Targeting essential pathways in trypanosomatids gives insights into protozoan mechanisms of cell death

An Aromatic Diamidine That Targets Kinetoplast DNA, Impairs the Cell Cycle in Trypanosoma cruzi, and Diminishes Trypomastigote Release from Infected Mammalian Host Cells

Trypanosoma cruzi is the etiological agent of Chagas disease, affecting approximately 10 million people in the Americas and with some 40 million people at risk. The objective of this study was to evaluate the anti-T. cruzi activity of three new diamidines that have a 3,4-ethylenedioxy extension of the thiophene core, designated MB17, MB19, and MB38. All three diamidines exhibited dose-dependent inhibition of epimastigote replication. The mechanisms of action of these diamidines were investigated. Unlike MB17 and MB19, MB38 exhibited a significant increase in the number of annexin-propidium iodide double-labeled cells compared to levels in control parasites. As MB17 had shown a lower 50% inhibitory concentration (IC50) against epimastigote growth, the mechanism of action of this drug was studied in more detail. MB17 triggered a decrease in the intracellular ATP levels. As a consequence, MB17 affected the genomic DNA and kinetoplast DNA (kDNA) and impaired the parasite cell cycle. Moreover, MB17 caused DNA fragmentation, with a more severe effect on kDNA than on nuclear DNA, resulting in dyskinetoplastic cells. MB17 was tested for toxicity and effectiveness for the treatment of infected CHO-K1 cells, exhibiting a 50% cytotoxic concentration (CC50) of 13.47 ± 0.37 μM and an IC50 of 0.14 ± 0.12 μM against trypomastigote release. MB17 also diminished the infection index by 60% at 0.5 μM. In conclusion, despite belonging to the same family, these diamidines have different efficiencies. To summarize, MB17 was the most potent of these diamidines against epimastigotes, producing DNA damage preferentially in kDNA, impairing the parasite cell cycle, and decreasing the infection index and trypomastigote release from infected mammalian host cells, with a high selectivity index (SI) (<90). These data suggest that MB17 could be an interesting lead compound against T. cruzi.

Flat oyster follows the apoptosis pathway to defend against the protozoan parasite Bonamia ostreae

The in vitro model Ostrea edulis hemocyte - Bonamia ostreae is interesting to investigate host-parasite interactions at the cellular level. Indeed, this unicellular parasite infects the flat oyster Ostrea edulis and multiplies within hemocytes, the central effectors of oyster defenses. Apoptosis is a mechanism used by many organisms to eliminate infected cells. In order to study the potential involvement of this mechanism in the oyster response to B. ostreae, in vitro experiments were carried out by exposing hemocytes from the naturally susceptible oyster O. edulis and a resistant oyster species Crassostrea gigas to live and heat-inactivated parasites. Hemocyte apoptotic response was measured using a combination of flow cytometry and microscopy analyses. Whatever the host species was, the parasite was engulfed in hemocytes and induced an increase of apoptotic parameters including intracytoplasmic calcium concentration, mitochondrial membrane potential or phosphatidyl-serine externalization as well as ultrastructural modifications. However, the parasite appears more able to infect flat oyster than cupped oyster hemocytes and the apoptotic response was more important against live than dead parasites in the natural host than in C. gigas. Our results suggest that O. edulis specifically responds to B. ostreae by inducing apoptosis of hemocytes.

Transcriptome and proteome analyses and the role of atypical calpain protein and autophagy in the spliced leader silencing pathway in Trypanosoma brucei

Under persistent ER stress, Trypanosoma brucei parasites induce the spliced leader silencing (SLS) pathway. In SLS, transcription of the SL RNA gene, the SL donor to all mRNAs, is extinguished, arresting trans-splicing and leading to programmed cell death (PCD). In this study, we investigated the transcriptome following silencing of SEC63, a factor essential for protein translocation across the ER membrane, and whose silencing induces SLS. The proteome of SEC63-silenced cells was analyzed with an emphasis on SLS-specific alterations in protein expression, and modifications that do not directly result from perturbations in trans-splicing. One such protein identified is an atypical calpain SKCRP7.1/7.2. Co-silencing of SKCRP7.1/7.2 and SEC63 eliminated SLS induction due its role in translocating the PK3 kinase. This kinase initiates SLS by migrating to the nucleus and phosphorylating TRF4 leading to shut-off of SL RNA transcription. Thus, SKCRP7.1 is involved in SLS signaling and the accompanying PCD. The role of autophagy in SLS was also investigated; eliminating autophagy through VPS34 or ATG7 silencing demonstrated that autophagy is not essential for SLS induction, but is associated with PCD. Thus, this study identified factors that are used by the parasite to cope with ER stress and to induce SLS and PCD.

1,3,4-Thiadiazole derivatives of R-(+)-limonene benzaldehyde-thiosemicarbazones cause death in Trypanosoma cruzi through oxidative stress

This work evaluated the in vitro and in vivo activity of TDZ 2 on Trypanosoma cruzi amastigotes and determined the possible mechanism of action of this compound on T. cruzi death. TDZ 2 inhibited T. cruzi proliferation in vitro and had low haemolytic potential. It also induced morphological and ultrastructural alterations. We observed a reduction of cell volume, the depolarization of the mitochondrial membrane, an increase in ROS production, lipoperoxidation of the cell membrane, lipid bodies formation and production of nitric oxide, a decrease in reduced thiols levels and, presence of autophagic vacuoles. The in vivo study found a reduction of parasitemia in animals treated with TDZ 2 alone or combined with benznidazole. Altogether, the alterations induced by TDZ 2 point to an oxidative stress condition that lead to T. cruzi cell death.

Molecular cloning, phylogenetic analysis and heat shock response of Babesia gibsoni heat shock protein 90

The Babesia gibsoni heat shock protein 90 (BgHSP90) gene was cloned and sequenced. The length of the gene was 2,610 bp with two introns. This gene was amplified from cDNA corresponding to full length coding sequence (CDS) with an open reading frame of 2,148 bp. A phylogenetic analysis of the CDS of HSP90 gene showed that B. gibsoni was most closely related to B. bovis and Babesia sp. BQ1/Lintan and lies within a phylogenetic cluster of protozoa. Moreover, mRNA transcription profile for BgHSP90 exposed to high temperature were examined by quantitative real-time reverse transcription-polymerase chain reaction. BgHSP90 levels were elevated when the parasites were incubated at 43°C for 1 hr.

Dibenzylideneacetones Are Potent Trypanocidal Compounds That Affect the Trypanosoma cruzi Redox System

Despite ongoing efforts, the available treatments for Chagas' disease are still unsatisfactory, especially in the chronic phase of the disease. Our previous study reported the strong trypanocidal activity of the dibenzylideneacetones A3K2A1 and A3K2A3 against Trypanosoma cruzi (Z. Ud Din, T. P. Fill, F. F. de Assis, D. Lazarin-Bidóia, V. Kaplum, F. P. Garcia, C. V. Nakamura, K. T. de Oliveira, and E. Rodrigues-Filho, Bioorg Med Chem 22:1121-1127, 2014, http://dx.doi.org/10.1016/j.bmc.2013.12.020). In the present study, we investigated the mechanisms of action of these compounds that are involved in parasite death. We showed that A3K2A1 and A3K2A3 induced oxidative stress in the three parasitic forms, especially trypomastigotes, reflected by an increase in oxidant species production and depletion of the endogenous antioxidant system. This oxidative imbalance culminated in damage in essential cell structures of T. cruzi, reflected by lipid peroxidation and DNA fragmentation. Consequently, A3K2A1 and A3K2A3 induced vital alterations in T. cruzi, leading to parasite death through the three pathways, apoptosis, autophagy, and necrosis.

Characterization of an African trypanosome mutant refractory to lectin-induced death

Incubation of African trypanosomes with the lectin concanavalin A (conA) leads to alteration in cellular DNA content, DNA degradation, and surface membrane blebbing. Here, we report the generation and characterization of a conA-refractory Trypanosoma brucei line. These insect stage parasites were resistant to conA killing, with a median lethal dose at least 50-fold greater than the parental line. Fluorescence-based experiments revealed that the resistant cells bound less lectin when compared to the parental line. Western blotting and mass spectrometry confirmed that the resistant line lacked an N-glycan required for conA binding on the cellular receptors, EP procyclin proteins. The failure to N-glycosylate the EP procyclins was not the consequence of altered N-glycan precursor biosynthesis, as another glycosylated protein (Fla1p) was normally modified. These findings support the likelihood that resistance to conA was a consequence of failure to bind the lectin trigger.

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Concanavalin A is toxic to Trypanosoma brucei.

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A mutant has been identified that is resistant to the concanavalin A killing.

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The mutant does not properly N-glycosylate the major lectin receptor, hence its resistance.

Sunlight inhibits growth and induces markers of programmed cell death in Plasmodium falciparum in vitro

Background

Plasmodium falciparum is responsible for the majority of global malaria deaths. During the pathogenic blood stages of infection, a rapid increase in parasitaemia threatens the survival of the host before transmission of slow-maturing sexual parasites to the mosquito vector to continue the life cycle. Programmed cell death (PCD) may provide the parasite with the means to control its burden on the host and thereby ensure its own survival. Various environmental stress factors encountered during malaria may induce PCD in P. falciparum. This study is the first to characterize parasite cell death in response to natural sunlight.

Methods

The 3D7 strain of P. falciparum was cultured in vitro in donor erythrocytes. Synchronized and mixed-stage parasitized cultures were exposed to sunlight for 1 h and compared to cultures maintained in the dark, 24 h later. Mixed-stage parasites were also subjected to a second one-hour exposure at 24 h and assessed at 48 h. Parasitaemia was measured daily by flow cytometry. Biochemical markers of cell death were assessed, including DNA fragmentation, mitochondrial membrane polarization and phosphatidylserine externalization.

Results

Sunlight inhibited P. falciparum growth in vitro. Late-stage parasites were more severely affected than early stages. However, some late-stage parasites survived exposure to sunlight to form new rings 24 h later, as would be expected during PCD whereby only a portion of the population dies. DNA fragmentation was observed at 24 and 48 h and preceded mitochondrial hyperpolarization in mixed-stage parasites at 48 h. Mitochondrial hyperpolarization likely resulted from increased oxidative stress. Although data suggested increased phosphatidylserine externalization in mixed-stage parasites, results were not statistically significant.

Conclusion

The combination of biochemical markers and the survival of some parasites, despite exposure to a lethal stimulus, support the occurrence of PCD in P. falciparum.

Novel Gallate Triphenylphosphonium Derivatives with Potent Antichagasic Activity

Chagas disease is one of the most neglected tropical diseases in the world, affecting nearly 15 million people, primarily in Latin America. Only two drugs are used for the treatment of this disease, nifurtimox and benznidazole. These drugs have limited efficacy and frequently induce adverse effects, limiting their usefulness. Consequently, new drugs must be found. In this study, we demonstrated the in vitro trypanocidal effects of a series of four gallic acid derivatives characterized by a gallate group linked to a triphenylphosphonium (TPP⁺) moiety (a delocalized cation) via a hydrocarbon chain of 8, 10, 11, or 12 atoms (TPP⁺-C₈, TPP⁺-C₁₀, TPP⁺-C₁₁, and TPP⁺-C₁₂, respectively). We analyzed parasite viability in isolated parasites (by MTT reduction and flow cytometry) and infected mammalian cells using T. cruzi Y strain trypomastigotes. Among the four derivatives, TPP⁺-C₁₀ and TPP⁺-C₁₂ were the most potent in both models, with EC₅₀ values (in isolated parasites) of 1.0 ± 0.6 and 1.0 ± 0.7 μM, respectively, and were significantly more potent than nifurtimox (EC₅₀ = 4.1 ± 0.6 μM). At 1 μM, TPP⁺-C₁₀ and TPP⁺-C₁₂ induced markers of cell death, such as phosphatidylserine exposure and propidium iodide permeabilization. In addition, at 1 μM, TPP⁺-C₁₀ and TPP⁺-C₁₂ significantly decreased the number of intracellular amastigotes (TPP⁺-C₁₀: 24.3%, TPP⁺-C₁₂: 19.0% of control measurements, as measured by DAPI staining) and the parasite’s DNA load (C₁₀: 10%, C₁₂: 13% of control measurements, as measured by qPCR). Based on the previous mode of action described for these compounds in cancer cells, we explored their mitochondrial effects in isolated trypomastigotes. TPP⁺-C₁₀ and TPP⁺-C₁₂ were the most potent compounds, significantly altering mitochondrial membrane potential at 1 μM (measured by JC-1 fluorescence) and inducing mitochondrial transition pore opening at 5 μM. Taken together, these results indicate that the TPP⁺-C₁₀ and TPP⁺-C₁₂ derivatives of gallic acid are promising trypanocidal agents with mitochondrial activity.

Depletion of the SR-Related Protein TbRRM1 Leads to Cell Cycle Arrest and Apoptosis-Like Death in Trypanosoma brucei

Arginine-Serine (RS) domain-containing proteins are RNA binding proteins with multiple functions in RNA metabolism. In mammalian cells this group of proteins is also implicated in regulation and coordination of cell cycle and apoptosis. In trypanosomes, an early branching group within the eukaryotic lineage, this group of proteins is represented by 3 members, two of them are SR proteins and have been recently shown to be involved in rRNA processing as well as in pre-mRNA splicing and stability. Here we report our findings on the 3rd member, the SR-related protein TbRRM1. In the present study, we showed that TbRRM1 ablation by RNA-interference in T. brucei procyclic cells leads to cell-cycle block, abnormal cell elongation compatible with the nozzle phenotype and cell death by an apoptosis-like mechanism. Our results expand the role of the trypanosomal RS-domain containing proteins in key cellular processes such as cell cycle and apoptosis-like death, roles also carried out by the mammalian SR proteins, and thus suggesting a conserved function in this phylogenetically conserved protein family.

The Oral Antimalarial Drug Tafenoquine Shows Activity against Trypanosoma brucei

The protozoan parasite Trypanosoma brucei causes human African trypanosomiasis, or sleeping sickness, a neglected tropical disease that requires new, safer, and more effective treatments. Repurposing oral drugs could reduce both the time and cost involved in sleeping sickness drug discovery. Tafenoquine (TFQ) is an oral antimalarial drug belonging to the 8-aminoquinoline family which is currently in clinical phase III. We show here that TFQ efficiently kills different T. brucei spp. in the submicromolar concentration range. Our results suggest that TFQ accumulates into acidic compartments and induces a necrotic process involving cell membrane disintegration and loss of cytoplasmic content, leading to parasite death. Cell lysis is preceded by a wide and multitarget drug action, affecting the lysosome, mitochondria, and acidocalcisomes and inducing a depolarization of the mitochondrial membrane potential, elevation of intracellular Ca(2+), and production of reactive oxygen species. This is the first report of an 8-aminoquinoline demonstrating significant in vitro activity against T. brucei.

Potential therapeutic targets and the role of technology in developing novel antileishmanial drugs

Leishmaniasis is the most prevalent pathogenic disease in many countries around the world, but there are few drugs available to treat it. Most antileishmanial drugs available are highly toxic, have resistance issues or require hospitalization for their use; therefore, they are not suitable for use in most of the affected countries. Over the past decade, the completion of the genomes of many human pathogens, including that of Leishmania spp., has opened new doors for target identification and validation. Here, we focus on the potential drug targets that can be used for the treatment of leishmaniasis and bring to light how recent technological advances, such as structure-based drug design, structural genomics, and molecular dynamics (MD), can be used to our advantage to develop potent and affordable antileishmanial drugs.

Isoforms of Hsp70-binding human LDL in adult Schistosoma mansoni worms

Schistosoma mansoni is one of the most common parasites infecting humans. They are well adapted to the host, and this parasite's longevity is a consequence of effective escape from the host immune system. In the blood circulation, lipoproteins not only help to conceal the worm from attack by host antibodies but also act as a source of lipids for S. mansoni. Previous SEM studies showed that the low-density lipoprotein (LDL) particles present on the surface of adult S. mansoni worms decreased in size when the incubation time increased. In this study, immunocytochemical and proteomic analyses were used to locate and identify S. mansoni binding proteins to human plasma LDL. Ultrathin sections of adult worms were cut transversely from the anterior, medial and posterior regions of the parasite. Immunocytochemical experiments revealed particles of gold in the tegument, muscle region and spine in male worms and around vitelline cells in females. Immunoblotting and 2D-electrophoresis using incubations with human serum, anti-LDL antibodies and anti-chicken IgG peroxidase conjugate were performed to identify LDL-binding proteins in S. mansoni. Analysis of the binding proteins using LC-MS identified two isoforms of the Hsp70 chaperone in S. mansoni. Hsp70 is involved in the interaction with apoB in the cytoplasm and its transport to the endoplasmic reticulum. However, further studies are needed to clarify the functional role of Hsp70 in S. mansoni, mainly related to the interaction with human LDL.

Preparation of rotenone derivatives and in vitro analysis of their antimalarial, antileishmanial and selective cytotoxic activities

Six derivatives of the known biopesticide rotenone were prepared by several chemical transformations. Rotenone and its derivatives showed differential in vitro antiparasitic activity and selective cytotoxicity. In general, compounds were more active against Plasmodium falciparum than Leishmania panamensis. Rotenone had an EC₅₀ of 19.0 µM against P. falciparum, and 127.2 µM against L. panamensis. Although chemical transformation does not improve its biological profile against P. falciparum, three of its derivatives showed a significant level of action within an adequate range of activity with EC₅₀ values < 50.0 µM. This antiplasmodial activity was not due to red blood cell hemolysis, since LC₅₀ was >>400 µM. On the other hand, all derivatives displayed a non-specific cytotoxicity on several cell lines and primary human cell cultures.

Additional evidence of the trypanocidal action of (-)-elatol on amastigote forms through the involvement of reactive oxygen species

Chagas’ disease, a vector-transmitted infectious disease, is caused by the protozoa parasite Trypanosoma cruzi. Drugs that are currently available for the treatment of this disease are unsatisfactory, making the search for new chemotherapeutic agents a priority. We recently described the trypanocidal action of (−)-elatol, extracted from the macroalga Laurencia dendroidea. However, nothing has been described about the mechanism of action of this compound on amastigotes that are involved in the chronic phase of Chagas’ disease. The goal of the present study was to evaluate the effect of (−)-elatol on the formation of superoxide anions (O₂^•−), DNA fragmentation, and autophagy in amastigotes of T. cruzi to elucidate the possible mechanism of the trypanocidal action of (−)-elatol. Treatment of the amastigotes with (−)-elatol increased the formation of O₂^•− at all concentrations of (−)-elatol assayed compared with untreated parasites. Increased fluorescence was observed in parasites treated with (−)-elatol, indicating DNA fragmentation and the formation of autophagic compartments. The results suggest that the trypanocidal action of (−)-elatol might involve the induction of the autophagic and apoptotic death pathways triggered by an imbalance of the parasite’s redox metabolism.

Ascorbate peroxidase, a key molecule regulating amphotericin B resistance in clinical isolates of Leishmania donovani

Amphotericin B (AmB), a polyene macrolide, is now a first-line treatment of visceral leishmaniasis cases refractory to antimonials in India. AmB relapse cases and the emergence of secondary resistance have now been reported. To understand the mechanism of AmB, differentially expressed genes in AmB resistance strains were identified by a DNA microarray and real-time reverse transcriptase PCR (RT-PCR) approach. Of the many genes functionally overexpressed in the presence of AmB, the ascorbate peroxidase gene from a resistant Leishmania donovani strain (LdAPx gene) was selected because the gene is present only in Leishmania, not in humans. Apoptosis-like cell death after exposure to AmB was investigated in a wild-type (WT) strain in which the LdAPx gene was overexpressed and in AmB-sensitive and -resistant strains. A higher percentage of apoptosis-like cell death after AmB treatment was noticed in the sensitive strain than in both the resistant isolate and the strain sensitive to LdAPx overexpression. This event is preceded by AmB-induced formation of reactive oxygen species and elevation of the cytosolic calcium level. Enhanced cytosolic calcium was found to be responsible for depolarization of the mitochondrial membrane potential and the release of cytochrome c (Cyt c) into the cytosol. The redox behavior of Cyt c showed that it has a role in the regulation of apoptosis-like cell death by activating metacaspase- and caspase-like proteins and causing concomitant nuclear alterations, as determined by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and DNA fragmentation in the resistant strain. The present study suggests that constitutive overexpression of LdAPx in the L. donovani AmB-resistant strain prevents cells from the deleterious effect of oxidative stress, i.e., mitochondrial dysfunction and cellular death induced by AmB.

Redox metabolism in Trypanosoma cruzi. Biochemical characterization of dithiol glutaredoxin dependent cellular pathways

In Trypanosoma cruzi, the modification of thiols by glutathionylation-deglutathionylation and its potential relation to protective, regulatory or signaling functions have been scarcely explored. Herein we characterize a dithiolic glutaredoxin (TcrGrx), a redox protein with deglutathionylating activity, having potential functionality to control intracellular homeostasis of protein and non-protein thiols. The catalytic mechanism followed by TcrGrx was found dependent on thiol concentration. Results suggest that TcrGrx operates as a dithiolic or a monothiolic Grx, depending on GSH concentration. TcrGrx functionality to mediate reduction of protein and non-protein disulfides was studied. TcrGrx showed a preference for glutathionylated substrates respect to protein disulfides. From in vivo assays involving TcrGrx overexpressing parasites, we observed the contribution of the protein to increase the general resistance against oxidative damage and intracellular replication of the amastigote stage. Also, studies performed with epimastigotes overexpressing TcrGrx strongly suggest the involvement of the protein in a cellular pathway connecting an apoptotic stimulus and apoptotic-like cell death. Novel information is presented about the participation of this glutaredoxin not only in redox metabolism but also in redox signaling pathways in T. cruzi. The influence of TcrGrx in several parasite physiological processes suggests novel insights about the protein involvement in redox signaling.

Genome evolution in trypanosomatid parasites

A decade of genome sequencing has transformed our understanding of how trypanosomatid parasites have evolved and provided fresh impetus to explaining the origins of parasitism in the Kinetoplastida. In this review, I will consider the many ways in which genome sequences have influenced our view of genomic reduction in trypanosomatids; how species-specific genes, and the genomic domains they occupy, have illuminated the innovations in trypanosomatid genomes; and how comparative genomics has exposed the molecular mechanisms responsible for innovation and adaptation to a parasitic lifestyle.

Identification of two novel HSP90 proteins in Babesia orientalis: molecular characterization, and computational analyses of their structure, function, antigenicity and inhibitor interaction

Background

HSP90 protects the cells from heat stress and facilitates protein maturation and stability. The full genome sequences of piroplasms contain two putative HSP90 proteins, which are yet uncharacterized. To this end, the two putative HSP90 proteins of Babesia orientalis were identified and characterized by molecular and in silico methods.

Methods

The two putative proteins in B. orientalis genome showing homology with putative HSP90 of other piroplasms were cloned and sequenced. A computational analysis was carried out to predict the antigenic determinants, structure and function of these proteins. The interactions of two HSP90 isoforms with respective inhibitors were also examined through docking analysis.

Results

The length of BoHSP90-A gene (amplified from gDNA) was 2706 bp with one intron from position 997 to 1299 bp. This gene amplified from cDNA corresponded to full length CDS with an open reading frame (ORF) of 2403 bp encoding a 800 amino acid (AA) polypeptide with a predicted size of 91.02 kDa. The HSP90-B gene was intronless with an ORF of 2349 bp, and predicted polypeptide comprised of 797 AA with a size of 90.59 kDa. The AA sequences of these two proteins of B. orientalis were the most identical to those of B. bovis. The BoHSP90-A and BoHSP90-B were recognized as 90 kDa in the parasite lysate by the rabbit antisera raised against the recombinant BoHSP90 proteins. The anti-B. orientalis buffalo serum reacted with the rBoHSP90s expressed in E. coli, indicating that these proteins might be secreted by the parasite before entry into host cells. The overall structure and functional analyses showed several domains involved in ATPase activity, client protein binding and HSP90 dimerization. Likewise, several HSP90 inhibitors showed binding to ATP binding pockets of BoHSP90-A and BoHSP90-B, as observed through protein structure-ligand interaction analysis.

Conclusions

The two putative HSP90 proteins in B. orientalis were recognized as 90 kDa. The rBoHSP90-A and rBoHSP90-B were reacted with the B. orientalis infected buffalo serum. The computational structure and functional analyses revealed that these two proteins may have chaperonic activity. The protein structure-ligand interaction analyses indicated that these two proteins had many drug target sites.

An inhibitor-driven study for enhancing the selectivity of indirubin derivatives towards leishmanial Glycogen Synthase Kinase-3 over leishmanial cdc2-related protein kinase 3

Background

In search of new antiparasitic agents for overcoming the limitations of current leishmaniasis chemotherapy, we have previously shown that 6-bromoindirubin-3'-oxime (6BIO) and several other 6-substituted analogues of indirubin, a naturally occurring bis-indole present in mollusks and plants, displayed reverse selectivity from the respective mammalian kinases, targeting more potently the leishmanial Cyclin-Dependent Kinase-1 (CDK1) homologue [cdc2-related protein kinase 3 (LCRK3)] over leishmanial Glycogen Synthase Kinase-3 (LGSK-3). This reversal of selectivity in Leishmania parasites compared to mammalian cells makes the design of specific indirubin-based LGSK-3 inhibitors difficult. In this context, the identification of compounds bearing specific substitutions that shift indirubin inhibition towards LGSK-3, previously found to be a potential drug target, over LCRK3 is imperative for antileishmanial targeted drug discovery.

Methods

A new in-house indirubin library, composed of 35 compounds, initially designed to target mammalian kinases (CDKs, GSK-3), was tested against Leishmania donovani promastigotes and intracellular amastigotes using the Alamar blue assay. Indirubins with antileishmanial activity were tested against LGSK-3 and LCRK3 kinases, purified from homologous expression systems. Flow cytometry (FACS) was used to measure the DNA content for cell-cycle analysis and the mode of cell death. Comparative structural analysis of the involved kinases was then performed using the Szmap algorithm.

Results

We have identified 7 new indirubin analogues that are selective inhibitors of LGSK-3 over LCRK3. These new inhibitors were also found to display potent antileishmanial activity with GI₅₀ values of <1.5 μΜ. Surprisingly, all the compounds that displayed enhanced selectivity towards LGSK-3, were 6BIO analogues bearing an additional 3'-bulky amino substitution, namely a piperazine or pyrrolidine ring. A comparative structural analysis of the two aforementioned leishmanial kinases was subsequently undertaken to explain and rationalize the selectivity trend determined by the in vitro binding assays. Interestingly, the latter analysis showed that selectivity could be correlated with differences in kinase solvation thermo dynamics induced by minor sequence variations of the otherwise highly similar ATP binding pockets.

Conclusions

In conclusion, 3'-bulky amino substituted 6-BIO derivatives, which demonstrate enhanced specificity towards LGSK-3, represent a new scaffold for targeted drug development to treat leishmaniasis.

The calpain inhibitor MDL28170 induces the expression of apoptotic markers in Leishmania amazonensis promastigotes

Background

Human cutaneous leishmaniasis is caused by distinct species, including Leishmania amazonensis. Treatment of cutaneous leishmaniasis is far from satisfactory due to increases in drug resistance and relapses, and toxicity of compounds to the host. As a consequence for this situation, the development of new leishmanicidal drugs and the search of new targets in the parasite biology are important goals.

Methodology/Principal Findings

In this study, we investigated the mechanism of death pathway induced by the calpain inhibitor MDL28170 on Leishmania amazonensis promastigote forms. The combined use of different techniques was applied to contemplate this goal. MDL28170 treatment with IC₅₀ (15 µM) and two times the IC₅₀ doses induced loss of parasite viability, as verified by resazurin assay, as well as depolarization of the mitochondrial membrane, which was quantified by JC-1 staining. Scanning and transmission electron microscopic images revealed drastic alterations on the parasite morphology, some of them resembling apoptotic-like death, including cell shrinking, surface membrane blebs and altered chromatin condensation pattern. The lipid rearrangement of the plasma membrane was detected by Annexin-V labeling. The inhibitor also induced a significant increase in the proportion of cells in the sub-G₀/G₁ phase, as quantified by propidium iodide staining, as well as genomic DNA fragmentation, detected by TUNEL assay. In cells treated with MDL28170 at two times the IC₅₀ dose, it was also possible to observe an oligonucleossomal DNA fragmentation by agarose gel electrophoresis.

Conclusions/Significance

The data presented in the current study suggest that MDL28170 induces apoptotic marker expression in promastigotes of L. amazonensis. Altogether, the results described in the present work not only provide a rationale for further exploration of the mechanism of action of calpain inhibitors against trypanosomatids, but may also widen the investigation of the potential clinical utility of calpain inhibitors in the chemotherapy of leishmaniases.

Selection of molecular targets for drug development against trypanosomatids

Trypanosomatid parasites are a group of flagellated protozoa that includes the genera Leishmania and Trypanosoma, which are the causative agents of diseases (leishmaniases, sleeping sickness and Chagas disease) that cause considerable morbidity and mortality, affecting more than 27 million people worldwide. Today no effective vaccines for the prevention of these diseases exist, whereas current chemotherapy is ineffective, mainly due to toxic side effects of current drugs and to the emergence of drug resistance and lack of cost effectiveness. For these reasons, rational drug design and the search of good candidate drug targets is of prime importance. The search for drug targets requires a multidisciplinary approach. To this end, the completion of the genome project of many trypanosomatid species gives a vast amount of new information that can be exploited for the identification of good drug candidates with a prediction of "druggability" and divergence from mammalian host proteins. In addition, an important aspect in the search for good drug targets is the "target identification" and evaluation in a biological pathway, as well as the essentiality of the gene in the mammalian stage of the parasite, which is provided by basic research and genetic and proteomic approaches. In this chapter we will discuss how these bioinformatic tools and experimental evaluations can be integrated for the selection of candidate drug targets, and give examples of metabolic and signaling pathways in the parasitic protozoa that can be exploited for rational drug design.

Turning up the heat: heat stress induces markers of programmed cell death in Plasmodium falciparum in vitro

Malaria is characterised by cyclical febrile episodes that result from the rupture of mature schizont-infected erythrocytes releasing merozoites. In patients infected with Plasmodium falciparum, fever may reach peak temperatures as high as 41 °C. Febrile episodes typically have a deleterious effect on parasites and probably benefit the host by aiding parasite clearance; however, the parasite may also gain advantage from limiting its burden on the host and prolonging infection to ensure development and transmission of slow-maturing gametocytes. Programmed cell death (PCD) may provide the parasite with a mechanism of self-limitation, although the occurrence and phenotype of PCD in the erythrocytic stages remain controversial due to conflicting data. This study aimed to characterise the cell death phenotype of P. falciparum in response to in vitro heat stress. A variety of biochemical markers of PCD, including DNA fragmentation, mitochondrial dysregulation and phosphatidylserine externalisation, as well as morphological studies of Giemsa-stained thin smears and real-time microscopy were utilised to characterise the phenotype. Heat stress decreased P. falciparum growth and development in vitro. Late-stage parasites were more susceptible, although early stages were more affected than expected. Early-stage parasites exposed to 41 °C exhibited markers of an apoptosis-like PCD phenotype, including DNA fragmentation and mitochondrial depolarisation. Heat-stressed late-stage parasites showed no significant DNA fragmentation or mitochondrial dysregulation; however, cytoplasmic vacuolisation was suggestive of an autophagy-like form of PCD. Our results therefore showed that biochemical and morphological markers of PCD varied with intra-erythrocytic parasite development and that P. falciparum exhibited facets of both apoptosis- and autophagy-like phenotypes after exposure to febrile temperatures, which may reflect a unique PCD phenotype.

Over-expression of 60s ribosomal L23a is associated with cellular proliferation in SAG resistant clinical isolates of Leishmania donovani

BACKGROUND

Sodium antimony gluconate (SAG) unresponsiveness of Leishmania donovani (Ld) had effectively compromised the chemotherapeutic potential of SAG. 60s ribosomal L23a (60sRL23a), identified as one of the over-expressed protein in different resistant strains of L.donovani as observed with differential proteomics studies indicates towards its possible involvement in SAG resistance in L.donovani. In the present study 60sRL23a has been characterized for its probable association with SAG resistance mechanism.

METHODOLOGY AND PRINCIPAL FINDINGS

The expression profile of 60s ribosomal L23a (60sRL23a) was checked in different SAG resistant as well as sensitive strains of L.donovani clinical isolates by real-time PCR and western blotting and was found to be up-regulated in resistant strains. Ld60sRL23a was cloned, expressed in E.coli system and purified for raising antibody in swiss mice and was observed to have cytosolic localization in L.donovani. 60sRL23a was further over-expressed in sensitive strain of L.donovani to check its sensitivity profile against SAG (Sb V and III) and was found to be altered towards the resistant mode.

CONCLUSION/SIGNIFICANCE

This study reports for the first time that the over expression of 60sRL23a in SAG sensitive parasite decreases the sensitivity of the parasite towards SAG, miltefosine and paramomycin. Growth curve of the tranfectants further indicated the proliferative potential of 60sRL23a assisting the parasite survival and reaffirming the extra ribosomal role of 60sRL23a. The study thus indicates towards the role of the protein in lowering and redistributing the drug pressure by increased proliferation of parasites and warrants further longitudinal study to understand the underlying mechanism.

Intracellular calcium channels in protozoa

Ca(2+)-signaling pathways and intracellular Ca(2+) channels are present in protozoa. Ancient origin of inositol 1,4,5-trisphosphate receptors (IP3Rs) and other intracellular channels predates the divergence of animals and fungi as evidenced by their presence in the choanoflagellate Monosiga brevicollis, the closest known relative to metazoans. The first protozoan IP3R cloned, from the ciliate Paramecium, displays strong sequence similarity to the rat type 3 IP3R. This ciliate has a large number of IP3- and ryanodine(Ry)-like receptors in six subfamilies suggesting the evolutionary adaptation to local requirements for an expanding diversification of vesicle trafficking. IP3Rs have also been functionally characterized in trypanosomatids, where they are essential for growth, differentiation, and establishment of infection. The presence of the mitochondrial calcium uniporter (MCU) in a number of protozoa indicates that mitochondrial regulation of Ca(2+) signaling is also an early appearance in evolution, and contributed to the discovery of the molecular nature of this channel in mammalian cells. There is only sequence evidence for the occurrence of two-pore channels (TPCs), transient receptor potential Ca(2+) channels (TRPCs) and intracellular mechanosensitive Ca(2+)-channels in Paramecium and in parasitic protozoa.

Trypanocidal activity of thioamide-substituted imidazoquinolinone: electrochemical properties and biological effects

Three thioamide-substituted imidazoquinolinone, which possess a heterocyclic center similar to tryptanthrin and are named C1, C2, and C3, were studied regarding (a) their in vitro anti-Trypanosoma cruzi activity, (b) their cytotoxicity and electrochemical behaviour, and (c) their effect on cell viability, redox state, and mitochondrial function. The assayed compounds showed a significant activity against the proliferative forms, but only C1 showed activity on the trypomastigote form (for C1, IC_{50  epi} = 1.49 μM; IC_{50  amas} = 1.74 μM; and IC_{50  try} = 34.89 μM). The presence of an antioxidant compound such as ascorbic acid or dithiotreitol induced a threefold increase in the antiparasitic activity, whereas glutathione had a dual effect depending on its concentration. Our results indicate that these compounds, which exhibited low toxicity to the host cells, can be reduced inside the parasite by means of the pool of low molecular weight thiols, causing oxidative stress and parasite death by apoptosis. The antiparasitic activity of the compounds studied could be explained by a loss of the capacity of the antioxidant defense system of the parasite to keep its intracellular redox state. C1 could be considered a good candidate for in vivo evaluation.

Further evidence of the trypanocidal action of eupomatenoid-5: confirmation of involvement of reactive oxygen species and mitochondria owing to a reduction in trypanothione reductase activity

Our group assays natural products that are less toxic and more effective than available nitroheterocycles as promising therapeutic options for patients with Chagas disease. Our previous study reported the trypanocidal activity of eupomatenoid-5, a neolignan isolated from the leaves of Piper regnellii var. pallescens, against the three main parasitic forms of Trypanosoma cruzi. The present study further characterizes the biochemical and morphological alterations induced by this compound to elucidate the mechanisms of action involved in the cell death of T. cruzi. We show that eupomatenoid-5 induced oxidative imbalance in the three parasitic forms, especially trypomastigotes, reflected by a decrease in the activity of trypanothione reductase and increase in the formation of reactive oxygen species (ROS). A reduction of mitochondrial membrane potential was then triggered, further impairing the cell redox system through the production of more ROS and reactive nitrogen species. Altogether, these effects led to oxidative stress, reflected by lipid peroxidation and DNA fragmentation. These alterations are key events in the induction of parasite death through various pathways, including apoptosis, necrosis, and autophagy.

Nuclear translocation and accumulation of glyceraldehyde-3-phosphate dehydrogenase involved in diclazuril-induced apoptosis in Eimeria tenella (E. tenella)

In mammalian cells, GAPDH (glyceraldehyde-3-phosphate dehydrogenase) has recently been shown to be implicated in numerous apoptotic paradigms, especially in neuronal apoptosis, and has been demonstrated to play a vital role in some neurodegenerative disorders. However, this phenomenon has not been reported in protists. In the present study, we report for the first time that such a mechanism is involved in diclazuril-induced apoptosis in Eimeria tenella (E. tenella). We found that upon treatment of parasites with diclazuril, the expression levels of GAPDH transcript and protein were significantly increased in second-generation merozoites. Then, we examined the subcellular localization of GAPDH by fluorescence microscopy and Western blot analysis. The results show that a considerable amount of GAPDH protein appeared in the nucleus within diclazuril-treated second-generation merozoites; in contrast, the control group had very low levels of GAPDH in the nucleus. The glycolytic activity of GAPDH was kinetically analyzed in different subcellular fractions. A substantial decrease (48.5%) in glycolytic activity of GAPDH in the nucleus was displayed. Moreover, the activities of caspases-3, -9, and −8 were measured in cell extracts using specific caspase substrates. The data show significant increases in caspase-3 and caspase-9 activities in the diclazuril-treated group.

The first suicides: a legacy inherited by parasitic protozoans from prokaryote ancestors

It is more than 25 years since the first report that a protozoan parasite could die by a process resulting in a morphological phenotype akin to apoptosis. Since then these phenotypes have been observed in many unicellular parasites, including trypanosomatids and apicomplexans, and experimental evidence concerning the molecular pathways that are involved is growing. These observations support the view that this form of programmed cell death is an ancient one that predates the evolution of multicellularity. Here we review various hypotheses that attempt to explain the origin of apoptosis, and look for support for these hypotheses amongst the parasitic protists as, with the exception of yeast, most of the work on death mechanisms in unicellular organisms has focussed on them. We examine the role that addiction modules may have played in the original eukaryote cell and the part played by mitochondria in the execution of present day cells, looking for examples from Leishmania spp. Trypanosoma spp. and Plasmodium spp. In addition, the expanding knowledge of proteases, nucleases and other molecules acting in protist execution pathways has enabled comparisons to be made with extant Archaea and bacteria and with biochemical pathways that evolved in metazoans. These comparisons lend support to the original sin hypothesis but also suggest that present-day death pathways may have had multifaceted beginnings.

Apoptotic marker expression in the absence of cell death in staurosporine-treated Leishmania donovani

The protozoan parasite Leishmania donovani undergoes several developmental transitions in its insect and vertebrate hosts that are induced by environmental changes. The roles of protein kinases in these adaptive differentiation steps and their potential as targets for antiparasitic intervention are only poorly characterized. Here, we used the generic protein kinase inhibitor staurosporine to gain insight into how interference with phosphotransferase activities affects the viability, growth, and motility of L. donovani promastigotes in vitro. Unlike the nonkinase drugs miltefosine and amphotericin B, staurosporine strongly reduced parasite biosynthetic activity and had a cytostatic rather than a cytotoxic effect. Despite the induction of a number of classical apoptotic markers, including caspase-like activity and surface binding of annexin V, we determined that, on the basis of cellular integrity, staurosporine did not cause cell death but caused cell cycle arrest and abrogated parasite motility. In contrast, targeted inhibition of the parasite casein kinase 1 (CK1) protein family by use of the CK1-specific inhibitor D4476 resulted in cell death. Thus, pleiotropic inhibition of L. donovani protein kinases and possibly other ATP-binding proteins by staurosporine dissociates apoptotic marker expression from cell death, which underscores the relevance of specific rather than broad kinase inhibitors for antiparasitic drug development.

Apoptosis and apoptotic mimicry in Leishmania: an evolutionary perspective

Apoptotic death and apoptotic mimicry are defined respectively as a non-accidental death and as the mimicking of an apoptotic-cell phenotype, usually by phosphatidylserine (PS) exposure. In the case of the murine infection by Leishmania spp, apoptotic death has been described in promastigotes and apoptotic mimicry in amastigotes. In both situations they are important events of the experimental murine infection by this parasite. In the present review we discuss what features we need to consider if we want to establish if a behavior shown by Leishmania is altruistic or not: does the behavior increases the fitness of organisms other than the one showing it? Does this behavior have a cost for the actor? If we manage to show that a given behavior is costly for the actor and beneficial for the recipient of the action, we will be able to establish it as altruistic. From this perspective, we can argue that apoptotic-like death and apoptotic mimicry are both altruistic with the latter representing a weaker altruistic behavior than the former.

Programmed cell death in Leishmania: biochemical evidence and role in parasite infectivity

Demonstration of features of a programmed cell death (PCD) pathway in protozoan parasites initiated a great deal of interest and debate in the field of molecular parasitology. Several of the markers typical of mammalian apoptosis have been shown in Leishmania which suggested the existence of an apoptosis like death in these organisms. However, studies to elucidate the downstream events associated with phosphotidyl serine exposure, loss of mitochondrial membrane potential, cytochrome c release, and caspase-like activities in cells undergoing such cell death remain an ongoing challenge. Recent advances in genome sequencing, chemical biology should help to solve some of these challenges. Leishmania genetic mutants that lack putative regulators/effectors of PCD pathway should not only help to demonstrate the mechanisms of PCD but also provide tools to better understand the putative role for this pathway in population control and in the establishment of a successful infection of the host.

Apoptosis-like programmed cell death induces antisense ribosomal RNA (rRNA) fragmentation and rRNA degradation in Leishmania

Few natural antisense (as) RNAs have been reported as yet in the unicellular protozoan Leishmania. Here, we describe that Leishmania produces natural asRNAs complementary to all ribosomal RNA (rRNA) species. Interestingly, we show that drug-induced apoptosis-like programmed cell death triggers fragmentation of asRNA complementary to the large subunit gamma (LSU-γ) rRNA, one of the six 28S rRNA processed fragments in Leishmania. Heat and oxidative stress also induce fragmentation of asrRNA, but to a lesser extent. Extensive asrRNA cleavage correlates with rRNA breakdown and translation inhibition. Indeed, overexpression of asLSU-γ rRNA accelerates rRNA degradation upon induction of apoptosis. In addition, we provide mechanistic insight into the regulation of apoptosis-induced asrRNA fragmentation by a 67 kDa ATP-dependent RNA helicase of the DEAD-box subfamily. This helicase binds both sense (s)LSU-γ and asLSU-γ rRNAs, and appears to have a key role in protecting rRNA from degradation by preventing asrRNA cleavage and thus cell death. Remarkably, the asrRNA fragmentation process operates not only in trypanosomatid protozoa but also in mammals. Our findings uncover a novel mechanism of regulation involving asrRNA fragmentation and rRNA breakdown, that is triggered by apoptosis and conditions of reduced translation under stress, and seems to be evolutionary conserved.

Spliced leader RNA silencing (SLS) - a programmed cell death pathway in Trypanosoma brucei that is induced upon ER stress

Trypanosoma brucei is the causative agent of African sleeping sickness. The parasite cycles between its insect (procyclic form) and mammalian hosts (bloodstream form). Trypanosomes lack conventional transcription regulation, and their genes are transcribed in polycistronic units that are processed by trans-splicing and polyadenylation. In trans-splicing, which is essential for processing of each mRNA, an exon, the spliced leader (SL) is added to all mRNAs from a small RNA, the SL RNA. Trypanosomes lack the machinery for the unfolded protein response (UPR), which in other eukaryotes is induced under endoplasmic reticulum (ER) stress. Trypanosomes respond to such stress by changing the stability of mRNAs, which are essential for coping with the stress. However, under severe ER stress that is induced by blocking translocation of proteins to the ER, treatment of cells with chemicals that induce misfolding in the ER, or extreme pH, trypanosomes elicit the spliced leader silencing (SLS) pathway. In SLS, the transcription of the SL RNA gene is extinguished, and tSNAP42, a specific SL RNA transcription factor, fails to bind to its cognate promoter. SLS leads to complete shut-off of trans-splicing. In this review, I discuss the UPR in mammals and compare it to the ER stress response in T. brucei leading to SLS. I summarize the evidence supporting the notion that SLS is a programmed cell death (PCD) pathway that is utilized by the parasites to substitute for the apoptosis observed in higher eukaryotes under prolonged ER stress. I present the hypothesis that SLS evolved to expedite the death process, and rapidly remove from the population unfit parasites that, by elimination via SLS, cause minimal damage to the parasite population.

Involvement of TatD nuclease during programmed cell death in the protozoan parasite Trypanosoma brucei

In this report, we describe the involvement of TatD nuclease during programmed cell death (PCD) in the human protozoan parasite Trypanosoma brucei. T. brucei TatD nuclease showed intrinsic DNase activity, was localized in the cytoplasm and translocated to the nucleus when cells were treated with inducers previously demonstrated to cause PCD in T. brucei. Overexpression of TatD nuclease resulted in elevated PCD and conversely, loss of TatD expression by RNAi conferred significant resistance to the induction of PCD in T. brucei. Co-immunoprecipitation studies revealed that TatD nuclease interacts with endonucleaseG suggesting that these two nucleases could form a DNA degradation complex in the nucleus. Together, biochemical activity, RNAi and subcellular localization results demonstrate the role of TatD nuclease activity in DNA degradation during PCD in these evolutionarily ancient eukaryotic organisms. Further, in conjunction with endonucleaseG, TatD may represent a critical nuclease in a caspase-independent PCD pathway in trypanosomatid parasites since caspases have not been identified in these organisms.

Trypanosomes and the solution to a 50-year mitochondrial calcium mystery

The ability of mitochondria to take up Ca(2+) was discovered 50 years ago. This calcium uptake, through a mitochondrial calcium uniporter (MCU), is important not only for the regulation of cellular ATP concentration but also for more complex pathways such as shaping Ca(2+) signals and the activation of programmed cell death. The molecular nature of the uniporter remained unknown for decades. By a comparative study of mitochondrial protein profiles of organisms lacking or possessing MCU, such as yeast in the former case and vertebrates and trypanosomes in the latter, two groups recently found the protein that possesses all the characteristics of the MCU. These results add another success story to the already substantial contributions of trypanosomes to mammalian biochemistry.

Tolerance to drug-induced cell death favours the acquisition of multidrug resistance in Leishmania

The control of the protozoan parasite Leishmania relies on few drugs with unknown cellular targets and unclear mode of action. Several antileishmanials, however, were shown to induce apoptosis in Leishmania and this death mechanism was further studied in drug-sensitive and drug-resistant Leishmania infantum. In sensitive parasites, antimonials (SbIII), miltefosine (MF) and amphotericin B (AMB), but not paromomycin (PARO), triggered apoptotic cell death associated with reactive oxygen species (ROS). In contrast, Leishmania mutants resistant to SbIII, MF or AMB not only failed to undergo apoptosis following exposure to their respective drugs, but also were more tolerant towards apoptosis induced by other antileishmanials, provided that these killed Leishmania via ROS production. Such tolerance favored the rapid acquisition of multidrug resistance. PARO killed Leishmania in a non-apoptotic manner and failed to produce ROS. PARO resistance neither protected against drug-induced apoptosis nor provided an increased rate of acquisition of resistance to other antileishmanials. However, the PARO-resistant mutant, but not SbIII-, MF- or AMB-resistant mutants, became rapidly cross-resistant to methotrexate, a model drug also not producing ROS. Our results therefore link the mode of killing of drugs to tolerance to cell death and to a facilitated emergence of multidrug resistance. These findings may have fundamental implications in the field of chemotherapeutic interventions.

Plasmodium falciparum metacaspase PfMCA-1 triggers a z-VAD-fmk inhibitable protease to promote cell death

Activation of proteolytic cell death pathways may circumvent drug resistance in deadly protozoan parasites such as Plasmodium falciparum and Leishmania. To this end, it is important to define the cell death pathway(s) in parasites and thus characterize proteases such as metacaspases (MCA), which have been reported to induce cell death in plants and Leishmania parasites. We, therefore, investigated whether the cell death function of MCA is conserved in different protozoan parasite species such as Plasmodium falciparum and Leishmania major, focusing on the substrate specificity and functional role in cell survival as compared to Saccharomyces cerevisae. Our results show that, similarly to Leishmania, Plasmodium MCA exhibits a calcium-dependent, arginine-specific protease activity and its expression in yeast induced growth inhibition as well as an 82% increase in cell death under oxidative stress, a situation encountered by parasites during the host or when exposed to drugs such as artemisins. Furthermore, we show that MCA cell death pathways in both Plasmodium and Leishmania, involve a z-VAD-fmk inhibitable protease. Our data provide evidence that MCA from both Leishmania and Plasmodium falciparum is able to induce cell death in stress conditions, where it specifically activates a downstream enzyme as part of a cell death pathway. This enzymatic activity is also induced by the antimalarial drug chloroquine in erythrocytic stages of Plasmodium falciparum. Interestingly, we found that blocking parasite cell death influences their drug sensitivity, a result which could be used to create therapeutic strategies that by-pass drug resistance mechanisms by acting directly on the innate pathways of protozoan cell death.

Identification and characterization of genes involved in leishmania pathogenesis: the potential for drug target selection

Identifying and characterizing Leishmania donovani genes and the proteins they encode for their role in pathogenesis can reveal the value of this approach for finding new drug targets. Effective drug targets are likely to be proteins differentially expressed or required in the amastigote life cycle stage found in the patient. Several examples and their potential for chemotherapeutic disruption are presented. A pathway nearly ubiquitous in living cells targeted by anticancer drugs, the ubiquitin system, is examined. New findings in ubiquitin and ubiquitin-like modifiers in Leishmania show how disruption of those pathways could point to additional drug targets. The programmed cell death pathway, now recognized among protozoan parasites, is reviewed for some of its components and evidence that suggests they could be targeted for antiparasitic drug therapy. Finally, the endoplasmic reticulum quality control system is involved in secretion of many virulence factors. How disruptions in this pathway reduce virulence as evidence for potential drug targets is presented.

Mitochondrial dysfunction in Trypanosoma cruzi: the role of Serratia marcescens prodigiosin in the alternative treatment of Chagas disease

BACKGROUND

Chagas disease is a health threat for many people, mostly those living in Latin America. One of the most important problems in treatment is the limitation of existing drugs. Prodigiosin, produced by Serratia marcescens (Rhodnius prolixus endosymbiont), belongs to the red-pigmented bacterial prodiginine family, which displays numerous biological activities, including antibacterial, antifungal, antiprotozoal, antimalarial, immunosuppressive, and anticancer properties. Here we describe its effects on Trypanosoma cruzi mitochondria belonging to Tc I and Tc II.

RESULTS

Parasites exposed to prodigiosin altered the mitochondrial function and oxidative phosphorylation could not have a normal course, probably by inhibition of complex III. Prodigiosin did not produce cytotoxic effects in lymphocytes and Vero cells and has better effects than benznidazole. Our data suggest that the action of prodigiosin on the parasites is mediated by mitochondrial structural and functional disruptions that could lead the parasites to an apoptotic-like cell death process.

CONCLUSIONS

Here, we propose a potentially useful trypanocidal agent derived from knowledge of an important aspect of the natural life cycle of the parasite: the vector-parasite interaction. Our results indicate that prodigiosin could be a good candidate for the treatment of Chagas disease.