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

Structural Rearrangement of the Serotonin Transporter Intracellular Gate Induced by Thr276 Phosphorylation

The reuptake of the neurotransmitter serotonin from the synaptic cleft by the serotonin transporter, SERT, is essential for proper neurological signaling. Biochemical studies have shown that Thr276 of transmembrane helix 5 is a site of PKG-mediated SERT phosphorylation, which has been proposed to shift the SERT conformational equilibria to promote inward-facing states, thus enhancing 5-HT transport. Recent structural and simulation studies have provided insights into the conformation transitions during substrate transport but have not shed light on SERT regulation via post-translational modifications. Using molecular dynamics simulations and Markov state models, we investigate how Thr276 phosphorylation impacts the SERT mechanism and its role in enhancing transporter stability and function. Our simulations show that Thr276 phosphorylation alters the hydrogen-bonding network involving residues on transmembrane helix 5. This in turn decreases the free energy barriers for SERT to transition to the inward-facing state, thus facilitating 5-HT import. The results provide atomistic insights into in vivo SERT regulation and can be extended to other pharmacologically important transporters in the solute carrier family.

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.

Serotonin transporter regulation by cholesterol-independent lipid signaling

Serotonin neurotransmission is largely governed by the regulation of the serotonin transporter (SERT). SERT is modulated in part by cholesterol, but the role of cholesterol and lipid signaling intermediates in regulating SERT are unknown. Serotonergic neurons were treated with statins to decrease cholesterol and lipid signaling intermediates. Contrary to reported decreases in 5-HT uptake after cholesterol depletion, biochemical and imaging methods both showed that statins increased 5-HT uptake in a fluoxetine-dependent manner. Simvastatin lowered the Km without changing Vmax for 5-HT or SERT distribution to the plasma membrane. Cholesterol repletion did not block enhanced 5-HT uptake by simvastatin but the enhanced uptake was blocked by lipid isoprenylation intermediates farnesyl pyrophosphate and geranylgeranyl pyrophosphate. Blockade of geranylgeranylation alone without statins also enhanced 5-HT uptake. Overall, this study revealed a specific neuronal effect of statin drugs and identified lipid signaling through geranylgeranylation within the isoprenylation pathway regulates SERT in a cholesterol-independent manner.

Cronobacter sakazakii Infection in Early Postnatal Rats Impaired Contextual-Associated Learning: a Putative Role of C5a-Mediated NF-κβ and ASK1 Pathways

This study was designed to test whether the Cronobacter sakazakii infection-impaired contextual learning and memory are mediated by the activation of the complement system; subsequent activation of inflammatory signals leads to alternations in serotonin transporter (SERT). To test this, rat pups (postnatal day, PND 15) were treated with either C. sakazakii (10⁷ CFU) or Escherichia coli OP50 (10⁷ CFU) or Luria bertani broth (100 μL) through oral gavage and allowed to stay with their mothers until PND 24. Experimental groups' rats were allowed to explore (PNDs 31-35) and then trained in contextual learning task (PNDs 36-43). Five days after training, individuals were tested for memory retention (PNDs 49-56). Observed behavioural data showed that C. sakazakii infection impaired contextual-associative learning and memory. Furthermore, our analysis showed that C. sakazakii infection activates complement system complement anaphylatoxin (C5a) (a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS1)) and mitogen-activated protein kinase kinase1 (MEKK1). Subsequently, MEKK1 induces pro-inflammatory signals possibly through apoptosis signal-regulating kinase-1 (ASK-1), c-Jun N-terminal kinase (JNK1/3) and protein kinase B gamma (AKT-3). In parallel, activated nuclear factor kappa-light-chain-enhancer B cells (NF-κB) induces interleukin-6 (IL-6) and IFNα-1, which may alter the level of serotonin transporter (SERT). Observed results suggest that impaired contextual learning and memory could be correlated with C5a-mediated NF-κβ and ASK1 pathways.

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.

Mutational Analysis and Modeling of Negative Allosteric Modulator Binding Sites in AMPA Receptors

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) constitute a subclass of the ionotropic glutamate receptor superfamily, which functions as glutamate-gated cation channels to mediate the majority of excitatory neurotransmission in the central nervous system. AMPARs are therapeutic targets in a range of brain disorders associated with abnormal glutamate hyperactivity. Multiple classes of AMPAR inhibitors have been developed during the past decades, including competitive antagonists, ion channel blockers, and negative allosteric modulators (NAMs). At present, the NAM is the only class of AMPAR ligands that have been developed into safe and useful drugs in humans in the form of perampanel (Fycompa), which was recently approved for treatment of epilepsy. Compared with the detailed understanding of other AMPAR ligand classes, surprisingly little information has been available regarding the molecular mechanism of perampanel and other classes of NAMs at AMPARs; including the location and structure of NAM binding pockets in the receptor complex. However, structures of the AMPAR GluA2 in complex with NAMs were recently reported that unambiguously identified the NAM binding sites. In parallel with this work, our aim with the present study was to identify specific residues involved in the formation of the NAM binding site for three prototypical AMPAR NAMs. Hence, we have performed a mutational analysis of the AMPAR region that links the four extracellular ligand-binding domains to the central ion channel in the transmembrane domain region. Furthermore, we perform computational ligand docking of the NAMs into structural models of the homomeric GluA2 receptor and optimize side chain conformations around the NAMs to model how NAMs bind in this specific site. The new insights provide potentially valuable input for structure-based drug design of new NAMs. SIGNIFICANCE STATEMENT: The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are glutamate-gated ion channels that mediate the majority of excitatory neurotransmission in the brain. Negative allosteric modulators of AMPA receptors are considered to have significant therapeutic potential in diseases linked to glutamate hyperactivity. The present work employs mutational analysis and molecular modeling of the binding site for prototypical NAMs to provide new molecular insight into how NAMs interact with the AMPA receptor, which is of potential use for future design of new types of NAMs.

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.

Regulation of monoamine transporters and receptors by lipid microdomains: implications for depression

Lipid microdomains ("rafts") are dynamic, nanoscale regions of the plasma membrane enriched in cholesterol and glycosphingolipids, that possess distinctive physicochemical properties including higher order than the surrounding membrane. Lipid microdomain integrity is thought to affect neurotransmitter signaling by regulating membrane-bound protein signaling. Among the proteins potentially affected are monoaminergic receptors and transporters. As dysfunction of monoaminergic neurotransmission is implicated in major depressive disorder and other neuropsychiatric conditions, interactions with lipid microdomains may be of clinical importance. This systematic review evaluates what is known about the molecular relationships of monoamine transporter and receptor regulation to lipid microdomains. The PubMed/MeSH database was searched for original studies published in English through August 2017 concerning relationships between lipid microdomains and serotonin, dopamine and norepinephrine transporters and receptors. Fifty-seven publications were identified and assessed. Strong evidence implicates lipid microdomains in the regulation of serotonin and norepinephrine transporters; serotonin 1A, 2A, 3A, and 7A receptors; and dopamine D1 and β2 adrenergic receptors. Results were conflicting or more complex regarding lipid microdomain associations with the dopamine transporter, D2, D3, and D5 receptors; and negative with respect to β1 adrenergic receptors. Indirect evidence suggests that antidepressants, lipid-lowering drugs, and polyunsaturated fatty acids may exert effects on depression and suicide by altering the lipid milieu, thereby affecting monoaminergic transporter and receptor signaling. The lipid composition of membrane subdomains is involved in localization and trafficking of specific monoaminergic receptors and transporters. Elucidating precise mechanisms whereby lipid microdomains modulate monoamine neurotransmission in clinical contexts can have critical implications for pharmacotherapeutic targeting.

Immune System Activation and Depression: Roles of Serotonin in the Central Nervous System and Periphery

Serotonin (5-hydroxytryptamine, 5-HT) has long been recognized as a key contributor to the regulation of mood and anxiety and is strongly associated with the etiology of major depressive disorder (MDD). Although more known for its roles within the central nervous system (CNS), 5-HT is recognized to modulate several key aspects of immune system function that may contribute to the development of MDD. Copious amounts of research have outlined a connection between alterations in immune system function, inflammation status, and MDD. Supporting this connection, peripheral immune activation results in changes in the function and/or expression of many components of 5-HT signaling that are associated with depressive-like phenotypes. How 5-HT is utilized by the immune system to effect CNS function and ultimately behaviors related to depression is still not well understood. This Review summarizes the evidence that immune system alterations related to depression affect CNS 5-HT signaling that can alter MDD-relevant behaviors and that 5-HT regulates immune system signaling within the CNS and periphery. We suggest that targeting the interrelationships between immune and 5-HT signaling may provide more effective treatments for subsets of those suffering from inflammation-associated MDD.

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.

PKCβ Inhibitors Attenuate Amphetamine-Stimulated Dopamine Efflux

Amphetamine abuse afflicts over 13 million people, and there is currently no universally accepted treatment for amphetamine addiction. Amphetamine serves as a substrate for the dopamine transporter and reverses the transporter to cause an increase in extracellular dopamine. Activation of the beta subunit of protein kinase C (PKCβ) enhances extracellular dopamine in the presence of amphetamine by facilitating the reverse transport of dopamine and internalizing the D2 autoreceptor. We previously demonstrated that PKCβ inhibitors block amphetamine-stimulated dopamine efflux in synaptosomes from rat striatum in vitro. In this study, we utilized in vivo microdialysis in live, behaving rats to assess the effect of the PKCβ inhibitors, enzastaurin and ruboxistaurin, on amphetamine-stimulated locomotion and increases in monoamines and their metabolites. A 30 min perfusion of the nucleus accumbens core with 1 μM enzastaurin or 1 μM ruboxistaurin reduced efflux of dopamine and its metabolite 3-methoxytyramine induced by amphetamine by approximately 50%. The inhibitors also significantly reduced amphetamine-stimulated extracellular levels of norepinephrine. The stimulation of locomotor behavior by amphetamine, measured simultaneously with the analytes, was comparably reduced by the PKCβ inhibitors. Using a stable isotope label retrodialysis procedure, we determined that ruboxistaurin had no effect on basal levels of dopamine, norepinephrine, glutamate, or GABA. In addition, normal uptake function through the dopamine transporter was unaltered by the PKCβ inhibitors, as measured in rat synaptosomes. Our results support the utility of using PKCβ inhibitors to reduce the effects of amphetamine.

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.

Amphetamine action at the cocaine- and antidepressant-sensitive serotonin transporter is modulated by αCaMKII

Serotonergic neurotransmission is terminated by reuptake of extracellular serotonin (5-HT) by the high-affinity serotonin transporter (SERT). Selective 5-HT reuptake inhibitors (SSRIs) such as fluoxetine or escitalopram inhibit SERT and are currently the principal treatment for depression and anxiety disorders. In addition, SERT is a major molecular target for psychostimulants such as cocaine and amphetamines. Amphetamine-induced transport reversal at the closely related dopamine transporter (DAT) has been shown previously to be contingent upon modulation by calmodulin kinase IIα (αCaMKII). Here, we show that not only DAT, but also SERT, is regulated by αCaMKII. Inhibition of αCaMKII activity markedly decreased amphetamine-triggered SERT-mediated substrate efflux in both cells coexpressing SERT and αCaMKII and brain tissue preparations. The interaction between SERT and αCaMKII was verified using biochemical assays and FRET analysis and colocalization of the two molecules was confirmed in primary serotonergic neurons in culture. Moreover, we found that genetic deletion of αCaMKII impaired the locomotor response of mice to 3,4-methylenedioxymethamphetamine (also known as "ecstasy") and blunted d-fenfluramine-induced prolactin release, substantiating the importance of αCaMKII modulation for amphetamine action at SERT in vivo as well. SERT-mediated substrate uptake was neither affected by inhibition of nor genetic deficiency in αCaMKII. This finding supports the concept that uptake and efflux at monoamine transporters are asymmetric processes that can be targeted separately. Ultimately, this may provide a molecular mechanism for putative drug developments to treat amphetamine addiction.

Substrate and Inhibitor-Specific Conformational Changes in the Human Serotonin Transporter Revealed by Voltage-Clamp Fluorometry

The serotonin transporter (SERT) regulates neurotransmission by the biogenic monoamine neurotransmitter serotonin (5-HT, 5-hydroxytryptamine) in the central nervous system, and drugs inhibiting SERT are widely used for the treatment of a variety of central nervous system diseases. The conformational dynamics of SERT transport function and inhibition is currently poorly understood. We used voltage-clamp fluorometry to study conformational changes in human SERT (hSERT) during 5-HT transport and inhibitor binding. Cys residues were introduced at 12 positions in hSERT to enable covalent attachment of a rhodamine-based fluorophore. Transport-associated changes in fluorescence from fluorophore-labeled hSERT expressed in Xenopus oocytes could be robustly detected at four positions in hSERT: endogenous Cys109 in the top of transmembrane domain (TM) 1b, Cys substituted for Thr323 in the top of TM6, Ala419 in the interface between TM8 and extracellular loop (EL) 4, and Leu481 in EL5. The reporter positions were used for time-resolved measurement of conformational changes during 5-HT transport and binding of cocaine and the selective serotonin reuptake inhibitors fluoxetine and escitalopram. At all reporter positions, fluorescence changes observed upon substrate application were distinctly different from those observed upon inhibitor application, with respect to relative amplitude or direction. Furthermore, escitalopram, fluoxetine, and cocaine induced a very similar pattern of fluorescent changes overall, which included movements within or around TM1b, EL4, and EL5. Taken together, our data lead us to suggest that competitive inhibitors stabilize hSERT in a state that is different from the apo outward-open conformation as well as inward-facing conformations.

Amphetamines, new psychoactive drugs and the monoamine transporter cycle

In monoaminergic neurons, the vesicular transporters and the plasma membrane transporters operate in a relay. Amphetamine and its congeners target this relay to elicit their actions: most amphetamines are substrates, which pervert the relay to elicit efflux of monoamines into the synaptic cleft. However, some amphetamines act as transporter inhibitors. Both compound classes elicit profound psychostimulant effects, which render them liable to recreational abuse. Currently, a surge of new psychoactive substances occurs on a global scale. Chemists bypass drug bans by ingenuous structural variations, resulting in a rich pharmacology. A credible transport model must account for their distinct mode of action and link this to subtle differences in activity and undesired, potentially deleterious effects.