Current developments in the therapy of protozoan infections

Long non-coding RNAs as possible therapeutic targets in protozoa, and in Schistosoma and other helminths

Long non-coding RNAs (lncRNAs) emerged in the past 20 years due to massive amounts of scientific data regarding transcriptomic analyses. They have been implicated in a plethora of cellular processes in higher eukaryotes. However, little is known about lncRNA possible involvement in parasitic diseases, with most studies only detecting their presence in parasites of human medical importance. Here, we review the progress on lncRNA studies and their functions in protozoans and helminths. In addition, we show an example of knockdown of one lncRNA in Schistosoma mansoni, SmLINC156349, which led to in vitro parasite adhesion, motility, and pairing impairment, with a 20% decrease in parasite viability and 33% reduction in female oviposition. Other observed phenotypes were a decrease in the proliferation rate of both male and female worms and their gonads, and reduced female lipid and vitelline droplets that are markers for well-developed vitellaria. Impairment of female worms' vitellaria in SmLINC156349-silenced worms led to egg development deficiency. All those results demonstrate the great potential of the tools and methods to characterize lncRNAs as potential new therapeutic targets. Further, we discuss the challenges and limitations of current methods for studying lncRNAs in parasites and possible solutions to overcome them, and we highlight the future directions of this exciting field.

Conus venom fractions inhibit the adhesion of Plasmodium falciparum erythrocyte membrane protein 1 domains to the host vascular receptors

Using high-throughput BioPlex assays, we determined that six fractions from the venom of Conus nux inhibit the adhesion of various recombinant PfEMP-1 protein domains (PF08_0106 CIDR1α3.1, PF11_0521 DBL2β3, and PFL0030c DBL3X and DBL5e) to their corresponding receptors (CD36, ICAM-1, and CSA, respectively). The protein domain-receptor interactions permit P. falciparum-infected erythrocytes (IE) to evade elimination in the spleen by adhering to the microvasculature in various organs including the placenta. The sequences for the main components of the fractions, determined by tandem mass spectrometry, yielded four T-superfamily conotoxins, one (CC-Loop-CC) with I-IV, II-III connectivity and three (CC-Loop-CX_aaC) with a I-III, II-IV connectivity. The 3D structure for one of the latter, NuxVA = GCCPAPLTCHCVIY, revealed a novel scaffold defined by double turns forming a hairpin-like structure stabilized by the two disulfide bonds. Two other main fraction components were a miniM conotoxin, and a O2-superfamily conotoxin with cysteine framework VI/VII. This study is the first one of its kind suggesting the use of conotoxins for developing pharmacological tools for anti-adhesion adjunct therapy against malaria. Similarly, mitigation of emerging diseases like AIDS and COVID-19, can also benefit from conotoxins as inhibitors of protein-protein interactions as treatment. BIOLOGICAL SIGNIFICANCE: Among the 850+ species of cone snail species there are hundreds of thousands of diverse venom exopeptides that have been selected throughout several million years of evolution to capture prey and deter predators. They do so by targeting several surface proteins present in target excitable cells. This immense biomolecular library of conopeptides can be explored for potential use as therapeutic leads against persistent and emerging diseases affecting non-excitable systems. We aim to expand the pharmacological reach of conotoxins/conopeptides by revealing their in vitro capacity to disrupt protein-protein and protein-polysaccharide interactions that directly contribute to pathology of Plasmodium falciparum malaria. This is significant for severe forms of malaria, which might be deadly even after treated with current parasite-killing drugs because of persistent cytoadhesion of P. falciparum infected erythrocytes even when parasites within red blood cells are dead. Anti-adhesion adjunct drugs would de-sequester or prevent additional sequestration of infected erythrocytes and may significantly improve survival of malaria patients. These results provide a lead for further investigations into conotoxins and other venom peptides as potential candidates for anti-adhesion or blockade-therapies. This study is the first of its kind and it suggests that conotoxins can be developed as pharmacological tools for anti-adhesion adjunct therapy against malaria. Similarly, mitigation of emerging diseases like AIDS and COVID-19, can also benefit from conotoxins as potential inhibitors of protein-protein interactions as treatment.

Alsinol, an arylamino alcohol derivative active against Plasmodium, Babesia, Trypanosoma, and Leishmania: past and new outcomes

Malaria, babesiosis, trypanosomosis, and leishmaniasis are some of the most life-threatening parasites, but the range of drugs to treat them is limited. An effective, safe, and low-cost drug with a large activity spectrum is urgently needed. For this purpose, an aryl amino alcohol derivative called Alsinol was resynthesized, screened in silico, and tested against Plasmodium, Babesia, Trypanosoma, and Leishmania. In silico Alsinol follows the Lipinski and Ghose rules. In vitro it had schizontocidal activity against Plasmodium falciparum and was able to inhibit gametocytogenesis; it was particularly active against late gametocytes. In malaria-infected mice, it showed a dose-dependent activity similar to chloroquine. It demonstrated a similar level of activity to reference compounds against Babesia divergens, and against promastigotes, and amastigotes stages of Leishmania in vitro. It inhibited the in vitro growth of two African animal strains of Trypanosoma but was ineffective in vivo in our experimental conditions. It showed moderate toxicity in J774A1 and Vero cell models. The study demonstrated that Alsinol has a large spectrum of activity and is potentially affordable to produce. Nevertheless, challenges remain in the process of scaling up synthesis, creating a suitable clinical formulation, and determining the safety margin in preclinical models.

Identification of Tight-Binding Plasmepsin II and Falcipain 2 Inhibitors in Aqueous Extracts of Marine Invertebrates by the Combination of Enzymatic and Interaction-Based Assays

Natural products from marine origin constitute a very promising and underexplored source of interesting compounds for modern biotechnological and pharmaceutical industries. However, their evaluation is quite challenging and requires specifically designed assays to reliably identify the compounds of interest in a highly heterogeneous and interfering context. In the present study, we describe a general strategy for the confident identification of tight-binding protease inhibitors in the aqueous extracts of 62 Cuban marine invertebrates, using Plasmodium falciparum hemoglobinases Plasmepsin II and Falcipain 2 as model enzymes. To this end, we first developed a screening strategy that combined enzymatic with interaction-based assays and then validated screening conditions using five reference extracts. Interferences were evaluated and minimized. The results from the massive screening of such extracts, the validation of several hits by a variety of interaction-based assays and the purification and functional characterization of PhPI, a multifunctional and reversible tight-binding inhibitor for Plasmepsin II and Falcipain 2 from the gorgonian Plexaura homomalla, are presented.

Antiprotozoal Nitazoxanide Derivatives: Synthesis, Bioassays and QSAR Study Combined with Docking for Mechanistic Insight

In view of the serious health problems concerning infectious diseases in heavily populated areas, we followed the strategy of lead compound diversification to evaluate the near-by chemical space for new organic compounds. To this end, twenty derivatives of nitazoxanide (NTZ) were synthesized and tested for activity against Entamoeba histolytica parasites. To ensure drug-likeliness and activity relatedness of the new compounds, the synthetic work was assisted by a quantitative structure-activity relationships study (QSAR). Many of the inherent downsides – well-known to QSAR practitioners – we circumvented thanks to workarounds which we proposed in prior QSAR publication. To gain further mechanistic insight on a molecular level, ligand-enzyme docking simulations were carried out since NTZ is known to inhibit the protozoal pyruvate ferredoxin oxidoreductase (PFOR) enzyme as its biomolecular target.

Finding of leishmanicidal activity of 14-hydroxylunularin in mice experimentally infected with Leishmania infantum

In this study, we report the in vivo efficacy of 14-hydroxylunularin evaluated in BALB/c mice experimentally infected with promastigotes of Leishmania infantum (syn L. chagasi), the major causative agent of visceral leishmaniasis in Latin America. Seven days post-infection, treatment with 14-hydroxylunularin started and it was administered by oral and subcutaneous routes in doses of 10 and 25 mg/kg of weight for ten days using Glucantime® as reference drug. In the liver, the evaluated compound showed parasite reduction above 90% by both administration routes being the oral route the most effective at both doses. Significant decreased numbers of parasites were also observed when the treated group was compared with the control group (p≤0.05). The subcutaneous route presented a remarkable difference with at least 80% parasite suppression in liver and spleen at 10 mg/kg dose and 90% in liver at 25 mg/kg. The leishmanicidal activity of 14-hydroxylunularin against L. infantum revealed by this study is another evidence in favor of this compound as a potential candidate for the development of a new oral treatment for leishmaniasis.

In vitro antigiardial activity of the cysteine protease inhibitor E-64

The quest for new antiparasitic alternatives has led researchers to base their studies on insights into biology, host-parasite interactions and pathogenesis. In this context, proteases and their inhibitors are focused, respectively, as druggable targets and new therapy alternatives. Herein, we proposed to evaluate the in vitro effect of the cysteine protease inhibitor E-64 on Giardia trophozoites growth, adherence and viability. Trophozoites (10⁵) were exposed to E-64 at different final concentrations, for 24, 48 and 72 h at 37 °C. In the growth and adherence assays, the number of trophozoites was estimated microscopically in a haemocytometer, whereas cell viability was evaluated by a dye-reduction assay using MTT. The E-64 inhibitor showed effect on growth, adherence and viability of trophozoites, however, its better performance was detected in the 100 µM-treated cultures. Although metronidazole was more effective, the E-64 was shown to be able to inhibit growth, adherence and viability rates by ≥ 50%. These results reveal that E-64 can interfere in some crucial processes to the parasite survival and they open perspectives for future investigations in order to confirm the real antigiardial potential of the protease inhibitors.

Development and validation of four Leishmania species constitutively expressing GFP protein. A model for drug discovery and disease pathogenesis studies

Green fluorescent protein (GFP)-parasite transfectants have been widely used as a tool for studying disease pathogenesis in several protozoan models and their application in drug screening assays has increased rapidly. In the past decade, the expression of GFP has been established in several Leishmania species, mostly for in vitro studies. The current work reports generation of four transgenic parasites constitutively expressing GFP (Leishmania mexicana, Leishmania aethiopica, Leishmania tropica and Leishmania major) and their validation as a representative model of infection. This is the first report where stable expression of GFP has been achieved in L. aethiopica and L. tropica. Integration of GFP was accomplished through homologous recombination of the expression construct, pRib1.2αNEOαGFP downstream of the 18S rRNA promoter in all species. A homogeneous and high level expression of GFP was detected in both the promastigote and the intracellular amastigote stages. All transgenic species showed the same growth pattern, ability to infect mammalian host cells and sensitivity to reference drugs as their wild type counterparts. All four transgenic Leishmania are confirmed as models for in vitro and possibly in vivo infections and represent an ideal tool for medium throughput testing of compound libraries.

Yeast-based automated high-throughput screens to identify anti-parasitic lead compounds

We have developed a robust, fully automated anti-parasitic drug-screening method that selects compounds specifically targeting parasite enzymes and not their host counterparts, thus allowing the early elimination of compounds with potential side effects. Our yeast system permits multiple parasite targets to be assayed in parallel owing to the strains’ expression of different fluorescent proteins. A strain expressing the human target is included in the multiplexed screen to exclude compounds that do not discriminate between host and parasite enzymes. This form of assay has the advantages of using known targets and not requiring the in vitro culture of parasites. We performed automated screens for inhibitors of parasite dihydrofolate reductases, N-myristoyltransferases and phosphoglycerate kinases, finding specific inhibitors of parasite targets. We found that our ‘hits’ have significant structural similarities to compounds with in vitro anti-parasitic activity, validating our screens and suggesting targets for hits identified in parasite-based assays. Finally, we demonstrate a 60 per cent success rate for our hit compounds in killing or severely inhibiting the growth of Trypanosoma brucei, the causative agent of African sleeping sickness.

Identification of Th1-responsive leishmanial excretory-secretory antigens (LESAs)

The objective of this study was to evaluate the immunomodulatory role of leishmanial excretory-secretory antigens (LESAs) released by in vitro cultured protozoan parasite Leishmania donovani promastigotes. A total of seventeen excretory-secretory proteins of relative molecular weights 11, 13, 16, 18, 21, 23, 26, 29, 33, 35, 42, 51, 54, 58, 64, 70 and 80 kDa were identified. The proteins were divided into five fractions (F1-F5) along with the whole LESAs, these fractions were evaluated for their potential antigenicity to induce macrophage effector functions, lymphoproliferation and cytokines production capabilities. Two fractions, F1 (11, 13 and 16 kDa) and F3 (26, 29 and 33 kDa), were found to be highly immunogenic as they significantly induced NADPH oxidase and SOD activities as well as NOx, TNF-α, IFN-γ and IL-12 production in stimulated RAW 264.7 macrophages. Further, these antigens also induced significant proliferation of human peripheral blood mononuclear cells along with increased production of IFN-γ and IL-12. The results strongly suggest the potential role of LESAs in the modulation of macrophage effector functions and Th1 immune response that gives a hope to develop potent vaccine for visceral leishmaniasis.

Microbial and fungal protease inhibitors--current and potential applications

Proteolytic enzymes play essential metabolic and regulatory functions in many biological processes and also offer a wide range of biotechnological applications. Because of their essential roles, their proteolytic activity needs to be tightly regulated. Therefore, small molecules and proteins that inhibit proteases can be versatile tools in the fields of medicine, agriculture and biotechnology. In medicine, protease inhibitors can be used as diagnostic or therapeutic agents for viral, bacterial, fungal and parasitic diseases as well as for treating cancer and immunological, neurodegenerative and cardiovascular diseases. They can be involved in crop protection against plant pathogens and herbivorous pests as well as against abiotic stress such as drought. Furthermore, protease inhibitors are indispensable in protein purification procedures to prevent undesired proteolysis during heterologous expression or protein extraction. They are also valuable tools for simple and effective purification of proteases, using affinity chromatography. Because there are such a large number and diversity of proteases in prokaryotes, yeasts, filamentous fungi and mushrooms, we can expect them to be a rich source of protease inhibitors as well.

Pathogen Genomics and the Potential for Understanding Diseases in the Developing World

Approximately 46% and 32% of deaths among children under five globally occur in sub-Saharan Africa and South Asia, respectively. Over 80% of the 4.2 million child deaths in Africa are caused by infectious diseases, sharply contrasted to Europe where 39% of the 0.15 million child deaths are attributable to infectious diseases (Fig. 5.1) (Black et al. 2010). Hence, despite the remarkable public health advancements in hygiene, sanitation, antimicrobial drugs and vaccine strategies of the twenty-first century, the burden of infectious diseases remains unacceptably high in the developing world.

4-Acetoxydolastane diterpene from the Brazilian brown alga Canistrocarpus cervicornis as antileishmanial agent

Natural marine products have shown an interesting array of diverse and novel chemical structures with potent biological activities. Our study reports the antiproliferative assays of crude extracts, fraction and pure compound (4R,9S,14S)-4α-acetoxy-9β,14α-dihydroxydolast-1(15),7-diene (1) obtained from brown alga Canistrocarpus cervicornis showing the antileishmanial activity. We showed that 1 had a dose-dependent activity during 72 h of treatment, exhibiting IC(50) of 2.0 μg/mL, 12.0 μg/mL, and 4.0 μg/mL for promastigote, axenic amastigote and intracellular amastigote forms of Leishmania amazonensis, respectively. A cytotoxicity assay showed that the action of the isolated compound 1 was 93.0 times less toxic to the macrophage than to the protozoan. Additionally, compound 1 induced ultrastructural changes, including extensive mitochondrial damage; decrease in Rh123 fluorescence, suggesting interference with the mitochondrial membrane potential; and lipid peroxidation in parasite cells. The use of 1 from C. cervicornis against L. amazonensis parasites might be of great interest as a future alternative to the development of new antileishmanial drugs.

Functional expression of parasite drug targets and their human orthologs in yeast

Background

The exacting nutritional requirements and complicated life cycles of parasites mean that they are not always amenable to high-throughput drug screening using automated procedures. Therefore, we have engineered the yeast Saccharomyces cerevisiae to act as a surrogate for expressing anti-parasitic targets from a range of biomedically important pathogens, to facilitate the rapid identification of new therapeutic agents.

Methodology/Principal Findings

Using pyrimethamine/dihydrofolate reductase (DHFR) as a model parasite drug/drug target system, we explore the potential of engineered yeast strains (expressing DHFR enzymes from Plasmodium falciparum, P. vivax, Homo sapiens, Schistosoma mansoni, Leishmania major, Trypanosoma brucei and T. cruzi) to exhibit appropriate differential sensitivity to pyrimethamine. Here, we demonstrate that yeast strains (lacking the major drug efflux pump, Pdr5p) expressing yeast (^ScDFR1), human (^HsDHFR), Schistosoma (^SmDHFR), and Trypanosoma (^TbDHFR and ^TcDHFR) DHFRs are insensitive to pyrimethamine treatment, whereas yeast strains producing Plasmodium (^PfDHFR and ^PvDHFR) DHFRs are hypersensitive. Reassuringly, yeast strains expressing field-verified, drug-resistant mutants of P. falciparum DHFR (^Pfdhfr^51I,59R,108N) are completely insensitive to pyrimethamine, further validating our approach to drug screening. We further show the versatility of the approach by replacing yeast essential genes with other potential drug targets, namely phosphoglycerate kinases (PGKs) and N-myristoyl transferases (NMTs).

Conclusions/Significance

We have generated a number of yeast strains that can be successfully harnessed for the rapid and selective identification of urgently needed anti-parasitic agents.

Parasites kill millions of people every year and leave countless others with chronic debilitating disease. These diseases, which include malaria and sleeping sickness, mainly affect people in developing countries. For this reason, few drugs have been developed to treat them. To make matters worse, many parasites are developing resistance to the drugs that are available. Thus, there is an urgent need to develop new drugs, but this is hampered by the fact that most parasites are difficult or impossible to grow in the laboratory. To address this, we have engineered baker's yeast to be dependent on the function of enzymes from either parasites or humans. In all, our engineered yeast constructs encompass six parasites (causing malaria, schistosomiasis, leishmaniasis, sleeping sickness, and Chagas disease) and three different enzymes that are known or potential drug targets. Further, we have increased yeast's sensitivity to drugs by deleting the gene for its major drug efflux pump. Because yeast is robust and easy to grow in the laboratory, we can use a robot to screen for drugs that will kill yeast dependent on a parasite enzyme, but not touch yeast dependent on the equivalent human enzyme.