DOPAC and HVA) in the nucleus accumbens and medial prefrontal cortex. Remarkably, DOV 216,303 abolished LPS-induced DOPAC and HVA formation in the nucleus accumbens, suggesting that LPS increases DAT activity in this brain area. DOV 216,303 also inhibited LPS-induced DOPAC and HVA formation in the medial prefrontal cortex. Since DAT density is very low in this brain structure, reuptake of DA predominantly takes place via NET, suggesting that LPS increases DAT and NET activity in the medial prefrontal cortex. Furthermore, DOV 216,303 pretreatment prevented LPS-induced 5-HIAA formation only in the medial prefrontal cortex, indicating that LPS increases prefrontal SERT activity. In conclusion, the present findings suggest that peripheral LPS increases DAT activity in the nucleus accumbens and increases NET and SERT activity in the medial prefrontal cortex of mice.

Arrestin-mediated activation of p38 MAPK: molecular mechanisms and behavioral consequences

Studies of kappa opioid receptor signaling mechanisms during the last decade have demonstrated that agonist activation of the receptor results in Gβγ-dependent signaling and distinct arrestin-dependent signaling events. Gβγ-dependent signaling results in ion channel regulation causing neuronal inhibition, inhibition of transmitter release, and subsequent analgesic responses. In contrast, arrestin-dependent signaling events result in p38 MAPK activation and subsequent dysphoric and proaddictive behavioral responses. Resolution of these two branches of signaling cascades has enabled strategies designed to identify pathway-selective drugs that may have unique therapeutic utilities.

Norepinephrine transporter heterozygous knockout mice exhibit altered transport and behavior

The norepinephrine (NE) transporter (NET) regulates synaptic NE availability for noradrenergic signaling in the brain and sympathetic nervous system. Although genetic variation leading to a loss of NET expression has been implicated in psychiatric and cardiovascular disorders, complete NET deficiency has not been found in people, limiting the utility of NET knockout mice as a model for genetically driven NET dysfunction. Here, we investigate NET expression in NET heterozygous knockout male mice (NET(+/-) ), demonstrating that they display an approximately 50% reduction in NET protein levels. Surprisingly, these mice display no significant deficit in NET activity assessed in hippocampal and cortical synaptosomes. We found that this compensation in NET activity was due to enhanced activity of surface-resident transporters, as opposed to surface recruitment of NET protein or compensation through other transport mechanisms, including serotonin, dopamine or organic cation transporters. We hypothesize that loss of NET protein in the NET(+/-) mouse establishes an activated state of existing surface NET proteins. The NET(+/-) mice exhibit increased anxiety in the open field and light-dark box and display deficits in reversal learning in the Morris water maze. These data suggest that recovery of near basal activity in NET(+/-) mice appears to be insufficient to limit anxiety responses or support cognitive performance that might involve noradrenergic neurotransmission. The NET(+/-) mice represent a unique model to study the loss and resultant compensatory changes in NET that may be relevant to behavior and physiology in human NET deficiency disorders.

The expression of platelet serotonin transporter (SERT) in human obesity

Background

Serotonin (5-HT) is a well-known modulator of eating behavior. However, the molecular mechanisms linking its action to body weight balance have been only partially elucidated. Since platelets are a suitable peripheral model to study 5-HT transport, metabolism and release, we herein evaluated the expression of the platelet 5-HT re-uptake system (SERT) by [³H]-paroxetine binding assay. A cohort of 114 unrelated individuals (34 males, 80 females; age, mean ± SD: 38.57 ± 12.47 years) without major psychiatric disorders, was recruited following a naturalistic design regarding age or gender and classified accordingly to their body mass index (BMI). Subjects were divided into 5 groups: normal-weight (NW), overweight (OW) and grade I-III obese (OB) individuals. For gender analyses, data were transformed into [³H]-paroxetine density (B_max)/BMI ratios to overcome both the disparity of women vs. men number and anthropometric differences between sexes.

Results

[³H]-paroxetine B_max (SERT density, fmol/mg proteins) was reduced in platelet membranes of grade II (p < 0.01) and III (p < 0.001) obese subjects vs. controls and in overweight subjects (p < 0.05) vs. grade III obese individuals. Considering all patients together, a strong negative correlation between B_max and BMI (r = −0.449; P < 0.0001) was demonstrated. Conversely, [³H]-paroxetine K_D (dissociation constant, nM) did not differ among groups. No gender-related variation concerning B_max/BMI ratios was observed in this cohort of subjects.

Conclusions

The down-regulation of SERT in platelet membranes of severe human obesity (BMI > 35 Kg/m²) confirms the involvement of 5-HT system in body weight gain. Moreover, this findings may help to elucidate those monoamine-endocrine networks acting on fat storage, adipocyte signaling and energy balance. Targeting 5-HT/5-HT-related markers will possibly uncover the existence of human obesity subtypes.

A review of the serotonin transporter and prenatal cortisol in the development of autism spectrum disorders

The diagnosis of autism spectrum disorder (ASD) during early childhood has a profound effect not only on young children but on their families. Aside from the physical and behavioural issues that need to be dealt with, there are significant emotional and financial costs associated with living with someone diagnosed with ASD. Understanding how autism occurs will assist in preparing families to deal with ASD, if not preventing or lessening its occurrence.

Serotonin plays a vital role in the development of the brain during the prenatal and postnatal periods, yet very little is known about the serotonergic systems that affect children with ASD. This review seeks to provide an understanding of the biochemistry and physiological actions of serotonin and its termination of action through the serotonin reuptake transporter (SERT). Epidemiological studies investigating prenatal conditions that can increase the risk of ASD describe a number of factors which elevate plasma cortisol levels causing such symptoms during pregnancy such as hypertension, gestational diabetes and depression. Because cortisol plays an important role in driving dysregulation of serotonergic signalling through elevating SERT production in the developing brain, it is also necessary to investigate the physiological functions of cortisol, its action during gestation and metabolic syndromes.

Rare coding variants of the adenosine A3 receptor are increased in autism: on the trail of the serotonin transporter regulome

BACKGROUND

Rare genetic variation is an important class of autism spectrum disorder (ASD) risk factors and can implicate biological networks for investigation. Altered serotonin (5-HT) signaling has been implicated in ASD, and we and others have discovered multiple, rare, ASD-associated variants in the 5-HT transporter (SERT) gene leading to elevated 5-HT re-uptake and perturbed regulation. We hypothesized that loci encoding SERT regulators harbor variants that impact SERT function and/or regulation and therefore could contribute to ASD risk. The adenosine A3 receptor (A3AR) regulates SERT via protein kinase G (PKG) and other signaling pathways leading to enhanced SERT surface expression and catalytic activity.

METHODS

To test our hypothesis, we asked whether rare variants in the A3AR gene (ADORA3) were increased in ASD cases vs. controls. Discovery sequencing in a case-control sample and subsequent analysis of comparison exome sequence data were conducted. We evaluated the functional impact of two variants from the discovery sample on A3AR signaling and SERT activity.

RESULTS

Sequencing discovery showed an increase of rare coding variants in cases vs. controls (P=0.013). While comparison exome sequence data did not show a significant enrichment (P=0.071), combined analysis strengthened evidence for association (P=0.0025). Two variants discovered in ASD cases (Leu90Val and Val171Ile) lie in or near the ligand-binding pocket, and Leu90Val was enriched individually in cases (P=0.040). In vitro analysis of cells expressing Val90-A3AR revealed elevated basal cGMP levels compared with the wildtype receptor. Additionally, a specific A3AR agonist increased cGMP levels across the full time course studied in Val90-A3AR cells, compared to wildtype receptor. In Val90-A3AR/SERT co-transfections, agonist stimulation elevated SERT activity over the wildtype receptor with delayed 5-HT uptake activity recovery. In contrast, Ile171-A3AR was unable to support agonist stimulation of SERT. Although both Val90 and Ile171 were present in greater numbers in these ASD cases, segregation analysis in families showed incomplete penetrance, consistent with other rare ASD risk alleles.

CONCLUSIONS

Our results validate the hypothesis that the SERT regulatory network harbors rare, functional variants that impact SERT activity and regulation in ASD, and encourages further investigation of this network for other variation that may impact ASD risk.

Chronic Ritalin administration during adulthood increases serotonin pool in rat medial frontal cortex

BACKGROUND

Ritalin has high tendency to be abused. It has been the main indication to control attention deficit hyperactivity disorder. The college students may seek for it to improve their memory, decrease the need for sleep (especially during exams), which at least partially, can be related to serotonergic system. Therefore, it seems worthy to evaluate the effect of Ritalin intake on mature brain. There are many studies on Ritalin effect on developing brain, but only few studies on adults are available. This study was undertaken to find Ritalin effect on serotonin transporter (SERT) density in medial frontal cortex (MFC) of mature rat.

METHODS

Thirty male Wistar rats were used in the study. Rats were assigned into five groups (n = 6 per group): one control, two Ritalin and two vehicle groups. Twelve rats received Ritalin (20 mg/kg/twice a day) orally for eleven continuous days. After one week of withdrawal and another two weeks of rest, in order to evaluate short-term effects of Ritalin, six rats were sacrificed. Another six rats were studied to detect the long-term effects of Ritalin; therefore, they were sacrificed 12 weeks after the previous group. The immunohistochemistry was performed to evaluate the results.

RESULTS

Immunohistochemistry studies showed a higher density of SERT in both 2 and 12 weeks after withdrawal from Ritalin intake in MFC of rat and there was no significant difference between these two groups.

CONCLUSIONS

Our findings demonstrated both short- and long-term effects of Ritalin on frontal serotonergic system after withdrawal period.

Lipopolysaccharide-induced anhedonia is abolished in male serotonin transporter knockout rats: an intracranial self-stimulation study

A growing body of evidence suggests that pro-inflammatory cytokines contribute to the pathogenesis of depression. Previously, it has been shown that cytokines (e.g. interferon-α therapy) induce major depression in humans. In addition, administration of the cytokine-inducer lipopolysaccharide (LPS) provokes anhedonia (i.e. the inability to experience pleasure) in rodents. Furthermore, serum pro-inflammatory cytokine levels are increased in depressed patients. Nevertheless, the etiology of cytokine-induced depression is largely unknown. Previously, it has been shown that selective serotonin re-uptake inhibitors decrease serum pro-inflammatory cytokine levels and that pro-inflammatory cytokines increase activity of the serotonin transporter (SERT). The purpose of this study was to explore the effect of partial and complete lack of the SERT in LPS-induced anhedonia assessed in the intracranial self-stimulation (ICSS) paradigm. A single intraperitoneal injection of LPS was used to induce a pro-inflammatory immune response in male serotonin transporter wild type (SERT(+/+)), heterozygous (SERT(+/-)) and knockout (SERT(-/-)) rats. Body weight and ICSS thresholds were measured daily. Although LPS reduced body weight in all genotypes, loss of body weight was less pronounced in SERT(-/-) compared to SERT(+/+) rats. Remarkably, LPS-induced anhedonia was totally abolished in SERT(-/-) rats and as expected was still present in SERT(+/+) and to a lesser extent in SERT(+/-) rats. Therefore, it is concluded that an intact SERT function is needed for pro-inflammatory cytokine-induced anhedonia and weight loss in rats.

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.

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

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