Binding of the multimodal antidepressant drug vortioxetine to the human serotonin transporter

Electrochemical oxidative thioetherification of aldehyde hydrazones with thiophenols

An electrochemically promoted oxidative dehydrogenation cross-coupling reaction between aldehyde hydrazones and thiophenols is demonstrated for the first time, which resulted in a variety of (Z)-thioetherified products in moderate to excellent yields. This strategy can be carried out under an air atmosphere, featuring scalability and excellent stereoselectivity. In addition, the transformation efficiently produces readily recyclable disulfide as a by-product with high yields, which significantly reduces the environmental pollution caused by thioetherification.

Establishment of a 3D multicellular placental microtissues for investigating the effect of antidepressant vortioxetine

The placenta is a unique organ with an active metabolism and dynamically changing physiology throughout pregnancy. It is difficult to elucidate the structure of cell-cell and cell-extracellular matrix interactions of the placenta in in vivo studies due to interspecies differences and ethical constraints. In this study, human umbilical cord vein cells (HUVEC) and human placental choriocarcinoma cells (BeWo) were co-cultured for the first time to form spheroids (microtissues) on a three-dimensional (3D) Petri Dish® mold and compared with a traditional two-dimensional (2D) system. Vortioxetine is an antidepressant with a lack of literature on its use in pregnancy in established cultures, the toxicity of vortioxetine was studied to investigate the response of spheroids representing placental tissue. Spheroids were characterised by morphology and exposed to vortioxetine. Cell viability and barrier integrity were then measured. Intercellular junctions and the localisation of serotonin transporter (SERT) proteins were demonstrated by immunofluorescence (IF) staining in BeWo cells. Human chorionic gonadotropin (beta-hCG) hormone levels were also measured. In the 3D system, cell viability and hormone production were higher than in the 2D system. It was observed that the barrier structure was impaired, the structure of intracellular skeletal elements was altered and SERT expression decreased depending on vortioxetine exposure. These results demonstrate that the multicellular microtissue placenta model can be used to obtain results that more closely resemble in vivo toxicity studies of various xenobiotics than other 2D and mono-culture spheroid models in the literature. It also describes the use of 3D models for soft tissues other than the placenta.

Role of vortioxetine in the treatment of neuropathic pain

Neuropathic pain is an important and disabling clinical problem, its management constitutes a challenge for healthcare professionals. Vortioxetine is a new antidepressant drug with multimodal action, which gives it a unique profile. Tricyclic antidepressants, in particular amitriptyline, and serotonin and norepinephrine reuptake inhibitors venlafaxine and duloxetine are first-line drugs in the treatment of neuropathic pain. The interaction between the pain and depression binomial is very frequent, being the most frequent psychological complication in patients with chronic pain. This comprehensive and descriptive review summarizes the most relevant pharmacological data on vortioxetine, as well as the specific literature on vortioxetine in neuropathic pain and chronic pain.

AlignMe: an update of the web server for alignment of membrane protein sequences

The AlignMe web server is dedicated to accurately aligning sequences of membrane proteins, a particularly challenging task due to the strong evolutionary divergence and the low compositional complexity of hydrophobic membrane-spanning proteins. AlignMe can create pairwise alignments of either two primary amino acid sequences or two hydropathy profiles. The web server for AlignMe has been continuously available for >10 years, supporting 1000s of users per year. Recent improvements include anchoring, multiple submissions, and structure visualization. Anchoring is the ability to constrain a position in an alignment, which allows expert information about related residues in proteins to be incorporated into an alignment without manual modification. The original web interface to the server limited the user to one alignment per submission, hindering larger scale studies. Now, batches of alignments can be initiated with a single submission. Finally, to provide structural context for the relationship between proteins, sequence similarity can now be mapped onto one or more structures (or structural models) of the proteins being aligned, by links to MutationExplorer, a web-based visualization tool. Together with a refreshed user interface, these features further enhance an important resource in the membrane protein community. The AlignMe web server is freely available at https://www.bioinfo.mpg.de/AlignMe/.

Binding and Activation of Serotonergic G-Protein Coupled Receptors by the Multimodal Antidepressant Vortioxetine

G-protein coupled receptors (GPCRs) are important pharmacological targets. Despite substantial progress, important questions still remain concerning the details of activation: how can a ligand act as an agonist in one receptor but as an antagonist in a homologous receptor, and how can agonists activate a receptor despite lacking polar functional groups able to interact with helix 5 as is the case for the related adrenergic receptors? Studying vortioxetine (VXT), an important multimodal antidepressant drug, may elucidate both questions. Herein, we present a thorough in silico analysis of VXT binding to 5-HT_1A, 5-HT_1B, and 5-HT₇ receptors and compare it with available experimental data. We are able to rationalize the differential mode of action of VXT at different receptors, but also, in the case of the 5-HT_1A receptor, we observe the initial steps of activation that inform about an activation mechanism that does not involve polar interaction with helix 5. The results extend our current understanding of agonist and antagonist action at aminergic GPCRs.

The antidepressant drug vilazodone is an allosteric inhibitor of the serotonin transporter

Depression is a common mental disorder. The standard medical treatment is the selective serotonin reuptake inhibitors (SSRIs). All characterized SSRIs are competitive inhibitors of the serotonin transporter (SERT). A non-competitive inhibitor may produce a more favorable therapeutic profile. Vilazodone is an antidepressant with limited information on its molecular interactions with SERT. Here we use molecular pharmacology and cryo-EM structural elucidation to characterize vilazodone binding to SERT. We find that it exhibits non-competitive inhibition of serotonin uptake and impedes dissociation of [³H]imipramine at low nanomolar concentrations. Our SERT structure with bound imipramine and vilazodone reveals a unique binding pocket for vilazodone, expanding the boundaries of the extracellular vestibule. Characterization of the binding site is substantiated with molecular dynamics simulations and systematic mutagenesis of interacting residues resulting in decreased vilazodone binding to the allosteric site. Our findings underline the versatility of SERT allosteric ligands and describe the unique binding characteristics of vilazodone.

Vilazodone (VLZ) is a drug for the treatment of major depressive disorders that targets the serotonin transporter (SERT). Here, the authors combine pharmacology measurements and cryo-EM structural analysis to characterize VLZ binding to SERT and observe that VLZ exhibits non-competitive inhibition of serotonin transport and binds with nanomolar affinity to an allosteric site in SERT.

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

Role of vortioxetine in the treatment of neuropathic pain

Neuropathic pain is an important and disabling clinical problem, its management constitutes a challenge for healthcare professionals. Vortioxetine is a new antidepressant drug with multimodal action, which gives it a unique profile. Tricyclic antidepressants, in particular amitriptyline, and serotonin and norepinephrine reuptake inhibitors venlafaxine and duloxetine are first-line drugs in the treatment of neuropathic pain. The interaction between the pain and depression binomial is very frequent, being the most frequent psychological complication in patients with chronic pain. This comprehensive and descriptive review summarizes the most relevant pharmacological data on vortioxetine, as well as the specific literature on vortioxetine in neuropathic pain and chronic pain.

Vortioxetine Derivatives with Amino Acid as Promoiety: Synthesis, Activity, Stability and Preliminary Pharmacokinetic Study

Vortioxetine (Vot) is an effective antidepressant with unique mechanisms exerting multi-target effects. However, severe side-effects such as nausea and vomiting are commonly experienced under conditions of long-term administration. Eight amino acid modified Vot derivatives were designed and prepared in this study. Similar or lower binding affinities of the modified compounds to the serotonin transporter (SERT) than Vot was observed in the 4-(4-(dimethylamino)-styrl)-N-methylpyridinium (ASP⁺) uptake assay on RBL-2H3 cells. Additionally, the majority of derivatives remained sufficiently stable in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF), indicating achievement of intestinal absorption in the modified form. Afterwards, all derived compounds exhibited slower hepatic clearance and a longer half-life, compared to the parent drug Vot. Notably, threonine-modified 3f exhibited significantly lower activity to SERT, serine-modified 3e showed the fastest degradation rate in rat plasma, with hydrolysis to an extent of 50% in 10 min, and better pharmacokinetic properties in rat, including C_max, t_1/2, and especially AUC_0-t, which was ~3-fold higher relative to the parent compound. Although, no clear understanding of SARs has been obtained, modification of Vot with amino acids containing hydroxyl groups may be beneficial to reduce the gastrointestinal side effect of Vot or obtain better pharmacokinetic properties, providing some ideas for the further study in the future.

Pharmaceutical salt and salt hydrates of vortioxetine with sulfonic acid to ameliorate the solubility and hygroscopicity via the charge-assisted hydrogen bond assembly

Salt forms of vortioxetine (VOT) with three sulfonic acids have been designed and prepared in 1 : 1 stoichiometry ratio, and their crystal structures and physicochemical properties have also been characterized in detail.

Focus on Human Monoamine Transporter Selectivity. New Human DAT and NET Models, Experimental Validation, and SERT Affinity Exploration

The most commonly used antidepressant drugs are the serotonin transporter inhibitors. Their effects depend strongly on the selectivity for a single monoamine transporter compared to other amine transporters or receptors, and the selectivity is roughly influenced by the spatial protein structure. Here, we provide a computational study on three human monoamine transporters, i.e., DAT, NET, and SERT. Starting from the construction of hDAT and hNET models, whose three-dimensional structure is unknown, and the prediction of the binding pose for 19 known inhibitors, 3D-QSAR models of three human transporters were built. The training set variability, which was high in structure and activity profile, was validated using a set of in-house compounds. Results concern more than one aspect. First of all, hDAT and hNET three-dimensional structures were built, validated, and compared to the hSERT one; second, the computational study highlighted the differences in binding site arrangement statistically correlated to inhibitor selectivity; third, the profiling of new inhibitors pointed out a conservation of the inhibitory activity trend between rabbit and human SERT with a difference of about 1 order of magnitude; fourth, binding and functional studies confirmed 4-(benzyloxy)-4-phenylpiperidine 20a–d and 21a–d as potent SERT inhibitors. In particular, one of the compounds (compound 20b) revealed a higher affinity for SERT than paroxetine in human platelets.

Recent Advances and Challenges of the Drugs Acting on Monoamine Transporters

Background:

The human Monoamine Transporters (hMATs), primarily including hSERT, hNET and hDAT, are important targets for the treatment of depression and other behavioral disorders with more than the availability of 30 approved drugs.

Objective:

This paper is to review the recent progress in the binding mode and inhibitory mechanism of hMATs inhibitors with the central or allosteric binding sites, for the benefit of future hMATs inhibitor design and discovery. The Structure-Activity Relationship (SAR) and the selectivity for hit/lead compounds to hMATs that are evaluated by in vitro and in vivo experiments will be highlighted.

Methods:

PubMed and Web of Science databases were searched for protein-ligand interaction, novel inhibitors design and synthesis studies related to hMATs.

Results:

Literature data indicate that since the first crystal structure determinations of the homologous bacterial Leucine Transporter (LeuT) complexed with clomipramine, a sizable database of over 100 experimental structures or computational models has been accumulated that now defines a substantial degree of structural variability hMATs-ligands recognition. In the meanwhile, a number of novel hMATs inhibitors have been discovered by medicinal chemistry with significant help from computational models.

Conclusion:

The reported new compounds act on hMATs as well as the structures of the transporters complexed with diverse ligands by either experiment or computational modeling have shed light on the poly-pharmacology, multimodal and allosteric regulation of the drugs to transporters. All of the studies will greatly promote the Structure-Based Drug Design (SBDD) of structurally novel scaffolds with high activity and selectivity for hMATs.

Targeting Gap Junctions: New Insights into the Treatment of Major Depressive Disorder

Background:

Major depressive disorder (MDD) is a multifactorial chronic and debilitating mood disease with high lifetime prevalence and associated with excess mortality. Treatments for this disease are not effective in all patients showing the need to find new therapeutic targets.

Objective:

This review aims to update our knowledge on the involvement of astroglial gap junctions and hemichannels in MDD and to show how they have become potential targets for the treatment of this pathology.

Methods:

The method applied in this review includes a systematic compilation of the relevant literature.

Results and Conclusion:

The use of rodent models of depression, gene analysis of hippocampal tissues of MDD patients and post-mortem studies on the brains from MDD patients suggest that astrocytic gap junction dysfunction may be a part of MDD etiologies. Chronic antidepressant treatments of rats, rat cultured cortical astrocytes and human astrocytoma cell lines support the hypothesis that the up-regulation of gap junctional coupling between astrocytes could be an underlying mechanism for the therapeutic effect of antidepressants. However, two recent functional studies suggest that connexin43 hemichannel activity is a part of several antidepressants’ mode of action and that astrocyte gap junctional intercellular communication and hemichannels exert different effects on antidepressant drug response. Even if they emerge as new therapeutic targets for new and more active treatments, further studies are needed to decipher the sophisticated and respective role of astrocytic gap junctions and hemichannels in MDD.

A structural model of the human serotonin transporter in an outward-occluded state

The human serotonin transporter hSERT facilitates the reuptake of its endogenous substrate serotonin from the synaptic cleft into presynaptic neurons after signaling. Reuptake regulates the availability of this neurotransmitter and therefore hSERT plays an important role in balancing human mood conditions. In 2016, the first 3D structures of this membrane transporter were reported in an inhibitor-bound, outward-open conformation. These structures revealed valuable information about interactions of hSERT with antidepressant drugs. Nevertheless, the question remains how serotonin facilitates the specific conformational changes that open and close pathways from the synapse and to the cytoplasm as required for transport. Here, we present a serotonin-bound homology model of hSERT in an outward-occluded state, a key intermediate in the physiological cycle, in which the interactions with the substrate are likely to be optimal. Our approach uses two template structures and includes careful refinement and comprehensive computational validation. According to microsecond-long molecular dynamics simulations, this model exhibits interactions between the gating residues in the extracellular pathway, and these interactions differ from those in an outward-open conformation of hSERT bound to serotonin. Moreover, we predict several features of this state by monitoring the intracellular gating residues, the extent of hydration, and, most importantly, protein-ligand interactions in the central binding site. The results illustrate common and distinct characteristics of these two transporter states and provide a starting point for future investigations of the transport mechanism in hSERT.

Substrate and inhibitor binding to the serotonin transporter: Insights from computational, crystallographic, and functional studies

The serotonin transporter (SERT) belongs to the monoamine transporter family, which also includes the dopamine and norepinephrine transporters. SERT is essential for regulating serotonergic signaling by the reuptake of serotonin from the synaptic cleft back into the presynaptic neuron. Dysregulation of SERT has been implicated in several major psychiatric disorders such as major depressive disorder (MDD). MDD was among the top five leading causes of years lived with disease in 2016 and is characterized as a major global burden. Several drugs have been developed to target SERT for use in the treatment of MDD, and their respective binding modes and locations within SERT have been studied. The elucidation of the first structure of a bacterial SERT homologue in 2005 has accelerated crystallographic, computational, and functional studies to further elucidate drug binding and method of action in SERT. Herein, we aim to highlight and compare these studies with an emphasis on what the different experimental methods conclude on substrate and inhibitor binding modes, and the potential caveats of using the different types of studies are discussed. We focus this review on the binding of cognate substrate and drugs belonging to the different families of antidepressants, including tricyclic antidepressants, selective serotonin reuptake inhibitors, serotonin-norepinephrine reuptake inhibitors, and multimodal drugs, as well as illicit drugs such as cocaine, amphetamines, and ibogaine. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Vortioxetine: A new alternative for the treatment of major depressive disorder

Major Depressive Disorder (MDD) is a serious psychiatric condition. Its treatment remains a challenge nowadays. Vortioxetine is a novel antidepressant with a unique profile, as it acts as a multimodal serotoninergic agent. Its efficacy in MDD has been established in many short- and long-term studies, with 7 positive, 4 negative and 1 failed randomized controlled trials. Moreover, its ability to modulate a wide range of neurotransmitters (serotonin, dopamine, norepinephrine, histamine, glutamate or GABA) confers vortioxetine pro-cognitive effects. Side effects are also different from conventional antidepressants, according to its low incidence of sexual dysfunction, weight gain or cardiovascular alterations. The aim of this systematic review is to describe the pharmacology, clinical efficacy and safety profile of vortioxetine, as well as its potential effectiveness in improving cognitive symptoms.

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.

Interrogating the Molecular Basis for Substrate Recognition in Serotonin and Dopamine Transporters with High-Affinity Substrate-Based Bivalent Ligands

The transporters for the neurotransmitters serotonin and dopamine (SERT and DAT, respectively) are targets for drugs used in the treatment of mental disorders and widely used drugs of abuse. Studies of prokaryotic homologues have advanced our structural understanding of SERT and DAT, but it still remains enigmatic whether the human transporters contain one or two high-affinity substrate binding sites. We have designed and employed 24 bivalent ligands possessing a highly systematic combination of substrate moieties (serotonin and/or dopamine) and aliphatic or poly(ethylene glycol) spacers to reveal insight into substrate recognition in SERT and DAT. An optimized bivalent ligand comprising two serotonin moieties binds SERT with 3,800-fold increased affinity compared to that of serotonin, suggesting that the human transporters have two distinct substrate binding sites. We show that the bivalent ligands are inhibitors of SERT and an experimentally validated docking model suggests that the bivalent compounds bind with one substrate moiety in the central binding site (the S1 site), whereas the other substrate moiety binds in a distinct binding site (the S2 site). A systematic study of nonconserved SERT/DAT residues surrounding the proposed binding region showed that nonconserved binding site residues do not contribute to selective recognition of substrates in SERT or DAT. This study provides novel insight into the molecular basis for substrate recognition in human transporters and provides an improved foundation for the development of new drugs targeting SERT and DAT.

SLC Transporters: Structure, Function, and Drug Discovery

The human Solute Carrier (SLC) transporters are important targets for drug development. Structure-based drug discovery for SLC transporters requires the description of their structure, dynamics, and mechanism of interaction with small molecule ligands and ions. The recent determination of atomic structures of human SLC transporters and their homologs, combined with improved computational power and prediction methods have led to an increased applicability of structure-based drug design methods for human SLC members. In this review, we provide an overview of the SLC transporters' structures and transport mechanisms. We then describe computational techniques, such as homology modeling and virtual screening that are emerging as key tools to discover chemical probes for human SLC members. We illustrate the utility of these methods by presenting case studies in which rational integration of computation and experiment was used to characterize SLC members that transport key nutrients and metabolites, including the amino acid transporters LAT-1 and ASCT2, the SLC13 family of citric acid cycle intermediate transporters, and the glucose transporter GLUT1. We conclude with a brief discussion about future directions in structure-based drug discovery for the human SLC superfamily, one of the most structurally and functionally diverse protein families in human.

Exploring the Inhibitory Mechanism of Approved Selective Norepinephrine Reuptake Inhibitors and Reboxetine Enantiomers by Molecular Dynamics Study

Selective norepinephrine reuptake inhibitors (sNRIs) provide an effective class of approved antipsychotics, whose inhibitory mechanism could facilitate the discovery of privileged scaffolds with enhanced drug efficacy. However, the crystal structure of human norepinephrine transporter (hNET) has not been determined yet and the inhibitory mechanism of sNRIs remains elusive. In this work, multiple computational methods were integrated to explore the inhibitory mechanism of approved sNRIs (atomoxetine, maprotiline, reboxetine and viloxazine), and 3 lines of evidences were provided to verify the calculation results. Consequently, a binding mode defined by interactions between three chemical moieties in sNRIs and eleven residues in hNET was identified as shared by approved sNRIs. In the meantime, binding modes of reboxetine's enantiomers with hNET were compared. 6 key residues favoring the binding of (S, S)-reboxetine over that of (R, R)-reboxetine were discovered. This is the first study reporting that those 11 residues are the common determinants for the binding of approved sNRIs. The identified binding mode shed light on the inhibitory mechanism of approved sNRIs, which could help identify novel scaffolds with improved drug efficacy.

Genetically encoded photocrosslinkers locate the high-affinity binding site of antidepressant drugs in the human serotonin transporter

Despite the well-established role of the human serotonin transporter (hSERT) in the treatment of depression, the molecular details of antidepressant drug binding are still not fully understood. Here we utilize amber codon suppression in a membrane-bound transporter protein to encode photocrosslinking unnatural amino acids (UAAs) into 75 different positions in hSERT. UAAs are incorporated with high specificity, and functionally active transporters have similar transport properties and pharmacological profiles compared with wild-type transporters. We employ ultraviolet-induced crosslinking with p-azido-L-phenylalanine (azF) at selected positions in hSERT to map the binding site of imipramine, a prototypical tricyclic antidepressant, and vortioxetine, a novel multimodal antidepressant. We find that the two antidepressants crosslink with azF incorporated at different positions within the central substrate-binding site of hSERT, while no crosslinking is observed at the vestibular-binding site. Taken together, our data provide direct evidence for defining the high-affinity antidepressant binding site in hSERT.

Molecular details of how antidepressant drugs bind to the human serotonin transporter are not currently clear. Here, the authors introduce photo-cross-linkers into the protein and map the binding site of several antidepressants.