In vitro treatment of Besnoitia besnoiti with the naphto-quinone buparvaquone results in marked inhibition of tachyzoite proliferation, mitochondrial alterations and rapid adaptation of tachyzoites to increased drug concentrations

A Theileria annulata parasite with a single mutation, methionine 128 to isoleucine (M128I), in cytochrome B is resistant to buparvaquone

Tropical theileriosis is a fatal leukemic-like disease of cattle caused by the tick-transmitted protozoan parasite Theileria annulata. The economics of cattle meat and milk production is severely affected by theileriosis in endemic areas. The hydroxynaphtoquinone buparvaquone (BPQ) is the only available drug currently used to treat clinical theileriosis, whilst BPQ resistance is emerging and spreading in endemic areas. Here, we chronically exposed T. annulata-transformed macrophages in vitro to BPQ and monitored the emergence of drug-resistant parasites. Surviving parasites revealed a significant increase in BPQ IC50 compared to the wild type parasites. Drug resistant parasites from two independent cloned lines had an identical single mutation, M128I, in the gene coding for T. annulata cytochrome B (Tacytb). This in vitro generated mutation has not been reported in resistant field isolates previously, but is reminiscent of the methionine to isoleucine mutation in atovaquone-resistant Plasmodium and Babesia. The M128I mutation did not appear to exert any deleterious effect on parasite fitness (proliferation and differentiation to merozoites). To gain insight into whether drug-resistance could have resulted from altered drug binding to TaCytB we generated in silico a 3D-model of wild type TaCytB and docked BPQ to the predicted 3D-structure. Potential binding sites cluster in four areas of the protein structure including the Q01 site. The bound drug in the Q01 site is expected to pack against an alpha helix, which included M128, suggesting that the change in amino acid in this position may alter drug-binding. The in vitro generated BPQ resistant T. annulata is a useful tool to determine the contribution of the various predicted docking sites to BPQ resistance and will also allow testing novel drugs against theileriosis for their potential to overcome BPQ resistance.

In vitro screening technologies for the discovery and development of novel drugs against Toxoplasma gondii

INTRODUCTION

Toxoplasmosis constitutes a challenge for public health, animal production and welfare. Since more than 60 years, only a limited panel of drugs has been in use for clinical applications.

AREAS COVERED

Herein, the authors describe the methodology and the results of library screening approaches to identify inhibitors of Toxoplasma gondii and related strains. The authors then provide the reader with their expert perspectives for the future.

EXPERT OPINION

Various library screening projects, in particular those using reporter strains, have led to the identification of numerous compounds with good efficacy and specificity in vitro. However, only few compounds are effective in suitable animal models such as rodents. Whereas no novel compound has cleared the hurdle to applications in humans, the few compounds with known indication and application profiles in human patients are of interest for further investigations. Taken together, drug repurposing as well as high-throughput screening of novel compound libraries may shorten the way to novel drugs against toxoplasmosis.

Toxoplasma gondii infection: novel emerging therapeutic targets

INTRODUCTION

Toxoplasmosis constitutes a challenge for public health, animal production, and welfare. So far, only a limited panel of drugs has been marketed for clinical applications. In addition to classical screening, the investigation of unique targets of the parasite may lead to the identification of novel drugs.

AREAS COVERED

Herein, the authors describe the methodology to identify novel drug targets in Toxoplasma gondii and review the literature with a focus on the last two decades.

EXPERT OPINION

Over the last two decades, the investigation of essential proteins of T. gondii as potential drug targets has fostered the hope of identifying novel compounds for the treatment of toxoplasmosis. Despite good efficacies in vitro, only a few classes of these compounds are effective in suitable rodent models, and none has cleared the hurdle to applications in humans. This shows that target-based drug discovery is in no way better than classical screening approaches. In both cases, off-target effects and adverse side effects in the hosts must be considered. Proteomics-driven analyses of parasite- and host-derived proteins that physically bind drug candidates may constitute a suitable tool to characterize drug targets, irrespectively of the drug discovery methods.

In Vitro versus in Mice: Efficacy and Safety of Decoquinate and Quinoline-O-Carbamate Derivatives against Experimental Infection with Neospora caninum Tachyzoites

The effects of decoquinate (DCQ) and three O-quinoline-carbamate-derivatives were investigated using human foreskin fibroblasts (HFF) infected with Neospora caninum tachyzoites. These compounds exhibited half-maximal proliferation inhibition (IC50s) from 1.7 (RMB060) to 60 nM (RMB055). Conversely, when applied at 5 (DCQ, RMB054) or 10µM (RMB055, RMB060), HFF viability was not affected. Treatments of infected cell cultures at 0.5µM altered the ultrastructure of the parasite mitochondrion and cytoplasm within 24 h, most pronounced for RMB060, and DCQ, RMB054 and RMB060 did not impair the viability of splenocytes from naïve mice. Long-term treatments of N. caninum-infected HFF monolayers with 0.5µM of each compound showed that only exposure to RMB060 over a period of six consecutive days had a parasiticidal effect, while the other compounds were not able to kill all tachyzoites in vitro. Thus, DCQ and RMB060 were comparatively assessed in the pregnant neosporosis mouse model. The oral application of these compounds suspended in corn oil at 10 mg/kg/day for 5 d resulted in a decreased fertility rate and litter size in the DCQ group, whereas reproductive parameters were not altered by RMB060 treatment. However, both compounds failed to protect mice from cerebral infection and did not prevent vertical transmission/pup mortality. Thus, despite the promising in vitro efficacy and safety characteristics of DCQ and DCQ-derivatives, proof of concept for activity against neosporosis could not be demonstrated in the murine model.

Proteomic characterization of Toxoplasma gondii ME49 derived strains resistant to the artemisinin derivatives artemiside and artemisone implies potential mode of action independent of ROS formation

The sesquiterpene lactone artemisinin and its amino-artemisinin derivatives artemiside (GC008) and artemisone (GC003) are potent antimalarials. The mode of action of artemisinins against Plasmodium sp is popularly ascribed to 'activation' of the peroxide group by heme-Fe(II) or labile Fe(II) to generate C-radicals that alkylate parasite proteins. An alternative postulate is that artemisinins elicit formation of reactive oxygen species by interfering with flavin disulfide reductases resposible for maintaining intraparasitic redox homeostasis. However, in contradistinction to the heme-activation mechanism, the amino-artemisinins are effective in vitro against non-heme-degrading apicomplexan parasites including T. gondii, with IC ₅₀ values of 50-70 nM, and induce distinct ultrastructural alterations. However, T. gondii strains readily adapted to increased concentrations (2.5 μM) of these two compounds within few days. Thus, T. gondii strains that were resistant against artemisone and artemiside were generated by treating the T. gondii reference strain ME49 with stepwise increasing amounts of these compounds, yielding the artemisone resistant strain GC003^R and the artemiside resistant strain GC008^R. Differential analyses of the proteomes of these resistant strains compared to the wildtype ME49 revealed that 215 proteins were significantly downregulated in artemisone resistant tachyzoites and only 8 proteins in artemiside resistant tachyzoites as compared to their wildtype. Two proteins, namely a hypothetical protein encoded by ORF TGME49_236950, and the rhoptry neck protein RON2 encoded by ORF TGME49_300100 were downregulated in both resistant strains. Interestingly, eight proteins involved in ROS scavenging including catalase and superoxide dismutase were amongst the differentially downregulated proteins in the artemisone-resistant strain. In parallel, ROS formation was significantly enhanced in isolated tachyzoites from the artemisone resistant strain and - to a lesser extent - in tachyzoites from the artemiside resistant strain as compared to wildtype tachyzoites. These findings suggest that amino-artemisinin derivatives display a mechanism of action in T. gondii distinct from Plasmodium.

Assessment of the Activity of Decoquinate and Its Quinoline-O-Carbamate Derivatives against Toxoplasma gondii In Vitro and in Pregnant Mice Infected with T. gondii Oocysts

The quinolone decoquinate (DCQ) is widely used in veterinary practice for the treatment of bacterial and parasitic infections, most notably, coccidiosis in poultry and in ruminants. We have investigated the effects of treatment of Toxoplasma gondii in infected human foreskin fibroblasts (HFF) with DCQ. This induced distinct alterations in the parasite mitochondrion within 24 h, which persisted even after long-term (500 nM, 52 days) treatment, although there was no parasiticidal effect. Based on the low half-maximal effective concentration (IC₅₀) of 1.1 nM and the high selectivity index of >5000, the efficacy of oral treatment of pregnant mice experimentally infected with T. gondii oocysts with DCQ at 10 mg/kg/day for 5 days was assessed. However, the treatment had detrimental effects, induced higher neonatal mortality than T. gondii infection alone, and did not prevent vertical transmission. Thus, three quinoline-O-carbamate derivatives of DCQ, anticipated to have better physicochemical properties than DCQ, were assessed in vitro. One such compound, RMB060, displayed an exceedingly low IC₅₀ of 0.07 nM, when applied concomitantly with the infection of host cells and had no impact on HFF viability at 10 µM. As was the case for DCQ, RMB060 treatment resulted in the alteration of the mitochondrial matrix and loss of cristae, but the changes became apparent at just 6 h after the commencement of treatment. After 48 h, RMB060 induced the expression of the bradyzoite antigen BAG1, but TEM did not reveal any other features reminiscent of bradyzoites. The exposure of infected cultures to 300 nM RMB060 for 52 days did not result in the complete killing of all tachyzoites, although mitochondria remained ultrastructurally damaged and there was a slower proliferation rate. The treatment of mice infected with T. gondii oocysts with RMB060 did reduce parasite burden in non-pregnant mice and dams, but vertical transmission to pups could not be prevented.

Transfection With Plasmid Causing Stable Expression of a Foreign Gene Affects General Proteome Pattern in Giardia lamblia Trophozoites

Giardia lamblia is an important causative agent of persistent diarrhea in humans, domestic animals, and cattle. Basic research is usually performed with the strain WBC6 and includes genetic manipulations such as transfections. Here, we investigate how transfection with a plasmid causing stable expression of a foreign gene affects the whole proteome pattern. Using shotgun mass spectrometry, we compare the proteomes of untransfected trophozoites to trophozoites transfected with Escherichia coli glucuronidase A (GusA). Besides GusA, which is detected in the transfected trophozoites only, the proteomes of untransfected and transfected trophozoites differ by 132 differentially expressed proteins. In particular, transfection induces antigenic variation. Since transfection causing stable expression affects the proteome pattern, transfection experiments should take into account this effect. Due to a unique peptide panel, GusA is an example for a suitable internal standard for experiments involving transfected cells. Data are available via ProteomeXchange with identifier PXD022565.

Isolation of Human Small Extracellular Vesicles and Tracking of their Uptake by Retinal Pigment Epithelial Cells In Vitro

Small extracellular vesicles (EVs) are among the most frequently investigated EVs and play major roles in intercellular communication by delivering various cargo molecules to target cells. They could potentially represent an alternative delivery strategy to treat ocular toxoplasmosis, a parasitosis affecting the retinal pigment epithelium (RPE). To date, the uptake of human small EVs by RPE cells has never been reported. In this study, we report on the intracellular uptake of fluorescently labelled human urine and fibroblast-derived small EVs by human RPE cells. In summary, both dye-labelled urinary small EVs and small EVs obtained from fibroblasts stably expressing membrane-bound green fluorescent protein were successfully internalized by RPE cells as revealed by immunohistochemistry. In recipient ARPE19 cells, BODIPY-labelled small EVs were found in close vicinity to the parasite Toxoplasma gondii. Additionally, an ultrastructural method was enabled to distinguish between labelled exogenous and endogenous small EVs within target cells.

Activities of Endochin-Like Quinolones Against in vitro Cultured Besnoitia besnoiti Tachyzoites

Endochin-like quinolones (ELQs) potently inhibit the proliferation of Plasmodium, Toxoplasma, Neospora, and Babesia by targeting the cytochrome b Qo and Qi sites and interfering with oxidative phosphorylation and pyrimidine biosynthesis. The activities of 14 different ELQs were assessed against B. besnoiti tachyzoites grown in human foreskin fibroblasts (HFF) by quantitative real time PCR. The values for 50% proliferation inhibition (IC50) of five ELQs were determined in a 3-days growth assay after an initial screen of 12 ELQs at 0.01, 0.1, and 1 μM. The IC50s of ELQ-121, -136, and -316 were 0.49, 2.36, and 7.97 nM, respectively. The IC50s of ELQs tested against B. besnoiti were higher than IC50s previously observed for P. falciparum and T. gondii. However, the B. besnoiti cytochrome b sequence and the predicted Qo and Qi ELQ binding sites in the Toxoplasma, Neospora, and Besnoitia cytochrome b are virtually identical, suggesting that the differences in ELQ susceptibility are not due to variations in the substrate binding sites. TEM of ELQ-treated parasites primarily demonstrated alterations within the parasite mitochondrion, profound thickening of the nuclear membrane, as well as increased vacuolization within the tachyzoite cytoplasm. Long-term treatment assays of intracellular B. besnoiti with ELQs for up to 20 days followed by the release of drug pressure caused a substantial delay in parasite growth and proliferation while ELQs were present, but parasite proliferation resumed days after ELQs were removed. Interestingly, structural alterations persisted after ELQ removal and parasite proliferation was slowed. These findings provide a basis for further in vivo studies of ELQs as therapeutic options against B. besnoiti infection.

A model for chronic bovine besnoitiosis: Parasite stage and inoculation route are key factors

In this work, an experimental model for chronic besnoitiosis in bovine was developed and characterized. Using a previously established calf model, two new variables (parasite stage and inoculation route) were combined and used. Twelve Holstein Friesian 3-month-old male calves were randomly divided into four groups of three animals each. Bradyzoites were obtained from a chronically infected bull and used for inoculation via three different inoculation routes. Three groups were inoculated with 10⁶ bradyzoites by intravenous (G1), subcutaneous (G2) and intradermal (G3) routes, and a non-infected control group (G4) was inoculated with PBS. The trial lasted for 90 days and included daily clinical monitoring as well as weekly skin biopsies and blood sampling. Sera were obtained to analyse both cellular and humoral responses. Once the calves were euthanized, tissues from the skin, eyes, respiratory and reproductive tracts, among others, were collected to study presence of the parasite. Clinically, the infection was classified as mild to moderate for the acute stage since all infected calves showed lymphadenopathy from four days post-infection (pi) and fever from one week pi until 24 days pi. However, the most relevant results were achieved during the chronic stage that was classified as moderate to severe. In fact, pathognomonic conjunctival cysts were observed in all infected calves from 40 days pi onwards and were more abundant in G3. Moreover, one calf from this group developed skin lesions (49 days pi). The microscopic tissue cysts and Besnoitia DNA were detected primarily in skin, reproductive tract and respiratory tissue samples, and parasite load was higher in G3. In conclusion, the parasite stage (bradyzoite) and the inoculation route are key factors that influence the outcome of an infection. In particular, the intradermal route led to more severe clinical signs of the chronic phase in the inoculated calves.

Antiparasitic Efficacy of Curcumin Against Besnoitia besnoiti Tachyzoites in vitro

Besnoitia besnoiti is the causative agent of bovine besnoitiosis. B. besnoiti infections lead to reduced fertility and productivity in cattle causing high economic losses, not only in Europe, but also in Asia and Africa. Mild to severe clinical signs, such as anasarca, oedema, orchitis, hyperkeratosis, and characteristic skin and mucosal cysts, are due to B. besnoiti tachyzoite and bradyzoite replication in intermediate host tissues. So far, there are no commercially available effective drugs against this parasite. Curcumin, a polyphenolic compound from Curcuma longa rhizome is well-known for its antioxidant, anti-inflammatory, immunomodulatory and also anti-protozoan effects. Hence, the objective of this study was to evaluate the effects of curcumin on viability, motility, invasive capacity, and proliferation of B. besnoiti tachyzoites replicating in primary bovine umbilical vein endothelial cells (BUVEC) in vitro. Functional inhibition assays revealed that curcumin treatments reduce tachyzoite viability and induce lethal effects in up to 57% of tachyzoites (IC50 in 5.93 μM). Referring to general motility, significant dose-dependent effects of curcumin treatments were observed. Interestingly, curcumin treatments only dampened helical gliding and twirling activities whilst longitudinal gliding motility was not significantly affected. In addition, curcumin pretreatments of tachyzoites resulted in a dose-dependent reduction of host cell invasion as detected by infections rates at 1 day p. i. These findings demonstrate feeding cattle with Curcuma longa rhizomes may represent a new strategy for besnoitiosis treatment.

Endochin-Like Quinolones Exhibit Promising Efficacy Against Neospora Caninum in vitro and in Experimentally Infected Pregnant Mice

We report on the efficacy of selected endochin-like quinolones (ELQs) against N. caninum tachyzoites grown in human foreskin fibroblasts (HFF), and in a pregnant BALB/c mouse model. Fourteen ELQs were screened against transgenic N. caninum tachyzoites expressing β-galactosidase (Nc-βgal). Drugs were added concomitantly to infection and the values for 50% proliferation inhibition (IC₅₀) were determined after 3 days. Three compounds exhibited IC₅₀ values below 0.1 nM, 3 ELQs had IC₅₀s between 0.1 and 1 nM, for 7 compounds values between 1 and 10 nM were noted, and one compound had an IC₅₀ of 22.4 nM. Two compounds, namely ELQ-316 and its prodrug ELQ-334 with IC₅₀s of 0.66 and 3.33 nM, respectively, were previously shown to display promising activities against experimental toxoplasmosis and babesiosis caused by Babesia microti in mice, and were thus further studied. They were assessed in long-term treatment assays by exposure of infected HFF to ELQs at 0.5 μM concentration, starting 3 h after infection and lasting for up to 17 days followed by release of drug pressure. Results showed that the compounds substantially delayed parasite proliferation, but did not exert parasiticidal activities. TEM of drug treated parasites detected distinct alterations within the parasite mitochondria, but not in other parasite organelles. Assessment of safety of ELQ-334 in the pregnant mouse model showed that the compound did not interfere in fertility or pregnancy outcome. In N. caninum infected pregnant mice treated with ELQ-334 at 10 mg/kg/day for 5 days, neonatal mortality (within 2 days post partum) was found in 7 of 44 pups (15.9%), but no postnatal mortality was noted, and vertical transmission was reduced by 49% compared to the placebo group, which exhibited 100% vertical transmission, neonatal mortality in 15 of 34 pups (44%), and postnatal mortality for 18 of the residual 19 pups during the 4 weeks follow-up. These findings encourage more research on the use of ELQs for therapeutic options against N. caninum infection.