Cell death and human intestinal protozoa: a brief overview

Protozoan programmed cell death or apoptosis is an important factor in the survival of the parasite and its pathogenicity. The most amazing aspect of protozoan cell death is in its molecular architecture. To date, protozoa lack most of the components of the highly complex cell death machinery studied in multicellular organisms. Hence the unique apoptotic machinery in protozoa can be exploited for the development of therapeutic drugs and diagnostic markers. This review focuses on human intestinal protozoa undergoing cell death and inducing or inhibiting host cell apoptosis. The first part of this review focuses on intestinal protozoa that undergo PCD under various stress conditions. The second part focuses on protozoa that induce or inhibit PCD in their host cell. Although these intestinal parasites differ in their mechanism of infection and intracellular localization, they may activate conserved cell death pathways within themselves and in the host cell. Understanding conserved cell death pathways in the intestinal protozoa and their host-parasite PCD relationship may lead to drug targets which can be used for a broad range of parasitic diseases.     

A proteomics screen implicates HSP83 and a small kinetoplastid calpain-related protein in drug resistance in Leishmania donovani clinical field isolates by modulating drug-induced programmed cell death

The therapeutic mainstay against the protozoan parasite Leishmania is still based on the antiquated pentavalent antimonials (Sb(V)), but resistance is increasing in several parts of the world. Resistance is now partly understood in laboratory isolates, but our understanding of resistance in field isolates is lagging behind. We describe here a comparative analysis of a genetically related pair of Sb(V)-sensitive and -resistant Leishmania donovani strains isolated from kala-azar patients. The resistant isolate exhibited cross-resistance to other unrelated Leishmania drugs including miltefosine and amphotericin B. A comparative proteomics screen has highlighted a number of proteins differentially expressed suggesting that programmed cell death (PCD) is modified in the resistant parasite. Indeed drug-induced PCD progression was altered in the Sb(V)-resistant strain as determined using early and late markers of apoptosis. Two proteins, the heat shock protein HSP83 and the small kinetoplastid calpain-related protein (SKCRP14.1) were shown to be intimately implicated in the drug-induced PCD phenotype. HSP83 increased drug resistance and reduced drug-mediated PCD activation by interfering with the mitochondrial membrane potential, whereas SKCRP14.1 promoted antimonial-induced PCD but protected against miltefosine-induced PCD. This study highlights the important role of PCD in drug susceptibility/resistance in the protozoan parasite Leishmania.     

Protozoan programmed cell death--insights from Blastocystis deathstyles

Programmed cell death (PCD) is an essential process in the growth and development of multicellular organisms. However, accumulating evidence indicates that unicellular eukaryotes can also undergo PCD with apoptosis-like features. The protozoan parasite Blastocystis hominis has been reported to exhibit both apoptotic and non-apoptotic features of PCD when exposed to a variety of stimuli. Recent observations of PCD pathways in Blastocystis suggest that this protozoan, as is the case with its multicellular counterparts, possesses complex cell-death mechanisms.     

Programmed cell death in Blastocystis hominis occurs independently of caspase and mitochondrial pathways

We demonstrated previously that a cytotoxic monoclonal antibody (MAb) 1D5 elicits a programmed cell death (PCD) response in Blastocystis hominis and showed that caspase-3-like protease influences but is not essential for PCD in MAb 1D5-treated B. hominis. We also showed that mitochondrial dysregulation played a role in cell death. In the current study, we further analyzed the signaling pathways involved in PCD mediated by MAb 1D5. B. hominis cells were treated with MAb 1D5 or control MAb 5, either with or without pretreatment with a pan-caspase inhibitor, zVAD.fmk, and/or a mitochondrial transition pore blocker, cyclosporine A (CA). Flow cytometric examination of cell size, mitochondrial membrane potential (delta psi(m)), caspase activation and in situ DNA fragmentation showed that zVAD.fmk and CA, used independently or in combination, failed to inhibit MAb 1D5-mediated PCD. Interestingly, cell exposure to either inhibitor resulted in partial inhibition of DNA fragmentation while combined exposure of cells to inhibitors abolished DNA fragmentation completely. This study sheds new light on the conserved nature of PCD pathways in parasitic protozoa and is also the first report describing caspase- and mitochondria-independent cell death pathways in a protozoan parasite.     

Programmed cell death in trypanosomatids: is it an altruistic mechanism for survival of the fittest?

The protozoan parasites Leishmania, Trypanosoma cruzi and Trypanosoma brucei show multiple features consistent with a form of programmed cell death (PCD). Despite some similarities with apoptosis of mammalian cells, PCD in trypanosomatid protozoans appears to be significantly different. In these unicellular organisms, PCD could represent an altruistic mechanism for the selection of cells, from the parasite population, that are fit to be transmitted to the next host. Alternatively, PCD could help in controlling the population of parasites in the host, thereby increasing host survival and favoring parasite transmission, as proposed by Seed and Wenk. Therefore, PCD in trypanosomatid parasites may represent a pathway involved both in survival and propagation of the species.     

Epidermal growth factor triggers an original, caspase-independent pituitary cell death with heterogeneous phenotype

Programmed cell death (PCD) is physiologically involved in the regulation of cell division and differentiation. It encompasses caspase-dependent mitochondrial and nonmitochondrial pathways. Additional caspase-independent pathways have been characterized in mitochondrial PCDs but remain hypothetical in nonmitochondrial PCDs. Epidermal growth factor (EGF) has been shown to inhibit division of pituitary somato-lactotrope cells occurring in parallel with EGF-mediated differentiation of these precursors into lactotrope cells. We show here that in somato-lactotrope pituitary cell line GH4C1, EGF triggers a PCD characterized by an apoptosis-like DNA fragmentation, insensitivity to broad-range caspase inhibitors, and absence of either cytochrome c or apoptosis-inducing factor release from mitochondria. Dying cells display loose chromatin clustering and numerous cytoplasmic vacuoles, a fraction of which are autophagic, thus conferring a heterogeneous phenotype to this PCD. Moreover, overexpression of cell death inhibitor Bcl-2 prevented not only the EGF-induced PCD but also its prodifferentiation effects, thus pointing to a mechanistic relationship existing between these two phenomena. Overall, the characterized differentiation-linked cell death represents an original form of caspase-independent PCD. The mechanisms underlying this PCD involve combinatorial engagement of discrete death effectors leading to a heterogeneous death phenotype that might be evolutionary related to PCD seen during the differentiation of some unicellular organisms.     

Programmed cell death pathways in cancer: a review of apoptosis,autophagy and programmed necrosis

Programmed cell death (PCD), referring to apoptosis, autophagy and programmed necrosis, is proposed to be death of a cell in any pathological format, when mediated by an intracellular program. These three forms of PCD may jointly decide the fate of cells of malignant neoplasms; apoptosis and programmed necrosis invariably contribute to cell death, whereas autophagy can play either pro‐survival or pro‐death roles. Recent bulk of accumulating evidence has contributed to a wealth of knowledge facilitating better understanding of cancer initiation and progression with the three distinctive types of cell death. To be able to decipher PCD signalling pathways may aid development of new targeted anti‐cancer therapeutic strategies. Thus in this review, we present a brief outline of apoptosis, autophagy and programmed necrosis pathways and apoptosis‐related microRNA regulation, in cancer. Taken together, understanding PCD and the complex interplay between apoptosis, autophagy and programmed necrosis may ultimately allow scientists and clinicians to harness the three types of PCD for discovery of further novel drug targets, in the future cancer treatment.     

PANoptosis in Viral Infection: The Missing Puzzle Piece in the Cell Death Field

In the past decade, emerging viral outbreaks like SARS-CoV-2, Zika and Ebola have presented major challenges to the global health system. Viruses are unique pathogens in that they fully rely on the host cell to complete their lifecycle and potentiate disease. Therefore, programmed cell death (PCD), a key component of the host innate immune response, is an effective strategy for the host cell to curb viral spread. The most well-established PCD pathways, pyroptosis, apoptosis and necroptosis, can be activated in response to viruses. Recently, extensive crosstalk between PCD pathways has been identified, and there is evidence that molecules from all three PCD pathways can be activated during virus infection. These findings have led to the emergence of the concept of PANoptosis, defined as an inflammatory PCD pathway regulated by the PANoptosome complex with key features of pyroptosis, apoptosis, and/or necroptosis that cannot be accounted for by any of these three PCD pathways alone. While PCD is important to eliminate infected cells, many viruses are equipped to hijack host PCD pathways to benefit their own propagation and subvert host defense, and PCD can also lead to the production of inflammatory cytokines and inflammation. Therefore, PANoptosis induced by viral infection contributes to either host defense or viral pathogenesis in context-specific ways. In this review, we will discuss the multi-faceted roles of PCD pathways in controlling viral infections.     

Lesion mimic mutants: keys for deciphering cell death and defense pathways in plants?

The identification of several lesion mimic mutants (LMM) that misregulate cell death constitutes a powerful tool to unravel programmed cell death (PCD) pathways in plants, particularly the hypersensitive response (HR), a form of PCD associated with resistance to pathogens. Recently, the characterization of novel LMM has enabled genes that might regulate cell death programmes to be identified as well as the dissection of defense signaling pathways and of crosstalk between multiple pathways in ways that might not be possible by studying the responses of wild-type plants to pathogens.     

Morphological classification of plant cell deaths

Programmed cell death (PCD) is an integral part of plant development and of responses to abiotic stress or pathogens. Although the morphology of plant PCD is, in some cases, well characterised and molecular mechanisms controlling plant PCD are beginning to emerge, there is still confusion about the classification of PCD in plants. Here we suggest a classification based on morphological criteria. According to this classification, the use of the term 'apoptosis' is not justified in plants, but at least two classes of PCD can be distinguished: vacuolar cell death and necrosis. During vacuolar cell death, the cell contents are removed by a combination of autophagy-like process and release of hydrolases from collapsed lytic vacuoles. Necrosis is characterised by early rupture of the plasma membrane, shrinkage of the protoplast and absence of vacuolar cell death features. Vacuolar cell death is common during tissue and organ formation and elimination, whereas necrosis is typically found under abiotic stress. Some examples of plant PCD cannot be ascribed to either major class and are therefore classified as separate modalities. These are PCD associated with the hypersensitive response to biotrophic pathogens, which can express features of both necrosis and vacuolar cell death, PCD in starchy cereal endosperm and during self-incompatibility. The present classification is not static, but will be subject to further revision, especially when specific biochemical pathways are better defined.     

细胞程序性死亡通路的研究进展

细胞程序性死亡(programmed cell death,PCD)一直被看做是细胞凋亡(apoptosis).随着细胞生物学研究的深入,新的细胞死亡途径逐渐被揭示出来,如胀亡、自噬、副凋亡等.这些通路有些是caspase依赖的,有些不依赖于caspase途径.在细胞程序性死亡过程中,各种通路不是单独起作用的,而是相互交联的,有彼此重叠的机制出现.目前,Clarke形态学分类法是得到大多数学者认可的细胞程序性死亡的分类方式.按照该分类法,可将PCD分为3大类,即:Ⅰ型细胞程序性死亡、Ⅱ型细胞程序性死亡和Ⅲ型细胞程序性死亡.     

Developmental programmed cell death in plants

Mechanisms of plant developmental programmed cell death (PCD) have been intensively studied in recent years. Most plant developmental PCD is triggered by plant hormones, and the 'death signal' may be transduced by hormonal signaling pathways. Although there are some fundamental differences in the regulation of developmental PCD in various eukaryotes of different kingdoms, hormonal control and death signal transduction via pleiotropic signaling pathways constitute a common framework. However, plants possess a unique process of PCD execution that depends on vacuolar lytic function. Comparisons of the developmental PCD mechanisms of plants and other organisms are providing important insights into the detailed characteristics of developmental PCD in plants.     

Programmed cell death in parasitic protozoans that lack mitochondria

In multicellular organisms and in all protozoans harbouring mitochondria, the pathways leading to programmed cell death (PCD) are localized in the mitochondria. Intriguingly, unicellular parasites devoid of mitochondria such as Trichomonas vaginalis and Giardia intestinalis undergo a form of cell death resembling apoptosis, the most frequent form of PCD. This reinforces the idea that PCD must have evolved before the evolution of multicellularity. Moreover, this leads to the hypothesis of an early emergence of death pathways in eukaryotes preceding mitochondrial endosymbiosis and brings into question the central role of mitochondria in PCD.     

Apoptotic-like regulation of programmed cell death in plants

In plants, apoptotic-like programmed cell death (PCD) can be distinguished from other forms of plant cell death by protoplast condensation that results in a morphologically distinct cell corpse. In addition, there is a central regulatory role for the mitochondria and the degradation of the cell and its contents by PCD associated proteases. These distinguishing features are shared with animal apoptosis as it is probable that plant and animal cell death programmes arose in a shared unicellular ancestor. However, animal and plant cell death pathways are not completely conserved. The cell death programmes may have been further modified after the divergence of plant and animal lineages leading to converged, or indeed unique, features of their respective cell death programmes. In this review we will examine the features of apoptotic-like PCD in plants and examine the probable conserved components such as mitochondrial regulation through the release of apoptogenic proteins from the mitochondrial intermembrane space, the possible conserved or converged features such as “caspase-like” molecules which drive cellular destruction and the emerging unique features of plant PCD such as chloroplast involvement in cell death regulation.     

Systems biology of yeast cell death

Programmed cell death (PCD) (including apoptosis) is an essential process, and many human diseases of high prevalence such as neurodegenerative diseases and cancer are associated with deregulations in the cell death pathways. Yeast Saccharomyces cerevisiae, a unicellular eukaryotic organism, shares with multicellular organisms (including humans) key components and regulators of the PCD machinery. In this article, we review the current state of knowledge about cell death networks, including the modeling approaches and experimental strategies commonly used to study yeast cell death. We argue that the systems biology approach will bring valuable contributions to our understanding of regulations and mechanisms of the complex cell death pathways.     

Pathological apoptosis in the developing brain

More than half of the initially-formed neurons are deleted in certain brain regions during normal development. This process, whereby cells are discretely removed without interfering with the further development of remaining cells, is called programmed cell death (PCD). The term apoptosis is used to describe certain morphological manifestations of PCD. Many of the effectors of this developmental cell death program are highly expressed in the developing brain, making it more susceptible to accidental activation of the death machinery, e.g. following hypoxia-ischemia or irradiation. Recent evidence suggests, however, that activation and regulation of cell death mechanisms under pathological conditions do not exactly mirror physiological, developmentally regulated PCD. It may be argued that the conditions after e.g. ischemia are not even compatible with the execution of PCD as we know it. Under pathological conditions cells are exposed to various stressors, including energy failure, oxidative stress and unbalanced ion fluxes. This results in parallel triggering and potential overshooting of several different cell death pathways, which then interact with one another and result in complex patterns of biochemical manifestations and cellular morphological features. These types of cell death are here called "pathological apoptosis," where classical hallmarks of PCD, like pyknosis, nuclear condensation and caspase-3 activation, are combined with non-PCD features of cell death. Here we review our current knowledge of the mechanisms involved, with special focus on the potential for therapeutic intervention tailored to the needs of the developing brain.     

Beyond apoptosis: nonapoptotic cell death in physiology and disease.

Apoptosis is a morphologically defined form of programmed cell death (PCD) that is mediated by the activation of members of the caspase family. Analysis of death-receptor signaling in lymphocytes has revealed that caspase-dependent signaling pathways are also linked to cell death by nonapoptotic mechanisms, indicating that apoptosis is not the only form of PCD. Under physiological and pathological conditions, cells demonstrate a high degree of flexibility in cell-death responses, as is reflected in the existence of a variety of mechanisms, including necrosis-like PCD, autophagy (or type II PCD), and accidental necrosis. In this review, we discuss recent data suggesting that canonical apoptotic pathways, including death-receptor signaling, control caspase-dependent and -independent cell-death pathways.     

Yeast programmed cell death: an intricate puzzle

Yeasts as eukaryotic microorganisms with simple, well known and tractable genetics, have long been powerful model systems for studying complex biological phenomena such as the cell cycle or vesicle fusion. Until recently, yeast has been assumed as a cellular 'clean room' to study the interactions and the mechanisms of action of mammalian apoptotic regulators. However, the finding of an endogenous programmed cell death (PCD) process in yeast with an apoptotic phenotype has turned yeast into an 'unclean' but even more powerful model for apoptosis research. Yeast cells appear to possess an endogenous apoptotic machinery including its own regulators and pathway(s). Such machinery may not exactly recapitulate that of mammalian systems but it represents a simple and valuable model which will assist in the future understanding of the complex connections between apoptotic and non-apoptotic mammalian PCD pathways. Following this line of thought and in order to validate and make the most of this promising cell death model, researchers must undoubtedly address the following issues: what are the crucial yeast PCD regulators? How do they play together? What are the cell death pathways shared by yeast and mammalian PCD? Solving these questions is currently the most pressing challenge for yeast cell death researchers.     

The Root Hair Assay Facilitates the Use of Genetic and Pharmacological Tools in Order to Dissect Multiple Signalling Pathways That Lead to Programmed Cell Death

The activation of programmed cell death (PCD) is often a result of complex signalling pathways whose relationship and intersection are not well understood. We recently described a PCD root hair assay and proposed that it could be used to rapidly screen genetic or pharmacological modulators of PCD. To further assess the applicability of the root hair assay for studying multiple signalling pathways leading to PCD activation we have investigated the crosstalk between salicylic acid, autophagy and apoptosis-like PCD (AL-PCD) in Arabidopsis thaliana. The root hair assay was used to determine rates of AL-PCD induced by a panel of cell death inducing treatments in wild type plants treated with chemical modulators of salicylic acid synthesis or autophagy, and in genetic lines defective in autophagy or salicylic acid signalling. The assay demonstrated that PCD induced by exogenous salicylic acid or fumonisin B1 displayed a requirement for salicylic acid signalling and was partially dependent on the salicylic acid signal transducer NPR1. Autophagy deficiency resulted in an increase in the rates of AL-PCD induced by salicylic acid and fumonisin B1, but not by gibberellic acid or abiotic stress. The phenylalanine ammonia lyase-dependent salicylic acid synthesis pathway contributed only to death induced by salicylic acid and fumonisin B1. 3-Methyladenine, which is commonly used as an inhibitor of autophagy, appeared to influence PCD induction in all treatments suggesting a possible secondary, non-autophagic, effect on a core component of the plant PCD pathway. The results suggest that salicylic acid signalling is negatively regulated by autophagy during salicylic acid and mycotoxin-induced AL-PCD. However, this crosstalk does not appear to be directly involved in PCD induced by gibberellic acid or abiotic stress. This study demonstrates that the root hair assay is an effective tool for relatively rapid investigation of complex signalling pathways leading to the activation of PCD.