Many ways to die: passive and active cell death styles.

In multicellular organisms, cells may undergo passive, pathological death in response to various environmental injuries, or actively decide to self-destroy in order to ensure proper physiological morphogenesis, preserve tissue homeostasis and eliminate abnormal cells. While the passive cell demise occurs in an accidental, violent and chaotic way, corresponding to "necrosis", the active auto-elimination, defined "programmed cell death" (PCD), is executed in planned modalities. Different PCD pathways have been described, such as apoptosis, autophagic death, para-apoptosis and programmed necrosis. However, death patterns may overlap or integrate, providing a variety of cellular responses to various circumstances or stimuli. The consequences for the whole organism of necrosis and PCD are quite different. In the case of classical necrosis, cytosolic constituents chaotically spill into extracellular space through damaged plasma membrane and provoke an inflammatory response, while in most PCDs the cellular components are safely isolated by membranes, and then consumed by adjacent parenchymal cells and/or resident phagocytes without inflammation. Thus, whereas the necrotic cell removal induces and amplifies pathological processes, the elimination of PCD debris may remain virtually unnoticed by the body. Otherwise, alterations of PCD controls may be involved in human diseases, such as developmental abnormalities, or neurodegenerative, autoimmune and neoplastic affections, whose treatment implies the complete understanding of cell suicide processes. In this review, the cellular death patterns are focused and their significance discussed.     

Approach to the early sporangial development in angiosperms considering meiosis control and cellular differentiation

 Applying current data on cell differentia- tion and meiosis control to the early sporangial development in angiosperms, a strict relationship between cell lineage and its differentiation fate is rejected. An evaluation of cytological features indicative of a meiotic (sporogenous) fate discards the sterilization phenomenon and introduces the premeiotic cellular differentiation (PCD) concept. The early sporangial development comprises 5 basic steps and 4 cellular stages, where PCD and meiosis extension and gradient are related to mechanisms of spore mother cell selection. Concepts here discussed explain the exceptions to the normal early sporangial development and allow a precise definition of archesporium and archespore. PCD and meiotic extension and gradient recover more information of the early sporangial development, distinguishing different developmental patterns leading to the same final result and retaining slight developmental differences. However, there are no early developmental characteristics distinctive of andro- or gynosporangia. Therefore, the heterosporic condition is not related to early developmental changes. Received January 5, 2001 Accepted August 29, 2001     

A proteomic analysis of Streptomyces coelicolor programmed cell death

Programmed cell death (PCD) is an active cellular suicide that occurs in eukaryotes and bacteria in response to both abiotic and biotic stresses. In contrast to eukaryotic apoptosis, little is known about the molecular machinery that regulates bacterial PCD. In a previous work, we described the existence of PCD phenomena in Streptomyces (Manteca et al., Res. Microbiol. 2006, 157, 143-152). In the present study, we performed a proteomic analysis of PCD in Streptomyces coelicolor, for which we developed a system to obtain dead and live cell-enriched samples. PCD in this filamentous bacterium is accompanied by the appearance of enzymes involved in the degradation of cellular macromolecules, regulatory proteins, and stress-induced proteins. We argue that some of these proteins have specific functions in the PCD pathway and putative roles for the identified proteins have been proposed. The increased amounts of several antioxidant proteins suggest oxidative stress as either the cause or consequence of the cell death.     

Polycomb Repressive Complex 2 and KRYPTONITE regulate pathogen-induced programmed cell death in Arabidopsis

The Polycomb Repressive Complex 2 (PRC2) is well-known for its role in controlling developmental transitions by suppressing the premature expression of key developmental regulators. Previous work revealed that PRC2 also controls the onset of senescence, a form of developmental programmed cell death (PCD) in plants. Whether the induction of PCD in response to stress is similarly suppressed by the PRC2 remained largely unknown. In this study, we explored whether PCD triggered in response to immunity- and disease-promoting pathogen effectors is associated with changes in the distribution of the PRC2-mediated histone H3 lysine 27 trimethylation (H3K27me3) modification in Arabidopsis thaliana. We furthermore tested the distribution of the heterochromatic histone mark H3K9me2, which is established, to a large extent, by the H3K9 methyltransferase KRYPTONITE, and occupies chromatin regions generally not targeted by PRC2. We report that effector-induced PCD caused major changes in the distribution of both repressive epigenetic modifications and that both modifications have a regulatory role and impact on the onset of PCD during pathogen infection. Our work highlights that the transition to pathogen-induced PCD is epigenetically controlled, revealing striking similarities to developmental PCD.

Changes in histone modifications mediated by Polycomb Repressive Complex 2 and KRYPTONITE play a regulatory role in pathogen-induced programmed cell death in Arabidopsis.     

Organisation and regulation of the cytoskeleton in plant programmed cell death

Programmed cell death (PCD) involves precise integration of cellular responses to extracellular and intracellular signals during both stress and development. In recent years much progress in our understanding of the components involved in PCD in plants has been made. Signalling to PCD results in major reorganisation of cellular components. The plant cytoskeleton is known to play a major role in cellular organisation, and reorganization and alterations in its dynamics is a well known consequence of signalling. There are considerable data that the plant cytoskeleton is reorganised in response to PCD, with remodelling of both microtubules and microfilaments taking place. In the majority of cases, the microtubule network depolymerises, whereas remodelling of microfilaments can follow two scenarios, either being depolymerised and then forming stable foci, or forming distinct bundles and then depolymerising. Evidence is accumulating that demonstrate that these cytoskeletal alterations are not just a consequence of signals mediating PCD, but that they also may have an active role in the initiation and regulation of PCD. Here we review key data from higher plant model systems on the roles of the actin filaments and microtubules during PCD and discuss proteins potentially implicated in regulating these alterations.     

Noncanonical cell death pathways act during Drosophila oogenesis

Programmed cell death (PCD) is a highly conserved process that occurs during development and in response to adverse conditions. In Drosophila, most PCDs require the genes within the H99 deficiency, the adaptor molecule Ark, and caspases. Here we investigate 10 cell death genes for their potential roles in two distinct types of PCD that occur in oogenesis: developmental nurse cell PCD and starvation-induced PCD. Most of the genes investigated were found to have little effect on late stage developmental PCD in oogenesis, although ark mutants showed a partial inhibition. Mid-stage starvation-induced germline PCD was found to be independent of the upstream activators and ark although it requires caspases, suggesting an apoptosome-independent mechanism of caspase activation in mid-oogenesis. These results indicate that novel pathways must control PCD in the ovary.     

Plant mitochondrial pathway leading to programmed cell death

Programmed cell death (PCD) is a finely tuned process of multicellular organisms. In higher plants, PCD regulates many developmental processes and the response of host plants to incompatible pathogens (hypersensitive response). Four types of PCD have been described in plants, mainly associated to vacuole rupture, that is followed by the appearance of the typical PCD hallmarks (i.e. nuclear DNA fragmentation and cell shrinkage). However, in some cases vacuole collapse is preceded by an early alteration of other subcellular organelles, such as mitochondria. In particular, the central role played by mitochondria in PCD has been largely recognised in animal cells. This review deals with the involvement of mitochondria in the manifestation of plant PCD, in comparison to that described in animal PCD. The main hallmark, connecting animal and plant PCD via mitochondria, is represented by the release of cytochrome c and possibly other chemicals such as nucleases, which may be accomplished by different mechanisms, involving both swelling and non-swelling of the organelles.     

Regulation of autophagy by polyphenolic compounds as a potential therapeutic strategy for cancer

Autophagy, a lysosomal degradation pathway for cellular constituents and organelles, is an adaptive and essential process required for cellular homeostasis. Although autophagy functions as a survival mechanism in response to cellular stressors such as nutrient or growth factor deprivation, it can also lead to a non-apoptotic form of programmed cell death (PCD) called autophagy-induced cell death or autophagy-associated cell death (type II PCD). Current evidence suggests that cell death through autophagy can be induced as an alternative to apoptosis (type I PCD), with therapeutic purpose in cancer cells that are resistant to apoptosis. Thus, modulating autophagy is of great interest in cancer research and therapy. Natural polyphenolic compounds that are present in our diet, such as rottlerin, genistein, quercetin, curcumin, and resveratrol, can trigger type II PCD via various mechanisms through the canonical (Beclin-1 dependent) and non-canonical (Beclin-1 independent) routes of autophagy. The capacity of these compounds to provide a means of cancer cell death that enhances the effects of standard therapies should be taken into consideration for designing novel therapeutic strategies. This review focuses on the autophagy- and cell death-inducing effects of these polyphenolic compounds in cancer.     

Antioxidant system in programmed cell death of sycamore (<Emphasis Type="Italic">Acer pseudoplatanus</Emphasis> L.) cultured cells

Reactive oxygen species (ROS) have pleiotropic effects in plants. ROS can lead to cellular damage and death or play key roles in control and regulation of biological processes, such as programmed cell death (PCD). This dual role of ROS, as toxic or signalling molecules, is possible because plant antioxidant system (AS) is able to achieve a tight control over ROS cellular levels, balancing properly their production and scavenging. AS response in plant PCD has been clearly described only in the hypersensitive response in incompatible plant–pathogen interactions and in the senescence process and has not been completely unravelled. In sycamore (Acer pseudoplatanus L.) cultured cells PCD can be induced by Fusicoccin (Fc), Tunicamycin (Tu), and Brefeldin A (Ba). These chemicals induce comparable PCD time course and extent, while H₂O₂ production is detectable only in Fc- and, to a lesser extent, in Ba-treated cells. In this paper the AS has been investigated during PCD of sycamore cells, measuring the effects of the three inducers on the cellular levels of non-enzymatic and enzymatic antioxidants. Results show that the AS behaviour is different in the PCD induced by the three chemicals. In Fc-treated cells AS is mainly devoted to decrease the concentration of toxic intracellular H₂O₂ levels. On the contrary, in cells treated with Tu and Ba, the cell redox state is shifted to a more reduced state and the enzymatic AS is partially down-regulated, allowing ROS to act as signalling molecules.     

Direct evidence of active and rapid nuclear degradation triggered by vacuole rupture during programmed cell death in Zinnia

Differentiation into a tracheary element (TE) is a typical example of programmed cell death (PCD) in the developmental processes of vascular plants. In the PCD process the TE degrades its cellular contents and becomes a hollow corpse that serves as a water conduct. Using a zinnia (Zinnia elegans) cell culture we obtained serial observations of single living cells undergoing TE PCD by confocal laser scanning microscopy. Vital staining was performed and the relative fluorescence intensity was measured, revealing that the tonoplast of the swollen vacuole in TEs loses selective permeability of fluorescein just before its physical rupture. After the vacuole ruptured the nucleus was degraded rapidly within 10 to 20 min. No prominent chromatin condensation or nuclear fragmentation occurred in this process. Nucleoids in chloroplasts were also degraded in a similar time course to that of the nucleus. Degradations did not occur in non-TEs forced to rupture the vacuole by probenecid treatment. These results demonstrate that TE differentiation involves a unique type of PCD in which active and rapid nuclear degradation is triggered by vacuole rupture.     

Apoptotic-like programmed cell death in plants

Programmed cell death (PCD) is now accepted as a fundamental cellular process in plants. It is involved in defence, development and response to stress, and our understanding of these processes would be greatly improved through a greater knowledge of the regulation of plant PCD. However, there may be several types of PCD that operate in plants, and PCD research findings can be confusing if they are not assigned to a specific type of PCD. The various cell-death mechanisms need therefore to be carefully described and defined. This review describes one of these plant cell death processes, namely the apoptotic-like PCD (AL-PCD). We begin by examining the hallmark 'apoptotic-like' features (protoplast condensation, DNA degradation) of the cell's destruction that are characteristic of AL-PCD, and include examples of AL-PCD during the plant life cycle. The review explores the possible cellular 'executioners' (caspase-like molecules; mitochondria; de novo protein synthesis) that are responsible for the hallmark features of the cellular destruction. Finally, senescence is used as a case study to show that a rigorous definition of cell-death processes in plant cells can help to resolve arguments that occur in the scientific literature regarding the timing and control of plant cell death.     

Programmed cell death of plant tracheary elements differentiating in vitro

Summary We used various microscopic and labeling techniques to examine events occurring during the programmed cell death (PCD) of plant tracheary elements (TEs) developing in vitro. TEs differentiating in vitro synthesize a secondary cell wall which is complex in composition and pattern at approximately 72 h after hormone manipulation. The timing of PCD events was established relative to this developmental marker. Cytoplasmic streaming continues throughout secondary wall synthesis, which takes 6 h to complete in a typical cell. Vital dye staining and ultrastructural analysis show that the vacuole and plasma membrane are intact during secondary cell wall synthesis, but the cytoplasm becomes less dense in appearance, most likely through the action of confined hydrolysis by small vacuoles which are seen throughout the cell at this time. The final, preeminent step of TE PCD is a rapid collapse of the vacuole occurring after completion of secondary cell wall synthesis. Vacuole collapse is an irreversible commitment to death which results in the immediate cessation of cytoplasmic streaming and leads to the complete degradation of cellular contents, which is probably accomplished by release of hydrolytic enzymes sequestered in the vacuole. This event represents a novel form of PCD. The degradation of nuclear DNA is detectable by TUNEL, an in situ labeling method, and appears to occur near or after vacuole collapse. Our observations indicate that the process of cellular degradation that produces the hollow TE cell corpse is an active and cell-autonomous process which is distinguishable morphologically and kinetically from necrosis. Although TE PCD does not resemble apoptosis morphologically, we describe the production of spherical protoplast fragments by cultured cells that resemble apoptotic bodies but which are not involved in TE PCD. We also present evidence that, unlike the hypersensitive response (HR), TE PCD does not involve an oxidative burst. While this evidence does not exclude a role for reactive oxygen intermediates in TE PCD, it does suggest TE PCD is mechanistically distinct from cell death during the HR.Abbreviations BA 6-benzylamino-purine - DAPI 4,6-diamidino-2-phenylindole diacetate - DCF 2,7-dichlorofluorescein diacetate - DPI diphenyleneiodonium - FDA fluorescein diacetate - HR hypersensitive response - NAA -naphthalene-acetic acid - PCD programmed cell death - ROI reactive oxygen intermediate - TE tracheary element - TUNEL TdT-mediated dUTP nick end labeling     

Redox regulation in plant programmed cell death

Programmed cell death (PCD) is a genetically controlled process described both in eukaryotic and prokaryotic organisms. Even if it is clear that PCD occurs in plants, in response to various developmental and environmental stimuli, the signalling pathways involved in the triggering of this cell suicide remain to be characterized. In this review, the main similarities and differences in the players involved in plant and animal PCD are outlined. Particular attention is paid to the role of reactive oxygen species (ROS) as key inducers of PCD in plants. The involvement of different kinds of ROS, different sites of ROS production, as well as their interaction with other molecules, is crucial in activating PCD in response to specific stimuli. Moreover, the importance is stressed on the balance between ROS production and scavenging, in various cell compartments, for the activation of specific steps in the signalling pathways triggering this cell suicide process. The review focuses on the complexity of the interplay between ROS and antioxidant molecules and enzymes in determining the most suitable redox environment required for the occurrence of different forms of PCD.     

Down the rabbit hole: Is necroptosis truly an innate response to infection?

Pathogenic microbes have evolved countless sophisticated mechanisms to subvert host immune responses and cause disease. Understanding evasion strategies employed by pathogens has led to numerous discoveries on specific host cell processes that are critical for controlling infection. Programmed cell death (PCD) is a key host defence to microbial infection, as well as being critical for organ development and cellular homeostasis in multicellular organisms. Much of our current understanding of PCD as a host response to infection has stemmed from the discovery and study of viral inhibitors of apoptosis, and more recently viral inhibition of the newly characterised from of PCD termed necroptosis, the mechanisms of which are still under intense investigation. Many bacterial pathogens also encode inhibitors of PCD, yet these discoveries are relatively more recent and thus the biological significance of such mechanisms is still under debate. In this viewpoint article, we will argue the concept that necroptosis is merely a “back‐up” mechanism in the event that apoptosis is inhibited, or whether it is a true host innate response to infection that has evolved in response to a growing arsenal of microbial evasion strategies.     

Anti-apoptosis and cell survival: A review

Type I programmed cell death (PCD) or apoptosis is critical for cellular self-destruction for a variety of processes such as development or the prevention of oncogenic transformation. Alternative forms, including type II (autophagy) and type III (necrotic) represent the other major types of PCD that also serve to trigger cell death. PCD must be tightly controlled since disregulated cell death is involved in the development of a large number of different pathologies. To counter the multitude of processes that are capable of triggering death, cells have devised a large number of cellular processes that serve to prevent inappropriate or premature PCD. These cell survival strategies involve a myriad of coordinated and systematic physiological and genetic changes that serve to ward off death. Here we will discuss the different strategies that are used to prevent cell death and focus on illustrating that although anti-apoptosis and cellular survival serve to counteract PCD, they are nevertheless mechanistically distinct from the processes that regulate cell death.     

Ultraviolet-C overexposure induces programmed cell death in Arabidopsis, which is mediated by caspase-like activities and which can be suppressed by caspase inhibitors, p35 and Defender against Apoptotic Death

Plants, animals, and several branches of unicellular eukaryotes use programmed cell death (PCD) for defense or developmental mechanisms. This argues for a common ancestral apoptotic system in eukaryotes. However, at the molecular level, very few regulatory proteins or protein domains have been identified as conserved across all eukaryotic PCD forms. A very important goal is to determine which molecular components may be used in the execution of PCD in plants, which have been conserved during evolution, and which are plant-specific. Using Arabidopsis thaliana, we have shown that UV radiation can induce apoptosis-like changes at the cellular level and that a UV experimental system is relevant to the study of PCD in plants. We report here that UV induction of PCD required light and that a protease cleaving the caspase substrate Asp-Glu-Val-Asp (DEVDase activity) was induced within 30 min and peaked at 1 h. This DEVDase appears to be related to animal caspases at the biochemical level, being insensitive to broad-range cysteine protease inhibitors. In addition, caspase-1 and caspase-3 inhibitors and the pan-caspase inhibitor p35 were able to suppress DNA fragmentation and cell death. These results suggest that a YVADase activity and an inducible DEVDase activity possibly mediate DNA fragmentation during plant PCD induced by UV overexposure. We also report that At-DAD1 and At-DAD2, the two A. thaliana homologs of Defender against Apoptotic Death-1, could suppress the onset of DNA fragmentation in A. thaliana, supporting an involvement of the endoplasmic reticulum in this form of the plant PCD pathway.     

Heat stress: an inducer of programmed cell death in Chlorella saccharophila

Programmed cell death (PCD) has been recognized as a fundamental cellular process conserved in metazoans, plants and yeast. However, the cellular mechanisms leading to PCD have not been fully elucidated in unicellular organisms. Evidence is presented that heat stress induces PCD in Chlorella saccharophila cells. Our results demonstrate that heat shock triggers a PCD pathway occurring with characteristics features such as chromatin condensation, DNA fragmentation, cell shrinkage and detachment of the plasma membrane from the cell wall, and suggest the presence of caspase 3-like activity. The caspase 3 inhibitor Ac-DEVD-CHO gave significant protection against heat shock-induced cell death. Moreover, a reduction in photosynthetic pigment contents associated with alteration of chloroplast morphology and a fairly rapid disappearance of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit and the light-harvesting complex of PSII have been observed. The timing of events in the signaling cascade associated with the C. saccharophila heat shock PCD response is discussed. Insights into this field may have general implications for understanding the pathway of cell death in unicellular green algae.     

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.     

Cell death in maize

Cell death occurs in plants as a part of normal development and as a response to toxins, pathogens and other environmental stimuli or insults. When cell death occurs as an orderly disassembly of the cell under the control of a genetically determined program, the process is referred to as programmed cell death (PCD). The PCD mechanisms of plants show many striking similarities to, but also intriguing differences from, those of animals. The extensive genetic, developmental and physiological characterizations of maize have made it an excellent system for the study of cell death. We describe the recent advances in the study of cell death in maize in light of what is known in plants and animals.