Disruption of a mitochondrial protease machinery in Plasmodium falciparum is an intrinsic signal for parasite cell death

Dinuclear and mononuclear metal(II) polypyridyl complexes against drug-sensitive and drug-resistant Plasmodium falciparum and their mode of action

BACKGROUND

Malaria remains one of the most virulent and deadliest parasitic disease in the world, particularly in Africa and Southeast Asia. Widespread occurrence of artemisinin-resistant Plasmodium falciparum strains from the Greater Mekong Subregion is alarming. This hinders the national economies, as well as being a major drawback in the effective control and elimination of malaria worldwide. Clearly, an effective anti-malarial drug is urgently needed.

METHODS

The dinuclear and mononuclear copper(II) and zinc(II) complexes were synthesized in ethanolic solution and characterized by various physical measurements (FTIR, CHN elemental analysis, solubility, ESI-MS, UV-Visible, conductivity and magnetic moment, and NMR). X-ray crystal structure of the dicopper(II) complex was determined. The in vitro haemolytic activities of these metal complexes were evaluated spectroscopically on B+ blood while the anti-malarial potency was performed in vitro on blood stage drug-sensitive Plasmodium falciparum 3D7 (Pf3D7) and artemisinin-resistant Plasmodium falciparum IPC5202 (Pf5202) with fluorescence dye. Mode of action of metal complexes were conducted to determine the formation of reactive oxygen species using PNDA and DCFH-DA dyes, JC-1 depolarization of mitochondrial membrane potential, malarial 20S proteasome inhibition with parasite lysate, and morphological studies using Giemsa and Hoechst stains.

RESULTS

Copper(II) complexes showed anti-malarial potency against both Pf3D7 and Pf5202 in sub-micromolar to micromolar range. The zinc(II) complexes were effective against Pf3D7 with excellent therapeutic index but encountered total resistance against Pf5202. Among the four, the dinuclear copper(II) complex was the most potent against both strains. The zinc(II) complexes caused no haemolysis of RBC while copper(II) complexes induced increased haemolysis with increasing concentration. Further mechanistic studies of both copper(II) complexes on both Pf3D7 and Pf5202 strains showed induction of ROS, 20S malarial proteasome inhibition, loss of mitochondrial membrane potential and morphological features indicative of apoptosis.

CONCLUSION

The dinuclear [Cu(phen)-4,4'-bipy-Cu(phen)](NO₃)₄ is highly potent and can overcome the total drug-resistance of Pf5202 towards chloroquine and artemisinin. The other three copper(II) and zinc(II) complexes were only effective towards the drug-sensitive Pf3D7, with the latter causing no haemolysis of RBC. Their mode of action involves multiple targets.

Dual role of an essential HtrA2/Omi protease in the human malaria parasite: Maintenance of mitochondrial homeostasis and induction of apoptosis-like cell death under cellular stress

Members of the HtrA family of serine proteases are known to play roles in mitochondrial homeostasis as well as in programmed cell death. Mitochondrial homeostasis and metabolism are crucial for the survival and propagation of the malaria parasite within the host. Here we have functionally characterized a Plasmodium falciparum HtrA2 (PfHtrA2) protein, which harbours trypsin-like protease activity that can be inhibited by its specific inhibitor, ucf-101. A transgenic parasite line was generated, using the HA-glmS C-terminal tagging approach, for localization as well as for inducible knock-down of PfHtrA2. The PfHtrA2 was localized in the parasite mitochondrion during the asexual life cycle. Genetic ablation of PfHtrA2 caused significant parasite growth inhibition, decreased replication of mtDNA, increased mitochondrial ROS production, caused mitochondrial fission/fragmentation, and hindered parasite development. However, the ucf-101 treatment did not affect the parasite growth, suggesting the non-protease/chaperone role of PfHtrA2 in the parasite. Under cellular stress conditions, inhibition of PfHtrA2 by ucf-101 reduced activation of the caspase-like protease as well as parasite cell death, suggesting the involvement of protease activity of PfHtrA2 in apoptosis-like cell death in the parasite. Under these cellular stress conditions, the PfHtrA2 gets processed but remains localized in the mitochondrion, suggesting that it acts within the mitochondrion by cleaving intra-mitochondrial substrate(s). This was further supported by trans-expression of PfHtrA2 protease domain in the parasite cytosol, which was unable to induce any cell death in the parasite. Overall, we show the specific roles of PfHtrA2 in maintaining mitochondrial homeostasis as well as in regulating stress-induced cell death.

The C-terminal segment of Leishmania major HslU: Toward potential inhibitors of LmHslVU activity

It is urgent to develop less toxic and more efficient treatments for leishmaniases and trypanosomiases. We explore the possibility to target the parasite mitochondrial HslVU protease, which is essential for growth and has no analogue in the human host. For this, we develop compounds potentially inhibiting the complex assembly by mimicking the C-terminal (C-ter) segment of the ATPase HslU. We previously showed that a dodecapeptide derived from Leishmania major HslU C-ter segment (LmC12-U2, Cpd 1) was able to bind to and activate the digestion of a fluorogenic substrate by LmHslV. Here, we present the study of its structure-activity relationships. By replacing each essential residue with related non-proteinogenic residues, we obtained more potent analogues. In particular, a cyclohexylglycine residue at position 11 (cpd 24) allowed a more than three-fold gain in potency while reducing the size of compound 24 from twelve to six residues (cpd 50) without significant loss of potency, opening the way toward short HslU C-ter peptidomimetics as potential inhibitors of HslV proteolytic function. Finally, conjugates constituted of LmC6-U2 analogues and a mitochondrial penetrating peptide were found to penetrate into the promastigote form of L. infantum and to inhibit the parasite growth without showing toxicity toward human THP-1 cells at the same concentration (i.e. 30 μM).

Mitochondrial dynamics in Angiostrongylus cantonensis-infected mouse brain

Angiostrongylus cantonensis is one of the most widespread parasites causing central nervous system (CNS) diseases in mammals. Since the mitochondrion is an essential cell organelle responsible for both physiological and pathological processes, its dysfunction might lead to inflammation and multiple disorders. In this study we aimed to investigate the changes in mitochondrial dynamics that occur in the mouse brain upon infection with A. cantonensis, using molecular biology techniques such as polymerase chain reaction (PCR), western blot analysis, transmission electron microscopy (TEM), and different staining methods. Here, we show that mouse brain infected with A. cantonensis exhibits altered mitochondrial dynamics, including fission, fusion, and biogenesis. Additionally, we demonstrate that caspases and B-cell lymphoma 2 (BCL-2) were significantly upregulated in A. cantonensis-infected brain. These results are indicative of the occurrence of apoptosis during A. cantonensis infection, which was further confirmed by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. These findings suggest the change in mitochondrial dynamics in A. cantonensis-infected brain, providing another point of view on the pathogenesis of meningoencephalitis caused by A. cantonensis infection.

Phosphatidylinositol 3-phosphate and Hsp70 protect Plasmodium falciparum from heat-induced cell death

Phosphatidylinositol 3-phosphate (PI(3)P) levels in Plasmodium falciparum correlate with tolerance to cellular stresses caused by artemisinin and environmental factors. However, PI(3)P function during the Plasmodium stress response was unknown. Here, we used PI3K inhibitors and antimalarial agents to examine the importance of PI(3)P under thermal conditions recapitulating malarial fever. Live cell microscopy using chemical and genetic reporters revealed that PI(3)P stabilizes the digestive vacuole (DV) under heat stress. We demonstrate that heat-induced DV destabilization in PI(3)P-deficient P. falciparum precedes cell death and is reversible after withdrawal of the stress condition and the PI3K inhibitor. A chemoproteomic approach identified PfHsp70-1 as a PI(3)P-binding protein. An Hsp70 inhibitor and knockdown of PfHsp70-1 phenocopy PI(3)P-deficient parasites under heat shock. Furthermore, PfHsp70-1 downregulation hypersensitizes parasites to heat shock and PI3K inhibitors. Our findings underscore a mechanistic link between PI(3)P and PfHsp70-1 and present a novel PI(3)P function in DV stabilization during heat stress.

Plasmodium chaperonin TRiC/CCT identified as a target of the antihistamine clemastine using parallel chemoproteomic strategy

The antihistamine clemastine inhibits multiple stages of the Plasmodium parasite that causes malaria, but the molecular targets responsible for its parasite inhibition were unknown. Here, we applied parallel chemoproteomic platforms to discover the mechanism of action of clemastine and identify that clemastine binds to the Plasmodium falciparum TCP-1 ring complex or chaperonin containing TCP-1 (TRiC/CCT), an essential heterooligomeric complex required for de novo cytoskeletal protein folding. Clemastine destabilized all eight P. falciparum TRiC subunits based on thermal proteome profiling (TPP). Further analysis using stability of proteins from rates of oxidation (SPROX) revealed a clemastine-induced thermodynamic stabilization of the Plasmodium TRiC delta subunit, suggesting an interaction with this protein subunit. We demonstrate that clemastine reduces levels of the major TRiC substrate tubulin in P. falciparum parasites. In addition, clemastine treatment leads to disorientation of Plasmodium mitotic spindles during the asexual reproduction and results in aberrant tubulin morphology suggesting protein aggregation. This clemastine-induced disruption of TRiC function is not observed in human host cells, demonstrating a species selectivity required for targeting an intracellular human pathogen. Our findings encourage larger efforts to apply chemoproteomic methods to assist in target identification of antimalarial drugs and highlight the potential to selectively target Plasmodium TRiC-mediated protein folding for malaria intervention.

Small Molecule Inhibitors Targeting the Heat Shock Protein System of Human Obligate Protozoan Parasites

Obligate protozoan parasites of the kinetoplastids and apicomplexa infect human cells to complete their life cycles. Some of the members of these groups of parasites develop in at least two systems, the human host and the insect vector. Survival under the varied physiological conditions associated with the human host and in the arthropod vectors requires the parasites to modulate their metabolic complement in order to meet the prevailing conditions. One of the key features of these parasites essential for their survival and host infectivity is timely expression of various proteins. Even more importantly is the need to keep their proteome functional by maintaining its functional capabilities in the wake of physiological changes and host immune responses. For this reason, molecular chaperones (also called heat shock proteins)-whose role is to facilitate proteostasis-play an important role in the survival of these parasites. Heat shock protein 90 (Hsp90) and Hsp70 are prominent molecular chaperones that are generally induced in response to physiological stress. Both Hsp90 and Hsp70 members are functionally regulated by nucleotides. In addition, Hsp70 and Hsp90 cooperate to facilitate folding of some key proteins implicated in cellular development. In addition, Hsp90 and Hsp70 individually interact with other accessory proteins (co-chaperones) that regulate their functions. The dependency of these proteins on nucleotide for their chaperone function presents an Achille's heel, as inhibitors that mimic ATP are amongst potential therapeutic agents targeting their function in obligate intracellular human parasites. Most of the promising small molecule inhibitors of parasitic heat shock proteins are either antibiotics or anticancer agents, whose repurposing against parasitic infections holds prospects. Both cancer cells and obligate human parasites depend upon a robust protein quality control system to ensure their survival, and hence, both employ a competent heat shock machinery to this end. Furthermore, some inhibitors that target chaperone and co-chaperone networks also offer promising prospects as antiparasitic agents. The current review highlights the progress made so far in design and application of small molecule inhibitors against obligate intracellular human parasites of the kinetoplastida and apicomplexan kingdoms.

Enhanced uptake, high selective and microtubule disrupting activity of carbohydrate fused pyrano-pyranones derived from natural coumarins attributes to its anti-malarial potential

Background

Malaria is one of the deadliest infectious diseases caused by protozoan parasite of Plasmodium spp. Increasing resistance to anti-malarials has become global threat in control of the disease and demands for novel anti-malarial interventions. Naturally-occurring coumarins, which belong to a class of benzo-α-pyrones, found in higher plants and some essential oils, exhibit therapeutic potential against various diseases. However, their limited uptake and non-specificity has restricted their wide spread use as potential drug candidates.

Methods

Two series of carbohydrate fused pyrano[3,2-c]pyranone carbohybrids which were synthesized by combination of 2-C-formyl galactal and 2-C-formyl glucal, with various freshly prepared 4-hydroxycoumarins were screened against Plasmodium falciparum. The anti-malarial activity of these carbohybrids was determined by growth inhibition assay on P. falciparum 3D7 strain using SYBR green based fluorescence assay. Haemolytic activity of carbohybrid 12, which showed maximal anti-malarial activity, was determined by haemocompatibility assay. The uptake of the carbohybrid 12 by parasitized erythrocytes was determined using confocal microscopy. Growth progression assays were performed to determine the stage specific effect of carbohybrid 12 treatment on Pf3D7. In silico studies were conducted to explore the mechanism of action of carbohybrid 12 on parasite microtubule dynamics. These findings were further validated by immunofluorescence assay and drug combination assay.

Results

2-C-formyl galactal fused pyrano[3,2-c]pyranone carbohybrid 12 exhibited maximum growth inhibitory potential against Plasmodium with IC₅₀ value of 5.861 µM and no toxicity on HepG2 cells as well as no haemolysis of erythrocytes. An enhanced uptake of this carbohybrid compound was observed by parasitized erythrocytes as compared to uninfected erythrocytes. Further study revealed that carbohybrid 12 arrests the growth of parasite at trophozoite and schizonts stage during course of progression through asexual blood stages. Mechanistically, it was shown that the carbohybrid 12 binds to α,β-heterodimer of tubulin and affects microtubule dynamics.

Conclusion

These findings show carbohydrate group fusion to 4-hydroxycoumarin precursor resulted in pyrano-pyranones derivatives with better solubility, enhanced uptake and improved selectivity. This data confirms that, carbohydrate fused pyrano[3,2-c]pyranones carbohybrids are effective candidates for anti-malarial interventions against P. falciparum.

The HslV Protease from Leishmania major and Its Activation by C-terminal HslU Peptides

HslVU is an ATP-dependent proteolytic complex present in certain bacteria and in the mitochondrion of some primordial eukaryotes, including deadly parasites such as Leishmania. It is formed by the dodecameric protease HslV and the hexameric ATPase HslU, which binds via the C-terminal end of its subunits to HslV and activates it by a yet unclear allosteric mechanism. We undertook the characterization of HslV from Leishmania major (LmHslV), a trypanosomatid that expresses two isoforms for HslU, LmHslU1 and LmHslU2. Using a novel and sensitive peptide substrate, we found that LmHslV can be activated by peptides derived from the C-termini of both LmHslU1 and LmHslU2. Truncations, Ala- and D-scans of the C-terminal dodecapeptide of LmHslU2 (LmC12-U2) showed that five out of the six C-terminal residues of LmHslU2 are essential for binding to and activating HslV. Peptide cyclisation with a lactam bridge allowed shortening of the peptide without loss of potency. Finally, we found that dodecapeptides derived from HslU of other parasites and bacteria are able to activate LmHslV with similar or even higher efficiency. Importantly, using electron microscopy approaches, we observed that the activation of LmHslV was accompanied by a large conformational remodeling, which represents a yet unidentified layer of control of HslV activation.

Pumilacidins from the Octocoral-Associated Bacillus sp. DT001 Display Anti-Proliferative Effects in Plasmodium falciparum

Chemical examination of the octocoral-associated Bacillus species (sp.) DT001 led to the isolation of pumilacidins A (1) and C (2). We investigated the effect of these compounds on the viability of Plasmodium falciparum and the mechanism of pumilacidin-induced death. The use of inhibitors of protein kinase C (PKC) and phosphoinositide 3-kinase (PI3K) was able to prevent the effects of pumilacidins A and C. The results indicated also that pumilacidins inhibit parasite growth via mitochondrial dysfunction and decreased cytosolic Ca²⁺.

The proteasome as a target to combat malaria: hits and misses

The proteasome plays a vital role throughout the life cycle as Plasmodium parasites quickly adapt to a new host and undergo a series of morphologic changes during asexual replication and sexual differentiation. Plasmodium carries 3 different types of protease complexes: typical eukaryotic proteasome (26S) that resides in the cytoplasm and the nucleus, a prokaryotic proteasome homolog ClpQ that resides in the mitochondria, and a caseinolytic protease complex ClpP that resides in the apicoplast. In silico prediction in conjunction with immunoprecipitation analysis of ubiquitin conjugates have suggested that over half of the Plasmodium falciparum proteome during asexual reproduction are potential targets for ubiquitination. The marked potency of multiple classes of proteasome inhibitors against all stages of the life cycle, synergy with the current frontline antimalarial, artemisinin, and recent advances identifying differences between Plasmodium and human proteasomes strongly support further drug development efforts.

Long circulatory liposomal maduramicin inhibits the growth of Plasmodium falciparum blood stages in culture and cures murine models of experimental malaria

Malaria continues to be one of the deadliest infectious diseases and a global health menace. The emergence and spread of drug-resistant strains of malaria parasites have further made the process of disease management grimmer. Thus, there is an urgent need to identify promising antimalarial strategies that can target the blood stages as well as block parasite transmission. Maduramicin is one such ionophore selected out of a recent screen of gametocytocidal compounds that exhibit potent antiplasmodial activity. However, maduramicin's strong hydrophobic nature and associated toxicity restrict its application in chemotherapy. To alleviate this problem, we have developed a liposomal formulation loaded with the ionophore maduramicin for the treatment of chloroquine sensitive and resistant Plasmodium infections. Here, we show that maduramicin in PEGylated liposomal formulations displayed enhanced antiplasmodial activity in vitro compared to free maduramicin. Significantly, four consecutive doses of 1.5 mg kg-1 body weight of PEGylated maduramicin loaded lipid vesicles completely cured cerebral and chloroquine resistant murine models of malaria without any obvious toxic effects and suppressed the key inflammatory markers associated with the progression of the disease. PEGylated liposomal maduramicin also exhibited a prolonged plasma clearance rate, implying a greater chance of interaction and uptake by infected RBCs. Furthermore, we also provide evidence that the detrimental effect of liposomal maduramicin on parasite survival is mediated by increased ROS generation and subsequent perturbation of parasite mitochondrial membrane potential. This study presents the first report to demonstrate the potent antimalarial efficacy of maduramicin liposomes, a strategy that holds promise for the development of successful therapeutic intervention against malaria in humans.

Novel Activity of a Synthetic Decapeptide Against Toxoplasma gondii Tachyzoites

The killer peptide KP is a synthetic decapeptide derived from the sequence of the variable region of a recombinant yeast killer toxin-like microbicidal single-chain antibody. KP proved to exert significant activities against diverse microbial and viral pathogens through different mechanisms of action, but little is known of its effect on apicomplexan protozoa. The aim of the present study was to evaluate the in vitro activity of KP against Toxoplasma gondii, a globally widespread protozoan parasite of great medical interest. The effect of KP treatment and its potential mechanism of action on T. gondii were evaluated by various methods, including light microscopy, quantitative PCR, flow cytometry, confocal microscopy, and transmission electron microscopy. In the presence of KP, the number of T. gondii tachyzoites able to invade Vero cells and the parasite intracellular proliferation were significantly reduced. Morphological observation and analysis of apoptotic markers suggested that KP is able to trigger an apoptosis-like cell death in T. gondii. Overall, our results indicate that KP could be a promising candidate for the development of new anti-Toxoplasma drugs with a novel mechanism of action.

Effect of tamoxifen on the sphingolipid biosynthetic pathway in the different intraerythrocytic stages of the apicomplexa Plasmodium falciparum

Parasites of the genus Plasmodium responsible for Malaria are obligate intracellular pathogens residing in mammalian red blood cells, hepatocytes, or mosquito midgut epithelial cells. Regarding that detailed knowledge on the sphingolipid biosynthetic pathway of the apicomplexan protozoan parasites is scarce, different stages of Plasmodium falciparum were treated with tamoxifen in order to evaluate the effects of this drug on the glycosphingolipid biosynthesis. Thin layer chromatography, High performance reverse phase chromatography and UV-MALDI-TOF mass spectrometry were the tools used for the analysis. In the ring forms, the increase of NBD-phosphatidyl inositol biosynthesis was notorious but differences at NBD-GlcCer levels were undetectable. In trophozoite forms, an abrupt decrease of NBD-acylated GlcDHCer and NBD-GlcDHCer in addition to an increase of NBD-PC biosynthesis was observed. On the contrary, in schizonts, tamoxifen seems not to be producing substantial changes in lipid biosynthesis. Our findings indicate that in this parasite, tamoxifen is exerting an inhibitory action on Glucosylceramidesynthase and sphingomyelin synthase levels. Moreover, regarding that Plasmodium does not biosynthesize inositolphosphoceramides, the accumulation of phosphatidylinositol should indicate an inhibitory action on glycosylinositol phospholipid synthesis.

Protein Degradation Systems as Antimalarial Therapeutic Targets

Artemisinin (ART)-based combination therapies are the most efficacious treatment of uncomplicated Plasmodium falciparum malaria. Alarmingly, P. falciparum strains have acquired resistance to ART across much of Southeast Asia. ART creates widespread protein and lipid damage inside intraerythrocytic parasites, necessitating macromolecule degradation. The proteasome is the main engine of Plasmodium protein degradation. Indeed, proteasome inhibition and ART have shown synergy in ART-resistant parasites. Moreover, ubiquitin modification is associated with altered parasite susceptibility to multiple antimalarials. Targeting the ubiquitin-proteasome system (UPS), therefore, is an attractive avenue to combat drug resistance. Here, we review recent advances leading to specific targeting of the Plasmodium proteasome. We also highlight the potential for targeting other nonproteasomal protein degradation systems as an additional strategy to disrupt protein homeostasis.

Combined antiparasitic and anti-inflammatory effects of the natural polyphenol curcumin on turbot scuticociliatosis

The histiophagous scuticociliate Philasterides dicentrarchi is the aetiological agent of scuticociliatosis, a parasitic disease of farmed turbot. Curcumin, a polyphenol from Curcuma longa (turmeric), is known to have antioxidant and anti-inflammatory properties. We investigated the in vitro effects of curcumin on the growth of P. dicentrarchi and on the production of pro-inflammatory cytokines in turbot leucocytes activated by parasite cysteine proteases. At 100 μm, curcumin had a cytotoxic effect and completely inhibited the growth of the parasite. At 50 μm, curcumin inhibited the protease activity of the parasite and expression of genes encoding two virulence-associated proteases: leishmanolysin-like peptidase and cathepsin L-like. At concentrations between 25 and 50 μm, curcumin inhibited the expression of S-adenosyl-L-homocysteine hydrolase, an enzyme involved in the biosynthesis of the amino acids methionine and cysteine. At 100 μm, curcumin inhibited the expression of the cytokines tumour necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) produced in turbot leucocytes activated by parasite proteases. Results show that curcumin has a dual effect on scuticociliatosis: an antiparasitic effect on the catabolism and anabolism of ciliate proteins, and an anti-inflammatory effect that inhibits the production of proinflammatory cytokines in the host. The present findings suggest the potential usefulness of this polyphenol in treating scuticociliatosis.

Plasmodium vivax Tryptophan Rich Antigen PvTRAg36.6 Interacts with PvETRAMP and PvTRAg56.6 Interacts with PvMSP7 during Erythrocytic Stages of the Parasite

Plasmodium vivax is most wide spread and a neglected malaria parasite. There is a lack of information on parasite biology of this species. Genome of this parasite encodes for the largest number of tryptophan-rich proteins belonging to ‘Pv-fam-a’ family and some of them are potential drug/vaccine targets but their functional role(s) largely remains unexplored. Using bacterial and yeast two hybrid systems, we have identified the interacting partners for two of the P. vivax tryptophan-rich antigens called PvTRAg36.6 and PvTRAg56.2. The PvTRAg36.6 interacts with early transcribed membrane protein (ETRAMP) of P.vivax. It is apically localized in merozoites but in early stages it is seen in parasite periphery suggesting its likely involvement in parasitophorous vacuole membrane (PVM) development or maintenance. On the other hand, PvTRAg56.2 interacts with P.vivax merozoite surface protein7 (PvMSP7) and is localized on merozoite surface. Co-localization of PvTRAg56.2 with PvMSP1 and its molecular interaction with PvMSP7 probably suggest that, PvTRAg56.2 is part of MSP-complex, and might assist or stabilize the protein complex at the merozoite surface. In conclusion, the PvTRAg proteins have different sub cellular localizations and specific associated functions during intra-erythrocytic developmental cycle.

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.

Wanted Plasmodium falciparum, dead or alive

Mechanisms of cell death in unicellular parasites have been subjects of debate for the last decade, with studies demonstrating evidence of apoptosis or non-apoptosis like mechanisms, including necrosis, and autophagy. Recent clarifications on the definition of regulated or accidental cell death by The Nomenclature Committee on Cell Death provides an opportunity to reanalyze some data, re-evaluate conclusions in the light of parasite diversity, and to propose alternative arguments in the context of malaria drug resistance, considering lack of really new drugs in the pipeline. Deciphering the mechanisms of death may help in detection of new drug targets and the design of innovative drugs. However, classifications have been evolving rapidly since initial description of "programmed cell death", leading to some uncertainty as to whether Plasmodium cell death is accidental or regulated.

Disruption of cellular homeostasis induces organelle stress and triggers apoptosis like cell-death pathways in malaria parasite

A regulated protein turnover machinery in the cell is essential for effective cellular homeostasis; any interference with this system induces cellular stress and alters the normal functioning of proteins important for cell survival. In this study, we show that persistent cellular stress and organelle dysfunction because of disruption of cellular homeostasis in human malaria parasite Plasmodium falciparum, leads to apoptosis-like cell death. Quantitative global proteomic analysis of the stressed parasites before onset of cell death, showed upregulation of a number of proteins involved in cellular homeostasis; protein network analyses identified upregulated metabolic pathways that may be associated with stress tolerance and pro-survival mechanism. However, persistent stress on parasites cause structural abnormalities in endoplasmic reticulum and mitochondria, subsequently a cascade of reactions are initiated in parasites including rise in cytosolic calcium levels, loss of mitochondrial membrane potential and activation of VAD-FMK-binding proteases. We further show that activation of VAD-FMK-binding proteases in the parasites leads to degradation of phylogenetically conserved protein, TSN (Tudor staphylococcal nuclease), a known target of metacaspases, as well as degradation of other components of spliceosomal complex. Loss of spliceosomal machinery impairs the mRNA splicing, leading to accumulation of unprocessed RNAs in the parasite and thus dysregulate vital cellular functions, which in turn leads to execution of apoptosis-like cell death. Our results establish one of the possible mechanisms of instigation of cell death by organelle stress in Plasmodium.

Diversity-oriented synthesis and activity evaluation of substituted bicyclic lactams as anti-malarial against Plasmodium falciparum

Background

Malaria remains the world’s most important devastating parasitic disease. Of the five species of Plasmodium known to infect and cause human malaria, Plasmodium falciparum is the most virulent and responsible for majority of the deaths caused by this disease. Mainstream drug therapy targets the asexual blood stage of the malaria parasite, as the disease symptoms are mainly associated with this stage. The prevalence of malaria parasite strains resistance to existing anti-malarial drugs has made the control of malaria even more challenging and hence the development of a new class of drugs is inevitable.

Methods

Screening against different drug resistant and sensitive strains of P. falciparum was performed for few bicyclic lactam-based motifs, exhibiting a broad spectrum of activity with low toxicity generated via a focussed library obtained from diversity oriented synthesis (DOS). The synthesis and screening was followed by an in vitro assessment of the possible cytotoxic effect of this class of compounds on malaria parasite.

Results

The central scaffold a chiral bicyclic lactam (A) and (A’) which were synthesized from (R)-phenylalaninol, levulinic acid and 3-(2-nitrophenyl) levulinic acid respectively. The DOS library was generated from A and from A’, by either direct substitution with o-nitrobenzylbromide at the carbon α- to the amide functionality or by conversion to fused pyrroloquinolines. Upon screening this diverse library for their anti-malarial activity, a dinitro/diamine substituted bicyclic lactam was found to demonstrate exceptional activity of >85% inhibition at 50 μM concentration across different strains of P. falciparum with no toxicity against mammalian cells. Also, loss of mitochondrial membrane potential, mitochondrial functionality and apoptosis was observed in parasite treated with diamine-substituted bicyclic lactams.

Conclusions

This study unveils a DOS-mediated exploration of small molecules with novel structural motifs that culminates in identifying a potential lead molecule against malaria. In vitro investigations further reveal their cytocidal effect on malaria parasite growth. It is not the first time that DOS has been used as a strategy to identify therapeutic leads against malaria, but this study establishes the direct implications of DOS in scouting novel motifs with anti-malarial activity.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-467) contains supplementary material, which is available to authorized users.

Cuprous oxide nanoparticles inhibit the growth and metastasis of melanoma by targeting mitochondria

Metal and its oxide nanoparticles show ideal pharmacological activity, especially in anti-tumor therapy. Our previous study demonstrated that cuprous oxide nanoparticles (CONPs) selectively induce apoptosis of tumor cells in vitro. To explore the anti-tumor properties of CONPs in vivo, we used the particles to treat mouse subcutaneous melanoma and metastatic lung tumors, based on B16-F10 mouse melanoma cells, by intratumoral and systemic injections, respectively. The results showed that CONPs significantly reduced the growth of melanoma, inhibited the metastasis of B16-F10 cells and increased the survival rate of tumor-bearing mice. Importantly, the results also indicated that CONPs were rapidly cleared from the organs and that these particles exhibited little systemic toxicity. Furthermore, we observed that CONPs targeted the mitochondria, which resulted in the release of cytochrome C from the mitochondria and the activation of caspase-3 and caspase-9 after the CONPs entered the cells. In conclusion, CONPs can induce the apoptosis of cancer cells through a mitochondrion-mediated apoptosis pathway, which raises the possibility that CONPs could be used to cure melanoma and other cancers.

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.

The prokaryotic ClpQ protease plays a key role in growth and development of mitochondria in Plasmodium falciparum

The ATP-dependent ClpQY system is a prokaryotic proteasome-like multi-subunit machinery localized in the mitochondrion of malaria parasite. The ClpQY machinery consists of ClpQ threonine protease and ClpY ATPase. In the present study, we have assessed cellular effects of transient interference of PfClpQ protease activity in Plasmodium falciparum using a trans-dominant negative approach combined with FKBP degradation domain system. A proteolytically inactive mutant PfClpQ protein [PfClpQ(mut)] fused with FKBP degradation domain was expressed in parasites, which gets stabilized by Shield1 drug treatment. We show that the inactive PfClpQ(mut) interacts with wild-type PfClpQ and associates within multi-subunit complex in the parasite. Stabilization of the PfClpQ(mut) and its association in the protease machinery caused dominant negative effect in the transgenic parasites, which disrupted the growth cycle of asexual blood stage parasites. The mitochondria in these parasites showed abnormal morphology, these mitochondria were not able to grow and divide in the parasite. We further show that the dominant negative effect of PfClpQ(mut) disrupted transcription of mitochondrial genome encoded genes, which in turn blocked normal development and functioning of the mitochondria.

Can a single "powerless" mitochondrion in the malaria parasite contribute to parasite programmed cell death in the asexual stages?

The protozoan pathogens responsible for malaria are from the Plasmodium genus, with Plasmodium falciparum and Plasmodium vivax accounting for almost all clinical infections. With recent estimates of mortality exceeding 800,000 annually, malaria continues to take a terrible toll on lives and the early promises of medicine to eradicate the disease have yet to approach realization, in part due to the spread of drug resistant parasites. Recent reports of artemisinin-resistance have prompted renewed efforts to identify novel therapeutic options, and one such pathway being considered for antimalarial exploit is the parasite's programmed cell death (PCD) pathway. In this mini-review, we will discuss the roles of the plasmodium mitochondria in cell death and as a target of antimalarial compounds, taking into account recent data suggesting that PCD pathways involving the mitochondria may be attractive antimalarial targets.

Radiation-induced cellular and molecular alterations in asexual intraerythrocytic Plasmodium falciparum

BACKGROUND

γ-irradiation is commonly used to create attenuation in Plasmodium parasites. However, there are no systematic studies on the survival, reversion of virulence, and molecular basis for γ-radiation-induced cell death in malaria parasites.

METHODS

The effect of γ-irradiation on the growth of asexual Plasmodium falciparum was studied in erythrocyte cultures. Cellular and ultrastructural changes within the parasite were studied by fluorescence and electron microscopy, and genome-wide transcriptional profiling was performed to identify parasite biomarkers of attenuation and cell death.

RESULTS

γ-radiation induced the death of P. falciparum in a dose-dependent manner. These parasites had defective mitosis, sparse cytoplasm, fewer ribosomes, disorganized and clumped organelles, and large vacuoles-observations consistent with "distressed" or dying parasites. A total of 185 parasite genes were transcriptionally altered in response to γ-irradiation (45.9% upregulated, 54.1% downregulated). Loss of parasite survival was correlated with the downregulation of genes encoding translation factors and with upregulation of genes associated with messenger RNA-sequestering stress granules. Genes pertaining to cell-surface interactions, host-cell remodeling, and secreted proteins were also altered.

CONCLUSIONS

These studies provide a framework to assess the safety of γ-irradiation attenuation and promising targets for genetic deletion to produce whole parasite-based attenuated vaccines.

Variation in apoptosis mechanisms employed by malaria parasites: the roles of inducers, dose dependence and parasite stages

Background

Plasmodium berghei ookinetes exhibit an apoptotic phenotype when developing within the mosquito midgut lumen or when cultured in vitro. Markers of apoptosis increase when they are exposed to nitric oxide or reactive oxygen species but high concentrations of hydrogen peroxide cause death without observable signs of apoptosis. Chloroquine and other drugs have been used to induce apoptosis in erythrocytic stages of Plasmodium falciparum and to formulate a putative pathway involving cysteine protease activation and mitochondrial membrane permeabilization; initiated, at least in the case of chloroquine, after its accumulation in the digestive vacuole causes leakage of the vacuole contents. The lack of a digestive vacuole in ookinetes prompted the investigation of the effect of chloroquine and staurosporine on this stage of the life cycle. Finally, the suggestion that apoptosis may have evolved as a strategy employed by ookinetes to increase the fitness of surviving parasites was explored by determining whether increasing the ecological triggers parasite density and nutrient depletion induced apoptosis.

Methods

Ookinetes were grown in culture then either exposed to hydrogen peroxide, chloroquine or staurosporine, or incubated at different densities and in different media. The proportion of ookinetes displaying positive markers for apoptosis in treated samples was compared with controls and results were analyzed using analysis of variance followed by a Turkey’s test, or a Kruskal-Wallis test as appropriate.

Results

Hydrogen peroxide below 50 μM triggered apoptosis but cell membranes were rapidly compromised by higher concentrations, and the mode of death could not be defined. Both chloroquine and staurosporine cause a significant increase in ookinetes with condensed chromatin, caspase-like activity and, in the case of chloroquine, phosphatidylserine translocation and DNA fragmentation (not investigated for staurosporine). However, mitochondrial membrane potential remained intact. No relationship between ookinete density and apoptosis was detected but nutrient depletion significantly increased the proportion of ookinetes with chromatin condensation in four hours.

Conclusions

It is proposed that both a mitochondrial and an amitochondrial apoptotic pathway may be involved, dependent upon the trigger that induces apoptosis, and that pathways may differ between erythrocytic stages and ookinetes, or between rodent and human malaria parasites.

The influence of ATP-dependent proteases on a variety of nucleoid-associated processes

ATP-dependent proteases are crucial components of all living cells and are involved in a variety of responses to physiological and environmental changes. Nucleoids are dynamic nucleoprotein complexes present in bacteria and eukaryotic organelles (mitochondria and plastids) and are the place where the majority of cellular responses to stress begin. These structures are actively remodeled in reaction to changing environmental and physiological conditions. The levels of nucleoid protein components (e.g. DNA-stabilizing proteins, transcription factors, replication proteins) therefore have to be continually regulated. ATP-dependent proteases have all the characteristics needed to fulfill this requirement. Some of them bind nucleic acids, but above all, they control and maintain the level of many DNA-binding proteins. In this review we will discuss the roles of the Lon, ClpAP, ClpXP, HslUV and FtsH proteases in the maintenance, stability, transcription and repair of DNA in eubacterial and mitochondrial nucleoids.

Can we teach an old drug new tricks?

Although resistance to chloroquine (CQ) has relegated it from modern chemotherapeutic strategies to treat Plasmodium falciparum malaria, new evidence suggests that higher doses of the drug may exert a different killing mechanism and offers this drug a new lease of life. Whereas the established antimalarial mechanisms of CQ are usually associated with nanomolar levels of the drug, micromolar levels of CQ trigger a distinct cell death pathway involving the permeabilization of the digestive vacuole of the parasite and a release of hydrolytic enzymes. In this paper, we propose that this pathway is a promising antimalarial strategy and suggest that revising the CQ treatment regimen may elevate blood drug levels to trigger this pathway without increasing the incidence of adverse reactions.