Unique pharmacological properties of serotoninergic G-protein coupled receptors from cestodes

Abstract

Background: Cestodes are a diverse group of parasites, some of them being agents of neglected diseases. In cestodes, little is known about the functional properties of G protein coupled receptors (GPCRs) which have proved to be highly druggable targets in other organisms. Notably, serotoninergic G-protein coupled receptors (5-HT GPCRs) play major roles in key functions like movement, development and reproduction in parasites.

Methodology/principal findings: Three 5-HT GPCRs from Echinococcus granulosus and Mesocestoides corti were cloned, sequenced, bioinformatically analyzed and functionally characterized. Multiple sequence alignment with other GPCRs showed the presence of seven transmembrane segments and conserved motifs but interesting differences were also observed. Phylogenetic analysis grouped these new sequences within the 5-HT7 clade of GPCRs. Molecular modeling showed a striking resemblance in the spatial localization of key residues with their mammalian counterparts. Expression analysis using available RNAseq data showed that both E. granulosus sequences are expressed in larval and adult stages. Localization studies performed in E. granulosus larvae with a fluorescent probe produced a punctiform pattern concentrated in suckers. E. granulosus and M. corti larvae showed an increase in motility in response to serotonin. Heterologous expression revealed elevated levels of cAMP production in response to 5-HT and two of the GPCRs showed extremely high sensitivity to 5-HT (picomolar range). While each of these GPCRs was activated by 5-HT, they exhibit distinct pharmacological properties (5-HT sensitivity, differential responsiveness to ligands).

Conclusions/significance: These data provide the first functional report of GPCRs in parasitic cestodes. The serotoninergic GPCRs characterized here may represent novel druggable targets for antiparasitic intervention.

Fig 1. Cladogram of cestode and invertebrate serotoninergic GPCRs.

The amino acid sequences of predicted cestode serotonin receptors were aligned with the repertoire of serotonin receptors cloned from various other flatworm and non-flatworm invertebrate model organisms; Dugesia japonica, 5-HT_Dj; Schistosoma mansoni, 5-HT_Sm; Caenorhabditis elegans, 5-HT_Ce; Drosophila melanogaster, 5-HT_Dm. The cestode GPCR sequences cloned and functionally expressed in this study (5-HT_7Egran1, 5-HT_7Egran2, 5-HT_7Mco1) cluster within a clade of 5-HT7 like receptors (green), as does an additional predicted M. corti sequence (5-HT_7Mco2). Other predicted cestode serotoninergic sequences (5-HT_1Egran1 and 5-HT_1Egran2) cluster within a clade of 5-HT1 like receptors (blue). See methods for complete list of UniProt and GenBank sequence identifiers used in this analysis. Analysis was bootstrapped with 500 replicates.

Fig 2. Molecular modeling of serotoninergic receptors from cestodes.

The structural similarities between three cestode receptors and the published structure of the serotoninergic receptor from human (4IAR) are shown. A) Model of the serotonin receptor 5-HT_7Egran1 (Brown) superposed to the published structure of the human serotonin receptor 5-HT1B (blue). B) Close view of the putative ergotamine interaction with residues of the transmembrane domains III and V from 5-HT_7Egran1 (lateral chains of the amino acids involved are indicated in red), molecular distances between atoms are indicated. C) Model of 5-HT_7Egran2 (violet) superposed to 5-HT1B (blue). D) Close view of the ergotamine interaction with some residues of the transmembrane domains III and V from 5-HT7_Egran2. E) Model of 5-HT_7Mco1 (Brown) superposed to 5-HT1B (blue). F) Close view of the ergotamine interaction with some residues of the transmembrane domains III and V from 5-HT_7Mco1. In all the representations, the molecule of ergotamine was marked in green and the residues in transmembrane domains potentially involved in ergotamine interaction were marked in red.

Fig 3. Heterologous expression of GPCRs reveals 5-HT evoked cAMP accumulation.

Time resolved measurements of cAMP accumulation in cells expressing individual cestode serotoninergic GPCRs (A) 5-HT_7Egran1, (B) 5-HT_7Egran2 and (C) 5-HT_7Mco1 before and after addition of IBMX (1^st arrow, 200μM) and different doses of 5-HT (2^nd arrow, doses indicated in legend). (D, E & F) dose response relationships to 5-HT measuring peak amplitude of 5-HT evoked luminescence change in cells expressing the indicated GPCR (solid circles), or untransfected HEK-293 cells (open circles).

Fig 4. Profiling neurotransmitter specificity against cestode serotoninergic GPCRs.

Responses (raw luminescence units, RLU) in cells transfected with (A) 5-HT_7Egran1, (B) 5-HT_7Egran2 and (C) 5-HT_7Mco1 to indicated neurotransmitters at the indicated time points in the arrow (10μM), with the exception of 5-HT (700nM in (A), 3nM in (B) and 2nM in (C)).

Fig 5. Ergot alkaloid activity at cestode 5-HT GPCRs.

(A-C) Dose response relationships describing sensitivity and magnitude of peak cAMP accumulation versus maximal 5-HT response to various doses of ergotamine at (A) 5-HT_7Egran1, (B) 5-HT_7Egran2 and (C) 5-HT_7Mco1 (D-F). Real time kinetic profiling of cAMP accumulation in response to an addition of either LSD and IBMX (10μM and 200μM, respectively, closed circles) or IBMX alone (200μM, open circles) at first arrow and then subsequent addition of 5-HT (5 μM, second arrow) for (D) 5-HT_7Egran1 or 5-HT (1 μM, second arrow) for (E) 5-HT_7Egran2 and (F) 5-HT_7Mco1. Dose response relationships for activation or blockade of 5-HT evoked cAMP accumulation by LSD at (G) 5-HT_7Egran1, (H) 5-HT_7Egran2 and (I) 5-HT_7Mco1.

Fig 6. Protoscoleces of Echinococcus granulosus stained with the fluorescent probe UCM120 under the confocal scanning laser microscope and effect of the parent compound UCM2550 on the motility.

- Protoscoleces were fixed, after washing they were incubated in the presence of the probe (100 μM) during several days, washed, fixed, washed again, mounted, and then observed by confocal microscopy. (A) Structure of the fluorescent probe UCM120 with the structure of the parent compound UCM2550 shown in black and the dansyl group shown in light brown. Images of two protoscoleces obtained from superficial (B) or from an entire stack of laser scanning (C). To assess specificity, protoscoleces were labeled under the same conditions with the probe (100 μM) in the presence of an excess (1000 μM) of 5-HT and the image projection was obtained from an entire stack of pictures (D). Relative motility observed in the presence of increasing concentrations of the parent compound UCM2550 in the WMicrotracker device measured after two hours of incubation (E); Image based motility quantification measured as a pixel change (F). Further details are provided in the Methods section. Asterisks indicate treatments found to be significantly different from the controls (*P ≤ 0.05) with t-test (E) or ANOVA and Dunnet post comparison tests (****P ≤ 0.0001, F). R: rostellum; S: suckers and Bo: body. The scale bar represents 50 μm in (A), (B) and (C) and 100 μm in (D). Arrowheads indicate localization of the probe.