Development of fluorescent probes that target serotonin 5-HT2B receptors

1 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 www.nature.com/scientificreports Development of Fluorescent Probes that Target Serotonin 5-HT2B Receptors Jhonny Azuaje1,2,3, Paula López1,2,3, Alba Iglesias3,4, Rocío A. de la Fuente3,4, José M. PérezRubio1,2, Diego García1, Tomasz Maciej Stępniewski5, Xerardo García-Mera2,3, José M. Brea3,4, Jana Selent5,6, Dolores Pérez1,2, Marián Castro3,4, María I. Loza3,4 & Eddy Sotelo1,2,3 Some 5-HT2B fluorescent probes were obtained by tagging 1-(2,5-dimethoxy-4-iodophenyl)-propan2-amine (DOI) with a subset of fluorescent amines. Some of the resulting fluorescent ligands showed excellent affinity and selectivity profiles at the 5-HT2B receptors (e.g. 12b), while retain the agonistic functional behaviour of the model ligand (DOI). The study highlighted the most salient features of the structure-activity relationship in this series and these were substantiated by a molecular modelling study based on a receptor-driven docking model constructed on the basis of the crystal structure of the human 5-HT2B receptor. One of the fluorescent ligands developed in this work, compound 12i, specifically labelled CHO-K1 cells expressing 5-HT2B receptors and not parental CHO-K1 cells in a concentration-dependent manner. 12i enables imaging and quantification of specific 5-HT2B receptor labelling in live cells by automated fluorescence microscopy as well as quantification by measurements of fluorescence intensity using a fluorescence plate reader. The biogenic amine serotonin, 5-hydroxytryptamine (5-HT), is one of the most versatile chemical messengers in the central and peripheral nervous systems1. In addition to its well-known role as a neurotransmitter2, regulating virtually all brain functions and neurophysiological processes, 5-HT controls critical functions3 within cardiovascular, pulmonary, gastrointestinal and genitourinary systems. Consequently, serotonin has been implicated in the etiology of numerous disease states4 (e.g., depression, anxiety, schizophrenia, obsessive-compulsive and panic disorders, migraine, hypertension, pulmonary hypertension, eating disorders, vomiting and irritable bowel syndrome). The large diversity of functions of serotonin is paralleled by the pharmacological complexity of serotonin receptors5. Of the 14 mammalian 5-HT receptor subtypes, all but one (5-HT3) belong to the super-family of G protein-coupled-receptors (GPCRs)5, 6, which mediate most of the serotonin-based signalling network by receiving and modulating complex information6, 7. The 5-HT2 receptor family comprises three closely related receptor subtypes8, 9, namely 5-HT2A, 5-HT2B, and 5-HT2C, that are the molecular targets of prominent drugs acting in different therapeutic areas1–4, 9–11 (e.g., schizophrenia, depression, hypertension, anxiety). 5-HT2 receptor subtypes mediate many of the central and peripheral physiological functions of serotonin1–4 and they couple preferentially to Gq/11 to increase the hydrolysis of inositol phosphates (IPs) and elevate cytosolic Ca2+. This similar functional behaviour is supported by a high structural homology8, 9. 5-HT2A, 5-HT2B, and 5-HT2C receptors share approximately 46–50% amino acid sequence identity, with the homology being higher than 70% within the transmembrane domains (which contain the 5-HT2 binding pocket)8, 9. Accordingly, the development of highly selective ligands that target a particular 5-HT2 receptor subtype constitutes a considerable challenge. The recent resolution of the crystal structure of the 5-HT2B receptor7 provides new opportunities for the rational discovery of novel small-molecule modulators of 5-HT receptors. 1Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain. 2Departamento de Química Orgánica, Facultad de Farmacia, Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain. 3Instituto de Farmacia Industrial (IFI), Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain. 4Centro Singular de Investigación en Medicina Molecular e Enfermidades Crónicas (CIMUS), Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain. 5PharmacoInformatics Group, Research Program on Biomedical Informatics (GRIB) PRBB, Barcelona, 08003, Spain. 6Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, 02-093, Warsaw, Poland. Correspondence and requests for materials should be addressed to E.S. (email: [email protected]) Received: 17 February 2017 Accepted: 23 August 2017 Published: xx xx xxxx OPEN www.nature.com/scientificreports/ 2 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 The 5-HT2B subtype remains one of the most attractive and enigmatic receptors amongst the 5-HT receptor superfamily12, 13, with key functional, signalling, and regulatory aspects remaining ambiguous. The involvement of this receptor in the development of migraine14, the modulation of the 5-HT transport system15, and the rewarding and reinforcing effects of the widely abused drug ecstasy (3,4-methylenedioxy-N-methylamphetamine, MDMA) has been validated16. 5-HT2B receptor also participate in other relevant processes, particularly in the cardiovascular system where it regulates cardiac development and cardiomyocyte proliferation and survival17, 18. 5-HT2B activation has been associated with diverse pathologies19, 20 (e.g., cardiac hypertrophy and pulmonary hypertension). It has been shown that 5-HT2B activation, along with the inhibition of serotonin transporters, plays a significant role in the pathogenesis of serotonin-induced valvular abnormalities20–22. In line with these observations, it has been demonstrated that norfenfluramine (a metabolite of the antiobesity drug fenfluramine that exhibits potent 5-HT2B agonist activity) and other 5-HT2B agonists used in the treatment of migraine induce valvular heart disease23–25. Similarly, it was recently verified that the use of the antiparkinsonian dopaminergic agonists cabergolide and pergolide (both of which also exhibit high 5-HT2B agonism) is associated with mitral, aortic, and tricuspid valvular heart disease26, 27. Accordingly, drugs that are able to activate 5-HT2B receptors and/ or increase circulating serotonin levels are considered to be potential valvulophatic inducers and, as a consequence, 5-HT2B agonism is considered a dangerous off-target effect22, 27–29, thus hampering the promising therapeutic potential of 5-HT2B receptor28. However, our understanding of the role of 5-HT2B receptor and an in-depth knowledge of the processes triggered by ligand−5-HT2B receptor interactions in living cells are still limited due to the lack of ad hoc molecular probes. The introduction of fluorescence-based techniques has progressed the study of GPCR pharmacology to the single cell level30. Fluorescent molecular probes have proven to be valuable tools that offer a wealth of relevant evidence31, 32, particularly the mapping or identification of ligand binding sites, ligand binding mechanisms, the physical nature of the binding pocket, the movement and internalization of receptors in living cells, and the localization and visualization of labelled receptors. Furthermore, fluorescent probes represent a safer, less expensive and faster alternative to radioligands. The extensive application of these techniques to GPCR research demands the development of fluorescently labelled GPCR ligands that have appropriate photochemical and pharmacological properties. A number of fluorescent probes have been described for 5-HT1A33, 5-HT334 and 5-HT635 receptors but, to the best of our knowledge, reports concerning fluorescently-tagged 5-HT2 ligands have not been published to date. As part of a project aimed at developing molecular probes for the study of the 5-HT2 receptor family, we report here the development and optimization of fluorescent tools that selectively target 5-HT2B receptors. The new ligands combine good fluorescence properties with satisfactory affinity and selectivity; accordingly, these molecular probes can contribute to a better understanding of the physiological and pathological implications of 5-HT2B receptors. Results and Discussion 1-(2,5-Dimethoxy-4-iodophenyl)-propan-2-amine (DOI)36 was selected as the reference ligand to develop the fluorescent probes designed within this study. DOI is a partial agonist derived from the amphetamine chemotype and it is one of the most useful and better pharmacologically characterized molecular tools for the study of the 5-HT2-receptor family37, 38. In addition, the [125-I]-R-DOI radioligand is recognized as a reference standard for high throughput screening campaigns at 5-HT2 receptors39, 40. Recent findings have shown that DOI produces a super-potent (≈15 pM) blockade of the pro-inflammatory effects of tumor necrosis factor alpha (TNF-α) in primary aortic smooth muscle cells and animal models41, 42. These findings, which provided new evidence about the role of 5-HT2A receptors in inflammation, increase the demand for DOI-based fluorescent probes. For this study, it was decided to employ racemic DOI as reference, accordingly all molecular probes were synthesized and tested as racemates. DOI is a well-recognized specific 5-HT2 ligand that, notwithstanding, exhibits weak selectivity within the 5-HT2 receptor family (5-HT2A, 5-HT2B, 5-HT2C)36–38. Previous reports have evidenced the introduction of functionalized alkyl chains on the methoxy group at position 2 of the phenyl ring in DOI produces an increased 5-HT2B selectivity43. This structure-activity relationship trend encouraged us to explore the use of DOI-based derivatives for the development of fluorescent ligands for the 5-HT2B receptor. A set of acid-functionalized DOI derivatives (7a–d) bearing variable spacers on the oxygen atom at position 2 was synthesized (Fig.1) according to previously described procedures44–48. With the aim of carrying out a preliminary evaluation of the biological repercussions of the different spacers, precursors 7a–d were transformed into the corresponding fluorescent derivatives 10a–d by reacting with a model dansyl amine (8a) and subsequent cleavage of the Boc groups (Fig.1). The fluorescent probes 10a–d contain the 1-(2,5-dimethoxy-4-iodophenyl)-propan-2-amine scaffold and the dansyl chromophore linked through variable spacers that differ in both length and composition (Fig.2). The results of the pharmacological evaluation of the model fluorescently-tagged amides 10a–d at the 5-HT2 receptor family (5-HT2A, 5-HT2B, 5-HT2C)49, 50 are presented in Fig.2. All compounds were evaluated in binding experiments (see experimental part) and the results were compared with the biological activity determined for the reference ligand (DOI). Additionally, the functional activities (IPs accumulation)51 of the compounds at the 5-HT2B receptor were also determined (Fig.2). In agreement with previous findings43, it was verified that the introduction of alkyl chains on the oxygen atom at position 2 of the phenyl ring of the amphetamine scaffold produces derivatives that exhibit incipient 5-HT2B subtype selectivity (10a–d, Fig.2). The drop observed in the 5-HT2A affinity is particularly noteworthy as this is in the micromolar range for all of the dansyl probes tested. Another remarkable trend identified on considering the biological data (Fig.2) obtained for DOI derivatives bearing dansyl as a fluorescent tag (10a–d) is the importance of the spacer group linking the pharmacophore and the fluorophore. It was observed that elongation of the spacer has a negative effect on both the affinity and activity at the 5-HT2B receptor (Fig.2), with the fluorescent probe bearing the propyl group (10a) displaying the most attractive affinity and selectivity profiles. www.nature.com/scientificreports/ 3 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 Figure 1. Synthesis of the target fluorescent DOI analogues44–48. Reagents and conditions: (a) K2CO3, R–X, MeCN, 80 °C, 8 h. (b) I2, THF, rt, 24 h. (c) Me-NO2, AcOH, 100 °C, 6 h. (d) Fe, AcOH, 100 °C, 12 h. (e) NH4OAc, NaCNBH4, THF, 2 h. (f) (Boc)2O, THF, 0 °C, 3 h. (g) DCC, DCM, rt, 12 h (70–93%). (h) HCl/Dioxane, DCM, 0 °C, 2 h (50–78%). Figure 2. Structural and pharmacological data at 5-HT2 receptors of model dansyl probes bearing different linkers (10a–d). www.nature.com/scientificreports/ 4 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 In an effort to shed light on the molecular determinants for the binding of the novel fluorescent probes, we used the recent crystallized structure of the 5-HT2B receptor (PDB ID: 4IB4)7 to create three-dimensional models of this receptor in complex with two fluorescent derivatives with different linker lengths: compound 10a (3 carbon linker) and 10b (5 carbon linker). The resulting complexes were embedded in a physiological environment consisting of a hydrated lipid bilayer and then subjected to extended molecular dynamics simulations with a total simulation time of 1.6 µs. A common binding pose was obtained for the DOI fragment for both simulated systems and this fragment is inserted deep into to the 5-HT2B receptor (Fig.3, inset top panel). The protonated nitrogen of DOI establishes a strong electrostatic interaction with D3.32 whereas the aromatic ring is sandwiched in a hydrophobic environment formed by a valine in position 3.33 and two phenylalanines in positions 6.51 and 6.52. This tight binding results in a stable pose along the simulation, as reflected by low RMSD values of 0.761 Å (compound 10a) and 0.703 Å (compound 10b) with respect to the average structure (Fig.3, lower panel). In contrast, the fluorescent tag that is linked to position 2 of the aromatic ring and reaches towards the extracellular side of the 5-HT2B receptor shows much higher dynamic variability around the average structure, with an RMSD of 2.144 Å for ligand 10a and 3.732 Å for compound 10b. The general higher fluctuation for both fluorescent probes is due to interaction with areas of higher flexibility in the extracellular loop region of the receptor. This situation Figure 3. Three-dimensional complexes of the fluorescent probes 10a (left) and 10b (right) bound to the 5-HT2B receptor obtained from extended molecular dynamics simulation with a total time of 1.6 μs (2 times × 800 ns). The upper panel highlights ligand-receptor interactions of a representative structure. The lower panel includes information about the dynamic properties and stability of the ligand binding by depicting frames each 50 ns along a total simulation time of 800 ns. The RMSD values are calculated for DOI or the fluorescent tag with respect to the average structure at 800 ns. www.nature.com/scientificreports/ 5 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 is inevitable as the fluorescent tag needs to reach sufficiently far out of the receptor in order to avoid undesired quenching interactions with the receptor, which would give rise to a non-functional fluorescence probe. Despite this higher fluctuation, it was found that during the course of the simulation compound 10a (3 carbon linker) formed frequent polar interactions with the backbone of ECL 2 and Q7.32 via the SO2 group (Fig.3, top left panel), which result in a stabilization of the fluorescent tag (RMSD 2.144 Å, bottom left panel). Such polar stabilization is reduced for compound 10b because the longer linker architecture (5 carbon linker) is not optimal for allowing this polar interaction. Such reduced stabilization could be a plausible explanation for the reduced binding affinity observed for compound 10b (5 carbon linker, Ki: 167.19 nM) compared to compound 10a (3 carbon linker, Ki: 111.70 nM). The model developed here predicts that compounds with a longer linker have reduced stabilizing interactions in the extracellular loop regions and therefore exhibit lower binding affinity. This prediction is consistent with the other experimental data. For example, compound 10c (9 carbon linker, Ki 806.04 nM) has a markedly lower binding affinity compared to compound 10a (3 carbon linker, Ki: 111.70 nM) and 10b (5 carbon linker, Ki 167.19 nM). The propyl spacer group present in the most pharmacologically appealing fluorescent probe identified (10a) was fixed for subsequent fluorophore exploration. Accordingly, the carboxylic acid 7a was combined with a set of 9 propylamine-functionalized fluorescent tags (Fig.4) by employing classical coupling experimental conditions (Fig.4)44–48. The selection of fluorescent amines (8) embraced not only classical fluorescent tags (e.g., 8a,b or 8i) but also several new fluorescent scaffolds (e.g., 8c–h). The employed tags (8) incorporate fluorescent groups with relatively small to large molecular volumes, thus providing a collection of probes with diverse structural and photophysical properties. Some of the non-conventional fluorophores were chosen on the basis Figure 4. Synthesis of the target fluorescent DOI analogues 12a–i and structures of the fluorescent amine precursors (8a–i). Reagents and conditions: (a) DCC, DCM, rt, 12 h (70–93%). (b) HCl/Dioxane, DCM, 0 °C, 2 h (50–78%). www.nature.com/scientificreports/ 6 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 of their complementary absorption/emission fluorescence spectra, since they could be employed as donor and acceptor pairs for the implementation of fluorescence resonance energy transfer (FRET) studies in the near future. Amines 8d–8g were prepared from the corresponding dimethyl phenanthrene-9,10-dicarboxylates (or pentaphene-6,7-dicarboxylate, for 8g), these aromatic o-diesters were synthesized by palladium-catalyzed [2 + 2 + 2] cycloaddition of arynes52, 53 with dimethyl acetylenedicarboxylate (see supporting information). The experimental details for the syntheses of compounds 12 are outlined in Fig.4. Briefly, the carboxylic acid 7a was reacted with different fluorescent amines (8a–i) employing DCC as a coupling reagent to give the amides 11. Finally, the protecting group (BOC) was removed by treatment with TFA to afford the fluorescent DOI-based probes 12a–i. Details on the synthesis, structural and photo-physical characterization of the obtained fluorescent probes are described in the experimental part and in Fig.4. The absorption and emission spectra of the fluorescent 5-HT2 ligands (10 and 12) were determined on 10 μM solutions. The most relevant fluorescent properties are collected in Fig.5. All of the synthesized derivatives emitted fluorescence in the visible region of the spectrum, with selected fluorescent tags (Fig.4) providing distinctive properties and excitation wavelengths. The excitation wavelengths varied from 305 to 555 nm, whereas those for emission varied from 398 to 580 nm. Most of the compounds showed satisfactory Stokes shift values (Fig.5). As for the first series described above, the pharmacological characterization of the resulting fluorescent ligands (12) was performed at two levels. Firstly, the in vitro affinity was assessed by radioligand binding assays at the three 5-HT2 receptor subtypes. Subsequently, the functional activity at the 5-HT2B receptors was evaluated (IPs accumulation). The data are collected in Fig.6 and some of the representative curves obtained are shown in Fig.7. It can be observed that all compounds stimulated IPs accumulation in a concentration-dependent manner, with EC50 values in the nanomolar range. These data confirm that the novel ligands retain the agonist behaviour exhibited by the model ligand (DOI). Examination of the pharmacological data contained in Fig.6 reveals the identification of some highly attractive fluorescent ligands (e.g., 12b–f). Most of the fluorescent probes retain the incipient 5-HT2B selectivity profile observed for the former member of the series (10a). Taking advantage of the existing X-ray structure, we performed docking followed of molecular probes from the 12 series (12a to 12i) into the 5-HT2B receptor (PDB ID: 4IB4). All probes have overlapping docking poses (Figs3 and 8) forming common interactions of the DOI fragment with residues D3.32, V3.33, F6.51 and F6.52 in the orthosteric binding pocket, as previously observed in our molecular dynamics simulations. In addition, we find that two residues, Q7.32 and the backbone of K211 at the extracellular loop clamp the polar part of the fluorophore tag (highlighted as red dashed lines in Fig.8). The linker length determines the position of the fluorophore and its polar region with respect to the polar extracellular loop region. As discussed above, it is likely that longer linkers limit such favourable polar interaction. The obtained binding modes (Fig.8) provide a potential explanation for affinity differences of studied compounds. According to our models, it appears that solvent exposure is an important factor for binding affinity towards the 5-HT2B receptor. Comparing dyes 12a to 12h (Fig.8) suggests that increased and unfavourable exposure of complex and hydrophobic probes that reach out into the solvent seems to be responsible for a drop of 5-HT2B affinity. Figure 5. Photophysical properties of fluorescent DOI probes 12a–i. www.nature.com/scientificreports/ 7 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 The obtained data support the benefits of introducing the acid-functionalized linker on the oxygen atom at position 2 of the phenyl ring of the model ligand (DOI), but also highlight the need for the optimized alkyl spacer (X = CH2−CH2−CH2). It should be noted that most of the fluorescent probes exhibited weak affinity (typically micromolar range) at the 5-HT2A and 5-HT2C receptors, while the functional experiments confirmed their agonistic behaviour (Fig.7). Moreover, a progressive drop in 5-HT2B affinity was observed when the molecular complexity of the dye increased (e.g. cpds 12g–i). Of the probes obtained, the 7-nitrobenz-2-oxa-1,3-diazole (NBD) labelled DOI ligand (12b) warrants particular attention as it is slightly less potent (Ki = 90.99 nM) than the reference ligand (DOI, Ki = 48.19 nM) while exhibit an excellent selectivity (≥20-fold) for the 5-HT2B receptor subtype. Although the NBD ligand (12b) has the most attractive pharmacological profile, the rhodamine derivative 12i showed the most promising properties of the series from the photophysical point of view. Having finished the preliminary photophysical and pharmacological characterization of the fluorescent derivatives 12a–i, their potential to specifically label 5-HT2B receptors in live cells was evaluated. Chinese hamster ovary K1 (CHO-K1) cells and CHO-K1 cells stably expressing the human serotonin 5-HT2B receptor Figure 6. Structures and pharmacological data at 5-HT2 receptors for the DOI-based fluorescent probes 12a–i. www.nature.com/scientificreports/ 8 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 (CHO-K1-5-HT2B), growing in 96-well plates, were incubated with at least three different concentrations of each compound for varying times and, after compound removal and washing of the cells, plates were subjected to fluorescence microscopy using a high content imaging instrument and the appropriate excitation and emission filter set for each compound. Unfortunately, substantial specific labelling of 5-HT2B receptors was not observed for compounds 12a–h under any of the conditions evaluated. For these compounds, the cellular fluorescence intensities were similar in CHO-K1-5-HT2B and in parental untransfected control CHO-K1 cells (data not shown). This non-specific labelling of cells independently of 5-HT2B expression might arise due to non-specific membrane binding or membrane penetration of the fluorescent ligands, as observed previously for other fluorescent probes for GPCRs54. While exhibiting a moderate 5-HT2B affinity/selectivity profiles compound 12i displayed a concentration-dependent specific labelling of CHO-K1-5-HT2B cells and this resulted in brighter fluorescence images obtained from these cells in comparison with those from control CHO-K1 cells (Fig.9). The best results were obtained for a 3 μM concentration of compound 12i, which yielded the maximum window of specific labelling of the cells for the concentrations assayed (Fig.9B). Under these conditions, the fluorescence intensity of the images collected from CHO-K1-5-HT2B cell wells was clearly superior to that observed for control CHO-K1 cell wells (***p < 0.001, two-way ANOVA and Bonferroni posttests), as quantified both by automated image analysis and by direct measurement of fluorescence emission on a fluorescence plate reader (Fig.9C,D). In the absence of compound, no substantial fluorescence signal was detected in the cells (Fig.9A), which indicates a lack of signal specificity due to cell autofluorescence. These results support the specific binding of 12i to 5-HT2B receptors. Image acquisition at higher magnification (40x) and analysis retrieved similar quantitative results (Supplementary FigureS1). Furthermore, confocal microscopy imaging of CHO-K1-5-HT2B cells labelled with compound 12i in the same conditions also allowed visualization of the probe fluorescence, mainly located in intracellular compartments after 10 min incubation at 37 °C (Supplementary FigureS2). The specificity and sensitivity of the 12i probe towards 5-HT2B receptors over other 5-HT2 receptor subtypes was evaluated by performing similar labelling experiments in CHO cells stably expressing 5-HT2A receptors (CHO-FA4-5-HT2A). In this case, no statistically significant specific labelling was detected when compound 12i was employed at the concentration of 3 μM (Supplementary FigureS3), and it was required to increase the 12i concentration to 20 μM in order to detect certain specific labelling of 5-HT2A receptors over background. However, even in these conditions (20 μM concentration of 12i) the signal window over background achieved for 5-HT2A labelling was considerably smaller than that detected in the case of 5-HT2B labelling at a concentration of 12i more than 6 times lower (3 μM) (Fig.9 and Supplementary FigureS3). These results were in good agreement with the affinity values of compound 12i obtained from our radioligand binding assays. Hence, compound 12i allows direct visualization of 5-HT2B receptors stably expressed in a cell line by live fluorescence microscopy imaging and constitutes a promising molecular probe for future studies. Conclusions A set of fluorescent ligands based on the 1-(2,5-dimethoxy-4-iodophenyl)-propan-2-amine (DOI) chemotype has been developed. Some of the novel fluorescent probes (e.g. 12b) show excellent affinity and selectivity profiles at the 5-HT2B receptors, while retaining the agonistic functional behaviour of DOI. The study highlighted the most salient features of the structure-activity relationship in this series and these were supported by a molecular modelling study based on a receptor-driven docking model constructed on the basis of the crystal structure of the human 5-HT2B receptor. One of the fluorescent ligands reported here enabled the visualization of 5-HT2B receptors in live cells. Compound 12i specifically labelled 5-HT2B receptors stably expressed in CHO-K1 cells in a concentration-dependent manner in fluorescence microscopy studies. Hence, the probes described here are the first examples of 5-HT2B selective fluorescent ligands and their availability should help to boost the GPCR biomolecular imaging field, thus enabling direct visualization and monitoring of spatiotemporal changes associated with 5-HT2B-related (patho)-physiological states. The ligand might be also useful for other assays where well characterized specific fluorescent probes and appropriate signal-to-background window are required, such as BRET-based binding assays. Figure 7. Concentration-response curves obtained for representative ligands and DOI in binding experiments (A) and in functional studies (B). www.nature.com/scientificreports/ 9 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 Methods Chemistry. Unless otherwise stated, all starting materials, reagents and solvents were purchased and used without further purification. The reactions were monitored by thin-layer chromatography (TLC) on 2.5 mm Merck silica gel GF 254 strips, and the purified compounds each showed a single spot; unless stated otherwise, UV light and/or iodine vapour were used to detect compounds. The purity and identity of all tested compounds were established by a combination of HPLC, elemental analysis, mass spectrometry and NMR spectroscopy Figure 8. (A) Superimposition of docking poses of compounds 12a–i for the 5-HT2B receptor (PDB ID: 4IB4) with following color code: 12a - red, 12b - pink, 12c - green, 12d - yellow, 12e - cyan, 12f - dark blue, 12g - purple, 12h - orange and 12i - silver. (B) Individual docking poses for the probes 12a–i bound to the 5-HT2B receptor (PDB ID: 4IB4). www.nature.com/scientificreports/ 16 Scientific RepoRts | 7: 10765 | DOI:10.1038/s41598-017-11370-2 Acknowledgements This research was carried out within the framework of the Cost Action GLISTEN and financially supported by the Spanish Government (grant numbers SAF2009-13609-C04-03 and GPC2014/003 (PS09/63) to E.S. and SAF201457138-C2-1-R to M.C. and M.I.L.). Authors also thanks financial support from Consellería de Cultura, Educación e Ordenación Universitaria of the Galician Government: (grant: GPC2014/03), Centro Singular de Investigación de Galicia accreditation 2016-2019 (ED431G/09). J. S. acknowledges financial support from Instituto de Salud Carlos III FEDER (CP12/03139 and PI15/00460). Author Contributions E.S. and M.I.L. conceived and designed the study. Supervision: E.S., J.S., M.C. Synthesis: J.A., P.L., J.M.P.-R., D.P., D.G., X.G.-M. Binding experiments: A.I., J.M.B. Docking experiments: T.M.S. and J.S. Labelling experiments: R.A.F., M.C. Writing: E.S., J.A., A.I., M.C. All authors read and approved the final version. Additional Information Supplementary information accompanies this paper at doi:10.1038/s41598-017-11370-2 Competing Interests: The authors declare that they have no competing interests. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. © The Author(s) 2017