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

Profile of metacaspase gene expression in Plasmodium vivax field isolates from the Brazilian Amazon

BACKGROUND

Metacaspases comprise a family of cysteine proteases implicated in both cell death and cell differentiation of protists that has been considered a potential drug target for protozoan parasites. However, the biology of metacaspases in Plasmodium vivax - the second most prevalent and most widespread human malaria parasite worldwide, whose occurrence of chemoresistance has been reported in many endemic countries, remains largely unexplored. Therefore, the present study aimed to address, for the first time, the expression pattern of metacaspases in P. vivax parasites.

METHODS AND RESULTS

P. vivax blood-stage parasites were obtained from malaria patients in the Brazilian Amazon and the expression of the three putative P. vivax metacaspases (PvMCA1-3) was detected in all isolates by quantitative PCR assay. Of note, the expression levels of each PvMCA varied noticeably across isolates, which presented different frequencies of parasite forms, supporting that PvMCAs may be expressed in a stage-specific manner as previously shown in P. falciparum.

CONCLUSION

The detection of metacaspases in P. vivax blood-stage parasites reported herein, allows the inclusion of these proteases as a potential candidate drug target for vivax malaria, while further investigations are still required to evaluate the activity, role and essentiality of metacaspases in P. vivax biology.

Metacaspase (Pf MCA-1) as antimalarial drug target: An in silico approach and their biological validation

AIMS

Acquired drug resistance of Plasmodium is a global issue for the treatment of malaria. There are various proteases in the genome of Plasmodium falciparum (P. falciparum) including metacaspase-1 (PfMCA-1) that are essential and are being considered as an attractive drug target. It is aimed to identify novel therapeutics against malaria and their action on PfMCA-1 along with other apoptotic pathway events.

MAIN METHODS

High throughput virtual screening of 55,000 compounds derived from Maybridge library was performed against PfMCA-1. Based on the docking score, sixteen compounds were selected for in vitro antimalarial screening against drug sensitive and resistant strains of P. falciparum using SYBR green-based assay. Subsequently, three lead molecules were selected and subjected to the evaluation of cytotoxicity, caspase like protease activity, mitochondrial membrane potential, ROS generation and DNA fragmentation via TUNEL assay.

KEY FINDINGS

The in silico and in vitro approaches have brought forward some Maybridge library compounds with antiplasmodial activity most likely by enhancing the metacaspase activity. The compound CD11095 has shown better antimalarial efficacy, and KM06591 depicted higher caspase mediated killing, elevated TUNEL positive cells and moderate ROS generation. Mitochondrial membrane depolarization was augmented by RJC0069. Exposure of P. falciparum to CD11095, KM06591 and RJC0069 has ended up in parasite growth arrest via multiple mechanisms.

SIGNIFICANCE

It is proposed that the Maybridge molecules CD11095, KM06591 and RJC0069 have antimalarial activity. Their mechanism of action was found to be by enhancing the metacaspases-like protease activity, mitochondrial depolarization and DNA fragmentation which stipulates significant insights towards promising candidates for drug development.

Dissecting The role of Plasmodium metacaspase-2 in malaria gametogenesis and sporogony

The family of apicomplexan specific proteins contains caspases–like proteins called “metacaspases”. These enzymes are present in the malaria parasite but absent in human; therefore, these can be explored as potential drug targets. We deleted the MCA-2 gene from Plasmodium berghei genome using a gene knockout strategy to decipher its precise function. This study has identified that MCA-2 plays an important role in parasite transmission since it is critical for the formation of gametocytes and for maintaining an appropriate number of infectious sporozoites required for sporogony. It is noticeable that a significant reduction in gametocyte, oocysts, ookinete and sporozoites load along with a delay in hepatocytes invasion were observed in the MCA-2 knockout parasite. Furthermore, a study found the two MCA-2 inhibitory molecules known as C-532 and C-533, which remarkably inhibited the MCA-2 activity, abolished the in vitro parasite growth, and also impaired the transmission cycle of P. falciparum and P. berghei in An. stephensi. Our findings indicate that the deletion of MCA-2 hampers the Plasmodium development during erythrocytic and exo-erythrocytic stages, and its inhibition by C-532 and C-533 critically affects the malaria transmission biology.

Plasmodium vivax metacaspase 1 (PvMCA1) catalytic domain is conserved in field isolates from Brazilian Amazon

In the present study, we investigated the genetic diversity of Plasmodium vivax metacaspase 1 (PvMCA1) catalytic domain in two municipalities of the main malaria hotspot in Brazil, i.e., the Juruá Valley, and observed complete sequence identity among all P. vivax field isolates and the Sal-1 reference strain. Analysis of PvMCA1 catalytic domain in different P. vivax genomic sequences publicly available also revealed a high degree of conservation worldwide, with very few amino acid substitutions that were not related to putative histidine and cysteine catalytic residues, whose involvement with the active site of protease was herein predicted by molecular modeling. The genetic conservation presented by PvMCA1 may contribute to its eligibility as a druggable target candidate in vivax malaria.

Identification of compounds active against quiescent artemisinin-resistant Plasmodium falciparum parasites via the quiescent-stage survival assay (QSA)

BACKGROUND

Quiescence is an unconventional mechanism of Plasmodium survival, mediating artemisinin resistance. This phenomenon increases the risk of clinical failures following artemisinin-based combination therapies (ACTs) by slowing parasite clearance and allowing the selection of parasites resistant to partner drugs.

OBJECTIVES

To thwart this multiresistance, the quiescent state of artemisinin-resistant parasites must be taken into consideration from the very early stages of the drug discovery process.

METHODS

We designed a novel phenotypic assay we have named the quiescent-stage survival assay (QSA) to assess the antiplasmodial activity of drugs on quiescent parasites. This assay was first validated on quiescent forms from different artemisinin-resistant parasite lines (laboratory strain and field isolates), using two reference drugs with different mechanisms of action: chloroquine and atovaquone. Furthermore, the efficacies of different partner drugs of artemisinins used in ACTs were investigated against both laboratory strains and field isolates from Cambodia.

RESULTS

Our results highlight that because of the mechanism of quiescence and the respective pharmacological targets of drugs, drug efficacies on artemisinin-resistant parasites may be different between quiescent parasites and their proliferating forms.

CONCLUSIONS

These data confirm the high relevance of adding the chemosensitivity evaluation of quiescent parasites by the specific in vitro QSA to the antiplasmodial drug development process in the current worrisome context of artemisinin resistance.

Anti-PfGARP activates programmed cell death of parasites and reduces severe malaria

Malaria caused by Plasmodium falciparum remains the leading single-agent cause of mortality in children¹, yet the promise of an effective vaccine has not been fulfilled. Here, using our previously described differential screening method to analyse the proteome of blood-stage P. falciparum parasites², we identify P. falciparum glutamic-acid-rich protein (PfGARP) as a parasite antigen that is recognized by antibodies in the plasma of children who are relatively resistant-but not those who are susceptible-to malaria caused by P. falciparum. PfGARP is a parasite antigen of 80 kDa that is expressed on the exofacial surface of erythrocytes infected by early-to-late-trophozoite-stage parasites. We demonstrate that antibodies against PfGARP kill trophozoite-infected erythrocytes in culture by inducing programmed cell death in the parasites, and that vaccinating non-human primates with PfGARP partially protects against a challenge with P. falciparum. Furthermore, our longitudinal cohort studies showed that, compared to individuals who had naturally occurring anti-PfGARP antibodies, Tanzanian children without anti-PfGARP antibodies had a 2.5-fold-higher risk of severe malaria and Kenyan adolescents and adults without these antibodies had a twofold-higher parasite density. By killing trophozoite-infected erythrocytes, PfGARP could synergize with other vaccines that target parasite invasion of hepatocytes or the invasion of and egress from erythrocytes.

Cell death in Leishmania

Leishmaniases still represent a global scourge and new therapeutic tools are necessary to replace the current expensive, difficult to administer treatments that induce numerous adverse effects and for which resistance is increasingly worrying. In this context, the particularly original organization of the Leishmania parasite in comparison to higher eukaryotes is a great advantage. It allows for the development of new, very specific, and thus non-cytotoxic treatments. Among these originalities, Leishmania cell death can be cited. Despite a classic pattern of apoptosis, key mammalian apoptotic proteins are not present in Leishmania, such as caspases, cell death receptors, and anti-apoptotic molecules. Recent studies have helped to develop a better understanding of parasite cell death, identifying new proteins or even new apoptotic pathways. This review provides an overview of the current knowledge on Leishmania cell death, describing its physiological roles and its phenotype, and discusses the involvement of various proteins: endonuclease G, metacaspase, aquaporin Li-BH3AQP, calpains, cysteine proteinase C, LmjHYD36 and Lmj.22.0600. From these data, potential apoptotic pathways are suggested. This review also offers tools to identify new Leishmania cell death effectors. Lastly, different approaches to use this knowledge for the development of new therapeutic tools are suggested: either inhibition of Leishmania cell death or activation of cell death for instance by treating cells with proteins or peptides involved in parasite death fused to a cell permeant peptide or encapsulated into a lipidic vector to target intra-macrophagic Leishmania cells.

Host-parasite interactions and ecology of the malaria parasite-a bioinformatics approach

Malaria remains one of the highest mortality infectious diseases. Malaria is caused by parasites from the genus Plasmodium. Most deaths are caused by infections involving Plasmodium falciparum, which has a complex life cycle. Malaria parasites are extremely well adapted for interactions with their host and their host's immune system and are able to suppress the human immune system, erase immunological memory and rapidly alter exposed antigens. Owing to this rapid evolution, parasites develop drug resistance and express novel forms of antigenic proteins that are not recognized by the host immune system. There is an emerging need for novel interventions, including novel drugs and vaccines. Designing novel therapies requires knowledge about host-parasite interactions, which is still limited. However, significant progress has recently been achieved in this field through the application of bioinformatics analysis of parasite genome sequences. In this review, we describe the main achievements in 'malarial' bioinformatics and provide examples of successful applications of protein sequence analysis. These examples include the prediction of protein functions based on homology and the prediction of protein surface localization via domain and motif analysis. Additionally, we describe PlasmoDB, a database that stores accumulated experimental data. This tool allows data mining of the stored information and will play an important role in the development of malaria science. Finally, we illustrate the application of bioinformatics in the development of population genetics research on malaria parasites, an approach referred to as reverse ecology.

Extracellular vesicles carrying lactate dehydrogenase induce suicide in increased population density of Plasmodium falciparum in vitro

Even with access to sufficient nutrients and atmosphere, Plasmodium falciparum can barely be cultured at maximum growth capacity in vitro conditions. Because of this behavior, it has been suggested that P. falciparum has self-regulatory mechanisms in response to density stress. Only recently has this process begun to be acknowledged and characteristics of a programmed cell death been assigned to the parasite at high parasitaemia in vitro cultures. In searching for death signals within the parasite community, we have found that extracellular vesicles (EVs) of P. falciparum from high parasitaemia cultures are able to induce programmed cell death processes in the population. A comparative proteomic analysis of EVs from low (EVL) and high (EVH) parasitaemia cultures was conducted, pointing to lactate dehydrogenase from P. falciparum (PfLDH) as the only parasite protein overexpressed in the later. Although the major function of P. falciparum lactate dehydrogenase (PfLDH) is the conversion of pyruvate to lactate, a key process in the production of energy in most living organisms, we investigated its possible role in the mechanism of parasite density control by intercellular signaling, given that PfLDH had already been listed as a component of extracellular vesicles of P. falciparum. In this study we present evidence of the EV-associated PfLDH regulation of parasite population by inducing apoptosis in highly parasitized cultures.

Induction of apoptosis-like cell death and clearance of stress-induced intracellular protein aggregates: dual roles for Ustilago maydis metacaspase Mca1

Metacaspases primarily associate with induction and execution of programmed cell death in protozoa, fungi and plants. In the recent past, several studies have also demonstrated cellular functions of metacaspases other than cell death in different organisms including yeast and protozoa. This study shows similar dual function for the only metacaspase of a biotrophic phytopathogen, Ustilago maydis. In addition to a conventional role in the induction of cell death, Mca1 has been demonstrated to play a key role in maintaining the quality of the cellular proteome. On one hand, Mca1 could be shown to bring about apoptosis-like phenotypic changes in U. maydis on exposure to oxidative stress, on the other hand, the protein was found to regulate cellular protein quality control. U. maydis metacaspase has been found to remain closely associated with the insoluble intracellular protein aggregates, generated during an event of stress exposure to the fungus. The study, therefore, provides direct evidence for a role of U. maydis metacaspase in the clearance of the stress-induced intracellular insoluble protein aggregates. Furthermore, host infection assays with mca1 deletion strain also revealed a role of the protein in the virulence of the fungus.

Proteases as antimalarial targets: strategies for genetic, chemical, and therapeutic validation

Malaria is a devastating parasitic disease affecting half of the world's population. The rapid emergence of resistance against new antimalarial drugs, including artemisinin-based therapies, has made the development of drugs with novel mechanisms of action extremely urgent. Proteases are enzymes proven to be well suited for target-based drug development due to our knowledge of their enzymatic mechanisms and active site structures. More importantly, Plasmodium proteases have been shown to be involved in a variety of pathways that are essential for parasite survival. However, pharmacological rather than target-based approaches have dominated the field of antimalarial drug development, in part due to the challenge of robustly validating Plasmodium targets at the genetic level. Fortunately, over the last few years there has been significant progress in the development of efficient genetic methods to modify the parasite, including several conditional approaches. This progress is finally allowing us not only to validate essential genes genetically, but also to study their molecular functions. In this review, I present our current understanding of the biological role proteases play in the malaria parasite life cycle. I also discuss how the recent advances in Plasmodium genetics, the improvement of protease-oriented chemical biology approaches, and the development of malaria-focused pharmacological assays, can be combined to achieve a robust biological, chemical and therapeutic validation of Plasmodium proteases as viable drug targets.

Genetic diversity of Plasmodium vivax metacaspase 1 and Plasmodium vivax multi-drug resistance 1 genes of field isolates from Mauritania, Sudan and Oman

BACKGROUND

Plasmodium vivax is the second most important human malaria parasite, widely spread across the world. This parasite is associated with important issues in the process toward malaria elimination, including potential for relapse and increased resistance to chloroquine. Plasmodium vivax multi-drug resistant (pvmdr1) is suspected to be a marker of resistance although definitive evidence is lacking. Progress has been made in knowledge of biological factors affecting parasite growth, including mechanisms of regulated cell death and the suspected role of metacaspase. Plasmodium vivax metacaspase1 (PvMCA1-cd) has been described with a catalytic domain composed of histidine (H372) and cysteine (C428) residues. The aim of this study was to test for a link between the conserved histidine and cysteine residues in PvMCA1-cd, and the polymorphism of the P. vivax multi-drug resistant gene (pvmdr1).

RESULTS

Thirty P. vivax isolates were collected from Mauritania, Sudan, and Oman. Among the 28 P. vivax isolates successfully sequenced, only 4 samples showed the conserved His (372)-Cys (428) residues in PvMCA1-cd. Single nucleotide polymorphisms observed were H372T (46.4%), H372D (39.3%), and C428R (85.7%). A new polymorphic catalytic domain was observed at His (282)-Cys (305) residues. Sequences alignment analysis of pvmdr1 showed SNP in the three codons 958, 976 and 1076. A single SNP was identified at the codon M958Y (60%), 2 SNPs were found at the position 976: Y976F (13%) and Y976V (57%), and 3 SNPs were identified at the position 1076: F1076L (40%), F1076T (53%) and F1076I (3%). Only one isolate was wildtype in all three codons (MYF), 27% were single MYL mutants, and 10% were double MFL mutants. Three new haplotypes were also identified: the triple mutant YVT was most prevalent (53.3%) distributed in the three countries, while triple YFL and YVI mutants (3%), were only found in samples from Sudan and Mauritania.

CONCLUSIONS

Triple or quadruple mutants for metacaspase genes and double or triple mutants for Pvmdr1 were observed in 24/28 and 19/28 samples. There was no difference in the frequency of mutations between PvMCA1-cd and Pvmdr1 (P > 0.2). Histidine and cysteine residues in PvMCA1-cd are highly polymorphic and linkage disequilibrium with SNPs of Pvmdr1 gene may be expected from these three areas with different patterns of P. vivax transmission.

Hydroxylated di- and tri-styrylbenzenes, a new class of antiplasmodial agents: discovery and mechanism of action

The first systematic evaluation of the antiplasmodial activity of the hydroxystilbene family of natural products and di/tristyrylbenzenes is described.

Structure-based drug design studies of the interactions of ent-kaurane diterpenes derived from Wedelia paludosa with the Plasmodium falciparum sarco/endoplasmic reticulum Ca²⁺-ATPase PfATP6

Malaria is responsible for more deaths around the world than any other parasitic disease. Due to the emergence of strains that are resistant to the current chemotherapeutic antimalarial arsenal, the search for new antimalarial drugs remains urgent though hampered by a lack of knowledge regarding the molecular mechanisms of artemisinin resistance. Semisynthetic compounds derived from diterpenes from the medicinal plant Wedelia paludosa were tested in silico against the Plasmodium falciparum Ca²⁺-ATPase, PfATP6. This protein was constructed by comparative modelling using the three-dimensional structure of a homologous protein, 1IWO, as a scaffold. Compound 21 showed the best docking scores, indicating a better interaction with PfATP6 than that of thapsigargin, the natural inhibitor. Inhibition of PfATP6 by diterpene compounds could promote a change in calcium homeostasis, leading to parasite death. These data suggest PfATP6 as a potential target for the antimalarial ent-kaurane diterpenes.

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.

Comparative structural analysis of the caspase family with other clan CD cysteine peptidases

Clan CD forms a structural group of cysteine peptidases, containing seven individual families and two subfamilies of structurally related enzymes. Historically, it is most notable for containing the mammalian caspases, on which the structures of the clan were founded. Interestingly, the caspase family is split into two subfamilies: the caspases, and a second subfamily containing both the paracaspases and the metacaspases. Structural data are now available for both the paracaspases and the metacaspases, allowing a comprehensive structural analysis of the entire caspase family. In addition, a relative plethora of structural data has recently become available for many of the other families in the clan, allowing both the structures and the structure-function relationships of clan CD to be fully explored. The present review compares the enzymes in the caspase subfamilies with each other, together with a comprehensive comparison of all the structural families in clan CD. This reveals a diverse group of structures with highly conserved structural elements that provide the peptidases with a variety of substrate specificities and activation mechanisms. It also reveals conserved structural elements involved in substrate binding, and potential autoinhibitory functions, throughout the clan, and confirms that the metacaspases are structurally diverse from the caspases (and paracaspases), suggesting that they should form a distinct family of clan CD peptidases.

Abnormalities of Endocytosis, Phagocytosis, and Development Process in Dictyostelium Cells That Over-Express Acanthamoeba castellanii Metacaspase Protein

BACKGROUND

Acanthamoeba castellanii forms a resistant cyst that protects the parasite against the host's immune response. Acanthamoeba Type-I metacaspase (Acmcp) is a caspase-like protein that has been found to be expressed during the encystations. Dictyostelium discoideum is an organism closely related to Acanthamoeba useful for studying the molecular function of this protozoan caspase-like protein.

METHODS

The full length of Acmcp and a mutated version of the same gene, which lacks the proline rich N-terminal region (Acmcp-dpr), were cloned into the pDneo2a-GFP vector separately. The pDneo2a-GFP-Acmcp and pDneo2a-GFPAcmcp-dpr were electro-transfected into wild type D. discoideum cells to create cell lines that over-expressed Acmcp or Acmcp-dpr.

RESULTS

Both cell lines that over-expressed Acmcp and Acmcp-dpr showed a significant increase in the fluid phase internalization and phagocytosis rate compared to the control cells. Additionally, the cells expressing the Acmcp-dpr mutant were unable to initiate early development and failed to aggregate or form fruiting bodies under starvation conditions, whereas Acmcp over-expressing cells showed the opposite phenomena. Quantitative cell death analysis provided additional support for these findings.

CONCLUSION

Acmcp is involved in the processes of endocytosis and phagocytosis. In addition, the proline rich region in Acmcp is important for cellular development in Dictyostelium. Given its important role in the development process, metacaspase protein is proposed as a candidate drug target against infections caused by A. castellanii.

A process similar to autophagy is associated with cytocidal chloroquine resistance in Plasmodium falciparum

Resistance to the cytostatic activity of the antimalarial drug chloroquine (CQ) is becoming well understood, however, resistance to cytocidal effects of CQ is largely unexplored. We find that PfCRT mutations that almost fully recapitulate P. falciparum cytostatic CQ resistance (CQR^CS) as quantified by CQ IC₅₀ shift, account for only 10–20% of cytocidal CQR (CQR^CC) as quantified by CQ LD₅₀ shift. Quantitative trait loci (QTL) analysis of the progeny of a chloroquine sensitive (CQS; strain HB3)×chloroquine resistant (CQR; strain Dd2) genetic cross identifies distinct genetic architectures for CQR^CS vs CQR^CC phenotypes, including identification of novel interacting chromosomal loci that influence CQ LD₅₀. Candidate genes in these loci are consistent with a role for autophagy in CQR^CC, leading us to directly examine the autophagy pathway in intraerythrocytic CQR parasites. Indirect immunofluorescence of RBC infected with synchronized CQS vs CQR trophozoite stage parasites reveals differences in the distribution of the autophagy marker protein PfATG8 coinciding with CQR^CC. Taken together, the data show that an unusual autophagy – like process is either activated or inhibited for intraerythrocytic trophozoite parasites at LD₅₀ doses (but not IC₅₀ doses) of CQ, that the pathway is altered in CQR P. falciparum, and that it may contribute along with mutations in PfCRT to confer the CQR^CC phenotype.

The Arabidopsis metacaspase9 degradome

Metacaspases are distant relatives of the metazoan caspases, found in plants, fungi, and protists. However, in contrast with caspases, information about the physiological substrates of metacaspases is still scarce. By means of N-terminal combined fractional diagonal chromatography, the physiological substrates of metacaspase9 (MC9; AT5G04200) were identified in young seedlings of Arabidopsis thaliana on the proteome-wide level, providing additional insight into MC9 cleavage specificity and revealing a previously unknown preference for acidic residues at the substrate prime site position P1'. The functionalities of the identified MC9 substrates hinted at metacaspase functions other than those related to cell death. These results allowed us to resolve the substrate specificity of MC9 in more detail and indicated that the activity of phosphoenolpyruvate carboxykinase 1 (AT4G37870), a key enzyme in gluconeogenesis, is enhanced upon MC9-dependent proteolysis.

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.

Features of autophagic cell death in Plasmodium liver-stage parasites

Analyzing molecular determinants of Plasmodium parasite cell death is a promising approach for exploring new avenues in the fight against malaria. Three major forms of cell death (apoptosis, necrosis and autophagic cell death) have been described in multicellular organisms but which cell death processes exist in protozoa is still a matter of debate. Here we suggest that all three types of cell death occur in Plasmodium liver-stage parasites. Whereas typical molecular markers for apoptosis and necrosis have not been found in the genome of Plasmodium parasites, we identified genes coding for putative autophagy-marker proteins and thus concentrated on autophagic cell death. We characterized the Plasmodium berghei homolog of the prominent autophagy marker protein Atg8/LC3 and found that it localized to the apicoplast. A relocalization of PbAtg8 to autophagosome-like vesicles or vacuoles that appear in dying parasites was not, however, observed. This strongly suggests that the function of this protein in liver-stage parasites is restricted to apicoplast biology.

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.

Deletion of mitochondrial associated ubiquitin fold modifier protein Ufm1 in Leishmania donovani results in loss of β-oxidation of fatty acids and blocks cell division in the amastigote stage

Recently, we described the existence of the ubiquitin fold modifier 1 (Ufm1) and its conjugation pathway in Leishmania donovani. We demonstrated the conjugation of Ufm1 to proteins such as mitochondrial trifunctional protein (MTP) that catalyses β-oxidation of fatty acids in L. donovani. To elucidate the biological roles of the Ufm1-mediated modifications, we made an L. donovani Ufm1 null mutant (Ufm1(-/-)). Loss of Ufm1 and consequently absence of Ufm1 conjugation with MTP resulted in diminished acetyl-CoA, the end-product of the β-oxidation in the Ufm1(-/-) amastigote stage. The Ufm1(-/-) mutants showed reduced survival in the amastigote stage in vitro and ex vivo in human macrophages. This survival was restored by re-expression of wild-type Ufm1 with concomitant induction of acetyl-CoA but not by re-expressing the non-conjugatable Ufm1, indicating the essential nature of Ufm1 conjugation and β-oxidation. Both cell cycle analysis and ultrastructural studies of Ufm1(-/-) parasites confirmed the role of Ufm1 in amastigote growth. The defect in vitro growth of amastigotes in human macrophages was further substantiated by reduced survival. Therefore, these studies suggest the importance of Ufm1 in Leishmania pathogenesis with larger impact on other organisms and further provide an opportunity to test Ufm1(-/-) parasites as drug and vaccine targets.

Autophagy in Apicomplexa: a life sustaining death mechanism?

Programmed cell death (PCD) pathways remain understudied in parasitic protozoa in spite of the fact that they provide potential targets for the development of new therapy. The best understood PCD pathway in higher eukaryotes is apoptosis although emerging evidence also points to autophagy as a mediator of death in certain physiological contexts. Bioinformatic analyses coupled with biochemical and cell biological studies suggest that parasitic protozoa possess the capacity for PCD including a primordial form of apoptosis. Recent work in Toxoplasma and emerging data from Plasmodium suggest that autophagy-related processes may serve as an additional death promoting pathway in Apicomplexa. Detailed mechanistic studies into the molecular basis for PCD in parasitic protozoa represent a fertile area for investigation and drug development.

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.

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.