Binding-induced fluorescence of serotonin transporter ligands: A spectroscopic and structural study of 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) and APP(+) analogues

Molecular docking and biochemical validation of (-)-syringaresinol-4-O-β-D-apiofuranosyl-(1→2)-β-D-glucopyranoside binding to an allosteric site in monoamine transporters

Albizia julibrissin Durazz is one of the most common herbs used for depression and anxiety treatment, but its mechanism of action as an antidepressant or anxiolytic drug have not been fully understood. We previously isolated and identified one lignan glycoside compound from Albizia Julibrissin Durazz, (-)-syringaresinol-4-O-β-D-apiofuranosyl-(1→2)-β-D-glucopyranoside (SAG), that inhibited all three monoamine transporters with a mechanism of action different from that of the conventional antidepressants. In this study, we generated homology models for human dopamine transporter and human norepinephrine transporter, based on the X-ray structure of Drosophila dopamine transporter, and conducted the molecular docking of SAG to all three human monoamine transporters. Our computational results indicated that SAG binds to an allosteric site (S2) that has been demonstrated to be formed by an aromatic pocket positioned in the scaffold domain in the extracellular vestibule connected to the central site (S1) in these monoamine transporters. In addition, we demonstrated that SAG stabilizes a conformation of serotonin transporter with both the extracellular and cytoplasmic pathways closed. Furthermore, we performed mutagenesis of the residues in both the allosteric and orthosteric sites to biochemically validate SAG binding in all three monoamine transporters. Our results are consistent with the molecular docking calculation and support the association of SAG with the allosteric site. We expect that this herbal molecule could become a lead compound for the development of new therapeutic agents with a novel mechanism of action.

Determining Ligand and Ion-Induced Conformational Changes in Serotonin Transporter with Its Fluorescent Substrates

Conformational changes are fundamental events in the transport mechanism. The serotonin transporter (SERT) catalyzes reuptake of the neurotransmitter serotonin after its release by serotonergic neurons and is the molecular target for antidepressant drugs and psychostimulants. Despite significant progress in characterizing the structure-function relationship of SERT, its conformational mechanism has not been fully understood. We present here a cell-based method for determining conformational changes in SERT with its fluorescent substrates by fluorescence imaging analysis. This method fluorometrically measures accessibility of strategically positioned cysteine residues in the substrate permeation pathway to calculate the rate constants of reactivity with MTS reagents in live or permeabilized cells. We validated this method by investigating ligand and ion-induced conformational changes in both the extracellular and cytoplasmic pathways of SERT. Furthermore, we applied this method for examining the influence of Cl^- binding and vilazodone inhibition on SERT conformation. Our results showed that Cl^- ion, in the presence of Na⁺, facilitates the conformational conversion from outward to inward open states, and that vilazodone binding stabilizes SERT in an outward open and inward-closed conformation. The present work provided insights into the conformational mechanism of SERT and also indicated that the cell-based fluorometric method is robust, straightforward to perform, and potentially applicable to any monoamine transporters in exploring the transport mechanism and mechanism of action of therapeutic agents for the treatment of several psychiatric disorders.

Two Lignan Glycosides from Albizia julibrissin Durazz. Noncompetitively Inhibit Serotonin Transporter

Albizia julibrissin Durazz. is one of the most common herbs used for depression and anxiety treatment, but its molecular basis and mechanism of action as an antidepressant or anxiolytic drug are not understood. In this study, we separated and identified two lignan glycosides that inhibit serotonin transporter (SERT) noncompetitively by decreasing Vmax with little change in Km for its fluorescence substrate. In addition, treatment with lignan glycosides did not alter total and cell surface expression levels of the transporter protein. The two compounds decreased the accessibility of a cysteine residue placed in the extracellular substrate permeation pathway by inducing a conformational shift toward an outward-closed state of SERT. These results are consistent with molecular docking for the association of the lignan glycosides to the allosteric site in SERT. The present work supports the proposal that these compounds act on SERT by a novel underlying mechanism of action different from that of conventional antidepressant drugs.

The effects of in-utero exposure to influenza on mental health and mortality risk throughout the life-course

Studies examining the later-life health consequences of in-utero exposure to influenza have typically estimated effects on physical health conditions, with little evidence of effects on mental health outcomes or mortality. Previous studies have also relied primarily on reduced-form estimates of the effects of exposure to influenza pandemics, meaning they are unlikely to recover effects of influenza exposure at an individual-level. This paper uses inverse probability of treatment weighting and "doubly-robust" methods alongside rare mother-reported data on in-utero influenza exposure to estimate the individual-level effect of in-utero influenza exposure on mental health and mortality risk throughout childhood and adulthood. We find that in-utero exposure to influenza is associated with small reductions in mental health in mid-childhood, driven by increases in internalising symptoms, and increases in depressive symptoms in mid-life for males. There is also evidence that in-utero influenza exposure is associated with substantial increases in mortality, although these effects are primarily driven by a 75% increase in the probability of being stillborn, with limited evidence of additional survival disadvantages at later ages. The potential for mortality selection implies that estimated effects on mental health outcomes are likely to represent a lower bound.

Limosilactobacillus reuteri ATCC 6475 metabolites upregulate the serotonin transporter in the intestinal epithelium

The serotonin transporter (SERT) readily takes up serotonin (5-HT), thereby regulating the availability of 5-HT within the intestine. In the absence of SERT, 5-HT remains in the interstitial space and has the potential to aberrantly activate the many 5-HT receptors distributed on the epithelium, immune cells and enteric neurons. Perturbation of SERT is common in many gastrointestinal disorders as well as mouse models of colitis. Select commensal microbes regulate intestinal SERT levels, but the mechanism of this regulation is poorly understood. Additionally, ethanol upregulates SERT in the brain and dendritic cells, but its effects in the intestine have never been examined. We report that the intestinal commensal microbe Limosilactobacillus (previously classified as Lactobacillus) reuteri ATCC PTA 6475 secretes 83.4 mM ethanol. Consistent with the activity of L. reuteri alcohol dehydrogenases, we found that L. reuteri tolerated various levels of ethanol. Application of L. reuteri conditioned media or exogenous ethanol to human colonic T84 cells was found to upregulate SERT at the level of mRNA. A 4-(4-(dimethylamino) phenyl)-1-methylpyridinium (APP+) uptake assay confirmed the functional activity of SERT. These findings were mirrored in mouse colonic organoids, where L. reuteri metabolites and ethanol were found to upregulate SERT at the apical membrane. Finally, in a trinitrobenzene sulphonic acid model of acute colitis, we observed that mice treated with L. reuteri maintained SERT at the colon membrane compared with mice receiving phosphate buffered saline vehicle control. These data suggest that L. reuteri metabolites, including ethanol, can upregulate SERT and may be beneficial for maintaining intestinal homeostasis with respect to serotonin signalling.

Handling of intracellular K+ determines voltage dependence of plasmalemmal monoamine transporter function

The concentrative power of the transporters for dopamine (DAT), norepinephrine (NET), and serotonin (SERT) is thought to be fueled by the transmembrane Na⁺ gradient, but it is conceivable that they can also tap other energy sources, for example, membrane voltage and/or the transmembrane K⁺ gradient. We have addressed this by recording uptake of endogenous substrates or the fluorescent substrate APP⁺(4-(4-dimethylamino)phenyl-1-methylpyridinium) under voltage control in cells expressing DAT, NET, or SERT. We have shown that DAT and NET differ from SERT in intracellular handling of K⁺. In DAT and NET, substrate uptake was voltage-dependent due to the transient nature of intracellular K⁺ binding, which precluded K⁺ antiport. SERT, however, antiports K⁺ and achieves voltage-independent transport. Thus, there is a trade-off between maintaining constant uptake and harvesting membrane potential for concentrative power, which we conclude to occur due to subtle differences in the kinetics of co-substrate ion binding in closely related transporters.

Fluorescent molecular rotors as sensors for the detection of thymidine phosphorylase

Three new fluorescent molecular rotors were synthesized with the aim of using them as sensors to dose thymidine phosphorylase, one of the target enzymes of 5-fluorouracil, a potent chemotherapic drug largely used in the treatment of many solid tumors, that acts by hindering the metabolism of pyrimidines. 5-Fluorouracil has a very narrowtherapeutic window, in fact, its optimal dosage is strictly related to the level of its target enzymes that vary significantly among patients, and it would be of the utmost importance to have an easy and fast method to detect and quantify them. The three molecular rotors developed as TP sensors differ in the length of the alkylic spacer joining the ligand unit, a thymine moiety, and the fluorescent molecular rotor, a [4-(1-dimethylamino)phenyl]-pyridinium bromide. Their ability to trigger an optical signal upon the interaction with thymidine phosphorylase was investigated by fluorescent measurements.

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.

Characteristics of the mitochondrial and cellular uptake of MPP+, as probed by the fluorescent mimic, 4'I-MPP

The discovery that 1-methyl-4-phenylpyridinium (MPP⁺) selectively destroys dopaminergic neurons and causes Parkinson’s disease (PD) symptoms in mammals has strengthened the environmental hypothesis of PD. The current model for the dopaminergic toxicity of MPP⁺ is centered on its uptake into dopaminergic neurons, accumulation into the mitochondria, inhibition of the complex-I leading to ATP depletion, increased reactive oxygen species (ROS) production, and apoptotic cell death. However, some aspects of this mechanism and the details of the cellular and mitochondrial accumulation of MPP⁺ are still poorly understood. The aim of this study was to characterize a structural and functional MPP⁺ mimic which is suitable to study the cellular distribution and mitochondrial uptake of MPP⁺ in live cells and use it to identify the molecular details of these processes to advance the understanding of the mechanism of the selective dopaminergic toxicity of MPP⁺. Here we report the characterization of the fluorescent MPP⁺ derivative, 1-methyl-4-(4'-iodophenyl)pyridinium (4'I-MPP⁺), as a suitable candidate for this purpose. Using this novel probe, we show that cytosolic/mitochondrial Ca²⁺ play a critical role through the sodium-calcium exchanger (NCX) in the mitochondrial and cellular accumulation of MPP⁺ suggesting for the first time that MPP⁺ and related mitochondrial toxins may also exert their toxic effects through the perturbation of Ca²⁺ homeostasis in dopaminergic cells. We also found that the specific mitochondrial NCX (mNCX) inhibitors protect dopaminergic cells from the MPP⁺ and 4'I-MPP⁺ toxicity, most likely through the inhibition of the mitochondrial uptake, which could potentially be exploited for the development of pharmacological agents to protect the central nervous system (CNS) dopaminergic neurons from PD-causing environmental toxins.

Functionalized lignin biomaterials for enhancing optical properties and cellular interactions of dyes

We report a library of functionalized lignins and demonstrate their utility as nanocontainers for organic dyes in biologically relevant applications. Kraft lignin was modified via S_N2 reaction at the phenolic -OH group utilizing a mild base, potassium carbonate, and various alkyl halides, several bearing additional functionalities, with dimethylsulfoxide as solvent. The resulting phenoxy ethers were characterized by ¹H-NMR and IR spectroscopy, as well as DLS and SEM to evaluate their morphology and supramolecular organization. Lignin modified with long-chain hydrocarbon tails was found to effectively encapsulate DiD, a cyanine dye, decrease aggregation, enhance optical transitions and exert a photoprotective effect. The dye-lignin assemblies were also examined as imaging agents, via confocal microscopy, and found to accumulate intracellularly with no leaching of the dye to hydrophobic subcellular components observed. Lignin functionalized with short chain carboxylic acids interacts with ligands directed at the norepinephrine transporter (NET), suggesting applications in sequestration of neuroactive compounds.

Pollen Aperture Factor INP1 Acts Late in Aperture Formation by Excluding Specific Membrane Domains from Exine Deposition

Accurate placement of extracellular materials is a critical part of cellular development. To study how cells achieve this accuracy, we use formation of pollen apertures as a model. In Arabidopsis (Arabidopsis thaliana), three regions on the pollen surface lack deposition of pollen wall exine and develop into apertures. In developing pollen, Arabidopsis INAPERTURATE POLLEN1 (INP1) protein acts as a marker for the preaperture domains, assembling there into three punctate lines. To understand the mechanism of aperture formation, we studied the dynamics of INP1 expression and localization and its relationship with the membrane domains at which it assembles. We found that INP1 assembly occurs after meiotic cytokinesis at the interface between the plasma membrane and the overlying callose wall, and requires the normal callose wall formation. Sites of INP1 localization coincide with positions of protruding membrane ridges in proximity to the callose wall. Our data suggest that INP1 is a late-acting factor involved in keeping specific membrane domains next to the callose wall to prevent formation of exine at these sites.

Binding-induced, turn-on fluorescence of the EGFR/ERBB kinase inhibitor, lapatinib

We report the photophysical properties, binding-induced turn-on emission, and fluorescence imaging of the cellular uptake and distribution of lapatinib, an EGFR/ERBB inhibitor. Lapatinib, a type II, i.e. inactive state, inhibitor that targets the ATP binding pocket of the EGFR family of receptor tyrosine kinases. DFT calculations predict that the 6-furanylquinazoline core of lapatinib should exhibit an excited state with charge transfer character and an S0 to S1 transition energy of 3.4 eV. Absorption confirms an optical transition in the near UV to violet, while fluorescence spectroscopy shows that photoemission is highly sensitive to solvent polarity. The hydrophobicity of lapatinib leads to fluorescent aggregates in solution, however, binding to the lipid-carrier protein, BSA or to the kinase domain of ERBB2, produces spectroscopically distinct photoemission. Confocal fluorescence microscopy imaging of lapatinib uptake in ERBB2-overexpressing MCF7 and BT474 cells reveals pools of intracellular inhibitor with emission profiles consistent with aggregated lapatinib.

Insights to ligand binding to the monoamine transporters-from homology modeling to LeuBAT and dDAT

Understanding of drug binding to the human biogenic amine transporters (BATs) is essential to explain the mechanism of action of these pharmaceuticals but more importantly to be able to develop new and improved compounds to be used in the treatment of depression or drug addiction. Until recently no high resolution structure was available of the BATs and homology modeling was a necessity. Various studies have revealed experimentally validated binding modes of numerous ligands to the BATs using homology modeling. Here we examine and discuss the similarities between the binding models of substrates, antidepressants, psychostimulants, and mazindol in homology models of the human BATs and the recently published crystal structures of the Drosophila dopamine transporter and the engineered protein, LeuBAT. The comparison reveals that careful computational modeling combined with experimental data can be utilized to predict binding of molecules to proteins that agree very well with crystal structures.