Salmonella-induced cell death: apoptosis, necrosis or programmed cell death?

Over the past several years, it has become apparent that enteropathogens activate cell death programs. For Salmonella and Shigella species, the induction of cell death is required for pathogenesis, and the mechanisms by which these bacteria induce cell death is an area of intense investigation. Although initial studies suggested that Salmonella induce cell death through an apoptotic pathway, recent studies demonstrate that cell death occurs through a unique caspase 1-dependent mechanism.     

Cell death: programmed, apoptosis, necrosis, or other?

There are at least two major types of active or physiological cell death. The most well-known form, apoptosis or Type I, involves early nuclear collapse, condensation of chromatin, generation of nucleosomal ladders, and cell fragmentation with little or no early alteration of lysosomes. It is most commonly seen in cells deriving from highly mitotic lines, and the cells are phagocytosed by neighboring cells or infiltrating macrophages. In metamorphosing or secretory cells, and under conditions where the majority of cells die, the bulk of the cytoplasm is consumed by expansion of the lysosomal system well before nuclear collapse is manifest. This form of cell death has been termed Type II cell death, and we revert to this terminology. The requirement for protein synthesis is more characteristic of Type II cell death in developmental situations than it is for Type I cell death. The variations seen force a reassessment of those aspects of physiological cell death that are truly universal, thereby focusing attention on the biology of the process. A better understanding of the biology and morphology of dying cells will help clarify the significance of the molecular and biochemical findings.     

Senescence and programmed cell death: substance or semantics?

The terms senescence and programmed cell death (PCD) have led to some confusion. Senescence as visibly observed in, for example, leaf yellowing and petal wilting, has often been taken to be synonymous with the programmed death of the constituent cells. PCD also obviously refers to cells, which show a programme leading to their death. Some scientists noted that leaf yellowing, if it has not gone too far, can be reversed. They suggested calling leaf yellowing, before the point of no return, 'senescence' and the process after it 'PCD'. However, this runs into several problems. It is counter to the historical definitions of senescence, both in animal and plant science, which stipulate that senescence is programmed and directly ends in death. It would also mean that only leaves and shoots show senescence, whereas several other plant parts, where reversal has not (yet) been shown, have no senescence, but only PCD. This conflicts with ordinary usage (as in root and flower senescence). Moreover, a programme can be reversible and therefore it is not counter to logic to regard the cell death programme as potentially reversible. In green leaf cells a decision to die, in a programmed way, has been taken, in principle, before the cells start to remobilize their contents (that is, before visible yellowing) and only rarely is this decision reversed. According to the arguments developed here there are no good reasons to separate a senescence phase and a subsequent PCD phase. Rather, it is asserted, senescence in cells is the same as PCD and the two are fully synchronous.     

Apoptosis or programmed cell death?

Although there are different ways in which cells may die, it is now thought that in a developmental context cells are induced to positively commit suicide whilst in a homeostatic context the absence of certain survival factors may provide the impetus for suicide. There appears to be some variation in the morphology and indeed the biochemistry of these suicide pathways; some treading the path of "apoptosis", others following a more generalized pathway to deletion, but both usually being genetically and synthetically motivated. There is some evidence that certain symptoms of "apoptosis" such as endonuclease activation can be spuriously induced without engaging a genetic cascade, however, presumably true apoptosis and programmed cell death must be genetically mediated. It is also becoming clear that mitosis and apoptosis are toggled or linked in some way and that the balance achieved depends on signals received from appropriate growth or survival factors.     

Can programmed cell death be induced in post-ejaculatory bull and stallion spermatozoa?

Apoptosis is common during spermatogenesis. Here, it was tested whether apoptosis could be induced in sperm after ejaculation. There were several lines of evidence to indicate that sperm are resistant to induction of apoptosis. First, incubation of bull sperm at temperatures characteristic of normothermia (38.5 °C) or heat shock (40 and 41 °C) for 4 h did not increase the proportion of sperm positive for the TUNEL reaction. There was also no reduction in mitochondrial polarity caused by exposure to 40 or 41 °C. Incubation at 38.5 °C (least-squares mean ± SEM = 4.0 ± 1.4%), 40 °C (6.2 ± 1.4%), and 41 °C (7.0 ± 1.4%) for 24 h did increase the proportion of sperm that were TUNEL positive slightly as compared to non-incubated control sperm (1.0 ± 1.4%). However, the increase in TUNEL labeling was not affected by incubation temperature and occurred even in the presence of the group II caspase inhibitor, z-DEVD-fmk. In addition, exposure of bull sperm to carbonyl cyanide 3-chlorophenylhydrazone (CCCP), which depolarizes mitochondrial membranes, did not increase TUNEL labeling. Stallion sperm were also resistant to increased TUNEL labeling in response to incubation at 41 °C for 4 h or exposure to CCCP. Western blotting was performed to determine whether failure of induction of apoptosis was due to aberrant caspase activation. Procaspase-9 was detected in bull sperm, but cleavage to caspase-9 was not induced by short-term aging at 38.5, 40, or 41 °C, or exposure to CCCP. Procaspase-3 was not detected in bull spermatozoa. In conclusion, post-ejaculatory bull and stallion sperm were resistant to induction of apoptosis; this resistance, at least in bulls, was due to refractoriness of mitochondria to heat shock-induced depolarization, lack of activation of procaspase-9, and an absence of procaspase-3.     

Calcium signalling and pancreatic cell death: apoptosis or necrosis?

Secretagogues, such as cholecystokinin and acetylcholine, utilise a variety of second messengers (inositol trisphosphate, cADPR and nicotinic acid adenine dinucleotide phosphate) to induce specific oscillatory patterns of calcium (Ca(2+)) signals in pancreatic acinar cells. These are tightly controlled in a spatiotemporal manner, and are coupled to mitochondrial metabolism necessary to fuel secretion. When Ca(2+) homeostasis is disrupted by known precipitants of acute pancreatitis, for example, hyperstimulation or non-oxidative ethanol metabolites, Ca(2+) stores (endoplasmic reticulum and acidic pool) become depleted and sustained cytosolic [Ca(2+)] elevations replace transient signals, leading to severe consequences. Sustained mitochondrial depolarisation, possibly via opening of the mitochondrial permeability transition pore (MPTP), elicits cellular ATP depletion that paralyses energy-dependent Ca(2+) pumps causing cytosolic Ca(2+) overload, while digestive enzymes are activated prematurely within the cell; Ca(2+)-dependent cellular necrosis ensues. However, when stress to the acinar cell is milder, for example, by application of the oxidant menadione, release of Ca(2+) from stores leads to oscillatory global waves, associated with partial mitochondrial depolarisation and transient MPTP opening; apoptotic cell death is promoted via the intrinsic pathway, when associated with generation of reactive oxygen species. Apoptosis, induced by menadione or bile acids, is potentiated by inhibition of an endogenous detoxifying enzyme NAD(P)H:quinone oxidoreductase 1 (NQO1), suggesting its importance as a defence mechanism that may influence cell fate.     

Nitric oxide: promoter or suppressor of programmed cell death?

Nitric oxide (NO) is a short-lived gaseous free radical that predominantly functions as a messenger and effector molecule. It affects a variety of physiological processes, including programmed cell death (PCD) through cyclic guanosine monophosphate (cGMP)-dependent and-independent pathways. In this field, dominant discoveries are the diverse apoptosis networks in mammalian cells, which involve signals primarily via death receptors (extrinsic pathway) or the mitochondria (intrinsic pathway) that recruit caspases as effector molecules. In plants, PCD shares some similarities with animal cells, but NO is involved in PCD induction via interacting with pathways of phytohormones. NO has both promoting and suppressing effects on cell death, depending on a variety of factors, such as cell type, cellular redox status, and the flux and dose of local NO. In this article, we focus on how NO regulates the apoptotic signal cascade through protein S-nitrosylation and review the recent progress on mechanisms of PCD in both mammalian and plant cells.     

Does the plant mitochondrion integrate cellular stress and regulate programmed cell death?

Research on programmed cell death in plants is providing insight into the primordial mechanism of programmed cell death in all eukaryotes. Much of the attention in studies on animal programmed cell death has focused on determining the importance of signal proteases termed caspases. However, it has recently been shown that cell death can still occur even when the caspase cascade is blocked, revealing that there is an underlying oncotic default pathway. Many programmed plant cell deaths also appear to be oncotic. Shared features of plant and animal programmed cell death can be used to deduce the primordial components of eukaryotic programmed cell death. From this perspective, we must ask whether the mitochondrion is a common factor that can serve in plant and animal cell death as a stress sensor and as a dispatcher of programmed cell death.     

Searching the point of no return in Helicobacter pylori life: necrosis and/or programmed death?

AIMS: Ultrastructural and molecular studies to support the hypothesis of programmed cell death in Helicobacter pylori were conducted. METHODS AND RESULTS: Evidence of programmed death in H. pylori is provided through electron microscopic detection and cytochemical labelling of electrondense bodies (EDB), containing packaged DNA in coccoid cells, resembling micronuclei of apoptotic eukaryotic cells. This morphological evidence is also supported by DNA cleavage in homogeneous fragments of about 100 base pairs. Programmed cell death was observed in H. pylori cultures at 37 degrees C, with a maximum of 37.5% of EDB coccoid cells after 7 days. The non-permissive temperature of 4 degrees C anticipated this process, with 40% of EDB coccoid forms within 3 days, and it remained substantially unaffected during the observation time of 14 days. CONCLUSION: In these experiments, deprivation of nutrients and a non-permissive temperature acted as a powerful trigger for programmed cell death. SIGNIFICANCE AND IMPACT OF THE STUDY: Helicobacter pylori bacterial populations, under stressing stimuli, can respond with programmed cell suicide as a means of species preservation.     

Virus-induced dysregulation of programmed immunocompetent cell death: pathology or adaptation?

The review summarizes information from recent literature and results of the authors' own investigations concerning dysbalance of programmed cell death in establishment of a long-term virus persistense. The article discusses molecular mechanisms of apoptosis modulation of immune cellls by persistent viruses.     

Glutathione half-cell reduction potential: A universal stress marker and modulator of programmed cell death?

The elucidation of factors that contribute to cell viability loss is presently compromised by the lack of a universal measure that quantifies “stress.” We have investigated mechanisms of viability loss in plant seeds to find a reliable marker of stress response. Oxidative damage has previously been correlated with degenerative processes and death, but how exactly this contributes to viability loss is unknown. We show in four species subjected to ageing or desiccation that seed viability decreased by 50% when the half-cell reduction potential of glutathione (E_GSSG/2GSH), a major cellular antioxidant and redox buffer, increased to −180 to −160 mV. We then conducted a metaanalysis of data representative of 13 plant and fungal orders to show that plant stress generally becomes lethal when E_GSSG/2GSH exceeds −160 mV. We put forward that this change in E_GSSG/2GSH is part of the signaling cascade that initiates programmed cell death (PCD), finally causing internucleosomal DNA fragmentation in the final, or execution phase, of PCD. E_GSSG/2GSH is therefore a universal marker of plant cell viability and allows us to predict whether a seed will live, germinate, and produce a new plant, or if it will die.     

Catalase and alternative oxidase cooperatively regulate programmed cell death induced by β-glucan elicitor in potato suspension cultures

In potato (Solanum tuberosum L.) suspension cells, the expression of the gene encoding alternative oxidase (AOX) and H₂O₂ accumulation were induced by treatment with -glucan elicitor. The inhibition of catalase activity enhanced both AOX mRNA expression and the production of H₂O₂, whereas the ascorbate peroxidase inhibitor did not have any effect on these responses. Simultaneous inhibition of catalase and AOX activities in elicited cells dramatically increased H₂O₂ accumulation, leading to the disruption of mitochondrial membrane potential (_m) and programmed cell death (PCD). The results demonstrate, for the first time, that not only AOX but also catalase plays a central role in the suppression of mitochondrial _m breakdown and PCD induced by -glucan elicitor.     

Cell death induced by ozone stress in the leaves of Populus deltoides × maximowiczii

When exposed to an acute ozone stress, cell death occurred in leaves of the O₃ sensitive Populus deltoides × maximowiczii clone Eridano. After treatment (5 h fumigation and 24 h recovery), the damaged areas covered more than 50 % of the leaf surface. At cellular level, these lesions were preceded by some apoptotic hallmarks and by biochemical and physiological alterations evoked by the apoplastic O₃ dissociation. The cell death pattern was highly localized and involved an increase of membrane permeability, externalization of phosphatidylserine, DNA fragmentation, callose accumulation, polyphenol production, and a biphasic oxidative burst accompanied by NO overproduction. These results indicate a process of programmed cell death that could have the biological significance of limiting the spreading the oxidative burst triggered by ozone dissociation in apoplastic environment. Moreover, materials derived from cell dismantling could be remobilized toward developing structures which can conclude their ontogenetic program after the stress.     

Ultrastructural lesions induced by neptunium-237: apoptosis or necrosis?

In this study, we are concerned with the 237 isotope of neptunium (237Np), which is a by-product of uranium in nuclear reactors. To study ultrastructural lesions induced by this element, a group of rats were injected with a solution of 237Np-nitrate once a day for 14 weeks. Lesions observed in liver and kidney are described using electron microscopy. Ultrastructural alterations of cellular membranes and intracellular organelles demonstrated the existence of neptunium toxicity. This toxicity was characterized by various lesions, such as cytoplasmic clarification, disappearance of mitochondrial cristae, swollen mitochondria, abnormal condensation of nuclear chromatin, and nuclear fragmentations. This study demonstrated the probable induction of apoptosis by neptunium both in liver and kidneys.     

Necrosis: a specific form of programmed cell death?

For a long time necrosis was considered as an alternative to programmed cell death, apoptosis. Indeed, necrosis has distinct morphological features and it is accompanied by rapid permeabilization of plasma membrane. However, recent data indicate that, in contrast to necrosis caused by very extreme conditions, there are many examples when this form of cell death may be a normal physiological and regulated (programmed) event. Various stimuli (e.g., cytokines, ischemia, heat, irradiation, pathogens) can cause both apoptosis and necrosis in the same cell population. Furthermore, signaling pathways, such as death receptors, kinase cascades, and mitochondria, participate in both processes, and by modulating these pathways, it is possible to switch between apoptosis and necrosis. Moreover, antiapoptotic mechanisms (e.g., Bcl-2/Bcl-x proteins, heat shock proteins) are equally effective in protection against apoptosis and necrosis. Therefore, necrosis, along with apoptosis, appears to be a specific form of execution phase of programmed cell death, and there are several examples of necrosis during embryogenesis, a normal tissue renewal, and immune response. However, the consequences of necrotic and apoptotic cell death for a whole organism are quite different. In the case of necrosis, cytosolic constituents that spill into extracellular space through damaged plasma membrane may provoke inflammatory response; during apoptosis these products are safely isolated by membranes and then are consumed by macrophages. The inflammatory response caused by necrosis, however, may have obvious adaptive significance (i.e., emergence of a strong immune response) under some pathological conditions (such as cancer and infection). On the other hand, disturbance of a fine balance between necrosis and apoptosis may be a key element in development of some diseases.