Bioactivity of Farnesyltransferase Inhibitors Against Entamoeba histolytica and Schistosoma mansoni

The protozoan parasite Entamoeba histolytica can induce amebic colitis and amebic liver abscess. First-line drugs for the treatment of amebiasis are nitroimidazoles, particularly metronidazole. Metronidazole has side effects and potential drug resistance is a concern. Schistosomiasis, a chronic and painful infection, is caused by various species of the Schistosoma flatworm. There is only one partially effective drug, praziquantel, a worrisome situation should drug resistance emerge. As many essential metabolic pathways and enzymes are shared between eukaryotic organisms, it is possible to conceive of small molecule interventions that target more than one organism or target, particularly when chemical matter is already available. Farnesyltransferase (FT), the last common enzyme for products derived from the mevalonate pathway, is vital for diverse functions, including cell differentiation and growth. Both E. histolytica and Schistosoma mansoni genomes encode FT genes. In this study, we phenotypically screened E. histolytica and S. mansoni in vitro with the established FT inhibitors, lonafarnib and tipifarnib, and with 125 tipifarnib analogs previously screened against both the whole organism and/or the FT of Trypanosoma brucei and Trypanosoma cruzi. For E. histolytica, we also explored whether synergy arises by combining lonafarnib and metronidazole or lonafarnib with statins that modulate protein prenylation. We demonstrate the anti-amebic and anti-schistosomal activities of lonafarnib and tipifarnib, and identify 17 tipifarnib analogs with more than 75% growth inhibition at 50 μM against E. histolytica. Apart from five analogs of tipifarnib exhibiting activity against both E. histolytica and S. mansoni, 10 additional analogs demonstrated anti-schistosomal activity (severe degenerative changes at 10 μM after 24 h). Analysis of the structure-activity relationship available for the T. brucei FT suggests that FT may not be the relevant target in E. histolytica and S. mansoni. For E. histolytica, combination of metronidazole and lonafarnib resulted in synergism for growth inhibition. Also, of a number of statins tested, simvastatin exhibited moderate anti-amebic activity which, when combined with lonafarnib, resulted in slight synergism. Even in the absence of a definitive molecular target, identification of potent anti-parasitic tipifarnib analogs encourages further exploration while the synergistic combination of metronidazole and lonafarnib offers a promising treatment strategy for amebiasis

Anti-schistosomal activities of quinoxaline-containing compounds:From hit identification to lead optimisation

Schistosomiasis is a neglected disease of poverty that is caused by infection with blood fluke species contained within the genus Schistosoma. For the last 40 years, control of schistosomiasis in endemic regions has predominantly been facilitated by administration of a single drug, praziquantel. Due to limitations in this mono-chemotherapeutic approach for sustaining schistosomiasis control into the future, alternative anti-schistosomal compounds are increasingly being sought by the drug discovery community. Herein, we describe a multi-pronged, integrated strategy that led to the identification and further exploration of the quinoxaline core as a promising anti-schistosomal scaffold

A Novel G Protein-Coupled Receptor of Schistosoma mansoni (SmGPR-3) Is Activated by Dopamine and Is Widely Expressed in the Nervous System

Schistosomes have a well developed nervous system that coordinates virtually every activity of the parasite and therefore is considered to be a promising target for chemotherapeutic intervention. Neurotransmitter receptors, in particular those involved in neuromuscular control, are proven drug targets in other helminths but very few of these receptors have been identified in schistosomes and little is known about their roles in the biology of the worm. Here we describe a novel Schistosoma mansoni G protein-coupled receptor (named SmGPR-3) that was cloned, expressed heterologously and shown to be activated by dopamine, a well established neurotransmitter of the schistosome nervous system. SmGPR-3 belongs to a new clade of “orphan” amine-like receptors that exist in schistosomes but not the mammalian host. Further analysis of the recombinant protein showed that SmGPR-3 can also be activated by other catecholamines, including the dopamine metabolite, epinine, and it has an unusual antagonist profile when compared to mammalian receptors. Confocal immunofluorescence experiments using a specific peptide antibody showed that SmGPR-3 is abundantly expressed in the nervous system of schistosomes, particularly in the main nerve cords and the peripheral innervation of the body wall muscles. In addition, we show that dopamine, epinine and other dopaminergic agents have strong effects on the motility of larval schistosomes in culture. Together, the results suggest that SmGPR-3 is an important neuronal receptor and is probably involved in the control of motor activity in schistosomes. We have conducted a first analysis of the structure of SmGPR-3 by means of homology modeling and virtual ligand-docking simulations. This investigation has identified potentially important differences between SmGPR-3 and host dopamine receptors that could be exploited to develop new, parasite-selective anti-schistosomal drugs

Functional expression of the <i>Schistosoma mansoni</i> SmGPR-3 receptor in yeast.

<p>(A) The full-length SmGPR-3 cDNA was expressed in <i>Saccharomyces cerevisae</i> strain YEX108 and grown in selective leu/histidine-deficient (leu⁻/his⁻) medium containing 2×10⁻⁴ M of each biogenic amine or vehicle (no drug control, ND). Yeast cells transformed with empty plasmid were used as a negative control (mock). Receptor activation was quantified from measurements of yeast growth in relative fluorescence units (RFU), using an Alamar blue fluorescence assay. The results are the means ± S.E.M. of 5–6 independent clones, each assayed in triplicate. The following biogenic amines were tested: adrenaline (A), noradrenaline (NA), dopamine (DA), epinine (EPN), serotonin (5-hydroxytryptamine, 5HT), octopamine (OA), tyramine (TA) and histamine (HA). (B) Functional assays were repeated with the same SmGPR-3-expressing yeast strain and variable concentrations of DA (△) or EPN (□). The mock control was tested with DA (•). EC₅₀ values for DA and EPN are 3.10×10⁻⁵ M and 2.85×10⁻⁵ M, respectively. The data are the means ± S.E.M. of two experiments, each in triplicate.</p

Dendogram analysis of biogenic amine (BA) G protein-coupled receptors (GPCR).

<p>A rooted phylogenetic tree was constructed from a ClustalW sequence alignment of vertebrate and invertebrate BA receptors, using MEGA 4 <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001523#pntd.0001523-Tamura1" target="_blank">[35]</a>. Included in the alignment are 15 predicted <i>Schistosoma mansoni</i> and <i>S. japonicum</i> BA GPCR sequences, of which nine clustered together into a separate clade (SmGPR). The receptor described in this paper, SmGPR-3 is identified by an open square (□). Other <i>S. mansoni</i> receptors are marked with solid squares (▪) and <i>S. japonicum</i> receptors are marked with solid triangles (▴). Sequences are identified by their accession numbers and the species names are abbreviated as follows: A.e. (<i>Aedes aegypti</i>), A.i. (<i>Agrotis ipsilon</i>), A.m. (<i>Apis mellifera</i>), B.m. (<i>Bombyx mori</i>), B.t. (<i>Bos taurus</i>), C.e. (<i>Caenorhabditis elegans</i>), C.f. (<i>Canis familiaris</i>), C.p. (<i>Cavia porcellus</i>), D.m. (<i>Drosophila melanogaster</i>), D.j. (<i>Dugesia japonica</i>), D.r. (<i>Danio rerio</i>), H.s. (<i>Homo sapiens</i>), H.v. (<i>Heliothis virescens</i>), M.b. (<i>Mamestra brassicae</i>), M.m. (<i>Mus musculus</i>), M.mul. (<i>Macaca mulatta</i>), P.a. (<i>Periplaneta americana</i>), P.x. (<i>Papilio xuthus</i>), R.n. (<i>Rattus norvegicus</i>), S.j. (<i>S. japonicum</i>), S.med. (<i>Schmidtea mediterranea</i>), S.l. (<i>Spodoptera littoralis</i>) and S.s. (<i>Sus scrofa</i>). Predicted <i>S. mansoni</i> coding sequences are identified by their “Smp” designation obtained from the <i>S. mansoni</i> Genome database (<i>S. mansoni</i> GeneDB) and the corresponding GenBank Accession number. H1–H4, histamine type 1–4 receptors; D1–D5, dopamine type 1–5 receptors; A, adrenergic receptors; 5HT, serotonin (5-hydroxytryptamine) receptors; mACh, muscarinic acetylcholine receptors; OA/TA, octopamine/tyramine receptors.</p

Sequence alignment of dopaminergic G protein-coupled receptors with <i>Schistosoma mansoni</i> SmGPR receptors.

<p>A ClustalW alignment was performed using representative examples of vertebrate dopaminergic GPCRs (D1–D5), the <i>S. mansoni</i> dopamine D2-like receptor (SmD2) and several members of the SmGPR clade. SmGPR sequences are boxed (horizontal box) and SmGPR-3 is marked by an arrow. Receptor sequences are identified by their accession numbers (brackets). The positions of the predicted seven transmembrane domains are marked by horizontal lines and the invariant residue in each TM segment <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001523#pntd.0001523-Ballesteros1" target="_blank">[37]</a> is identified by an asterisk (*) Other conserved residues of functional relevance are marked by circles (•) and conserved motifs are boxed (vertical boxes). Residues discussed in this study, R^2.64 (Arg96), D^3.32 (Asp117), S^5.42 (Ser198), T^7.39 (Thr462) and Y^7.43 (Tyr466) are identified by vertical arrows.</p

Antagonist effects on SmGPR-3 activity.

<p>(A) Yeast YEX108 auxotrophic <i>his</i> strain expressing SmGPR-3 was incubated with agonist (DA, 100 µM) and test antagonist or vehicle. Antagonists were tested at 100 µM except for flupenthixol, which was used at 10 µM. The data were normalized relative to the control sample that contained 100 µM DA but no antagonist. To test for drug induced toxicity, assays were repeated in the presence of 100 µM test antagonist in histidine-supplemented (<i>his+</i>) medium, which enables the cell to grow irrespective of receptor activation (His +ve control; see text for details). Abbreviations are as follows: SPIP, spiperone; PROP, propanolol; CLZP, clozapine; BUSP, buspirone; MINS, mianserin; CPRH, cyproheptadine; FLPX, flupenthixol; PRMZ, promethazine; HLRD, haloperidol. B–F. Dose-dependent inhibiton by haloperidol (IC₅₀ = 1.4 µM), flupenthixol (IC₅₀ = 3.9 µM), promethazine (IC₅₀ = 28.0 µM), mianserin (IC₅₀ = 45.0 µM) clozapine (IC₅₀>100 µM). The error bars are the means ± SEM for 3–4 experiments and at least 2 clones (in triplicates).</p

Immunolocalization of SmGPR-3 in larval <i>Schistosoma mansoni</i>.

<p><i>S. mansoni</i> cercaria were probed with affinity purified anti-SmGPR-3 antibody, followed by fluorescein isothiocyanate (FITC)-labelled secondary antibody. (A) Immunoreactivity (green) can be seen along the major longitudinal nerve cords (solid arrowheads) and in transverse commissures (open arrowhead), including the posterior transverse commissure near the base of the tail (open arrow). (B) No significant immunoreactivity was observed in negative controls probed with anti-SmGPR-3 antibody that was pre-adsorbed with peptide antigens or (C) controls probed with secondary antibody only. (*) non-specific labelling.</p

Effects of dopamine and related substances on schistosome motility.

<p>(A) <i>In vitro</i> transformed 3-day-old schistosomula were incubated with test drug, dopamine (DA) or epinine (EPN), each at (10⁻⁴ M) or vehicle (CT, control). Animals were treated for 5 min at room temperature, after which they were examined with a compound microscope equipped with a digital video camera and SimplePCI (Compix Inc.) for image acquisition. Images were recorded for 1 minute (∼3 frames/second) and an estimate of body length in µm was obtained for each animal in every frame. Each tracing shown is of an individual animal and is representative of 12–15 larvae per experiment and 3–4 independent experiments per treatment. (B) Experiments were repeated with various concentrations of test agonist in a range of 10⁻⁷ M–10⁻⁴ M, or in the absence of test substance (CT, control). Images were recorded as above and body length was measured for each frame. Motility is defined as the frequency of length changes (shortening and elongation) per minute of observation, as described in the Methods. The data are presented as the means and SEM of three separate experiments each with 12–15 animals. (C) Schistosomula were treated with test substances at a single concentration or in the absence of drug (CT, control) and motility was measured as above. Dopamine (DA), epinine (EPN), flupenthixol (FLPX), promethazine (PRMZ) were each tested at 50 µM. The remaining substances, adenaline (A), metanephrine (MTN) and haloperidol (HLRD) were tested at 500 µM. The data are the means and SEM of three separate experiments each with 12–15 animals. * Significantly different from the no drug control at P<0.05.</p