Novobiocin induces apoptosis-like cell death in topoisomerase II over-expressing arsenite resistant Leishmania donovani

Type II DNA Topoisomerases in trypanosomatid and apicomplexan parasites

Diseases caused by trypanosomatid parasites have no commercially available vaccines for human application. Treatment modalities completely rely on chemotherapeutics strategies that often exhibit clinical drawbacks, like host toxicity, side effects and treatment failure for drug resistance. These, in many instances, are costly, making them unaffordable for certain groups of beneficiaries. To find reasonable solutions, researchers are attempting to identify and validate new drug targets that would offer parasite specificity. DNA topoisomerases in parasites present a consolidated class of drug targets due to their multiple structural and functional differences with host homologs. Type II DNA topoisomerases in these parasites, in particular, have been attracting interest of scientific community attributable to their pivotal role in the replication of the atypical DNA. In this article, we present a detailed review of structural and functional features of type II DNA topoisomerases of clinically-relevant trypanosomatid and apicomplexan parasites. Also, we provide up-to-date information on different molecules that target these enzymes. Altogether, the review will largely help in understanding the rationale for exploiting type II DNA topoisomerases in these groups of parasites as drug targets.

Potential molecular targets of Leishmania pathways in developing novel antileishmanials

The illness known as leishmaniasis has not become a household name like malaria, although it stands as the second-largest parasitic disease, surpassed only by malaria. As no licensed vaccine is available, treatment for leishmaniasis mostly relies on chemotherapy. Inefficiency and drug resistance are the major impediments in current therapeutics. In this scenario, identification of novel molecular drug candidates is indispensable to develop robust antileishmanials. The exploration of structure-based drugs to target enzymes/molecules of Leishmania which differ structurally/functionally from their equivalents in mammalian hosts not only helps in developing a new class of antileishmanials, but also paves the way to understand Leishmania biology. This review provides a comprehensive overview on possible drug candidates relating to various Leishmania molecular pathways.

Systematic in silico Evaluation of Leishmania spp. Proteomes for Drug Discovery

Leishmaniasis is a group of neglected infectious diseases, with approximately 1. 3 million new cases each year, for which the available therapies have serious limitations. Therefore, it is extremely important to apply efficient and low-cost methods capable of selecting the best therapeutic targets to speed up the development of new therapies against those diseases. Thus, we propose the use of integrated computational methods capable of evaluating the druggability of the predicted proteomes of Leishmania braziliensis and Leishmania infantum, species responsible for the different clinical manifestations of leishmaniasis in Brazil. The protein members of those proteomes were assessed based on their structural, chemical, and functional contexts applying methods that integrate data on molecular function, biological processes, subcellular localization, drug binding sites, druggability, and gene expression. These data were compared to those extracted from already known drug targets (BindingDB targets), which made it possible to evaluate Leishmania proteomes for their biological relevance and treatability. Through this methodology, we identified more than 100 proteins of each Leishmania species with druggability characteristics, and potential interaction with available drugs. Among those, 31 and 37 proteins of L. braziliensis and L. infantum, respectively, have never been tested as drug targets, and they have shown evidence of gene expression in the evolutionary stage of pharmacological interest. Also, some of those Leishmania targets showed an alignment similarity of <50% when compared to the human proteome, making these proteins pharmacologically attractive, as they present a reduced risk of side effects. The methodology used in this study also allowed the evaluation of opportunities for the repurposing of compounds as anti-leishmaniasis drugs, inferring potential interaction between Leishmania proteins and ~1,000 compounds, of which only 15 have already been tested as a treatment for leishmaniasis. Besides, a list of potential Leishmania targets to be tested using drugs described at BindingDB, such as the potential interaction of the DEAD box RNA helicase, TRYR, and PEPCK proteins with the Staurosporine compound, was made available to the public.

Heat Shock Proteins as the Druggable Targets in Leishmaniasis: Promises and Perils

Leishmania, the causative agent of leishmaniasis, is an intracellular pathogen that thrives in the insect gut and mammalian macrophages to complete its life cycle. Apart from temperature difference (26 to 37°C), it encounters several harsh conditions, including oxidative stress, inflammatory reactions, and low pH. Heat shock proteins (HSPs) play essential roles in cell survival by strategically reprogramming cellular processes and signaling pathways. HSPs assist cells in multiple functions, including differentiation, adaptation, virulence, and persistence in the host cell. Due to cyclical epidemiological patterns, limited chemotherapeutic options, drug resistance, and the absence of a vaccine, control of leishmaniasis remains a far-fetched dream. The essential roles of HSPs in parasitic differentiation and virulence and increased expression in drug-resistant strains highlight their importance in combating the disease. In this review, we highlighted the diverse physiological importance of HSPs present in Leishmania, emphasizing their significance in disease pathogenesis. Subsequently, we assessed the potential of HSPs as a chemotherapeutic target and underlined the challenges associated with it. Furthermore, we have summarized a few ongoing drug discovery initiatives that need to be explored further to develop clinically successful chemotherapeutic agents in the future.

Antileishmanial drug discovery: comprehensive review of the last 10 years

This review covers the current aspects of leishmaniasis including marketed drugs, new antileishmanial agents, and possible drug targets of antileishmanial agents.

Trypanosomatids topoisomerase re-visited. New structural findings and role in drug discovery

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There is an urgent need of new treatments against trypanosomatids-borne diseases.

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DNA topoisomerases are pointed as potential drug targets against unicellular parasites.

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Trypanosomatids have a full set of DNA topoisomerases in both nucleus and kinetoplast.

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TopII and TopIII are located in the kinetoplast and fully involved in kDNA replication.

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Tritryps TopIB differ in structure from mammalian’s pointing to an attractive target.

The Trypanosomatidae family, composed of unicellular parasites, causes severe vector-borne diseases that afflict human populations worldwide. Chagas disease, sleeping sickness, as well as different sorts of leishmaniases are amongst the most important infectious diseases produced by Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp., respectively. All these infections are closely related to weak health care services in low-income populations of less developed and least economically developed countries. Search for new therapeutic targets in order to hit these pathogens is of paramount priority, as no effective vaccine is currently in use against any of these parasites. Furthermore, present-day chemotherapy comprises old-fashioned drugs full of important side effects. Besides, they are prone to produce tolerance and resistance as a consequence of their continuous use for decades. DNA topoisomerases (Top) are ubiquitous enzymes responsible for solving the torsional tensions caused during replication and transcription processes, as well as in maintaining genomic stability during DNA recombination. As the inhibition of these enzymes produces cell arrest and triggers cell death, Top inhibitors are among the most effective and most widely used drugs in both cancer and antibacterial therapies. Top relaxation and decatenation activities, which are based on a common nicking–closing cycle involving one or both DNA strands, have been pointed as a promising drug target. Specific inhibitors that bind to the interface of DNA-Top complexes can stabilize Top-mediated transient DNA breaks. In addition, important structural differences have been found between Tops from the Trypanosomatidae family members and Tops from the host. Such dissimilarities make these proteins very interesting for drug design and molecular intervention. The present review is a critical update of the last findings regarding trypanosomatid’s Tops, their new structural features, their involvement both in the physiology and virulence of these parasites, as well as their use as promising targets for drug discovery.

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.

Natural product based leads to fight against leishmaniasis

The growing incidence of parasitic resistance against generic pentavalent antimonials, specifically for visceral disease in Indian subcontinent, is a serious issue in Leishmania control. Notwithstanding the two treatment alternatives, that is amphotericin B and miltefosine are being effectively used but their high cost and therapeutic complications limit their use in endemic areas. In the absence of a vaccine candidate, identification, and characterization of novel drugs and targets is a major requirement of leishmanial research. This review describes current drug regimens, putative drug targets, numerous natural products that have shown promising antileishmanial activity alongwith some key issues and strategies for future research to control leishmaniasis worldwide.

Developments in diagnosis and treatment of visceral leishmaniasis during the last decade and future prospects

Human visceral leishmaniasis (VL) continues to be a life-threatening neglected tropical disease, with close to 200 million people at risk of infection globally. Epidemics and resurgence of VL are associated with negligence by the policy makers, economic decline and population movements. Control of the disease is hampered by the lack of proficient vaccination, rapid diagnosis in a field setting and severe side effects of current drug therapies. The diagnosis of VL relied largely on invasive techniques of detecting parasites in splenic and bone marrow aspirates. rK39 and PCR, despite problems related to varying sensitivities and specificities and field adaptability, respectively, are considered the best options for VL diagnosis today. No single therapy of VL currently offers satisfactory efficacy along with safety. The field of VL research only recently shifted toward actively identifying new drugs for safe and affordable treatment. Oral miltefosine and safe AmBisome along with better use of amphotericin B have been rapidly implemented in the last decade. A combination therapy will substantially reduce the required dose and duration of drug administration and reduce the chance of the development of resistance. In addition, identification of asymptomatic cases, vector control and treatment of post-kala-azar dermal leishmaniasis would allow new perspectives in VL control and management.

Mitochondria Superoxide Anion Production Contributes to Geranylgeraniol-Induced Death in Leishmania amazonensis

Here we demonstrate the activity of geranylgeraniol, the major bioactive constituent from seeds of Bixa orellana, against Leishmania amazonensis. Geranylgeraniol was identified through (1)H and (13)C nuclear magnetic resonance imaging and DEPT. The compound inhibited the promastigote and intracellular amastigote forms, with IC(50) of 11 ± 1.0 and 17.5 ± 0.7 μg/mL, respectively. This compound was also more toxic to parasites than to macrophages and did not cause lysis in human blood cells. Morphological and ultrastructural changes induced by geranylgeraniol were observed in the protozoan by electronic microscopy and included mainly mitochondria alterations and an abnormal chromatin condensation in the nucleus. These alterations were confirmed by Rh 123 and TUNEL assays. Additionally, geranylgeraniol induces an increase in superoxide anion production. Collectively, our in vitro studies indicate geranylgeraniol as a selective antileishmanial that appears to be mediated by apoptosis-like cell death.

Leishmaniasis: current status of available drugs and new potential drug targets

The control of Leishmania infection relies primarily on chemotherapy till date. Resistance to pentavalent antimonials, which have been the recommended drugs to treat cutaneous and visceral leishmaniasis, is now widespread in Indian subcontinents. New drug formulations like amphotericin B, its lipid formulations, and miltefosine have shown great efficacy to treat leishmaniasis but their high cost and therapeutic complications limit their usefulness. In addition, irregular and inappropriate uses of these second line drugs in endemic regions like state of Bihar, India threaten resistance development in the parasite. In context to the limited drug options and unavailability of either preventive or prophylactic candidates, there is a pressing need to develop true antileishmanial drugs to reduce the disease burden of this debilitating endemic disease. Notwithstanding significant progress of leishmanial research during last few decades, identification and characterization of novel drugs and drug targets are far from satisfactory. This review will initially describe current drug regimens and later will provide an overview on few important biochemical and enzymatic machineries that could be utilized as putative drug targets for generation of true antileishmanial drugs.

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.

Theiler's murine encephalomyelitis virus infection induces a redistribution of heat shock proteins 70 and 90 in BHK-21 cells, and is inhibited by novobiocin and geldanamycin

Theiler's murine encephalomyelitis virus (TMEV) is a positive-sense RNA virus belonging to the Cardiovirus genus in the family Picornaviridae. In addition to other host cellular factors and pathways, picornaviruses utilise heat shock proteins (Hsps) to facilitate their propagation in cells. This study investigated the localisation of Hsps 70 and 90 in TMEV-infected BHK-21 cells by indirect immunofluorescence and confocal microscopy. The effect of Hsp90 inhibitors novobiocin (Nov) and geldanamycin (GA) on the development of cytopathic effect (CPE) induced by infection was also examined. Hsp90 staining was uniformly distributed in the cytoplasm of uninfected cells but was found concentrated in the perinuclear region during late infection where it overlapped with the signal for non-structural protein 2C within the viral replication complex. Hsp70 redistributed into the vicinity of the viral replication complex during late infection, but its distribution did not overlap with that of 2C. Inhibition of Hsp90 by GA and Nov had a negative effect on virus growth over a 48-h period as indicated by no observable CPE in treated compared to untreated cells. 2C was detected by Western analysis of GA-treated infected cell lysates at doses between 0.01 and 0.125 μM, suggesting that processing of viral precursors was not affected in the presence of this drug. In contrast, 2C was absent in cell lysates of Nov-treated cells at doses above 10 μM, although CPE was evident 48 hpi. This is the first study describing the dynamic behaviour of Hsps 70 and 90 in TMEV-infected cells and to identify Hsp90 as an important host factor in the life cycle of this virus.

99mTc-Novobiocin: a~novel radiotracer for infection imaging

The radiosynthesis of 99mTc-Novobiocin (99mTc-NBN) complex and its suitability as a radiotracer for infection imaging was assessed. The radiochemical purity (RCP) of the 99mTc-NBN complex was determined using radio-TLC and radioactive HPLC and biodistribution was studied in artificially infected (A.I.) rats and rabbit, using single well gamma counter (SWGRC) interface with scalar count rate meter (SCRM) and Gamma Camera (γ-CM). The maximum RCP observed for the preparation having 2 mg of NBN, 111 MBq of sodium pertechnetate (Na99mTcO4) and 125 μL of SnF2 (1 μg/μL in 0.01 N HCl) at a pH 5.6 was 98.97±0.40% and remained stable >90% up to 120 min. The activity of the 99mTc-NBN in the infected muscle (TI) was significantly increased from 6.50±0.15 to 19.00±0.17% and decreased in the inflamed muscle (TII), normal muscle (NT), blood, liver, spleen, stomach and intestine within 120 min. The TI/NT and TII/NT ratios were 7.60±1.08 and 1.60±1.14. The Whole Body Static (WBS) images of A.I. rabbit were obtained at 30, 40, 50 and 60 min after the I.V. administration of 111 MBq of 99mTc-NBN to the A.I. rabbit. The stability in saline and serum, higher TI/NT, lower TII/NT ratios and WBS images confirmed the feasibility of the 99mTc-NBN complex as an infection imaging agent.

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

Apoptosis is a normal component of the development and health of multicellular organisms. However, apoptosis is now considered a prerogative of unicellular organisms, including the trypanosomatids of the genera Trypanosoma spp. and Leishmania spp., causative agents of some of the most important neglected human diseases. Trypanosomatids show typical hallmarks of apoptosis, although they lack some of the key molecules contributing to this process in metazoans, like caspase genes, Bcl-2 family genes and the TNF-related family of receptors. Despite the lack of these molecules, trypanosomatids appear to have the basic machinery to commit suicide. The components of the apoptotic execution machinery of these parasites are slowly coming into light, by targeting essential processes and pathways with different apoptogenic agents and inhibitors. This review will be confined to the events known to drive trypanosomatid parasites to apoptosis.

Apoptotic markers in protozoan parasites

The execution of the apoptotic death program in metazoans is characterized by a sequence of morphological and biochemical changes that include cell shrinkage, presentation of phosphatidylserine at the cell surface, mitochondrial alterations, chromatin condensation, nuclear fragmentation, membrane blebbing and the formation of apoptotic bodies. Methodologies for measuring apoptosis are based on these markers. Except for membrane blebbing and formation of apoptotic bodies, all other events have been observed in most protozoan parasites undergoing cell death. However, while techniques exist to detect these markers, they are often optimised for metazoan cells and therefore may not pick up subtle differences between the events occurring in unicellular organisms and multi-cellular organisms.

In this review we discuss the markers most frequently used to analyze cell death in protozoan parasites, paying special attention to changes in cell morphology, mitochondrial activity, chromatin structure and plasma membrane structure/permeability. Regarding classical regulators/executors of apoptosis, we have reviewed the present knowledge of caspase-like and nuclease activities.

Berberine chloride causes a caspase-independent, apoptotic-like death in Leishmania donovani promastigotes

Berberine chloride, a quarternary isoquinoline alkaloid, is a promising anti-leishmanial compound, IC(50) being 7.1 microM in L. donovani promastigotes. This leishmanicidal activity was initiated by its pro-oxidant effect, evidenced by enhanced generation of reactive oxygen intermediates that was accompanied by depletion of thiols; pre-incubation in N-acetyl cysteine, attenuated its cell viability, corroborating that generation of free radicals triggered its parasiticidal activity. Externalization of phosphatidylserine and elevation of intracellular calcium preceded depolarization of the mitochondrial membrane potential, which translated into an increase in the sub G(0)/G(1) population and was accompanied by DNA laddering, hallmarks of apoptosis. Berberine chloride failed to induce caspase activity and anti-leishmanial activity in the presence of a pan caspase inhibitor, Z-Val-Ala-DL-Asp (methoxy)-fluoromethylketone remained unchanged, which indicated that the apoptosis was caspase independent. Collectively, the data indicates that Berberine chloride triggers an apoptosis-like death following enhanced generation of reactive oxygen species, thus meriting further pharmacological investigations.

An orally effective dihydropyrimidone (DHPM) analogue induces apoptosis-like cell death in clinical isolates of Leishmania donovani overexpressing pteridine reductase 1

The protozoan parasite Leishmania donovani is the causative agent of visceral leishmaniasis. The enzyme pteridine reductase 1 (PTR1) of L. donovani acts as a metabolic bypass for drugs targeting dihydrofolate reductase (DHFR); therefore, for successful antifolate chemotherapy to be developed against Leishmania, it must target both enzyme activities. Leishmania cells overexpressing PTR1 tagged at the N-terminal with green fluorescent protein were established to screen for proprietary dihydropyrimidone (DHPM) derivatives of DHFR specificity synthesised in our laboratory. A cell-permeable molecule with impressive antileishmanial in vitro and in vivo oral activity was identified. Structure activity relationship based on homology model drawn on our recombinant enzyme established the highly selective inhibition of the enzyme by this analogue. It was seen that the leishmanicidal effect of this analogue is triggered by programmed cell death mediated by the loss of plasma membrane integrity as detected by binding of annexin V and propidium iodide (PI), loss of mitochondrial membrane potential culminating in cell cycle arrest at the sub-G0/G1 phase and oligonucleosomal DNA fragmentation. Hence, this DHPM analogue [(4-fluoro-phenyl)-6-methyl-2-thioxo-1, 2, 3, 4-tetrahydropyrimidine-5-carboxylic acid ethyl ester] is a potent antileishmanial agent that merits further pharmacological investigation.

Naphthoimidazoles promote different death phenotypes in Trypanosoma cruzi

SUMMARY

In a screening of 65 derivatives of natural quinones using bloodstream trypomastigotes of Trypanosoma cruzi, the 3 naphthoimidazoles derived from beta-lapachone - N1, N2 and N3--were selected as the most active. Investigation of their mode of action led to the characterization of mitochondrion, reservosomes and DNA as their main targets, and stimulated further studies on death pathways. Ultrastructural analysis revealed both autophagic (autophagosomes) and apoptotic-like (membrane blebbing) phenotypes. Flow cytometry analysis showed, in N2-treated trypomastigotes, a small increase of phosphatidylserine exposure, and a large increase in the percentage of necrosis, caused by N1 or N2. These death phenotypes were not detected in treated epimastigotes. The strong increase in labelling of monodansyl cadaverine, the inhibition of the death process by wortmannin or 3-methyladenine, the overexpression of ATG genes in treated epimastigotes, together with ultrastructural evidence point to autophagy as the predominant phenotype induced by the naphthoimidazoles. However, there are other pathways occurring concomitantly with variable intensities, justifying the need to detail the molecular features involved.

Inhibition of active nuclear transport is an intrinsic trigger of programmed cell death in trypanosomatids

The link between nucleocytoplasmic transport and apoptosis remains controversial. Nucleocytoplasmic exchange of molecules seems indeed essential for the initiation and execution of the apoptotic programme; but inhibition of nuclear transport factors may also represent a powerful apoptotic trigger. The GTPase Ran (together with its partners), first discovered to be essential in nucleocytoplasmic transport, has multiple key functions in cell biology, and particularly in spindle assembly, kinetochore function and nuclear envelope assembly. Among the Ran partners studied, NTF2 appears to be solely involved in nucleocytoplasmic transport. Here, we localised Ran and several of its partners, RanBP1, CAS and NTF2, at the nuclear membrane in the trypanosomatid Leishmania major. Remarkably, these proteins fused to GFP decorated a perinuclear 'collar' of about 15 dots, colocalising at nuclear pore complexes with the homologue of nucleoporin Sec13. In the other trypanosomatid Trypanosoma brucei, RNAi knockdown of the expression of the corresponding genes resulted in an apoptosis-like phenomenon. These phenotypes show that Ran and its partners have a key function in trypanosomatids like they have in mammals. Our data, notably those about TbNTF2 RNAi, support the idea that active nucleocytoplasmic transport is not essential for the initiation and execution of apoptosis, and, rather, the impairment of this transport constitutes an intrinsic signal for triggering PCD.

Induction of apoptosis-like cell death by pentamidine and doxorubicin through differential inhibition of topoisomerase II in arsenite-resistant L. donovani

The current study has been undertaken to investigate the sensitivity of the topoisomerase II (topo II) of wild type (Ld-Wt) and arsenite-resistant (Ld-As20) L. donovani to an anti-leishmanial agent pentamidine and an anti-cancer drug doxorubicin. We demonstrate that the cross resistance to pentamidine and doxorubicin in Ld-As20, was in part implicated through differential inhibition of topo II in Ld-Wt and Ld-As20. Further, the treatment of promastigotes at drug concentrations inhibiting 50% of topo II activity inflicted a regulated cell death sharing several apoptotic features like externalization of phosphatidylserine, loss of mitochondrial membrane potential, cytochrome C release into the cytosol, activation of cellular proteases and DNA fragmentation. The cytotoxic potential of pentamidine and doxorubicin in L. donovani has been shown to be mediated through topoisomerase II inhibition and results in inciting programmed cell death process.

Characterization of metacaspases with trypsin-like activity and their putative role in programmed cell death in the protozoan parasite Leishmania

In this report, we have characterized two metacaspases of Leishmania donovani, L. donovani metacaspase-1 (LdMC1) and LdMC2. These two proteins show 98% homology with each other, and both contain a characteristic C-terminal proline-rich domain. Both genes are transcribed in promastigotes and axenic amastigotes of L. donovani; however, LdMC1 shows increased mRNA levels in axenic amastigotes. An anti-LdMC antibody was obtained and showed reactivity with a single approximately 42-kDa protein band in both promastigote and axenic amastigote parasite whole-cell lysates by Western blotting. Pulse-chase experiments suggest that LdMCs are not synthesized as proenzymes, and immunofluorescence studies show that LdMCs are associated with the acidocalcisome compartments of L. donovani. Enzymatic assays of immunoprecipitated LdMCs show that native LdMCs efficiently cleave trypsin substrates and are unable to cleave caspase-specific substrates. Consistently, LdMC activity is insensitive to caspase inhibitors and is efficiently inhibited by trypsin inhibitors, such as leupeptin, antipain, and N(alpha)-tosyl-L-lysine-chloromethyl ketone (TLCK). In addition, our results show that LdMC activity was induced in parasites treated with hydrogen peroxide, a known trigger of programmed cell death (PCD) in Leishmania and that parasites overexpressing metacaspases are more sensitive to hydrogen peroxide-induced PCD. These findings suggest that Leishmania metacaspases are not responsible for the caspase-like activities reported in this organism and suggest a possible role for LdMCs as effector molecules in Leishmania PCD.

Miltefosine induces apoptosis in arsenite-resistant Leishmania donovani promastigotes through mitochondrial dysfunction

The control of leishmaniasis in absence of vaccine solely depends on the choice of chemotherapy. The major hurdle in successful leishmanial chemotherapy is emergence of drug resistance. Miltefosine, the first orally administrable anti-leishmanial drug, has shown the potential against drug-resistant strains of Leishmania. However, there are discrepancies regarding the involvement of P-glycoprotein (Pgp) and sensitivity of miltefosine in multiple drug-resistant (MDR) cell lines that overexpress Pgp in Leishmania. To address this, the effect of miltefosine in arsenite-resistant Leishmania donovani (Ld-As20) promastigotes displaying an MDR phenotype and overexpressing Pgp-like protein was investigated in the current study. Results indicate that Ld-As20 is sensitive to miltefosine. Miltefosine induces process of programmed cell death in Ld-As20 in a time-dependent manner as determined by cell shrinkage, externalization of phosphatidylserine and DNA fragmentation. Miltefosine treatment leads to loss of mitochondrial membrane potential and the release of cytochrome C with consequent activation of cellular proteases. Activation of cellular proteases resulted in activation of DNase that damaged kinetoplast DNA and induced dyskinetoplasty. These data indicate that miltefosine causes apoptosis-like death in arsenite-resistant L. donovani.

Phenotypical characteristics, biochemical pathways, molecular targets and putative role of nitric oxide-mediated programmed cell death in Leishmania

Nitric oxide (NO) has been demonstrated to be the principal effector molecule mediating intracellular killing of Leishmania, both in vitro and in vivo. We investigated the type of cell death process induced by NO for the intracellular amastigote stage of the protozoa Leishmania. Specific detection methods revealed a rapid and extensive cell death with morphological features of apoptosis in axenic amastigotes exposed to NO donors, in intracellular amastigotes inside in vitro - activated mouse macrophages and also in activated macrophages of regressive lesions in a leishmaniasis-resistant mouse model. We extended our investigations to the dog, a natural host-reservoir of Leishmania parasites, by demonstrating that co-incubation of infected macrophages with autologous lymphocytes derived from dogs immunised with purified excreted-secreted antigens of Leishmania resulted in a significant NO-mediated apoptotic cell death of intracellular amastigotes. From the biochemical point of view, NO-mediated Leishmania amastigotes apoptosis did not seem to be controlled by caspase activity as indicated by the lack of effect of cell permeable inhibitors of caspases and cysteine proteases, in contrast to specific proteasome inhibitors, such as lactacystin or calpain inhibitor I. Moreover, addition of the products of two NO molecular targets, cis-aconitase and glyceraldehyde-3-phosphate dehydrogenase, also had an inhibitory effect on the cell death induced by NO. Interestingly, activities of these two enzymes plus 6-phosphogluconate dehydrogenase, parasitic enzymes involved in both glycolysis and respiration processes, are overexpressed in amastigotes selected for their NO resistance. This review focuses on cell death of the intracellular stage of the pathogen Leishmania induced by nitrogen oxides and gives particular attention to the biochemical pathways and the molecular targets potentially involved. Questions about the role of Leishmania amastigotes NO-mediated apoptosis in the overall infection process are raised and discussed.

Alterations on the growth and ultrastructure of Leishmania chagasi induced by squalene synthase inhibitors

Leishmaniasis is an important disease in widely dispersed regions of the world. In South America, visceral leishmaniasis (VL) is mainly caused by Leishmania chagasi. The morbidity associated with the infection is high, and death may occur in some untreated patients. Treatment has been based upon pentavalent antimonial drugs for more than half a century and problems, including development of resistance to antimonials and lack of efficacy against VL/HIV co-infections, have emphasized the need for new drugs. Squalene synthase (SQS) is an essential enzyme for the biosynthesis of protozoal sterol molecules. In this work, nineteen synthetic quinuclidines, potentially inhibitors of SQS, were tested against promastigote forms of L. chagasi and the IC50 values of the compounds were determined. The most active compounds had IC50 values of around 30 nM and induced complete growth arrest and cell lysis at sub-micromolar concentrations. We analyzed the morphological structure of the parasites treated with these compounds by transmission electron microscopy of thin sections. Treated parasites showed significant ultrastructural changes, which varied from discrete alterations to total destruction of the cells, depending on the drug concentration and the time of incubation. One important change observed was a typical swelling of the unique and highly branched mitochondrion, where the inner membrane lost its organization. There was an increase in the number of autophagosomal structures. Changes in the organization of the nuclear chromatin and alterations in the flagellar pocket and flagellar membrane were also observed.

Death of a trypanosome: a selfish altruism

African trypanosomes and some related parasitic protozoa are affected by a form of programmed cell death (PCD) that shows typical hallmarks of apoptosis. Although it has been speculated that PCD has a function in life-cycle progression and the struggle for survival of these parasites, no satisfactory model has yet been proposed for the molecular mechanism(s) of PCD in protozoa, raising questions about its physiological relevance in these organisms. As we discuss here, the most important point that needs to be addressed is whether a single-celled organism can undertake a process that is considered altruistic.

Leishmania donovani: intracellular ATP level regulates apoptosis-like death in luteolin induced dyskinetoplastid cells

Leishmaniasis presents a spectrum of diseases ranging from benign cutaneous lesions to the often-fatal visceralizing form. Luteolin, a dietary flavone induces apoptosis-like death in both promastigote and amastigote forms of Leishmania, the causative agent of the diseases. Here, we have elucidated the mechanism of action of luteolin by analyzing the mitochondrial and cytosolic changes associated with apoptosis-like death of leishmanial cells. In Leishmania donovani, treatment with luteolin induces the loss of both maxicircles and minicircles which resulted in the formation of dyskinetoplastid cells. The loss of mitochondrial DNA causes reduction in the activities of complex I, II, III, and IV of electron transport chain. However, the mitochondrial ATPase activity of complex V remains almost unaltered during treatment with luteolin but the sensitivity to oligomycin is lost. The inactivation of ETC complex is associated with decrease in mitochondrial as well as glycolytic ATP production, which is responsible for depolarization of Deltapsi(m) and alteration in mitochondrial structure. This event is followed by the release of cytochrome c from mitochondria in mt-DNA depleted leishmanial cells and causes an activation of caspase like proteases. Collectively our results provide the first insight into the mechanistic pathway of apoptosis-like death where inhibition of glycolytic ATP production is an essential event responsible for depolarization of Deltapsi(m) in mt-DNA depleted cells to propagate apoptosis-like death in leishmanial cells.