Autophagy in protists

Regulation of ATG8 membrane association by ATG4 in the parasitic protist Toxoplasma gondii

In the process of autophagy, the Atg8 protein is conjugated, through a ubiquitin-like system, to the lipid phosphatidylethanolamine (PE) to associate with the membrane of forming autophagosomes. There, it plays a crucial role in the genesis of these organelles and in autophagy in general. In most eukaryotes, the cysteine peptidase Atg4 processes the C terminus of cytosolic Atg8 to regulate its association with autophagosomal membranes and also delipidates Atg8 to release this protein from membranes. The parasitic protist Toxoplasma gondii contains a functional, yet apparently reduced, autophagic machinery. T. gondii Atg8 homolog, in addition to a cytosolic and occasionally autophagosomal localization, also localizes to the apicoplast, a nonphotosynthetic plastid bounded by four membranes. Our attempts to interfere with TgATG8 function showed that it appears to be essential for parasite multiplication inside its host cell. This protein also displays a peculiar C terminus that does not seem to necessitate processing prior to membrane association and yet an unusually large Toxoplasma homolog of ATG4 is predicted in the parasite genome. A TgATG4 conditional expression mutant that we have generated is severely affected in growth, and displays significant alterations at the organellar level, noticeably with a fragmentation of the mitochondrial network and a loss of the apicoplast. TgATG4-depleted parasites appear to be defective in the recycling of membrane-bound TgATG8. Overall, our data highlight a role for the TgATG8 conjugation pathway in maintaining the homeostasis of the parasite's organelles and suggest that Toxoplasma has evolved a specialized autophagic machinery with original regulation.

Environmentally regulated glycosome protein composition in the African trypanosome

Trypanosomes compartmentalize many metabolic enzymes in glycosomes, peroxisome-related microbodies that are essential to parasite survival. While it is understood that these dynamic organelles undergo profound changes in protein composition throughout life cycle differentiation, the adaptations that occur in response to changes in environmental conditions are less appreciated. We have adopted a fluorescent-organelle reporter system in procyclic Trypanosoma brucei by expressing a fluorescent protein (FP) fused to a glycosomal targeting sequence (peroxisome-targeting sequence 2 [PTS2]). In these cell lines, PTS2-FP is localized within import-competent glycosomes, and organelle composition can be analyzed by microscopy and flow cytometry. Using this reporter system, we have characterized parasite populations that differ in their glycosome composition. In glucose-rich medium, two parasite populations are observed; one population harbors glycosomes bearing the full repertoire of glycosome proteins, while the other parasite population contains glycosomes that lack the usual glycosome-resident proteins but do contain the glycosome membrane protein TbPEX11. Interestingly, these cells lack TbPEX13, a protein essential for the import of proteins into the glycosome. This bimodal distribution is lost in low-glucose medium. Furthermore, we have demonstrated that changes in environmental conditions trigger changes in glycosome protein composition. These findings demonstrate a level of procyclic glycosome diversity heretofore unappreciated and offer a system by which glycosome dynamics can be studied in live cells. This work adds to our growing understanding of how the regulation of glycosome composition relates to environmental sensing.

Apis mellifera venom induces different cell death pathways in Trypanosoma cruzi

Chagas disease chemotherapy is based on drugs that exhibit toxic effects and have limited efficacy, such as Benznidazole. Therefore, research into new chemotherapeutic agents from natural sources needs to be exploited. Apis mellifera venom consists of many biologically active molecules and has been reported to exhibit remarkable anti-cancer effects, often promoting an apoptosis-like death phenotype. This study demonstrates that A. mellifera venom can affect the growth, viability and ultrastructure of all Trypanosoma cruzi developmental forms, including intracellular amastigotes, at concentrations 15- to 100-fold lower than those required to cause toxic effects in mammalian cells. The ultrastructural changes induced by the venom in the different developmental forms led us to hypothesize the occurrence of different programmed cell death pathways. Autophagic cell death, characterized by the presence of autophagosomes-like organelles and a strong monodansyl cadaverine labelling, appears to be the main death mechanism in epimastigotes. In contrast, increased TUNEL staining, abnormal nuclear chromatin condensation and kDNA disorganization was observed in venom-treated trypomastigotes, suggesting cell death by an apoptotic mechanism. On the other hand, intracellular amastigotes presented a heterogeneous cell death phenotype profile, where apoptosis-like death seemed to be predominant. Our findings confirm the great potential of A. mellifera venom as a source for the development of new drugs for the treatment of neglected diseases such as Chagas disease.

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.

Genome of Acanthamoeba castellanii highlights extensive lateral gene transfer and early evolution of tyrosine kinase signaling

BACKGROUND

The Amoebozoa constitute one of the primary divisions of eukaryotes, encompassing taxa of both biomedical and evolutionary importance, yet its genomic diversity remains largely unsampled. Here we present an analysis of a whole genome assembly of Acanthamoeba castellanii (Ac) the first representative from a solitary free-living amoebozoan.

RESULTS

Ac encodes 15,455 compact intron-rich genes, a significant number of which are predicted to have arisen through inter-kingdom lateral gene transfer (LGT). A majority of the LGT candidates have undergone a substantial degree of intronization and Ac appears to have incorporated them into established transcriptional programs. Ac manifests a complex signaling and cell communication repertoire, including a complete tyrosine kinase signaling toolkit and a comparable diversity of predicted extracellular receptors to that found in the facultatively multicellular dictyostelids. An important environmental host of a diverse range of bacteria and viruses, Ac utilizes a diverse repertoire of predicted pattern recognition receptors, many with predicted orthologous functions in the innate immune systems of higher organisms.

CONCLUSIONS

Our analysis highlights the important role of LGT in the biology of Ac and in the diversification of microbial eukaryotes. The early evolution of a key signaling facility implicated in the evolution of metazoan multicellularity strongly argues for its emergence early in the Unikont lineage. Overall, the availability of an Ac genome should aid in deciphering the biology of the Amoebozoa and facilitate functional genomic studies in this important model organism and environmental host.

Structural characterization and inhibition of the Plasmodium Atg8-Atg3 interaction

The autophagy-related proteins are thought to serve multiple functions in Plasmodium and are considered essential to parasite survival and development. We have studied two key interacting proteins, Atg8 and Atg3, of the autophagy pathway in Plasmodium falciparum. These proteins are vital for the formation and elongation of the autophagosome and essential to the process of macroautophagy. Autophagy may be required for conversion of the sporozoite into erythrocytic-infective merozoites and may be crucial for other functions during asexual blood stages. Here we describe the identification of an Atg8 family interacting motif (AIM) in Plasmodium Atg3, which binds Plasmodium Atg8. We determined the co-crystal structure of PfAtg8 with a short Atg3¹⁰³⁻¹¹⁰ peptide, corresponding to this motif, to 2.2 Å resolution. Our in vitro interaction studies are in agreement with our X-ray crystal structure. Furthermore they suggest an important role for a unique Apicomplexan loop absent from human Atg8 homologues. Prevention of the protein-protein interaction of full length PfAtg8 with PfAtg3 was achieved at low micromolar concentrations with a small molecule, 1,2,3-trihydroxybenzene. Together our structural and interaction studies represent a starting point for future antimalarial drug discovery and design for this novel protein-protein interaction.

Autophagy-related Atg8 localizes to the apicoplast of the human malaria parasite Plasmodium falciparum

Autophagy is a membrane-mediated degradation process, which is governed by sequential functions of Atg proteins. Although Atg proteins are highly conserved in eukaryotes, protozoa possess only a partial set of Atg proteins. Nonetheless, almost all protozoa have the complete factors belonging to the Atg8 conjugation system, namely, Atg3, Atg4, Atg7, and Atg8. Here, we report the biochemical properties and subcellular localization of the Atg8 protein of the human malaria parasite Plasmodium falciparum (PfAtg8). PfAtg8 is expressed during intra-erythrocytic development and associates with membranes likely as a lipid-conjugated form. Fluorescence microscopy and immunoelectron microscopy show that PfAtg8 localizes to the apicoplast, a four membrane-bound non-photosynthetic plastid. Autophagosome-like structures are not observed in the erythrocytic stages. These data suggest that, although Plasmodium parasites have lost most Atg proteins during evolution, they use the Atg8 conjugation system for the unique organelle, the apicoplast.

Autophagy in trypanosomatids

Autophagy is a ubiquitous eukaryotic process that also occurs in trypanosomatid parasites, protist organisms belonging to the supergroup Excavata, distinct from the supergroup Opistokontha that includes mammals and fungi. Half of the known yeast and mammalian AuTophaGy (ATG) proteins were detected in trypanosomatids, although with low sequence conservation. Trypanosomatids such as Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. are responsible for serious tropical diseases in humans. The parasites are transmitted by insects and, consequently, have a complicated life cycle during which they undergo dramatic morphological and metabolic transformations to adapt to the different environments. Autophagy plays a major role during these transformations. Since inhibition of autophagy affects the transformation, survival and/or virulence of the parasites, the ATGs offer promise for development of drugs against tropical diseases. Furthermore, various trypanocidal drugs have been shown to trigger autophagy-like processes in the parasites. It is inferred that autophagy is used by the parasites in an-not always successful-attempt to cope with the stress caused by the toxic compounds.

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.

Autophagy across the eukaryotes: is S. cerevisiae the odd one out?

Autophagy is conserved throughout the eukaryotes and for many years, work in Saccharomyces cerevisiae has been at the forefront of autophagy research. However as our knowledge of the autophagic machinery has increased, differences between S. cerevisiae and mammalian cells have become apparent. Recent work in other organisms, such as the amoeba Dictyostelium discoideum, indicate an autophagic pathway much more similar to mammalian cells than S. cerevisiae, despite its earlier evolutionary divergence. S. cerevisiae therefore appear to have significantly specialized, and the autophagic pathway in mammals is much more ancient than previously appreciated, which has implications for how we interpret data from organisms throughout the eukaryotic tree.

Metamorphoses of malaria: the role of autophagy in parasite differentiation

Several protozoan parasites undergo a complex life cycle that alternates between an invertebrate vector and a vertebrate host. Adaptations to these different environments by the parasites are achieved by drastic changes in their morphology and metabolism. The malaria parasites must be transmitted to a mammal from a mosquito as part of their life cycle. Upon entering the mammalian host, extracellular malaria sporozoites reach the liver and invade hepatocytes, wherein they meet the challenge of becoming replication-competent schizonts. During the process of conversion, the sporozoite selectively discards organelles that are unnecessary for the parasite growth in liver cells. Among the organelles that are cleared from the sporozoite are the micronemes, abundant secretory vesicles that facilitate the adhesion of the parasite to hepatocytes. Organelles specialized in sporozoite motility and structure, such as the inner membrane complex (a major component of the motile parasite's cytoskeleton), are also eliminated from converting parasites. The high degree of sophistication of the metamorphosis that occurs at the onset of the liver-form development cascade suggests that the observed changes must be multifactorial. Among the mechanisms implicated in the elimination of sporozoite organelles, the degradative process called autophagy contributes to the remodelling of the parasite interior and the production of replicative liver forms. In a broader context, the importance of the role played by autophagy during the differentiation of protozoan parasites that cycle between insects and vertebrates is nowadays clearly emerging. An exciting prospect derived from these observations is that the parasite proteins involved in the autophagic process may represent new targets for drug development.

Autophagy: mechanism and physiological relevance 'brewed' from yeast studies

Autophagy is a highly conserved process of quality control occurring inside cells by which cytoplasmic material can be degraded and the products recycled for use as new building blocks or for energy production. The rapid progress and 'explosion' of knowledge concerning autophagic processes in mammals/humans that has occurred over the last 15 years was driven by fundamental studies in yeast, principally using Saccharomyces cerevisiae, leading to the identification and cloning of genes required for autophagy. This chapter reviews the role of yeast studies in understanding the molecular mechanisms of autophagic processes, focusing on aspects that are conserved in mammals/humans and how autophagy is increasingly implicated in the pathogenesis of disease and is required for development and differentiation.

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

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

Which roles for autophagy in Toxoplasma gondii and related apicomplexan parasites?

Autophagy is a life-sustaining process by which cytoplasmic components are sequestered in double-membrane vesicles called autophagosomes, and degraded after fusion with a lytic compartment. This process can be triggered under cellular stress conditions in order to recycle damaged organelles or provide nutrients to the cell, but may also be involved in cell remodelling during normal development. This catabolic process is conserved among most eukaryotes and characterisation of its molecular machinery has benefited greatly from functional genetic studies in yeast and mammalian models. Until recently, not much was known about the functions of autophagy in Apicomplexa, but recent data obtained in Toxoplasma have shed light on a very important role for this machinery, potentially at the crossroads between life and death decisions for the parasite. The possible roles for autophagy during the life cycles of other medically important apicomplexan parasites and the perspectives for discovering new drug targets in this pathway for combating these parasites are discussed in this review.

Autophagy is a cell death mechanism in Toxoplasma gondii

Nutrient sensing and the capacity to respond to starvation is tightly regulated as a means of cell survival. Among the features of the starvation response are induction of both translational repression and autophagy. Despite the fact that intracellular parasite like Toxoplasma gondii within a host cell predicted to be nutrient rich, they encode genes involved in both translational repression and autophagy. We therefore examined the consequence of starvation, a classic trigger of autophagy, on intracellular parasites. As expected, starvation results in the activation of the translational repression system as evidenced by elevation of phosphorylated TgIF2α (TgIF2α-P). Surprisingly, we also observe a rapid and selective fragmentation of the single parasite mitochondrion that leads irreversibly to parasite death. This profound effect was dependent primarily on the limitation of amino acids and involved signalling by the parasite TOR homologue. Notably, the effective blockade of mitochondrial fragmentation by the autophagy inhibitor 3-methyl adenine (3-MA) suggests an autophagic mechanism. In the absence of a documented apoptotic cascade in T. gondii, the data suggest that autophagy is the primary mechanism of programmed cell death in T. gondii and potentially other related parasites.

Guidelines for the use and interpretation of assays for monitoring autophagy

In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.

Molecular and cellular mechanisms involved in the Trypanosoma cruzi/host cell interplay

The protozoan parasite Trypanosoma cruzi has a complex biological cycle that involves vertebrate and invertebrate hosts. In mammals, the infective trypomastigote form of this parasite can invade several cell types by exploiting phagocytic-like or nonphagocytic mechanisms depending on the class of cell involved. Morphological studies showed that when trypomastigotes contact macrophages, they induce the formation of plasma membrane protrusions that differ from the canonical phagocytosis that occurs in the case of noninfective epimastigotes. In contrast, when trypomastigotes infect epithelial or muscle cells, the cell surface is minimally modified, suggesting the induction of a different class of process. Lysosomal-dependent or -independent T. cruzi invasion of host cells are two different models that describe the molecular and cellular events activated during parasite entry into nonphagocytic cells. In this context, we have previously shown that induction of autophagy in host cells before infection favors T. cruzi invasion. Furthermore, we demonstrate that autophagosomes and the autophagosomal protein LC3 are recruited to the T. cruzi entry sites and that the newly formed T. cruzi parasitophorous vacuole has characteristics of an autophagolysosome. This review summarizes the current knowledge of the molecular and cellular mechanisms of T. cruzi invasion in nonphagocytic cells. Based on our findings, we propose a new model in which T. cruzi takes advantage of the upregulation of autophagy during starvation to increase its successful colonization of host cells.

Trichomonas vaginalis pathobiology new insights from the genome sequence

The draft genome of the common sexually transmitted pathogen Trichomonas vaginalis encodes one of the largest known proteome with 60,000 candidate proteins. This provides parasitologists and molecular cell biologists alike with exciting, yet challenging, opportunities to unravel the molecular features of the parasite's cellular systems and potentially the molecular basis of its pathobiology. Here, recent investigations addressing selected aspects of the parasite's molecular cell biology are discussed, including surface and secreted virulent factors, membrane trafficking, cell signalling, the degradome, and the potential role of RNA interference in the regulation of gene expression.

Protease signalling: the cutting edge

Protease research has undergone a major expansion in the last decade, largely due to the extremely rapid development of new technologies, such as quantitative proteomics and in-vivo imaging, as well as an extensive use of in-vivo models. These have led to identification of physiological substrates and resulted in a paradigm shift from the concept of proteases as protein-degrading enzymes to proteases as key signalling molecules. However, we are still at the beginning of an understanding of protease signalling pathways. We have only identified a minor subset of true physiological substrates for a limited number of proteases, and their physiological regulation is still not well understood. Similarly, links with other signalling systems are not well established. Herein, we will highlight current challenges in protease research.

Autophagy protein Atg3 is essential for maintaining mitochondrial integrity and for normal intracellular development of Toxoplasma gondii tachyzoites

Autophagy is a cellular process that is highly conserved among eukaryotes and permits the degradation of cellular material. Autophagy is involved in multiple survival-promoting processes. It not only facilitates the maintenance of cell homeostasis by degrading long-lived proteins and damaged organelles, but it also plays a role in cell differentiation and cell development. Equally important is its function for survival in stress-related conditions such as recycling of proteins and organelles during nutrient starvation. Protozoan parasites have complex life cycles and face dramatically changing environmental conditions; whether autophagy represents a critical coping mechanism throughout these changes remains poorly documented. To investigate this in Toxoplasma gondii, we have used TgAtg8 as an autophagosome marker and showed that autophagy and the associated cellular machinery are present and functional in the parasite. In extracellular T. gondii tachyzoites, autophagosomes were induced in response to amino acid starvation, but they could also be observed in culture during the normal intracellular development of the parasites. Moreover, we generated a conditional T. gondii mutant lacking the orthologue of Atg3, a key autophagy protein. TgAtg3-depleted parasites were unable to regulate the conjugation of TgAtg8 to the autophagosomal membrane. The mutant parasites also exhibited a pronounced fragmentation of their mitochondrion and a drastic growth phenotype. Overall, our results show that TgAtg3-dependent autophagy might be regulating mitochondrial homeostasis during cell division and is essential for the normal development of T. gondii tachyzoites.

Autophagy is a catabolic process involved in maintaining cellular homeostasis in eukaryotic cells, while coping with their changing environmental conditions. Mechanistically, it is also a process of considerable complexity involving multiple protein factors and implying numerous protein-protein and protein-membrane interactions. The cellular material to be degraded by autophagy is contained in a membrane-bound compartment called the autophagosome. We have characterised the formation of autophagosomes in the protozoan parasite Toxoplasma gondii by following the relocalisation of autophagosome-bound TgAtg8. Thus, exploiting GFP-TgAtg8 as a marker, we showed that it is a process that is regulated and can be induced artificially by amino acid starvation. Autophagic vesicles were also observed in normally dividing intracellular parasites. Depleting Toxoplasma of the TgAtg3 autophagy protein led to an impairment of TgAtg8 conjugation to the autophagosomal membrane and, at the cellular level, to a fragmentation of the single mitochondrion of the parasite and to a severe growth arrest. We have thus found that TgAtg3-dependent autophagy is essential for normal intracellular development of T. gondii tachyzoites.

When, how and why glycolysis became compartmentalised in the Kinetoplastea. A new look at an ancient organelle

A characteristic, well-studied feature of the pathogenic protists belonging to the family Trypanosomatidae is the compartmentalisation of the major part of the glycolytic pathway in peroxisome-like organelles, hence designated glycosomes. Such organelles containing glycolytic enzymes appear to be present in all members of the Kinetoplastea studied, and have recently also been detected in a representative of the Diplonemida, but they are absent from the Euglenida. Glycosomes therefore probably originated in a free-living, common ancestor of the Kinetoplastea and Diplonemida. The initial sequestering of glycolytic enzymes inside peroxisomes may have been the result of a minor mistargeting of proteins, as generally observed in eukaryotic cells, followed by preservation and its further expansion due to the selective advantage of this specific form of metabolic compartmentalisation. This selective advantage may have been a largely increased metabolic flexibility, allowing the organisms to adapt more readily and efficiently to different environmental conditions. Further evolution of glycosomes involved, in different taxonomic lineages, the acquisition of additional enzymes and pathways - often participating in core metabolic processes - as well as the loss of others. The acquisitions may have been promoted by the sharing of cofactors and crucial metabolites between different pathways, thus coupling different redox processes and catabolic and anabolic pathways within the organelle. A notable loss from the Trypanosomatidae concerned a major part of the typical peroxisomal H(2)O(2)-linked metabolism. We propose that the compartmentalisation of major parts of the enzyme repertoire involved in energy, carbohydrate and lipid metabolism has contributed to the multiple development of parasitism, and its elaboration to complicated life cycles involving consecutive different hosts, in the protists of the Kinetoplastea clade.

A novel triazolic naphthofuranquinone induces autophagy in reservosomes and impairment of mitosis in Trypanosoma cruzi

Chagas' disease, caused by the protozoan Trypanosoma cruzi, represents a serious health problem in Latin America, and the available chemotherapy, which is based on 2 nitro-derivatives, is not satisfactory. In folk medicine, natural products including naphthoquinones have been employed for the treatment of different parasitic diseases. In the pursuit of alternative drugs for Chagas' disease, we investigated the mechanism of action of the triazolic naphthoquinone (TN; 2,2-dimethyl-3-(4-phenyl-1H-1,2,3-triazol-1-yl)-2,3-dihydronaphtho[1,2-b]furan-4,5-dione), which is the most active compound against T. cruzi trypomastigotes among a series of naphthofuranquinones. TN was active against the 3 parasite forms producing a dose-dependent inhibitory effect. In epimastigotes, TN induced reservosome disruption, flagellar blebbing, Golgi disorganization, the presence of cytosolic concentric membrane structures and abnormal multiflagellar parasites. The treatment also led to the appearance of well-developed endoplasmic reticulum profiles surrounding organelles that associated with an increase in monodansylcadaverine labelling, suggesting autophagy as part of the TN mechanism of action. Interestingly, no ultrastructural damage was detected in the mitochondria of naphthoquinone-treated epimastigotes. Flow cytometric analysis demonstrated an impairment of mitosis, an increase in ROS production and the maintenance of mitochondrial membrane potential. TN could be a good starting point in the investigation of a chemotherapeutic approach for the treatment of Chagas' disease.

Lysosomes and lysosomal cathepsins in cell death

Lysosomes are the key degradative compartments of the cell. Lysosomal cathepsins, which are enclosed in the lysosomes, help to maintain the homeostasis of the cell's metabolism by participating in the degradation of heterophagic and autophagic material. Following the targeted lysosomal membrane's destabilization, the cathepsins can be released into the cytosol and initiate the lysosomal pathway of apoptosis through the cleavage of Bid and the degradation of the anti-apoptotic Bcl-2 homologues. Cathepsins can also amplify the apoptotic signaling, when the lysosomal membranes are destabilized at a later stage of apoptosis, initiated by other stimuli. However, the functional integrity of the lysosomal compartment during apoptosis enables efficient autophagy, which can counteract apoptosis by providing the energy source and by disposing the damaged mitochondria, which generate the ROS. Impairing autophagy by disabling the lysosome function is being investigated as an adjuvant therapeutic approach to sensitize cells to apoptosis-inducing agents. Destabilization of the lysosomal membranes by the lysosomotropic detergents seems to be a promising strategy in this context as it would not only disable autophagy, but also promote apoptosis through the initiation of the lysosomal pathway. In contrast, the impaired autophagy and lysosomal degradation linked with the increased oxidative stress underlie degenerative changes in the aging neurons. This further suggests that lysosomes and lysosomal cathepsins have a dual role in cell death. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Plant NBR1 is a selective autophagy substrate and a functional hybrid of the mammalian autophagic adapters NBR1 and p62/SQSTM1

(Macro)autophagy encompasses both an unselective, bulk degradation of cytoplasmic contents as well as selective autophagy of damaged organelles, intracellular microbes, protein aggregates, cellular structures and specific soluble proteins. Selective autophagy is mediated by autophagic adapters, like p62/SQSTM1 and NBR1. p62 and NBR1 are themselves selective autophagy substrates, but they also act as cargo receptors for degradation of other substrates. Surprisingly, we found that homologs of NBR1 are distributed throughout the eukaryotic kingdom, while p62 is confined to the metazoans. As a representative of all organisms having only an NBR1 homolog we studied Arabidopsis thaliana NBR1 (AtNBR1) in more detail. AtNBR1 is more similar to mammalian NBR1 than to p62 in domain architecture and amino acid sequence. However, similar to p62, AtNBR1 homo-polymerizes via the PB1 domain. Hence, AtNBR1 has hybrid properties of mammalian NBR1 and p62. AtNBR1 has 2 UBA domains, but only the C-terminal UBA domain bound ubiquitin. AtNBR1 bound AtATG8 through a conserved LIR (LC3-interacting region) motif and required co-expression of AtATG8 or human GABARAPL2 to be recognized as an autophagic substrate in HeLa cells. To monitor the autophagic sequestration of AtNBR1 in Arabidopsis we made transgenic plants expressing AtNBR1 fused to a pH-sensitive fluorescent tag, a tandem fusion of the red, acid-insensitive mCherry and the acid-sensitive yellow fluorescent proteins. This strategy allowed us to show that AtNBR1 is an autophagy substrate degraded in the vacuole dependent on the polymerization property of the PB1 domain and of expression of AtATG7. A functional LIR was required for vacuolar import.

The peptidases of Trypanosoma cruzi: digestive enzymes, virulence factors, and mediators of autophagy and programmed cell death

Trypanosoma cruzi, the agent of the American Trypanosomiasis, Chagas disease, contains cysteine, serine, threonine, aspartyl and metallo peptidases. The most abundant among these enzymes is cruzipain, a cysteine proteinase expressed as a mixture of isoforms, some of them membrane-bound. The enzyme is an immunodominant antigen in human chronic Chagas disease and seems to be important in the host/parasite relationship. Inhibitors of cruzipain kill the parasite and cure infected mice, thus validating the enzyme as a very promising target for the development of new drugs against the disease. In addition, a 30kDa cathepsin B-like enzyme, two metacaspases and two autophagins have been described. Serine peptidases described in the parasite include oligopeptidase B, a member of the prolyl oligopeptidase family involved in Ca(2+)-signaling during mammalian cell invasion; a prolyl endopeptidase (Tc80), against which inhibitors are being developed, and a lysosomal serine carboxypeptidase. Metallopeptidases homologous to the gp63 of Leishmania spp. are present, as well as two metallocarboxypeptidases belonging to the M32 family, previously found only in prokaryotes. The proteasome has properties similar to those of other eukaryotes, and its inhibition by lactacystin blocks some differentiation steps in the life cycle of the parasite. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.