Investigating the evolution of apoptosis in malaria parasites: the importance of ecology

Drug-induced ER stress leads to induction of programmed cell death pathways of the malaria parasite

The rapid emergence of drug resistance against the mainstream antimalarial drugs has increased the need for development of novel drugs. Recent approaches have embarked on the repurposing of existing drugs to induce cell death via programmed cell death pathways. However, little is known about the ER stress response and programmed cell death pathways of the malaria parasite. In this study, we treated ex vivo Plasmodium berghei cultures with tunicamycin, 5-fluorouracil, and chloroquine as known stress inducer drugs to probe the transcriptional changes of autophagy and apoptosis-related genes (PbATG5, PbATG8, PbATG12, and PbMCA2). Treatments with 5-fluorouracil and chloroquine resulted in the upregulation of all analyzed markers, yet the levels of PbATG5 and PbATG12 were dramatically higher in chloroquine-treated ex vivo cultures. In contrast, tunicamycin treatment resulted in the downregulation of both PbATG8 and PbATG12, and upregulation of PbMCA2. Our results indicate that the malaria parasite responds to various ER stressors by inducing autophagy- and/or apoptosis-like pathways.

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.

Discrimination Experiments in Entamoeba and Evidence from Other Protists Suggest Pathogenic Amebas Cooperate with Kin to Colonize Hosts and Deter Rivals

Entamoeba histolytica is one of the least understood protists in terms of taxa, clone, and kin discrimination/recognition ability. However, the capacity to tell apart same or self (clone/kin) from different or nonself (nonclone/nonkin) has long been demonstrated in pathogenic eukaryotes like Trypanosoma and Plasmodium, free-living social amebas (Dictyostelium, Polysphondylium), budding yeast (Saccharomyces), and in numerous bacteria and archaea (prokaryotes). Kin discrimination/recognition is explained under inclusive fitness theory; that is, the reproductive advantage that genetically closely related organisms (kin) can gain by cooperating preferably with one another (rather than with distantly related or unrelated individuals), minimizing antagonism and competition with kin, and excluding genetic strangers (or cheaters = noncooperators that benefit from others' investments in altruistic cooperation). In this review, we rely on the outcomes of in vitro pairwise discrimination/recognition encounters between seven Entamoeba lineages to discuss the biological significance of taxa, clone, and kin discrimination/recognition in a range of generalist and specialist species (close or distantly related phylogenetically). We then focus our discussion on the importance of these laboratory observations for E. histolytica's life cycle, host infestation, and implications of these features of the amebas' natural history for human health (including mitigation of amebiasis).

Transmission traits of malaria parasites within the mosquito: Genetic variation, phenotypic plasticity, and consequences for control

Evaluating the risk of emergence and transmission of vector‐borne diseases requires knowledge of the genetic and environmental contributions to pathogen transmission traits. Compared to the significant effort devoted to understanding the biology of malaria transmission from vertebrate hosts to mosquito vectors, the strategies that malaria parasites have evolved to maximize transmission from vectors to vertebrate hosts have been largely overlooked. While determinants of infection success within the mosquito host have recently received attention, the causes of variability for other key transmission traits of malaria, namely the duration of parasite development and its virulence within the vector, as well as its ability to alter mosquito behavior, remain largely unknown. This important gap in our knowledge needs to be bridged in order to obtain an integrative view of the ecology and evolution of malaria transmission strategies. Associations between transmission traits also need to be characterized, as they trade‐offs and constraints could have important implications for understanding the evolution of parasite transmission. Finally, theoretical studies are required to evaluate how genetic and environmental influences on parasite transmission traits can shape malaria dynamics and evolution in response to disease control.

Emerging functions of helicases in regulation of stress survival in malaria parasite Plasmodium falciparum and their comparison with human host

The cellular response to various stresses is a universal phenomenon and involves a common set of stress responses that are largely independent of the type of stress. The response to stress is complex and cells can activate multiple signaling pathways that act in concert to influence cell fate and results in a specific cellular outcome, including reduction in macromolecular synthesis by shared pathways, cell cycle arrest, DNA repair, senescence and/or apoptosis. Whether cells mount a protective response or die depends to a great degree on the nature and duration of the stress and the particular cell type. Helicases play essential roles in DNA replication, repair, recombination, transcription and translation, and also participate in RNA metabolic processes including pre-mRNA processing, ribosome biogenesis, RNA turnover, export, translation, surveillance, storage and decay. In order to survive in the human host, the malaria parasite Plasmodium falciparum has to handle variety of stresses, which it encounters during the erythrocytic stages of its life cycle. In recent past the role of helicases in imparting various stress responses has emerged. Therefore in the present review an attempt has been made to highlight the emerging importance of helicases in stress responses in malaria parasite and their comparison with human host is also presented. It is noteworthy that PfDHX33 and PfDDX60 are larger in size and different in sequence as compared to the HsDHX33 and HsDDX60. The study suggests that helicases are multifunctional and play major role in helping the cells to combat various stresses.

The role of models in translating within-host dynamics to parasite evolution

Mathematical modelling provides an effective way to challenge conventional wisdom about parasite evolution and investigate why parasites ‘do what they do’ within the host. Models can reveal when intuition cannot explain observed patterns, when more complicated biology must be considered, and when experimental and statistical methods are likely to mislead. We describe how models of within-host infection dynamics can refine experimental design, and focus on the case study of malaria to highlight how integration between models and data can guide understanding of parasite fitness in three areas: (1) the adaptive significance of chronic infections; (2) the potential for tradeoffs between virulence and transmission; and (3) the implications of within-vector dynamics. We emphasize that models are often useful when they highlight unexpected patterns in parasite evolution, revealing instead why intuition yields the wrong answer and what combination of theory and data are needed to advance understanding.

A stochastic microscopic model for the dynamics of antigenic variation

We present a novel model that describes the within-host evolutionary dynamics of parasites undergoing antigenic variation. The approach uses a multi-type branching process with two types of entities defined according to their relationship with the immune system: clans of resistant parasitic cells (i.e. groups of cells sharing the same antigen not yet recognized by the immune system) that may become sensitive, and individual sensitive cells that can acquire a new resistance thus giving rise to the emergence of a new clan. The simplicity of the model allows analytical treatment to determine the subcritical and supercritical regimes in the space of parameters. By incorporating a density-dependent mechanism the model is able to capture additional relevant features observed in experimental data, such as the characteristic parasitemia waves. In summary our approach provides a new general framework to address the dynamics of antigenic variation which can be easily adapted to cope with broader and more complex situations.

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.

Injury and immune response: applying the danger theory to mosquitoes

The insect immune response can be activated by the recognition of both non-self and molecular by-products of tissue damage. Since pathogens and tissue damage usually arise at the same time during infection, the specific mechanisms of the immune response to microorganisms, and to tissue damage have not been unraveled. Consequently, some aspects of damage caused by microorganisms in vector-borne arthropods have been neglected. We herein reassess the Anopheles-Plasmodium interaction, incorporating Matzinger's danger/damage hypothesis and George Salt's injury assumptions. The invasive forms of the parasite cross the peritrophic matrix and midgut epithelia to reach the basal lamina and differentiate into an oocyst. The sporozoites produced in the oocyst are released into the hemolymph, and from there enter the salivary gland. During parasite development, wounds to midgut tissue and the basement membrane are produced. We describe the response of the different compartments where the parasite interacts with the mosquito. In the midgut, the response includes the expression of antimicrobial peptides, production of reactive oxygen species, and possible activation of midgut regenerative cells. In the basal membrane, wound repair mainly involves the production of molecules and the recruitment of hemocytes. We discuss the susceptibility to damage in tissues, and how the place and degree of damage may influence the differential response and the expression of damage associated molecular patterns (DAMPs). Knowledge about damage caused by parasites may lead to a deeper understanding of the relevance of tissue damage and the immune response it generates, as well as the origins and progression of infection in this insect-parasite interaction.

War and peace: social interactions in infections

One of the most striking facts about parasites and microbial pathogens that has emerged in the fields of social evolution and disease ecology in the past few decades is that these simple organisms have complex social lives, indulging in a variety of cooperative, communicative and coordinated behaviours. These organisms have provided elegant experimental tests of the importance of relatedness, kin discrimination, cooperation and competition, in driving the evolution of social strategies. Here, we briefly review the social behaviours of parasites and microbial pathogens, including their contributions to virulence, and outline how inclusive fitness theory has helped to explain their evolution. We then take a mechanistically inspired ‘bottom-up’ approach, discussing how key aspects of the ways in which parasites and pathogens exploit hosts, namely public goods, mobile elements, phenotypic plasticity, spatial structure and multi-species interactions, contribute to the emergent properties of virulence and transmission. We argue that unravelling the complexities of within-host ecology is interesting in its own right, and also needs to be better incorporated into theoretical evolution studies if social behaviours are to be understood and used to control the spread and severity of infectious diseases.

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.

Evolution of the animal apoptosis network

The number of available eukaryotic genomes has expanded to the point where we can evaluate the complete evolutionary history of many cellular processes. Such analyses for the apoptosis regulatory networks suggest that this network already existed in the ancestor of the entire animal kingdom (Metazoa) in a form more complex than in some popular animal model organisms. This supports the growing realization that regulatory networks do not necessarily evolve from simple to complex and that the relative simplicity of these networks in nematodes and insects does not represent an ancestral state, but is the result of secondary simplifications. Network evolution is not a process of monotonous increase in complexity, but a dynamic process that includes lineage-specific gene losses and expansions, protein domain reshuffling, and emergence/reemergence of similar protein architectures by parallel evolution. Studying the evolution of such networks is a challenging yet interesting subject for research and investigation, and such studies on the apoptosis networks provide us with interesting hints of how these networks, critical in so many human diseases, have developed.

Costs of crowding for the transmission of malaria parasites

The utility of using evolutionary and ecological frameworks to understand the dynamics of infectious diseases is gaining increasing recognition. However, integrating evolutionary ecology and infectious disease epidemiology is challenging because within-host dynamics can have counterintuitive consequences for between-host transmission, especially for vector-borne parasites. A major obstacle to linking within- and between-host processes is that the drivers of the relationships between the density, virulence, and fitness of parasites are poorly understood. By experimentally manipulating the intensity of rodent malaria (Plasmodium berghei) infections in Anopheles stephensi mosquitoes under different environmental conditions, we show that parasites experience substantial density-dependent fitness costs because crowding reduces both parasite proliferation and vector survival. We then use our data to predict how interactions between parasite density and vector environmental conditions shape within-vector processes and onward disease transmission. Our model predicts that density-dependent processes can have substantial and unexpected effects on the transmission potential of vector-borne disease, which should be considered in the development and evaluation of transmission-blocking interventions.

Apoptosis of Ascogregarina taiwanensis (Apicomplexa: Lecudinidae), which failed to migrate within its natural host

Sexual reproduction of Ascogregarina taiwanensis (Apicomplexa: Lecudinidae), a parasite specific to the mosquito Aedes albopictus, in Malpighian tubules is initiated by the entry of the trophotozoites developed in the midgut shortly after pupation (usually <5 h). However, only a low proportion of trophozoites are able to migrate; others end up dying. In this study, we demonstrated that those trophozoites that failed to migrate eventually died of apoptosis. Morphological changes such as shrinkage, chromatin aggregations and formation of blunt ridges on the surface were seen in moribund trophozoites. In addition, DNA fragmentation of trophozoites isolated from the midgut of pupae was demonstrated by the presence of DNA ladders, Annexin V staining and TUNEL assays. Detection of caspase-like activity suggests that apoptosis of those trophozoites may have occurred through a mechanism of an intrinsic or mitochondrial-mediated pathway. Although apoptosis has been observed in various protozoan species, it is not clear how apoptosis in single-celled organisms might result from evolution by natural selection. However, we speculate that apoptosis may regulate the parasite load of A. taiwanensis within its natural mosquito host, leading to an optimized state of the survival rate for both parasite and host.

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.

Apoptosis and apoptotic mimicry in Leishmania: an evolutionary perspective

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

Enhanced transmission of drug-resistant parasites to mosquitoes following drug treatment in rodent malaria

The evolution of drug resistant Plasmodium parasites is a major challenge to effective malaria control. In theory, competitive interactions between sensitive parasites and resistant parasites within infections are a major determinant of the rate at which parasite evolution undermines drug efficacy. Competitive suppression of resistant parasites in untreated hosts slows the spread of resistance; competitive release following treatment enhances it. Here we report that for the murine model Plasmodium chabaudi, co-infection with drug-sensitive parasites can prevent the transmission of initially rare resistant parasites to mosquitoes. Removal of drug-sensitive parasites following chemotherapy enabled resistant parasites to transmit to mosquitoes as successfully as sensitive parasites in the absence of treatment. We also show that the genetic composition of gametocyte populations in host venous blood accurately reflects the genetic composition of gametocytes taken up by mosquitoes. Our data demonstrate that, at least for this mouse model, aggressive chemotherapy leads to very effective transmission of highly resistant parasites that are present in an infection, the very parasites which undermine the long term efficacy of front-line drugs.

On Programmed Cell Death in Plasmodium falciparum: Status Quo

Conflicting arguments and results exist regarding the occurrence and phenotype of programmed cell death (PCD) in the malaria parasite Plasmodium falciparum. Inconsistencies relate mainly to the number and type of PCD markers assessed and the different methodologies used in the studies. In this paper, we provide a comprehensive overview of the current state of knowledge and empirical evidence for PCD in the intraerythrocytic stages of P. falciparum. We consider possible reasons for discrepancies in the data and offer suggestions towards more standardised investigation methods in this field. Furthermore, we present genomic evidence for PCD machinery in P. falciparum. We discuss the potential adaptive or nonadaptive role of PCD in the parasite life cycle and its possible exploitation in the development of novel drug targets. Lastly, we pose pertinent unanswered questions concerning the PCD phenomenon in P. falciparum to provide future direction.

The meaning of death: evolution and ecology of apoptosis in protozoan parasites

The discovery that an apoptosis-like, programmed cell death (PCD) occurs in a broad range of protozoan parasites offers novel therapeutic tools to treat some of the most serious infectious diseases of humans, companion animals, wildlife, and livestock. Whilst apoptosis is an essential part of normal development, maintenance, and defence in multicellular organisms, its occurrence in unicellular parasites appears counter-intuitive and has proved highly controversial: according to the Darwinian notion of “survival of the fittest”, parasites are expected to evolve strategies to maximise their proliferation, not death. The prevailing, and untested, opinion in the literature is that parasites employ apoptosis to “altruistically” self-regulate the intensity of infection in the host/vector. However, evolutionary theory tells us that at most, this can only be part of the explanation, and other non-mutually exclusive hypotheses must also be tested. Here, we explain the evolutionary concepts that can explain apoptosis in unicellular parasites, highlight the key questions, and outline the approaches required to resolve the controversy over whether parasites “commit suicide”. We highlight the need for integration of proximate and functional approaches into an evolutionary framework to understand apoptosis in unicellular parasites. Understanding how, when, and why parasites employ apoptosis is central to targeting this process with interventions that are sustainable in the face of parasite evolution.

Stilbene-chalcone hybrids: design, synthesis, and evaluation as a new class of antimalarial scaffolds that trigger cell death through stage specific apoptosis

Novel stilbene-chalcone (S-C) hybrids were synthesized via a sequential Claisen-Schmidt-Knoevenagel-Heck approach and evaluated for antiplasmodial activity in in vitro red cell culture using SYBR Green I assay. The most potent hybrid (11) showed IC(50) of 2.2, 1.4, and 6.4 μM against 3D7 (chloroquine sensitive), Indo, and Dd2 (chloroquine resistant) strains of Plasmodium falciparum, respectively. Interestingly, the respective individual stilbene (IC(50) > 100 μM), chalcone (IC(50) = 11.5 μM), or an equimolar mixture of stilbene and chalcone (IC(50) = 32.5 μM) were less potent than 11. Studies done using specific stage enriched cultures and parasite in continuous culture indicate that 11 and 18 spare the schizont but block the progression of the parasite life cycle at the ring or the trophozoite stages. Further, 11 and 18 caused chromatin condensation, DNA fragmentation, and loss of mitochondrial membrane potential in Plasmodium falciparum, thereby suggesting their ability to cause apoptosis in malaria parasite.

Evolution of apoptosis-like programmed cell death in unicellular protozoan parasites

Apoptosis-like programmed cell death (PCD) has recently been described in multiple taxa of unicellular protists, including the protozoan parasites Plasmodium, Trypanosoma and Leishmania. Apoptosis-like PCD in protozoan parasites shares a number of morphological features with programmed cell death in multicellular organisms. However, both the evolutionary explanations and mechanisms involved in parasite PCD are poorly understood. Explaining why unicellular organisms appear to undergo 'suicide' is a challenge for evolutionary biology and uncovering death executors and pathways is a challenge for molecular and cell biology. Bioinformatics has the potential to integrate these approaches by revealing homologies in the PCD machinery of diverse taxa and evaluating their evolutionary trajectories. As the molecular mechanisms of apoptosis in model organisms are well characterised, and recent data suggest similar mechanisms operate in protozoan parasites, key questions can now be addressed. These questions include: which elements of apoptosis machinery appear to be shared between protozoan parasites and multicellular taxa and, have these mechanisms arisen through convergent or divergent evolution? We use bioinformatics to address these questions and our analyses suggest that apoptosis mechanisms in protozoan parasites and other taxa have diverged during their evolution, that some apoptosis factors are shared across taxa whilst others have been replaced by proteins with similar biochemical activities.

Impact of protozoan cell death on parasite-host interactions and pathogenesis

PCD in protozoan parasites has emerged as a fascinating field of parasite biology. This not only relates to the underlying mechanisms and their evolutionary implications but also to the impact on the parasite-host interactions within mammalian hosts and arthropod vectors. During recent years, common functions of apoptosis and autophagy in protozoa and during parasitic infections have emerged. Here, we review how distinct cell death pathways in Trypanosoma, Leishmania, Plasmodium or Toxoplasma may contribute to regulation of parasite cell densities in vectors and mammalian hosts, to differentiation of parasites, to stress responses, and to modulation of the host immunity. The examples provided indicate crucial roles of PCD in parasite biology. The existence of PCD pathways in these organisms and the identification as being critical for parasite biology and parasite-host interactions could serve as a basis for developing new anti-parasitic drugs that take advantage of these pathways.

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.